The development of charge-selective monolayers is pivotal for advancing the performance and stability of perovskite photovoltaics. In this contribution, we will report on the latest developments by the team at Karlsruhe Institute of Technology on self-assembled monolayers (SAMs) with carbazole-based compounds for charge transport layers in perovskite-based single junction solar cells and tandem devices. This contribution encompasses results on interfacial optimization, evaporated SAMs, device stability as well as performance.

In 2022, we reported for the first time that evaporation is a suitable method to realize high performance SAM hole transport layers (SAM-HTLs) for perovskite photovoltaics [1]. Here, we will present a follow-up study that reports on the critical role of SAM-HTLs for the growth of co-evaporated perovskite thin films and the amount of organic materials required for stoichiometric film growth [2]. Through simulations, material analysis and X-ray spectroscopy, we demonstrate how exposed phosphonic acid functional groups impact the perovskite crystal structure and material usage. Our findings pave the way for the controlled design of substrates to optimize interfacial interactions and material consumption while enhancing the electrical properties of the films.

Additionally, we will discuss a study that reports on the use of SAM-HTLs and a bilayer passivation strategy for reducing interfacial non-radiative recombination at the perovskite/electron-transport-layer interface. This interfacial design strategy leads to improved charge extraction and a high power conversion efficiency (>31%) as well as good stability in perovskite/Si tandem solar cells after 1,000 hours of maximum power point tracking at 1 Sun [3].

[1] Farag, A.; et al. Evaporated Self‐Assembled Monolayer Hole Transport Layers: Lossless Interfaces in p‐i‐n Perovskite Solar Cells. Adv. Energy Mater. 2023, 13.

[2] Feeney, T.; et al. Understanding and Exploiting Interfacial Interactions Between Phosphonic Acid Functional Groups and Co-Evaporated Perovskites. Matter 2024, 7, 2066–2090.

[3] Fang, L.; Interfacial Design Strategies for Stable and High-Performance Perovskite/Silicon Tandem Solar Cells on Industrial Silicon Bottom Cells. Under Review.

The stability of perovskite solar cells under working conditions is the main challenge to their practical application. The defect passivation in the bulk and at the surface of perovskite films has been applied as the state-of-the-art strategy in enhancing stability in perovskite solar cells. The interfacial energy alignment is well known to maximise efficiency but not stability. No direct experimental evidence provides insight into stability and energy alignment. The misunderstanding may have originated from the abundant literature on surface treatments and the impact of those on stability. However, the surface treatments so far reported in the literature simultaneously modify the defect density and the energy alignment of the perovskite. We disentangled the defect density by the energy alignment, providing direct evidence for stability and energy alignment. In this talk, we shall summarize that an optimal energy level alignment and consequent selective charge extraction at the interface with the contacts are equally necessary for stability, on top of passivating the defects within the perovskite.

Several recently reported high-efficiency devices incorporate 2D layers to passivate 3D perovskites, leading to improved performance metrics such as open-circuit voltage (Voc) and long-term stability. Motivated by these advances, we investigated the growth of oriented 2D Ruddlesden–Popper metal halides by pulsed laser deposition on various substrates. While oriented growth is found to be substrate-independent, the stability of the 2D phase is strongly influenced by the underlying substrate. Specifically, (PEA)₂PbI₄ films grown on strained epitaxial MAPbI₃ remain stable for over 184 days, showing no signs of cation exchange. In contrast, when deposited onto single crystals of MAPbI₃, the (PEA)₂PbI₄ films undergo a phase transition from n = 1 to n > 3, driven by cation exchange. The optoelectronic properties of these 2D/3D interfaces were analyzed both in situ and ex situ, offering insights into the growth mechanisms and material characteristics required to achieve stable vapor-phase heterojunctions. We discuss how these findings can be extended to a broader range of 2D/3D material systems and explore their implications for future device applications.

Ref. https://doi.org/10.1038/s41699-025-00571-3

Hole-selective monolayers have become a versatile tool for fabricating both single-junction perovskite and tandem perovskite/silicon solar cells. Although these monolayer materials and their formation methods have been employed for several years, aspects of their behavior and design remain poorly understood.

In this presentation, I will discuss the questions of the molecular design principles of monolayer-based selective contact layers. First, I will examine the success of materials featuring a carbazole chromophore backbone, highlighting how variations in linker fragments, functional groups, and anchoring moieties influence performance of the devices. Next, I will explore mixed-monolayer strategies, demonstrating how co-adsorption of complementary molecules enables tuning of interfacial properties such as wettability. Finally, I will present our efforts in developing electron-selective monolayers, describing the design strategies and synthetic routes we have employed to achieve electron selectivity. Through these insights, I aim to highlight the unresolved questions and suggest future advances in selective-contact engineering for perovskite photovoltaics.

Tin-lead perovskite solar cells have emerged as an alternative to pure lead analogues, reducing the toxicity and allowing for ideal bandgaps (~1.3 eV). The optimization of the interface between the perovskite and the charge carrier-selective layer plays a pivotal role in improving the device performance and stability, the main bottlenecks towards commercialization. In this regard, self-assembled monolayers (SAMs) have gained popularity as hole-selective layers to adjust the band energy alignment. In our research group, we have demonstrated that tin-lead perovskite solar cells with carbazole-based SAMs exhibit higher power conversion efficiencies (PCE) than those with commonly used PEDOT:PSS [1]. However, the stability of SAMs remains a debate in the research field. Herein, we investigate the role of the halogen atom of the SAMs into the device life-shelf stability and photostability by employing different halogenated SAMs. In particular, we show that Cs_0.25FA_0.75Pb_0.5Sn_0.5I₃ solar cells with iodide-2-(9H-carbazol-9-yl)ethyl)phosphonic acid (I-2-PACz) retain ~90% of the initial PCE after 7944 hours of storage in nitrogen atmosphere and without encapsulation. This outstanding life-shelf stability is attributed to defect passivation at the buried perovskite interface related to the halogen atom.

The buried interface between the perovskite absorber and the hole-transporting layer (HTL) has emerged as a critical bottleneck that limits both the power conversion efficiency (PCE) and long-term operational stability of inverted perovskite solar cells (PSCs)[1,2]. We propose a rational interfacial engineering strategy involving the incorporation of a thin interlayer composed of arylphosphonic acids at the buried perovskite/HTL interface. This molecular layer facilitates chelating interactions with undercoordinated Pb²⁺ ions at the perovskite surface, thereby effectively passivating interfacial defects. Additionally, the bulky aryl groups and multidentate coordination introduce steric hindrance, which modulates perovskite crystallization dynamics and promotes the formation of uniform, defect-suppressed films. The resultant inverted PSCs demonstrate remarkable photovoltaic performance, achieving a PCE of 26.8% on a small-area device (0.053 cm²) with an exceptional fill factor (FF) exceeding 87%. On a larger active area of 1.08 cm², the device maintains a high PCE of 25.6% with an open-circuit voltage (Voc) of 1.21 V and a FF of 80.3%. Furthermore, these devices exhibit high operational stability under both indoor conditions (including room and elevated temperatures) and prolonged outdoor maximum power point tracking, highlighting this interfacial modification approach in advancing the practical deployment of high-efficiency inverted PSCs.

 Perovskite solar cells (PSCs) have attracted worldwide attention due to their high power conversion efficiency (PCE) and low-cost solution processing. Besides the film and interfacial engineering of the perovskite layer, the development of hole-collecting materials (HCMs) is a critical key to boosting the performance of PSCs.

 In p-i-n PSCs, the use of chemically adsorbed monolayers that can efficiently collect photo-generated holes from the perovskite layer and transport them to a transparent conducting oxide (TCO) electrode as the hole-collecting layer (HCL) has become one of the mainstream directions.^[1]

 Recently, we developed a tripodal molecule composed of a triazatruxene core connected with three phosphonic acid anchoring groups (PATAT).^[2] We demonstrated that after being chemically adsorbed on the TCO surface, PATAT molecules tend to form a monolayer with a face-on orientation, resulting in improved hole-collection compared to their monopodal counterpart, and the corresponding p-i-n PSCs using the PATAT monolayer as HCL exhibited PCEs up to 23%. However, PATAT still has its own drawbacks that need to be improved, such as its difficulty in further functionalization and the hydrophobicity of its monolayer.

 In this work, we designed a series of isotriazatruxene derivatives bearing three phosphonic acid anchoring groups, iso-PATAT and its halogen-substituted analogues, as hole-collecting monolayer materials. Isotriazatruxene is an isomer of triazatruxene, which has two indole moieties facing each other, leading to a steric hindrance when bulky substituents such as alkyl phosphonic acid groups were introduced into –NH positions. Taking advantage of this point, after being chemically adsorbed, instead of binding to the TCO surface, some phosphonic acid groups would point upward, leading to a hydrophilic monolayer and improved surface wettability.

 P-i-n PSC devices using these iso-PATAT molecules as hole-collecting monolayers were fabricated and evaluated. The single-junction and monolithic all-perovskite tandem solar cells fabricated with iso-PATAT derivatives showed a champion efficiency approaching 26 and 29%, respectively.

 In this presentation, molecular design, characterization, and device evaluation will be discussed in detail.

Perovskite based solar cells (single- and multijunction) have emerged as highly promising candidates for next-generation photovoltaic technologies, offering low-cost fabrication, high efficiencies, and tunable bandgaps. In particular, perovskites with a bandgap of 1.68 eV are ideally suited as top cells in perovskite/silicon tandem solar cells, which gained relevance in both research and industry.

A critical challenge on the path to further efficiency improvements and industrial implementation lies at the interface between the perovskite absorber and the electron transport layer, typically C₆₀. In this talk, I will present a detailed analysis of a loss mechanism that occurs when C₆₀ is exposed to air. This degradation process not only impacts device performance but also has important implications for the design of manufacturing environments and for the accurate evaluation of interfacial engineering strategies: For example, the apparent effectiveness of an interlayer can be misinterpreted if C₆₀ degradation is not properly accounted for.

Furthermore, I will demonstrate two approaches to reduce interfacial losses:
(1) The incorporation of functional interlayers between perovskite and C₆₀, reduce non-radiative recombination by passivation and improved bandalignment.
(2) The use of an evaporable industrially relevant alternative to C₆₀ that shows reduced interfacial losses.

The results presented are relevant for both p-i-n single-junction and perovskite-based multi-junction solar cells and offer concrete strategies for improving device architectures and manufacturing processes-paving the way for stable, scalable, and high-efficiency perovskite photovoltaic technologies.

For highest efficiency potential and compatibility with industrial silicon surface texture, our work focusses on the fabrication of perovskite top solar cells that conformally coat µm-sized random pyramid silicon texture. For that purpose, the hybrid co-evaporation/ wet chemical route is applied to ensure controlled perovskite thin film formation over these relatively large silicon pyramids. Similarly, suitable deposition methods for selective contacts are chosen. Resulting perovskite interfaces to the hole and electron selective contacts are studied in detail and suitable passivation schemes are proposed. Supported by opto-electronic simulation and device characterization, we gain insights to the underlying working mechanisms and find that the large improvement in V_OC and FF of the applied passivation approach at the perovskite/C₆₀ interface is driven by a field-effect reducing interfacial recombination and increasing electron concentration within the perovskite bulk which enhances conductivity. Based on these findings, we were able to yield high open circuit voltage, closing the gap to one-step wet chemically processed tandem devices, and reach a certified power conversion efficiency of 31.6% for fully-textured perovskite/silicon tandem solar cells.

In recent years, inverted (p-i-n) perovskite solar cells (PSCs) employing self-assembled monolayers (SAMs) at their bottom interface have been widely investigated owing to their low-temperature processing, high fabrication reproducibility, ever-increasing power conversion efficiency (PCE) and long-term damp-heat stability.[1,2] In comparison to the functionalization of the bottom contact interface by SAMs, the top interface presents more challenging issues because solution-processed perovskite thin films tend to lost volatile species from the upward side during high-temperature annealing. These compositional variations lead to defect generation and, in turn, to non-ideal charge carrier dynamics. This, in addition to the formation of energetic barriers due to poor energy level alignment, hinder efficient electron extraction from the perovskite to the electron transport layer (ETL). Therefore, tailored interlayers at the top interface between the perovskite and the ETL, C60 or its derivatives (e.g., [6,6]-phenyl-C₆₁-butyric acid methyl ester, PCBM), are crucial for improving the performance and stability of inverted perovskite solar cells (PSCs).

Indeed, surface modification plays a pivotal role in developing state-of-the-art inverted perovskite solar cells (PSCs).[3-5] However, the functional specificity of amine-based compounds often results in selective passivation, leading to inconsistent device performance improvements. In this study, by combining the analysis of the quasi-Fermi level splitting (QFLS) [6] and surface photovoltage (SPV) by absolute photoluminescence and Kelvin probe measurements, we systematically investigate three cyclic-structured amine salts (benzene-, thiophene-, and piperazine-based) to elucidate their distinct enhancement mechanisms via defect passivation and thus suppression of non-radiative recombination. Our results reveal that the benzene and thiophene functional groups effectively passivate defects at grain interiors and boundaries owing to their conjugated structure, thereby improving the interfacial contact and reducing the defect density. Meanwhile, the piperazine-based molecules exhibit a superior hole-blocking capability, effectively repelling minority carriers back into the perovskite bulk and thus improving charge transfer kinetics. These mechanistic insights inspired the development of a bimolecular passivation strategy that simultaneously addresses surface defect mitigation and charge carrier management. By implementing this bifunctional approach combining piperazine- and thiophene- iodate, we achieved a champion device efficiency exceeding 25% (mask area: 0.152 cm²) with exceptional operational stability. The encapsulated devices maintained over 85% of their initial performance after 600 hours of continuous operation at 50°C under ambient conditions (ISOS-L-1 protocol).

Perovskite solar cells have attracted substantial attention due to their promising efficiency, but their operational stability, particularly at the interfaces with charge transport layers, remains a key limitation. This issue is especially critical in inverted (p-i-n) architectures, where fullerene-based electron transport layers often compromise long-term device stability. In response, the use of low-dimensional perovskite interlayers with hydrophobic organic spacers was explored to improve stability by templating the perovskite structure. ^[1] However, conventional organic cations used in these interlayers are typically electronically inert, which hampers charge extraction and reduces overall device efficiency. In this work, we introduce low-dimensional perovskites incorporating electroactive napthalimide- and napthalenediimide-based spacers.^[2,3] These functional organic moieties also enable modification or replacement of the fullerene-based electron transport layers, producing passivated interfaces that support efficient charge transport. This strategy led to improved photovoltaic performance, achieving power conversion efficiencies exceeding 20% along with enhanced device stability, underlining the value of electroactive interlayers in optimizing inverted perovskite solar cells.

Commercialization of perovskite photovoltaics (PPVs) hinges on the successful transition from laboratory-scale fabrication to industrial-scale manufacturing. Gas-quenching-assisted blade coating (GABC) has emerged as a promising strategy for high-throughput deposition of perovskite films. However, challenges remain in controlling crystallization kinetics and achieving uniform film morphology. Particularly, the formation of voids at the substrate/perovskite interface hinders the fabrication of thick perovskite layers compatible with fully printable devices with non-reflective carbon electrodes. This talk focuses on advancing fab-scale production of PPVs, showcasing a successful transition from lab-scale fabrication to GABC. By integrating in situ optical spectroscopy with doctor-blade coating, we first reveal the critical role of gas quenching in modulating nucleation kinetics and enabling compact, dense perovskite layers. Furthermore, we identify the gas-quenching stage as pivotal for α-phase FAPbI₃ nanocrystal nucleation under ambient conditions, which is essential for producing highly crystalline and stable perovskite films after annealing. Spontaneous cesium ion incorporation from cesium chloride further facilitates nucleation and nanocrystal growth, enabling fully printed devices with power conversion efficiencies (PCEs) of up to 19.36% and minimodules reaching 16.23%. Finally, to address the optical limitations of carbon electrodes in light recycling, we introduce a two-dimensional perovskite layer-assisted growth (2D-LAG) strategy. This approach mitigates void formation in perovskite films over 1 μm thick by promoting heterogeneous nucleation and suppressing solvent entrapment. The resulting monolithic, void-free films achieve PCEs of 19.9% on rigid substrates and 17.5% on flexible substrates in fully printed perovskite solar cells with non-reflecting carbon electrodes.

Lead-Sn (Pb-Sn) perovskites are highly promising for single and tandem solar cells owing to their reduced toxicity and narrower bandgap. However, achieving high-quality films remains a challenge due to the rapid crystallization and self-doping of tin, especially under scalable fabrication techniques. Here, we present a scalable two-step blade coating technique involving solvent and spacer engineering to enhance Pb-Sn perovskite crystallization. In the second step, a mixed solvent system of isopropanol and 2-methyl-2-butanol was employed to facilitate precursor diffusion and promote complete conversion to perovskite. Introducing organic spacers such as phenylethylammonium (PEA) is an effective strategy to enhance passivation of grain boundaries and improve stability by forming low-dimensional/3D hybrid structures. This approach enabled power conversion efficiencies (PCE) exceeding 16% with good operational stability.^[1] In this work we show that organic cations based on the thiophene moiety can reach PCE of 17.7%. Our results demonstrate substantial improvements in both efficiency and stability, marking a significant step toward the industrial-scale fabrication of high-performance Pb–Sn perovskite solar cells.

Perovskite-based multijunction solar cells are rapidly approaching the stage of commercial deployment, exhibiting extraordinary advancements in their energy conversion efficiencies over the past five years. For example, the most efficient two-terminal configuration that has been developed to date is a tandem structure combining perovskite materials with silicon (Si). Despite these impressive efficiency gains, the long-term reliability and stability of these solar devices remain significant concerns.

One of the primary reasons for this uncertainty is the relatively brief period since the inception and development of perovskite-based multijunction solar cells. This short timeframe poses substantial challenges for conducting long-term, decade-spanning reliability tests, primarily due to the inherent time constraints associated with such extensive evaluations. Additionally, there is a notable lack of established accelerated aging protocols, which are essential for predicting the long-term performance and durability of these devices. This gap exists because the fundamental failure mechanisms of perovskite-based multijunction solar cells are not yet fully understood, making it difficult to develop standardized testing methods that can reliably simulate prolonged operational conditions.

In the upcoming presentation, I will elaborate on our recent research findings that investigate the behavior of perovskite cells when integrated into multijunction configurations. First, I will explore how defects at the nanometric scale can initiate and propagate degradation within the solar cells (starting from single-junctions), ultimately impacting their overall performance and longevity. Furthermore, I will discuss the various strategies we have developed to mitigate these nanometric defects, thereby enhancing the stability and reliability of the devices.[1,2]

Beyond the microscopic level, I will also address the effects observed at larger scales, such as the micrometer (µm) and centimeter (cm) levels. For instance, the choice of texturing applied to the silicon cells plays a crucial role in determining both the performance and stability of the solar cells, however by adjusting fabrication parameters, µm-sized textures can be covered without performance loss. [3]  Moreover, I will highlight how precise control of passivation over various length scales has enabled us to fabricate wafer-scale tandem devices with high performance [4]. Finally, we extend our reach beyond tandem devices and I will showcase our latest developments in perovskite-perovskite-Si triple junction solar cells. [5]

Over the past decade, perovskite photovoltaics have made remarkable progress, with the best lab-scale cells now achieving close to 27% power conversion efficiency. These gains come from a deeper ability to engineer and passivate the interfaces inside the cell. Yet high efficiency is only half the story: the very passivation strategies that unlock record voltages and fill factors can introduce new routes to degradation, leaving stability lagging behind. In this talk, I will trace how light- and heat-induced stresses activate instabilities at critical interfaces. One example is the thermal decomposition of ammonium-based passivation ligands that were initially chosen to suppress non-radiative recombination. Another example is halide ions migrating across the perovskite/transport-layer junction and penetrating as far as the electrode. I will also examine chemical mismatches—such as acidic hole-transport layers contacting lead-halide perovskites—that accelerate interface breakdown. Finally, I will outline emerging strategies to arrest these pathways, including diffusion barriers, thermally robust molecular passivants, and interface-benign transport materials.

Perovskite solar cells (PSCs) have rapidly gained prominence in the field of photovoltaics due to their outstanding optoelectronic properties and ease of fabrication. In less than a decade, these devices have evolved from modest efficiencies of 3.8% to rivalling state-of-the-art silicon-based cells at 27.0%, positioning them as a leading candidate for next generation solar energy technologies. As performance gains begin to plateau near the thermodynamics efficiency threshold, the need to innovative light management techniques becomes increasingly critical to achieve further improvements. To overcome the optical losses, nanoscale surface structuring has emerges as a promising strategy to increase photo harvesting. However, traditional approaches often involve intricate, multi-step processes that can damage the perovskite material and hinder large-scale production. [1][2]

This work presents a novel solution based on femtosecond laser technology for surface nanopatterning of metal halide perovskite (MHP) films. The technique explores the formation of Laser-Induced Periodic Surface Structures (LIPSS) which is a rapid, single-step, and scalable solution compatible with industrial fabrication [3]. These periodic line structures, with tuneable periods, could significantly enhance light trapping and absorption within the perovskite thin film. We report the successful fabrication of LIPSS with periodicities around 300 nm in CsPbI₃ thin films. Detailed optical, electronic and structural characterization reveals enhanced properties, including increased photoluminescence intensity, longer carrier lifetimes, and improved stability of the black phase compared to pristine surface.

These findings highlight a powerful route toward integrating nanostructures into PSCs to improve light absorption and reduce losses. The proposed laser-based technique holds strong potential for boosting efficiency while meeting the demands of scalable, low-cost solar cell manufacturing.

Fully printed flexible perovskite solar modules (f-PSMs) offer strong potential for the commercialization of perovskite photovoltaics (PV) due to their compatibility with roll-to-roll (R2R) manufacturing. However, challenges remain in suppressing interfacial recombination losses and improving the mechanical reliability of microcrystalline perovskite films, leading to significant efficiency losses in large-scale production. Here, self-assembled monolayers (SAMs) are employed to modify or replace SnO₂, effectively reducing interfacial energy barriers and recombination losses. The resulting flexible devices achieve a PCE of 17.0% with negligible hysteresis, retain 95% of initial efficiency after 3000 bending cycles, and exhibit a T₉₅ lifetime of 1200 hours under 1 sun at 65 °C. Furthermore, thiol vapor annealing is introduced to treat freshly printed perovskite films in a fully R2R process. Thiol molecules facilitate solvent removal, passivate under-coordinated Pb²⁺ ions, and promote grain growth, relieving lattice strain and enhancing mechanical resilience. Treated films show reduced modulus, increased fracture toughness, and minimal cracking, with devices retaining 93% efficiency after 25000 bending cycles. This process also improves film uniformity and interfacial contact, yielding record PCEs of 12.1% (20.25 cm²) and 10.1% (100 cm²) for fully R2R-printed f-PSMs.

A straightforward, solution-processable method facilitates the production of high-performance perovskite solar cells at a relatively low temperature and cost. Nevertheless, defect-induced charge traps are unavoidable in three-dimensional (3D) active-layer perovskite films, particularly at their surfaces and grain boundaries, which can affect their long-term stability. A comprehensive understanding of the fundamental characteristics of 3D perovskite films and their interfaces is essential for their effective fabrication and device engineering. In this talk, we examine strategies to mitigate defects by transforming defective surfaces, such as residual non-active phases δ-FAPbI3 and PbI2, into low-dimensional perovskite (LDP) structures. The incorporation of LDP into both interfaces of 3D perovskite films is anticipated to reduce non-radiative interface recombination and enhance their formation energy. For instance, large alkyl-amine ligands from an LDP layer can passivate a 3D perovskite film through field-effect passivation and robust chemical bonding. Nonetheless, the development of a pure-phase homogeneous LDP capping layer with an adjusted crystal orientation remains challenging. To address these challenges, we successfully demonstrated that by designing the ligand chemistry and optimizing the fabrication engineering of LDP, it is possible to achieve the desired high-quality LDP capping layers with high phase purity, good orientation, and full coverage. This approach also considers how our findings can improve the long-term stability of devices using scaled-up technologies. Ultimately, we expect that our methods will contribute to meeting the industrial stability criteria for photovoltaic modules, considering industrial aspects.

Understanding and controlling interface chemistry is key to enhancing the performance and stability of halide perovskite (MHP) semiconductor devices. However, analyzing buried interfaces remains challenging due to their complex chemical reactivity and sensitivity to external stimuli. In this work, we employ a combination of hard X-ray photoelectron spectroscopy (HAXPES) and operando X-ray photoelectron spectroscopy (opXPS) to investigate the chemical and electronic structure of MHP interfaces with adjacent functional layers.[1]

Focusing on atomic layer deposited (ALD) SnO₂ and NiO layers on mixed-cation mixed-halide perovskite films, we demonstrate how advanced photoemission techniques enable the detection of buried chemical species and subtle changes in energy level alignment that are critical to device operation.[2] In particular, our HAXPES measurements reveal the formation of new interfacial species that can detrimentally affect charge transport, while opXPS provides insights into the dynamic chemical evolution of these interfaces under light and bias. Furthermore, we explore the impact of introducing protective organic interlayers between the perovskite and oxide layers, which can mitigate interfacial reactions and preserve favorable band alignment.

In this framework, we further investigate nickel oxide (NiOₓ), a widely used inorganic hole transport layer (HTL) in inverted MHP solar cells. Despite its advantageous optical and electronic properties, NiOₓ suffers from high surface reactivity and defect formation at the NiOₓ/MHP interface, limiting device performance. We show that ultraviolet-ozone (UVO) treatments, commonly employed to improve wettability and surface activation, can in fact enhance NiOₓ surface reactivity and introduce additional defect states. By systematically comparing pristine and UVO-treated NiOₓ interfaces, we identify the chemical nature of these defects and demonstrate that the insertion of a MeO-2PACz organic interlayer effectively mitigates interfacial reactions. Importantly, neither the UVO treatment nor the grafting of the organic interlayer significantly alters the bulk properties of NiOₓ, emphasizing the interfacial specificity of these modifications.

This methodological approach highlights the strength of combining synchrotron-based and lab-based XPS to gain a comprehensive understanding of interface stability and defect formation in MHP-based optoelectronics.

Identification and development of various novel charge transport and photoactive materials, additive engineering, and controlling the absorber’s morphology mainly helped to achieve efficient perovskite and organic solar cell (PSC and OSC) devices. However, one of the dominant factors in improving cell Power Conversion Efficiency (PCE) is the charge-carrier extraction from the perovskite or organic photo-absorber material. It has been observed that the improvement in PCE of the PSCs and OSCs depends very much on the hole-transport layer (HTL) of the device. Recently, self-assembled monolayers (SAMs), notably [2-(9H-carbazol-9-yl)ethyl] phosphonic acid (2PACz) and its derivatives have gained attention as interfacial modifiers for tuning the work function (WF) of indium tin oxide (ITO) substrate in both inverted PSC and conventional OSC configurations.

This work explores the interfacial behaviour of several donor polymers- PFO, P3HT, PM6, PCE10, and PCE13 used as hole-injection layers, in combination with blended SAMs of varying dipole strengths. Through ultraviolet photoelectron spectroscopy (UPS), we examined that the strategy of blending SAMs altered the ITO work function by more than 1 eV. Our results, however, demonstrate that the optimal behaviour predicted by the existing integer charge transfer (ICT) model [1,2] is not always shown by polymer/SAM/ITO interfaces. Rather, large interfacial dipoles predominate, which hinders alignment of the vacuum level and reduces the impact of the underlying SAM/ITO WF on the polymer WF. When donor polymer generations are compared, clear patterns emerge that the older materials, such PFO and P3HT, have greater SAM compatibility and are less sensitive to variations in ITO WF. The surface potential of the electrode is more important for newer donor polymers (such PM6, PCE10, and PCE13), hence SAMs with higher intrinsic dipole moments are required to minimize energy losses. This knowledge is particularly important for perovskite devices, as the interface between the HTL and the perovskite layer has a big impact on the effectiveness of hole extraction, determining V_OC, [3,4] and the overall functionality of the device.

Overall, this study underscores the need for deliberate interface design in both OSCs and PSCs. By understanding the interfacial energetics shaped by blended SAMs and donor materials, we can more effectively engineer contacts to enhance charge transfer, reduce recombination, and support the advancement of efficient next-generation solar technologies.

Organic Photovoltaics (OPVs) have received widespread attention and research, owing to their unique advantages like inexpensive, light weight, flexible, processable and less environmental pollution. In the past decades, due to the efforts made in device engineering and material replacement, bulk heterojunction (BHJ) OPVs has developed rapidly with power conversion efficiency (PCE) exceeding 20 % in recently developed non-fullerene small-molecules acceptors (NFAs) to replace fullerene-counterpart¹.

To further improve the performance of NFA-OPV, a ternary strategy was evolved by adding a proper third component to the binary system. High potential PCEs are offered by ternary system where either the third component acts as an additional donor or additional acceptor that serves to extend the range of absorption and can also tune material properties. Interestingly, the incorporation of the third counterpart that serves to extend the range of absorption and can also tune material properties. It can regulate the accumulation and orientation of the molecule, as well as the phase separation of donor and acceptor, providing high crystallinity and ordered molecular stacking that can improve the charge transport and inhibit the bimolecular recombination through well optimized phase separation^2,3.

However, most research on ternary strategies is based on the bulk hetero junction (BHJ) system. This makes it difficult to control other important morphological parameters, such as molecular orientation and domain purity as it further complicates the morphological regulation. Accordingly, to tailor vertical phase distribution efficiently, the Layer-by-Layer (LBL) deposition approach of the layers is considered as a promising alternative to the BHJ. The sequential deposition method allows the carefully controlled arrangement and orientation of the organic molecules, leading to improved photovoltaic performance. It avoids the difficulty of controlling the bulk morphology through forming a proper vertical phase separation that can be controlled, which is efficient for the charge transportation and collection at the corresponding electrodes. Furthermore, p–i–n-like bilayer structure enables easier exciton dissociation at the D/A interface and can reduce charge carrier recombination loss^4,5.

Hence, in this piece of work, I have developed a novel OPV structure, introducing a perovskite quantum dots (PQDs) interlayer sandwiched between the organic semiconductor donor and the NFA layers using LBL deposition approach, resulting in enhancement in the performance of  QDs based OPV devices by 11% ( from PCE of 16.6 % for the pristine Binary OPV to PCE = 18.8%  for the QDs-based Ternary OPV) along with 99 % performance retention after 3 months of storage compared to only 30% for the bilayer devices without PQDs. This unique structure allows for the LBL preferred vertical phase separation and well-controlled D/A interface film morphology, exhibiting efficient transport and extraction properties. Moreover, the incorporation of PQDs to create alloy structure with the NFA was beneficial for reducing bimolecular and trap-assisted recombination at the interface, improving exciton separation, and charge transfer, resulting in the higher, V_OC, J_SC, and FF of the PQDs based device. Finally, our findings evidence that introducing the PQDs as a third component in the interface between the donor and the NFA via LBL deposition approach is an effective strategy to improve the interfacial microstructure of the active layers, resulting in high-performance OPV devices.

Electrical SPM operation modes like Kelvin probe force microscopy (KPFM) or conductive atomic force microscopy are ideally suited to do nanoscale photovoltaic measurements. In this presentation, I will present some of our recent activities in the development of specialized scanning probe microscopy methods to study hybrid perovskite materials. Solar cells based on metal halide perovskite (MHP) materials will enable cheaper and more energy-efficient photovoltaic and optoelectronic devices compared to current silicon-based technologies. To advance MHP technology further, however, will require a better understanding of the fundamental processes leading to energy losses, unstable operation conditions and premature aging. The macroscopically observed properties of optoelectronic MHP materials and devices are the result of the complicated interplay between nanoscale structure and function. Thus, the key to understanding MHP materials is the micro- and nanoscale characterization of the many nano- and microscale structures; from sub-granular twin domains, over grain boundaries and interfaces to lateral variations in crystal grains orientations and facets.

Using static and time-resolved Kelvin probe force microscopy (KPFM), we have developed unique techniques for mapping the surface potential and photovoltage. Using cross sectional measurements, we map and record the potential distribution across different layers of solar cell devices under operating conditions [1–3]. Here, the key to a reliable cross-sectional KPFM measurement is the connection of a high-resolution quantitative KPFM operation mode [4] and the preparation of a smooth surface through the solar cell. We use a combination of mechanical fracturing of the cells with a complex polishing process using ion beams. Using a pointwise spectroscopy technique, we can record and map the surface photovoltage (SPV) dynamics with 10-20 nm lateral resolution. With this Nano-SPV technique, we revealed local SPV overshoots in the vicinity of grain boundaries following an illumination pulse [5]. The overall aim of our research is to address some of the key challenges of MHP research, such as phase segregation, degradation and interface heterogeneity, to enable a deeper understanding of the different loss mechanisms and intrinsic instabilities that currently limit the application of MHP solar cells.

[1]         V. W. Bergmann, S. A. L. Weber, F. Javier Ramos, M. K. Nazeeruddin, M. Grätzel, D. Li, A. L. Domanski, I. Lieberwirth, S. Ahmad, and R. Berger, Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell, Nat Commun 5, 6001 (2014).

[2]         S. A. L. Weber, I. M. Hermes, S.-H. Turren-Cruz, C. Gort, V. W. Bergmann, L. Gilson, A. Hagfeldt, M. Graetzel, W. Tress, and R. Berger, How the formation of interfacial charge causes hysteresis in perovskite solar cells, Energy Environ. Sci. 11, 2404 (2018).

[3]         I. M. Hermes, Y. Hou, V. W. Bergmann, C. J. Brabec, and S. A. L. Weber, The Interplay of Contact Layers: How the Electron Transport Layer Influences Interfacial Recombination and Hole Extraction in Perovskite Solar Cells, J. Phys. Chem. Lett. 9, 6249−6256 (2018).

[4]         A. Axt, I. M. Hermes, V. W. Bergmann, N. Tausendpfund, and S. A. L. Weber, Know your full potential: Quantitative Kelvin probe force microscopy on nanoscale electrical devices, Beilstein J. Nanotechnol. 9, 1809 (2018).

[5]         Y. Yalcinkaya, P. N. Rohrbeck, E. R. Schütz, A. Fakharuddin, L. Schmidt-Mende, and S. A. L. Weber, Nanoscale Surface Photovoltage Spectroscopy, Adv. Opt. Mater. 12, 2301318 (2024).

Colloidal nanocrystals of lead halide perovskites (LHP NCs) have a long history. They were initially conceived in Pb-doped CsBr crystals and films 30 years ago. About a decade ago, they were produced as colloids in apolar solvents. Importantly, the fast structural dynamics, essentially entropically stabilized lattice, do not seem to be detrimental to exhibiting the textbook optical quality of a semiconductor. To date, they have become the most widely researched quantum dot material. They have challenged the ethos of this field in nearly every aspect. For instance, they are bright emitters without ever being coated with an epitaxial shell. They are the first colloidal quantum dot (QD) material that exhibited excitonic coherence on a timescale comparable to their radiative rates. They are the first colloidal QD material demonstrating collective and hence accelerated radiative decay of tens of picoseconds – superfluorescence - in the periodic ensembles (known as NC superlattices). LHP NCs exhibit a giant oscillator strength effect, allowing extremely fast emission rates (lifetimes down to 60ps) at large NC sizes, and at a single particle level in the single-photon emission regime – single-photon superradiance. The exciton fine structure of LHP NCs is readily engineerable through the shape anisotropy. We also find that, by simple proximity to highly chiral plasmonic nanostructures, the otherwise linearly polarized emission becomes fully chiral; the transcribed chirality is also manifested in their absorption (dichroism), making them the first fully chiral single-photon emitters. We will review the diverse opportunities that LHP NCs increasingly offer as classical and quantum light sources. The presentation will encompass the work of my interdisciplinary team and diverse international collaborators, whose names will be appropriately mentioned in the presentation and footnotes.

[1] V. Morad, A. Stelmakh, M. Svyrydenko, L.G. Feld, S.C. Boehme, M. Aebli, J. Affolter, C.J. Kaul, N.J. Schrenker, S. Bals, Y. Sahin, D.N. Dirin, I. Cherniukh, G. Raino, A. Baumketner, M.V. Kovalenko Nature, 2024, 626, 542–548

[2] C. Zhu, S.C. Boehme, L.G. Feld, A. Moskalenko, D.N. Dirin, R.F. Mahrt, T. Stöferle, M.I. Bodnarchuk, A.L. Efros, P.C. Sercel, M.V. Kovalenko, G. Rainò. Nature, 2024, 626, 535–541

[3] I. Cherniukh, G. Rainò, T. Stöferle, M. Burian, A. Travesset, D. Naumenko, H. Amenitsch, R. Erni, R.F. Mahrt, M.I. Bodnarchuk & M.V. Kovalenko.  Nature 2021, 593, 535–542

[5] T.Kim, R. M. Kim, J. H. Han, M. Svyrydenko, M. Bodnarchuk, G.Raino, K. T. Nam, M. V. Kovalenko et  al. submitted

High environmental stability and surprisingly high efficiency of solar cells based on 2D perovskites have renewed interest in these materials. These natural quantum wells consist of planes of metal-halide octahedra, separated by organic spacers. The unique synergy of soft lattice and opto-electronic properties are often invoked to explain superior characteristic of perovskites materials in applications. At the same time such unique synergy creates fascinating playground for exciton physics which challenges our understanding of this elementary excitation. I will demonstrate that even after decade of intense investigation the notation” unique” so often used in case of perovskites deserves serious scrutiny.

I will explore the excitonic landscape in 2D semiconductors. First, I will highlight the controversy surrounding the unexpectedly high light emission efficiency of this material and show that it can be explained by the interplay between phonons and the exciton fine structure. I will demonstrate that the soft lattice can suppress relaxation of excitons to dark state making 2D perovskites great light emitters. Moreover, I will discuss the exciton fine structure measured for multiple 2D layered perovskites characterized by a different lattice distortions imposed by organic spacers. Surprisingly, it has a non-trivial impact on the exchange interaction allowing the energy spacing between dark and bright excitons to be tuned. This tuning knob, not available in classic semiconductors, makes 2D perovskites a unique material system where the exciton manifold can be controlled via the steric effect. Finally, I will demonstrate alternative approach to the injection of spin-polarized carriers in 2D perovskites bu building Van der Waals heterostructures. 

The benefits of vacuum processed perovskite solar cells will be discussed. Co-sublimation of perovskite precursors leads to high efficiency solar cells albeit with somewhat low open circuit voltages. Using a home build setup to probe the photoluminescence of the perovskite films while they are grown on a rotating substrate in a high vacuum chamber allows us to see surprising evolution of the photoluminescence.

We will show the effect of seed-layers on the evolution of the photoluminescence of the perovskite film as well as the addition of passivating agents. 

This will be shown for perovskites with different compositions and film thicknesses. The best perovskite are used to prepare thin film solar cells reaching 23 % power conversion efficiency.

This work is done by the postdocs Vladimir Held and Yunseong Chin 

Solvent engineering in metal halide perovskite (MHP) precursor inks is critical for controlling the perovskite thin film crystallization process. This coordination engineering approach is widely used to tune film structure and morphology, yet significant knowledge gaps exist regarding the solution-phase structures present in MHP inks, specifically the coordination chemistry of solvated polynuclear halodoplumbate complexes. Although various structures have been proposed in literature, the definitive nature of these halodoplumbate complex ions remains elusive. We develop a systematic framework to characterize and differentiate the various structural motifs present in these solution-phase complexes. Utilizing a comprehensive set of X-ray scattering and spectroscopy techniques, we have examined the representative structure of the solution-phase, identifying a spectrum of motifs from mononuclear to polynuclear. This deeper insight into MHP solution chemistry equips the scientific community with vital information to guide the strategic development of novel MHP chemistries through informed coordination engineering.

Two-dimensional lead halide perovskites (2DHPs) with a general structural formula A2A'(n-1)PbnX3n+1 (A = bulky spacer, A'= Small monovalent cation, X = I, Br, Cl) have emerged as a promising class of materials for next-generation optoelectronic devices. Suppressed photoinduced halide-ion segregation (PHS) was reported in 2D mixed halide perovskites (2DMHPs) due to suppressed ion migration, and strong excitonic effects resulting from this unique layered structure. However, 2DMHPs remain a major argument in both theoretical estimation and experimental observations. Notably, all previous reports of PHS studies in MH2DHPs are based on polycrystalline thin films[1, 2], which could be an important reason for inconsistent results due to the big variations in defect density. There is a lack of observation for the formation of iodide-rich domains under photoexcitation using photoluminescence tools, which is a signature feature in 3D PHS studies [3, 4].

Here, we demonstrate that BA2PbBrxI4-x (BA = butylammonium, x=1, 2, 3) 2DMHP single crystals have highly ordered halide stacking preference with bromide and iodide exclusively occupying B and T-sites respectively in thermal equilibrium. With hyperspectral PL and absorption imaging methods, we directly reveal the formation of a new phase upon exposure to above-bandgap excitations, which can be attributed to a photo-induced halide switching process, whereas T-site iodide switches its position with B-site bromide with local PbX64- octahedral. No PHS induced I-rich domains are observed in both PL and absorption mappings in those single-crystalline 2DMHPs, which is due to such photoisomerization does not involve any multiple unit cell mass transfer. A schematic and photoluminescence imaging of this photoisomerization of 2DMHP are presented in Figure 1. We conducted temperature dependent single crystal X-ray diffraction (SCXRD) and powder XRD (PXRD) with in situ photoexcitation measurements to reveal change in lattice constant during photoisomerization. We find that the halide switching results in an expansion of in-plane lattice constant while a reduction of interlayer distance. The new photo-switched structure and its dark form are chemically inequivalent structural isomers which exhibit distinctively different physical properties. The ability to alter local halide distributions with light while not causing phase inhomogeneity could be a key to enable a new pathway for in quantum technologies, optical switching and memory applications.

Phase change materials (PCMs) have a sizeable latent heat associated with a first-order phase transition (solid-solid, solid-liquid), which renders them excellent candidates for thermal management applications, such as thermal energy storage and solid-state heat pumps and air- conditioners.[1] Among different PCMs, those undergoing solid-solid transitions are the most desirable due to the absence of fluid leakages.[2] To be implemented in most thermal management technologies, solid-solid PCMs should display an elusive combination of i) high thermal conductivity (>10¹ W m^-1 K^-1), which allows a rapid thermal exchange, and ii) high latent heat (> 10² J g^-1), which minimizes the amount used in the devices.[3] Additionally, the phase change should occur at the desired temperature. Solid-solid PCMs displaying such a combination of properties have not been realized so far.

Two-dimensional (2D) halide perovskites are receiving renewed attention as an emerging class of solid-solid PCMs. Because the two sublattices are individually tunable, it has been proposed that these materials could exhibit a combination of high latent heat and thermal conductivity. In this talk, we will discuss our advancements in achieving Cu-based perovskites with the formula (C_nH_2n+1NH₃)₂CuX₄ (X = Cl^–, Br^–), with even n = 16–22 that show a remarkable latent heat (67 J g^-1) at relatively high temperatures (80–120 C). Additionally, we will show our recent efforts to increase the electrical and thermal conductivity of halide perovskite via internal charge-transfer doping.

Perovskite solar cells (PSCs) offer high power conversion efficiency and low production costs [1], but their commercial potential is still constrained by insufficient long-term stability. The incorporation of two-dimensional (2D) cationic layers onto three-dimensional (3D) perovskite frameworks has emerged as a promising strategy to mitigate instability in perovskite solar cells (PSCs) [2]. While this approach enhances moisture resistance and suppresses interfacial recombination, the relationship between spacer chemistry, concentration, and phase behavior remains not fully understood [3]. In this work, we present a systematic comparison of two alkylammonium spacers—butylammonium iodide (BAI) and butyl-1,4-diammonium diiodide (BDAI₂)—which form Ruddlesden–Popper (RP) and Dion–Jacobson (DJ) 2D phases, respectively, onto CH₃NH₃PbI₃ 3D perovskite.

By varying spacer concentration under ambient conditions, we accessed distinct structural regimes. At high concentrations ([BAI] = 50 mmol L⁻¹ and [BDAI₂] = 5 mmol L⁻¹), it is observed that BAI facilitated the evolution from n = 1 to n = 2 RP phases, suggesting dynamic structural reorganization, while using BDAI2 DJ structures showed rapid degradation without observable phase progression. At lower concentrations, both spacers acted primarily as passivation agents, but only BAI significantly enhanced environmental stability without disrupting charge transport.

Characterizations via UV–vis, XRD, SEM, c-AFM, PL, and EIS demonstrated a strong correlation between dimensionality, interfacial conductivity, and long-term device performance. Notably, DJ-phase samples exhibited promising initial optoelectronic properties but suffered from severe degradation due to humidity.

Our findings highlight critical trade-offs between rigidity and ionic migration in hybrid perovskite systems. This study offers insights into interface engineering and dimensional control, paving the way for the design of robust, high-efficiency perovskite solar cells (PSCs) with improved operational lifetimes.

Photocatalytic hydrogen production using 2D Ruddlesden-Popper tin iodide perovskites emerges as a promising approach for sustainable energy conversion. However, a key challenge associated with these materials is their susceptibility to degradation due to the oxidation of tin and iodide. In this study, microcrystals of 4-fluorophenethylammonium tin iodide perovskite were synthesized in a mixture of hydroiodic acid (HI) and water, demonstrating long-term photostability and robust photocatalytic hydrogen production via HI splitting. Intermittent light irradiation was found to enhance hydrogen evolution by promoting more efficient charge separation and reducing the accumulation of trapped charge carriers that would otherwise lead to recombination. Notably, reused samples exhibited improved photocatalytic performance over time. Furthermore, degraded samples could be easily regenerated through a simple chemical treatment, restoring their hydrogen production capability. In addition, the use of Sn-based perovskite solar cells as photocathodes in aquous solution for hydrogen generation will be also reported, highlighting the interest of Pb-free perovskites for photocatlytic applications.

Polycrystalline perovskite thin films are consistently associated with high performance optoelectronic devices. The main challenges currently under the spotlight lie with the stability and reliability of these devices, requiring a better understanding of the thin film surfaces and interfaces. However, very little is known about them. Given the soft nature of these materials, there is a struggle to directly assign the origin of the electronic features and to correlate them to the chemical nature of the surfaces. Here, by exploiting a multimodal approach, we measure for the first time, at sub-micron scales, the diffraction patterns of the thin film grain surface, which provide the material fingerprint, and its related electronic properties. Armed with this knowledge, we are eventually able to unambiguously assign the origin of the photoemission spectra for a wide library of metal halide perovskites. In lead halide perovskites, we identified their photoemission spectra to be the convolution of three different spectra, that of pristine perovskite, lead halide inclusions, and metallic lead. In particular, metallic lead is identified to be the origin of the often-reported mid-gap photoemissive states, generally assigned to deep electronic states within the halide perovskite semiconductor band gap. The chemical composition of the metal halides, the photo-degradation of the perovskites under visible light, and the quality of the precursors heavily define the presence of such states. Overall, the achievement of such understanding elucidates the origin of carrier loss, photo-degradation, and sample reproducibility, aiding in the targeted improvement of the stability and reliability of these metal halide perovskites and their associated optoelectronic devices.

Mixed-metal halide perovskites, with the general formula ABX₃ - where A is a cation (methylammonium, formamidinium, Cs), B is a metal (Sn, Pb) and X is an anion (I, Br, Cl) – are integral to all-perovskite tandem solar cells. Although groundbreaking research has pushed efficiencies to 30.1% so far, this is still just a fraction of fundamental limits (>40%). Device parameters such as the open-circuit voltage (V_OC) and fill-factor (FF) trace back directly back to the photoluminescence quantum yield.[1] This picture is complicated in Sn-containing perovskites given that the presence of Sn⁴⁺ impurities in the precursor solution leads to both the introduction of background hole dopants (serving to increase the PLQY) and non-radiative recombination centers (serving to decrease the PLQY). [3] Nevertheless, these fundamental relationships make PL microscopy a powerful technique for understanding mixed-metal halide perovskites - particularly when correlated with other compositional, structural, or topographical imaging modes.

To probe the compositional origins of photophysical heterogeneity in halide perovskites, we correlate steady-state & time-resolved, wide-field PL microscopy with nanoprobe X-ray fluorescence (n-XRF). We find that what appear to be micron sized grains in atomic force microscopy images are in fact highly heterogeneous in Sn/Pb composition. From PL microscopy, we observe that Sn-rich regions are red-shifted and higher intensity in steady-state PL images. Time-resolved photoluminescence microscopy shows that while the initial photocarrier distribution is homogeneous, these carriers funnel into more localized regions. Finally, the combination of time-resolved and steady-state photoluminescence microscopy allows us to obtain the spatial dopant density on the microscale.[3] Together, these results suggest that Sn-rich regions have a higher dopant density and play a role as recombination centres. We expect these results to be informative for the design of next-generation dopant management strategies in mixed-metal halide perovskites to supress, or even harness, self-doping.

[1] Giles E. Eperon, Maximilian T. Hörantner & Henry J. Snaith, Metal halide perovskite tandem and multiple-junction photovoltaics, Nature Reviews Chemistry, 2017, 1, 0095

[2] Kunal Datta, Junke Wang, Dong Zhang, Valerio Zardetto, Willemijn H. M. Remmerswaal, Christ H. L. Weijtens, Martijn M. Wienk, René A. J. Janssen, Monolithic All‐Perovskite Tandem Solar Cells with Minimized Optical and Energetic Losses, Advanced Materials, 2021, 34 (11), 2110053

[3] Robert J. E. Westbrook, Margherita Taddei, Rajiv Giridharagopal, Meihuizi Jiang, Shaun M. Gallagher, Kathryn N. Guye, Aaron I. Warga, Saif A. Haque & David S. Ginger, Local Background Hole Density Drives Non-Radiative Recombination in Tin Halide Perovskites, ACS Energy Lett., 2024, 9 (2), 732-739

The hard X-ray In-situ Nanoprobe (ISN) beamline at 19-ID is one of the new feature beamlines built as part of the Advanced Photon Source Upgrade (APS-U) at Argonne National Laboratory. It is specifically designed to enable hierarchical, multimodal characterization of complex materials and devices under in situ and operando conditions, with high spatial resolution (down to 25–30 nm) and near-atomic sensitivity. The ISN is optimized to address key research challenges across a wide range of material systems—such as stability, degradation, (in)homogeneity–performance correlations, defect dynamics, and charge/discharge processes—with direct relevance to photovoltaics, energy storage, catalysis, and microelectronics.

The endstation employs Kirkpatrick–Baez (K–B) mirrors to achieve a diffraction-limited spot at 25 keV and covers a tunable X-ray energy range of 4.8–30 keV, enabling deep penetration and element-specific contrast. A 55 mm working distance allows flexible sample integration under realistic conditions. Available sample environments include temperature control (heating/cooling), gas and liquid flow, and applied electric fields, facilitating advanced functional material studies. Multiple contrast mechanisms are supported, including X-ray fluorescence (XRF) imaging for elemental and trace contaminant analysis, X-ray beam induced current (XBIC) and X-ray beam induced voltage (XBIV) for mapping electronic properties, auxiliary X-ray excited optical luminescence (XEOL) for optical characterization, X-ray diffraction (XRD) for structural insights, and ptychography for high-resolution imaging.

The ISN beamline offers a powerful and versatile platform for the correlated, nanoscale characterization of functional materials, providing unprecedented opportunities to unravel complex structure–property relationships in real-world operating environments.

Vapor-phase deposition techniques are widely used in the semiconductor industry. In the case of metal halide perovskites (MHPs), vapor deposition is also being explored as a solvent-free method, avoiding the use of toxic solvents. However, the complex compositions of MHPs, often involving precursors with vastly different volatilities, require strategies where each precursor is evaporated independently or alternative approaches that enable out-of-equilibrium vaporization of all precursors simultaneously. Among physical vapor deposition (PVD) methods, pulsed laser deposition (PLD) stands out as a promising yet underexplored technique for MHPs. PLD utilizes laser energy to eject material from a target through both thermal and non-thermal processes. This enables high versatility in target composition, allowing the deposition of complex thin films from a single-source target. In this presentation, we highlight recent advances in the PLD of MHPs. We discuss methods enabling compositional flexibility, ranging from inorganic to hybrid compounds, as well as optimization strategies for polymorph control. These include the transition from polycrystalline to epitaxial monocrystalline layers, the development of 2D structures, and the fabrication of porous scaffolds for hybrid vapor-vapor or vapor-solution growth. We also address the challenges of PLD and present approaches for scaling up this method. Furthermore, we explore how insights gained from PLD can be transferred to more industry-standard techniques such as sputtering deposition.

Ref.

https://doi.org/10.1021/acsenergylett.4c01466

Metal iodide thin films have gained a lot of attention during the recent decade. Although their applications cover a variety of technological fields, majority of the research is motivated by photovoltaics. Halide perovskites are the most studied metal iodides for photovoltaics. Halide perovskite solar cells are made from abundant and low-cost materials, yet they show high solar conversion efficiencies. Unfortunately, some of the halide perovskites show poor chemical and thermal stability. In addition, the presence of lead in the best-performing materials causes concern. These issues have recently drawn attention to other types of materials including Ag₂BiI₅ and Cs₃Bi₂I₉, for example. Halide perovskites are expected to find their main application in tandem solar cells with silicon, and the same may be true also for Ag₂BiI₅, Cs₃Bi₂I₉ and related materials.

Large-scale applications of halide perovskite and other metal iodide thin films require scalable and well-controllable deposition methods. The currently used methods are simple and low-cost but are difficult to scale up for industrial mass production of solar cells. Atomic layer deposition (ALD) is well known for its unique controllability and excellent scalability and has therefore a lot to give also in the field of metal iodide films. We have developed, as the first team in the world, ALD processes for various metal iodides. We started by developing processes for the binary iodides PbI₂ [1], CsI [2], and SnI₂ [3]. All these processes use metal silylamides as the metal precursors and SnI₄ as the iodine precursor. The binary processes can be combined to make more complex materials: so far we have made the inorganic halide perovskites CsPbI₃ [2] and CsSnI₃ [3] by combining CsI with PbI₂ and SnI₂, respectively. CH₃NH₃PbI₃ can be prepared as well by exposing PbI₂ to CH₃NH₃I vapor [1]. We recently designed a new iodine source that produces anhydrous HI vapor on-site and overcomes thus the limitations of SnI₄ such as high cost and tin contamination in the deposited films. We have demonstrated the feasibility of the source by depositing CsI.

Our most recent efforts are directed towards Ag₂BiI₅, Cs₃Bi₂I₉ and related materials. As the first steps, we have developed ALD processes for the silver halides AgCl, AgBr and AgI. We are currently working on an ALD process for bismuth iodide BiI₃. Also, the first experiments aiming to combine AgI and BiI₃ to ternary iodides are underway.

Halide perovskite (HaP)-silicon tandem solar cells received considerable interest from the scientific community, which resulted in a substantial increase in power conversion efficiency (PCE) up to 34.9 %,[1] considerably higher than the individual single-junction cells. The increase in efficiency has led to an enhanced interest in the industrialization of this technology. However, the most efficient devices are fabricated using deposition techniques that are not compatible with large scale production, making the technology transfer more challenging. Co-evaporation is an up-scalable technique, which offers good thickness control and conformal coverage of micro-size pyramidal textures on silicon bottom cells,[2] making it particularly suitable for HaP-silicon tandem devices. It was shown that the growth of co-evaporated HaP films can be modified using seed layers [3,4] or by rinsing the self-assembled monolayer (SAM) hole-extraction layer (HTL) interface with ethanol.[5,6] Still, it remains unclear how these modifications influence the early HaP film formation. Previous studies have shown that incomplete SAM coverage can lead to variations in HaP film quality and, ultimately, to degraded device performance.[7] In the present study, we use synchrotron-based surface-sensitive techniques, in particular x-ray photoemission electron microscopy (XPEEM) and infrared scattering type scanning near-field optical microscopy (IR s-SNOM). Both XPEEM and IR s-SNOM techniques enable experiments with a lateral resolution of 20 nm, allowing us to analyze the influence of SAM inhomogeneities and CsCl seed layer on the composition of 5 and 20 nm thick co-evaporated FA_0.8Cs_0.2PbI_2.7Br_0.3 (FA⁺ = formamidinium cation, C₆H₅N₂⁺) HaP films. The measurements are complemented with cathodoluminescence, photoluminescence, and x-ray diffraction analyses of both thin (5, 20 nm) and thick (100, 500 nm) films, as well as the characterization of the buried interface by film delamination. We use a model system to study the influence of inhomogeneities in [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) SAM coverage where we identified regions with monolayer MeO-2PACz, as well as regions with thicker MeO-2PACz layer containing unbound molecules. The incorporation of FA⁺ is impeded in regions with a monolayer MeO-2PACz, unless a CsCl seed layer is employed. We show how inhomogeneities in MeO-2PACz typically occur when spin-coating SAM on textured silicon bottom cells, and their influence on HaP formation can be mitigated using CsCl seed layer, allowing for ~30 % (certified) efficient HaP-silicon tandem solar cells, the highest value for fully vacuum-processed HaP films to the best of our knowledge. Our study provides a deep understanding of co-evaporated HaP film formation at the nanoscale, allowing for future evidence-based buried interface optimization.

The black-phase cesium lead iodide (CsPbI₃) perovskite is a highly promising material for next-generation optoelectronic devices due to its optimal bandgap, high carrier mobility, and excellent light absorption properties. However, its practical application remains significantly limited by its structural instability under ambient conditions, where it tends to rapidly convert into the non-perovskite yellow δ-phase. Various strategies have been explored to stabilize the black phase, including A-site and X-site doping, surface passivation, laser writing, and strain engineering. While these approaches have shown partial success, they also present significant limitations. A-site doping often involves volatile organic cations, which can introduce chemical instability [1]. X-site halide substitution (e.g., Br^-, Cl^-) tends to alter the bandgap [2], which is undesirable for certain optoelectronic applications. Additionally, methods such as surface passivation [3] and laser-induced [4] stabilization can create by-products or defects that impair charge transport, limiting device performance.

Among the emerging approaches, strain engineering has demonstrated significant potential for stabilizing the black phase. Interface strain, typically induced by annealing thin films on substrates with differing thermal expansion coefficients, has been shown to suppress the phase transition to the yellow phase [5]. However, this effect is confined to the interface region and effective only in ultrathin films (<100 nm), which are insufficient for high-performance optoelectronic devices like solar cells and photodetectors that require thicker active layers (300–500 nm).

In this work, we propose a novel approach to induce bulk-localized nanoscale strain through partial B-site substitution in CsPbI₃. By incorporating a small amount of dopant cations at the Pb²⁺ site, we introduce localized lattice distortions throughout the bulk of the film. This nanoscale strain reduces the spontaneous orthorhombic distortion of the crystal lattice and increases the Pb–I–Pb bond angle, both of which are critical factors in stabilizing the black perovskite phase. Unlike interface-limited strain methods, our strategy enables stabilization throughout the entire film thickness, thereby overcoming the primary limitation of previous strain engineering techniques.

We demonstrate that the optimized B-site substitution by Sn²⁺, Bi³⁺, and Cd²⁺ (up to 20%) does not significantly alter the optical bandgap of CsPbI₃, preserving its desirable optoelectronic properties. Furthermore, this method enhances phase stability under ambient conditions without compromising film quality or charge transport. Devices fabricated using this strain-stabilized CsPbI₃ exhibit superior performance, as evidenced by enhanced photodetector metrics, including higher responsivity, faster response times, and improved operational stability.

This study establishes a alternative route to achieving long-term black-phase stability in thicker perovskite films by leveraging atomic-level strain engineering via targeted B-site doping. Our findings provide a pathway toward the practical deployment of CsPbI₃-based optoelectronic devices with improved reliability and performance.

Quantum cutting represents a transformative strategy to mitigate thermalization losses that typically occur when high-energy photons are absorbed by semiconductors.[1,2] Recent advances have extended this concept from rare-earth doped crystals to semiconductor–rare-earth hybrid systems, particularly those utilizing halide perovskite absorbers,[2] thereby exploiting their exceptional optoelectronic properties.

In this study, we focus on Ytterbium (Yb)-doped CsPb(Cl_1-xBr_x)₃, a metal halide perovskite that absorbs in the visible and exhibits intense near-infrared (NIR) photoluminescence (PL) — a clear signature of quantum cutting. We first optimized the composition of Yb-doped CsPb(Cl_1-xBr_x)₃ by tuning both the Br content (x = 0.2 - 0.65) and [Yb:Cs] ratio (0.4 - 1.2). We observe a gradual blue shift of the absorption peak as the Br content decreases, and a red shift as the Yb concentration increases. Upon absorption of photons with wavelength < 500 nm, the doped perovskite converts the absorbed energy into NIR photons. The NIR PL signal appears at ~985 nm (1.26 eV), characteristic of Yb³⁺ ²F₅/₂ → ²F₇/₂ f-f transitions, confirming the occurrence of quantum cutting. The highest PL intensity is observed for (Cl_0.6Br_0.4) and a [Yb:Cs] ratio of 0.6. The NIR emission energy is slightly higher than the energy gap of Sn–Pb based perovskites (MA(Pb_1-ySn_y)I₃), which exhibit an optical bandgap around 1.21 eV when y = 0.65 - 0.85. This spectral alignment would be critical for enabling efficient energy transfer between the quantum cutting layer and the absorber layer.

Both doped and undoped perovskite films are synthesized following a double-step deposition from solution.[2] We use a suite of advanced spectroscopic techniques, including Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), PL, ultraviolet photoelectron and inverse photoemission spectroscopies (UPS/IPES), to systematically investigate the elemental composition and electronic structure of Yb-doped CsPb(Cl_0.6Br_0.4)₃. RBS and XPS depth profiling provides insights into the composition and elemental distribution of the films. Both techniques reveal an Yb enrichment near the surface of the film, corroborated by PL measurements, which show a stronger NIR 985 nm emission when the film is illuminated from the surface side than from the bottom side. We discuss the potential integration of Yb-doped CsPb(Cl_0.6Br_0.4)₃ with Sn–Pb based perovskite absorbers, offering a pathway to surpass conventional efficiency limits while providing a cost-effective strategy for enhanced energy conversion.

Lead (Pb) based perovskites with the general formula ABX₃ have attracted significant attention in providing desirable optoelectronic devices, such as solar cells, light-emitting diodes (LEDs), and photocatalysis. However, the high toxicity and low stability of Pb are a big concern for health and the environment, as well as commercialization, leading to extensive research to replace lead with less toxic elements. Recent works have been exhibiting tin (Sn²⁺) as a promising alternative due to comparable ionic radius and in a group electronic configuration to Pb⁺. Nevertheless, Sn²⁺ is prone to oxidation, easily transforming into Sn⁴⁺ in ambient conditions, which creates a destabilized perovskite structure and significantly reduces its functional performance. To overcome this issue, this work explores an alternative strategy by inserting the Sn²⁺ octahedron into a layered double perovskite (LDP) with the formula Cs₄M(II)M(III)₂Cl₁₂. In this LDP structure, Sn²⁺ is placed on the M(II) site, while the M(III) site is occupied by different trivalent metal cations such as In³⁺ or Sb³⁺. The two M(III) octahedrons in a unit cell serve as top and bottom protective layers, effectively suppressing the oxidation sensitivity of Sn²⁺ by minimizing the contact with any degradation factors, such as oxygen and water molecules leads to the formation of Sn⁴⁺.

Our x-ray diffraction (XRD) and elemental analysis confirm the successful formation of LDP Cs₄M(II)M(III)₂Cl₁₂ nanocrystals (NCs) via the modified hot injection method, showing well-defined and stable crystal structures. The as-synthesized perovskite NCs demonstrate a morphology of hexagonal nanoplatelets with diverse size distributions based on the selected M (III) cations. Specifically, In-based nanoplatelets show an average size of 41.4 nm, while Sb cases exhibit average sizes of 58.8 nm. 

Compared to conventional lead-based perovskite, these Sn-based LDP nanocrystals reveal significant enhancements in the structural stability. The long-term stability test upon the tracking of XRD patterns reveals that these NCs could retain their original structural integrity for more than 300 days, a remarkable improvement over Pb or other Sn(II)-based perovskite, which latter case normally degrades even within hours. In addition, the optical stability of the LDP NCs, as regularly measured by UV-visible absorption spectroscopy, maintained the original feature for over 40 days, indicating a hint of their potential for stable optoelectronic applications.

Besides, this nanomaterial demonstrates tunable electronic properties by substituting distinct trivalent metal cations at the M(III) site, which means the bandgap could be adjusted. For instance, the presence of In³⁺ shows a relatively wide bandgap at 3.54 eV, indicating a good fit for a wide scale of bandgap semiconductors. Interestingly, once Sb³⁺ is placed at the M(III) site, the bandgap could be narrowed to 1.89 eV. This characteristic of tunable bandgap is considered a good platform for customizing the material’s properties for corresponding needs. We further evaluated the photoelectrochemical performance through chronoamperometry (I-t curves) under 1 sun illumination. The samples exhibited rapid photo responses upon light switching on/off, with steady-state photocurrent densities of 35.31 µA cm⁻² and 49.66 µA cm⁻², respectively.

In summary, this work demonstrates a novel Sn(II)-based LDP NCs with different trivalent metal ions at the M(III) site, showing significantly enhanced structural stability and tunable optical properties, all which make them highly versatile for a wide range of application in future, such as solar cells, light-emitting devices, and photocatalysis.

The ideality factor n_id quantifies the deviation of the current-voltage characteristics of a solar cell from the ideal diode equation, excluding the influence of series and shunt resistances. It can be influenced by two effects: the scaling of the electron and hole density with injection level and the scaling of the dominant recombination mechanism with the electron and hole density. In an extrinsically-doped semiconductor in low-level injection (e.g. n << p = N_A), the majority carrier density is independent of the injection level which leads to an ideality factor of n_id = 1, independent of the dominant recombination mechanism. In the case of an intrinsic semiconductor (i.e. high-level injection, n = p), the ideality factor serves as tool to distinguish between recombination mechanisms: n_id = 1 is typically associated with radiative recombination or recombination via shallow traps; n_id = 2 with recombination through deep trap states; n_id = 2/3 with Auger recombination; and interface recombination can span a range of values depending on injection conditions and transport asymmetries.

Lead halide perovskites neither perfectly fall in the doped nor the intrinsic category. They have very low doping densities in the dark but show photodoping effects under illumination due to trapping of charge carriers. This situation is uncommon in semiconductor physics and requires innovative ways of analyzing and understanding charge carrier dynamics in general and ideality factors in particular.

Here, we discuss how photodoping in otherwise intrinsic semiconductors affects the ideality factor. At low injection levels, the trap occupation is limited by minority carrier detrapping. The trap behaves as a shallow trap yielding an ideality factor of n_id = 1, just as in the intrinsic case. At higher injection levels however, we find that the trap occupation is now limited by the majority carrier trapping, interestingly leading to an ideality factor of n_id = 1.5. At very high injection levels, the electron and hole density surpass the trap density, leading to the intrinsic case of a deep trap with n_id = 2. We point out that the ideality factor of n_id = 1.5 is a normal outcome of SRH theory and does not require interface recombination. We discuss the consequence of this effect on the photoluminescence quantum yield in steady state and on the transient photoluminescence behavior and compare the theory to measurements on perovskite thin films.

Solution-processable semiconductors like halide perovskites and certain molecules are promising for next-generation spin-optoelectronic applications [1]. Yet, we don’t fully understand what governs spin and light polarization in these materials, and even less how these are affected by chirality [2].

In this talk, I will give an overview of our recent efforts to understand the spin-optoelectronic performance of these materials through time-, space- and polarization-resolved spectroscopy and microscopy.

For investigating halide perovskite films, we pushed broadband circular dichroism to diffraction-limited spatial and 15 fs time resolution for creating a spin cinematography technique to witness the ultrafast formation of spin domains due to local symmetry breaking and spin-momentum locking [3].

I will then briefly explain the fundamentals and artefacts involved in measuring circularly polarized luminescence reliably and introduce an open-access methodology and code to do so [4]. Finally, I will show our most recent development of a transient sensitive broadband full-Stokes spectroscopy with unprecedented time- and polarization resolution to track the emergence of chiral light emission [5].

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

[2] Nature Reviews Chemistry 9, 208 (2025).

[3] Nature Materials 22, 977 (2023).

[4] Advanced Materials 35, 2302279 (2023).

[5] Nature, https://doi.org/10.1038/s41586-025-09197-3 (2025).

Halide perovskites are a uniquely versatile class of materials, exhibiting highly tunable photonic, electronic, and spin-dependent properties. Their soft, ionically conductive lattice and broad chemical flexibility allow for precise control over structure and composition, making them promising candidates for applications ranging from solar energy conversion and light-emitting devices to spintronics and quantum information technologies. These materials offer a compelling platform for designing multifunctional, adaptive interfaces, and for integrating advanced physical phenomena into scalable device architectures.

Our group combines first-principles simulations, data-driven modeling, and machine learning to unravel the fundamental structure–property relationships that govern performance, instability, and emergent phenomena in halide perovskites. A major research thrust is the chemistry and dynamics of point defects, which are central to charge recombination, ion migration, and long-term degradation. We investigate defect formation energies, migration barriers, and electrostatic interactions under realistic conditions, and develop mitigation strategies including compositional engineering, strain modulation, and surface passivation.

In parallel, we explore the growing frontier of chiral hybrid perovskites, where the incorporation of chiral organic ligands breaks inversion symmetry and enables spin-selective charge transport via the chiral-induced spin selectivity (CISS) effect. We examine how chirality, spin–orbit coupling, and lattice dynamics together shape chiroptical responses, opening new directions for spin-optoelectronic devices, polarized light detection, and quantum spin filtering.

Together, our work aims to establish a unified, multiscale framework for understanding and engineering halide perovskites as intelligent materials — where light, charge, spin, and lattice degrees of freedom can be co-optimized for next-generation optoelectronic and quantum technologies.

Incorporating chiral ligands into a perovskite structure induces overall chirality, enabling unique properties such as interaction with circularly polarized light [1] and chiral-induced spin selectivity [2]. We investigate the transfer of chirality from chiral organic ligands to the inorganic framework in 2D chiral perovskites with different metal cations, i.e. MBA₂SnI₄ and MBA₂PbI₄. Using structural descriptors, we analyze how the presence of chiral ligands distorts the inorganic layers and how these distortions relate to the overall structural chirality.
By comparing the Sn- and Pb-based perovskites, we find that chirality transfer manifests differently in the two systems [3-4]: it is more pronounced in the asymmetry of Sn–I bonds, while in the Pb structures, it is more evident in the asymmetry of I–Pb-I bond angles. These differences are attributed to the distinct assembly of the chiral cations and the stereochemical differences between Sn and Pb.
Furthermore, we explore the temperature dependence of chirality using molecular dynamics simulations with machine-learned force fields. Our results show that structural chirality decreases with increasing temperature, negating the low-temperature structural differences. This is consistent with previous findings that attribute this loss to the reorientation of the ammonium group that links the ligand to the framework [3].

Chiral organic–inorganic hybrid metal halides are emerging as a promising class of materials for spin-controlled optical and optoelectronic applications. Their lattice chirality can be tuned via the incorporation of enantiopure organic cations.^1-3 However, materials crystallizing in enantiomorphic space groups, where the R- and S-configured structures are non-superimposable mirror images, remain rare. In my talk, I will present novel chiral zero-dimensional (0D) (R-/S-MBA)₂SnBr₆ (MBA = methylbenzylammonium) crystals with enantiomorphic helical space groups P6₅ (M-helix) and P6₁ (P-helix). The helicity of the inorganic framework arises from 60° helical rotational alignment along the six-fold screw axis, stabilized by strong N–H···Br and C–H···π interactions. This cooperative assembly induces long-range chirality in the lattice, supported by DFT calculations showing strong electronic coupling between MBA ligands and SnBr₆²⁻ units. The chiral crystals and thin films exhibit mirrored circular dichroism (CD) spectra with a relatively high g_CD factor of 3.5 × 10⁻². Interestingly, the crystals does not the Cotton effect a rare phenomenon in chiral metal halides. Additionally, the materials demonstrate broadband second harmonic generation (SHG) from 875 to 1200 nm, with a dissymmetry factor (g_CP–SHG) up to 0.44 under 1030 nm excitation. Alloying with Pb induces a structural transition to a non-helical chiral space group (P2₁2₁2₁), highlighting that helicity is intrinsic to specific metal–halide combinations. These results introduce a new family of chiral halides with unique linear and nonlinear optical activity, opening exciting opportunities in spin-optoelectronics and chiroptical photonics.

Keywords:

Chiral metal halides, enantiomorphic space group, helical framework, circular dichroism, second harmonic generation

Chiral two-dimensional (2D) perovskites, derived from hybrid organic-inorganic halide perovskites (HOIPs), offer key advantages for optoelectronics, including defect tolerance, strong light absorption, compositional tunability, and low-cost synthesis. Their 2D structure leads to strong quantum confinement, evident in blue-shifted absorption/emission and elevated exciton binding energy, resulting in excitonic optical and photoluminescence behavior even at room temperature. Incorporating chiral cations distorts the inorganic framework, enabling circular dichroism, circularly polarized photoluminescence, and chiral-induced spin selectivity. These properties are of high interest for optoelectronic and spintronic technologies, including circularly polarized light sources [1], detectors [2], and spin-polarized current control [3]. Since such applications rely on thin films, understanding the impact of grain size on performance is essential.

In this study, we examined the optical properties—absorption, PL, and CD—of (R-3BrPEA)₂PbI₄ thin films based on (1R)-1-(3-bromophenyl)ethanamine [1]. Hypophosphorous acid (HPA) was used as a precursor additive to modulate surface roughness [4], a method previously shown to enhance grain size in MAPbI₃ by adjusting pH, polarity, and surface tension. We demonstrate that varying HPA concentration allows control over surface roughness, film density, and crystalline strain. The results reveal strain as a key determinant of photoluminescence yield, highlighting this approach’s potential for optoelectronic applications.

Ultraflexible perovskite solar cells (PSCs) have attracted significant attention as lightweight and flexible power sources due to their high efficiency and the ultrathin, lightweight nature of plastic substrates with thicknesses on the order of one micron. Although ultraflexible PSCs offer great potential as high power-per-weight (ppw) energy harvesters, their ppw values are still limited by relatively low device efficiencies. This limitation is primarily attributed to the poor thermal stability of conventional ultrathin plastic substrates and the common use of p-i-n architectures in ultraflexible PSCs, where p-type polymer bottom layers are employed to enable low-temperature fabrication.

In this study, we developed highly efficient ultraflexible PSCs with an n-i-p structure on a newly designed 1.5  μm-thick thermally stable plastic substrate composed of parylene and SU-8. The resulting ultraflexible PSCs achieved a power conversion efficiency of 18.2% while maintaining excellent mechanical flexibility, with stable operation under a bending radius as small as 500  μm.

Furthermore, we fabricated an ultraflexible PSC module by connecting six individual cells in series for energy harvest under indoor lighting conditions. By integrating this PSC module with perovskite nanocrystal LEDs, we successfully demonstrated perovskite LED operation powered solely by the harvested energy from indoor light¹.

Perovskite solar cells have rapidly transitioned from laboratory research to a commercially promising photovoltaic technology within just over a decade, driven by their high power conversion efficiency, low-cost production, and excellent performance in low-light conditions. Hybrid organic-inorganic perovskites combine the solution-processability of organic materials with the robust optoelectronic properties of crystalline inorganic semiconductors, creating a versatile material platform that outperforms conventional photovoltaic technologies in specific applications.

To enable scalable and environmentally responsible production, sustainable and industry-compatible processing methods are essential. This begins with the development of green synthesis routes for perovskite precursors. Solaveni, a German innovator, has introduced sustainable synthesis techniques for organic alkylammonium halides and metal halides using novel green halide chemistry. This presentation evaluates these synthesis routes through a life cycle assessment (LCA), comparing their environmental impacts—resource use, energy consumption, and emissions—against traditional methods. The LCA provides critical insights into the sustainability of these precursors, informing strategies to minimize the ecological footprint of perovskite photovoltaics. Additionally, innovative recycling and upcycling methods for perovskites are introduced, enabling rapid material recovery and fostering a circular economy. These advancements pave the way for sustainable, high-performance perovskite solar technologies with reduced environmental impact.

Perovskite solar cells (PSCs) are at the forefront of next-generation photovoltaics, combining high power conversion efficiency with low-cost and scalable fabrication. Their unique optoelectronic properties and potential for flexible, lightweight applications make them strong candidates for future solar energy systems. However, key challenges, particularly related to long-term stability, scalable manufacturing, and consistent film quality, must be addressed to enable commercial deployment [1,2]. To tackle these issues, we present an automated platform that integrates machine learning (ML) workflows with high-resolution image analysis for real-time process optimization. As illustrated in Figure 1, this platform combines computational tools and data-driven methods to study and impove the crystallization dynamics and morphological quality of PSC films.

Our approach leverages microscopic imaging acquired in situ or post-deposition, which is processed using state-of-the-art segmentation frameworks, including the Segment Anything Model (SAM) and Detectron2. These tools enable the precise identification of morphological features such as crystal domains, grain boundaries, and nuclei formation sites. To further classify and characterize these regions, we employ a ResNet152 convolutional neural network (CNN), which supports detailed recognition of grain size, shape, and orientation.
From the segmented images, we extract a set of quantitative descriptors to assess film homogeneity and microstructural quality. These include crystal size distributions, aspect ratios, and novel information-theoretic metrics such as Shannon entropy and Computable Information Density (CID). These metrics enable a deeper understanding of the crystallization process, revealing correlations between microstructural patterns and device performance. [3].
Applied across multiple fabrication batches, our pipeline captures subtle morphological variations and identifies key trends linked to processing conditions. In particular, we show that entropy and CID serve as sensitive, compact indicators of film homogeneity, complementing physical measurements and aiding in the detection of suboptimal growth regimes.

This methodology provides a scalable and automated framework for image-based quality assessment in PSC fabrication. While real-time control is not yet implemented, the demonstrated analytical capacity lays the groundwork for future integration into closed-loop synthesis platforms, ultimately supporting more consistent and efficient manufacturing.

We present a scalable and eco-friendly inkjet printing strategy for fabricating broadband photodetectors based on hybrid nanocrystalline perovskites (CsPbBr₃-Cs₄PbBr₆) integrated with CVD-grown single-layer graphene (SLG). The hybrid architecture leverages the exceptional light absorption of CsPbBr₃ nanocrystals and the ultrahigh carrier mobility of graphene to overcome the mobility limitations typical of all-inorganic perovskites. Through controlled post-deposition annealing at 120 °C, we achieved a dual-phase morphology that broadens spectral sensitivity and enhances ambient stability.

The resulting devices, printed directly onto SLG-FET platforms, exhibit peak responsivity exceeding 57x10³ A/W at 312 nm and detectivity above 10¹³ Jones—values that surpass most solution-processed perovskite photodetectors to date. The photodetectors show reliable performance across 270–700 nm, maintain ~250 μA photocurrent over six months without encapsulation, and operate with no external gate bias. This performance stems from efficient photogating mechanisms at the perovskite–graphene interface and the “raisin-bread” architecture that embeds CsPbBr₃ nanocrystals within a Cs₄PbBr₆ matrix.

Our work demonstrates how inkjet printing enables reproducible, mask-free, and vacuum-free processing of hybrid perovskite devices, with clear implications for roll-to-roll manufacturing of next-generation UV–visible optoelectronics, including flexible and wearable sensors.

Halide perovskites (HPs) continue to revolutionize optoelectronics due to their outstanding properties—tunable bandgaps, high absorption coefficients (10⁴–10⁵ cm⁻¹), and remarkable defect tolerance. These features have enabled advances in photodetectors (PDs) and light-emitting diodes (PeLEDs), with recent external quantum efficiencies (EQE) in PeLEDs exceeding 26%, surpassing those of planar OLEDs [1-4]. Nevertheless, the field demands scalable, material-efficient, and environmentally responsible manufacturing processes.

Inkjet printing has emerged as a transformative technology for perovskite optoelectronics, offering digital patterning, low material waste, and compatibility with flexible substrates. Since early implementations yielded modest efficiencies below 10% [3,4], the technique has matured toward fully inkjet-printed PeLEDs [5], integration with sustainable solvent systems [6], and novel device functionalities. Recent efforts by our group include:

​​​
(i) Fully inkjet-printed green PeLEDs using CsPbBr₃ nanocrystal inks, where post-print annealing was found to modulate structural dimensionality and increase photoluminescence by over 70-fold [5].

(ii) Red-emitting TEA₂SnI₄ and PEA₂SnI₄ PeLEDs fabricated with eco-friendly DMSO inks, demonstrating excellent emission stability and environmental compliance with EU RoHS directives [6].

(iii) Photodetectors based on 2D/3D tin perovskite (PEA₀.₅BA₀.₅)₂FA₉Sn₁₀I₃₁, showing high responsivity (up to 50 A/W), broadband detection from UV to near-infrared, and improved performance over time under effective encapsulation strategies [7].

(iv) Single-mode random lasing in inkjet-printed FASnI₃ integrated into vertical cavities, reaching Q-factors up to 1000 and demonstrating low-threshold, spectrally stable lasing [8].

This body of work underscores the convergence of scalable inkjet printing with advanced functional materials, offering new opportunities in optoelectronic design. Detailed evaluation of ink formulation, printing parameters, and annealing protocols reveals pathways to control crystallization dynamics, mitigate printing artifacts (e.g., coffee-ring effects), and optimize film morphology. Additionally, green chemistry approaches—including the replacement of lead with tin and DMF with DMSO—are shown to substantially reduce environmental impact without compromising device performance.

The integration of light emission and photodetection capabilities in similar material platforms further supports the vision of multifunctional, monolithically integrated perovskite optoelectronics. These advances position inkjet printing as a key enabling technology for future applications in wearable electronics, flexible photonics, smart displays, and sustainable photonic systems.

[1] Zhao, B. et al., Nature Nanotechnology 18, 981–992 (2023).

[2] Wenger, B. et al., Nat Commun 8, DOI 10.1038/s41467-017-00567-8 (2017).

[3] Shen, W. et al., ACS Appl. Mater. Interfaces 14, 5682-5691 (2022).

[4] Hermerschmidt, F. et al., Mater. Horizons 7, 1773-1781 (2020).

[5] Vescio, G. et al., Adv. Eng. Mater. 25 (2023).

[6] Vescio, G. et al., ACS Energy Lett. 7, 3653-3655 (2022).

[7] Vescio, G. et al., Small Science (2025).

[8] H. P. Adl et al. Advanced Materials 36 (2024).

Halide perovskite solar cells combine record‐high power conversion efficiencies with low‐cost, solution‐based processing. and have rapidly emerged as a leading candidate in next-generation photovoltaics also in combination with silicon photovotlaics. However, their commercial viability remains challenged by environmental and stability concerns. In this work, we present a comprehensive approach to the sustainable development of halide perovskite photovoltaics through environmentally conscious materials and processes on both glass and flexible sustrates. Key strategies include the use of green solvents for perovskite film deposition, enabling safer and more scalable fabrication routes. We demonstrate the fabrication of fully flexible perovskite solar cells under ambient air conditions using low-toxicity, eco-friendly solvents via scalable printing technologies. Additionally, we replace conventional noble metal electrodes with carbon-based back contacts, offering a cost-effective and sustainable alternative with good conductivity and stability, showcasing potential for full roll-to-roll production. Finally, we address the critical issue of lead toxicity by partially substituting lead with tin in mixed Pb-Sn perovskite compositions, achieving reduced environmental impact while maintaining promising photovoltaic properties as single junction as well as in perovskite/perovkite tandem. Together, these innovations mark significant progress toward greener, safer, and more sustainable perovskite solar technologies suitable for widespread deployment.

In 2025, certified efficiency up to 27% has been demonstrated using Perovskite Solar Cells.[1] Also, many progresses have been done on encapsulation to improve their stability.[2] These results are showing their potential for the next generation of photovoltaic device.  This explain why some industrials are now investigating this field and start to build their pilot and fabrication lines. So far most of high efficiency devices have been achieved using the DMF / DMSO solvent system with an antisolvent step using chlorobenzene.[3] This approach is very sensitive to the antisolvent step and has a narrow processing window which might make it difficult to scale up. Other approaches rely on the use of nitrogen or vacuum quenching which might be costly due to the high amount of high purity gas use during the nitrogen quenching step and the slow speed of the vacuum quenching respectively.[4] Here we present our work on inkjet printed perovskite using a relatively unexplored solvent with low toxicity, 1-methoxy-2-propanol. By using different additive we show that we can control the crystallization kinetic to make compact perovskite layer without any quenching and achieve device efficiency over 20%.

The highly sought after optoelectronic properties of metal halide perovskites and their straightforward solution-processability offers a promising way ahead to realizing high-performance devices through additive approaches like printing.[1,2] The composition of inks used in printing plays a vital role in functional thin film formation which is critical for optical conversion and electrical transport in devices. However, understanding and controlling crystallization of perovskite layers from printed wet films on substrates for device applications remains largely elusive. In this study, the influence of graphene nanosheets on the crystallization dynamics of inkjet-printed methyl ammonium lead bromide (MAPbBr₃) thin films is presented and leveraged for device application in photodetection.  

The centers of heterogeneous nucleation could be ascribed to graphene nanosheets, resulting in highly textured films while retaining optical properties such as the absorption and emission features of the MAPbBr₃. Stark differences in morphology ranging from poorly connected island films to dendritic networks were observed through a variety of microscopic techniques and correlated to the crystalline orientation from x-ray diffraction studies. Additionally, a correlation with time resolved photoluminescence exhibited an interesting evolution in the decay profile that could be linked to the crystallinity of the films, as well as with electrical measurements. To profit from the control on crystallinity to the electro-optical characteristics of these films, printed photodetectors were investigated and found to exhibit strong selective photoresponse above 2.3 eV without the use of external filtering. Benefitting from the influence of graphene on the optoelectronic properties of the films, devices with responsivities of 2 A W^-1 and detectivities of nearly 1.9×10¹⁰ Jones were obtained. Similarly, flexible devices were also fabricated on polymer substrates and found to have good retention of photoresponse to cyclical mechanical stress. These findings point to the crucial role of tuning ink composition with nano-additives such as graphene, its impact on film formation, and implication in the optoelectronic performance of printed devices.  

Perovskite solar cells (PSCs) represent a third-generation solar cell technology and a promising alternative to traditional photovoltaic technologies. One of the key advantages of this technology is solution processability which enables fabrication on flexible substrates using roll-to-roll (R2R) applicable methods such as slot die coating, gravure printing, and rotary screen printing. R2R manufacturing methods offer significant advantages in terms of material utilization and production speed, making them ideal for large-scale, portable, wearable and other applications where lightweight and flexibility are essential [1, 2, 3, 4].

The commercialization of PSCs hinges on the development of scalable, low-temperature, and solution-processable materials compatible with high-throughput R2R fabrication techniques. The R2R manufacturing methods require specific ink compositions and rheological properties, which our research has addressed previously. Tin dioxide (SnO₂) is a leading candidate for the electron transport layer (ETL) in PSCs due to its excellent electronic properties, transparency, and chemical robustness. Yet conventional colloidal SnO₂ dispersions designed for laboratoryspin‑coating translate poorly to industrial processes because of inadequate viscosity which causes uncontrollable ink spreading. We have demonstrated that transferring SnO2 deposition from laboratory to larger scale is achievable through ink formulation [2].

In this work, we present the development of a R2R-compatible SnO₂ ink for gravure printing. The formulation is based on a solvent blend containing water and alcohol. Alcohol is introduced as a rheology modifier to ensure suitable particle size distribution and viscosity leading to shear-thinning behavior that ensures clean ink release from the gravure cells and prevents ink bleeding after printing. This resulted in uniform deposition and film formation at low annealing temperatures (<140 °C) in ambient conditions, suitable for flexible substrates. The resulting SnO₂ layers demonstrate high optical quality, low surface roughness, and compatibility with perovskite absorbers. Devices fabricated using R2R printed SnO₂ ETLs in an n-i-p architecture with printed perovskite layer achieved power conversion efficiencies up to 11 % with excellent reproducibility and operational stability under continuous illumination (nearly 10 % PCE). This work highlights the importance of ink rheology and process compatibility in transitioning PSCs toward industrial-scale production via roll-to-roll manufacturing.

The evolution of inkjet-printed electronics, from resistive memories to multifunctional optoelectronic systems, has opened transformative pathways for neuromorphic computing, photonic logic, and intelligent sensing platforms. The unique advantages of inkjet printing, including additive patterning, scalable fabrication, and compatibility with flexible substrates, enable the precise engineering of complex heterostructures that integrate charge, light, and memory functionalities.

Our work began with the development of fully inkjet-printed metal-insulator-metal (MIM) structures using high-k HfO₂ dielectrics. These devices demonstrated low-power, non-volatile switching, strong retention, and uniformity suitable for passive memory and selector applications, establishing a scalable platform for printed memory arrays. Building upon this, we transitioned to 2D materials, particularly hexagonal boron nitride (h-BN), achieving devices with high endurance, reproducibility, and tolerance to stochastic variation. These characteristics enabled reliable operation of logic-in-memory (LiM) architectures, including MAGIC gates and current-controlled logic implemented on printed substrates, supported by Monte Carlo modeling and experimental validation.

This foundation set the stage for the next leap: the integration of halide perovskites as multifunctional active layers. Leveraging the photoelectric tunability of Sn-based and CsPbBr₃ nanocrystal perovskites, we demonstrated a new generation of printed devices capable of modulating light in response to electrical and optical history. In particular, TEA₂SnI₄, PEA₂SnI₄, and FASnI₃-based devices were inkjet-patterned within vertical cavity and multilayer structures to exhibit persistent photoconductivity, state-dependent photoluminescence, and feedback-tunable random lasing—hallmarks of optical memristors and photonic synapses.

Through careful control of ink formulation, solvent selection (e.g., DMSO over DMF), annealing conditions, and dimensionality engineering, these perovskite systems revealed behaviors akin to memristive switching, but with the added versatility of light-assisted logic, programmable spectral response, and in-memory photonic modulation. Importantly, their environmental compatibility, mechanical flexibility, and seamless integration with previously demonstrated memory stacks affirm their suitability for monolithic optoelectronic platforms.

This research trajectory, from printed HfO₂ and h-BN memories to light-responsive perovskite-based systems, illustrates the maturation of inkjet printing into a core enabler for future electronics. By uniting memory, emission, detection, and logic on a single substrate, we envision adaptive, multifunctional, and sustainable devices for applications in neuromorphic computing, wearable photonics, and reconfigurable metasurfaces.

Metal halide perovskite solar cells have become a viable option for future renewable energy. Record single and tandem junction all-perovskite solar cells already provide power efficiencies of over ~26% and ~30%, respectively. The next target in photovoltaic energy conversion can possibly be met by developing all-perovskite multi-junction solar cells. These require highly efficient and stable perovskite sub-cells with bandgaps in the range between 1.2 and 2.3 eV. Especially for narrow and wide bandgap perovskites challenges remain in reducing the energy loss between bandgap and open-circuit voltage. Guided by ultra-sensitive photocurrent spectroscopy, absolute and time-resolved photoluminescence spectroscopy, and in combination with bulk and interface passivation strategies, it is possible to reduce non-radiative losses in each of the bandgap regions and achieve open-circuit voltages that approach and sometimes exceed 90% of the detailed balance limit. By monolithically stacking multiple perovskite sub cells with complementary bandgap using recombination junctions designed to provide near-zero electrical and optical losses, it is possible to fabricate monolithic multi-junction configurations with high power conversion efficiencies.

Metal-halide perovskite (MHP) photovoltaic (PV) modules are progressing towards commercialization. One major hurdle that remains is establishing confidence in long-term field performance and durability of MHP modules. Field testing, failure analysis, and understanding of degradation mechanisms are essential to advancing this technology toward commercialization. Here, I will present a brief overview of recent results from the PV Accelerator for Commercializing Technologies, PACT, program. In addition, I will dig into performance data from up to 1 year of outdoor testing of MHP modules in Golden, Colorado and Albuquerque, New Mexico, USA. This talk will cover nondestructive and destructive characterization of field-tested modules points to use to determine the root cause of the observed failures. We find module scribes and interfaces as areas of potential mechanical weakness and chemical migration, resulting in shunt pathways and increased series resistance. Finally, we perform indoor accelerated stress testing with light and elevated temperatures, demonstrating failure with similar scribe degradation signatures as compared to the field-tested modules highlighting this as a possible screening test for field failures.

Perovskite solar cells have demonstrated impressive efficiencies and affordable manufacturing costs, able to either compete with silicon solar cells or enhance them with tandem designs. [1] However, a well-known challenge with perovskite solar cells is their limited lifetime. The short life-cycle is further compounded by a lack of clear recycling strategies for perovskite solar cells. [1] Our goal in this study is to address the twin problem of short lifetime and lack of recycling methodology by demonstrating a way to revive degraded perovskite solar cells with green solution-based recycling technique.

Typically, the degradation of perovskite solar cells is caused by the deterioration of the perovskite layer itself. [2] The idea behind perovskite solar cell revival is to remove the degraded perovskite layer with a suitable solvent, in a way that the surrounding scaffold structure remains intact. The perovskite layer is then regenerated by infiltrating the scaffold with a fresh perovskite solution and annealing to remanufacture the cell. The main challenges in the revival process is thorough removal of degraded perovskite without damaging the scaffolding structure and ensuring sufficient revival rate.

We used perovskite solar cells with carbon electrodes due to their high stability and scalability. [3] The recycling was performed with green and non-toxic γ-Valerolactone (GVL) solvent [4] to maximise the sustainability of the process. Our preliminary results show that full 100 % revival rate is possible in champion cases but the technique requires further optimisation to reach these results consistently. Another important observation is that incorporating zirconia nanoparticles into the solar cell scaffold improves its robustness during degraded perovskite flushing. By embracing circular economy techniques, we can strive for more sustainable PV technologies.

Perovskite solar cells (PSCs) exhibit excellent efficiencies, but face challenges related to interfacial and environmental stability.¹ Here, we present two quantum dot (QD)-based strategies to address these limitations. In the first, CdS QDs are introduced as an interfacial layer between SnO₂ and the perovskite absorber. This approach reduces surface oxygen vacancies and hydroxyl groups, as confirmed by XPS, while Kelvin probe force microscopy reveals enhanced surface potential uniformity. Perovskite films grown on CdS-passivated SnO₂ show larger grain sizes and reduced PL intensity, suggesting improved charge extraction. Time-resolved PL confirms a significant increase in electron transfer rate leading to ~15% higher device efficiency. Previously, we also investigated halide exchange at the heterojunction between perovskite QDs and 3D perovskite films using in situ photoluminescence.² By extracting the activation energy of the Br-to-I exchange, we demonstrate its role in defect passivation and suppression of bimolecular recombination. QDs also enable favorable energy level alignment, enhancing hole extraction and stability under thermal stress. Their hydrophobic ligands further protect the perovskite against moisture ingress. In summary QD-based interfacial engineering strategies can improve both performance and durability of PSCs.

Halide perovskites are exciting new semiconductors that show a great promise in low cost and high-performance optoelectronics devices. However, the poor stability of conventional halide perovskites is limiting their practical use. In this talk, I will present a molecular approach to the synthesis of a new family of hybrid material – Organic Semiconductor-incorporated Perovskite (OSiP), which are more versatile and intrinsically stable. Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically flat interface and short interlayer distance. In addition, the rigid conjugated ligand design dramatically enhances their chemical stability, suppresses solid-state ion diffusion, and modulates electron-phonon coupling, making them useful in many applications, particularly solid-state lighting. Using these stable hybrid materials, we demonstrate efficient light emission and amplification in single crystalline nanostructures, epitaxial heterostructures, and polycrystalline thin films.

With the advent of metal halide perovskites into the world of emerging sustainable semiconductors, a host of previously unprecedented applications has materialised. Aside from their raging success in photovoltaics, perovskites as applied to light emitting diodes (LEDs) and displays have gained particular interest due to their extremely facile bandgap tunability, directional and narrow-linewidth emission characteristics, high brightness, superior efficiencies and colour purity among other desirable features. However, most of the world records in terms of device efficiency and stability have been achieved on lab-scale pixels processed using solution-based techniques, primarily spin-coating. However, with its inherent advantages of scalability, reproducibility and precise thickness control, vacuum-based thermal evaporation provides an edge over solution-processing for all optoelectronic applications. Thus, as we move towards integrating sustainable semiconductors in consumer electronics, it is important to optimise thermal evaporation-based device fabrication, especially for large-area, flexible and niche applications for light sources and displays.

Following the recent advancements in vacuum-evaporated perovskite solar cells in our group, we have employed this technique to fabricate highly luminescent all-inorganic CsPbI₂Br perovskite films for red light emitting diodes (LEDs). By optimizing the growth conditions, we have been able to achieve unprecedented photoluminescence quantum efficiencies (PLQE) close to 20% under 1-sun equivalent conditions. Moreover, no sign of unwanted halide segregation has been observed under continuous illumination, thereby resulting in a stable PL emission in the wavelength range of 630-640 nm (pure-red emission). To understand the effect of deposition conditions on the resulting optoelectronic properties of evaporated perovskites, a range of fundamental characterization including intensity-dependent PLQE, fluence-dependent TRPL, widefield hyperspectral imaging, temperature-dependent PL, THz spectroscopy and transient photoconductivity measurements have also been conducted. Furthermore, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) has been employed to obtain information on the chemical composition and electronic properties of the evaporated films. There is very little existing information on the structure-property relationships in evaporated films tailored to luminescence. To our knowledge, our work is the first of its kind to take into account various factors related to the unique morphology of these films and to study their concomittant effect on their optoelectronic behaviours. Finally, by extensive screening of the charge injection layers, we have been able to demonstrate proof-of-concept LEDs with external quantum efficiency >3% with turn-on voltage ~ 3V, which is a world record for evaporated red perovskite LEDs as of now.

Overshoot pulses of perovskite light-emitting diodes reveal two their operation regimes controlled by variations of driving voltage.

Rokas Gegevičius¹, Ignas Ledzinskas¹, Jevgenij Chmeliov¹, Iakov Goldberg², Robert Gehlhaar² Karim Elkhouly² and Vidmantas Gulbinas¹

¹Center for Physical Sciences and Technology, Sauletekio Ave. 3, LT-10257 Vilnius, Lithuania;

²IMEC, 3001 Leuven, Belgium;

In some applications, perovskite light-emitting diodes (PeLEDs) are expected to operate in pulsed mode. The generation of high-intensity light pulses requires substantial electrical pumping power, which can lead to deterioration of PeLED performance, its degradation or even damage. Contrarily, PeLEDs operating in a non-conventional regime, based on the so-called overshoot effect enables the generation of short, high-intensity, perfectly electrically synchronisable optical pulses while maintaining relatively low electrical pumping power. Here we demonstrate the generation of overshoot pulses (OSP) by FAPI PeLEDs with different perovskite layer thickness and analyse their dependence on alternating driving voltage and temperature.   The intensity and shape of the OSPs are determined not only by the voltage and duration of the pump pulses, but also by the offset voltage applied between the pump pulses, as well as the afterpulse voltage applied immediately after the end of the pump pulse. The offset voltage determines the distribution of the mobile ions, strongly affects the strength and spatial distribution of the internal electric field during the pump pulse action and thus is a crucial parameter determining evolution of the conventional electroluminescence intensity and generation of the OSPs. Meanwhile, the afterpulse voltage controls the intensity and duration of the OSPs.  The intensity of the OSPs increases strongly at temperatures below ~ 200 K. Mathematical modelling makes it possible to reproduce the electroluminescence dynamics and identifies two distinct PeLED operation regimes: one that facilitates OSP generation and another that prevents it.

Herein, we report mixed Ruddlesden–Popper (RP) (quasi-2D) perovskite–perovskite planar heterojunctions at the hole transport layer (HTL) and perovskite emission layer (EML) interface, facilitating efficient perovskite light emitting diode (Pe-LED) devices. A chloride-based quasi-2D layer of formamidinium lead chloride (FA_xPbCl₃) acts as an HTL, and a bromide-based quasi-2D layer of PEA₂(FAPbBr₃)_n−1PbBr₄ with potassium bromide (KBr) additive acts as a light emission layer (EML). The solution-processed quasi-2D perovskite–perovskite planar heterojunction between the HTL and EML interface has an ultrathin interlayer of poly(9,9-bis(3′-(N,N-dimethyl)-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) (PFN-P1), which prevents either halide intermixing at the perovskite–perovskite interface or any dissolution of the underneath perovskite HTL by polar solvents used in processing the EML, resulting in a conformal quasi-2D perovskite–perovskite heterojunction. More importantly, the mixed dimensional perovskite HTL transmittance spectral range can be extended, which corresponds to low absorption especially in the deep blue and UV region of the electromagnetic spectrum, by introducing an excess amount of formamidinium chloride (FACl). When translated to a device, the optimized Pe-LED devices having a quasi-2D perovskite–perovskite planar heterojunction between the HTL and EML have superior hole transport and hole injection due to a higher charge carrier mobility and favorable energetic alignment, outperforming their counterparts based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The optimized Pe-LED devices have current and power efficiencies of 38 cd A^–1 and 31 lm W^–1, an EQE of 9.3%, a turn-on voltage of 3 V, and a high brightness of 11 500 cd m^–2, together with high reproducibility. They emit a saturated green electroluminescence (EL) with an fwhm of 24 nm, having CIE coordinates (0.17, 0.75). Significantly, the Pe-LED devices with a quasi-2D perovskite HTL/EML heterojunction show remarkable EL stability of about ∼30 min at 100 cd m^–2, which is 4 times higher than that of Pe-LEDs with a PEDOT:PSS HTL/perovskite EML heterojunction.

Perovskite nanocrystals for bright and stable PeLEDs and LED arrays

Sarika Kumari,¹  Rafael S. Sánchez,¹ and Iván Mora-Seró^1, *

¹ Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castelló, Spain.

Corresponding authors: (rasanche@uji.es, sero@uji.es)

Perovskite materials have been exploited for their applications in light emitting devices for several years since their first application in 2014¹. All inorganic PeNCs have been considered suitable materials for the light emitting diodes because of their properties like high colour purity, high PLQY, high carrier mobility and direct bandgap. High brightness is achieved by these colloidal Perovskite nanocrystals. We have aimed to fabricate the PeLEDs array for writing different characters by using the different PeLEDs pixels on the substrate. The PeLEDs were fabricated with high brightness up to 96,720 cd/m², T₅₀ of 90 min with 23 % EQE. In this work, we have used our already established synthesis protocol of high purity CsPbBr₃ NCs and the device fabrication process with architecture of ITO/PEDOT: PSS/Poly-TPD/CsPbBr₃ NCs/POT2T (40 nm) /LiF (1.0 nm)/Al (100 nm) is described in this report². 

The LED array devices containing 64 pixels by using the substrates of the area of 25 mm*25 mm as shown in Fig 1. In this stage, we have encountered the problem of turning up of the random pixels when trying to turn on one pixel which means that the device pixels were uncontrollable.

Lead-free halide perovskite quantum dots (QDs), including Sn-based and double perovskite QDs, have gained increasing attention as environmentally benign alternatives to lead-based perovskites for optoelectronic applications. These materials offer tunable bandgaps, strong light absorption, and excellent solution processability. However, achieving high crystal quality, defect tolerance, and efficient charge transport remains critical to unlocking their full potential.

In our recent studies, we have successfully synthesized phase-stable, low-defect Sn-based, Sn–Pb alloyed, and double perovskite QDs with enhanced photoluminescence quantum yields (PL QYs) and long carrier lifetimes [1–6]. Strategies such as Sn(IV) suppression and metal ion doping effectively minimized trap-mediated recombination and lattice distortion. Ultrafast transient absorption (TA) and time-resovled photoluminescence (TRPL) further revealed negligible electron or hole trapping, consistent with enhanced PL QY in optimized samples.

Focusing on double perovskite QDs, we developed Sb³⁺/Mn²⁺ co-doped Cs₂NaInCl₆ systems that exhibit efficient broadband white-light emission via self-trapped excitons. This co-doping approach not only induces white emission but also suppresses cation disorder, leading to PL QYs approaching 100%. To improve device performance, we replaced long-chain ligands with short-chain alternatives[8], which increased film conductivity by nearly 20-fold and reduced the hole-injection barrier by 0.4 eV. These improvements enabled light-emitting diodes (LEDs) with an external quantum efficiency (EQE) of 0.86%—the highest reported to date for double perovskite QD-based LEDs.

In this talk, we will present our recent progress in the synthesis, surface passivation, and photophysical characterization of lead-free perovskite QDs, and discuss their promising applications in optoelectronic devices such as LEDs. Our findings offer new insights into the design principles for achieving high-efficiency, stable, and environmentally friendly perovskite-based devices.

The presentation will review some recent results on the effect of carrier-lattice coupling and lattice matching on the optoelectronic properties of halide perovskites and their heterostructures and nanostructures. The early theoretical prediction and experimental demonstration of screening of electron-hole interactions by charge carrier-lattice coupling in 3D perovskites, was crucial to explain why photocarrier collection is possible at RT and how solar cell architectures could evolve toward much thicker active layers. The presentation will introduce recent methodological developments for the description of electron-coupling in the high temperature regime where lattice fluctuations are dominated by slow structural relaxations and strong anharmonicity. For the low-temperature regime, the importance of excitonic polarons will be stressed and new empirical and semi-empirical approaches will be described to account for the complexity of the carrier-lattice coupling in halide perovskites. Finally, the concept of lattice-matching will be presented and its predictive power will be demonstrated through a few experimental examples related to 2D/3D and multilayered halide perovskite heterostructures.  

Exciton-polaritons are solid-state quasi-particles presenting properties lying in between light and matter such as non-linear interactions (from the excitonic Coulomb repulsion), fast propagation (arising from the photonic component). The interest of polaritons¹ spans from the fundamentals of correlated light-matter interaction to emergent applications such as ultra-thin lasers, optical amplifiers, logic gates and even emulators of lattice-like Hamiltonians.

We implement a monolithic microcavity composed of two mirrors, a bottom distributed Bragg reflector made of 10 alternating SiO₂ and TiO₂ layer pairs, and a top silver mirror of ~20 nm thickness. Between them, we deposit 2D layered phenethylammonium (PEA₂PbI₄) perovskites of ~100 nm thickness. As a cavity spacer, we use PMMA polymer, whose thickness is controllably tuned to bring the photonic and excitonic modes in energy resonance. The vertical design of the planar microcavity system is guided by transfer matrix method simulations. We measure the dispersion relation of polaritons under white light (left-hand side of Fig. 1 panels) and weak, non-resonant driving for three exciton-photon energy detunings (right-hand side of Fig. 1 panels)  resulting from the different PMMA thicknesses.

Interestingly, under circularly polarized, non-resonant (3.06 eV) CW laser driving, the lower polariton branch emission follows the circular pump polarization. Under the same excitation conditions, the exciton does not exhibit a circular degree of polarization, due to the slower decay of bare excitons, as opposed to exciton-polaritons. Finally, photostability experiments for long exposure excitation time scales (min) also reveal that the polariton emission is more resilient than bare excitons, indicating that strong coupling serves as a protection for excitons².

Screening the Local Structural Space in Lead Halide Perovskites 
 
Milos Dubajic, Samuel D. Stranks
 
 
 
Recent investigations have revealed that halide perovskites are best described at equilibrium as containing short-lived (picosecond), nanoscale (a few unit cells) domains of lower symmetry than the bulk average structure, where the corner-sharing lead halide octahedra are tilted relative to each other. In this study, we employ Brillouin spectroscopy together with X-ray diffuse scattering to systematically screen the local structural space across various perovskite compositions. We demonstrate that, in selected compositions, finite polarization vector fields are generated in the vicinity of these nanodomains. Furthermore, by controlling the temperature, we modulate both the size and the dynamics of the nanodomains, a change that is directly observable as a softening of the elastic stiffness tensor upon cooling. Our precise determination of the symmetry and shape of these nanodomains across different average phases highlights the tunability achieved through A-site cation and halide anion substitutions. Given the strong correlation between these local structures and the macroscopic performance of these materials, our findings pave the way for the future tuning of optoelectronic properties in lead halide perovskite devices.

Hybrid organic-inorganic perovskites (HOIPs) are a highly promising class of materials for advanced applications in nonlinear optics, especially for second-harmonic generation (SHG), a process that occurs only in materials with a noncentrosymmetric crystal structure. The breaking of inversion symmetry is not only crucial for SHG but also enables other important functionalities, including ferroelectricity and the bulk photovoltaic effect (BPVE). The BPVE can produce ultrafast, dissipation-less photocurrents without the need for heterostructures or interfaces, making these materials highly attractive for next-generation photovoltaics and self-powered photodetectors

A sure way to achieve noncentrosymmetry is through the incorporation of homochiral organic ligands, which inherently forces the material to crystallize in one of the chiral Sohncke space groups. While this is a guaranteed route to an SHG-active material, it limits the structural possibilities to chiral space groups, among which only a part is polar. Thus, non-chiral acentric structures can crystallize in a broader range of noncentrosymmetric space groups, potentially offering greater chance for e.g. ferroelectric properties.

This presentation will provide an overview of rational design strategies to engineer SHG-active hybrid perovskites and key techniques and challenges for comprehensive characterization of their temperature-dependent SHG responses. We will focus on crystal engineering techniques that induce noncentrosymmetry in achiral systems. Ligand halogenation, also known as halogen engineering, has proven to be a potent tool for inducing polar distortions and symmetry breaking, often through the formation of halogen bonds.[1] The introduction of specific organic cations, such as methylhydrazinium (MHy⁺), while being a nonchiral molecule, represents another powerful approach known to promote the formation of noncentrosymmetric strcture in lead halide perovskites.[2-4] These organic cations can induce structural distortions and break inversion symmetry through their unique geometric and electronic properties.

Through a series of case studies, this talk will demonstrate how these synthetic approaches can be leveraged to design and obtain novel hybrid perovskites with tailored SHG activity, feature multinoncentrosymmetry (the presence of multiple distinct temperature-dependent noncentrosymmetric crystal phases within a single material system) and other noncentrosymmetry-induced functionalities. The combination of rational crystal engineering strategies and comprehensive variable-temperature characterization provides a pathway toward developing next-generation nonlinear optical materials with enhanced performance characteristics.

Understanding ionic dynamics in halide perovskites is critical for improving the performance and stability of devices, including solar cells, X-ray detectors, and memristors [1-3]. Conventional frequency-domain optoelectrical techniques often suffer from contact-related effects and interfacial recombination, which can obscure the signatures of ionic dynamics [4-6]. Optical approaches such as time-resolved photoluminescence can avoid these limitations, but are rarely used to study ionic responses due to the difficulty of separating overlapping contributions with similar timescales in the time domain [7].

In this talk, we present intensity-modulated photoluminescence spectroscopy (IMPLS) as a fully optical method to probe dynamic behavior across a broad range of timescales. By analyzing the phase and amplitude of the PL response as a function of the modulated excitation frequency, IMPLS enables the identification of distinct mechanisms based on their characteristic times. We demonstrate its use on halide perovskite films and compare the results to standard optoelectronic techniques [8]. This approach provides new insights into slow processes such as ion migration and defect dynamics, and opens possibilities for broader material and device characterization.

The efficiency of halide perovskite solar cells has been continuously rising over the past decade to values above 26%. Future technological development will have to deal with issues of device stability but also thrive to further minimize efficiency-limiting loss processes in the bulk and at interfaces within the cell stack. The identification and understanding of electrical losses will require the ability to characterize solar cells and multilayer stacks with a variety of steady-state, time-domain and frequency-domain techniques that are sensitive to the transport and recombination of charge carriers. Especially, time- and frequency-domain techniques offer a large amount of information on dynamic processes in the solar cell, while posing a substantial challenge in terms of the complexity of data analysis.¹ Here, I discuss our recent work related to transient photoluminescence (TPL) applied to halide perovskites. I show that by using extremely low repetition rates and a gated CCD camera, we can obtain high dynamic range TPL data with continuously changing decay times that exceed 100µs.^2-3 Furthermore, I show that by changing the repetition rate, basically any decay time can be extracted from one sample, whereby the extracted decay time is approximately the inverse repetition rate. I explain why this is the case both mathematically and physically. Further, I present recent results on the determination of diffusion lengths as a function of injection level⁴ using the reabsorption effect.

In the recent years, chiral hybrid organic-inorganic perovskites (HOIPs) have gained a huge interest for applications in optoelectronics, spintronics, photodetection, energy harvesting and beyond, allowing for the absorption and subsequent emission of polarized light with enhanced tunability across the electromagnetic spectrum [1,2]. So far the research has widely centered on low dimensional systems, such as 2D and quasi-2D HOIPs, with fewer examples of 1D and 0D ones, demonstrating significant chiroptoelectronic and spin-polarization features. However, expanding the corner-sharing interconnection of the inorganic motif to the three dimensions is highly demanded for practical applications where a isotropic charge transport is demanded, since the organic layers usually behave as dielectrics in low-dimensional systems [3]. However, the steric constrains imposed by the bulky chiral cations usually prevent the accordance with the Goldschmidt tolerance factor.
In this scenario, we have developed novel chiral HOIPs derivatives displaying a 3D corner-sharing octahedral interconnection closely resembling that of prototypical perovskites [4]. This architecture is attained by integrating the relatively small ditopic cation R/S-3-aminoquinuclidine (R/S-3AQ) yielding the (R/S-3AQ)Pb₂Br₆ materials, featuring a direct bandgap and a isotropic electronic band structure in agreement with a 3D delocalized excitation, in stark contrast with the Ruddlesden-Popper counterpart (R/S-3AQ)₂PbBr₄·2Br showing typical 2D characteristics. The experimentally determined chiral anisotropy factor aligns well with theoretical predictions based on first-principles calculations for this type of chiral structure, and a pronounced Rashba-type spin splitting is detected in the conduction band, expected for a non-centrosymmetric semiconductor and driven by the combined effects of spin-orbit coupling and structural chirality. A reduced exciton binding energy was determined in the 3D material, index of a more stable excitonic population and favorable for an increased charge transport. Conductivity and spin-relaxation measurements are currently in progress to further assess the influence of these electronic properties on the material's potential for optoelectronic and spintronic applications. Thanks to their structural chirality and broad chemical tunability, these 3D chiral HOIP derivatives emerge as a promising foundation for the development of next-generation nonlinear functional materials.

Zero-dimensional copper halides Cs₃Cu₂X₅ (X = Cl, Br, I) are promising materials for optoelectronic applications due to their high photoluminescence efficiency, stability, and large Stokes shifts [1]. In this work, we use density functional theory to uncover the chemical bonding origin of the Stokes shift in these materials.

Upon excitation, the [Cu₂X₅]³⁻ cluster undergoes strong local distortions, including shortened Cu–Cu and Cu–X bonds. These structural changes are driven by the formation of a self-trapped exciton, where a hole localizes on Cu(d) orbitals [2-3]. Analysis of the electronic structure and -pCOHP reveals reduced antibonding interactions and enhanced bonding character in the excited state, stabilizing the distorted geometry.

Our results establish a direct link between orbital-specific hole localization, bonding rearrangement, and the resulting Stokes shift. This provides a fundamental understanding of the excitation mechanism in Cs₃Cu₂X₅ and offers design principles for tuning optical properties in 0D copper halides.

Synthesizing perovskite quantum dots (PQDs) within nanoporous matrices offers a promising alternative to traditional colloidal methods [1]. These matrices feature a controllable network of voids, acting as nanoreactors in which precursor solutions can be infiltrated. Further thermal or chemical treatments convert them into nanocrystals. This confinement enables precise control over PQD dimensions, size distribution, and crystallinity without requiring stabilizing ligands.

This talk highlights the method's potential to enhance PQD stability and functionality and to study fundamental properties of both individual QDs and QD networks. Key advantages include: (i) Stabilization of metastable phases, like the α-phase of CsPbI₃ even at room temperature [2]; (ii) Control over the environmental responsiveness [3]; (iii) High optical performance, with quantum yields exceeding 85% for specific compositions such as FAPbBr₃ [4]; (iv) Versatility for low-dimensional Structures, as the approach extends to fabricating low-dimensional perovskite structures, enabling remarkable blue photoluminescence with quantum yields surpassing 40% [5] or the formation of exciton-polaritons; Efficient charge transport within the embedded QD network, which depends critically on achieving PQD interconnectivity to enable dot-to-dot charge transfer, central for successful device operation [6].

Mechanochemistry has a great potential for the solvent-free synthesis of complex multinary metal halides. [1, 2] Indeed, typical bottlenecks related to the different solubilities of metal halide precursors and the difficulty in ensuring a precise stoichiometry in final crystals when utilizing solution processes can be overcome by dry mechanochemical synthesis. Herein I will show how this strategy can be employed to achieve high-entropy 3D halide perovskites for the first time. [3]

More precisely, we have achieved substantial simultaneous alloying in three of the four sublattices of halide double perovskites with general formula Cs₂(B1:B2)(C1:C2)(X1:X2)₆ by means of ball-milling and thermal annealing. This leads to highly relevant properties such as pronounced bandgap bowing and full visible light absorption even for pure-chloride compositions.

The fundamental processes involved in such high-entropy alloying are revealed through a combination of detailed structural characterization and DFT calculations, highlighting the different ion-exchange kinetics and how these are linked to defect formation energies in different phases.

Eventually, I will show how these materials can be implemented as photoelectrocatalysts in the highly-sought after oxygen evolution reaction which is a key process in many energy-related applications.

Metal halide perovskite materials with the chemical formula ABX₃, where A is an organic or inorganic cation, B is a divalent metal and X is a halide anion have emerged as a leading class of semiconductors, offering exceptional structural and electronic properties for diverse applications. Furthermore, their reversible electrical or optical property changes in response to oxidizing or reducing environments make them prospective materials for gas detection technologies. The perovskite-based sensors operate efficiently at room temperature, offering an advantage over the traditional metal oxide gas sensors, eliminating additional energy consumption and enabling the development of portable gas sensing devices.^[1],[2] However, the presence of lead (Pb) in these materials poses a serious challenge for their widespread application and commercialization, owing to its toxicity. To address this issue, lead-free perovskite materials have recently gained increasing attention due to their environmental sustainability and potential for safer applications. The lead-free composition of these perovskites mitigates environmental and health risks associated with traditional lead-based materials and provides a sustainable platform for advanced sensing technologies. Their structural adaptability, coupled with room temperature operability, positions these materials as promising candidates for next generation medical diagnostics and environmental monitoring systems.

This study investigates lead-free Cs₂AgBiBr₆ perovskite synthesis and doping strategies to develop high-performance gas sensors for detecting volatile organic compounds (VOCs), as well as oxidizing and reducing gases. These sensors can be regarded as eco-friendly for the following reasons: they are lead-free; synthesized at room temperature without the use of hazardous organic solvents; function and recover efficiently at room temperature without the need for heating or UV irradiation; and operate under very low input voltage, significantly reducing overall energy consumption.

Lead-free Cs₂AgBiBr₆ perovskites were synthesized using precipitation method under ambient conditions and systematically doped with selected metals such as Zinc (Zn), Manganese (Mn), and Tin (Sn).  Due to doping, structural characterization through X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed significant improvements in crystallinity, grain size, and surface morphology. Energy dispersive spectroscopy (EDS) further validated the successful integration of dopants into the perovskite matrix. Additional analysis using UV-Vis spectroscopy provided critical insights into the material's optical properties, including light absorption characteristics and band gap energy, further showcasing the impact of doping on their performance.

The gas sensing capabilities of doped Cs₂AgBiBr₆ perovskites were assessed for VOCs such as Acetone and Limonene, which are established biomarkers for metabolic disorders, including diabetes and liver dysfunction. Moreover, detecting oxidizing and reducing gases in ambient air, such as Ozone (O₃) and Hydrogen (H₂), is crucial for environmental monitoring and safety applications. The sensors demonstrated sensitivity, selectivity, and rapid response times, even at low gas concentrations. Notably, all measurements were conducted at room temperature, eliminating the need for external heating and thereby reducing energy consumption while maintaining the integrity of the breath samples. This operational advantage makes the sensors both practical and efficient for real-world applications.

In summary, this work demonstrates the potential of lead-free, doped perovskites as sustainable and effective sensors for VOCs and inorganic gases, offering promising solutions for healthcare and environmental monitoring.

Halide perovslkites are well known for their excellent optoelectronic properties that has enabled them to create record breaking performances in areas of solar harvesting, light emitting diodes and detectors. In this talk we will focuss on the ionic effects within this family of semiconductors which is normally as its weakness. The talk will cover our efforts to modulate and utilise the ionic properties of halide perovskites. 

The significant ionic activity in these materials enables their use in memory devices, where the interplay between ionic and electronic transport gives rise to resistive switching behavior. These distinctive characteristics make halide perovskites highly promising for neuromorphic applications. Their strong light absorption and coupled ionic–electronic transport allow them to be stimulated by both electrical and optical inputs. In this presentation, I will discuss various approaches to modulate the material compositions, interfaces to tailor memristive behavior in a wide variety of architectures including memristive PV devices as well as perovskite-based LEDs with integrated memory functions. These devices can emulate a wide variety of functions such as human visual processing, enabling functions such as contrast enhancement, feature extraction, and other forms of sensory pre-processing. Finally, I will also touch on how perovskite-based memory devices can be engineered to emulate artificial neurons, paving the way for highly integrated and multifunctional neuromorphic architectures.

The dual electronic-ionic nature of perovskite solar cells has complicated the interpretation of almost all the standard PV characterisation techniques. For example, when ions move on the timescale of current-voltage measurements, they can act to modify carrier recombination rates and carrier extraction, influencing the shape of the response. Ions can also modify fast measurements, where the ‘frozen in’ ion distribution impacts the electronic response of the device. On the flip side, the ions can act as probes, giving us useful information about how well a cell is operating.

One technique we have used is impedance spectroscopy, a very common characterisation technique. The Nyquist plots measured for PSCs show a wide variety of different shapes, and many different interpretations of these spectra can be found in the literature. We recently showed that all of these experimentally observed shapes can be reproduced by a standard three layer drift diffusion model with a single mobile ion species, without the need to invoke any exotic physics within the device. The low frequency regime contains a wealth of information about the internal workings of the cell that can be obtained purely from shape recognition of the Nyquist plot, without any modelling expertise. This presentation will cover our recent work measuring and modelling a wide variety of perovskite solar cells (PSCs) and using ion migration to diagnose device physics.

Tin-based halide perovskites promise lead-free optoelectronic devices, but surface degradation remains the critical barrier to commercialization. Here, we combine density functional theory calculations with thermodynamic modeling to reveal the atomic-scale mechanisms governing stability in the 2D hybrid perovskite 4FPSI. By systematically investigating point defects including vacancies and interstitials in both in-plane and out-of-plane configurations, we uncover how defect spatial distribution controls surface reactivity and device performance.

Our calculations identify two competing degradation pathways. Sn(II) oxidation to Sn(IV) via O₂ exposure dominates under ambient conditions, while I⁻/I₃⁻ redox processes prevail in solution environments. Surface stoichiometry critically determines defect energetics. Sn-poor terminations generate deep acceptor V_Sn traps within the bandgap, whereas Sn-rich surfaces promote interstitial formation that disrupts the SnI₆ octahedral framework. Organic cation coverage provides crucial protection: complete 4F-PEA⁺ layers create substantial water diffusion barriers, but partial coverage exposes reactive Sn sites with significantly lower activation energies.

Most remarkably, we discover intrinsic self-healing through surface reconstruction and interstitial migration. This finding explains experimentally observed performance recovery after rest periods and fundamentally reframes our understanding of tin perovskite stability. The dynamic equilibrium positions Sn(IV) as both a degradation product and a healing intermediate. Our mechanistic insights reveal that surface engineering, not bulk composition, holds the key to stable lead-free photovoltaics. These results provide clear design principles for defect passivation and interface optimization in next-generation solar cells.

Thin film transistors (TFTs) built using lead-tin perovskite have gained immense attention due to its excellent device mobilities along with high on-off ratios, which makes such TFTs perfect devices for switches and digital applications. However, facile oxidation of tin (Sn) from +2 to +4 state leads to undesired doping, resulting in a loss of channel modulation. In this work, the influence of A-site cation on the performance and stability of TFT is evaluated. We report an ambient stable (RH: >70%, RT: 25°c) TFT with a threshold voltage (V_th) of 4.7 V and an on-off ratio of nearly 10⁶ ; stable for an hour of exposure without encapsulation. The stability of the devices were evaluated by observing the shift in transfer characteristics of the control and target composition systems as shown in TOC Figure. The gate modulation in control device is lost within 5 minutes of air exposure (TOC Figure (a)) while target device shows gate modulation even after an hour of air exposure (TOC Figure (b)). This stability is attributed to the substitution of appropriate A-site cation, which may have led to increased defect formation energy and thus lowered oxidation and doping, as evidenced by XPS and Hall measurements, respectively. Devices with such low V_th and high on-off ratios help in realizing circuits with lower operating power and well-defined, wide range on-off states.

Tin-based perovskite solar cells (Sn-PSCs) represent a promising alternative to their lead-based counterparts, offering reduced toxicity and appealing optoelectronic properties. However, their widespread application remains limited by poor operational stability, mainly driven by the oxidation of Sn²⁺ to Sn⁴⁺ under ambient conditions[1]. In this work, we explore the dynamic behavior of Sn-PSCs during operation, focusing on the interplay between degradation and self-healing processes. By incorporating thiophene-2-ethylammonium iodide (TEAI) as an additive in FASnI₃ devices, we observe a spontaneous recovery of photovoltaic performance under continuous light and ambient exposure in unencapsulated conditions.

To gain deeper insight into the underlying mechanisms, we combine electrical and optical characterization techniques, including impedance spectroscopy and photoluminescence measurements. This talk will focus on the processes governing this unusual behavior and will discuss the possible chemical pathways involved in the observed self-healing effect, aiming to contribute to a better understanding of stability challenges in lead-free perovskite solar cells.

Hybrid organic inorganic perovskites (HOIPs) have started to emerge as leading materials for optoelectronic technologies including solar cells, photodetectors, lasers, and light‐emitting diodes. Within this family, two-dimensional (2D) layered HOIPs have attracted growing attention in recent years owing to their intrinsically superior environmental stability compared to most 3D HOIPs, as well as their remarkable structural and compositional tunability. The choice of organic ammonium cations, drawn from a vast library of candidate molecules, governs the assembly of these hybrid materials. Typically, this organic component does not directly influence the hybrid’s optical or electronic behavior; however, the incorporation of so-called electroactive organic cations has begun to receive considerable research interest.[1] For example, through the incorporation of such tailored organic cations, 2D HOIPs with an extended absorption spectrum, enhanced (out-of-plane) charge carrier transport, and reduced exciton binding energy have been obtained.

In 2023, we [2] showed that carbazole-based organic ammonium cations with different alkyl spacer lengths can be used to tune the optical and electronic properties of 2D lead iodide HOIPs. With decreasing spacer length, there was evidence for enhanced electronic coupling between the organic and inorganic layers. For all spacer lengths (3, 4, and 5 carbon spacers), light-induced charge transfer from the organic to the inorganic layer was detected. Specifically for the carbazole with the shortest spacer (Cz-3), an organic-inorganic (interlayer) charge transfer state was observed. The out-of-plane charge carrier transport was enhanced for all the carbazole-containing 2D HOIPs compared to that of the reference 2D HOIP containing an electronically inactive phenylethylammonium (PEA) cation, with the 2D HOIP based on Cz-3 possessing the highest charge carrier mobility.

In recent research, we build further on this work to gain a deeper understanding of the influence of molecular design on the optical and electronic properties of low-dimensional HOIPs. We compare the properties of two low-dimensional hybrids containing the same carbazole-inspired organic cation but with a different lead iodide inorganic framework. Depending on the connectivity of the octahedra, differences in the photoinduced charge transfer dynamics between the organic and inorganic layers are obtained, and other transfer pathways become available because of changes in relative energy alignment between organic and inorganic states. In other recent work, we studied 2D HOIPs containing electroactive organic cations for which we were able to determine the crystal structures to deduce detailed structure-property relationships. In a combined experimental-computational study, we show that the organic-inorganic interlayer electronic coupling is highly sensitive to the orientation of the organic core with respect to the inorganic framework.

The discussed work deals with the control and characterization of spin degrees of freedom of photo-generated carriers in halide perovskite materials via magnetic doping. The materials under consideration, CsPbBr3 and PEA2PbI4, include the diluted concentration of Ni2+ or Mn2+ ions at a metal substitution position. Magnetic doping was implemented extensively in colloidal quantum dots, but with a limited way in the perovskite semiconductors, wherein the latter possesses other significant properties (e.g., dopant-carrier spin-exchange interactions, a large g-factor and extension of spin relaxation time).

Our work reports a thorough investigation of spin degrees of freedom in the mentioned materials, monitored by magneto-photoluminescence and optically detected magnetic resonance (ODMR) spectroscopies, which provide a significant information on exact location of host carriers and dopants, as well as examine interactions between them.

specifically, one project focused on a thorough investigation of the influence of Ni2+ dopants on the optical and magneto-optical properties of CsPbBr3 nano-cubes. The study implemented methodologies that are applied to halide perovskites for the first time, like steady-state and transient optically detected magnetic resonance (ODMR) spectroscopy, leading to significant advances that address long-standing debates in this field: (a) A direct identification of defect centers, in conflict with the widely assumed defect-free behavior of halide perovskites; (b) Direct observation of spin-exchange interactions between dopant unpaired electrons and photo-generated carriers, which has not been resolved previously. The extracted physical parameters from the ODMR experiments included: g-factors and their anisotropy, spin exchange interactions, angular momentum, carrier-dopant coupling constants, radiative and spin-lattice relaxation times. A second project focused on the influence of Mn2+ dopants in 2D MA2PbI4 and PEA2PbI4 single crystals, implementing the mentioned magneto-optically spectroscopies, focusing on the added values of the dopant to the so-called Rashba effect (an ongoing project).

The spin properties mentioned here undoubtedly can play an important role in the development of new spin-based technologies.

Lead halide perovskites are in the limelight due to their excellent semiconductor properties, making them ideal for photovoltaic and optoelectronic applications.[1] The emergence of perovskites as sought-after semiconductors owe to their unique electronic band structure, inherent defect tolerance, increased conversion efficiencies, synthetic flexibility, and easy and cost-effective processibility.[2] Lead halide perovskites show near-unity PL quantum yield, narrow emission bandwidth, high solar cell efficiencies, etc. All these properties are due to the dynamic nature of the ground and excited states. These hybrid systems have strong electron-phonon interactions which manifest strongly in their optical and photovoltaic behaviours.[3] We have used ultrafast femtosecond impulsive vibrational spectroscopy (IVS) to detect the low-frequency phonon modes in an interesting 2D/3D perovskite heterostructure to understand the impact of these electron-phonon coupling interactions on the charge transfer properties between the high gap 2D to the lower gap 3D system. [4] The observation of these phonon modes will help us understand the charge funnelling for a quantum well system ideal for efficient perovskite-based LEDs.

References:

[1]. Stranks, S. D. et.al., Metal-halide perovskites for photovoltaic and light-emitting devices. Nature Materials, (2015),17, 928-938.

[2]. Yin, W. J., et.al., Unusual defect physics in CH₃NH₃PbI₃ perovskite solar cell absorber. Applied Physics Letters,(2014), 104(6), 063903.

[3]. Miyata, A., et.al., Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites. Nature Physics, (2015), 11(7), 582-587.

[4]. Tsai, H., et.al., High-efficiency two-dimensional Ruddlesden–Popper perovskite solar cells. Nature, (2016),536(7616), 312-316.

Trap states in 2D metal-halide perovskites strongly affect optoelectronic performance [1]. Understanding and distinguishing between the different types of energy carriers, such as excitons, free carriers, and trap-mediated states is essential, as these species govern charge carrier dynamics and determine emission efficiency [2]. Power-dependent studies, when combined with lifetime and spectrally resolved photoluminescence measurements, are key tools to disentangle the contributions of each carrier type. While excitons and free carriers are associated with radiative recombination and efficient light emission, trap states introduce non-radiative losses and reduce device performance. A clear spatial and spectral separation of these carriers is therefore critical for targeted material optimization and precise control of optoelectronic properties [3].

Our study focuses on two-dimensional metal-halide perovskite flakes, which exhibit strong excitonic emission and pronounced carrier trapping phenomena, making them ideal systems for investigating power-dependent spectral and lifetime dynamics. The micron-sized flakes are synthesized via solution-based methods and exfoliated onto transparent substrates for optical characterization.

We combine hyperspectral and Fluorescence Lifetime Imaging Microscopy with direct spatial visualization of carrier dynamics. By systematically evaluating the power dependence of the spectral, temporal, and spatial characteristics of the optical excited state, we can map spectral shifts, intensity changes, and the emergence of distinct emissive components with high fidelity. Backed up by advanced numerical modeling, this multimodal approach enables the precise localization and mapping of charge carrier species in perovskite materials at the micrometer scale. Such fine-grained control is essential to probe and decouple overlapping excitonic, free carrier, and trap-state contributions [2]. This capability is critical for guiding synthesis strategies and improving the optoelectronic quality of low-dimensional perovskites.

Two-dimensional perovskite materials have attracted increasing interest as active materials in photonic and optoelectronic applications, primarily due to their intriguing optical and electronic properties. Among their most striking properties is pronounced exciton-phonon coupling, which is believed to be relevant to a variety of key (opto)electronic parameters, including carrier mobility, photo- and electroluminescence, and exciton binding energies. However, the interplay between the structure of the perovskite and electron-phonon coupling effects remains poorly understood; understanding the relationship between these effects is therefore essential to the continued development of high-performance perovskite materials.

To this end, we employ advanced optical methods to investigate a series of 2D Ruddleston-Popper perovskites, employing derivatives of the chiral methylbenzylammonium (MBA) molecule as organic spacer ligands. By fine-tuning the structure of the MBA derivatives—leveraging both substitution and stereochemical effects—we demonstrate control over the strength of electron-phonon coupling. We link these observations to the mechanical properties of the inorganic lattice, which are modulated by the organic spacer ligands. 

Our results reveal a clear pathway to control the dynamics of the inorganic lattice of 2D perovskites through molecular design of the organic spacer ligand, and further highlight the importance of stereochemical and enantiomeric effects in controlling electron-phonon coupling in these materials.

The benefits of vacuum processed perovskite solar cells will be discussed. Different vacuum based methods are used ranging from thermal co-sublimation, close-space sublimation and vapor assisted flash evaporation.

I will also discuss the benefit of using these perovskites in substrate configuration as well as semitransparent cells for bifacial operation and for building integrated PV. The use of sublimed perovskite in perovskite-silicon tandem cells using fully textured Si bottom cells will be presented, including methods to identify the shortcomings of these tandem cells. 

Using a home build setup to determine the stability for both indoor and outdoor applications we are able to monitor the solar cells stability. First results on outdoor and indoor stability measurements will also be presented demonstrating the importance of encapsulation methods. Best devices have a T95 over 1000 hours mpp at 75 degrees C and 1 sun illumination.

Vapour deposition is a versatile, solvent-free method for fabricating metal halide perovskite (MHP) thin films, gaining increasing attention for its potential to produce high-efficiency perovskite solar cells. Despite its many advantages, vapour deposition of MHPs faces a number of challenges preventing its wide scale adaptation on an industrial scale, such as incomplete conversion of the precursors to the perovskite phase and slow deposition rates. In this talk, I will highlight how the use of additives compatible with vapour deposition can help address these challenges. I will also discuss the opportunities offered by vapour deposition to the fabrication of microstructured 2D chiral perovskites for photonic applications. 

Current research in thin-film deposition emphasizes compatibility with large-scale manufacturing, reproducibility, and uniformity. Although thermal vacuum deposition has long been established in the semiconductor and OLED industries, applying this technique to hybrid perovskite materials remains particularly challenging. The complexity arises primarily from their unconventional sublimation behaviors, characterized by abnormal adsorption dynamics and difficulties in precise process control. In this presentation, I will first elucidate the distinctive sublimation mechanisms and deposition kinetics associated with hybrid perovskites, highlighting how these differ fundamentally from traditional thermal evaporation of conventional inorganic materials. Subsequently, I will introduce our novel solutions to overcome these challenges, including the implementation of a meticulously designed deposition protocol combined with a specifically tailored evaporator system capable of real-time, precise process control. Leveraging these advancements, we successfully fabricated perovskite solar cells with power conversion efficiencies exceeding 25% in conventional n-i-p architectures, demonstrating remarkable batch-to-batch reproducibility. Additionally, I will discuss our recent progress on vacuum-deposited modules exceeding 200 cm², illustrating the significant industrial interest and considerable potential of vacuum-based techniques for perovskite photovoltaic commercialization.

Cesium lead mixed halide (CsPbI₂Br) perovskite solar cells (PSCs) have attracted significant interest due to their exceptional thermal stability and optimal 1.9 eV wide-bandgap, ideal for perovskite/perovskite tandem applications. However, achieving high-performance films under low-temperature processing remains a critical challenge. While previous studies on thermally evaporated CsPbI₂Br have relied on either high post-annealing temperatures (>260 °C) [1, 2] or complex multi-source deposition setups [3] to achieve desirable film morphology, we demonstrate that applying 100 °C of substrate heating during co-evaporation, combined with a mild 150 °C post-annealing step, enables the formation of stoichiometrically balanced films with enhanced crystallinity and optimized morphology. Using phenethylammonium chloride (PEACl) for surface passivation further suppresses non-radiative recombination, improving film quality and device performance. The resulting inverted (p–i–n) perovskite solar cells achieved a power conversion efficiency (PCE) of 13.21%, alongside excellent operational stability—retaining 80% of their initial efficiency after 450 hours under continuous 1-sun illumination. This work establishes a new benchmark for vacuum-deposited CsPbI₂Br PSCs and underscores the potential of this low-temperature, co-evaporation-based strategy for integration into tandem and flexible photovoltaic applications.

 Tin-based metal halide perovskites are emerging as a compelling class of materials in the pursuit of efficient and eco-friendly photovoltaics. Although lead triiodide perovskites have recently achieved exceptional performance in optoelectronic applications, concerns about their wide bandgap (1.5–1.6 eV), lead toxicity, and environmental impact continue to fuel the search for safer alternatives.

 Tin perovskites have gained attention as a less hazardous option, but their development has been hindered by chemical instability and defect formation. Conventional approaches relying on solvent and additive engineering often suffer from chemical instability—primarily due to electron donating solvents that accelerate the oxidation of Sn²⁺ to Sn⁴⁺ [1], leaving behind detrimental by-products and high background hole concentration. [2] To overcome these limitations, solvent-free deposition strategies, which have demonstrated to be advantageous in lead-based systems, are being explored for tin perovskites. Techniques such as thermal evaporation offer precise control over stoichiometry, uniformity over large areas, and compatibility with scalable manufacturing—all under inert, vacuum conditions ideal for handling oxidation-sensitive tin compounds.

 Despite these advantages, the fabrication of high-quality tin halide perovskite films via vacuum methods remains in its infancy, with only a few successful demonstrations and limited understanding of the resulting materials’ intrinsic properties. [3] [4] [5]

In this study, we first report the fabrication of additive-free formamidinium tin triiodide (FASnI₃) thin films via co-evaporation under vacuum. The resulting films display a substantial suppression of Sn⁴⁺ species, indicating reduced oxidation and intrinsic semiconducting characteristics with supressed tendency toward self-doping effects. The measured optical bandgap (~1.31 eV) closely aligns with theoretical predictions, affirming the structural integrity of the material. These findings position co-evaporation as a viable and scalable pathway for advancing lead-free perovskite technologies in next-generation optoelectronic applications.

Metal halide perovskite solar cells have gained significant attention over the last decade due to their low-cost fabrication methods and high efficiency potential. Typically, perovskite films are prepared by solution-based deposition techniques, which offer short deposition times and a broad range of compositions.¹ However, achieving conformal coverage of textured surfaces, highly relevant for monolithic perovskite/silicon tandem solar cells,²  or compositional gradients in the absorber material, to achieve graded Fermi levels,³ can be challenging with these techniques. These limitations can be overcome by co-evaporating the perovskite precursor materials.

Our work focuses on the vacuum-based preparation of perovskite absorbers with a band gap of about 1.68 eV, optimized for monolithic perovskite/silicon tandem solar cells. We show how the choice of hole-transporting material affects the composition of perovskite films in p-i-n solar cells. Our findings reveal that perovskites co-evaporated on spin-coated [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) contain a significantly smaller amount of FAI if the MeO-2PACz layer is washed with ethanol before the perovskite deposition. The reduced FAI content leads to a different morphology and alters the bromine to iodine ratio, impacting the solar cell performance and the band gap of the films.

We use two approaches, namely “seed layers” ⁴ and “loaded hole transport layers” ⁵, for tuning co-evaporated perovskite films through modifying the initial growth stage, yielding larger process windows, reduced sensitivity to substrate properties and improved process stability.

We study how the perovskite composition differs between planar and textured surfaces with random pyramids on silicon heterojunction bottom cells. Finally, fully textured perovskite/silicon tandem solar cells with ~30% PCE (certified) are demonstrated.

Our results illustrate multiple factors influencing the perovskite growth and highlight the potential of co-evaporation processes for the preparation of efficient perovskite/silicon tandem solar cells on textured substrates.

Perovskite-silicon tandem solar cell are highly promising candidates for the industrialization of perovskite based solar cells. We employ several evaporation techniques such as co-evaporation, sequential evaporation, and the hybrid route to fabricate the uniform perovskite films needed for such tandem devices. Each of these approaches has its own challenges and opportunities. In this talk I will share the progress we have made to gain understanding of the mechanisms, crystallization behavior, and the influencing factors that govern the evaporation process. We were able to use these perovskite films in tandem solar cells reaching efficiencies above 31% on 1 cm² cells. Nevertheless it is not fully clear which evaporation approach is best suited for the scaling of perovskite-silicon tandems. Full evaporation approaches like co-evaporation or sequential evaporation have the advantage of having all steps in the same atmosphere, whereas hybrid evaporation / wet-chemical approaches avoid the challenging evaporation of organic halides but introduce additional complexity in the wet-chemical 2^nd step. In this talk I will compare what we have so far learned about all three approaches and will highlight key gaps to be addressed in the future.    

Low-dimensional Ruddlesden-Popper (RP) structures are becoming ubiquitous in the field of halide perovskites optoelectronics. These materials are tunable between 2D (n = 1) and quasi-2D (1 < n < inf.) phases which determine their optical (bandgap), electronic (binding energy, conductivity) and chemical properties (hydrophobicity) [1]. The tunability has enabled their use as the active layer for light absorption, detection and emission, or at interfaces for efficient charge transport and improved stability. However, solution-processing of such films is challenging since interactions between precursors (organic and metal cations, halide anions) and solvents (such as DMF, DMSO) cause competing crystallization reactions that undermine dimensional purity and impact the optoelectronic properties and device stability [2]. We developed a co-evaporation method to deposit RP films with high control on crystallization by eliminating these precursor-solvent interactions [3,4]. Here, we deposit prototypical 2D (PEA₂PbI₄) and quasi-2D (PEA₂FAPb₂I₇) thin films using phenethylammonium (PEA+) as the organic spacer cation and formamidinium (FA+) as the organic cation for the quasi-2D phase and study the interactions that determine film formation.

The formation of the bare 2D layer (PEA₂PbI₄) and the development of an interface on top of a 3D perovskite film was studied using X-ray diffraction, X-ray photoelectron spectroscopy, electron microscopy and photoluminescence microscopy. The co-evaporated 2D structures were then used as interfacial layers at the hole-transporting interface to yield approx. 22% efficient perovskite solar cells, driven by improvements in the open-circuit voltage and fill factor [4]. Notably, this performance gain is observed over a large RP thickness range, owing to the improved control on RP phase through co-evaporation.

We then studied the crystallization of quasi-2D structures (PEA₂FAPb₂I₇) and observed the impact of phosphonic acid substrate modification using density functional theory calculations, synchrotron-based X-ray scattering and ultrafast pump-probe spectroscopy [5]. Here, favorable interactions between unbound solution-processed  propylphosphonic acid (PPAc) and PEA⁺ increases the relative uptake of PEA⁺ compared to FA⁺, favoring the formation of the 2D phase over the quasi-2D phase. This allows the formation of heterostructures between 2D and quasi-2D phases, controlled by the PPAc concentration and the co-evaporated film thickness.

Taken together, our work demonstrates a new synthesis method to overcome a critical challenge in coating RP perovskites with high phase purity. We demonstrate the applicability of this approach in solar cells as well as present a new way to synthesize controlled RP heterostructures. This work also complements ongoing research in the field of halide perovskites on vapor-based coating methods and presents a scalable method to coat bulk films as well as interfacial layers for high-performance devices.

Within the research field of single- and multi-junction solar cells the metal halide perovskites are becoming increasingly appealing for commercialization, primarily due to their record power conversion efficiencies (PCEs). These PCE records are a result of several attractive qualities inherent to perovskites. One of these qualities is its compositional flexibility. A large variety of hybrid compositions, including organic and inorganic precursors and dopants, have led to a range of different band gap semiconductors with increased optoelectronic performance and stability. These improved properties come accompanied by an increased production complexity due to, among other things, the different volatility and solubility of its components.

The most explored production method, with the largest number of publications and the current PCE record, is solution-based deposition. Solution-based deposition techniques allow relativity easy inclusion of compatible precursors without the use of specialized or expensive equipment.

Another method, vapor-phase deposition, in particular co-sublimation of perovskite precursors, has also been successfully shown to lead to high quality perovskite films and solar cells. These perovskite films have the added benefit of the absence of trace amounts of solvents or the necessity of all precursors to be soluble in the same solvent. Additionally, this method can be more seamlessly transferred into already established production lines in the semiconductor industry. However, the number of precursors that can controllably and reproducibly be co-sublimed is limited. Moreover, relatively slow sublimation rates are required to maintain control over the perovskite stoichiometry, resulting in deposition times in the order of hours.

In this presentation a novel gas flow assisted flash evaporation (GFAFE) method will be introduced. This method allows for a high degree of freedom in precursors as the system flash evaporates all tested materials without losing its stoichiometry. The most important novelty of this system is its unprecedented deposition speed of over 1.0 µm/min. This can be used for complex, complete perovskite deposition from a single source, or in a sequential manner, where it tackles the specific need for high deposition speed of organic components as formulated from industry.

This system has initially been used for the production of perovskite solar cells which has led to proof-of-concept devices made with both single source and sequentially deposited perovskites. The performance of the solar cells is enhanced by the use of dopants and additives from the singular powder source.

The system uses a 10x10 cm² sample holder and is designed with upscaling for wafer sized devices in mind by using a showerhead design to homogeneously expand the gas flow. The system uses a powder reservoir that holds up to 5 grams of powder, which has been found to be sufficient for over 10 production batches, making the total production throughput very high, especially for a lab-scale setup. The combination of the qualities above enables us to make high quality, complex perovskites, faster than with conventional methods while decreasing the complexity of perovskite deposition procedure.

Halide perovskites exhibit superior optoelectronic properties and hold great promise in advanced optoelectronic devices. However, they lack precise thickness and interfacial structure control in heterojunctions, critical for modular multilayer architectures such as multiple quantum wells. Vapor-phase deposition method have great advantage in thickness control for perovskites. As a dry processing method, it is also capable of preserving the interfacial structure especially for perovskite heterojunctions. In this work, we demonstrate layer-by-layer heteroepitaxial growth CsPbBr₃ deposition on 2D perovskites by thermal evaporation. The strict heteroepitaxial templating, revealed by reciprocal space maps, significantly improved perovskite film quality by reducing energetic disorder, enhancing photoluminescence quantum yield, and improving carrier transport. Angstrom-level thickness control and sub-Angstrom smooth layers enable quantum-confined photoluminescence of CsPbBr₃ from monolayer, bilayer, and through to bulk.

Futhermore, we demonstrate that the interfacial structure could be tuned by deposition parameters, controls the electronic structure between type-I and type-II heterojunctions. The interfacial-tunable band offset shift could reach much higher values than what has been achieved in III-V semiconductors. Electron transfer from CsPbBr₃ to 2D perovskite for the type-II heterojunction results in observation of charge separation and delayed electron-hole recombination, which is absent in the type-I heterojunction. Our results show that the precise quantum confinement control and large band offset tunability unlock perovskite heterojunctions as platforms for scalable, low-cost modular quantum and superlattice-based optoelectronic applications.

Halide perovskites deposited by physical or chemical vapour deposition techniques are significantly less explored than their solution-processed counterparts. In this talk I will cover our contributions to understanding the hybrid vapour-vapour deposition approach, where one first needs to understand the caveats of the first step (i.e., strong vacuum ~10^-6 mbar PVD of the inorganic template) [1], before studying the transformation of the inorganic template into a 3D perovskite upon exposure to organohalide vapours (e.g., formamidinium iodide or methylammonium bromide) in a close-space-sublimation setting in weak vacuum conditions ~ 0.1 mbar [2]. By tracking X-ray diffraction and photoluminescence in-situ, we identify four different regimes of the transformation and expose critical, previously overlooked, external factors that determine the optoelectronic quality of the perovskite thin-films. In the final part, I will present a new degradation-inversion approach in which we synthesize formamidinium iodide (FAI) in-situ in ambient conditions directly near the inorganic template by applying the principle of Le Chatelier and reacting the as-formed FAI with the inorganic template to form the first proof-of-principle devices with this fast, new, low-cost synthesis approach. [3] 

Metal halide perovskite-based solar cells have transitioned over the past decade from a research innovation to a commercially viable technology. Vapor phase deposition methods for perovskite solar cells (PSCs) are gaining increasing interest in both academia and industry, holding great promise for the commercialization of perovskite-based photovoltaics. Despite these advances, current laboratory-scale vapor-based processes are limited by low deposition rates, creating a significant barrier to achieving the production throughput necessary for industrial adoption. In this contribution, we report on the latest developments and investigations by the team at the Karlsruhe Institute of Technology on advancing vapor phase deposition processes for perovskite photovoltaics. Our research aims to provide practical guidelines to support the transition from research-scale methods to scalable and cost-effective manufacturing.

In 2024, we presented a perspective that conveys a balanced viewpoint from both industry and academia on the prospects of vapor phase deposition of perovskite photovoltaics as part of a large global consortium [1]. Building on this perspective, earlier this year we reported on strategies for achieving high-throughput vapor deposition processes for the industrialization of PSC fabrication in collaboration with industry partners [2]. The latter study addresses the critical challenges of scaling vapor-based processes by evaluating the thermal stability of perovskite precursors, analyzing deposition modes, and conceptualizing a linear sublimation source for production throughput analysis. Together, these studies offer a comprehensive framework for advancing vapor phase deposition methods and accelerating the commercialization of perovskite-based photovoltaic technologies.

Furthermore, this contribution will report on the latest developments of our team on vapor-based perovskite absorber layers and their application in perovskite single junction solar cells as well as perovskite/Si tandem solar cells. This encompasses our recent study on sequential evaporation of inverted formamidinium lead triiodide (FAPI) PSCs that highlights the impact of different hole transport layers (HTLs) on the crystallization and film formation of FAPI perovskite thin films [3]. This study reveals significant changes in PbI₂ crystal orientation depending on the HTL, which in turn affects the subsequent conversion and crystallization processes. We achieve power conversion efficiencies (PCEs) of more than 17%, the highest reported for fully vacuum-processed pure FAPI PSCs in the p-i-n architecture.

1. Abzieher, T. et al. Vapor phase deposition of perovskite photovoltaics: short track to commercialization? Energy Environ. Sci. (2024) doi:10.1039/D3EE03273F.

2. Petry, J. et al. Industrialization of perovskite solar cell fabrication: strategies to achieve high-throughput vapor deposition processes. EES Sol. 1, 404–418 (2025).

3. Diercks, A. et al. Sequential Evaporation of Inverted FAPbI3 Perovskite Solar Cells - Impact of Substrate on Crystallization and Film Formation. ACS Energy Lett. 1165–1173 (2025) doi:10.1021/acsenergylett.4c03315.

Hybrid perovskites semiconductors provide an opportunity to create highly efficient low-cost, lightweight, photovoltaics that have a high power-to-weight ratio. Physical vapor deposition is the technique that enables the creation of a multilayer heterostructure that contains hybrid perovskites semiconductors, small-molecule organic and metal oxide semiconductors. However, to develop such an intricate architecture that can reach close to the theoretical limit, the metal-halide perovskite semiconductors need to have minimal defects and phase stability. We have shown that by manipulating, the substrate material [(Patel et al., 2017)] the temperature [(Lohmann et al., 2020)] and the addition of additives [(Lohmann et al., 2022)] can aid in growing highly crystalline, low defect density perovskite thin films.

In this presentation, we will show our progress in developing high quality thermally evaporated perovskite semiconductors and our advancements of the evaporation technique to develop triple junction perovskite photovoltaics.

References

[1] Lohmann, K. B., Motti, S. G., Oliver, R. D. J., Ramadan, A. J., Sansom, H. C., Yuan, Q., Elmestekawy, K. A., Patel, J. B., Ball, J. M., Herz, L. M., Snaith, H. J., & Johnston, M. B. (2022). Solvent-Free Method for Defect Reduction and Improved Performance of p-i-n Vapor-Deposited Perovskite Solar Cells. ACS Energy Letters, 7(6), 1903–1911. https://doi.org/10.1021/acsenergylett.2c00865

[2] Lohmann, K. B., Patel, J. B., Rothmann, M. U., Xia, C. Q., Oliver, R. D. J., Herz, L. M., Snaith, H. J., & Johnston, M. B. (2020). Control over Crystal Size in Vapor Deposited Metal-Halide Perovskite Films. ACS Energy Letters, 5(3), 710–717. https://doi.org/10.1021/acsenergylett.0c00183

[3] Patel, J. B., Wong‐Leung, J., van Reenen, S., Sakai, N., Wang, J. T. W., Parrott, E. S., Liu, M., Snaith, H. J., Herz, L. M., & Johnston, M. B. (2017). Influence of Interface Morphology on Hysteresis in Vapor‐Deposited Perovskite Solar Cells. Advanced Electronic Materials, 3(2), 1600470. https://doi.org/10.1002/aelm.201600470

Vapor phase deposition offers a scalable pathway for translating perovskite solar cell fabrication from laboratory-scale to continuous industrial manufacturing. Successful scale-up necessitates precise control of process parameters as well as robust process repeatability and reproducibility. Controlling the sublimation behavior and deposition kinetics of organic precursor materials remains a significant challenge in vapor-based fabrication processes. While the sublimation properties of methylammonium iodide have been extensively studied, [1-5] analogous studies on formamidinium iodide (FAI) remain scarce. [6,7] This study is the first to systematically investigate how the FAI precursor particle size and the geometry of the sublimation crucible influence the directionality of the emitted vapor flux during deposition.

We demonstrate that conical crucibles lead to beam-focusing of the emitted vapor flux, while cylindrical crucibles exhibit a broader and less directional emission profile. Furthermore, in the case of conical crucibles we observe a strong impact of the FAI particle size on the directionality of the vapor flux, whereas this effect does not occur for cylindrical crucibles. We show that such variations in emission profiles significantly impact the deposited film thickness uniformity, especially with respect to lateral source-to-substrate distance. The inconsistent particle size distribution found in commercial FAI powders in combination/in conjunction with conical crucibles therefore represents a significant challenge for reproducible and repeatable sublimation processes under laboratory conditions. Finally, analysis of commonly used inorganic materials reveals that effusion characteristics are highly material-dependent, adding to the complexity of multi-material deposition processes.

Our findings highlight the critical role of precursor particle size, choice of crucible geometry and spatial arrangement within the vacuum chamber to improve film homogeneity and ensure repeatability and reproducibility of laboratory-scale sublimation processes.

The thermal decomposition of formamidinium iodide (FAI) during vacuum deposition poses a major challenge in the fabrication of high-quality perovskite thin films. In this study, we present a strategy to suppress FAI decomposition by incorporating a minor additive during precursor preparation. The additive modulates the chemical environment, effectively suppressing the formation of volatile decomposition byproducts during deposition, and thereby stabilizes the working pressure during the process. In addition, this approach enabled the formation of wide-bandgap perovskite films with high crystallinity. As a result, single-junction devices based on 1.68 eV bandgap perovskites achieved power conversion efficiency (PCE) of 20.5%, while silicon/perovskite tandem devices exhibited efficiencies of approximately 30%. Our findings provide a practical method for mitigating the thermal instability of organic halide precursors during vacuum deposition. This strategy is expected to advance the advancement of high-efficiency perovskite solar cell technologies using vacuum deposition and facilitate their further commercialization.

In the last ten years, tandem solar cells based on perovskite (PK) materials have shown promising results, surpassing the theoretical limits of single junction Silicon (Si) solar cells. Even though PK/Si tandem solar cells appear capable of achieving >35% of power conversion efficiency, many challenges need to be overcome in order to upscale the PV devices from 1 cm² laboratory scale to larger areas. With that perspective, vapor deposition of the absorber layer seems promising in order to elaborate a conformal and high quality perovskite on top of textured industrial silicon wafers. In the literature, the main industrially compatible techniques to grow the PK layer can be divided into two axes: i) PK deposition by full vapor deposition techniques and ii) PK deposition by hybrid processes (mix of dry and wet processes).

In a first part, we propose pulsed laser deposition (PLD) as full vacuum scalable method to fabricate uniform black phase inorganic perovskites and charge transport layers on 707 cm² substrates (> G12 area). We first developed several PLD-grown contact layers (ITO, SnO2 and NiOx), and then demonstrated the deposition of CsPbI₂Br and CsPbI₃ on 300 mm wafers, exhibiting PL peaks at λ=648 nm (~1.91 eV) and 700 nm (~1.77 eV), with FWHM of 28.9 and 36.5 nm, respectively. The films show excellent uniformity: 0.8% in thickness and 0.2% in PL wavelength. Final device integration and performance measurements are currently underway. Looking ahead, we aim to scale-up the growth rate from the current 8 nm/min at 20 Hz to 125 nm/min using an industrial PLD system operating at 300 Hz, further highlighting PLD as a promising route for large-scale PSC manufacturing.

In a second part, we focus on a hybrid deposition process combining thermal co-evaporation, close space sublimation (CSS) and solvent step for the elaboration of organic-inorganic perovskite layer. Using different characterization techniques (X-ray Diffraction, X-Ray Photoelectron Spectroscopy, Scanning Electron Microscope, etc.), we firstly investigate the structural and chemical properties of the inorganic scaffold. The goal of this study is to understand how the homogeneity, the porosity and the composition of the first layer affect the growth of the final PK film. Simultaneously, we examine the key factors that influence the crystallization mechanism during the second wet step.

Final part will be dedicated to the stability assessment of perovskite/silicon tandem architecture under outdoor and accelerated ageing conditions.

As the efficiency and stability of perovskite solar cells have progressed over time, the scalability and throughput of the manufacturing processes used to fabricate devices remains as a critical hurdle to solve before perovskites reach mass adoption in the photovoltaic market. Record efficiency perovskite solar cells are still largely produced by spin coating techniques, and scaling up production to larger areas and higher throughputs remains challenging. One promising alternative technique from a manufacturing perspective is two-step vapor processing via close space sublimation (CSS), which is a low-vacuum technique capable of high deposition rates. To date, the performance of CSS perovskites has largely been limited without a clear indication of the fundamental reason. Particularly for p-i-n configurations, there are few reports and only a handful of reports with efficiencies >16%. In this work, we build upon previous work demonstrating easy bandgap tunability using CSS with mixed halide organic reaction sources. We analyze the sources of losses in p-i-n perovskite solar cells produced by CSS and put in place reaction optimization and surface passivation strategies that enable >20% PCE for pure iodide perovskites and >18% for a range of tandem relevant bandgaps. This is a step towards closing the gap between close space sublimated and spin coated perovskite solar cells and paves the way for future investigations into scaleup and integration of these absorbers into perovskite silicon tandem solar cells.

Semi-transparent solar cells have attracted massive industry interest as they offer tri-functionality by forming the skin of the building, enabling natural light illumination, and generating power. Cu(In,Ga)Se₂ (CIGSe) photovoltaics are a promising technology option for such applications. There are two different approaches to achieve transparent CIGSe solar cells. One involves the fabrication of ultra-thin CIGSe absorbers on see-through transparent back contacts. This method, however, is limited by the formation of GaO_x parasitic layers at the back contact interface, which leads to efficiency losses. An alternative approach involves spatially segmented full-thickness solar cells with transparent parts separating them to give the semi-transparent characteristic.

As proof of concept, we present a top-down micro-structuring approach, where an opaque CIGSe solar cell is spatially segmented into micro-sized line-shaped solar cells. By varying the lines’ width and spacing, it is possible to control the window’s average visual transparency (AVT), making it suitable for different applications. The fabrication process begins with a photolithography step, where the selected pattern of micro-lines is designed on top of a complete CIGSe solar cell stack. An aqueous bromine solution etches the developed solar cell areas, while the photoresist protects the solar cell lines [1]. The bromine solution results in a slight over-etch of the window layer by about 20-30 µm. This over-etch reduces the active device area, leading to a performance loss, which is particularly pronounced in narrower lines (narrower than 200 µm). After the bromine etching, the sample was dipped in sodium hypochlorite (commercial bleach) to remove the exposed molybdenum, thereby making the areas in between the solar cell lines fully transparent. To ensure proper charge carrier collection, we performed simulations to design a metallic front contact grid that minimizes shadowing. Narrow, tapered aluminum electrical contacts were evaporated on top of the full length of the line solar cells, with a width between 30 mm and 1 mm. The top-down, materials inefficient process resulted in a semi-transparent CIGSe solar cell with a total area efficiency of 6.1 % and an AVT of 49 %.

Developing a more sustainable and materials-efficient fabrication approach, we use sputter-deposition and lift-off processes to achieve selective deposition of the CIGSe micro-striped solar cells. Here, Cu-In-Ga is deposited onto a patterned photoresist, followed by a lift-off process. The Cu-In-Ga lines are subsequently transformed into CIGSe lines by a selenization process. Solar cell devices are then completed by depositing a buffer layer in a chemical bath and by sputtering a i-ZnO/ZnO:Al window layer. With this approach, we have achieved a power conversion efficiency of approximately 6% for individual lines.

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

The goal for TPV is to have a device that absorbs in the visible range as little as possible while it absorbs the remainder part of the spectrum (i.e. UV/IR) in order to be visible transparent to the human eye, and as so, it can be split into two main categories or approaches, namely wavelength-selective and non-wavelength-selective. These two approaches are being actively investigated using different materials, such as organic materials and perovskites. However, inorganic-based structures constitute a very attractive prospect as they can be integrated as different functional layers in the solar cell architecture (i.e. as absorber, Charge Transport Layer and transparent electrical contacts). Additionally, many inorganic materials present high bandgap, tuneable conductivity, low deposition temperatures and can be deposited by a plethora of techniques that are possible to upscale for industrial purposes. Another key aspect is that these materials are stable, have low thermal budgets and are CRM-free. Given these aspects the challenge is on how to combine them in advanced device architectures to develop a final device that is efficient, transparent and aesthetically pleasing that can be integrated in architectural components (windows, canopies, façades) and/or on devices that present low power draws such as smartphones, wearables and IoT devices and sensors.

Herein, we will discuss the basic principles and figures of merit in TPV, as well as the state of the art for inorganic-based strategies. The talk will also focus into two main approaches that we are developing: ZnO1-xSx UV-selective absorbers and on the optimisation of oxide-based architectures integrating nanometric a-Si:H layers as a non-wavelength-selective approach. Main challenges and late results attained with both strategies will be reviewed, including the achievement of record devices with Light Utilisation Efficiency (LUE) up to 2.3%, transparency in the range between 30% and 70% and photoconversion efficiencies up to 5%.

Development of new photovoltaic (PV) technologies based on novel green materials with higher efficiency and lower cost options could create a new industry with independent supply chains, foster IoT market diversification, and attenuate market volatility. Sb-chalcogenide compounds have recently gained increasing attention as defect tolerant, non-toxic and highly stable materials for earth abundant thin film polycrystalline PV technology.  Despite of their recent addition to the thin film device value chain, efficiency values of these materials have climbed rapidly and are now approaching 11%. Yet despite of this rapid progress, there is still headroom to increase performance significantly by addressing both, fundamental material challenges and innovation in device designing and development. This presentation will review the most relevant progress achieved for Sb-chalcogenides thin film PV, with the emphasis on key processing strategies to optimize absorber material and thin film solar cells properties (doping and alloying), understanding of buried interfaces and push the boundaries of understanding and performance. Based on the state-of-the art progress in performance of emerging Sb-chalcogenide PV materials, a roadmap of customized PV application variability, including semitransparent and tandem PV devices for BIPV, PIPV and IoT markets will be presented.

Sb₂(S,Se)₃ has attracted increasing attention as a photovoltaic absorber material due to its high absorption coefficient, excellent stability, and RoHS-compliant composition. However, current high-efficiency Sb₂(S,Se)₃ solar cells are typically limited to opaque, monofacial configurations, creating a technological gap for tandem integration with silicon solar cells or for use in semi-transparent solar windows. Here, we demonstrate a single-junction, bifacial, and semi-transparent Sb₂(S,Se)₃ solar cell, enabled by employing an indium tin oxide (ITO) back electrode atop a MnS hole-transporting layer (HTL). The ultrathin and fully depleted absorber layer, fabricated via hydrothermal synthesis, facilitates carrier drift rather than diffusion toward the functional layers, significantly enhancing bipolar transport and bifacial absorption. Under AM1.5 illumination, the device achieves an impressive bifaciality of 0.86, despite a slight open-circuit voltage loss caused by the MnS/ITO Schottky junction. To further improve performance, we optimized the Sb₂(S,Se)₃ absorber by introducing a targeted additive into the precursor solution. This strategy promotes more favourable band alignment and effectively suppresses bulk defect formation, thereby enhancing material quality and increasing device efficiency to 10.7%. More systematically investigation of the correlation among transparency, bandgap, and efficiency underscores the significant promise of Sb₂(S,Se)₃ solar cells.

The development of TPV devices has gained significant attention in recent years due to their potential for seamless integration into Building-Integrated Photovoltaics (BIPV). BIPV has been identified as a key enabling technology for the development of "Near Zero Energy Buildings" (NZEB), achieved through the integration of a new generation of PV modules capable of replacing traditional architectural elements. TPV devices are also of interest in other applications, such as Product-Integrated Photovoltaics (PiPV), and any scenarios where aesthetics are important. TPV devices have the potential to revolutionize photovoltaic technology by enabling on-site generation while minimizing visual impact.

One of the most promising approaches to achieve TPV is to utilize the ultraviolet region of the solar spectra, while avoiding absorption in the visible range and more specifically on the photopic response range of the human eye, thus attaining complete apparent transparency. One promising approach to achieve high transparency on solar cells is by using wide bandgap materials with bandgap energy (E_g) above 2.7 eV. However, the choice of absorber material is critical, due to the lower number of photons in this spectral region, making it a must to have a good spectral match with the bandgap and the UV onset to maximize photon absorption. In this way, ZnO_1-xS_x has been proposed as an ideal UV absorber thanks to a composition-tunable bandgap that can shift from 3.2 eV (pure ZnO) down to 2.7 eV (x=0.5) by anionic substitution of oxygen by sulfur. ZnO_1-xS_x is a material composed of earth-abundant raw elements, compatible with scalable fabrication processes and can be synthesized at low temperatures, which potentially allows minimizing the carbon footprint, economic costs and energy expenditure associated with device manufacturing.

In this work we present a fully oxide based solar cells with structure SGL/FTO/MoO_x/ZnO_1-xS_x/ZnO/AZO. This device is a full ALD-compatible process, which enables high process reproducibility and conformality for the coverage of 3D structures. We explore the different compositional ranges from sulfur rich to sulfur poor and how it affects to performance, also crystal formation and phase segregation is explored at different synthesis temperatures. In a second level, two samples with different HTL are prepared and studied, one with MoO_x and another with NiO. Based on these prelimienary results, we explore an aproach to significantly increase the Voc of the ZnO_1-xS_x TPV solar cells by the fine tuning of the MoO_x HTL thickness, preparing two 5x5 cm² samples with a gradient of the MoO_x HTL layer. The complete optimized structure demostrated an Average Photopic Transmittance of 73%, showing indeed excelent transparency, while achieving colore rendering Index (CRI) of 97, coriming its high color neutrality and achieving record-high Voc values close to 800 mV. 

Selenium (Se) holds historical significance as the first material used in a solar cell, in 1883, marking the initial exploration of materials capable of harnessing solar energy. However, it took over a century before being seriously considered for photovoltaics, achieving at this time an efficiency of 5% with the following Au/Se/TiO2/FTO architecture. In 2017, IBM improved the device architecture with more suitable selective contacts (Au/MoOx/Se/ZnMgO/FTO) and an impressive efficiency of 6.5% was achieved. This breakthrough reignited interest in selenium for photovoltaics, resulting in many recent research publications, with a particular focus on its potential use as a top cell in tandem configurations, and much more recently as indoor devices. During the last year, two new world-record efficiencies have been reported: 7.2% and 8.1% under AM1.5 illumination, and more than 20% under indoor conditions. However, all these results were achieved for thick absorber layer (higher than 1 um) except for the IBM device, where the impressive 6.5% efficiency was achieved with a thickness of just 100 nm.

Selenium benefits from a low temperature processing (melting point around 200 ºC) and a direct bandgap of approximatively 1.95 eV. Although this bandgap may initially not seem ideal for semi-transparent applications, recent findings suggest that very thin (less than 50 nm) amorphous silicon devices can achieve high visible transparency and impressive efficiencies, with promising light utilization efficiencies (LUE) of 1%. An initial optical simulation of a complete device architecture using a 100 nm thick selenium indicated an average visible transmittance (AVT) of 51% across the visible spectrum. This could potentially lead to LUE values higher than 2.5%, which is an outstanding result, suggesting that c-Se is inherently compatible for semi-transparent applications.

To check the viability of Se for semi-transparent PV, different strategies for the synthesis of Se layers have been established. A baseline processing technology allowing the fabrication of state-of-the-art Se devices has been developed and optimized. Subsequently, strategies to get transparency have been implemented and developed. All the results of this work will be presented at the Matsus Fall 25 conference where the viability of selenium for semi-transparent photovoltaics will be discussed.

Transparent photovoltaics (TPV) is a disruptive approach in which the solar cells can selectively transmit the visible light to human eyes harvesting UV and/or NIR photons.[1] TPV is attractive as it widens the deployment of PV into new sectors, like building integrated photovoltaics (BIPV), greenhouses, car windows and sunglasses, thus providing an immense potential to generate solar electricity beyond the conventional rooftops and solar power plants. One possible approach to TPV is based on wavelength-selective absorbers where the chromophore requires an absorption far from the photopic response of the human eye.

DSSCs offer a sustainable solution for transparent and colorless photovoltaic cells, thanks to the exceptional tunability of both dyes and electrolytes. However limited classes of dyes possess energetic levels that can ensure an efficient injection while having a bandgap sufficiently narrow to selectively absorb the NIR region.[2] Among these classes, polymethine dyes (cyanines and squaraines) are promising for their high molar extinction coefficient and easily tunable properties through modification of central core or lateral units. Cyanines in particular have already been investigated for dye sensitized solar cell (DSSC) devices with promising results in terms of transparency and performance.[3]

In recent years, our research group has developed a series of polymethine-based NIR dyes for wavelength-selective DSSCs, as well as different colorless redox shuttle. Most of these molecules were synthesized using microwave-assisted procedures aligned with green chemistry principles, enabling efficient and cost-effective production. This work presents the design, synthesis, and device integration of NIR dyes that achieve high transparency coupled with acceptable performances. Additionally, we report on the development of compatible colorless redox mediators and transparent electrodes. A comprehensive discussion is provided on the interplay of materials and the current technological challenges in realizing fully transparent and colorless NIR-DSSCs for BIPV applications.

These new molecules have been deeply characterized in terms of their optical, photophysical and electrochemical properties, showing interesting structure/property relationships. Finally, photovoltaic performances have been evaluated in lab-scale DSSCs and optimized by different anode modifications and electrolyte formulations.

Among the diverse array of photovoltaic technologies, organic and perovskite photovoltaics stand out as particularly promising candidates for transparent solar applications, each offering unique advantages. Organic photovoltaics (OPVs), based on an excitonic semiconductor, inherently provide true transparency while simultaneously enabling the harvesting of near-infrared (NIR) light, making them ideal for integration into windows and building facades. Perovskite solar cells, characterized by their direct bandgap and exceptionally high open-circuit voltages (Voc), stand out in capturing high-energy photons in the blue region of the spectrum with very low Voc losses, positioning them most suitable for tandem configurations.

The combination of perovskites with organic PVs into tandem architectures emerges as a natural approach, leveraging the complementary spectral absorption and electronic properties of both materials. Such tandem systems not only improve overall efficiency but also offer tunability of transparency and color rendering index (CRI) through bandgap engineering of the individual layers. This flexibility enables the customization of optical appearance to meet aesthetic and functional requirements.

In this contribution, we outline the fundamental design principles for developing highly efficient, transparent and fully solution processed organic PV modules. Additionally, first fully solution processed transparent wide bandgap perovskite modules will be shown. We further introduce an innovative voltage-matching concept tailored to reconcile the differing voltages of wide-bandgap perovskite and low-bandgap organic cells, addressing a key challenge in tandem device fabrication. Finally, we present the first demonstrators of transparent perovskite-organic tandem modules developed within the European project Citysolar, showcasing their potential for sustainable, aesthetically pleasing, and high-performance building-integrated photovoltaics.

In this work, we present the development of tandem solar cells that integrate a perovskite-based top module (PSC-NUV) with a dye-sensitized solar module (DSSC-NIR) serving as the bottom sub-cell. The aim of this configuration is to create semi-transparent photovoltaic (PV) modules specifically tailored for architectural applications, with a focus on building-integrated photovoltaics (BIPV). The integration of these two distinct solar technologies was achieved through an innovative coupling strategy involving specially engineered interlayers. These interlayers were designed to facilitate optimal adhesion and minimize optical losses by matching the refractive indices of the different layers.

Comprehensive optical characterization of the tandem solar cells was conducted, including assessments of Average Visible Transmittance (AVT) and Colour Rendering Index (CRI). These parameters are critical for determining the suitability of such devices in architectural contexts where light quality and transparency are essential. In parallel, photovoltaic performance metrics were measured for both individual sub-cells as well as the complete tan]dem device to evaluate their efficiency and functional compatibility.

For the fabrication process, advanced scalable printing technologies were employed. Blade coating [1] was utilized for producing the PSC-NUV module, while screen printing [2] was used to fabricate the DSSC-NIR sub-cells. Each type of module was produced independently on substrates with an area exceeding 100 cm². These large-area modules were subsequently integrated into full tandem configurations using custom-designed lamination and electrical interconnection techniques. Laser patterning was implemented within the PSC-NUV modules to enable accurate series interconnections, with particular attention given to optimizing the layout of these interconnects to reduce the so called "dead zones." A notable achievement in this work was the fabrication of semi-transparent PSC-NUV modules over a 100 cm² area with geometric fill factors (GFF) surpassing 92%, a critical benchmark that significantly contributes to maximizing overall device PCE.

In terms of performance, the PSC module exhibited a power conversion efficiency (PCE) of 4.38% over the 100 cm² area, while the DSSC module achieved a PCE of 1.04%. After filtering through the DSSC layer, the efficiency decreased to 0.68%, reflecting the energy absorbed in the tandem configuration. The fully assembled tandem device reached a combined PCE of 5.06%. Importantly, the AVT values for both the PSC-NUV and DSSC-NIR modules were measured -separately- to be approximately 60%, underlining their potential as viable semi-transparent PV solutions.

This work clearly demonstrates the technical feasibility and potential of integrating high-performance, semi-transparent tandem solar modules—combining PSC and DSSC technologies—into the fabric of buildings. The resulting modules not only contribute to energy generation but also support aesthetic and functional requirements, marking a significant step forward in the development of next-generation, energy-harvesting materials for sustainable and visually appealing urban environments.

The solar cell technology is experiencing tremendous growth globally, as well as the building integrated photovoltaics (BIPV) field [1-4]. The latter is growing extremely fast because the integration of photovoltaics (PV) into building roofs and façades provides cost-effective energy solutions, as modules can substitute building envelopes, such as roofing or glass windows. Windows make up a large percentage of modern building real estate, therefore transforming them into PV devices they would drastically increase the available area for on-site electricity generation.
However, the adoption of this technology depends on its ability to transmit light in terms of average visual transmittance (AVT) coupled with a reasonably high photo-conversion efficiency (PCE) [1-4].
Currently, most commercially available devices consist of patterned crystalline silicon (c-Si) wafer technology [1]. Although Si-based PV offers both high PCE and AVT, it results in an unpleasant and impeded view. On the other hand, amorphous silicon (a-Si) can enable a homogeneous appearance for semi-transparent photovoltaic (STPV) by decreasing the thickness of the light-absorbing material such that semi-transparency is achieved. However, the latter result in windows with an inherent low color rendering index (CRI).
Perovskite PV technology has taken giant steps from fundamental science to device engineering, achieving up to 26% photo-conversion efficiency [5-7] in almost a decade time. The possibility to exploit this technology on glass substrates gives an unbeatable power to weight ratio in comparison to similar photovoltaic systems, thus opening new possibilities and new integration concepts in BIPV.
Also, perovskite solar cells (PSCs) hold an advantage over traditional silicon solar cells in the simplicity of their manufacturing processing [6-7]. While silicon cells require expensive, multi-step processes, at high temperature and under high vacuum in cleanroom facilities, PSC materials can be realized in lab environment using a variety of inexpensive, simpler, and low-temperature solution processing and deposition techniques with the potential to be scaled up for large-area device fabrication [7].
Moreover, there is an enormous effort to push PSC from research and development at the lab-scale level to a  large-scale industrial level, making PSC an outstanding contender for STPV. So far, top-performing STPV using thin perovskites have reached 12.6% PCE with 21.5% AVT, and 2.7% LUE [6].
Here, we developed a STPV technology with a relatively high AVT (43%), PCE (6%), LUE (3%), and CRI (89%) based on laser patterning of thin-film PSC. Our design philosophy is based on micro-striped solar cells separated by a fully transparent gap, providing high transparency. The device architecture is a thin-film, planar, n-i-p configuration with a commercial transparent conductive oxide-coated glass bottom contact electrode, an inorganic electron-transport layer, a hybrid halide perovskite absorbing layer, an organic hole-transport layer, and a thermally evaporated or sputtered top contact electrode. The material layers are fabricated by inexpensive solution processing methods and deposited onto substrates at low temperature by scalable and rapid printing techniques, such as spincoating. All developments target material-efficient and sustainable fabrication approaches for technology transfer to industry.

With the world's growing population, the demand for food and electricity is increasing rapidly. To meet these demands, the agricultural and energy sectors need to work together to achieve sustainable development. Agrivoltaics is one of the promising approaches that combines the production of crops and renewable energy generation in the same land footprint. However, the use of conventional opaque solar panels in agrivoltaic systems creates shading effects, being unable to share the solar spectrum, given that plants absorb only >1.7 eV photons whilst silicon (or other inorganic) solar cells absorb all photon energies above the near IR. To overcome this issue, low-cost semi-transparent photovoltaic modules produced using scalable manufacturing offer an exciting alternative. Such cells enable wavelengths between 400 and 700 nm, which is referred to as photosynthetically active radiation (PAR) to pass through to the crop. Thus, Organic Photovoltaics (OPV) have gained significant attention due to their ability to harness photons in the near-infrared and ultraviolet spectra while allowing visible light to pass through, providing high transparency.

The current state-of-the art power conversion efficiency (PCE) of OPVs exceeds 20% for a single junction OPV¹and reaches 11% for semitransparent based OPV devices², following rapid improvement in recently developed non-fullerene small-molecules acceptors (NFAs) to replace fullerene-counterpart. Moreover, high potential PCEs are offered by a ternary system where either the third component acts as an additional donor or additional acceptors that serve to extend the range of absorption and can also tune material properties. Interestingly, the incorporation of the third counterpart can regulate the accumulation and orientation of the molecule, as well as the phase separation of donor and acceptor, providing high crystallinity and ordered molecular stacking that can improve the charge transport and inhibit the bimolecular recombination through well optimized phase separation³.

Furthermore, most research on ternary strategies is based on the bulk hetero junction (BHJ) system, which is sensitive to material properties and processing conditions. This makes it difficult to control other important morphological parameters, such as molecular orientation and domain purity as it further complicates the morphological regulation, especially the D:A orientation in the vertical direction of blend films which is mainly related to the charge transport and collection⁴. Thus, to tailor vertical phase distribution efficiently, the sequential deposition or named layer by layer (LBL) deposition approach of the D and A materials is considered as a promising alternative to the BHJ⁵.

Hence, I have developed a novel OPV structure, introducing a perovskite quantum dots (PQDs) interlayer sandwiched between the organic semiconductor donor and the NFA layers using LBL deposition approach, resulting in enhancement in the performance of OPV devices by 11% ( from PCE=16.6% for the pristine binary OPV to PCE= 18.8% for the QDs-based ternary OPV) along with 99 % performance retention after 3 months of storage compared to only 30% for the bilayer devices without PQDs. This unique structure allows for the LBL preferred vertical phase separation and well-controlled D/QDs/A interface film morphology, exhibiting efficient charge transport and extraction properties. Interestiingly, from the perspective of the agrivoltaic application, the PQD interlayer (third component) does not absorb much light in the visible region, allowing it to be transmitted to the plants without influencing the crop yields.

In summary, the semitransparency and promising PCE along with the cost-effective, eco-friendly processing, are promising selling points of this OPV technology for the agrivoltaic system relative to traditional PV technologies.

The study of perovskite solar cells degradation is a complex issue due to the multitude of phenomena that can contribute to it. Recently, we have obtained new insights using a model that combines several electronic and ionic processes, that can produce capacitive and inductive response in different circumstances.¹ These models are very successful to describe huge memory effects and hysteresis in perovskite memristors, by the combination of different techniques: current-voltage scan, time transients, and impedance spectroscopy.^2-4 Here we show the changes of impedance spectroscopy and time transients as a diagnosis of hysteresis, time constants and evolution of degradation in the perovskite solar cells. Normally the degradation can produce two main impacts, decrease of charge collection (lowering photocurrent) or increase of recombination (lowering photovoltage). We need to find dynamical signatures of the phenomena causing these effects to discover the physical reasons for the devaluated performance.

Despite their relatively emerging degree of development, metal halide perovskite light-emitting diodes (PeLEDs) have reached outstanding brightness and radiative efficiency levels that roughly graze the maximum theoretical limits. Unfortunately, the complete understanding of their working principles and the photo-electrochemical mechanisms involved in the charge carrier injection/recombination dynamics are still a conundrum. Additionally, the strong ionic character of perovskites enables the migration of ions and the gradual formation of crystalline defects upon exposure to light and/or to an external electric field, which aggravates the complexity of these systems. In fact, these ionic processes are apparently coupled with those electrical involved in the generation of light and seem to be also connected with the widely reported limited long-term stability of the devices. Here, I will discuss the exploitation of a new methodology based on the combination of three frequency-domain modulated techniques, i.e. impedance spectroscopy (IS), voltage-modulated electroluminescence spectroscopy (VMELS) and current-modulated electroluminescence spectroscopy (CMELS), aimed at extracting values of characteristic thermodynamic constants and at reaching a full understanding of the PeLEDs technology. We propose a new theoretical model and an equivalent circuit that unifies these three techniques, which consider both the non-radiative and radiative contributions, as a powerful tool for the advanced characterization of any light-emitting device, being especially useful for the study of perovskite-based optoelectronic devices due to their inherent complexity. Particularly important is the deconvolution of the electrical, optical and ionic processes that are involved in the current-to-photon conversion, heat generation and/or degradation of the light-emitting material/device, as well as the elucidation of how all these phenomena mutually interact.

Hyperspectral spectral photoluminescence (HPL) imaging is a powerful, contactless, and spatially-resolved technique widely employed to characterize solar cells at various stages of their development and operation. However, this method is time-consuming due to the sequential acquisition across multiple wavelengths and the subsequent analysis of large, high-dimensional data cubes. We propose an advanced framework for HPL data analysis in perovskite solar cells. We first investigate the application of deep learning (DL) techniques to significantly reduce both acquisition and processing times, paving the way for faster and more scalable solar cell analyses, and then explore the correlation between electrical and optical characteristics of the cells.

We perform a DL-based local analysis of HPL data cubes to extract maps of Urbach energy (Eu), quasi-Fermi level splitting (QFLS) and bandgap energy (Eg). A multilayer perceptron (MLP) is trained on synthetic data generated using the generalized Planck’s law, combined with a logistic absorptance model, and simulated additive noise. The performances of the MLP are compared to non-linear least squares (NLLS) fitting applied to HPL spectra, following the approach introduced by Laot et al. [1].

Our results show that, while the MLP does not significantly increase the prediction error, it substantially reduces the computation time. The average absolute relative error (ARE) across more than 13 000 spectra - between the NLLS fitting results and the MLP prediction - is of 1.76% for Eu, 0.07% for Eg and 0.12% for QFLS. The total prediction time of for all three parameters across 13 000 voxels is under 1 second using MLP, compared to approximately 10 minutes using NLLS fitting.

We investigate how increasing the wavelength step size - thereby reducing the number of sampling points in the spectrum - impacts the prediction accuracy on the three variables of interest (Eg, Eu, QFLS). This approach enables a significant reduction in acquisition time, which not only accelerates the characterization process but also minimizes the degradation of perovskite materials that may occur during prolonged measurements. Furthermore, we apply transfer learning methods to adapt DL models trained on densely sampled spectra for use with sparsely sampled spectra, thereby reducing the training time required for the new models.

Using the theoretical framework proposed by Kirchartz et al. [2] we compute the local photocurrent, recombination current and open-circuit voltage, under the approximation that the quantum efficiency is equivalent to the previously calculated absorptance of the cells. We finally correlate the optical characteristics of the cells with their electrical parameters by analyzing a dataset of more than 100 p-i-n halide perovskite solar cells.

We explore how local parameter heterogeneity influences the global electrical figures of merit.

The extraction of photogenerated charge carriers and the creation of a photovoltage are essential functions of any solar cell. These processes do not occur instantaneously but have specific time constants, such as one associated with the increase of the externally measured open circuit voltage after a brief light pulse and another related to its decrease. In this presentation, I will introduce a method for analyzing transient photovoltage measurements at various bias light intensities by combining the rise and decay times of the photovoltage. I will demonstrate how to distinguish between recombination and extraction using transient photovoltage measurements, along with an analysis approach based on determining the eigenvalues of a 2 × 2 matrix. The model yields two time constants (the inverse eigenvalues), one for the voltage rise and one for its decay after the pulse. These time constants can be experimentally measured as a function of light intensity. By comparing the model with experimental data, I can derive a time constant for recombination and another for charge extraction, with the ratio of these time constants directly correlating with solar cell efficiency.¹ This general approach is applicable in situations where the Fermi level splitting within the solar cell absorber can be approximated by a single value, and its depth dependence can be ignored. In such cases, it can be applied not only to transient photovoltage but to any small signal optoelectronic technique in the time or frequency domain.²

Despite the rapid development of perovskite solar cells several challenges remain. A deeper understanding of the main losses, and how to mitigate them, is needed to make targeted improvements possible. In this talk, I will discuss our drift-diffusion modelling approach to shed new light on these fascinating solar cell materials.

Drift-diffusion techniques make it possible to connect and explain the fundamental generation, transport and extraction processes to macroscopic device performance. If done right, one can also do the opposite: By reverse-engineering current-voltage measurements on actual solar cells, one can identify the limiting factors. As an example, we show how current-voltage data and electroluminescence measurements help us identify the voltage losses in a series of co-evaporated FACsPbIBrCl perovskite solar cells with organic transport layers.

The physical processes underlying the impedance response of perovskite solar cells are not well understood. Typically, the low-frequency peak in such impedance spectra is attributed to ion dynamics, while the high-frequency peak is associated with electronic processes.

We introduce a new formula which enables us to directly derive the ion diffusion coefficient from the impedance response of perovskite solar cells. The validity of this formula is confirmed through extensive drift-diffusion simulations.

Upon demonstrating this, we determine the ion diffusion coefficients of a MAPI and a FAMAPI solar cells. The obtained diffusion coefficients are consistent with previously reported values from other characterization techniques. The advantage of this method is that it facilitates the precise, rapid, and straightforward determination of the ion diffusion coefficients.

Next, we introduce an experimental method for identification of the limiting aspect of perovskite solar cells under operating conditions in terms of recombination losses. We illuminate a bifacial cell from both sides separately with either red, blue or white light, each absorbed differently in the cell depending on the position in the device. Using the fill factor from the device characteristics for each case taking into account the direction of illumination we are able to accurately identify which part of the cell is limiting the performance. We show that this holds for many typical perovskite solar cells using drift-diffusion simulations. Finally, we issue a protocol to determine the dominant recombination channel under operating conditions in full device configuration.

Outdoor stability testing under natural sunlight provides the most relevant test of solar cell stability under operational conditions [1]. Understanding perovskite-based solar cells’ (PSCs’) recovery properties under natural diurnal light-dark cycling can point to methods to extend its lifetime [2, 3]. We studied the effect of climate conditions on perovskite solar cell lifetime, which showed that outdoor T80 is climate dependent with the ambient temperature being the dominant factor [4]. Based on this understanding, we designed an agro-photovoltaic system, where semi-transparent PSCs were coupled to a photobioreactor held at a constant temperature. Cultivation of photosynthetic microorganisms is a sustainable approach for producing feed, food and high-value compounds, and excess light-caused photosaturation and photoinhibition can be limited by light filtration by the semi-transparent PSCs. The synergy between photovoltaics – improved outdoor stability of PSCs - and photosynthesis – with improved yields with the proper light filtration - in this system will be presented [Gupta, R.K., M. B. Maung, V. Dubovsky, G. Ziskind, N. Kamenaya and I. Visoly-Fisher, in preparation].

Emerging photovoltaic technologies are commonly processed using solution deposition methods. Layers deposited in this way often suffer from non-uniformities in thickness and composition, resulting in locally varied cell performance. Such inhomogeneities and defects can be visualized by various imaging techniques, such as photo- and electroluminescence (PL/EL), dark and illuminated lock-in thermography, as well as optical imaging. Such non-uniformities can be challenging for upscaling emerging solar cells, as they result in performance losses and may lead to local hot-spots, which are the origin of degradation.

In this contribution, we first present EL images of carbon-based perovskite solar cells with a mesoporous layer stack which exhibit locally varying temporal evolutions. The scan-rate dependent current-voltage characteristics of perovskite solar cells and the temporal evolution of the EL signal are generally associated with the presence of mobile ions in the cell.[1] To analyse the transient EL images, we set up a device model in the drift-diffusion simulation software Setfos which quantitatively reproduces a set of steady-state and transient measurements. By employing this model, we are able to attribute the inhomogeneities in EL intensity to spatially varying ion densities. We further show how the mobile ion density influences the reverse bias breakdown behaviour in perovskite solar cells due to the strongly varying potential at layer interfaces, which facilitates tunnelling current.[2] Reverse bias conditions are imposed on shaded sub cells in modules or can be induced by current mismatch situations in monolithically stacked tandem devices. Non-uniformities caused by a spatially varying ion density and consequently reverse bias breakdown voltages result in strongly varying (reverse) bias potentials, inducing current (and temperature) hot-spots.[2]

The effect of reverse bias stressing on perovskite solar cells is further assessed by a combination of characterization techniques. To this end, cells are stressed below their breakdown voltage while the EL or PL signal is recorded. During regular intermittent measurements, current-voltage scans, impedance measurements, as well as (forward) electroluminescence images are taken to assess the underlying degradation mechanisms induced by reverse bias conditions.

Understanding the physics and origins of degradation mechanisms in solar cells is a challenging task. As a result, research often concentrates on a few model systems, relying on extensive and often costly characterization to track how material properties evolve over time.

This approach is particularly limiting for technologies like organic and perovskite solar cells, which can use a wide variety of materials. To unlock their full potential, accelerated characterization methods are needed. Without these, building quantitative structure-property relationships in the hope of designing bespoke materials for targeted applications (such as indoor, semi-transparent, and agrivoltaics) will fall short.

In this talk, I will present how modeling and machine learning (ML) can be coupled with high-throughput data from accelerated aging experiments to address this challenge. I will introduce optimPV, a fully open-source framework that integrates multiple physical modeling tools (e.g., PDE solvers, optical simulations, drift-diffusion modeling) with ML-based optimization methods (including Bayesian optimization, Bayesian inference, and genetic algorithms).

I will show how optimPV enables the identification of the dominant degradation process and the extraction of key material parameters, such as charge carrier mobilities and recombination rates. The effectiveness of this framework will be illustrated through a large-scale degradation study on organic solar cells, featuring 25 donor-acceptor blends processed under varied conditions. This analysis revealed key degradation trends and identified problematic material combinations to avoid in future formulations.

Lastly, I will discuss how the framework can be extended to other systems and case studies, incorporating a range of material types (organic and perovskite), physical models (PDE-based and drift-diffusion), and experimental techniques, including: (i) transient absorption spectroscopy, (ii) transient photoluminescence and microwave conductivity, and (iii) light-intensity-dependent current–voltage measurements.

Perovskite solar cells (PSCs) are promising candidates to reach the market to complement the current offer of photovoltaic cells although for such a thing they still must overcome some challenges such as long-term stability. The hole transporting layer (HTL) is a crucial component in n-i-p PSC, since it must favor an adequate movement of charges and protect the perovskite layer from environmental conditions. In this sense, the commonly used HTL, spiro-OMeTAD, does not provide PSCs with sufficient stability and is too expensive. Cheaper molecular materials such as metallophthalocyanines are proving to be a good alternative, as they provide greater stability.[1]

In this communication, we will present novel ZnPcs and CuPcs monomers [2] (see as examples Figure 1), among others, as efficient, stable, and low cost HTMs in PSCs. The MPcs are substituted with functional groups that possess a very good solubility in a wide range of organic solvents, adequate HOMO LUMO levels and their photovoltaics performance as high stable solution processing in a wide range of perovskite solar cell devices.

Ensuring high performance and long-term stability of perovskite solar cells (PSCs) is essential for their transition to large-scale industrial deployment. However, device-level quality of PSCs is often limited by microscopic inhomogeneities within the active perovskite layer - such as local variations in composition,  structure, and optoelectronic response - which can adversely affect both device performance and stability.

Identifying and understanding these local defects is critical for improving device reliability. Given the complex composition and polycrystalline nature of perovskite materials, imaging techniques with high spatial resolutions are essential for identifying and characterizing such local variations [1,2].

In this talk, I will discuss how advanced imaging and spectroscopic techniques - ranging from ultraviolet photoemission to infrared scanning probe microscopies - can shed light on the microscopic origins of performance losses and degradation in PSCs. These insights provide a deeper understanding of the detrimental roles of microscopic inhomogeneities in shaping macroscopic device behavior of PSCs, informing strategies to improve their performance and stability.

Since the emergence of the Perovskite Solar Cells (PSC) in 2009[1], significant progresses have been made, with high-efficiency devices reaching up to an outstanding 27% [2] in just 16 years. However, despite this quick development in a relatively short time frame, one of the main limiting factors hindering the industrialization of these photovoltaic technologies is their limited operational stability.

To mitigate the fast degradation of these photovoltaic devices, several strategies has been explored, with encapsulation emerging as one of the most effective[3]. These methods play a vital role in delaying the rapid degradation of the perovskite photovoltaic devices protecting them from environmental factors as heat, moisture and oxygen. However, ensuring the long-term reliability of these devices not only require of stable materials and hermetic encapsulations, but also robust testing protocols to evaluate the inner stability of the device and the effectiveness of the encapsulation protocols of the devices.

In this context, standardized stressing tests -performed both in outdoor and indoor conditions- are key[4,5]. Outdoor tests track devices under real operation conditions, while during indoor tests the degradation processes are accelerated by exposing devices to controlled stressing factors, such as elevated temperatures, continuous illumination or light and dark cycles[5]. However, reliable stability data remains scarce in the literature. This is, in part due to the lack of well-equipped testing facilities with the infrastructure needed to perform these tests.

One notable location for outdoor tests is Valencia, which offers extreme climatic conditions- high humidity, high temperatures and a large number of sunny days, particularly during the summer time- making it ideal for outdoor stress testing. Moreover, the University of Valencia and in particular the MOED group has become an established group in this regard, having experience in vacuum-processed perovskite solar cells, in encapsulation of PSC, and in conducting indoor and outdoor tests.

Beyond infrastructure, several technical issues must be addressed to ensure reliable stability data. These include the right measurement of the light, the correct calculation of the active area of the devices, and the gathering of representative and consistent data from each pixel or device over time. Post-treatment of the data is also essential, not only to condense the information but also to reveal correlations between the stability data with variables that would otherwise remain hidden.

This presentation will highlight the results and the insights gained over the past year by the MOED group on encapsulation methods, stability testing under various conditions, and advanced data post-processing methods for analysing stability data.

Photoluminescence imaging techniques are commonly used to investigate the optoelectronic and transport properties of halide perovskite absorbers and devices [1]. However, obtaining precise spatial maps of key physical parameters—such as carrier lifetime, quasi-Fermi level splitting (QFLS), and bandgap energy (Eg)—requires a high local signal-to-noise ratio (SNR), typically achieved through long acquisition times. Prolonged measurements can compromise data integrity due to changes occurring during acquisition. This limitation is particularly critical in operando experiments conducted under elevated humidity or temperature, where shortened acquisition times are essential to track dynamic changes in the material’s properties in real time.
To address this challenge, we previously demonstrated a denoising strategy based on Total Variation Regularization (TVR) [2], enabling the extraction of high-quality lifetime images from quickly acquired, noisy time-resolved fluorescence imaging (TR-FLIM) datasets [3]. In this work, we present a significant advancement by applying a tailored version of the Noise2Noise (N2N) algorithm [4] to denoise multidimensional datasets. The main advantage of N2N lies in its unsupervised learning framework, which makes it particularly well-suited to complex, real-world image denoising situations compared to TVR.
Using this approach, we performed in-situ TR-FLIM data acquisitions on halide perovskite thin films—specifically triple-cation compositions—under controlled humidity conditions (relative humidity XX%). This allowed, for the first time, micrometer-scale tracking of local carrier lifetime degradation during environmental exposure. The reduced acquisition time made possible by N2N denoising enabled the resolution of spatial heterogeneity in perovskite degradation.
In conclusion, coupling advanced PL imaging techniques with unsupervised denoising approaches like N2N opens new avenues for accelerated, high-resolution characterization of halide perovskites. This methodology not only deepens our understanding of material stability under realistic conditions but also holds broader potential for studying other beam-sensitive materials and for developing fast, reliable imaging workflows for operando and accelerated experiments. 

Hybrid perovskite absorbers are poised to become a crucial part of next-generation photovoltaics (PVs) for mitigating the impact of energy production on the climate. In one projected application, two-terminal silicon-perovskite tandem PVs, a high-bandgap perovskite absorber is coated on top of a silicon absorber (with intermediate layers) - achieving power conversion efficiencies superior to the Shockly-Queisser limit of single-junction silicon PVs. One of the major advantages of the two-terminal tandem architecture is that the perovskite deposition step can be integrated conveniently into the high-throughout production of mono-crystalline silicon modules that currently dominate the global market. In practice, these modules are assembled from individual silicon wafers in batch-to-batch operation. Consequently, for seamless integration, the perovskite absorber must be deposited onto a surface of silicon-wafer size, which are typically from about 20 cm to 30 cm wide. There are two principal routes of perovskite absorber deposition: Solvent-free vapor deposition and coating or printing from solution. While the first route offers advantages in homogeneity and compatibility with textured silicon substrates, the second route is more cost-effective and less operationally complex. However, solution processing of perovskite thin films is challenging to control on the targeted substrate scales for high-throughput coating techniques such as slot-die coating and spray coating (spin coating is most likely not an option for commercial PVs due to excessive material waste and throughput limitations). Therefore, there is a great merit in detailed investigation of how to deposit perovskite thin films homogeneously on substrates of typical silicon-wafer sizes.

For perovskite deposition from solution, not only the homogeneity of the coated wet films, but also the homogeneity of the drying process comes into play. This is because perovskite crystallization is highly drying-rate dependent. Typical perovskite precursor solutions require very high drying rates at moderate temperatures, which is why slot-nozzles purged by pressurized air or nitrogen, so-called “air knives” are often employed for drying. It is common knowledge that the mass transport under these air knives is highly inhomogeneous. That is to say that a liquid film under an air knife will dry much faster directly under the nozzle opening than on the edges of the film farther away from the nozzle. To counter-balance this inhomogeneity, technologists typically move the wet solution film under the air knife linearly. Still, the part of the substrate that experiences the air stream first, will dry differently than the part of the film situated at the center of the substrate. In turn, the part of the film that passes under the air knife last will have different drying dynamics from the other two parts mentioned before. Conclusively, it is challenging to homogenize drying dynamics of thin films moving under an air knife. In this talk, we investigate how this problem can be addressed by a systematic parameter variation during drying - the available paramters being the air flow velocity, the air knife distance, slot-width and mounting angle as well as the movement speed of the air knife. The investigation is implemented by a) developing a suitable and computationally efficient drying model of a high number of positions on the substrate b) defining how a homogeneous drying process is characterized and c) optimizing the available parameter space as a function of time for obtaining homogeneous drying. The presented results showcase the high potential and prediction power of employing drying models to homogenize perovskite drying. Further, the dependence of the controllability of drying on the homogeneity of the coated wet film as well as the time and film composition at crystallization onset is demonstrated.

The experimental research carried out by the DELFO group can be summarised in three main areas. The first area focuses on the study of the degradation mechanisms and performance optimization of perovskite-based optoelectronic devices under various conditions. First, we investigate the impact of ultraviolet (UV) light on perovskite solar cells with the structure ITO/PTAA/CsMAFAPbIBr/PCBM/BCP/Ag. Devices were exposed to continuous 385 nm UV light at intensities ranging from 1.5 to 30 mW/cm² in an inert N₂ atmosphere. Periodic J–V measurements reveal that UV exposure primarily affects the short-circuit current (JSC), while the open-circuit voltage (VOC) remains stable. The efficiency loss is attributed to reduced charge extraction, with a logarithmic trend observed in T80 versus UV power density. High-speed J–V scans suggest that JSC degradation is mainly driven by UV-induced ionic migration. Second, we compare inverted (p-i-n) perovskite solar cells using NiOX as the hole transport layer, with and without a self-assembled monolayer (Me-PACz). SAM-modified devices show improved VOC and efficiency due to reduced interfacial recombination, as confirmed by temperature-dependent electrical characterization. However, long-term outdoor testing reveals that SAM-based minimodules suffer from greater JSC degradation, likely due to SAM deterioration. These findings contribute to a deeper understanding of stability and performance in perovskite devices.Finally, we present the development of MAPbI₃-based memristors, focusing on the influence of buffer layers, perovskite thickness, and electrode materials. Optimized devices demonstrate excellent resistive switching behavior, with high retention (>2×10⁵ s), endurance (30,000 cycles), and ON/OFF ratios (~10⁶), making them promising candidates for non-volatile memory applications.

The rising demand for low-power, on-device intelligence in edge computing is driving the development of neuromorphic systems that combine novel algorithms with emerging materials and device geometries. Memristive technologies, particularly those based on solution-processed mixed halide perovskites, offer promising routes toward energy-efficient, scalable alternatives to traditional CMOS architectures. In this presentation, we showcase two complementary efforts that advance the use of perovskite memristors in neuromorphic computation.

First, we introduce a large, image-based dataset comprising thousands of experimental current–voltage (I–V) curves from printable perovskite memristors. [1] A convolutional neural network (CNN) is trained on this dataset to recognize and classify seven distinct memristive switching behaviors. This machine learning (ML)-based approach not only automates the assessment of device quality—achieving up to 91% accuracy in binary switching performance classification—but also establishes a foundation for predictive modeling of optimal operation conditions.

Building on this, we address the challenge of hardware-efficient multiclass classification by proposing a novel Outcome-Driven One-vs-One (ODOvO) algorithm, implemented using optoelectronic perovskite memristors as synaptic elements.[2] The light modulation of synaptic weights, fed in our algorithm from experimental data, is a key enabling parameter that permits classification without modifying further applied electrical biases. By integrating the algorithmic advantages of One-vs-One and One-vs-Rest schemes, our method reduces synaptic resource requirements by at least 10× (only 196 synapses) and achieves competitive accuracy on benchmark datasets like MNIST—all while significantly lowering power and iteration costs through light-based modulation of synaptic weights. Consequently, our approach constitutes a feasible solution for neural networks where key priorities are the minimum energy consumption i.e., small iterations number, fast execution, and the low hardware requirements allowing experimental verification.

Together, these studies illustrate how the synergistic integration of device-level advances and algorithmic innovation can pave the way toward scalable, low-energy neuromorphic platforms. The approaches presented offer a path toward practical, experimentally verifiable AI systems that meet the demands of next-generation edge IoT intelligence.

The A-site cation in lead-halide perovskite nanocrystals (NCs) plays a pivotal role in fine-tuning their structural and electronic properties. The presently available chemical space remains minimal since, thus far, only three A-site cations, cesium, formamidinium, and methylammonium, have been reported to favor the formation of stable lead-halide perovskite NCs. Inspired by recent reports on bulk single crystals with aziridinium (AZ) as the A-site cation, we present a straightforward colloidal synthesis of AZPbBr₃ NCs with a narrow size distribution and size tunability down to 4 nm, producing quantum dots (QDs) in the regime of strong quantum confinement. NMR and Raman spectroscopies confirm the stabilization of the AZ cations in the locally distorted cubic structure. AZPbBr₃ QDs exhibit bright photoluminescence with quantum efficiencies of up to 80%. Stabilized with cationic and zwitterionic capping ligands, single AZPbBr₃ QDs exhibit stable single-photon emission at both room and cryogenic temperatures, reduced blinking, and high single-photon purity, comparable to the best-reported values for MAPbBr₃ and FAPbBr₃ QDs of the same size.

Beyond compositional engineering, QD shape engineering offers an additional and powerful tool for further fine-tuning and improvement of optical properties, allowing the manipulation of features that are inaccessible by keeping the shape isotropic. In the case of perovskite QDs, shape anisotropy can enable, for example, directional emission, spatial confinement of excitons in one or two dimensions, tuning of exciton fine structure, and radiative decay. To systematically explore shape-dependent properties of one-dimensional CsPbBr3 perovskite structures, we developed a synthetic approach toward stable, size- and shape-uniform nanorods with tunable thickness (5-24 nm) and aspect ratio (1-16, larger for thinner nanorods). By exploiting the difference between {110} and {001} facets of the orthorhombic perovskite structure, we achieved precise control over nanorod morphology. With the use of ligands providing sufficient stability, we performed comprehensive optical characterization, paving the way for advanced optical functionalities in perovskite nanostructures.

For semiconductors, precise tuning of the bandgap holds the key to unlocking technologies. For halide perovskites, such tunability is possible by alloying halides into effective solid solutions. But what are the stability boundaries of such a mixture? In metallurgy, Hume-Rothery rules ascribe the radius ratio of cations to anions as a guiding principle for stability, which also holds for oxide-perovskites. We use lead halide perovskite nanocrystals as a model system to experimentally validate the boundaries of crystal stability and contrast it with the fundamental theories. We base the experiment on the ability to conduct halide exchange post-synthetically, effectively tuning their composition from single-halide into ternary solid solution alloys. The halide composition determines the band-gap, and a spectroscopic readout is used to interrogate the resulting crystals. To map such a vast space of composition and sizes, thousands of exchange reactions were conducted. To execute such vast experimental task a high-throughput robotic anion exchange and spectroscopic process was developed. We mapped the size-dependent behavior across the ternary halide composition of these materials by synthesizing and spectroscopically characterizing over 3000 successful experiments. The resulting maps allow for the first time to point to a stable ternary CsPb(ClₓBr_yI_z)_₃ halide perovskite domain. We showcase the stabilizing effect surface energy has on ternary solid solution compositions, and that smaller nanocrystals demonstrate a smaller miscibility gap. The vision is to extend our understanding of sable perovskite compositions also to bulk, influencing the design of future optoelectronic devices.

Ordered arrays of nanocrystals, called supercrystals, have attracted significant attention owing to their unique collective quantum effects arising from the coupling between neighboring nanocrystals. In particular, lead halide perovskite nanocrystals are widely used because of the unique combination of the optical properties and faceted cubic shape, which enables the formation of highly-ordered supercrystals. The most frequently used method for the fabrication of perovskite supercrystals is based on self-assembly of nanocrystals from solution via slow evaporation of the solvent. However, the supercrystals produced with this technique grow in random positions on the substrate. Moreover, they are mechanically too soft to be easily manipulated with microgrippers, which hinders their use in applications.

This presentation will detail how mechanically robust supercrystals built from cubic lead halide perovskite nanocrystals are synthesized that can easily be relocated over macroscopic distances into positions and substrates of choice. X-ray nanodiffraction provides details about the local structure of the supercrystals, and fluorescence laser scanning microscopy under applied bias reveals the effect of strong electric fields on the (collective) optical properties of the supercrystals.

X-ray imaging technologies play a vital role across a wide range of fields, from materials science and high-energy physics to medical diagnostics and security screening. However, conventional imaging screens are hindered by their rigidity, brittleness, and high cost, making them unsuitable for the growing demand for flexible, eco-friendly, and cost-effective imaging solutions.

In this work, we introduce a systematic study on the synthesis and fabrication of highly efficient and stable zero-dimensional (0D) Mn²⁺-activated organic–inorganic zinc halide systems. By utilizing two carefully selected organic spacers, we engineered a series of 0D luminescent systems exhibiting intense green emission, with photoluminescence quantum yields (PLQY) reaching up to 98%. Comprehensive experimental and theoretical studies were conducted to uncover the mechanisms underlying their optical response and to examine the long-term stability of these 0D systems.

Moreover, the radioluminescence and scintillation performance of the 0D Mn²⁺-activated organic-inorganic zinc halide systems were evaluated to identify the factors influencing their efficiency. Our rational design approach led to an improved light yield of up to 31,000 photons/MeV and an ultralow detection limit of only 112 nGy/s, which is 50 times lower than the dose typically required for standard medical diagnostic procedures. To test their practicality for real-life applications, the scintillators were embedded in flexible PDMS matrices, enabling the capture of high-resolution X-ray images of various objects.

This work presents a promising path toward the development of flexible, scalable, and high-performance Mn²⁺-activated orgnic-inorganic zinc halide screens for next-generation X-ray imaging technologies.

Colloidal nanocrystals (NCs) have inorganic cores and organic or inorganic ligand shells. They are prized for their size- and shape-dependent properties and serve as building blocks of artificial materials and unconventional devices. Here, we describe NC-based, three-dimensional optical metamaterials constructed using imprinting techniques single- and multiple-types of metal, metal oxide, and semiconductor NCs. We focus on the chemical and thermal addressability of NCs, i.e., the ability to select, exchange, strip, or add atoms, ions, and molecules during or post-deposition, that is not accessible in bulk materials, and allows the control of metamaterial structure and properties. Through ligand engineering we tailor the dielectric function of metal NC assemblies through an insulator-to-metal transition.¹ By juxtaposing NC assemblies and bulk thin films to make bilayer heterostructures, we exploit ligand exchange to trigger folding of two- into three-dimensional structures,² which we use to achieve broadband^3,4 and reconfigurable⁵ 3D chiral optical metamaterials and 3D chiral luminescent materials.

Mastering light-matter interaction lies at the heart of quantum technologies – the Purcell effect, for instance, has been instrumental in generating indistinguishable photons with high quantum efficiency [1]. A promising frontier in this field aligns towards the chiroptical regime which may unlock unconventional nonreciprocity and chiral quantum photonics [2,3], while achieving a full control over single-photon chirality remains a major challenge. Here, we demonstrate a highly efficient chiral single-photon source by transducing the strong local optical helicity of the gold helicoid into a single perovskite quantum dot (PQD) that serve as a nanoprobe, in a non-invasive and non-averaged manner. By fine-tuning the near-field coupling and the excitonic and plasmonic spectral resonances, we achieved single-photon chirality as high as 80% and an excitation dissymmetry factor reaching 90%, achieving a full optical chirality (excitation-emission) with a near-unity efficiency. These results are accompanied by rapid radiative decay (~300 ps) and a Purcell enhancement factor of approximately 3. Electromagnetic simulations reveal that chiral field enhancement arises from circulating surface currents unique to the helicoid nanogeometry. Furthermore, we present a prototype of a switchable circular dichroism probe that leverages excitation-wavelength-dependent enantiospecificity. Our results set a new benchmark in solid-state chiral quantum light sources and offer a versatile platform for next-generation on-chip quantum photonics and nanoscale chiroptical sensing.

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 fabricate photonic structures out of 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 2D arrays, exhibiting strong circular dichroism and intense circularly polarized photoluminescence (CPL) combined with a wide range of emitters, making them seamlessly applicable in various practical applications.^1,2

1.             Qi, X. et al. Chiral plasmonic superlattices from template-assisted assembly of achiral nanoparticles. Nat. Commun. 16, 1687 (2025).

2.             Mendoza-Carreño, J. et al. A single nanophotonic platform for producing circularly polarized white light from non-chiral emitters. Nat. Commun. 15, 10443 (2024).

The design of dendrimers and pro-mesogenic ligands enables new strategies to create nanocrystals (NC) liquid crystal hybrids. The use of these supramolecular ligands systems can direct the assembly of nanocrystals (NCs) is to 3 dimensionally ordered superlattices allowing multicomponent and the multiscale organization of nanocrystals (NCs) with controlled composition, size, and shape. These NCs, acting as 'artificial atoms' with tunable electronic, optical, and magnetic properties, pave the way for the development of a new periodic nanomaterials. The resulting superlattices are ideal building blocks for incorporation into new thin films, and integrated devices. The scalability of this process ensures its feasibility for large-scale applications. It is possible to control the formation and phase transformations in deposited NC superlattices by adjusting the thermal energy of a nanocrystal dispersion. These structural changes can allow the solids to anneal an improve their organization post-deposition. The modular assembly of these NCs allows the desirable features of the underlying quantum phenomena to be captured stimuli-responsive thin films.

Topochemical reactions are chemical reactions of solids that enable transforming their chemical compositions and crystal structures, with short movements of the constitutive atoms. When applied to nano-objects, these approaches enable to reach complex compositions, heterostructures and shapes.[1] Topochemical reactions of nano-objects however rely on colloidal syntheses below 300 °C in usual solvents, which limits their use to ionic or metallic objects. Post-modifications of more covalent materials would pave the way to specific properties, related to (electro)catalysis, magnetism, and hardness, for instance, but they require higher temperatures.[2-4] In this work, we will show how to trigger and control topochemical reactions of covalent nanoparticles by using liquids stable at relatively high temperatures, thereby giving access to new compositions. We will focus on molten salts as thermally stable liquids, and focus on topochemical reactions involving both cation exchange and galvanic replacement.

Cation exchange in indium pnictide nanocrystals dispersed in molten KGaI₄ and KAlI₄ has been very recently unveiled to tune the optical properties of these III-V nanocrystals.[1,2] With the aim of targeting more covalent materials and possibly modulating (electro)catalytic properties versus CO₂ and CO electroreduction (CO₂R and COR), we have focused on cation exchange on CuSi₂P_3. This compound crystallizes in a distorted zinc blende structure composed of corner-sharing [CuP₄] and [SiP₄] tetrahedra.[5] The material encompasses Cu⁺ cations in a covalent [Si₂P₃]^- framework. It shows high performances as electrocatalyst for COR into acetate. We hypothetize that partial exchange of Cu⁺ and Si by Zn²⁺ and Ga^3+/2+/+ would enable adjusting the catalytic selectivity and activity, also probably impacting optoelectronic properties. The existence of zinc blende-related Zn_1.5Si_1.5P₃, and Ga_1.5Si_1.5P₃ [6,8] with [MP₄] (M=Zn or Ga) and [SiP₄] tetrahedra hints at the possibility to exchange Cu and Si in CuSi₂P₃ nanoparticles.

In this talk, we will demonstrate successful topochemical reactions of covalent silicophosphide nanoparticles in molten salts.  By using synchrotron radiation-based in situ X-ray diffraction, during the reactions in molten salts, we unveiled minute scale transformations of CuSi₂P₃ into Zn_1.5Si_1.5P₃ or into a bimetallic copper zinc silico-phosphide (Cu_0.9Zn_0.2Si_2.0 P_2.4) by reaction with Zn²⁺ precursors. The selectivity of the reaction can be tune by adjusting the amount of Zn reagent. On the other hand, we succeeded to react CuSi₂P₃ nanoparticles with different gallium precursors (GaCl₃, GaI₃ and Ga₂I₃ [2]) to yield bimetallic copper gallium silicophosphides Cu_0.8Ga_0.2Si_2.0P_2.3, Cu_0.6Ga_0.4Si_2.0P_1.9 and Cu_0.7Ga_0.4Si_2.0P_2.2 via a similar process. Partial substitution of Cu by Zn or Ga into the silicophosphide is accompanied by a phase transition from a cubic structure to a tetragonal one. TEM confirmed that the morphology and size of the particles did not evolve significantly, while TEM-EDS mapping confirmed the homogenous distribution of the elements at the particle scale. EXAFS at the Cu-K edge showed the preservation of [CuP₄] tetrahedra, while EXAFS at the Ga-K and Zn-K edges indicated the formation of [ZnP₄] and [GaP₄] tetrahedra, as expected for an exchange reaction, which was confirmed by solid-state ³¹P, ⁷¹Ga and ²⁹Si NMR. XANES demonstrated that cationic exchange with Zn or Ga is accompanied by galvanic reactivity, initial Cu⁺ gets oxidized to an average oxidation state Cu^1.5+ meanwhile Ga and Zn species are reduced during the topochemical reaction.The evaluation of the electrocatalytic properties of these new bimetallic silicophosphides for CO reduction is under way.

In the current context of escalating climate change and all of its related problems, innovative solutions are needed through the whole range of energy related technologies. In its specific field, smart windows devices stand out for their ability to reduce energy consumption and CO₂ emissions by controlling dynamically the light and heat transmittance in buildings. In this context, nanostructured TiO₂ crystals (NCs) emerge as a versatile platform because of its excellent energy conversion and electrochromic properties.

In our group, one of the research lines exploits the aliovalent doping of the nanostructured TiO₂ crystals to improve their electrical conductivity, specific capacitance and their ability to modulate optical transmittance further and in a more selective manner: in previous works V- and Nb-doped TiO₂ nanocrystals have already been successfully synthetized and deeply characterized to unveil the role of the dopants^1,2.

In my contribution, I will present the results coming from the synthesis and in situ characterization of the TiO₂ nanocrystals doped with W: the amount of doping (10%) is based on previous studies where the optimization of the localized surface plasmon resonance (LSPR) properties was addressed3. The successful synthesis was proved by Raman microscopy and XRD, which confirmed the presence of the anatase phase and effective W-doping. An UV-VIS-NIR absorption spectrum showed that the LSPR absorption mechanism is present in NIR region for these NCs. NCs thin films were prepared by doctor blading an organic viscous and then calcinating it. The morphology of the films is characterized by FESEM and also TEM images were taken, from which is clear that the NCs have an average diameter of 5nm.

The electrochemical properties were tested through a set of cyclic voltammetries at different scan rates from which it is evident a change in the current shape of the redox pair and an increase in the charge capacitance of the W doped TiO₂ NCs, resulting in an improvement of the electrochemical behavior.

The spectroelectrochemical behavior was tested through UV-VIS absorption measurements at different applied voltages: the LSPR and plasmonic light absorbing mechanisms characteristic of this W doping are revealed, resulting in a partial filtration of the light spectrum. This suggests its potential in smart windows applications.

Finally, a deep in situ analysis comprehending x-ray absorption spectroscopy (XAS), x-ray photoemission spectroscopy (XPS) spectro-electrochemistry (SEC) electrochemical impedance spectroscopy (EIS) techniques have been used to characterize the real-time electronic and structural changes during operation allowing to give insights into the electrochemical reaction mechanisms.

Magic-sized clusters (MSC) are identical CdS 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. In this talk I will highlight some remarkable behavior we have recently found in their ability to isomerize and their ability to self-organize into hierarchical assemblies with optical activity. We have found that MSCs have the ability to undergo a chemically-induced, reversible isomeric transformation between discrete states. The diffusionless reconfiguration of the inorganic core follows a first order kinetic rate driven by a distortion of the ligand binding motifs. Additionally, these MSCs display a surprising ability to organize into films with hierarchical assembly that spans over seven orders of magnitude in length scale. Meniscus-guided evaporative assembly results in large-scale homochiral domains with anisotropy values (g-factors) near 1.1 and areas surpassing 6 mm². These g-factors are among the highest reported for all semiconductor particles. Through Mueller matrix polarimetry spatial mapping we unravel the mechanism behind the formation of the self-organized chiral domains. Beyond optical properties, the multiscale self-organization behavior of these MSCs displays similarities to biosystems, providing a new platform for the design and study of materials.

The precision of nanocrystal shapes is crucial to tailor the functionalities of nanomaterials. Traditional molecular dynamic simulations are computationally too expensive to unveil the multiscale nature of nanocrystal synthesis from the potential energy of an atom to the mesoscales of a nanocrystal composed of tens of millions of atoms. To overcome this issue, we implement rejection-free kinetic Monte Carlo simulations using the semi-Gibbs ensemble sampling solid-to-liquid energy variations to grow and dissolve atoms at the nanocrystal surface. This allows the simulation of realistically sized nanocrystals coupled with the energetics of atoms. We discuss the growth of symmetry-preserving shapes, such as cubes, octahedra, rhombic dodecahedra, and their truncations. We show the importance of surface site kinetics associated with adatom nucleation on facets of different crystallographic directions, leading to the entrapment of nanocrystal shapes in metastable equilibrium. We then discuss the spontaneous symmetry breaking of shapes due to the dynamics of surface defects. The multiscale simulations reproduce the emergence of nanocrystal shapes inaccessible to other computational tools.

Plasmonic hot carriers have garnered considerable attention in photovoltaics and photocatalysis, yet their full potential is limited by the challenge of harvesting both positive and negative polarity hot carriers at the same time. Here, we demonstrate an unprecedented plasmonic hot carrier device capable of extracting both types of hot carriers simultaneously. This scheme involves generating and harnessing plasmonic hot electrons and holes concurrently using a lateral Si p-n junction diode coupled to Ag nanoprisms. Our experimental and numerical results jointly reveal precise control of the generation and injection of plasmonic hot carriers, stemming from differing injection probabilities of each type of hot carriers into the substrates. We show that the bipolar plasmonic photodetector exhibits outstanding performance compared to plasmonic devices utilizing single-polarity hot carriers, attributed to the simultaneous participation of plasmonic hot carriers in the photoconductivity nature of the diode. We believe this strategy of harnessing bipolar hot carriers will pave the way for the rational design of future plasmonic applications by providing significantly improved photoconductivity and flexible utilization of hot carriers.

Nanocrystal-based solids represent a versatile class of materials whose collective properties can be finely adjusted by tuning parameters such as shape, size, chemical composition, and surface ligands. These materials are particularly relevant for advancing plasmonic, optoelectronic, and thermoelectric technologies. Controlling thermal transport within such systems is crucial, as local heating—whether induced by optical absorption or electrical current—can impair performance, cause instability, or trigger undesirable reactions. In this contribution, I will discuss recent findings on the heat transport properties of superlattices composed of gold nanospheres, nanorods, and nano-bipyramids. Using correlative scanning electron microscopy and spatio-temporally resolved thermoreflectance techniques, we accessed thermal dynamics with nanosecond resolution and sub-micron spatial detail. In polymer-ligand-capped gold nanosphere assemblies, we observed that monolayer configurations exhibit faster thermal diffusion compared to multilayers. Monte Carlo simulations incorporating quasi-ballistic phonon transport suggest this behavior arises from the interplay between extended phonon mean free paths and ligand interdigitation. In assemblies of gold nanorods and bipyramids, our results show that heat preferentially propagates along the nanoparticles’ longitudinal axis, maintaining directional flow even in bent or curved configurations. In ordered superlattices, this results in pronounced anisotropic heat conduction, with higher diffusivity along the particles' elongation. Finite element analysis and effective medium theory confirm that this directional transport can be tuned by modulating particle shape, aspect ratio, and packing geometry. Harnessing such anisotropy offers new strategies for improving heat dissipation and directing thermal flow within functional devices, all while preserving tunable optical and electronic properties.

The synthesis and characterization of thin-film plasmonic supercrystals of gold nanoparticles will be discussed.[1,2] The dense packing of the nanoparticles in the supercrystals leads to emergent optical properties due to extreme light-matter interactions.[3] The resulting enhanced near-fields in the structures can be exploited for surface-enhanced spectroscopies but also for plasmonic photocatalysis. [4,5] Towards such applications it is, in turn, also important to understand the thermal properties of the materials on relevant timescales.[6,7] The correlation of electron microscopy, spectroscopy and small-angle X-ray-scattering helps to understand how nanoparticle surface chemistry affects structure formation and how structure dictates the emerging properties. This is in particular interesting for anisotropic nanoparticles where the interplay of shape anisotropy and ligand properties leads to interesting new structures and near-field distributions which can be related to polarization-dependent optical properties.[8] With binary mixtures even more complex geometries can be obtained, providing plenty of room to explore for these polaritonic materials.