Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies of single-junction devices now exceeding 26%. However, degradation, defective interfaces with charge extraction layers, the low hurdle for ionic migration, and the structural flexibility of the perovskite structure still pose both opportunities and challenges to their commercialization in light-harvesting applications. Combinatorial characterization approaches are vital for probing and analysing such instabilities.

We demonstrate a combined modelling and experimental approach[1] towards exploring the effects of energy-level alignment at the interface between wide-bandgap mixed-halide perovskites and charge-extraction layers, unravelling separate loss factors and highlighting avenues for improving open-circuit voltage.

We further highlight the potential of optical-pump THz-probe spectroscopy following controlled intervals of air exposure as an ideal technique to monitor air-induced degradation of optoelectronic parameters such as charge-carrier mobilities and recombination rates in low-bandgap lead-tin iodide perovskites.[2][3] We explore the best choice of A-cation in lead-tin iodide perovskites with intermediate lead-tin ratios and find that FASn0.5Pb0.5I3 emerges as the most promising contender.

In addition, we utilize a combination of ultra-low frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination[4,5] of the ultra-low frequency vibrational response for a wide range of metal-halide semiconductors, showing that such effects results from high anharmonicity of specific Raman-active modes.

We further probe the charge-carrier transport in layered, two-dimensional (2D) metal halide perovskites that have been found to improve the stability of metal halide perovskite thin films and devices. We demonstrate unexpectedly high densities of sustained populations of free charge carriers, surpassing the Saha equation predictions,[6] and demonstrate a high degree of transport anisotropy in highly oriented thin films.[7]

Our recent study on charge generation and recombination in bulk-heterojunction and planar-mixed heterojunction blends comprising a crystalline polymer donor with Se-containing Y6-derived non-fullerene acceptors has shown both high photovoltaic internal quantum efficiencies and high external electroluminescence quantum efficiencies. Crystallographic and spectroscopic studies reveal that the pseudo-2D, fused-ring molecular acceptors are not only intrinsically highly luminescent but also meets the criteria in achieving intrinsically radiative recombination within the blend by promoting delocalized excitons with much longer luminescent lifetime and reduced exciton binding energies.

These results provide the important demonstration of efficient OPV blends to achieve PCEs over 20%. Regarding the development of perovskite solar cells (PSCs), several novel interface and additive engineering approaches have been employed to enable PSCs to show very high PCE (26.9%) and stability in the inverted architecture devices. Moreover, new multifunctional redox mediators have also been developed to overcome the halide segregation issues that strongly hinder the development of highly efficient and stable large-bandgap PSCs. The resulting devices showed very low photovoltage loss and high PCE >20%. Their integration with OPV to form 2-T tandem devices has shown record-high PCE of 26.2% with good operational stability.

“Redox Mediator Stabilized Wide Bandgap Perovskites for Monolithic Perovskite-Organic Tandem Solar Cells”, S. Wu, Y. Yan, J. Yin, K. Jiang, F. Li, Z. Zeng, S. Tsang, and A. K-Y. Jen, Nature Energy 2024, 9, 411.

“Advances in inverted perovskite solar cells”, X. Zhang, S. Wu, H. Zhang, A. K.-Y. Jen, Y. Zhan & J. Chu, Nature Photonics, 2024, 18, 1243.

“Molecularly tailorable metal oxide clusters ensured robust interfacial connection in inverted perovskite solar cells”, F. Li, C. Zhao, Y. Li, Z. Zhang, X. Huang, Y. Zhang, J. Fang, T. Bian, Z. Zeng, A. K.-Y. Jen, Sci. Adv., 2024, 10, eadq1150.

“Hydrogen Bond-Bridged Intermediate for Perovskite Solar Cells with Enhanced Efficiency and Stability”, F. Li, X. Deng, Z. Shi, S. Wu, Z. Zeng, D. Wang, Y. Li, F. Qi, Z. Zhang, Z. Yang, S-H. Jang, F. R. Lin, S-W. Tsang, X. K. Chen, and A. K.-Y. Jen, Nature Photonics, 2023, 17, 478.

“Advances and Challenges in Understanding the Microscopic Structure–Property–Performance Relationship in Perovskite Solar Cells”, Y. Zhou, L. Herz, A. K-Y. Jen, and M. Saliba, Nature Energy, 2022, 7, 794.

“Dilution Effect for Highly Efficient Multiple-Component Organic Solar Cells”, L. Zuo, S. B. Jo, Y. K. Li, Y. Meng, R. J Stoddard, Y. Liu, F. Lin, F. Liu, D. S. Ginger, H-Z. Chen, A. K-Y. Jen, Nature Nanotech, 2022, 17, 53.

“Planar-Mixed Heterojunction Organic Photovoltaic Suppresses Recombination Loss”, K. Jiang, J, Zhang, C. Zhong, F. Lin, F. Qi, Q. Li, Z. Peng, W. Kaminsky, S. H. Jang, J. Yu, X. Deng, H. Hu, D. Shen, F. Gao, H. Ade, M. Xiao, C. Zhang, and A. K-Y. Jen, Nature Energy, 2022, 7, 1076.

“2D Metal–Organic Framework for Stable Perovskite Solar Cells with Minimized Lead Leakage”, Shengfan Wu, Zhen Li, Mu-Qing Li, Yingxue Diao, Francis Lin, Jie Zhang, Peter Tieu, Wenpei Gao, Feng Qi, Xiaoqing Pan, Zhengtao Xu, Zonglong Zhu, Alex K.-Y. Jen, Nature Nanotech, 2020, 15, 934.

Metal-halide perovskites (MHPs) have emerged as a transformative class of materials in optoelectronics, celebrated for their remarkable properties and flexible fabrication methods. These materials are central to the development of high-performance solar cells, advanced optoelectronic devices, and cutting-edge quantum emitters. Among the various fabrication techniques, thermal evaporation is particularly advantageous, providing exceptional control over film thickness, stress-free deposition, composition fine-tuning, surface modification, and scalability—qualities that are essential for producing large-area devices and enabling sophisticated device customization【1, 2】.

In my recent work, I have focused on overcoming the scalability and reproducibility challenges in perovskite solar cell (PSC) manufacturing, which are essential for their commercialization. A major achievement has been the sixfold acceleration of deposition rates through the optimization of the co-evaporation process【3】. This method allows for the production of high-quality films with outstanding power conversion efficiency (PCE), eliminating the need for post-annealing treatments, and simplifying the manufacturing process, thereby improving cost-efficiency for large-scale PSC production.

Furthermore, our research has explored the use of thermally evaporated perovskite-based Multiple Quantum Wells (MQWs), which utilize quantum mechanical effects to enhance optoelectronic properties【4】. We have demonstrated that MAPbI3-based MQWs offer significant improvements in photoluminescence, charge separation, and near-infrared photodetection【5】. These findings are pioneering in advancing light-emitting devices and photodetectors, contributing to the next generation of optoelectronic technologies.

This work not only addresses key obstacles in scaling PSC production but also highlights the versatility and transformative potential of thermal evaporation in rethinking device design. These advancements position thermally evaporated perovskites as a crucial technology for the future of sustainable energy and next-generation optoelectronic applications.

References

Min, H., et al., Nature, 2021, 598, 444.; Yoo, J.J., et al., Nature, 2021, 590, 587.

J. Li et al., Joule, 2020, 4, 1035; H.A. Dewi et al., Adv. Funct. Mater., 2021, 11, 2100557; J. Li et al., Adv. Funct. Mater., 2021, 11, 2103252.

H.A. Dewi et al., ACS Energy Lett., 2024, 9, 4319−4322.

Advanced Materials, 2021, 33, 2005166; L. White et al., ACS Energy Lett., 2024, 9, 835–842.

L. White et al., ACS Energy Lett., 2024, 9, 4450–4458.

Perovskite-based multi-junction solar cells allow to overcome efficiency limits of single-junction cells by reducing thermalization losses. Even though, impressive progress has been achieved by perovskite/silicon and all-perovskite tandem devices, significant challenges persist, including the substantial carbon emissions due to energy-intensive silicon wafer production or fundamental stability concerns due to the oxidation of Sn²⁺ to Sn⁴⁺ in narrow-gap perovskites. On the contrary, organic solar cells (OSCs) based on narrow-gap non-fullerene acceptors (NFA) present an attractive alternative as rear cells in perovskite-based tandem devices. In our previous work, the benchmark PM6:Y6:PC₆₁BM ternary OSCs maintained approximately 95% of its efficiency after 5000 hours of continuous operation under irradiation with low-energy photons (l = 850 nm), but some notable degradation was found when illuminated with a white light-emitting diode (LED), indicating that photons in the visible spectral region infer device degradation.^[1] In a perovskite-organic tandem solar cell, the wide bandgap perovskite sub-cell serves as a low-pass filter that protects the organic sub-cell against high-energy photons.^[2] While the photostability of the perovskite-organic tandem devices is still limited by the photostability of the wide gap perovskite, for the narrow-gap sub-cell, NFA based organic solar cells might be a better choice compared to narrow-gap Pb-Sn perovskite solar cells. 

In this work we generalize our study to include a wider range of Y-type acceptors (Y18 (E_g = 1.31 eV), CH1007 (E_g = 1.30 eV), mBzS-4F (E_g = 1.25 eV)), that we identified to show great promise in our perovskite/organic tandem solar cells. Most importantly, we could evidence that the remarkable photostability found for the Y6 NFA is generally valid for the entire Y-family. Using monochromatic light sources covering the ultraviolet, visible, and near-infrared spectral regions we are able to identify in detail the influence of photon energy on device stability. By varying the device architecture (e.g. hole extraction layer either MoO3 or PEDOT-F) we are able to identify the impact of the photoactive organic absorber in the degradation. Under continuous operation in the maximum-power point under irradiation with low-energy photons (λ > 590 nm), the devices show excellent long-term stability (well above 1000 hours), while higher-energy photons (λ < 530 nm) infer increasingly severe degradation. Combining this wavelength-selective degradation studies with in-situ photoluminescence, Raman spectroscopy, photoluminescence quantum yield investigations, transient absorption, and suns-Voc device analytics, we systematically investigate the degradation pathway.

The need for renewable energy is increasing due to various reasons, such as climate change and reducing environmental pollution. As for solar cells, one of the representative types of renewable energy, silicon-type solar cells are currently the mainstream, but they have already achieved an efficiency close to theoretical efficiency and have reached a technical limit in which efficiency is hard to be increased any further. Perovskite solar cells, which have been drawing attention as next-generation solar cells, have the advantage of having an absolute efficiency value of about 15% higher than that of silicon solar cells. Perovskite/Si tandem solar cells have achieved an efficiency of 28.6%[1] in M10-sized (330.56 cm²) perovskite/Si tandem solar cells, and major solar cell manufacturers in China and around the world are accelerating the commercialization of perovskite solar cells.

The basic structure of a perovskite solar cell generally consists of a perovskite active layer, charge transporting layers, and a transparent conducting layer. Recent research suggests various approaches to improve the performance of perovskite solar cells as well as the reliability of the devices. Compared to silicon solar cells, the materials that make up perovskite solar cells have the advantage of having a high degree of freedom in material development and that they can be developed at low cost. Due to these advantages, the speed of development of perovskite solar cells was able to accelerate, and research and development to synthesize and apply new materials is currently being actively conducted at various research institutes and industries.

In this presentation, I will introduce Hanwha QCells’ business and R&D activities and present prospects for future research and commercialization.

Our research group focuses on the development of functional polymer materials and hybrid perovskite materials for various kinds of optoelectronic devices, including thin-film transistor (TFT), (photo-)memory, light-emitting diode (LED), and solar cell devices. We are particularly interested in exploring the structure-performance relationships of polymers and hybrid perovskites. In addition to advances in the controlled synthesis of organic semiconductors, we also explore innovative interfacial and device engineering to optimize the device performance. In this presentation, we will highlight our works in the past few years on interface engineering for organic, perovskite and perovskite/organic tandem solar cells. An integrated study of combining material synthesis, interface engineering, and morphology analyses will be introduced and discussed to explore the full promise of the devices, with a particular focus on long-term device stability (see Figure 1).[1-3] We will start with our recent interface engineering works on organic solar cells and extend to single-junction perovksite solar cells and perovksite-organic tandem cells.

Semitransparent perovskite solar cells (ST-PSCs) are critical components for tandem applications, where their integration with other photovoltaic technologies, resulting in perovskite/silicon, all-perovskite, or perovskite/CIGS tandem devices, can overcome the single junction limit.[1, 2] A key challenge in fabricating ST-PSCs is protecting the underlying layers from damage caused by the sputtering process used to deposit transparent electrodes. This protection has conventionally been achieved using buffer layers deposited via atomic layer deposition (ALD).[3, 4] However, ALD requires specialized equipment not available in all laboratories and represents an expensive, time-consuming process, limiting its widespread adoption.

In this study, we present a cost-effective, universally applicable solution-processed buffer layer based on metal-oxide nanoparticles. To identify the optimal buffer layer, we systematically evaluated a variety of metal-oxide nanoparticles for their ability to mitigate sputtering-induced damage. Photoluminescence (PL) quenching measurements were employed to assess damage levels, revealing that certain materials exhibited higher defect densities after the sputtering process. In addition, absorbance measurements provided insights into the UV shielding performance of these materials during sputtering. Analysis of their crystal structures further revealed either preferential crystal growth, which enhanced charge transport, or amorphous structures, depending on the nanoparticle type. These combined insights enabled the selection of an optimal solution-processed buffer layer with superior protective and functional properties.

We demonstrate the efficacy of this buffer layer in monolithic tandem device configurations, including perovskite-CIGS tandems, all-perovskite tandems, and perovskite/silicon tandems. For the latter, we integrate an optimized ST-PSC with a polished front-side/unpolished rear-side p-type silicon heterojunction (SHJ) solar cell. The intrinsic roughness of the unpolished rear-side significantly enhances light absorption, eliminating the need for a dedicated texturization step. This innovation enables a final tandem efficiency of 25.3%, illustrating the potential of combining solution-processed buffer layers with unpolished silicon wafers for higher photocurrent generation.

Our findings highlight the scalability and versatility of solution-processed buffer layers as a practical solution for perovskite-based tandem solar cells. The universal nature of this buffer layer allows it to be employed in various tandem configurations, including perovskite-CIGS and all-perovskite tandem solar cells, broadening its applicability and impact across tandem technologies.

References:

[1]    W. Shockley and H. J. Queisser, J. Appl. Phys. (1961), vol. 32, no. 3, pp. 510–519.

[2]    A. De Vos, J. Phys. D. Appl. Phys. (1980), vol. 13, no. 5, pp. 839–846.

[3]    S. Mariotti et al, Science (2023)., vol. 381, no. 6653, pp. 63–69.

[4]    E. Aydin et al., Nature (2023), vol. 623, no. 7988, pp. 732–738.

All-perovskite tandem solar cells (APTSCs) have the potential to exceed the detailed balance limit of single‑junction solar cells, and, being a thin-film technology, promise a cost-effective large-scale production. APTSCs combine a wide bandgap (WBG) top cell (E_g ≈ 1.80 eV) with a narrow bandgap (NBG) perovskite bottom cell (E_g ≈ 1.25 eV). In the past years, the power conversion efficiency (PCE) of 2-terminal (2T) APTSCs increased steadily, now surpassing 30% [1]. In this contribution, we will present our recent results on APTSCs, focusing on both, the reduction of electrical and optical losses.

To increase the current density of APTSCs, we apply different strategies to minimize optical losses. The optimization of TCOs allows for the tuning of layer thicknesses to spectrally shift the interferences. Moreover, we implement nanotextures into our APTSCs (Figure 1a) and make use of the reduced parasitic absorption in the NBG’s hole-selective layer. These measures allow to increase the photogenerated current density by more than 1 mA/cm² under current-matching conditions.

For the improvement of the optoelectronic properties of APTSC, we studied the suppression of non-radiative recombination losses through surface treatments at the interface between perovskite and electron transport layer (ETL) in both, WBG and NBG subcell. We found that a treatment with piperazinium iodide (PI) virtually eliminates non-radiative recombination losses at this interface (Figure 1b), enabling V_OCs up to 1.36 V [2]. Further efforts address the replacement of the standard hole-transport layer (HTL) PEDOT:PSS, by self-assembling monolayer (SAM) materials in the NBG perovskite cell [3]. We find that the pseudo-halide additive lead-thiocyanate (Pb(SCN)₂) has a significant influence on the performance for both, PEDOT:PSS- and SAM-based NBG perovskite solar cells. While additive engineering with SCN‑compounds is highly important for PEDOT:PSS-based NBG perovskite solar cells, our study demonstrates that it strongly limits the performance of SAM-based devices. In a next step, to further improve the performance of NBG subcells, we improve the binding of SAMs to the substrate, which yields V_OCs >0.85 V and thereby surpasses the photovoltaic performance of PEDOT:PSS-based devices (Figure 1c).

Finally, the combined efforts described above enable APTSCs with a champion PCE of 27.5% (Figure 1d). Our contribution will also address pathways to improve the optical and electronic quality of APTSC and to enhance their PCE beyond 30%.

Halide perovskites have rapidly become the focal point of research in the development of innovative materials for cost-effective and highly efficient photovoltaic technologies. The journey began with the groundbreaking demonstration by Prof. Miyasaka and his team in 2007, which sparked a surge of exploration into perovskite-based solar cells. Since then, numerous advancements have been made, with researchers achieving certified power conversion efficiencies that are continuously approaching the theoretical maximum. Recent studies have shown that these perovskites not only rival but often exceed the performance of traditional photovoltaic materials, positioning themselves as game-changers in the solar energy landscape.
However, the most durable and efficient perovskite materials currently known contain lead, a substance recognized as one of the most toxic elements on the planet. In response to environmental concerns, researchers have shifted focus towards lead-free alternatives, particularly tin-based perovskites. Progress in enhancing the power conversion efficiency of these tin-based options has been promising. Nevertheless, a critical aspect that requires further investigation is the long-term stability of tin-based perovskite solar cells, which remains largely unexplored. In this presentation, we will delve into the factors affecting the stability of these innovative lead-free perovskite solar cells and assess their potential for widespread application in the future.

The world record efficiency of the Pb-free Sn-perovskite solar cells in the paper is 15.7%. We now report 15.3% efficiency. According to the calculation, in the grain boundary of Sn-perovskite layer, p-type defects such as Sn vacancy are apt to form. It is shown that Lewis base passivation, such as S atm, N atm, O atom, and so on, is effective for decreasing the defect density1). We proved the calculation results experimentally by using ethylene diamine, PCBM, and 5-mercapto-1-methyltetrazole tin salt, giving 15.3% efficiency 2,3,4). These additives made the energy band alignment optimized and make the crystal size larger, in addition to lower the defect concentration. The durability of the Sn based perovskite is another research item. We improved the thermal stability at 85 ℃ by suppressing ion migration by introducing ion-migration blocking layer. We analyzed these items which cause thermal degradation. The thermal stability of SnPb alloyed perovskite solar cells was also improved in the same way5). Finaly, we show the direction to improve these efficiencies and stabilities.

Sn-based perovskites have a large processing window, rapid crystallization, and variable bandgap. This makes them an ideal material to function as the low-bandgap cell for roll-to-roll flexible all perovskite tandem foils. However, Sn-based halide perovskites struggle with rapid oxidation of Sn²⁺ to Sn⁴⁺. The formation of Sn⁴⁺ leads to rapid and severe degradation of the semiconductor quality. To produce scalable and affordable perovskites, roll-to-roll (R2R) processing is the most promising production method. For R2R processing, ambient conditions are preferable in terms of scalability and cost. However, in order to perform roll-to-roll processing in ambient conditions, a strategy to prevent the oxidation of Sn during deposition is necessary, as ambient air would oxidize conventional perovskite precursors. Roll-to-roll processing also requires the use of non-toxic solvents, excluding DMF as a possible solvent. Therefore, there is a need for air and water stable perovskite precursors.

Here we use Sn(II)EDTA as a water- and air-stable perovskite precursor. Chelating agent EDTA prevents Sn²⁺ oxidation and is stable in both water and air. We show vapor conversion to pure phase Sn(II) perovskite in a simple one-step procedure, using water as solvent. The resulting perovskite has strong PL and a tunable bandgap via compositional variations, incorporating MA/FA. Additionally, mixed Pb/Sn perovskites can be formed by using Pb(II)EDTA.

The main objective of this study is to develop a sustainable approach for reviving perovskite solar cells (PSCs) to achieve the highest-value product, not merely the recovery of elements. PSCs have emerged as an innovative technology in photovoltaics, offering high efficiency and cost-effective fabrication. Despite their potential, widespread adoption is hindered by challenges related to instability and the lack of effective recycling strategies [1]. Therefore, as PSC technology approaches commercialisation, scalable and environmentally responsible recycling methods are essential to ensure long-term viability and minimize environmental impact.

In this contribution, we introduce a green solution-based recycling methodology specifically designed to revive PSCs with carbon electrodes (C-PSCs), yielding the highest value product while preserving the device architecture. C-PSCs stand out for their superior stability and scalable fabrication [2]. Furthermore, their fully porous structure and fabrication method (infiltrating the perovskite in the porous scaffold) enable in-situ reloading or regenerating the perovskite material. According to our holistic assessment of different recycling opportunities in PSCs [3], revival through re-infiltration of fresh perovskite material into the porous scaffold provides the highest value product.

The degradation of C-PSCs is primarily attributed to the perovskite material [4], which can be selectively removed and replaced while maintaining the integrity of the porous scaffold. We developed a solution-based recycling technique utilising a non-toxic solvent, γ-Valerolactone (GVL), which has demonstrated a competitive performance in PSC fabrication [5]. GVL provides a green alternative to toxic solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), making it a promising candidate for sustainable PSC recycling.

Currently, the efficiencies of the revived C-PSCs showed an average revival rate of 65% when compared to the fresh devices, and we are now working on different strategies to significantly enhance the rate. Our findings highlight the importance of scaffold stability during the recycling process. For example, optimized C-PSC structures incorporating zirconia nanoparticles exhibit enhanced mechanical robustness, ensuring durability during solvent exposure in the washing and re-infiltration steps. It is hypothesized that zirconia nanoparticles enhance cohesion among carbon particles through sintering, providing robust structural integrity to withstand solvent interactions. The solution-based revival method demonstrates a sustainable pathway for recycling PSCs to achieve maximum value product. By addressing key challenges in PSC sustainability, this work represents a step toward scalable and eco-friendly revival techniques promoting circular economy in photovoltaics.

Perovskite-based photovoltaics (PV) are increasingly recognized as a pivotal technology in the pursuit of climate neutrality by 2050. Their customizable optoelectronic characteristics, combined with straightforward, low-cost manufacturing methods, make them ideal candidates for a wide array of applications. These include tandem solar cells, space applications that leverage their favourable power-to-weight ratio, and even harvesting ambient indoor light. [1] While significant attention is directed toward optimizing their efficiency, the critical issue of environmental sustainability is often ignored. For perovskite-based PV to achieve large-scale commercial deployment, it is crucial that sustainability becomes a central aspect of its development.

Over the last decade, perovskite solar cells (PSCs) have undergone a remarkable increase in efficiency, recently reaching a 26.1% power conversion efficiency (PCE), rivalling traditional silicon-based systems. [2] Despite this progress, comprehensive strategies to mitigate the environmental impact of PSCs are still lacking. High-performance devices are typically produced with solvents that are toxic and potentially harmful to both the environment and the large-scale manufacturing processes, such as Roll-to-Roll production. [3] Additionally, the reliance on scarce raw materials, and the high cost of organic hole transport layers (HTLs) – due to their complex and inefficient synthesis methods – limits the scalability of this technology. [4, 5]

To address the gap between performance and sustainability, we developed a range of affordable, novel HTLs, founded from the well-known poly(triarylamine) (PTAA) [6] that can be processed with more environmentally benign solvents. This was accomplished by modifying the polymer backbone with a phenothiazine scaffold, improving its solubility in common organic solvents. [7] The impact of methyl substitutions on the TPA phenyl group was also evaluated, exploring the trade-offs between solubility and overall device performance. Additionally, a benzothiadiazole unit was considered, given its promising role in organic semiconductors. [8]

These modified polymers demonstrated excellent solubility in tetrahydrofuran (THF), a cost-effective, non-aromatic, halogen-free solvent that is environmentally friendly and poses low toxicity risks. [9] A full set of structural, optoelectronic, and thermal analyses of the resulting polymers (P1-4) confirmed their viability as HTLs. These materials were incorporated into flexible n-i-p devices, using PTAA as a benchmark. Among the polymers, P1 exhibited competitive efficiency when PTAA is processed with conventional toluene, and even surpassing the latter when processed with THF. Additionally, P1 showed notable improvements in light soaking stability in unencapsulated devices compared to PTAA. A solid-state film study revealed that the structural modifications were key to improving device performance.

To further optimize device performance, we used a multivariate analysis approach (Design of Experiment) to fine-tune the balance of HTL and dopant concentrations, resulting in improved PCEs and more efficient material use. Additionally, we revamped the synthetic methods to prioritize sustainability. By replacing traditional solvents with water-based processes, we achieved high-yield, fast, and scalable reactions that are environmentally friendly, cost-effective, and suitable for advancing to higher technology readiness levels (TRLs).

Cost-efficient climate change mitigation requires the continued rapid expansion of the photovoltaic (PV) system production to the multi-TW level. The PV industry is currently transforming into a key industry sector, which implies that sustainability aspects will have increasingly wide-ranging impacts. We identify four paradigm shifts that can enable a sustainable TW-scale transformation and that address power conversion efficiency, materials, circularity, and social aspects. It will be discussed if and how these can be accomplished with state-of-the-art silicon-based PV technologies and which transformative potential is offered by novel perovskite PV technologies:

A change to highly efficient multijunction architectures can reduce materials consumption but implies higher materials complexity. By replacing fossil fuel-based technologies and also by switching from wafer-based to thin film technologies, PV will reduce global mining activities but may create new supply risks and resource complexities (Fig. 1a). Although end-of-life material streams lag behind PV production by several decades, material circularity needs to be anticipated already in the R&D stage to manage huge future waste streams efficiently. Finally, social aspects especially during production but also in serving all humanities energy needs are an integral part of sustainability and are essential for a wide acceptance of the technology.

In the temporal scale, these sustainability challenges fall into two distinct phases (Fig. 1b): a first phase, until mid-century, when rapid PV capacity expansion to mitigate climate change needs to be the main focus, and a second phase where material circularity will become critical. These phases need to be understood and addressed in a multi-dimensional, path dependent optimization approach. Thereby perovskite PV can play a pivotal role to achieve optimum sustainable yield for multi-TW scale photovoltaics.

Solution processing from nanoparticle dispersions allows the use of eco-friendly processing agents for the deposition of organic semiconductor thin-films for photovoltaic and other optoelectronic applications. Omitting surfactants to stabilize the dispersions is essential to not jeopardize the solar cell performance. Instead, in this work, the nanoparticles are stabilized by charging through electrical doping of the polymers, which creates a repellent electrostatic potential. Novel surfactant-free nanoparticle dispersions from high-performance organic semiconductors are synthesized, e.g., in alcohols by nanoprecipitation. Design criteria will be discussed how to select the components of the dispersion. The role of the ionization potential of donors, the miscibility of donors and acceptors as well as the properties of the dispersion medium are elucidated. The corresponding solar cells achieve power conversion efficiencies beyond 10%, demonstrating the general feasibility of this alternate, all-eco-friendly processing route. Finally, the concept is translated to water-based dispersions. Due to the poor wetting of water on many surfaces, the deposition processes are revisited.

Organic solar cells have recently broke the power conversion efficiencies limit of 20%. This puts extra attention to increasing their stability as their last remaining weak point. Since they consist of organic molecules, they are susceptible to oxygen, light, heat and humidity, which are commonly found stresses in their working environment.

Different tabilizing additives (antioxidants, radical scavengers, hydroperoxide decomposers, UV absorbers) can be incorporated in active layers of organic solar cells to inhibit degradation. This is both inexpensive and easily upscalable, and it does not introduce further complexity into the device architecture.

Here we will present our recent results on long-term stability improvement using naturally occurring antioxidants that act as singlet oxygen quenchers and radical scavenging compounds in highly efficient non-fullerene based organic solar cells.

references:

1. Atajanov R, Turkovic V et al.  The mechanisms of degradation and stabilization of high-performing non-fullerene acceptor based organic solar cells. (in preparation)

2. Balasubramanian S, Turkovic V et al. Vitamin C for Photo-Stable Non-fullerene-acceptor-Based Organic Solar Cells. ACS Appl Mater Interfaces 2021; dx.doi.org/10.1021/acsami.3c06321

3. Prete M, Turkovic V et al. Synergistic effect of carotenoid and silicone-based additives for photooxidatively stable organic solar cells with enhanced elasticity. J Mater Chem C 2021; dx.doi.org/10.1039/D1TC01544C

The key requirements of current technology, aimed at reducing the environmental impact of excessive fossil fuel consumption through the effective integration of clean renewable energy conversion devices, have recently sparked increased interest in the development of semi-transparent solar cells (STCs) for direct incorporation into energy-sustainable buildings. In this context, organic solar cells (OSCs) emerge as a promising and innovative technology for the fabrication of highly efficient STCs for building-integrated photovoltaics (BIPV) applications. A comprehensive understanding of the physical and chemical properties of the active materials at the micro/nanoscale is essential for the fabrication OSCs with the desired operational efficiency, which can be achieved through precise design and optimization of their active components and mutual interfaces.  Indeed, as typical OSC designs consist of multi-layered structures, where the absorber material—processed as a thin film—is sandwiched between charge-transporting layers and electrodes, a deep understanding of the interface properties is also crucial. Importantly, photovoltaic devices need to operate under external stress or harsh environmental conditions such as light, heat, humidity, and oxidative agents. Valuable insights into the relationship between microstructural characteristics and failure mechanisms can be gained by simulating the devices operational conditions while simultaneously monitoring in-situ the changes in their structural and morphological properties under various stress factors. In this context, we present here an alternative approach for the study of the morphological and structural properties of multi-layered ST-OSCs for BIPV. The OSCs under investigation consist of a PM6:Y6 photoactive layer, a ZnO electron-transporting layer, an organic HTL-X hole-transporting layer (poly(3,4-ethylenedioxythiophene) (PEDOT)-based ionomer) and a transparent electrode made of silver nanowires (AgNWs). The work addresses stability issues related to both bulk and interface properties under prolonged heating and illumination conditions. Our experimental methodology combines in-situ Energy Dispersive X-ray Reflectometry (EDXR) with complementary ex-situ techniques, such as Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), and micro-Raman spectroscopy. The combination of these techniques provided valuable insights into the chemical, structural, and morphological degradation processes affecting the stability of the multi-layer devices [1]. Specifically, an increased roughness at the ZnO/PM6:Y6 interface was observed by EDXR, although evidence of substantial structural stability under illumination was found by XRD. In contrast, the system exhibited overall stability when subjected to prolonged heating in the dark, suggesting the photo-induced origin of the observed degradation phenomenon. Such effect is related to a photo-oxidation process of the active material occurring during continuous illumination of the device due to the use of an the hygroscopic organic HTL under ambient moisture conditions, as confirmed by micro-Raman measurements. This process may also be activated by a photocatalytic role of the ZnO layer. The results obtained were highly valuable in designing an alternative cell configuration, where the organic hygroscopic HTL-X was replaced by the inorganic MoOx compound. Our findings indicate that the device in this alternative configuration was stable under light stress, suggesting that the use of the inorganic HTL MoOx can limit the photo-oxidation of the PM6:Y6 active material, thereby preventing the cell from degradation [2].

[1] B. Paci et al. Adv. Funct. Mater., 2011, 21: 3573-3582, https://doi.org/10.1002/adfm.201101047

[2] B. Paci et al. Nanomaterials, 2024, 14(3), 269, https://doi.org/10.3390/nano14030269

Fundings: Funded by the European Union’s Horizon 2020 research and innovation program under Grant Agreement No 101007084 (CITYSOLAR).

Understanding and controlling the dynamics and interactions of photoinduced charged states between photoexcitation and charge extraction is key to improving Organic Solar Cell (OSC) efficiency. The two main processes following charge separation are charge transport and recombination; both involve unpaired electrons and are spin-dependent: to hop to the next localized state or to recombine with an opposite charge, the unpaired electrons’ spins must have a singlet (antiparallel) configuration. If they are in a triplet (parallel) configuration, transport or recombination is blocked.[1] Spin manipulation with microwaves allows controlled switching between these configurations, directly affecting device current, and is at the basis of Electrically Detected Magnetic Resonance (EDMR), an operando technique enabling valuable molecular-level insights into spin-dependent charge transport and recombination. [2]

EDMR measurements are performed by detecting changes in current flowing through a fully assembled miniature solar cell in an external magnetic field, following excitation with microwave pulses. By exploiting differences in resonance conditions for spins in different molecular environments, charged states on donor and acceptor molecules can be selectively probed and characterised, and their role in charge transport and recombination processes can be investigated.

In this work, we use EDMR to investigate organic solar cells based on two non-fullerene acceptor blends (PBDB-T:ITIC and PM6:Y6) and their corresponding fullerene analogues (PBDB-T:PCBM and PM6:PCBM). Through Rabi nutation experiments [3], in which spins are coherently manipulated by microwave pulses of increasing length, we detect clear spin-locking signatures characteristic of coupled pairs of interacting spins on donor and acceptor molecules, encountering to recombine and leading to transient current quenching. We find that this bipolar recombination, rather than unipolar charge transport, is the dominant process leading to the observed EDMR signal.

Hall effect is a valuable and widely used semiconductor characterization technique in the microelectronic industry, giving access to information on charge carrier type (n- or p-), density and mobility. Photo-Hall effect was further developed over the last few years to extend the pool of parameters accessible and probe minority carrier properties. Combined knowledge of majority and minority carrier mobility, density, lifetime and diffusion lengths are particularly sought after for the development of solar and photodetection technologies. Yet, the use of Hall and photo-Hall effect with organic semiconductors is faced with important challenges, including the need for heterojunctions to dissociate tightly bound excitons and, therefore, generate a photo-Hall signal.

In this study, we develop a Hall device merged with a solar cell architecture to probe photocarrier properties in the organic semiconductor of interest. Polycrystalline rubrene is selected for this study, benefiting from a band-like transport facilitating the interpretation of the Hall signal. Moreover, crystalline rubrene exhibits photocurrent generation in its pristine form, providing an ideal test bed for photo-Hall comparison with and without heterostructures. The ‘gated’ heterostructure is compared with simple mono- and bi-layer Hall devices, demonstrating a two-order of magnitude increase in photoconductivity upon appropriate source and gate bias. While the use of a gated heterostructure was aimed at facilitating exciton dissociation, our photo-Hall data reveal that the increase in photoconductivity is primarily associated with an impact of the device structure on charge carrier mobility. We observe a decrease in mobility with increasing light intensity in pristine rubrene, potentially associated with Coulomb scattering with nearby negative charges in a low dielectric environment. Spatial separation of holes and electrons enabled by the gated bilayer heterostructure however maintains hole mobility at its highest level throughout the light intensity range studied (up to ~100 mW/cm², equivalent to 1 Sun AM 1.5). Further analysis of the photocarrier density evolution with light intensity reveals strong deviations with Langevin theory, as expected for high mobility semiconductors and strong mobility imbalance between holes and electrons.

In summary, through this work we demonstrate that photo-Hall measurements can be performed in organic semiconductors, giving access to valuable information on charge carrier transport, generation and recombination. Importantly, our photo-Hall results reveal that strong spatial separation of holes and electrons is crucial for efficient photocarrier transport and extraction, suggesting the need to further explore crystalline bilayer heterostructures. Contrary to traditional bulk heterojunctions, such device would combine spatial separation of charges with high exciton and charge carrier diffusion lengths.

In 2001 I spent a summer at the Chemistry Department of Princeton University (US) thanks to a short-term mobility grant to investigate the electronic and optical properties of a Ruthenium based dye which was been employed in Grätzel or Dye-sensitized solar cells.¹ That study was the first of a long series where Density Functional Theory (DFT) and its Time Dependent extension (TDDFT) was used to describe the electronic and optical properties of a metal-based dye and simulate its spectrum in solution. Since that pioneering research to the present, computational modelling based on TDDDFT methods has played a central role in understanding the properties of materials involved in DSSCs and the mechanisms underlying the DSSC functioning. Computational modelling has thus become crucial both in the design of new materials for DSSCs and in their characterization. We focused on metal-organic dyes of Ru(II), Os(II), Cu(I), Fe(II) evaluating the role of solvent in the line-up of molecular orbitals, in the energy of the excited states w.r.t the ground state and in the simulated Uv-vis spectrum.² The role of spin orbit coupling (SOC) has been explored for Os(II) based dyes and for dyes where Ru(II) experiences a peculiar ligand environment.³ In addition to metallorganic dyes, organic dyes and other constituent materials of DSSCs, such as the electron-hole conductor (Spiro-OMeTAD)⁴ used in the solid-state version and the dye-sensitized TiO₂ nanoparticles⁵ were investigated. It has been a long and fruitful journey in which interdisciplinary skills have been merged to develop a new technology, and it was from this stimulating environment that perovskite-based solar cells were born.

Metal-halide perovskites (MHPs) have emerged as leading candidates for optoelectronic devices [1] and show great potential for photocatalytic applications [2]. Their bulk properties are significantly influenced by atomic motions at finite temperature, including anharmonic effects, ionic migration, and even phase segregation under illumination. At MHP surfaces, where the bonding network is disrupted and dangling bonds are present, such atomic motions are intensified and can be expected to severely impact the stability and the performance of MHPs.

In this talk, two fundamental aspects of MHP surfaces will be addressed: firstly, the origins of (in-)stability in MHP surfaces will be revisited and compared for tin- and lead-halide perovskite surfaces from molecular dynamics simulations [3]. The degradation mechanism of MHP surfaces and their chemical origins will be discussed. Additionally, the impacts of water and oxygen on the surface degradation will be disentangled, and their detrimental synergistic effects will be highlighted [4]. Second, the dynamical origin of benign electronic surface states in MHPs will be demonstrated, as revealed by machine-learning molecular dynamics simulations [5]. Best practices for modeling MHP surfaces will be showcased, and atomistic insights into the crucial role of the soft lead-halide scaffold in suppressing deep surface states will be derived.

[1] S. Stranks, H.J. Snaith, Nat. Nanotech. 2015, 10, 391–402

[2] L. Romani, et al. Angew. Chem. 2021, 60, 7, 3611-3618

[3] W. Kaiser, et al. J. Phys. Chem. Lett. 2022, 13, 10, 2321–2329

[4] J. Hidalgo, et al. J. Am. Chem. Soc. 2023, 145, 45, 24549-24557

[5] F. Delgado, F. Simoes, L. Kronik, W. Kaiser, D.A. Egger, et al. In preparation

Formamidinium-lead-iodide (FAPbI₃) has established itself as the state of the art for high solar-energy conversion efficiency in perovskite-based solar cells. FAPbI₃ has a rich phase diagram, and it has been noted that long-range correlation between organic and lattice dipoles can influence phase transitions and, consequently, optoelectronic properties. In this regard, system size effects can play a crucial role for an appropriate theoretical description of FAPbI₃. In this context, we perform a systematic study on the structural and electronic properties of the photoactive phase of FAPbI₃ (α-FAPbI₃) as a function of system size. Utilizing ab initio molecular dynamics at 300 K and first-principles calculations, we demonstrate that the selection of the computational system/setup must satisfy three criteria concurrently to ensure an accurate theoretical description: the (correct) value of the band gap, the extent (or the absence of) structural distortions, and the zeroing out of the total dipole moment. We demonstrate that the net dipole moment vanishes as the system size increases due to PbI₆ octahedra distortions rather than due to FA⁺ rotations. Additionally, we show that thermal band gap fluctuations are predominantly correlated with octahedral tilting. The optimal agreement between simulation results and experimental properties for FAPbI₃ is only achieved by system sizes approaching the nanoscale.

Inverted perovskite solar cells (PSCs) have emerged as a promising alternative to conventional structures, offering advantages such as simplified fabrication and improved stability. However, commonly used electron transport layers (ETLs) in p-i-n devices, such as fullerene-based materials, suffer from stability issues and induce a highly defective perovskite/ETL interface, featuring trap-mediated non-radiative recombination, which adversely impacts both the efficiency and stability of the resulting solar cells. The introduction of organic spacer moieties at the perovskite/ETL with stabilization effects has been proposed as an effective strategy to mitigate surface-assisted recombination and increase the stability of the device. 

However, conventional molecular passivators are electronically insulating, resulting in charge confinement with respect to the light-absorbing material, thus limiting their functionality. In this context, optimizing the electronic properties of the passivating layer material offers a promising approach to overcome performance limitations by enhancing charge transport to the ETL.  

In this contribution, we present the results of an experimental and theoretical study concerning the introduction of electroactive spacer moieties at the interface between the perovskite absorber and fullerene-based ETLs in inverted PSCs. We employ ab-initio molecular dynamics and density functional theory to elucidate the experimental findings and to unveil the underlying mechanisms connected to the interfacial charge transfer and improved stability brought about by the electroactive spacers. The use of electroactive spacers as ETL in fullerene-free devices is also explored.

We examine trivalent doping of tin-halide perovskites as a method for reducing p-doping and controlling defect activity. Using density functional theory (DFT) calculations and experimental characterisation, we show that doping with scandium, lanthanum, and cerium efficiently raises the Fermi level, lowering background carrier concentrations and defect densities and thereby enhancing material performance.To further enhance the Fermi energy shift upon tin substitution with trivalent ions, we incorporate the allyoung I-Br ratio technique. This method optimizes the iodine-to-bromine ratio in the perovskite structure, fine-tuning the electronic properties and band alignment. Combining trivalent doping with the allyoung I-Br technique results in a more significant Fermi level upshift, further suppressing p-type conductivity and defect activity. Solar cell fabrication and testing confirm the effectiveness of this approach, with scandium-doped devices showing increased photocurrent and open-circuit voltage compared to undoped controls, even without full optimization. This work highlights the potential of combined cation doping and compositional engineering to advance the performance of tin perovskites for optoelectronic applications.

Increasing power consumption and need for the green, renewable and cost-efficient energy source draw lot of attention to metal halide perovskites in past two decades. Metal halide perovskites have shown exceptional potential in converting solar energy to electric power in photovoltaics with very high efficiency, they are composed of earth abundant materials and the production cost is very low, yet their application is hampered by limited operational stability. This stimulated the development of hybrid layered (two-dimensional, 2D) halide perovskites based on hydrophobic organic spacers, templating perovskite slabs. Incorporation of the small organic spacers into the perovskite layers leads to larger stability and disfavours transition into the non-photoactive phase. However, conventional organic spacer cations are electronically insulating, resulting in charge confinement within the inorganic slabs, thus limiting their functionality. This can be ameliorated by extending the π-conjugation of the spacer cations. We demonstrate the capacity to access Ruddlesden-Popper and Dion-Jacobson 2D perovskites incorporating for the first time aryl-acetylene-based (4-ethynylphenyl)‍methylammonium (BMAA) and buta-1,3-diyne-1,4-diylbis‍(4,1-phenylene)‍dimethylammonium (BDAA) spacers, respectively. We assess their unique opto‍(electro)‍ionic characteristics by a combination of techniques and apply them in mixed-dimensional perovskite solar cells that show superior device performances with a power conversion efficiency of up to 23% and higher operational stability, opening the way for multifunctionality in layered hybrid materials and their application. At the interface between perovskite active layer and charge transport layers, the aryl-acetylene-based spacers serve not only as passivating layer, but they are also actively involved in the charge extraction from the perovskite to the hole transport material. We show two different mechanisms of the charge extraction for the studied organic spacers and explain the enhanced power conversion efficiency compared to the untreated pure 3D perovskite. Moreover, theoretical analysis suggests that the BDAA spacer presents a very promising system for interfacial modulation and active charge transport with both electron and hole transport layers. Efficient charge transport properties are enabled by favourable band alignment of the BDAA spacer with the perovskite absorber and by the extended π-conjugated aryl-acetylene core. The π orbitals delocalised through the entire system could effectively overlap with both perovskite and charge-transport layers, offering a new approach to designing multifunctional materials and interfaces. 

Perovskite-based multijunction solar cells are on the brink of commercialization, with efficiencies skyrocketing in the past 5 years. E.g., the most efficient two-terminal configuration ever made is a perovskite/Si tandem. Yet, the reliability of these devices is still a big question mark. This is in part due to the short time since their development hindering decade-long tests due to sheer time constraints, but also because there is a lack of established accelerated aging protocols due to a lack of understanding of primary failure modes. In the presentation, I will cover our recent results looking at the perovskite cells in multijunctions and how nanometric defects trigger device degradation and how they can be mitigated. I will further discuss effects on the um- and cm-scale, e.g., how the choice of the silicon cell texturing affects performance and stability, as well as long-distance ionic effects and how precise control over different length scales enabled us to fabricate excellent wafer-scale devices beyond the state-of-the-art.

Photovoltaics (PV) is a major actor of the ongoing energy transition towards a low- carbon- emission society. More than 1.6 TW of PV systems were operational at the beginning of the year 2024, producing more than 2135 TWh of electricity, or 8.3% of the global electricity demand. Several factors lie behind the plummeting cost and fast ramp up of this technology. One factor is the fact that PV is modular: Identical solar panels of hundreds of watts are combined, by the dozens in rooftop installations, or by the millions in utility-scale power plants. Another successful factor relies on the possibility to adapt PV devices to different requirements realizing for example high-power modules, semitransparent PV modules, flexible products, etc..

Several semiconductor materials, realized with different processes, have been used as absorber of solar cells: today more than 98% of the overall cell production is made with crystalline silicon, while the remaining part of the production is obtained with inorganic thin film materials (manly CdTe, CIGS, and CIS). Organic and hybrid materials can contribute to the realization of the next generation of photovoltaic modules with improved performance or which can be used to integrate photovoltaics in various contexts such as the building-integrated PV or the agrivoltaics.

In this presentation an overview of the work done on the development of innovative materials and solar cells in Solar Photovoltaic Division of ENEA is reported.

Starting from our expertise on silicon-based solar cells, the research on hybride halide perovskite solar cells for the development of perovskite/silicon tandem devices will be discussed. As for the bottom cell, heterojunction silicon solar cells (SHJ) realized on both p-type and n-type silicon wafers have been considered, studying also selective contacts alternative to the doped silicon thin films generally used in the current SHJ device architectures. n-i-p or p-i-n perovskite solar cells have been used for the top component, studying different carrier transporting layers and developing appropriate transparent front electrodes for perovskite/Si tandem cell.

On the other hand, the research on semitransparent spectrally-selective thin film PV will be discussed as a promising approach for agrivoltaic application and in particular for PV greenhouses with reduced energy demand. The approach aims at an integrated complementary use of solar light for PV and photosynthesis, by tuning the transmission of the PV modules on the absorption spectrum of the plants. We studied two possible implementations, one based on organic PV and one on thin film silicon PV. In particular, the organic approach will be presented, where spectral selectivity can be achieved by tailoring the absorption characteristics of the active materials.

The winning future of the emerging perovskite (PSK) solar cells is closely linked to the dimension scalability and the possibility to boost the existing photovoltaic (PV) technology employing tandem architectures.[1] The synergetic development of large area PSK devices fitting the standard silicon wafer dimensions and the optimization of PSK/silicon tandem architectures can definitively open up new horizons for winning the commercialization challenges. To this end, bi-dimensional (2D) materials recently demonstrated their effectiveness in boosting the PSK PV device efficiency and stability by mitigating the performance drop when scaling the cell dimensions up to module size.[2] In particular, transition metal carbides, nitrides and carbonitrides (MXenes) having a general formula M_n+1X_nT_x (n = 1, 2, 3), where M represents an early transition metal, X is carbon and/or nitrogen, and T_x stands for surface terminations (such as OH, O, F or alternatively Cl) have been used in perovskite-based devices with different aims. During their synthesis, their surfaces are naturally functionalized, by significantly shifting the WF in a broad range form 1.6 to 6 eV.[3] Recently we demonstrated a general approach where Ti₃C₂T_X MXenes can be employed as dopant of the perovskite precursor solution to shift the perovskite layer work function by achieving a proper energy level alignment at perovskite/charge transporting layer (CTL) interfaces, eventually passivating perovskite bulk and interfacial defects.[4][5] This resulted in Mxene-engineered large area perovkite opaque module (PSM) demonstring PCE overcoming 17% and 15% over 121 cm² and 240 cm² substrates area respectively.[6]. When moving from the opaque to semi-transparent (ST) devices employed in tandem configuration, the absence of the light backscattering by the metal counter electrode (here replaced by a ST one) and to an enlarged band-gap (from 1.68 to 1.72 in case Si-based cells are employed as bottom technology) led to a not-negligible reduction of the ST-perovskite cell (ST-PSC) short circuit current (J_SC).[7] In this case, we proposed the use of Ti₃C₂Cl₂ MXenes where, a strong coordination between Cl atoms and Pb²⁺ ions was established, resulting in a reduction of metallic lead clusters (Pb⁰) responsible for the formation of deep defects and trap free charge carriers in the perovskite films. Moreover, the interaction between the MXene Cl terminations and the perovskite Pb²⁺ ions formed an adduct with the perovskite precursor, acting as heterogeneous nucleation site for the perovskite film. The resulting perovskite film showed a more pronunced n-character (WF tuning) and enlarged grain size with increased absorbance, translating in ST-PSCs with increased J_SC. The as optimized ST-PSC showed PCE exceeding 18% while ST large area modules (ST-PSM) achieved PCE surpassing 16% over 90 cm² substrate area.

The MXene-based ST-PSMs (4 parallel-connected modules) have been coupled with wafer sized (15.7x15.7 cm²) commercial silicon heterojunction (Si-HJT) bifacial cells, in a 4 terminal (4T) architecture by realizing a 20x20cm² PSK/silicon tandem mini-panel with PCE above 20%. Moreover, the power generated density (PGD) has been estimated to be > 23 mW/cm² considering a typical radiation of 30%. The modular architecture proposed for the tandem mini-panel, can represent a building block to develop larger 4T tandem panels, with minimized PCE losses. Following this approach, on one side the PSK solar modules can be independently optimized, realized and stacked atop the commercial Si-HJT cells employing an ad-hoc developed lamination process. On the other side, the as-proposed panel architecture does not require any modification in the Si production lines, making the tandem technology appealing for the already exiting Si cell producers.

Currently, a perovskite/silicon tandem solar cell with 34.6% power conversion efficiency (PCE) is the most efficient dual‑junction solar cell under the AM1.5g spectrum [1]. Following this impressive efficiency improvement, perovskite/perovskite/silicon triple-junction solar cells have now gained significant attention and are rapidly developing [2–9] . In our work, we address some of the main aspects to realize monolithic two-terminal perovskite/perovskite/silicon triple‑junction solar cells.

First, in two-terminal multijunction solar cells, high open-circuit voltage (V_OC) is a key characteristic, as the voltages from different subcells add up. Therefore, minimizing voltage losses in individual subcells as well as the recombination layer between the cells is crucial.

To maximize V_OC, we identified the high-bandgap perovskite top cell as the main source of voltage loss in our triple-junction solar cell. We addressed this by optimizing both, the bulk quality of the perovskite and the interface between the perovskite and the electron transport layer (here C₆₀). Specifically, we replaced our reference triple-cation double-halide perovskite with a triple-cation triple-halide composition, which exhibits superior bulk quality. Additionally, a piperazinium iodide passivation layer [10] is introduced between the perovskite and C₆₀ to reduce the non-radiative recombination loss at this interface. These optimizations led to a high V_OC exceeding 3.0 V. Furthermore, we optimized the recombination layer between the two-perovskite sub cells leading to a V_OC of ∼3.1 V, which is among the highest values reported in literature for this structure. Finally, to investigate the remaining voltage losses in our structure, we conducted electrical simulations. The results indicate that by employing perfectly band aligned charge transport layers, the V_OC of our triple-junction cell has the potential of up to ~3.4 V [11].

Secondly, in a two-terminal multijunction solar cell, the sub-cell generating the lowest current limits the current of the whole device. Therefore, maximizing the current density (j_SC) requires current matching between the sub cells. This can be achieved through optimization of bandgaps and thicknesses of the perovskite middle cell and top cell.

To enhance the j_SC, we conducted optical simulations that revealed the middle cell as a significant limiting factor in our current triple-junction structure (~ 8.3 mA/cm² j_SC). Ideally, lowering the middle cell bandgap to 1.47 eV, would result in a current-matched triple-junction cell (at ~13 mA/cm²) [11]. However, this approach requires partially substituting lead with tin in the perovskite composition, which introduces significant stability challenges [12]. Therefore, we kept the bandgap of the perovskite middle cell at 1.55 eV and focused on increasing this absorber thickness with the potential of reaching > 11 mA/cm² j_SC for the triple-junction device [11]. In this regard, careful optimization of the crystallization process is necessary to ensure high bulk quality in the thick perovskite layer. For this purpose, we used a chloride-containing perovskite composition with a thickness of more than 750 nm. Our results highlight that optimizing the hole transport layer (HTL) and processing conditions, such as the choice of the antisolvent, is crucial for achieving a thick high-quality perovskite layer. Moving forward, we plan to implement this optimized middle cell in our triple-junction solar cell.

Lead halide perovskite-based photovoltaics has attracted great attention in the last decade since silicon photovoltaic as the dominating technology is approaching its theoretical efficiency limit. Combined in a monolithic perovskite/Si tandem solar cell, they promise a substantial leap in power conversion efficiency beyond the single-junction limit without increasing cost significantly. However, realizing high performance in module-sized formats is limited by the practical challenge of producing uniform high-quality perovskite layers onto large-scale silicon bottom cells. The polycrystalline and mechanically soft nature of perovskite thin films lead to a high defect concentration and inefficient carrier extraction if the crystallization is not properly controlled.

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

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

Wide-bandgap perovskite solar cells, which have an energy bandgap greater than 1.65 eV, show promise for constructing tandem solar cells. However, high bromide content in these cells can lead to photostability and halide segregation problems that affect performance and cause voltage loss. Therefore, minimizing voltage loss caused by non-radiative recombination at the interface is crucial to improving device performance. In current literature, various aromatic ammonium halides have been commonly used to passivate the interfacial region between the perovskite surface and the electron/hole selective layer. These materials enhance photovoltaic performance while also improving stability. However, most studies have focused on the passivation effects depending on the different structures of cations. Since the properties of wide-bandgap perovskite with mixed halide composition can be affected by post-reaction with other halide ions, investigating passivation effects depending on the different halogenates of the passivation molecules is highly required.

Our study involved treating a small amount of fluorinated aromatic ammonium halides(FAAX, X=Cl, Br, I) on the perovskite surface. The results showed that the treated film displayed suppressed nonradiative recombination and reduced trap density, leading to significantly reduced voltage loss. It was found that the degree of defect passivation and resulting device performances were changed depending on the different halides. The optimal device achieved a high power conversion efficiency close to 22% with an excellent improvement in voltage from 1.13V to 1.21V, demonstrating the great potential of the material in devices. Finally, based on the wide-band gap perovskite top cell, we realized a high-performance four-terminal perovskite/Cu(In,Ga)Se2 tandem devices. These findings emphasize the importance of carefully designing of passivation molecules for the wide-bandgap perovskite, as it could lead to significant improvements in PCE and the stability of these devices.

Improved reverse bias stability and UV resistance are recognized as needs to improve long-term viability of halide perovskite semiconductors in photovoltaic applications. In this talk we will discuss our work on improving stability under both reverse bias stress and reverse current stress, including how interfacial and architecture engineering can have a significant impact on the reverse bias behavior of perovskite solar cells. We show that by controlling both electrode interfaces we can realize average breakdown voltages comparable to those of silicon cells, demonstrating cells that can survive reverse bias under partial shading for hours at a time.  We also explore two pathways for cell failure under reverse current stress and demonstrate that some cells can pass reverse current densities corresponding to those at the max power point without permanent damage. Finally, we explore metrology methods for characterizing both cells and modules that allow real-time analysis of failure modes under various stress conditions.

In the evolving field of photovoltaics, hybrid and organic solar cells, particularly organic-inorganic hybrid perovskite solar cells (PSCs), have shown remarkable progress in achieving power conversion efficiencies close to traditional silicon-based systems. Despite this, the long-term stability of PSCs remains a significant hurdle, limiting their commercial potential. This presentation will focus on the innovative use of interface engineering to enhance the durability and performance stability of PSCs. By examining the root causes of instability and the latest strategies for interface manipulation, we aim to illuminate how these techniques not only mitigate surface defects but also protect the perovskite core from environmental degradation. We have explored a variety of interface modification agents and their roles in stabilizing the perovskite structure, including the application of low-dimensional perovskite capping layers and the design of advanced charge transport layers. The presentation will delve into the fundamental insights and mechanisms behind device degradation, presenting cutting-edge methods to counteract these challenges. Additionally, we will discuss the broader implications of these developments for the future of hybrid and organic photovoltaics, emphasizing the critical role of interface engineering in paving the way towards more reliable and efficient solar technologies.

Achieving stable formamidinium-caesium lead perovskite solar cells under light and heat is one of the major challenges hindering the commercialization of perovskite solar cells (PSCs), while the mechanism of device degradation, especially at the interface, is unclear and concealed.[1] Herein, by utilizing a novel polymer and organic Benzothiophene molecule (BT) blend, we construct a phase-separation hole transport layer (HTL) designed to compare with polymer PTAA and self-assembled monolayer Me-4PACz. We revealed that the efficiency difference in fresh samples arises from the crystallinity and orientation of the near HTL perovskite region, which result in defects and non-radiative recombination. However, this fails to provide guidance on the degradation pathway when aging at open-circuit condition with heat. We found that the behaviour of charge percolating and dissipating under open-circuit conditions impacts the change in the space charge region at the buried interface and the generation of mobile ions, thereby contributing to the loss of short-circuit current after aging. Devices incorporating our BT phase-separation HTL retained 85% of initial MPP efficiency for over 1,500 hours under full-spectrum simulated sunlight at 85 degrees Celsius and Open-Circuit Conditions in ambient air with a relative humidity of 50 to 60% (ISOS-L-3), five times longer than other HTL alternatives. [2]

Traps and structural defects at the hole and electron transport interfaces of the microcrystalline absorber limits the efficiency and long-term stability of perovskite solar cells (PSCs) due to accumulation of the ionic clusters, non-radiative recombination and electrochemical corrosion. Surface engineering using self-assembled monolayers (SAM) was considered as an effective strategy for modification of charge-collection junctions. We have investigated how flourinated and non-flourinated triphenylamine based SAMs affect surface properties, charge transport, efficiency and stability in inverted perovskite solar cells and modules. We tested different SAMs for interface stabilization both from p- and n-junction of p-i-n PSCs. While p-side integration of FTPATC reduced the strain in the lattice of the perovskite layer, the n-side configuration was characterized by chemical interaction via bonding with A-site cations, which stabilized the interfaces. With this, the double-side passivation strategy with FTPATC has a synergetic effect: p-side modification gains the PCE up to 22.2 %, the incorporation of FTPATC to electron-transport boosts the stability of the devices under harsh conditions at the elevated temperatures (90 °C). Comparing our results with the benchmark material 2PACz highlighted a critical specificity in ETL interface modification. 

         Transitioning global energy production away from carbon-emitting sources to renewable technologies is one of the foremost issues faced by the scientific community. Given the vast amount of solar energy incident on the planet, photovoltaic technologies will need to play a crucial role in decarbonization. To this end, metal halide perovskite solar cells (PSCs) have emerged as promising candidates for next-generation commercial photovoltaics primarily thanks to the exceptional power conversion efficiencies (PCE) achieved (>26%).

         Defect-induced non-radiative recombination and energy-level misalignment at the perovskite/electron transport (ETL) interface remain as key bottlenecks to further improving PSC performance. To systematically address these issues, we developed a novel class of highly tuneable ferrocene-based organometallic interlayers.

         Our first report focused on the excellent efficiency (25%) and stability (meeting IEC61215 standards) achieved using the FcTc₂ molecule and was largely attributed to surface passivation through C=O substituents.¹ Aiming to better understand the structural features responsible for the exceptional performance, oligo-ferrocene analogues Fc₂Tc₂ and Fc₃Tc₂ were investigated.² It was found that further improvements in performance (26.1%) could be achieved with additional ferrocene units. However, a direct correlation between molecular properties and their effects on the perovskite surface and photovoltaic performance remained unclear.

         In our latest research, we identified a novel avenue for tuneability in these ferrocene-based molecules which has gone unnoticed in the literature, electrochemical potential (Fc-E_HOMO). The E_HOMO of ferrocene can be tuned through substitution, so we introduced two novel molecules, FcFk₂ and FcFc₂, featuring structurally similar carboxyl-furyl substituents, responsible for surface passivation, whilst differing significantly in E_HOMO. This allowed us to isolate the structural features responsible for the energetic and passivating interaction. It was found that only when the Fc-E_HOMO (as in FcFc₂) lies above the perovskite VB is a proportional decrease in the surface work function observed. A decrease in surface work function promotes interfacial band bending, reducing minority (hole) charge carrier concentration, and thus improving open circuit voltage and fill factor in the resulting PSCs. When probing the literature, this unrecognized trend holds true for all published examples. Furthermore, the unique chemical interactions between the Fc molecules and PbI₂ were probed and the E_HOMO energies of the compounds involved were again found to play a key role. Ultimately, excellent performance (>25%) and stability (T₉₀>1000 h at 65 °C MPPT) were reached by adopting these strategies.

         This work serves to outline design rules for the directed synthesis of next-generation multifunctional ferrocene-based interlayers featuring optimized passivating groups and energetic alignment.

Figure 1: a) PSC structure and key structural features of the developed organometallic interlayers; b) relation of PVK surface work function with Fc-EHOMO; c) Surface potential distribution extracted from KPFM mapping; d) Unrealised correlation between surface work function and EHOMO; e) PSC JV data highlighting the differences in key photovoltaic parameters.

This work: Francesco Vanin, William D. J. Tremlett, Danpeng Gao, Qi Liu, Bo Li, Shuai Li, Jianqiu Gong, Xin Wu, Zhen Li, Ryan K. Brown, Liangchen Qian, Chunlei Zhang, Xianglang Sun, Xintong Li, Xiao Cheng Zeng, Zonglong Zhu, and Nicholas J. Long

Recent progress on organic photovoltaic (OPV) materials has enabled solar cells with impressive power conversion efficiencies exceeding 19% to date at lab scale. However, large area industrial OPV modules are still far behind in terms of performances today. Also, the promise of low-cost for this printed solar cells technology is far from being achieved today. It is important to develop scalable materials, environmentally friendly processed and simplified device structures. Minimizing the number of required layers is one of the key to simplify the OPV module fabrication. In this communication, strategies to suppress charge transport layers, respectively Hole-Transport-Layers (HTL) and/or Electro-Transport-Layers (ETL) will be discussed. For example, we successfully suppressed the need of HTL by simply blending an additional molecule with the bulk heterojunction. Upon mild thermal annealing, the molecules migrates to the silver top electrode to bind with the metal, thus, forming in situ, post-fabrication, the required hole-transport interface. The resulting solar cells shows exceptional thermal stability.

Polycyclic aromatic molecules, especially perylene diimide based dyes, are widely used as electronic materials in photovoltaic devices.¹ Our team at the University of Calgary within the department of chemistry has been actively developing modified perylene diimides for use in a range of solution processed electronic devices including organic solar cells,² organic light emitting diodes,³ transistors and sensors,⁴ electrochromic films,⁵ and photo/electro cathodes.⁶ This presentation will detail the design and synthesis of several classes of modified perylene diimides for specific use in large area organic photovoltaic devices and modules, primarily as cathode interlayers in both conventional and inverted type cells, but also as non-fullerene acceptors. Structure-property relationships, the formation of functional large area films via slot-die coating using green solvents, and utility both small area devices and flexible modules will be discussed. Applications as indoor light recycling devices and integration into operational greenhouses will be touched on.

The emergence of nonfullerene acceptors (NFAs) has triggered a rapid advance in the performance of organic solar cells (OSCs), endowing OSCs to arise as a promising contender for 3^rd generation photovoltaic technologies. Meanwhile, the ultimate goal of OSCs is to deliver affordable, stable, and efficient solar-to-power products contributing to global carbon neutrality. However, simultaneously balancing these critical factors of OSCs toward commercialization is extremely challenging. In this presentation, I will first introduce the self-assembly strategy we developed to bridge the gap between high power conversion efficiency (PCE), long-term stability, green-solvent processability, scalability, and low-cost manufacturing. Our approach demonstrates green-solvent-processable and open-air-printable OSCs with a simplified device architecture, achieving enhanced PCE alongside improved shelf life, thermal stability, and light illumination durability. Subsequently, I will present our recent findings on elucidating material combinations for maximum industrial viability in OSCs. Building on this research, I will discuss the development of high-commercial-viability OSCs tailored for large-scale production and practical applications. Finally, I will summarize our efforts and key insights into advancing the commercial viability of OSCs, paving the way for the realization of affordable, stable, and efficient OSC technologies.

The development of high-performance organic photovoltaic materials has gained significant attention due to their potential for low-cost, flexible, and lightweight solar energy solutions, including semi-transparent photovoltaics for building-integrated applications.^[1] Central to this effort is the optimization of donor-acceptor blends, where efficient charge transfer and exciton dynamics are critical for enhancing device efficiency.^[2]

Among the promising materials, the blend of PCE10, a polymer donor, and FOIC, a non-fullerene acceptor, has shown considerable potential due to its strong near-infrared absorption and favorable energy level alignment.^[3] In this work, we present a comprehensive investigation into the hole transfer dynamics and optoelectronic properties of a blend material for organic photovoltaic applications. Through a combination of theoretical modeling and experimental analysis, we aim to deepen the understanding of the role of the electronic and excitonic structures in the dynamics that govern the charge separation.

We calculated the energy levels and the absorption spectra by DFT for the individual PCE10 and FOIC molecules as well as their blended configurations. In parallel, we performed extensive experimental investigations, including photoelectron spectroscopy (PES) and femtosecond transient absorption spectroscopy, to explore the photo-physical properties of PCE10, FOIC, and their blend. PES measurements allowed us to estimate the ionization energy and electron affinity of the materials, which are critical for understanding the energy level alignment in the blend. The temporal dynamics of the excitons in the blend were further analyzed to unravel the recombination mechanisms that were dominated by the exciton-exciton annihilation (EEA). By comparing the decay times with different probe energies, we show how the hole transfer processes from acceptor to donor within the blend affect the efficiency of the EEA mechanism. These findings deepen our understanding of the complex interactions between donor and acceptor materials in organic photovoltaic systems, providing valuable insights into the recombination processes and charge transfer mechanisms in organic blends.

Photoluminescence (PL) measurements of solar cells under operating conditions provide a powerful tool for determining their upper performance limits. In organic solar cells, however, PL typically consists of overlapping contributions from radiative free charge carrier recombination and non-dissociated radiative exciton recombination, with the latter often obscuring the former. As a result, steady state PL measurements only capture the combined effects of both radiative recombination processes. To overcome this limitation, an adaptation of time-resolved photoluminescence (tr-PL) is introduced. For this method, an electric voltage is applied in addition to the light pulse. Upon shutoff of the laser, the solar cell is also disconnected from the voltage source and the PL decay under open circuit condition is recorded. By exploiting the significantly different lifetimes of non-dissociated excitons (pico- to nanoseconds) and free charge carriers (microseconds), the PL contributions of each species can be identified via fitting and extrapolation of the decays [1,2]. Hence, the free charge carrier PL and non-dissociated exciton PL can be determined for a range of applied voltages. Using a state-of-the-art D18:Y6 organic solar cell (power conversion efficiency: 16.2%) we demonstrate that the implied voltage (i.e., the quasi-Fermi level separation in the absorber layer) can be determined from the free charge carrier PL under operation conditions revealing the effects of transport losses both in steady-state and time-resolved. By leveraging the measured current, a PL-based pseudo current-voltage curve is constructed, showing an implied efficiency (free of transport losses) of 18.1%. Unlike conventional methods to obtain a pseudo current-voltage curve (e.g. intensity dependent V_OC measurements), this approach eliminates the need for assumptions about the photogenerated current density. In fact, when combined with electroluminescence measurements, the method enables a quantification of the photogenerated current density in the voltage range between the maximum power point and V_OC, revealing a 5% reduction compared to the short-circuit current density. Analysis of the PL originating from non-dissociated excitons suggests field-dependent exciton dissociation to be the cause of the measured voltage dependence of the photogenerated current. Therefore, this novel method represents a significant advancement in the characterization of organic solar cells, specifically for understanding exciton dissociation dynamics in low offset organic absorber systems. 

Solar energy harvesting and conversion represent a compelling scientific, technological and societal to move away from the exploitation of fossil fuels. In this context, Dye-sensitized solar cells (DSSCs) are viable and cheap alternatives to conventional silicon-based cells with advantages in terms of transparency and efficiency in indoor conditions.[1] Ruthenium and polypyridine complexes holds the golden standard in this field, as they possess ideal characteristics such as long-lasting metal-ligand charge transfer (MLCT) states and efficient charge separation, limiting recombination at the dye-TiO₂ interface. However, ruthenium is a rare and expensive metal, and the development of more sustainable energy devices based on earth-abundant metals is now a must. A quick glance at the periodic table reveals iron as a potential good candidate. However, striking photophysical differences exist between ruthenium(II) polypyridyl complexes and their Fe(II) analogues, the latter suffering from short-lived MLCT states resulting of their ultra-fast relaxation into metal-centered (MC) states. [2] Pyridyl-N-heterocyclic carbenes (pyridylNHC) brought a strong s-donor character required to promote a higher ligand field splitting of the iron d orbitals, resulting in a destabilization of the MC states over the MLCT manifold with slowdown of the excited state deactivation providing iron(II) complexes with tens of picoseconds lifetimes making them more promising for applications in DSSCs. [3] In this contribution I will present our recent advances in the development of photoactive iron-carbene dye sensitizers and characterization of iron-sensitized solar cells[4] with a focus on the computationally driven design of efficient sensitizers going from homoleptic to heteroleptic complexes (bearing different anchoring groups) and on electrolyte contents. Our synergistic computational and experimental approach led to the best photocurrent and efficiency ever reported for an iron sensitized solar cell (2% PCE and 9 mA/cm²) using a co-sensitization process.

Hybrid AMX₃ perovskites (A=Cs, CH₃NH₃; M=Sn, Pb; X=halide) have in the last years revolutionized the scenario of photovoltaic technologies. Despite the extremely fast progress, the materials electronic properties which are key to the performance are relatively little understood. We developed an effective GW method incorporating spin-orbit coupling [1] which allows us to accurately model the electronic, optical and transport properties of halide perovskites, opening the way to new materials design. In parallel, a series of different strategies will be reported to increase the device stability and efficiency.[2] While instability in aqueous environment has long impeded employment of metal halide perovskites for heterogeneous photocatalysis, recent reports have shown that some particular tin halide perovskites (THPs) can be water-stable and active in photocatalytic hydrogen production. To unravel the mechanistic details underlying the photocatalytic activity of THPs, we compare the reactivity of the water-stable and active DMASnBr₃ (DMA = dimethylammonium) perovskite against prototypical MASnI₃ and MASnBr₃ compounds (MA = methylammonium), employing advanced electronic–structure calculations. We find that the binding energy of electron polarons at the surface of THPs, driven by the conduction band energetics, is cardinal for photocatalytic hydrogen reduction.[3] In this framework, the interplay between the A-site cation and halogen is found to play a key role in defining the photoreactivity of the material by tuning the perovskite electronic energy levels. Our study, by elucidating the key steps of the reaction, may assist in development of more stable and efficient materials for photocatalytic hydrogen reduction. Finally, we report a report is made on a composite system including a double perovskite, Cs₂AgBiCl₆/g-C₃N₄, used in parallel for solar-driven hydrogen generation and nitrogen reduction, quantified by a rigorous analytical approach. [4] The overall picture of our theoretical investigations underlines a crucial role of computational investigation, casting the possibility of performing predictive modeling simulations, in which the properties of a given system are simulated even before the materials laboratory synthesis and characterization. At the same time, computer simulations are shown to offer the required atomistic insight into hitherto inaccessible experimental observables. 

The field of organic photovoltaics research has witnessed a renaissance in recent years.
In particular, the introduction of non-fullerene acceptors (NFA) in organic heterojunction
solar cells has alleviated the issue of large voltage losses in traditional fullerene-based junctions[1,2]
resulting in a major boost of power conversion efficiency from 11% to >19%[3] starting to rival those made
of perovskites. New regimes of photophysics are reached in these new materials that are currently not well understood
suggesting that more powerful experimental measurements and computational models
are urgently needed to rationalize, explain and further build on these advances.

Our group has contributed to this objective by developing a powerful non-adiabatic molecular dynamics simulation
tool in recent years. In our approach, the wavefunction of charge carriers (electrons or holes)[4,5] or electronic
excitations [6] is progapaged in nanoscale organic materials (10-100 nm) on the 10-100 ps timescale by solving the
time-dependent electronic Schrödinger equation coupled to intramolecular and lattice vibrations. The method has led to a paradigmatic
shift of our mechanistic understanding of charge[4,5] and exciton transport processes[6] in ordered (high mobility)
organic semiconductors, transient quantum delocalization.

Here we report on a timely extension of our methodology, termed excitonic state-based surface hopping (X-SH), that now allows us to simulate
the quantum dynamical dissociation of excitons to charge carriers in truly nanoscale organic materials interfaces.[*] We apply this new methodology
to study exciton dissociation at the interface between the donor material alpha-hexathiophene and the
non-fullerene acceptor perylene diamide. We find that for this system exciton dissociation proceeds
via the generally accepted picture: fast relaxation of the initial band-like electronic excitation to
a localized Frenkel exciton, diffusion of the Frenkel exciton to the interface via hopping followed by formation
of interfacial (``cold") charge transfer state that dissociates thermally to free carriers or recombines on long time scales[*].

Intriguingly, as we increase the electronic coupling between the molecules (or, equivalently, increase the electronic band width
or charge mobility of donor and acceptor), we increasingly observe a second, much more efficient ``hot" exciton dissociation channel[*].
Here Frenkel excitons convert to hybrid exciton-charge transfer states that directly form free carriers. Remarkably, the hot exciton
dissociation process is observed to occur at distances of up to several nanometers away from the interface owing to the delocalized nature
of the hybrid exciton-charge transfer states[*]. This way the formation of kinetically slow interfacial charge transfer
states that are prone to recombination is avoided. Similar observation are made when the dielectric constant of the donor and
acceptor materials are increased in place of the electronic coupling[*]. Both modifications result in a better energetic alignment of excitonic
and charge transfer states that leads to the emergence of hybrid exciton-charge transfer states as
gateways for efficient hot exciton dissociation.

Our study uncovers an important design principle for efficient hot exciton dissociation in organic materials interfaces. Moreover
our simulations may help rationalise contrasting experimental findings in regard with the nature of the exciton dissociation mechanism
at these interfaces (hot vs cold).

[*] F. Ivanovic, W.-T. Peng, S. Giannini, J. Blumberger, manuscript in preparation.

A fundamental understanding of the photophysics of organic photovoltaics (OPV) at the molecular level remains a major challenge, limiting the rational design of novel materials with enhanced properties. To help bridge this knowledge gap, we developed a novel multi-scale methodology that integrates Quantum Mechanics (QM) calculations with Classical Molecular Dynamics (CMD) simulations in a sequential QM/CMD framework to explore the ground- and excited-state properties of OPV materials. Our approach begins with CMD simulations of macromolecules (oligomer models) in solution, followed by simulating film formation via solvent evaporation. Further CMD simulations are then conducted on the resulting films to generate uncorrelated configurations for subsequent QM calculations. These calculations employ density functional theory (DFT), time-dependent DFT (TD-DFT), and the wavefunction-based ADC(2) (second-order algebraic-diagrammatic construction) method, together with an electronic embedding scheme to explicitly account for environmental effects. We have applied this multi-scale methodology to study (i) the PF5-Y5 polymer(1,2), a model system for covalently bound donor-acceptor interfaces, and (ii) water/alcohol-processable quinoxaline (Qx)-based polymer donors(3), such as P(Qx8O-T). Key findings include the influence of molecular structure (e.g., OEG vs. alkyl side chains) on solution dynamics and intermolecular interactions, as well as the stabilization of π-π stacking conformations in films after solvent evaporation. Our analysis quantifies the effects of molecular dynamics and environment on electronic transitions, providing an improved description of optical absorption. Notably, double-hybrid functionals, incorporating a second-order perturbation (MP2) contribution in their formulation, delivered the most accurate TD-DFT predictions of singlet-triplet energy gaps, validated by experimental data. This study highlights the importance of incorporating disorder, dynamics, and molecular environment effects to accurately model the electronic properties of OPV materials, offering insights for the design of next-generation photovoltaic systems.

We unravel the atomistic mechanisms that govern the crystallization process of methylammonium lead iodide through the application of microsecond timescale molecular dynamics simulations based on the recently developed MYP2[1] extension of the MYP0[2,3] force-field for hybrid perovskites. The findings indicate that methylammonium iodide (MAI) and lead iodide (PbI2) precursors exhibit a propensity to aggregate into a disordered film, which ultimately undergoes a thermally activated disorder-to-order transformation to achieve crystallization. Notably, the crystal evolution during the annealing process reveals morphological characteristics consistent with the Stranski-Krastanov growth mode. The activation energy of 0.37 eV of the crystal growth may be ascribed to the energy required to dissociate defective Pb-I bonds and facilitate Pb diffusion. We also discuss the mechanisms underlying the spontaneous generation of lead vacancies and cation-cation antisites.

We conclude our talk by discussing perspectives in the study of crystallization by means of physics-based models as well as machine learning force-fields.[4]

Perovskite solar cells exhibit degradation when exposed to ambient (oxygen, humidity), illumination, elevated temperature, and bias. In recent years, there has been more effort in studying the photo/electrochemical reactions in 3D perovskites, which has led to increased understanding of their degradation under illumination and/or bias. The degradation is initiated by oxidation of iodide by photogenerated or injected holes, and resulting highly mobile oxidized species (interstitial iodide defects, I₂, and I₃^-) can then participate in additional redox reactions, as well as react with the organic cation. Compared to 3D perovskites, photo/electrochemistry of 2D perovskites is less well understood, despite the fact that these materials are commonly used in 3D/2D active layers in perovskite solar cells in order to improve device efficiency and stability. Furthermore, due to large number of possible 2D lead halide perovskite materials, relationships between 2D material structure and its properties are not sufficiently well understood. Here we will discuss the photostability of different 2D perovskites, and show that the stability is closely related to the formation of spacer cation vacancies [1]. As the spacer cation vacancies are less likely to form in Dion-Jacobson (DJ) 2D perovskites compared to Ruddlesden-Popper (RP) 2D perovskites [1], DJ perovskites exhibit better stability under illumination and yield better stability of perovskite solar cells. The implications of spacer cation vacancy formation on ion migration, photoinduced halide segregation, and stability under illumination in general are discussed.

Three-dimensional (3D) and two-dimensional (2D) halide perovskites have emerged as exceptional semiconducting materials over the past decade, renowned for their superior carrier lifetimes and structural versatility. However, a deeper understanding of the role of Pb²⁺ and Sn²⁺ ions, as well as the influence of organic spacers on the structure, properties, and performance of these materials, is critical for advancing their applications. In parallel, perovskitoids, a distinct but related class of materials, offer even greater structural and compositional diversity. Recent insights have illuminated how specific organic spacer cations can effectively stabilize various perovskitoid structures. We hypothesized that perovskitoids, with their robust organic-inorganic networks, could suppress ion migration in solar cells, thereby enhancing stability and performance.

By investigating perovskitoids across varying dimensionalities, we demonstrated that cation migration within perovskitoid-perovskite heterostructures is effectively suppressed, resulting in significantly improved long-term stability. Increased dimensionality in perovskitoids enhances charge transport, octahedral connectivity, and out-of-plane orientation. Notably, the 2D perovskitoids (Organic cation)₈Pb₇I₂₂ provide efficient surface passivation and enable the fabrication of uniform, large-area films. These properties have led to perovskite solar cells achieving a certified power conversion efficiency of 24.6% (Nature, 2024, 633, 359–364). This presentation will delve into the latest understanding of structure-property relationships in halide perovskites and perovskitoids, offering practical guidelines for selecting and incorporating organic spacers into crystalline materials and optoelectronic devices.

Although often quoted as ’defect-tolerant’, understanding and control of the many defects in the polycrystalline perovskite thin films are crucial for the achievement of stable and highly performant photovoltaic devices. Photoemission from within the perovskite band gap has often been attributed to the presence of various deep electronic defects on the perovskite surfaces. As such, ultraviolet photoelectron spectroscopy and microscopy are routinely used to directly probe the presence, spatial distribution, and trapping dynamics of these defects. However, the relatively high photoemission intensity and the broad spectral distribution of these mid-gap photoemissive states seem to suggest the presence of a hidden contributor in addition to potential localized electronic states within the semiconductor bandgap.

Here, we exploit a multimodal approach that is capable of measuring locally, at sub-micrometer scale, the surface crystalline and electronic properties of individual perovskite grains. For the first time the crystalline structure of the perovskite surface is reported and by correlating the crystalline and electronic properties of various perovskite compositions, we reveal the chemical nature of mid-gap photo-emissive states.

In this work, we present our most recent work related to the enhancement of perovskite solar cells stability via the application of MXene transport layers. We have modified the halide perovskite abdorber, the transport layers and the device interfaces via several methods such as additive engineering and the application of novel electron and hole transport layers like MXenes. For example, we use the 2D Ti₃C₂ MXene in normal PSC configuration of the type: FTO / c-TiO₂ / m-TiO₂ / halide perovskite (HP) / MXene / Spiro-OMeTAD / Au. We enployed the quadruple halide perovskite (HP) Rb₀CsMAFAPb(I_xBr_y)₃ and the DMA_1-XMA_XPbI₃ as the absorbers. The MXene (Ti₃C₂-T_x) was employed as the interface of HP and the hole transport layer (HTL) to fabricate HP/MXene heterojunctions. Our champion solar cells resulted in PCE above 22 %. Both indoor stability studies under ISOS-L protocol (continuous MPP tracking under N₂ atmosphere for 1000 h) and outdoor stability analyses under the ISOS-O protocol (MPP tracking, encapsulated devices) demonstrated the superior stability of PSCs when the MXene is employed. We also disclose the analysis of PSC stability via in-situ characterization where the effect of bias voltage, temperature or both is analyzed. Our results demonstrate the effect of strain in the crystal structure of halide perovskite sand its relation to stability.

Scalable manufacturing techniques are key to the commercialisation of perovskite solar cells (PSCs), with roll-to-roll (R2R) processes like slot-die coating emerging as transformative technologies. This talk explores advancements in depositing ultra-thin self-assembled monolayers (SAMs) via slot-die coating, demonstrating their potential to replace traditional hole transport layers in flexible PSCs. Through a combination of experimental optimisation and non-destructive spectroscopic analysis, we quantify deposition uniformity and, in particular, material coverage, demonstrating the precision and scalability of this approach.

The talk will address the significant challenges of transitioning to roll-to-roll (R2R) printing, including maintaining coating uniformity across large areas, controlling defect formation during high-speed deposition, and ensuring environmental stability under ambient conditions. It will also explore specific solutions developed for SAM deposition and their implications for the R2R fabrication of perovskite solar cells and modules.

The increasing demand for sustainable energy solutions has fueled significant progress in perovskite solar cells (PSCs), a promising photovoltaic technology with efficiencies exceeding 25%. Despite these advancements, achieving long-term operational stability and optimal device performance remains a critical challenge. This study focuses on both crystal growth control and interface passivation as dual strategies to address these challenges. First, the crystal growth mechanisms of hybrid halide perovskites were explored, with an emphasis on reducing charge recombination at grain boundaries and achieving controlled grain size distribution and large-area uniformity with enhanced crystallinity. By incorporating functional organic additives, the structural and optoelectronic properties of perovskite films were significantly enhanced, leading to improved device efficiency and stability. In addition to optimizing the perovskite layer, interfacial engineering played a pivotal role in this study. The tailored buffer materials, specifically bathocuproine (BCP) derivatives, were employed in the p-i-n PSC architecture to overcome instability of BCP. Modifying the functional groups of BCP improved molecular planarity, charge transport, and device durability. These optimized derivatives reduced recombination losses and extended device lifetimes, highlighting their importance in advancing PSC performance. This comprehensive approach, combining crystal growth control and interface passivation, demonstrates a pathway for achieving high-efficiency and stable PSCs, offering valuable insights for the commercialization of next-generation photovoltaic technologies.

Perovskite solar cells (PSCs) have attracted significant attention from both academia and industry, primarily due to their exceptional optoelectronic properties which enable high power conversion efficiencies (PCEs) of up to 26.7%¹. Additionally, their abundance of constituent elements and the potential for low-temperature, solution based thin-film processing make them attractive for cost-efficient, high-throughput production. Flexible PSCs, with their mechanical resilience and compatibility with roll-to-roll (R2R) fabrication, offer a promising pathway for large-scale applications. Despite advancements at lab scale, scaling PSC technology to industrial levels remains challenging. Key issues include adapting fabrication processes to industrial equipment while maintaining efficiency, achieving long-term operational stability comparable to commercial photovoltaic technologies with lifespans exceeding 20 years, and ensuring economic and environmental viability.

In this study, we address these obstacles by demonstrating the scalability of PSC technology through the use of R2R-compatible processing equipment. Our approach utilizes flexible aluminum foils with unique benefits such as high flexibility, lightweight properties, low cost, and excellent durability². Additionally, their high-temperature processing capability facilitates the use of advanced materials, including indium-free fluorinated tin oxide (FTO) electrodes and nickel oxide (NiO_x) hole transport layers (HTL), which are challenging to implement on conventional flexible substrates.

Perovskite solar cell stacks were deposited on various aluminum (Al) foil substrates using R2R-compatible methods and solvents. Prototype devices processed at the sheet-to-sheet (S2S) scale on 15 × 15 cm² Al/FTO substrates achieved efficiencies of up to ~15% with high reproducibility. Preliminary results of PSCs processed on Al/FTO substrates via the R2R method further underscore the strong scalability potential of this approach. Additionally, S2S-processed Al/ITO devices, fabricated using R2R-compatible solvents  under ambient conditions, showed efficiencies exceeding 15%, with outstanding stability.

Stability tests focused on PSC devices fabricated on Al/ITO substrates, where non-encapsulated devices exhibited thermal stability for over 3000 h at 85°C. To further assess device stability, damp heat test (85°C/85% RH) and light-soaking test, with additional applied heat (55°C), were conducted on encapsulated devices. The tested devices maintained visually stable for over 3000 h further supporting the robustness of these flexible perovskite solar cells.

Moving forward, we aim to enhance the efficiency, yield, stability and scalability of PSCs. Specifically, our focus will be to improve device efficiencies to levels comparable to lab-scale cells, approaching 20%. Additionally, we will work on achieving improved statistical distribution of efficiencies across larger areas, ensuring higher yield and reproducibility. Stability will be further evaluated through extended light-soaking tests under elevated temperatures, as well as rigorous indoor and outdoor performance testing. To advance scalability, we plan to transition from small-area prototypes to the fabrication of larger-area modules, paving the way for industrial-level production.

¹ https://www.nrel.gov/pv/interactive-cell-efficiency.html

² https://www.hyetsolar.com

The solution processing route of perovskite (PVK) photovoltaic (PV) fabrication has a very high potential for commercialization of the emerging technology. In the last years formamidiunium-based perovskite is leading the efficiency charts in single junction configuration for PVK PV reaching efficiencies above 26%.[1] The major challenges with those formulations are the stabilization of the photoactive α-phase, the buried, bulk and top interfaces passivation and the upscaling with industrial compatible techniques in ambient air environment in which the humidity can promote the δ-phase quite easily.[2]–[8]
Here we present a study on nickel-porphyrin additive as a bulk passivator for Cs-FAPI perovskite made in ambient air with scalable techniques (meniscus coating) and green antisolvent quenching. Three different substitution patterns with meso-phenyl and beta-phenylethynyl or beta-phenylimidazo-/-linker are explored as substituent in nickel-porphyrin structure to improve the dipole moment and the charge extraction consequently. Moreover, the terminal ammonium group in the above-mentioned porphyrin’s substituent can easily interact with the PVK lattice enabling the incorporation of metal-porphyrin system into the absorber layer. Coupling the bulk passivation effect of porphyrins and Cs-FAPI PVK trap states evidenced by one of the highest Voc (1.162) ever recorded to our knowledge with only 368 mV loss for a PVK band gap of 1.53 eV. The bulk and the top-interface passivations were studied in detail and optimized for small area cells (0.5 cm2 active area) and minimodules (10 cm2 active area). In both cases the maximum recorded efficiency was above 22% with enhanced light soaking (ISOS-L1) and shelf-life (ISOS-D1) stability above 1000h.

Perovskite solar cells (PSCs) have rapidly progressed among emerging photovoltaic (PV) technologies, evolving from proof-of-concept devices to high-performance modules within a decade[1]. Their key advantages are high defect tolerance, high absorption coefficient, and low-cost solution-processed fabrication techniques. These enabled roll-to-roll (R2R) manufacturing of PSCs, offering a pathway to scalable and high-throughput production. However, progress has primarily focused on planar architectures, which introduces fabrication complexity that limits scalability. For instance, transparent conductive oxides are comprised of rare elements such as indium, introducing a high risk in the photovoltaics supply chain [2].

We introduce Power Roll’s innovative microgroove-based architecture, a breakthrough designed to fully exploit R2R manufacturing. This approach employs a flexible substrate embossed with micro-grooves, enabling selective deposition of charge transport layers on the opposite groove faces and concomitant cell interconnection into modules [3]. This design simplifies manufacturing by eliminating the need for conventional transparent conductive oxides and multi-step scribing, enabling the production of lightweight, low-cost, and low-carbon-footprint perovskite solar modules. In addition, this leads to exceptional power/weight ratio, opening up new market opportunities.

This work aims to highlight the main advantages and challenges of manufacturing efficient and stable fully R2R processed perovskite solar modules by utilising the micro-groove architecture.

Perovskite-based solar cells have been extensively studied by the scientific community over the past decade and they are currently a very promising technology to be integrated into tandem PV module, for example associated with silicon solar cells [1]. However, one of the challenges lies in the up-scaling of the production of perovskite solar cells from small laboratory-scale cells (< 1 cm²) to larger modules [1,2]. In this context, there is considerable interest in extending the analysis previously conducted on a micrometer or millimeter scale [3-6] to a larger scale. Our work introduces, for the first time, full-sample size hyperspectral absolutely calibrated photoluminescence (PL) imaging applied to 16 cm² perovskite semi-transparent mini-modules.

We conducted HI photoluminescence acquisitions on semi-transparent perovskite mini-modules composed of glass/FTO/TiO2/triple-cation-perovskite/PTAA/ITO and followed their evolution over time for two weeks. In parallel, IV measurements were performed on the same semi-transparent module to track the corresponding evolution of electrical properties. We observe the presence of two categories of cells: cells that have local PL drops and relatively low PL intensity on average, and cells with relatively high PL intensity. The spectral information shows that their PL peak position is also different. By fitting the absolute spectra, we obtain the maps of local quasi-Fermi level splitting (QFLS), band gap value (E_g) and voltage loss (V_loss = E_g/q-QFLS), shown in the TOC graphic. This allows us to distinguish PL intensity variations caused by bandgap fluctuations from those due to defect-induced voltage losses.

In low-PL-intensity cells, local PL drops suggest probable shunt pathways. We found that these cells exhibit higher quasi-Fermi level splitting (QFLS) due to a better light-induced defect passivation but greater voltage losses due to larger bandgaps. In overall, they caused 2% of QFLS losses as compared to a theoretically homogeneous module without shunts. However, as supported by numerical simulations, they had the benefit of delaying the ageing of the device by screening ion migration.

When following the evolution over time of the mini-module, we observe a decrease in the bandgap and thus voltage loss, as the sum of the QFLS of the cells remains relatively stable (as shown in the TOC graphic). By comparing to the open-circuit voltage (V_oc) extracted from IV measurements, we could estimate that initially, the voltage loss due to the perovskite absorber and its interfaces (difference between blue and black curves) represented 92% of the total loss (difference between the blue and the red curves), while the remaining 8% are due to the collection properties and band alignment (difference between black and red curves). Over the two weeks of experiment the total V_loss of the module was reduced by 330 mV, with 220 mV (67%) coming from the passivation of the bulk and its interfaces, and 110 meV (33%) resulting from improvement in other layers properties and/or their band alignment with the perovskite absorber.

This work paves the way for a wide range of characterization studies to address for example the stability or scalability of many kinds of large size thin film devices. Probing compositional inhomogeneities and device performance at each step of the fabrication process of perovskite-based modules will be a powerful tool to accelerate their industrialization. It also opens the door for hyperspectral electroluminescence, which will provide a better understanding of charge transport properties at the module scale. Finally, it can also be applied to tandem characterization, in which additional layers complicate charge transport and may induce additional strain potentially leading to phase segregation.

This work has just been published in Solar RRL [7].

Polymer solar cells (PSCs) have become a promising technology for sustainable energy harvesting due to their flexibility, low cost, and potential for large-scale applications. The development of non-fullerene acceptors has enabled PSCs to achieve a power conversion efficiency exceeding 20%, highlighting the excellent industrial potential of this emerging solar technology. However, despite these efficiency gains, the long-term stability of PSCs remains a significant challenge that hinders their commercial viability.

We has been recently working on the intrinsic degradation mechanisms of polymer solar cells under light, heat and electric field stress. Especially, we recently demonstrated that unexpected interfacial degradation at the photoactive layer/MoO₃ interface is responsible for the thermal degradation of polymer:non-fullerene solar cells.[1-3] By proper interfacial protection of the anode interfaces, the cells showed an excellent thermal stability under 85 °C or even 150 °C thermal annealing.[2-3] Our findings underscore the effectiveness of our approach and provide valuable insights for the design of more stable polymer solar cells.

Reference:

Qin, X.; Yu, X.; Li, Z.; Fang, J.; Yan, L.; Wu, N.; Nyman, M.; Österbacka, R.; Huang, R.; Li, Z.; et al. Thermal-Induced Performance Decay of the State-of-the-Art Polymer: Non-Fullerene Solar Cells and the Method of Suppression. Molecules 2023, 28 (19), 6856.

Yu, X.; Wu, N.; Österbacka, R.; Ma, C.-Q. et al. Unexpected anode interfacial reaction lowering the performance of organic solar cells upon thermal annealing and method for suppression, manuscript under submission

Xi, Q.; Qin, J.; Wu, N.; Österbacka, R.; Ma, C.-Q. et al. Thermal Stability Improvement of Inverted Organic Solar Cells by Mitigating the Undesired MoO₃ Diffusion Towards Cathode with a High-Ionization Potential Interface Layer, manuscript under submission

Organic photovoltaics (OPVs) have seen tremendous progress after introduction of non-fullerene acceptors (NFAs) and recently achieved certified power conversion efficiency (PCE) 19.2% for small scale devices and 14.5% for the minimodules.^1&2 OPVs became materials of choice as organic active layers are easy to solution process in green solvents and easy to transition to roll-to-roll (R2R) manufacturing on flexible substrates using different printing techniques like, slot-die coating, blade coating etc. They have enormous potentials to be used in different targeted applications, e.g. indoor photovoltaics to power internet-of-things (IoTs), building integrated photovoltaics and in agrivoltaics by achieving semi-transparency. Zinc Oxide (ZnO) is the most commonly used electron transport layer (ETL) in OPVs but needs high temperature processing which limits its use on flexible substrates that are damaged by the required annealing temperature.

We are working towards development of process for R2R manufacturing of OPVs, and here we report comparison of different ETLs to achieve similar performance at low temperature processability, that can be implemented on ITO coated flexible substrates, specifically Polyethylene Terephthalate (PET). Different metal oxide ETLs compared in the study are Tin Oxide nanoparticles (NPs) from two commercial suppliers and Zinc Oxide from sol-gel recipe. Laboratory scale semi-transparent devices using PTQ10: BTP-4F-12 (Y12) active layer in a green solvent o-xylene were fabricated, elsewhere reported comparing different green solvents.³ We achieved average transmittance above 70% and comparable performance about 7.5% has been achieved using both Zinc Oxide (sol-gel) and Tin Oxide (SnO₂ NPs) processed 150^oC and 120^oC respectively. The process then transferred slot-die coating using progressive layer deposition, where initially just ETL and active layer slot-die coated, and hole transporting layer spin coated and then all three layers. i.e. ETL, active layer and hole transporting layers slot-die coated. Silver back contacts were evaporated to complete the device in both cases. Slot-die coated devices with 60% average transparency showed 9.3% PCE, with short circuit current density (Jsc) reaching to 19.9 mA/cm² with open circuit voltage (Voc) 0.85V and fill factor (FF) close to 55%, as shown in figure 1.

Further research towards translating these S2S printed devices to realise a complete module and translating the process to our state-of-the-art R2R fabrication facility to develop flexible modules is ongoing and will be the part of this talk.

Conventional silicon-based (c-Si) photovoltaic (PV) modules present limitations when used in agrivoltaic systems. They compete with crops for sunlight and create uneven shading, which can disrupt consistent plant growth. Alternative thin-film PV technologies, such as lead-based perovskites or cadmium telluride (CdTe), raise environmental concerns due to toxicity, while options like copper indium gallium selenide (CIGS) depend on scarce resources. Organic photovoltaics (OPVs) present a promising solution, offering environmentally friendly, thin-film technology with exceptional flexibility in light absorption. These features allow customization of transparency and color and enable seamless integration into various shapes and surfaces, overcoming the drawbacks of traditional silicon panels.

This ongoing project focuses on transforming agrivoltaics by incorporating transparent OPV modules with tailored light absorption into agricultural structures. The goal is to optimize conditions for plant growth by managing light intensity, diffusion, and spectrum, while providing mechanical protection against severe weather, such as hail, wind, and rain. By combining power generation and agriculture, the project aims to establish sustainable systems that benefit both sectors.

Therefore, the project investigates combinations of organic semiconductor materials to simultaneously enhance device performance and selective transparency in a scalable fabrication process. The research focused on optimizing donor-acceptor blends, photoactive layer materials and thicknesses tailored to the light requirements of seedling growth of model plants. Additionally, the study explores various hole transport layers, transparent top electrodes, and encapsulants to improve solar cell performance and stability. Both rigid and flexible devices are examined, offering versatility for integration in greenhouses and other agrivoltaic systems. Results using the PTB7-Th:IEICO-4F active layer blend demonstrate power conversion efficiencies of 5.2% in rigid, semi-transparent modules on glass, with ongoing optimization for flexible modules. Furthermore, a ternary blend of PTB7-Th:IEICO-4F:PC70BM was successfully implemented to enhance performance and stability,  confirmed through light soaking tests. The study also monitors the variations in device performance that result from the upscaling processes to evaluate and mitigate efficiency losses. The current phase of the project involves large-scale production of flexible, semi-transparent OPV modules with a commercial partner, aiming for integration into agricultural protective nets. A 10 m² prototype is planned for installation in an apple orchard experimental station by April 2025. In conclusion, with this study we aim to find a layer stack and substrate combination that results in a working OPV module suitable for modern agrivoltaics and in a manner that has the potential to revolutionize the field.

Recent advances of polymer solar cell materials and technologies allow the development of semitransparent devices, where the transmittance is optimised n order to maximise the transparency normalised to the overall A and B chlorofill spectrum.
In analogy to Average Visible Transmittance (AVT)  for Building Integrated PhotoVoltaics (BIPV), Agrivoltaic (AgriPV) performances [1] of Organic Photo-Voltaic (OPV) cells and modules can be measured in terms of ACT (Average Chlorofill Transparency) and Light Utilization Efficiency (LUE) for AgriPV.
Scalable approaches are investigated, in order to avoid processing in protected atmosphere and chlorinated solvents.
Recent results on PTQ0:DTY6 in o-Xy processed in air will be shown.
We demonstrate how this donor-acceptor system is an ideal candidate  for AgriPV.
Our analysis will be based on detailed morphological characterisation on small area test cells, and on large area area modules fabrication and characterization. A preliminary set of growth test on selected crops for AgriPV tests will be also shown.

Semitransparent organic photovoltaics (STOPVs) hold significant promise for applications such as power-generating windows in buildings and agricultural greenhouses, owing to their tunable optical properties and ability to harvest near-infrared light while remaining semitransparent in the visible range.^[1–3] Despite recent advancements, upscaling STOPVs from small laboratory cells to large-area modules remains a significant challenge. Maintaining high performance while ensuring a uniform, aesthetically pleasing appearance for architectural applications often results in compromises, such as losses in power conversion efficiency (PCE) and average visible transmission (AVT). Additionally, large-scale fabrication processes must meet industry standards, such as avoiding the use of halogenated solvents.

In this study, we present two semitransparent organic modules (STOMs) with a device area of 11.4 cm² that address these challenges by optimizing optical and electrical properties while eliminating the use of halogenated solvents. Both modules exhibit a visually homogeneous appearance, suitable for architectural integration, and retain up to 92% of the respective PCE of their small-area counterparts. The first module uses a multilayered aluminum-doped zinc oxide and silver back electrode combined with a conductive, metal-free top electrode from PEDOT: PSS. A laser-structured direct contact between this top electrode and the silver layer within the back electrode enables efficient monolithic interconnection, achieving a PCE of 6.1% and an AVT of 47.5%. A second design incorporates an extended back electrode with TiO₂ and SiO₂ as dielectric bragg reflector, further enhancing optical and electrical performance. This improved architecture achieves an unprecedented AVT of 50.8% and a PCE of 7.9%, yielding a record-high light utilization efficiency (LUE = AVT × PCE) for modules of 4.0%.

The unproven durability of perovskite photovoltaics (PV) is likely to pose significant technical hurdles in the path towards the widespread deployment of this burgeoning thin-film PV technology. The overall durability and reliability of perovskite PVs, which is related to the operational-stability, is directly affected by the mechanical properties of metal-halide perovskite materials, cells, and modules, but this connection has been largely overlooked. This is particularly important in flexible perovskite PV which are subjected to higher mechanical stresses. Thus, there is a sense of urgency for addressing the mechanical reliability issue comprehensively, and help perovskite PV reach their full potential. To address these final technical hurdles, several rationally-designed interfacial tailoring approaches are used to enhance the mechanical properties. Most importantly, these approaches are designed such that they also increase efficiency of perovskite PV. The important challenges and opportunities, together with best practices, pertaining to the three key interrelated elements that determine the mechanical reliability of perovskite PV are discussed: (i) driving stresses, (ii) mechanical properties, and (iii) mechanical failure. The scientific rationales for these approaches to improve the mechanical properties are also discussed, together with the presentation of examples where failure-mitigation results in more efficient, durable, and reliable perovskite PV.

In this presentation the potential of metal halide perovskite solar cells for space power applications will be presented. This presentation will focus on our recent work on the feasibility of metal halide perovskites (MHPs) for space, which includes the assessment of the systems in low-intensity-low-temperature (LILT) conditions for outer planetary missions, in addition to exposure to high temperatures and variable radiation conditions that directly and independently impact the absorber and transporting layers of the devices.  Here, the discussion will include a number of perovskite systems such as solution processed mixed Pb-Sn systems and the FAMACs family of MHPs, as well as blade-coated architectures. It will be shown that perovskites display remarkable tolerance to high radiation exposure and that while measurements do suggest high energy radiation negatively affects the transporting layers and interfaces in the devices: the perovskite absorber is not affected in any significant way. Moreover, these systems are observed to self-heal under ambient conditions in the dark demonstrating the unique behavior of perovskite solar cells and their potential for future space applications. This is further substantiated by high temperature measurements that indicate specific triple cation perovskites displays no appreciable or permanent degradation up to 500 K, supporting in particular their potential as candidate systems for future lunar missions. Finally, our recent work on multiple stressors and the effects of MHP solar cells on the ISS orbit will be presented which provide a more practical assessment of the systems in space.

Flexible perovskite solar cells (f-PSCs) are particularly well-suited for a range of applications, including outdoor consumer products (e.g., portable chargers, wearables, tents, backpacks, deployable roll-ups, vehicles, drones, sails, etc.),([1]-[3]) indoor internet-of-things (IoT) devices,[4] and space technology.[5] Additionally, lightweight and manufacturability (e.g. roll-to-roll) are attractive features of f-PSCs for residential rooftop [6] and utility-scale PV applications.([7], [8])

Typically, f-PSCs are subjected to much higher applied mechanical stresses (stretching, bending, twisting) during manufacturing and operation, compared to their rigid counterparts on glass substrates.([9], [10]) Thus, the mechanical reliability of f-PSCs plays an outsized role in determining their durability.([9], [10]) Although numerous studies have documented cracking in the brittle, thin layers of multilayer f-PSCs, such as transparent-conducting oxide (TCO) electrode and perovskite, during bending tests, ([11], [12]) effects on polymer substrates in f-PSCs, and other flexible electronic devices, remain underexplored. This oversight likely stems from the assumption that polymer substrates, given their high toughness and substantial thickness relative to the other layers, are unlikely to crack. Contrary to this assumption, here we reveal pervasive, severe, and extensive cracking of polymer substrates in f-PSCs subjected to bending. Importantly, we show that such cracking of polymer substrates in the simple TCO/polymer bilayer, which is widely used in flexible electronic devices, is a general phenomenon. Substrate cracking undermines the mechanical integrity and reliability of the entire device, making it susceptible to cyclic fatigue and other time-dependent failure mechanisms, such as creep and environment-assisted cracking or degradation.

Based on in-situ experiments and modeling studies, we identify the substrate-cracking mechanisms specific to f-PSCs, which are related to the film/substrate elastic mismatch. Based on this understanding, we design and demonstrate a substrate-cracking mitigation strategy that relies on interlayer engineering. This approach is generic and holds promise for application to not only f-PSCs and OPVs but also myriad other flexible devices.

The advancement of flexible photovoltaic (PV) technologies needs the establishment of standardized protocols for their characterization, particularly concerning mechanical performance and durability. Here we show newly proposed standard procedures such as ISOS-B, ISOS-M and a new protocol for measuring PCE over 1000 bending cycles under 1% strain, which aim to complete existing guidelines such as IEC 61215 and IEC 61646.
ISOS-B focus on bending stress testing. This protocol is crucial for accurately assessing the mechanical stability of flexible PV devices, which can exhibit negligible degradation even under extreme bending conditions. ISOS-M addresses mechanical and long-term stability under various environmental conditions, suggesting tests in controlled ambient conditions to isolate the effects of mechanical stress from other environmental factors. This is essential for understanding how flexible devices perform over time, especially when exposed to real-world conditions. We further evaluate the need to measure the mechanical stability of a flexible device independently from the thickness and on the material properties of the substrate, by calculating its strain and defining a new figure of merit called fatigue factor. The introduction of a fatigue factor quantifies the mechanical performance of flexible PV devices, facilitating objective comparisons of durability and efficiency. This factor allows researchers to evaluate the relationship between mechanical stress and electrical performance. In conclusion, by improving upon existing IEC guidelines, these standards provide a robust framework for evaluating the mechanical and environmental performance of flexible PV technologies, ultimately contributing to their commercialization and integration into different applications. The integration of these standardized procedures fosters advancements in flexible photovoltaics, leading to improved device reliability and performance in practical applications.

The study of perovskite solar cells degradation is a complex issue due to the multitude of phenomena that can contribute to it. 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. 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 evolution of degradation in the perovskite solar cells. We discuss the capacitive and inductive effects that contribute to increased recombination and reduced charge collection.

Perovskite solar cells (PSCs) typically feature a simple planar structure, where the perovskite photo-absorber layer is sandwiched between two electrical contacts and their respective charge-selective layers [1]. This configuration enables efficient extraction of photo-generated charge carriers from the perovskite layer, leading to remarkably high power conversion efficiencies (PCEs) exceeding 26% in state-of-the-art PSCs [2]. However, further improvements in PCE are partially constrained by inherent light transmission losses. These losses stem from reflection at material interfaces and parasitic light absorption by the substrate, the transparent conducting electrode, and the charge-selective layers [3].

In my presentation, I will address this challenge by exploring an alternative device architecture known as the back-contact (BC) structure [4]. In BC PSCs, all electrode components are positioned on one side of the perovskite active layer, allowing the perovskite photo-absorber layer to be directly illuminated. This design effectively eliminates transmission losses associated with traditional planar structures. Moreover, the BC structure offers enhanced access to the perovskite photo-absorber layer, enabling further electro-optic optimizations such as surface trap state passivation and the application of anti-reflective coatings [5,6].

From a device physics perspective, the unique geometry of BC PSCs decouples the properties of the photo-absorber from those of the electrodes. This distinction is invaluable for investigating the degradation mechanisms of the perovskite photo-absorber layer under environmental stressors. These studies can be conducted through in situ measurements during exposure or through post-exposure analyses, providing critical insights into the stability and performance of BC PSCs.

Textured interfaces have demonstrated to be an effective strategy to enhance light harvesting in solar devices. Different texture schemes applied to the many types of solar cell technologies have been reported over the years, demonstrating their potential as light trapping structures. Nevertheless, the effect of a particular texture design may not be restricted to enhance light absorption, but also to induce favorable conditions for the fabrication, performance and stability of the subsequent layers in the device. Consequently, in certain solar cell technologies, a textured device may outperform the planar configuration due to the combination of several factors.

During the talk, different texture arrays applied to either rigid or flexible substrates will be analyzed along with their effect in the perovskite-based solar cells performance. Moreover, some of such arrays were applied to multi-junction configurations such as Si-perovskite or all-perovskite tandem devices, expanding their application beyond standard single-junction devices. Apart from the expected increase in the current delivered by the device, some textures also enhanced other properties such as wettability, perovskite growth or bending performance. Finally, the unification of certain textures to induce hierarchal arrangements will be discussed as a technique to retain the benefits from more than one texture configuration.

One of the key properties of hybrid perovskites is ambipolar transport, the ability to transport equal electric currents with both electrons and holes. Such feature is commonly attributed to pure, intrinsic semiconductors and ultimately descends from their proverbial defect tolerance, that makes their optoelectronic properties minimally affected by defects in their crystal structure or composition. A current significant challenge is to get reliable n-type or p-type doping in hybrid perovskites, which would be crucial to unlock improvements in devices requiring unipolar transport, such as field effect transistors or light emitting devices, as well as to limit charge recombination at interfaces in perovskite solar cells, in order to enhance their power conversion efficiency. Here we show experimental study of wide-bandgap perovskite solar cells with the inclusion of a controlled amount of two-dimensional transition metal carbides (MXenes Ti3C2Cl) for interface engineering [1]. The photocarrier dynamics in the active area of the full device was explored with ultrafast optical spectroscopy by combining time resolved photoluminescence and differential transient transmission, in a configuration analogous to that as reference [2]. Our technique disentangles photoexcited electron and hole dynamics, providing direct access to their separate trapping rates, their concentrations and their bimolecular recombination rates.

The efficiency of p-i-n perovskite solar cells has recently seen a significant boost, primarily due to the integration of self-assembling molecules (SAMs) as a hole-transporting layer (HTL). SAMs offer several advantages over traditional HTLs, including minimal material usage, low cost, and ease of processing. Current efforts are largely focused on carbazole-based phosphonic acids such as 2PACz and its derivatives. However, the flexibility of organic chemistry enables the creation of phosphonic acids with alternative organic cores that may provide unique benefits.

We designed and synthesized three novel phosphonic acids for use as an HTL in p-i-n perovskite solar cells, each featuring an aromatic core commonly found in organic semiconductors. The resulting SAMs differ in their energy level alignment with the perovskite active layer. Among them, a pyrene-based phosphonic acid (4PAPyr) achieves optimal alignment, leading to solar cells that outperform the commercially available 2PACz. After device optimization, a power conversion efficiency of 22.2% was attained with 4PAPyr. Solar cells using 4PAPyr possess increased stability under maximum power point tracking under continuous 1-sun light soaking as compared to those using 2PACz. Furthermore, improved surface coverage on ITO with 4PAPyr results in a notably higher yield of functional solar cell devices and a reduced spread in device efficiencies as compared to 2PACz, a crucial factor toward the development of large-area solar cells using SAMs as an HTL. Our study highlights the critical role of diversifying phosphonic acid design to drive further advancements in the efficiency, stability, and scalability of perovskite solar cells.[1]

Transition metal oxides have attracted increasing attention for applications in electron and hole transport layers of hybrid and organic photovoltaic devices. These layers present complex electronic structures due to the presence of narrow electronic d-bands within the bandgap. Such bands are expected to significantly influence the carrier transport on the layers, thereby affecting their role in solar cells.
In this context, the optimization of depositions and accurate optical characterization of oxide electron and hole transport layers were performed. The depositions were performed by reactive sputtering of metallic targets using variable O₂/Ar flow rates. Detailed plasma characterization included optical emissions and self-bias voltages as a function of the O₂ flow and deposition powers.
The optical analysis was based on approximation free expressions, using spectral transmittance, reflectance, and ellipsometry data. It allowed accurate determinations of the refractive index, absorption coefficient, bandgap, and layer thickness (20 ≤ h ≤ 1200 nm). The focus was on MoO_x and Nb₂O₅ electron transport and NiO_x and CoO_x hole transport results. Except for Nb₂O₅ layers, which display classical characteristic semiconducting absorption edges, the spectra show clear absorption effects inside the bandgap associated with the presence of the d-states. The relationships between the optical results and electronic structure besides the structural and morphological characteristics and deposition parameters were analyzed. 
The effective Nb₂O₅ ^[1,2] and CoO use of the layers to lead-halide perovskite cells are presented, and the potential applications of MoO_x and NiO_x layers are discussed.  
 

Non-fullerene acceptors have revolutionised organic photovoltaics. However, greater fundamental understanding is needed of the crucial relationships between molecular structure and photophysical mechanisms. Herein, we use a combination of spectroscopic, morphology, and device characterisation techniques to explore these relationships for a high performing non-fullerene acceptor, anti-PDFC. We focus on transient absorption spectroscopy across multiple timescales and ultrafast time-resolved vibrational spectroscopy to acquire the “holy grail” of simultaneous structural and dynamic information for anti-PDFC and its blend with the well-known conjugated polymer PM6. Most significantly, we observe that the singlet exciton of anti-PDFC is localised on the perylene diimide central core of the molecule, but the radical anion is primarily localised on the fluorinated indene malonitrile terminal units (which are common to many state-of-the-art non-fullerene acceptors, including the Y6 family). This electron transfer from the central core to the termini of an adjacent molecule is facilitated by a close interaction between the termini and the central core, as evidenced by single crystal diffraction data and excited state calculations. Finally, the very efficient charge extraction measured for PM6:anti-PDFC photovoltaic devices may be correlated with this anion localisation, enabling effective charge transport channels and thus enhancing device performance.

Ultrafast transient absorption (TA) technique allows unique investigation of perovskite material close to the interfaces with electron and hole transporting layers (ETL and HTL, respectively). It can be realized when the excitation pulse has short enough wavelength with respect to the perovskite absorption onset (yielding penetration depth < 50 nm) and when the probe pulse delay is short enough (<100 ps) that photoexcited electrons and holes are not yet redistributed by the diffusion over the whole perovskite thickness. As we have shown earlier, different charge cooling, charge recombination dynamics and different charge extraction rates to ETL and HTL can be observed [1,2].

However, such selective probing can also give valuable and insightful information about the structural differences at both interfaces, as well as photoinduced changes occurring close to ETL and HTL. It is possible due to intense and narrow TA bleach signal observed in perovskites [3] which are sensitive to the bandgaps of the bulk phase or low-dimensional phases. We will show our new findings related to photoinduced ion segregation in triple cation mixed halide perovskite as well low-dimensional phase distribution in quasi-2D perovskites, recently reported [4,5] or currently developed.

Regarding the ion segregation, we observed the strongest photoinduced redistribution of the halides near ETL, especially when mesoporous TiO₂ is used as an ETL. A surprising dependency of the ion segregation rate and extent on the excitation conditions (pulsed or continuous), pump fluence and temperature was revealed. For example, the segregation time can increase from 30 minutes at single μJ/cm² fluence to 100 minutes at 0.1 mJ/cm² fluence, and it can be ~3 times slower when the sample temperature is increases from room temperature by ~30K [4]. The results can be explained by the model including the local electric field and local heating effects.

As for 2D perovskites, an asymmetry of the quasi-2D phases distribution close to ETL and HTL was found (typically more low-dimensional phases are near the bottom layer [6]). The asymmetry depends on the preparation conditions of perovskite layer and can be correlated with the efficiency of the cell. Moreover, adding passivation MXene layer to HTL decreases the amount of quasi-2D phases in perovskite close to this layer. For both triple cation and 2D perovskites, the photostability and damage thresholds were also found.

Finally, novel findings related to the observation of coherent acoustic phonons will be also shown [4]. Upon the ultrashort pulse photoexcitation close to ETL or HTL, at certain conditions (e.g high pump fluences of single mJ/cm²) an acoustic wave propagation in perovskite [3] can be observed and the sound velocity in perovskite material can be determined from the oscillation pattern in TA signal. The differences in the TA oscillation mechanism between the bulk and 2D phases was also revealed.

In conclusion, our results show the great potential of using ultrafast TA to obtain many information in perovskite solar cells beyond the typical time-resolved findings (e.g. charge cooling, recombination and extraction times). Important phenomena related to the structure and its photoinduced modifications of perovskite near ETL and HTL interfaces can be exclusively investigated using ultrafast TA technique. Many of them are hard or impossible to be studied by other commonly used structural techniques that only probe the average response of the whole perovskite layer.

Lead halide perovskites have been in the lime light of emerging photovoltaic materials the last decade, due to their high absorption coefficient, high defect tolerance and charge mobility, and high power conversion efficiency in solar cell devices. The photoexcited charge density response is here important for understanding lead halide perovskites during operation, and is in turn related to the material structure [1,2], photoinduced response, and subsequent electronic and lattice relaxation in the system. In this contribution, we present investigations of the photoinduced ion migration mechanism and nature of the excited state [3-6] and their relation to pathways for electronic and lattice relaxations with both experimental and theoretical probes. In particular, we present how the A-site cation and type of halide affect the chemical bonding and photoinduced ion movement in the system. We show that the excess energy after thermalization into phonons under blue-light illumination is large enough to overcome the activation energy for iodide displacement, and can thus trigger vacancy formation and ion movement in contrast to red-light illumination [4,5]. Here, a dipolar A-site cation would decrease the energy of defect formation, but instead impede defect migration [6]. We highlight the high thermal expansion of the system compared to common substrate materials [7], an effect that recently has been found to be crucial to mitigate in order to achieve high-efficiency solar cells. [8]. Recent collaborative work has exploited this knowledge to fabricate solar cells with certified efficiencies above 26% [9, 10] with high tolerance to thermal stress [10]. We will briefly also report how the type of halide and number of monolayers affect the excitonic and vibrational properties in 2D perovskites. By addressing ion migration, interfacial strain management, and quantum tuning through halide selection and layer control, the findings contribute to the development of high-performance, stable perovskite-based technologies with tailored optical and electronic properties.

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

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

Using a careful low-dose scanning TEM (STEM) protocol, we are also able to image these materials in their thin-film form with atomic resolution. [4] Our images enable a wide range of previously undescribed phenomena to be observed, including a remarkably highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI₂ interfaces, with a striking absence of long-range disorder in the crystal. These findings explain why inter-grain interfaces are not necessarily detrimental to perovskite solar cell performance, in contrast to what is commonly observed for other polycrystalline semiconductors.

Combining broad beam electron diffraction with 4D STEM (or scanning electron diffraction), I will also show why certain 2D perovskite materials are better than others at improving device performance and stability.

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

Current silicon photovoltaics are approaching the single-junction efficiency limit, which is largely imposed by the inevitable thermalization losses caused as above-gap photocarriers relax to the band-edge prior to extraction. These losses could be mitigated if the excess energy of a photon could instead be used to produce an additional excitation, and organic molecular singlet fission has been touted as a highly efficient method of multi-exciton generation towards this goal. Harvesting the nascent triplets in a silicon cell via transfer across an appropriately designed interface could then lead to improved photocurrent. An alternative approach is to radiatively couple a chromophore to silicon via quantum cutting, wherein the absorption of a high-energy photon leads to emission of two down-converted photons closer to the silicon band gap. Here, I will discuss our efforts to design augmented silicon devices via singlet fission, with an emphasis on the effect of the interface between the organic chromophore and silicon. Through magnetic field-dependent photocurrent and photoluminescence measurements, we identify systems which allow for triplet energy transfer into silicon solar cells. We further identify the band alignment requirements necessary to enable triplet energy transfer and how this progresses towards commericalising singlet fission solar cells. In addition our efforts in developing singlet fission enhanced silicon photovoltaics, I will discuss how charge and energy transfer at hybrid inorganic-organic interfaces can lead to new classes of optoelectronic devices, with an emphasis on 2D materials.

Single-junction organic photovoltaics (OPVs) nowadays have reached promising power conversion efficiencies, around 20%. Besides new materials, going beyond the current efficiencies could, in principle, be achieved by multi-junction devices, which promise a reduction in thermalization and absorption losses [1]. In this talk, we will present a multi-junction in-plane spectral splitting geometry that we call Rainbow solar cells and that aims at overcoming the limitations of stacked solution processed devices [2]. In the Rainbow geometry, a series of sub-cells are placed next to each other laterally, and illuminated through an optical component that splits the incoming white beam into its spectral components, thus matching local spectrum and absorption for each sub-cell. The fabricated n-terminal devices are capable of extracting the maximum power of each sub-cell without the need for current matching nor processing challenges. We demonstrate the concept for a high and low band-gap sub-cells, obtaining an efficiency increase of around 30% of the Rainbow geometry with respect to our best single junction device [2]. Then, we use high throughput methods based on gradients on the parameters of interest and blade coating [3-5] to screen tens of materials exhibiting either wide band gap [4] or narrow bandgap [5], and thus push the efficiency of the Rainbow multi-junction further up. Combining experiments and simulations we provide material design rules for this type of device both, for binary and ternary based sub-cells. Finally, the major limitations will be discussed.

The industrial deployment of organic photovoltaics (OPV) is currently constrained by various factors, such as the scalability of manufacturing techniques and the integration of OPV into practical applications [1]. One of the most effective methods for fabricating interconnections in OPV modules is short-pulse laser patterning [2], as this allows for high precision interconnections with minimum dead area in the module [3]. Nevertheless, this technique is challenging to optimize due to the wide variety of materials used in OPVs and the broad range of laser ablation parameters. The work presented here proposes a universal methodology to optimize the laser ablation parameters for a wide range of laser characteristics and materials. The methodology has been successfully tested with various materials in the OPV stack with nanosecond lasers, which, although more challenging to utilize than femtosecond lasers, offer greater industrial interest due to their lower cost [4]. Semitransparent OPV modules on both rigid and flexible substrates have been fabricated using laser ablation, with efficiencies reaching 4.3 % for a module area of 164 cm², underlining the promise of the technique for large-scale manufacturing and paving the way for further work to enhance the performance of these modules.

Additionally, the study addresses aspects related to the integration of OPV modules into plastic components, thereby enhancing their market value, potentially useful in different applications. The thermoplastic injection technique is employed to integrate the modules directly into plastic pieces. This process not only enables the 3D shaping of the modules, but it also imparts specific mechanical properties [5]. In this work we demonstrate for the first time the incorporation of new surface and optical properties to injection moulded OPV modules through micro and nanotexturing. First, we combine nanosecond laser ablation and thermoplastic injection to modify the surface properties of the OPV module. Microstructures are successfully replicated onto polycarbonate and polymethyl methacrylate plastic parts, and their properties are characterized by means of confocal microscopy, optical transmission, as well as contact angle measurements to confirm the enhancement in surface hydrophobicity from ~90º to ~135º. Then, light management nanotextures are successfully transferred to in-mold OPV modules, thereby enhancing their transmittance by 4.5 %.

To accelerate the optimization process of solar cells, a deep understanding of material systems is essential. This is particularly important for emerging materials such as organic semiconductors and halide perovskites, which offer a larger degree of variation with respect to the stoichiometry and crystallinity. The downside of this chemical versatility is the larger amount of unknown optical and electronic properties of these emerging optoelectronic materials that triggers the need for more extensive efforts towards material and device characterization as compared to e.g. crystalline silicon. However, the strong correlation between material properties makes it challenging to isolate individual parameters using experimental data.

In the past, numerical simulations such as drift-diffusion simulations have enabled researchers to reconstruct characterization data using known material parameters (e.g., mobilities, recombination coefficients, band gaps) and compare the results with experimental data to analyze solar cells fabricated in a lab.^[1-3] Nevertheless, the traditional fitting routine for inferring parameters from one or several measurements is time-consuming and relies on a deterministic approach that lacks any quantification of the confidence in the uniqueness of the resulting fit.

To address these limitations of traditional fitting of numerical models to experimental data in the context of photovoltaics, we applied a methodology from the field of machine learning. We use a neural network as a surrogate model for the device simulation. The role of the neural network is to speed up (by a factor of 10⁵) the process of device simulation within a range of predefined parameters.^{[4, 5]} To include information on the confidence in and the uniqueness of the resulting fits, we employ the framework of Bayesian Parameter Estimation (BPE)^[6-8] methods. In this work, we have adapted this workflow to the parameter inference problem in organic solar cells. By leveraging the power of machine learning, we can efficiently explore the complex parameter space and provide a more accurate and robust characterization of organic solar cells. By analyzing JV curves of a certain blend of organic solar cells at different light intensities and for different thicknesses, we were able to infer electronic properties such as mobilities, recombination coefficients, and defect densities. Moreover, this approach provides a posterior probability distribution after observing an JV curve, allowing us to quantify the information gained per additional unit of experimental information considered for the inference process.

The application of hybrid organic-inorganic perovskite materials is hampered by issues related to their operational stability in response to external stimuli, such as voltage bias and light, which is associated with their mixed ionic-electronic conductivity.1–3 This can, to an extent, be overcome by incorporating tailored organic moieties within hybrid perovskite frameworks that can provide impermeability to ion migration and enable the formation of low-dimensional architectures with enhanced stability and functionality.2–3 We explore their dynamic control under operating conditions in response to voltage bias and light4–5 to enable long-term stabilization while opening perspectives toward advanced functionality beyond photovoltaics.6–7

Hybrid halide perovskite has established its credibility as high performance thin film photovoltaic technology. In only one-decade, the hybrid organic-inorganic halide perovskite solar cell achieved to compete with all mature crystalline technologies, by reaching a certified 26.7 % power conversion efficiency (PCE) on cells and 20.6 % PCE on small modules. Perovskite’s strength stem from their remarkable opto-electronic properties. However, the technology still requires significant attentions regarding stability, in particular rapid structural and electronic degradation can be engendered when exposed to various external stressors (temperature^[1,2], humidity^[3-5], light^[6-7], electrical bias^[8]). 

To cope with the long-term stability issue, it is a paramount to precisely understand the multiple degradation pathways of the perovskite upon and during the external stressing. To this end,  in situ or operando characterization techniques are central tools. In this communication, we will be discussing the degradation of different perovskite composition on the basis of humidity or temperature-controlled in situ x-ray diffraction and corroborated with in situ electron spin resonance spectroscopy and in situ transmission electron microscopy. For example, one key finding which we will discuss is that α-FAPbI₃ degradation is substantially accelerated when temperature is combined to illumination and when it is interfaced with the extraction layers, and, second the existence of a temperature gap region which takes place only under illumination involving an intermediate stage between the thermal-induced perovskite degradation and the formation of PbI₂ by-product.^[9]

Many efforts are being made to upscale perovskite solar cells (PSCs) and modules while retaining the characteristics of the solar cell at the lab scale. Unfortunately, the efficiency of these devices drops significantly with increasing active area¹. This decline can be attributed in part to the resistance of the electrodes but also to layer inhomogeneities leading to areas of poorer efficiency and shunt paths². One attractive way to investigate non-uniformities and their origins in mid- to large-area perovskite solar cells is through imaging techniques.

Using an in-house developed multispectral imaging setup, we took electroluminescence (EL), photoluminescence (PL), and lock-in thermography (LIT) images of carbon-based perovskite solar cells and modules produced by Solaronix SA. Steady-state and transient images show different types of inhomogeneities originating from various fabrication issues. By combining the experimental characterizations with FEM and drift-diffusion simulations, we are able to understand better the origin and nature of these defects.

In particular, we measured and modelled carbon perovskite solar cells with macroscopic defects, appearing as hotspots in LIT measurements. In contrast to the traditional dark lock-in thermography (DLIT) method, which uses a large voltage modulation, we developed a small-signal LIT imaging (SS-DLIT) technique together with a frequency and thermal-dependent FEM model which is capable of simulating these experiments. Fitting thermal AC simulations to bias voltage-dependent measured data allowed us to quantify the diode-like behaviour of the hotspots and examine their origin.

Additionally, we assess slow transient effects (from seconds to several hours) seen in the EL images of encapsulated carbon PSCs. To better understand local and temporal variations in the EL signal, we modelled the complete device stack with drift-diffusion simulations, fitting the material parameters to steady-state and impedance data and using a recombination-coupled emission model to obtain the transient EL signal. We assign the temporal turn-on dynamics to the migration of two mobile ions with different mobilities, and we clarify the effect of their distribution and local concentration on the radiative recombination.

The widespread commercialization of metal halide perovskites in semiconductor applications is still hindered due to the instability of the perovskite itself[1,2]. The intrinsic instability primarily arises from the influence mobile ions in the film, which cannot be trivially circumvented[3]. However, ionic reactivity is not always detrimental; these ions are responsible for the well-known “perovskite healing” effects and can even result in perovskites exhibiting memory-like behaviour[4-6]. Understanding the precise mechanisms and influences of the different ionic species is therefore crucial for optimizing device performance for PV and LEDs, and can open up new technological pathways which exploit the positive ionic effects (examples include perovskite computing and perovskite memory).  

To date, key techniques used to study ion dynamics are predominantly electrical; techniques include impedance spectroscopy, transient ion drift, etc.[7-9]. These methods – indeed, all electrical methods - are inherently limited by the requirement of electrical contacts, restricting their applicability exclusively to operational devices. This is a significant drawback as the electrical response in perovskite devices often is dominated by the interfaces and contacts, rather than from the ions in the bulk[10,11]. This makes it challenging to disentangle the intrinsic ionic properties from surface and interfacial effects. Moreover, such electrical methods are not applicable for contact-free applications of perovskites, such as phosphors. Finally, it is challenging to conduct spatial mapping of properties with electrical measurements, which can be useful to understand local variations and inhomogeneities.

To overcome such drawbacks, we present an alternative fully optical technique which is sensitive to ionic processes across all relevant timescales. We present an overview of our technique and the results obtained for a triple-cation, double-halide perovskite. We show how the ionic properties, such as the characteristic lifetimes, resistances and more can be extracted using our method. Furthermore, we present the added benefits of analyzing ionic processes purely optically; mapping is possible using our technique, and half-stacks or bare films can be measured. These benefits greatly simplify the interpretation of the data, which is otherwise generally complex in the electrically equivalent techniques. With our approach, we resolve at least two processes in our perovskite film, with measured lifetimes of 2 ms and 6 s, likely corresponding to the diffusion times of the iodide vacancies and a mobile cation species, respectively[10]. Finally, compare our results with electrical measurements on corresponding perovskite solar cells and find that the ionic features we observe optically correlate to those seen with impedance spectroscopy. 

The recent developments in the metal halide perovskite solar cells were able to achieve power conversion efficiencies comparable to silicon solar cells. Perovskite materials have emerged as promising contenders in the field of photovoltaic technology, offering a cost-effective and energy-efficient alternative to traditional silicon-based solar cells. Their low-temperature fabrication processes and earth-abundant precursor materials position them as highly attractive for scalable solar energy solutions [1]. Despite achieving an impressive performance, perovskite solar cells still face significant challenges for outdoor implementation due to limited reliability. Although there are many external factors at play such as humidity, temperature and even the prolonged sun exposure, the interfaces between the halide perovskite absorber layer and adjacent charge transport films play a big part in the inherent stability of the cell component [2].

Here, we studied the effect of external stressors such as the exposure to air and humidity on double cation, Cs_0.3FA_0.7Pb(Br_0.2I_0.8)₃  and  triple cation, Cs_0.05(MA_0.17FA_0.83)_0.95Pb(Br_0.2I_0.8)₃  perovskites, where MA and FA stand for methylammonium (CH3NH3) and formamidinium (CH₂(NH₂))₂ as well as perovskite crystallisation by in-situ Grazing Incidence Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) and photoluminescence (PL) spectroscopy measurements [3]. We carried out GIWAXS measurements whilst depositing perovskite on different substrates (e.g. SnO₂ and NiO) to study perovskite crystallisation, where we were able to detect differences in perovskite crystallisation. Furthermore, to advance our understanding of degradation, we carried out real-time GIWAXS and concomitant PL measurements as the perovskite degrades under different conditions where it was possible to detect different perovskite crystal structures, present heterogeneously, diminish with time as the PbI2 concentration increased at the interface. We observed that different conditions triggered unique defect routes with different reaction kinetics. In air, we observed phase instabilities such as the breakdown of both double and triple cation perovskites into non-perovskite organic iodide species (CH₃NH₂I) as well as cesium iodide, CsI. These phase instabilities were not present under 100% humidity without oxygen. Whilst the phase instabilities were present in air for both double cation and triple cation, we observed no intrinsic phase instabilities when 2D interlayer (4-FPEAI) was deposited on top of the perovskite. For 100% humidity, we observed  partially reversible electron transfer from I^-  to Pb²⁺ leading to irreversible  Pb⁰ formation  for double and triple cation perovskite cells.

 Recently, monolayers of carbazole derivatives have emerged as a promising replacement for the conventional polymer hole collecting layer materials such as PTAA and PEDOT:PSS in the inverted structure perovskite solar cells[1]. The hole collecting monolayer (HCM) has advantages in electric conductivity, optical transparency and device stability.

 The orientation of the HCM molecules is essential to achieve high hole collection efficiency. Since the HCM molecule has a permanent dipole moment, the molecular orientation affects the energy levels. This also affects the orbital overlap between the p orbitals of HCM molecules responsible for hole correction and the perovskite layer. Further, the orientation of the HCM molecules changes surface free energy leading to the morphology of the perovskite films formed on it. In fact, molecules with extended p-conjugated backbone and those with three anchor groups that bind to the substrate, facilitating parallel alignment, have been developed[2],[3].

 However, it is difficult to determine the molecular orientation on the conductive metal oxide electrode (ITO) due to its roughness. For example, spectroscopic ellipsometry, near edge X-ray absorption fine structure (NEXAFS) and multiple-angle incident resolution spectrometry provide only information on the molecular orientation with respect to the direction of light incidence, rather than the molecular orientation with respect to the inclined surface of the ITO.

 In this study, we measured the molecular orientations of HCM molecules on the ITO substrates using the ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). UPS is a standard technique to examine the valence electronic structure, where the electrons are excited by ultraviolet photons and kinetic energy of photoelectrons is analyzed. The probing depth of UPS is a few molecular layers. If the excitation source is replaced by a metastable helium atom, only the outermost molecular orbitals are detected, i.e. MAES is extremely surface sensitive. By comparing the UPS and MAES spectra, we can determine the molecular orientations.  

 We applied UPS/MAES to 2PACz and MeO-2PACz which have one anchor group to bind to the ITO substrate, and 3PATAT-C3[3], which has three anchor groups. At first, we assigned the observed peaks with the aid of the density functional theory calculation. Then, by comparing the spectra peak intensities of the UPS and MAES spectra, we determined the molecular orientations. If the molecules adopt the parallel orientation, the peak of p orbitals are strongly observed in MAES. If the molecules orientate perpendicular to the substrates, the s orbitals are strongly observed in MAES. From the analysis, we determined that 2PACz is tilted, MeO-2PACz is perpendicular and 3PATAT-C3 is parallel to the electrode. In this study, we demonstrate that the combination of UPS and MAES is a powerful technique to determine the molecular orientation on the conductive oxide electrode that could not be detected by other spectroscopic techniques due to the surface roughness.

Although HCM molecules with functional groups designed to alter molecular orientation for enhanced orbital overlap and modulation of surface free energy have been developed, precise measurements have not been achieved, and no definitive evidence linking these characteristics to improvements in power conversion efficiency has been established.

With the capability of greatly increasing power conversion efficiency (PCE) due to their potential to exceed the Shockley-Queisser limit of single-junction solar cells, perovskite/silicon tandem solar cells (TSCs) have emerged as a promising candidate among PV technologies that could enhance the adoption of solar energy across various applications (1,2). However, their transition from lab-scale prototypes to industrial-scale manufacturing poses some concerns in terms of environmental sustainability that, together with limitations to the durability of perovskite materials, stand as significant barriers to their widespread commercial deployment [1,4].

Stemming from harmonized life cycle data inventories and by modelling future-oriented scenarios, this study presents a prospective life cycle assessment of four potential perovskite-silicon tandem designs (TSC 1, TSC 2, TSC 3, TSC 4) produced and operated currently to 2050. The analysis incorporates adjustments to the TSC architecture to address material and processing scalability, projections of operational parameters, and changes in the electricity mix used in the tandem solar devices manufacturing supply chain. Four main LCA metrics are screened to investigate the environmental and energy performances of the devices: greenhouse gas emissions (GHG), cumulative energy demand (CED), energy payback time (EPBT) and carbon payback time (CO₂PBT).

The analysis outcomes indicate that the carbon footprint and energy consumption of tandem solar devices are expected to decrease over time. Furthermore, TSC are anticipated to demonstrate better eco-profiles compared to single-junction silicon devices in the future, as a higher efficiency overcompensates the higher environmental burden of production assuming the same lifetime for both technologies.

Printed photovoltaics, particularly organic and perovskite-based technologies, present promising pathways for affordable and flexible solar energy solutions. Their lightweight nature and low-cost production methods make them ideal for diverse applications, ranging from portable electronics to building-integrated photovoltaics.

Despite significant progress, several challenges continue to impede their broader adoption. Organic photovoltaics, for instance, exhibit lower power conversion efficiencies compared to conventional silicon-based solar cells due to limitations such as reduced charge carrier mobility and high recombination rates. Meanwhile, perovskite-based solar cells, which have achieved remarkable efficiencies of up to 26% in laboratory-scale, struggle with stability issues, particularly under humid and high-temperature conditions. Both organic and perovskite photovoltaics face durability challenges, including degradation over time and susceptibility to moisture and oxygen ingress. Overcoming these obstacles is essential to fully realize the potential of printed photovoltaic technologies in sustainable energy generation.

Antimony chalcogenide solar cells emerge as a compelling thin-film solar technology, offering a tunable bandgap, high inherent stability, and a large absorption coefficient. However, despite their high theoretical potential, their practical power conversion efficiencies remain relatively low (around 10%), limiting their competitiveness with other photovoltaic technologies. Their solution-processable nature enables straightforward integration of chemical additives to improve the performance. Yet, while various additives have been employed, the underlying chemical mechanisms remain insufficiently understood.

This presentation will delve into the role of additive engineering in enhancing the performance and stability of solution-processed photovoltaics. Our focus will centre on unravelling the intricate relationships between microstructure, charge transport mechanisms, and their impact on device performance and longevity. Ultimately, we aim to leverage this understanding to design novel, more efficient additive systems tailored for improved photovoltaic technologies.

All-inorganic CsPbIxBr3-x perovskite materials have been a promising research subject in emerging perovskite solar cells (PSCs) due to their excellent thermal stability and rapid progression in photovoltaic performance.[1] However, there are still some foremost challenges such as limited absorption range, poor phase stability, and serious defect-traps, hindering further developments of inorganic perovskite materials. Recently, partial substitution of Pb2+ (B-site cation) with other metal ions has been shown to tune the energy levels, align the tolerance factor, and reduce the defect-states of all inorganic perovskite thin films. In line with these studies, the B-site doping strategy can be considered as one of the most important lattice engineering approaches to not only improve the photovoltaic performance but also prolong the stability of all-inorganic PSCs.

Herein, a compositional engineering approach to tune the CsPbI2Br crystallization by incorporating various B-site metal dopants will be presented.[2] Special attention will be put on doping CsPbI2Br perovskite film with novel palladium (Pd2+) complexes. The optimized amount of Pd2+ complex not only stabilize the black α-phase but also improves the morphology and optoelectronic properties of perovskite film. Besides, the proposed modification thoroughly aligns the energy level, promotes the built-in potential, and reduces the defect states in the perovskite, resulting in a high power conversion efficiency (PCE) of 16.4% with a remarkable open-circuit voltage (VOC) of 1.27 V. Moreover, the fabricated CsPbI2Br PSCs deliver remarkable improvement in environmental and operational stabilities with decent PCE. Finally, challenges and cutting-edge engineering approaches for optimizing the PV performance of all-inorganic PSCs will be presented.

Photovoltaics play a crucial role in the transition to renewable energy. However, conventional rigid and bulky solar cells fail to meet the demands of emerging applications, where mechanical compliance and high specific power are essential.

Recent advancements in perovskite solar cells demonstrate significant potential for seamlessly integrating solar power into diverse applications, such as wearables, biomedical, and aerospace systems, transforming how we interact with technology and utilize energy in daily life.

This presentation will explore the potential of small-scale, thin, lightweight, and flexible perovskite solar cells in advancing next-generation electronics. Through meticulous precursor solution engineering, diligent optimization of device interfaces, and careful substrate selection, we have significantly enhanced the performance and stability of lightweight photovoltaic cells. These advancements have also enabled their integration into complex systems that would otherwise be challenging for traditional photovoltaic technologies.

Case studies—including a solar-powered wearable metabolic analysis platform [1] and a lightweight quadcopter drone [2]—highlight the transformative potential of small scale lightweight perovskite photovoltaics by enabling innovative applications and reshaping our relationship with technology and energy consumption.

The growing demand for polycarbonate (PC) has highlighted its versatility across various applications. However, its inherent surface roughness and limited chemical resistance have hindered its integration as a substrate in solar cell technologies. Here, we present the first successful fabrication of perovskite solar cells (PSCs) on PC films, enabled by a solution-processed planarization layer that reduced surface roughness from 1.46 µm to 23 nm while enhancing chemical resistance. This approach improved the water vapor transmission rate (WVTR) of PC films by 47%, with an additional 81% reduction achieved through ITO sputtering. The optimized PSCs achieved a power conversion efficiency of 13.0% and demonstrated excellent mechanical stability, retaining over 87% of their performance after 1000 bending cycles. Stability tests (ISOS-D-1, ISOS-T-1) showed promising T80 values of 1776 h and 144 h, respectively. This work paves the way for integrating perovskite photovoltaics into flexible applications such as smart ID cards, self-powered packaging, and energy-harvesting windows, offering new possibilities for lightweight and versatile solar technology.

IoT applications demand powering lots of wireless sensor which strengthens the necessity of solar cells working under low light condition that helps to get rid of usage of single time use batteries. Fluorescent lamp and LED are the popular light sources inside buildings with their emission spectra in the visible Range of Electromagnetic spectrum. High Absorption coefficient and bandgap in the range of visible spectra makes perovskite a natural choice for indoor photovoltaics. Perovskites solar cells show promising performance in indoor Photovoltaics. A dual additive passivation strategy with gold as counter electrode reached a power conversion efficiency (PCE) of 44.72% + [1].  CuPc: CuSCN as HTL (Hole Transporting Layer) with Carbon electrode was used to obtain PCE of 32.1% for small area devices [2]. Despite carbon electrode was considered the best candidate to get high performance and stable perovskite solar cells at low-cost budget [3], the best efficiency reported using this electrode for modules in low light environments is 18% at 1000 lux% [4]. In this work, we used fully printable technique from ETL (Electron Transporting Layer) to carbon deposition to realise carbon-based perovskite mini modules in ambient atmosphere. With efficient surface passivation by HTAB on FAPbI3 perovskite and P3HT as HTL, we achieved unprecedented 25% PCE with Maximum power density of 72 mW/cm2 over an active area of 30 cm2 with Carbon as a counter electrode. LCA results suggest that these indoor carbon-based solar cells are promising to replace batteries for powering IoT devices

Halide perovskites are dominating the field of next generation solar cells. Beyond traditional “3D” perovskites, low dimensional perovskites (LDPs) – intended to be single/few inorganic layers (typically <3) spaced by large organic cations – have recently attracted wide interest for their added their higher structural stability. Used as interfacial layers on top of 3D perovskites, they empowered perovskite solar cell stability¹. Besides that, LDPs have not found an application yet mainly due to their wider band gap and disordered morphology which negatively impact charge transport (happening along the inorganic backbone). Lack of knowledge on how to manipulate the orientation of the inorganic sheets is the actual bottleneck. This limits vertical charge percolation and efficiency, ultimately making them uncompetitive in the solar cells arena.

Here, I will present the main applications of LDP in highly efficient solar cells, and I will present a new interfacial design using ferroelectric LDP as a mean to boost charge extraction². Beyond their role as interfacial layers, I will also discuss a novel effective strategy to implement LDP as active layer for wide band gap solar cells. We uplifted the charge transport barrier being able of inducing a vertical growth of the inorganic framework. As a result, we break LDPs efficiency limits, triggering vertical charge percolation, allowing us reaching a “record” efficiency of 9.4 % with a device Voc of 1.4 eV, the highest so far for LDPs solar cells. Such proof of concept intimately demonstrates the potential of controlling the crystalline orientation of LDPs providing an essential strategy for boosting their performances^{3, 4}. The results pave the way for their implications into emerging field, from indoor PV for IoT powering, to building integration or agrivoltaics.

Perovskite-inspired materials (PIMs) have received increasing attention due to their potential to replace halide perovskites in scenarios where the presence of toxic lead is undesired or not accepted. The typical wide bandgap of PIMs makes them ideally suited for indoor light harvesting, which could enable a battery-less and sustainable Internet-of-Things in the future.

PIMs include a broad family of low-dimensional semiconductors with different stoichiometries [1]. One example is the vacancy-ordered pnictogen-based halides. These materials are intrinsically air stable and can be solution processed. Despite theoretical indoor efficiencies approaching 50% or more, current performance has reached initial efficiencies of ⁓5–10% under 1000 lux indoor lighting using bismuth (Bi)- and antimony (Sb)-based PIMs. Two main challenges associated with PIMs for IPVs are: 1) The inherent low-dimensional nature and high defect densities, which pose challenges such as carrier localization in achieving very high IPV efficiencies, and 2) the lack of unified measurement protocols leading to short-circuit current density overestimation or conducting IPV measurements under illumination intensities different from the recently reported standard test conditions (⁓300 µW cm⁻² at 1000 lux).

In this talk, I report our recent efforts on two-dimensional (2D) PIMs with adaptable structural and photophysical properties for sustainable IPV applications [2-4]. One example is a new vacancy-ordered PIM, Cs₂AgBi₂I₉, exhibiting weak electron-phonon coupling and large polaron formation [3,4]. Cs₂AgBi₂I₉ can overcome the inherent performance loss pathways found in other PIMs due to enhanced electronic dimensionality. The Cs₂AgBi₂I₉ IPV devices achieved a power conversion efficiency of ⁓7.6% under approximately 300 µW cm⁻² WLED illumination [4]. This is a record PCE for IPVs based on halide PIMs. Additionally, the IPV devices of Cs₂AgBi₂I₉ maintained consistent performance under different light color temperatures, showing the versatility of Cs₂AgBi₂I₉ as a reliable IPV absorber in various indoor environments.

Organic and perovskite solar cells face complex degradation mechanisms driven by environmental stressors, intrinsic material instabilities, and interfacial failures. In this talk, I will dissect the fundamental degradation pathways that limit operational lifetimes, from phase segregation and ion migration in perovskites to photochemical instability and morphological evolution in organics. By methodically studying these processes, we can develop strategies that slow down aging, extend lifetime without compromising efficiency. Through our latest research, I will highlight how targeted stabilization strategies, including compositional engineering and interfacial modifications, are assisting for durable, high-performance organic and perovskite solar cells.

This presentation will argue that ‘slow science’—carefully designed studies, FAIR data reporting, mechanistic insights and long-term studies—are critical in an era of accelerated technological development.

The recent advances in increasing the power conversion efficiency (PCE) of organic photovoltaics (OPVs) have been driven by the advent of new active organic materials and reduced performance losses associated with traditional cell structures. As the maximum PCE values of organic solar cells have now exceeded 20%, the importance of further cell engineering —including the bulk heterojunction, charge extraction interlayers, and light management — becomes critical and, in most cases, determines the overall performance. In this presentation, I will discuss our latest efforts to increase the efficiency of OPVs to well beyond 20%. I will show how the use of innovative charge-extracting interlayers can help increase the overall performance of the cells as well as their operating lifetime. Moreover, I will describe how using antireflection coatings and molecular dopants can help further enhance the PCE and show how their combination with innovative interlayers can improve overall material utilisation and device sustainability. 

Recent advancements in the integration of dye-sensitized solar cells (DSCs) with hybrid photocapacitors have been achieved, paving the way for sustainable energy solutions in Internet of Things (IoT) applications. These innovative devices have been engineered to efficiently harvest and store energy from ambient light sources, addressing the growing demand for autonomous power in smart ecosystems. DSCs incorporating copper-based coordination complexes have been developed, exhibiting power conversion efficiencies exceeding 38% under typical indoor illumination levels of 1000 lux. These complexes, featuring copper redox mediators with low reorganization energies, have been shown to enhance charge transport and minimize recombination losses, resulting in improved photovoltages and rapid dye regeneration.^1,2

The integration of DSCs with asymmetric supercapacitors has been demonstrated, achieving high photocharging voltages approaching ~ 1V and photocharging efficiencies ~20% with capacitance retention of 100% observed.. The use of earth-abundant materials and the optimization of light absorption through novel dye formulations have been emphasized in the development of these systems.The practical viability of these technologies has been validated through the successful powering of  IoT networks for extended periods using ambient light alone. These systems have outperformed commercial amorphous silicon modules by a factor of 4 in inference throughput, showcasing their potential for autonomous, energy-efficient smart devices. Machine learning algorithms have been integrated to enable dynamic energy management, allowing IoT devices to adapt their computational load based on real-time energy availability.³  

Emphasis is being placed on developing scalable manufacturing techniques and standardizing testing protocols to ensure consistent evaluation of indoor photovoltaic performance under diverse lighting conditions. By bridging the gap between materials science, device engineering, and IoT applications, these advancements are paving the way for truly sustainable, maintenance-free electronic ecosystems capable of supporting the exponential growth of IoT devices in indoor environments.⁴

The current R&Ds of perovskite solar cells (PSCs) has created many unique methods in molecular engineering, using organic molecules, for defect passivation at grain boundaries and heterojunction interfaces in PSCs.¹ Our group has focused on the method of interfacial passivation with functional organic molecules, which enables high voltage output (close to theoretical limit) in photovoltaic performance.² For practical applications, the stability of the devices still remains a major challenge. Organic cations in halide perovskites and diffusible dopants in hole transport materials (HTMs) are responsible for low stability at high temperatures (>120^oC). To solve this, all-inorganic compositions of perovskite and use of dopant-free HTMs are highly desired. CsPbI²Br PSCs (bandgap 1.9eV) achieved high Voc over 1.4V with PCEs of >17%. Under indoor LED illumination, it works at PCE >34% being supported by high Voc.³ Passivation of iodine defects with 2,5-thiophenedicarboxylic acid (TDCA) achieved Voc of 1.54V for CsPbIB2.⁴ As inorganic perovskites, lead-free compositions such as Ag-Bi halides also become an important target for achieving environmentally kind PSCs.⁵ AgBiS_² as a sulfur-based IR-absorbing material achieves high photocurrent densities (>34mA/cm²).⁶

For implementation of PCSs in society, lightweight modules are demanded. Thin plastic film-based PCSs are manufactured by low-temperature material preparation.⁷ Ink-jet coating process is applicable for device fabrication. This topic will also be introduced in the lecture.

As a technology metal halide perovskite (MHP) , photovoltaics continue to rapidly advance and offer a unique opportunity to impact electrical generation at scale.  While advances in efficiency, both for single junction and tandem devices, continue there is a need for more rapid improvements in  stability to enable commercialization.  Studies examining stability have longer learning cycles and are challenging to understand in part due to a myriad of experimental protocols and conditions.  We will discuss work examining operational stability of fielded devices and connections with device stability tested indoors.  We also will indicate the challenge associated with sample variation and a lack of reproducibility which create challenges in understanding of basic degradation modes.  The extent to which these can be overcome by aggregating data and improving device process reproducibility is an ongoing activity at NREL and we will discuss some of our efforts in this regard.  The basic considerations associated with the material defect chemistry and its links to the device physics will also be discussed.

Perovskite solar cells (PSCs) have emerged as a promising alternative to traditional silicon-based solar cells, but significant challenges persist in developing perovskite inks and scalable fabrication processes suitable for large-scale production. This study presents a novel approach to address these challenges by formulating colloidal inks that utilize toluene (TL) and chlorobenzene (CB) as co-antisolvents, enabling efficient PSC fabrication through a slot-die (SD) coating process. The engineered colloidal inks exhibit improved rheological properties, which enhance wettability and facilitate the formation of high-quality perovskite films. The presence of large colloidal structures, including α-cubic perovskite, δ-hexagonal perovskite, and intermediate transition phases, promotes heterogeneous nucleation while lowering the activation energy required for crystallization. This leads to superior crystal growth and improved film morphology. Additionally, the use of co-antisolvents strengthens the binding energy between formamidinium (FA) and PbI3 while weakening the coordination of dimethyl sulfoxide (DMSO), creating a thermodynamically favorable environment for perovskite crystallization. This colloidal strategy achieves devices with a maximum power conversion efficiency (PCE) of 21.32% and outstanding long-term stability, maintaining 77% of the initial efficiency over 10,115 hours. Furthermore, the scalability of this method is demonstrated through the fabrication of lab-scale minimodules with an efficiency of 20.26% and larger-area minimodules with 19.15% efficiency. These results provide critical insights into the interplay between ink composition, rheological behavior, film quality, crystallization kinetics, and overall device performance. This work highlights a scalable and efficient pathway for advancing the commercial viability of PSCs.

As the global demand for sustainable energy continues to rise, hybrid and organic photovoltaics (PV) are emerging as transformative solutions.

These technologies offer unique advantages, such as lightweight designs, flexibility, and the ability to be manufactured using cost-effective and scalable processes. Such qualities make them well-suited for diverse applications ranging from wearable devices to building-integrated photovoltaics.

Coatema Coating Machinery is at the forefront of this evolution, bridging the gap between laboratory innovations and industrial-scale roll-to-roll (R2R) production.

This presentation will explore Coatema’s "lab-to-fab" approach, highlighting contributions to advancing PV manufacturing worldwide.

Key focus areas will include an overview of Coatema’s PV machinery deployed globally and insights into the systems that enable efficient, large-scale production.

We will provide a breakdown of essential machine components while addressing the challenges inherent in such complex production systems, to guarantee a stable and reproducible process.

Particular attention will be given to in-line monitoring technologies, which are critical for ensuring product quality during production. These include real-time measurement of coating thickness, sheet resistance, contamination, and defect detection.

Finally, we will provide an outlook on future trends in PV manufacturing.

Mesoporous carbon-based (mC) hole-transporting-layer-free architectures offer a cost-effective solution for the commercialization of perovskite solar cells (PSCs). Adding 5-aminovaleric acid (AVA) to MAPbI₃ reduces defect concentration and enhances pore filling, while Eu enrichment in CsPbI₃ reduces cation migration and enables device reusability [1].

In this study, AVA-MAPbI₃ mC-PSCs were encapsulated at room temperature (RT) with a solvent- and water-free polyurethane (PU) resin [2]. Under continuous ambient light, RT, and 40% relative humidity (RH), PU encapsulant acts as a barrier to extend device durability and enable reusability. The formation of a bump in the J-V curve after ~250 h, already reported at low scan-rate but here observed at 50 mV/s, strongly reduces the photovoltaic performances. We demonstrate that the bump is not linked to the formation of PbI₂ but is explained by a water-vacancy interaction that increases cation mobility and enhances screening effects near the electron-transport layer. The photovoltaic performances are fully restored by drying the devices under N₂ flow for ~48 hours.

A further addition of a hydrophobic Kapton tape interlayer between the PU and the device mitigates bump formation, boosts t₉₀ to ~6000 h and projects t₈₀ to ~10,800 h. Differently from the Kapton tape used alone, PU provides an effective sealing all around the devices, ensuring stability in 100% RH at 90 °C and even underwater.

For indoor applications, Eu:CsPbI₃ mC-PSCs typically degrade from γ- to δ-phase within ~1 h in air, whereas PU-encapsulated devices achieve t₈₀ ~250 h, extendable to 1250 h with an additional closure glass slide[3].

The development of stable perovskite solar cells (PSCs) is crucial for their breakthrough in the photovoltaic (PV) market. While PSCs offer high power conversion efficiencies (PCEs), low-cost and straightforward fabrication, their stability is lower than traditional silicon PVs. PSCs are highly susceptible to degradation caused by intrinsic factors, such as the structural and chemical stability of the materials used, as well as by external factors like moisture, oxygen, light, and temperature. Hermetic encapsulation, offered by laser-assisted glass frit sealing, provides effective protection against the external environment, with proven stability enhancement of n-i-p and HTM-free structures.[1, 2] However, light-induced degradation remains a challenge, particularly in triple mesoscopic structures where mesoporous TiO₂ (ETL) accelerates degradation under UV light and oxygen. Replacing this mesoporous TiO₂ layer with non-photocatalytic alternatives, incorporating UV filters, or using insensitive to photocatalytic degradation perovskites are some pathways to mitigate the UV-driven degradation.[3] However, perovskite degradation can also be induced by the atmosphere inside the hermetic cell cavity due to the presence of moisture and/or oxygen. Most of the studies that investigate the light stability of PSCs under controlled atmospheres have also not been conducted in encapsulated cells, as they would be in actual working conditions.[4, 5]

The present study addresses the challenges of achieving long-term stability in perovskite solar devices by assessing the impact of laser-assisted glass frit encapsulation under different atmospheres – non-controlled (air), CO₂, and N₂ - on the stability of 5-AVAxMA1-xPbI3 perovskite films and HTM-free solar cells to light. Using a hermetic chamber filled with a controlled atmosphere and equipped with a glass lid, both films and cells were encapsulated via the laser-assisted glass frit process. The encapsulated devices were subjected to continuous 1-sun illumination at 45 °C for more than 1000 hours of light exposure. After monitoring the absorbance of the perovskite films over the exposure period in conjunction with Scanning Electron Microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis of perovskite films with different aging conditions, i.e. fresh, aged in dark and in light, the results show that films glass encapsulated under a nitrogen atmosphere exhibited the best stability while the films encapsulated under non-controlled atmosphere (air) suffered extensive degradation. Moreover, these results are aligned with the photovoltaic performance of PSCs exposed to the same aging conditions. Further investigation is necessary to assess the role of the carbon counter electrode in the stability of the cells, which is related to the ability of this carbon layer to adsorb moisture and reactive gases. Overall, these findings highlight the importance of laser-assisted glass frit encapsulation as an effective strategy for improving the stability and performance of perovskites. Furthermore, the study emphasizes the importance of using a controlled atmosphere during the encapsulation process.

During operation, perovskite solar cells (PeSCs) are subjected to continuous heating (55–65°C) under 1-sun illumination, with additional heat generated internally through mechanisms such as Joule heating, recombination, and photon thermalization. Effective thermal management or heat dissipation is crucial to minimize heat-induced damage. This study introduces a fullerene derivative, PC61B-TEG, as a multifunctional interlayer between the perovskite and C60 electron transport layer (ETL) in inverted PeSCs. To enhance heat dissipation and charge extraction, PC61B-TEG was doped with N-DMBI, an n-type dopant. The incorporation of n-doped PC61B-TEG significantly improved device performance, achieving a PCE of 24.42%. The device also demonstrated exceptional thermal stability, retaining 90% of its initial PCE after 2400 hours at 85°C (under N₂ without encapsulation) and 80% of its initial PCE after 1180 hours under 1-sun illumination at room temperature and 25% RH (with encapsulation). The improved thermal conductivity (κ) of the n-doped PC61B-TEG enabled efficient heat transfer from the perovskite layer, while enhanced electrical conductivity (σ) and an upshifted Fermi level (EF) facilitated superior charge transport and increased quasi-Fermi level splitting. This resulted in higher open-circuit voltage (V_OC), short-circuit current density (J_SC), and fill factor (FF). The study underscores the importance of doping in enhancing both performance and stability, demonstrating its potential for diverse perovskite optoelectronic applications.[1,2]

Mixed Sn-Pb perovskites are promising solar cell materials for single- and multi-junction devices thanks to the possibility of tuning the bandgap energy down to 1.2-1.3 eV. However, tin-containing perovskites are adversely affected by multiple factors leading to doping. In this work, we investigated how doping in Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃ is induced by the presence of Sn⁴⁺ in the spin-coating solvent and secondly by exposure of these layers to oxygen. For these measurements we used time-resolved and steady-state microwave conductivity techniques (TRMC and SSMC), structural and optical characterization.

First, we performed a quantitative analysis how the back ground doping and defect density in these spin-coated layers is affected by addition of SnF₂ ranging from 0 to 20 mol%. Optical spectroscopy is used to determine the fraction of Sn⁴⁺ to Sn²⁺ in the spin-coating solution, which varies from 0.012% to 0.032%. By applying SSMC, we observe a large decrease in dark conductivity from ~ 100 to < ~ 1 S m^-1 in the spin-coated layers on going from 0 to 2 mol% SnF2, with no further measurable reduction for higher SnF₂ concentrations. We demonstrate that the minimum SnF₂ addition required to achieve this reduction in dark conductivity is not absolute, but highly dependent on the extent of oxidation of the SnI2 precursor. The dynamics of laser-induced excess carriers show progressively longer carrier lifetimes with higher SnF2 concentrations. By fitting intensity-dependent photoconductivity signals, we find that upon SnF₂ addition the concentrations of doping and defects concomitantly decrease by an order of magnitude. It is inferred that in the spin-coating solution a ~ 100 times excess of SnF₂ is required to scavenge all Sn4+ (SnI4) and obtain nearly intrinsic lead tin perovskites.

Then we observed that exposure of these layers to oxygen leads to progressively higher dark conductivities, which slowly decay back to their original levels over days when the layers were stored under N₂. Allegedly, oxygen acts as an electron acceptor, leading to tin oxidation from Sn²⁺ to Sn⁴⁺ and the creation of free holes which effectively p-dope the perovskite. Additionally, the metastable oxygen-induced doping is enhanced by exposing the perovskite simultaneously to oxygen and light. We emphasize that, although exposure to oxygen is relatively short, this is sufficient to cause immediate and permanent changes in the charge carrier dynamics measured by TRMC. Basically, the defect density arising from short-term exposure to oxygen immediately impairs the solar cell properties, while changes in the structural and optical properties only emerge upon prolonged exposure leading to accumulation of oxidation products.

Understanding the charge carrier dynamics in advanced photovoltaic materials is crucial for improving their efficiency and stability [1]. Transient absorption (TA) spectroscopy is a vital tool for investigating ultrafast photophysical processes in heterojunction, that provides insights into phenomena like hot carrier thermalization, nonlinear recombination, and electron-phonon coupling [2]. TA spectroscopy also reveals recombination pathways, and highlights the role of charge transport layers in accelerating recombination over time, ranging from picoseconds to nanoseconds, offering a deeper understanding of the mechanisms influencing perovskite solar cells efficiency [3].

In this study, we investigate the ultrafast charge carrier dynamics in quasi-2D perovskite/MXene heterostructures, a novel class of hybrid materials with promising applications in optoelectronics. Mxenes have already shown their versatility in enhancing charge transport, light absorption, and stability makes them an exciting material for next-generation solar cells [4]. By combining stationary absorption, transient absorption, and photo luminescence spectroscopy with in-depth structural analysis, we unveil the interplay between exciton dissociation, charge transfer, and recombination processes at the perovskite/MXene interface. Our results highlight the role of MXene as an efficient charge carrier, significantly suppressing non-radiative recombination pathways and enhancing charge carrier lifetimes. Additionally, the heterostructure exhibits strong interfacial coupling, facilitating rapid exciton separation and efficient charge transport. These findings not only provide fundamental insights into the light-matter interactions at the nanoscale but also pave the way for the design of high-performance, stable, and flexible optoelectronic devices.

Mixed tin–lead (Sn–Pb) halide perovskites have emerged as promising candidates for next-generation photovoltaic technologies and near-infrared optoelectronic applications, owing to their narrow bandgaps and superior optoelectronic characteristics. Nevertheless, their vulnerability to oxidative degradation poses a significant challenge to their commercial viability. In this investigation, we demonstrate that the selection of the A-site cation critically influences the oxidation stability of Sn–Pb perovskites. Through a comparative analysis of perovskite thin films and solar cells utilizing different A-site cations—methylammonium (MA⁺), formamidinium (FA⁺), and cesium (Cs⁺)—we establish that Cs-containing perovskites exhibit markedly improved resistance to oxidative stress relative to MA-based counterparts.

Our study uncovers that degradation in MA-rich perovskites is predominantly driven by the formation of triiodide (I₃⁻), a potent oxidizing agent derived from native iodine (I₂) species. The hydrogen bonding interactions between MA⁺ cations and I₂ facilitate the generation of I₃⁻, thereby accelerating the oxidation of Sn(II) to Sn(IV) and consequent perovskite degradation. In stark contrast, Cs⁺ cations, with their strong polarizing capabilities, effectively sequester I₂, thereby inhibiting I₃⁻ formation and enhancing oxidative stability.

Leveraging these mechanistic insights, we propose two strategic approaches to bolster the stability of MA-based Sn–Pb perovskites against oxidation. Firstly, we enhance the polarizing environment of surface A-site cations through the application of CsI and RbI coatings, which effectively reduce triiodide formation and mitigate oxidative pathways. Secondly, the incorporation of sodium thiosulfate (Na₂S₂O₃) as an I₂ scavenger within the perovskite matrix significantly suppresses the deleterious effects of native I₂.

Our findings underscore the critical importance of A-site cation selection and surface engineering in managing oxidative degradation mechanisms in Sn–Pb perovskites. These advancements pave the way for the development of highly efficient and durable Sn–Pb perovskite-based solar cells and optoelectronic devices, facilitating their transition toward practical and long-term applications.

Researchers are making significant strides in the search for new photovoltaic materials to enhance the stability of highly efficient hybrid perovskite solar cells (PSCs), as the highly efficient hybrid PSCs demonstrate susceptibility to degradation under elevated humidity.^[1] All-inorganic perovskites, particularly black-phase (α, β, or γ) CsPbI₃, show promise with a 1.70 eV bandgap, superior thermal stability, and up to 22% efficiency.^[2,3] However, the structural instability of CsPbI₃ remains a major challenge, as its photoactive phases (α, β, γ) spontaneously transition to the photoinactive δ-phase at room temperature. This instability is attributed to its borderline tolerance factor (~0.8) and phase transitions triggered by moisture and ambient conditions.^[4]

Incorporating additives like dimethylammonium iodide (DMAI) has improved phase stability but requires high processing temperatures (>200 °C) and controlled (inert or dry) environments.^[5] Addressing these limitations, this study explores butylammonium acetate (BAAc) to enhance β-CsPbI₃ film formation at 160 °C under ambient conditions. BAAc improves crystallization by interacting with DMAPbI₃, facilitating DMA+ removal and producing more stable devices. The resulting PSCs achieved a power conversion efficiency (PCE) of 18.6% in ambient air and retained over 80% of their initial PCE after 500 hours of illumination (ISOS-L-1). Additionally, remarkable shelf-life storage stability was observed, with over 95% of the initial PCE retained after 10000 hours of storage.  This cost-effective, environmentally friendly approach not only broadens fabrication conditions and improves device stability but also enhances the potential for scalable PSC manufacturing, reassuring the scientific community of its practicality.

Metal halide perovskites (MHP) semi-conductors have recently been the source of an intense interest in the broad scientific community but more specifically in the photovoltaics community.  They are notabily appealing  due to their exceptional optoelectronic features, resistance to defects, relative ease of fabrication and extreme tunability regarding their physical and chemical properties.

The interaction between external stimuli, such as light, and the crystal lattice is still being investigated, as it links processes at timescales ranging from femtosecond to hours. Indeed, under light illumination photo-segregation, i.e. change of the local stoichiometry, or photo-brightening, photo-darkening and many others can occur, altering the potential energy landscape and effectively the photoluminescence quantum yield (PLQY).

Those processes and their effects are reversible over a long timescale, but their magnitude depends on the hysteresis of the sample. This has recently been taken advantage of to create memory cells with MHP polycrystalline thin films with fJ switching energy [1].

In our work we leverage the interplay between different timescales and define a framework which agnostically finds the optimal light cycle maximizing the PLQY. Each cycle is then defined by a series of steps, characterized by the pulse peak energy and the time of exposure. Our experiment shows that the optimal cycle is highly non-linear regarding the average energy injected into the system, highlighting the role of traps and their interplay with the slow lattice reorganization. This can be taken advantage of by varying the composition and frequency to alter different physical and chemical processes revealing long lasting effect on the photoluminescence. We believe that our approach paves the way for a general framework to quickly enhance MHP thin film optoelectronic performance.

The recent emergence of the Internet of Things paved the way for the development of smart items and self-powered devices. This new approach for domotics induced a need for efficiently powering these appliances under low light conditions, which gave rise to indoor photovoltaics. Among all the existing technologies available, 3^rd generation photovoltaics proved to be highly efficient for this purpose. DSSCs showed to be particularly interesting ^[1] as they are an excellent compromise between efficiency and processability. In consequence, the industrial development of DSSCs is growing more and more, bringing a necessity for fabricating DSSCs stable for long periods of time.

SL9 and SL10 are two recently published dyes ^[2] that have demonstrated high Power Conversion Efficiency (PCE) in co-sensitized devices in indoor and outdoor conditions. In this work, these dyes were synthetised and characterized using NMR, HRMS, UV-Vis spectroscopy and cyclic voltammetry. They were then integrated in industrially processed DSSCs to evaluate their long-term stability and performance under white LED light. As a result, co-adsorbed SL9 and SL10 displayed high PCE at 1000 lux of more than 25% after 3000 hours, making them comparable to benchmark sensitizers. Even at 130 lux of illumination, more than 22% PCE was recorded, exceeding most of other commercially available technologies.

Literature has shown that the preliminary adsorption of bufexamic acid on the photoanode allows to improve the dense packing of the sensitizer grafted on the surface of the mesoporous film of titania ^{[2, 3]}. This method was applied to SL9/SL10 co-sensitized cells and contributed to increase the J_SC and fill factor by a few percent, which is consistent with previous findings.

Finally, a set of SL9/SL10 cells with and without pre-adsorption were subjected to accelerated stability tests to evaluate the influence of the acid pre-adsorption on the stability of the cells. The cells were then monitored under white LED light at low and moderate intensity for 1000 hours. After leading several tests, there was no clear evidence of a detrimental effect after bufexamic acid treatment. Indeed, both typologies of cells demonstrated similar trends for the decrease of the V_OC and J_SC. We assume that while the density of the dye monolayer does have a positive effect on the quantity of light absorbed and converted, there is no evident related mechanism influencing the degradation of the cells overtime. This makes bufexamic acid pre-adsorption an excellent option for improving the cell efficiency without compromising the stability in the long run.

Photovoltaic energy sources have emerged as a viable alternative to meet the world’s growing energy needs. However, current commercial technologies, such as silicon-based solar cells, require energy-intensive manufacturing processes, resulting in longer Energy Payback Time.[1] Among other emerging photovoltaic technologies,[2] Dye-Sensitized Solar Cells (DSSCs) stand out for their promising properties, including low-cost fabrication, easy tunability of absorption properties to produce solar cells in different colours, and high stability.[3,4]

Given the possibility of working with transparent substrates with DSSCs, the idea of using them as smart windows has driven research into dyes with a wide range of colours and on the implementation of photochromic dyes to obtain devices with self-regulating coloration.[5,6] To this end, different families of photochromic dyes based on diphenyl-naphthopyran moiety have been intensively studied, with efficiencies above 4% and fast coloration/discoloration kinetics.[7]

The fast response time and strong fatigue resistance of naphthopyran-based photochromic dyes make them highly valuable for various optical and smart applications. A key feature of these molecules is their tunability: by exploiting advances in organic dye research, modifications can be made to their donor, acceptor, or pi-conjugated systems. These adjustments allow precise control of their optoelectronic properties, leading to improved solar cell efficiencies and optical performance.

In this work, we will present new structures of photochromic dyes for dye-sensitised solar cells by implementing different donor or acceptor units. We will discuss the theoretical investigation carried out by DFT calculations, the synthesis of the dyes and their characterisation, and finally their implementation in devices and their characterisation to determine their photovoltaic properties. We will show that these new dyes can be used to develop solar cells with panchromatic absorption and high colour rendering index, opening up new applications in glazing.

Dye-sensitized solar cells (DSSCs) have emerged as a promising solution for renewable energy, offering an affordable, efficient, and flexible way to convert sunlight into electricity[1]. Co-sensitization involves the use of multiple dyes is a promising approach to enhance light absorption and improve the overall device efficiency[2]–[4]. The synergistic interaction between these dyes leads to improved light harvesting by broadening the solar spectrum coverage, while minimizing dye aggregation. It also helps in reducing energy losses which thereby improves the device’s overall performance. Using photovoltaic characterization[5], stationary absorption, and ultrafast transient absorption (TA)[6], [7] techniques, we investigate the mechanisms driving performance of the systems.

In this study, we examine the systems featuring squaraine (SQ258)[8] and triphenylamine (D35, L1, XY1b) dyes in conjunction with cobalt- and copper-based electrolytes. The co-sensitization of dye pairs: XY1b  with L1, and SQ258 with D35, which possess complimentary absorption spectra and unique electronic characteristics are selected. A key finding is the discovery of novel “co-Stark shift effect”, where the absorption of D35 shifts due to the electric field created by electron injection from SQ258 into titania, and the absorption of L1 shifts due to electron inject from XY1b. These interactions highlight the significant role of electron coupling in co-sensitized systems and can suggest a new method to directly estimate electron injection quantum yield from the co-sensitized dye. Co-sensitization for squaraine dyes shows substantial reduction in aggregation, thereby improving electron injection quantum yield. In contrast, no changes in the absorption of individual triphenylamine dyes were observed, resulting in their inherently stable behaviour.

TA studies further reveal that triphenylamine dyes demonstrate efficient electron injection due to the absence of fast internal conversion seen in squaraine dyes. However, they exhibit charge recombination on the hundreds-of-picoseconds timescale, whereas squaraine dye avoid such recombination, balancing the trade-off between injection efficiency and recombination suppression. Additionally, thin TiO₂ layers combines with these dye mixtures enable the construction of semi-transparent, bi-facial solar cells in various colours, offering both efficiency and aesthetic flexibility[9].

This study provides new insights into the interplay of dye interactions and their impact on DSSC performance, paving the way for the development of advanced co-sensitized systems with optimized efficiency and design potential.

In the realm of third-generation molecular light-harvesting technologies, our focus is on efficiently capturing and recycling diverse light sources, including indoor, artificial, ambient and diffused sunlight using custom engineered dye-sensitized solar cells (DSCs). DSCs stand out for their high efficiency, exceeding 40%, and their suitability for indoor use due to their lower cost, stability and ease of production.^1-2 Recent innovations, such as co-sensitization approach, introduction of dual-species copper-based electrolytes replacing traditional iodide systems, use of bilayer TiO₂-ZnO nanostructured electrodes, have addressed recombination issues, enhancing performance of these innovative nano-photovoltaic devices under indoor and ambient lighting conditions.^1-6 These advancements not only improve efficiency but also promote environmentally friendly practices, positioning DSCs as a viable option to replace conventional one-time-use primary batteries for powering electronic devices facilitating self-powered applications thereby reducing the carbon footprint.^1-3.

My presentation will highlight CSIR’s pursuit of self-reliance in indoor light-harvesting technologies underscored by advancements in the domain of DSCs and the fascinating lab to land transition being realized developing innovative self-powered products in my research lab at NIIST over the past decade. At NIIST, our endeavors extend to the custom design and optimization of these indoor light harvesters, utilizing tailor-made molecules, materials, and device architectures realizing efficiencies of 40% and above.¹ By nurturing capabilities, CSIR strives to establish a formidable position in the global indoor photovoltaic landscape, and propelling India towards self-sufficiency in emerging photovoltaic sectors.

When assembling a DSSC, two different electrolytes were applied and evaluated: Iodolyte AN50 (Solaronix, 50 mM 1,2-dimethyl-3-propylimidazolium iodide in acetonitrile) and one named "ACVAL" (LiI 0.8 M + I2 0.05 M in a mixture 85/15 acetonitrile/ valeronitrile).

We tested both electrolytes in DSSC containing anthocyanins (cyanidin 3-O-glucoside and delphinidin-3-glucoside) or carotenoids (myxo xanthophyll-like derivates, aphanizophyll and zeaxanthin).

The DSSC's efficiencies increased when using ACVAL, particularly for those cells sensitized with anthocyanins, but not consistently when using carotenoids. Other parameters of the DSSC, particularly the FF and reproducibility, were also highly affected when using this electrolyte.

Is the iodide concentration the cause of this different behavior? Is the solvent?

Different experimental routines were applied to answer these questions. In addition to the current density vs. potential profiles, electrochemical impedance spectroscopy measurements, spectroscopic techniques, and thermodynamic considerations arising from analyzing the involved redox couples were considered.

The iodide concentration and adsorption to the semiconductor used in the DSSC photoanode could affect the visible spectra of the pigments. Therefore, the overall efficiency of the assembled cell could also be affected. As assessed by EIS measurements, the balance between the recombination and the electron transport through the semiconductor changed when using ACVAL or Iodolyte AN50.

Thermodynamic calculations, considering the redox-measured potential for the iodine-based couples and the dyes, were also helpful in understanding the observed differences.

Physical vapor deposition (PVD) has long been a cornerstone in semiconductor technology, offering advantages such as conformal deposition, precise thickness control, and scalability for applications like thin-film photovoltaics (PV). Among PVD techniques, pulsed laser deposition (PLD) has emerged as a promising yet underexplored approach for the fabrication of metal halide perovskites (MHPs). Recent advancements demonstrate that PLD enables single-source vapor-phase growth with precise stoichiometry control, achieving single-junction solar cell efficiencies exceeding 19%.1PLD’s ability to control film thickness and deposition rates further allows for the epitaxial growth of halide perovskites on lattice-matched substrates, highlighting its potential for high-performance optoelectronic devices.2 This work discusses the latest developments in PLD for MHPs, spanning from mechanosynthesis of perovskite precursors to controlled thin-film growth, solar cell integration and opportunities beyond solar. We address key challenges of PVD methods for MHPs such as high deposition rates, which can be leverage for one-step growth of perovskites, or even for rapid deposition of the inorganic components for the fabrication of MHP via the hybrid route for tandem devices. We will furthermore discuss opportunities for advancing other PVD methods based on learnings from PLD, towards scalable and versatile PVD methods for next-generation optoelectronic technologies.

References

https://doi.org/10.1016/j.joule.2024.09.001

https://doi.org/10.1038/s44160-024-00717-z

Hybrid Organic-Inorganic Perovskites are largely investigated worldwide for Photovoltaics[1] today, due to their unique properties. We have previously developed a patented [A. Alberti, E. Smecca, A. La Magna, S. Perugini, M. Abbiati. Method and apparatus for deposition of a layer of perovskite on a substrate. IT20210001898 (granted) - EP4284969 (granted) - US20240117524A1 (filed)] innovative vacuum deposition method called Low-Vacuum Proximity-Space-Effusion (LV-PSE)[2] to prepare CH₃NH₃PbI₃ (MAPbI₃) thin film for semitransparent perovskite solar cells. The method requires lower investment cost for equipment than conventional evaporation and consists of a two-step deposition under low vacuum conditions to produce high-quality thin layers of phase-pure MAPbI_3. They reached an average efficiency of 14.4% with 150 nm-thick active layers in p-i-n devices. An in-depth study of the deposition process and conversion mechanism from PbI₂ to MAPbI₃ has been carried out to further improve the quality of the films. In particular, we investigated how the working pressure and substrate temperature of the first step impact on the quality not only of the PbI₂ film but also of the final MAPbI₃. X-Ray diffraction (XRD) and Spectroscopic Ellipsometry (SE) were used to investigate the structural and optical properties of the deposited films, while the morphology has been studied by Scanning Electron Microscopy. We found that using a working pressure of 2x10^-2 mbar, the prepared PbI₂ films are more oriented along the [001] direction, as attested by the lower full width at half maximum values of the rocking curves. This perovskite structure is also more reproducible than in samples prepared at a pressure of 6x10^-3 mbar, as attested by the narrower spread of the collected data. We suggest that the higher reproducibility at the higher working pressure benefits from the lower deposition rate (~40nm/min) that helps the atoms’ arrangement after they reach the substrate. At the lower pressure, a higher PbI₂ deposition rate (~60nm/min) is achieved. The second step, consisting of methylammonium iodide (MAI) deposition, has been optimized to define the process window for the complete conversion of PbI₂ into MAPbI₃. We found that the MAI deposition time is strictly dependent on the first-step process, currently enabling a net maximum deposition rate for the complete conversion of ~30nm/min. This value is comparable with the highest value reported in literature for MAPbI₃ co-evaporation[3]. As an advantage, our two-step deposition still leaves room for improvement based on deposition pressures and temperature. Device preparation using LV-PSE layers deposited with the two different pressures is ongoing.

Perovskite solar cells (PSCs), advancing solar technology with remarkable photoconversion efficiency (PCE) and stability, typically use hybrid organic-inorganic lead halide perovskites. However, concerns remain about the organic component's impact on degradation. Transitioning to all-inorganic cesium perovskites is an alternative route to tackle the long-term stability challenges in PSCs.

Within inorganic perovskites, CsPbI₃ suffers from polymorphism ranging from the photoactive α-phase to the inactive δ-phase. In contrast, CsPbBr₃ perovskites offer robust thermal, humidity, light stability and do not suffer from polymorphism. With a Shockley-Queisser single-junction limit of ~ 16% and a wide bandgap of 2.3eV, it is attractive for semi-transparent, building-integrated photovoltaics and multijunction applications. Many CsPbBr₃ works are based on solution-processing using conventional spin-coating technique limiting uniformity over large areas. Also, dissolving the precursors in solution, which frequently comes with toxicity concerns, can be challenging.

Alternatively, thermal evaporation offers a solvent-free, industry-compatible fabrication method, enabling precise thickness control, conformal and uniform coverage over large substrates.

Here, we fabricate a solvent-free CsPbBr₃ PSC via dual-source sequential evaporation. CsPbBr₃ films deposited on compact SnO₂ electron transport layer, are pinhole-free and exhibit phase purity with reduced defects. Thin film annealing studies using X-ray diffraction, conducted alongside device investigations, revealed a decrease in phase transition temperature from 300°C to 250°C. Finally, the fabricated device results in a PCE of 7.16% with an open-circuit voltage of 1.31V. An all-inorganic PSC with a vacuum-processed absorber layer is demonstrated to achieve a phase-pure, compact film of desired thickness, paving the way for exploring CsPbBr₃ active layer.

All-inorganic perovskite solar cells (PSCs) utilizing CsPbI₂Br have emerged as promising candidates for next-generation photovoltaic technologies, owing to their superior stability against humidity, thermal fluctuations, and ultraviolet radiation [1], as well as their ideal bandgap of 1.9 eV [2]. These properties make CsPbI₂Br particularly suitable for tandem solar cells with silicon counterparts. Nevertheless, challenges such as rapid crystallization inherent to the spin-coating process often result in non-uniform morphologies and defect-rich films, thereby compromising device performance and reliability.

To address these limitations, we employed physical vapor co-deposition (PVD) to fabricate CsPbI₂Br thin films under controlled conditions, which allows precise control over deposition parameters and substrate temperature to fabricate high-quality films. Initially, thin films were deposited via co-evaporation of CsBr and PbI₂ onto glass substrates at substrate temperatures (Tsub) of 23°C and 100°C. Films deposited at 100°C exhibited superior optical and structural qualities, including enhanced absorption coefficients, compared to those produced via spin-coating [3] or PVD methods without substrate heating [4]. Building on these results, CsPbI₂Br thin films were subsequently deposited on FTO/PTAA (hole transport layer) substrates under identical conditions to construct inverted solar cells with the architecture FTO/PTAA/CsPbI₂Br/C₆₀/BCP/Ag. Following deposition, the films were annealed at 300°C for 10 seconds in nitrogen and cooled either rapidly or slowly (~0.051°C/s). Rapid cooling produced irregular surfaces and high defect densities, resulting in a maximum PCE of 4.21%. In contrast, slow cooling enhanced surface smoothness and crystallinity, as corroborated via SEM analysis, leading to an improved PCE of 5.82%. Additionally, slow cooling caused a color shift from pale brown to rich dark brown, indicating enhanced crystallinity. This study underscores the importance of substrate heating and controlled post-annealing cooling to achieve high-quality CsPbI₂Br films with improved photovoltaic performance and stability, providing valuable insights for further improvements in CsPbI₂Br-based inverted PSCs.

Among the various deposition techniques available for perovskite thin-film fabrication, thermal evaporation stands out for its ability to produce uniform and high quality films with precise control over thickness and composition. However, it is known that this method comes with its challenges, such as the decomposition of organic components and limited control over morphology. While most studies on thermal evaporation focus on pure iodide systems, like MAPbI₃ and FAPbI₃, the challenges related to mix-halide perovskites, which are crucial for bandgap tuning and device stability, remain elusive. These challenges are further compounded by the critical role of substrates, which play a critical role in determining the morphology, crystallinity, and optoelectronic properties of the resulting films.

For this purpose, we systematically investigated the influence of various substrates, such as PTAA, NiOx, PEDOT:PSS and the self-assembled monolayer (SAM) MeO-2PACz, on the formation of thermally evaporated FAPbI₁Br₂ perovskite films with thicknesses ranging from 3 nm to 200 nm. Characterization techniques including X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and  scanning electron microscopy (SEM) were utilized to analyze the surface properties and the morphology of the evaporated films. Bulk properties, like crystal structures and optical absorption characteristics of the films, are investigated using X-ray diffraction (XRD) and UV-vis spectroscopy.

Our results reveal distinct substrate-dependent effects on the formation and composition of resulting perovskite thin films. For instance, on PEDOT:PSS  and NiOx we initially only observe FAPbI₃ formation for low coverages, and only after the deposition of ~45 nm the bromide becomes incorporated and the desired FAPbI₁Br₂ forms. This delayed incorporation is attributed to the formation of volatile bromine species, such as HBr or Br₂, during co-evaporation, triggered by the interaction of the precursors with the substrate surface at the interface. In contrast, on PTAA substrates, bromide is incorporated right away, forming the intended FAPbI₁Br₂ composition, which indicates the absence of reactions leading to volatile bromine species. Maybe the most surprising results were obtained for the SAM substrates, which are commonly and successfully employed in solution-processed perovskite devices. Here, no bromide incorporation was observed for any layer thickness up to 200 nm and overall the perovskite formation was hindered. This highlights the significant challenges SAM layers may present in thermal evaporation and that the choice of substrate can influence the film growth not only at the interface but also across device-relevant thicknesses.

In conclusion, this study provides valuable insight into the critical role of the chosen substrate in the thermal evaporation of FAPbI₁Br₂ perovskite thin-film, with the challenges of achieving consistent halide incorporation being particularly evident through the formation of volatile bromide species. Understanding the relationship between substrate properties and perovskite film characteristics is crucial for optimizing device performance and advancing the development of efficient and stable perovskite solar cells.

The integration of metal halide perovskites (MHPs) into perovskite-silicon tandem solar cells offers significant potential for achieving high efficiencies (>33%) while utilizing low-cost materials.^1,2 A critical challenge in realizing this potential is determining the optimal fabrication technology for scalable, high-throughput and cost-effective production. Among the various deposition methods, physical vapor deposition (PVD) techniques, remain relatively underexplored in the context of perovskite top cell fabrication.^3-7

In this work, we explore the use of pulsed laser deposition (PLD) as a single-source PVD technique to form the inorganic scaffold in the hybrid sequential method, a widely adopted approach for fabricating perovskite top cells. A PbI₂:CsBr layer, with a 10:1 ratio, is deposited at an accelerated rate of ~ 55 nm/min, five times faster than traditional PVD techniques.^3-7 The deposition is followed by spin-coating an organic cation solution containing formamidinium iodide (FAI) and bromide (FABr) in ethanol, and annealing to form Cs_xFA_1-xPb(Br_yI_1-y)₃ absorbers. X-ray photoelectron spectroscopy confirms the stoichiometric transfer of the scaffold compounds, while X-ray diffraction reveals the polycrystalline nature of the films. Scanning electron microscopy demonstrates a porous morphology that facilitates efficient solution diffusion and complete conversion. As a result, the perovskite films exhibit a polycrystalline α-phase structure with tuneable bandgaps, which can be modulated by the Br ratio in the precursor solution. Preliminary single-junction solar cells achieve efficiencies of ~18%, with tandem devices under further development. With the added benefits of bandgap tuning, precise thickness control, and conformal coverage, this method shows promise for achieving efficient current matching conditions in monolithic perovskite/silicon tandem solar cells.

In conclusion, this work highlights the potential of PLD to advance PVD-based fabrication methods for perovskite-based monolithic tandem solar cells, offering high deposition rates, tuneable bandgaps, and precise control over thickness and coverage. Additionally, we address challenges related to achieving very high deposition rates of >100 nm/min, contributing to the development of scalable vapor deposition techniques for perovskite top cell fabrication.

References:

[1] Best Research-Cell Efficiency Chart  (https://www.nrel.gov/pv/cell-efficiency.html) [Access: 18th May 2024]

[2] S. De Wolf, E. Aydin, Tandems have the power, Science 381, 30-31 (2023)

[3] T. Abzieher, et al., Energy Environ. Sci. 17, 1645-1663 (2024)

[4] M. Roß, et al., Adv. Energ. Mater. 35, 11, 2101460 (2021)

[5] F. Sahli, et al., Nature Mater. 17, 820–826 (2018)

[6] T. Soto Montero, W. Soltanpoor, M. Morales-Masis, APL Matter. 8, 110903 (2020)

[7] T. Soto-Montero, et al., Adv. Funct. Mater, 2300588 (2023)

Outdoor stability testing under natural sunlight provides the most relevant test of solar cell stability under operational conditions [1]. Understanding perovskite-based solar cells’ recovery properties under natural diurnal light-dark cycling can point to methods to extend its lifetime [2, 3]. We systematically studied various aspects of such testing including: (i) 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]; (ii) the effect of perovskite solar cell architecture and component materials on outdoor lifetimes, showing that the perovskite material degradation is NOT the determining factor; (iii) optimized cell encapsulation in terms of the cell lifetime, showing preference for encapsulation schemes with Al₂O₃ thin films and glass-on-glass with butyl rubber-based sealant compared to other studied encapsulations; and (iv) prediction of the outdoor degradation behavior from accelerated indoor stability analyses enabled by machine learning algorithms and mathematical decompositions, which can be used to determine the most relevant stress factors affecting outdoor stability  [5].

Surface passivation is a critical strategy for enhancing the efficiency and stability of perovskite solar cells (PSCs). While exposed surface passivation has been extensively explored, the buried interface remains underutilized. This study systematically investigates thiocyanate salts with varying cations—ammonium (NH₄⁺), methylammonium (MA⁺), ethylammonium (EA⁺), and n-butylammonium (n-BA⁺)—for their role in facilitating bifacial surface passivation. The thiocyanate anion (SCN⁻) was found to significantly improve perovskite precursor solubility, enabling recrystallization and passivation at both exposed and buried interfaces. Our findings reveal that the cation type influences recrystallization and device performance. NH₄⁺, in combination with 4-methoxy-phenylethylammonium iodide (MEO-PEAI), yielded optimal results, achieving a power conversion efficiency (PCE) of 24.3% (VOC = 1.17 V, JSC = 25.1 mA/cm², FF = 82.9%). This performance is attributed to the suppression of non-radiative recombination and improved film morphology. The treated devices demonstrated enhanced thermal and photostability, maintaining 80% of their initial PCE after 437 hours of thermal stress at 65 °C and 503 hours under continuous illumination. These results highlight the potential of thiocyanate-assisted passivation strategies for advancing PSC technology.

Metal halide perovskites exhibit exceptional optical and electrical properties, positioning them as promising materials for a range of optoelectronic applications, including solar cells and light-emitting diodes (LEDs). Their solution-processable nature enables the fabrication of high-quality polycrystalline films. However, their susceptibility to degradation under various stress conditions remains a significant challenge, highlighting the importance of understanding their structural and optoelectronic properties at the nano- and microscale, particularly at individual grains and grain boundaries.

In this study, we investigate multi-crystalline CsPbBr₃ films deposited on silicon substrates to explore the influence of grain orientation and boundaries on their optical properties. We utilise electron backscatter diffraction (EBSD) and cathodoluminescence (CL) spectral imaging to determine the crystal orientation and the optical properties of the same sample area, respectively. Both techniques raster-scan a high-energy electron beam of a few keV over the sample surface while collecting either EBSD patterns or CL spectra.

By combining EBSD and CL spectroscopy, we achieve, for the first time, a direct correlation between crystal orientation and optical signature at the nanoscale (CL) of perovskite films. Our findings reveal that (1) the CL intensity and spectral characteristics are independent of the crystal orientation within individual grains, and (2) the CL intensity decreases significantly at grain boundaries. Depth-resolved CL analysis provides further insights into reabsorption processes and light out-coupling mechanisms. Notably, no electron beam-induced degradation was observed, even after repeated scans at 15 keV for EBSD and 5 keV for CL.

We complement the experimental results with optical near-field simulations to gain deeper understanding of the dominating effect of light (CL) outcoupling as well as carrier diffusion upon electron exposure. Furthermore, we will utilise time-resolved photoluminescence and CL to correlate carrier lifetime and diffusion depending on the position of the electron beam, namely within a perovskite grain or boundary.

Additionally, we fabricated LEDs with spin-coated CsPbBr₃ films as the top layer, enabling in-situ CL and secondary electron (SE) mapping under electrical bias. This configuration leverages the high spatial resolution of SE imaging and the spectral precision of CL to examine the effects of electrical bias on device performance. Interestingly, we observe that the recorded electroluminescence (EL) spectrum is red-shifted and broadened in comparison to the CL spectrum.

In-situ and operando studies of perovskite films and devices will deepen the understanding of their degradation processes under electron excitation and electrical bias as well as the role of grain orientation and boundaries at the nanoscale.

The photoactive α-phase of formamidinium lead triiodide (α-FAPbI₃) is thermodynamically unstable under ambient conditions, limiting its use in perovskite solar cells. However, tilting of the [PbI₆] octahedra comprising the cubic α-FAPbI₃ structure increases resistance to its transformation into the photoinactive 2H-phase. This tilting is typically induced by substituting FA⁺ or I^- for Cs⁺, methylammonium, or Br^-. However, such substitutions increase the semiconductor bandgap and risk halide or cation segregation, leading to formation of local FAPbI₃-rich regions that rapidly transform to the undesirable 2H-phase. To demonstrate this, we compare β-alanine and L-arginine addition to FAPbI₃ precursor solutions using liquid state nuclear magnetic resonance (NMR) spectroscopy and find that: (a) both additives undergo in-situ reaction with FA⁺ which makes the effective additive chemically different than the pristine additives, and (b) this solution chemistry inhibits zwitterion formation in β-alanine, but not in L-arginine. After reaction L-arginine retains its zwitterionic form and therefore its carboxylate (COO^-) functionality, which is key to its α-FAPbI₃ stabilization activity. This enables COO^--Pb²⁺ interactions in solution, which we show using solid-state NMR are preserved into the solid thin films and are critical for inducing octahedral tilt in α-FAPbI₃ thin films fabricated in the presence of L-arginine, and therefore α-phase stabilization. Consequently, such films exhibit over 1,000 hours of photoactive phase stability under ambient conditions. Using nano infra-red mapping, we show that incorporated L-arginine is concentrated at the domain boundaries of α-FAPbI₃ thin films, indicating perovskite phase surface-binding and consistent with additive-templated octahedral tilting.

SnO₂ is the most popular electron transport layer (ETL) for n-i-p perovskite solar cells. The good performance of SnO₂ ETLs is surprising, as the conduction band minimum is expected to be well below that of lead halide perovskite, which should lead to considerable voltage losses in the solar cell.

In order to investigate processes at the SnO₂ / perovskite interface we performed simple electrochemical measurements such as cyclic voltammetry and chronoamperometry on complete perovskite solar cell devices in the dark. The device structure used was ITO/SnO₂/FAPbI₃/spiro-OMeTAD/Au.   

Time-resolved potential steps in forward direction (cathodic) displayed a biphasic current response, where the faster part can be attributed to ionic displacement (with charge: mC cm^-2 regime) and the slower part (seconds) to an electrochemical reaction (charge: 100s mC cm^-2). Cyclic voltammetry displays a quasi-reversible electrochemical reduction reaction occurring at about -0.8 V (vs the spiro-OMeTAD/Au contact, which serves as both counter and reference electrode), see Figure 1. We attribute the reduction to proton insertion into the SnO₂, where the protons are provided by formamidinium cations (H₂NCHNH₂⁺):

SnO₂ + H₂NCHNH₂⁺ + e^-  →  SnO₂-H + H₂NCHNH

The reversibility of this electrochemical reaction is poor, as electrons in SnO₂ can react with holes in the perovskite:

SnO₂-H + H₂NCHNH + h⁺  →  SnO₂ + H₂NCHNH₂⁺

The reactions observed here explain the significant charging effect that is required before n-i-p perovskite solar cells attain their maximum photovoltage upon illumination, a process that can require several seconds.

Perovskite solar cells (PSCs) have emerged as a leading photovoltaic technology due to their remarkable efficiency and improved stability. However, the intricate interplay of electronic and ionic processes that dictate their performance remains poorly understood. Impedance spectroscopy (IS) provides a robust characterization approach to probe these processes across multiple time scales. Despite its potential, the coupling of recombination, charge transport, and extraction phenomena within IS responses complicates the accurate identification of dominant resistive mechanisms.

This study introduces a novel method for evaluating recombination resistance using current-voltage (j–V) curve reconstruction as a diagnostic tool. Our analysis includes four PSC configurations with varying charge extraction and recombination characteristics. By correlating fitted parameters from IS spectra with experimental j–V data, we demonstrate that recombination resistance can reliably be distinguished in systems with optimized charge extraction. Conversely, hindered charge extraction introduces significant coupling effects, masking recombination resistance within the IS-derived resistive parameters.

We validated our approach across devices employing different transport layers and selective contacts, including PEDOT:PSS and MeO-2PACz. The reconstructed j–V curves align with experimental measurements in configurations where recombination dominates, highlighting the efficacy of the method in decoupling resistive contributions. For devices with hindered charge extraction, such as those incorporating ICBA as the electron transport material, discrepancies between reconstructed and experimental j–V curves underscore the influence of transport resistance.

These findings emphasize the necessity of precise identification and decoupling of physical processes within IS spectra for advancing PSC diagnostics. The study contributes to a deeper understanding of the operational dynamics in perovskite photovoltaics, paving the way for enhanced device design and optimization.

This research represents a significant step toward the commercialization of PSCs, offering tools for improved characterization and performance evaluation critical to their long-term viability.

Metal halide perovskites have emerged as a class of semiconductors that challenges the dominance of conventional materials in applications such as solar cells. However, some of their distinctive characteristics, including ionic behavior, a soft lattice, and low formation energy, present additional challenges for stability and performance.

In this work, we explore postsynthetic strategies to harness such dynamic chemical nature of these materials.

We describe the mechanism that enables the fabrication of highly luminescent 3D lead-based perovskite nanoparticles [1] from non-emissive 0D material composite films. In this process, hydroxide ions play a pivotal role, reversibly binding to the perovskite particles, passivating traps, and enhancing both stability and optical properties. The matrix’s basicity is critical in generating OH⁻ ions, which facilitate surface passivation and improve the overall performance of the perovskite nanocomposites. [2]

Additionally, we highlight advancements in Sn-based perovskites, where additives induce superior crystallization, resulting in lead-free films with enhanced stability. These improvements translate into photovoltaic devices that maintain performance for over 2000 hours of continuous operation in an inert atmosphere, [3] and more importantly, they exhibit unconventional reactions to humidity exposure.

These approaches underscore the transformative potential of postsynthetic modifications in overcoming the inherent limitations of perovskites, paving the way for stable, high-performance optoelectronic devices.

The analysis of fundamental properties, such as energy level positions and bandgaps, are important to enhance our understanding of semiconducting materials such as halide perovskites. Here, most commonly spectroscopic tools such as ultraviolet photoelectron spectroscopy or UV-vis measurements are exploited to measure the density of states or band-to-band transitions.

In this talk, I will show data measured by reflection electron energy loss spectroscopy (REELS), which is a measurement technique that records electrons which are inelastically scattered from a surface. The energy losses can be analyzed to give insights into a variety of surface excitations, most notable for the current study are electronic transitions from conduction to valence band states.

I will show REELS measurements obtained for a wide range of halide perovskites and compare this data to the more conventional analysis performed by UV-vis spectroscopy. The most important difference between these two techniques is the surface sensitivity. Notably, in REELS we can tune the probing depth from around 1 to 10 nm by varying the excitation energy. This allows us to investigate the effect of surface modification on the optical gap of perovskite surfaces. This is particularly interesting for the investigation of the formation of 2D perovskite films on top of 3D layers, for which I will show various examples.

Shortwave infrared (SWIR) light emitters and detectors are indispensable across various fields, yet conventional technologies based on epitaxially grown semiconductors like InGaAs are costly and complex to integrate with CMOS technology. Colloidal quantum dots (CQDs) offer a promising alternative through solution-based processing, but prevalent SWIR-active CQD systems often involve heavy metals, limiting their widespread adoption. Here we present significant strides in InAs colloidal nanorods technology aimed at overcoming these limitations. Initially, we address the challenge of synthesizing SWIR-active InAs nanorods by developing a controlled synthesis method using safer, more readily available precursors. This approach enables the production of monodisperse InAs nanorods with tunable bandgaps 2000 nm, expanding their applicability into the extended-SWIR spectrum. Furthermore, we introduce a novel surface-passivation technique with InAs/ZnSe core/shell colloidal nanorods. These nanorods exhibit exceptional emissive properties, demonstrating high photoluminescence quantum yields of up to 60% across the entire technologically crucial SWIR range (1200–1800 nm). Leveraging these advancements, we showcase a SWIR-active InAs nanorod photodetector achieving a record external quantum efficiency of ∼15% at ∼1450 nm. Our findings underscore the potential of InAs CQDs as robust candidates for next-generation SWIR optoelectronic devices, combining high performance with environmentally friendly materials and scalable synthesis methods suitable for industrial applications.

The commercialisation of perovskite photovoltaic (PV) technologies requires advancements in large-area module efficiency, scalable and cost-effective manufacturing, and long-term operational stability. Stability issues in perovskite solar cells (PSCs) often stem from the materials used in the hole-transporting layer (HTL). Innovative, sustainable, and affordable hole-transport materials (HTMs) are crucial to address these challenges. Cu₂ZnSnS₄ (CZTS), an earth-abundant p-type semiconductor traditionally used as a light-absorbing material in heterojunction solar cells, has recently gained attention as an HTL for PSCs due to its desirable electronic properties.

This study explores the synthesis and application of CZTS nanoparticles (NPs) as an HTM in PSCs. The nanoparticles were produced using a hot-injection method under an oxygen-free environment and subsequently processed into an ink formulation for spin-coating. The resulting CZTS thin films, approximately 50 nm thick, were annealed to ensure structural and optical transparency in the visible solar spectrum. Comprehensive material characterisation—including transmittance, Raman spectroscopy, UV-Vis spectroscopy, scanning electron microscopy, and X-ray diffraction—confirmed the quality and stability of the CZTS layers.

Preliminary findings indicate that PSCs employing CZTS as an HTL demonstrate superior stability compared to conventional organic HTM devices. Over one month, the CZTS-based devices retained or even improved their photovoltaic efficiency, unlike organic HTL-based devices, which experienced significant degradation. Current-voltage measurements, external quantum efficiency and photoluminescence spectroscopy analyses revealed enhanced charge injection in the CZTS HTL. These results suggest that CZTS is a robust and sustainable alternative to traditional HTMs, paving the way for more durable perovskite PV technologies.

Do mobile ions help or hinder performance? Mobile halide ions in perovskite solar cells (PSCs) are often linked to negative effects such as degradation and hysteresis. However, this study demonstrates that mobile ions alter photovoltaic design parameters, enabling devices with mobile ions to achieve higher maximum efficiencies than those without.

Recent advancements in PSCs have been driven by surface treatments that reduce recombination and enhance photovoltages. These photovoltages often exceed the cells’ built-in potentials, with significant energetic offsets reported between the band edges of the perovskite and transport layers. This contradicts conventional photovoltaic design principles. This work attributes such tolerance for energetic offsets to mixed ionic and electronic conduction within the perovskite layer. Using the novel Stabilise and Pulse (SaP) technique, combined with drift-diffusion simulations, we explore how ionic charge distribution impacts performance. At steady-state, electrostatic redistribution of ions significantly reduces surface recombination currents, increasing photovoltage by tens to hundreds of millivolts. These findings reveal that mobile ions reduce the sensitivity of photovoltage to energetic misalignments at interfaces, ultimately improving device efficiency.

Building on these insights, we outline photovoltaic design principles that account for the effects of mobile ions and highlight the SaP method's capability to measure band offsets across different transport layers. These findings provide a new framework for optimising PSC design and performance.

Transparent photovoltaics (TPV) is an emerging and disruptive technology in which the solar cells 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 dye requires an absorption far from the photopic response of the human eye.
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. Fully transparent and colorless DSSC were built reaching 80 % transmittance in complete devices. [3,4]
The synthesis of new series of cyanines and squaraines have been performed in a one-step reaction under microwave heating, saving time and money in the process, and even increasing yields and purity. A simple crystallization of the crude products yielded very low cost and industrially scalable products.
These new sensitizers 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.

Singlet fission (SF) is a spin allowed process that allows the sharing of energy from a singlet exciton with a near neighbouring chromophore to generate two triplet excitons, through a correlated triplet pair and a coupled triplet pair. When the triplet energy is matched to the bandgap energy an underlying solar cell it is theoretically possible to increase the solar cell efficiency from 33% to 45%, however only modest gains have been demonstrated to date.

New SF chromophores, especially those energy matched to silicon, are required. We have demonsrated tuning of the absolute and relative energy levels in SF chromophores by controlling the captodative stabilization of diketopyrrolopyrrole chromophore.  

In Addition, many potentially important molecular chromophores do not support singlet fission in the solid state due to poor crystal packing. We have demonstrated that by introducing a bridge, to promote secondary self-assembly in the solid state, between two potential SF chromophores we can turn on SF in otherwise SF inactive molecules. In this talk I will discuss molecular design for SF chromophores energy matched to silicon, and crystal engineering, molecular aggregation and molecular design in new SF systems.

Perovskite absorbers attract huge interest in the scientific community thanks to their outstanding optoelectronic properties demonstrating high potential on the development of solar cells, light emitting diodes and X-Ray/Particle Detectors. A class of lead halide perovskite compounds based on bromide halogen (MAPbBr₃, FAPbBr₃ and CsPbBr₃) have been recently studied as UV-vis light absorber with wide bandgap (>2.3eV) for the development of semi-transparent perovskite solar cells (ST-PSC) for building integrated PV field¹ . Furthermore, they have been tested as X-Ray detectors for medical imaging application. Although the demonstration of excellent key performing indexes (KPIs), long-term stability remains the main goal to achieve. In this talk, we reported the strategy to improve the performance and the stability of ST-PSC and X-Ray detectors working on hybrid (MA/FAPbBr₃, FAPbBr₃) and inorganic (CsPbBr₃) perovskite compositions, novel 3D/2D perovskite passivation scheme, light management tools and scalable deposition techniques. Several in-situ and ex-situ characterization tools have been performed in order to understand the impact of the stability test on morphological, structural and optoelectronic properties of the device under study. Regarding ST-PSC, state-of-art KPIs have been reached showing PCE of 8.4%, AVT of 68% and LUE above 5.7% using 150nm-thick FAPbBr₃ perovskite and sputtered ITO top electrode. Environmental, light and thermal stress have been evaluated using ISOS stability protocols showing improved T80 parameters thanks to the device optimization¹. In parallel, self-powered thin-film based X-Ray detectors have been demonstrated for direct X-ray conversion at 0V bias using ST-PSC-like architecture showing high linearity to the radiation dose, surface sensitivity of 185.64 μC Gy^-1 cm^-2, bulk sensitivity of 3700 μC Gy^-1 cm^-3 and LoD of 133 nGy s^-1 with maximum photon energy at 40KeV. The X-Ray detectors have been tested under continuous X-Ray irradiation for 600 hours absorbing a cumulative radiation dose equal to 189.46 Gy without showing any performance degradation².    

Although silicon-based photovoltaic technology currently leads the market, it is hindered by challenges such as limited raw material availability and reduced performance under low or diffuse light. Dye-sensitized solar cells (DSSCs) present a compelling alternative, offering low cost, simple fabrication, and enhanced versatility, including the potential for flexibility and transparency. DSSCs rely on a photoelectrochemical reaction involving photoanode, dye, redox electrolyte, and counter-electrode (CE). While most of the research focuses on investigating and optimizing dye and electrolyte, the CE plays a critical role in determining the overall photovoltaic performance; yet the most commonly used CEs are based on Platinum and PEDOT. Broadening the range of CEs is essential for improving DSSC efficiency and facilitating the development of novel redox couples and dyes [1]. In this contribution, a series of metal-sulfur coordination polymers have been synthesized, characterized and successfully adopted as counter-electrodes for DSSCs [2]. Even if insoluble, such systems have been effectively processed from liquid dispersions without additives and have been subjected to thorough chemical and physical characterization. These materials appear as highly delocalized systems featuring low activation energies, high electrical conductivity, and good thermo-oxidative behaviour. They have been adopted as CEs in DSSCs showing comparable photovoltaic performance to standard Pt and PEDOT based CEs and exceeding 10% conversion efficiencies. This research paves the way for the development of innovative metal-coordination polymers for high-performing counter electrodes in solar cells.

Smart storage solutions are crucial for balancing the supply and demand of renewable energy to combat climate change. My work explores integrating energy-storage capabilities into renewable energy devices like solar cells, using sustainable, stable, and tunable materials. I will present Metal-organic nanosheets (MONs) comprising earth-abundant non-toxic metals, self-assembled with an organic linker to define their conductivity as viable candidates for energy storage applications. MONs are hybrid materials composed of an organic linker and a metal ion forming robust two-dimensional networks. With molecular modifications tuning their bulk electrical properties, MONs offer a broad design space for energy storage. I will demonstrate that when interfaced with thin film photovoltaic devices, MONs can enable a new generation of energy storable solar cells. 

To sustainably meet the growing energy demands of the world, a substantial increase in the deployment of energy storage technologies is needed. With the increase in the number of deployed devices arises the need to evaluate their end-of-life, transition to a resource efficient circular economy. In this talk, I will also evaluate each component of my device design from an environmental point of view. I will present the various material design routes being developed to enable a sustainable approach both during fabrication and as readily ‘refurbish-able/upgradable’ devices, leveraging the ability to replace MONs repeatedly with no observable loss in functionality over many product generations. My talk will identify features which are conducive to circular economy and identify barriers to resource efficiency for these technologies and suggest some potential solutions and priority areas for future research.

Flexible electronic devices allow for the seamless integration of components with different functions, e.g. energy harvesting and storage, enabling self-powered portable and wearable devices. In this talk, we focus on flexible perovskite solar cells (f-PSC) for energy harvesting and flexible supercapacitors (f-SC) for energy storage.

Beside their remarkable efficiency, as high as 25% [1], f-PSCs present many appealing features, such as flexibility, conformability, high power-to-weight ratio, making them the perfect candidates for several applications: from IoT and portable/wearable electronics, to space [2]. Also, f-PSC fabrication is based on abundant materials and low-cost manufacture via solution processes. Still, most reports focus on lab scale processing, i.e. spin-coating, hazardous solvents, and expensive synthesis routes for crucial materials, all hardly compatible with industrialization.

Supercapacitors (SC) have become promising candidates in diverse fields that require high energy throughput (e.g. hybrid electric vehicles) and stable energy throughput (e.g. sensitive automation, computer chips, portable electronics), due to fast storage capability (i.e. low discharge time, SC: 1–10 s vs Li-ion battery: 10–60 min) and enhanced cyclic stability (SC > 30,000 h vs battery > 500 h) [3]. Technical challenges to increase the still low energy density include the development of advanced materials for electrodes with appropriate design, the choice of electrolyte, and the potential window of the electrodes.

We present our latest results on f-PSC reliable and sustainable fabrication routes for both materials and devices, compatible with high throughput roll-to-roll manufacture [4, 5]. We also report our latest results on f-SCs on unconventional substrates, i.e. paper, fabricated with sustainable, low-cost materials and large-area printing techniques [6].

Finally, we show the integration of a f-PSC mini-module and a f-SC on paper into a hybrid photo-supercapacitor [7], which combines energy conversion and storage in a single device. The device quickly reaches the saturated voltage under various light intensities and displays a self-discharge of >2 minutes, with overall and storage efficiencies of 2.8% and 23% respectively, with a broad potential window of 3.8 V [8].

Machine learning is profoundly reshaping how we approach scientific research, offering new opportunities to both accelerate and enhance traditional methodologies. This shift is also significant in materials science, where the complexity of understanding material properties and optimizing manufacturing processes has long posed challenges. The application of machine learning techniques is now enabling faster discovery, more accurate predictions, and better-informed decision-making [1]. In the context of photovoltaic devices—where the efficiency of material properties directly impacts the device performance—this technological revolution is proving to be especially valuable.

In this presentation, I will delve into the application of generative models, a subset of machine learning, and their role in enhancing the speed and accuracy of numerical simulations aimed at exploring material properties, in particular dynamical properties. These machine learning models can significantly accelerate numerical simulations by identifying internal relevant material degrees of freedom [2]. Additionally, I will discuss how machine learning techniques are being used to analyze and process experimental data [3,4]. Often, experimental data can be noisy or incomplete, and integrating it with simulation results can be a complex task. However, by leveraging machine learning algorithms, it becomes possible to identify hidden patterns, fill in missing information, and reconcile experimental measurements with theoretical models. This integration enables the creation of a unified data stream that provides a comprehensive view of material performance, encompassing both simulated predictions and real-world observations. The goal of combining generative models with data analysis is to streamline the entire research process, from simulation to experimentation [5]. This approach reduces the time and resources traditionally required to design, test, and optimize new materials, especially in fields like photovoltaics, where improving efficiency is crucial. By using machine learning to accelerate these steps, we can significantly shorten the cycle time for the development of advanced materials, driving innovation in energy technologies and beyond.

Achieving long term operational stability of perovskite cells under real-world conditions continues to be a major concern. Many mechanisms have been shown to cause degradation, recent examples being exposure to both water and oxygen [1], screening of the internal field driving charge extraction through ion migration [2] and interfacial recombination at the SnOx/bathocuproine interface in the hole blocking layer [3].

Here, we show how degradation mechanisms in a solar cell can be identified from experimental measurements by creating a digital twin, a virtual representation of an object designed to produce an accurate reflection of a physical object [https://www.ibm.com/think/topics/what-is-a-digital-twin]. Through simulations performed in real time, our digital twin can analyse performance changes due to degradation under operation and suggests potential mitigations. Our digital twin is a combination of the device transport model IonMonger [4] and machine learning [5]. It can be used to test hypotheses about the physical processes responsible for degradation. These processes include the role of mobile iodide vacancies in influencing charge transport across the interface through charge accumulation/depletion at interfaces, trap assisted recombination at the interfaces, and contributions of other impurities. The TOC Figure shows example results from our digital twin along with perovskite solar cells fabricated in the Cameron lab https://people.bath.ac.uk/chppjc/research.html.

TOC Figure Left: Perovskite solar cells fabricated in the Cameron lab. Right: Illustrative example joint distributions of two IonMonger input parameters for a degrading device as derived by Bayesian Parameter Estimation.

This presentation will cover our recent work measuring and modelling the impedance spectra of a wide variety of perovskite solar cells (PSCs); and introduce the wealth of information that can be obtained from the mid- and low- frequency impedance response. The low frequency impedance response gives insight into which part of a fully operational device is limiting cell efficiency [1]; and our recent modelling suggests that the mid-frequency response may be able to rapidly predict the long-term stability of perovskite solar cells; saving time by suggesting which cells to submit for long term testing [2].

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 [3]. Impedance spectroscopy is a common characterisation technique used to probe the physics of the device on different timescales. 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 [2]. Furthermore, we show that the low- and mid-frequency responses convey 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. More recently we have looked at the effect of cell degradation (in the form of an increasing recombination rate) on the impedance spectrum, and the results suggest that impedance is a powerful tool for rapidly predicting the long term stability of perovskite solar cells.

Hybrid perovskite photovoltaics (PVs) are poised to impact future’s renewable energy production and contribute significantly to the reduction of global CO2 emissions. Power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) were increased almost up to the level of silicon PV, while stable power outputs of over 10,000h of operation were demonstrated. Further, the bandgap of hybrid perovskites can be tuned to match optimal values for tandem PVs. However, the scalability of the technology still falls behind the industry standard for concurrent PV technologies, that is the PCE of perovskite modules falls off steeply with increasing aperture area.

There are two fundamentally different routes for fabricating perovskite thin-films, coating of solution films that are subsequently dried and crystallized or deposition from a vapor at low pressures. While solution coating, for example by spray coating or slot-die coating, is more cost-effective due to its operation at atmospheric pressure and its compatibility with roll-to-roll machines, trials on vacuum evaporation indicate superior conformity and adhesion on small- to medium-sized substrates, which are important characteristics when depositing on rigid, fixed-size silicon solar cells for tandem application. However, due its high economical prospects, solution processing on small to medium-sized substrates via intermittent coating should not be discarded per se.

In this work, we will investigate slot-die coating on medium-sized substrates (25cm-300cm) from a practical and theoretical viewpoint. The main challenge for this process arises from two aspects: 1) The difficulty to achieve homogeneous coating over the whole substrate length and 2) the difficulty to dry the precursor solution in homogeneous way over the limited substrate length. We first address the latter issue by making use of state-of-the-art drying models in a simulative sandbox. In particular, we show that Bayesian optimization can find effective parameters for drying of perovskite precursor films on medium substrate sizes. Furthermore, we show that by making use of pre-existing knowledge of the coated thickness inhomogeneity from ex-situ or in-situ measurements, the same algorithms can be exploited for drying films with an inhomogeneous thickness distribution in an optimal way such that the perovskite crystallization occurs a the optimal time and position during the drying. We further outline the interdependencies of the different coating parameters and demonstrate how these interdependencies can be exploited for achieving more homogenous perovskite coatings. In a final perspective, we showcase how these insights can be leveraged in automatically controlled perovskite coaters and dryers.

Fabricating thick (1000 nm) solution-processed perovskite layers is expected to increase the efficiency of carbon-contact-based solar cells compared to thinner (500 nm) films. However, increasing only the deposited layer thickness often results in buried voids inside the dry film. This is detrimental to the efficiency of the device. Recently, we have developed a theoretical framework based on Phase Field simulations[1]. It is capable of describing the main physical processes determining the morphology: evaporation, diffusion, spontaneous nucleation, crystal growth, and advection[2]. With the help of the simulations, it is possible to explain why voids form in the film. The crystals nucleate at random spots inside the liquid film. The movement of the condensed-vapor interface, due to evaporation, leads to an agglomeration of the crystals at the film surface. The crystals block further evaporation and the remaining solvent is the origin of the buried voids inside the dry film. We explain how adding seeds on the substrate before coating the thick film can prevent this. In this case, processing conditions have to be modified compared to standard operating procedures for thin films. The theoretical expectations can be verified experimentally, leading to a performance improvement of the devices.

On the value of device characterization for the optimization of solar cells

The biggest advantage of organic photovoltaic, namely the flexibility of organic synthetic chemistry is also the biggest challenge for material selection and process optimization. To find the best process in a highly multidimensional process parameter and material space, traditional Edisonian optimization methods are insufficient.

Sophisticated optimization algorithms that help to explore the high-dimensional process parameter space have recently been applied. Thanks to increasing computing power and modern approaches in the field of machine learning, these methods are continuously improving [1 - 3].

Many of the optimization approaches are black-box optimization methods based on purely mathematical models and do not, or only partially, consider the physical domain knowledge underlying organic photovoltaics. In this case, it is sufficient to measure only the efficiency of the respective devices.

In this contribution, we want to quantify how far, domain knowledge about the material parameters and the device physics of the organic solar cells is beneficial to the actual optimization process. Therefore, we have implemented a virtual laboratory to compare different process optimization strategies based on different amounts of device characterization. The virtual laboratory is essentially a benchmark function for the optimization that is based on actual data obtained from several hundred organic solar cells and associated characterization data.

Within the virtual laboratory, we compare black-box optimization with an optimization algorithm that is virtually fed characterization data and that can make predictions in the process parameter space using drift-diffusion simulation as well as optical modeling. We show that prior knowledge of optical constants combined with optical modeling is particularly advantageous for identifying local maxima for larger layer thicknesses.