3D and 2D metal halide perovskites (MHPs) and perovskite derivatives have provoked a substantial revolution in the field of photovoltaics and optoelectronics because of their superior optical characteristics, physical properties’ tunability, ease of fabrication, and low cost. In very recent times, MHPs have attracted a significant interest for their possible use in various photocatalytic applications. The suitability of MHPs in relevant solar-driven reactions comes essentially from their highly tunable and narrow band gap, long carrier lifetimes, and high mobilities, together with a good defect tolerance. In addition, their band alignment, relative to the potentials of common redox half-reactions, indicates the thermodynamic suitability of these materials to effectively run reduction reactions (H₂ generation, CO₂ reduction) and even oxidation reactions for MHPs with higher band gap values (e.g., chloride-based).¹ In this presentation, we will provide an overview of the research activity currently running in our research group aiming at exploiting the full potential of MHPs for solar fuel production. In particular, we will show how a rational materials chemistry engineering can be applied to modulate the photocatalytic properties for the intended solar-drive reaction. In addition, strategies to boost the activity through heterojunction design will be presented. Finally, through a combined experimental and computational modelling work details about the underlying mechanisms for hydrogen photogeneration and nitrogen fixation will be presented.^2–4

Mono- or di-ammonium cations are commonly used to enhance the performance and stability of perovskite solar cells (PSCs) via surface defect passivation.^1,2 However, their effectiveness is still limited by the little understanding of the structure-property-performance relationship of the capping layer/3D perovskite stack.³ This work explores how the molecular geometry of diamine spacers affects the structure, properties, and performances of low dimensional (LD) capping layers on top of 3D perovskites, and their impact in solar cell devices. Two diamine spacers with similar chemical composition but different molecular geometry are tested: 4,4′-Dithiodianiline (2S) and 4,4’-Ethylenedianiline (ET). In 2S, the two amine groups are spatially close owing to a torsion in the backbone of the molecule. Instead, in ET the amine groups are at the maximum distance. The torsion allows 2S to bind to neighboring vacancy sites at the surface of the perovskite lattice, enhancing its passivation capabilities with respect to ET. The 2S spacer forms a 2D metal halide phase at the perovskite surface, which offers better charge extraction properties than the 1D phase induced by ET spacer. In solar cells incorporating 2S, these properties result in a power conversion efficiency (PCE) of 20.72%, improved from the 18.36% PCE of the reference. The ET spacer lowers the PCE to 15.67% due to less effective interaction with defect sites and lower charge extraction efficacy. Our results suggest that the double amine binding by the 2S spacer stabilizes the performance of the solar cells, enabling almost no loss of efficiency after 1000 hours under constant illumination in inert atmosphere.

Lead halide perovskite nanocrystals (LHP NCs) exhibit instability due to the dynamic and labile nature of both their inorganic core and the organic-inorganic interface, adversely impacting their optical and electronic properties [1]. The quest for novel capping ligands has not stopped, rather contrary [2], given the ever-expanding expectations for LHP NCs' deployment as classical and quantum light sources [3, 4]. We hypothesized that the facile molecular engineering of sulfonium salts as X-type ligands could enable highly customized surface chemistries for LHP NCs. Molecular dynamics simulations indicated that sulfonium ligands with diverse tail and headgroup structures exhibit equal or greater affinity to CsPbBr3 surfaces compared to their broadly studied ammonium counterparts. CsPbBr3 NCs capped with sulfonium bromides exhibit photoluminescence quantum yields exceeding 90% in colloids and enhanced durability in the typical purification processes. The compactness of the headgroup and tail branching significantly govern the long-term colloidal stability and resilience towards dilution and concentration. Further molecular engineering of sulfonium ligands allowed venturing into more demanding MAPbBr3 and FAPbBr3 NCs (MA, methylammonium; FA, formamidinium).

Metal halide perovskite nanocrystals (NCs) have emerged as a promising material for various applications due to their unique optoelectronic properties. Traditional fabrication methods for these NCs rely on colloidal chemistry, which can be time-consuming and may require complex purification steps. In this presentation, we will discuss the use of ultrafast laser processing as a versatile tool for the controlled modification of metal halide perovskite NCs. This technique offers several advantages, including rapid processing times, control over NC morphology, and the ability to conjugate NCs with 2D materials.

We will present our work on the laser-induced morphological and structural changes of cesium lead bromide nanocrystals. By varying the laser fluence and wavelength, we have achieved different transformations in these NCs, including exfoliation, fragmentation, and oriented attachment leading to the formation of nanoplatelets and nanosheets. These transformations are accompanied by partial or complete anion exchange, which can be controlled by the choice of solvent.

Furthermore, we will discuss our findings on the laser-assisted fabrication of metal halide perovskite-2D nanoconjugates. By irradiating a solution containing both perovskite NCs and graphene-based materials with femtosecond laser pulses, we have successfully decorated the 2D flakes with perovskite NCs without affecting their primary morphology. The density of anchored NCs can be finely tuned by adjusting the number of irradiation pulses, allowing for precise control over the properties of the resulting nanoconjugates.

Finally, we will present our latest research on the application of laser-processed perovskite-rGO conjugates as electrodes in Zn-ion capacitors. The laser-induced conjugation method enables the uniform distribution of perovskite NCs on reduced graphene oxide (rGO) sheets, resulting in enhanced electrochemical performance. The resulting electrodes exhibit high specific capacitance and excellent stability, demonstrating the potential of this approach for energy storage applications.

Overall, our work highlights the versatility of ultrafast laser processing as a powerful tool for the controlled synthesis and modification of metal halide perovskite nanocrystals. This technique offers a rapid and efficient route to fabricate perovskite-2D nanoconjugates with tailored properties, opening up new possibilities for their application in various fields, including optoelectronics, energy conversion, and energy storage.

The optimization of the perovskite solar cell has been considered one of the important technological challenges. Since the device can be developed through various combinations of materials and experimental conditions, efficient use of computational techniques for device optimization is highly demanded to overcome the problem of the so-called "combinatorial explosion" in experimental studies. In our laboratory, we are trying to collectively apply various computational tools in computer simulations and machine learning techniques. In this presentation, we will report on our recent trial to predict device characteristics of perovskite solar cells using machine learning and computer simulations. One challenge is predicting device performances such as photo conversion efficiency (PCE) from experimentally obtained cross-sectional scanning electron microscope (SEM) images. By applying convolutional neural network algorithms, we could have achieved moderate success in so-called image regression, in which SEM images are used as input to predict PCE. Another challenge is to discover an optimal material and device configuration through device simulation and machine learning (regression) modeling. In the presentation, we will discuss our computational method and tools with some predictive results toward optimizing the perovskite solar cell.

Hybrid perovskites are emerging as top contenders in the next generation of photovoltaics, yet their stability under operational conditions remains a significant challenge. Specifically, degradation occurs at the interface with charge transport layers in perovskite solar cells during operation.^[1] To address this, we explore using supramolecular interfacial modulators at the interface of charge-transport layers with the purpose of suppressing degradation without interfering with the photovoltaic performance.^[2,3] This included functionalised triarylamine-based modulators, which are known to form hole-transporting supramolecular stacks,^[2] as well as chiral P,M-(1-methylene-3-methyl-imidazolium)[6]helicene iodides, which could contribute to the charge transport through chiral-induced spin-selectivity (CISS) effects. These modulators were applied at the interface between the perovskite active layer and the hole-transporting material in conventional perovskite solar cells (Figure 1). We have investigated their impact on the structural characteristics and optoelectronic properties via a combination of techniques, including X-ray diffraction, UV-vis and both steady-state and time-resolved photoluminescence spectroscopy, complemented with the further analysis of photovoltaic devices. Our investigation challenges the role of chirality in perovskite photovoltaics and reveals the contribution to the improvement of operational stabilities without compromising device performance, offering promising new strategies for advancing perovskite photovoltaics.^[3]

Over the last decade, halide perovskites have undoubtedly become one of the most promising materials or set of materials for a plethora of optoelectronic applications. However most synthetic and thin film deposition and growth approaches currently employed are difficult to upscale.

In this talk I will discuss some simple and upscalable methods for halide perovskites:

In the first part, I will present solvent-free mechanochemical grinding by ball-milling as a way to obtain high-purity perovskite bulk and nano powders on a gram scale in a short time. Moreover I will show how thermal vacuum deposition allows for the transformation of this bulk powders into high quality thin films with excellent optoelectronic properties in a completely solvent-free approach.

In the second part of the talk, I will discuss a simple spray-coating deposition method based on commercial and cost-effective airbrush technology. This methodology employs pre-synthesized colloidal nanoinks formed by room-temperature, short-ligand-assisted reprecipitation route. Eventually, spray-coated films are employed in proof-of-concept photodetectors with significant responsivity.

For researchers, it's a long road from the idea to the published article. Producing great research results does not necessarily mean they will automatically be appreciated by the community. Choosing the right journal, convincing editors as well as reviewers, and making work visible to others are essential steps on the way to success.

In this tutorial talk, Dr. Anja Wecker, Editor in Chief of Advanced Optical Materials at WILEY, will give an insight into publishing opportunities in relevant journals and the related peer review process. She started as an editor 12 years ago and since then has worked for a number of journals within WILEY’s materials science portfolio, such as Advanced Materials, Advanced Functional Materials, Advanced Photonics Research, Laser & Photonics Reviews, and Small Methods. From an editorial perspective, she will provide some guidance on how to best pass peer review with a well-prepared submission and maximize success in scientific publishing.

Lead halide perovskite nanocrystals (LHP NCs) are an emerging class of light-emitting semiconductors owing to their remarkable optoelectronic properties such as their high photoluminescence quantum yield (PLQY), tunable emission across the visible spectrum emission, and facile synthesis, which make them excellent candidates in high-performance devices such as LED lights and photovoltaic devices. One of the most interesting features of halide perovskites in comparison to classical semiconductor materials is their high defect tolerance, I.e., the uncoordinated ions and surface dangling bonds results can only form states within or close to valance or conduction bands. However, chlorine-based perovskites possess low defect tolerance and thus the surface defects result in the formation of deep traps that significantly reduce their photoluminescence quantum yield (PLQY).¹ Consequently, the PLQY of as-synthesized Cl-perovskite NCs is much lower (1-5%) than Br or I-perovskite NCs (80-100%).

In this work, I will discuss a comprehensive study of post-synthetic surface passivation of CsPbCl₃ NCs with metal halides and molecular ligands to improve the PLQY of NCs. A large variety of ligands with different functional groups such as quaternary amines, sulfonates, and phosphonates were screened to study the enhancement of PLQY according to their binding ability to the NC surface. In addition, different metal halides are screened to fill surface vacancies and thus improve the PLQY. These studies revealed that the chloride vacancies are the main reason for the low PLQY of Cl-perovskite NCs. Furthermore, we demonstrated the direct synthesis of CsPbCl₃ NCs with relatively high PLQY using the strongly binding ligands obtained from the screening of post-synthetic ligand passivation. Our results not only provide an in-depth understanding of the type of trap states in CsPbCl₃ NCs but also unravel the ligand types for effective passivation of CsPbCl₃ NCs, thus offering a new avenue to synthesize high-quality luminescent NCs either by direct synthesis or post-synthetic passivation.

To realize the large-scale production and commercialization of perovskite solar cells, development of scalable and sustainable manufacturing processes is required. Various large-area deposition methods such as blade-coating, slot-die coating, spray-coating and ink-jet printing have been reported as viable options for perovskite solar cell fabrication [1], [2]. Gravure printing as a deposition method offers an appealing alternative due to the high processing speed and possibility for “patterning by deposition” [3].

In this work, the development and optimization of the rheological and corresponding optoelectrical properties of gravure printing inks are presented. The tuning of commercial tin-oxide (SnO₂) nanoparticle inks for electron transporting layers in a n-i-p architecture is described. Also, the optimization of the perovskite inks with variable polymer additives is specifically addressed. As the performance of the printed PCSs is improved with material, ink, and process tuning, so will the opportunities for system integration and exploitation. In this work, in addition to the perovskite solar cell developments, the recent achievements in variable applications where the integration of flexible perovskite devices is required, is introduced [4].

Developing highly sensitive X-ray detectors with high atomic-number elements is crucial for reducing dose rates in medical applications. An X-ray detector with high sensitivity can accurately and efficiently detect even small amounts of X-ray radiation and convert it into an electrical signal, effectively implying that smaller doses can be applied in X-ray instruments in the medical field. Traditional X-ray detector materials such as Si and CdTe suffer from disadvantages such as low-to-moderate atomic numbers, high thermal noise and crystal impurities, all negatively impacting the overall detector performance. Bismuth (Bi)-based materials offer significant advantages, including excellent stability and high atomic numbers, to name a few, making them highly interesting candidates for applications such as X-ray detection.¹ This study investigates fully inorganic compounds, such as Cs₂AgBiBr₆ and other fully inorganic materials, as well as hybrid organic-inorganic Bi-based materials.

Mechanosynthesis is showcased as an effective method for fabricating new materials with precise stoichiometries, with the possibility for swift upscaling.^2,3 We aim to explore mechanosynthesis for the targeted synthesis of Bi-based materials and then utilize these synthesized powders for different applications, such as X-ray detection. These materials were synthesized using, for instance, dry mechanochemical ball-milling under various reaction conditions. The study emphasizes the impact of different reaction conditions, material composition, additives, and post-treatment processes on the performance of the detectors.

X-ray diffraction (XRD) was utilized to analyze the structural properties and composition of the synthesized pellets. The performance of the detectors was evaluated by measuring their photocurrent response under X-ray illumination. Our results indicate that Bi-based materials significantly outperform conventional commercial X-ray detectors in terms of sensitivity and detection limit. Optimizing the mechanosynthesis reaction conditions and carefully selecting additives and post-treatments further enhances the properties and performance of the Bi-based detectors.

This research demonstrates that Bi-based perovskite-inspired materials, in both single-crystal and pellet forms, have significant potential as effective X-ray detectors. These findings pave the way for developing safer, more stable, and high-performance alternatives to current commercial X-ray detectors. Additionally, this work highlights mechanosynthesis as a swift and viable route for fabricating these materials, contributing to more environmentally friendly and commercially viable solutions, for instance, in photovoltaics, LEDs, and X-ray detection.

Colloidal heterostructures are nanoparticles composed of two materials connected at an interface, that can exhibit unique properties different from those of their individual components. When two materials with suitable band alignment are chosen, the resulting heterostructures can convert sunlight into electron – hole pairs, which are then separated by the electronic structure of the junction. This can efficiently inhibit recombination, making the photogenerated carriers available for exploitation in photocatalytic reactions.

Our recently reported CsPbX₃/Pb₄S₃X₂ heterostructures ^[1] (where X= Cl, Br, I) are a promising example of such architectures, where two semiconductors with different crystal structures are connected by an epitaxial interface. Notably, the composition of the two domains can be independently tuned by exploiting a combination of direct-synthesis protocols and post-synthetic halide exchanges, leveraging on the different halide mobility in the two domains.

In this work, we first employed a combination of absorption and ultraviolet photoelectron spectroscopy to explore the band alignment of CsPbX₃/Pb₄S₃X₂ heterostructures, which we can now synthesize on the 100 mg scale. Based on these results, we identified a potential application in the photocatalysis of biomass conversion, like the oxidation of 5-hydroxymethylfurfural ^[2],[3]. Unlike more popular processes like CO₂ reduction these reactions can take place in organic solvents, which are more compatible with metal halides, and still hold significant relevance thanks to the production of chemicals with added economical value.

So far, our preliminary tests demonstrate excellent in-operando stability, and show that heterostructures are significantly more active as photocatalysts than their individual constituting materials, which we attribute to the efficient carriers separation at the junction. These promising results pave the way to further photocatalysis studies, currently ongoing, and are a significant first step towards the real-world application of these complex yet fascinating heterostructures.

Lead halide perovskite (LHP) nanocrystals have recently attracted considerable attention as a promising class of materials for optoelectronic applications. Their ionic lattice and labile surface chemistry require new strategies in ligand development. Recently, a range of ligands have been developed that significantly improve the colloidal stability of LHP NCs. This was mainly achieved by using zwitterionic ligands. To drive further development of ligands for LHP NCs, we sought to rationalize the binding behavior of commonly used ligands. Our study shows that zwitterionic phosphocholine- and phosphoethanolamine-based ligands bind statically to the LHP NC surface and exhibit the highest stability. Sulfobetaine-based ligands offer a good balance between stability and binding dynamics. Several ammonium-based ligands exhibit high dynamics but often have insufficient stability due to their pH sensitivity or suboptimal binding to the A-site pockets.

In this work, we propose a new family of long-chain guanidinium-based ligands that, without compromising stability, provide dynamic binding to the surface of NCs and thus easily accessible active sites on the surface. A library of guanidinium-based ligands was synthesized and applied to LHP NCs of different sizes and compositions, such as CsPbBr₃, FAPbBr₃ and CsPbI₃. The new synthetic approach enables the preparation of LHP NCs with quantum yields of up to 95% in colloidal solutions and up to 80% in compact films. This combination of properties makes LHP NCs capped with guanidinium-based ligands very promising for photocatalytic applications.

Utilizing machine learning in developing perovskite solar cells significantly reduces the need for extensive experimental trials, saving time and resources. This approach enhances the efficiency of perovskite solar cells by enabling the prediction of optimal material combinations and processing conditions based on existing data, leading to improved performance and reduced material usage. This study uses Machine Learning (ML) techniques to optimize the performance of Perovskite Solar Cells (PSCs) by determining the ideal thickness of the electron transport layer and the best coating parameters for high Power Conversion Efficiency (PCE). ML models are trained on extensive experimental data to predict outcomes based on varying electron transport layer thicknesses and coating conditions. A key benefit of this method is its ability to reduce the number of experiments needed to find the optimal parameters in solar cell production. This study utilizes a device configuration of ITO/SnO₂/FAMAPbI₃/Spiro-OMeTAD/MoO₃/Ag. For optimizing the electron transport layer (SnO₂) thickness in PSCs, a database was created from a series of experimental studies on coating parameters and environmental conditions. Three ML algorithms, Random Forest (RF), eXtreme Gradient Boosting (XGBoost), and Category Gradient Boosting (CatBoost), were chosen to optimize PCE. Hyperparameter optimization was performed using Bayesian optimization. Root Mean Square Error (RMSE) and Pearson Correlation Coefficient (r) were the metrics used to evaluate the models. The best results were achieved with XGBoost, showing an RMSE of 0.455 and a Pearson coefficient of 0.958. Consequently, the highest PCE obtained from the experimental studies has been increased from 13.2% to 14.26% with an 8% improvement under the predicted conditions.

Perovskites are a class of materials with a distinctive crystal structure ABX3. They have
garnered significant attention as promising candidates for solar cell applications. These
materials, typically consisting of a hybrid organic-inorganic lead or tin halide-based
structure, offer exceptional light absorption, charge-carrier mobility, and tunable band
gaps. The ease of fabrication and potential for low-cost production, combined with rapidly
improving power conversion efficiencies that now rival traditional silicon-based solar cells,
underscore their potential in the photovoltaic industry. Challenges remain, particularly
concerning stability and toxicity. In our current study simple and hybrid perovskites and
their properties have been studied through machine learning (ML) models, density
functional theory (DFT) calculations, and synthesis. Addressing stability challenges, as a
first step we concentrated on lead containing organic-inorganic perovskites, trying to find
stable compositions, while the challenging toxicity is our next goal. We started with data
mining, then used 7 ML algorithms to predict band gap energies for generated perovskites
compositions. Based on the gathered data the best model for the prediction was chosen
Linear Regression model. Due to it’s band gap energy FAPbBr_1.125I_1.875 was chosen as one
of the best compositions for solar cell applications. The ML results were confirmed through
DFT calculations and experiments. Band gap energies were theoretically calculated
through DFT and hybrid methods. Hartree–Fock method is used for hybrid functionals. The
synthesis was implemented thought solvothermal method and the uniform thin films were
obtained trough spin coating.

Perovskite solar cells (PSCs) have achieved impressive power conversion efficiencies (PCE) of up to 26.1%,^[1] however their commercialization is hindered by significant stability challenges, particularly in the presence of oxygen/moisture, and under operating conditions such as voltage bias, light exposure, and temperature fluctuations. These issues are often associated with migration of ions, which can lead to a degradation of optoelectronic properties, and, in turn can adversely affect both the performance and long-term stability of PSCs. Inherent defects and grain boundaries are the main ion migration pathways within the perovskite layer. Notably, unreacted lead iodide (PbI₂) at these sites, including surface of perovskite grains, and the interface between the perovskite layer and the charge transport layer, is a major cause of intrinsic instability under illumination. PbI₂ can break down into iodine and metallic lead, acting as recombination centers, further promoting ion migration, and accelerating degradation.^[2] In stark contrast for the operation of PSCs, it has been shown that enhancing device performance often involves adding a small excess of PbI₂ to the precursor solution.

The conflicting observations that PbI₂ can both enhance performance but also degrade stability suggests a need for research efforts directed toward simultaneously mitigating crystallization of residual PbI₂ while improving device stability, ensuring that efficiency gains do not compromise the long-term reliability of perovskite solar cells.

This presentation addresses these challenges through supramolecular complex engineering by introducing beta-cyclodextrin (β-CD) into a triple cation perovskite layers to effectively prevent the crystallization of residual PbI₂. This approach results in uniform crystal growth and the passivation of undercoordinated lead cation defects, as confirmed by XRD and SEM analyses. The use of β-CD leads to a PSC with an improved PCE of 21.36%, surpassing the control (19.4%), and enhanced stability against aggressive thermal stress and high humidity (85% RH), likely attributed to defect passivation as evidenced by PL, SCLC and the dependence of V_OC on the incident light intensity studies. Notably, in comparison to the β-CD-free control, the β-CD-treated sample exhibited minimal optical bandgap shifts of 3 meV after 1170 hours of moisture exposure. In addition, the devices treated with a 0.5% of β-CD showcased improved stability, maintaining over 73% of their initial PCE even after undergoing 320 hours of testing at 50-60℃. XPS and NMR observations reveal that β-CD effectively anchors uncoordinated Pb²⁺ ions, preventing the formation of metallic Pb and enhancing film stability under environmental stress. Furthermore, this method not only passivates unreacted PbI₂ but also provides valuable insights into the role of β-CD in PSCs. Additional tests with Maltose as a non-cyclic control were conducted and confirm the superior ability of β-CD to enhance perovskite film stability under harsh conditions.^[3] The formation of a supramolecular system between β-CD and perovskite holds promise as a strategy to control perovskite precursor chemistry, material structure, and subsequent device performance and stability.

Energy-harvesting materials are currently on the rise, as solar energy provides the Earth with a considerable amount of energy.[1] Designing hybrids that harness the synergistic power of both systems is of great significance in such a context. Hence, the use of nanohybrids based on perovskite nanocrystals and photoactive organic molecules such as BODIPY dyes triggers the properties of wide and strong light absorption in the visible range and the generation of reactive species such as triplet excited states, respectively, which could potentially be used for organic photocatalysis.[2] In this contribution, we investigated the photophysical properties of nanocrystal-organic nanohybrids at the interface. For this end, CsPbBr₃ perovskite nanocrystals (NCs) have been merged with BODIPY dyes, specifically 8-(4-carboxyphenyl)-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP) and 8-(4-carboxyphenyl)-2,6-diiodo-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (I2-BDP). Through the use of steady-state and time-resolved absorption and photoluminescence techniques, we unveiled the sensitization mechanism of BODIPY dyes in the corresponding nanohybrids by selective excitation of the NCs. Spectroelectrochemical analysis and theoretical calculations confirm the generation of transient intermediate species, which are found to be crucial in the photosensitization mechanism. The efficient generation of triplet reactive species in I2-BDP allows the NC@I2-BDP nanohybrid to be used as a potential photocatalyst for the oxidation of α-terpinene to ascaridole via singlet oxygen formation.[3]

Tuning the crystallinity, stability, and optoelectronic properties of tin halide perovskite-based semiconductors is crucial for advancing these promising materials toward practical device applications. Embedding organic semiconductor cations within the layered structure of tin perovskites can offer an effective means to attenuate the dielectric and quantum confinement effects by enabling the charge transfer between the organic and inorganic layers. However, incorporating bulky pi-conjugated organic semiconductor cations in layered perovskites is a challenge. Herein we leverage our previously demonstrated molecular engineering strategy^[1] to afford the incorporation of napththalene diimmide (NDI) cations with alkyl chain lengths of 6 carbons into two-dimensional (2D) tin halide perovskites that show enhanced electron mobility and air stability. We show the importance of both cation molecular structure and processing conditions in obtaining high quality thin films. Additionally, we discuss the role of additives in facilitating the formation of quasi-2D tin halide perovskites with enhanced crystallinity and elucidate the importance of the additive molecular structure—suggesting this strategy can generally advance the structural qualities of solution-processed perovskite materials, and consequently enhance device performance and stability.

Bifaciality is a salient sub-field for perovskite solar cells (PSCs), both due to reaching higher efficiency values for single-junction devices and applying them to tandem devices with other technologies or PSCs [1]. For this purpose, transparent electrodes (TEs) have been investigated, including TCOs, metal nanowires, ultrathin films, and 2D materials such as graphene [2]. TEs with noteworthy bendability properties are also riveting to be used in lightweight-flexible devices for novel areas like portable devices or aerospace applications [3]. In this context, we present bifacial perovskite solar cells, suggesting alternative transparent electrodes composed of three ultrathin layers, called OMOs (oxide-metal-oxide). The main material of the study is nickel oxide/silver/nickel oxide (NAN) [4] layers where a silver layer is sandwiched between NiO films, with thicknesses of 35 and 8, for NiO and Ag layers respectively. The triple layers are fabricated with sequential e-beam deposition, consisting of only one evacuation process. A comprehensive characterization of NAN layers providing the transmittance and conductivity properties and their performance when applied to perovskite solar cells, with remarkable Voc of 1.11 V using an active layer bandgap of 1.6 eV, will be presented in which the charge transport and absorber layers of the devices are fabricated via solution-based methods on rigid and flexible substrates.

Halide perovskite solar cells have revolutionized the photovoltaic field in the last decade. In a decade of intensive research it has been a huge improvement in the performance of these devices, however, the two main drawbacks of this system, the use of hazardous Pb and the long term stability, still to be open questions that have not been fully addressed. Sn-based perovskite is the most obvious alternative to Pb, producing degradation materials less toxic and presenting the highest performance among the different Pb-free halide perovskites, but presenting a lower stability than their Pb-containing counterparts. In this talk, we highlight how the use of proper additives and light soaking for defect engineering can increase significantly the stability of formamidinium tin iodide (FASnI3) solar cells, and discuss about the different mechanism affecting this stability, beyond the oxidation of Sn2+, and how they can be countered, analyzing specially the light soaking treatment. In addition, we report alternative ways to prepare 2D Sn-perovskite powders, that can be used as precursors for fabrication of Sn-based LEDs or as by themselves to drive photocatalytical reactions. We will discuss on the rational design of halide perovskites containing non-critical raw materials towards photoelectrochemical processes, and the importance of extracting basic electronic and optical information to understand the carrier dynamics to maximize the performance and stability of these materials. Moreover, proper interrogation tools are needed to validate their photo(electro)catalytic activity and selectivity.

Metal-halide perovskites exhibit bright and sharp luminescence, with properties that can be tuned over a wide range through solution processing. The first half of this talk examines realising polarised luminescence. Here, CsPbI3 nanoplatelets are self-assembled into an edge-up orientation. Through strong dielectric and quantum confinement, there is a large exciton fine structure splitting. As a result, we achieve strong emission from out-of-plane dipoles for the optically bright excitons in these superlattices. In light-emitting diodes, this leads to a high degree of polarisation of 74.4% in electroluminescence, without requiring any photonic structures [1].

The second half of this talk makes use of the versatility of metal-halide perovskites to examine hot carrier cooling [2]. In particular, although defect tolerance has been widely quoted as a key enabling properties of band-edge (cold) carriers, it is unknown whether this can be extended to hot carriers. Through interband and intraband femtosecond spectroscopy, along with excitation-energy-dependent photoluminescence quantum efficiency measurements and kinetic modelling, we show that hot carriers are not universally defect tolerant, but depend on the energy of the traps. The trap density in CsPbX3 (X is Br, I or both) is intentionally tuned by washing the surface with polar antisolvents [3], and we show that hot carriers are directly trapped by defects, without going through a cold carrier intermediate. By showing how defects affect hot carriers, this work leads to design principles that could be used to realise hot carrier solar cells.

Metal halide perovskites (MHPs) exploitation represents the next big frontier in photovoltaic technology with power conversion efficiencies above 26 %. However, two main drawbacks limit the potential application and commercialization of the technology: toxicity, due to the presence of lead, and instability under moisture, light and temperature, among other factors.  Currently, research efforts are dedicated to the synthesis of novel Pb-free MHPs with elements such as tin (Sn), germanium (Ge), bismuth (Bi), Titanium (Ti) or antimony (Sb) as substitutes for lead (Pb). Although the tin-based perovskite solar cell is the most promising candidate, other promising PSCs with elements such as Bi (with current efficiencies around 5 %) are being highly studied. Among the different approaches employed to boost the stability of halide perovskite solar cells, A-site substitution using organic ligands, has been the centre of research worldwide. In lead perovskite solar cells, A-site substitution with various organic and inorganic materials has been commonly explored to control the optoelectronic properties and consequently improve device performance. Cations such as methylammonium (MA+), formamidinium (FA+), phenethylammonium (PEA+), ethane- 1,2-diammonium (EDA+), and guanidinium (GA+) ions have shown positive outcomes when used at optimized stoichiometries in perovskite solar cell devices. Recently, a novel A-site cation, dimethylammonium (DMA+) cation, has been introduced in Pb-containing perovskite solar cells, which enhanced the photovoltaic performance and device stability. DMA can act as an A-site cation to incorporate with CsPbI3, creating a hybrid perovskite of CsxDMA1-xPbI3, which is more stable in ambient conditions than pure CsPbI3. This enhancement in stability is also observed when partial substitution of the large dimethylammonium (DMA) cation at the A site of FAxCs1–xPbIyBr3–y perovskites, results in a bandgap increase and it is accompanied by an expansion of the crystal lattice. The solar cells resulted extremely stable, retaining 96% of their original efficiency over 2200 h at 85 °C in the dark and 92% of their original efficiency after operation at 60 °C for 500 h. It is particularly noteworthy the high stability and hydrophobicity of DMASnX3 and PEA2SnX4, the Pb-free MHP do not dissolve in water, but it rather remains dispersed in solution, maintaining unchanged the crystal structure and the optical properties. The latter makes this perovskite an interesting candidate as photocatalyst. In this presentation, I will show our most recent results on the synthesis of Pb-based and Pb-free halide perovskites and their application as solar cells and potential photo(electro)catalyst. The materials were fabricated as heterojunctions with MXene thin films.

Quaternary Mixed-Metal Chalcohalides (MMCs), composed of Group IV and V elements such as Pb, Sn, Bi, and Sb, along with halide and chalcogenide ions, are an emerging class of perovskite-inspired ns² semiconductors (1). These materials possess promising optical and electronic properties, which are due to two key features: the presence of two ns² cations and spontaneous polarization. These result in a lower probability of carrier capture (2,3). While these materials are highly desirable for various applications in energy conversion and storage devices, their potential use may be limited by the challenges associated with synthesizing chalcohalide thin films. For instance, they often require non-solution methods or multi-step processes (1,4).

In my presentation, I will discuss our most recent research on MMCs. The initial portion of my talk will focus on the single-step, solution-based fabrication and the characterization of Pb-Sb-S-I and Sn-Sb-S-I MMC compositions. I will also highlight the impressive stability of these MMC compositions, both in terms of material and device performance under ambient air, humidity, and operation. Towards the end, I will present some of our preliminary findings related to the light-harvesting capabilities of these MMCs.

Most semiconductor technologies rely on the intentional engineering of charge transport. In the case of halide perovskites — a semiconductor class currently developed for critical components in solar cells and light-emitting devices —reliable control of their charge transport is still needed. Such advances may help improve the performance of these semiconductors in current applications and open myriad new possibilities. In this talk, I will describe two routes to enhance the conductivity of halide perovskites. We first describe how introducing charge reservoirs inside expanded analogs of halide perovskites allows controlling their electrical doping.¹ Secondly, we describe how mixed-valence in halide perovskite can be harnessed to enhance conductivity.²

References

1) Matheu, R.; Ke, F.; Breidenbach, A.; Wolf, N. R.; Lee, Y.; Liu, Z.; Leppert, L.; Lin,Y.; Karunadasa, H. I. Charge Reservoirs in an Expanded Halide Perovskite Analog:Enhancing High-Pressure Conductivity through Redox-Active Molecules. Angew. Chemie Int. Ed. 2022, 61, e202202911.

2) Li, J.; Matheu, R.; Ke, F.; Liu, Z.; Lin, Y.; Karunadasa, H. I. Mosaic CuI−CuII−InIII 2D Perovskites: Pressure-Dependence of the Intervalence Charge Transfer and a Mechanochemical Alloying Method. Angew. Chemie Int. Ed. 2023, 62, e202300957.

Halide perovskite semiconductors can merge the highly efficient operational principles of conventional inorganic semiconductors with the low‑temperature solution processability of emerging organic and hybrid materials, offering a promising route towards cheaply generating electricity as well as light. Following a surge of interest in this class of materials, research on halide perovskite nanocrystals (NCs) has gathered momentum in the last decade. This talk will highlight several findings of our group on their synthesis, for example our recent study on the influence of various exogenous cations and acid-based equilibria on the growth of NCs, and the preparation of NCs in the strong quantum confinement regime. Our group is pioneering syntheses of NC heterostructures in which a perovskite domain is interfaced either with a metal chalcogenide or a chalcohalide. This talk will discuss how we can computationally predict the feasibility of such heterostructures, based on interfacial matching. We will also overview selective anion/cation exchange reactions that can take place on a starting heterostructure, from which a series of second-generation heterostructures can be generated. 

In two-dimensional hybrid organic-inorganic metal halide perovskites (2D-HOIPs), the stacking setting of the adjacent inorganic sheets is influenced by the nature of the incorporated organic cation. [1-2] The photophysical properties of 2D-HOIPs can be accordingly tuned. However, the structural properties of 2D-HOIPs are more complex than the parent 2D metal oxide perovskites because of the larger size of the organic cations, their different chemical and structural properties. For instance, an order-disorder transition of the cation may result to a ferroelectric phase, [3] while octahedral distortion can modulate photophysical and electrical properties of the HOIP. [4] In this context, octahedral distortion as well as cation’s distortion urges for a systematic understanding of the crystallographic properties of 2D-HOIPs. [5]

In recent years, implementation of iodine-terminated alkylammonium cations is attracting a lot of interest, since it has been proposed that the development of halogen-halogen supramolecular interactions between the iodine atom of the organic cation (acting as halogen bond, XB, donor) and the apical inorganic iodine atoms of the inorganic sheets (acting as halogen bond, XB, acceptors) suppress phase transitions towards undesired lower-symmetry phases. [6] The rich halogen chemistry of metal halide perovskite semiconductors brings such halogen bond (XB) strategies under the spotlight, [7] while halogen-halogen interactions, in general, may lead also to unprecedented properties. [8] Herein, a bimodal iodine-terminated alkylammonium cation has been studied as a supramolecular modulator of the crystalline organization of a 2D-HOIP. Utilizing heat as an external stimulus a controlled and reversible single crystal-to-single-crystal phase transition was observed. Controlling the conformation of the cation, the modulation of the interlayer XB formation was achieved. The new perovskite phase has discrete structural and photophysical properties, as evidenced by variable temperature investigation by single-crystal X-ray diffraction, solid-state NMR, UV-Vis and photoluminescence spectroscopies.

Halide perovskite materials have garnered significant attention for their potential in diverse applications such as solar cells, photodetectors, lasers and photocatalysis. CsPbBr₃, an all-inorganic metal halide perovskite, exemplifies this potential with its impressive stability against moisture and light [1]. With a direct band gap of 2.3 eV, large atomic number and high light-electron conversion efficiency, CsPbBr₃ is particularly suitable for high-energy radiation detection [2]. The material’s versatility extends to photovoltaics [3] and multicolour LEDs [4] through partial or total substitution of the bromide anion with iodide and/ or chloride. Device performance in polycrystalline films, which are typically produced via solution-based or vapour deposition methods, is greatly affected by the density of grain boundaries. In general a negative effect is attributed to grain boundaries due to an increased number of recombination sites, pathways for ion migration and openings for perovskite degrading species like oxygen [5]. Conversely, Luchkin et al. reports grain boundaries to be the primary location for photocarrier generation and transport, which can enhance device performance [6]. Therefore, controlling the deposition process to manage grain morphology and relating to the film performance is essential for the device optimization.   

In this study, we employ the hardly explored close space sublimation (CSS), a fast and scalable physical vapour deposition technique, to fabricate polycrystalline CsPbBr3 films [7]. Understanding the growth of CsPbBr₃ can serve as a model for other perovskite materials, contributing to advancements in thin film solar cells and light emitting applications. This solvent free synthesis route enables rapid growth rates up to 6 µm/min with a high material utilization of 98%. Single-phase CsPbBr₃ films are achieved across a wide range of deposition parameters. By separately adjusting the deposition conditions (substrate and source temperature, pressure), we access different growth regimes, enabling tunable grain sizes in the films without additional treatment.

Our study focuses on characterizing films with thicknesses of 1 – 10 µm and varying grain sizes. Using electrical and optical characterization methods like current – voltage characteristics, conductive atomic force microscopy, scanning electron microscopy, Raman and photoluminescence spectroscopy we correlate the optoelectronic properties with film morphology and grain structure. Taking advantage of the grain tunability offered by CSS, we aim at exploring the effect of grain size on the optoelectronic properties of CsPbBr₃, which is crucial for the optimization of perovskite-based device performances.

Over the last ten years, the role of hybrid metal halide perovskites has received significant attention as suitable materials for various electronic applications. Indeed, their outstanding optoelectronic properties such as high-power conversion efficiency, tunable bandgap, and high absorption coefficient, make them suited for several applications in different devices as photovoltaic cells, photodetectors, light emitting diodes, and sensors [2]. Starting from the hybrid organic-inorganic perovskites (HOIPs), the introduction of a chiral molecule as organic cation leads to the breaking of the spatial inversion symmetry, allowing new possible designs based on the combination of polarity and chirality [1,3].  In the scientific scene, this opened plenty of novel applications provided by outstanding chiroptical properties, such as circular dichroism, circular polarized emission, chiral induced spin selectivity and so on. To extend the actual knowledge of these chiral systems, it is important to investigate those parameters which have a major impact on the chirality transfer mechanism, with the final aim to unveil it. From a material chemistry point of view this involve an important work on materials’ structure, involving several modulations/substitutions on the latter. More specifically, in this contribution we will present the results of the role of the central metal, showing the modulation of the optoelectronic properties with two different metals. Firstly, we prepare an initial series of novel chiral metal halide maintaining the chiral cation (4-Chlorophenyl)ethylenimine (Cl-MBA), accompanied by different central metal, namely Pb, Sn and Ge. This approach brought to the discovery of (R/S/rac-ClMBA)₂SnI₄ and (R/S/rac-ClMBA)₂GeI₄ compositions, and to another interesting Ge-containing 1D system, namely (R/S/rac-ClMBA)₃GeI_5. The work provides a comparison not only in terms of structural features and chiroptical properties but also in terms of computational modelling, which helps us to deeply understating the role of central cation and the difference in terms of efficiency moving from a Pb-based perovskites to a Pb-free one. Final aim of this work is to unveil the impact of chemical degrees of freedom on the chirality transfer between the organic cation and the inorganic framework to provide tuning strategies for materials engineering.

The structural and thermodynamic stability of pure FAPbI3 perovskite films has been a topic of extensive research and discussion in the field of perovskite solar cells. While previous studies have primarily focused on the transition of X-ray diffraction (XRD) patterns from an α-FAPbI3 cubic phase to the more stable delta phases, our research takes a novel approach. We present a comprehensive structural and symmetry analysis that uncovers the tetragonal nature of the perovskite FAPI phases processed with Flash IR annealing (FIRA), offering a fresh perspective on this intriguing subject.

In phase transformation and crystallization, the film's crystal structure might be affected by the strain field, especially if the film undergoes structural distortions or tilting of lattice planes. Therefore, by controlling the FIRA process parameters (such as temperature and heat rate), it is possible to finely tune the properties of the film, including strain-related properties, as shown. [1] These processes lead to highly stable perovskite devices, where we further show that molecular additives contribute more to the crystallization dynamics as a mass transport diffusion process than to the atomic defect level. We also present a new methodology based on large dataset computational image processing correlated with advanced characterization techniques.

Operando Perovskite Interface Characterization using Photoemission Spectroscopy To enhance and refine perovskite optoelectronic devices, understanding the atomic and electronic structures of perovskite material interfaces is essential. X-ray-based spectroscopies, including photoelectron spectroscopy and X-ray absorption spectroscopy, provide element-specific, non-destructive chemical and electronic structure insights at the atomic level. With recent advancements at synchrotron facilities and methodologies developed by our team and others, these tools are rapidly advancing for energy material system studies. [1-3] They facilitate the examination of interface chemistry, energy alignment, valence and conduction band properties, and charge dynamics. In this presentation, we share our latest results using X-ray absorption spectroscopy and energy-dependent photoelectron spectroscopy on perovskite interfaces. Our research covers single crystal surfaces and interfaces to functional device structures. We illustrate how to analyze interface chemistry and determine energy alignment in perovskite/hole-conductor/metal interfaces, both in the dark and under visible light. Specifically, we will discuss operando measurements using hard X-ray photoelectron spectroscopy.

References:
1. Svanström, S et al ACS Appl. Mater. Interfaces 15, 12485–12494 (2023).
2. Man, G. J. et al. Phys. Rev. B 2021, 104, L041302

3. Garcia-Fernandez, A. et al., SMALL 2022, 18, 2106450

The integration of plasmonic nanoparticles into perovskite-based optoelectronic devices represents a promising frontier in enhancing device efficiency for both light-emitting diodes (PeLEDs) and solar cells (PSCs). Perovskite materials, known for their large absorption, long diffusion lengths, tunable bandgap, high quantum yield, and narrow emission bandwidths, are ideal candidates for next-generation optoelectronic applications. Despite these advantages, PeLEDs still face challenges in optimizing quantum yield, color purity, and angular control. Similarly, PSCs, specifically tandem solar cells with narrow bandgap perovskites, suffer from limited absorption capacities, hindering their potential efficiency.

Through rigorous simulation design and experimental synthesis, our research demonstrates that embedding plasmonic nanoparticles randomly distributed within these perovskite structures can significantly enhance their photophysical properties. By optimizing parameters such as metal type (Ag, Au, Cu), shape, and volume filling concentration, we establish guidelines for the future development of plasmonic resonance-based optoelectronic devices [1-3].

Specifically, we achieve a three-fold enhancement in the integrated emission of thin CsPbBr3 films by incorporating precisely engineered spherical Ag nanoparticles, allowing for controllable directionality in the forward direction or at desired larger angles. Similarly, we demonstrate that, through rigorous design, it is possible to provide a realistic prediction of the magnitude of the absorption enhancement that can be reached for perovskite films embedding metal particles. In all-perovskite tandem solar cells, we maximize light harvesting while minimizing parasitic absorption, providing an absolute power conversion efficiency enhancement of 2% in Sn-based perovskite compositions through synergistic near- and far-field plasmonic effects [4]. This approach allows for thinner perovskite films, facilitating photocarrier collection and reducing the amount of potentially toxic lead in the devices.

Our findings underscore the transformative potential of plasmonic nanoparticles in enhancing the efficiency of perovskite-based optoelectronic devices, paving the way for advanced applications in lighting, displays, and renewable energy solutions.

Photocatalysis has emerged as a highly valuable and economic method that was used extensively in the last few decades for the synthesis of important organic molecules and facilitating crucial bond formations such as C-C, C-N, and C-O bonds.[1-3] Although significant development has been made in the field, conventional photocatalysts usually comprise expensive transition metals such as Ru[4], Ir[5], or Au[6], and are often hard-to-prepare, require air-free conditions for obtaining high conversion rates, and may be of limited reusability. A promising alternative for solar- or LED-driven photocatalysis lies in lead halide perovskite quantum dots (LHP-QD) as recent developments highlighted their strong and broadly tunable absorption with high coefficients (~10⁶ M^-1cm^-1), near unity quantum yields (>90%)[7], highly dynamic and accessible surface, and low electron-hole binding energy causing facile charge generation/transfer. These properties overall indicate the high potential of lead halide perovskites as sustainable heterogeneous photocatalysts.   

In this work, we showcase the use of LHP-QDs as efficient photocatalysts in various organic transformations. First, we demonstrate the impact of designer ligands on the catalytic activity at extremely low catalyst loadings (<10*10^-6 mol%, <100 ppb) for α-, and γ- bromination of the β-ketoesters, ketones, and various benzyl derivatives reaching turnover numbers over 9,000,000. Investigating the effect of solvent on the performance as well as doing a comprehensive mechanistic study allowed us to rationalize the catalyst properties. Next, we show the development of thin-inorganic coatings (AlO_x, TiO_x, and ZrO_x) on LHP QDs for efficient photocatalysis. We explore the dimerization of benzyl bromide derivatives as model systems using surface-engineered LHP-QDs, achieving high conversions to the desired products in short reaction times (<4h) and with low catalyst loadings. Our findings confirm that a properly designed surface passivation and balancing of the oxidation and reduction reaction rates can successfully resolve the stability issues anticipated for such ionic compounds in reaction conditions. Photocatalysts can be separated from the reaction mixture, regenerated, and still exhibit >95% conversion efficiency during the second cycle of the reaction.

Overall, our efforts reveal that LHP QDs enable photocatalyzed transformations that are otherwise not achievable with the established photocatalysts, and we believe the engineering of the surface in these materials will allow the community to advance in the context of photoredox catalysis.

Vacuum deposition emerges as a pivotal method for scaling up perovskite solar cells (PSCs) in commercial applications, offering conformal coatings on various surfaces and precise layer control, aligning with large-scale manufacturing needs. Atmospheric conditions, particularly moisture, significantly impacts perovskite layer quality, requiring controlled environments. Through in-situ grazing incidence wide angle X-ray scattering (GIWAXS) and 3D time of flight secondary ion mass spectroscopy (ToF-SIMS) we show that annealing in the presence of humidity, is essential for the interdiffusion of inorganic and organic precursors necessary to achieve high quality thermally evaporated perovskite absorbers via sequential deposition. Notably, spectroscopy measurements also indicate high non-radiative bulk recombination in the absorbers photoluminescence when processed in the absence of relative humidity, bolstering the importance of interdiffusion for the device's overall performance. This has resulted in the development of high-performance p-i-n perovskite solar cells (PSCs) exclusively processed through vacuum techniques, achieving an impressive power conversion efficiency of 20.45%. Stability tests reveal an initial burn-in phase during accelerated aging at 85°C and 1 sun illumination using full spectrum while holding the device at the open-circuit condition. Despite this phenomenon, the devices exhibit remarkable stability, remaining functional for additional 500 hours post the initial challenge. This finding highlights the resilience and potential durability of the newly developed vacuum-processed PSCs, marking a significant advancement in the field of perovskite solar cell technology.

Modern color image sensors and cameras face challenges in improving sensitivity and color fidelity due to the inherent inefficiencies in light utilization. A major factor contributing to these inefficiencies is the use of passive optical filters, which absorb a significant portion of incoming light, thereby diminishing the sensors' quantum efficiency. We present an innovative architecture for color detector arrays, leveraging multilayer monolithically stacked lead halide perovskite thin-film photodetectors. By utilizing the tunable bandgap of perovskites, we selectively absorb the red, green, and blue regions of the visible light spectrum, eliminating the need for traditional color filters. We achieved external quantum efficiencies of 50%, 47%, and 53% for the red, green, and blue channels and color accuracy of 4.5ΔE_Lab, outperforming state-of-the-art color-filter array and Foveon-type photosensors while avoiding demosaicing artifacts. Our study lays the groundwork for the next generation of image sensors, employing conceptually novel architectures with enhanced light utilization.

The recent surge in interest towards antimony- and bismuth-based perovskite-inspired materials (PIMs) is attributed to their potential as sustainable and air-stable absorbers for photovoltaic applications.^[1]

Lately, we have presented the first 2D triple-cation antimony-based PIM (CsMAFA-Sb),^[2] which incorporates inorganic cesium alloyed with organic methylammonium (MA) and formamidinium (FA) cations at the A-site of the A₃Sb₂X₉ structure. The inclusion of the hybrid A-site cations proved to be crucial in reducing trap-assisted recombination pathways, thereby enhancing the performance of both outdoor and indoor photovoltaics. Furthermore, through a careful doping engineering of the hole-transport layer, the devices remained stable after nearly 150 days of storage in dry air.

While promising shelf-lifetimes of PIM-based photovoltaics have been often reported, the operational stability of the devices remains so far largely overlooked. To address this, we have incorporated Bi(III) ions into the CsMAFA-Sb structure while at the same time introducing dimethyl sulfoxide (DMSO) into the solvent system of the precursor solution. This resulted in a novel PIM, CsMAFA-Sb:Bi, with enhanced film morphology and large crystalline domain size. The corresponding photovoltaics have demonstrated a slightly increased power conversion efficiency (PCE) and significant improvements in the stability under operational conditions at the Maximum Power Point (MPP) compared to the control devices based on CsMAFA-Sb. Remarkably, under a constant 0.1-Sun illumination at the MPP for over 100 hours, the CsMAFA-Sb:Bi devices not only sustained the initial PCE, but even witnessed an over 10% PCE increase. Under constant 1-Sun MPP for over 100 hours, the CsMAFA-Sb:Bi devices still retained over 75% of the initial PCE. The stability enhancement is attributed to the reduced ion migration upon Sb:Bi co-alloying. Our findings represent the most extensive stability study reported for PIM-based photovoltaics to date, which demonstrates their potential for commercial applications.

The request for sustainable and renewable energy sources has motivated a significant shift toward innovative methods for producing and storing clean energy through green solar fuels. Photocatalytically active heterojunctions based on metal halide perovskites (MHPs) are drawing important interest for their tuneable ability to promote several redox reactions. This study investigates two composite systems: a classic double perovskite-based, Cs₂AgBiCl₆/g-C₃N₄,¹ and a vacancy-ordered perovskite-based, Cs₂SnBr₆/g-C₃N₄. The first system has been studied and employed for both solar-driven hydrogen generation from chloride media and nitrogen reduction and the second one for nitrogen reduction. The efficiency of the Cs₂AgBiCl₆/g-C₃N₄ system depends on the relative amounts of perovskite and carbon nitride to promote the two reactions. Spectroscopic investigations and density functional theory (DFT) modelling reveal that perovskite halide vacancies are primary reactive sites for hydrogen generation and can be, together with g-C₃N₄ nitrogen vacancies crucial also for nitrogen reduction.¹ On these bases, again through a combined experimental and computational approach, we provide a detailed framework of the mechanism supporting the efficient nitrogen reduction reaction to ammonia observed in Cs₂SnBr₆/g-C₃N₄ system. The vacancy-ordered perovskite is characterized by a higher surface density of halide vacancies than the double perovskite and then the Cs₂SnBr₆/g-C₃N₄ system achieves the highest ammonia evolution rate of 266 μmol g^-1 h^-1. Photoluminescence studies and differential transmission measurements stress the importance of compositional engineering in enhancing photocatalysis efficiency for both reactions. This work represents a significant advancement in photocatalytic green fuels production, especially in the field of nitrogen photofixation, proposing materials and structures that can potentially improve sustainable ammonia production, thereby contributing to energy independence and reduced carbon emissions.

Perovskite solar cells (PCSc) have emerged as a highly promising photovoltaic technology due to their cost effective fabrication processes and remarkable power conversion efficiencies. In 2009, PSCs were initially proposed as visible light sensitisers for photovoltaics, with 3.8% power conversion efficiency (PCE) at the laboratory scale to date, reaching an extraordinary 26.1%. However, the environmental and health concerns are associated with lead (Pb) in high performing PCSs. Tin perovskite solar cells (Sn-PSCs) are considered as eco friendly alternatives to Pb based PSCs. Ploy (3,4-ethylenedioxythiophene) polystyrene sulphonic acid (PEDOT.PSS) as hole transport layer (HTL) and dimethyl sulfoxide (DMSO) employed for Sn-PCSs approaching 15% but they are leading towards fast Sn oxidisations. Here we presented the PEDOT.SS and DMSO free compositions FA_0.95EDAI_0.05SnI₃ in p-i-n structure for Sn-PSCs. The perovskite film achieved good morphology using ploy (3,4-ethylenedioxythiophene) as HTL along with Al₂O₃ nanoparticles as inter layer in a DMSO-free solvent system. The fabricated device achieved a remarkable PCE of 8.72 % under simulated AM 1.5G illumination.

Halide perovskites have emerged as promising materials for optoelectronic devices due to their exceptional optoelectronic properties. These materials have found many applications such as light-emitting diodes, photodetectors, lasers, and solar cells[1–6]. Among them, inorganic halide perovskites CsPb₂Br₅, CsPbBr₃, as well as dual-phase perovskites, are particularly promising. However, achieving the formation of thin films by simple and scalable fabrication methods under ambient conditions with low temperature deposition remains a challenge.

Traditional techniques like spin-coating[1], vapor deposition [7], thermal vacuum deposition [8] or electrospining [9] are difficult to upscale and often require inert atmospheres or high temperatures, limiting their practicality. We address this challenge by developing a simple approach for the fabrication of CsPbBr₃ and CsPb₂Br₅ nanocrystals inks at room temperature, which are then used for the one-step deposition of thin films, using readily available commercial airbrush technology.

This approach offers significant advantages in scalability compared to existing techniques. We demonstrated the deposition of thin films with tunable properties from these nanoinks by a scalable spray-coting process using a simple commercial airbrush under near-ambient conditions. The nanoinks were placed in the airbrush cup, a nitrogen gas flow with a pressure of 0.4 bar was used, and the flow was controlled through nozzle opening. The substrates were heated on a hot plate to allow for instant solvent evaporation.

We investigated the effects of deposition temperature and the number of deposited layers. We observed crystallographic differences with increasing temperature. At room temperature, we obtained phase-pure orthorhombic CsPbBr₃ and tetragonal CsPb₂Br₅. At higher deposition temperatures, traces of PbBr₂ were observed alongside CsPb₂Br₅ using the CsPb₂Br₅ nanoink. Additionally, CsPbBr₃ nanoink deposited at higher temperature exhibited the presence of a dual-phase CsPbBr₃/ CsPb₂Br₅. The optical properties were studied. We noticed similar photoluminescence for all conditions with an emission peak centered around 520 nm characteristic of this perovskite and as has already been noted by others [10-13].

Eventually, we considered the photocurrent properties of the spray-coated films. With optimized deposition parameters, the photoresponse under different lighting conditions (white light and monochromatic light) shows up to four orders of magnitude differences between photocurrent and dark current for dual-phase CsPb₂Br₅/CsPbBr₃ thin films.

Halide perovskite nanocrystals are promising materials for optoelectronic applications. Metal doping provides an avenue to boost their performance further, e.g., by enhancing light emission, or to provide additional functionalities, such as nano-scale magnetism and polarisation control. However, the synthesis of widely size-tuneable nanocrystals with controlled doping levels has been inaccessible using traditional hot injection synthesis, preventing systematic studies on dopant effects device application.

Here, we report a versatile synthesis method for metal-doped perovskite nanocrystals with precise control over size and doping concentration under ambient conditions. Our room temperature approach results in fully size-tuneable isovalent doping of CsPbX₃ nanocrystals (X = Br, Cl) with various transition metals M²⁺ tested (M = Mn, Ni, Zn). This gives for the first time access to small, yet precisely doped quantum dots beyond the weak confinement regime reported so far. It also enables a comparative study of the photophysics across multiple size and dopant regimes, where we show dopant-induced localisation to dominate over quantum confinement effects. This generalisable, facile synthesis method thus provides a toolbox for engineering perovskite nanocrystals toward light-emitting technologies under industrially relevant conditions. 

Lead halide perovskite nanocrystals (NCs) have emerged recently inspired by the surge of the efficiencies of the solar cells based on thin film perovskites. Two critical problems hindering their widespread application are yet to be resolved: (i) high toxicity due to the presence of heavy metal lead, (ii) relative instability on the presence of various environmental factors. To address these problems, recently, significant efforts to develop lead-free colloidal NCs have been made.[1] Some of the most promising alternative, less toxic, and more stable materials are bismuth halide perovskites (BiHPs), A₃Bi₂X₉. However, there is still a lot of confusion around the synthesis and properties of BiHP NCs for several fundamental and practical reasons: variety of possible structures, potential luminescent impurities, questionable presence of the quantum confinement in such 0D materials, and rare reliable synthetic reports on small BiHP NCs.

In our work, we try to address the above problems using two approaches. First, we have developed rational design to synthesize small and monodisperse colloidal BiHP NCs in a reproducible manner. We study the hot injection synthesis, which allowed obtaining <10 nm-sized NCs by optimizing halide precursors. Then, we will focus on the original method of templated synthesis of BiHP NCs first synthesizing small cesium halide NCs and then converting them by cation insertion into Cs₃Bi₂X₉ NCs with low size dispersion (manuscript in preparation).

In the second part, we will discuss a powerful method for studying the fundamental properties of BiHPs and other Pb-free perovskites by encapsulating them into the mesoporous silica with ordered pores of 2-9 nm.[2] This offers an appropriate means to control the size of the NCs precisely, access small sizes in the quantum confinement regime, and systematically study the effect of size on their properties. The fitted optical absorption spectra revealed that the bandgap of BiHP NCs scale with the pore size. In addition, their exciton binding energies were calculated to be 70-400 meV. It is unambiguously demonstrated for the first time that the 0D BiHPs exhibit the quantum confinement effect. This conclusion is strongly supported by DFT calculations on BiHP clusters with varying dimensions.

Mixed tin-lead (Sn-Pb) perovskite solar cells are promising candidates for next-generation photovoltaics due to their tunable bandgap and potential for high efficiency. Mixed Sn-Pb perovskites possess a close to ideal narrow bandgap for constructing all-perovskite tandem cells to potentially surpass the theoretical efficiency limit of single junction solar cells. One of the main obstacles that need to be overcome is the-oftentimes-low quality of the mixed Sn−Pb perovskite films, largely caused by the facile oxidation of Sn(II) to Sn(IV), as well as the difficulties in controlling film crystallization dynamics. Furthermore, one of the conventional solvent used to process these materials is Dimethyl Sulfoxide (DMSO), which was found to be reduced in the presence of iodide ions, generating in turn iodine-based oxidant species that can degrade Sn(II) perovskite material [1-2]. To address this issue, we investigated the DMSO-free processing of mixed Sn-Pb perovskites.

In our study, we employed methylammonium chloride (MACl) as an additive in a dimethylformamide (DMF)-only solvent system. The introduction of MACl significantly enlarges the grain size, it improves the crystallinity of the perovskite films without introducing new crystalline phases and enhances the optoeelctronic properties of the material. The resulting perovskite solar cells demonstrate efficiencies comparable to control samples processed with traditional DMF/DMSO mixed solvents without additives. More importantly, the devices incorporating MACl exhibit considerably enhanced stability under maximum power point tracking (MPPT), with stable efficiencies for more than 250 hours of continuous operation in N₂ environment. In contrast, control cells reach 80% of the initial power conversion efficiency after only 10 hours, due to a signficant loss in both the open circuit voltage and short circuit current. 

Our findings highlight the potential of DMSO-free processing of mixed Sn-Pb perovskites with MACl additives to overcome the limitations posed by DMSO induced oxidation, paving the way for more stable narrow bandgap perovskite solar cells. This approach provides a promising pathway for the production of high-performance and stable perovskite photovoltaics.

Further breakthroughs in halide perovskite solar cells require advances in new compositions and underpinning materials science. Indeed, a deeper understanding of these complex hybrid perovskite materials requires atomic-scale characterization of their transport, electronic and stability behaviour. This presentation will describe recent combined modelling and experimental studies on metal halide perovskites [1,2] in two fundamental areas related to improving operational stability in optoelectronic devices: (i) Iodide ion transport and the effects of using different sized A-cations and mixed Pb-Sn compositions as there is limited understanding of the impact of Sn substitution on the ion dynamics of Pb halide perovskites; (ii) Insights into passivating perovskites with molecular amino-silane compounds including surface interactions of additives; here we find that strong binding of amino-silanes at undercoordinated surface Pb ions adjacent to iodide vacancies and thereby promoting surface passivation.

[1] Y.H. Lin, M.S. Islam, H.J. Snaith et al., Science, 384, 767 (2024); R. Wang, B. Saunders et al., Energy Environ. Sci., 16, 2646 (2023).

[2] K. Dey, M.S. Islam, S.D. Stranks et al., Energy Environ. Sci., 17, 760 (2024); C. Kamaraki et al., Adv. Energy Mater., 14, 2302916 (2024).

Tin halide perovskites (chemical formula ASnX₃, with A=methylammonium MA⁺, formamidinium FA⁺ or cesium Cs⁺ and X= iodide I^- and/or bromide Br^-) have emerged as promising materials for next-generation photovoltaic and optoelectronic applications. Despite the promise, their power conversion efficiency has not yet matched that of lead-based solar cells (>25%). The soft nature of the tin-halide lattice and the facile oxidation of tin(II) to tin(IV) result in high probability of defect formation and increased background doping levels, thereby leading to significant carrier recombination and low power conversion efficiencies in devices.[1] Recent research has focused on understanding and enhancing carrier dynamics, particularly examining the effects of electronic doping and defect states on the quality and optoelectronic properties of the semiconductor. Optimizing doping levels and minimizing defect concentrations are critical for improving device performance.

By combining simulations with spectroscopy measurements, the role of defects in polycrystalline thin films of tin-based perovskites has been rationalized. Through chemical composition tuning, the chemistry of defects can be altered. Very high background doping limits carrier dynamics due to enhanced recombination of carriers through Auger processes. [2] Electronic doping can be controlled by adding excess tin in the precursor solution, such as extra SnF₂, or by partially substituting iodine with bromine. Furthermore, fluoride can passivate Sn surface defects,[2] while halide alloying tailors the bandgap of the material and improves structural stability.[3] Importantly, both single-halide and mixed-halide tin perovskites exhibit excellent photostability.[4-5] The central cation does not directly affect electronic doping, it significantly influences the morphology of the polycrystalline film, which in turn affects mobility and conductivity.

Integrating these findings provides a comprehensive understanding of how doping, defect states, and chemical composition interact to influence the performance of tin halide perovskites. Leveraging these insights can pave the way for the development of highly efficient and stable perovskite-based devices, advancing the field of renewable energy and optoelectronics.

 Lead halide perovskite (HaPs) shows fast technological progress in photovoltaic and other devices due to their attractive optoelectronic properties. However, the stability of perovskite films remains a major roadblock to their practical implementation which creates concern and calls for advanced characterization methods. The intrinsic self-healing (SH) property of halide perovskites, owing to their strong lattice dynamics, offers a potential solution to this issue. SH means that a material can recover from damage autonomously. Till today, though the mechanism(s) of SH in HaPs and the experimental and material parameters and properties for it are still mostly a black box.

We will show how the SH dynamics can be measured by combining Fluorescence Recovery After Photobleaching (FRAP) and PL imaging, for MAPbI₃ (MAPI) and γ-CsPbI₃ (CsPI) [1-2].  FRAP quantifies fluorescence recovery after photobleaching (by a high-intensity laser source) in a particular region of interest (ROI). Here we use FRAP to resolve the diffusion kinetics of SH-related species.  Specifically, we damaged at different power densities for 3 seconds at 8 different ROIs. For both CsPI and MAPI, we observe a combination of fast (few seconds or less) and slow (tens of minutes) kinetics, both at the illuminated spot and in regions of the film surrounding it. In the periphery of the excitation spot, CsPI shows photo-darkening and MAPI shows photo-brightening, immediately following damage. During the self-healing process, of the directly illuminated spot, MAPI peripheral fluorescence returns to its initial level, whereas CsPI exhibits gradual photo-brightening to above the original level.

We attribute these spatio-temporal effects to a combination of fast carrier diffusion following photoexcitation and slower ionic diffusion that kick in at later stages.  The role of carrier migration in emission dynamics following photoexcitation has generally been neglected but may be an important factor in experiments studying the system response under illumination. Assessment of the diffusion coefficients for the slower process from the self-healing rate is commensurate with halide ion diffusion. This is partially supported by Raman spectroscopy of damaged films. Overall, our work provides visual evidence for the spatiotemporal dynamics of photo-damage and SH of HaPs, an understanding of which will aid the development of long-term device applications.

Further improvements in the performance of perovskite solar cells requires a deep understanding of their device physics and the different power loss mechanisms. In the literature, defect densities at the perovskite/transport layer interfaces, ion-mediated non-radiative recombination and imperfect charge extraction have been cited as the main sources of these power losses. However, the magnitudes of the parameters of these loss mechanisms, such as defect densities, ion densities, capture coefficients and time constants/lifetimes are not well known. This is due to the difficulty of discriminating between multiple mechanisms that respond simultaneously in a typical characterization measurement, in addition to the lack of optoelectronic models describing these loss mechanisms clearly. In this talk, I will provide improved data analysis methods for typical optoelectronic methods (both in the time and frequency domain) used to characterize perovskite solar cells, to accurately discriminate between the loss mechanisms and determine their parameters.

In the case of capacitance measurements to determine defect/doping/ion densities, I will show that the capacitance response of the perovskite layer is hidden by the response of the electrodes and the resistive transport layers. This effect dominates the response in several capacitance measurements reported in literature, leading to the calculated defect densities and related parameters largely being artefacts of measurement. Analytical resolution limits are derived for these techniques to distinguish a real defect response from a measurement artefact. Finally, I will show experimental measurements on single crystal devices that overcome these limits and allow identifying the actual defect density in the device.

I will furthermore develop an optoelectronic model that explicitly accounts for the extraction of charge carriers through the resistive transport layers, and apply it to the analysis of small perturbation methods in the time and frequency domain. This model predicts the existence of a time constant for charge carrier extraction, in additional to bulk and electrode-mediated recombination time constants. By developing modified transfer functions in the frequency domain, the charge extraction and recombination time constants can be accurately extracted using these methods. I will also develop a figure of merit that determines the efficiency of charge extraction in the perovskite solar cell. This figure of merit depends on the time constants of charge extraction and recombination, and can be calculated at each bias point of the current-voltage curve. Figure of merit values between 0.7–0.95 at or close to the 1 sun open-circuit voltage are obtained experimentally, indicating a significant electric field exists in the transport layers in these conditions.

Understanding the atomic-scale crystallographic properties of photovoltaic semiconductor materials such as silicon, GaAs, and CdTe has been essential in their development from interesting materials to large-scale energy conversion industries. However, studying photoactive hybrid perovskites by transmission electron microscopy (TEM) has proved particularly challenging due to the large electron energies typically employed in these studies. [1,2] In particular, the very close structural relationship between a number of crystallographic orientations of the pristine perovskite and lead iodide has resulted in severe ambiguity in the interpretation of EM-derived information, severely impeding the advance of atomic resolution understanding of the materials.

In this talk, I will outline how to reliably study hybrid organic-inorganic perovskite materials using electron microscopy. With the ability to image the pristine phase of these beam-sensitive materials, we are able to obtain highly localised crystallographic information about technologically relevant materials. Using low-dose selected area electron diffraction, I will show how mixing the archetypal CH(NH2)2PbI3 (FAPbI3) and CH3NH3PbI3 (MAPbI3­) improves solar cell device performance through the elimination of twin domains and stacking faults. [3]

Using a careful low-dose scanning TEM (STEM) protocol, we are also able to image these materials in their thin-film form with atomic resolution. [4] Our images enable a wide range of previously undescribed phenomena to be observed, including a remarkably highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI2 interfaces, with a striking absence of long-range disorder in the crystal. These findings explain why inter-grain interfaces are not necessarily detrimental to perovskite solar cell performance, in contrast to what is commonly observed for other polycrystalline semiconductors. Additionally, we observe aligned point defects and dislocations that we identify to be climb-dissociated, and confirm the room-temperature phase of CH(NH2)2PbI3 to be cubic. We further demonstrate that degradation of the perovskite under electron irradiation leads to an initial loss of CH(NH2)2+ ions, leaving behind a partially unoccupied, but structurally intact, perovskite lattice, explaining the unusual regenerative properties of partly degraded perovskite films. Our findings thus provide a significant shift in our atomic-level understanding of this technologically important class of lead-halide perovskites.

Finally, I will show how we can use gentle conductive AFM to probe the IV-characteristics of mixed halide perovskite thin films very locally, showing how we can control the local hysteresis behaviour by controlling the presence of extended intragrain defects using additives. This allows us to highlight the influence of ion migration on the optoelectronic properties of perovskite thin films with very high resolution. [5]

Our findings thus provide a significant shift in our atomic-level understanding of this technologically important class of lead-halide perovskites.

Ion migration is usually viewed as detrimental to the performance of perovskite solar cells. In particular, the high mobility of the halide anions can initiate phase separation and other material changes which lead to bulk and interface degradation and loss of cell efficiency.

On the other hand, it is becoming clear that if ion mediated degradation can be prevented, then there are many benefits to the presence of mobile ions in perovskites.

Recently it has been demonstrated computationally and experimentally that mobile ions improve the performance of perovskite cells with non-ideal interfacial band offsets [1][2]. The interplay between ion location and charge recombination can also be used to gain key information about the band off-sets in fully operational devices [3]. The ion mediated low frequency impedance response can tell us whether a cell is electron or hole recombination limited and indicate which interface is limiting cell performance [4]. In this presentation the benefits and disadvantages of mobile ions will be discussed; with a particular focus on the key information the ionic response can give us in common characterisation techniques such as JV curves and impedance spectroscopy.

Hybrid organic inorganic metal halide perovskites (MHPs) denote a family of compound semiconductors, which established a novel class of optoelectronics, most prominently known for the perovskite solar cell. While the power conversion efficiency of these photovoltaic devices saw a steep rise in the past decade, tailoring the interfaces between the MHP film and charge transport layer became the major control lever to enhance performance and stability. The use of photoemission spectroscopy to analyze the chemical and electronic properties of these interfaces has been challenging due to many possible chemical reactions at the buried interfaces.¹ However, the formation of a stable interface between the perovskite film and adjacent functional layers is critical to enable effective protection coatings.

Here, I will discuss the use of synchrotron- and lab-based X-ray photoelectron spectroscopy (XPS) experiments to address the particular chemistry of MHP interfaces to adjacent oxide charge transport and pre-encapsulation layers (CTL). At the example of SnO₂ and NiO layers grown by atomic layer deposition (ALD) on top of a double cation mixed halide perovskite film investigated by hard X-ray photoelectron spectroscopy (HAXPES), we find evidence for the formation of new chemical species and changes in the energy level alignment at the interface detrimental to cell performance.²

I will conclude with a general discussion on the use of PES methods for the analysis of MHP layers and in particular the effect of irradiation-induced damage via synchrotron and lab-based X-ray sources,³ which we also use to track unique physicochemical phenomena such as stimulated self-healing in formamidinium lead bromide.⁴

Hybrid metal halide perovskites have given rise to some of the most exciting developments in optoelectronics over the last decade, and are the most promising candidates for high-performance multi-junction solar cells that surpass the fundamental efficiency limits of traditional devices. Furthermore, their bandgap tunability, high radiative quantum yields, and defect tolerance also make them excellent light emitters. Two-dimensional (2D) perovskite semiconductors have promising prospects for enhancing the stability of perovskite-based photovoltaic devices. In addition, these low-dimensional materials with electronic confinement offer further opportunities in light emission and quantum technologies. However, their technological applications still require a comprehensive understanding of the nature of charge carriers and their transport mechanisms. This talk will show how time-resolved optical spectroscopy can be employed to investigate charge-carrier dynamics, exciton formation dynamics, charge-carrier mobilities, and charge-phonon coupling in perovskite semiconductors. I will discuss the impact of heterogeneities and low dimensionality, and the peculiar softness of the lattice. Our work reveals band transport with high in-plane mobilities that give rise to efficient long-range conductivity in 2D perovskites. We show how the organic cation moderates the coupling of charge carriers to optical phonon modes, impacting the charge-carrier mobilities. Furthermore, we demonstrate a new experiment for simultaneously recording the terahertz and optical transmission transients, thus allowing us to monitor the exciton formation dynamics over the picosecond timescale. The observed dynamics reveal a long-living population of free charge-carriers that greatly surpasses the theoretical predictions of the Saha equation even at temperatures as low as 4K. Our findings provide new insights into the temperature-dependent interplay of exciton and free charge carriers in 2D Ruddlesden-Popper perovskites. Furthermore, the sustained free charge-carrier population and high mobilities revealed by this work demonstrate the potential of these semiconductors for applications that require efficient charge transport, such as solar cells, transistors, and electrically driven light sources.

All-inorganic halide perovskite nanoplatelets (PNPls) have recently gained significant attention in the field of materials science. These materials exhibit remarkable optoelectronic properties, making them promising candidates for applications in solar cells, LEDs, and photodetectors. The interest in PNPls stems from their exceptional electronic and optical properties, which can be tuned over a wide range that are deeply intertwined with their unique structural characteristics. Tuning the properties of PNPls has relied at first on changing of the elemental compositional of the perovskite structure, whether the halide part and/or the 1^st cation from organic to inorganic, or between different organic molecules. Over the last few years, studies have shown that optical tuning of PNPls based on the number of monolayers (MLs) is a viable strategy, where the monolayer corresponds to a single microscopic layer of inorganic metal-halide octahedrons surrounded by large organic cations or ligands. While extensive research has explored the optical properties of these systems, the role of phonons (lattice vibrations) in different dimensionalities remains underexplored. This is important for a fundamental understanding of the collective carrier-phonon coupling in the excited states, thermalization process, initial charge separation and final transport that includes the mobility of electrons and holes and their interconnection to the charge carrier-lattice interactions. Since all the above-mentioned phenomena are dimensionality- and phase-related, where the coupling between the excited state and phonons changes dramatically with both the dimensionality and phase, the vibrational properties over different system dimensionalities and/or crystallographic phases should be revealed for a better understanding of the role of phonon in the materials.

In this work, we employed optical and non-resonant Raman spectroscopy for the realization of, firstly, a model for the vibrational properties of 2-6 MLs of CsPbBr₃ NPls in comparison with larger nanocrystals. Our Raman measurements showed that, by systematically varying the number of monolayers of the nanoplatelets, there is distinct changes in the relative intensities of the vibrational modes that are sensitive to the number of monolayers. These observations can be attributed to the quantum confinement effect, which becomes more pronounced as the thickness of the nanoplatelets decreases. In addition, there is strain generated in the materials upon the formation of lower thickness NPls. This can be identified through the shift of Pb-Br vibrational mode. Secondly, we established a model of the vibrational properties over a wide temperature range to identify changes in phase, strain, and anharmonicity for different system dimensionalities. Raman measurements revealed the tetragonal phase formation at room temperature for all prepared nanocrystal systems is dominant with the orthorhombic and cubic phases at low and high temperatures, respectively. This in addition to the phase related and/or the anharmonicity of the shift of phonon modes with temperature. Furthermore, there were changes in the peak intensities’ ratios, which provide valuable insights into the structural variations induced by the number of monolayers.

Strain, phase transitions, anharmonicity, and their dependence on dimensionality are important for electrical and thermal conductivity. Moreover, it helps to clarify the mechanism and pathways of electronic energy into heat in 2D lead halides. Therefore, these results provide an essential initial overview of the crucial vibrational properties of the system, paving the way for a better understanding of carrier-phonon coupling in these materials. The ability to determine these structural parameters via Raman spectroscopy establishes it as an indispensable characterization technique for rapid and accurate analysis of thickness and confinement regimes in perovskite nanoplatelets and suggests its potential for dimensionality analysis in other nanocrystal families.

Three-dimensional (3D) and low-dimensional (LD) perovskite solar cells (PSCs) are among the most promising strategies for achieving highly efficient and stable perovskite solar cells. Despite being popular and effective, the necessity of the surface LD perovskite (LDP) layer remains uncertain [1]. The use of organic salts without forming an LDP raises questions about the need for this capping layer. I will compare recent results obtained with the 3D/LDP configuration to those using surface cation passivation and 3D/LDP bilayers, both of which can achieve the highest efficiency values in perovskite solar cells. This comparison will provide a comprehensive perspective on the benefits of these two strategies, including examples where both approaches have achieved efficiencies exceeding 23%[2,3].

Reference
[1] Sam Teale, Matteo Degani, Bin Chen, Edward Sargent, Giulia Grancini, Nat Energy, 2024.
[2] M. Degani, Q. An, M. Albaladejo-Siguan, Y. J. Hofstetter, C. Cho, F. Paulus, G. Grancini and Y. Vaynzof, Sci. Adv, 2021, 7, 7930
[3] Matteo Degani, Riccardo Pallotta, Giovanni Pica, Masoud Karimipour, Alessandro Mirabelli, Kyle Frohna  Miguel Anaya, Samuel D. Stranks, Monica Lira Cantù, Giulia Grancini, submitted

To make better electronic and light-based devices using advanced materials, we need to understand how their structure affects their properties. Hybrid organic/inorganic perovskites are promising new materials, which are both soft and ionic, i.e. atoms can wiggle around their equilibrium positions and they are highly charged¹. Transitions of electrons to different bands will distort the crystal lattice, and it will do so along specific vibrational coordinates. While there is consensus that electron-phonon interactions in these materials are crucial in determining their optoelectronic properties, i.e. EPC limits the maximum charge-carrier mobility², it has been extremely challenging to obtain direct- and mode specific information on these interactions.

The big question is: which vibrational modes are coupled how strong to which electrons? And how do we measure this?

Here, we will dive into the world of (multidimensional) THz spectroscopy to answer the above questions. I will highlight our recently developed 2D-electron-phonon-coupling spectroscopy (2D-EPC), which can extract direct information on mode-specific electron-phonon interactions. We benchmarked this technique on prototypical methylammonium lead iodide, and show there is distinct behavior of the coupling of electrons between two different vibrational modes. Temperature dependent experiments allow us to follow the EPC over the tetragonal-to-orthorhombic phase transition, and polarization dependence allows us to study mode-specific and direction-dependent dispersion of this coupling³.

We also varied the A-site cation in three prototypical ‘black-phase’ perovskites: methylammonium (MA) lead iodide, formamidinium (FA) lead iodide and Cesium lead iodide, and demonstrate differences in the coupling between vibrational modes of the inorganic lattice and electrons with different energies.

Lead halide perovskite-based photovoltaics has attracted great attention in the last decade since silicon PV as the dominating technology is approaching its theoretical efficiency limit. Combined in a silicon-perovskite (Si-Pero) tandem solar cell, they promise a substantial leap in power conversion efficiency beyond the single-junction limit without increasing cost significantly. However, realizing high performance in module-sized formats is, in part, held back by the practical challenge of producing uniform high-quality perovskite layers onto large scale silicon bottom cells. Perovskite thin films are highly poly-crystalline and mechanically soft resulting in high defect concentration and inhibiting efficient carrier extraction if not properly controlled.

In pursuit of a non-invasive and robust in-line imaging method that locally resolves the quality of the perovskite thin film absorber processed over a silicon bottom solar cell, we advanced the k-imaging method developed by Hacene et al. [1] also for Si-Pero tandem solar cells. K-imaging is based on intensity dependent perovskite photoluminescence, eliminating the effect from constant optical in- and out-coupling effects. With a basic power law model a single effective parameter k is extracted which reflects the complex superposition of competing recombination processes, i.e. radiative band-to-band, non-radiative trap-assisted bulk (Shockley-Read-Hall) and interface recombination. This parameter k allows for quantitative analysis simplifying the interpretation significantly in contrast to qualitative PL imaging, while barely raising setup complexity and cost.

We show that k-imaging reproducibly identifies general quality discrepancies as well as local inhomogeneities, which clearly correlate with typical defects in the thin film. Further, we proved its applicability regardless of specific perovskite processing techniques and its compatibility with flat as well as industrially relevant textured Silicon bottom cells. Overall, k-imaging is a valuable and unique technique meeting the requirements for efficient optimization in academic research as well as quality assessment in large-scale industrialized production of Si-Pero tandem solar cells.

Despite the remarkable optoelectronic efficiencies of hybrid perovskite based devices heterogeneity in both the performance and stability is observed at the nanoscale [1,2]. Furthermore, improvements in the performance of solar cells, LEDs and X-ray detectors are often achieved through empirical findings, with a full understanding of the underlying structural mechanisms yet to be elucidated. Considering this fact, we present how correlative multimodal microscopy, enabled by computer vision algorithms, has allowed for structural insights into: the intrinsic quantum confinement phenomena observed in FAPbI₃ [3]; and the origins of photo-induced halide segregation observed in mixed halide compositions [4].

Scanning electron diffraction (SED), a variant of 4D-STEM, allows for information on crystallographic phase, orientation, and defects to be uncovered; whereas hyperspectral photoluminescence (PL) mapping provides information on optoelectronic characteristics. The spatial correlation between the two techniques therefore provides a direct link between structure and performance. Similarly to the work of Jones, Osherov and Alsari et al. [5] to find common areas between separate tecniques, gold fiducial markers are synthesized and deposited onto a thin film. Due to the resolution differences between SED (typical probe size ∼5 nm) and hyperspectral PL mapping (resolution diffraction limited to ∼100s of nm) multiple contiguous SED scans are recorded before being stitched together during post processing. To do this robustly a keypoint detection and matching algorithm is applied to virtual bright field images created from the SED data [6]. Once the keypoints are detected the random sample consensus algorithm, or variants thereof, is used to define an affine transform which stitches one image onto the other, thus correcting for differences in rotation, shear, and translation [6]. To finally coregister the hyperspectral PL and stitched SED datasets several options are available, however common approaches operate on the principle of maximising metrics such as the normalised cross correlation or mutual information between datasets [7]. Importantly, as all image transforms are known we can then calculate where each ‘pixel’ from a SED scan corresponds to in the hyperspectral PL data.

Due to the size and complexity of the resulting datasets interpretation of the data is challenging. For instance, if only 50 SED scans are considered, each of which contains 512 x 512 diffraction patterns (typical during one experimental session), this amounts to a total of 13,107,200 individual patterns and hundreds of gigabytes of data. To mitigate this problem and rapidly reduce the dimensionality of the data we have adapted the simple linear iterative clustering (SLIC) algorithm, prevalent in the field of remote sensing [8]. This approach allows us to reduce hundreds of thousands of individual diffraction patterns to ∼100 single crystal patterns obtained by averaging over individual grains of the polycrystalline film. Remarkably, once parallelised, this approach proves exceptionally computationally efficient with typical compute times taking approximately a minute using a standard desktop machine (32Gb RAM, 11th Gen Intel(R) Core(TM) i5-11400 CPU). An automated indexing procedure of the clustered SED patterns can then be employed to obtain phase and orientation maps over large areas (typically ∼20x20 μm) at low computational cost.

This approach has allowed the structural causes of the intrinsic quantum confinement phenomena observed at cryogenic temperatures in FAPbI₃ to be attributed to {111}_c type nanoscale twinning, and an understanding of the structural origins of photo-induced halide segregation to be obtainable. We hope this toolkit, with data analysis pipelines which are open source, enables correlative microscopy experiments with other instruments and on other material systems. Furthermore, we anticipate the insights provided on hybrid perovskites will allow for more directed modifications to the fabrication of devices, thereby accelerating improvements in device efficiency and stability.

Even though the efficiency of perovskite solar cells has increased significantly in recent years, their long-term stability is still poor, hindering commercial applications. One of the main reasons for this poor stability is mobile ionic carriers, which can migrate within the perovskite crystal lattice and accumulate at the adjacent charge transport layers, reducing the extraction efficiency of electronic carriers. To compare strategies to mitigate ion migration, reliable ways of quantifying the density, mobility, and activation energy of mobile ions are necessary. Here, we propose an updated way of characterizing mobile ions based on capacitance transients. In capacitance transients, we generally measure the modulation of the electronic capacitance due to mobile ions drifting through the perovskite. In our updated approach, we first approximate the time-dependent ionic carrier, electronic carrier, and potential distribution within a perovskite solar cell after applying a voltage pulse. Subsequently, we calculate the capacitance from these distributions using a small-signal approximation of the drift-diffusion equations, resulting in capacitance transients. By fitting capacitance transients generated from drift-diffusion simulations, we show that an accurate extraction of the density, mobility, and activation energy of mobile ions within the perovskite is possible. Lastly, we apply the proposed model to measured capacitance transients of p-i-n perovskite solar cells and approximate their ion density, ionic mobility, and activation energy.

A recent classification of multilayered perovskites as Ruddlesden-Popper, Dion-Jacobson and "Alternative cations in the interlayer" was introduced in relation with the chemistry of the compounds or the crystallographic order along the stacking axis. 2D multilayered perovskites  exhibit  indeed attractive features related to tunable quantum and dielectric confinements, strong lattice anisotropy, more complex combinations of atomic orbitals and lattice dynamics than 3D perovskites, extensive chemical engineering possibilities. But more important for photovoltaic applications, 2D multilayered perovskites have exhibited improved device stability under operation [1]. Combined in quasi-lattice matched [2] 2D/3D thick bilayer structures, excellent solar cell device stability can be achieved. Band alignment calculations nicely explain the difference of performances for n-i-p or p-i-n devices [3]. The lattice mismatch concept [2] can also provide guidance for the choice of the proper 2D/3D combination, with the prospect of using 2D perovskite as a template during the growth of a 3D perovskite, leading also to enhanced stability of 3D-based solar cells. This was demonstrated recently the stabilization of MA-free, solar cells based on pure FAPbI3 [4]

Nanoscale resolved imaging and spectroscopy using scattering-type Scanning Near-field Optical Microscopy (s-SNOM) or tapping AFM-IR (local detection of photothermal expansion) bypasses the diffraction limit of light to achieve a wavelength-independent spatial resolution of < 20 nm in the infrared (IR) frequency range [1,2]. A wide range of analytical capabilities have been demonstrated, e.g. nanoscale chemical mapping and material identification [3], conductivity profiling [4,5], determination of secondary structure of individual proteins [6] and vector field mapping [7], making them a trusted tool for surface analysis in many branches of sciences and technology. Applications are often limited by a lack of suitable light sources, preventing studies of low energy phonons, polaritons, and molecular vibrations. Here we demonstrate s-SNOM and tapping AFM-IR imaging and spectroscopy based on a fully integrated and automated commercial OPO laser source, covering the spectral range 1.4 – 18 μm (ca. 7140 – 550 cm-1) with narrow linewidth < 4 cm-1 in the entire tuning range. Sweeping the laser frequency enables nano-spectroscopy with unprecedented spectral coverage, enabling studies of fundamental molecular resonances and quantum states in the long wavelength IR spectral range, which until now was not possible.

The performance of metal halide perovskite (MHP) thin films in optoelectronic devices is heavily influenced by their final crystal structure and nanomorphology. Therefore, understanding growth processes during deposition and post processing or ageing is important to control the performance in optoelectronic devices. I will show examples of the application of advanced in-situ characterization techniques to gain real-time insights into the formation and evolution of perovskite films. By monitoring the growth dynamics of perovskite thin films during solution processing, we can identify critical factors that govern the nucleation, intermediate phase formation, and final crystallization of the material. In-situ techniques such as grazing incidence wide angle X-ray scattering (GI-XRD), optical microscopy, and photoluminescence (PL) spectroscopy are employed to capture transient phases and intermediate states that are otherwise challenging to detect in ex-situ studies. Machine Learning approaches may help us in future to reduce characterization cost. This research contributes to the broader understanding of MHP thin film fabrication, offering guidelines for the scalable production of high-performance optoelectronic devices.

Stereochemically active lone pair (SCALP) cations are attractive units for realizing optical anisotropy. Antimony(III) chloride perovskites with the SCALP have remained largely unknown to date. We synthesized a new vacancy ordered Cs₃Sb₂Cl₉ perovskite single crystals with SbCl₆ octahedral linkage containing the SCALP. Remarkably, all-inorganic halide perovskite Cs₃Sb₂Cl₉ single crystals exhibit an exceptional birefringence of 0.12 ± 0.01 at 550 nm. The SCALP brings a large local structural distortion of the SbCl₆ octahedra promoting birefringence optical responses in Cs₃Sb₂Cl₉ single crystals. Theoretical calculations reveal that the considerable hybridization of Sb 5s and 5p with Cl 3p states largely contributes to the SCALP. Furthermore, the change in the Sb−Cl−Sb bond angle creates distortion in the SbCl₆ octahedral arrangement in the apical and equatorial directions within the crystal structure incorporating the required anisotropy for the birefringence. This work explores pristine inorganic halide perovskite single crystals as a potential birefringent material with prospects in integrated optical devices.

The ideal high-bandgap partner for a tandem solar absorber using c-Si as the low-bandgap bottom cell must have a bandgap of 1.6 eV to 1.9 eV and excellent optoelectronic properties.

Selenium, perhaps the very first material to be studied for its photovoltaic properties, is emerging as an interesting candidate for this application. Indeed Se has a bandgap of around 1.95 eV [1] and a steep increase in absorption above this photon energy. Todorov et al showed [2] in 2017 that single-junction Se solar cells could reach 6.5% efficiency using a new cell architecture with a very thin absorber layer. Recently we have increased the OCV to a new record of 991 mV [2] and mapped the main shortfalls of state-of-the-art Se-cells preventing them from approaching their theoretical potential OCV. The talk will introduce Se as a solar absorber and discuss we know about its properties and progress towards use in solar devices such as tandem photovoltaics and PEC stacks [3].

The high efficiency thin film technologies available or emerging (e.g., CIGS, CdTe or halide perovksites) have all issues in terms of cost, element abundance or long-term stability. Finding new solar absorber is a cumbersome process involving complex synthesis and characterization. First principles computations on the other hand can asses important solar absorber properties such as band gap, mobilities or defects and offer an attractive way to speed up this process. Here, we will report on a large scale high-throughput computational search for new solar absorbers among known inorganic materials. Importantly, the need for high carrier lifetime is taken into account by including in the screening intrinsic defects and their role as potential Shockley-Read-Hall recombination centers. Screening 30,000 known inorganic compounds, we identify a handful very promising solar absorbers. I will discuss the chemistries that we identified and highlight a few interesting new materials. I will especially focus on BaCd₂P₂, a new phosphide where our experimental follow-up work confirms the promising properties including adequate band gap but also long carrier lifetime and very high stability. Beyond BaCd₂P₂, our work highlights the discovery of an entire family of Zintl phosphides with exciting recent results on CaZn₂P₂ thin films. I will finish my talk highlighting the opportunities and challenges ahead in computationally-driven discovery of new solar absorbers.

The synthesis of multinary semiconductors for solar energy conversion applications such as kesterite (Cu₂ZnSn(S,Se)₄, CZTSSe) is extremely challenging due to the complexity of this type of compounds. Having multiple elements in their structure the formation of secondary phases, punctual or extended detrimental defects, and/or singular interfaces is commonly very problematic. In particular, quaternary kesterite-type compounds are not the exception, and all these detrimental issues explain why during almost 10 years the world record efficiency was unchanged. But, the very recent development of molecular inks route with special precursors, allows the accurate control of single kesterite phase with high crystalline quality. In addition, the use of selective diluted alloying has shown a high potential for minimizing detrimental punctual defects formation, contributing to increase the conversion efficiency record of kesterite based solar cells up to 15% in a short time.

This presentation will be focused first in demonstrating how the molecular inks synthesis route was of key relevance for the control of high quality single phase kesterite, through the modification of the synthesis mechanisms. The relevance of the composition of the ink, the precursor salts, and the interaction between the solvent and the cations in the solution is key for a reliable and reproducible high efficiency kestetite production baseline. Then, diluted alloying/doping strategies will be presented including Cu, Zn and Sn partial substitution with elements such as Ag, Li, Cd or Ge [1]. The positive impact of these cation substitutions will be discussed in regards of their impact on the kesterite quality, as well as on the annihilation of detrimental punctual defects, allowing for new efficiency records at 15% level.

Finally, very recent and innovative interface passivation strategies will be discussed, showing the pathway to increase the record efficiency beyond 20%.

The Internet of Things (IoT) is a rapidly growing ecosystem of billions of smart devices connected together via the cloud, embedding intelligence into infrastructure. But a significant challenge is the reliance of these devices on batteries as the power supply [1]. This talk explores the use of indoor photovoltaics (IPVs) as an alternative, either directly powering the IoT devices, or working in synergy with energy storage devices. Lead-halide perovskites currently demonstrate the highest power conversion efficiency (PCE), now approaching 45% [2]. However, the high lead content could act as a barrier to use in consumer electronics. This talk explores the development of IPVs from lead-free perovskite-inspired materials. We start with our early efforts on bismuth- and antimony-halide materials [3], discussing the challenges and opportunities with silver bismuth halide compounds [4], before covering our recent work with Sb₂S₃ IPV [5]. In the latter case, we achieve 17.55% PCE under indoor lighting, the highest yet reported for this material, and develop 5 cm² minimodules as a prototype to power a multisensory device. We finish with the broader challenges and opportunities of the emerging area of perovskite-inspired materials for IPVs.

Solar energy has experienced remarkable growth in the past decades, but innovative PV technologies and architectures must be developed to reduce the cost/watt utilization. Thin-film solar cells with high absorption coefficients and direct band gaps have emerged as promising candidates for achieving high efficiency at low cost. Thin-film solar cells, usually necessitate light-trapping strategies to reduce optical losses at the nanoscale, but similar nanoscale strategies can be used to reduce also the electrical losses. Recombination losses, Shunt, and Series resistances can be controlled by engineering the surface contact areas with appropriate nanopatterning designs. In this work, we explore the effects of reducing the junction area in thin film solar cells by using coupled 3D simulations based on FDTD, Schrodinger-Poisson solver, and Drift diffusion models to predict the performance of different stacks and 3D patterns.  As a case study, we focus on Zinc-Phosphide (Zn₃P₂) as an emerging earth-abundant, p-type absorber material with a direct band gap. Different n-type heterojunction partners such as Indium Phosphide (InP), Silicon  (Si), and Titanium Oxide (TiO₂) have been explored, and an intermediate Silicon oxide (SiO₂) layer with Zn₃P₂-filled holes in a periodic configuration have been introduced to modulate the n-contact area. The junction area fraction has been varied from 100% (no openings) down to 1.4% for each configuration, showing an overall improvement of the cell performance in efficiency due to a strong increase in the open circuit voltage. The dark JV curves, simulated and modeled with a 2/3Diode model, showed the linear relationship between saturation current and junction area fraction. The reverse saturation current, mainly represented by recombination mechanisms, scales down with the contact area. The beneficial effect of this method has been proved independently by the heterojunction contact leading to an increase of the V_oc from 0.07V up to 0.12 V. With this method Zn₃P₂ based solar cell with a record efficiency of 5.96%[1] can be potentially improved up 12-13% trying different kind of junctions and demonstrating the versatility of this approach.

The mitigation of climate change requires major transformations in the ways we generate energy and

operate technologies that release CO2. Photonic concepts and novel light-driven technologies provide many opportunities to mitigate CO2 emissions, transforming our current modes of energy use into more effective and sustainable ones. In a recent review paper, we describe several of these concepts that are in the early stage of scientific discovery, with at the same time great technological potential.1

In this presentation, we focus on how to create photovoltaics with improved properties that have potential for large-scale implementation. I will present an integrated near field/far-field multiple scattering formalism to control the absorption of light in multijunction solar cells. As a model system we use III-V/Si multi-junction solar cell and enhance the light trapping inside the silicon bottom cell by multiple scattering, creating a record photovoltaic energy conversion efficiency for silicon-based multijunction solar cells of 36.1%.2 A similar light trapping concept can be applied in other multijunction solar cell geometries, such as perovskite/silicon tandem solar cells.

We then present a study on the nanoscale incoupling of light in textured perovskite/silicon solar cells, and show how optical Mie resonances create strong light inhogeneities in the tandem solar cell that can affect its performance. In addition we create micron-scale light scattering structures in solar cells to enhance emission in the (far-)infrared to create passive radiative cooling, enhancing the efficiency of and long-term stability of the solar cell. Light-driven processes can also help fabricate novel photovoltaics materials, and we show our most recent work on laser-induced crystallization of methyl-ammonia lead iodide perovskite directly from solution, with the crystal formation monitored in-situ through photoluminescence and Raman spectroscopy.

I will also present the 900 M€ Dutch national research, innovation and industrial development program SolarNL, in which universities, research institutes, and companies work together to develop photovoltaics technology and industry to help create a fully sustainable energy generation system in our society by 2040.3

References

1)   Photonic solutions to fight climate change, G. Tagliabue, H.A. Atwater, A. Polman, and E. Cortes, Nature Photon. (2024), in press. See here for a preprint of this article.

2)   Wafer-bonded two-terminal III-V//Si triple-junction solar cell with power conversion efficiency of 36.1 % at AM1.5g, P. Schygulla, R. Müller, O. Höhn, M. Schachtner, D. Chojniak, A. Cordaro, S. Tabernig, B. Bläsi, A. Polman, G. Siefer, D. Lackner, and F. Dimroth, Progr. Photovolt. 32, 1-9 (2023); Nano-patterned back-reflector with engineered near-field/far-field light scattering for enhanced light trapping in silicon-based multi-junction solar cells, A. Cordaro, R. Müller, S. Tabernig, N. Tucher, P. Schygulla, O. Höhn, B. Bläsi, and A. Polman, ACS Photon. 10, 4061 (2023)

3)  SolarNL: www.solarnl.eu

Transparent Photovoltaic (TPV) technologies represent a promising branch within photovoltaics, seeking to expand their applications by overcoming challenges related to on-site integration, especially within architectural elements related to BIPV, and more recently, also in the areas of IoT and Agrivoltaics. Unlike conventional approaches solely focused on efficiency, TPV introduces two additional dimensions: transparency and aesthetics, which pose added challenges to the device architecture. Moreover, for TPV technologies to be translated into competitive products it is critical to work on sustainable and stable materials that at least meet the stringent requirements for any PV technologies in conjunction with the transparency and aesthetic value that allow for seamless integration.

This challenge is being actively investigated using different materials, such as organic materials and perovskites. However, oxide-based structures constitute a very attractive prospect as they can be integrated as different functional layers in the solar cell architecture (i.e. as absorber, Charge Transport Layer and transparent electrical contacts). Additionally, many binary or ternary oxide compounds present high bandgap, tuneable conductivity, low deposition temperatures and can be deposited by a plethora of techniques that are possible to upscale for industrial purposes. Another key aspect is that many oxide materials are stable, cheap and CRM-free. Given these aspects the challenge is on how to combine them in advanced device architectures (with other oxides or materials) to develop a final device that is efficient, transparent and aesthetically pleasing that can be integrated in architectural components (windows, canopies, façades) or even on devices that present low power draws such as smartphones or screens and IoT devices and sensors.

Herein, we will discuss the basic principles of TPV as well as the state of the art of oxide-based strategies. This includes two main approaches that are based on either the development of Zn(O.S) UV-selective absorbers and on the optimisation of oxide-based architectures integrating nanometric a-Si:H layers. Main challenges and late results achieved in both strategies will be reviewed, including the achievement of record devices with Light Utilisation Efficiency up to 1.3%, transparency in the range between 30% and 70% and photoconversion efficiencies up to 5%.

A method for improving the quality of heterointerfaces which has been increasingly investigated is selective area epitaxy (SAE) as it can help reduce interface defect formation and defect propagation between layers. SAE relies on a mask layer, such as silicon dioxide, patterned with (nano)holes where the growth is limited to under appropriate growth conditions. First, by reducing the area of the holes down to the nanoscale it can have a significant impact in the defect formation mechanisms, such as misfit dislocations, during epitaxial growth. Moreover, any threading dislocations will also be stopped by the mask layer, significantly reducing the amount propagating into the epilayer. SAE therefore holds potential in facilitating the epitaxial growth of materials lacking a lattice matched substrate as well as allowing for new material combinations for enhanced photovoltaic device performance. However, most of the techniques used for SAE using nanoscale holes rely on low-throughput techniques, such as molecular beam epitaxy and electron beam lithography. In our recent work we have shown how to overcome these limitations using e.g. metalorganic chemical vapour deposition (MOCVD) and Talbot Displacement lithography for the case of the earth-abundant photovoltaic absorber zinc phosphide (Zn₃P₂).

Zn₃P₂ is an emerging photovoltaic absorber for single-junction devices with a direct bandgap (1.5 eV) and other promising optoelectronic properties. However, its large lattice parameter and coefficient of thermal expansion has complicated its incorporation in heterojunctions, while the lack of controlled n-type doping has hindered the creation of homojunctions. Previous work has shown that SAE allows for high quality epitaxial growth of Zn₃P₂ nanopyramids and textured thin films. Unfortunately, the approach used relied on the aforementioned low-throughput techniques in addition to the use of scarce elements (In) in the substrate. In our recent work we have demonstrated how to overcome the first two limitations through the compatibility of SAE grown Zn₃P₂ with MOCVD, as well as scaling the nanopatterned areas using Talbot Displacement lithography. Through a combinatorial study we have explored the effect of temperature, precursor partial pressures and pitch on factors such as growth selectivity, defect formation and functional properties that were evaluated using a range of microscopy and spectroscopy techniques.

Zinc phosphide (Zn₃P₂) constitutes as a promising solar absorber material due to its high absorption and carrier moblity as well as due to the abundance of zinc and phosphorous in the earth crust. Since the first published studies few decades ago, the efficiencies of Zn₃P₂-based solar cells have remained below its potential. We believe this is mostly due to the limited understanding of how to tune its optoelectronic properties.

In this talk we report on the progress towards the understanding of the growth and functional properties of the material such as light absorption, carrier concentration and mobility [1-5]. We report selective area growth as the method that results into the highest quality material as well as with the highest conversion efficiency, beyond the previous published record ~6%. We finalize by providing the main design rules for next-generation Zn₃P₂-based heterojunction solar cells, which should allow us to go beyond the current conversion values.

Certain phosphorus-containing III-V semiconductors (GaP, InP and related alloys) are among the best-performing PV absorbers. Yet, there is hardly any other phosphide material that has received extensive attention for applications in PV or optoelectronics in general. Exciting progress has been reported within the family of Zn-based phosphide compounds (Zn₃P₂, ZnGeP₂, ZnSnP₂ etc.), but high PV efficiencies are yet to be demonstrated. In this talk, I will discuss two radically different classes of semiconductors that harness the unique ability of phosphorus to exist in a broad range of oxidation states.

The first class is “P-rich phosphides”. In stark contrast to almost any other compound semiconductor previously investigated for PV applications, P-rich phosphides contain bonds between nonmetallic atoms (in their specific case, phosphorus-phosphorus bonds). I will present the first successful thin-film synthesis [1] of any polycrystalline P-rich phosphide. The synthesized material is CuP₂, a 1.5 eV band gap semiconductor with strong optical absorption and native p-type doping in an attractive range for thin-film heterojunction solar cells.

A second intriguing family of materials can be obtained by combining phosphorus with a more electropositive and a more electronegative species. Of particular interest are “phosphosulfides”, where sulfur is the more electronegative species. Many phosphosulfides are predicted to be stable semiconductors with direct band gaps in the visible and disperse band edges. Yet, there are less than five reports of phosphosulfides in thin-film form and hardly any optoelectronic characterization [2].

Backed by a unique suite of combinatorial thin-film deposition setups with access to S and P sources, we have explored three ternary phosphosulfide phase diagrams by high-throughput experiments. In this process, we have synthesized several semiconducting compounds that were previously unknown or that had only been synthesized in bulk form. We will show that the photoluminescence decay time of some of these phosphosulfides is already above 100 ns, demonstrating that phosphosulfides also deserve close attention by the PV research community.

To understand how good (or bad) these early-stage PV materials are at their current development stage, I will discuss a recently proposed figure of merit to assess the quality of a generic PV absorber [3], [4].

From solar photovoltaics to batteries to sustainable fuels, many advances in renewable energy technology are enabled by the availability of high-quality functional materials. In the past decade, innovations such as computational materials design and combinatorial synthesis techniques have rapidly expanded our ability to predict new materials, screen for targeted properties, and identify promising candidates to accelerate the energy transition.

However, although the goal of many of these studies is to contribute to global sustainability, rarely do scientists explicitly include sustainability metrics in early-stages of the materials design process. Often, life cycle assessments (LCAs) are not performed until after a material or device has been scaled up to high technology readiness levels (TRLs), at which it may be too late to make significant changes (this is a phenomenon known as “technology lock-in”). The exclusion of life cycle design at early stages is likely not intentional, but rather because we have not yet established the tools and infrastructure to do so, and because of the disconnects between the fields of materials science and life cycle assessment.

In this talk, I will discuss the promises and challenges of integrating life cycle assessment into early-TRL materials design and discovery. I will give a brief overview of what LCA can and cannot do, share tools I have developed to connect the brightway LCA infrastructure to materials design infrastructure, and discuss case studies from my research on inorganic solar materials (focusing specifically on the role of process uncertainty). Lastly, we’ll look towards the future and explore how the materials scientist community can center life cycle thinking in our own research.

   Selenium is an elemental semiconductor with a wide bandgap appropriate for a range of optoelectronic and solar energy conversion technologies [1]. Developing high-performance selenium-based devices requires an in-depth understanding of both majority and minority carriers [2]. However, characterizing these carrier properties necessitates a wide range of experimental techniques with different sample configurations and illumination levels, complicating the analysis. This often results in discrepancies in the literature and values that fail to accurately reproduce experimental performance in device simulations. Thus, more reliable methods for extracting charge carrier information are highly sought after in the study of emerging optoelectronic materials.

   We study the properties of both carriers in selenium simultaneously using a high-sensitivity, variable temperature photo-Hall system with a rotating parallel dipole line (PDL) magnet [3]. These results are compared with those from other advanced characterization tools, including transient THz spectroscopy, capacitance-based techniques, and voltage-dependent quantum efficiency measurements. To address discrepancies, we construct semiconductor physics models to account for non-idealities at interfaces and surfaces, and assess the validity of commonly used assumptions in standard analysis models, such as complete ionization of acceptors and donors at room temperature. This study is complemented by device simulations, resulting in a unique combination of material properties for high-performance selenium photoabsorbers that accurately reproduce experimental JV-curves and EQE-spectra.

Developing new photovoltaic materials historically has been a time-consuming process, and only few materials have cleared the path into commercialization. It has also been shown that increasing device efficiency is strongly correlated with the number of publications [1] and thus also with the overall research effort in a particular material class. High-throughput synthesis and characterization can be a path to accelerate material development. Recently an empirical figure of merit (FOM) based on 8 material parameters has been proposed by Crovetto [2], to predict potential device efficiency based on measured or estimated materials. This can be useful tool to screen potential materials or direct research efforts. In all of this the application advanced characterization and interpretation of data plays an important role. Determining reliable numbers for carrier lifetime, doping density [3,4], mobility, or band offsets (among others) can be challenging, and misinterpretation of data or unsuitable measurement conditions may result in misguided conclusions about research priorities. We propose that development and publication of best practices [5] as well as numerical simulation-aided analysis methodologies in advanced characterization techniques can help to reduce unwanted variance in results and focus research efforts.

[1] Dale and Scarpulla, Solar Energy Materials and Solar Cells, 251 (2023) 112097

[2] Crovetto, J. Phys. Energy 6 (2024) 025009

[3] Hages, et al, Adv. En. Mat. 7 (2017), 1700167S

[4] Ravishankar, Unold, Kirchartz, Science 371 (2021), eabd8014

[5] Hempel et al, Adv. En. Mat. 12 (2022) 2102776

We fabricate Cu(In,Ga)(S,Se)2 (CIGS) solar cells using the two step-selenization method. First a 500 nm thick 10x multilayer of CuGa/In is sputtered on a soda-lime glass substrate with a Mo back contact layer, followed by evaporation of a 2 µm thick Se layer. This multi-stack is then annealed in a graphite box in a rapid-thermal annealing oven. During the anneal H₂S can be added and removed at different times during the anneal. In a first step we add H₂S in the first phase of the anneal to reduce Ga-diffusion towards the backside of the absorber layer and to include S in the bulk of the absorber layer.  We also add H₂S at the end of the anneal to create a higher band gap S-rich CIGS layer to reduce the front interface recombination. The final absorber layers show a minority carrier recombination time as measured by time resolved photoluminescence measurements of up to 100 ns. The absorber layers are then stored for a few days. Then they are cleaned in an ammonium sulfide solution 1 hour before the CdS top layer is deposited by chemical bath deposition. The solar cells are finished with sputtered ZnO and ITO transparent conductive oxides. The finished solar cells show a best total area efficiency of 16% under AM1.5G illumination. To find out where the major recombination in the devices is located, we performed bias- and temperature-dependent admittance spectroscopy measurements on the cells and compared the results to simulated admittance profiles of CIGS cells [1]. In addition to these devices, the admittance response of devices from Avancis was measured as well.

The results show two distinct recombination patterns, one present at room temperature in forward bias and the second becoming visible in the measurement range only at lower temperatures. The room temperature response showed a bias- and frequency-dependency which in simulations could only be replicated with interface defects. The bias and frequency-dependency of the second recombination response could be replicated with either bulk recombination or a barrier at the CIGS-CdS interface. A clear distinction between the two recombination channels could not be made as these two responses are very similar in the bias voltage versus measurement frequency space. Considering that the series resistance of the devices shows an exponential increase as the temperature is reduced, the barrier at the CIGS-CdS interface seems to be more likely as a cause. The activation energy of the interface defect is of the order of 200 meV, whereas the activation energy of the second recombination response was measured to be about 100 meV deep.

It appears therefore that further improvements to the power conversion efficiency of the devices should involve an improvement of the CIGS-CdS interface properties. This is also in agreement with observations that show that a short anneal at about 200°C after the CdS deposition can improve the fill factor of the devices strongly, likely due to some limited interdiffusion at the interface reducing the recombination behavior at that location.

Engineering of Single atom photocatalysts is a novel yet very challenging pathway that serves the frontline of catalysis field for over the last 10 years. As a result, intriguing performances have been seen due to their unique electronic structures and maximized atomic utilization. However, the major challenge lies in forming a desirable support surface that can allow stable single atom trapping with isolated dispersion of those active sites. So far many strategies for either single atom anchoring or support system have been reported but often with special requirements. Here in, we report, a simple electrochemical deposition approach for anchoring the single atoms in a controlled and desirable way. Titanium dioxide (TiO₂) nanotubes [TiNT] in general on annealing are known to have surface defects (like Ti³⁺-O_v [oxygen vacancy]) are capable of acting as a suitable traps for single atoms. Therefore, different amount of single atom dispersion have been achieved by electrochemically depositing copper single atoms (CuSA) on the TiNTs using CuCl₂ solution of concentration as low as 0.1 mM. Such SA decorated TiNTs have exhibited stronger driving force for photogenerated charge carrier separation and transfer, as a function of the amount of SA dispersion and its deposition condition. Consequently, CuSA/TiNTs have led to maximum photocurrent of 20 mA/cm², thereby attaining significant photoelectrochemical efficiency of 6% which is 1.2 times higher than the so far reported non noble metal based single atom photocatalysts. This study not only reveals the excellent ability of CuSAs to boost the overall charge carrier kinetics but also paves the way for designing advanced non noble metal based single atom photocatalysts that can attain remarkable efficiencies.

Colloidal quantum dots (CQDs) represent a groundbreaking technology reshaping the landscape of commercial imaging and display solutions. This talk explores the rapid advancements in quantum-dot semiconductors, examining their unique properties and addressing the challenges they face in competing with state-of-the-art epitaxially grown compound semiconductors.  Among the critical factors for commercial viability are material selection, suitable photodiode architecture, and scalable production methods. The presentation will address the important question: "How can we create a high-performance semiconductor from quantum dots?" Special attention will be given to manufacturing techniques, discussing the economic feasibility of scaling up to large wafer processing. Highlighting recent breakthroughs, we will showcase successful integrations of PbS QDs into compact, lightweight, low-power, and cost-effective imaging systems, emphasizing their potential as ideal candidates for next-generation shortwave infrared (SWIR) cameras and advanced imaging applications. The talk will also cover the challenges and opportunities in the commercialization process, including regulatory nuances, safety considerations, and strategies for reducing production costs.

Near- and short-wave infrared (NIR/SWIR) active quantum dots have attracted considerable interest for applications in biotechnology, energy conversion and optoelectronics. Most research has been conducted on binary lead- and mercury-based QDs due to the comparable simplicity of their synthesis and their outstanding properties. RoHS (Restriction of Hazardous Substances in Electrical and Electronic Equipment)-compliant III-V compounds such as InAs and InSb are currently emerging, but their more covalent character, high oxidation sensitivity and scarcity of appropriate group-V precursors make it much more challenging to achieve precise control of their size, shape and surface state.

This presentation will focus on novel types of PbS-based core/shell structures. For the PbS core QD synthesis, a library of thioureas was synthesized as the sulfur precursors, which gave rise to a broad size range, excellent monodispersity and high reaction yield. This approach was also adapted to the continuous flow synthesis of larger amounts of QDs. A widely adopted strategy to enhance the PLQY of PbS QDs consists of their overcoating with a CdS shell using Pb/Cd cation exchange. Nonetheless, this procedure leads to a continuous blue-shift of the emission wavelength with proceeding cation exchange, while the PL intensity goes through a maximum, making it difficult to optimize the emission for a desired wavelength. To avoid cation exchange, we added an intermediate buffer shell of ZnS on the PbS core QDs before growing the CdS shell. We present the optical and structural properties of the novel PbS/ZnS/CdS core/shell/shell QDs as a function of the different shell thicknesses and compare them with results from ab-initio simulations on slabs of these structures. In an extension of this study, PbS/CdS QDs obtained via cation exchange have been overcoated using a reactive monomolecular precursor to obtain PbS/CdS/CdS thick-shelled QDs. Finally, recent advances in the synthesis of InAs-based core/shell structures will be presented.

Photodetectors in the short-wave infrared (SWIR: 1-1.7 µm) range have garnered significant attention due to the growing demand for 3D imaging and facial recognition technologies. The inherent transparency of silicon above 1 µm wavelength and the intricate integration processes of III-V materials restrict conventional technologies from effectively addressing this spectral region. In this context, it has become essential to explore alternative photodetector materials. Among the candidates for SWIR sensing, lead sulfide quantum dots (PbS QD) are very promising for integration as active material due to their strong, size-tuneable absorption at targeted infrared wavelengths. However, when exceeding a specific nanocrystal size, these QD are prone to surface oxidation and aggregation limiting their performances and integration in sensor devices. [1]

Among several strategies developed to passivate and deposit these QDs as thin layer, one of the most promising is to combine them with halogenated perovskites. [2] Inspired by this approach, we investigated and optimized all the steps, from the synthesis of the PbS QD to their integration in the perovskite matrix. An efficient solution exchange of long-chain ligands on the surface of QDs with perovskite precursors is performed and confirmed through FTIR, NMR, and XPS measurements. The relevance of this perovskite shell around the QD is explored optically (PL, Abs spectroscopy) revealing significantly improved luminescence stability under ambient atmosphere for more than 2 months in thin films. To facilitate thin film fabrication, we developed a stable ink based on the ligand-exchanged QDs and perovskite precursors. The optimized inks are stable for days in suitable solvents for film deposition. This new approach enables one-step thin film deposition compared to the multi-step approach required to manage organic ligand exchange. The solution-processed perovskite-exchanged PbS QD strategy was devised to produce reproducible and homogenous thin films absorbing in the desired NIR spectral region which are further integrated into devices. This study covers fundamental understanding of the exchanged quantum dot system and explores its photodetector performance.

The colloidal synthesis of two-dimensional (2D) lead chalcogenide semiconductors yields near-infrared emissive materials with strong excitonic contribution at room temperature.^[1-4] They are model systems for efficient charge carrier multiplication and hold potential as intriguing candidates for fiber-based photonic quantum applications. However, synthetic access to the third family member, 2D lead telluride (PbTe), remains elusive due to a challenging precursor chemistry. Here, we report a direct synthesis for 2D PbTe nanoplatelets (NPLs) with tunable photoluminescence (PL, 910 – 1460 nm (1.36 – 0.85 eV), PLQY 1 – 15 %), based on aminophosphine precursor chemistry.^[1] Our NMR study underpins the synthetic importance of an ex-situ transamination of tris(dimethylamino)phosphine with octylamine to yield a reactive tellurium precursor for the formation of 2D PbTe NPLs at temperatures as low as 0 °C. Associated GIWAXS measurements confirm the 2D geometry of the NPLs and the formation of superlattices. The importance of a post-synthetic passivation of PbTe NPLs by PbI₂ to ensure colloidal stability of the otherwise oxygen sensitive samples is supported by X-ray photoelectron spectroscopy. Our results expand and complete the row of lead chalcogenide-based 2D NPLs, opening up new ways for further pushing the optical properties of 2D NPLs into the infrared and toward technologically relevant wavelengths.

Core/shell (c/s) semiconductor nanocrystals (NCs) are key building blocks in modern optoelectronic devices. The specific core and the shell materials determine the alignment between valence- (VB) and conduction-band (CB) energy levels. Because photogenerated electrons (e^-) and holes (h⁺) relax to their lowest and highest available energy levels, respectively, these carriers are confined to the core and away from the surface in type-I c/s NCs, resulting in high photoluminescence (PL) stability and efficiency. In contrast, c/s NCs with a reverse-type-I configuration are highly susceptible to the environment as both carriers localize on the shell and are easily extractable by charge scavengers. In a type-II configuration, one of the semiconductors has both higher (or lower) VB and CB values, resulting in one carrier being confined to the core and the other to the shell. The presence of physically separated e^-–h⁺ pairs (excitons) gives type-II c/s NCs long PL lifetimes. In the presence of quantum confinement, the exact size of the core and shell open a continuum between quasi-type-II—with only partial delocalization of one of the carriers, for example—and true type-II configurations.

Here, we report the synthesis and structural characterization of PbCh/AeCh core/shell nanocrystals (Ae = Ca, Sr, Ba; Ch = S, Se). Using a new synthesis developed by us, we have successfully passivated PbS or PbSe cores with alkaline-earth (Ae) chalcogenide shells. For example, PbS/SrS and PbSe/SrS are near-IR active NCs with PL maxima ranging between ~1000–2000 and 1500–2300 nm, respectively. Colloidal epitaxy in this system is possible thanks to both core and shell materials adopting identical rock salt crystalline structures, with a lattice parameter mismatch of ≤ 5% for SrS, CaSe, and SrSe (Figure). We predict that these materials will be more robust and, because the lead-based core is buried inside the NCs, potentially less toxic and more biocompatible compared to other bare lead-based materials.

In recent years, metal chalcogenides have emerged as prominent candidates as low-cost catalysts in renewable green technology applications. The scientific community has expressed significant interest in hydrogen production due to its potential as a clean alternative to fossil fuels, devoid of CO₂ emissions, and its impressive energy density. ¹

This project focuses on investigating NiSe, CoSe, and their ternary compounds, selected for their noteworthy electro-catalytic and photothermal properties, which raise the ability to generate a substantial increase in temperature, and enhance reaction kinetics (as per Arrhenius). ²

Previous efforts in this project concentrated on synthesizing, characterizing, and examining the basic electrochemical and photothermal properties of nanostructures. Synthesis of NiSe, CoSe, and ternary compounds with varying metal ratios yielded hexagonal nanoparticles in the 15-30 nm size range. These nanoparticles exhibited enhanced electro-catalytic activity in the hydrogen evolution reaction and demonstrated significant heating performance across different solvents and electrolytes. Furthermore, these nanoparticles were employed in advancing solar-driven membrane distillation technology, as part of a collaborative endeavor with Italian researchers. ³

Subsequently, the focus shifted to developing stable and suitable fluorine-doped tin oxide (FTO) electrodes for a new system, allowing the investigation of how the photothermal properties of nanoparticles influence the kinetics of the electrochemical hydrogen evolution reaction under illumination. A comprehensive analysis of kinetic parameters, including the activation energy of nanoparticles under various conditions, was conducted. These values offer insights into the degree to which temperature variations can impact the kinetics of an electrochemical reaction. Moreover, they can provide additional explanations for the differences in the activity observed among various catalysts. ⁴ In some instances, the nanoparticles exhibited lower activation energy than platinum (Pt). Additionally, an initial experiment was conducted integrating the new electrodes and the new system. Future work will focus on optimizing the new setup and exploring catalytic activity under illumination by leveraging the photothermal effect.

Wet chemical synthetic routes for transition metal dichalcogenides (TMDCs) of the composition MX₂ (M = Mo, W; X = S, Se), which have been studied extensively in the past, yield colloidal inks of ultrathin (mono- and bilayer) laterally smaller nanoplatelets (NPLs) and laterally larger nanosheets (NSs) with interesting photophysics.[1-5] In contrast, the telluride analogues remain elusive, with only MoTe₂ nanocrystals being synthetically accessible in the semimetallic 1T' phase,[6] despite the fact that their direct band gap in the NIR region (0.95 eV[7]) make semiconducting MoTe₂ monolayers a promising candidate for fibre-optic applications in the O-band.

One major challenge in synthesizing semiconducting MoTe₂ is the small difference of only 35 meV between the semiconducting (stable) 2H phase and the semimetallic 1T' phase,[8] making it difficult to target one phase over the other during the reaction. Meanwhile the transformation between the 1T' and 2H phase is unfavored at typically accessible reaction temperatures in colloidal systems (~ 300 °C).[6,8]

In addition, reactive tellurium sources are scarce and often require more sophisticated tailoring of the precursor chemistry in comparison to sulfur and selenium. Here we adapt a synthesis using TeO₂ and thiol precursor chemistry[9] to yield MoTe₂ and WTe₂ NPLs primarily consisting of the 2H phase, which we substantiate using insights from spectroscopic and microscopic analyses.

The electrochemical reduction of nitrate (NO_₃^⁻) to ammonia (NH_₃) offers a sustainable approach to minimize nitrate pollution while generating valuable chemicals. This study focuses on the functionality of copper-based catalysts, specifically Cu₃N, Cu₂O, CuO, and Cu₃P, in promoting this reaction. Utilizing 0.1 M phosphate-buffered saline (PBS) as the electrolyte, a systematic investigation of the performance and stability of these catalysts is conducted. It was revealed that Cu₃N’s surface was modified during catalysis to CuO. Post-catalysis characterizations were conducted to understand these transformations, revealing significant insights into the stability and activity of the oxidized forms. The findings indicate that Cu_₂O and CuO exhibited comparable activity to Cu_₃N after the oxidation process. Additionally, the relatively unexplored realm of transition-metal nitrides and phosphides presents a fertile ground for further research. The inclusion of Cu_₃P highlighted its distinct potential in nitrate reduction applications, demonstrating greater activity in hydrogen evolution compared to the other copper species. The comprehensive evaluation of Cu_₃N, Cu_₂O, CuO, and Cu_₃P provided a nuanced understanding of copper-based catalysts, laying the groundwork for future advancements in electrochemical nitrate reduction studies.

Group V (bismuth and antimony) containing copper chalcogenide-based nanostructures are an environmentally benign class of compounds for employment in potential energy storage and conversion applications. We investigate the mechanistic insights in the colloidal synthesis of this class of materials and harness them in various applications. The colloidal approach for synthesis of most heterostructured/ multicomponent nanocrystals (NCs) typically proceeds via the formation of binary semiconductor NC seeds. Contrary to this, a lesser explored pathway involves liquid droplets for catalysing the growth of the semiconductor NCs, known as the solid-liquid-solid (SLS) mechanism This offers facile control over the reaction kinetics with the variation of the nature of the metal seed catalysing the growth process. Using this approach, we synthesized Bi-Cu_2-xS heterostructures with tuneable Cu_2-xS stem. The stability of the Cu thiolate intermediate formed in the reaction could be varied by modification of the alkyl phosphonic acid used, which in turn controls the number of Cu_2-xS stems in the heterostructures. The advantages of the branched morphology were examined by assessing the electrochemical performance of the single stem and multi stem Bi-Cu_2-xS NCs anodes in K⁺ ion batteries. Multiple stem NCs show enhanced cycling stability and rate capability with higher specific capacity (∼170mAh·g−1 after 200 cycles) than the single pods (∼111mAh·g−1 after 200 cycles).[1] Following this work, we synthesized multinary anisotropic Cu-Bi-Zn-S nanorods (NRs) via the SLS mechanism wherein in situ generated Bi NCs catalyses the formation of Bi-Cu_2-xS heterostructures which eventually transforms into homogeneously alloyed quaternary Cu-Bi-Zn-S NRs. We observe that the reaction proceeds through the dissolution of the metallic bismuth seed into a trisegmented heterostructure with a Bi-rich BixCuySz phase, a Cu-rich BixCuySz stem, and an alloyed transitional BixCuySz segment present at the heterointerface. Finally, the formation of the homogenous NRs is facilitated by the gradual dissolution of the Bi-rich seed and recrystallization of the Cu-rich stem into the transitional segment. The NRs exhibit promising thermoelectric properties with very low thermal conductivity values of 0.45 and 0.65 W/mK at 775 and 605 K, respectively, for Zn-poor and Zn-rich NRs.[2]

Our interest in exploring multinary group V containing colloidal nanostructures also led to the direct synthesis of quaternary compositions of Cu-Sb-S-based NCs. We developed a hot injection synthetic pathway for three substituted tetrahedrites compositions i.e., Cu₁₀Zn₂Sb₄S₁₃, Cu₁₀Ni₂Sb₄S₁₃, Cu₁₀Co₂Sb₄S₁₃. Balancing the precursor reactivities of constituent species was crucial for obtaining phase pure and better size distribution of the NC ensemble. All the synthesized substituted tetrahedrites exhibited lower thermal conductivity while Cu₁₀Ni₂Sb₄S₁₃ exhibited the highest electrical conductivity thus making them promising candidates for thermoelectric applications. [3]

Our quest for developing compositionally complex nanostructures led to pushing our limits from quaternary nanostructures to emerging high entropy materials. High entropy materials are defined as materials containing more than 5 constituent elements with 5-35% of each element and crystallizing in a single phase stabilized by a high configurational entropy. The presence of multiple cations with different reactivities towards the chalcogenide species results in the formation of additional byproducts in the reaction leading to the emergence of a multiphase system in a conventional hot injection or heat-up colloidal synthesis pathway for high entropy materials. Therefore, the design of these nanostructures requires strategic techniques to avoid phase separation. Using cation exchange as a tool we use our preformed multicomponent group V containing copper chalcogenide-based NCs as templates for the subsequent diffusion of additional cations in the chalcogenide phase resulting in the synthesis of the target single-phase high entropy NCs.

Colloidal semiconductor Quantum Dots (QDs), often considered as artificial atoms, have reached an exquisite level of control, alongside gaining fundamental understanding of their size, composition and surface-controlled properties, as recognized by the Nobel prize in Chemistry 2023. Their tuned characteristics and scalable bottom-up synthesis accompanied by the applicability of solution based manipulation, have led to their wide implementation in displays, lasers, light emitting diodes, single photon sources, photodetectors and more.

For the next step towards enhancing their functionalities, inspired by molecular chemistry, we introduce the controlled linking and fusion of two core/shell quantum dots creating an artificial molecule manifesting two coupled emitting centers. Accordingly, the coupled colloidal quantum dot molecules (CQDMs) present novel behaviors differing than their quantum dot building blocks. First, two types of biexcitons coexist as observed via heralded spectroscopy. Moreover, such CQDMs open the path to a novel electric field induced instantaneous color switching effect, allowing color tuning without intensity loss, that is not possible in single quantum dots. All in all, such quantum dot molecules, manifesting two coupled emission centers, may be tailored to emit distinct colors, opening the path for sensitive field sensing and color switchable devices such as a novel pixel design for displays or an electric field color tunable single photon source

Semiconductor quantum dots and quantum shells with complex confinement potentials are promising scintillators. They can demonstrate intense, fast, and durable scintillation under X-ray or electron excitation. Photon yields can reach greater than 100 photons/keV, exceeding typical scintillation standards, driven by structurally-engineered slowed Auger recombination and consequent radiative biexciton emission. This remains true even with several electron-hole pairs in the nanoshell. The single nanosecond lifetime of the quantum dot and quantum shell scintillators is much faster than typical standards with no afterglow. Fast scintillation improves frame rates in imaging, potential in tomography imaging, and typically also improves energy resolution. The samples can maintain bright scintillation performance under intense synchrotron X-ray excitation for at least 10 hours, equivalent to c. 1 million hours with a normal laboratory source. Using micrometer thick films to perform imaging, resolution of 20 lines/mm can be achieved. Based upon these results and performance metrics reported in literature, some general design strategies will be discussed with particular attention to materials which are strong performers as laser media and single photon sources. Furthermore, the predictive capacity of using more accessible laser-based spectroscopy experiments will be highlighted.

The direct wet-chemical synthesis of 2D lead chalcogenide nanoplatelets yields photoluminescent materials with strong excitonic contribution at room temperature. [1-4] In the spirit of the recent Nobel prize for the discovery and synthesis of quantum dots (QDs) we report herein on our studies of strongly confined wet-chemically synthesized flat 2D PbSe QDs. These 2D nanocrystals have lateral dimensions of e.g. 6 x 5 nm^2, a thickness of 1-3 PbSe monolayers and exhibit PL in the NIR between 860 – 1510 nm with a PL quantum yield of up to 60 %. [2,3] The highly efficient PL at fiber-optics-relevant telecommunication wavelengths renders colloidal lead chalcogenide 2D semiconductors intriguing materials for future solution-processable optics. Scanning tunnelling spectroscopy (STS) of single flat PbSe QDs revealsa conduction and valence band density of states that is typical for QDs rather than a steplike function linked to 2D nanoplatelets and substantiates the strong confinement in the flat PbSe QDs. Our experimental observations are supported by theoretical calculations of the electronic band structure using the tight-binding approach. 

In the second part of the talk I will focus on colloidal 2D PbS nanoplatelets (NPLs) with a thickness of 1-2 nm. [4] The PbS NPLs exhibit excitonic PL at 720 nm, directly tying to the typical PL limit of unmodified CdSe NPLs. In the first comprehensive study of the low-temperature PL from PbS NPLs we observe unique PL features in single PbS NPLs at 4 K, including narrow zero-phonon lines widths down to 0.6 meV. Time-resolved measurements identify trions as the dominant emission source with a 2.3 ns decay time. Sub-meV spectral diffusion and no immanent blinking over minutes is observed, as well as discrete jumps without memory effects. These findings advance the understanding and underpin the potential of colloidal PbS NPLs for optical and quantum technologies. [5]

Transition metal dichalcogenides (TMDCs) are highly researched photonic two-dimensional (2D) semiconductors. Among other members of the TMDC group, WS₂ shows remarkably high nonlinear susceptibility owing to its lack of inversion symmetry.^[1] This effect is observed for all odd-numbered WS₂ layers but is especially strong in monolayers and can be further increased by strain,^[2] while defective crystal structures lead to inversion symmetry breaking of even-layered WS₂.^[3]
The ability of TMDCs to yield stable monolayers with rich exciton physics^[4] inspired the wet-chemical synthesis of MoS₂^[5,6,7] and WS₂^[6,8] as well as their respective Mo1-xWxS2 alloys^[9] and heavier chalcogenides (Se ^[10] and Te). With the scalable bottom-up approach, predominantly atomically thin^[9] nanosheets (NSs) with controlled crystal phase-^[5,6], size^[5]- and composition^[9] are obtained and can be readily processed in solution or after precipitation. Comprehensive characterization via HR-TEM, Raman spectroscopy, and steady-state absorption spectroscopy confirms the synthesis of primarily monolayered semiconducting NSs with a narrow lateral size distribution (5-25 nm).

Here, we investigate the non-linear optical response of colloidal WS₂ with a femtosecond-laser-pulsed confocal mirror microscope. Our study includes one- and two-photon photoluminescence and efficient second harmonic generation (SHG) of colloidal WS₂ monolayers. The power-dependent SHG intensities from the colloidal WS₂ monolayers show steeper slopes than the commercially available CVD-grown WS₂ flakes, indicating a higher non-linear susceptibility for colloidal WS₂.
These results highlight the exceptionally high non-linear response in colloidal WS₂, underscoring the potential of colloidally synthesized WS₂ as functional 2D semiconductors.

Forming complex structures of functional materials in a controlled and reproducible fashion is a well-known challenge. [1,2] Specifically, bimetallic phosphides are of interest for energy-related applications; however, a satisfactory structure-function relationship has not been fully deciphered yet. In this work, we show that a colloidal chemistry approach produces bimetallic phosphides electrocatalysts of Co and Cu, size range 40-100 nm, where segregation and phase transformation induce significant changes in morphology compared to solid solutions. Their complexity permits the tuning of the catalytic sites to the hydrogen and oxygen evolution reactions (HER and OER), allowing the bimetallic phosphides to catalyze the full water splitting reaction. The experimental results show that in alkaline medium water cleavage is particularly favorable on CuxCoyP catalysts (and especially when x = 50%), enhancing their HER performance with an overpotential of 184 mV @ 10 mA/cm2. As for the OER enhancement, the results show that the bimetallic phosphides undergo a surface transformation during the OER, whereby (oxy)hydroxides form at anodic potentials in alkaline solution and serve as the actual electrocatalysts. The best OER performance was displayed by Cu25Co75P having an overpotential of 283 mV @ 10 mA/cm2. [3] Additionally, these CuxCoyP catalysts offer promising functionality towards methanol oxidation reaction (MOR) without fully oxidizing it to CO2 and rather producing beneficial product formate (HCOO-), displaying a lower overpotential by up to 180 mV as compared to OER and a higher mass activity. At 1.52 V and on passing 300 C charge the Faradaic efficiency for formate production is 100% in case of both the bimetallic and monometallic phosphides. This kind of selective oxidation of methanol is highly desired in direct alcohol fuel cell applications. [4]

The safety and stability concerns with liquid electrolytes of Li batteries are prompting their replacement by solid-state conductors; however, ion mobilities of conventional solid electrolytes do not match up to their liquid counterparts. To address this challenge, we have been exploring fast-ion or superionic solids based on earth abundant selenides and sulfides, where cations form a “liquid-like” network within a rigid anion cage allowing cation mobilities rivaling those of liquid electrolytes. Whereas superionicity is attained in these materials only at elevated temperatures, we find that in nanocrystals of copper selenide and sulfide the superionic phase is attained at lower temperatures than in the bulk. From electronic structure investigations and in-situ electron microscopy studies of copper selenide, we find that the key factor in this effect is compressive strain prevalent in nanocrystals, which also makes ion-transport pathways energetically feasible. Superionic transport achieved in nanostructures can be extended to macroscopic length scales by assembling solids from the nanostructures. Copper selenide nanowires exhibit an ionic conductivity of 4 S/cm which is an order-of-magnitude higher than that in bulk copper selenide. This record high conductivity results from the combination of crystalline paths for conduction in the axial direction with nanoscale confinement in the radial direction. These advances pave the way for fast-ion solid electrolytes.

Electrochemical nitrogen reduction reaction (NRR) is an environmentally friendly alternative strategy to the high energy consumed and high carbon released Haber–Bosch process for NH₃ production. Nevertheless, it is limited by a low ammonia yield and faradaic efficiency (FE) due to (i) the complexity associated with the breaking of the N≡N bond and (ii) the competing hydrogen evolution reaction. The primary challenge for NRR is the development of highly efficient electrocatalysts which will be able to tackle these hindrances effectively and convert N₂ to NH₃ under benign conditions, with minimal energy input. This presentation will dive into the concept of exploring surface engineering strategies of 2D materials such as defect engineering and heterojunction formation that can enhance the catalytic activity of the NRR process. We will showcase two main examples: the electrochemical dealloying of exfoliated 2D-PdBi₂ nanoflakes into palladium hydride (PdH_x, x ≤ 2) in addition to the formation of the MoS2/rGO heterostructure which significantly enhance NRR and dwell on the mechanisms. The applied design ideas, synthetic methods, and catalytic performance of the 2D catalysts will be described with the fabrication of mechanisms to inspire more practical design strategies for NRR electrocatalysts.

In the last decade, lead halide perovskites have revolutionized material science with their remarkable charge mobility, light absorption, and adjustable band gaps, all achieved through low-temperature processing.[1] However, their instability and toxicity limit their broader application. Despite these challenges, perovskites hold great potential for future energy technologies, spurring the development of "perovskite-inspired" materials (PIMs). In this aspect, the scientific interest has recently shifted to alkali metal-based chalcogenides, which represent a new category of semiconducting inorganic compounds and are being explored as potential candidates in the quest for novel energy materials. Recently, the focus has shifted to alkali metal-based chalcogenides as promising semiconductors for energy materials. Ternary alkali-metal dichalcogenides {AMeE (A = Li, Na, K, Rb, Cs; Me= Metals E = S, Se, Te} are identified as potential candidates for energy conversion and storage.[2] While high-temperature solid-state synthesis often results in limited phase control, wet chemical synthesis offers a promising alternative by producing uniform nanoscale particles and providing insights into their formation.[3,4]

Building on several theoretical and experimental studies on cesium copper-based chalcogenides, we present the synthesis of cesium copper selenide on a nanoscale regime with precise control over dimension, morphology, and phase.[5,6] The influence of key reaction variables, such as precursor’s reactivity, ligands, reaction temperature, and reaction time, on the size and shape of the nanocrystals was demonstrated, showcasing the flexibility of the wet chemical synthesis. An ex-situ mechanistic investigation reveals that NC formation is driven by the dissolution of binary Cu_2-xSe, followed by incorporating Cs⁺ to form the ternary CsCu₅Se₃. The current study also reveals that variation in the alkyl chain length of amines influences the size, shape, and formation of distinct phases. The structural, electronic, and thermoelectric characteristics were experimentally evaluated and further corroborated by computational analysis. The experimental results revealed the material's ultralow thermal conductivity of 0.6 W.M^-1.K^-1 and a good thermoelectric figure of 0.3 at 720 K, providing concrete evidence of its potential. The detailed mechanistic insights presented in this study will significantly advance the development of cutting-edge functional materials in the field of alkali metal chalcogenides for various applications.

A thermoelectric cooler is a solid-state device that transfers heat from one side to another when an electrical current passes through it. This technology is appealing because it can provide precise and localized cooling and heating without using hazardous liquids or gases commonly found in traditional vapor compression refrigeration. These devices are compact, customizable in size, work in any orientation, operate noiselessly, and require minimal maintenance. Even though thermoelectric coolers could be transformative for many advanced thermal management applications, their widespread adoption is hindered by the low efficiency of the thermoelectric materials and costly manufacturing processes.
 In this work, we use extrusion-based 3D printing techniques to fabricate high-performance thermoelectric materials using nanomaterial-based ink. The ink formulation is optimized to ensure structural integrity and particle interfacial bonding during annealing, providing p- and n-type materials with record-high zT values of 1.46 and 1.35 at room temperature, respectively. Moreover, we integrate the printed materials into a 32-pair device and achieve a significant cooling temperature gradient of 50 °C and a coefficient of performance of 3.8, comparable to best-performing thermoelectric coolers, avoiding material waste, and the energy-intense and inefficient steps, such as high-temperature synthesis, pressure-assisted sintering, and cutting and dicing ingots, commonly used in conventional manufacturing processes.

Metal chalcogenides are key material enablers for renewable energy technologies to decrease the current global reliance on fossil fuels and reduce greenhouse gas emissions.^[1,2] However, there is a persistent demand for the development of advanced materials that offer improved efficiency and performance. These innovations are essential for enhancing sustainability and tackling critical global challenges. In the current decade, high entropy chalcogenide nanocrystals (HECh NCs) have emerged as fascinating new materials with remarkable mechanical properties, offering significant potential for a wide range of sustainable energy applications. ^[3] This class of advanced nanomaterials typically comprise five or more principal elements with nearly- equimolar compositions, that utilizes a high configurational entropy to stabilize multiple elements within a single crystal lattice or sublattice. ^[4]

Motivated by the growing demand for HEChs, we present the synthesis of 5 and 6 component high entropy metal telluride with Pd_xCu_yTe₂ as the principal base. Using Pd_xCu_yTe₂ as seed, we design the dimension-controlled colloidal synthesis of (Ni,Cu,Pd,In,Sb)Te and (Ni,Cu,Pd,Ga,Sb,Sn)Te by two-pot diffusion-mediated synthesis at relatively low temperature. The key reaction variables such as precursor reactivity, capping ligands, reaction temperature, and reaction time have been shown to influence the size and shape of the NCs, highlighting the flexibility of the wet chemical synthesis method. The structural and electronic attributes of these materials were evaluated experimentally by XRD, TEM, XPS and UV-Vis spectroscopy. Combined instrumental analyses aided in elucidating the atomic-scale nucleation and growth mechanisms. The synthesized NCs demonstrated exceptional catalytic performance for hydrogen evolution reaction in acidic electrolyte, achieving a current density of 10 mA/cm² at much lower overpotential. The mechanistic insights detailed in this study expands the scope of synergistic properties exhibited by HEChs and will be crucial for advancing the development of innovative, functional HEChs for diverse applications.

Electrochemical water splitting represents one of the most advanced technologies to provide clean and renewable energy. [1] In this process, oxygen evolution reaction (OER) requires four electrons,[2] which is known as a complex and pivotal step that controls the water splitting efficiency. Up to now, the most popular and efficient catalyst for OER is an Iridium-based electrode. [3] However, the cost of Ir makes the process expensive, and finding cheaper material based on earth-abundant metal is mandatory. In this context, we present the development of a high-entropy layered double hydroxide electrocatalyst for the oxygen evolution reaction. MgAl-based LDH is a kind of material with a higher number of hydroxyl groups, which are expected to boost OER. [4] Here, we proceed to substitute the Mg cation with four other cations at different molar fractions using a metastable entropy-stabilized solution. The characterization of high entropy LDHs using XRD, XPS, TEM, and UV-visible spectroscopy indicates the effective substitution of Mg by different cations. We found that HE-LDHs material performs better than commercial IrO₂, and the Mg substitution leads to an optimal composition, which can lower the d-band center position as an effective method to improve the OER performance.

Optoelectronics applications require control over the generation, separation, and extraction of photoexcited charges and control over heat. Yet, in many material systems, energy carrier transport must navigate defects of various natures over a broad range of length and time scales. There are many approaches to inferring microscopic energy transport through energetic, temporal, or spatial markers, but each faces limitations. Moreover, heterogeneous systems are often elusive to simple kinetic models that reveal fundamental transport parameters. To understand the principles that govern electronic and thermal relaxation dynamics in complex systems relevant to optoelectronic applications, advanced experimental techniques and theoretical models rooted in fundamental physical phenomena are needed. This presentation will focus on the following questions: How do heterogeneous environments and interfaces impact microscopic energy transport? How can we access information about energy carriers that traditionally do not have clear spectroscopic signals? How can we control the directionality of energy carrier flow? I will describe pump–probe optical measurements and modeling of both charge-carrier and thermal transport in nanocrystal assemblies.

Colloidal perovskite nanocrystals exhibit many intriguing properties, one of which is collective behavior. Assemblies of close-packed nanocrystals in the form of continuous films and individual mesocrystals (superlattices) attract attention because of reported enhancements in exciton diffusion, energy transport, light amplification, and quantum phenomena. To deepen our understanding of these materials, it is important to study changes in the photophysics of perovskite nanocrystals as they transition from a liquid dispersion to a disordered glassy film and an ordered superlattice.

In this contribution, I will discuss our efforts to study the optical properties of single-component all-inorganic and hybrid organic-inorganic perovskite nanocrystal superlattices. First, we will consider the possibility of collective response in CsPbBr₃ nanocrystals from a theoretical perspective of single-excitation superradiance, discussing factors that may weaken or strengthen it. Second, we will examine experimentally achievable CsPbBr₃ nanocrystal superlattices, focusing on changes in their steady-state and time-resolved spectroscopic observables in response to nanocrystal synthesis origin and environmental conditions such as sample aging. Lastly, we will highlight other compositions of single-component perovskite nanocrystal superlattices and the prospects of achieving a tunable collective response.

Absorption of light via interband optical transitions constitutes a primary process in nature, e.g., in photosynthesis, as well as in applied technologies, e.g., in solar photovoltaic cells, photodetectors, or (quantum) light-matter interfaces. In cavity-free systems, engineerability of the rate of absorption has thus far been limited, consistent with the wide-spread belief that the coupling strength between initial and final state (described by the square of the matrix element of the light-matter interaction in Fermi’s golden rule) is an intrinsic parameter of the employed material. However, enhanced absorption rates could be realized via giant-oscillator-strength (GOS) transitions, leveraging coherent oscillations of the electron polarization in a volume significantly larger than a unit cell.[1] While experimental evidence for such a superradiance phenomenon has indeed already been provided in emission processes, realizations in an absorption process, i.e., in the form of “superabsorption”, have been sparse and/or require complicated excited-state engineering approaches.[2]

Here, employing colloidal CsPbBr₃ perovskite QDs, we demonstrate a robust and straightforward implementation of superabsorption as a time-reversal process of single-photon superradiance.[3][4] Optical spectroscopy reveals that the band-edge absorption in large CsPbBr₃ perovskite QDs exhibits a superlinear increase with QD volume, consistent with the 3D delocalization of a giant exciton wavefunction. Calculations based on the configuration-interaction framework attribute this behavior to strong electronic correlations, and fully corroborate the experimental findings. Our results shed light on a process as fundamental as light absorption, in a new class of commercially relevant direct semiconductors, and may facilitate the development of more efficient optoelectronic devices and new quantum light-matter interfaces.

Indium phosphide (InP)-based quantum dots (QDs) are the most industrially relevant Cd- and Pb-free QDs for photonic applications due to their excellent optoelectronic properties, including a tunable band gap, tolerance to dopants, high absorption coefficients, near-unity photoluminescent (PL) quantum yields (QYs), and narrow PL linewidths.[1] In recent years, research efforts have successfully improved the quality of InP-based QDs.[1] However, their long-term stability and the deterioration of their optoelectronic properties are not yet well understood. In this work, we systematically investigate how the optoelectronic properties of colloidal InP/ZnSe/ZnS core/shell QDs in solution are affected by continuous white LED light exposure under a nitrogen atmosphere (< 0.1 ppm O₂, < 0.1 ppm H₂O).

Characterization of the absorption spectrum over time shows that the QDs are colloidally stable in solution and do not photodegrade. Remarkably, the PLQY of the InP/ZnSe/ZnS QDs decreased rapidly over time and did not recover after light exposure, indicating irreversible photodarkening of the QDs. A control experiment conducted in the dark demonstrated no degradation of the QDs or reduction in their PLQY throughout the duration of the experiment. The photodarkening effect was mitigated by reducing the incoming photon flux. However, when the changes in QY are plotted against the absorbed photon dose, the data collapses in a master curve with a logarithmic dependence on the dose. These ensemble-level results are further complemented by single-particle measurements for a comprehensive interpretation. In summary, our findings provide new insights into the photodarkening of InP-based core/shell QDs relevant for photonic applications.

Colloidal semiconductor nanocrystals have long been considered a promising source of time-correlated and entangled photons via the cascaded emission of multiexcitonic states. The realization and spectroscopy of such cascaded emission, however, is strongly hindered by the highly-efficient, nonradiative Auger process, which renders multiexcitonic states non-emissive. Here we present a room-temperature heralded spectroscopy study of three-photon cascades from triexcitons in giant CsPbBr₃ nanocrystals. Single particle heralded spectroscopy combines the power of the temporal correlation of photon detections with an added information of spectral resolution. In this technique the photoluminescence of a single nanocrystal is collected through a spectrometer coupled to a single-photon avalanche diode array detector, so that each detected photon is time-stamped according to its arrival time, and energy-stamped according to the array pixel it was detected in. This is the first study to fully characterize these types of cascades from quantum dots at room temperature, a task which is difficult due to the significant broadening of emission lines as well as due to temporal fluctuations in the emission. Our results show that the emission pathway of triplets of photons in these particles is dominated by the lowest excited state, and that multiexcitons (biexcitons and triexcitons) are extremely weakly bound, in contrast with low temperature observations. In addition, we aim at elucidating the underlying properties or processes that can lay in the basis of observed differences in blinking statistics and try to correlate these with either intrinsic or surface related properties. This presents interesting opportunities in using emission cascades as deterministic few-photon sources at room temperature that could have important consequences in the development of colloidal quantum light sources.

The ability to efficiently up-convert broadband, low-intensity light would be an enabling technology for volumetric 3D printing, background-free biomedical imaging, and sensitizing silicon-based cameras to the short-wave infrared. Our approach uses colloidal quantum dots to absorb low-energy photons and sensitize the spin-triplet excitonic states of nearby conjugated molecules.[1-3] Once there, pairs of these long-lived excitations can combine via triplet fusion to generate shorter-wavelength fluorescence.

We recently harnessed high-quality, ultra-small (d:1.7-2.8 nm) PbS quantum dots[4] to generate photochemically active blue light (λ~420nm) from continuous-wave red (λ: 635nm) excitation.[5] However, this performance was somewhat unanticipated, because the large ‘Stokes shift’ in most ultra-small nanocrystals appears to herald an unacceptable loss of incident photon energy. Intriguingly, we inferred from the quasi-equilibrium dynamics of triplet energy transfer that the chemical potential of photoexcited, ultra-small PbS quantum dots is surprisingly high—completing an advantageous suite of photophysical properties, but reinforcing fundamental questions regarding their emissive state(s).[5,6]

Accordingly, I will present a photophysical effort to relate these anomalous behaviours to the long-discussed, surface-linked ‘trap’ emission from canonical Cd-chalcogenide quantum dots. We show that this non-ideal emission can be observed, intermittently, from individual nanocrystals, and is consistent with the occasional formation of defect states, shallow within the gap, on timescales that align with rare, photoinduced structural reorganization.

Semiconductor nanocrystals have promise in many optoelectronic and electronic technologies due to their tunable electronic structure and facile colloidal processing. However, one major factor limiting their widespread adoption is inherent heterogeneity within an ensemble. For this reason, magic sized clusters, with atomically precise structures and negligible heterogeneous broadening, have drawn significant attention as a potential system with ensemble level homogeneity. However, many of the most common nanocrystal materials, such as Cd-chalcogenides and In-pnictides, form clusters with bandgaps far to the blue, limiting potential applications. We illustrate the colloidal synthesis of nontoxic, earth abundant, iron sulfide clusters with narrow and invariant absorption features and a ~700 nm band-gap which allows for absorption across the visible spectrum. These ~2nm diameter particles exhibit quantum confinement, with a blue shift from the expected 0.95 eV bandgap. Furthermore, through controlled surface coordination via ligand exchange, we can promote band-edge photoluminescence. These results suggest surface defects play a major role in determining the nonradiative processes present in the system. Further growth in polymerizing media (e.g. oleyl amine) facilitates the formation of long-range order into quasi-1D fibrils. The formation of these fibrils coincides with a red-shift to the absorption spectrum while maintaining the apparent morphology of the individual nanocrystals. These iron sulfide clusters show promise as a platform for future device engineering due to the unique combination of optical properties and material availability and safety.

Photoluminescence (PL) down conversion has been a major success for wide band gap colloidal material, now being commercially available. More recently large efforts have been devoted to infrared (IR) detection using nanocrystal as active material [1]. This combination of cost-effective production and high performances further raise the interest for using the PL of such nanocrystals in the IR range. In particular, incoherent sources are missing beyond telecom wavelengths. Targeted applications relate to active imaging (machine vision for material sorting, damage inspection on food), LIDAR, or airfield lightning. The main challenge relates to the dop of PL efficiency as wavelength is increased. Part of this drop relates to intrinsic behaviour (lengthening of radiative lifetime) and some to the presence of non-radiative process that have to be correlated with the less mature growth of heterostructure using narrow band gap core. In this talk I will review our recent effort to push the electroluminescence of such IR nanocrystals beyond 2 µm [2] and then to further address limitation relates to their surface passivation. Another direction of interest is the coupling of such IR material to photonic structure. This effort has been very successful for photodetection to enhance the light absorption. Here, I will show that the coupling to plasmonic cavity or dielectric cavity is highly effective to shape the PL spectrum and enhance the PL directivity [3-5]. Last, I will show that the benefit of such cavity can also be applied to heavy metal free IR nanocrystals.

Monolithic integration of thin-film photodiode with the Si read-out circuit (ROIC) offers a low-cost alternative to the conventional flip-chip bonded III-V near-infrared (NIR) and short-wave infrared (SWIR) imagers. Thin film absorber materials have already enabled advanced SWIR imagers featuring the smallest pixel pitch and highest resolution. Attractive optoelectronic properties such as spectral tunability, quantum confinement, large area solution processability, ligand dependent energy band structure and electronic properties tunability make colloidal quantum-dot (CQD) as one of the most promising candidates for thin film NIR and SWIR photodetector absorber materials. Thanks to the massive advancement towards synthesis of the CQDs, these materials now include a wide range of semiconductors to choose from for applications like photodetectors, phototransistors, light-emitting diodes and solar cells.

To engineer thin-film CQD based optoelectronic devices it is crucial to obtain a complete energy band structure of CQD with various ligand types, exchange methods, and sizes of these quantum confined nanocrystals. However, the effort to design and simulate efficient devices is constrained by the lack of systematic studies and the inconsistencies found in the literature regarding reported energy band structures of CQDs. We demonstrate that the accurate characterization of energy band structure by ultraviolet photoelectron spectroscopy lies at the heart of the film preparation process and largely depends on the distribution and packing density of the deposited CQDs. Transmission electron microscopy images confirm that our proposed multi-step coating technique ensures over 90% CQD coverage (both PbS and InAs) within probing area. This is well supported by X-ray photoelectron spectroscopy, atomic force microscopy and variable angle spectroscopic ellipsometry measurements. Extracted energy band structures are further validated by fabricating SWIR PbS and InAs CQD thin film photodiodes.

Our comprehensive energy band characterization of SWIR PbS and InAs CQDs with various ligands by both solid state and liquid phase ligand exchange processes showcases an accurate and reproducible scheme to achieve complete Fermi reference energy band structure modelling of thin film photodiode stack. The insight on the overestimation of Fermi and valence band maximum |E_F-E_VBM| due to poor packing density and the summary of the energy band structures of both PbS and InAs CQDs for various ligands will advance the energy landscaping of thin film CQDs. With this methodology, we can better choose the proper ligand and size of the CQDS to efficiently design thin film devices.

Combining integrated optical platforms with solution processable semiconductor materials offers a clear path towards miniaturized and robust light sources, including lasers. Semiconducting colloidal quantum dots present a unique platform to realize this by combining tunable properties and high luminescence efficiency with solution processing. A limiting aspect for both red and green emitting materials remains the drop in efficiency at high excitation density due to non-radiative quenching pathways, such as Auger recombination. Next to this, lasers based on such materials remain ill characterized, leaving questions on their ultimate performance.

Here, we show that weakly confined ‘bulk’ colloidal quantum dots offer a unique solution processable materials platform to circumvent the long-standing material issues. First, we demonstrate that optical gain in such systems is mediated by a 3D plasma state of unbound electron-hole pairs which gives rise to broadband and sizable gain across the full red spectrum with record gain lifetimes and low threshold. As proof of concept, the nanocrystals are integrated on a silicon nitride platform enabling high spectral contrast, surface emitting and TE polarized PCSEL – type lasers with ultra-narrow beam divergence across the visible (green, red) spectrum from a small surface area. Our results prime these 'bulk' nano-materials as excellent materials platform to realize highly performant and compact on-chip light sources.

Colloidal nanoplatelets (NPLs) are promising materials for lasing applications. The properties are usually discussed in the framework of 2D materials, where strong excitonic effects dominate the optical properties near the band edge. At the same time, NPLs have finite lateral dimensions such that NPLs are not true extended 2D structures. Here we study the photophysics and gain properties of CdSe/CdS/ZnS core–shell–shell NPLs upon electrochemical n doping and optical excitation. Steady-state absorption and PL spectroscopy show that excitonic effects are weaker in core–shell–shell nanoplatelets due to the decreased exciton binding energy. Transient absorption studies reveal a gain threshold of only one excitation per nanoplatelet. Using electrochemical n doping, we observe the complete bleaching of the band edge exciton transitions. Combining electrochemical doping with transient absorption spectroscopy, we demonstrate that the gain threshold is fully removed over a broad spectral range and gain coefficients of several thousand cm–1 are obtained. These doped NPLs are the best performing colloidal nanomaterial gain medium reported to date, with the lowest gain threshold and broadest gain spectrum and gain coefficients that are 4 times higher than in n-doped colloidal quantum dots. The low exciton binding energy due to the CdS and ZnS shells, in combination with the relatively small lateral size of the NPLs, results in excited states that are effectively delocalized over the entire platelet. Core–shell NPLs are thus on the border between strong confinement in QDs and dominant Coulombic effects in 2D materials. We demonstrate that this limit is in effect ideal for optical gain and that it results in an optimal lateral size of the platelets where the gain threshold per nm2 is minimal.

Colloidal cadmium chalcogenide-based 2D nanoplatelets (NPLs) display exceptionally narrow absorption and photoluminescence bands, large one- and two-photon absorption cross sections [1] as well as low Auger recombination rates [2] and low gain thresholds [3]. Following the major strides made in the colloidal synthesis of tailored NPLs and related heterostructures, the current research focus has shifted to the incorporation of such NPLs into optical setups and devices, e.g. LEDs and lasing [4]. To date, such application-oriented setups often rely on NPL thin films [5]. Recently, however, an alternative approach using colloidal NPLs in solution gained traction, e.g. in short capillaries with the promise of higher photostability and integration into cavities [6].

Here, we demonstrate optical gain in hollow fused silica liquid-core fibers (LCFs, 20 µm core diameter) filled with a colloidal solution of 4.5 monolayer thick core-crown CdSe/CdS NPLs. The fibers are transversally excited at 480 nm in a stripe geometry (ca. 60 mm) by a 4 ns optical parametric oscillator. Aside from monoexcitonic spontaneous emission, we also observe amplified spontaneous emission (ASE), showing a characteristic bathochromic shift and peak sharpening due to its biexcitonic nature. Importantly, the arising ASE (pump energy threshold of 65 µJ) could only be observed when enabling the LCF waveguiding properties by utilizing a high refractive index solvent, like tetrachloroethylene. If a solvent with a lower refractive index than fused silica is used, e.g. hexane, which suppresses waveguiding, no ASE threshold is reached.

In conclusion, our findings indicate that NPL-filled LCFs offer a viable and efficient approach to achieving visible lasing from fused silica fibers. Incorporating colloidal semiconductor nanostructures into LCFs enables a pathway towards visible-range fiber lasers and offers integrability and flexibility, including tunable optical properties by simple replacement of the lasing medium.

The development of high-quality QDs with tunable visible emission started with CdSe. QD designs containing Cd or Pb remain those with the brightest emission and most precise control over properties. However, as consumer applications demand QDs free of toxic elements, alternative materials have drawn considerable attention in recent years. InP-based QDs now offer photoluminescence efficiencies and a color tunability that are on par with those of high-quality Cd- and Pb-containing designs. This makes InP-based QDs an ideal phosphor for displays and lighting.

In stark contrast to these successes, InP-based QDs struggle in more demanding optical applications. In particular, the development of QD lasers using InP-based QDs lags behind other QD materials by more than two decades. Lasing from InP-based QDs has been reported only once, while the vast majority of the QD lasing literature has successfully used Cd-based or Pb-halide perovskite QDs. The near-total absence of InP-based QDs in the lasing literature is consistent with spectroscopic measurements, which have highlighted difficulties to achieve population inversion and gain from an ensemble of InP-based QDs.

In this presentation, I will show ensemble and single-particle experiments to investigate why InP-based QDs are not yet suitable for lasing applications, despite their high brightness, and how their performance may be improved. On the ensemble level, we find a correlation between the magnitude of charge-carrier losses on the sub-ps timescales and slow delayed emission on the ns-to-μs timescales. From single-particle measurements, we find a cause–effect relationship between hot-carrier trapping and delayed trap emission. Based on the characteristics of the trap-related emission, we propose that hot-carrier traps are most likely internal defects, for example located on the InP/ZnSe interface. This highlights the direction into which InP-based QDs should be improved for next-generation applications.

Artificial photosynthesis is considered a promising method for achieving carbon-neutral targets. The hydrogen evolution reaction (HER) from the photoelectrolysis of water and the photoelectrochemical (PEC) CO2 reduction have gathered significant attention as an effective way to store intermittent solar energy in fuels and chemicals, as well as closing the chemical carbon cycle. Unfortunately, the photoelectrode materials used in these reactions are often unstable or exhibit insufficient activity or selectivity for the CO2 reduction reaction (CO2RR). To show the technological path of this approach, we need to focus on addressing stability and efficiency of these systems.

In this context, we present a few examples of how light-absorbing materials can be utilized in integrated photoelectrochemical cells or when directly interfaced with the electrolyte for HER and CO2RR. Specifically, we demonstrate how we can analyze and enhance the stability and performance of various photoelectrode materials used in these reactions focusing on different classes of materials.

Electrocatalysis research has gained momentum recently owing to its crucial role in the conversion and storage of renewable energy. Numerous material science advancements have been demonstrated in fuel cells, water electrolysis, carbon dioxide, and nitrogen reduction technologies. Materials with high intrinsic catalytic activity and selectivity have been looked for in such research, aiming at improved performance and maximal product yields. However, approaching the commercialization stage, the research focus has shifted to the stability of electrocatalysts. Only materials that can demonstrate reliable operation over thousands to tens of thousands of hours are considered at this stage. Since typical testing time ranges are much shorter, a question arose – how can we quantify the degree of degradation and use it to predict stable operation over the years?

Numerous accelerated stress tests (AST) have been proposed to address this issue. Such tests imply that the underlying degradation governing mechanisms and their dependence on the device's operational conditions changes are well-known. Unfortunately, this is not the case for many technologies, sparking advancements in testing instrumentation development. Thus, the introduction of online inductively coupled plasma mass spectrometry (online ICP-MS) in the electrocatalysis research has assisted in resolving the mechanism of Pt dissolution in fuel cells and Ir dissolution in water electrolysis [1]. The application of identical location transmission electron microscopy (IL-TEM) has been crucial in understanding the morphological and compositional changes in the catalyst during the operation [2]. These techniques can also be combined with complementary surface science tools in in-situ and in-operando modes. When the catalyst layer rather than catalyst properties are to be studied, e.g., mass transport effects, gas diffusion electrode (GDE) half-cell setups are more suitable [3].

Despite the demonstrated benefits of such tools in electrocatalysis, their penetration in photoelectrocatalysis (PEC) research is still limited. This talk aims to change this status quo and motivate PEC researchers to adopt existing electrocatalysis techniques. To this end, a short overview will be given of how online ICP-MS, IL-TEM, GDE, and related methods have been used in fuel cell and water electrolysis research. A summary of our recent studies on the dissolution of representative photoabsorbers, such as Fe₂O₃, WO₃, BiVO₄, etc, and co-catalysts will follow this [4, 5]. The talk will be summarized by comparing and contrasting the state of the art in electrocatalysis and PEC research and discussing further tentative directions for the latter.

The development of photoelectrochemical (PEC) systems for solar fuel production has been focused mainly on the fabrication and study of highly efficient materials. However, when the main target is to increase performance in terms of photocurrent densities, the selection of operating conditions becomes inconsistent due to ad hoc approaches preferred by researchers to maximise photocurrent densities at the expense of stability, scalability and practical application. The operating conditions for increased stability and for improved solar-to-fuel efficiency are usually at odds. For example, BiVO₄ photoelectrodes report higher stability when used in near-neutral electrolytes, although photocurrent densities are higher when alkaline electrolytes are used [1]. Similarly, chronoamperometries performed at low electrode potentials usually register more stable, although lower, photocurrent densities compared to those at high electrode potentials. For BiVO₄ photoelectrodes, the effect of different operating condition has been studied separately, e.g. irradiance [2], temperature [3] and pH [4], although with a focus on performance instead of stability. More recently, the authors decoupled the effect of increased irradiance and temperature on the dissolution of BiVO₄ by examining the competing behaviour between these two factors [5]. However, the effect of other relevant operating conditions such as electrolyte concentration, pH, electrode potential and electrolyte flow, and their interactions, has yet to be reported.

In this study, we realised a systematic experimental study of different factors, including irradiance, temperature and electrolyte nature (pH, concentration and flow) and its effect on the degradation of spray pyrolyzed BiVO₄ photoelectrodes. For this, a 4-cell array was developed to perform simultaneous tests under different conditions. Fresnel lenses were used to achieve irradiances between 1 and 110 suns, and an external water bath to control the electrolyte temperature (25 to 50 °C). KPi buffer solution was used as electrolyte at different pHs (5 to 11) and concentrations (0.1 to 1.0 M). Chronoamperometries were performed for long term tests, while electrochemical impedance spectroscopy was used to assess in-operando the charge transfer processes at different stages during the degradation and to quantify the effect of operating conditions on the properties of the semiconducting material, i.e. flat band potential and donor density. The array allowed to investigate a wide range of operating conditions, and its combinatorial effect on the dissolution of BiVO₄. Additionally, seldom reported reproducibility and sensitivity studies were also performed.

It was found that increased temperature impacts negatively the stability of the photoelectrodes, while increasing the irradiance limits the amount of charge towards the dissolution process. However, increased irradiance also induces concomitant effects, that is, increased temperature and lowered pH at the surface of the oxygen-evolving photoanode. These effects were successfully decoupled by using the above-described experimental design. It was also found that the electrolyte concentration, and respective buffer capacity, have a significant effect on the stability of the photoelectrode, although to a lesser extent compared to pH, while the electrolyte flow has a minor but noticeable effect on the of the photoelectrochemical performance.

This study contributes to the broader field of solar fuel production by highlighting the importance of stability alongside efficiency in the development of new photoelectrode materials. Understanding the complex interaction between operating conditions and degradation mechanisms can prove useful in future research to tailor these conditions to optimize the longevity and efficiency of PEC systems. Additionally, the methodology discussed here can serve as a framework for upcoming systematic studies on photoelectrode materials.

The generation of hydrogen by renewable processes is a current challenge for researchers around the world [1, 2]. Photocatalytic water splitting is a method in which photocatalytically active particles are dispersed in an aqueous solution and illuminated by sunlight, resulting in the evolution of gaseous hydrogen and oxygen [3]. It is considered a promising approach, especially because of the low complexity of its technological implementation, which holds the promise of cost-competitive hydrogen [4].
To be suitable as a photocatalyst a material is required to fulfill criteria, such as a suitable band gap size, band edges that straddle the water splitting reaction potentials and good stability under reaction conditions. The n-type semiconductor BiVO₄, with a band gap of 2.4 V, is considered to fullfil these criteria for the oxygen evolution reaction.
One of the most investigated methods to modify the optical and electronic properties to enhance the photocatalytic performance is doping [5]. For example Wang et al. have shown an increased photocatalytic performance for cationic substitution of BiVO₄ with Mo [6] and Rohloff et al. have shown an increased photocurrent density for BiVO₄ photoelectrodes by anionic doping with fluorine [7].
In this study the influence of the band gap shift due to anionic substitution of chlorine into BiVO₄ is investigated. Therefore, BiVO₄ particles are prepared via hydrothermal synthesis with the addition of halide salts, leading to partial substitution of oxygen in the lattice. The crystal structure is characterized by X-ray diffraction and the morphology by scanning electron microscopy. The chemical composition is analyzed by energy dispersive x-ray spectroscopy and electron energy loss spectroscopy. UV-vis spectroscopy is performed to investigate the optical properties. The oxygen evolution rate is determined by gas chromatography during the photocatalytic reaction in the presence of a sacrificial reagent.

Life cycle assessment (LCA) studies clearly indicate that a minimum 10% solar to hydrogen (STH) conversion efficiency and a 10-year lifetime are required for a positive energy return on energy invested (ERoRI) for photoelectrochemical (PEC) water splitting [1-3]. Analysis of published reports reveals that many lab-scale and larger demonstrations satisfy the STH criterion, but no demonstration comes within an order of magnitude of the lifetime requirement. A drastic improvement in the demonstrated/projected lifetime of PEC water splitting schemes is urgently needed if this approach is to compete with hydrogen generation via renewables (wind/solar) coupled to electrolyzers. Although the LCA and technoeconomic analysis (TEA) of PEC CO₂ reduction (CO₂R) are less numerous, examination of the related electrocatalytic literature points to minimum lifetimes of five years or more [4].

Two experimental systems which address the PEC CO₂R stability challenge will be presented.

Many promising light absorbers are not stable under the conditions of PEC CO₂R reduction. Use of a transparent electron transport layer (ETL) can address this challenge, but the ETL must have good electronic transport, be stable under CO₂R conditions, and, ideally, be catalytically inert for the competing hydrogen evolution reaction (HER). Oxygen-deficient TaO_x satisfies these criteria, and p-Si/TaO_x/Cu photocathodes have CO₂R faradaic efficiencies (FE) >50% at photocurrent densities up to 8 mA cm^-2 under 1 sun illumination with multi-hour stability [5]. Dissolution of the Cu co-catalyst appears to be the dominant degradation mechanism, suggesting that redeposition of the catalyst could extend the lifetime.

Are there any materials which have intrinsic activity/stability (that is, without the aid of electron transport/protection layers and/or co-catalysts) as photocathodes for CO₂R? This question is especially pertinent if operation in aqueous media is considered. Reports of photocatalytic CO₂R using metal sulfides are suggest a starting point for materials discovery [6]. More specifically, the Cu(In,Ga)(S,Se)₂ (CIGS) alloy family is interesting due to the extensive study of its properties as photovoltaic materials and its wide bandgap tuning range. Indeed, co-catalyst free Cu(In,Ga)S₂ (CIGS) thin-film photocathodes (Eg ~1.8 eV) reduce CO₂ to CO and HCOO^- in aqueous media at FEs of 28-32% and 14%, respectively. Extensive structural characterization (Raman, ambient pressure XPS, XAS) shows that Cu (In,Ga)S₂ photocathodes are stable for at least a few hours. Interestingly, as would be predicted by considerations of equilibrium (Pourbaix) stability, Se-alloyed photocathodes corrode rapidly. Additionally, Cu(In,Ga)S₂ films with lower bandgaps also appear to be unstable. These findings suggest that the previously unexplored Cu-deficient surface composition and specific surface defects, especially deep anti-site defects, might be playing a key role in governing the PEC CO₂R stability of CIGS-based photocathodes..

Among the various strategies proposed for enhancing the photoelectrochemical (PEC) performance of state-of-the-art photoelectrodes, the construction of heterostructures based on semiconductors and co-catalysts is widely acknowledged as one of the most versatile and reliable. This approach is often associated with higher quantum efficiency for photoinduced redox processes, due to improved interfacial charge transfer dynamics, and reduced degradation due to photocorrosion, as the photoinduced charge is less susceptible to this less competitive pathway. However, numerous studies[1-3] have demonstrated that this simplistic view conceals a much more complex microscopic nature, often significantly dependent on the specific characteristics of the interfaces. Therefore, understanding the redox dynamics in such complex systems is essential to direct the carriers along the desired pathways. Several indirect methods are available for this purpose, such as electrochemical impedance spectroscopy (EIS) and transient absorption spectroscopy (TAS), but operando X-ray absorption spectroscopy (XAS) stands out as the most promising technique[4,5]. Operando XAS can provide direct, element-selective information about changes in the redox state and the local environment of the co-catalyst metal centers.

In line with this aim, we report our recent advancements in the development of operando PEC-XAS experimental techniques to understand the redox dynamics of various semiconductor/co-catalyst assemblies, fully replicating the operating conditions of a PEC cell. This tool is employed to investigate potential and light-induced processes in i) BiVO4/WO3 heterostructures coated with Co-based co-catalysts, providing evidence for specific electronic interactions with the semiconductor, and ii) Ni-based co-catalysts on hematite photoanodes, offering insights into the rate-determining step of PEC oxidation of biomass derivatives by such systems. Additionally, operando PEC-XAS has proven to be an invaluable tool for monitoring degradation processes, as it enables structural understanding and real-time monitoring of the dissolution processes of the catalysts' active elements.

Transition metal nitrides and oxynitrides are a promising class of functional materials with tunable electronic and optical properties based on anion and cation composition. As such, they offer significant potential for custom-designed applications in photoelectrochemical (PEC) energy conversion. This contribution summarizes our group’s recent advancements and insights into transition metal nitride and oxynitride thin films, emphasizing their potential for solar fuels synthesis. Despite their promise, transition metal nitrides and oxynitrides have been less explored than their oxide counterparts due to complex synthesis requirements. We address these synthetic challenges using non-equilibrium reactive sputter deposition, allowing us to deposit well-controlled thin films within the Ti-Ta-N, Zr-Ta-N, and Zr-O-N composition spaces. Our investigations start with orthorhombic Ta₃N₅, the most established nitride-based photoanode material. With a highly controlled synthesis approach and tailored defect concentrations, we examine the impact of shallow and deep trap states on the operational stability of Ta₃N₅.[1] By comparing water and ferrocyanide oxidation conditions, we observe that shallow oxygen donors within Ta₃N₅ can kinetically stabilize the photoelectrode|electrolyte interface. In contrast, deep-level defects are generally detrimental to its PEC performance. We demonstrate, though, that the concentration of such deep-level defects can be dramatically reduced by controlled Ti doping.[2] While Ti doping minimally affects the structure and band gap of Ta₃N₅, it significantly impacts surface photovoltage, band bending, and photogenerated charge carrier lifetimes. Comprehensive characterization reveals that Ti⁴⁺ ions substitute for Ta⁵⁺ lattice sites, introducing compensating acceptor states and reducing concentrations of deleterious nitrogen vacancies and Ta³⁺ states. This reduction of deep-level defects suppresses trapping and recombination, leading to enhanced PEC activity. Beyond orthorhombic Ta₃N₅, we have explored novel photoanodes with cubic bixbyite-type structures, such as ZrTaN3 and Zr₂ON₂, featuring multiple cation and anion identities. We have recently identified the ternary nitride ZrTaN₃ as a strong visible light absorber and functional photoanode thin film.[3] Complementary density functional theory (DFT) calculations indicate that ZrTaN₃ has a direct band gap that can be tuned by modulating the elemental occupancy of inequivalent cation sites. Finally, changing the anion composition in the Zr-O-N space allows us to tune the band gap in the UV-visible range, achieving PEC activity for oxidation reactions.[4] For crystalline Zr₂ON₂, we observe mild surface oxidation beneficially passivates the surface, while too-thick surface oxides suppress charge transfer and PEC activity. This observation highlights an appealing feature of oxynitrides as photoanodes: the formation of self-passivating surface oxide layers. This passivation makes them suitable for integration with ultrathin atomic layer deposition protection layers, whose major drawbacks are pinholes that can potentially be tolerated by (oxy)nitride semiconductors through the passivating surface oxide layers. Overall, our results underscore the potential of the defect engineering of established and the development of novel transition metal nitride and oxynitride semiconductors for achieving robust and stable materials in solar energy conversion.

The conversion of energy from renewable and CO₂ free energy sources into chemical energy, has the potential to contribute significantly to cover our energy needs [1]. Photoelectrochemical (PEC) water splitting, with its simpler setup compared to combining a photovoltaic cell and an electrolyser, shows promise as a cost-effective method for producing green hydrogen in the future [2]. A PEC system consists of two separate electrodes connected by an ohmic contact, each providing a site for one of the two water-splitting half-reactions [2].

Research over the past decades has primarily focused on improving the photoanode efficiency, leading to a thorough understanding of the material properties required for high conversion efficiency [3-5]. However, for PEC systems to be viable on an industrial scale, the long-term stability of the photoelectrode materials is essential. These materials must withstand degradation  by the electrolyte and resist photo corrosion [2, 6] Investigating the degradation processes of photoelectrodes is therefore important from both scientific and practical perspectives, as it is crucial for developing photoelectrodes with longer lifespans [3, 7].  

In photoanodes, photogenerated holes  are known to participate either in the oxygen evolution reaction, to be lost to recombination, or to cause the oxidation of anions in the photocatalyst [6]. This oxidation process can lead to the dissolution of metal ions into the electrolyte, as the crystal lattice becomes destabilized due to changes in the local structure around the oxidized ions [8, 9].

Since oxynitrides are known for their good performance with respect to efficiency, LaTiO₂N based photoanodes were selected for this study on degradation. The LaTiO₂N was synthesized via solid state synthesis followed by thermal ammonolysis. The photoanodes were prepared by electrophoretic deposition followed by TiCl₄ necking and cocatalyst application via dip coating. 

The degradation of the LaTiO₂N based photoanodes was investigated as a function of the illumination conditions and the electrolyte temperature using chronoamperometry in combination with linear scan voltammetry. IPCMS was used to detect potential dissolution of cations into the electrolyte. The photoanodes were investigated postmortem with respect to compositional, morphological and optical changes using STEM-EDX, SEM and UV-Vis-Spectroscopy.

A large body of literature exists on photocatalytic H2 generation using a wide variation of semiconductors, morphologies, and strategies to split water using the semiconductors suspended in an aqueous solution (with or without sacrificial agents). Many semiconductors have in common that for an efficient transfer of photogenerated charge carriers, a co-catalyst is required. For electron transfer and H2 generation mostly Pt nanoparticles are used that are deposited onto the semiconductor surface by various techniques. Due to the precious nature of Pt, over the years, numerous efforts have been devoted to the shrinkage of the particle size and thus to enhance the utilization of the noble metal – in the most extreme case down to an insulated single atom of Pt.

In the presentation we discuss the use of Pt dispersed and anchored as single atoms (SAs) on TiO2 surfaces and the activation to a most efficient use for photocatalytic H2 generation. We discuss various trapping and stabilization approaches of SAs on photocatalysts that prevent agglomeration (and accordingly the deactivation of SA Pt). Moreover, we show that only a very small amount of Pt (loading density of SAs ) is needed to achieve a maximum activity of a semiconductor surface.

The climate crisis is one of the most pressing challenges of our time. Global and coordinated action to reduce greenhouse gas emissions and mitigate their impacts is indispensable. Proposed solutions to address this crisis include the adoption of renewable energy, increased energy efficiency, reforestation, and the use of innovative technologies. In the technology sector, photocatalysis is attracting growing interest, fueling intense research into new materials.¹ In our study, we examined the photocatalytic activity of heterojunctions between Bi₂WO₆ and Cs₃(Bi_1-xSb_x)₂Br₉. Bi₂WO₆ is a widely recognized graphitic material in the field of photocatalysis, while perovskites, known for their excellent tunability, prove to be outstanding co-catalysts.^2,3 Lead-free inorganic perovskites have been the subject of numerous studies for years.^4,5 Of particular interest is the introduction of different metals into the B-site, introducing a high entropy approach which is still in its infancy.⁶ In our study, we analyzed how the structural and optical properties change with increasing doping rates. The formation of a heterojunction between the two materials could create a more complex charge carrier transport pattern than with single materials, increasing their lifetime and thus improving photocatalytic results.⁷ The heterojunction is facilitated by the innovative in-situ synthesis adopted, in which perovskite is formed within a solvent in which Bi₂WO₆ has been dispersed. The aim of the project is the photocatalytic splitting of water and CO₂, underlining the importance of property-structure relationships also in the photocatalytic field. The project aims to highlight the differences in various set-ups, whether liquid-liquid or gas-solid. The discovery of new materials and the acquisition of new knowledge about photocatalysts are key to the development of advanced technologies for CO₂ reduction. Photocatalytic methane evolution paves the way for a circular ‘waste to fuel’ economy in which waste molecules such as CO₂ regain value.

Photoelectrochemical (PEC) devices can mimic photosynthesis and show great promise for sustainable fuel production. These artificial leaves integrate light absorbers with suitable catalysts to directly harness, convert, and store abundant solar energy in the form of value-added chemical fuels.^[1] However, most conventional prototypes employ wide bandgap semiconductors, moisture-sensitive inorganic light absorbers, expensive materials or corrosive electrolytes. Here, we introduce the design and assembly of PEC devices that contain an organic π-conjugated donor-acceptor bulk heterojunction (PCE10:EH-IDTBR) with sufficient photovoltage for both proton reduction and CO₂-to-syngas conversion.^[2] The rational combination of design strategies from organic photovoltaic (OPV) and inorganic PEC fields, coupled with a carbon-based encapsulant, promoted long-term H₂ production over 12 days in benign aqueous media. Given the modular nature of our device design, interfacing the devices with a molecular cobalt porphyrin catalyst allowed for tunable and selective CO production under 0.1 sun. Further assembly of these OPV photocathodes with BiVO₄ in a standalone artificial leaf demonstrated unassisted concurrent CO₂ reduction and water oxidation over 4 days. This establishes a new path for organic semiconductors, as we approach the composition, function, and efficiency of natural leaves.

The pursuit of sustainable green fuel production through photocatalysis is a central theme in modern scientific exploration. The focus lies in developing an active photocatalytic architecture capable of efficiently converting light into energy. Plasmonic metals, especially gold, have garnered attention due to their unique optical properties, known by tuneable localized surface plasmon resonance (LSPR).¹ However, a significant trouble in achieving overall efficiency is the limited lifetime of photogenerated charge carriers, known as hot carriers.²

Addressing this challenge involves strategically incorporating a metal or semiconductor near the plasmonic entity. This approach aims to extend the recombination time of hot carriers, enhancing the overall efficiency of the photocatalytic system. While considerable developments have been made, a systematic exploration into the deposition of metals, semiconductors, and hybrid systems remains crucial for advancing plasmonic photocatalysis.³

This research depicts the key strategies and motivations driving the engineering of efficient architectures in plasmonic photocatalysis, including metal-metal, metal-semiconductor, and metal-semiconductor-metal configurations. The presented research involves photocatalytic applications of diverse plasmonic nanostructures, such as gold@gold-silver alloy nanostructures, gold@manganese oxide core-shell nanostructures, and gold@platinum core-shell nanorods deposited on ceria particles. These nanostructures showcase the potential for light-enhanced photocatalysis, offering valuable insights into sustainable energy solutions.^4-6 Through a detailed understanding of plasmonic-metal-semiconductor interactions, this research contributes to the ongoing efforts to harness renewable energy sources and advance green fuel production via innovative photocatalytic architectures.

Polymeric carbon nitride (CN) materials have emerged as metal-free, low-cost, and environmentally friendly semiconductors in various applications, including photoelectrodes in photoelectrochemical water-splitting. Unlike CN powder, which is used as a dispersed photocatalyst, for applications such as photoelectrochemical cells, light-emitting diodes, and solar cells, the deposition of the CN on a conductive substrate is required. The deposition of CN layers on different substrates can be divided into two main categories: 1) ex-situ deposition of prepared CN powder and 2) in-situ growth of a CN layer directly on the substrate. Generally, the in-situ methods comprise two steps: the first one is the deposition or growth of nitrogen-rich monomers (such as melamine, urea, thiourea, dicyanamide, etc.), forming a film of the monomers on the substrate. The following step is the calcination process, in which these monomers polymerize, forming the final CN electrode. Most in situ preparation techniques differ in the deposition or growth method of the monomers' film but maintain a similar 'standard' calcination process of slowly heating this film to 500–550 °C and keeping the final temperature for several hours, usually under an inert atmosphere. A significant drawback of a long heating process is the possible sublimation and decomposition of the CN monomers and final layer, which may lead to a less uniform film. In addition, from an economical point of view, a long calcination process at a high temperature consumes much energy.

In this talk, I will introduce a facile, scalable, energy-saving, and reproducible synthesis of CN layers on conductive substrates using a fast heating method. In this method, the predesigned CN monomer films are subjected for several minutes (5-20 min) to higher temperatures than the 'standard' calcination procedure (650–680 °C). The high-temperature process enables fast condensation of the monomers, and negligible degradation is obtained thanks to the short time at the target temperature. As a result, the formation of a uniform CN layer with excellent contact with the substrate and good activity as a photoanode in PEC is achieved. The optimal CN photoanode reaches photocurrent densities of ~200 μA cm⁻² at 1.23 vs. RHE in neutral and acidic solutions and 120 μA cm⁻² in a basic solution.^[1]

While III-V semiconductors have achieved the highest photo-electrochemical solar-to-hydrogen conversion efficiencies, they are remarkably unstable during operation in a harsh electrolyte. The first half of this talk will focus on the degradation mechanism of III-V cells and surface modification strategies aimed at protecting them from photocorrosion. We applied noble metal catalysts, oxide coatings by atomic layer deposition, and MoS₂ in an effort to protect the GaInP₂ surface that was in contact with acidic electrolyte. We also grew epitaxial capping layers from III-V alloys that should be more intrinsically stable than GaInP₂. The ability of the various modifications to protect semiconductor surfaces was evaluated by operating each photoelectrode at short circuit for extended periods of time.

In the second part of this talk, I’ll discuss potential pitfalls to consider when characterizing semiconductor materials for durability. Measurements of photoelectrode durability in three-electrode (half-cell) configurations are typically not representative of the results obtained for nominally the same electrodes/materials when tested in a two-electrode (full-cell) configuration. While full-cell measurements are the best proxy to predict performance in a deployed photo-electrochemical water-splitting system, there are few materials that can drive both the water reduction and oxidation half-reactions. Component-level or half-cell testing is the only option for materials unable to perform unassisted water splitting. During this talk the results of multiple durability tests, some with and some without a reference electrode, will be used to evaluate key differences between the two types of testing in an effort to elucidate what leads to the incongruity between half- and full-cell measurements. It is anticipated that with this understanding, the photoelectrochemical water-splitting community can begin to move towards an accepted protocol for long-term durability testing that will have more predictive relevance to realistic device configurations.

In this talk, I will present recent advances of artificial photosynthesis utilizing gallium nitride (GaN), which is the second most produced semiconductors next only to silicon. Through nanoscale, quantum, and catalyst engineering, conventional GaN-based semiconductors can be transformed to be efficient and stable photocatalyst materials for a broad range of artificial photosynthesis reactions, including solar water splitting, carbon dioxide reduction, methane oxidation, and nitrogen reduction to ammonia. By growing GaN nanostructures under N-rich conditions, the nonpolar surfaces can be transformed to be gallium oxynitride during harsh photocatalysis reaction, which not only protects the surfaces of the light absorber but leads to significantly enhanced photocatalytic and photoelectrochemical performance. Moreover, we have developed unique photocatalytic processes wherein high efficiency solar hydrogen can be produced utilizing tap water, or seawater, without any wire connection, or electricity input. The demonstration of large-scale solar water splitting systems and the performance will be discussed and reported, together with advances in carbon dioxide and nitrogen reduction to clean chemicals and fuels.

One of the current issues in the electrochemical CO2 reduction reaction (CO2RR) is the stability of the catalyst. [1] Copper is the most promising material for producing desirable C2+ chemicals at reasonable rate, yet this metal reconstructs during operation. [1] A solution is to either block this reconstruction or to learn how to direct it towards structures with the desired selectivity. [2]  

Herein, we propose to explore a different class of materials. These materials are liquid metal Ga-based nanoparticles (NPs). We propose them as alternative to traditional solid catalysts with great potential for stable CO2RR thanks to their self-regenerating dynamic surface. [3,4] Yet, we are still learning about their chemistry, which is at its infancy compared to other NPs. [5,6] Developing their chemistry is important to further explore them for selectivity in CO2RR and eventually for other reactions.

In this talk, we will focus on the most recent work where we synthetize tunable and monodisperse Ga-based NPs incorporating a variety of different metals through colloidal chemistry. In particular, we prepare solid-liquid-solid Ga-M NPs (M= Ag, Cu, Au, Pd) wherein a solid metal is encapsulated within a liquid Ga NP confined by its oxide skin. We elucidate the formation mechanism of these unique nanostructures through a combination of state-of-the-art in-situ techniques, including electron microscopy and X-ray absorption spectroscopy. Finally, we demonstrate the functionality of these NPs as CO2RR electrocatalysts.

Coupled multi-physics modeling can reveal new insights into the processes that govern the performance and degradation of photoelectrodes under varying operation conditions [1]. While many subprocesses have been studied to a great extent, a unified model for PEC device operation is still a work in progress [2]. For example, recent studies use simplified models for the charge transport in the semiconductor [3] or neglect the electrolyte species transport [4]. Also, charge transfer theory at the semiconductor-electrolyte junction (SCEJ) coupled to the solid and liquid has been seldom investigated [5]. Furthermore, the modeling of (time-dependent) degradation through (electro-)chemical corrosion has been initiated recently yet with simplified charge transfer and neglected species transport [6].

Herein, new physics relevant for photoelectrode operation have been included to predict the performance and stability of photoelectrode material systems. Transport of electrons and holes in the semiconductor were modeled with Poisson-drift-diffusion equations including carrier generation and recombination terms. Special attention was put on the surface recombination by trap states. Band edge unpinning was introduced through surface state (dis-)charging, which allowed to study a surface state-passivating co-catalyst. Transport of species in the electrolyte was modeled in a multi-modal manner including diffusion, drift in electric field and drift in boundary layer flow. Charge transfer modeling at the SCEJ based on Marcus Theory was implemented to study simultaneously multiple redox reactions of different energetics and kinetics. Degradation was modeled as competition between charge transfer to electrolyte species and charge capture by surface bonds with time-dependent photoelectrode dissolution.

A BiVO₄ photoanode with CoPi co-catalyst for water splitting served as case study for model validation and demonstration. Parameters of the SCEJ and charge transfer were determined through first principle calculations and with dedicated experiments. Voltammograms were used for validation by variation of operational conditions (pH, temperature and irradiance) and its effect on, otherwise, inaccessible material properties was assessed by sensitivity analysis. Voltammograms showed a mild pH-dependence resulting from changes in the surface state charging and electrolyte species concentrations. Under concentrated sunlight (> 10 kW m^-2), a cathodic shift in the onset potential was observed, dependent on surface state charging and recombination. The effect of mass transport limitation on the photocurrent caused by sluggish species near the SCEJ was also quantified. A photocurrent dependence on the illumination direction (front vs. back), typical for BiVO4 due to limited majority carrier transport, was confirmed in the model. The dissolution-based degradation model was able to capture characteristics of the photocurrent decrease experimentally observed in BiVO4 photoanodes.

In-depth parametric studies on photoelectrode operation (including degradation) were conducted with a coupled multi-physics model. The model has capabilities to be expanded to include multiple dimensions and complex photoelectrode structures to provide more insights into the heterogeneity and local variation of operation condition in a photoelectrochemical device or component. Moreover, additional degradation mechanisms, e.g. surface passivation, will permit stability studies of a broad range of photoelectrode materials.

The atomistic understanding of complex reaction mechanisms in (photo-)electrocatalysis aids not only the discovery of improved catalytic materials but also the choice of ideal reaction environments for tailored products.

In my talk, I will present density functional theory-based studies on electrocatalytic reaction mechanisms with a special focus on electrochemical CO₍₂₎ reduction and biomass valorization. I will describe how the combination of constant-potential DFT approaches and transition state theory-based considerations allow us to explicitly study the potential, pH and electrolyte dependence of multistep reaction networks relevant for the green transition [1].

Further, I will discuss general trends in the thermodynamic and kinetic preferences of the competing elementary reactions in electrocatalytic reductions. Here, I will show how the potential and pH response of specific reaction pathways can be exploited for tuning product selectivity [2]. Finally, I will show the generality of the found trends by extrapolating from electrochemical CO₍₂₎ reduction to the electrochemical reduction of Furfural [3].

In most photo-electrochemical devices the kinetic phenomena does not only imply the reactions we are aiming at but also some others that are linked to the dynamics of the material acting as catalysts. Some of them are positive for the performance of the material increasing the number or improving the nature of active sites. Still, some others are detrimental for the system and can cause failure. In the talk I will address what can we do from the point of view of atomistic simulations to start understanding this phenomena so that we can control the nature and prolong the stability of the active sites under true reaction conditions. I will elaborate on the gaps in our understanding and the role of accelerators to investigate this long-term effects in the active materials. Finally,  I will indicate the most relevant challenges in the field highlighting what is needed to progress further in this area.

Oxide and oxynitride oxygen-evolution (OER) photoelectrodes in contact with an electrolyte and under an applied bias and/or light irradiation undergo often irreversible changes in surface structure and composition. Computational determination of the energetically most favored changes as a function of experimental parameters constitutes a powerful complementary technique to experimental spectroscopy and local-probe studies. Perovskite tantalate photoelectrocatalysts exist as bulk perovskite oxides, layered oxides, and bulk perovskite oxynitrides. Based on density functional theory calculations, we determine likely alterations under application conditions and examine their effect on the catalytic activity. For the layered Sr₂Ta₂O_7, we predict a partial dissolution of the surface that deactivates the catalyst. A similar structural change on the NaTaO₃ surface, however, leads to enhanced catalytic activity by enabling an alternative OER mechanism. For SrTaO₂N we predict a loss of nitrogen from the surface layers, the resulting electron doping of the surface also leading to a deactivation of the OER. These results show that compositionally similar materials can undergo very diverse changes under OER application conditions that, while mostly detrimental, can - in select cases - also improve the catalytic activity.

Understanding the dynamics at catalyst-liquid interfaces is crucial for gaining insights into degradation/deterioration of catalysts and complex reaction such as the oxygen evolution reaction (OER). Notwithstanding LaTiO₂N and BiVO₄ are gaining attention as active photocatalysts for OER, a lot of questions are still elusive such as the identification of active catalytic surface sites, their catalytic properties as well as the organization of water at the catalyst-water interface. Spin-polarized density functional theory-based molecular dynamics (DFT-MD) enables extensive investigations of various catalytic systems, explicitly considering environmental factors such as solvents under ambient conditions. For instance, possible degradation mechanisms of LaTiO₂N (100) surface have been investigated by DFT-MD. Notably, we identified the transfer of oxygen atoms from the sub-layer to the surface as a potential degradation process for LaTiO₂N. The generated oxygen vacancy in the sublayer can potentially propagate into the bulk and break the lattice. For BiVO₄ we investigated the desorption process of surface vanadium atoms from surface towards the solvent employing enhanced sampling techniques, which is powerful tool for the understanding of reaction mechanisms and calculation of free energy surfaces.  Pourbaix diagrams have been calculated in order to understand the stable LaTiO₂N and BiVO₄ surface configuration at different electrochemical conditions. Preliminary catalytic activity of LaTiO₂N and BiVO₄ in catalyzing the OER have been elucidated by DFT-static calculations for free-energy change, obtaining a good agreement with the available experimental results. Our research has also focused on the implementation of grand-canonical-ensemble approaches in the CP2K simulation software. Testing and applications have been applied on LaTiO₂N and BiVO₄ systems as well.

Halide perovskites and organic bulk heterojunction semiconductors have attracted significant interest for photovoltaic applications due to their excellent optoelectronic properties, such as strong solar absorption, wide defect tolerance, and long charge diffusion lengths. These properties are also crucial in the development of photoelectrochemical devices for solar fuels and chemicals. However, the instability of these devices in aqueous environments must be addressed. In this talk, I will present our team's recent progress in protecting CsPbBr₃ halide perovskite and PM6:D18:L8-BO and PTQ10:GS-ISO organic bulk heterojunctions with various carbon allotrope layers and sheets. These include mesoporous carbon, graphite, glassy carbon, and boron-doped diamond, which are decorated with electrocatalysts. These layers and sheets offer protection and catalytic activity but can degrade under harsh conditions, such as those required for oxygen evolution—a bottleneck reaction in many solar fuel and chemical productions. The addition of Ni and NiFeOOH is crucial to ensuring photocurrent stability, maintaining photocurrents above 6 mA cm^-2 with projected stability of months under harsh +1.23 V vs RHE applied bias on CsPbBr₃ photoanodes. A similar approach provides PM6:D18:L8-BO photoanodes that achieve 25 mA cm^-2 at +1.23 V_RHE and monolithic tandem organic photoanodes with PM6:D18:L8-BO and PTQ10:GS-ISO with 5% unassisted solar-to-hydrogen efficiency, both showing days-long stability. In these cases, the stability is mainly limited by the morphological instability of organic bulk heterojunctions. Oxygen bubble accumulation on the surface of these devices is also a limiting factor for photocurrent stability. These and other challenges in achieving stable photocurrents in perovskite and organic bulk heterojunction photoanodes will be discussed.

Gas diffusion electrodes, crucial for fuel and electrolysis cells using gas-phase reactants, are often made from materials like graphitic carbon or metals, which block light and hinder photoelectrode membrane assemblies for solar fuel production. This presentation introduces transparent gas-diffusion layers using F-doped SnO₂ (FTO) coated SiO₂ fiber felt substrates for solar H₂ production and water splitting from humid air. The initially demonstrated substrates exhibited a porosity of 90%, a roughness factor of 15.8, and a Young’s Modulus of 1 GPa.[1] FTO coating provides a sheet resistivity of 20 ± 3 Ω sq⁻¹ and 50% transmittance in the 300nm-800nm range for the transparent conductive porous substrates (TPCSs). Various semiconductors, including Fe₂O₃, BiVO₄, Cu₂O, and semiconducting polymers, were deposited on the TPCSs, showing superior photoelectrochemical performance compared to flat FTO photoelectrodes. Moreover, strategies to improve the robustness, tune the porosity, and scale up the production of the TPCSs are herein presented, and the challenges and limitations of gas-phase PEM-PEC water splitting are discussed. Finally, unassisted gas-phase water splitting is demonstrated with a PEM-PEC cell at 1-sun, achieving a photocurrent density on the order of 1 mA cm^–2 with a system integrating a BiVO₄ photoanode and a polymer semiconductor photocathode for complementary light absorption together with a polymer electrolyte membrane.

Membrane-photoelectrode assemblies, i.e. photoelectrodes on porous substrates coated with an ionomer and directly integrated with a polymeric ion-exchange membrane, are a promising design configuration to perform photoelectrochemical water splitting [1]. This compact and modular design inspired by commercial electrolyzers allows to feed the system with pure water (in liquid or gaseous phase) minimizing the products crossover.

The most common proton-exchange membranes (PEMs) absorb light only in the UV region (for λ< 400 nm) but they impose an acidic pH which causes the corrosion of the majority of the catalysts. The most used anion-exchange membranes (AEMs) are partially opaque in the visible range but the alkaline environment assures the chemical stability of a large variety of materials. Although multiple demonstrations of proton-exchange and anion-exchange membrane-photoelectrode assemblies have been reported [2], the effects of (photo)corrosion of the semiconductors and/or of the co-catalyst used were not investigated in-depth.

Here, we studied the dynamics and the effects of photocorrosion of molybdenum-doped bismuth vanadate (Mo:BiVO₄) photoanodes with cobalt phosphate (CoPi) co-catalyst integrated in proton-exchange and anion-exchange membrane-photoelectrode assemblies with commercial Pt/C cathodes. BiVO₄ suffers from (photoelectro)chemical instability in neutral and alkaline solutions, leading to homogeneous film dissolution [3]. We investigated for the first time the (photo)corrosion of the Mo:BiVO₄ photoanode with CoPi co-catalyst in contact with hydrated ionomers.

The photoanodes were deposited on porous metallic felts through a dip coating technique followed by annealing and the photoelectrodeposition of CoPi co-catalyst. The preliminary optimization of the dopant concentration, semiconductor loading and co-catalyst deposition time was done testing the performance of the coated felts in a simplified cell configuration with liquid electrolyte. The optimal photoelectrodes on metallic felts were placed in thin layers of ionomer solutions obtained by a doctor blade method. After the evaporation of the solvents, the ionomer-coated felts were hot pressed to form membrane-photoelectrode assemblies.        

Chronoamperometric durability tests of the membrane-photoelectrode assemblies with liquid water and humid air at different temperatures were interspersed with cyclic voltammetries and impedance spectroscopies to determine the degradation rates in these different operating conditions. Scanning electron microscopy, energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy of the aged samples with inductively-coupled plasma mass spectroscopy of the liquid solutions were used to observe the effects of Mo:BiVO₄ and CoPi photocorrosion in PEM and AEM assemblies.

The methodology introduced and the setup developed allow to investigate the photostability of membrane photoelectrode-assemblies with other semiconductors and co-catalysts with the aim to develop efficient, sustainable but also durable materials and devices for the conversion of solar light into fuels.

BiOBr is used as a promising material in various photoelectrocatalytic reactions, but its application in hydrogen evolution reaction (HER) is not commonly reported, because the instability under photoelectrochemical environment has become a drawback for BiOBr as a photoelectrocatalyst for reduction reactions.¹ To solve this problem, MoS2 is induced to form a van der Waals heterojunction to stabilize the crystal structure of BiOBr in HER.² By DFT calculation, we state that the active sites in the heterojunction are on sulfur centers, which also combines BiOBr by van der Waals force. Moreover, different MoS2/BiOBr ratios can result in different structure tolerance of BiOBr: heterojunction with 1% MoS2 can increase the stability of BiOBr while 50% MoS2 even accelerate the reduction of BiOBr. By performing the in situ wide-angle X-ray diffraction (WAXD) on MoS2/BiOBr with 1% and 50% of MoS2, respectively, we monitored the phase transfer speed of BiOBr during the HER. Interestingly, when UV light is induced, there is less amount of BiOBr reduced under negative potential due by the photogenerated holes that could react with extra electrons from negative bias or photo energy and prevent BiOBr to be further reduced.³

Johnson Matthey’s (JM) vision is for a world that is cleaner and healthier, today, and for future generations. As global leader in sustainable technologies, we apply science to catalyse the net zero transition for our customers. Through science we enhance life for millions of people, all over the world. We are making it our business to help address the four essential transitions: driving down transport emissions, transforming our energy emissions, decarbonising chemicals production, and creating a truly circular economy. If the transport transition is all about moving people and goods while lowering emissions, the energy transition is about finding sustainable ways to power the world. Hydrogen has a huge role to play because when it is used as fuel, the only by-product is water. Low-carbon hydrogen is essential for a viable hydrogen economy.

Because chemical and materials design for real world applications are a multiscale problem, in JM we have experts in different length scales (from Å to m) and we use state-of-the-art methods, such as high-throughput calculations and machine learning (ML) techniques. We perform 1) Nanoscale: Electronic and atomic modelling, 2) Micro/Mesoscale: Kinetic data and modelling, pore-scale modelling, and 3) Macroscale: Reactor scale modelling (e.g., CFD, process modelling). Theoretical work together with experimental measurements guide the development of real-world catalysts, driving innovation and progress in sustainable technologies. Several use cases will be demonstrated coming from our four business areas: Clean Air (CA), Platinum Group Metal Services (PGMS), Catalyst Technologies (CT) and Hydrogen Technologies (HT) where theoretical and computational tools are used to understand complex materials and guide experiment.

A fundamental study employing Density Functional Theory has been performed to study the deactivation of the industrially relevant Cu/ZnO/Al₂O₃ catalysts for methanol synthesis. It has been shown that the catalyst is impacted strongly by the behaviour of the zinc oxide (ZnO) component, particularly with regards to sintering. Although the copper component also sinters, the overall deactivation observed is largely driven by changes in the ZnO moieties.[1]

The addition of small amounts of a silicon-based promoter[2] has previously been identified as a promising low-cost, readily available, and non-toxic additive that slows down the rate of deactivation.[3] The presence of silicon improves the aging characteristics and slows down the sintering of ZnO. Improving and slowing down the sintering of the standard methanol catalyst is of great industrial relevance. This is particularly important with regards to sustainable methanol production from CO₂, due to the significantly higher quantities of by-product water produced compared to conventional methanol synthesis conditions. Therefore, gaining fundamental understanding by looking at the stability promoter role of Si is important.

In this study, two main questions were addressed: 1) What is the effect of Si in both bulk and surface? and 2) How does water interact with bare ZnO and Si_xZn_y-2xO_y (x=1,2,3)? Finally, the results will be compared to experimental data.

The green transition requires discovery and development of new catalyst materials for sustainable production of chemicals and fuels. However, it is difficult to predict a material, which might have a high catalytic activity for a given reaction, therefore the development of catalysts up until now has been driven mainly by trial and error. It would increase the pace of development, if we could predict a range of promising materials or if we at least could understand the limitations of catalysis. In this context high entropy alloys offer a chemical space of possible materials where the composition can be smoothly varied and where the properties also might vary in a seamless manner. This is good news for catalysis as such a smooth space is easier to explore to determine the interesting regions in composition space. Furthermore, the highly heterogeneous nature of a high entropy alloy surface reveals fundamental effects which are important for chemistry on surfaces in general, but are overlooked in the classic mean field view on catalysis. 

Artificial nitrogen fixation is essential to provide food security. Today ammonia is produced through the Haber-Bosch process from nitrogen and hydrogen gases whereas nitrates are produced through the Ostwald process from ammonia and oxygen. From these, various nitrogen containing fertilizers are produced depending on the application of use. The Haber-Bosch process is, however, not sustainable since it relies on natural gas resources for hydrogen generation, and at the same time highly polluting of CO2 emission. It is therefore necessary to develop alternative routes for ammonia and nitrate synthesis. One of the most attractive solution would be to have a heterogenous electrolytic cell with an aqueous electrolyte that works at ambient conditions, where a nitrogen fertilizer can be produced on-site. There are, however, several factors that make it difficult to accomplish this, mainly because the N2 molecule is inert and difficult to reduce and because of the side reaction, the hydrogen evolution reaction (HER), which usually takes place more easily than the nitrogen reduction reaction (NRR). It has been predicted that all the transition metals will much more easily catalyze HER than NRR.
 
Over the last few years we have been searching, using density functional theory (DFT) calculations, for alternative materials that can catalyze NRR while suppressing HER. The class of materials we have investigated are transition metal ceramics of e.g. nitrides, oxides, sulfides, carbides, oxynitrides and carbonitrides. Several promising candidates are predicted within each class of materials and we have tested several of them experimentally. There, we grow the catalysts in thin-films using magnetron sputtering, which are then tested in a micro reactor for electrocatalytic performance. The electrochemical micro reactor is connected in-line with the ammonia detection unit, preventing any possible contamination which makes the results reliable and robust. Experiments are done both in N2 saturated electrolyte and in Ar saturated electrolyte and isotope labelled 15N2 is used to proof catalysis. In this presentation, I will discuss both the theoretical predictions and the experimental performance of several candidates for NRR.

Electrocatalytic systems are crucial in various renewable energy conversion and storage technologies, forming a foundational basis for our sustainable future. Realizing their full potential requires advancements e.g., in catalytic materials to achieve better catalytic efficiencies, higher stability, and lower costs. This necessitates an atomic-level understanding of electrocatalytic systems, particularly the complex electrocatalyst-electrolyte interface, which involves numerous components and processes. Moreover, the interface properties can vary substantially depending e.g. on solvent and electrode potential and the variations can, in turn, have direct impact on electrocatalytic behaviour. The theoretical and computational methods are pivotal, as they can offer atomic level insight into interface chemistry even under realistic reaction conditions, but this calls constant development of methods and approaches.

The grand-canonical ensemble (GCE) DFT calculations [1] offer a robust framework for modelling electrochemical interfaces and reactions at the atomic level, while maintaining fixed electrode potentials. In my presentation, I will cover our recent developments in GCE-DFT [2], which make the method applicable to systems beyond the reach of the standard GCE-DFT approach. The examples of GCE-DFT calculations to be presented include computing Pourbaix diagrams for metals under realistic reaction conditions [3,4], demonstrating how pH and potential can strongly influence the state of the catalyst, and N₂ reduction to ammonia on graphene-based material, highlighting the potential dependency of reaction thermodynamics and kinetics and the role of explicit water molecules in these calculations [5]. Finally, the advantages and limitations of this method will be discussed and compared to standard DFT calculations.

Electrochemistry holds the promise to be a cornerstone for the sustainable production of fuels and chemicals. However, these catalytic reactions are increasingly complex to understand and hereby also improve. In particular, reactions that suffer from selectivity challenges such as CO₂ reduction

In this talk, I will discuss how experiments and computational simulations can support each other. I will focus on electrochemical reduction of NO_x, CO₂, N_2, and the combinations. Importantly, all these reactions share a direct competition with hydrogen, and furthermore, several products are formed from each reactant of these reactants. I will give minimalistic models that do not overfit or over-interpretate experimental data. Examples are:

- Electrochemical CO₂ reduction show multiple different products depending on metal catalyst [1]. I will show why copper is unique as catalyst with a multiple-carbon product distribution [2]. Following I will discuss data analytics on copper facets to steer the product distribution [3].

- Electrochemical NO_x reduction, also multiple products are formed; N₂O, N₂ and NH₃[4]. Several catalyst enable reductions to ammonia amongst them copper [5] and recently also Co and Fe based catalysts close to their reduction potential [6].

- Electrochemical N₂ reduction to ammonia (NH₃) at ambient conditions is burgeoning [7-8]. Most interesting in aqueous there is not a “copper” catalyst [9]. While in non-aqueous, the univocally working system is a Li-mediate system [10]. For this system, I will show that varying multiple experimental parameters display similar performance characteristics [11] and I will discuss systems beyond lithium.

Finally, I will discuss how one can use these insights to establish predictive schemes for products beyond the typical reduction reaction products, hereunder synthesis of urea [12].

[1] Hori et. al., Journal of the Chemical Society, Faraday Transactions, 1989, 85, 2309-2326.

[2] Bagger, Ju, …, Strasser, Rossmeisl, ChemPhysChem, 2017, 18, 3266–3273.

[3] Bagger, Ju, …, Strasser, Rossmeisl, ACS Catalysis, 2019, 9, 7894−7899

[4] Rosca, Duca, Groot, Koper, Chem. Rev. 2009, 109, 2209–2244

[5] Wan, Bagger, Rossmeisl, Angewandte Chemie, 2021, 133 (40), 22137-22143

[6] Carvalho, …, Stoerzinger, JACS 2022, 144, 14809 14818.

[7] Andersen et al. Nature, 2019, 570, 504-508.

[8] Lazouski et al, Nature Catalysis, 2020, 3, 2520-1158.

[9] Bagger, Wan, Stephens, Rossmeisl, ACS Catalysis, 2021, 11 (11), 6596-6601

[10] Tsuneto et al. Journal of Electroanalytical Chemistry (1994)

[11] Spry, Westhead, Tort, Titirici, Stephens, Bagger, ACS Energy Letters, 2023, 8 (2), 1230-1235

[12] Wan, Wang, Tan, Filippi, Strasser, Rossmeisl, Bagger, ACS Catalysis, 2023, 13, 1926-1933

Understanding selectivity trends is a crucial hurdle in the developing innovative catalysts for generating hydrogen peroxide through the two-electron oxygen reduction reaction (2e-ORR). The adsorption free energy of O* and OOH* intermediate and the degree of O2's adsorption to the surface and have been suggested as selectivity descriptors for 2e-ORR.[1] These approaches have been the main guide for understanding and predicting selectivity for 2e-ORR catalysts over the past decade. Yet, none of them has yielded an appropriate selectivity descriptor capable of quantifying selectivity, thereby serving as a metric for describing trends in selectivity. To resolve this issue, we identify a thermodynamically derived selectivity parameter (ΔΔG) based on computational hydrogen electrode (CHE) model [2] which allows to quantify selectivity using predicted adsorption free energies of ORR intermediates (OOH* and O*) and free energy of H2O2 (Figure 1). [3] We validate the efficacy of this parameter, across a wide spectrum of reported binary alloys [4] and demonstrate that only a small number of binary alloys with a single active site that have been reported to have high activity are selective for 2e-ORR. [5] These findings highlight the potential of ΔΔG as a selectivity parameter for identifying high-performance 2e-ORR catalysts. It also demonstrates the significance of concurrently considering both selectivity and activity trends. This holistic approach is crucial for obtaining a comprehensive understanding in the identification of high-performance catalyst materials for optimal efficiency in various applications.

The two-electron oxygen reduction reaction (2e^- ORR) is an appealing alternative to produce hydrogen peroxide (H₂O₂) for isolated communities, where water treatment infrastructure is rudimentary or non-existent [1]. Notwithstanding, to efficiently carry the 2e^- ORR, stable and selective electrocatalysts are needed that circumvent the complete reduction to water (4e^- ORR). As pure noble metals and their alloys generally display the best performance, affordable and active materials are intensively sought after [2,3].

Computational models to design ORR electrocatalysts extensively rely on DFT-calculated adsorption energies of key intermediates, such as *OOH and *OH. To avoid the ill-defined energy of O₂, water is used as reference, which is suitable for the 4e- ORR. However, when applied to the 2e^- ORR, it is often overlooked the fact that H₂O₂ is seriously misdescribed by density functional theory calculations, potentially harming the conclusions of widely used routines to design improved electrocatalysts [4,5].

In this presentation, I will show that the DFT energies of O₂ and H₂O₂ entail large errors for several exchange-correlation functionals and that these errors are correlated. I will also explain how this prevents the calculation of accurate equilibrium potentials and distorts the free-energy diagram of the ideal catalyst, even when water is used as a reference. Finally, I will detail how Sabatier-type plots are affected when incorrect energies of molecules are used for the 2e^- ORR, emphasizing that experimental trends of real materials are matched when both O₂ and H₂O₂ energy are rectified.

References

[1]        S. Brueske, C. Kramer, A. Fisher, Bandwidth Study on Energy Use and Potential Energy Saving Opportunities in U.S. Chemical Manufacturing, (2015). https://www.osti.gov/biblio/1248749.

[2]        S.C. Perry, D. Pangotra, L. Vieira, L.-I. Csepei, V. Sieber, L. Wang, C. Ponce De León, F.C. Walsh, Electrochemical synthesis of hydrogen peroxide from water and oxygen, Nat Rev Chem 3 (2019) 442–458. https://doi.org/10.1038/s41570-019-0110-6.

[3]        S. Siahrostami, A. Verdaguer-Casadevall, M. Karamad, D. Deiana, P. Malacrida, B. Wickman, M. Escudero-Escribano, E.A. Paoli, R. Frydendal, T.W. Hansen, I. Chorkendorff, I.E.L. Stephens, J. Rossmeisl, Enabling direct H2O2 production through rational electrocatalyst design, Nature Mater 12 (2013) 1137–1143. https://doi.org/10.1038/nmat3795.

[4]        M.O. Almeida, M.J. Kolb, M.R.V. Lanza, F. Illas, F. Calle‐Vallejo, Gas‐Phase Errors Affect DFT‐Based Electrocatalysis Models of Oxygen Reduction to Hydrogen Peroxide, ChemElectroChem 9 (2022) e20220021 (1-7). https://doi.org/10.1002/celc.202200210.

[5]        R. Urrego-Ortiz, M. Almeida, F. Calle-Vallejo, Error awareness in the volcano plots of oxygen electroreduction to hydrogen peroxide, ChemSusChem (2024) e202400873. https://doi.org/10.1002/cssc.202400873.

Improving the efficiency of electro-catalysts for the Oxygen Evolution Reaction (OER) is key for the energy transition. RuO2 and IrO2 are considered the gold standards for OER. In recent years, it has been suggested that the OER mechanism on these oxides could involve the formation of unconventional intermediates, -O-H and -OO-H, formed by the direct interaction of -O and -OO species on a coordinatively unsaturated metal site and a proton bound to a surface oxygen [1,2,3]. These species are competitive with the classical -OH and -OOH OER intermediates. Solvation is a key ingredient to describe interfacial electrochemical processes and can affect the relative stability of these intermediates. Here we present a comparative study on the nature of key intermediates of OER on TiO2, RuO2, and IrO2 (110) surfaces, by means of Density Functional Theory (DFT) calculations in conjunction with Ab-Initio Molecular Dynamics (AIMD). We first rationalize the nature of the species and the relative stability trends in vacuum. Then, we discuss the effect of including water solvation by means of static solvation schemes. The results indicate that -OO-H is preferred in place of -OOH for all surfaces considered, and -OH is preferred over -O-H except for RuO2. Finally, we investigated the nature of the catalyst/water interfaces as well as the interaction of intermediates with liquid water based on AIMD. On RuO2 -OH and -O-H display a very different interaction with water, resulting in distinct hydrogen bond networks [4]. Interestingly, -OO-H is quite rigid on RuO2, while it has a dynamic behavior on IrO2 as the proton is shared between the -OO species and a surface oxygen atom. This study provides critical insights on the role of solvation to the nature of key intermediates of OER, a key aspect for providing a fundamental understanding of OER on catalytic surfaces.

Single atom catalysts (SACs) have emerged as a new class for the development of active and selective catalysts. These materials are commonly based on anchoring a noble transition metal to some kind of carrier. In the present lecture, we demonstrate that MXenes — two-dimensional materials with application in energy storage and conversion — spontaneously form SAC sites under anodic polarization conditions, using the applied electrode potential as a probe to transform the surface into an SAC-type structure. Combining ab initio molecular dynamics simulations and electronic structure calculations in the density functional theory framework, we demonstrate that only the SAC sites rather than the basal planes of MXenes are highly active and selective for the oxygen evolution or chlorine evolution reactions, respectively. Our findings may simplify synthetic routes toward the formation of active and selective SAC sites and could pave the way for the development of smart materials by incorporating fundamental principles from nature into materials discovery: while the pristine form of the material is inactive, the application of an electrode potential activates the material by the formation of active and selective single-atom sites.

A key strategy for decarbonising our economy is electrifying industrial syntheses using 'power-to-X' technologies.[1,2] These methods, marrying electrocatalysis with renewable energy sources such as solar or wind power, show great promise for generating platform chemicals and high-value products. For instance, the increasing demand for Nylon-6 has intensified the need for greener routes to synthesize cyclohexanone oxime, a precursor to this polymer. Current methods rely on hydroxylamine derived from environmentally demanding processes, often involving harsh conditions, acidic or alkaline environments, and hazardous reagents. To address these issues, we developed a novel one-pot electrochemical synthesis of cyclohexanone oxime using aqueous nitrate as the nitrogen source under ambient conditions.[3]

Our approach utilizes Zn-Cu alloy catalysts to drive nitrate electroreduction, forming a hydroxylamine (*NH₂OH) intermediate that subsequently reacts with cyclohexanone in the electrolyte to yield the oxime. Optimal performance was achieved with a Zn₉₃Cu₇alloy, which reached a 97% yield and a 27% Faradaic efficiency at 100 mA/cm². Mechanistic insights from in situ Raman spectroscopy and density functional theory (DFT) calculations highlighted the importance of binding strength of key reaction intermediates in controlling product selectivity within the electrochemical-chemical (EChem-Chem) tandem process. Specifically, weak surface adsorption (e.g., pure Zn) requires higher potentials for the EChem step, whereas stronger binding (e.g., pure Cu) facilitates this process at lower potentials but leads to the complete reduction of *NH₂OH to NH₃ rather than oxime formation.

This presentation will focus on our computational DFT studies, which provided detailed insights into the resting states of the electrocatalysts, the reaction mechanisms, and how surface interactions dictate catalytic activity and selectivity. Overall, this work introduces a sustainable pathway for organonitrogen synthesis via electrochemical nitrate reduction, demonstrating how tuning surface properties enables selective production. These findings are expected to inspire novel approaches for nitrate utilization and the development of other EChem-Chem processes for environmentally friendly organic synthesis.

Understanding molecular adsorption behavior on electrode surfaces is crucial for optimizing electrocatalytic processes. Since electrode reactivity is significantly influenced by varying experimental conditions, there is a pressing need for adaptable theoretical frameworks that provide in-depth insights into these phenomena. Modeling surface coverage offers a good balance between accuracy and computational cost in capturing the complexity of the solid-liquid interface [1,2].

In this oral presentation, I will introduce an automated workflow developed in Python for the high-throughput analysis of key molecular adsorbates—specifically hydrogen, oxygen, and hydroxide—that may be present during electrochemical oxidation and reduction reactions in aqueous electrolytes. This approach enables predictive assessments of the resting state of metallic electrode surfaces at different applied potentials and pH values by leveraging the Computational Hydrogen Electrode (CHE) method, which is essential prior to any reactivity study.

This framework is driven by a machine learning calculator trained on DFT data, generated using the Cluster Expansion method [3]. It can predict the surface coverages of pristine Cu, Ag, Au, Ni, and Pt metal surfaces [4]. Future work will explore mixed coverages and extend the model to different surface facets and metal alloys. The model's extrapolating capabilities, powered by a representative featurization of the systems, should allow for the screening of the electrocatalyst resting state across the entire composition range of alloying elements. If the predictions prove accurate, this automated scheme could serve as a rapid Pourbaix diagram generator for any metallic electrode surface.

Functional materials are important for applications in the fields of catalysis and renewable energy. Specific functionalities include charge transport through electronic material components as well as catalytic reactivity on material surfaces. In the talk, I will advocate that there is a relation between charge transport efficiency and reactivity, and therefore developing novel algorithms that calculate both are important for better understanding of intrinsic material limitations. We cover our latest results in developing and using charge transport calculation methods and demonstrate them on catalytic materials. Our home code is developed on a user-friendly GUI and enables to use widely available Density Functional Theory results as input. The methodology is demonstrated on several generic materials such Fe2O3 on top of graphene, a well-studied water oxidation catalyst. We have calculated the cumulative probability of a charge to reach hematite's surface using a wave propagation simulator. Graphene supported hematite has higher cumulative probability of charge transfer than bare hematite. Graphene supported hematite having carbon vacancies in graphene shows higher cumulative probability than its pristine counterpart. These indicatives for improved carrier transport and catalysis are beneficial for water splitting. 

The electrochemical reduction of CO₂ into industrially valuable chemicals like ethylene and ethanol is one of the most promising strategies to mitigate the greenhouse gas effect and tackle global energy challenges. Currently, the production of these crucial C₂ molecules is only achieved using Cu-based electrocatalysts. However, obtaining high selectivity and efficiency towards specific reduction products remains challenging due to the complex reaction pathways involved. In this regard, the adsorption of CO onto the catalyst surface represents a crucial step since it corresponds to a key intermediate for the formation of C-C bonds.

In this work, we first employ machine learning models (ML) to predict the adsorption energies of CO on Cu/M alloy surfaces, where M indicates different types of metal atom impurities. These ML models, trained on a dataset of adsorption energies calculated via Density Functional Theory (DFT) calculations, help us understand the interaction of CO with various alloy compositions and identify potential candidates for high-performance catalysts. Secondly, we investigate the kinetics and thermodynamics of CO dimerization on these alloy surfaces using DFT simulations at constant potential, by including an explicit water layer wetting the catalyst surface. Our results enable the identification of scaling relations for this critical reaction step by varying the metal species in the alloy, which in turn facilitates the rational design of more efficient and selective copper-based catalysts.

The structure–activity relationship is a cornerstone topic in catalysis, which lays the foundation for the design and functionalization of catalytic materials. Of particular interest is the catalysis of the hydrogen evolution reaction (HER) by palladium (Pd), which is envisioned to play a major role in realizing a hydrogen-based economy. Designing Pd-based catalysts with optimal activity and selectivity relies on a thorough understanding of the surface structure under reaction conditions. Herein, first-principles density functional theory calculations are employed to investigate the stability of Pd-hydride/Pd interfaces under electrochemical conditions and the effect of Pd-hydride formation on the HER activity. Based on calculated Pourbaix diagrams, we can identify the relevant regions close to the equilibrium electrode potential and pH for the HER, where the Pd surfaces start to be covered by hydrogen adatoms, and when the electrode potential is decreased, there are clear thermodynamic indications for more and more subsurface hydride layers [1-2]. The formation of subsurface hydride results in a compressive strain that lowers the magnitude of the H adsorption free energy on Pd surfaces, thereby increasing the HER activities. Our results reveal an activity trend following Pd(111) > Pd(110) > Pd(100) and that the formation of subsurface hydride layers causes morphological changes and strain, which affect the activity for proton electroreduction and HER, as well as the nature of active sites [3]. Further, computational approaches to elucidate the reconstruction processes on these low-index Pd surfaces during proton electroreduction will be discussed and insights corroborated by experimental electrochemical scanning tunneling microscopy and on-line electrochemical inductively coupled plasma mass spectrometry during cyclic voltammetry. Reconstruction phenomena, creation of defects, phase transformations, and dislocations within the Pd subsurface layers upon the hydride formation, will be discussed in detail on the basis of molecular dynamics simulations. Summarizing, significant insights into the role of hydride formation on the structure–activity relations toward the design of efficient Pd-based nanocatalysts for HER.

     Computational modeling of electrochemical systems has gained significant interest over the past decade. In this talk, we will focus upon how experimental data can be used in continuum modeling, best-practices and lessons learned, as well as emergent continuum-scale approaches that resolve pore-level phenomena and provide macroscopic design and operation recommendations.

     To provide context for some of these ideas, some of our past models that utilize experimental/computational collaborations and/or novel computational approaches will be highlighted. In particular, we will discuss our recently developed pore-scale model of an ion-exchange membrane. Ion exchange membranes (IEMs) are crucial to the efficient operation of many electrochemical devices but detailed understanding of the microscopic transport mechanisms within an IEM remain elusive. Volume-averaged continuum modeling approaches have typically been applied to the entire IEM domain and are useful for macroscopic properties, however the water domains thought to be responsible for the bulk of ionic transport have rarely been modeled explicitly. In this contribution, we build upon previous modeling efforts and assume that water domains can be modeled as cylindrical, charged pores. We develop a generalizable, two-dimensional continuum model of a water domain through an ion exchange membrane using a modified Poisson-Nernst-Planck framework. Our model incorporates solvent transport, migration, diffusion, adaptive permittivity and viscosity models, and the finite-size effects of co- and counter-ions. By using our model to simulate transport under different operating conditions, we can visualize resultant spatial profiles of concentration and potential within a nanopore.  Additionally, we quantify the relative contribution of each transport mechanism to the flux of co- and counter-ions through pores of varying properties and demonstrate the utility of this model through its adaptability to its many applications for electrosynthesis, carbon removal, and fuel cell technologies.

     Finally, as electrochemical processes are inherently multi-scale, we will provide our perspective and upcoming work on opportunities for coupling continuum modeling with other time- and length-scales for a deeper understanding of electrochemical transport phenomena.

To become more CO₂ neutral, Europe’s energy supply system and its chemical industry are getting more and more electrified. H₂ and products from CO₂ are envisioned as a suitable long-term energy storage option. They can be produced with high efficiencies by electrolysis. Yet, electrolysis processes account for only a small fraction of all chemical production processes. Over the next few decades, we expect to see a systematic and large-scale ramp-up of electrolysis for the production of various e-Fuels and chemicals.

A requirement for the establishing competitive electrolysis processes is a thorough, quantitative understanding and optimization of catalyst and electrode processes. This includes the identification of experimentally validated electrolysis models and, in particular, the model-based analysis and optimization of the electrodes. To date, there are few continuum-level models that include adequate reaction kinetics to describe electrochemical performance and product selectivity.

This presentation will show how suitable kinetic models and model parameters can be determined for a wide range of electrosynthesis processes: PEM water electrolysis [1-3], CO₂ reduction in aqueous [4], and organic electrolyte [5]. In some of these applications, electrochemical reaction kinetics play a major role, whereas others suffer from slow sorption processes carbonation reactions in the electrolyte, gas/liquid phase equilibria or slow transport processes.

Dynamic measurements, such as cyclic voltammetry and chronoamperometry/potentiometry, are well reproduced by the models, and the underlying processes that cause a characteristic dynamic response are revealed. Crucially, the models are then used to identify the performance-limiting processes among the various reaction and transport processes, and measures are proposed to improve the performance of the electrodes. The kinetic models are essential tools and building blocks for the model-based design and optimization and condition diagnosis of electrolysis cells.

Zero-gap membrane electrode-based CO/CO₂ electrolysis, powered by renewable energy sources, presents a promising avenue for achieving sustainable production of key building-block chemicals, including ethanol and ethylene. Nevertheless, achieving the capability to operate at industrially relevant current densities exceeding 200 mA/cm² and maintaining stable performance for extended periods up to 1000 hours demands substantial further development and understanding.

This presentation will focus on the application of in-operando X-ray technology to elucidate degradation mechanisms within a zero-gap membrane electrode CO/CO₂ electrolysis system. ^[1-3] In the context of CO₂ electrolysis, the transport mechanisms of cations and water inside a membrane electrode assembly during operation will be discussed. The dynamic behavior of cations and water is linked to flooding issues and salt precipitation in the gas diffusion layer (GDL), which leads to performance degradation. ^[1-2] For CO electrolysis, our findings reveal that GDL flooding, the potential presence of metal contaminants at the cathode, and the anodic oxidation of liquid products at the anode cause changes in selectivity during long-term stability tests.^[3] Some appropriate strategies are demonstrated to mitigate some of these issues.

Adsorption of ions and hydrophobic solutes are key processes in electrochemistry. The former regulates the Electric Double Layer and dictates important quantities, e.g. capacitance. The latter has major implications for heterogeneous catalysis, where small hydrophobic molecules are commonly involved as reactants and products. Despite solvation of charged and hydrophobic species are remarkably different, I will show in this presentation that the exotic way hydrophobicity arises at electrified metal-water interfaces influences both of them with a common underlying molecular mechanism.1-3 

I will first show from classical molecular dynamics1 that the peculiar molecular arrangement of electrified gold/water interfaces induces atypical fluctuations of the liquid water density, resulting in a hydrophobic water-water interface formed close to the metal. I will then illustrate how such hydrophobicity dictates solvation free energy and regulates the accumulation of hydrophobic solutes (e.g. CO)1,3, as well as some ions (e.g. Cl-)2, at the interface. I will finally discuss some implication of these findings for electrochemical reactions involving hydrophobic molecules, with examples for CO2 and N2 reduction, as well as acid-base chemistry.1,5

[1] A. Serva, M. Salanne, M. Havenith, S. Pezzotti. PNAS 2021,118, e2023867118.

[2] S. R. Alfarano, S. Pezzotti, C. Stein, et al. PNAS 2021, 118, e2108568118.

[3] A. Serva, S. Pezzotti, J. Chem. Phys. 2024, 160, 9.

[4] A. Serva, M. Havenith, S. Pezzotti. J. Chem. Phys. 2021, 155, 204706.

[5] S. Murke, W. Chen, S. Pezzotti, M. Havenith. J. Am. Chem. Soc 2024, 146, 12423.

Ammonia (NH₃) is a crucial feedstock used across many sectors, ranging from the production of fertilizers to fine chemicals, and it is also a promising hydrogen carrier for decarbonizing hard-to-abate sectors. An alternative to its main production route, the Haber-Bosch process, is the electrochemical reduction of nitrate (NO₃⁻), which is a pervasive water pollutant. Cu-based electrodes have demonstrated excellent activity and selectivity towards ammonia, preventing undesired pathways such as dinitrogen (N₂) production. However, the cathodic potentials applied during operation induce significant reconstructions on the electrode, which are exacerbated by nitrate's strong oxidizing nature [1,2], thus adding substantial challenges to their accurate modelling and understanding. In this presentation, I will discuss our advances in elucidating the reaction mechanism of nitrate reduction to ammonia on Cu-based catalysts. To this end, we first mapped the complete reaction network, including key adsorbed intermediates such as nitrite (NO₂*), nitrogen (N*), and hydroxylamine (NH₂OH*). Then, by using ab-initio methods based on Density Functional Theory, we obtained the full energy profile including all intermediates and relevant transition states. These elements were wrapped up into a transient-state microkinetic model to identify the dominant reaction pathways and assess the influence of both the reaction environment (including solvent, pH, and electric potential) and the catalyst's history. This study paves the way for a comprehensive understanding of complex reaction networks and their interactions with the reaction environment.

Molecular dynamics can be a powerful tool to study the properties of electrochemical interfaces.

In this talk, I will take you on a walk through the atomic scale world that unfolds at the surface of metallic electrodes. We will start by visiting a forest of adsorbates, including H and OH. In this context, I will discuss why even adsorbed hydrogen on platinum surfaces, which is probably one of the most studied electrochemical interfaces, still bares its secrets. We will then go further and view the influence of adsorbed OH on the operando electrochemical interface. Here, I will ask the question why the presence of OH can (not) change everything. Finally, we will look further up from the surface and scrutinize the behavior of ions at the interface. Can molecular dynamics help us unravelling at which height the ions reside?  What happens to the solvation structure of the ions as they approach the interface? And how closely can ions pack at charged metal surfaces?

All this information can be used to better understand electrochemical processes at the interface.

Under electrochemical conditions the catalyst might change composition, phase and deactivate. Although all these processes are crucial to the understanding of the materials and processes and the success of the technologies little is known of the detailed processes taking place. In the presentation I will revise a few cases of rearrangement of materials under reaction conditions and electrochemical environment. These will include the activation of materials for the oxygen evolution reaction (OER) and the rearragement copper based catalysts in CO2 reduction (eCO2RR). The changes in the properties and the pretreatment of the materials can control the selectivity thus calling for more advanced simulations that can take into account these structural modifications. This opens the way to a new, more complex view on the catalytic activity under true reaction conditions and also provides information on how to avoid deactivation paths that might lead to catastrophic failure in the actual devices.

Oxide-water interfaces host many chemical reactions in nature and industry. There, reaction free energies markedly differ from bulk. While we can experimentally and theoretically measure these changes, we are often unable to address the fundamental question: what catalyses these reactions? Recent studies suggest that surface and electrostatics contributions are insufficient to answer. The interface modulates chemistry in subtle ways. Revealing them is essential to understanding interfacial reactions, hence improving industrial processes. Here, we introduce a thermodynamic approach combined with cavitation free energy analysis to disentangle the driving forces at play. We find water dictates chemistry via large variations of cavitation free energies across the interface. The resulting driving forces are both large enough to determine reaction output and highly tunable by adjusting interface composition, as showcased for silica-water interfaces. These findings shift the focus from common interpretations based on surface and electrostatics, and open exciting perspectives for regulating interfacial chemistry.

Electrochemical CO₂ reduction (eCO₂R) is a promising technology to store renewable energy into chemical bonds and close the carbon cycle. However, to date its industrial exploitation is limited to the production of CO and HCOO^–, whilst CO₂ conversion to C₂₊ chemicals is possible only on the laboratory scale.¹ Low C₂₊ activity and selectivity are due as well to poor understanding of the CO₂ reduction pathways beyond C₁ products, and consequently of the active sites responsible for multi-carbon products, mainly ethanol, ethylene, and n-propanol.² Spectroscopy study can significantly enhance this understanding, by allowing the detection of reaction intermediates on the surface, and density functional theory (DFT) can support the assignment of these spectroscopic signals.³

Here, by means of DFT simulations, we unveil reaction mechanisms and active sites to produce ethylene and ethanol during the electrochemical reduction of CO₂ by supporting the assignment of Raman spectroscopic signals.^4,5 In the first study,⁴ we assessed CO coverage effects on Cu(100). At high CO surface coverage (> 0.5 ML), average CO binding energy decreases due to CO-CO repulsive interactions and the abundance of weakly adsorbed CO on atop sites. Besides, vibrational analysis shows that the Cu-CO stretching peak increases to the detriment of the C=O rotation peak at high CO coverages, in line with analogous experimental observations within the ethylene selectivity window. By employing the Hammer’s decomposition scheme, we confirmed that surface atop CO intermediates are the most reactive species to form the CO-CO dimer, having a very endothermic rebond energy (i.e. proxy of dimer dissociation). In the second work,⁵ we focused on the selectivity switch between ethanol and ethylene occurring during eCO₂R at around –0.8 V vs RHE on oxide-derive copper nanocubes. At this applied potential, four vibrational fingerprints are detected via surface-enhanced Raman spectroscopy (SERS), namely 1182, 1318, 1453, and 1595 cm^–1. Density functional theory simulations and further reduction studies of selective precursors attribute these vibrations to the *OCHCH₂ intermediate, the first selective precursor to ethanol and n-propanol. The formation of this intermediate is favored by distorted Cu active sites with low s-band states, which stabilize the terminal oxygen.

Based on the insights here proposed, we call for an updated reaction mechanism for CO₂ reduction to ethylene and ethanol. Both these products require high CO coverage and undercoordinated sites, which facilitates the CO-CO dimerization step, while distorted Cu atoms stabilize *OCHCH₂, opening the selective route to ethanol and n-propanol. These guidelines support the rational design of electrocatalysts to maximize ethanol and ethylene selectivity independently.

The conversion of CO₂ via electrochemical processes is a promising technology to close the carbon cycle, especially when combined with renewable energy sources. Given their high market value and energy density, significant efforts are currently dedicated to designing copper (Cu)-based catalysts for converting CO₂ into multicarbon molecules. By integrating concepts from molecular catalysts, the engineering of Cu-based catalysts aims to finely tailor the behavior of active sites on metallic surfaces, which remains a long-standing interest for the controlled design of novel electrocatalytic materials. In this context, we have recently explored strategies to enhance the conversion of CO₂ into hydrocarbon molecules with two or more carbon atoms (C₂₊) via molecular doping or metal alloying.

Despite impressive progress achieved through the development of flow cells with improved gas/liquid/solid interfaces, the realistic development of CO₂ electrolyzers is still hindered by several fundamental challenges. These include understanding the local microenvironment, reducing the large operating voltage, and improving CO₂ utilization efficiency.

In my presentation, I will review our recent progress in understanding and controlling the interface at various levels within CO₂ electrolyzers, from the molecular scale to the complete electrolysis system.

Electrochemical CO₂ conversion can result in a variety of products, often as a mixture, and controlling the product selectivity remains a key challenge. It has been shown that pulsing the electrochemical potential can lead to altered product distributions, influenced by effects on, e.g., transport, double-layer rearrangement, adsorption/desorption, and changes to electrode structure and composition [1].

Herein we report our observations using metal electrodes normally selective for the 2-electron formation of CO as major product under steady-state (potentiostatic) conditions, finding that they can produce significant amounts of higher-order products (including methane and ethylene) under the application of pulse potential waveforms. We confirm this is not due to metal impurities in the system, but is significantly affected by phenomena such as surface restructuring and accumulation of liquid products. Furthermore, time-resolved differential electrochemical mass spectrometry (DEMS) measurements reveal distinctly different transient behaviors between the different gaseous products, providing key new mechanistic insight for clarifying the roles of pulsing.

Electrolysis is usually performed under constant potential or current, but pulsed electrolysis has received growing attention over the last years, especially in the context of CO₂ reduction where favourable impacts on product selectivity are observed. However, the mechanism at play remains under debate.^1,2 Among the most often reported contributions are the modified mass-transport of reactants at the electrode, the oxidation of metal catalyst during anodic steps or the morphological reconstruction of the surface under dynamic conditions. Much less explored are the (micro-)kinetics events during transient operation. Herein, we address some specific cases following classical theory of electron transfer and chemical reaction rate. Upon conditions where mass-transport effect, metal oxidation and surface reconstruction are not interfering, we predict large selectivity improvements (exceeding 5000% for some cases), resulting only from (micro-)kinetics effects. For the specific reaction framework investigated, we provide rational explanations for the selectivity switch, derive a method for pulsed program optimization and show that the high selectivity improvements are compatible with high current density operation, relevant for commercially-viable CO₂ reduction.

Correlating activity, selectivity and stability with the structure and composition of catalysts is crucial to advance the knowledge in chemical transformations which are essential to move towards a more sustainable economy. Among these, the electrochemical CO2 reduction reaction(CO2RR) and, more recently, CO reduction reaction (CORR) hold the promise to close the carbon cycle by storing renewable energies into chemical feedstocks. Although notable progress has been made in understanding the parameters which govern activity and selectivity in CO2RR, CORR is still in its infancy. Catalyst stability remains a less explored property for both reactions.

In this talk, I will show how well-defined copper nanocrystals (NCs) synthesized via colloidal synthesis can be used as model system to establish unambiguous structure/property relations in CO2RR and CORR.

First of all, I will illustrate how tailor made copper NCs have revealed synergy between shape and size, thus the importance of facet ratio in CO2RR.[1] Secondly, I will show that these relationships hold when these catalysts are integrated in a gas-fed electrolyzers at technologically relevant conditions with currents up to 300 mA/cm2.[2]

I will then present our recent advancement in understanding structural dependence relationship in CORR. Specifically, I will discuss about the importance of lattice strain for the production of alcohols in this reaction as revealed by a combination of theoretical simulation and advanced characterization techniques. I will close with a discussion of stability of these catalysts and compare CO2RR/CORR conditions. [3]

Carbon capture, utilization, and storage (CCUS) strategies are gaining attention as effective methods to achieve carbon dioxide neutrality while creating value-added products through CO₂ conversion. Among these strategies, electrochemical CO₂ reduction stands out due to its low temperature and pressure requirements and its ability to store energy from renewable sources like solar and wind in the form of valuable chemicals such as formic acid or formate [1].

The “Development of Chemical Processes and Pollution Control” (DePRO) research group at the University of Cantabria, Spain, has been actively working on the continuous electrochemical reduction of CO₂ to formate. Various electrocatalysts for both the cathode and anode, along with different electrode configurations, have been investigated. This communication focuses on the recent advancements and challenges in the lab’s research on continuous CO₂ electrocatalytic reduction to formate. The experiments were conducted using a consistent setup and operating conditions, with variations in cathodic electrocatalysts, including Pb-, Sn-, and Bi-based materials, and different cathode configurations like plate electrodes, particulate electrodes (PE), Gas Diffusion Electrodes (GDE), Catalyst Coated Membrane Electrodes (CCME), and Membrane Electrode Assembly (MEA). We have also explored various anodes, such as DSA/O₂ and Ni-based electrodes, and different ion exchange membranes, including Nafion cationic exchange membranes (CEM) and Sustainion anionic exchange membranes (AEM), while performing the Oxygen Evolution Reaction in the anodic compartment [2].

Notable achievements were made using a Bi MEA configuration with a CO₂-humidified input stream [3]. These results include formate concentrations of up to 337 g·L^-1, Faradaic Efficiencies of 89 %, and energy consumption values as low as 180 kWh·kmol^-1, representing one of the best trade-offs reported in the literature, marking noteworthy progress in this field.

Further research is essential to scale up this process industrially by developing more stable electrocatalysts for both the cathode and anode. Recent work by the research group has focused on optimizing the manufacturing of GDEs for CO₂ electroreduction to formate using the automatic spray pyrolysis technique [4]. This approach has yielded valuable insights into creating more efficient electrodes for CO₂ reduction.

Additionally, replacing the Oxygen Evolution Reaction at the anode with more valuable oxidation reactions is crucial. The research group has explored the coupling of the Glycerol Oxidation Reaction with CO₂ electroreduction to formate, resulting in valuable products in both compartments of the electrochemical reactor. Recent results include high formate concentrations of up to 359 g·L⁻¹ with Faradaic efficiencies up to 95% at the cathode, along with dihydroxyacetone production at a rate of 0.434 mmol·m⁻²·s⁻¹ [5]. This represents a significant advancement in the development and application of this technology.

The most challenging deal we face these years is the need to lower greenhouse gas (GHG) emissions and tackle climate change; though calls to reduce it are growing louder yearly, emissions remain high. CO₂ is the key contributor. For this reason, synthesising high-added-value products from CO₂ conversion is a promising approach to mitigate the problem. [1] Among the different alternatives, exploiting CO₂ via electrochemical reduction under mild conditions (ambient pressure and temperature) represents an opportunity to support a low-carbon economy.[2] The electrocatalytic (EC) CO₂ reduction (CO₂R) driven by renewable energy can be exploited for the future energy transition, for the carbon storage into valuable products like syngas (H₂/CO mixtures), organic acids (formic acid) and chemicals/fuels (C₁₊ alcohols). A big challenge for the industrialisation of this technology is to find low-cost electrocatalysts, efficient reactors and process conditions. In the efforts to develop efficient, selective and stable materials, we have exploited the current knowledge of thermocatalytic CO₂ hydrogenation to develop noble-metal-free CO₂R electrocatalysts. For instance, Cu/Zn/Al synthesised catalyst producing methanol and CO from the CO₂ thermocatalytic (TC) hydrogenation (at H₂ pressure (P) of 30 bar and temperature (T) > 200 ^oC) promotes the formation of methanol (⁓32% of FE) during the EC CO₂R in a gas-diffusion-electrode system; while operating in the liquid phase, the same catalyst produces syngas with a tunable composition (95% of FE at the most positive applied potential) and other liquid C₂₊ products (in both cases at ambient T, P).[3]  Conversely, Cu/ZnO electrocatalyst has also been tested at industrially relevant current densities in liquid phase configuration.[4]  We demonstrated through ex-situ characterisations that the presence of ZnO nanoparticles in the mixed Cu/ZnO catalyst plays an important role in forming and stabilising mixed oxidation states of copper and Cu¹⁺/Cu⁰ interfaces in the electrocatalyst (in bulk and surface). These interfaces seem to promote CO dimerisation to ethanol. Indeed, ethanol was produced with the Cu/ZnO catalyst, reaching ethanol productivity of about 5.3 mmol∙g_cat^-1∙h^-1 in a liquid-phase configuration at ambient conditions. The Cu/Zn/Al and Cu/ZnO electrocatalysts have also been tested in a catholyte-free configuration with an increased selectivity to ethylene, reaching approx. 60% and 70% of FE, respectively. Here, Cu catalyst structure transformed, on average, completely to metallic with a very thin layer of Cu¹⁺ during testing, which seems to promote the selectivity towards C₂H₄, demonstrating that the reaction pathways for EC CO₂R are determined mainly by transport limitations rather than only by the intrinsic properties of the electrocatalysts. Our results open a promising path for the prospective implementation of metal-oxide nanostructures for CO₂ conversion to the chemicals and fuels of the future.

Previous electrochemical CO₂ reduction studies have shown alkaline electrolytes favor the production of acetaldehyde over acetaldehyde and ethanol, and proposed hypothesis of their generation pathways accordingly [1], [2], [3]. However, our work shows acetate can also come from the fast non-faradaic chemical oxidation of acetaldehyde in alkaline solutions [4]. This could lead to an overestimated acetate productivity and correspondingly an underestimated acetaldehyde productivity, and thus mislead following investigations on the reaction mechanisms of CO₂ reduction reaction conducted in alkaline environment.

In the presented work, we will first systematically demonstrate how and why misleading acetaldehyde and acetate production could be caused, and propose suggestions on accurately measuring their productivities in alkaline electrolytes. In addition, we will present real-time detection of gas (hydrogen, methane and ethylene) and volatile liquid (acetaldehyde) being produced during electrochemical CO reduction on polycrystalline and various single crystal ((100), (110), (111), and (211)) Cu electrodes at low overpotentials (< 0.65 V ). Results reveal that acetaldehyde production as well as the production rate of acetaldehyde to ethylene are both potential- and facet-dependent. The quantified acetaldehyde-to-ethylene production ratio will provide insightful information for understanding the bifurcation point of acetaldehyde/ethylene production from electrochemical CO₂ reduction in a mechanistic perspective.

Highly porous ZIF-8 and ZIF-67 synthesized by an environmentally friendly steam-assisted dry-gel technique are investigated as potential catalysts for electrochemical reductive reactions. The synthesis conditions play a crucial role in the growth of the metal-organic frameworks (MOFs). Tuning water content, in particular, significantly influences the morphological and structural properties of ZIF-8. These properties show significant effects in CO₂ electroreduction, producing syngas with different CO: H₂ ratios. The CO selectivity changed by variation in the volume of water during the synthesis and significantly affects the crystal size and morphology of the ZIF-8 particles. An optimum CO selectivity of 50% is obtained at -1.2 V_RHE by increasing the water content to 20mL during the dry-gel synthesis. The tunability of CO selectivity influenced by the synthesis conditions and the reductive potential used during the CO₂ electroreduction process is advantageous for producing syngas in different CO: H₂ ratios. In addition, despite exhibiting similar textural and structural properties, ZIF-8 and ZIF-67 show distinctive CO₂ electroreductive performance, which is assigned to the vital role of their respective metal center.

Carbon monoxide is an important raw material for the synthesis of different bulk chemicals (e.g., phosgene and different products thereof). Considering that it can be a building block for the synthesis of virtually any organic compound, carbon monoxide might play an important role in the production of synthetic fuels and other typical petrochemical products. This can be achieved at high temperatures and pressure in the Fischer-Tropsch process, rendering this a widely investigated topic. As an alternative, electrochemical methods offer a way of forming different chemicals from carbon monoxide under considerably milder conditions.

In this study, we aimed to explore the effect of different experimental parameters on the rate and selectivity of the electrochemical reduction of CO (CORR), using cell components and catalysts available from commercial sources. Studying the reaction in different electrolyzers, we highlight the dual effect of the electrolyte solution that separates the membrane and the catalyst layer. On the one hand, it limits the product crossover to the anode, but on the other hand, gives ground to electrode flooding, hence also limiting the maximum achievable current density. The zero-gap electrolyzer structure is therefore most beneficial in terms of both achievable reaction rate and energy efficiency, but product accumulation in the anolyte is a hurdle to get over, as will be further discussed in my talk.

The phase boundary between the catalyst particles, the liquid phase, and the gas reactant can be engineered by using functional catalyst layer additives. In the second part of my talk the effect of the used gas diffusion layer, the catalyst layer composition, and the catalyst additive will be discussed. Various, systematically chosen polymeric materials were tested as catalyst binders to reveal any possible contribution of certain molecular motifs (e.g., functional groups, fluorinated backbone, etc.), present in the polymer additives. The electrochemical results are contrasted with the detailed physico-chemical characterization of the catalyst layers. Building on these optimization studies, experiments were also performed in a scaled-up electrolyzer stack (3×100 cm² geometric area) to evaluate the possible industrial applicability of this process.

Global warming has received a lot of attention from all over the world. The development of earth-abundant catalysts for highly selective electrochemical CO₂ reduction reaction (CO₂ RR) is a  promising way to mitigate the increasing amount of CO₂ in the atmosphere. Here, we have obtained non-noble metals consisting of Cu and Sn for the highly selective reduction of CO2 through a facile method. The morphology and particle size of the as-prepared CuSn catalyst were very different from the Cu catalyst, which was transformed into nanometers after Sn doping. Compared to the faradic efficiency of Cu₂O, the total selectivity of the nanocatalyst towards CO₂ RR was improved by 25% at a current density of −200 mA cm⁻² in 1 M KOH electrolyte, and its selectivity was shifted towards formate. The high performance is attributed to the synergistic catalytic effect between Cu and Sn, which is still under investigation. This approach could also be used to design and develop high performance electrocatalysts for the selective conversion of CO₂ to other products.

Theoretical atomic scale calculations of the electrochemical reduction of CO₂ and the competing hydrogen evolution reaction are presented. The calculations include evaluation of the activation energy of the various elementary steps as a function of applied voltage based on efficient methods for finding saddle points on the energy surface that represent transition states for the reactions. The energy and atomic forces are calculated using density functional theory (DFT). Copper is found to be special among the transition metals in that the activation energy for CO₂ reduction becomes lower than that of hydrogen evolution reaction (HER) within a certain window of applied voltage [1]. The fact that the onset potential of formate and CO formation is similar can be explained by the fact that the energy barrier for these two competing processes turns out to be similar [2]. The most likely step for reduction of CO, which also turns out to be the rate limiting step for methane formation, involves a Heyrovsky mechanism to form COH, rather than formation of CHO. The rate of C-C bond formation is strongly dependent on the surface structure, Cu(100) being the most active facet, and it can be affected by H-adatom coverage. The optimal mechanism for C-C bond formation is found to involve a nearly simultaneous electron-proton transfer to form *OCCOH. Calculations of CO adsorption on doped copper surfaces reveal multiple CO molecules adsorbed on a single surface impurity [3]. The calculations have mostly been carried out by explicitly including a few (4 or 5) water molecules around the reacting surface species while the rest of the electrolyte is described with an implicit solvent approach. Proper inclusion of a liquid electrolyte at the surface of the electrode is a challenge as it makes the DFT calculations too heavy. Ongoing methodology development based on a hybrid simulation approach where the liquid electrolyte is fully represented will be introduced [4].

Multi-physical transport processes on multiple scales occur in electrochemical devices and components for CO₂ electroreduction. These complex coupled transport processes determine the local environment in the catalyst layer and subsequently also the reaction rates at the catalytic sites. Experiments have difficulties to provide locally resolved information within a working cell, but can provide important insight into catalytic mechanisms or provide macroscopic performance characteristics (current-voltage behaviour, selectivity’s, etc.). The multi-physics and multi-scale models can provide locally resolved insights, starting from the double layer [1,2], the pore-scale [2,3], all the way to the volume-averaged continuum-scale [4], but typically rely on experimental input for model parameters or validation. Operational conditions (e.g. steady state vs. transient) can further provide interesting insights into limiting phenomena. I will discuss how combined experimental and computational approaches can help provide relevant insights into (photo)electrochemical CO₂ reduction to improve activity and selectivity utilizing multi-scale and transient models that are fed by dedicated experimental data.

Copper-based electrocatalysts are showing great promise for converting CO2 into valuable products through electrochemical processes. However, achieving high selectivity for higher carbon number (C2+) products is still a major hurdle for their commercial use. In our study, we have developed a range of electrocatalysts using octadecylamine (ODA) coated Cu2O nanoparticles. High-resolution transmission electron microscopy (HRTEM) has shown that these coatings vary in thickness from 1.2 to 4 nanometers. Density functional theory (DFT) calculations indicate that with low coverage, ODA molecules tend to spread out on the surface of Cu2O, exposing hydrophilic areas. In contrast, at higher coverage, the ODA molecules pack closely together, which can hinder mass and charge transfer. This variation in the arrangement of ODA molecules on the nanoparticles significantly impacts the selectivity of the products.

Further insights were gained through in situ Raman spectroscopy, which demonstrated that the ideal ODA thickness helps stabilize important intermediates in the production of C2+ products, particularly ethanol. Additional tests using electrochemical impedance spectroscopy and pulse voltammetry have shown that thicker ODA coatings increase resistance to charge transfer, whereas thinner ODA layers facilitate quicker desorption of intermediates. The optimal thickness of the ODA layer results in the slowest rate of intermediate desorption, correlating with the highest observed concentration of these intermediates via in situ Raman spectroscopy. Consequently, this leads to a Faradaic efficiency exceeding 73% for ethanol and ethylene production. This study highlights the critical role of molecular coating thickness in tuning the performance of Cu-based electrocatalysts for efficient and selective CO2 conversion.

It is experimentally known that electrochemical CO₂ reduction (eCO₂R) does not take place on Cu, Ag and Au without a cation near the electrode surface, and that both organic and inorganic cations modulate eCO₂R activity and selectivity. The rational optimization of microenvironments in eCO₂R requires understanding of the intricate chemical and transport phenomena taking place across length scales.

We modeled CO₂ reduction to CO on Ag via a multiscale approach, accounting for the role of the cation at all scales. The transport is modeled by generalized modified Poisson-Nernst-Planck equations and a microkinetic model is used to calculate the current densities of CO₂ reduction to CO and the competing reduction of water (H₂OR) and H⁺ to H₂ based on the local conditions. Kinetic rate constants are obtained from atomic-scale calculations (DFT and AIMD). In particular, the number of active sites (the microenvironments) depends on the local cation concentration.

We considered different buffers, including alkali (Li⁺, K⁺ and Cs⁺), alkali-earth (Mg²⁺ and Ba²⁺) and organic (tetramethylammonium - TMA⁺) cations, and we evaluated different concentrations (from 1 mM to 2 M) over a large potential window (-0.4 to -1.6 V vs RHE). We observed consistent behaviors across cations of different nature: at slightly negative applied potentials, higher cation concentration leads to improved activity, despite the lower CO₂ solubility in higher ionic strength buffers. At strongly negative applied potentials, cation accumulation due to double layer charging leads to transport limitations and the CO current density is lower at higher cation concentrations. Cs⁺ leads to higher CO current density and high CO selectivity for a combination of favorable chemical and transport properties. We applied the same framework to a TMA⁺ - based anion exchange membrane (AEM) in the vicinity of an Ag electrode and evaluated the effect of a fixed background charge at different hydration levels on eCO₂R activity and selectivity. We showed that, when the AEM is in contact with an electrolyte, the inorganic cation is still present at the AEM-electrolyte interface and contributes to the CO₂R and H₂OR microenvironments.

The model consistently reproduces experimental trends and helps to elucidate optimal local condition for eCO₂R on Ag in aqueous electrolytes and ionomers. Our work paves the way for a coupled multiscale electrolyte and ionomer design for electrochemical interfaces.

Local CO₂ availability at the catalyst is an important limitation for low temperature CO₂ electroreduction in aqueous media. Integrated CO₂ capture and electrochemical utilization is energetically and economically advantageous, as it combines sorptive and electrolytic functions [1]. Previously chemicals used as sorbent and organic electrolyte include monoethanolamine and ionic liquids [2]. Improved uptake capacity, kinetics, and selectivity as well as good chemical and thermal stability are essential criteria in the sorbent electrolyte selection process [3]. A common employed strategy to overcome the mass transport-limited CO₂ supply in liquid electrolyte systems is to provide CO₂ in its gaseous form to the electrode, leading to record performances in such systems [4].

Integration of sorbents in gas-fed electrolyzers enables to locally increase CO₂ availability and thereby boost the maximum reduction rate [5]. Typical Membrane Electrode Assembly (MEA) designs are composed of multiple layers (e.g., hydrophobic porous transport layer, carbon nanoparticles or fibers, ionomer, catalyst nanoparticles, …). Studying the effect of the sorbent and its integration in the assembly is impeded by the stack complexity and therefore hard to decouple from other phenomena. In this work, a planar interdigitated electrode assembly was developed to test sorbent electrolytes for CO­₂ reduction in gas-fed environments under well-defined conditions. Ionic liquid-silica nanocomposites were successfully tested as sorbent electrolyte coatings for CO₂ reduction in such a gas-fed model system, providing a proof-of-concept for the enhanced CO₂ electroreduction by increasing reactant availability . Next to the experimental approach, kinetic parameters for the CO₂ uptake, transport and electroreduction were modeled in a multiphysics model and compared to experiments. The extracted kinetic parameters obtained from the interdigitated electrode model system were implemented in a model of a MEA configuration to determine optimum sorbent placement.