The toxicity and bioavailability of lead (Pb) in halide perovskites motivates the search of Pb-free perovskite-inspired materials (PIMs), which would replicate the great optoelectronic properties of the traditional Pb-based counterparts. PIMs employing pnictogen cations from Group VA of the periodic table (e.g., antimony (III) (Sb³⁺) and bismuth (III) (Bi³⁺)) have recently emerged as low-toxicity alternatives for Pb-based halide perovskites, with applications in photovoltaics and beyond [1]. In particular, Bi-based PIMs are interesting because can lead to shallow traps and high defect tolerance in some compositions, while being eco-friendly. This field of research is, however, still in its early infancy with a growing number of efforts to improve the device efficiencies and lifetimes, with the majority of the studies focusing on the shelf-life rather than the operational stability. 

In this talk, I will summarize key recent examples of bismuth-based PIMs studied by us for a range of applications, from photovoltaics for outdoor and indoor, non-linear optics, photocatalysis. In particular, I will summarize our recent findings on CU₂AgBiI₆ PIM and the compositional engineering efforts to improve its morphology and charge carrier transport [2,3]. I will also highlight novel Bi-based compositions for solar cells and indoor photovoltaics.

Solar cells are among the most developed technologies in the renewable energy sector, not only academically, but also commercially. In this context, thin-film photovoltaics (based on emerging absorbers like perovskites, Sb₂Se₃, CZTS etc.) provide an excellent opportunity to expand the reach of solar energy production, thanks to the promising efficiencies combined with light-weight and flexibility. However, for many of these emerging materials, moderate/high temperature annealing (150-300 °C) is required, and the necessary temperature often limits the usable substrates. Photonic curing (also known as flash annealing) could represent a way to overcome this limitation and really unlock the potential of thin film photovoltaics. In this technique, heat is created by the absorption of strong and ultrarapid light flashes that permits to reach very high temperature on the top layer while keeping cold the bottom of the substrate. While this technique has widely been used in silicon,[1] very few reports are present for these emerging materials (mainly on perovskites), [2-4], and fundamental investigation is still missing.

In this talk I will present the most recent results collected on the use of photonic curing on different thin film absorbers, analyzing the structural, morphological and optoelectronic properties and comparing it with traditional thermal annealing.

Perovskite solar cells in which methylammonium lead iodide (MAPbI₃) is used as a solar absorber material, have reached maturity in the last years owing to a concerted effort to optimize material synthesis, stability, and device architectures and performance. However, the halide perovskite family features thousands of stable members beyond MAPbI₃, with broad compositional tunabilty and a wide range of optoelectronic properties. With this talk, I will provide an overview of our current understanding of the electronic and excited-state structure of several classes of perovskites and perovskite-like structures beyond ABX₃ featuring double perovskites, extended perovskites, and chalcohalide materials. Our first-principles calculations based on the GW and Bethe-Salpeter Equation approaches, allow us to map the complex landscape of electronic properties and excitons, understand the impact of chemical and structural heterogeneity, dimensionality, and temperature effects, and provide chemically intuitive rules for when to trust canonical textbook models for excitons in these materials.

Metal halide semiconductors have emerged as attractive materials for solar cells with power-conversion efficiencies now exceeding 26%, however, these record efficiencies have all relied on incorporation of lead as the metal. The search for less toxic ingredients has led to the emergence of a plethora of new bismuth-based semiconductors, including bismuth halides and chalcogenides. Power conversion efficiencies around 6% have been realised for such materials, triggering new research efforts to explore and eliminate current limitations to performance.

Here, we show that an ultrafast charge-carrier self-trapping process limits long-range charge-carrier transport in most bismuth-based semiconductors.[1-7] We have examined the evolution of photoexcited charge carriers in the double perovskite Cs₂AgBiBr₆ using a combination of temperature-dependent photoluminescence, absorption and optical pump−terahertz probe spectroscopy.[1] We observe rapid decays in terahertz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of 1.0 ps to an intrinsic self-trapped small polaronic state. Alloying Cs₂AgBiBr₆ with Cs₂AgSbBr₆ on the trivalent metal site interestingly leads to significantly stronger self-localisation,[2] which we attribute to self-localised charge carriers probing the energetic landscape more locally thus turning an alloy’s low-energy sites (here, Sb sites) into traps, which dramatically deteriorates transport properties. We further demonstrate the novel lead-free semiconductor Cu₂AgBiI₆ which exhibits a low exciton binding energy of ~29 meV and a lower and direct band gap near 2.1 eV,[3,4] making it a significantly more attractive lead-free material for photovoltaic applications. However, charge carriers in Cu₂AgBiI₆ are found to exhibit similarly strong charge-lattice interactions[4,5]. Further work examining five compositions along the AgBiI₄–CuI solid solution line (stoichiometry Cu_4x(AgBi)_1−xI₄) shows that increased Cu+ content enhances the band curvature around the valence band maximum, resulting in lower charge-carrier effective masses, reduced exciton binding energies, and higher mobilities, as well as partly mitigating the extent of such ultrafast self-localisation.[5] Interestingly, we show that thin films of BiOI lack of such self-trapping, with good charge-carrier mobility maintained over longer time scales, reaching ∼3 cm² V^–1 s^–1 at 295 K and increasing gradually to ∼13 cm² V^–1 s^–1 at 5 K, indicative of prevailing bandlike transport.[6] Finally, we examine thin films of AgBiS₂ nanocrystals as a function of Ag and Bi cation-ordering,[7] which is modified via thermal-annealing. We show that homogeneous cation disorder reduces charge-carrier localization, most likely because cation-disorder engineering flattens the disordered electronic landscape, removing tail states that would otherwise exacerbate Anderson localization of small polaronic states.[7]

Overall, self-trapping of charge carriers therefore emerges as a clear challenge for this class of materials. Our findings explore the parameter space governing such self-localization, highlighting the effects of local energetic disorder as an exacerbating factor that may pose new challenges to alloying strategies. In addition, or findings show that cation-disorder engineering may partly mitigate such effects through flattening of the local energy landscape.

Following the emergence of lead halide perovskites (LHPs) as materials for efficient solar cells, research has progressed to explore stable, abundant and non-toxic alternatives. However, the performance of such lead-free perovskite-inspired materials (PIMs) still lags significantly behind that of their LHP counterparts. For bismuth (Bi)-based PIMs, one significant reason is a frequently observed ultrafast charge-carrier localization (or self-trapping). It has been suggested that self-trapping in Cs₂AgBiBr₆ originates from strong lattice deformation potential imparted by bismuth along with the presence of soft silver-halide bonds. Given that self-trapping places a fundamental limit on the performance of materials, investigating the influence of chemical composition on the charge-carrier dynamics is highly topical.

This work aims to understand whether the presence of Bi or Ag-halide bonds is in fact crucial to the emergence of self-trapped states. This was done through a dual study of charge-carrier dynamics in BiOI and (AgI)_x(BiI₃)_y thin films. Our investigation reveals that despite possessing a low and strong electron-phonon coupling, there is no ultrafast localisation in BiOI. Instead, by unravelling the early and long-time charge-carrier dynamics in BiOI, we find that the material performance is limited by the presence of multi-phonon emission mediated non-radiative channels in the material.

On the other hand, ultrafast localisation is persistent across both mixed Ag-Bi iodides and BiI₃. Thus, the presence of Bi and/or Ag-halide bonds alone cannot account for self-trapping in these materials. We find that a delicate interplay between chemical composition and crystal and band structures determine the charge-carrier dynamics in (AgI)_x(BiI₃)_y. Overall, our dual study addresses crucial gaps in understanding the limitations of Bi-based PIMs and educate future material design.  

Halide double perovskites, such as Cs₂AgBiX₆ (X = Br, Cl), are emerging semiconductors for optoelectronic applications due to their lower toxicity.^[1] Although exhibiting an indirect bandgap, colloidal Cs₂AgBiCl₆ nanocrystals (NCs) show bright dual-peak photoluminescence spectra.^[2] However, the origin of both emission bands is still under debate. Here, we show that silver (Ag) plays a crucial role in both emissions of these double perovskite NCs. The trapping of holes in Ag vacancies leads to a spatial localization of the hole wave function on the scale of the lattice constant. This provides k-values for the hole wave function at all boundaries of the Brillouin zone, resulting in localized bound excitons with a red emission at 650 nm with an activated temperature dependence. The blue emission (425 nm) stems from the lecithin ligands of the NCs spectrally overlapping with the plasmon resonance of the surface-attached Ag nanoclusters. This work is of importance for the surface chemistry of Cs₂AgBiCl₆ double perovskites and their optoelectronic applications.

Metal halide perovskites emerged as outstanding materials for optoelectronics due to their excellent optoelectronic properties, such as direct and tunable band gaps, large absorption cross sections and long lifetimes and diffusion paths of the charge carriers.[1] So far, the field has been dominated by lead-based perovskites, but issues concerning lead toxicity and stability have led to explore new possible perovskite candidates. In this context tin and double perovskites of different dimensionalities have attracted increasing interest.[2,3] The optoelectronic features, as well as the efficiencies of these materials in optoelectronic devices, is largely influenced by the nature of the chemical bond, the defects activity and the coupling of photogenerated charges with the lattice. 

In this presentation a theoretical perspective is provided about the influence of chemical composition and dimensionality on the defect activity and the charge carrier photophysics in perovskites beyond the APbX3 composition, by focusing on less-toxic Sn and Bi/Ag double perovskites. The defect chemistry and photophysics of tin and double perovskites will be discussed, by keeping a parallelism between 3D and 2D, i.e. MASnI3 vs PEA2SnI4 and Cs2AgBiBr6 vs (BA)4AgBiBr8 phases, as test cases.[4,5] The analysis aims to highlight the effects of chemical composition and quantum confinement (QC) on several key properties of these materials. The origin of the self p-doping, strongly limiting the efficiency of tin perovskites, will be discussed, as well as computationally designed doping strategies aimed to reduce it. Hence, discussion will move to analyze the charge carrier photophysics in tin and double perovskites, by focusing on the  processes possibly originating the sub-gap emissive features in these materials. Specifically, the competition between the exciton self-trapping and trapping/emission at defect centers vs the chemical composition will be discussed. 

This contribution aims to provide a theoretical framework guiding experimentalists in the design of stable and efficient lead-free perovskite materials. 

In this talk I will discuss recent progress in our group on the development of a new class of thin film solar cells employing Silver Bismuth Sulfide as an emerging absorber for solution processed eco-friendly solar cells. I will first introduce our first report on AgBiS2 colloidal nanocrystal solar cells reporting power conversion efficiency of ~6% [1]. Then I will discuss on the opportunities of tuning the optical properties of this ternary compound via controlling cation disorder homogenization. We demonstrated that by homogenizing cation disorder in this compound we can drastically increase the absorption coefficient of this material as one with the high absorption amongst the semiconductors considered for photovoltaics. This taken together with advances on the device architecture led us to reach power conversion efficiencies of ~9% albeit using an absorber of only 35 nm [2]. In the last part of my talk I will describe our initial efforts on developing AgBiS2 nanocrystal inks and their use with environmentally friendly solvents that led us to achieve efficiencies in excess of 7% [3]. I will conclude my talk with our most recent findings towards an improved passivation strategy of AgBiS2 nanocrystal inks along with the formation of a double heterojunction in the device stack that led to power conversion efficiencies in excess of 10% with Voc of 0.5V, FF of 0.75 and Jsc of 28 mA/cm2.

[1] Solution-processed solar cells based on environmentally friendly AgBiS₂ nanocrystals, M Bernechea, N Cates, G Xercavins, D So, A Stavrinadis, G Konstantatos, Nature Photonics 10 (8), 521-525, 2016

[2] Cation disorder engineering yields AgBiS₂ nanocrystals with enhanced optical absorption for efficient ultrathin solar cells, Y Wang, SR Kavanagh, I Burgués-Ceballos, A Walsh, DO Scanlon, G Konstantatos, Nature Photonics 16 (3), 235-241, 2022

[3] Environmentally Friendly AgBiS₂ Nanocrystal Inks for Efficient Solar Cells Employing Green Solvent Processing Y Wang, L Peng, Z Wang, G Konstantatos, Advanced Energy Materials 12 (21), 2200700, 2022

Non-toxic, all-inorganic perovskite light absorbers received great attention in recent years due to their potential to replace lead-containing perovskite materials in photovoltaics. Regardless of the potential of lead-based halide perovskites, their application on large scale is challenging, mainly due to the toxicity of lead and stability issues. Lead-free absorbers, such as the double perovskite Cs₂AgBiBr₆ as a prominent representative, offer high stability and auspicious optoelectronic properties. Cs₂AgBiBr₆ has been studied intensively in the past few years. Various researchers pre-synthesize this absorber as powder by a synthesis method first suggested by Slavney et al. in 2016 [1] and the resulting absorber powder is further dissolved to render a thin film deposition. Although this method is well reproducible, it is only suitable for the lab-scale synthesis of Cs₂AgBiBr₆, with yields usually in the range of a few grams. Besides this rather small yield, the synthesis requires exceptional safety precautions.

Within this work, we present an alternative synthesis approach for Cs₂AgBiBr₆ powder, namely via spray-drying [2]. During spray-drying, the perovskite phase forms by in-situ crystallization upon drying of the atomized precursor solution. The product is compared to the conventionally synthesized absorber. XRD and Raman analyses confirm the product’s phase-purity, whereas UV-Vis spectroscopy confirms the desired bandgap. Furthermore, absorber films in single-junction perovskite solar cells are successfully produced from both absorber powders, clearly demonstrating the suitability of spray-drying as an alternative synthesis method.

Moreover, we investigated the Bi-substitution by Sb to reduce the perovskite’s original bandgap of ~2.2 eV achieving better suitability as top-cell absorber in perovskite-silicon tandem devices [3]. Spray-drying enabled a fast screening and identification of the substitution limit, and could overcome limitations of the conventional synthesis for the Sb-substituted Cs₂AgBiBr₆.

As a side notice, we can tell that we additionally investigated the lead-free perovskites Cs₃Bi₂Br₉ and Cs₃Bi₂I₉. Besides these, lead-based halide perovskites have been synthesized successfully via spray-drying, too. With the prospect of an industrial scale photovoltaic industry, the demand for high-quality perovskite materials will most likely increase in future. Therefore, spray-drying could pave the way for an easy adaptation to increasing production volumes of perovskite powders in general owing to its suitability as screening method and its capability of producing large amounts of powders.

Thin-film PV is a key technology for low cost and low environmental impact solar energy conversion. Metal halide perovskites are leading among thin film PV technologies, but challenges remain regarding toxicity of Pb and stability. Motivated by the search for low cost, non-toxic and Earth abundant materials solutions for thin film PV, new inorganic semiconductors with complex compositions are being explored. One of the challenges with material discovery and new material compositions, is their fabrication in thin film form. Challenges such as volatility incompatibility or solvent incompatibility, hinders progress in either high quality material demonstration or functionality via device integration.

In this presentation, we will discuss how the combination of mechanochemical synthesis and pulsed laser deposition (PLD), allows the exploration of novel Earth abundant and non-toxic semiconductors materials. We will explore the development, growth and understanding of semiconductor materials such as Pb-free double perovskites (e.g. Cs2AgBiBr6), new photoactive light-absorbers such as Ag3SI and p-type transparent electrodes such as S-doped and Cs-doped CuI.

In summary, the presentation will highlight how controlled synthesis and material design can make a significant contribution to the emerging generation of efficient and sustainable thin film-based solar cell devices.

In the field of perovskite solar cells, explorations of new lead-free all-inorganic perovskite materials are of great interest to address the instability and toxicity issues of lead-based hybrid perovskites. Recently, copper-antimony-based double perovskite materials have been reported with ideal band gaps, which possess great potential as absorbers for photovoltaic applications.^1-2 Here, we synthesize Cs₂CuSbCl₆ double perovskite nanocrystals (DPNCs) at ambient conditions by a facile and fast synthesis method, namely, a modified ligand-assisted reprecipitation (LARP) method. We choose methanol as solvent for precursor salts as it is less toxic and easily removed in contrast to widely-used dimethylformamide. Our computational structure search shows that the Cs₂CuSbCl₆ structure containing alternating [CuCl₆]^5- and [SbCl₆]^3- octahedral units is a metastable phase that is 30 meV/atom higher in energy compared to the ground state structure with [CuCl₃]^2- and [SbCl₆]^3- polyhedra. However, this metastable Cs₂CuSbCl₆ double perovskite structure can be stabilized through the solution-based nanocrystal synthesis. Using an anion-exchange method, Cs₂CuSbBr₆ DPNCs are obtained for the first time, featuring a narrow band gap of 0.9 eV. Finally, taking advantage of the solution processability of DPNCs, smooth and dense Cs₂CuSbCl₆ and Cs₂CuSbBr₆ DPNC films are successfully fabricated.

Ultrafast transient spectroscopy is a vital tool to investigate the photophysical processes taking place in energy harvesting materials. In particular, the findings provide valuable insights into the efficiency limiting processes in solar cells. The focus of this talk will be on solution-processed metal-halide perovskite solar cells as they have received immense attention in the field of photovoltaic research due to their outstanding power conversion efficiency, which has surpassed 26% in a relatively short time. Understanding carrier losses at metal halide perovskite/charge transport layer interfaces is a pre-requisite to bring the efficiency closer to the Shockley-Queisser limit. Interfacial recombination can to some extent be accessed through time resolved techniques, however identifying this is often a challenge due to the complexity associated to the interpretation and modelling of the extracted charge carrier transients.

approach is to utilize complementary transient spectroscopic techniques, namely transient absorption spectroscopy and transient photoluminescence spectroscopy, not only to unravel the processes limiting the solar cell’s short circuit density and the open circuit voltage but also to evaluate different charge recombination channels and extraction. Herein, we limit our focus to examine half-stacks comprised of the photoactive layer and the respective hole transport layers. Several different hole transport materials (PTAA, NiOx,,and 4PACz) are adjacent to the photoactive layer with a bandgap of 1.53 eV. We report on challenges faced during performing complementary spectroscopic techniques and interpretation of the extracted transients. Specifically, differentiation between photophysical processes such as charge extraction and interfacial charge recombination.

BiOI is a nontoxic, stable and polar semiconductor, which shows high conversion efficiencies in photocatalytic water splitting. This is due to an ultrafast and effective charge separation also launching coherent phonons after short laser pulse excitation [1]. The coupling of electronic excitations and phonons should also manifest itself in polaronic effects. To gain unambiguous experimental evidence for polaronic effects we have carried out time-resolved photoemission electron microscopy (TR-PEEM) experiments. This technique offers the unique possibility to learn about the dispersion E(k) of the conduction band (CB) and its occupation with electrons as a function of time after pulsed optical excitation. In order to interpret our data it is not sufficient to describe merely the temporal evolution of the electron distributions in a static CB. We observe that the dispersion of the CB changes itself in time. This is in particular monitored around the Gamma point of the CB. This combined temporal change in band-structure and electron distribution can be explained by the formation of a polaronic excitation.

Two-dimensional transition metal carbides (MXenes) are of great interest for a range
of applications in electronics, including solar cells due to their tunable optoelectronic
characteristics, strong metallic conductivity, and attractive solution processability. In this study,
we used photo-conductive atomic force microscopy (pcAFM) to map the local (nanoscale)
photovoltaic performances of the Ti 3 C 2 T x MXene integrated TiO 2 electron transport layer (ETL)
based perovskite solar cells (PSCs) to determine the treatment's impact on the microscopic
charge flow inside the devices. The nanoscale photovoltaic performances of the devices with
MXene integrated ETLs is first studied by using the pcAFM technique. The underlying PV
mechanisms and their localized dependency on the interfacial modification across the layers
must be understood through the investigation of these photoresponses at the nanoscale. The
morphology and photocurrent maps with different applied voltages have been simultaneously
measured with nanoscale resolution from the top surfaces of the devices without back contacts.
Compared to the as-deposited samples, Ti 3 C 2 T x MXene based PSCs show a more uniform and
improved current flow throughout the film. Average local photocurrent for MXene-induced
PSCs is significantly larger than that of as-deposited PSCs at zero applied bias and steadily drops
as positive bias is raised until it reaches the open circuit voltage. Large variations in short-circuit
current were also observed at different locations across the film that appeared identical in
topography images. Our study reveals that MXene-integrated ETLs have the potential to improve
the polycrystalline photovoltaic devices' performance by enhancing the active layers' intrinsic
properties and nanoscale photoconduction.

Metal halide perovskite (MHPs) solar cells represent a promising newcomer in the front of emerging photovoltaic technologies to address the dramatic energy crisis and climate change that we are facing. The exceptional properties of MHPs derive from their hybrid organic-inorganic nature, which allows also for low-cost and straightforward processing. Solar cells containing MHPs as absorbing layer have already achieved a power conversion efficiency of about 25,7 %, close to the efficiency of silicon-based devices. Nevertheless, a major limitation, still preventing the uptake of the technology, is related to the reduced stability of these materials when exposed to operative conditions, namely temperature, light, and moisture. Herein, an effective defect passivation of MHP surfaces is a key strategy to tackle both the stability and the enhancement of solar cell performances. Although many solution-based approaches ¹ have been tested, we propose here an innovative use of plasma, as a solvent-free, scalable, and non-invasive promising strategy to boost MHP solar cells performances ². As benchmark material we used Methylammonium Lead Iodide perovskite; thus we have exposed the surface of polycrystalline thin films made of this material to different plasma conditions implying the variation of power, gas, and treatment time, both for low-pressure (LP) and atmospheric pressure plasmas (APPs). The impact of Ar,  N₂, H₂ and O₂ LPPs on MAPbI₃ optochemical properties and morphology was correlated to the performance of the photovoltaic devices and rationalized by density functional theory calculations³. An interesting improvement in photoluminescence was observed for the Ar and H₂ treated films, while an improvement in PCE was observed only for the Ar treated device. This result was ascribed to the efficient removal of the superficial organic component, revealed through X-ray photoelectron spectroscopy (XPS), following suitable surface passivation by the electron transporting layer deposition. APP treatments, fed with He gas, were tested on the surface of MAPbI_3, too. A milder morphological modification than LPP treatments was observed in this case, but withstanding good surface passivation, as confirmed through the improved photoluminescence intensity, while the effect in terms of photovoltaic devices is still under investigations. Moreover, starting from these encouraging results, new plasma surface processes are now object of evaluation, such as the plasma-deposition of thiophene-like films and the study of LPP treatments applied to tin-based perovskite solar cells, approaches which already show the great versatility and potential of plasma-based techniques.

Recently, inorganic anti-perovskites with the formula X₃AN (X = Ba, Sr, Ca, Mg; A = As, Sb) have been reported to exhibit excellent optoelectronic properties like small carrier effective masses, suitable direct bandgaps, high optical absorption coefficients as well as allowed optical transitions at band edges. These properties can be tuned depending on the X and A site. Extending the composition to quaternary anti-perovskites (X₆AA′N₂) enables the synthetic possibilities for new materials.[1,2]

Herein we report on the ammonothermal synthesis of EA₅Pn₂(NH)₂. The three newly synthesized compounds Ca₅AsSb(NH)₂, Ca₅AsBi(NH)₂ and Sr₅AsBi(NH)₂ crystallize in the tetragonal space group P4/mmm. Their crystal structure was solved and refined by scXRD. Raman spectroscopy was used to further determine structural elements and verify the presence of imide-groups. Further investigations were carried out using powder X-ray diffraction, UV/Vis-spectroscopy, and density functional theory calculations.

The materials exhibit direct bandgaps in the range between 1.90 eV and 1.14 eV. By composition variation of the A-site elements as well as the X-site elements, the effective band gaps can be tuned. Hybrid density functional theory calculations verify the direct nature of the band gap, indicating large band dispersions through the enhanced covalency of the pnictides, benefiting the carrier transport. In summary, these new Imide-anti-perovskite materials exhibit interesting properties as efficient light harvesting materials for single junction solar cells.

The shift towards an energy mix with more renewable sources have resulted in massive quantities of solar photovoltaics being installed over the worls. This is a growing electronic waste stream that needs to be planned for. Present appropaches to solar recycling involve high-temperature burning for separation and release of the PV cells for metal recovery processes. However, such processes can release gaseous by-products that can cause serious health and environmental issues as well as deprioritises the recovery of pure silicon. This talk will cover our group's efforts in developing new extraction techniques that eschew burning as well as incorporate green solvents. Through an incorporation of such novel techniques along with hydrometallurgy, we demonstrate separation of glass, extraction of the various metallic elements such as Ag, Cu and Al as well as silicon at good purity levels. Our efforts in upcycling the extracted elements into other products such as thermoelectrics as well as battery anodes will also be described. 

The current models of production and consumption are no longer deemed adequate because of the inefficient use of resources (especially energy) and its environmental consequences. To sustainably meet the growing energy demands of the world, a substantial increase in the deployment of photovoltaic and energy storage technologies is needed. Dye-sensitized solar cells (DSCs) have the potential to provide a viable source of renewable energy to help decarbonize our economy and power wearable devices. With their exceptional performance in diffused light under indoor conditions, DSCs remain competitive for powering the next digital revolution forming the internet of things (IoT). They can also be interfaced with storage materials to enable a new generation of energy storable solar cells. While this is all encouraging, with the increase in the number of deployed devices arises the need to evaluate their end-of-life, transition to a resource efficient circular economy. In this talk, I will evaluate each component of a dye-solar cell from an environmental point of view. I will present the various material design routes being developed to enable a sustainable approach both during fabrication and as readily ‘refurbishable/upgradable’ devices, leveraging the ability to replace dyes and electrolytes repeatedly with no observable loss in functionality over many product generations. My talk will identify features which are conducive to circular economy and identify barriers to resource efficiency for these technologies and suggest some potential solutions and priority areas for future research.

References: 1. R. G. Charles, M. L. Davies and P. Douglas, "Third generation photovoltaics — Early intervention for circular economy and a sustainable future," 2016 Electronics Goes Green 2016+ (EGG), Berlin, Germany, 2016, pp. 1-8, doi: 10.1109/EGG.2016.7829820.

IRENA estimates that 70 million tonnes PV waste will have been generated by 2050. Furthermore, we are already seeing challenges in the supply chains of critical materials for renewable energy and electronics. Emerging solar technologies involve printable devices which have low embedded energy and are fully recyclable at the end of their life. Using recycled materials in these devices could lead to further enery savings and reduced environmental impact, as well as providing a versatile, low cost technology for clean energy. We present the outcomes of a proof-of-concept to provide data to assess the potential for printable electronic, electrochemical and photovoltaic devices using waste materials. Carbon black has numerous applications, but production leads to 2.4 kg CO₂ emissions per kg virgin carbon black. Our project seeks to identify opportunities for the recycled carbon black as a higher-value product in electronic and electrochemical devices, such as printable photovoltaics. Most current research into printable solar cells using the triple mesoscopic stack configuration uses commercial carbon inks which is based on virgin carbon black. We have developed conductive inks from recovered carbon black from waste tyres, tested them in devices (indoors and outdoors) and then shown that the device itself can be regenerated and re-used.

In this talk I will present our latest discoveries in converting bio and plastc waste into high value chemicals and H2 via electrooxidation reactions including catalyst design, mechanism and upscale . We will target prescursors such as ethylene glycol or glycerol.  I will also talk about upgrading plastic into carbon composites for Na ion batteries. In particular I will talk about some high energy density C/Sn  cpmpsotes and the alloying mechanism of Na with Sn.

In both cases i will talk about in operando techiques development such as XAS or XRD-PDF. Initial results on LCA will also be presented from the perspective of economic viability and   environmental footprint.

Givcen the importance of net zero technologies, this talk will provide insights into waste utilisation towads high value added products.  Products such as glycolic acid or lactic  acid deriving from such processes can be used in  cosmentic and renwable plastic . H2 represents a huge market and  technology, where I still have a lot  to learn

Recycling processes for lithium-ion batteries (LIBs) have been developed to remove chemical compounds from these batteries for secondary uses, as well as resynthesizing cathode material using direct or short-loop recycling, for use in cathodes of fresh batteries. The reason for extracting compounds from the cathode of discarded LIBs is that they are composed of high value and critical minerals such as Li, Co, Ni and Mn. In particular the cathodes with compositions such as LiCoO₂, LiNi_yMn_zCo_xO₂, and LiMn₂O₄ are important to evaluate for reclamation of these metals and re-use. It is known from the literature that end-of-life LiNi_yMn_zCo_xO₂ contains around 15 to 20% less lithium than fresh cathode and therefore cannot be directly reuse without further lithiation. It is a challenge to return this end of life and recovered black mass, containing NMC materials to form that can be reused in a new cell. In this work we describe a direct recycling route for the discarded NMC532 using a low cost, and facile relithiation process. The cathode material was extracted from a disassembled end of life or scrap NMC vs graphite pouch cell, and then delaminated from aluminium using a 3.5M NaOH solution that selectively dissolved the Al foil. The recovered powder was analyzed by XRF, XRD, CHNS and ICP-OES techniques. After characterization, solid state relithiation was carried out with the lithium source LiOH at 700 °C and the product of this reaction was analyzed again using the above techniques and evaluated electrochemically.

The aviation industry is responsible for 2% of the total GHG emissions and 10% of the fuel consumption worldwide, with a predicted market growth of 3.7% a year. It is expected that a large portion of the current global fleet will continue to be operational until 2040-2050 calling for the development of drop-in alternatives based on sustainable resources [1].

Significant research efforts have been devoted to identifying routes to produce liquid fuels from CO₂ and H₂ leveraging the Fisher-Tropsch (FT) synthesis. One option is to feed CO₂ and H₂ in a reverse water gas shift reactor (RWGS) and use the resulting syngas in a traditional FT process [2]. The limitations of this approach are: (i) the RWGS reactor operates at high temperatures (600-1000 °C); and (ii) the FT reactor produces a significant amount of wax, a mixture of heavy hydrocarbons that requires additional upgrading units, such as a hydrocracking (HC) reactor to improve the yield of liquid fuels. The direct hydrogenation of CO₂ to liquid fuels in a single step is more appealing, but usually the formation of short chain hydrocarbons is favoured [3]. Yao et al. (2020) [4] synthesized a novel Mn-Fe-K based catalyst capable of converting CO₂ and H₂ with excellent yield and selectivity towards liquid hydrocarbons in the jet fuel range (C8–C16) and minimal wax production.

In this work, we carry out a techno-economic analysis and life-cycle assessment (LCA) of two SAF production processes: a one-step process (1sFT) based on the above-mentioned catalyst; and a two-step process (2sFT) based on the work of [2]. Both processes are fed with CO2 from direct air capture (DAC) and green H2. 1sFT and 2sFT are simulated at scale using Aspen HYSYS to evaluate their economic performances and obtain inventories for the LCA. The environmental assessment is conducted in SIMAPRO 9.3, using Ecoinvent 3.8 Cut-Off database for the background process inventories. The fuel obtained from 1sFT and 2sFT is compared against the fossil-based alternative considering different environmental KPIs, as for SAF to be truly sustainable the reduction is GHG emission should not be associated with high collateral ecological damage, a problem often referred to as burden shifting. ReCiPe2016 is used as life-cycle impact assessment method [5] and GWP100 is evaluated alongside the monetized endpoint impacts to human health, ecosystem quality and resource scarcity. 

Our results show that the one-step process is superior both in economic and environmental terms to the two steps process, due to a lower capital cost, higher selectivity towards liquid hydrocarbons and lower energy requirements. On a well-to-wake basis the 1sFT process is predicted to reduce GHG emissions by 75% and the 2sFT process by 58%. The analysis of the endpoint environmental impacts confirms a better environmental performance of the synthetic fuels compared to the fossil counterpart, with the 1sFT outperforming the 2sFT by a larger extent, with 1sFT and 2sFT having a total externality cost 44% and 22% lower that the fossil fuel, respectively. At current CO2 and H2 prices the productions cost of low-carbon synthetic fuels is predicted to be 6-8 times higher than fossil-based aviation fuels, which is in agreement with the literature, and a combination of feedstock cost reduction and policy intervention (e.g. carbon taxation) is necessary for synthetic fuels to become cost competitive.

The Internet of Things (IoT) has recently emerged as a technology with immense potential societal impacts. However, powering IoT devices presents a key challenge. Indoor photovoltaics (IPV), offering the potential for self-powered and low-maintenance IoT devices, may be a promising solution. Concurrently, driven by the need for tandem structure, wide-gap perovskite semiconductors are emerging. The optimal bandgap for IPV devices (1.8 – 1.9 eV) is considerably wider than most traditional solar materials but in accordance with existing wide-gap perovskites.

In this talk we discuss the performance of various wide-gap perovskite IPVs as model systems under spectrophotometrically calibrated indoor light conditions and compares them to thermodynamic performance predictions. Computational simulations indicate trap-assisted recombination as a primary loss mechanism in current perovskite IPVs. To support further advancement in the field, a computational tool is provided to extrapolate calibrated solar (AM1.5G) measurements for benchmark performance estimates at a range of indoor conditions. This work highlights the need for standardized characterization methods and suggests pathways for device optimization to unlock the full potential of perovskite IPVs for a future of sustainable, self-powered IoT devices

Metal halide perovskite semiconductors hold great promise for future photovoltaic applications. The efficiency of metal halide perovskites solar cells (PSCs) is determined by several parameters, one of them is the open circuit voltage (V_OC). In order to achieve high efficiency solar cells, it is important to reach a high V_OC. Consequently, we want to understand the loss mechanisms limiting the V_OC. It is known that the V_OC loss is dominated by non-radiative recombination occurring e.g. in the bulk of the perovskite, or at the interfaces. Measuring the V_OC of a PSC in order to understand what exactly causes the non-radiative recombination and where in PSC, will not provide the answer, because it is just the overall result. Therefore, we try to understand the V_OC loss in perovskite solar cells via absolute photoluminescence intensity analysis. With this analysis, the quasi Fermi level splitting (QFLS) of different components in a PSC can be determined_. Since the QFLS is related to the maximum V_OC achievable for a photovoltaic device, this relation can be used for predicting device properties.

In this work, the effect of passivation on defects and the non-radiative recombination of charge carriers in mid to wide bandgap perovskite solar cells at the perovskite and electron transport layer interface is examined with absolute PL to understand the detrimental processes happening in the PSCs. We begin by analyzing the voltage loss for p-i-n devices and by describing the passivation of surface defects at the interface of the mid-bandgap perovskite with the electron transport layer using a solution-processed quaternary ammonium halide (choline chloride, CCl) or an evaporated ultrathin lithium fluoride (LiF) layer, that translates to solar cells with a high V_OC. We extend this passivation evaluation to an array of wide-bandgap perovskite devices. After this analysis, we demonstrate that non-radiative recombination is primarily located in the film and that it increases with increasing bandgap. The other prominent loss is located near the interface of the perovskite with the electron transport layer and is passivated equally for both passivation techniques and for all bandgaps investigated in this study. This implies that the surface treatment with two different materials result in the same gain. To further understand the working principle of the passivation strategies, we extend the passivation study by increasing the amount of passivation material in between the perovskite-ETL interface. It is found that upon application of increasing thicknesses of LiF or increasing concentration of CCl in between the perovskite/ETL interface, the gain in QFLS and V_OC starts plateauing and an optimum thickness or concentration should be used.

In summary, by combining the absolute photoluminescence spectroscopy results, it is found that by using a LiF or CCl a significant reduction of the non-radiative recombination losses that currently limit wide-bandgap PSCs is achieved. And that this gain is primarily due to minimizing the loss at the perovskite-ETL interface. These results contribute to solving the main challenges of the wide-bandgap perovskite in achieving a high V­_OC.

In conventional flexible perovskite solar cells (FPSCs) produced on plastic substrates such as polyethylene terephthalate (PET) and featuring a conductive transparent oxide layer like indium-doped tin oxide (ITO), the ITO and perovskite layers are susceptible to cracking when the devices are moderately bent. While changes in device series resistance can be indicative of cracks in the ITO layer, characterizing cracks in the perovskite layer is more complex.

At a microscopic level, perovskite layer cracking implies an upsurge in the density of bulk and surface defects, leading to an increase in trap states within the layer. This, in turn, can amplify the loss of photo-generated charge carriers through trap-assisted carrier recombination processes. However, comprehending and quantifying the impact of applied mechanical stress (MS) on device properties and behavior necessitates a sophisticated analysis of charge carrier generation, recombination, and extraction processes both before and after MS application [1].

This study is dedicated to examining the properties and behavior of FPSCs pre- and post-application of MS to unravel the dynamics of recombination losses induced by MS, providing insights into the mechanisms contributing to performance degradation in perovskite-based solar cells. The investigation involves a comprehensive analysis of device properties and behavior through experimental and theoretical approaches. The degradation of photoelectrical parameters in fabricated PFPSCs due to applied MS is elucidated by meticulously scrutinizing various recombination pathways within the perovskite layer.

Interfacial charge carrier recombination is currently one of the major performance bottlenecks in single- and multi-junction metal halide perovskite (MHP) solar cells. In our work, we investigate interfacial charge carrier recombination processes in state-of-the-art MHP thin films and device structures by transient spectroscopies including transient reflection, transient absorption, time-resolved photoluminescence, and time-domain terahertz spectroscopy across a wide dynamic range from femto- to microseconds. The MHPs investigated are multi-cation (mixed) halide perovskites as neat MHP thin films, passivated MHP thin films, perovskite films adjacent to charge transport layers (CTLs), MHP films adjacent to CTLs with additional interlayers (ITLs) and in the presence of electrodes, as well as MHP films on transparent conductive oxides (TCOs) with and without common self-assembled monolayers (SAMs). Organic CTLs are also used, since they allow direct probing of the carrier dynamics in the CTL (not only in the perovskite film), thereby allowing to distinguish between carrier extraction and interfacial recombination. Our spectroscopic experiments are supported by computational studies providing insight into the role of (bulk and surface/interface) defects on carrier recombination, the chemistry of defect passivation at interfaces, and interfacial carrier extraction and recombination dynamics. Our studies provide in-depth insight into interfacial charge carrier recombination processes at various types of interfaces in perovskite devices and reveal pathways to mitigate those losses to enhance the device Voc and quantum efficiency in both single- and multijunction photovoltaic solar cells.

Metal-halide perovskites (MHP) stand out as highly promising and cost-effective optoelectronic materials due to their exceptional optoelectronic properties and versatile fabrication methods [1-6]. These materials find applications in diverse fields such as solar cells, light-emitting diodes, photodetectors, and even quantum emitters. Quantum confinement can unveil unexpected and advantageous characteristics, leading to the development of high-performance devices.

One approach to induce quantum confinement involves creating layers of quantum-confined materials through the deposition of multiple thin films. Thermal evaporation emerges as a particularly promising technique for fabricating halide perovskite films, offering precise control over layer thickness, fine-tuning of composition, stress-free film deposition, and the ability to modify surface properties. Thermal evaporation in perovskite fabrication has expanded the possibilities of thin film production, showcasing its capability to generate ultrathin perovskite films that serve as the foundation for multi-quantum well structures.

This method enables the manipulation of growth properties, influencing the optoelectronic characteristics of nanoscale thin films and inducing quantum confinement effects within the structure. The precise control over photoluminescence through quantum confinement opens up a wide array of possibilities for unconventional optoelectronic properties and novel applications of perovskites [7-10].

1. J. Li et al; , Joule 2020, 4, 1035

2. H.A. Dewi et al., Sust. Energy & Fuels. 2022, 6, 2428

3. E. Erdenebileg, et al Solar RRL, 2022, 6, 2100842

4. HA Dewi, et al., Adv. Funct. Mater. 2021, 11, 2100557

5. J.Li et al., Adv. Funct. Mater. 2021, 11, 2103252

6. E. Erdenebileg et al., Material Today Chemistry, 2023, 30, 101575

7. E. Parrott et al. Nanoscale, 2019, 11, 14276

8. KJ Lee et al., Nano Letters, 2019, 19, 3535

9.  KJ Lee et al., Advanced Materials, 2021, 33, 2005166

10. T. Antrack et al., Adv. Sci. 2022, 9, 2200379

Perovskite photovoltaics stand out as a top contender in the quest for new solar energy harvesting technologies. However, the presence of toxic lead in perovskite devices necessitates exploration of alternatives. The most popular of which has been tin-perovskites, known for their extremely fast and difficult to control crystallization. In this talk, a novel method of printing of tin-perovskite films will be presented, involving a crystallization trigger and subsequent control of nucleation and growth through solvent engineering. It was developed using an in-line optical spectroscopy characterization equipment providing analysis of the crystallization process in-situ. Photoluminescence, Raman and reflectance probes were installed and probed the corresponding signals to give information of film changes after printing and during post-printing treatment. The new method for printing tin-perovskite films enabled the fabrication of the first slot-die coated solar cell device, opening avenues for the development of fully printed, lead-free perovskite photovoltaics at an industrial scale.

Hybrid perovskites are one of the most promising materials for next-generation optoelectronic properties, including solar cells. However, the solution-based fabrication process brings to the formation of polycrystalline films, resulting in defects, which have a detrimental effect on the perovskite solar cell performance because they are expected to trap charge carriers, facilitating nonradiative electron-hole recombination. Hence, the investigation of the impact of defects on fundamental photo-physical properties becomes crucial for the optimization of the device.

In this framework, I have investigated the optoelectronic properties of three differently defective materials. More in details, I employed different amount of antisolvent and the passivation procedure to finely tune the defects concentration. In order to accurately evaluate defects concentration, I performed transient absorption measurement, and I analyzed the results considering the Burnstein-Moss effect. With the aim of investigating the correlation between defects and fluence regime, I used time resolved techniques, i.e. time resolved photoluminescence and transient absorption spectroscopy. In particular, time resolved photoluminescence highlighted that, by increasing the trap states concentration in the material, a shift in the onset for radiative regime arises at progressively higher fluences. On the other hand, transient absorption spectroscopy evidenced the strong correlation between the organic cation used for the passivation and the occurrence of the Auger regime. Therefore, for the material with lower trap states concentration, the range in which radiative recombination dominates is enlarged. Ultimately, this is reflected in the solar cell efficiency which is enhanced by 22% with respect to the higher defective material.

In organic solar cells, the charge-transfer (CT) electronic states are crucial in the charge-generation process, significantly influencing the overall device performance. They are formed at the interface between the electron-donor (D) and electron-acceptor (A) material, which usually exhibit significant electric fields.

In this study, we use a dedicated D-A system to tune intrinsic and extrinsic interface electric fields. It consists of two doped organic layers on top of each other, forming a planar organic p-n junction. By applying increasing voltages up to 6 V and introducing different thicknesses of intrinsic layers between 0 nm and 20 nm at the interface, the electric field at the interface can be deliberately varied. We observe substantial shifts in the CT-state peak emission, approximately 0.5 eV (150 nm), causing a transition from red to green color. This effect can be explained in a classical electrostatic picture, as the interface electric field superimposes the Coulomb interaction between the electron-hole pair. Our investigations illustrate the extent to which CT-state energies are influenced by their immediate electric environment. While CT state energy is often referred to as a fixed quantity, we want to emphasize that this understanding needs to be revised, especially for planar systems with a high degree of interfacial dipole alignment.

Photonic architectures offer a promising avenue for optimizing the performance of various optoelectronic devices. Nevertheless, the future of optoelectronic devices hinges on cost-effective and large-scale manufacturing techniques that can reduce expenses and boost production efficiency. To harness the remarkable attributes of photonic structures for enhancing these devices, they must undergo a processing approach akin to that of the devices themselves.

In our research group, we employ soft nanoimprinting lithography, a versatile, rapid, and cost-efficient method for crafting nanostructures from a diverse variety of materials. In soft nanoimprinting lithography, we make use of pre-patterned soft elastomeric stamps to corrugate materials such as resists, biopolymers, colloids and nanomaterials in general. In all cases, the resulting photonic architectures can exhibit a resolution below 100 nm while covering areas up to 1 cm². During this presentation, I will demonstrate our utilization of pre-patterned stamps to induce the long-range alignment of different colloids, including gold colloids and perovskite nanocrystals, to attain distinct optical properties, such as lattice resonances with high Q-factors. Moreover, I will showcase how elastomeric molds pre-patterned with chiral motifs can lead to chiral metasurfaces, exhibiting strong circular dichroism and intense circularly polarized photoluminescence (CPL), making them seamlessly applicable in various practical applications.

Chiral plasmonic nanostructures have attracted much interest in recent years as new platforms for modulating the polarization of light. Other than photonic devices, chiral plasmonic nanostructures facilitate the ultrasensitive detection of chiral biological materials, e.g. DNA and proteins. To synthesize chiral plasmonic nanostructures, the most well-established method is chirality transfer from biological materials to inorganic nanostructures. Alternatively, circularly polarized light can be used to control the formation of the plasmonic enantiomeric nanostructures, where the chirality of the product solely defined by the polarization of the light. This synthetic method may be more cost-effective while allowing the wavelength and intensity of light to be used as tunable parameters. Here we investigate the use of circularly polarized light to create chiral gold nano bipyramids. Hot charge carriers can be generated with light and consumed to fabricate chiral plasmonic structures. Scanning electron microscopy (SEM) coupled with cathodoluminescence measurements comparing the nano bipyramids before and after circularly polarized light illumination is used for characterization. We will show that both dielectric or plasmonic chiral additions can be deposited depending on the identity of reagents. Circular dichroism (CD) spectra of the chiral plasmonic nanostructures further demonstrate the polarization dependent light-matter interactions of the materials synthesized.

Can we generate and transfer chirality?

Chirality is an important structural property: right-handed and left-handed chiral materials have identical chemical composition and connectivity, but they are related by mirror transformation, forming a couple of enantiomers.

Chiral materials show unique features: the intrinsic non-centrosymmetry leads to optical rotation, circular dichroism (CD), second-harmonic generation (SHG), piezoelectricity, pyroelectricity, ferroelectricity, and topological quantum properties.

In this talk, I will discuss the intriguing interplay between chirality and physical properties in innovative materials, ranging from twisted bilayers (non-magnetic as well magnetic) to chiral hybrid organic-inorganic perovskites.

About Figure 1: Generation of chirality by twisting in bilayer graphene and corresponding spin-
texture switching.

References: 
[ 1 ] K. Yananose, G. Cantele, P. Lucignano, S.-W. Cheong, J. Yu, and A. Stroppa, Phys. Rev. B. 104
(2021) 075407.
[ 2 ] K. Yananose, P. G. Radaelli, M. Cuoco, J. Yu, and A. Stroppa, Phys. Rev. B. 106 (2022) 184408.

Hybrid semiconductor materials are predicted to lock chirality into place and encode asymmetry into their electronic states, while the softness of their crystal lattice accommodates lattice strain and maintains high crystal quality with the low defect densities necessary for high luminescence yields. The realization of chiral bulk emitters with bright circularly-polarized luminescence from such materials is desired for the design of chiroptical photonic and opto-spintronic applications. Here, we report the fabrication of novel chiral layered hybrid lead-halide perovskites with high photoluminescence quantum efficiencies and large degrees of circularly polarized photoluminescence at room temperature.[1] Using state-of-the-art transient chiroptical spectroscopy, we rationalize the excellent photoluminescence yields from suppression of non-radiative loss channels and very high rates of radiative recombination. We further find that photo-excitations sustain polarization lifetimes that exceed the timescales of radiative decays, which rationalize the high degrees of polarized luminescence. We postulate that the superior optoelectronic properties of the layered hybrid perovskites arise from their special tolerance to crystal structure chirality, which we carefully designed by cation engineering. We demonstrate that our materials can be used in electroluminescent devices and polarized-light detectors. Our findings pave the way towards high-performance solution-processed photonic systems for chiroptical applications and chiral-spintronic logic at room temperature.

The significant growth, development, and evolution of technologies such as optoelectronics and spintronics have always been accompanied by access to materials with specific and extraordinary properties. Among these materials, recently, chiral trigonal tellurium (Te) stands out since it exhibits electrical magneto-chiral anisotropy and spin polarization,[1-3] electrical conductivity anisotropy and intrinsic polarized photoresponse,[3-5 tunable Rashba spin-orbit coupling,[6] optical activity[7,8] and bulk photovoltaic effect (BPVE)[9]. However, since its properties depend on its crystal structure, for its successful integration into devices and the development of new applications, it is key to determine its crystallographic orientation and handedness and its interaction with light. In this work, using bulk single crystals, we show how the response of Te to polarized light depends on the crystal orientation which has implications for optical and electrical transport studies. By linearly polarized Raman spectroscopy we identify different crystal faces (1 0 0), (1 1 0) and (0 0 1) and the orientation of the trigonal axis corresponding to the helical Te chains. Moreover, we correlate the angle-resolved experimental patterns derived from the data analysis with the symmetry of the crystal. Furthermore, by circularly polarized measurements, we highlight that only for incidence parallel to the trigonal axis, i.e. in the (0 0 1) face, is possible to determine the handedness. In this case, we observe different peaks shift for left- and right-handed crystals in the corresponding cross-helicity Raman spectra. We support our findings with X-ray diffraction and chirality- and orientation-sensitive chemical etching, providing robust insights for the analysis of chiral and low-dimensional materials.[10]

In recent years, photoactive chiral materials are attracting considerable interest owing to relevant applications in optoelectronics as well as high resolution imaging [1]. In these regards, hybrid materials are skyrocketing the field of material science for optoelectronics because they can tune the properties of soft and inorganic assemblies [2]. A recent interesting class of luminescent chiral materials is represented by chiral hybrid perovskites, since they are showing prominent circularly polarized emissions without any need of expensive ferromagnets or extremely low temperatures [3]. Indeed, the chiral source impacts specific non-covalent interactions occurring within the chiral scaffold, which in turn affect the efficiency of the chiral emissions [4]. Modern multiscale modeling and simulations nowadays have an unprecedented level of accuracy, enabling an efficient chiral design of luminescent materials. The chiral design concepts of low-dimensional perovskites herein discussed are based on enhanced sampling simulations and TD-DFT calculations [5] from the predicted free-energy basins. This simulation strategy enables to consider a variety of contributions including molecular rotations within the chiral framework, that may affect the generated chiroptical properties.

Chiral solution-processable semiconductors based, for example, on small molecules, polymers or halide perovskites offer an exciting new avenue to simultaneously control charge, spin and light using a single material. This could enable efficient spin-optoelectronic devices ranging from displays and holography to detectors, and even applications in quantum information technology.[1] In this talk, I will give an overview of our recent efforts to understand the underlying mechanisms by developing novel time-resolved chiroptical spectroscopy techniques.

By pushing broadband circular dichroism to diffraction-limited spatial and 15 fs time resolution, we create a spin cinematography technique to witness the ultrafast formation of spin domains in halide perovskite films due to local symmetry breaking and spin-momentum locking.[2]

In terms of circularly polarized photoluminescence (CPL), I will first explain the fundamentals and artifacts involved in measuring CPL reliably and introduce an open-access methodology and code to do so [3].

I will conclude by showing our most recent development of a transient broadband full Stokes vector polarimetry with unprecedented time and polarization resolution to track the emergence of chiral light emission [to be submitted].

[1] Nature Reviews Materials 8, 365 (2023)

[2] Nature Materials 22, 977 (2023)

[3] Advanced Materials 44, 2302279 (2023)

Magic-sized clusters (MSC) are identical inorganic cores that maintain a closed-shell stability, inhibiting conventional growth processes. Because MSCs are smaller than nanoparticles, they can mimic molecular-level processes, and because of their small size and high organic-ligand/core ratio, MSCs have “softer” inter-particle interactions, with access to a richer phase diagram beyond the classical close packed structures seen with larger particles. These MSCs display a surprising ability to self-organize into films with hierarchical assembly that spans over seven orders of magnitude in length scale. The films are optically active with g-factors among the highest reported for all semiconductor particles. In this talk I will highlight some remarkable behavior we have recently found in magic sized clusters, with a focus on the symmetry breaking and chirality transfer that occurs during their self-organization into thin films. And I will discuss our method for extracting the chiroptic-CD signal from the raw data that contains highly linear anisotropic effects, derived using Mueller matrix and Stokes vector conventions, and our extension of that expression using a more accurate third order Taylor series expansion to reveal "pairwise interference" contributions to CD spectra that, unlike LDLB contributions, cannot be averaged out of the signal.

Spin-crossover (SCO) complexes are promising building blocks for spintronic and high-density memory devices as they can be reversibly switched between two distinct spin states, low-spin (LS) and high-spin (HS) using a variety of external inputs (i.e. temperature, light, pressure, etc…). Additionally, the spin transition goes with change in other properties like volume, conductivity, or color. One step forward, chiral SCO compounds have been proposed as active components of magneto-optical devices.[1 Interestingly, when electrons are injected through chiral molecules, one electronic spin is preferably transmitted, working as spin filters at room temperature. This fascinating outcome is called Chiral Induced Spin Selectivity (CISS) effect.[2] Few years ago, we observed a CISS enhancement when adding paramagnetic centers in chiral self-assembled monolayers (SAMs).[3] Now, we want to push the field further by taking benefit from SCO bistability to modulate the magnitude of CISS effect. In this scenario, we pursue using chiral SCO compounds self-assembled on surfaces to further elucidate the role of the metallic center in the CISS performance. To reach this final goal, we have synthetized chiral SCO complexes and prepared self-assembled monolayers (SAMs) by a solution approach (preliminary step to have enhanced CISS effect). Specifically, in this project we have prepared chiral SCO complexes with tetradentate chiral Schiff base ligands (L) of formula [Fe(L)(NCX)₂] (X = S, Se).

In this presentation I will discuss our studies of controlling the spin currents using chiral metal-halide hybrid semiconductors thru the chiral induced spin-selectivity (CISS) effect. Chirality is introduced through an enantiomerically pure organic ammine as the A-site in layered 2D Ruddlesden-Popper type structures. The inorganic component make up the layers and are either lead or tin halide. Chirality is introduced into the inorganic component thru structural distortions and chiral arrangement of the organic components. We have studied this chiral transfer in the 2D layered systems, 0D dimer inorganic structures, and in metal-halide semiconductor nanocrystals with chiral ligands attached.

These systems exhibit CISS whereby only one spin sense can transport across the chiral layer and the other spin sense is blocked for one handedness of the chiral perovskite layer. We show that chiral perovskite layers are able to achieve > 80% spin-current polarization using magnetic conductive probe AFM. We have demonstrated the CISS effect in a half spin-valve where only one of the contacts is a ferromagnet and these results agree with the magnetic conductive probe AFM measurements.  However, to enable a broader range of opto-electronic functionality achieving spin accumulation in traditional semiconductor structures at room temperature and without magnetic fields is key.  Current efforts that employ ferromagnet/semiconductor interfaces is limited due to the conductivity mismatch.  We have demonstrated spin injection across chiral halide perovskite/III-V interfaces achieving spin accumulation in a standard semiconductor III-V [MH1] (AlxGa1-x)0.5In0.5P multiple quantum well (MQW) light emitting diode (LED). The MQW is recieved as a working LED and it's internal structure was not modified.  The spin accumulation in the MQW is detected via emission of circularly polarized light with a degree of polarization of up to ~15%. Characterization of the chiral perovskite/III-V interface demonstrates a clean conformal semiconductor/semiconductor electrical contact where the fermi-level can equilibrate. Our findings demonstrate chiral perovskite semiconductors can transform well-developed semiconductor platforms to ones that can also control spin at room temperature.

Organic or perovskite photovoltaics poses a multi-objective optimization problem in a
large multi-dimensional parameter space. Massive progress was achieved in developing
methods to accelerate solving such complex optimization tasks. We have demonstrated
for both types of semiconductors, that the combination of Gaussian Process Regression
(GPR) and Bayesian Optimization (BO) are most efficient in predicting new materials,
identify optimized processing conditions or invent alternative device architectures in
larger parameter rooms. For a 4 dim space (solvent, donor-acceptor ratio, spin speed,
concentration) with about 1000 variations in a 10 % grid space, 30 samples are sufficient
to find the optimum. For 6 dimensional spaces, the possible variations go into the
millions and billions. Nevertheless, our automated lines, being operated in an
autonomous optimization mode, were able to identify globalized optima within several
hundred´s of experiments. In the outlook we discuss whether these autonomously
operated research lines can as well handle unorthodox optimization problems such as
the recycling of organic or perovskite solar cells

Narrow bandgap non-fullerene acceptors (NFAs) offer the semitransparency feature that makes organic photovoltaic (OPV) technology attractive for application in niche markets, such as building-integrated photovoltaic and agrivoltaics. However, evaluating each photoactive material system following classical experimentation methodologies and advancing it towards mass production presents a complex multiparametric challenge that requires intensive time and resources. In this talk, we propose the use of high-throughput lateral gradient methodology to evaluate the photovoltaic potential and semitransparency of narrow bandgap material systems.  More than two thousand devices with different narrow bandgap material systems were manufactured using a platform that combines doctor blade coating technique with controllable velocity profiles to create thickness gradients of photoactive layer. As a result, the power conversion efficiency of the blade-coated devices varied between 0.06 to 10.45% in the champion devices of the screened material system. The large data pool collected for all samples was employed to perform statistical analysis to reveal the most influential parameters for narrow bandgap devices. We have found that the electronic descriptors (i.e. electron affinity, ionization energy, and optical band gap) had a more prominent impact on the final device performance than the processing conditions. Finally, we have shown the potential application of each narrow bandgap blend system for semitransparent devices by calculating its average visible transmittance as a function of photoactive layer thickness. Our study paves the way for high-throughput experimentation to accelerate the mass production and materials screening of narrow bandgap OPVs for semitransparent photovoltaic applications.

Semi-transparent organic photovoltaics are potential candidates for making smart windows in buildings and for agrivoltaics. The challenge here is to achieve high light utilization efficiency (LUE) without compromising on average visible transmittance (AVT). In this work, we designed and fabricated selective spectral tuned Distributed Bragg Reflectors (DBR) using alternating layers of TiO₂ and SiO₂, high and low refractive index oxide materials. The reactive sputtered layers on the back side of a pre-coated ITO glass exhibited close to 90% reflection between 650nm to 900nm wavelength range (near infra-red) with an outstanding AVT of around 90%. To test the superiority of these DBR integrated substrates for organic photovoltaics, we first optimized fully slot die coated organic solar cells to achieve above 14% PCE with blend PM6:Y7 systems using an opaque device configuration. Following that, the DBR integrated semi-transparent devices are made with blend PM6:Y7 and AgNWs top transparent electrodes, fully slot die coated. The resulting solar cells show 20% improvement in current density J_sc from the DBR substrates, compared to devices without integrated DBR, and LUE values above 3%, with an AVT around 40% and PCE around 8.3%. These results show the potential of achieving improved semi-transparent photovoltaics with industrial compatible printing processes.

Device modeling is extensively used in solar cell research going from simple models such as the Shockley diode equation or partial differential equations (PDE) to more complex models like Monte-Carlo or drift-diffusion (DD). These models are often used as a diagnostic tool to understand and quantify the main losses for a given device by reproducing experimental measurements.

However, one of the main criticisms about using complex models as a means to quantify material properties is the many fitting (10 to 40) parameters that need to be estimated. Many argue that with so many fitting parameters one could fit almost any model to the experimental data.

In this presentation, the challenges of using high-dimensional physical models to quantify material properties accurately will be addressed. As well as, how machine learning methods such as Bayesian optimization (BO) combined with high-throughput (HT) experimental data can be leveraged to study material properties at scale.

The potential of using modeling, BO and HT data will be illustrated by several case studies using different physical models (PDE or DD) and experimental methods such as (i) transient absorption spectroscopy, (ii) transient photoluminescence and microwave conductivity, (iii) light-intensity-dependent current-voltage characteristics.

The open-source package BOAR (Bayesian Optimization for Automated Research) will be introduced here as a flexible package to address many challenges of solar cell research such as smart experimental planning or fitting of high-dimensional models to experimental data. This package combined with PDE solvers and open-source DD package SIMsalabim provides an easy-to-use and flexible toolbox for solar cell research.

The need of scalable fabrication of high-efficiency organic photovoltaic cells and modules has gradually emerged. In particular, indoor organic photovoltaics (IOPV) constitutes an attractive energy harvesting solution to power IoT devices, given its deployability, reliability, and power density. A substantial portion of the billions of new IoT devices that will be installed in the coming years are expected to be located inside buildings. Such devices like environmental sensors, can have several shapes and sizes, hence the need to develop custom-made conformable photovoltaic devices to facilitate their integration into the final product. In this context, inkjet printing has become a very attracted printing technology for large-scale printed flexible cells and modules with freedom of shapes and designs.

Herein we demonstrate the challenge to go from lab-scale to industrial scale to achieve highly efficient fully inkjet printed IOPV cells and module. To prove the great advantage of inkjet printing as a digital technology allowing freedom of forms and designs, particular OPV modules with different shapes are demonstrated and integrated into different IoT devices to operate autonomously without using batteries or connections to the grid to ensure sufficient flexibility in their placement.

Inverted Organic Photovoltaics (OPVs) allowed more flexibility on designing the roll-to-roll (R2R) production process and thus provided technological opportunities to OPV technology [1,2]. In addition, inverted OPVs are exhibiting significantly longer lifetime performance compared to normal structure OPVs, another important parameter for OPV commercialization [3]. The presentation will cover recent advances in developing high-performance metal oxides carrier selective contacts and Indium-tin-oxide (ITO)-free electrodes for non-fullerene acceptors (NFA) based Inverted OPVs. High-performance NFA inverted OPVs incorporating solution processed metal oxide hole selective contact (HSC) providing similar PCE and light stability  performance to that achieved with the commonly used thermally evaporated MoO₃ HSC will be presented [4]. Furthermore, the presentation will highlight the systematic understanding of the relationship between doped metal-oxide and Ag nanowires (NWs) interfaces that allowed the development of Ag-NWs based bottom transparent electrodes for inverted configuration OPVs with simplified processing [5]. Finally, laser printed nanoparticle-based metal grids for the development of  efficient ITO-free Inverted OPVs with up-scalability perspectives will also be presented [6].

ZnO has been widely used as an electron transport layer (ETL) in organic photovoltaics (OPVs) to facilitate photo-generated electrons extraction due to its appropriate band alignment with various active absorbing layers. Typically, ZnO ETLs are prepared by solution-based techniques such as spin coating. However, with spin coating, it is difficult to control the oxygen stoichiometry, which has great impacts on the defects formation and hence materials properties of ZnO. For example, carrier mobility and optical transparency would affect charge transport and light utilization of the active layer. Moreover, surface defects at the interface between ZnO and the active layer could have great impacts on the stability of the devices.

In this work, we deposited ZnO thin films by radio frequency magnetron sputtering and studied the effects of deposition conditions on the materials properties of the ZnO. Furthermore, the sputtered ZnO were integrated in OPVs with a non-fullerene acceptor-based absorber, PM6:Y7. Our results show that sputtered ZnO on indium tin oxide (ITO) coated glass are polycrystalline and have high optical transparency of >85% in the visible region, which is comparable to a bare ITO glass substrate. Substrate temperature, oxygen flow rate ratio of the sputtering gas ([O₂]/[Ar+O₂]) and thickness of the ZnO all have significant effects on the OPV device performance. The roles of oxygen stoichiometry on OPV device performance were investigated by photoemission spectroscopy and J-V measurements. Our results indicate that a higher [O₂]/[Ar+O₂] effectively reduces the concentration of oxygen-related defects, resulting in improved device performance. Utilizing a two-step sputtering process to suppress interfacial defects, solar cells using optimized sputtered ZnO deliver a higher power conversion efficiency than using spin-coated ZnO. Lifetime measurements under controlled environments were carried out to evaluate the stability of the devices. The difference in stability is explained by the ZnO/PM6:Y7 interfacial quality, and can be related to the difference in defect concentration at the ZnO surface.

In summary, we have investigated the effects of different sputtering conditions of ZnO ETLs on PM6:Y7 device performance. Devices using optimized sputtered ZnO ETLs outperform those using spin-coated ZnO ETLs. Moreover, our results suggest that the tuning of ZnO surface defects through the manipulation of oxygen stoichiometry plays a crucial role on the interfacial quality between ZnO ETL and the active layer, which in turn can be used to optimize the interface stability and device lifetime. Our findings highlight the potential of sputtered ZnO as a promising ETL in highly efficient non-fullerene acceptor based organic photovoltaics.

This research involved the creation via the coaxial electrospinning process of flexible core/shell nanofibrous mats with high porosity that can serve as alternative porous structures to the brittle porous TiO₂-based photoelectrodes currently in use. The core was made from polyethylene terephthalate (PET), while the shell consisted of a PET-Titanium dioxide (PET-TiO₂) nanocomposite. Two different shapes of TiO₂ (nanobars and nanorhombics) were synthesized via the solvothermal method and then incorporated at various concentrations (0-20 wt.%) into the shell layer of the nanofibers to enhance their optical, thermal, and mechanical properties. The study investigated the surface morphology, porosity, specific surface area, thermal stability, dye adsorption, optical, and mechanical properties of the developed nanofibrous mats. Particular emphasis was placed on understanding the impact of solvent miscibility and solvent composition on the morphology of the fibrous mats. Various characterization techniques confirmed the successful incorporation of TiO₂ nanoparticles into the shell layer of the nanofibers. It was observed that adding up to 15 wt.% TiO₂ within the shell layer enhanced light harvesting and shifted the UV-Visible absorption edge of the nanofibrous mats to larger wavelengths. Moreover, PET/(PET-TiO₂) core/shell nanofibrous mats obtained with nanobars TiO₂ showed higher thermal stability due to a better TiO₂/PET interfacial interaction. Photoluminescence results confirmed the efficient charge transfer between TiO₂ nanoparticles and PET with a minimum recombination of the photogenerated charges. Furthermore, the high porosity of the coaxial electrospun mats, ranging from 75% to 81%, had a positive impact on the diffusion of dye molecules and their subsequent adsorption on the surface of the mats.

Finally, we believe that this detailed study on PET-TiO₂ core-shell nanofibrous structures provides new insight for further development of optimum DSSC photoanode microstructure at low temperature and contribute towards achieving efficient flexible DSSCs.

The performance optimization of organic solar cells is influenced not only by the design of the molecular structures but also by the regulation of the blend morphologies. Currently, there is a well-established understanding of how material design can regulate energy levels, film absorption, carrier mobility, and molecular crystallinity, leading to efficient device performance through the aggregation regulation of active layers. However, there is a lack of effective methods and investigations on the regulation of the metastable state of the active layer, the analysis of degradation mechanisms, and the improvement of operational stability. At present, organic solar cells are not only facing the basic problem of unclear metastable decay mechanism, but also facing the important challenge of synergistic development of "efficiency and stability". The development of physical and chemical methods of metastable regulation is an important breakthrough point for future work. On the basis of levelized cost of electricity analysis, this report will briefly indicate the performance requirements of organic photovoltaic materials and a brief analysis of performance improvement strategies and introduce the metastable morphology characteristics of active layer. By analyzing and revealing the attenuation mechanism of metastable morphology, combined with the intrinsic characteristics of small molecule materials, this report is used to understand the morphologic changes of the active layer under light/thermal stress, and explore the physical attenuation dynamics. Finally, the recent progress made by our research group in regulating the metastable state through physical and chemical methods, as well as the coupling strategies employed. This report aims to provide theoretical support for a deeper understanding of the mechanisms behind metastable morphology attenuation, and to offer valuable references for fellow researchers in the field.

Abstract

Recently, non-fullerene acceptor based organic solar cells show a significant improvement in power conversion efficiency. However, rapid degradation occurs under illumination, particularly when photocatalytic metal oxide electron transport layers are used in these devices.[1,2] We introduced vitamin C (ascorbic acid) into the organic solar cells as a photostabilizer and systematically studied its photostabilizing effect on inverted PBDB-T:IT-4F devices. The presence of vitamin C as an antioxidant layer between the ZnO electron transport layer and the photoactive layer strongly suppressed the photocatalytic effect of ZnO that induces NFA photodegradation. Upon 96 h of exposure to AM 1.5G 1 Sun irradiation, the reference devices lost 64% of their initial efficiency, while those containing vitamin C lost only 38%. The UV–visible absorption, impedance spectroscopy, and light-dependent voltage and current measurements reveal that vitamin C reduces the photobleaching of NFA molecules and suppresses the charge recombination. This simple approach using a low-cost, naturally occurring antioxidant, provides an efficient strategy for improving photostability of organic semiconductor-based devices.

The synthesis of carbon nano-rings and related structures opens a new field in the chemistry of cyclic conjugated systems with structural, dynamic and optical properties different from their counterparts, the linear conjugated systems.They possess unique zise-dependent optoelectronic properties that make them ideal candidates for use in nanoelectronics and photovoltaics. For this reason, the simulation of photoinduced processes is a substantial and complementary contribution to a better understanding and control of these systems.

I will present the results of our study of the structure-dynamics-optical properties relationship in carbon nano-rings and other cyclic conjugation systems. We performed the simulation of photoinduced dynamics in them. It was possible to find quantum confinements, electronic delocalization, thermal fluctuations, structural deformations and vibrational couplings during the non-radiative relaxation process, which are responsible for the changes observed in the optical properties.

The correct description of these processes requires atomistic simulations of non-adiabatic molecular dynamics (i.e., more than the Born-Oppenheimer approximation) considering multiple coupled excited electronic states. Using the NEXMD package it is possible to perform the calculation of the electronic structure, “on the fly”, i.e., the energies, energy gradients and non-adiabatic
couplings of the different excited electronic states are calculated at each step of the dynamics. In addition, nuclear propagation is performed using the hybrid classical-quantum hybrid method of quantum jumps. This method identifies the electric couplings during nuclear propagation.

tbc

The efficiency of poly(3-hexylthiophene) (P3HT) – nonfullerene acceptor (NFA) based bulk heterojunction (BHJ) solar cells lacks considerably behind many other polymer donor:NFA systems. For reasons which are yet incomprehensible. Here, we report on a series of P3HT:NFA solar cells, and  elucidate the origin of performance losses in terms of the photophysical processes. It is a matter of fact that the interfacial ionization energy (IE) offset is a critical parameter in NFA-based blends in determining the efficiency of the exciton-to-charge transfer (CT) state conversion. We show that while large IE offsets in excess of >0.9 eV still facilitate complete exciton quenching, the device internal quantum efficiency (IQE) is limited by geminate and / or non-geminate recombination processes in P3HT-based photoactive blends. Our finding shows a drop in IQE when the diagonal bandgap of the photoactive blend i.e the difference between the IE of the donor and the electron affinity (EA) of the acceptor is small irrespective of the IE offset. Understanding the relationship between the IE offsets, EA offsets at the interface of donor and acceptor materials, and the performance of organic solar cells (OSC) could improve the charge generation efficiency. Thus, enables us to understand the relation between small diagonal bandgap and the decrease of the IQE in energy gap law framework.

Compared to single junction solar cells, multi-junction architectures can harvest a wider range of the sun´s spectrum while reducing thermalization losses, thus potentially leading to larger efficiencies [1, 2]. Regardless of the specific geometry, i.e. stack, spectral splitting or rainbow, material systems with band gaps wider and narrower than the single junction optimum should be developed to fully exploit the multi-junction concepts [2]. Importantly, the specific materials combination, as well as device parameters (e.g. thickness) are strongly correlated in a multiparametric optimization problem.

In this contribution, we will show our current strategy to tackle this challenge: combining high throughput material screening methods [3] with a spectrum on demand light source [4]. In particular, we focus on materials with narrow bandgap, for which we have screened more than 15 systems, and wide bandgap for which we investigated 9 material combinations and two solvents. Then, we use the best systems to build Rainbow solar cells, i.e. an in-plane spectral splitting geometry in which a series of sub-cells are placed next to each other laterally, and illuminated through an optical component that splits the incoming white beam into its spectral components, thus matching local spectrum and absorption for each sub-cell [5].

[1]        D. CarloRasi and R. A. J. Janssen, “Advances in Solution-Processed Multijunction Organic Solar Cells,” Advanced Materials, 31 (2019) 1806499.

[2]       I. M. Peters et al, “Practical limits of multijunction solar cells”, Prog. In Photovolt. 31 (2023) 1006.

[3]       A. Harillo-Baños et al, “Efficient Exploration of the Composition Space in Ternary Organic Solar Cells by Combining High-Throughput Material Libraries and Hyperspectral Imaging”, Adv. Ener. Mater., 10 (2020) 1902417.

[4]        M. Casademont-Viñas et al “Spectrum on demand light source (SOLS) for advanced photovoltaic characterization”, Rev. Sci. Instrum. 94 (2023) 103907.

[5]       M. Gibert-Roca et al, “RAINBOW Organic Solar Cells: Implementing Spectral Splitting in Lateral Multi-Junction Architectures” Adv. Mater (2023) 2212226.

Technological deployment of organic photovoltaics (OPV) requires improvements in device light-conversion efficiency and stability while keeping material costs feasible. Non-fullerene small molecule acceptors emerged as a new horizon pushing the boundaries of OPV and bring them back to competition with other solution processed photovoltaics with unique properties such as semitransparency and power per weight. The power conversion efficiency of OPV has reached 19% in just a couple of years after replacing fullerene alternatives. Non fullerene acceptors are inherently more stable than fullerene derivatives. However, they do still suffer from long term photo and thermal stabilities. There is an understanding gap where we look into acceptors and donot structure-proprty relationships to understand the performance-stability relationship.

In this talk, I will talk how NFAs exceeded the limits of fullerenes and share approaches to overcome photo and thermal stability issues. Finally, I will share state-of-the-art activities on transparent OPV and their scale-up and deployment in potential areas such as greenhouses.

Recent advancements in Organic Solar Cells (OSCs) have led to peak efficiencies of up to 19%, primarily due to the development of novel organic molecules for active and transport layers. Despite this progress, OSC technology still encounters commercialization challenges. These are largely attributed to the extensive range of potential materials and the intricate interplay between their structures and properties, which complicates efficiency predictions. Addressing this, the materials science field is increasingly utilizing machine learning (ML) to decipher structure-property correlations.

Our research addresses this challenge by exploring the relationship between the electrical properties of OSCs, such as Power Conversion Efficiency (PCE) and Fill Factor (FF), and their structural features, including band center position and crystallinity. These are determined through the deconvolution of UV-Visible (UV-VIS) spectroscopy data. By employing Gaussian Process Regression, we have devised a method that predicts solar cell efficiency using only UV-VIS spectra. This approach potentially eliminates the need for evaporation and IV measurements, streamlining the process and reducing costs.

To ensure predictive accuracy, we compiled a dataset of 25 donor-acceptor combinations, yielding a total of 200 cells with intentional variations in film thickness and crystallinity. We ensured exceptional homogeneity in layer preparation using the Autonomous Materials and Device Application Platform (Amanda) ¹. Analysis of this dataset enables us to foresee the performance of new material combinations. This significant breakthrough in OSC research could expedite the development of efficient and commercially viable solar cell materials.

Implementation of 2D materials in organic photovoltaics (OPV) is one of the promising routes to modify fundamental device properties, and potentially improve device efficiency and stability due to their unique properties such as tunable electronic structure, high charge carrier mobility and high optical transparency. For MXenes, a family of 2D transition metal carbides and nitrides, the ability to tune work function via surfaces termination routes have made it especially promising as contact layer material in solar cell devices. In this work, we employ such 2D MXene, Ti₃C₂T_x, in electron transport layers (ETL) to develop composite 2D based ETL, and demonstrated their high performance for non-fullerene acceptor (NFA) based OPV. The composite 2D-ETL based OPV resulted in robust ETL-organic interfaces with efficient electron extraction and transport properties for high device performance (~14% using green solvents), and importantly, significantly prolonged device lifetime when compared to conventional 2D-free ETL based OPV. The integration of the 2D MXene interlayers in OPV is explored in terms of morphological, optical, and electrical properties along with ISOS-L device lifetime measurements. Importantly, photoelectron spectroscopy results provide insights into the modified surface defect state concentration on the ETL surface, which can directly be related to the improved device stability. The work demonstrates the unique potential of MXene as contact layer material in high performance organic photovoltaics with prolonged device lifetimes.

While multi-junction geometries have the potential to boost the efficiency of organic solar cells, the experimental gains yet obtained are still very modest.[1,2] This work proposes an alternative spectral splitting device concept in which various individual semiconducting junctions with cascading bandgaps are laid side by side, thus the name RAINBOW. Each lateral sub-cell receives a fraction of the spectrum that closely matches the main absorption band of the given semiconductor. Here, simulations are used to identify the important material and device properties of each RAINBOW sub-cell. Using the resulting design rules, three systems are selected, with narrow, medium, and wide effective bandgaps, and their potential as sub-cells in this geometry is experimentally investigated. With the aid of a custom-built setup that generates spectrally spread sunlight on demand, the simulations are experimentally validated, showing that this geometry can lead to a reduction in thermalization losses and an improvement in light harvesting, which results in a relative improvement in efficiency of 46.6% with respect to the best sub-cell. Finally, a working proof-of-concept monolithic device consisting of two sub-cells deposited from solution on the same substrate is fabricated, thus demonstrating the feasibility and the potential of the RAINBOW solar cell concept.

Organic Photovoltaics have seen a significant increase in power conversion efficiency (PCE) recently, approaching 20% on small lab cells. However, the record efficiencies on module level are still substantially lower (~30%, relatively). Hence, it is necessary to perform focused upscaling research to reduce the gap between small-area cells and large-area modules. In this work, we present successful upscaling of devices based on PM6:Y6-C12:PC₆₁BM, processed in air from non-halogenated solvents, from cells (0.1 cm²) to large-area modules (>200 cm²) with barely any performance losses. High PCEs of >14% on total module area (>15% with respect to active area) are achieved by utilizing accelerated blade coating to eliminate thickness gradients in the layers. Inactive (interconnect) areas are significantly reduced by high-resolution short-pulse (nano- and femtosecond) laser patterning to achieve high geometric fill factors exceeding 97%. The results are confirmed by independent certified measurements.Organic Photovoltaics have seen a significant increase in power conversion efficiency (PCE) recently, approaching 20% on small lab cells. However, the record efficiencies on module level are still substantially lower (~30%, relatively). Hence, it is necessary to perform focused upscaling research to reduce the gap between small-area cells and large-area modules. In this work, we present successful upscaling of devices based on PM6:Y6-C12:PC₆₁BM, processed in air from non-halogenated solvents, from cells (0.1 cm²) to large-area modules (>200 cm²) with barely any performance losses. High PCEs of >14% on total module area (>15% with respect to active area) are achieved by utilizing accelerated blade coating to eliminate thickness gradients in the layers. Inactive (interconnect) areas are significantly reduced by high-resolution short-pulse (nano- and femtosecond) laser patterning to achieve high geometric fill factors exceeding 97%. The results are confirmed by independent certified measurements.

The pursuit of more efficient buildings is driving digitalization, leading to a rapidly growing need for connected sensors. These sensors typically rely on batteries, necessitating costly maintenance to replace them every few years and contributing to a significant environmental impact.

By harnessing ambient energy, the lifespan of batteries can be extended, and in many cases, batteries may become unnecessary. Circumventing the issues with batteries will enable much easier deployment of distributed sensing, facilitating data collection that, in turn, yields a more energy-efficient society. Light is a primary source of ambient energy in buildings, and organic solar cells can be tuned to harvest that light energy with high efficiency. Examples of applications for connected devices suitable for light energy harvesting include electronic shelf labels, asset tracking labels, and consumer electronics such as remote controls.

Epishine, an innovative Swedish company, has developed a scalable manufacturing process for organic solar cells using roll-to-roll printing and lamination technologies. This technology puts Epishine in a favorable position to address the increasing demand for high-performance indoor solar cells. In this presentation, I will provide an overview of Epishine's technology and manufacturing process, as well as present some of Epishine’s internal research efforts based on our unique approach to producing OPV [1]

Excited triplets of organic molecules have the interesting features of long excitation lifetime and spin polarization. Among various functions of excited triplets, our recent works on the development of chromophore assemblies exhibiting photon upconversion and singlet fission will be presented.

Triplet-triplet annihilation based photon upconversion (TTA-UC) from visible light to UV light is useful for various photocatalysts including artificial photosynthesis, but its low efficiency (<10%) has been a problem for several years. We have developed a chromophore pair that exhibits a high TTA-UC efficiency of over 20%, which is two times higher than the previous record for Vis-to-UV TTA-UC. The Ir coumarin complex shows strong absorption in the visible range and relatively weak absorption in the UV range and does not quench the upconverted UV emission. By combining the Ir coumarin complex with TIPS-naphthalene, we have achieved a high TTA-UC efficiency of over 20% and a very low threshold excitation intensity I_th below solar irradiance.^1,2 Furthermore, we have recently developed bicontinuous porous monoliths showing visible to UV TTA-UC and will introduce the details in the talk.

We are also interested in the spin degree of freedom of singlet fission in the context of quantum information science (QIS).³ We developed chromophore assemblies and dimers exhibiting singlet fission, and successfully observed quantum coherence of quintet triplet dimer state at room temperature for the first time. We found that the suppressed chromophore dynamics is crucial for achieving both of quintet formation and quantum coherence.

Photon upconversion offers the prospect of combining low energy near-infrared radiation to make high energy photons in the visible wavelengths. The goal is to improve photovoltaics and introduce new bioimaging techniques. In order to harness the intrinsic ability of colloidal semiconductor nanocrystals to couple strongly with light, it is important to efficiently outcouple energy from photoexcited quantum dots (QDs), much like how nature uses molecular antennas to direct light during photosynthesis. This talk focuses on the organic-inorganic interface for triplet-fusion based photon upconversion, where orbital overlap between the QD donor and molecular acceptor is critical for efficient energy transduction. In the past 8 years, we have learnt a great deal about how the electronic interactions between chalcogenide nanocrystals and acene conjugated molecules affect the photon upconversion quantum yield. We apply the lessons learnt to extending photon upconversion to silicon nanocrystal light absorbers, addressing synthetic challenges with invaluable insight from time-resolved spectroscopy.

Quantum dot-organic hybrids have become popular as triplet-photon converting systems, for example as triplet sensitizers for triplet-triplet annihilation photon upconversion and photon multiplication based on singlet fission.[1] The organic ligand in such hybrid structures plays a crucial role in mediating triplet exciton transfer between the organic molecules and inorganic quantum dots in solution, and many studies have studied the intricate details of triplet mediation via the ligand.[2] Here I will higlight another aspect of the ligand, namely as a directing agent in bulk organic-semiconductor/quantum-dot blend films. These blends, comprising organic semiconductors and inorganic quantum dots  are relevant for many optoelectronic applications and devices. However, the individual components in organic-QD blends have a strong tendency to aggregate and phase separate during processing to form films, compromising both their structural and electronic properties. Here, I will explain how a suitable organic ligand can achieve well-dispersed inorganic-organic blend films, as characterised by X-ray and neutron scattering and electron microscopy. As proof-of-concept I will show two examples of these films applied to singlet fission based photon multiplication[3] and triplet-triplet annihilation based photon upconversion. Due to the optimal blend morphology triplet excitons can be transferred with near unity efficiently across the organic-inorganic interface, while the TIPS-tetracene films maintain efficient SF (190% yield) in the organic phase, resulting in 95% of the triplet excitons generated via SF in the organic phase being harvested by the QDs. By changing the relative energy between organic and inorganic components yellow upconverted emission is observed upon 790 nm NIR excitation. Overall, this talk will exemplify a highly versatile approach to overcome long-standing challenges in the blending of organic semiconductors with QDs with relevance for many optical and optoelectronic applications.

The conversion of low-energy light into photons of higher energy based on sensitized triplet–triplet annihilation (sTTA) upconversion in bicomponent systems is emerging as the most promising wavelength-shifting methodology to recover sub-bandgap solar photons, because it operates efficiently at excitation powers as low as the solar irradiance. Excellent efficiencies have been obtained in liquid environments as well as in prototype upconversion enhanced solar cells, but the research is still focused on the realization of affordable solid states upconverters suitable to be implemented in current solar technologies. We show here that controlled confinement of the upconverting materials in nanostructured or nanosized materials can improve the material performance at low powers. [1] [2] [3] [4] The result presented will show demonstrate how this strategy can represent a crucial guideline for the future development of upconverting photonic devices operating at subsolar irradiances suitable for technological implementation.

[1]  Mattiello et al. Diffusion-free intramolecular triplet–triplet annihilation in engineered conjugated chromophores for sensitized photon upconversion. ACS Energy Letters 7 (8), 2435-2442 (2022)

[2] A.Ronchi & A. Monguzzi; Developing solid-state photon upconverters based on sensitized triplet–triplet annihilation. J. Appl. Phys. 129 (5), 050901 (2021)

[3] Saenz et al. Nanostructured Polymers Enable Stable and Efficient Low‐Power Photon Upconversion, Adv. Funct. Mater. 31, 1, 2004495 (2021) 

[4] Meinardi et al. Quasi-thresholdless photon upconversion in metal–organic framework nanocrystals, Nano Letters 19 (3), 2169-2177 (2019)

The upconversion of light has numerous potential applications ranging from photocatalysis, photovoltaics, imaging, and advanced manufacturing. The strategies being investigated to achieve this include energy transfer upconversion in lanthanide nanoparticles, and sensitized triplet fusion, also known as photochemical upconversion or triplet-triplet annihilation upconversion (TTA-UC). The latter involves the generation of triplet excited states by a sensitizer and their subsequent transfer to an emitter. Emitter triplets annihilate (fuse) to generate an emissive singlet state from which a higher energy photon is emitted. This strategy is spectrally adaptable, and high efficiencies are routinely achieved in solution, approaching the quantum yield ceiling of 50%.

Recently, there has been progress in translating triplet fusion upconversion into the solid state, which is desirable from a device engineering standpoint.[1] There are many possible solid-state device architectures. Broadly these can be categorized into materials wherein the sensitizer and emitter are evenly distributed, and heterogeneous devices where triplet generation is spatially separated from triplet fusion. The latter strategy is attractive, as the upconverted singlet state is protected to an extent from Förster resonance energy transfer back to the sensitizer. A simple concept would be to lay down a monolayer of sensitizer overlaid with a thin film of emitter. However, such devices suffer from poor ligh absorption.

In dye-sensitized solar cells (DSSCs), the problem of poor light absorption by a monolayer of chromophores is addressed with a nanostructured metal oxide support, such that incoming light passes through a multitude of chromophore-bearing interfaces. Indeed, DSSCs incorporating TTA have been demonstrated.[2]  As there are many steps in photochemical upconversion, it is desirable to investigate certain aspects in isolation. To concentrate on the solid-state sensitization process, here we present a strategy where the sensitizer is chemisorbed on a nanoporous Al₂O₃ support, but the emitter remains in solution.[3]

Using time-resolved spectroscopies, we show that the sensitizer triplets are long-lived, do not suffer significant aggregation-induced quenching and can be efficiently transferred to the emitter chromophores. We demonstrate devices with internal upconversion quantum yields as high as 9.4%, which is 19% of the maximum achievable value and 38% of what can be achieved with the diphenylanthracene emitter. This shows that using dyes bound to nanoporous Al₂O₃ films is a viable strategy to sensitize upconversion within a solid state material, and bodes well for future development with solid state emitters. Initial work on liquid-chromophore and fully solid state systems based on the same Al₂O₃ films will also be presented.

Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, an ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV) is demonstrated in an OLED device. This OLED reaches 100 cd/m2, which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and the triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons. We found that the essential component for efficient blue emission is a smaller energy difference between the CT state and triplet exciton, accelerating the energy transfer between the two states and achieving the optimal performance by avoiding direct decay from the CT state to the ground state. Our study demonstrates that the developed OLED allows for a much longer operation lifetime than that from a typical blue phosphorescent OLED because the blue emission originates from a stable low-energy triplet exciton that avoids degrading the constituent materials.

The development of green, sustainable, and economical chemical processes represents a cornerstone challenge within chemistry today. Semiconductor heterogeneous photocatalysis is currently utilized within a wide variety of societally impactful processes, spanning reactions such as hydrogen production and CO2 conversion, to the organic transformation of raw materials for value-added chemicals. Metal halide perovskites (MHPs) have recently emerged as a new promising class of cheap and easy to make photocatalytic semiconductors, though their unstable ionically bound crystal structure has thus far restricted widespread application. Recently years, we examine the issues hampering MHP-based photocatalysis and proposal a general approach being taken to achieve promising and stable photocatalytic reaction environments. Specifically, we utilize relatively low-polarity solvents such toluene and trifluorotoluene for CO2 photoreduction and organic transformations over MHPs. However, the effectiveness of single MHP photocatalysts is relatively poor. We further introduction of a second component to form a heterojunction, including Schottky, type II, and Z-scheme heterojunction, to accelerate carrier migration and boost reaction rates, thus increasing the photoactivity.

Lead halide perovskites (LHPs) have demonstrated enormous potentials in a wide range of application including optoelectronic devices and photocatalysis due to their exceptional optical and electronic properties. However, long-term stability and toxicity of LHPs are the main limiting factor towards their practical application. In this view, lead-free halide double perovskites (DPs)_, are under spotlight due to their superior material stability and attractive optoelectronic properties. Nevertheless, relatively larger band gap (> 4eV) of Cs₂AInCl₆ (A = Ag Na) DPs materials are the main bottlenecks towards their light harvesting application including photocatalysis.^[1,2] Moreover, the severe charge recombination and presence of fewer catalytic active sites are considered as main limiting factor towards efficient photocatalytic CO₂ reduction.^[3] The doping and compositional engineering of Cs₂AInCl₆ (A = Ag, Na) DPs are under spotlight due to their dopant-induced extended light harvesting abilities and enhanced self-trapped excitons (STEs) emission for a wide range of applications including optoelectronic devices and photocatalysis.^[1,2,4,5] Furthermore, the catalytic performances of lead-free DP NCs can be further improve by coupling them with two-dimensional (2D) nanosheet (NSs) with desirable energy offset. The  type II heterojunction between DP NCs and 2D NSs can enhance the charge separation and inhibit the charge recombination. It is well established that 2D ultrathin g-C₃N₄ NSs possess large surface area, rich density of catalytic active sites, and superior charge transport. All these features are advantageous for photocatalytic CO₂ reduction. In particular, the interface between Cs₂AgBiBr₆ DP NCs/ g-C₃N₄ 2D NSs would significantly facilitate the charge transfer and separation of photogenerated excitons.

In this talk, I will present the colloidal synthesis of iron-doped Cs₂AInCl₆ (A = Ag, Na) DP NCs, which resulted in a significant extension of absorption edge into the visible part of spectrum. Furthermore, I will discuss how iron doping allows precise tuning of the optical band gap and electronic band structures of the resulting DP NCs and their application in photocatalytic CO₂ reduction. Next, I will also explore our recent results on multifacet spheroidal Cs₂AgBiBr₆ DP NCs/g-C₃N₄ NSs heterostructure, including colloidal synthesis, optical properties and their application in visible-light driven photocatalytic CO₂ reduction.

Lead halide perovskite nanocrystals (LHP NCs) are a state-of-the-art light-harvesting material, prized for their outstanding optoelectronic properties—high absorption coefficient, extensive charge carrier diffusion lengths, and low exciton binding energy. Despite achieving remarkable photovoltaic performance with a photoconversion efficiency exceeding 25% within a decade, the transition of LHP to industrial-scale development encounters obstacles due to moisture intolerance and ambient instability [1].

The inorganic/hybrid core of LHP NCs contributes to their unique photophysical properties, while surface ligands play a crucial role in stabilizing and influencing charge transfer dynamics in diverse environments [2,3]. Therefore, understanding the complex NC-ligand interfacial chemistry is vital for achieving colloidal stability in aqueous and polar media.

Our solution involves optimizing hydrophilic and hydrophobic interactions on the LHP NC surface using engineered multidentate ligands. We developed NKE-12, a bolaamphiphilic ligand derived from lysine (K) and glutamic acid (E), featuring multiple cationic (NH₃⁺) and anionic (COO^-) anchoring groups. Applied in a ligand exchange treatment on CsPbBr₃ NCs, NKE-12 achieved effective surface passivation and water molecule localization away from the inorganic core through hydrophilic interactions [4]. Confirmation of successful ligand exchange was obtained through Fourier transform infrared and X-ray photoelectron spectroscopy. Structural integrity was preserved and validated by transmission electron microscopy and X-ray diffraction patterns. NKE-12 binding interactions on the NC surface were elucidated through multidimensional nuclear magnetic resonance ligand spectroscopy [5].

Deconstructing NKE-12 into NK-12 and NE-12 fragments provided insights into the individual roles of cationic and anionic terminal ends in passivation and solvent phase transfer. The synergistic effect of robust ligand binding and increased hydrophilicity resulted in significant two-week-long water stability for CsPbBr₃/NKE-12 NCs compared to oleylamine/oleic acid capped NCs. Enhanced charge separation and transport capabilities observed through electrochemical measurements prompted the exploration of photocatalytic attributes. Water-stabilized CsPbBr₃/NKE-12 NCs exhibited excellent photocatalytic activity for acrylamide polymerization, showcasing their potential for diverse applications.

Presently, we are expanding ligand engineering, exploring different multi-ionic ligands to further extend water stability and deepen our understanding of photogenerated charge extraction processes. Our findings on ligand design principles for developing moisture-resistant perovskite NCs hold significant promise for applications in photovoltaics and photocatalysis.

Over the past decade, metal halide perovskites (MHP, CsPbX₃: X = Cl, Br, I) have been widely investigated as promising materials for optoelectronics, achieving a record-breaking efficiency in solar cells.^[1] From a fundamental point of view, MHP could be excellent candidates for photocatalysis due to their high photogenerated charge-carrier production and mobility as well as their narrow and tunable bandgap energy.^[2] MHP with tuneable bang-gap energy could be obtained through fast substitution of bromide by iodine or chloride (CsPbBr_3-yX_y : X = Cl, Br, I).^[3]

In this presentation, the electronic properties of MHPs were encapsulated by TiO_2, and their electronic properties were tuned by anion substitution at room temperature. Then, charge-carrier lifetime and dynamics were investigated in CsPbBr_3-yX_y along with interfacial electron transfer from CsPbBr_3-yX_y to TiO₂ by means of time-resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS). The results show that the bandgap engineering and the position of the conduction band and valence band level in both materials are detrimental for optimal interfacial charge transfer. In addition, the optimal bandgap configuration for the most efficient charge injection positively affects the photocatalytic activity. The encapsulation of MHPs by TiO₂ did improve the stability for hydrogen generation from aqueous solution.

Metal halide perovskites (MHPs) are extremely appealing emerging materials for photocatalytically active heterojunctions, thanks to their ability to promote light-driven reactions. However, a comprehensive understanding of photoexcitation and charge transfer mechanisms, as well as predictive models for photocatalytic performances are still missing. We present here a robust method to investigate photocatalytic activity of MHP heterojunctions in a variety of reactions, that combines in synergy a wide array of experimental techniques with advanced computational modelling. For all the studied compounds, we were able to assess a clear link between the photocatalytic activity and optoelectronic properties of the materials, in particular by exploiting ultrafast spectroscopy techniques to study available transitions and carrier dynamics as a function of the perovskite loading and finding a connection with the performance of the material.

More in detail, we studied heterojunctions obtained by combining graphitic carbon nitride (g-C3N4) with different kinds of lead-free perovskites to enhance selected features. We focused first on double perovskite Cs3Bi2Br9, where hydrogen photogeneration rate was successfully controlled by tuning the perovskite loading in the Cs3Bi2Br9/g-C3N4 mixture.[1] We also studied Ge-based layered 2D perovskite and g-C3N4 for light-induced hydrogen evolution: we showed how, thanks to organic cation engineering, perovskites based on 4-phenylbenzilammonium (PhBz), such as PhBz2GeBr4 and PhBz2GeI4, result in hydrogen production with promising air and water stability.[2] Recently, we have extended our study to a double perovskite mixture, namely Cs2AgBiCl6/g-C3N4, used both for solar-driven hydrogen generation and nitrogen reduction, with an activity strongly depending on the perovskite loading.[3] Through advanced spectroscopic investigation and density function theory (DFT) modelling we have identified the perovskite loading that allows the best performance of the heterojunction and, most importantly, accounted for the microscopic processes responsible for the photocatalytic performance. Our results are showing a systematic approach of MHP-based heterojunctions that is of crucial importance to get the ability to engineer and optimize novel materials for photocatalysis.
 

Metal halide perovskites are arising a great interest of the scientific community because of their opto-electronic properties, as the high extinction coefficient, the low exciton binding energy, the excellent charge transport properties and optimal band gap in the visible range that make them good candidates for photocatalytic applications. Among the different perovskite materials, Cs₃Bi₂Br₉ (CBB) is a particularly interesting lead-free halide perovskite that shows excellent stability and has already been used as a visible light-driven photocatalyst in dye degradation experiments. However, the understanding of the interaction between dyes and the CBB and the mechanism underlying the behavior of the CBB in the photocatalytic system still needs to be fully elucidated [1]. Here, the chemical co-precipitation method was successfully applied to synthesize CBB powder that was then used as photocatalyst for the dye degradation. XRD investigation showed that the prepared Cs₃Bi₂Br₉ powder materials undergoes to structural evolution when exposed to different environment (water, isopropanol, air). Namely, we detected a systematic structural evolution of Cs₃Bi₂Br₉ to the kinetically stable non-perovskite BiOBr phase. In addition, such a transformation was found faster in water, where the conversion in BiOBr was already complete within 24 hours, than in isopropanol and in ambient air [2]. On the basis of the proved CBB higher stability in isopropanol, such a solvent has been selected as reaction medium for the heterogenous photocatalytic experiments carried out monitoring the decoloration of structurally different azodyes, cationic Methylene Blue (MB) and anionic Methyl Orange (MO), at different concentration. Preliminarily, the adsorption of each dye on the CBB under dark condition has been spectroscopically monitored to clarify the interaction between dyes and CBB powder samples. Finally, the photocatalytic experiments have been performed monitoring the dyes decoloration as a function of the catalyst conversion of the Cs₃Bi₂Br₉ in BiOBr, finally proving the beneficial effect of BiOBr in the degradation of both types of cationic MB and anionic MO dyes [3].

Photocatalytically active heterojunctions based on metal halide perovskites (MHPs) are drawing significant interest for their chameleon ability to foster several redox reactions. The lack of mechanistic insights into their performance, however, limits the ability of engineering novel and optimized materials. Herein, we report on a composite system including a double perovskite, Cs2AgBiCl6/g-C3N4, used in parallel for solar-driven hydrogen generation and nitrogen reduction. The composite efficiently promotes the two reactions, but its activity strongly depends on the perovskite/carbon nitride relative amounts. Through advanced spectroscopic investigation and density function theory modelling we studied the H2 and NH3 production reaction mechanisms, finding perovskite halide vacancies as the primary reactive sites for hydrogen generation, withstanding a positive contribution of low loaded g-C3N4, in reducing carrier recombination. For nitrogen reduction, instead, the active sites are g-C3N4 nitrogen vacancies, and the heterojunction best performs at low perovskites loadings, as the composites maximizes light absorption and reduced carrier losses. We believe these insights are important add-ons towards universal exploitation of MHPs in contemporary photocatalysis.

Photocatalytic conversion of water and carbon dioxide using solar energy offers a clean solution to the world energy requirements of a sustainable future. Achieving its full potential depends on developing inexpensive photocatalysts that can efficiently absorb solar light and drive separated photoinduced charges to react with water and carbon dioxide. In this talk, I will present recent developments we have achieved in the preparation of inexpensive photocatalyst composites with enhanced charge transfer separation and photocatalytic performance. For example, I will present fast and convenient mechanochemical syntheses of halide perovskite nanocrystals of CsPbBr₃ and Cs₂AgBiBr₆, together with copper-loaded reduced graphene oxide. I will also present our developed antisolvent crystallizations to prepare composites such as Cs₂AgBiBr₃/bismuthene and Cs₃Bi₂Br₉/g-C₃N₄, all of which are active in the reduction of CO₂ to CO and CH₄. Extensive characterizations of these materials have enabled us to gain insights into their physical and charge-transfer properties and relate them to their photocatalytic activity. These characterizations underline the importance of surface area, reactant adsorption, and charge separation to achieve best performances. Some of these composites such as Cs₃Bi₂Br₉/g-C₃N₄ separate charges via a direct Z-scheme heterojunction, boosting charge separation and photocatalytic conversions.  Altogether, these results will contribute to the rational design and application of halide perovskites for solar fuels and chemicals.

Halide perovskite solar cells have revolutionized the photovoltaic field in the last decade. Nevertheless, 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. Beyond perovskite solar cells halide perovskite are outstanding for the development of other optoelectronic device, causing that currently the research with these materials widespread to different optoelectronic fields and photocatalytic applications, where again stability and reduction Pb content still at the center of research effort. In the present talk, we will describe our efforts towards the application of these materials for solar-driven hydrogen production coupled to other processes like organic transformations and waste valorization. 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. The need for integration of the developed materials into tailored photoelectrochemical devices highlight the urgent need for stabilization strategies to move beyond the proof-of-concept stage to relevant technological developments.

Metal halide perovskites nanocrystas (NCs) have promising applications as heterogeneous photocatalysts due to their strong visible-light absorption, band-gap engineering, charge transfer ability and easy recovery. There are several factors that influence the photocatalytic performance, such as NCs sizes, co-catalyst nature, solvent, and atmosphere. Although there are several examples in their use as photocatalyst for the CO₂ reduction and H₂ generation a more limited information is available for organic chemical transformations. Here, the photocatalytic activity of colloidal CsPbBr₃ NCs and Cs₃Sb₂Br₉ NCs for the photoreduction of p-substituted benzyl bromides under visible-light excitation, using a sacrificial electron donor (N,N-diisopropylethylamine or methanol), will be discussed. Interesting the electron donating and electron withdrawing ability of the substituent determines the selectivity of the reaction towards C-C formation or reductive dehalogenation. The role of the organic ligands to drive forward the C-C coupling and the affinity of the benzyl radicals to the metal halide NC surface will be illustrated.

The chemical instability of halide perovskites (HaP) in protic solvents remains the main obstacle to their utilization in photoelectrochemical devices. Despite the implementation of protective strategies for PEC applications[1], [2], systematic studies of (photo)electrochemical processes and the potential corrosion of perovskite/electrolyte interfaces and their impact on energy conversion and stability are still lacking [3].

In this study, we examined the photoelectrochemical behavior of 2D di-phenylethylammonium lead tetra iodide (PEAH₂PbI₄) thin films as a model system to study fundamental photoelectrochemical processes of metal halide perovskites. We found the use of excess PEAI in solution necessary to stabilize the thin films like by chemical equilibrium and a surfactant effect. Introducing the Fe(CN)₆ ^3-/4- redox couple electroactive species and considering PEAI a supporting electrolyte and stabilization agent [4],[5], we studied photo-electrochemical processes at the semiconductor/electrolyte interface by monitoring the open-circuit potential (OCP). Under illumination, a potential inversion was observed for different equilibrium concentrations of the redox probe. In-situ UV-Vis and XRD analyses provided compelling evidence indicating that the potential inversion, indicating either electron or hole accumulation on the PEAH₂PbI₄, leads to different photo-corrosion phenomena. Lead iodide, PbI₂, as well as a phenylethylamine-lead iodide intercalation compound, (PEA)-PbI₂ , are observed as degradation products. Surface photovoltage (SPV) studies revealed that the redox probe ratio determines carrier separation dynamics at the 2D perovskite / electrolyte interface. Based on the SPV results, we conclude that electron transfer to the redox couple is a critical step in suppressing photodegradation of the material in aqueous solutions. These findings shed light on the intricate interplay between carrier dynamics and chemical changes in the context of semiconductor/electrolyte interfaces, providing valuable insights for the development of design strategies to effectively utilize halide perovskites as photoelectrodes. In addition, our findings provide fundamental insight into the photo-corrosion processes of HaPs, which are currently of high interest for many applications foremost solar cells.

Hybrid AMX₃ perovskites (A=Cs, CH₃NH₃; M=Sn, Pb; X=halide) have in the last years revolutionized the scenario of photovoltaic technologies. Despite the extremely fast progress, the materials electronic properties which are key to the performance are relatively little understood. We developed an effective GW method incorporating spin-orbit coupling [1] which allows us to accurately model the electronic, optical and transport properties of halide perovskites, opening the way to new materials design. In parallel, a series of computational simulation carried out using Car-Parrinello molecular dynamics have been performed investigating the nature of the perovskites/TiO₂ interface, the role of moisture in the perovskite degradation process and the effect of the defect on the device working mechanism. Finally, a series of different strategies will be reported to increase the device stability and efficiency.[2] While instability in aqueous environment has long impeded employment of metal halide perovskites for heterogeneous photocatalysis, recent reports have shown that some particular tin halide perovskites (THPs) can be water-stable and active in photocatalytic hydrogen production. To unravel the mechanistic details underlying the photocatalytic activity of THPs, we compare the reactivity of the water-stable and active DMASnBr₃ (DMA = dimethylammonium) perovskite against prototypical MASnI₃ and MASnBr₃ compounds (MA = methylammonium), employing advanced electronic–structure calculations. We find that the binding energy of electron polarons at the surface of THPs, driven by the conduction band energetics, is cardinal for photocatalytic hydrogen reduction.[3] In this framework, the interplay between the A-site cation and halogen is found to play a key role in defining the photoreactivity of the material by tuning the perovskite electronic energy levels. Our study, by elucidating the key steps of the reaction, may assist in development of more stable and efficient materials for photocatalytic hydrogen reduction. The overall picture of our theoretical investigations underlines a crucial role of computational investigation, casting the possibility of performing predictive modeling simulations, in which the properties of a given system are simulated even before the materials laboratory synthesis and characterization. At the same time, computer simulations are shown to offer the required atomistic insight into hitherto inaccessible experimental observables.

Two-dimensional hybrid perovskites are a highly intriguing class of materials, composed of alternating inorganic and organic molecular layers. Their reduced dimensionality combined with weak dielectric screening leads to the formation of tightly bound excitons that efficiently absorb and emit radiation. A central questions for excitons in perovskites from the perspectives of both fundamental physics and applications is their mobility. In addition, the external control of their optical response has proven difficult due to challenges to introduce electrical doping into this class of materials.

In this talk I will focus on studies of optically detected exciton transport in 2D perovskites via ultrafast microscopy, discussing different regimes of propagation including free and localized states. I will show how the excitons behave at the structural phase transition and how their transport is determined by strong interactions with the lattice vibrations. In the second part, I will illustrate how the formation of electrically tunable trions, bound states of excitons in free carriers can be achieved in these hybrid semiconductors to optically detect doping. The trions in 2D perovskites are stable for both n- and p- doped regimes, feature unusually large binding energies and a substantial mobility at elevated temperatures.

Two-dimensional (2D) metal halide 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.

In this work, we employ ultrafast optical and terahertz spectroscopy to investigate the exciton formation dynamics, the charge-carrier mobilities, and the charge-phonon couplings of 2D Ruddlesden-Popper perovskites (PEA)₂PbI₄ and (BA)₂PbI₄ (where PEA is phenylethylammonium and BA is butylammonium). We reveal band transport with high in-plane mobilities that give rise to efficient long-range conductivity. 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.

Metal-halide perovskite materials demonstrated extraordinary performance in solar cells and light emission in recent years, and their layered low-dimensional counterparts promise even greater tunability due to the huge variety of molecules available for the organic phase. [1, 2] Single octahedra-layer structures act as two-dimensional quantum wells, showing strong confinement and large exciton binding energies. Band gap and light emission can be designed by the choice of the organic cations.[3-5]

We investigate the emission from single plates of two-dimensional Ruddlesden-Popper metal-halide perovskites with different organic cations by angle-resolved polarized photoluminescence (PL) spectroscopy. The choice of the organic cation strongly influences the number of PL peaks that we observe in the band edge emission, and the different PL peaks manifest a strong angle dependence in their linear polarization, manifesting an angle-dependent intensity maximum along one of the principal axes of the octahedra lattice. Such a marked intensity maximum along one in-plane direction is surprising, since theory predicts isotropic in-plane exciton wave function distributions [6]. Therefore, electron-phonon coupling could be at the origin of the distinct anisotropy of the emission polarization, since a large number of phonon bands in such materials show strong directionality. [7, 8] We investigate the angle resolved phonon response together with the emission polarisation of the single plates with resonant and non-resonant Raman spectroscopy, which allows for a detailed correlation of the directionality of the emission with the phonons. The combined angle-resolved emission and Raman spectroscopy allows for a detailed investigation of the exciton fine structure and assignment of confined exciton states and phonon side bands.

Layered perovskites are quasi-twodimensional crystals in which n layers of metal-halide octahedra are spatially separated by organic molecular layers. A wide variety of organic molecules can be used in the fabrication of these materials, allowing for variation of the interlayer distance and orientation, as well as functionalization, for example by introduction of chiral or electroactive organic molecules. Furthermore, chemical substitution at the metal and halide sites can drastically change the band gap size and symmetry in these materials. In this talk, I will discuss how chemical substitution affects the exciton fine structure of layered perovskites, focusing on metal site substitution and different organic interlayers. Our first-principles calculations based on the GW and Bethe-Salpeter Equation approaches demonstrate the importance of chemical, structural, and dielectric heterogeneity for controlling the nature and binding energy of excitons and the need for first-principles calculations to understand the electronic and excited state structure of this complex material class.

Perovskite-derived two-dimensional (2D) materials are emerging as an excellent combination to the three-dimensional (3D) metal halide perovskites for enhancing its stability.[1] In this 2D/3D approach, bulky ammonium organic cations are deposited on top of the 3D perovskite active layer forming a very thin coating of a low dimensional 2D perovskite.[2] 2D/3D inverted perovskite solar cells (PSCs) have achieved to retain >95% of their initial value after >1000 hours at damp-heat test conditions (85 ºC and 85% RH), while power conversion efficiencies (PCE) above 25% have been recently reported in 2D/3D PSCs.[3]

In this work, we investigate the deposition of two diammonium spacers with similar chemical composition (4,4′-Dithiodianiline, 2S, and 4,4’-ethylenedianiline, ET) but with a totally different molecular geometry on top of a 3D perovskite with Cs_0.09FA_0.91PbI₃ formula to manipulate the efficiency and stability of the PSCs. Our results demonstrate an improved PCE of 21% when using the 2S spacer in opposition to an inferior PCE of 16% in the cells covered with the ET spacer. The stability tests display no loss in the PCE upon constant illumination at RT at the MPPT when using the 2S spacer vs. a significant drop in the PCE for the cells with the ET spacer. This divergent behavior is ascribed to the formation of a parallel oriented layer of a 2D perovskite with the 2S spacer that facilitates the charge extraction in the PSC in opposition to the isotropically layer of a 1D perovskite detected upon addition the ET spacer.

Perovskite materials have gained attention in the field of photovoltaics due to their high conversion efficiency, tunable bandgaps, high charge carrier mobility, low exciton binding energy, and ease of synthesis [1]. Perovskite Solar Cells (PSCs) have demonstrated promising Power Conversion Efficiencies (PCE) of up to 25\%, comparable to the widely commercialized crystalline silicon counterparts. However, achieving highly efficient PSCs requires careful optimization of the perovskite absorbing layer and interfaces with charge-selective contacts.

This study focuses on interface passivation strategies, with a particular emphasis on the use of organic molecules for surface treatment. Organic molecules, especially those containing ammonium (NH3$^+$), show promise in passivating surface trap states [2-3]. The introduction of ammonium-based organic molecules in post-surface treatments leads to the formation of low-dimensional 2D perovskite structures, impacting the quantum confinement regime.

The investigation approaches the correlation between molecular flexibility and the formation of 2D/3D perovskite heterointerfaces, exploring their subsequent effects on solar cell performance. The study synthesizes two variations of a well-studied phenylethylammonium iodide (PEAI): a more rigid trans-2-phenylcyclopropylammonium iodide (PCPEAI) and a more flexible cyclohexylethylammonium iodide (CHEAI).

Detailed analyses, including SEM-Cathodoluminescence [4], reveal that the 2D phases present a heterogeneous distribution over the 3D surface. The study also highlights that the absence of $\pi$-$\pi$ stacking interactions and a higher degree of freedom in the more flexible CHEAI molecule prevent undesirable aggregation, resulting in improved device efficiency. The CHEAI-based devices exhibit a commendable efficiency of close to 21\%. The results indicate that molecular flexibility plays a crucial role in facilitating the formation of 2D phases.

This research contributes valuable insights into the intricate relationship between molecular flexibility, 2D/3D perovskite heterointerfaces, and PSC performance. The findings offer guidance for designing more efficient and stable perovskite solar cells, making progress in next-generation photovoltaic technologies.

Halide perovskites have garnered significant attention due to their unique properties and potential applications in optoelectronics and energy-related fields. However, the reduction in crystal dimension of perovskite from 3D to low dimensional structures introduces challenges, particularly regarding quantum and dielectric confinements, resulting in larger band gaps and exciton-binding energies. The low dimensional structures imply intrinsically bigger unit cells which makes challenging the application of standard density functional theory (DFT) calculation in terms of computational cost. Furthermore, DFT tends to significantly underestimate band gaps, which is extremely problematic for optoelectronic materials. Density Functional Tight-Binding (DFTB) is a flexible, semi-empirical method based on DFT which capable of simulating large system sizes and offers the possibility of accurate band gap prediction with low computational cost.^1,2

In this work, we highlight the application of DFTB methodology for studying the electronic structure, effective masses, and charge density localization in low-dimensional perovskite materials. By employing empirical fitting and parameterization, the DFTB method captures the electronic band structures of model 2D halide perovskites (e.g., Cs₂PbI₄, BA₂PbI₄, PEA₂PbI₄, and BA₂PbBr₄). We show good agreement between DFTB results and experimental electronic band gaps, as well as reduced effective masses. This first attempt is promising for further applications to other low-dimensional (1D, 0D, hollow) perovskite nanostructures or 2D/3D perovskite heterostructures with large sizes and complexity, which demonstrated excellent operational stability in solar cell architectures.

: A wide range of physical properties associated with layered (two-dimensional) perovskites has been realized due to the vast composition space of the inorganic layer and molecular diversity in organoammonium cations. A new class of layered halide perovskites recently discovered by our group, termed “mosaic” perovskites, form through alloying of single and double layered perovskites and feature three distinct metal ions. The first example of these disordered alloys comprised Cu(I), Cu(II), and In(III) in the perovskite layer, leading to emergent optoelectronic properties owing to intervalence charge transfer between Cu ions.

Here, we discuss facile synthetic methods to produce mosaic layered perovskites, the role of metal-cation ordering in optoelectronic and magnetic properties, and the interplay between stability and disorder in these complex mixtures. Moreover, we expand the diversity of mosaic layered perovskite alloys through exploration of transition-metal ion pairs that engender strong intra-layer coupling. We demonstrate that features of the known Cu(I)-Cu(II)-In(III) mosaic alloy can be used to systematically evaluate candidates and experimentally realize new examples of mosaic alloys.

Overall, these mosaic halide-perovskite alloys represent a platform for rational design of intra-layer coupling interactions within layered halides beyond the conventional compositional limitations of single or double perovskites.

The development of efficient solar cells based on perovskite materials has emerged as a promising path for advancing photovoltaic technology. Specifically, 2D perovskites (2D-PVKs) have been found to exhibit higher stability properties against their 3D counterparts [1]. However, many factors are essential for an effective construction of these devices. Solvent engineering plays a critical role in controlling the crystalline structure and optoelectronic properties of 2D perovskites [2]. However, the existing and research-dominant perovskite solar cells (PSCs) base their formulations on highly toxic solvents such as DMF, increasing potential health risks for researchers and the environment [3].

Herein, we mainly focused on the development of novel solvent formulations, tailored to the specific requirements of 2D perovskite fabrication. Following solvent’s solubility parameters, we predict novel and green solvents that significantly enable the formation and growth of perovskite crystals. These new solvents along with the state-of-the-art fabrication processes aim to provide an enhanced control over crystal growth, minimizing defects, while meeting existing efficiencies of solar cells. All while prioritizing sustainable and environmentally green practices.

Our research extends beyond solvent considerations, seeking to use these new formulations to not only enhance efficiencies but also to contribute to the long-term stability that 2D PVKs offer. The strategic selection of solvents aims to mitigate degradation mechanisms, such as ion migration and moisture-induced instabilities, which are well-known challenges in perovskite solar cell technology.

In conclusion, this project underscores the role of solvent engineering and the selection of non-hazardous solvents in the fabrication of 2D perovskite films, and its further use in solar devices. The ongoing efforts to develop novel solvent formulations represent a stride towards a deeper understanding of perovskite crystallization and heighten the potential of these materials for its widespread adoption in energy conversion technologies.

Two-dimensional (2D) organic-inorganic metal halide perovskites are persistently emerging as a promising class of semiconductors, not only due to their superior intrinsic and extrinsic stability but also due to their virtually endless compositional possibilities. Especially the versatile organic cations offer many options to study the underlying physics of these materials as well as to modify key properties on a device and application oriented research path. Looking for more stable and less toxic alternatives to 2D Pb-based perovskites, double perovskites based on Ag, Cu, Bi and Sb are at the focus of the most recent research efforts.

We synthesized 2D n=1 Ruddlesden-Popper double perovskite based on large, conjugated cations, creating different types of band gap alignments to break the quantum confinement of the 2D layered crystal structure. We discuss the structural and optical properties, the photoluminescence, the electronic structure, the charge-carrier mobilities and the mixed- and out-of-plane conductivities for 8 new materials. We isolate the most promising organic cation and successfully incorporate it into the first pure n=1, parallel oriented, 2D RP double perovskite solar cell.

Two-dimensional layered metal-halide perovskites (2DLPs) represent an emerging class of materials where a semiconducting metal-halide octahedral layer is sandwiched between two layers of bulky organic cations. The distinctive structural characteristic of these materials results in high in-plane mobility of excitons and charge carriers, but at the same time, hinders the out-of-plane mobility due to the presence of mostly isolating organic cations [1], [2]. This limits the possibility of studying charge and energy transfer processes in vertical heterostructures of this material class. Instead, lateral heterostructures, wherein the composition changes along the in-plane direction, offer an intriguing approach for investigating potential transfer processes at the junction region.

We studied the formation of lateral heterostructures in the system PEA₂PbBr₄-PEA₂PbI₄ by developing a facile room-temperature anion exchange method in solution. Ion exchange methods are a common tool used to tune the optical properties in 3D metal-halide perovskites but are underrepresented in the case of 2DLPs [3]. The approach takes advantage of the highly anisotropic structure of 2DLPs, where the bulky organic cations suppress the vertical diffusion of the ions while lateral diffusion is preferred [4]. We observed that 2DLPs can be stabilized in polar solvents such as octanol by exposing them to the corresponding halide salt of the organic cation. Introducing a different halide salt of the cation compared to the parent 2DLP leads to the formation of lateral heterostructures, which initiate at the edges of the microcrystals and propagate toward the center. This process ultimately results in a core-crown-like microstructure containing 2DLPs of two different halides, each contributing to a distinct emission profile. We demonstrate the influence of different processing parameters like the type of ion source and the solvent on the microstructure and optical properties of the resulting heterostructure. Furthermore, we observe a strong dependency on the direction of the exchange process. Treating PEA₂PbBr₄ with an iodine source results in the predominant formation of nearly phase-pure PEA₂PbI₄ along the microcrystal peripheries. Conversely, subjecting PEA₂PbI₄ to a bromide source consistently yields an alloyed phase, with no discernible presence of a pure bromide phase. This behavior stands in stark contrast to that of 3D perovskites and is attributed to the preferential occupancy of halides in specific positions within the [PbX₆]^4- octahedral [5].

The formation of such a heterojunction in the in-plane direction of the semiconducting layer provides an opportunity to control the directionality of the charge carrier or energy flow toward the edges or the center of these microstructures. This, in turn, contributes significantly to a better understanding of the optoelectronic properties in heterostructured 2DLPs.

Halogen perovskites represent a family of semiconductors with optical and transport properties that can be widely modified through element substitutions or crystal structure distortions. They are processable in solution and find application in solar cells, LEDs, lasers and detectors. Despite the high efficiency of 3D perovskites, 2D halide perovskites are more stable, durable, and chemically versatile. These materials, characterized by a two-dimensional crystal structure, are composed of thin layers of perovskite separated by organic cations and specifically the single crystal atomic configuration confers several advantageous properties and characteristics. Indeed, in 2D single crystals the absence of grain boundaries enhances electronic properties, such as charge transport.
Achieving the right crystal thickness is crucial for device performance too, preventing carrier loss, and ensuring effective light absorption to unlock the full potential of carrier transport. This can be attained by a successful space-confined method, which allows to control the thickness during perovskite preparation. The last one is crucial for 2D and quasi-2D perovskites, since it is essential for perovskites in solar cells to align their crystallography orientation with the carrier collection direction into the device, to permit the conduction. Indeed, 2D perovskites have to deal with their preferential orientation growth, which is typically parallel to the substrate with which they are in touch, insulating the carriers into the layers and causing transport carrier problems into the device. To avoid this problem, the task is to modify the orientation growth from parallel to vertical to the substrate.

In this contribution my work related to space-confined and orientation-growth techniques will be explained, in which the details of the processes and results obtained will be explored. Specifically, by confining the growth within specific spaces introducing modifications to confinement surfaces, it was possible to dictate the size and shape of the resulting single crystal. Moreover, to change the crystallography orientation, we worked with the addition of additives and using functionalized cations as spacers, in order to favor the vertical layer disposition. The samples obtained have been analyzed by different instruments, such as AFM, confocal microscope, XRD and others, investigating their thickness, morphology and orientation.

Two dimensional perovskites 2DPs – intended to be single/few inorganic layers (typically <3) spaced by large organic cations – have attracted a wide interest for their modular wide band gap and proved stability, but their application in solar cells has been so far unsuccessful. Lack of knowledge on how to manipulate the orientation of the inorganic sheets during solution-based film deposition only enabling a parallel orientation to the substrate is the actual bottleneck. This limits vertical charge percolation and efficiency, ultimately making them uncompetitive in the solar cells arena.

Here I will discuss a novel effective strategy to break such efficiency limit by controlling the orientation of the inorganic backbone of thiophene-based 2DPs (with n<3) inducing a vertical growth of the inorganic framework with respect to the substrate. Integrated in a solar cell, we enable vertical charge percolation, reaching a “record” efficiency of 9.4 %. Such proof of concept intimately demonstrates the potential of controlling the crystalline orientation of 2DPs providing an essential strategy for boosting their performances. Beyond such demonstration and its validation monitoring the crystal orientation at the nanometric scale, we reveal the fundamental mechanism behind. Two key ingredients play a concomitant role: 1) inducing the formation of nanometer-size crystallites already in the precursor solution and 2) creating a Cl-rich 2DP phase at the bottom interface where Chlorine – differently from what commonly happens in 3D perovskite- substitutes Iodine entering in the structure and ultimately inducing the vertical growth.

This talk summarizes recent advances in tuning layered perovskite materials from a first-principles perspective, covering energy level alignment, spin character of energy levels, manipulating carrier concentrations by heteroatom doping as well as by coherent stimulation of charge oscillations between the organic and inorganic components. Energy level alignments between the organic and inorganic components are satisfactorily covered by large-scale, spin-orbit coupled hybrid density functional theory (HSE06) calculations, with simulation sizes up to several thousand atoms enabled by the FHI-aims code in an accurate all-electron approach. Using chiral organic cations, chirality is imparted to the inorganic part of the system via structural asymmetries and hydrogen bonds, leading to conduction band spin splitting that is characterized by a simple structural descriptor. In order to utilize this and similar effects in devices, manipulating carrier concentrations in the conduction bands and valence bands is an important prerequisite. We study Bi as a potential n-dopant in Pb based layered perovskites, as well as Sn as an effective p-dopant. Direct, large supercell calculations of these substitutions show that n-doping by Bi should be weak but attainable (with 0.55 eV offset to the conduction band edges in phenethylammonium lead iodide, Bi-induced doping levels are not shallow). However, a population of compensating deeper defects appears to limit the experimentally attainable doping efficiency. For Sn, we show that an enhanced tendency of forming Pb vacancies nearby is likely responsible for experimental observations of slight p doping. As a final point, we rationalize experimental observations of laser-controlled coherent charge oscillations in a layered perovskite in terms of electron-phonon coupling, offering a potential model for direct, picosecond control of carrier populations in perovskite materials.

The family of hybrid organic-inorganic lead-halide perovskites are the subject of intense interest for optoelectronic applications, including solar cells, light-emitting diodes and detectors. Interest in the sub-class of 2D layered hybrid perovskites has been increasing due to their structural and compositional versatility. This versatility allows for the incorporation of spacer cations with an extended conjugated system that are electronically active, as opposed to the electronically inactive alkylammonium- and phenylethylammonium-derived spacer cations that are most often used.[1]

I will present our work on 2D layered perovskites containing optically and electronically active carbazole-based Cz-Ci molecules [2-3], where Ci indicates an alkylammonium chain and i indicates the number of CH2 units in the chain, varying from 3−5. These 2D perovskites, with a formula of (Cz-Ci)2PbI4, show a tunable electronic coupling between the inorganic lead-halide and organic layers. The strongest interlayer electronic coupling was found for (Cz-C3)2PbI4, containing the carbazole spacer with the shortest alkyl chain length. Using ultrafast spectroscopy, we measure ultrafast hole transfer from the photoexcited lead-halide layer to the Cz-Ci molecules, the efficiency of which increases by varying the chain length from i=5 to i=3. The charge transfer results in long-lived carriers (10 – 100 ns) and quenched emission. Electrical charge transport measurements using single-carrier devices show markedly increased out-of-plane carrier mobilities compared to the reference 2D perovskite (PEA)2PbI4, with carrier mobility increasing from i=5 to i=3.[4]

Quasi-2D perovskites with Ruddlesden-Popper structure, with the formula C₂A_n-1Pb_nBr_3n+1 , where C is a bulky spacer cation, A is a small organic cation (methylammonium (MA), or formamidinium (FA)) or Cs⁺, and n is the number of octahedral layers between the spacer cation bilayers, have been attracting increasing attention for applications in light emission in blue and green spectral ranges. Quasi-2D perovskites with different spacer cations exhibit vastly different crystallization, photoluminescence, and stability. One of the methods to adjust crystallization and achieve favourable energy landscape for efficient funnelling to result in bright sky-blue or green emissions is to use a mixture of spacer cations. In this work, we investigated the use of different carboxylic group containing spacers, including 5-ammonium valeric acid (5AVA), 4-ammonium butyric acid (4ABA), 3-ammonium propionic acid (3APA) and their mixtures, for preparation of n=2 and n=3 quasi-2D perovskites with MA and FA small cations. The films generally exhibited bright emission, which could be further increased with different additives, and the cations used affected the ratios of different n phases present in the film, and ultimately emission colour. However, the films exhibited strong tendency toward disproportionation upon exposure to elevated temperature or ambient. Thus, we explored mixing the carboxylic group containing spacers (5AVA was selected as it produces very bright films) with a spacer cation forming films with exceptional stability, 2-thiopheneethylammonium, namely TEA. For the optimal 5AVA-TEA ratio, we can obtain films with predominantly n=2 phase even after ambient exposure for n=2 solution stoichiometry, very different from pure 5AVA-based films (entirely dominated by n=1 phase after 3 min ambient exposure), as well as pure TEA-based films (showing prominent presence of n=1 and n=2 phases with no significant change with ambient exposure). For n=3, pure TEA-based films again show negligible change with atmosphere exposure, while for pure 5AVA there is a significant presence of 3D perovskite.  In addition, for the same solution stoichiometry, changing the small cation affects the crystallization and consequently light emission, with sky blue emission achievable only for MA small cation and n=2 solution stoichiometry. The effect of solution composition (spacer cations used and their ratios to small cations) on the phase composition of resulting film and consequently light emission is discussed.

Hybrid organic-inorganic perovskites have become one of the leading materials in
photovoltaics, although their instability under operating conditions interferes with practical
applications. This can be overcome by using tailored organic systems that interact with hybrid
perovskite frameworks through various noncovalent interactions, which predominantly involve
aromatic π-based systems as interfacial modulators or spacers forming layered hybrid
perovskites. We demonstrate the unique capacity to broaden this material space by relying on
unorthodox σ-hole-based interactions that can enhance the functionality of hybrid perovskites,
improving their operational stability without compromising photovoltaic performances,
providing a versatile supramolecular strategy for advancing hybrid photovoltaics.

1. J. V. Milić*, Chimia 2022, 76, 784.
2. W. Luo, G. AlSabeh, J. V. Milić*, Photochemistry 2022, 50, 342.
3. J. V. Milić, J. Phys. Chem. Lett. 2022, 13, 9869.
4. M. A. Ruiz Preciado, D. J. Kubicki, A. Hofstetter, L. McGovern, M. H. Futscher, A.
Ummadisingu, R. Gershoni-Poranne, S. M. Zakeeruddin, B. Ehrler, L. Emsley, J. V. Milić, M.
Grätzel, J. Am. Chem. Soc. 2020, 142, 1645.
5. W. Luo, J. V. Milić*, et al. 2023, manuscript submitted

Biological nervous systems possess versatile attributes, serving diverse functions; for instance, the central nervous system (CNS) governs learning and memory, while the peripheral nervous system (PNS) is responsible for sensory perception. Consequently, there is a need to engineer artificial synapses tailored to adapt to performance requirements in various applications. Brain-inspired neuromorphic computing aims to emulate the learning and memory capabilities of the CNS, being inspired from the long-term potentiation (LTP) observed in biological synapses. The application of artificial synapses in the fields of nervetronics and neuroprosthetics requires the emulation of the short-term plasticity (STP), enabling the rapid signal transmission and fast responses akin to those in the biological PNS. To demonstrate the broad applicability, spanning areas including neuromorphic computing and bio-inspired nervetronics, our study has explored the modulation of STP and LTP using ion-gel gated polymer synaptic transistors (IGPSTs). We have modulated the polymer semiconductor (PSC) film’s crystallinity through post-deposition film annealing, self-assembly monolayer treatment, and the introduction of various sidechain length, leading to the conversion between STP and LTP properties in IGPSTs. Moreover, we have utilized a straightforward yet effective approach by blending two PSCs with the same backbone but different sidechains. This blend strategy has led to a substantial improvement in LTP characteristics, whereas IGPSTs employing each PSC individually show only STP properties. IGPSTs with enhanced LTP properties demonstrate their potential as neuromorphic computing devices, effectively simulating learning processes in artificial neural networks. Concurrently, IGPSTs featuring STP capabilities are used to demonstrate various artificial nervous systems, such as artificial reflex arcs, neuromuscular systems, and neuro-prosthetic nerves incorporating artificial proprioceptors. These pioneering studies on neuromorphic devices have expanded the scope of applications for artificial synapses and validate the feasibility of these innovative technologies.

Electrochemical random access memory (ECRAM) and organic electrochemical transistors (OECT) are gaining significant attention due to the unique properties introduced via the mobile ions. Despite the progress in device fabrication, there is very little in terms of device models.[1, 2] Here, we will describe results obtained using a 2D semiconductor device model that incorporates ions and reactions in a self-consistent manner.

For example, although OECTs electrolytes contain both cations and anions, it is common to consider only the primary ion (as a cation in a normally-off p-type transistor). We simulate normally-on and normally-off transistors showing that the normally-on is highly affected by the counter ions (anions) and field-screening at the contacts.

Inorganic ECRAM transistor devices have a structure similar to OECT, where the memory (ion retention) is provided by the diffusion's very high electric field activation. Comparing measured multi-level potentiation [3] to detailed device simulation, we reveal the role of electrolyte polarisation. We also show that sublinear potentiation response can be explained by an electrochemical reaction similar to that of lithium plating in batteries. Namely, electrochemical reactions must be considered when dealing with electrochemical devices.

Neuromorphic sensors, designed to emulate natural sensory systems, hold the promise of revolutionizing data extraction by facilitating rapid and energy-efficient analysis of extensive datasets. However, a significant challenge lies in accurately distinguishing specific analytes within mixtures of chemically similar compounds using existing neuromorphic chemical sensors. In this study, we present an artificial olfactory system (AOS), which is developed through the integration of human olfactory receptors (hORs) and artificial synapses, for the first time. This AOS is sophisticatedly engineered by interfacing an hOR-functionalized extended-gate with an organic synaptic device. The AOS generates clearly distinct patterns for odorants and mixtures thereof, at the molecular chain length level, attributed to specific hOR-odorant binding affinities. This approach enables precise pattern recognition via training and inference simulations. These findings establish a solid foundation for the development of high-performance sensor platforms and artificial sensory systems, which are ideal for applications in wearable and implantable devices.

Developing high-mobility p-type semiconductors that can be grown using cost-effective scalable methods at low temperatures, has remained challenging in the electronics community for the integration of complementary electronics with the well-developed n-type metal oxide counterparts. Tin (Sn²⁺) halide perovskites emerge as promising p-type candidates but suffer from low crystallisation controllability and high film defect density, which result in uncompetitive device performance (1). In this talk, I would like to introduce a general overview and recent progress of our group of p-type Sn-based metal halide perovskites for the application of field-effect transistors (FETs). In the first part of the talk, I will mainly address inorganic perovskite thin-film transistors with exceptional performance using high-crystallinity and uniform cesium-tin-triiodide-based semiconducting layers with moderate hole concentrations and superior Hall mobilities, which are enabled by the judicious engineering of film composition and crystallization. The optimized devices exhibit high field-effect hole mobilities of over 50 cm² V⁻¹ s⁻¹, large current modulation greater than 108, and high operational stability and reproducibility (2). In the second part of the talk, I will introduce A-site cation engineering method to achieve high-performance pure-Sn perovskite thin-film transistors (TFTs). We explore triple A-cations of caesium-formamidinium-phenethylammonium to create high-quality cascaded Sn perovskite channel films, especially with low-defect phase-pure perovskite/dielectric interface. As such, the optimized TFTs show record hole mobilities of over 70 cm² V⁻¹ s⁻¹ and on/off current ratios of over 10⁸, comparable to the commercial low-temperature polysilicon technique level (3). The p-channel perovskite TFTs also show high processability and compatibility with the n-type metal oxides, enabling the integration of high-gain complementary inverters and rail-to-rail logic gates.

Halide perovskites have been attracted as potential candidates in various electronic devices such as memristors and neuromorphic devices with superior resistive switching properties such as low power consumption, scalability, and compositional and mechanical flexibility as well as photovoltaics, LED, and so on. Among them, we report here low-dimensional lead-free halide perovskites for the resistive switching memory devices and propose their potential applicability in multi-functional electronic devices using the resistive switching characteristics. Although most studies have been focused on the lead-based halide perovskites, it is necessary to focus the research on lead-free halide perovskites without toxicity of lead for various viable electronic applications such as tin, copper, antimony, or bismuth-based materials. Here we found that copper (Cu²⁺)- and bismuth (Bi³⁺)-based halide perovskites can be suitable candidates for a variety of applications, especially memristor devices due to their visible characteristic electrical traits and improved stability. With these reports, we also suggest materials and devices design perspectives for multi-functional electronic devices including next generation memristors. For copper(II)-based halide perovskites, 2-dimensional (2D) (BzA)₂CuBr₄ (BzA = benzylammonium) perovskite was synthesized on Pt substrate, which retained a higher ON/OFF ratio over 10⁸ even at lower operational voltage around +0.2 - -0.3 V. Longer endurance over 2,000 cycles was observed with Schottky conduction at HRS. [1] For bismuth (III)-based halide perovskites, layered double perovskite with chemical formula of BA₂CsAgBiBr₇ was synthesized on Pt substrate by phase conversion of 3D Cs₂AgBiBr₆, which showed endurance of 1,000 cycles and retention time over 2×10⁴ s at operational voltage as low as 0.5 V, along with ON/OFF ratio as high as ~10⁷. [2] Both the previously reported 3D counterparts on ITO substrate [3] and 2D Ag-Bi system [2] here showed excellent memristor properties, but especially the ON/OFF ratio of 2D Ag-Bi system was five orders of magnitude larger than those of 3D counterparts. Modification of Schottky conduction at high resistance state (HRS) is found to be responsible for high ON/OFF ratio in low-dimensional structure. In addition, as another study of bismuth-based materials, we fabricated a low-dimensional Cs₃Bi₂Br₉-based memristor with a thin film less than 1 micrometer, where synthetic Cs₃Bi₂Br₉ powder precursor enabled uniform thin film formation.[4] Thin film also exhibited low operating voltages as +0.44 V with gradual current increase at the SET process with reliable cell-to-cell and device-to-device resistive switching behavior. It was also found that the 2D low-dimensional copper and bismuth-based halide perovskites studied here showed much better stability under high humidity and at elevated temperature than the 3D and/or lead-based counterparts, which is beneficial for practical applications. Therefore, it was confirmed that low-dimensional lead-free halide perovskite materials are promising candidates for resistive switching memory devices, and we also expect that various halide perovskites can be derived and applied to multi-functional electronic devices including memristors, neuronic or synaptic devices, and so on.

Doping of organic semiconductor films enhances their conductivity for applications in organic electronics, thermoelectrics and bioelectronics. However, much remains to be learnt about the properties of the conductive charges in order to optimize the design of the materials. Electrochemical doping is important for organic electrochemical transistors (OECTs) used in neuromorphic systems. Benefits of doping via electrochemistry include controllable doping levels, reversibility and high achievable carrier densities. We introduce a new technique, applying in-situ terahertz (THz) spectroscopy directly to electrochemically doped polymers in combination with time-resolved spectro-electrochemistry, chronoamperometry and OECT device measurements. We evaluate the intrinsic short-range transport properties of the polymers (without the effects of long-range disorder, grain boundaries and contacts), while precisely tuning the doping level via the applied oxidation voltage. Moreover, temperature-dependent measurements allow to extract the thermodynamic and activation parameters of the electrochemical processes. Results will be shown for a variety of polymers based on polythiophene backbones with different sidechains, aligned polymer chains and novel 2D organic films.

Organic electrochemical transistors (OECTs) prove to be effective devices in various applications such as neuromorphic functionalities, bioelectronics, and sensors. Analyzing these mixed ionic-electronic devices is often complex due to the coupling of hole transport along the channel with ion insertion from the electrolyte. Numerous literature reports highlight persistent dynamical hysteresis effects in current-voltage curves, attributed to the gradual ionic charging of the channel under applied gate voltage.

We introduce a model that takes into account the primary electrical and electrochemical operational aspects of the device. This model is based on a thermodynamic function of ion insertion, allowing for the convenient classification of hysteresis effects. Such hysteresis can be categorized as capacitive or inductive. We specifically identify the volume capacitance as the derivative of the thermodynamic function, linked to the chemical capacitance of the ionic-electronic film.

Our findings reveal that the inductor response observed in impedance spectroscopy is associated with ionic diffusion from the surface, filling the channel up to the equilibrium value. The model uncovers multiple dynamical features tied to specific kinetic relaxations that govern the transient and impedance response of the OECT.

In biological tactile somatosensory system, the cooperation of mechanoreceptors, neurons and synapses allows human to efficiently detect, transmit and process the tactile information.  Emulation of the tactile sensory nerve to achieve advanced sensory functions in robotics with artificial intelligence is of great interest. Here, we report an artificial organic afferent nerve (AOAN) by integrating novel pressure-activated organic electrochemical synaptic transistor (OEST) and artificial mechanoreceptors. Owing to the effect of electrochemical ion doping and ion trapping in bulk conjugated semiconductor, external mechanical stimulation enables activation and modulation of OEST, endowing the system with the recognition/sensation of spatiotemporal tactile information and a low retention loss during signal transmission. Dendritic integration function for neurorobotics is achieved to perceive directional movement of object. An intelligent robot with our system, coupling with a closed-loop feedback program is demonstrated for slip detection and prevent slippage of objects. This work provides a promising approach towards next-generation intelligent neurorobotics and low power biomimetic electronics.

Halide perovskites have been widely explored for numerous optoelectronic applications among which phototransistors appeared as one of the most promising light signal detectors. However, it is still a great challenge to endow halide perovskites with both mobility and high photosensitivity because of their high sensitivity to moisture in ambient atmosphere, which limits the efficiency of transporting and collecting charge carrier. Here, we explore FAPbBr₃ perovskite quantum dots (QDs) phototransistor with band-like charge transport and measure a dark hole mobility of 14.2 cm²V^-1s^-1 at ambient atmosphere which is about an order of magnitude higher than solution processed perovskite QDs devices. Attaining both high mobility and good optical figures of merit, including photoresponsivity and detectivity, a detectivity of ~10¹⁶ Jones is achieved, which is a record for halide perovskite nanocrystals. Simple A-site salt (FABr) treatments offer a mechanism for connecting between perovskite QDs for better charge transfer in high-quality devices. All these important properties are superior to most advanced inorganic semiconductor phototransistor, indicating a promising future in optoelectronic applications.

Future brain-computer interfaces will necessitate electronic circuits capable of processing signals in a localized and highly individualized manner within the nervous system and other living tissues. However, traditional neuromorphic implementations based on silicon have limitations in bio-integration due to poor biocompatibility, circuit complexity, and low energy efficiency. Organic mixed ionic-electronic conductors (OMIECs) are an emerging technology that has the potential to overcome these limitations. OMIECs enable efficient signal transduction by tightly coupling ions and electrons, making them ideal for interfacing electronics with biological systems. Here, we explore the use of OMIECs to develop organic electrochemical neurons and synapses that can be modulated using different types of chemical signals. These soft and flexible organic electrochemical neurons and synapses operate at low voltage and respond to multiple stimuli, signaling a new era for printed organic electronics [1]. We will discuss their ease of integration with biological nerves [2] and demonstrate the potential for OMIECs to enable highly localized and precise signal processing in brain-computer interfaces.

An iontronic-based artificial tactile nerve is a promising technology for emulating the tactile recognition and learning of human skin with low power consumption. However, its weak tactile memory and complex integration structure remain challenging. We present an ion trap and release dynamics (iTRD)-driven, neuro-inspired monolithic artificial tactile neuron (NeuroMAT) that can achieve tactile perception and memory consolidation in a single device. Through tactile-driven release of ions initially trapped within iTRD-iongel, NeuroMAT only generates non-intrusive synaptic memory signals when mechanical stress is applied under voltage stimulation. The induced tactile memory is augmented by auxiliary voltage pulses independent of tactile sensing signals. We integrate NeuroMAT with an anthropomorphic robotic hand system to imitate memory-based human motion; the robust tactile memory of NeuroMAT enables the hand to consistently perform reliable gripping motion. We utilized the augmented tactile memory of NeuroMAT to develop a NeuroMATICS-based robotic hand system, which consistently and reliably emulated tactile-memory-driven human motion. Our theoretical study and practical demonstration of the iTRD behavior can provide an inspiration with artificial nerve disciplines and can be extended to in the field of neuromorphic electronic skin and machine learning.

Halide perovskite materials are mixed electronic and ionic conductors that find use in several applications. The ionic conductivity is responsible for a memory effect that leads to undesirable hysteresis in the solar cell configuration but it is a requirement in their use as a resistive memory. In this presentation, it is shown how conductive and insulating states are formed via migration of halide vacancy and electrochemically active metals halide perovskite useful as memristors. We show that the working mechanism and performance of the memory devices can be tuned and improved by a careful selection of each structural layer. Several configurations are  evaluated in which structural layers are modified systematically: formulation of the perovskite,¹ the nature of the buffer layer^2,3  and the nature of the metal contact⁴. We show that in order to efficiently promote migration of metal contact the use of pre-oxidized metals greatly enhance the performance of the memristor and reduces the energy requirements. Overall, we provide solid understanding on the operational mechanism of halide perovskite memristors that has enabled increased stabilities approaching 10⁵ cycles with well separated states of current and further improvements expected.⁵

The escalating demand for solar energy highlights the critical need for advancing more efficient next-generation photovoltaic (PV) technologies. Currently, perovskite-based single junction and tandem solar cells are reaching or even exceeding the performance of established technologies like three-five tandem and GaAs solar cells. In the case of perovskite versus GaAs, employing nanophotonics to fine-tune the PV bandgap emerges as a viable strategy, especially when reaching the limits of composition engineering of FAPbI3. Tandem solar cells present another practical approach to surpass the single-junction Shockley-Queisser limit. Our initial efforts concentrated on solution-coating thick perovskite layers onto small, textured silicon substrates, successfully proving the concept's viability. In Singapore, we are actively working to shift this technology from laboratory research to manufacturing, focusing on applying perovskite layers to industrial Czochralski (CZ) silicon wafers. With cooperative efforts, our goal is to propel this technology from lab-scale to full-scale fabrication, aiming to surpass the performance of three-five PVs in full module size, thereby addressing the growing global energy demands with more efficient solar solutions.

Integrating metal halide perovskite top cells with bottom cells formed by crystalline silicon or low band gap perovskites into monolithic tandem devices has recently attracted increased attention due to the high efficiency potential and application relevance of these cell architectures. Here we present our recent results on monolithic tandem combinations of perovskite top-cells with crystalline silicon, and Sn-Pb perovskites as well as tandem relevant aspects of perovskite single junction solar cells. 

In 2020, we have shown that self-assembled monolayers (SAM) could be implemented as appropriate hole selective contacts. The implementation of new generation SAM molecules enabled further reduction of non-radiative recombination losses with high open circuit voltages and fill factor. By fine-tuning the SAM molecular structure even further, the photostability of perovskite composition with tandem-ideal band gaps of 1.68 eV could be enhanced by reduction of defect density and fast hole extraction. That enabled a certified efficiency for perovskite/silicon tandems at 29.15% [1].

By optical optimizations, we could further improve this value to 29.80% in 2021. Periodic nanotextures were used that show a reduction in reflection losses in comparison to planar tandems, with the new devices being less sensitive to deviations from optimum layer thicknesses. The nanotextures also enable a greatly increased fabrication yield from 50% to 95%. Moreover, the open-circuit voltage is improved by 15 mV due to the enhanced optoelectronic properties of the perovskite top cell on top of the nanotexture [2].

In the end of 2022, we enabled a new world record for perovskite/silicon tandem solar cells at 32.5% efficiency. We demonstrated that an additional surface treatment strongly reduces interface recombination and improves the band alignment with the C60 electron transporting material. With these modifications, single junction solar cells show high open circuit voltages of up to 1.28 V in a p-i-n configuration, and we achieve 2.00 V in monolithic tandem solar cells [3]. A comparable surface treatment was also applied to 1.80 eV band gap perovskites to enable Voc values of 1.35 V and these were integrated into monolithic all-perovskite tandem solar cells enabling a certified efficiency of 27.5% [4].

In addition to the experimental material and device development, also main scientific and technological challenges and empirical efficiency limits as well as advanced analysis methods will be discussed for perovskite based tandem solar cells. In addition, results on upscaling and stability of these industrial relevant tandem solar cells by thermal evaporation will be shown.

Perovskite-based tandem solar cells, with their compelling performance and cost-effective fabrication, stand at the forefront of photovoltaic technologies. Nevertheless, reduction in energy loss and enhancements in robustness of the front wide-bandgap perovskite sub-cell remain crucial, aiming for an efficient and stable perovskite-based tandem device. Surface passivation using large organic spacer cations on the top of perovskite films is a promising strategy to address the issues mentioned above, as it can effectively diminish the defect density. Simultaneously, the introduction of low-dimensional phases into perovskites has been demonstrated to decrease the minority concentration at the interface, effectively reducing nonradiative recombination and mitigating the mismatch between the open-circuit voltage (V_OC) and the quasi-Fermi level splitting (QFLS), particularly for wide-bandgap-based perovskites.[1-4] In some other instances, the enhancement in VOC following surface passivation was attributed to the dipole moment of specific spacer cations that did not give rise to low-dimensional phases.[5,6] Moreover, the reduction in energy offsets of conduction bands (CBs) between the perovskite layer and the transporting layers also directly results in a V_OC enhancement.[6,7] Uniquely separating passivation from energy alignment remains challenging, especially for multifunctional cations that can form 2D phases but also generate a dipole moment. Therefore, to achieve further enhancements in V_OC and fill factor (FF) for wide bandgap perovskite sub-cells, it is essential to develop a comprehensive understanding of the underlying mechanisms.

As it will be discussed at the conference, the impacts of surface treatments with guanidinium bromide (GABr), 4-fluorophenylammonium iodide (F-PEAI) and their mixture on the formation of low-dimensional phases, device performances, as well as underlying loss mechanisms. Based on a hybrid experimental (QFLS) - theoretical (drift-diffusion simulation) approach, it reveals that the reductions in surface recombination velocity and energy level offsets between perovskite and electron transporting layer are the major contributors to a record V_OC × FF production for 1.80 eV perovskite solar cell, which exhibited notably reproducibility transferring to another laboratory. After integrating with a narrow-bandgap perovskite rear cell, we demonstrated an efficient all-perovskite tandem solar cell with stable output of 27.2% under maximum power point.

Tandem technology is the most promising route to further enhance the power conversion efficiency of solar cells. In recent years perovskite/silicon tandem solar cells have demonstrated an unprecedented rise of efficiencies far beyond the silicon single-junction efficiency limit. Further, all-perovskite multijunction solar cells gained increasing interest due to its compatibility with a fully printable, flexible, and lightweight technology platform.

In this contribution, optical aspects of perovskite tandem solar cells are discussed. This comprises 1) “classical” light management issues such as the minimization of reflection losses and the improvement of light trapping in thin absorber layers for the full spectral range. To achieve this, usually nano- and or micro-textures are implemented into the device coming along with special technological challenges concerning the compatibility of the textures with the respective perovskite deposition method. Further, we discuss 2) current-matching constraints in monolithic perovskite tandem devices. While under standard texting conditions there is little tolerance concerning the engineering of the perovskite bandgap(s), there is evidence that outdoor weather conditions, bifacial illumination as well as luminescent coupling can strongly relax current-matching constraints. Finally, we will give an outline how 3) optical engineering of luminescence and re-absorption processes can not only influence the short-circuit current density but also the electronic performance of the tandem solar cell device.

Halide perovskites overwhelmed the field of photovoltaics with unprecedented progress in efficiency. Their facile bandgap tunability renders perovskite solar cells excellent building-blocks for multi-junction architectures, that provide the prospect to overcome fundamental efficiency limits of single-junctions. Combinations of perovskite wide-gap cells exist with a large variety of narrow-gap technologies, such as silicon or CIGS. Furthermore, low-cost tandem technologies are particularly interesting, such as all-perovskite tandems. However, stability concerns exist especially for narrow-gap perovskite cells, that typically contain large amounts of tin (instead of lead). The notorious oxidation of the Sn²⁺ to Sn⁴⁺ infers a detrimental self-doping [1]. Since this issue might impose a fundamental stability limit, organic solar cells are an attractive alternative as a narrow-gap subcell to form perovskite/organic tandems. Since the introduction of non-fullerene acceptors has revived the field, organic solar cells now reach efficiencies >19% and express absorption spectra extending well into the infrared. Organic and perovskite semiconductors share similar processing technologies, which makes them attractive partners in multi-junction architectures. I will discuss the prospects and challenges of perovskite-organic tandem solar cells by highlighting the key aspects of the individual building blocks and their interplay in the tandem. Specifically, the role of non-fullerene acceptors in efficient narrow-gap organic solar cells with high operational stability is discussed. A further focus is the wide-gap perovskite solar cell, where long term stability is the most pressing issue that needs attention. Eventually, the design and functionality of high-quality interconnects are outlined along with a view on its impacts on the characteristics of the tandem. I will present a specific example of a perovskite/organic tandem using an ALD grown InO_x interconnect yielding a very promising efficiency of 24% with prospects reaching well beyond 30%.[2] In the end, I will benchmark perovskite-organic tandem solar cells against other emerging tandem solar cell technologies.

Tandem photovoltaic with complementary absorbing sub-cells is a promising pathway to overcome the thermalization loss and surpass the detailed balance limit of single junctions, stimulating intense interest. In recent years, the integration of wide-bandgap perovskite solar cells with narrow-bandgap organic solar cells represents a more prospective route in constructing tandem solar cells. However, for n-i-p type perovskite/organic tandems, the crucial interconnection layer (ICL) has been limited to the MoO_x/metal nanoparticles/ZnO structure. As is well-known, this ICL has several limitations, leading to undesired optical and energy losses within the tandem's ICL, as well as high costs and time consumption from a fabrication perspective. In our study, we introduced a solution-processed hole transporting material to replace the traditional thermally evaporated MoO_x in building the ICL for inorganic-perovskite/organic tandems. The optimized ICL exhibited significantly improved transmittance, particularly in the near-infrared region, which was conducive to enhancing the efficiency of the rear organic cell in harnessing light and achieved an increase in short-circuit current density (J_SC) of approximately 1.5 mA/cm². Additionally, owing to the favorable energy level alignment in the optimized ICL, the tandem device achieved a recorded open-circuit voltage of up to 2.3 V, along with an efficiency of 21% and enhanced stability.

The efficiency of perovskite solar cells has surpassed 26%, approaching the fundamental efficiency limits for single-junction cells. Tandem technology, combining two solar cells with different bandgaps, offers a solution to overcome detailed-balance limits by reducing thermalization loss. Although the best perovskite/silicon and all-perovskite tandem devices have exceeded 33% and 29% efficiencies respectively, the high energy consumption of silicon wafers leads to significant carbon emissions, and the self-doping oxidation effect (Sn²⁺ to Sn⁴⁺) in narrow-bandgap perovskites poses an instability issue. These challenges can be addressed by changing the narrow bandgap cells to non-fullerene organic photovoltaics (OPVs). In such structures, solution-processed OPVs are energy-friendly and do not have stability concerns related to self-doping oxidation. The recent emergence of near-infrared acceptor Y6 further positions perovskite/organic tandem solar cells as a promising technology.

In previous work, we integrated PM6:Y6:PC₆₁BM ternary OPVs into a perovskite/organic tandem devices, yielding a record efficiency of 24%. Notably, OPVs maintained approximately 95% original efficiency after 5000 hours of continuous operation under irradiation with low-energy photons (850 nm), but degraded rapidly when illuminated with a white light-emitting diode (LED), indicating that the visible spectral region could be responsible for device degradation^[1]. In this work, we utilize monochromatic LED sources covering ultraviolet, visible, and near-infrared spectral region to systematically explore the photostability of Y6 ternary OPVs. Our results reveal that under continuous operation in the maximum-power point under irradiation with low-energy photons (λ > 590 nm), the devices show long-term stability (>1000 hours), while high-energy photons (λ < 530 nm) infer degradation, with the device's T90/T80 lifetime strongly correlated with the photon energy. Additionally, the distinct degradation behaviors of single polymers or NFAs under monochromatic light exposure elucidate their respective contributions to degradation in bulk heterojunction devices.

Interestingly, ternary OPVs featuring other Y-family NFAs with various  energy gaps (Y18 (1.31 eV), CH1007 (1.30 eV), mBzS-4F (1.25 eV)) exhibit similar degradation behaviors under the same illumination conditions,  verifying that the photo-degradation behavior evidenced above is generally valid. The distinctive photostable property of OPVs under low-energy photons can eliminate concerns regarding the stability of the rear cell in perovskite-based tandem structures, because the front perovskite cells function as spectral low-pass filters. We integrates these OPVs as sub-cells in a tandem structure with a wide-bandgap perovskite cells, the corresponding photostability of perovskite/organic tandem devices strongly depends on that of the wide-bandgap perovskite sub-cells, highlighting the advantage of organic rear cells and directing community focus more towards the stability of wide-bandgap perovskite in the future.

Stacking two perovskite layers with complementary bandgap in tandem device promise to exceed the efficiency limit of single-junction solar cells. In addition to high-efficiency potential, perovskite-perovskite (all-perovskite) thin-film tandems can be fabricated on flexible and lightweight substrates with very high power-to-weight ratios, thus opening numerous applications where flexibility and lightweight are important considerations. Leveraging low-temperature coating methods and high throughput roll-to-roll manufacturing, perovskite-based thin-film tandems promise very low manufacturing costs and CO₂ footprint. Working towards our vision of roll-to-roll (R2R) manufacturing of flexible all-perovskite tandem solar modules, we present our latest progress on the development of highly efficient flexible all-perovskite tandem devices (Fig. 1). With interface and additive engineering, we demonstrate 25.4 % power conversion efficiency in a flexible 2-terminal all-perovskite tandem solar cell. Furthermore, we will present the first flexible all-perovskite tandem mini-modules and discuss the challenges of homogenous coating of perovskite on large areas using both scalable solution processes and vapor processes. Finally, we will discuss the potential and future challenges of developing highly efficient, lightweight, and stable all-perovskite tandem photovoltaics.

Metal halide perovskite solar cells have advanced into a viable option for future renewable energy. Record single and tandem junction all-perovskite solar cells already provide power efficiencies of over ~26% and ~28%, respectively. A next target in photovoltaic energy conversion can possibly be met by developing perovskite triple or even quadruple junction solar cells. While these hold a promise to afford higher efficiencies, they require developing stable perovskite sub cells with bandgaps in the range of 1.8 to 2.3 eV, i.e., a range that has not received much attention so far. These wide-bandgap perovskites often suffer from more pronounced voltage losses due to non-radiative bulk and interfacial charge recombination. In developing new perovskite sub cells, photocurrent spectroscopy and absolute photoluminescence spectroscopy are used in combination with bulk and interface passivation strategies to eliminate these losses. This has enabled to reduce the voltage deficit over a wide range of bandgap. Guided by optical modeling, monolithic multi-junction solar cells have been fabricated by stacking two and three different bandgap perovskite sub cells in series using recombination junctions designed to provide near-zero electrical and optical losses. Collectively, these strategies enable monolithic tandem and triple junction solar cells with a power-conversion efficiency of over 26%.

Understanding the performance limiting factors of organic solar cells (OSCs) with very small optical bandgaps is crucial for the development of novel tandem photovoltaics (PVs), such as OSC/OSC or perovskite/OSC tandems. For example, by combining a low-bandgap OSC ~ 1.1 eV with a high-bandgap absorber ~ 1.7 eV into a tandem solar cell, a power conversion efficiency (PCE) as high as 31.0% can be theoretically achieved, according to the detailed balance (DB) limit.

In this study, we investigate the loss mechanisms of ultra-low-bandgap OSCs based on the blend of the donor polymer PTB7-Th with the non-fullerene acceptor (NFA) COTIC-4F, with a bandgap of 1.15 eV, a promising candidate for tandem PV. In a conventional device structure, we reach a record PCE value of 8.55% with an enhanced short-circuit current density (J_SC) and fill factor (FF) compared to previous records that relied on an inverted device geometry. While setting a record for this system, our PCE metrics still fall behind higher-bandgap OSC systems.

To guide further improvements, we investigate the various loss mechanisms and recombination processes using photoluminescence (PL) measurements, bias-dependent time-delayed collection field (TDCF) measurements, bias-assisted charge extraction (BACE), fluence-dependent photoinduced absorption (PIA), as well as light intensity-dependent open-circuit voltage (V_OC). Complemented with optical and electrical device simulations, we show that J_SC loss can be largely attributed to inefficient exciton dissociation in combination with geminate recombination of the charge transfer state. Further, we identify the fairly high bimolecular recombination coefficient as the main reason for the poor performance, while surface recombination is shown to mainly affect V_OC. Finally, our simulations show that the simultaneous reduction of bimolecular recombination (e.g. by ternary blends), surface recombination (e.g. by self-assembled monolayers), exciton and charge transfer recombination (e.g. by vapor annealing) would enable efficiencies of >15% in the PTB7-Th:COTIC-4F system.

Mobile ions play a significant role in perovskite photovoltaics (PV), yet their impact on the overall performance and stability of tandem solar cells (TSCs) remains largely unexplored. Moreover, the effects of hysteresis in the current-voltage (JV) characteristic and ionic field screening are usually not considered to be a major problem anymore in high-performance tandem cells due to the introduction of comparatively stable and well-performing pin-type perovskite cells. This conclusion is based on the established practice of using a relatively slow JV scanning rate for the characterization of these devices. Here, based on recent work,[1-4] I will present a comprehensive study that combines an experimental analysis of ionic losses in Si/perovskite and all-perovskite TSCs during device aging with drift-diffusion simulations. Our findings demonstrate that mobile ions have a significant influence on the hysteresis of both tandem cells at high JV scan speeds (e.g. 400 V/s) as well as on performance degradation due to field screening. Additionally, subcell-dominated “fast-hysteresis” measurements on all-perovskite tandems reveal more pronounced ionic losses in the wide-bandgap subcell during aging, which we attribute to its tendency for halide segregation. Drift-diffusion simulations fully corroborate the results. Finally, I will discuss how we can use the obtained ionic properties as an early fingerprint to predict the long-term stability of perovskite cells. Overall, our research provides valuable insights into how ion migration influences the energy-lifetime yield of perovskite PV and highlights new strategies to improve the stability of all perovskite-based single- and multi-junction cells.

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 years. This talk will highlight several recent findings of our group on how the growth of NCs can be influenced by various exogenous cations and acid-based equilibria. We will also discuss our current understanding of halide exchange reactions involving perovskite nanocrystals as well as their heterostructures with other materials. We will outline new synthesis routes to strongly confined perovskite nanocrystals and their optical properties. Also, while most of the emphasis has been put on CsPbX₃ perovskite NCs, more recently the so-called double perovskite NCs, having chemical formula A⁺₂B⁺B³⁺X₆, have been identified as possible alternative materials, together with various other metal halides structures and compositions, often doped with different elements. This talk will also discuss the research efforts of our group on these materials. I will highlight how for example halide double perovskite NCs are less surface tolerant than the corresponding Pb-based perovskites.

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 providing novel design tools by combining polarity and chirality [1,3]. This in turn open the way to explore several appealing chiroptoelectronic properties such as circular dichroism, circularly polarized emission, chiral-induced spin selectivity and so on. To extend the actual knowledge of these chiral systems and being able to properly engineer optimized phases it is important to carefully investigate the parameters affecting the chiral-induced response. From a materials chemistry point of view, this implies the ability of properties tuning by cation(s) and halide modulation/substitution. In this contribution we will present the results of the investigation of the role of the central metal and chiral cation, showing the modulation of the optoelectronic properties in two different approaches. On one side, we explored the preparation of a series of novel chiral metal halides including the same chiral cation, namely (4-Chlorophenyl)ethylenimine (Cl-MBA), but with a different central metal, namely Pb, Sn and Ge. This led to the discovery of the (R/S/rac-ClMBA)₂SnI₄ [4] and (R/S/rac-ClMBA)₂GeI₄ compositions showing also another structural 1D topology in presence of Ge, namely (R/S/rac-ClMBA)₃GeI₅ [5].  A comparison between these systems in terms of crystal structure and optical properties, coupled to computational modelling, sheds light on the tuning effect of central metal on the chiroptical properties. Another approach explore, relies on the properties modulation by including ad hoc synthesized bifunctional non-commercial chiral cations such as ((R)-1-amminobutan-2-ol) [6], thus exploring the impact of amine chemistry on the structural and optical properties. Final aim is to understand the impact of chemical degrees of freedom on the chirality transfer between the organic cation and the inorganic framework in order to provide tuning strategies for materials engineering.

Future quantum technologies promise to deliver unprecedented computing power, secure communications and ultra-high precision measurements. A very promising way for realising these applications is via quantum optics, and single photon emitters are the key element of quantum optical technologies [1]. Among the various known methods for generating single photons, the use of optical transitions between quantum confined and isolated electronic states in semiconductor quantum dots (QDs) is one of the most promising approaches. Nowadays, after more than two decades of intensive development, epitaxial QDs have reached high fidelity performance; however, it appears that the best performance of these materials as quantum light sources is only achieved when operated at cryogenic temperatures. Instead, colloidal QDs have been much less explored as single photon emitters, but have the potential for operation beyond cryogenic temperatures (>4 K) due to stronger quantum confinement of carriers. Colloidal QDs can be synthesized in a variety of compositions, in particular metal chalcogenides such as CdSe, CdS, ZnSe, etc. and perovskite QDs are the most promising in terms of single photon emission properties. One of the fundamental technological difficulties for the realisation of single photon sources based on colloidal QDs is their slow decay rate at room temperature, since core-shell cadmium chalcogenide QDs have a typical lifetime in the range of tens of ns and perovskite QDs in the order of a few ns [2]. However, these lifetimes are too long to achieve ultrafast emission rates in the range of 100 Mph/s [1]. So far, various methods have been tried to increase the decay rate of colloidal QDs, e.g. by using a nanophotonic approach; experimental studies have shown an increase of the decay rate by a factor of 6 for a QD encapsulated in a plasmonic shell [3] and about 80 in plasmonic nanogaps [4] and patch antennas [5]. Another interesting method is based on exciton charging to enhance the QD emission rate itself, which can be realised by electrochemical or photochemical charge injection. Recent work by Morozov et al. [6] has shown a dramatic increase in the recombination rate for CdSe/CdS QDs using an electrochemical cell, from an initial lifetime of about 100-200 ns down to 1 ns at the highest applied voltage bias. Our study aims to exploit these results to further increase the room temperature decay rate of colloidal QDs by applying an exciton charging approach to CsPbBr₃ perovskite QDs. Since perovskite QDs are not stable in polar solvents, an electrochemical method as used by Morozov et al. [6] cannot be applied here. For this reason, we investigate a novel approach based on a solid state device where an external electric field is applied directly to the perovskite QDs using a parent structure of a field effect transistor (FET) to generate highly charged excitons with a fast decay rate. Two alternative structures will be explored: one with an indium tin oxide (ITO) electrode to inject holes into the QDs, and the other with an aluminium layer to introduce electrons into the QDs. This will allow us to study the recombination of excitons in two different scenarios, the first with an excess of holes and the second with an excess of electrons. So far, we have investigated the performance of the device structure using a thin film of CsPbBr₃ perovskite QDs. By applying a voltage bias to the device up to 40 V, we have observed a decrease in the lifetime of about 20% of the perovskite QDs that is reversible by returning to 0 V. These preliminary results are promising for moving on to the study of individual QDs.

In this presentation I will discuss our studies of controlling spin populations in metal-halide organic/inorganic hybrid systems. Lower dimensional perovskites are of particular interest since the lower degree of symmetry of the metal-halide connected octahedra and the large spin-orbit coupling can potentially lift the spin degeneracy. To achieve their full application potential, an understanding of spin-relaxation in these systems are needed. Here, we report an intriguing spin-selective excitation of excitons in a series of 2D lead iodide perovskite (n = 1) single crystals by using time- and polarization-resolved transient reflection spectroscopy as well as transient circular dichroism. Exciton spin relaxation times are studied at room temperature for a range of 2D lead iodide perovskite systems and correlated to the exciton binding energy and degree of distortion in the metal halide framework. We also studied the spin-dynamics using THz emission spectroscopy and report on the circular photogalvanic and spin photogalvanic effect. Finally we are also investigating a novel class of chiral hybrid semiconductors based upon layered metal-halide perovskite 2D Ruddlesden-Popper type structures. These systems exhibit chiral induced spin selectivity (CISS) whereby only one spin sense can transport across the chiral layer and the other spin sense is blocked for one handedness of the chiral perovskite layer. We show that chiral perovskite layers are able to achieve > 80% spin-current polarization. I will discuss studies of spin-injection into a chiral-layer.

In the last years, the hybrid metal-halide perovskites have attracted a great interest in the scientific community due to their excellent photo-physical properties and their outstanding optoelectronic and photovoltaic properties. In particular, formamidinium lead triiodide (FAPbI₃) is the best candidate for single-junction HMP photovoltaics. It shows the highest theoretical photoconversion efficiency due to his energy bandgap, that is the lowest in the family of Pb-based halide perovskites (HPs). The very promising a-phase of FAPbI₃ is however difficult to stabilize at room temperature. Recently, different groups work to implement a strategy to make it stable and some of them have obtained promising results [1]. The difficulties to stabilize the material have so far prevented exploring experimentally the whole peculiarities and characteristics of FAPbI₃ beyond photovoltaic applications. However, FAPbI₃, as other member of HP family, are highly promising material in the spintronic domain due to their large spin-orbit coupling and its spin-dependent optical selection rules.

In this work, we have studied, at 2K, the coherent evolution of the electronic spins in a polycrystalline film of FAPbI₃ by using a picosecond pump-probe experimental set-up and measuring the photo-induced Faraday rotation technique (PFR) in transverse magnetic field. We identified two contributions to the PFR signal that we associated to localized electrons and holes respectively. We measured long spin coherence times of localized electrons (holes) of 3.1ns (1.6 ns) at 1.49 eV. These values are shorter than the values obtained in FA_0.9Cs_0.1PbI_2.8Br_0.2 [2], MAPI [3, 4,5] and CsPbBr₃ [6] but are of the same order of magnitude. We have also obtained the value of g Landé factors for electrons 3.479 and for holes 1.137 that compares very well with previous results on FA_0.9Cs_0.1PbI_2.8Br_0.2 [2]. From the measured hole g factors and ellipsometry results on FAPbI₃ we are able to obtain the Kane energy parameter of FAPbI₃, Ep=13.5 eV, that is a central parameter to describe the band-structure and optical properties in the framework of the effective mass approximation [7]. This experimentally estimated value is smaller than the theoretical value [8]. All this results are of prime importance to evaluate the potentialities of the FAPbI₃ as future actor in engineered HP spin-optronic devices.

Lead-free double perovskites are emerging as an alternative to toxic lead-based perovskites for both light-emitting and light harvesting applications. Owing to the soft and polar nature, lattice deformations in metal halide perovksites can strongly couple to charge carriers upon photoexcitation. In contrast to lead-based perovskites, double perovskites exhibit intrinsic defects (Ag-vacancies), resulting in bound excitons with giant oscillator strengths. We utilize femtosecond pump-probe spectroscopy to investigate the dynamics of bulk and localized coherent phonons in spherical Cs₂AgBiBr₆ double perovskite nanocrystals at cryogenic temperatures. In this study, we observe that optically launched bulk lattice vibrations modify the spectral position of the bound excitonic resonance. Surprisingly, we found that this Ag-vacancy bound exciton is modulated in its giant oscillator strength. We argue that this is a consequence of an optically generated vibrational wave-packet localized at this very Ag-vacancy. This optically induced and spatially localized lattice shaking could become potentially useful for initiating photochemical or photocatalytic reactions with atomic precision.

We provide a survey of our experimental studies of coherent spin dynamics of electrons and holes in lead halide perovskite crystals, nanocrystals and 2D materials. Time-resolved Faraday/Kerr rotation is used as technique in combination with magnetic fields up to 10 T and cryogenic temperatures. We measure spin relaxation and spin dephasing times, evaluate electron and hole Lande g-factors, demonstrate the hyperfine interaction of carrier spins with nuclear spin system. In perovskite nanocrystals grown both by colloidal chemistry and synthesized in glass matrix the electron spin coherence is detected even at room temperature and unusual temperature dependence of the electron g-factor is found. We establish experimentally and approve theoretically universal dependences of the electron, hole and exciton Lande g-factors on the bandgap energy, which in nanocrystals are further modified by carrier confinement. Spin mode locking effect based on spin synchronization under periodic laser excitation is found in nanocrystals in glass. Experimental approaches of spin physics give reach information about these materials.

Novel growth and fabrication technologies allow for an unprecedented level of control over
nanoscale semiconductor quantum devices. Recently, several groups have demonstrated
samples where multiple quantum dots can be tuned into resonance [1,2]. When multiple
quantum emitters become indistinguishable, collective quantum effects like superradiance
and measurement-induced cooperative emission [2] emerge due to entanglement between
the emitters.
A detailed understanding of the signatures of cooperative emission is challenging for
several reasons: First, close-to-identical emitters operate in a regime where non-degerate
perturbation theory breaks down. Second, real-world quantum devices typically strongly
interact with local phonon baths as well as with a global photonic environment, and
the different environments generally influence each other. Moreover, the presence of ad-
ditional imperfections like spectral wandering further obfuscates the physical picture.
Nevertheless, for few emitters, the problem of cooperative emission in the presence of
multiple environments can be solved numerically exactly using the process tensor (PT)
formalism [3,4].
Here, I report on recent theoretical and experimental findings and summarize the
current understanding of cooperative emission in semiconductor nanostructures. This in-
cludes the discussion of how genuine superradiance can be distinguished from measurement-
induced cooperative emission, how to interpret peaks in g⁽²⁾ photon correlation experi-
ments, and why inter-emitter correlations remain strong for long times despite strong
interactions with phonons.

Lead halide perovskites have attracted tremendous attention, initially for their performance in thin film photovoltaics, and more recently for a variety of remarkable optical properties. Defect tolerance through polaron formation within the ionic lattice is a key aspect of these materials. In recent studies [1,2], some of us have applied two-dimensional electronic spectroscopy (2DES) to probe the timescale and amplitude of the electronic gap correlations in lead halide perovskite nanocrystals via homogeneous lineshape dynamics. The 2DES data reveal irreversible, diffusive dynamics that are consistent with liquid-like structural dynamics on the 100 femtosecond timescale. These dynamics were assigned to the optical signature of polaron formation, the conceptual solid-state analogue of solvation.

To complement the 2DES data, we have employed the method of femtosecond electron diffraction (FED) to directly measure the sub-picosecond lattice dynamics of weakly confined CsPbBr3 nanocrystals following above-gap photoexcitation. The data reveal a light-induced structural distortion appearing on a time scale varying between 380 and 1200 fs depending on the excitation fluence. We attribute these dynamics to the effect of exciton–polarons on the lattice and the slower dynamics at high fluences to slower sub-picosecond hot-carrier cooling, which slows down the establishment of the exciton–polaron population. Further analysis and simulations show that the distortion is consistent with motions of the [PbBr3]− octahedral ionic cage, and closest agreement with the data is obtained for Pb–Br bond lengthening. 

Together, the 2DES and FED data offer a unique and complementary view of exciton-polaron dynamics in lead halide perovskite nanocrystals.

3D halide perovskites are direct band gap semiconductors, with low effective masses, sizeable optical oscillator strenghts. In relation with an original crystallographic structure for a semiconductor, electronic band edges at the R point define 3D halide perovskites as a novel semiconductor class characterized by Sz=1/2 for the top of the valence band and Jz=1/2 for a split-off band at the bottom of the conduction band due to a giant spin-orbit coupling [1,2]. Excitonic properties have been at the center of discussions since the beginning of the hype on photovoltaic in 2012. Following a prediction in 2014 [2], the direct measurement of the exciton binding energy in 2015 [3], led to values strongly reduced with respect to accepted values so far reported. Perovskite photovoltaics is since considered closer to the field of classical semiconductor solar cells than initially expected. The understanding of the excitonic properties, especially the exciton fine structure and the nature of exciton complexes, was further questioned with the advent of perovskite nanostructures leading to prospects for quantum light emission. A debate between an initial proposition of a fine structure related to a singlet dark ground state [4,5] and an inverted dark-bright ordering [6] , was closed recently in favour of the initial proposition [7,8,9]. Several open questions are remaining, especially about the nature of the coupling between the charge carriers and the lattice dynamics, which is unusual with respect to classical semiconductors [10]. Direct experimental investigations on the low energy lattice dynamics including neutron scattering are in progress [11]. State of the art abinitio simulations of the lattice dynamics and electron-phonon coupling will be shown [12]. The regime of low-temperature Fröhlich polaronic coupling in bulk 3D perovskites and quantum dots is also explored.

The search for material platforms that ensure low cost, ease of fabrication, usability (room temperature) and nonlinearity in the same device is a serious challenge for today's photonics. Our work presents a first step towards a major breakthrough in the field of perovskite integrated photonics. For this purpose, we developed a versatile, template-assisted method for fabricating perovskite microstructures of any arbitrary, pre-defined shape. Our structures demonstrate waveguiding capabilities and facilitate the formation of spatially extended condensates of coherent exciton-polaritons.

Our microwires, characterized by their ability to bend without compromising optical quality, are formed from CsPbBr3 crystals and can be deposited on any substrate, enhancing compatibility with existing photonic devices. Notably, our method overcomes the limitations of traditional waveguiding setups by eliminating the need for extrinsic cavity mirrors. Our approach significantly simplifies the fabrication process, making on-chip polaritonic devices more accessible and cost-effective.

We demonstrate polariton lasing from the interfaces and corners of the microwires, with large blueshifts observed with excitation power. The high mutual coherence between different edge and corner lasing signals, evidenced in the far-field photoluminescence and angle-resolved spectroscopy, indicates the formation of a coherent, extended over macroscopic distance polariton condensate. This condensate is capable of propagating over long distances within the wires and even coupling between neighbouring wires through air-gaps.

The simplicity and scalability of our platform, combined with its compatibility with standard photonic components, pave the way for future large-scale, integrated polaritonic circuitry. Our findings not only demonstrate the potential of CsPbBr3 perovskites in photonic applications but also provide a more accessible path for the development of advanced on-chip optical devices with build-in nonlinearities.

In recent years, lead halide perovskite (LHP) quantum dots (QDs) have attracted much attention for their unique optical properties. In contrast to conventional III-V and II-VI semiconductors with a rather complex valence band structure, LHPs are free of band degeneracies around the band gap. This reduces the complexity of their optical spectra around the band gap. Furthermore, the rather small effective electron and hole mass in LHPs generates a light exciton, which leads to significant confinement energies. The resulting well-separated energy levels simplify the assignment of electronic transitions, making LHP QDs an ideal model system to study quantum confinement effects.

We discuss highly monodisperse [1] spherical-like LHP QDs with diameters in the range of the exciton Bohr diameter, which feature a multitude of distinct resonances in their absorption spectra. By employing a simplified analysis within the envelope function approximation [2], we are able to assign the absorption resonances to center-of-mass motion confined excitons. Femtosecond transient absorption measurements on these QDs reveal bleaching of the exciton levels as well as a series of induced absorption features which we attribute to biexcitonic transitions [3]. In the context of light emitting devices, the emission and lasing properties of confined systems are of special interest. We therefore use our well-defined excitonic QDs to study their optical gain and amplified spontaneous emission characteristics. Our work shows the first insight into confined excitons in LHP QDs and provides a better understanding of their linear and nonlinear absorption and emission spectra as well as their polarization-dependent properties.

Synthesis of tin (Sn) halide perovskite nanostructures has been challenging as the high surface to volume ratio promotes fast Sn²⁺ to Sn⁴⁺ oxidation and facilitates the production of polymorphs with different optoelectronic properties. Recently, an optimized synthetic route was developed, allowing the production of robust and monodisperse three-dimensional (3D) CsSnI₃ nanocrystals (NCs) with sizes in the 6 to 10 nm range [1]. The synthetic process produces also small amounts of two-dimensional (2D) (R-NH3⁺)₂SnI_4, Ruddlesden–Popper nanosheets, that form stable colloidal mixtures with the 3D NCs [1].

Herein, the spontaneous and stimulated emission properties of thin films of the coexisting 3D-2D nanostructures are discussed. The films emission is dominated by the NC component, as transient spectroscopy indicates the presence of efficient energy funneling from the nanosheets to the nanocrystals. At cryogenic temperatures, two CsSnI₃ NC luminescence species exist, identified as surface bound excitons and free excitons. The relative population of the two species depends on temperature, aging, thermal cycling and surface passivation.

Upon intense photoexcitation with nanosecond pulses, stable amplified spontaneous emission (ASE) is observed from the CsSnI₃ NCs at temperature up to 150K. Fabrication of NC-polymer multilayer structures allows the reduction of the ASE threshold and the observation of ASE at temperatures approaching 300 K.

By embedding optically active materials with high oscillator strength in optical microcavities, the strong light-matter interaction regime can be reached where exciton-polariton quasiparticles are formed that are part photon and part exciton. At sufficiently high excitation density, eventually a non-equilibrium Bose-Einstein condensate can form, exhibiting nonlinear behavior, macroscopic coherence and quantum fluid properties. Bulk crystalline thin films of CsPbBr3 in microcavities have shown to allow room temperature polariton condensation. Nanocrystals have the promise of enhanced polariton interactions due to their spatial confinement.

Here we report on strong light-matter coupling of colloidal CsPbBr3 nanocrystals and polariton condensation at ambient conditions, as evidenced by nonlinearly increasing emission, line narrowing and coherence measurements. We use a tunable open microcavity based on distributed Bragg reflectors to tune the polariton energy. By precise nanofabrication, we create tiny indentations in the mirrors to effectively produce potential landscapes for the polariton condensate, which opens perspectives to use the polariton quantum fluid as analogue simulator for interesting hamiltonians.

In recent years, hybrid organic-inorganic two-dimensional perovskites (2D PVKs) have attracted great interest due to the unique combination of intriguing properties at room temperature, making this class of semiconductors suitable for various optoelectronic applications. [1]

The alternating of organic and inorganic layers induces the formation of a natural multiple quantum well (MQW), offering the possibility to tailor the optical and structural properties of 2D PVKs acting directly on the chemical structure. In fact, by changing the elements of the metal-halide octahedra, it is possible to tune the band gap over a wide energy range, while varying the organic components enables a fine tuning of the MQW structure, introducing organic building blocks with new functionalities for the development of unexplored hybrid semiconductors. [2,4]

In this work, we modulate the structural distortion of novel 2D perovskites to design their optical characteristics and electrical band structure. We investigate the impact of the cation length and the ammonium binding group on the crystalline structure using interlayer cations with different characteristics. By doing so, we show that the exciton characteristics can be easily modified through careful organic cation design.

Furthermore, we integrate the developed PVKs single crystals into nanophotonic devices by synthesising microwires [5] that act as waveguides, in which strong light-matter coupling occurs between the optical modes of the crystal and the PVK exciton, with the generation of exciton−polaritons, the half-light half-matter quasi-particles that inherit unique properties from both excitons and photons.

The understanding of the structure-property relationship and the demonstration of new features can be exploited for the realization of compact, integrated optical devices with wide tunability.

The production of single indistinguishable photons is important for linear optical operations, in the realization of photonic-based quantum computational circuits.  The present work aims to investigate the use of the exciton recombination in single CsPbBr₃ and CsPb(Br/CL)₃ perovskite nanocrystals as a source of single indistinguishable photon emission. Using the HONG-OU-MANDEL(HOM) interference technique, two sequentially produced photons are overlapped to investigate the interfering visibility [1,2]. The nanocrystals are synthesized by the group of M. Kovalenko (ETH Zürich & EMPA Dübendorf) to have a rod-like symmetry. By breaking the symmetry of the confinement potential, the exciton emission spectra reduce to a pure singlet line. This eases the collection of the emitted photons, as filtering of orthogonal polarization states is not needed and a Bandpass filter can be employed (mainly to suppress biexciton light). The nanocrystals show ~50 ps lifetime [Fig.] and a narrow spectral line at cryogenic temperatures. I will present our HOM results and compare them with previous work[3].  We achieve low g²(0) values around 0.10 [Fig.] and uncorrected HOM interference visibility up to 0.44.

Self-assembled nanocrystals (NCs) into superlattices (SLs) are alternative materials to polycrystalline films and single crystals, which can behave very differently from their constituents, especially when they interact coherently with each other, leading to collective emission, as superfluorescence [1]. In this work we concentrate on the Superradiance (SR) emission observed in SLs formed by CsPbX3 NCs synthesized by hot injection with synthesized with oleic acid and oleylamine (OA/OAM) and didodecyldimethylammonium bromide (DDAB) ligands. These SLs were studied by Photoluminescence (PL), Time Resolved PL (TRPL) and Micro-PL/TRPL spectroscopy at low and high temperatures to extract information about the SR states and uncoupled domains of NCs. In the case of CsPbBr3 SLs formed by large sized NCs we have measured very narrow (1-5 meV) emission lines on the low energy side of their µ-PL spectra, which can be considered near homogeneous, and characterized by lifetimes as short as 160 ps. The enhancement factor estimated for these low energy narrow lines can reach values as high as 10, like the value estimated from the inverse ratio of their lifetimes over that in a film, which is associated to domains of nearly identical NCs formed by 1000 to 40000 NCs within the micrometric SLs [2]. Therefore, the nature of these emission lines is consistent with a SR exciton state whose origin is due to the coupling of optical dipoles in perovskite NCs; this origin is consistent with our estimate of the correlation length for these SL domains with correlated NCs. The coherence of the SR exciton state has been studied in the temperature region dominated by a radiative recombination dynamics (from 4 to 70-80 K). The importance of thermal decoherence for the SR state is observed above 25 K and due to its coupling with an effective phonon energy of around 8 meV. The emission of high energy states in the SL spectra are also characterized by short lifetimes, because of Förster transfer of energy (from small to large NCs of the distribution or from high to low emission energies). More recent work on perovskite SLs is being directed to a better control of the SL growth and evaluating the effect of a solid-state anion state procedure [3] on initially formed CsPbBr3 SL and the optical properties of the SLs, including the superriant state.

The brightness of an emitter is ultimately described by Fermi’s golden rule, with the decay rate of the emitter’s excited state proportional to the product of its oscillator strength and the local density of photonic states (LDOS). Since the former is an intrinsic material property and, hence, already set by the choice of the employed compound, the quest for ever brighter emission has mainly focused on enhancing the LDOS using dielectric or plasmonic resonators to achieve a large Purcell effect or strong light-matter interactions^1,2. A much less explored avenue is to boost the oscillator strength, and hence the emission rate, via a collective behavior termed superradiance. Recently, it had been proposed that the latter can be realized via the giant oscillator-strength transitions of a weakly confined exciton in a quantum dot (QD) when its coherent motion extends over many unit cells. Here we demonstrate single-photon superradiance in perovskite quantum dots (QDs) with a record-low sub-100 ps radiative decay time, almost as short as the reported exciton coherence time³. The characteristic dependence of radiative rates on QD size, composition, and temperature suggest the formation of giant dipoles, as confirmed by effective-mass calculations. The results aid the development of ultrabright, coherent quantum-light sources and attest that quantum effects, e.g. single-photon emission, persist in nanoparticles ten times larger than the exciton Bohr radius.

Perovskite solar cells have come to the forefront of solar research in the last decade with certified efficiencies of now >26%. This is approaching rapidly the Shockley-Queisser limit for single-junction solar cells, implying that the main breakthroughs for perovskites were achieved with relatively narrow bandgaps.[1a,1b] Less progress, however, was made for wider bandgap perovskites, which are of interest for multijunction photovoltaics, detector applications, or water splitting. These wide bandgap perovskites are often comprised of fully inorganic components, which are hard to dissolve in conventional solvent systems and require more sophisticated synthesis as well as crystallisation techniques.

In this talk, I will discuss strategies to address these challenges by providing a library of hitherto unexplored wider bandgap perovskites using combinatorics. Mechanosynthesis is then studied to attain otherwise inaccessible liquid precursors permitting the realization, e.g., of “triple cation” wide bandgap perovskites.[2]

Unfortunately, the newly formulated liquid precursors often exhibit complex crystallization behaviour struggling to expel the typically used DMSO solvent. To delay the crystallization time, two strategies are proposed to remove the strongly complexating DMSO molecules through a) modified processing of the liquid thin-film[3] and b) a coordination solvent with a high donicity and a low vapor-pressure[4] leading to a marked improvement in the overall film quality.

Lastly, interface manipulation, especially on top of the formed perovskite, is becoming a central topic to advance further. Typically, this involves chemical surface treatments with a complex interaction. Here, light annealing is introduced as a universal, non-chemical approach to modify the perovskite surface resulting in a reduced surface recombination.[5]

[1a] Saliba et al. Energy & Environmental Science (2016), [1b] Turren-Cruz, Hagfeldt, Saliba; Science (2018)

[2] Ferdowsi,…, Saliba ; Chemistry of Materials (2021)

[3] Byranvand,…, Saliba; One‐Step Thermal Gradient‐and Antisolvent‐Free Crystallization of All‐Inorganic Perovskites for Highly Efficient and Thermally Stable Solar Cells, Advanced Science (2022)

[4] Zuo,…, Saliba; Coordination Chemistry as a Universal Strategy for a Controlled Perovskite Crystallization, Advanced Materials (2023)

[5] Kedia,…, Saliba; Light Makes Right: Laser Polishing for Surface Modification of Perovskite Solar Cells, ACS Energy Letters (2023)

     Perovskite nanocrystals (NCs) such as those made from CsPbBr₃ have numerous uses. Most involve exploiting their light emission and are motivated by their defect tolerance and large, as-made emission quantum yields (QYs). For the latter, near unity QYs are possible. This leads to ready applications as light emitters but also intriguingly to possible material platforms for demonstrating semiconductor-based optical refrigeration.

     Absent, though, is a fundamental understanding of perovskite NC emitting states, central to these applications. Of particular note are observations of near-universal, size-, temperature-, and composition-dependent absorption/emission Stokes shifts where observed energy differences are those that separate absorbing and emitting states. Moreover, efficient (near-unity efficiency), photoluminescence up-conversion, induced by exciting NCs below gap, motivates better understanding the microscopic nature of perovskite band edge states.

     I will explain work we have done recently within the context of attempts to optically refrigerate a semiconductor. These studies now provide new insight into the band edge states of CsPbBr₃ (and possibly CsPbX₃) NCs. Starting with a microscopic mechanism for explaining how it is possible to obtain efficient, near-unity efficiency photoluminescence up-conversion to we extend the conclusions of these measurements to explain the origin of observed, size-dependent absorption/emission Stokes shifts. Here, despite some initial studies we and others have conducted, very little is known about their true origin. This is to be contrasted to more conventional NCs such as CdSe where band edge exciton fine structure quantitatively accounts for both global and resonant Stokes shifts, with emission emerging from a dark exciton. Although similar perovskite NC fine structure might account for their shifts, both theory and experiment predict bright/dark fine structure splittings easily an order of magnitude too small to account for experiment.

     We now propose that perovskite NC emitting states are polarons, which result from the lattice accommodation of photogenerated charges. Polaron binding energies and lifetimes are, in turn, suggested to be the origin of observed l-, T-, and composition-dependent absorption/emission Stokes shifts and excited state lifetimes. This represents a significant departure from more conventional descriptions of NC band edge states, which exclusively involve exciton fine structure and dark exciton emitting states.

Halide Perovskite Photovoltaics are on the brink of commercialization and after having achieved high performance, the stability, scalability, and sustainability of the technology are now the most important technological challenges to overcome. Halide perovskites being versatile in terms of their processability, there are many options for the development of scalable processing technology. Solution processing based on slot-die coating and inkjet printing are two promising options with their respective design advantages and opportunities.

The development of precursor inks that allow reliable and sustainable processing is one of the most important challenges we are tackling. Our team's current efforts in this domain focus on three major aspects:

Accelerated ink and process optimization by combinatorial processing approaches: We are employing innovative combinatorial processing techniques based on inkjet printing and slot-die coating to expedite the optimization of precursor inks and associated manufacturing processes.

Exploration of Green Solvent Alternatives with Extended Ink Shelf-Life: In our commitment to sustainability, we are investigating environmentally friendly solvent alternatives. Our focus extends beyond performance considerations to include the development of green solvents that not only meet performance standards but also contribute to an extended ink shelf-life. This dual emphasis on performance and longevity is crucial for practical implementation and large-scale production.

Collaborative Data Management: Based on ongoing efforts of the PerovskiteData base Project, we are introducing Research Data Management Tools that can easily be adapted and multiplied by the entire research community. This collaborative approach facilitates efficient decision-making and accelerates the iterative optimization process.

By addressing these challenges head-on, our research aims to contribute to the acceleration of the deployment and utilization of Perovskite Photovoltaics for power generation.

Hybrid organic-inorganic perovskite materials have gained emphasis in worldwide photovoltaic (PV) research, particularly due to their impressive power conversion efficiencies (PCEs) above 26% as well as their potential of becoming a candidate for inexpensive mass production¹. Perovskite-based research today is mostly on thin layer solution based deposition techniques given their potential for integration into semitransparent. However, integration of this approach into windows and buildings is challenging as homogeneity, reproducibility, yield, and other aesthetic relevant characteristics required for building integration market have not been demonstrated to a satisfactory level. As an alternative, three-layered dielectric/metal/dielectric (DMD) thin film sequences have been successfully employed in perovskite-based research resulting in comparable PCEs up to 15%², superior average visible transmittance (AVT), and good light utilization efficiency (LUE)³, defined as the product between AVT and PCE, highlighting the great promise of transparent electrodes. Here, we have studied semi-transparent DMD electrodes based on WO₃ and Ag, to boost the PCE in semi-transparent formamidinium lead halide solar cells. A screening upon the mixed halide perovskite, from pure bromide to pure iodide has been made, demonstrating that the PCE increases with the semi-transparent electrode compare to the conventional semi-transparent Au. This work demonstrates the potential use of semi-transparent electrode to improve the light harvesting of the perovskite layer, by using optical methods to compensate chemical limitation, paving the way for application in building integration. It is proved herein that semitransparent electrodes with WO3 of 40 nm allows for the fabrication of solar cells covering a wide bandgap regime ranging from materials with suitable bandgaps for single-junction solar cells (FAPbI3, Eg = 1.5 eV) to wide bandgap materials (FAPbBr3, Eg = 2.3 eV). The solar cells prepared with this approach give promising results that compete well with more intricate fabrication approaches previously reported, while at the same time current and aesthetic parameter like Average Visible Transmittance (AVT) and Light Utilization Efficiency (LUE) are increased to real building relevant values.

Halide perovskites have been under the focus for photovoltaic applications where their power conversion efficiencies have soared to efficiencies exceeding 26%. They have also garnered tremendous research interest over recent years for the development of next-generation light-emitting diodes and detectors. The spotlight on this class of semiconducting material stems from several of its enviable traits such as long carrier diffusion length, defect tolerance, high colour purity, and spectral bandgap tunability which spans across the visible and infrared spectrum. This talk will cover our recent efforts in utilising the ionic characteristics of halide perovskites to enable memristive devices that can be employed for neuromorphic applications. The talk will cover 2 and 3 terminal memristive devices as well as our efforts to utilise colour changing LEDs and photovoltaic devices as unconventional computing elements.

The family of Sn-perovskite has recently emerged as a nontoxic compound for a future ecofriendly photonic technology. With an extraordinary quantum yield of emission at room temperature, bandgap tunability with the composition, and straightforward incorporation in optical architectures by chemistry methods, the ASnX₃ (A is an organic/inorganic anion and X is a halide) perovskite represents a suitable nontoxic material to implement lasers, optical amplifiers or light emitting diodes, among other devices. However, since Sn²⁺ is easily oxidized into Sn⁴⁺ losing its optoelectronic properties, there are few reports where a stable ASnX₃ device is provided. In this work, high-quality FASnI₃ (FA, formamidinium) polycrystalline thin films are successfully fabricated to demonstrate optical amplification and lasing. With an appropriate low temperature treatment and a polymethylmethacrylate(PMMA) clapping, the FASnI₃ films exhibit a remarkable stability and an efficient generation of amplified spontaneous emission (ASE) under nanosecond optical pumping. First, these films are deposited on a SiO₂ substrate to conform an optical waveguide whose geometrical parameters (i.e. thicknesses of the films) are properly designed to optimize the excitation and to enhance the generation of the photoluminescence. As a result, ASE is demonstrated with an extremely low threshold, about 100 nJ/cm², and a strong polarization anisotropy preferable to the transverse electric (TE) polarization. Moreover, the waveguide exhibits narrow lasing lines (< 1 nm) caused by the formation of random cavity loops in the polycrystalline grains. Then, the same structure is implemented in a flexible device using polyethylene terephthalate (PET) substrates. Here, despite that the formation of films formation of films on a flexible platform is always challenging, the flexible waveguide also shows ASE and RL lines with only one fold higher threshold (1 µJ/cm²). Finally, the FASnI₃ films are also incorporated as active medium in a Distributed Feedback Laser properly designed to achieve optical resonance at the ASE wavelength. This architecture  conforms an optical cavity able to demonstrate the generation of lasing action with peaks narrower than 1 nm. The proposed devices represent an important step towards the development of future cheap and green photonic technology based on Sn perovskites.

Metal halide perovskites are of immense importance due to their excellent optoelectronic properties, holding great promise in the field of advanced and clean energy technologies. One of the fundamental mechanisms governing their physical properties is the interplay of electron-phonon coupling with the strong lattice anharmonicity and their inherent locally disordered structure [1,2]. To understand the consequences of these effects, we have recently introduced a powerful theoretical framework that allows to capture anharmonic electron-phonon coupling in these intrinsically polymorphous compounds [3,4]. In this talk, I will demonstrate our methodology for both inorganic and hybrid halide perovskites and show that (i) local disorder is at the origin of overdamped and strongly coupled anharmonic phonons, (ii) low-energy optical vibrations dominate electron–phonon renormalized band gaps, departing from a simplified picture of a Fröhlich interaction, and (iii) local disorder is the key to explain the monotonic increase of the band gap across phase transitions. Our work provides a new perspective for the interpretation of perovskite materials fundamentals as well as opens the way for efficient simulations of halide perovskites' key properties, like carrier mobilities, excitonic spectra, and polaron physics.

Lead and tin halide perovskites are currently under intense investigation for their potential applications in optoelectronics, due to their favorable and adjustable semiconducting properties. One of the key advantages of perovskites lies in their ability to be deposited as thin films using energy-effective techniques, resulting in high-quality semiconductors. Despite the potential for widespread adoption in the industrial sector, vacuum deposition of perovskite films and devices remains a specialized area. Here we will examine the latest developments in the vacuum deposition of perovskite films, focusing on methods to manipulate their morphology and structure. Specifically, we will highlight the impact of factors such as composition, deposition rate, and substrate temperature on properties like luminescence quantum yield and recombination lifetime. Lastly, we will explore the use of multilayer semiconducting structures in single-junction solar cells and photodetectors.

The dynamic response of metal halide perovskite devices shows a variety of physical responses that need to be understood and classified for enhancing the performance and stability and for identifying physical behaviours that may lead to developing new applications. A multitude of chemical, biological, and material systems present an inductive behavior that is not electromagnetic in origin, termed a chemical inductor. We show that the structure of the chemical inductor consists of a two-dimensional system that couples a fast conduction mode and a slowing down element. The impedance spectra announce the type of hysteresis, either regular for capacitive response or inverted hysteresis for inductive response. We develop the methods to characterize time transient and photocurrent response to a voltage pulse. We can obtain important control of the time constant that determine hysteresis. We apply these insights in kinetics processes to the development of memristors and neurons. Overall we provide a method to determine important kinetic responses of the halide perovskites that can be aused in many different applications.

Perovskites, especially Pb-free tin halide perovskites, show promise for efficient optoelectronic/electronic microsystems and image sensors, fabricated through simple techniques like inkjet printing. This approach aims to reduce device costs globally, cater to various applications, and decrease reliance on current semiconductor technologies. In this work we anticipated innovations with 2D and 2D/3D tin halide perovskites which have the potential to become commercially viable products, addressing the evolving needs of a digital society while promoting sustainability and reducing dependence on existing semiconductor technologies.

Along the work we have proposed and tested experimentally (chemical synthesis and basic structural and optical properties) many perovskite and perovskite-like materials. From them, we focused on several of them: i) 2D/3D perovskite compounds (R0.5,BA0.5)2FA9Sn10I31 with  R = PEA, TEA, DIP cations for allowing crystallization and stabilize the tin-perovskite phase for inkjet printing deposition techniques; ii) FASnI3  with additives and light soaking was further used for spin-coated films used in the optimization of vertical lasing structures;  iii) PEA2SnI4SnI4 and TEA2SnI4 for red emitting LEDs and photonics (emitting devices and photodetectors); Cs3Cu2Cl5 and Rb3InCl6:Sb for blue and green LEDs.

In photovoltaic devices, important advances were achieved: world record efficiency for flexible lead-free perovskite solar modules of 5.7 % under 1-sun and an impressive 9.4% for 2000 lx, indicating their potential for indoor applications. This was possible through the in-situ synthesis of SnI2 from metallic tin and iodine in a DMF:DMSO solvent.

For lead-free LEDs: (i)-(ii) First inkjet-printed Pb-free perovskite LED emitting at red wavelengths and first on a flexible substrate with commom DMF and sustainable DMSO solvents.

In photonics applications several advances were achieved: (i) ASE/lasing action and near single-mode operation were observed over a threshold of 200 µJ/cm2 for inkjet-printed films of 2D/3D G-LFPs, which is close to that obtained with the counterpart device implemented with the 3D FASnI3 film deposited by spin-coating; (ii) This 2D/3D G-LFPs also exhibits good performances for low-level light photodetection: responsivity as high as 50 A/W was measured after one month at 450 nm in encapsulated devices, with an estimated (using dark current value) detectivity better than 8.1010 Jones; flexible photodetectors exhibit comparable performances. (iii) inkjet-printed films of 2D tin-perovskites (TEA2SnI4 among them) are exhibiting efficient two photon absorption mechanism at room temperature and also leading to efficient photodetectors in glass and PET substrates with very low dark currents.

The proposed eco-friendly devices should be taken into account in future advances for scalable flexible optoelectronic Sn-based applications.

The pursuit of efficient and stable perovskite solar cells (PSCs) has propelled investigations into the nuanced realm of charge extraction dynamics. This presentation delves into the comprehensive exploration of charge extraction effects on the efficiency and stability of PSCs, encompassing the intricacies associated with mixed organic-inorganic, inorganic, and lead-free perovskite types. Special attention is devoted to the burgeoning topic of self-assembled monolayers (SAMs), with a focus on the critical role played by hole and electron-selective SAMs in tailoring charge extraction processes [1].

A focal point of our study involves the application of time-resolved surface photovoltage (TR-SPV) as the principal methodology. TR-SPV emerges as a novel and powerful technique, enabling the elucidation of charge dynamics within the remarkably short time scale of 1 nanosecond to 100 milliseconds. Through this method, we unravel the subtle intricacies governing charge extraction, shedding light on the temporal evolution of carrier behavior at the interface [1,2].

The discourse spans an investigation into the influence of various perovskite compositions and structures on charge extraction mechanisms. Our exploration includes a detailed analysis of mixed organic-inorganic, purely inorganic, and lead-free perovskite solar cells [3]. Additionally, we delve into the role of SAMs in modulating charge extraction dynamics, offering insights into their tailored design for enhanced PSC performance.

As a testament to the practical implications of our findings, this presentation culminates in the demonstration of a PSC device exhibiting exceptional stability. The device, subjected to rigorous testing, showcases remarkable resilience, maintaining its structural and functional integrity for an unprecedented 3000 hours without any discernible signs of degradation. This endeavor not only advances our understanding of charge extraction dynamics but also underscores the potential for achieving prolonged stability in perovskite solar cell technology.

Recent discoveries have revealed a breakthrough in the field using inorganic-organic hybrid layers called perovskites as the light harvester in the solar cell. The inorganic-organic arrangement is self-assembled as alternate layers, being a simple, low cost procedure. These organic-inorganic hybrids promise several benefits not delivered by the separate constituents.

In this work we present a fully printable mesoporous indium tin oxide (ITO) perovskite solar cell. The solar cell structure consists of triple-oxide screen-printed mesoporous layers. In this structure, the perovskite is not forming a separate layer but fills the pores of the triple-oxide structure. The perovskite is utilized as both the light harvester and a hole transporting material. One of the advantageous of this solar cell structure is the transparent contact (mesoporous ITO) which permit the use of this cell structure in bifacial configuration without the need for additional layers or thinner counter electrode. We performed photovoltaic (PV) measurements on both sides (i.e. ITO-side and glass-side), where the glass side show 15.3% efficiency compare to 3.8% of the ITO-side. Further study of the mechanism shows that the dominant mechanism when illuminating from the glass-side is Shockley-Read-Hall recombination in the bulk, while illuminating from the ITO-side show recombination in multiple traps and inter gap defect distribution which explain the poor PV performance of the ITO-side. Electrochemical impedance spectroscopy shed more light on the resistance and capacitance. Finally, we demonstrate 18.3% efficiency in bifacial configuration. This work shows a fully printable solar cell structure which can function in bifacial configuration.

Halide perovskite solar cells (PSCs) have already demonstrated power conversion efficiencies above 26%, which makes them one of the most attractive photovoltaic technologies. However, there are several challenges must be overcome for the technology to be competitive and commercially available. One of the main bottlenecks towards their commercialization is their long-term stability, which should exceed the 20-year mark. Several are the strategies currently employed to stabilize PSCs, for example, the use of complex metal oxides as transport layers, the passivation of defects in the halide perovskite layer through additive engineering, or the replacement of metal electrodes by carbon-based electrodes, etc. Among these, additive engineering is an effective pathway for the enhancement of device lifetime. Additives applied as organic or inorganic compounds, improve crystal grain growth enhancing power conversion efficiency. The interaction of their functional groups with the halide perovskite (HP) absorber, as well as with the transport layers, results in defect passivation and ion immobilization improving device performance and stability. In this talk, we will briefly summarize the different types of additives recently applied in PSC to enhance not only efficiency but also long-term operational stability. We discuss the different mechanism behind additive engineering and the role of the functional groups of these additives for defect passivation. Special emphasis is given to their effect on the stability of PSCs under environmental conditions such as humidity, atmosphere, light irradiation (UV, visible) or heat, taking into account the recently reported ISOS protocols. We also discuss the relation between deep-defect passivation, non-radiative recombination and device efficiency, as well as the possible relation between shallow-defect passivation, ion immobilization and device operational stability. We will also show our most recent results applying additives in PSC where we have been able to obtain efficiencies above 21 % and highly stable devices showing null degradation after more than 10000 h under continuous light irradiation of 1 sun.

REFERENCES

[1]     M. V. Khenkin, E.A. Katz, et al., M. Lira-Cantu, Nat. Energy 5 (2020) 35–49.

[2]     H. Xie, M. Lira-Cantu, J. Phys. Energy 2 (2020) 24008.

[3]     A. Mingorance, et al., Adv. Mater. Interfaces 5 (2018).

[4]     M. Lira-Cantú, Nat. Energy 2 (2017) 17115.

[5]     A. Pérez-Tomas, Sustain. Energy Fuels 3 (2019) 382–389.

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Perovskite solar cells (PSC) are highly promising candidates for future space photovoltaics due to their high specific power potential. Yet, for successful application, devices need to resist several sets of extremes in space. In this presentation we will discuss their extraordinary radiation tolerance [1]–[3] and then move on to extreme temperatures and temperature cycles, two extremes that perovskites are well suited to. Atomic oxygen (AtOx), on the other hand, corrodes unencapsulated PSCs swiftly, as we identify. And while we find that this can be avoided using ultrathin evaporated 0.7 µm silicon oxide (SiOx) barriers, we set out to understand the AtOx-induced degradation mechanisms of phenethylammonium iodide (PEAI)-2D passivated and non-passivated devices. Surprisingly, degradation is more severe in 2D passivated PSCs. To disentangle damage mechanisms between 2D passivated and non-passivated devices we apply injection-current-dependent electroluminescence (EL) and intensity-dependent photoluminescence quantum yield (IPLQY) measurements. These allow us to derive pseudo-JV curves that are independent of parasitic resistance effect from damaged transport layers. We quantify an implied FF, that remains high after degradation suggesting that the perovskite bulk is not severely degraded. On the other hand, GIWAX studies reveal a degraded surface that limits charge extraction, and thus leads to lower performance metrics. This surface degradation is severely accelerated for 2D passivations and proceeds to areas that are covered by copper contacts due to lateral diffusion of AtOx though the 2D surface owing to the large interplanar distance of 2D perovskites.

[1]      F. Lang et al., “Efficient minority carrier detrapping mediating the radiation hardness of triple-cation perovskite solar cells under proton irradiation,” Energy Environ. Sci., vol. 12, no. 5, pp. 1634–1647, 2019, doi: 10.1039/C9EE00077A.

[2]      F. Lang et al., “Proton‐Radiation Tolerant All‐Perovskite Multijunction Solar Cells,” Adv. Energy Mater., vol. 11, no. 41, p. 2102246, Nov. 2021, doi: 10.1002/aenm.202102246.

[3]      F. Lang et al., “Radiation Hardness and Self-Healing of Perovskite Solar Cells,” Adv. Mater., vol. 28, no. 39, pp. 8726–8731, Oct. 2016, doi: 10.1002/adma.201603326.

All-inorganic cesium lead triiodide (CsPbI3) perovskites show high potential for photovoltaic applications due to their excellent thermal stability and suitable bandgap (Eg≈1,72 eV) ideal for tandem device. However, the photovoltaic performances of CsPbI3 perovskite solar cells (PSCs) are significantly restricted by a scarcely controllable crystal quality leading to high nonradiative recombination processes [1]. Therefore, the potential of CsPbI3 is overshadowed by the black-phases (α, β, and γ) structural instability, prone to spontaneous evolution into the photoinactive δ-phase at room temperature. This transition can be accelerated by ambient moisture, resulting in diminished PCE. Consequently, stabilizing black CsPbI3 at room temperature has become a critical topic. The regulation of the nucleation and growth rates has been identified as an efficient strategy for enhanced film coverage [2]. CsPbI3 perovskite films are produced via a cost-effective and straightforward solution-based manufacturing process. However, the nucleation and growth processes are largely uncontrollable, leading to inevitable defects stemming from conventional coating methods. Understanding the fundamental mechanism underlying film formation, specifically in terms of nucleation and growth, is crucial. Identifying this mechanism is a necessary step for precisely adjusting film crystallization kinetics. The optimization of additive engineering has proven to be a successful and straightforward approach in developing Cs-based photovoltaic devices with enhanced efficiency and stability. Consequently, it has emerged as a significant focus in PSC (perovskite solar cell) research. The aim of this work is to obtain a stable CsPbI3 polycrystalline thin film modulating the different parameters involved in the solution processing of the films, spanning from the employed solvent to the use of additives modifying the solution chemistry, to the use of alternative Pb sources DMAPbI3 [3]. The chemistry involving additives currently revolves around the utilization of HI and DMAI [4,5] within the precursors salts solution. Experimental evidence in both scenarios substantiates the formation of the intermediate DMAPbI3 within the solution, HI reacts with DMF and DMAI with PbI2. By synthesizing DMAPbI3 through hydrolysis and utilizing it directly as a lead source in the solution, it's possible to achieve better stoichiometric control involving DMA+ and to produce higher-quality films.The formation of intermediate complexes during the crystallization process assumes a pivotal role. For instance, the inclusion of additives, such as Cl− ions in the precursor solution has a positive impact on the nucleation of perovskites. Importantly, these Cl− ions aid in the nucleation process without becoming part of the perovskite lattice structure itself. This, in turn, leads to the production of perovskite films with enhanced optoelectronic properties, suggesting an improvement in their ability to interact with and respond to light. The presented strategies allow to investigate the effect on the nucleation process and how the existence of solute–solvent crystalline intermediate phases have an impact on chemical reaction kinetics. Finally, the optimized films have been integrated and tested in solar cells.

Metal halide perovskite (MHP) semiconductors are excellent candidates for contemporary optoelectronics innovation, particularly for photovoltaics.[1] The advantages of this class of materials derive from their hybrid nature, allowing for straightforward fabrication processes, and from their unique optoelectronic properties. A typical 3D organic-inorganic perovskite has a chemical formula of ABX3, where A is an organic cation (such as MA [methylammonium] or FA [formamidinium]), B is a metal cation (such as Pb2+), and X is a halogen anion (such as I or Br).[2] However, recent advances have also explored more complex compositions embedding diverse cations/anions.[3] These materials are prepared by simple and straightforward solution processing, the material precursors dissolved in a solvent undergoes self-assembly into a perovskite structure during spin-coating onto a substrate under mild thermal annealing.

As the technology continues to mature, this still is a key advantage, allowing for affordable and scalable processing. Understanding perovskite ink properties is therefore a fundamental requirement towards industrialization, with special regards to their evolution over time.

It has been demonstrated that even for the simplest system, the precursor solution is a complex – and dynamic – dispersion which contains not only solvated ions but also lead halide complexes, colloids and aggregates of different natures and dimensions. In these complex dispersions, multiple chemical species are present and can interact – or react – between each other or with the solvent.

We have proved the existence of a reactivity between two of the perovskite components – MA and FA – in the precursors solutions, that leads to the formation of a novel condensation product, methylformamidinium (MFA). We have studied different parameters that affects such reactions kinetics, therefore modifying the ink composition over time, and proposed solutions to overcome these issues.[4]  With the aim of correlating the solution chemistry with the film structural properties, through the synergic use of solution Nuclear Magnetic Resonance (NMR) spectroscopy, X-ray Diffraction and Density Functional Theory (DFT) calculations, we have recognized and explained for the first time a correlation between the aging of perovskite precursor solutions, the presence of MFA species in solution and the emergence of photoinactive hexagonal polytypes (6H and 4H) [5]

Starting from the known reactivity of the chemical species present in ink solutions, we outline the directions towards which future research efforts should be directed.[6]

Perovskite on silicon (Si) tandem solar cells are predicted by the ITRPV to be the first Si-based tandem solar cells to enter the market. Their superior power conversion efficiency promises a step change in module efficiency that is not possible with silicon only technologies, as shown by Oxford PV’s recent certified world record of 28.6% in a full area cell. With its integrated pilot line at Brandenburg, Germany, Oxford PV is currently ramping up to high-volume production of commercial M6 tandem wafer product.

Perovskite tandem commercialisation has taken place in a very short period of time compared to other photovoltaic technologies. This rapid progress has been possible due to the development of in-house protocols and the statistical analysis of the large datasets generated. Some of the advanced characterisation at Oxford PV and how they inform decision making on process and technology designs in view of cell efficiency and longevity targets will be presented. The measurements can be categorised according to their speed, the complexity of data analysis and the value of the measured parameters – increasing measurement speed and analysis allows for more insights on more samples more frequently, and so a key part of industrial solar R&D involves innovations on measurement protocols. In this talk, there will be examples of the advanced characterisation suite that have developed at Oxford PV to maximise the learning for as many samples as possible whilst focusing on the most important parameters, and eliminating redundancies. Upon generation of high quality datasets, machine learning tools are used to improve our statistical understanding of trends we see with development of the Oxford PV tandem cell technology. As well as cell efficiency enhancements, these complimentary research methods are just as suited for improving cell reliability, for which the perovskite field has made good steps in the last 12 months in aligning on predictive accelerated stressing protocols. The big data approach is also suitable for identifying process sensitivities in view of designing scalable processes, a critical part of R&D in an industrial setting.

Finally, sustainability, both in financial and ecological terms, is a crucial aspect of manufacturing at large scale. Leveraging advanced characterisation and machine learning can expedite from small scale proof of concepts towards achieving comparable KPIs with high-TRL production ready processes. Oxford PV is committed to bring to the market the most sustainable technology in the most sustainable way; examples of the initiatives Oxford PV has implemented to make that happen will be presented.

Organic photodetectors (OPDs) with a performance comparable to that of conventional inorganic ones have been demonstrated for the visible wavelength regime. However, near-infrared (NIR) photodetection has proven to be challenging and, to date, the true potential of organic semiconductors in this spectral range (800–2500 nm) remains largely unexplored. In this talk, I will introduce new device concepts for organic NIR detectors, based on resonant optical cavities and doped photo-active layers. Design rules and optimization strategies will be discussed, yielding wavelength selective devices (20 nm resolution) with a tunability of the detection wavelength over several hundreds of nanometers, allowing the printing of miniature NIR spectrometers. In a second part of the talk, we explore the performance limitations of organic NIR detectors: A relation between open-circuit voltage, dark current, and noise current is demonstrated for OPDs with detection wavelengths beyond 1100 nm. Based on these findings we estimate an upper limit of achievable specific detectivity values for organic photodiodes as a function of their longest NIR detection wavelength.

Near-infrared organic photodetectors (OPDs) experienced a fast growth in recent years owing to their advantages over traditional inorganic photodetectors. By fine-tuning the optical bandgap of the organic material, OPDs can be engineered for high and specific light detection. In particular, the advent of non-fullerene acceptors (NFAs), with tunable energy levels, allowed OPDs with light detection in the near-infrared (NIR) region.

Analyzing the structures of reported NFAs, many of the best performing contain thiophene-based fused aromatics within the donor core. A strategy to shift the absorption windows is to replace the thiophene moiety with selenophene. Selenophene has lower aromatic stabilization energy than thiophene and will increase the quinoidal character of the material.

Until now, few fused selenophene containing NFAs containing straight chain alkyl groups have been reported. The replacement of the alkyl aryl groups with straight chain alkyl groups will change the solid-state microstructure of thiophene-based NFAs and then enhance the power conversion efficiency (PCE). In this work, we report the first NFA containing a fused indacenodiselenophene with octyl sidechains at the bridgehead positions. This is the direct selenophene analogue of the previously indacenodithiophene acceptor known as IDIC, which has been widely reported in the OPV community.

Herein we show that the selenophene analogue IDSe exhibits a red-shifted absorption compared to the thiophene analogue IDIC, and when paired with a suitable wide band gap donor PTQ10, organic photodetector devices using a bulk heterojunction configuration have an ultralow dark current density (J_d) of 1.65×10^-9 A cm^-2 at -2V and a specific detectivity (D*) of 10¹² Jones at -2V bias. With a combination of optoelectronic measurements, transient analyses and morphological investigations based on GIWAXS measurements, we attributed the low J_d of the PTQ10:IDSe blend to the higher and more balanced carrier mobilities compared to the thiophene benchmark. In addition, we fabricated large-area OPDs (5x5 cm substrates) using doctor blade coating in air and obtained extraordinarily low dark current, indicating its ability to be large-scaled processed in the future.

Planner hetero-junction (PHJ) structure is one of the key device fabrication strategies to increase specific detectivity by reducing dark current in organic photodiodes (OPDs). We demonstrated that efficient free charge generation in partially chlorinated subphthalocyanine molecules (Cl6-SubPc) can assist PHJ OPDs to achieve detectivity up to 1013 Jones and dark current below 10-7 A cm-2 up to -5 V. The photophysical analysis of Cl6-SubPc based devices clearly show that device photocurrent is primarily generated through direct charge generation within the Cl6-SubPc layer, rather than exciton separation at layer interfaces. Molecular modelling suggests that direct charge generation is facilitated by Cl6-SubPc’s high octupole moment which generates a shift in molecular energetics ­­[1,2] (Fig(1(a)). Also, lower trap densities analysed from capacitance-frequency response [3] in thick PHJ device helps in further minimising leakage currents as shown in Fig(1(b)). In summary, this study provides clear indication that Cl6-SubPc is promising material for single component OPD devices.

Photodiodes based on organic semiconductors exhibit many advantageous properties including tailorable light absorption, low embodied energy manufacturing, structural conformality, and low material toxicity. These properties make organic photodiodes attractive for emerging photodetector applications, especially for novel applications requiring different optoelectronic and mechanical properties than provided by conventional photodiodes based on inorganic semiconductors.[1] However, thus far the specific detectivity of organic photodiodes has remained subpar relative to conventional photodiodes. A critical parameter limiting the specific detectivity of photodiodes is the dark saturation current. In organic photodiode devices, the dark saturation current is strongly limited by non-radiative processes resulting in dark saturation currents several orders of magnitude higher than expected for radiative band-to-band transitions; however, the origin of these non-radiative processes is still debated. In this work, we show that the dark saturation current, along with the specific detectivity, in organic photodiode devices is fundamentally limited by transitions via mid-gap trap states.[2] This new insight is generated by a universal trend observed for a large set of organic bulk heterojunction systems and substantiated by sensitive external quantum efficiency and temperature-dependent current measurements. These findings have important implications for organic photodiodes, providing new insight into the origin of non-radiative losses and the associated noise, establishing the performance limits for these devices. 

Solution-processed metal halide perovskite photodetectors (PPDs) have a great potential in visible and near-infrared light sensing and X-ray imaging applications, owing to their excellent optoelectronic properties. This presentation will consist of two parts. After a brief survey of the advantages of solution-processed perovskites in high-resolution, flexible large-area imagers we will focus on better understanding the physical mechanisms that determine the dark current density and detectivity of thin-film photodetectors in the first part [1].

We will demonstrate that extrinsic leakage current at the periphery of the bottom pixel electrode is greatly reduced by introducing a so-called pixel edge cover layer. When such an edge cover layer is inserted, the dark current density decreases to ~10−6 mA cm−2 at −0.5 V, which is among the lowest values reported for polycrystalline PPDs.1 Moreover, the dark current density becomes independent of pixel size, up to 1000 ppi. To reduce dark current density even further, we carefully investigated a range of charge blocking layers, effectively reducing the dark current density even further.

By combining the appropriate blocking layer with the edge cover, a perovskite thin-film photodiode was fabricated that has a spectral responsivity of 0.5 A/W at 950 nm (Fig. 1a), combined with an ultralow dark current density of 5 × 10−8 mA cm−2 (Fig. 1b) and noise current of 2 × 10−14 A Hz−1/2 .

In the second part we will present recent developments in realizing three prototypical applications, i.e. a paper-thin photo-imager [2], a direct conversion X-ray detector [3] and a novel narrowband optical sensor that can measure a person’s heartbeat and respiration rate from a distance of over one meter using NIR light [4].

Including thin-film encapsulation layers - required to guarantee a long lifetime - the photo-imager [2] is only ca. 100 mm thick and can be wrapped around a non-flat object (radius of curvature of 0.6 cm). The wrapping ensures the close proximity between sensor surface area and a curved object, effectively increasing the area of the object that can be imaged with high-resolution. This is relevant for instance for nail-to-nail fingerprint imaging. Biometric fingerprinting is demonstrated with liveness detection.

The resulting direct-conversion X-ray detector [3] has a low detection limit of 0.22 nGyair frame-1, a sensitivity of 1060 µCGyair-1cm-2 (at a bias of 0.03 V µm−1) with a high spatial resolution of 6 line-pairs mm-1.

Finally, we report an inherently-narrowband solution-processed, thin-film photodiode with ultrahigh and controllable NIR responsivity based on a tandem-like perovskite-organic architecture [4]. The device possesses low dark currents (< 10−6 mA cm−2), linear dynamic range > 150 dB, and operational stability over time (> 8 h). With a narrowband quantum efficiency that can exceed 200% at 850 nm and intrinsic filtering of other wavelengths to limit optical noise, the device exhibits higher tolerance to background light than optically-filtered silicon-based sensors.

Formamidinium lead iodide (FAPbI₃) has gained considerable interest as a promising photoactive layer for optoelectronic devices due to its broad spectrum of light absorption and increased thermal stability when compared to its conventional methylammonium counterpart (MAPbI₃). Recent developments in substituting formamidinium (FA) with smaller Cs cations have further accelerated its growth in the photovoltaic (PV) community by enhancing phase stability and power conversion efficiency (PCE). However, only a few studies have been reported on perovskite photodetectors (PPDs). Here, we investigate the influence of Cs incorporation on PD performance in the Cs_XFA_1-XPbI₃ system (X = 0–0.25). We demonstrate that A-site substitution with Cs reduces microstrain within the crystal lattice and promotes the formation of a compact, well-ordered surface with improved morphology. To evaluate the performance of the Cs_XFA_1-XPbI₃ system, we have prepared p-i-n photodiode-type PDs using a one-step spin-coating method. We find that the lattice strain relaxation considerably reduces the Jd in the device, which may be partially due to the reduction in lattice strain which reduces trap density and improves charge transport. Particularly, the lowest Jd value of 3.3x10^-9 A cm^-2 is achieved for Cs_0.05FA_0.95PbI₃ at -0.5 V, which also greatly contributes to its highest specific detectivity of 6.1x10¹¹ A Hz^-1/2 and widest linear dynamic range of 135 dB. Furthermore, when this mixed-cation perovskite material is integrated into a device, long-term stability without sacrificing performance is observed, which holds great promise for future optoelectronics applications.

Lead halide perovskite and organic semiconductors have been promising classes of materials for photodetector (PD) applications. However, despite their high performances with low noise and specific detectivity (D*) values exceeding the one of silicon detectors, perovskite PDs working regime is limited to the visible and the first fraction of near-infrared (NIR) spectrum due to their bandgap.1,2 Differently, organic PDs have the advantage of tunable absorption thanks to their chemical design. In this work, we fabricated perovskite-organic heterojunction (POH) PDs with extended absorption up to 900 nm, thanks to the deposition of a donor:acceptor bulk-heterojunction (BHJ) layer on top of the perovskite. We systematically studied the effect of the energetic of the donor materials on the dark current (Jd) of the device by using polymers from the PBDB-T family (PCE12, PM6 and PM7). By using this approach, PM7-based POH delivered ultra-low noise of 2 x 10-14 A Hz-1/2 and high specific detectivity D* of 4.7 x 1012 Jones (-0.5 V) at 840 nm. This was achieved by increasing the energetic barrier between the HOMO of the donor and the LUMO of the acceptor as explained by our analytical model.3 We found that the donor sites effectively act as recombination centres at the perovskite/BHJ interface, affecting Jd. In particular, PM7 with a deeper HOMO level, formed a larger LUMO(Y6)-HOMO(D) gap compared to PM6. This resulted in a smaller contribution of the HOMO of the donor to the dark current, similar to the intrinsic traps introduced by Y6 at perovskite/BHJ interface.

Printing technology is set to enable the high-throughput low-cost and customized fabrication of optoelectronic and sensors devices. For this goal to become a reality, this functional printing approach should encompass a material process development which enables high device performance and industrial compatibility, thus enabling a facile transfer of research results into consumer applications.

In this contribution, I will present the fabrication of fully printed organic photodiodes (OPDs) by digital printing techniques (e.g. aerosol and inkjet). The devices show mechanical flexibility, semitransparency and an excellent reproducibility. I will discuss the use of the ink formulation as way to access and tailor material optoelectronic properties. In this direction, we have focused on the control of the molecular order in organic semiconductors though an inkjet printing process. This enabled us to deposit functional polymers with a high degree of alignment and explore its use in polarization sensitive OPDs. Secondly, I will outline our recent efforts in the fabrication of inkjet printed OPDs based on novel non-fullerene acceptors (NFAs). The devices show a photo-response up to 800nm and reach record responsivities of 400mA/W as well as cut-off frequencies surpassing 2MHz. Furthermore, we demonstrate the successful decoupling of the optical and rheological properties by using a visibly transparent polymer donor and color-selective NFAs. This approach offers spectral flexibility without the need for a variation in process parameters. The choice of NFA enabled devices with color selectivity in the ranges of 400-600nm and 500-800nm enabling a proof of concept filterless visible-light communication system. Finally, I will present the fabrication printed passive and active matrix OPD arrays.

Photodetectors that are sensitive in shortwave infrared (SWIR) range (1 µm - 2 µm) are of significant interest for applications in 3D, night and adverse weather imaging, machine vision, autonomous driving, among others. Currently available technologies in the SWIR rely on costly epitaxial semiconductors that are not monolithically integrated with CMOS electronics. Solution-processed quantum dots have been developed to address this challenge enabling low-cost manufacturing and facile monolithic integration on silicon in a back-end-of-line (BEOL) process. To date, colloidal quantum dot (CQD) materials to access the SWIR have been based on lead sulfide (PbS) and mercury telluride (HgTe) compounds imposing major regulatory concerns and potentially impeding their deployment in consumer electronics. In this talk I will present a novel synthetic method of environmentally friendly silver telluride quantum dots and their application in high-performance SWIR photodetectors. The CQD photodetector stack is entirely based on Restriction of Hazardous Substance (RoHS) compliant materials and exhibits a spectral range from 350 nm - 1600 nm, with room-temperature detectivity of the order 10¹² Jones, 3dB bandwidth in excess of 100 KHz and linear dynamic range of over 118 dB. We further demonstrate, for the first time, a monolithically integrated SWIR imager that is based on solution processed, heavy-metal-free materials, paving the way of this technology to consumer electronics market [1]. In the second part of the talk I will turn to our recent activities on the development of III-V CQDs and their applicaiton in photodetectors aiming for speeds suited for gated imaging. I will dicuss the challenges on the synthesis of InSb CQDs in terms of surface passivation and environmental stability and present our approach to overcome those challenges reporting InSb CQD SWIR photodetector that exhibits external quantum efficiency (EQE) of 25% at 1240 nm, a wide linear dynamic range exceeding 128 dB, fast photoresponse time of 70 ns, and specific detectivity of 3.6 × 10¹² Jones.

[1] Silver Telluride Colloidal Quantum Dot Infrared Photodetectors and Image Sensors, Yongjie Wang¹, Lucheng Peng¹, Julien Schreier², Yu Bi², Andres Black², Aditya Malla¹, Stijn Goosens², Gerasimos Konstantatos^1,3*, Nature Photonics, accepted.

The ultraviolet (UV) part of spectrum is of paramount importance for a large range of applications from health and safety to process and environmental monitoring. Yet the complexity of integrating UV sensitive optoelectronics on standard complementary metal–oxide–semiconductor (CMOS) technology has curtailed to a large extent the use of UV spectrum, especially in consumer electronics. Currently UV photodetectors rely on III-V nitride semiconductors that are not monolithic to silicon and are typically fabricated off-die.

    We overcome this challenge by developing RoHS compliant environmentally friendly colloidal quantum dots (QDs) (zinc magnesium oxide , ZnMgO) which has compositionally tunable absorption across UV and high photoluminescence quantum yield (> 90%) in the visible.[1] Herein, we will present a non-toxic, visible blind QD based down-converting thin-film technology that expands the spectral coverage of Si-CMOS-sensors into the UV, enabling efficient UV detection without affecting the sensor performance in the visible and NIR. A Si-photodetector (PD) integrated with the QDs results in a record 9-fold improvement (800% relative enhancement) in photoresponsivity from 0.83 to 7.5 mA W⁻¹ at 260 nm. Leveraging the tunability of these QDs, we will show a simple UV-band identification scheme (a sensor), which uses two distinct-bandgap ZnMgO QDs stacked in a tandem architecture whose spectral emission color depends on the UV-band excitation light. The downconverting stack enables facile discrimination of UV light (different UV bands) using a standard CMOS image sensor (camera) or by the naked eye and avoids the use of complex optics.

 Colloidal infrared (IR) quantum dots (QDs) have been studied with many advantages, such as tunable absorption and fluorescence emission spectra, high molar extinction coefficients, high photoluminescence (PL) quantum yield, high stability, cost-efficient and scalable synthesis, and solution processability. Most of the reported IR-active QDs are based on materials containing toxic heavy metals, such as PbS and HgTe QDs. III- V semiconductors containing one group-13 and one group-15 element of the periodic table are highly promising Pb- and Hg-free alternative materials.^[1] Colloidal InSb QDs have been considered as a low-toxic alternative to Pb or Hg chalcogenide QDs. The InSb has a large excitonic Bohr radius of ~60 nm, and it allows the band gap of InSb QDs to be widely tuned in the IR spectral range through size control. However, only a few chemical synthesis methods have been reported due to the limited precursors and the synthesized InSb QDs have featureless or broad excitonic absorption peaks showing extremely broad size distribution so far.

 Here, we report the chemical synthesis and surface engineering of colloidal InSb QDs. In the synthesis part, various synthesis parameters such as precursors, temperature, time, and an introduction of additional ions are explored to produce InSb QDs with unprecedented optical quality. The optical properties of InSb QDs are studied mainly based on the wavelength and the width of the excitonic absorption features. We demonstrate full tunability of the first excitonic absorption peak in the IR range from 700 to 2000 nm and narrow line widths by virtue of an improved size homogeneity of the InSb QDs. The effect of each synthesis parameter on the optical properties of InSb QDs and their growth mechanism will be discussed. In the surface engineering part, the surface ligands of InSb QDs are altered from the original long-chain organic ligands to short-chain ligands to afford conductive InSb QDs films. Several ligands were tested and their optical/structural properties of InSb QDs were compared before and after surface modification. With the thus optimized experimental conditions, we obtain InSb QDs films with high stability, high conductivity, and suitable band energy levels for application as environmentally benign IR photodetectors.

Photodetectors on the basis of transition metal dichalcogenides (TMDCs) have let to impressive performance numbers with regard to gain and responsivity.¹ In contrast, their speed, parametrized by the response time and 3dB electrical bandwidth, has been mostly disappointing so far with response times often ranging in the milli- to microsecond regimes. The corresponding 3dB bandwidths seldomly exceed a few MHz, and this is too slow to be competitive with established, fast photodetection materials that typically operate with GHz speed.

This presentation details how rational tuning of the substrate material,² the contact geometry,³ the photoresistance and the capacitance⁴ of lateral photodetectors based on mechanically exfoliated WSe₂ layers affects the 3dB bandwidth and enables photodetection with a speed in excess of 230 MHz. We show that optical switching with such devices can be carried out at zero bias, requiring just 27 fJ per switching event. Reducing the detector thickness to mono- and bilayers of WSe₂ only weakly decreases the response speed, rendering WSe₂ advantageous over MoS₂ for fast optical communication. Further miniaturizations of the device geometry have the potential for Gigahertz photodetection with such easily fabricated TMDC photodetectors which exhibit long-term stability under ambient conditions.

Organic semiconductors have raised much attention due to their many attractive properties, like low-cost, light weight, mechanical flexibility, and abundant availability. Despite that, they still lack high mobility compared to their inorganic counterparts as a consequence of their often amorphous nature.
An exception to this is rubrene, which can form highly ordered phases, demonstrating an extraordinarily high charge carrier mobility for holes (>10 cm²/(Vs)^-1) even in thin films. Depending on the post-treatment of our vacuum-deposited films, we can control different crystalline phases, like the orthorhombic and triclinic ones.
For fast-response OPDs, the triclinic phase is a promising candidate since it exhibits high hole mobility in the vertical direction. However, the high surface roughness is a crucial reason why these devices fall short in specific detectivity up to now. In this work, we employ different strategies to reduce the impact of Ohmic shunts within the device to minimize the noise current of our devices. Optimizing the concentration of rubrene and C₆₀ in the active layer and adjusting the electron transport material and its thickness are essential building blocks for achieving low noise currents. However, the most promising approach has been to establish a suppressing layer for the post-treatment of the film. This suppressing layer allows to reduce the dark current by four orders of magnitude. We characterize the morphology of our films with atomic force microscopy and grazing-incidence wide-angle X-ray scattering and investigate the performance parameters of fully working devices. In addition to a low noise current, a high external quantum efficiency is crucial for achieving high specific detectivity values. We can accomplish a competitive specific detectivity of 10¹³ with the employed strategies based on thermal noise domination at 0 V. Speed, noise, and linear dynamic range are investigated to complement the study.

Metal halide perovskites (MHPs) have shown excellent results as X-ray scintillation detectors. X-ray tomography requires many projections and therefore scintillators with excellent stability, which is challenging for MHPs that often suffer from fast degradation under X-ray irradiation and ambient conditions. MHP nanowires could offer improved sensitivity and resolution by exploiting nanophotonic light guiding, and the stability could be improved by growing physically protected nanowires directly in anodized aluminum oxide (AAO) nanopore templates.

We have developed a one-step solution method to grow arrays of up to 30 μm long single-crystalline CsPbBr₃ nanowires in an AAO template, with controlled diameters ranging from 30 to 360 nm [1,2]. The CsPbBr3 nanowires in AAO (CsPbBr3 NW/AAO) show increasing X-ray scintillation efficiency with decreasing nanowire diameter, with a maximum photon yield of ∼5 300 ph/MeV at 30 nm diameter. 2D X-ray images can distinguish line pairs with a spacing of 2 μm and slanted edge measurements show a spatial resolution of ∼160 lp/mm at modulation transfer function (MTF) = 0.1 [1]. The stability was tested over 2 weeks of X-ray exposure [3]. The X-ray scintillation unexpectedly correlated positively with room humidity, but showed no systematic degradation. The resolution was stable at (180 ± 20) , i.e., about 2.8 micron.

For 3D microscopy, a tomogram with 600 projections was taken with a Cu X-ray source over 41 h. The scintillation variations remained below 5% during the acquisition, which allowed a successful 3D reconstruction with high spatial resolution [3]. The combination of high spatial resolution, radiation stability, and easy fabrication makes these CsPbBr3 NW/AAO scintillators a promising candidate for high-resolution X-ray imaging applications.

The development of detectors for high energy photons, protons and heavy particles is a long-lasting research topic not only for fundamental applications but also, more recently, for medical applications in radio and hadron therapy. There is an increasing demand for sensors able to provide, ideally in-situ and in real-time, an accurate recording and mapping of the dose delivered during a treatment plan. The development of novel high performing, thin and flexible sensors for the detection of ionizing radiation in real-time at affordable costs is rapidly increasing, as the technology currently available still fails to address the requirements of large-area, conformability and portability, lightweight and low power operation.

Organic small molecules and polymers are promising active layers for advanced dosimetry purposes, as their mechanical features allow the development of devices able to adapt to complex contoured surfaces with outstanding portability (low power operation) and lightweight. They also provide the unique possibility to develop human-tissue-equivalent detectors, thanks to their density and composition, which makes them ideal candidates for medical dosimetry applications. Their low average atomic number and density, also grants a low absorption of the incoming radiation, making them extremely radiation-tolerant. The physical process of radiation detection for organic thin- film based detectors will be discussed in two different configurations: 1) the direct one, based on a simple planar device with an organic thin film as active conversion layer, and 2) the indirect one, based on a polysiloxane-based scintillating layer effectively coupled to an organic phototransistor (OPT).

We report on their performance under exposure to intense photons and MeV protons radiation fields and will discuss how to detect and exploit the energy absorbed both by the organic semiconducting layer and by the plastic substrate, allowing to extrapolate information on the irradiation history of the detector. A new kinetic model has been developed to describe the detector response mechanism, able to precisely reproduce the dynamic response of the device under photon/proton irradiation and to provide further insight into the physical processes controlling its response.

Organic semiconductors have proven to be a versatile technology platform and are compatible with high-scale mass production. Vacuum-deposited organic light-emitting diodes (OLEDs) dominate mobile display applications and are superior due to their mechanical flexibility, high contrast, low-cost, and form-free production. Recently, organic photodetectors have attracted much interest since they are directly compatible with the OLED display technology and have excellent potential to complement it. Furthermore, their physical properties, such as semitransparency, flexibility, narrowband detection, and, most importantly, high detectivity, offer new perspectives for optoelectronic sensing and will stimulate new consumer electronics applications.

Typically, organic solar cells and photodetectors are comprised of an electron donating and accepting material to facilitate efficient charge carrier generation. This approach has proven successful in achieving high-performance devices but has several drawbacks. For example, creating the proper donor-acceptor microstructure is critical to achieve the desired performance. However, this is challenging when it comes to upscaling and is considered one of the significant degradation pathways regarding the stability of these devices. Therefore, we investigate a vacuum-deposited oligothiophene A-D-A molecule in a single-component device and find that it can generate free charge carriers with an internal quantum efficiency of 20% already at zero bias. Optimizing the device structure, we achieve specific detectivities of 10¹³ Jones (based on noise measurements), high speed (f_-3dB = 330 kHz), and linear dynamic range (190 dB). To unveil the charge carrier generation within this material system, we employ ultrafast transient absorption spectroscopy and quantum chemical calculations and find that free charge carriers are already formed at time scales below one picosecond. Exhibiting this excellent performance and using the simple device structure, such single-component devices are perfect candidates for application.

Methylammonium lead bromide (MAPbBr₃) have emerged as the next-generation materials for self-powered photodetectors due to its easy fabrication protocol and better stability. However, there is still a lack of sufficient understanding of the effect of electrode distance, metal contacts, irradiation power and applied temperature on the performance of MAPbBr₃ SC-based perovskite PDs. Here, we demonstrated the impact of different electrode materials, variable electrode distance,¹ light intensities and temperatures on the performance of MAPbBr₃ SC-based perovskite PDs and analyse them with the help of transient photoresponse and impedance spectroscopy.² This study reveals that the key performance parameters of PD decrease with increasing irradiation intensity as well as increase in Schottky barrier height (SBH) between the metal and the semiconductor, due to changes in charge recombination and carrier lifetime.^2,3 On the other hand, the temperature-dependent study revealed that the performance of PD was found to be related to the increasing scattering of impurities, temperature activated ion accumulation, and phonons, change in conductivity and band gap rather than the change in charge recombination. Next, we fabricated a planar-type PD based on metal doped p-type MAPbBr₃/n-type MAPbBr₃ SC junction photodiode using asymmetric electrode and use the combined effect of electric field due to asymmetric Schottky barrier formation and additional built-in electric field in the p-n junction depletion region demonstrate for the first time to achieve excellent self-powered photodetection properties.⁴ The p-n junction on the MAPbBr₃ single crystal was formed by a controlled epitaxial grow of Ag⁺ doped MAPbBr₃ SC (p-type) on the facet of Sb³⁺ doped MAPbBr₃ SC (n-type). The as-integrated p-n junction device with asymmetric electrodes shows a typical photovoltaic behaviour with a high open circuit voltage of 0.95 V and great sensitivity to 530 nm illumination at zero bias with a responsivity of up to 0.41 A W^-1 and a specific detectivity of 6.39 × 10¹¹ Jones, which are among the highest values reported for MAPbBr₃ single crystal based self-powered photodetectors. In addition, the p-n junction device maintained 80% of the initial performance after being exposed under continuous operation illumination for 12 h in atmospheric condition and restored 94% performance of its initial value after 36 h of storage in the dark. This work provides a new way for designing a high performance self-powered perovskite-based p-n junction photodiodes.

The rapid development of industry in field of machine vision systems, the Internet of Things (IoT) and smart houses rises the demand for scalable bifacial photodetectors [1-3]. Perovskite photodetectors (PPDs) hold a great potential to dominate photodetector market due to their high detectivity, fast response, and cost-effectiveness. However, unlocking the full potential of PPDs for the industry requires overcoming critical technological challenges, notably in scalability, robustness, weight, and semitransparency.

To bring PPDs towards commercialization, we have fabricated scalable self-powered PPDs using the industrially compatible slot-die coating technique on flexible plastic substrates. The integration of thin dielectric-metal-dielectric layers as transparent top contacts has granted the devices with an impressive bifaciality factor of 78% and light-to-dark current ratios reaching an order of magnitude of 10⁴. Alongside their high detectivity, fabricated PPDs possess the capability to harness solar energy, exhibiting reasonable power conversion efficiencies (PCEs) from both, the bottom- and top-side illuminations.

In this presentation, we will showcase the advancements we made in scalable PPD technology. We will discuss the role of selection of top contacts to optimize the performance of bifacial optoelectronics and conclude the key aspects of fabrication of scalable devices.

The very rapid growth of internet-connected devices brings two important challenges.  The first is how will all these devices communicate?  The second is how will they be powered? Optical communications can help, particularly in the form of “Li-Fi” which modulates indoor lighting to transmit information.  It is important to supplement Wi-Fi because the radio frequency spectrum is now very crowded.  Organic semiconductors have been shown to be useful as both transmitters and receivers for Li-Fi, and also for indoor power harvesting.

This talk will give an overview of Li-Fi using organic semiconductors and then focus on their use as receivers.   In particular it will show how organic photovoltaics can be used to receive data and generate power simultaneously.   By using an array  of four detectors in a multiple input multiple -output (MIMO) setup data rates up to 363 megabits per second were achieved together with simultaneous harvesting of power.  These results show that organic photovoltaics can be used for wireless data communication while prolonging the battery life of a mobile device.

The design and implementation of complete, operational photo-electrochemical devices requires a careful consideration of material requirements and their interplay in an integrated device. Multi-physical, multi-dimensional and multi-scale modeling tools are essential for the investigation of feasibility, optimization and implementation of such devices. Modeling activities however always have to be fed and, ultimately, validated by experimental data of material and device characteristics. In this talk, I will discuss the development of predictive modeling tools for photo-electrochemical water and CO2 reduction. These models account for the atomistic-scale via density functional theory, molecular-scale by micro-kinetic and molecular dynamics models, nm-scale by double layer transport models, meso-scale by tomography-based direct numerical simulations, and device scale by volume-averaged device models. I will review each of these modeling steps and highlight the importance of interfacing them. I will then show what experimental data is needed to run these simulations and how these models can be validated. I will end with some device-level demonstrations of photo-electrochemical devices that follow the design guidelines developed by the multi-scale models.

Unlimited access to clean, sustainable and renewable source of energy seems to be a dream scenario, however with an assistance of machine learning (ML) approach the experimental work which actually reveals the right descriptors may gain a powerful tool allowing to omit a time consuming trials path. As a matter of fact, the ML has already shown the potential to significantly reduce the time and effort required for developement of new and efficient photocatalysts. 

However, precise definition of descriptors will require an identification of the bottleneck materials parameters, features and processes, thus different architectures and working arrangements will be shown and disscused in this presentation in order to test their usfulness for prediction of novel and better arrangements for hydrogen and other carbon based solar fuels production. 

Over 400 samples based on the semiconductor oxides have been prepared and measured in order to bulid a pre-base for ML tests. Different materials and working systems, their compositions, geometry, properties will be disscused in view of their impact on the final prediction. In this presentation we  will discuss the strengths and weaknesses of different architectures and working arrangements for hydrogen and carbon-based solar fuels production.

Nowadays, the existence of high entropy perovskite oxides is well established and, since their discovery, their fields of application have been continuously studied. The main advantage provided by these types of materials with general formula ABO₃ is the high possibility of tuning their properties by acting on the chemical compositions. The study of stability, fields of existence, and solubility limits in a systematic way remains lacking in comparison with continuous application research, despite the presence of large datasets and computational studies.¹ In our work, we investigated the structure and solubility limits using a chemometric approach for two families of perovskite oxides with the following compositions: on the site A we chose to keep lanthanum due to its stability, while, on the B site, we explored different cation mixtures based on Cr, Mn, Fe, Co, Ni and Zn, to determine an experimental domain by including various experimental data, i.e. crystal structure, oxygen vacancies amount and temperature-dependance, and composition.² The synthesized samples were subjected to x-ray diffraction (XRD) analysis followed by Rietveld refinement to determine crystallographic parameters. Through multivariate analysis, elemental concentrations were correlated to phase stability and nature, and cell parameters. The structural study aims at highlight possible correlations between the crystal symmetry and the catalytic characteristics of the material. Finally, through thermogravimetric analysis, it is possible to study non-stoichiometry and phase transitions to be integrated within the previously designated experimental domains.³ The overall dataset will allow to define the best compositions to be used for the desired applications such as heterogeneous catalysis, solid-oxide fuel cells, and oxygen transport membranes.⁴

Luminescent solar concentrators (LSCs) are a promising technology to help integrate photovoltaic (PV) cells as functional architecture in the urban environment.[1] LSCs typically consist of a waveguide slab, which is doped or coated with a luminescent species, or luminophore.[2] Incident light is absorbed by the luminophores and re-emitted at longer wavelengths through photoluminescence. A large portion of the emitted light is concentrated by the waveguide via total internal reflection to the edges of the LSC, where a strip of PV cells may be retrofitted attached to collect the spectrally converted light. While LSCs have traditionally been cuboidal in geometry, in recent years, new architectures have been explored for example, circular, curved, polygonal, wedged, and leaf-shaped designs.[3,4] These new
designs can help reduce optical losses, enable integration within existing infrastructure, or unlock new applications.

However, fabrication of complex LSC geometries can be both time- and resource-intensive. LSCs are commonly made using conventional manufacturing methods such as casting, which either limits possible architectures to the availability of suitable moulds or leads to material waste as designs are cut to shape. Moreover, it is not possible to introduce secondary internal structure within LSCs by casting. To tackle these limitations, in this talk the use of 3D printing (specifically fused deposition modelling) as a tool to fabricate complex prototype LSC architectures for rapid optical evaluation will be described. It will be shown that the print pattern (e.g. line thickness, build-up of layers) introduces internal structure within the LSCs, affecting the optical pathways for light absorption and emission. Moreover, we will demonstrate how we have implemented upgrades to the Monte Carlo ray-trace software pvtrace, to enable advance prediction of the optical efficiency of 3D-printed LSCs.[5] The more versatile computational workflow afforded by our upgrades, coupled with 3D-printed prototypes, will enable rapid screening of more intricate LSC architectures, while reducing experimental waste.

The search for new photoactive materials able to efficiently produce solar fuels is a matter of growing interest due to the current global energetic crisis. In response to this situation, the generation of solar fuels has appeared as a sustainable alternative. In last years, extensive efforts have been made to develop efficient catalytic systems capable of harvesting light absorption and reducing CO₂ especially when using water as the electron donor.

Herein, we report different strategies and modifications of photocatalysts to increase process performance. Among them, an interesting approach to improve charge separation in photocatalytic systems is the use of heterojunctions. In this line, the combination of different semiconductors with noble metal nanoparticles or organic semiconducting polymers leads to a separation of the photogenerated charge carriers to increasing their life time, facilitating charge transfer to adsorbed molecules.

Organo-inorganic hybrid materials show a dramatic reactivity improvement in CO₂ photoreduction, enhancing methane selectivity. Reaction pathways are not well defined for this reaction and several uncertains are still unsolved. To explain this behavior a combination of in-situ NAP-XPS, FTIR, TAS spectroscopies and theoretical tools has been used, showing a more efficient light absorption and charge transfer in the hybrid photocatalyst compared with bare materials.

In photocatalytic systems for solar energy conversion, a key challenge for efficient operation is the efficient separation and stabilisation of photogenerated charges, and thereby the minimisation of undesired recombination losses. This challenge is particularly great for photocatalytic systems because of the long charge lifetimes required to drive catalysis A further consideration is to minimise the energy losses required to drive this charge separation. My talk will cover examples of recent work from my group addressing aspects of this challenge. I will start by considering the role of polaron localisation / relaxation in driving charge separation in metal oxides, and quantification of the energetic loss associated with this polaron formation. Kinetic competion between polaron formation and ligand field state mediated charge recombination will be highlighted as the key challenge limiting the performance of many visible light absorbing metal oxides. The roles of metal oxide dielectric constant, surface facet energies, traps, ‘photocharging’ and heterojunctions in aiding charge separation in metal oxides will be discussed, drawing on examples from BiVO₄ and SrTiO₃. I will then go on to discuss charge separation and stabilisation in alternative photocatalyst materials, including organic semiconductor nanoparticles and metal-organic frameworks, highlighting the observation of remarkably long lived charge photogeneration in both systems and their relevance to the efficiency photocatalytic performance.