Session 1A - Emerging materials and technologies (EPV) Chair Opening - Giulia Grancini

Bulk heterojunction organic solar cells (BHJ OSCs) potentially can offer low cost, large area, flexible, light-weight, clean, and quiet alternative energy sources for indoor and outdoor applications. OSCs using non-fullerene acceptors (NFAs) have garnered a lot of attention during the past few years and shown dramatic increases in the power conversion efficiency (PCE). PCEs higher than 19% for single-junction systems have been achieved, but the device lifetime is still too short for practical applications. Thus, understanding the degradation mechanisms in an OSC is crucial to improve its long-term stability. In this talk, I will discuss the degradation mechanisms in BHJ OSCs. We investigated the impact of different blend materials and device structures on the device stability. A combination of characterization methods such as solid state Nuclear Magnetic Resonance (NMR), resonant soft X-ray scattering (RSoXS), AFM, X-ray photoelectron spectroscopy (XPS), Electron paramagnetic resonance (EPR) spectroscopy, and capacitance spectroscopy are employed to gain insight into the device degradation mechanisms. We propose strategies to improve the device stability.

Tokyo Chemical Industry (TCI) is a global supplier of laboratory chemicals and specialty materials, and also leading the next-generation solar technology by providing high-quality and reliable materials, including metal halide perovskite precursors and materials for carrier transporting.

Organic-Inorganic Perovskite Precursors

https://www.tcichemicals.com/assets/brochure-pdfs/Brochure_F2035_E.pdf

Perovskite Precursor for Solar Cell Purified Lead(II) Iodide

https://www.tcichemicals.com/assets/brochure-pdfs/Brochure_FF031_E.pdf

High Purity Perovskite Precursor Purified Lead(II) Bromide

https://www.tcichemicals.com/assets/brochure-pdfs/Brochure_FF088_E.pdf

Perovskite Precursors Tin(II) Iodide, Tin(II) Bromide

https://www.tcichemicals.com/assets/brochure-pdfs/Brochure_FF075_E.pdf

Perovskite Precursor: Lead Acetate Anhydrous

https://www.tcichemicals.com/assets/brochure-pdfs/Brochure_FF070_E.pdf

Hole Selective Self-Assembled Monolayer (SAM) Forming Agents: PACzs

https://www.tcichemicals.com/assets/brochure-pdfs/Brochure_FF092_E.pdf

SAM Formation Reagent with Face-on Orientation to Substrate Surface: 3PATAT-C3

https://www.tcichemicals.com/assets/brochure-pdfs/Brochure_FF169_E.pdf

Hole Transport Materials For Stable and Practical Perovskite Solar Cells: TOP-HTMs

https://www.tcichemicals.com/assets/brochure-pdfs/Brochure_FF090_E.pdf

High Quality Hole Transport Material: Spiro-OMeTAD

https://www.tcichemicals.com/assets/brochure-pdfs/Brochure_FF158_E.pdf

Dopants for Organic Electronics Research

https://www.tcichemicals.com/JP/en/product/topics/dopants_for_organic_electronics_research

Solar Cell Materials (PSC, OPV, DSSC Materials)

https://www.tcichemicals.com/assets/brochure-pdfs/Brochure_F2033_E.pdf

In this short talk, these highly useful materials will be introduced including most recently commercialized new products.

Metal halide perovskites (chemical formula ABX₃, A = formamidinium (FA+), methylammonium (MA+), or Cs+ , B = Pb2+, Sn2+, X = I−, Br−, or Cl−) have demonstrated their potential as a material platform for a new generation of optoelectronic technology. One superior feature of metal halide perovskites is their continuously tunable bandgap from near infrared to ultraviolet by designing the chemical composition of the semiconductor crystalline unit. This enables them to provide top absorbers with matched bandgaps for tandem solar cells and emissive layers for colorful light-emitting diodes (LEDs)  just to mention the most common applications.

In this lecture I will assess the most recent advances in elucidating the (photo)chemistry of defects related to the chemical composition of the perovskite crystalline unit. I will show how they define the charge carrier dynamics in the semiconductor. Based on such understanding I will discuss the main electrical and spectroscopic features related to the activity of defects and how to interpret them. Eventually I will correlate the nature of defects and their photo-physics the the figures of merit of optoelectronic devices and their stability.

Hybrid organic-inorganic perovskite (HOIP) semiconductors based on metal halide frameworks offer unprecedented opportunity to tailor structural and materials properties using the full flexibility afforded by the associated inorganic and organic components, and such tunability offers wide-ranging potential for applications including solar cells, light-emitting devices, detectors, transistors and advanced computing devices [1]. In this talk we will focus on examining the role that the organic cation can play in introducing or impacting chirality, symmetry breaking and associated phase transitions within HOIPs, as well as on the resulting chiroptical (e.g., circularly dichroism [2,3]), spin-selective charge transport (e.g., chirality-induced spin selectivity [4]), electronic structure (e.g., spin splitting of the conduction band [5-7]), and melting (e.g., allowing for melt-processed film formation and reversible glass-crystal transitions [8,9])  characteristics of these systems. Recent examples of such symmetry-related tunability highlight the promise of using the organic component to control light, charge and spin within the wide-ranging HOIP family.

Session 1B1 - Material design and modelling: Chair introduction - Virginia Carnevali

Machine learning (ML) is a powerful tool to accelerate the development of halide perovskite materials and devices. Because this burgeoning class of material for photovoltaics entails a colossal chemical composition space, ML is very suitable to replace the conventional trial-and-error approach used in their characterization. Thus, there has been a pressing need within the materials research community to identify ML models that can be implemented to inform the physical and chemical behavior of the perovskites. We apply ML models varying from echo state networks to statistical models to classify and predict physical properties such as hole transport layer electrical conductivity, halide perovskite photoluminescence response, the power conversion efficiency of photovoltaic devices, etc. Specifically, we use in situ environmental optical measurements to predict the optical behavior of Cs-FA perovskites for 50+ hours, upon materials’ exposure to moisture. Here, we compare linear regression, echo state network, and seasonal auto-regressive integrated moving average with eXogenous regressor algorithms and attain accuracy of >90% for the latter. Our high-throughput measurements and ML-supported analyses validate the potential of ML to detect and forecast hybrid perovskites’ response with a variety of chemical compositions.

Metal halide perovskites, notable for their unique properties, have gained attention for optoelectronic applications including solar cells, LEDs, and photodetectors. Our research employs a blend of computational techniques, encompassing electronic structure calculations (DFT and tight binding) and reactive molecular dynamics simulations, to explore the electronic and dynamical properties of halide perovskites. 

A key focus is defect analysis in these perovskites, essential for enhancing solar cell device efficiency and longevity. We investigate electronic energy levels and dynamic properties, identifying defects causing recombination losses and chemical degradation, and develop mitigation strategies through composition engineering, passivation, and film quality optimization. 

Our second focus delves into the chirality of perovskites, where we introduce chiral organic ligands to induce unique properties like chiral-induced spin selectivity and chiroptical activity. Using first-principles methods, we calculate circular dichroism and electron/spin transport, especially under varying temperatures. This research aims to identify structural features affecting optoelectronic responses, guiding the design of novel chiral perovskites for advanced optoelectronics, such as spin LEDs and chiral photodetectors.

The translation of statistical techniques from the artificial intelligence community to materials science and engineering is helping to bridge the divide between traditional modelling and measurements [1]. In the study of metal halide perovskites, data-driven machine learning (ML) workflows are being used for diverse tasks ranging from novel materials discovery to closed-loop accelerated device optimisation. I will provide an introduction to this topic, with a focus on how the limits of materials modelling are being extended by incorporating ML techniques to develop deeper insights into the behaviour of perovskites across time and length scales [2,3]. In particular, I will discuss our latest understanding of compositional and structural disorder at the nanoscale, linked to multi-modal experimental characterisation [4]. The overarching goal is to shed light on the origins of the exceptional performance of these systems, as well as to identify routes to develop the next generation of perovskite-inspired materials. 

Session 1B2 - Breaking efficiency limits Chair introduction - Nakita Noel

Triplet generation at the hybrid inorganic/organic semiconductor interface is a promising approach to extend the (photo-)excited state lifetime, and thus, facilitate solar energy harvesting. The generated triplet states can be easily utilized in triplet-triplet annihilation (TTA) upconversion (UC), where two low energy triplet states are combined to form one high energy singlet state. With this approach, it is possible to harness a greater portion of the solar spectrum and e.g., increase solar cell efficiencies. While substantial UC efficiencies have been demonstrated in solution-based UC systems, the required performance in their solid-state counterparts has not been achieved. Typically, excimer formation or intermolecular interactions result in an unfavorable singlet and triplet energy surface, disallowing efficient TTA.

While rubrene has been the most commonly studied annihilator in solid-state devices to date, it is not well matched with the perovksite sensitizer. We have recently placed an effort on replacing rubrene in the UC devices to increase the achievable apparent anti-Stokes shift. We have demonstrated successful upconversion using 9,10-Nanoscale, 2022,14, 17254-17261bis(phenylethynyl)anthracene and its derivatives and naphtho[2,3-a]pyrene.

Here, I will present the current understanding of triplet generation at the bulk perovskite/organic interface and discuss the role of molecular aggregation and intermolecular coupling on the energy landscape underlying the upconversion process.

Recent announcements from top tier crystalline silicon (c-Si) solar cells manufacturers report record certified power conversion efficiencies (PCE) more than 26.8% for architectures based either on poly-silicon (poly-Si) or silicon heterojunction (SHJ) in front/back contacted (FBC) configuration [1][2]. Also in case of interdigitated back contacted (IBC) configuration, several industrial players have announced in-house measured or certified PCE well above 26.7% and up to 27.1% at cell level [3][4][5] or certified PCE up to 24.9% at module level [6], which very likely implies the deployment of high-efficiency solar cells exhibiting PCE above 26.5%. PERC architecture, which currently dominates the photovoltaic (PV) market, is therefore poised to lose market share soon, first, in favor of the so-called FBC industrial TOPCon architecture [7], then, either to FBC SHJ architecture or to architectures with IBC configuration (high-thermal budget, low-thermal budget, or so-called hybrid architecture). This market dynamics is mostly driven by the lower CAPEX of FBC industrial TOPCon with respect to FBC SHJ architecture or other IBC architectures despite all these newer architectures being capable of exhibiting PCE very close to 27%. Modern manufacturing techniques and outstanding wafer quality enable such record c-Si solar cells, which behave close to the Auger limit [8]. Still within the highest attainable theoretical efficiency [8][9], there is still room to demonstrate efficiencies well above 27%. In the long run, the market will be disrupted by double-junction perovskite/c-Si tandem solar cells, given the quick rise of their conversion efficiency up to 33.9% [10]. However, the current need to move beyond 27% in c-Si single junction solar cells is to cover the time needed to upscale and opportunely industrialize perovskite/c-Si tandem solar cell technology. Therefore, it is of great interest to explore novel c-Si solar cell architectures which can exhibit PCE well above 27%. In this contribution, with the support of advanced numerical simulations [11-14], we study several high-thermal budget, low-thermal budget, and hybrid architectures in both FBC and IBC configurations and discuss their efficiency drivers.

REFERENCES:

[1] Jinko, press release October 2023

[2] H. Lin, et al., Nat. En., 8 (2023)

[3] K. Yoshikawa, et al., SOLMAT, 173 (2017)

[4] Longi, AIKO, GS @ Asian PVSEC 34

[5] LONGi, press release December (2023)

[6] Maxeon, press release March 2024

[7] A. Richter, et al., Nat. En., 6 (2023)

[8] L. Black & D. Macdonald, SOLMAT, 246 (2022)

[9] M. A. Green, Nat. En., 8 (2023)

[10] LONGi, press release November 2023

[11] Synopsys TCAD (2015)

[12] P. Procel, et al., SOLMAT, 186 (2018)

[13] P. Procel, et al., IEEE JPV, 9 (2019)

[14] P. Procel, et al., PiP, 28 (2020)

Singlet fission (SF), an exciton multiplication process occurring in organic semiconductors, offers a mechanism to break the singlet-junction limit in photovoltaics. If the triplet excitons generated by SF can be transferred to inorganic quantum dots (QDs), where they radiatively recombine, SF based photon multiplication is achieved, converting a single high-energy photon into two low-energy photons. Such a SF photon multiplication film (SF-PMF) could raise the efficiency of the best Si photovoltaics from 26.7% to 32.5%. In this talk I will outline the basic photophysics of singlet fission and triplet transfer to QDs and then discuss recent results on singlet fission/QD blend films in which support efficient SF (190% yield) in the organic phase and quantitative triplet energy transfer across the organic-QD interface, resulting in 95% of the triplet excitons generated in the organic phase being harvested by the QDs. I will outline the potential benifits and limiations of this approach and the way forward to implementing it with Si PVs.

Will Vapor Phase Deposition of Perovskite Photovoltaics Accelerate Commercialization?

Ulrich W. Paetzold^{* 1,2}

Karlsruhe Institute of Technology, Institute of Microstructure Technology Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen Germany

Karlsruhe Institute of Technology, Light Technology Institute Engesserstrasse 13, 76131 Karlsruhe, Germany

Mail: Ulrich.Paetzold@kit.edu

Vapor phase deposition of organic-inorganic perovskite solar cells (PSCs) is raising increasing interest in academia and industry, holding great promise for commercializing perovskite-based photovoltaics (PV). Despite the well-established use of vapor phase deposition processes in commercial manufacturing of thin-film photovoltaics and other optoelectronic applications, research on vapor phase processed PSCs is still underrepresented compared to their solution-based counterparts. Solution-processed PSCs still dominate the academic research, benefiting from fast optimization feedback and straightforward integration in modern research laboratories.

This contribution will present a recently published perspective (Ref 1) by an international team that conveys a balanced viewpoint from industry and academics on the prospects for vapor phase deposition of perovskite-based PV. The perspective highlights strategic opportunities of vapor phase deposition for the its commercialization of perovskite-based PV. In addition, the latest developments at the Karlsruhe Institute of Technology on vapor phase deposited PSCs and perovskite/Si tandem solar cells will be presented. With regard to co-evaporated PSCs, we have recently developed a comprehensive understanding of how interfacial hydrogen bonding impacts bulk perovskite properties, highlighting the potential for rational design of substrates to control organic cation incorporation (Ref. 2), and enabling us to incorporate our evaporated self-assembled monolayer hole transport layers (Ref. 3) into all-evaporated PSCs. In addition, our recent progress on sequentially evaporated and hybrid-processed PSCs will be summarized at the conference.

(1) Vapor Phase Deposition of Perovskite Photovoltaics: Short Track to Commercialization?
T. Abzieher, D. More, …, and U.W. Paetzold.
Energy & Environmental Science 2024 (DOI: 10.1039/D3EE03273F)

(2) Understanding and exploiting interfacial interactions between phosphonic acid functional groups and co-evaporated perovskites
T. Feeney, J. Petry, …, and U.W. Paetzold.
Matter 2024 – Accepted not published.

(3) Evaporated Self-Assembled Monolayer Hole Transport Layers: Lossless Interfaces in p-i-n Perovskite Solar Cells
A. Farag, T. Feeney, …, and U.W. Paetzold.
Advanced Energy Materials 13(8), 2203982 2024 (DOI: 10.1002/aenm.202203982)

Epitaxial semiconductors involving III-V compounds and silicon promise the highest performance levels in PV applications such as in solar cells and photoelectrochemical cells [1-3]. However, highest performance in solar energy conversion can only be achieved, when using optimum absorber layers and advanced contact formation for electronic and chemical passivation, i.e. for the protection of the solid-liquid interface against corrosion as well as impeding interfacial non-radiative recombination. In order to address the surface and interface properties of III-V semiconductor layer structures in relation to their performance, we present the synthesis, theoretical modelling and properties of critical and well-defined interfaces such as GaInP/AlInP [4] or GaInP/GaN [5]. Here, lattice matched n-type GaN or AlInP(100) charge selective contacts are prepared on n-p GaInP(100) top absorbers in highest-efficiency III–V multijunction solar or photoelectrochemical cells, where the cell performance can be greatly limited by missing electron selectivity and detrimental valance band offsets. Hence, understanding of the atomic and electronic properties of the heterointerfaces, for instance, is crucial for the reduction of photocurrent losses in III–V multijunction devices. We discuss the essential considerations on the properties of critical interfaces in relation to photoelectrochemical cells from a conceptual and from a theoretical modeling point of view assuming mostly idealized surface conditions. We also address latest progress on the important III-V/Si interface, modifications by fine-tuning of the preparation and describe experimental model experiments on the surface reactivity of III-phosphide surfaces to H2O exposure. These different surface science approaches are then related to photoelectrochemical cells for H2 evolution and CO2 reduction using different III-V based tandem cells and providing highest conversion yields.

Session 1B3 - Operational and material stability Chair introduction - Sofia Masi

MXene have been applied in Perovskite solar cells (PSCs) demonstrating the improvement of their power conversion efficiency. However, device stability studies are still missing. Especially under real outdoor conditions where devices are subjected to the synergy of multiple stressors. In this work, additive engineering has been applied to 2D Ti₃C₂ MXene and applied in normal PSCs configuration. The MXene was applied at the interface between the halide perovskite and the hole transport layer. The functionalization of the Ti₃C₂ MXene was made utilizing the same organic additive passivating the halide perovskite layer. Our functionalizing strategy creates a continuous link between the MXene and the halide perovskite layer to obtain MXene-based PSCs with a ~22 % power conversion efficiency, in comparison with the control device showing 20.56 %. Stability analyses under different conditions (dark, continuous light irradiation and real outdoor analysis) reveal that the enhancement of the PSCs lifespan is always observed when the MXene layer is employed. To our knowledge, this is the first report of the stability analysis of MXene-based PSCs carried out under real outdoor (ISOS-O) conditions.

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

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

Halide perovskite solar cells have revolutionized the photovoltaic field in the last decade. In a decade of intensive research it has been a huge improvement in the performance of these devices, however, the two main drawbacks of this system, the use of hazardous Pb and the long term stability, still to be open questions that have not been fully addressed. Sn-based perovskite solar cells are the devices presenting the highest performance after Pb-based but significantly below them. In addition, Sn-based perovskite solar cells exhibit a long term stability lower than their Pb containing counterparts, making stability their main problem. In this talk, we highlight the use of additives to increase significantly the stability of formamidinium tin iodide (FASnI3) solar cells, and discuss about the different mechanism affecting this stability, beyond the oxidation of Sn2+. Another important challenge for the commercialization of this technology is the up-scaling of the Sn-perovskite deposition especially due to the fast crystallization producing inhomogeneous thin films. Here, we will analyze the up-scaling of these devices by the use of blade coating and analyze the important effect of additives in the control of Sn-perovskite film crystallization.

During my lecture, I will present our latest results on characterizing different types of Self-Assembled Molecules (SAMS) for perovskite solar cells using advanced photo-induced time-resolved techniques. Using PICE (Photo-induced charge extraction), PIT-PV (Photo-induced Transient PhotoVoltage), and other techniques, we have been able to distinguish between capacitive electronic charge and a more significant amount of charge because of the intrinsic properties of the perovskite material and it's dependence on the different selective contacts based on SAMs. Moreover, the results allow us to compare different materials used as hole transport materials (HTM) and the relationship between their HOMO and LUMO energy levels, the solar cell efficiency, and the charge losses because of interfacial charge recombination processes occurring at the device under illumination. These techniques and measurements are crucial to understanding the device's function and further improving the efficiency and stability of perovskite MAPI-based solar cells. The need for novel materials that make more stable devices without compromising efficiency is critical for commercializing these types of solar cells. We will show some examples of those future materials that will soon be present on the market.

High radiation hardness is the primary requirement for application of lead halide perovskite semiconductors in X-ray detectors for medical diagnostics and solar panels for space missions. Multiple reports show that perovskite absorber films and solar cells indeed could successfully tolerate high electron, proton and neutron fluences as well as gamma rays and x-rays [1-2]. Among different types of ionizing radiation, gamma rays have very high penetration ability and hence could hardly be mitigated using simple shielding used to suppress the damage from proton and also electron fluences. Thus, the investigation of the radiation hardness of lead halide perovskites with respect to gamma rays is essentially important from fundamental point of view and also in the context of the emerging applications.

Herein, we present the results of our systematic study of model lead halide perovskite materials: MAPbI₃, FAPbI₃, (CsMA)PbI₃ and (CsMAFA)PbI₃, where MA and FA are methylammonium and formamidinium cations, respectively [3]. We show that among the studied materials FAPbI₃ is the only one which does not degrade after receiving the ultrahigh radiation doses up to 20 MGy and thus represents highly promising absorber material for radiation-tolerant solar cells. Other complex lead halides produce different aging products upon exposure to gamma rays including metallic lead and PbI₂.

 Infrared near-field optical microscopy revealed the radiation-induced depletion of organic cations from the grains of MAPbI₃ and their accumulation at the grain boundaries. Using a set of complementary techniques, we evidenced that multication (CsFA)PbI₃ and (CsMAFA)PbI₃ perovskites undergo a facile phase segregation to domains enriched with Cs, MA and FA cations. This new degradation pathway is quite similar to the gamma-ray-induced halide phase segregation we observed previously for Cs_0.15MA_0.10FA_0.75Pb(Br_0.17I_0.83)₃ material [4-5]. The revealed aging pathways could be successfully mitigated through the rational compositional engineering of complex lead halides using (1) partial lead substitution and (2) the formation of the mixed dimensional 2D/3D absorber materials. The perovskite solar cells maintained 80-90% of their initial performance after exposure to extreme doses of gamma rays approaching 1 MGy, which is unprecedented result for all types of photovoltaic cells.

To summarize, our findings suggest that the radiation hardness of the rationally designed perovskite semiconductors could go far beyond the impressive threshold of 20 MGy we set herein for FAPbI₃ films and 1 MGy we demonstrate for completed perovskite solar cells. Thus, the unique radiation hardness of complex lead halides opens many exciting opportunities for practical implementation of these materials in detectors for medical diagnostics and solar cells operating in harsh radiation environments.

HOPV Rising Stars: Emerging photovoltaics - Chair introduction - Jovana Milic

Halide double perovskite semiconductors such as Cs2AgBiBr6 are widely investigated as a more stable, less toxic alternative to lead–halide perovskites in photoconversion applications including photovoltaics and photoredox catalysis. However, the relatively large (~2.1 eV) and indirect bandgap of Cs2AgBiBr6 limits efficient sunlight absorption. Similar to lead–halide perovskites, the bandgap of double perovskites can be manipulated through (partial) substitution of metals or halides with similarly charged ions. However, commonly used solvent-based synthesis routes often lead to the formation of domains or side phases, rather than solid solutions with controlled properties. This results in an inhomogeneous electronic landscape which is detrimental for photoconversion applications. Here, we show that mechanochemical synthesis methods, such as ball milling, are a valid route to synthesize phase-pure double perovskites. With the use of synchrotron radiation we followed the formation mechanisms during mechanochemical synthesis of Cs2Ag[BiM]Br6 (with M = Sb, In, or Fe or X = Cl, Br, or I), and identified new intermediate phases, providing insights into the reaction kinetics. We find that mechanochemical synthesis is a successful approach to make compounds that have not been reported via solution-based synthesis routes, such as Cs2AgBi0.5In0.5Br6, and Cs2AgBi1-xFexBr6. Where substitution with In3+ increases the band gap energy, it is lowered when replacing Bi3+ with Fe3+ or Br− with I−. Hence, the optical bandgap of Cs2AgBiBr6 can be tuned over the entire visible spectrum when partly substituting Bi3+ or Br−. For instance, we find that controlled replacement of Bi3+ with Fe3+ via mechanochemical synthesis results in a remarkable tunability of the absorption onset between 2.1 to ~1 eV. In addition, with synchrotron-based high pressure X-ray diffraction (XRD), we find that the softness of these materials varies with its chemical composition. Our first-principles density functional theory (DFT) calculations demonstrate that this bandgap reduction originates from a lowering of the conduction band minimum upon introduction of Fe3+, while the valence band remains constant. Additionally, our DFT calculations suggest that the bandgap becomes direct when 50% of Bi3+ is replaced with Fe3+. Finally, we find that the tunability of the conduction band minimum is reflected in the photoredox activity of these semiconductors. The improved understanding of the reaction mechanism of alloyed-AgBi double perovskites might help to overcome the current challenges faced with solution processing methods. Hence, opening up new avenues for enhancing the visible light absorption of double perovskite semiconductors and for harnessing their full potential in sustainable energy applications.

Halide perovskites and elpasolites are key for optoelectronic applications due to their exceptional performance and adaptability. However, understanding their crucial elastic properties for synthesis and device operation remains limited. We performed temperature- and pressure-dependent synchrotron-based powder X-ray diffraction at low pressures (ambient to 0.06 GPa) to investigate their elastic properties in their ambient-pressure crystal structure. We found common trends in bulk modulus and thermal expansivity, with an increased halide ionic radius (Cl to Br to I) resulting in greater softness, higher compressibility and thermal expansivity in both class of materials. For non-cubic systems, in which the elastic properties are anisotropic, we obtained axis-dependent compressibility. The A cation has a minor effect, and mixed-halide compositions show intermediate properties. Notably, thermal phase transitions in MAPbI₃ and CsPbCl₃ induced lattice softening and negative expansivity for specific crystal axes, even at temperatures far from the transition point. These results emphasize the significance of considering temperature-dependent elastic properties, which can significantly impact device stability and performance during manufacturing or temperature sweeps.

Mixed ionic-electronic conductivity is a defining feature of hybrid perovskites, and its understanding is key to open new frontiers in the design and optimization of perovskite based optoelectronics. [1] Progress in this direction relies on the development of appropriate experimental methods able to probe mixed conduction in these materials, and of models that can provide an accessible, yet accurate, interpretation of device function.

In this contribution, we present a systematic approach to the investigation of mixed conduction in methylammonium lead iodide (MAPI) thin films based on horizontal device structures. [2] The results highlight the importance of interfacial charging in the long time scale polarization behavior of MAPI devices close to equilibrium. Such polarization behavior is consistent with the dynamics of electric field screening in MAPI devices probed with non-destructive spectroscopic and optoelectronic methods. [3] Next, we address the interpretation of perovskite solar cells’ electrical response under light and/or voltage bias, a long-standing question in the field. We propose an equivalent circuit model that can describe the impedance of mixed conducting perovskite devices under bias, based on the previously introduced concept of ionic-to-electronic current amplification. [5] We demonstrate the analytical accuracy of the proposed model, by comparing results obtained with drift-diffusion simulations. [6] Based on the description of the ionic and electronic charge carrier dynamics, our analysis of calculated impedance spectra allows the clarification of several anomalous experimental observations. These include inductive behavior and multiple low frequency impedance features reported for perovskite solar cells. [7] Our study contributes to enabling the use of impedance spectroscopy as a routine characterization technique in perovskite photovoltaics.

We conclude by providing an outlook on possible strategies to address the defect chemical quasi-equilibrium of halide perovskites under light. By combining models describing their equilibrium defect chemistry [8] [9] with the relevant description of their optoelectronic properties, we are able to comment on the effect of light and halogen partial pressure on the steady-state defect concentrations. The results highlight the potential of controlling defect chemistry for the optimization of solar energy conversion devices based on mixed conductors.

Metal halide perovskites (MHPs) have gained significant scientific interest due to their outstanding optoelectronic properties and high efficiencies in solar cell photoconversion. There are many ways to prepare MHPs of reasonable quality, the solution process is still the best in this manner. But the vacuum-based methods, which are relevant for industry and large area deposition, are not lacking far behind. The final product is always the MHP thin film of similar morphology and structure. The only substantial difference is the dimension of the grains, which used to be larger for solution processed films. Therefore, logical question arises, what are the differences in the MHP film formation?

Recently, with the insight of in-situ photoluminescence (PL) and GIWAXS measurements, we have studied the MHP film deposition by solution processes  [1]. Based on the results depicted bellow, we divided the MHPs growth into three stages. In short, we see fast growth of MHPs grains from GIWAXS in the first growth stage, accompanied by proportional increase in PL signal. However, in the second stage the growth speed significantly decreases, and the PL signal is highly quenched. This observation means that the free grown grains (stage I) are of low defect densities, but once they start to connect (stage II) the grain boundaries are formed, where defects are concentrated. This leads to rapid decrease of the PL signal. The last stage documents partial degradation at long deposition times only. Surprisingly, very similar observation was done by in-situ measurement of evaporated MHPs [2] and films prepared by pizza oven deposition [3]. Preliminary results shows the same tendencies for pulsed laser deposited MPHs as well.

Deeper understanding to this universal formation mechanism of MPHs thin films give us a unique opportunity to enhance its optoelectronic quality. One critical aspect is formation of large grains, the second one is the passivation process mainly at the grain boundaries. In-situ PL characterization is able to guide us through these processes.

The small-perturbation analysis of perovskite solar cells (PSCs) highlights a fundamental conundrum – while time domain measurements yield two time constants corresponding to the rise and subsequent decay of the photovoltage or photocurrent, the corresponding frequency domain methods only yield one time constant from the analysis of the negative imaginary part of the transfer function. To solve this problem, we propose a modification of the frequency domain transfer function that focusses on the transition of its real part to negative values at high frequencies. After verification using drift-diffusion simulations and equivalent circuit analysis, the application of the method to experimental intensity-modulated photovoltage spectroscopy data of a PSC allows calculation of the hidden rise time constant, showing a good agreement with rise time constants obtained from transient photovoltage measurements. Combining these measurements with transient photoluminescence measurements allows calculation of the figure of merit (FOM) that determines the charge collection efficiency. We determine large FOM values between 0.7–0.95 at or close to the 1 sun open-circuit voltage, indicating a significant electric field exists in the transport layers that allows fast charge collection in these conditions.

Further breakthroughs in halide perovskite solar cells require advances in new compositions and underpinning materials science. Indeed, a deeper understanding of these complex hybrid perovskite materials requires atomic-scale characterization of their transport, electronic and stability behaviour. This presentation will describe combined atomistic modelling and experimental studies on metal halide perovskites [1-4] in two fundamental areas related to improving stability in optoelectronic devices: (i) Iodide ion transport and the effects of using different sized A-cations and mixed Pb-Sn compositions; the motivation here is that there is limited understanding of the impact of Sn substitution on the ion dynamics of Pb halide perovskites. DFT modelling and impedance spectroscopy measurements indicate suppressed ion migration in Pb–Sn materials.  (ii) Insights into passivating perovskites with molecular compounds including surface binding interactions of additives; here we find that since surface Pb ions adjacent to iodide vacancies are severely undercoordinated, molecular adsorption through bonds to Pb acts by increasing the Pb coordination and thereby promoting surface passivation.

This talk will cover our recent work combining machine learning and simulation methodologies to produce much faster and more direct characterisation of materials and devices. We have created a virtual model through a combination of device transport models and machine learning. This combination can be used to test hypotheses about the physical processes within these devices. These processes include the role of interfaces where charge accumulation/depletion can occur and traps where nonradiative recombination takes place are often located. I will show how we address outstanding questions on charge transport in lead halide perovskites.

My main topic will concern how machine learning can solve the inverse parameter problem, where a device model (here the drift-diffusion code IonMonger) is combined with Bayesian parameter estimation to deduce the input parameters for the device model from experimental measurements of the device characteristics [1]. This approach allows us to pinpoint causes of features seen in the measurements using only a few hours of computation. The virtual model can be continuously updated to reflect the current output of a fabricated laboratory device. Through these updates the materials processes underlying changes in the devices’ outputs can be identified. Accurately and rapidly simulating the function and performance of the lab device opens up the possibility of pinpointing the origins of degradation and allows improvements to be made much more quickly in future device iterations.

The talk will also describe how we efficiently search the input space using Bayesian optimization to minimize the difference between the simulation output from our mesoscale simulations with the code BoltMC, and a set of experimental results. BoltMC uses Boltzmann transport theory implemented via ensemble Monte Carlo to provide insight into mobility-limiting mechanisms [2]. From our analysis, we conclude that the formation of large polarons, quasiparticles created by the coupling of excess electrons or holes with ionic vibrations, cannot explain the experimentally observed temperature dependence of electron mobility.

Ab initio calculations are becoming more and more efficient and have emerged as an indispensable tool to model, characterise and understand complex systems like halide perovskites and perovskite-like materials. In particular, over the last decade, such computational approaches have been extensively employed and successfully unveiled the underlying atomic-scale physical mechanisms of these exciting materials. In this talk, I will overview our most recent results on the electronic structure of prototypical structures of layered halide perovskites, vacancy ordered double perovskites, and low dimensional halide perovskite-like materials. I will present the key details of their electronic structure for each type of system that define their experimentally observed optical properties and achieved performances. Our results show how well (or how bad) these different types of materials can perform for different opto-electronic applications ranging from indoors and outdoors PV, light emitters. Furthermore, I will present results the substitutional engineering for the class of halide double salts most promising for low-light PVs. Finally, in the last part of my talk, I will focus on our latest state-of-the-art ab initio calculations of the charge carrier transport properties when comparing three-dimensional ABX3 and layered halide perovskites. Our results explore directly the effects of structural dimensionality on the carrier mobilities of a selection of prototypical layered perovskites and identify the importance of the intrinsic carrier density in layered compounds to the exhibited transport properties.

Understanding how process conditions influence nanostructure formation in solution-processed photoactive layers is crucial in order to improve the power conversion efficiency of organic solar cells. To this end, a simulation framework was developed to visualize bulk heterojunction morphology formation driven by crystallization, liquid-liquid demixing, and evaporation. Both the thermodynamic and the kinetic mechanisms related to these distinct processes can interact in complex ways, which result in a wide variety of possible structures.

In this talk, an overview of morphology formation pathways simulated for binary systems that can be subject to spontaneous phase separation and crystallization, such as donor-acceptor films, or solvent-solute blends, is presented. A comprehensive description of the interplay of physical phenomena which shape the structural evolution is provided. Particular morphological features encountered in different crystallization regimes (e.g. diffusion-limited, two-step, or demixing-assisted crystallization) are discussed as well. Finally, comparisons between predictions from simulations and experimental characterizations of organic photoactive mixtures are also undertaken.

Perovskite solar cells (PSCs) are one of the most promising candidates for next-generation photovoltaics. Due to their exceptional increase in power conversion efficiencies (PCEs) over the last 15 years and their excellent suitability for tandem device architectures, the emerging technology has reached the brink of commercialization. This achievement was enabled by immense efforts in fast, manual prototyping based on a culture of apprenticeships, where each team develops fabrication processes customized for their specific equipment. While  this approach was very successful for improving the state-of-the-art PCEs, researchers should now consider focusing more on rigorousness of scientific reporting. The reason is that the field suffers from immense issues with reproducibility, leaving a huge potential for standardization and refining of routines. A simple indicator for the rigorousness of scientific reporting is the extent to which process parameters are controlled and provided in the description of experiments.

Leveraging extensive experience with modelling of perovskite processing[1], we perform a meta survey of the process parameters provided in published literature on perovskite solution processing. For conducting the analysis, ways of sampling representative meta data are developed. Previously, meta data was collected in an extensive effort establishing the perovskite open source database[2][3]. However, we found that the database prioritizes frequently reported over rarely reported parameters. Additionally, the acquisition of these data took between 5.000 to 10.000 of volunteer work hours. To shortcut the process of data acquisition, we leverage recent progress from the field of deep machine learning to collect data on perovskite solution processing on a representative sample of around 1.500 publications with minimal need for human intervention. However, this approach comes with novel challenges. Common deep learning algorithms do not have an understanding of the technology, such that the potential for misinterpretation and error is high. Therefore, the algorithm must be carefully assessed in comparison to human reading performance. The exact formulation of instructions, the choice of the specific model and its hyperparameters, as well as pre-training are critical indicators for increasing model performance. We find that deep leaning models can be on par with human reading performance (that is not error-free either), when given simple instructions. It is however an open question, if these models can be scaled to advanced meta data extraction projects like the perovskite database. Furthermore, a strategy on how to publish these models must be developed as the trained models could potentially be exploited to access copyright-protected content.

After conducting the analysis for each of the 10 process parameters, we identified as critically important for reproducing perovskite solution processing, we find that there is a great potential for improving the rigorousness of the description of experiments in perovskite solar cell fabrication. Furthermore, there are clear differences in the likelihood of certain parameters to be provided. For example, the spin speed (or the sheering velocity) of the perovskite solution deposition is provided in almost all cases, while the temperature of the atmosphere during processing is almost never reported. As consequence of our analysis, we propose that the specification of experiments in perovskite processing should be given more attention, in particular when reviewing and writing articles on perovskite solar cells (we do not exclude ourselves from this imperative). Because the technology is so successful that it is in the process of being passed on to industry, researchers now have the privilege of prioritizing rigorousness of reporting and process analysis over device optimization speed. This further opens up new opportunities for publishing studies on fundamental process physics and chemistry, differentiating technology development carried out at rapid pace in industry from research on fundamentals conduced by publicly funded research institutions.

Thin film and surface technologies are an important basis for applications in modern life. The Fraunhofer Institute for Electron Beam and Plasma Technology FEP in Dresden has expertise in the development and application of electron beam and plasma technologies for vacuum coatings and surface treatments, and we work closely with academic and industrial partners to scale-up laboratory solutions to meet commercial demands for applications in the fields of energy and environment, life sciences, and microelectronics. Our aim is to overcome bottlenecks in cost, reliability, and performance towards reliable and scalable solutions for surface engineering, film deposition, and system integration.

In this talk, results of vacuum technology process development for photovoltaics will be presented. A particular focus will be on processes for large area transparent high-barrier layers and transparent conductive electrodes using vacuum roll-to-roll coating processes. Examples from current industry focused projects for the development of perovskite photovoltaics will complete this presentation.

Si-based multi-junction architectures are hindered by incomplete harvesting in the near-infrared spectral range when the Si bottom cell is flat, without the conventional macroscopic light trapping surface texture. This is the case for e.g. III-V/Si multijunction solar cells where the III-V layers are wafer-bonded on a flat Si bottom cell, and for some perovskite/silicon tandem designs where the perovskite layers are ideally grown on a flat Si surface.

Here we present a novel nanostructured Ag back contact design that creates strong light trapping in a Si bottom cell with a flat top surface. We design a diffractive silver back-reflector featuring a near-infrared light scattering matrix that optimizes trapping of multiply-scattered light into a range of diffraction angles. By integrating near-field and far-field simulations we minimize reflection and parasitic plasmonic absorption by engineering destructive interference in the patterned back contact.

We test the new design on flat single-junction Si TOPCon solar cells and find a strongly improved near-infrared external quantum efficiency using the nano-backpattern. We then fabricate nanopatterned metagratings on GaInP/GaInAsP//Si two-terminal triple-junction solar cells via substrate conformal imprint lithography and characterize them optically and electronically, demonstrating a certified record efficiency of 36,1% for Si-based multijunction solar cells. The results are relevant for several other tandem cell designs.

Formamidinium lead iodide (FAPbI₃) emerges as one of the most promising materials for perovskite solar cells (PSCs) with high power conversion efficiency (PCE) and good stability. However, (i) the scalability lags behind and only a few reports have been dedicated so far towards the scalable processing of FAPbI₃ perovskite solar modules; (ii) FAPbI₃ has not been applied in multi-junction solar cells and the performance of perovskite–perovskite–silicon triple-junction solar cells lag considerably behind with only a limited number of reports on prototypes. First, this study reports void-free, α-phase, and high-quality FAPbI₃ thin films processed via vacuum-assisted growth (VAG) method in p-i-n-based PSCs. VAG eliminates interfacial voids at the buried interface of hole-transport layer (HTL)/FAPbI₃, enabling a high PCE of 22.3% for p-i-n-based PSCs. We demonstrate that the voids result in non-radiative recombination loss (i.e., open-circuit voltage (V_OC) loss) and poor charge extraction (i.e., low current density (J_SC)). An innovative combination of employing methylammonium chloride (MACl) as an additive and applying a moderate N₂-flow during the VAG process facilitates blade coating homogeneous large-area FAPbI₃ thin films without interfacial voids. As a result, scalable PSCs (0.105 cm²) and mini-modules (aperture area of 12.25 cm², geometrical fill factor of 96.3%) with PCEs of 20.0% and 18.3% are achieved, respectively. Second, this study addresses significant challenges in processing triple junctions in which the most critical junction is the middle perovskite sub-cell. We present triple-junction perovskite–perovskite–silicon solar cells achieving an unprecedented PCE of 24.4%. Through the optimization of light management for each perovskite sub-cell (with bandgaps of ~1.84 eV and ~1.52 eV for the top and middle cells, respectively), the current generation is maximized to 11.6 mA cm^–2. The key to this achievement is the development of a high-performance middle perovskite sub-cell, utilizing a stable pure-α-phase FAPbI₃ perovskite thin film that is free of wrinkles, cracks, and pinholes. This enables a high V_OC of 2.84 V in the triple-junction architecture. Notably, non-encapsulated triple-junction devices retain up to 96.6% of their initial efficiency when stored in the dark at 85°C for 1081 hours. These are remarkable advances in upscaling FAPbI₃-based PSCs and multi-junction PVs.

Metal halide perovskite solar cells have gained significant attention over the last decate due to their low-cost fabrication methods and high efficiency potential. Typically perovskite films are prepared by solution-based depositon techniques, which allow a rapid deposition and a wide variety of compositions.[1] However, conformal coverage of textured surfaces, higly relevant for monolithic perovskite/silicon tandem solar cells, or compositional gradients in the absorber material can only be realized with considerable effort with solution‑based deposition techniques. These limitations can be overcome by co‑evaporating the perovskite precursor materials.[2,3]

In contrast to solution-based deposition processes, where the stoichiometry is determined by the weight of the individual precursors in the solution, the composition in co-evaporation processes is controlled by the evaporation rates of the precursors, which are typically measured using quartz crystal microbalances (QCMs). However, determining the composition from these rates is not always straightforward and accurate, as the rates of the materials can overlap or the incorporation of precursors can change due to complex chemical reactions.[4] In this work, we analyze in detail the composition of perovskite films prepared via simultaneous co-evaporation of PbI₂, PbBr₂, formamidinium iodide (FAI) and CsI and correlate the film stochiometry to the optoelectronic properties of the resulting solar cells. We show that changes in the evaporation rates are not necessarily transferred proportionally into the final film composition.

Furthermore, we analyze how the composition of the perovskite films can be influenced by the choice of the hole transporting material for cells in p-i-n configuration. We observe that perovskites co-evaporated on spin-coated MeO‑2PACz ([2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid) contain a significantly smaller amount of FAI if the MeO-2PACz layer is washed with ethanol before the perovskite depostion. The gained knowledge is used to tune the co-evaporation process to enable perovsktie solar cells with band gaps between 1.65 eV and 1.70 eV, optimized for monolithic perovskite/silicon tandem solar cells. Overall, our study provides valuable insights into the co-evaporation process and demonstrates the importance of composition control for achieving efficent perovskite solar cells.

The bandgap (E_g) tunability of metal halide perovskites over a wide spectral range from 1.2 eV to 3 eV through compositional engineering makes them particularly attractive for both single and multi-junction solar cells. Leveraging the extensive data available in the Perovskite Database¹, we have performed a meta-analysis of over 40,000 sets of solar cell device metrics from peer-reviewed publications to elucidate the current state of broad E_g MHP semiconductor devices.

By comparing and contrasting intrinsic and optimization limitations across a wide range of E_g values from 1.2 eV to 3 eV, we find that while a wide variety of MHP absorbers have been developed, material quality across the E_g spectrum remains suboptimal. The most efficient solar cells are still achieved with structures close to the MAPbI3 archetype, due to a predominant optimization of the entire device to an absorber bandgap of 1.55 to 1.6 eV.

Our analysis reveals significant contributions of at least three primary factors to the degradation of device performance relative to the theoretical limit of the Shockley-Queisser model: 1) Mismatches in the energy levels of the selective transport materials for wide E_g MHPs. The losses at the material interfaces depend on the band offset and increase continuously with the energetic distance from the band energy for which the layer stack was optimized. 2) compromised optoelectronic quality of wide E_g MHP absorbers and 3) dynamic compositional heterogeneity induced by light-induced phase segregation phenomena. Above a bromide content of x=0.5, the device performance almost collapses. The contribution of light-induced phase segregation and decrease in PLQY occurring in this range are discussed as possible causes of this performance collapse.

Our meta-analysis underscores the significant progress made in MHP technology while highlighting the ongoing challenges in maximizing device performance across diverse E_g values. Insights from this study provide valuable guidance for future research efforts aimed at overcoming the identified limitations and realizing the full potential of MHP semiconductor devices.

Narrow-bandgap perovskite solar cells have revolutionized photovoltaics, achieving remarkable power conversion efficiencies (PCE) up to 25%. However, their opaque and dark color has hindered their widespread adoption in building-integrated photovoltaic (BIPV) applications due to sustainability in our everyday surroundings. By manipulating the chemical composition of the perovskite materials, a new frontier of transparency in perovskite solar cells has been unlocked. By widening the bandgap of perovskite, reaching up 2.6 eV, researchers have successfully bridged the gap between transparency and efficiency. This breakthrough paves the way for a new generation of BIPVs that blend seamlessly into architectural designs while delivering substantial energy savings. By using a transparent conductive oxide as top electrode, three generation of wide-bandgap perovskite solar cells, hybrid halide perovskite (Ma⁺ and Fa⁺), inorganic halide (Cs⁺) and inorganic Pb-free perovskite have been studied and developed in a full-semitransparent stack with the aim to meet harmony between conversion efficiency and transparency. These devices exhibit impressive transmittances, reaching a staggering 70%. FaPbBr₃ attained an impressive PCE of 8% with a remarkable V_OC up to 1.7 V while also exhibiting suitability for blade coating on glass [1] and plastic substrates, paving the way for large-scale production. CsPbBr_3, a fully inorganic perovskite, has been scrutinized in an attempt to bolster its α-phase, and a Pb-free Cs₂AgBiBr₆, holistic inorganic wide band gap perovskite solar cell has been successfully fabricated maintaining this high level of transparency reaching 8% at low light illumination. Furthermore, we are presenting for the first time a 25 cm² module for a blade-coated, fully inorganic, lead-free perovskite-like material capable of powering electronic devices in the IoT field. This advancement eliminates concerns about the potential environmental impact of lead in perovskite structures. The development of wide-bandgap perovskite solar cells represents a significant leap forward in the field of photovoltaics. These transparent devices hold immense promise for seamlessly integrating solar energy into buildings, reducing our reliance on fossil fuels and promoting a sustainable future.

Bulk perovskite films have proved to be excellent candidates for photovoltaic devices due to their remarkable optoelectrical properties. However, a major drawback is their relative instability due to rapid degradation and defect formation in ambient conditions. One method of countering these drawbacks is to utilise a thin, 2D perovskite passivating layer on top of a bulk perovskite film, effectively combining the high efficiencies seen in 3D films with the increased stability of 2D films [1]. Whilst such alterations have shown promising results for photovoltaic devices, the feasibility of utilising passivated perovskite films for other applications such as solar energy conversion have not yet been studied.

Incorporating excitonic processes demonstrated by certain organic semiconductors could potentially bypass the detailed balance limit imposed on traditional single junction solar cells [2]. These processes include multiple exciton generation by high energy photons reducing thermalisation losses or by converting multiple low energy, below band-gap photons into one high energy photon. The latter process is commonly referred to as “photon upconversion” and can be achieved in certain organic semiconducting materials via sensitised triplet-triplet annihilation where two spin-1 triplet excitons combine to form one emissive spin-0 singlet exciton.

Due to the high absorption cross-section of bulk perovskite films at near infrared wavelengths they have emerged as promising solid-state sensitisers to generate triplet excitons in an adjacent organic semiconductor film [3], where direct optical generation of triplet excitons is spin-forbidden. In this presentation we present the first observation of bulk perovskite sensitised upconversion using a 2D passivation layer. We investigate the role that the 2D perovskite intermediary layer plays in the upconversion process is in both the impact of the passivating layer on the bulk perovskite and for the upconverting performance of the system. This presentation explores the inherent balance between reduced transport across the interface due to the introduction of a potential barrier versus the reduction of parasitic back transfer by increasing the distance between the strongly absorbing sensitiser and annihilator. Such experimental results raise important questions on the nature of energy transfer across a bulk perovskite/2D perovskite/organic semiconductor interface. This observation holds implications not only for the emerging field of perovskite-sensitised upconversion but also for broader research endeavours exploring interfaces between hybrid and organic semiconductors.

Singlet fission offers a way to overcome the detailed balance limit in silicon photovoltaics by addressing thermalisation losses.[1] This process, which occurs in adjacent chromophores, describes the splitting of the initial singlet excited state into two low-energy triplet excitons via a triplet pair intermediate ¹(TT). If the triplet state energy is greater than the 1.1 eV bandgap of silicon, the triplet excitons in the organic semiconductor can transfer to silicon. Such a device could realise efficiencies of 45%.[2]

Tetracene was recently shown to undergo triplet transfer to silicon.[3] This demonstration was a major step towards the realisation of a working singlet fission device. However, tetracene is not an ideal chromophore as the singlet fission yield is not unity (c.a. 120% out of 200%) and the triplet energy (1.1 eV) is on the silicon band edge. The field therefore requires new chromophores that have yields closer to unity and greater triplet energies.

To design new chromophores a clear understanding of the role of the excimer and the triplet-pair state in singlet fission is required. This presentation will discuss our recent results in resolving the emissive intermediate state in singlet fission.[4] I will present transient photoluminescence and spectrally resolved magneto-photoluminescence data which identifies an emissive species that is distinct from the excimer state. Our results confirm that the excimer acts as a trap and unites the view that the ¹(TT) state is an emissive intermediate in singlet fission.

Perovskite solar cells remain prone to stability issues and slow transients either being present already directly after fabrication or occurring during operation for longer times. Additionally, metastabilities can be observed, which allow for resistive switching and applications beyond solar cells.

In this talk, I address these topics from various angles. These include the question whether conclusions regarding solar-cell stability can be drawn from the perovskite data base [1] by using advanced data analysis; and how performance-limiting factors can be identified at current-voltage data using machine-learning based methods [2].

In the second part I will present high performance perovskite resistive switches, which show an on-off current ratio in the order of 1e10 and are stable for millions of cycles. With the aid of transient and temperature-dependent measurements, the factors determining the switching behaviour are unraveled. They are complemented by photoluminescence mapping and nanoscale measurements shedding light on the switching mechanism in the device.

Recently perovskite semiconductors have triggered a revolution in solar cell research, however, the actual contribution of mobile ions to the total degradation loss remains poorly understood. Here, we reveal that the initial degradation of perovskite solar cells is largely the result of mobile ion-induced internal field screening - a phenomenon that has been poorly addressed in relation to degradation. The increased field screening leads to a decrease in the steady-state efficiency, often due to a large reduction in current density, while the efficiency at high scan speeds (>1000 V/s) where the ions are immobilized is much less affected. We also show that interfacial recombination does not increase upon aging, yet the open-circuit voltage decreases due to an increase in the mobile ion density upon aging. This work reveals a key degradation mechanism before chemical or extrinsic mechanical degradation effects manifest, and it highlights the critical role mobile ions play therein.

In the domain of perovskite solar cells (PSCs), the imperative to reconcile impressive photovoltaic performance with lead-related issue and environmental stability has driven innovative solutions. This study pioneers an approach that not only rectifies lead leakage but also places paramount importance on the attainment of rigorous interfacial passivation. Crown ethers, notably benzo-18-crown-6-ether (B18C6), were strategically integrated at the perovskite-hole transport material interface. Crown ethers exhibit a dual role: efficiently sequestering and immobilizing Pb2+ ions through host-guest complexation and simultaneously establishing a robust interfacial passivation layer. Selected crown ether candidates, guided by density functional theory (DFT) calculations, demonstrated proficiency in binding Pb2+ ions and optimizing interfacial energetics. Photovoltaic devices incorporating these materials achieved exceptional power conversion efficiency (PCE), notably 21.5% for B18C6, underscoring their efficacy in lead binding and interfacial passivation. Analytical techniques, including time-of-flight secondary ion mass spectrometry (ToF-SIMS), ultraviolet photoelectron spectroscopy (UPS), time-resolved photoluminescence (TRPL), unequivocally affirmed Pb2+ ion capture and suppression of non-radiative recombination. Notably, these PSCs maintained efficiencies even after enduring 300 hours of exposure to 85% humidity.

Achieving the long-term stability of perovskite solar cells is arguably the most critical challenge to enabling their widespread commercialisation. Understanding the perovskite crystallisation process and its direct impact on device stability is essential to achieve this goal. Surprisingly, we find that the intermediate phases that occur during the crystallization process strongly influence the long-term perovskite device stability. The commonly employed dimethyl formamide/dimethyl sulfoxide (DMF/DMSO) solvent system preparation method results in poor crystal quality and microstructure of the polycrystalline perovskite films. In this work, we introduce a high-temperature DMSO-free processing method that utilizes dimethylammonium chloride (DMACl) as an additive to control the perovskite intermediate precursor phases. By precisely controlling the 2H to 3C perovskite phase crystallization sequence, we tune the grain size, texturing, orientation (corner-up vs face-up), and crystallinity of the formamidinium (FA)yCs1-yPb(IxBr1-x)3 perovskite system. Encapsulated devices show significantly improved operational stability, with a champion device showing a T80 of 490 hours under simulated sunlight at 85 °C in air, under open circuit conditions. Our work introduces a new processing method that allows higher overall perovskite device stability by controlling the intermediate phase domains during the perovskite formation. This work highlights the importance of material quality to achieve long-term operational stability of perovskite optoelectronic devices. [1]

Narrow bandgap perovskite solar cells based on mixed lead-tin perovskites tend to suffer from poor stability under operating conditions. This impedes the development of stable all-perovskite multi-junction solar cells.  We explore the causes of this instability under extended periods of combined 65°C thermal and illumination stressing using a range of structural, optical, and electronic characterization techniques on lead-tin perovskite films, half-stacks and devices.

We show that the bulk phase, absorbance, mobility and background carrier density of lead-tin perovskite films are stable on timescales that exceed those of device degradation. We also find that non-radiative recombination rates in the perovskite increase moderately during the first few hundred hours of stressing. However, through a combination of device simulations and variable rate current-voltage (J-V) scanning we demonstrate that this change can only account for a small portion of the observed device performance loss.

Ultimately, we identify a rapidly increasing impact of mobile ions during aging as the major cause of the observed device degradation. A close investigation of the J-V characteristics of devices reveals the formation of a charge extraction barrier during aging, which is hugely reduced in very fast J-V scans where mobile ions are less able to aggregate at interfaces. We quantify the increasing impact of mobile ions on device performance during aging, and furthermore demonstrate that this impact can be significantly mitigated by an alternative hole transport layer choice.

Over longer timescales, we additionally identify the growth of a non-perovskite degradation phase as well as HTL-dependent changes in optoelectronic properties. We quantify the impact of these changes on device performance in comparison to the effects from mobile ions, demonstrating that the dominant effect can change with aging time and device architecture. Thus, we are able to closely identify the various processes that limit the stability of lead-tin perovskite solar cells and make recommendations to overcome these challenges.

Following advancements to increase the efficiency of perovskite solar cells, the current emphasis is mainly on enhancing their operational stability. Various methods of encapsulation have been employed to safeguard perovskite solar cells from environmental factors and preserve their efficiency. Recent research studies have primarily focused on using organic/inorganic multilayers to create a more robust barrier against the degradation of perovskite solar cells. [1-3]However, the commercial feasibility of employing inorganic materials might be limited due to their high-cost fabrication process and low flexibility. The encapsulation techniques using polymers stand out for their versatility in material selection and functionality, making them suitable for manufacturing flexible devices. Polymers efficiently act as encapsulants, preventing the infiltration of water and oxygen into the perovskite layer while also inhibiting the release of perovskite composition. One such polymer, parylene-C, offers cost-effective extrinsic protection against environmental harm, mainly humidity and oxygen, to uphold the performance and reliability of perovskite solar cells. [4-5] In our study, we utilized a multilayer deposition of parylene-C with a high light and low water vapor transmission rate, uniformly applied across the surface of the perovskite solar cells. To assess the operational stability of these devices, we employed ISOS-D1 and D2 protocols including conditions such as ambient/ambient and 85ºC/ambient, pertaining to temperature/relative humidity. The obtained results underscore the robustness of inverted control perovskite solar cells coated with parylene-C. These cells reached T80, lasting over 210 hours at an accelerated temperature of 85ºC and 30-35% relative humidity, in contrast to their unencapsulated counterparts, which failed after undergoing the accelerated test for 98 hours. Furthermore, we conducted a study on the operational mechanisms of the devices influenced by degradation using impedance spectroscopy measurements at open-circuit conditions with various irradiances, and dark conditions at different biases.

The understanding of interfacial properties in perovskite devices under irradiation is crucial for their engineering, for instance in devices where electrodes are in direct contact with the perovskite such as certain scintillator designs.

We show how the electronic structure of the interface between CsPbBr₃ perovskite nanocrystals (PNCs) and Au is affected by irradiation of X-rays, near infrared (NIR) and ultraviolet (UV) light. The effect of X-ray and light exposure could be differentiated by employing low-dose X-ray photoelectron spectroscopy (XPS) available at the Bessy II synchrotron in Berlin. A combination of a pulsed, low flux X-ray source, and a high transmission electron analyzer allow recording of XPS spectra without damaging even radiation sensitive samples. A coupled laser provides both NIR and UV light making it possible to isolate the effects of the light exposure. 

Apart from the common degradation product of metallic lead (Pb⁰), a new intermediate component (Pb^int) was identified in the Pb 4f XPS spectra after exposure to high intensity X-rays or UV light. The Pb^int component is determined to be metallic Pb on-top of the Au substrate formed from underpotential deposition (UPD, a surface-limited redox process) of Pb induced from damage of the perovskite allowing for migration of Pb²⁺. This has implications on all devices where the perosvkite could come in contact with gold, and potentially also for other metals used as electrode materials. 

Over the last decade metal halide perovskites have attracted a tremendous amount of attention owing to their favorable intrinsic optoelectronic properties, such as high absorption coefficients, suitable carrier mobilities, remarkable defect tolerance, as well as ease of fabrication. The rapid progress in the development of single-junction perovskite solar cells achieved a certified power conversion efficiency (PCE) of 26.1%. These high PCEs are generally found in FA-rich MHPs combined with a small amount of Cs and MA. Although these state-of-the-art triple cation, mixed halide perovskites have been extensively studied, an in-depth fundamental understanding of how the phase behavior in Cs_0.05FA_0.85MA_0.10Pb(I_0.97Br_0.03)₃ (CsMAFA) affects the optoelectronic properties is still lacking. Here temperature-dependent XRD, photoluminescence combined with electrodeless microwave photoconductivity measurements (TRMC) were carried out. The refined unit cell parameters a and c in combination with the thermal expansion coefficients derived from XRD patterns reveal that CsMAFA undergoes an alfa-beta phase transition at ~ 280 K and another transition to the gamma-phase at ~ 180 K. From the analyses of the TRMC measurements we show that shallow traps only in the gamma-phase negatively affect the charge carrier dynamics. Most importantly, CsMAFA exhibits the lowest amount of microstrain in the beta-phase at around 240 K, corresponding to the lowest amount of trap density, which translates into the longest charge carrier diffusion length for electrons and holes. Below 200 K we find a considerable increase in deep trap states most likely related to the temperature-induced compressive microstrain leading to a huge imbalance in charge carrier diffusion lengths between electrons and holes.

To increase the open-circuit voltage in solar cells based, we investigated the charge carrier dynamics in bi- and tri-layers using TRMC. From the results of the bilayers, we find almost balanced mobilities for electrons and holes in CsMAFA, and carrier extraction is nearly quantitative. For the n-i-p and p-i-n triple layers, both carriers are extracted at low laser intensities independent of the configuration, which is based on the small, rapidly decaying TRMC signal. An important remaining open question in such systems is the fate of light induced carriers after collection by the TL. Here we demonstrate long-lived charge separation over the n-i-p and p-i-n structures from the fact that on applying bias illumination (BI) the photoconductance signal is much higher than without BI. In the former case charge extraction is retarded, which is attributed to the electric field that is build up from the extracted electrons and holes oppressing further charge collection. Finally, we want to see how charge collection and recombination at the interface affects the quasi Fermi level splitting (QFLS) which is a measure of the possible attainable open circuit voltage. For those bilayers showing short-lived charge separation such as in CsMAFA/C60, implying electron collection is followed by rapid interfacial recombination, the corresponding QFLS is reduced with respect to the pristine CsMAFA layer. Most importantly for the other bilayer combinations including in C60/CsMAFA long-lived charge separation is observed which translates in an increase of the QFLS with respect to the neat CsMAFA layer.

Interest in wet chemical synthesis to produce metal halide perovskite powder has increased in recent years because it subsequently allows for easy production of large quantities of perovskite crystals. These crystals can then be used to produce thin perovskite films for solar cells which exhibit high stability and performance [1,2]. The stoichiometric homogeneity achieved by the crystallisation process in the alcohol dispersions simplifies thin film deposition and therefore improves photovoltaic device performance over large areas. In this study we characterize the formation of metal halide perovskite in environmentally friendly alcohols using in situ wide and small angle X-ray scattering to compare the perovskite formation in methanol, ethanol, propan-2-ol and pentanol. The different alcohols are used as solvents for precipitation reactions forming methyl ammonium lead iodide MAPbI₃ and methyl ammonium formamidinium lead iodide MA_0.5FA_0.5PbI₃ perovskites. Time resolved in-situ small and wide X-ray scattering (SAXS and WAXS) permits the reaction of the dissolved methylammonium iodide and formamidinium iodide with the suspended lead iodide to form perovskite to be followed in real time, and provides evidence of the mechanism by which the structural assembly from precursors to perovskite occurs. These measurements led to a deeper understanding of intermediate steps during the reaction, where particle morphology control is possible during the perovskite material assembly. The production of metal halide perovskite by precipitation as described in this work is cost-effective and has fewer safety issues due to the low solvent toxicity. This work outlines an alternative processing route to make stable perovskites for photovoltaics.

Halide perovskites (HPs) have drawn the attention of the scientific community, not only for their steep increase in power conversion efficiency during the last decade, up to 26.1% in 2023, but also for using low-cost solution-based processing methods.

Despite the popularity of HPs as an absorber material, it is not fully understood the role that anions, cations and the solvent play during the early stages of the crystallisation process. For this reason, we investigated the precursor solution of different HPs (MAPbI₃, MAPbBr₃, MAPbCl₃, FAPbI₃, CsPbI₃, RbPbI₃, KPbI₃ and NaPbI₃) at room temperature using small angle X-ray scattering (SAXS) as well as the precursor solution of MAPbI₃ at increasing temperature in-situ (from room temperature up to 120°C). The binary precursors (e.g. MAI and PbI₂ to synthesise MAPbI₃) were dissolved in common solvents to synthesise HPs such as γ-butyrolactone (GBL), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP) and mixtures thereof.

SAXS is a non-destructive technique based on the scattering length difference between the scattering objects in a solution. Applying SAXS, the size and shape of nanoscale particles (scattering objects) can be investigated, as well as their adjacent distance and interactions with each other.[2] We performed SAXS experiments at BESSYII, at the PTB’s four-crystal monochromator beamline[3] using the ASAXS endstation.[4]

The SAXS patterns of all the measured samples show a peak in the scattered intensity at q-values between 2.5 and 3.3 nm^-1, except for MAPbCl₃ in DMF:DMSO 1:1, which did not show any peak. The maximum holds two essential pieces of information: it demonstrates the agglomeration of scattering objects and the peak position corresponds to the average distance between scattering objects (d_exp) in a range of 2-3 nm. In a previous study, we developed a core-shell model with [PbX₆] (X = I, Br) octahedra arranged as single or corner-sharing octahedra as the core surrounded by solvent molecules for HPs with molecular A-cation (MA⁺, FA⁺). This shows that the size of the agglomerates changes with the composition of HPs precursors and with the solvent, but not with a molecular A-cation. However, when alkali metals are used as A-cation instead (Na⁺, K⁺, Rb⁺, Cs⁺), we can demonstrate that d_exp not only depends on the solvent but also on the A-cation. This is explained by the smaller ionic radius of alkali metals compared to the molecular cations [5,6] therefore the charge density is higher. For this reason, we extended the previous model to take this phenomenon into account. Based on this information, the extended core-shell model assumes that the A-cation and the solvent molecules compete to surround the [PbI₆] octahedra. In this A-cation core-shell model, the core is composed of [PbI₆] octahedra, which can be arranged as a single octahedron or a corner-sharing octahedra. The [PbI₆] octahedra of adjacent scattering objects are surrounded by a solvent shell with molecules or by an A-cation shell. The SAXS data analysis (using SASfit[7]) shows higher polydispersity as the previous model, which indicates an increase in the heterogeneity of the solution, this is in agreement with the proposed extended model.

We will discuss the influences of the A-cation and solvent on the core as well as the solvent shell of the scattering objects since they have the potential to influence the crystallization process of the HP and therefore the performance of a device produced from solution processing.

Perovskite solar cells (PSCs) are receiving renewed interest since they have reached high power conversion efficiency and show potential for application not only on rigid and flexible substrates but also on mechanically deformable substrates for integration on non-planar curvilinear surfaces. Here we demonstrate PSCs fabricated on transparent conducting oxide (TCO) free ultra-thin polyethylene terethapthalate (PET) substrates capable of efficiently harvesting indoor light even under compressive strain. Interface engineering with PTAA improved the shunt resistance and band alignment at perovskite-hole transport layer interface which resulted in enhanced charge extraction, leading to 114% improvement in PCE from 5.57 % to 11.91% under 500 lux indoor LED (4000K) illumination.  The champion device exhibited a PCE of 18.37% under 250 lux cool white LED (4000K) light. The maximum power output (P_max) of the devices varied from 13.78 to 25.38 µW/cm² by changing the indoor light illumination from 250 to 1000 lux, respectively. Moreover, the devices showed impressive performance even after mechanical deformation and retained 83% and 76 % for 1 sun and indoor light, respectively, under 30% compressive strain. Our approach pays way for fabrication of efficient indoor light harvesting PSCs on mechanically deformable substrates for integration on non-planar surfaces prone to compressive strain.   

Multi-junction tandem solar cells, utilizing complementary bandgaps, offer the potential to surpass the detailed balance limit for single-junction perovskite solar cells (PSCs). Through the tunability of perovskite bandgaps and recent advancements in mixed lead-tin (Pb:Sn) narrow bandgap PSCs, it is now possible to create highly efficient multi-junction solar cells solely using perovskites, achieving certified PCEs of up to 26.4% in all-perovskite tandem solar cells.

However, there are two main challenges when using Pb:Sn perovskites. Firstly, methylammonium (MA) has been traditionally used in the most efficient Pb:Sn PSCs, but its thermal and chemical stability concerns prompt the search for MA-free alternatives. Second, conventional solution processing methods remain prevalent in lab-scale Pb:Sn PSCs fabrication. There were many attempts to thermal evaporate Pb:Sn as this method presents significant advantages, including high-quality thin film fabrication, precise thickness control, elimination of toxic solvents, large-scale compatibility, and reproducibility. Vacuum evaporation can also be applied to the fabrication of all-perovskite tandem solar cells without damaging underlying layers. Although vacuum deposition has proven successful in achieving high-efficiency and large-area PSCs for Pb-based perovskites, its application to Pb:Sn perovskites is less unexplored. Ball et al. and Igual-Munoz et al. are among the few to report Pb:Sn PSCs using vacuum evaporation.

In this work, we demonstrate that through careful control of the environment and some fine-tuning, it is possible to deposit Pb:Sn perovskite films of high quality through thermal evaporation. We compare devices made using this method to those made from similar state-of-the-art solution processed methods. Our work demonstrates that it is possible to approach the performance of solution processed devices within a workable processing window using vacuum evaporation. Our champion devices reach PCEs of 17%, surpassing previously set records of 14% for thermally coevaporated Pb:Sn devices. This was done without requiring additional passivation or bulk additive, which are used in the previous reports.

The pursuit of highly efficient and cost-effective photovoltaic materials has led to the emergence of organic-inorganic metal halide perovskites. However, their inherent instability poses challenges to their practical application, limiting their lifespan and scalability. Mixed tin-lead compositions have garnered significant attention due to their unique optoelectronic features and small bandgaps, offering promise for various applications. Nonetheless, their low ambient stability presents a significant hurdle that requires a thorough investigation into their degradation mechanisms. This study aims to understand the degradation mechanisms of tin-lead perovskites, with a specific focus on the role of halide chemistry and the impact of iodine on their stability. Our findings reveal a cyclic degradation process, where iodine and SnI4 act as key degradation products, compromising the stability of the perovskite material. Furthermore, the presence of triiodide, derived from native iodine oxidants, exhibits a strong correlation with degradation. We observe that the selection of A-site cations significantly influences the oxidation stability of Sn-Pb perovskites. Cesium-rich phases and solar cells demonstrate superior resistance to oxidative stress compared to their methylammonium-based counterparts, primarily due to the limited formation of triiodide. Leveraging this insight, we successfully stabilize sensitive methylammonium-based Sn-Pb perovskite films and devices against oxidation by employing CsI coatings. This practical approach provides essential guidelines for enhancing the stability of perovskite materials and devices. The significance of this study lies in its contribution to the design and engineering of perovskite materials and devices. Understanding the role of iodine in perovskite deterioration is crucial for improving their stability and durability, thereby paving the way for their commercialization. By elucidating the degradation mechanisms of tin-lead perovskites, we can develop effective strategies to mitigate their degradation, enhance their stability and lifespan, and unlock their full potential for various photovoltaic applications. This work aligns with the objectives of the scientific program it is part of, as it addresses the challenges associated with the stability of perovskite solar cells. Stable and efficient perovskite solar cells play a vital role in renewable energy production, contributing to a more sustainable and environmentally conscious future. By overcoming the degradation issues and enhancing the stability of tin-lead perovskite materials, this research contributes to the development of advanced photovoltaic materials.

Dye-sensitized solar cells (DSSCs) offer cost-effective and versatile energy generation. Due to their exceptional power conversion efficiency (ηPCE) under artificial light and indirect daylight, they are promising candidates for indoor photovoltaic applications. The devices' counter electrode (CE) determines the overall ηPCE. It should possess several critical qualities, including electrical conductivity and electrocatalytic activity, affordability, high electrochemically active surface area, corrosion resistance, and multi-process deposition [1]. Platinum (Pt) is the most used material for CEs in liquid-junction DSSCs due to its exceptional electrical conductivity and catalytic activity; ηPCE above 12% has been reported for the devices with Pt CE. Nevertheless, Pt presents several challenges, including its high cost, limited stability over extended periods of device operation, detachment of the nanoparticles, and migration to the photoanode, provoking recombination on the photoanode and photocurrent degradation [2].

Carbon-based materials are emerging as strong candidates for CE materials in DSSCs due to their low cost, abundance, extensive surface area, electrical conductivity, corrosion resistance, and reactivity for redox mediator reduction [3]. The most common way to implement carbon CE in the DSSC is to deposit it onto a conductive substrate, such as fluorine-doped tin oxide (FTO) glass, and encapsulate it with an electrolyte gap between the working electrode (WE) or by placing the carbon on the TiO2 using an insulating layer to prevent physical contact with the WE. However, they often suffer from poor adhesion, leading to electrode degradation. Alternatively, pre-made materials in sheet form carbon papers can easily be processed as CE with minimal impact on the DSSCs structure. The carbon paper porous structure enhances the surface area and enables electrolyte loading; excellent electrical conductivity facilitates swift electron transfer within the electrode [4]. The dispute with pre-made carbon sheets is incorporating them into the DSSC structure with guaranteed electrical contact with the FTO layer or on top of the sensitized TiO2 layer without causing a short-circuit.

In the present work, we studied the I−/I3− mediated liquid-junction DSSC assembly by pressing a carbon paper composite against the TiO2 WE and evaluated three assembly combinations: a single macro-porous carbon fiber (Carb); a bilayer of the Carb structure pressed against TiO2, and, on the CE-FTO side, a micro-porous carbon-based with PTFE layer (Carb/PTFE); and a compact ultrathin TiO2 blocking layer (BL) was added to the macro-porous structure of the Carb/PTFE bilayer by spray pyrolysis (TiO2BL/Carb/PTFE). Current-voltage performance from carbon-based DSSCs and conventional devices with Pt CE (Ref) were measured and revealed that the current density and fill factor (FF) were significantly reduced in DSSCs with Carb (12 mA·cm-2 and 0.48). This is likely due to recombination losses caused by poor electrical contact between the carbon sheet and the FTO and an insufficient rate of I3- reduction, resulting in electron and hole recombination [5]. This effect was lower in Carb/PTFE devices, where current density and FF increased significantly (15 mA·cm-2 and 0.58). The difference is explained by the porosity of each layer and their respective orientation; the micro-porous structure provides good electrical contact to the FTO, while the macro-porous is better for redox mediator infiltration when pressed against TiO2. However, the FF remained very low compared to Ref (0.73), most likely caused by a short-circuit from the charge transfer competition between WE and CE. This issue was mitigated using the TiO2BL/Carb/PTFE, rendering a superior FF (0.62) and a competitive ηPCE of 6.9 %, corresponding to 97% obtained with Pt-CE. With ongoing research, carbon papers could significantly advance the commercial prospects of DSSCs for flexible devices and module manufacturing. Moreover, its integration aligns with sustainability goals and reduces the carbon footprint in clean energy applications.

In 1991 a paradigm shift of photovoltaics occurred with the publication of a high efficiency dye-sensitized solar cells (DSSC) utilizing a high surface area nanostructured photoelectrode [1]. In my talk I will briefly overview the development of DSSCs to the present state-of-the art. The development of new photo-sensitizers for DSSC led to the breakthrough of perovskite solar cells (PSC) in 2012 [2, 3]. With efficiencies at present above 26% and close to 34% for PSC/Si tandem devices, PSC is the frontrunner of emerging photovoltaic technologies.

My talk will focus on the longterm stability of PSCs as the key challenge for future industrialization. In particular, I will discuss different types of molecules that synergistically improve both power conversion efficiency and stability. For example, we have developed a class of molecules to post-treat formamidinium lead iodide (FAPbI₃) perovskite films, which remains in black-phase after 2 years ageing under ambient condition without encapsulation. The treated perovskite solar cell devices show high efficiencies with less than 1% performance loss after more than 4500 h at maximum power point tracking, yielding a theoretical T₈₀ of over 9 years under continuous 1-sun illumination [4].

The rapid proliferation of Internet of Things (IoT) devices and wireless sensor networks heralds a potential digital revolution, contingent upon overcoming the energy consumption and sustainability barriers.¹  Our research focuses on the development of high-efficiency ambient photovoltaic (PV) cells, utilising sustainable, non-toxic materials, aimed at powering IoT devices in indoor environments. We introduce a novel copper(II/I) electrolyte-based dye-sensitized PV cell demonstrating unprecedented power conversion efficiency of 38% under standard indoor illumination conditions.² This work includes a comprehensive implementation of a long short-term memory (LSTM) based energy management system that leverages on-device prediction to dynamically adjust computational loads, ensuring continuous operation without power losses. Our approach not only addresses the energy demand of IoT devices but also aligns with environmental sustainability goals by reducing electronic waste and eliminating the need for batteries.  This comprehensive approach not only addresses the current limitations posed by inefficient charge transport materials but also opens new avenues for sustainable, high-performance indoor photovoltaic solutions. Through collaborative theoretical and experimental efforts, we showcase the development of novel coordination polymers that exhibit exceptional hole mobility and conductivity, providing a sustainable alternative to traditional charge transport materials.³ The integration of these materials into photovoltaic devices showcases a significant leap toward achieving perpetual, intelligent IoT devices, thereby driving the future of indoor energy harvesting and AI integration towards a sustainable, digitally revolutionised future.

In this presentation we will follow the history of spiro-OMeTAD as a high-performance solid-state charge-transfer material which was central to the development of solid-state dye-sensitized solar cells and perovskite solar cells. The original objective was to replace the liquid, corrosive iodide-based electrolyte in dye-sensitized solar cells with a more benign solid material. Spiro-linked solid-state hole transport materials were originally developed for their application in OLEDs. Spiro-OMeTAD was a slightly modified version of the original molecules and featured eight methoxy-groups, following the rational that these groups would help to shift the energy levels to facilitate efficient hole-transfer from a photooxidised dye. Doping through partial oxidation and blending with lithium salts and pyridine derivatives further helped to boost the performance of these materials. In 2012 the use of spiro-OMeTAD enabled the fabrication of the first solid-state perovskite solar cells, overcoming stability and performance limitations of the photoelectrochemical perovskite solar cells used hitherto. Twelve years later spiro-OMeTAD still remains the most-commonly used hole transport material in perovskite solar cells.

Session 2A - Organic photovoltaics (OPV) Chair Opening - Derya Baran

An ever increasing interest in the development and application of innovative optical and optoelectronic devices places greater emphasis for the advancement of new smart and functional materials that are readily processable. Significant progress has already been realized in the fields of organic light-emitting diodes (OLEDs) and photovoltaic cells (OPVs) through development of novel semiconducting materials. Here we discuss developments and advancements in materials design towards photonic structures that aid and improve light management in organic and inorganic/organic hybrid devices, with focus on solar cells. Specifically, we present a novel inorganic/organic hybrid material that is produced in a one-pot synthesis and is solution processable, and can manipulated to exhibit a refractive index of 1.5 to up to 2.05. The material is of very low optical loss and, thus, can be readily integrated into efficient photonic structures. We cover systems targeted for use in light in-coupling structures, anti-reflection coatings, and beyond. Extension to architectures for heat management, important for a broad range of photovoltaic device platforms, including inorganic, inorganic/organic hybrid and organic devices, will also be presented.

Royal Society of Chemistry Industry talk

The performance of efficient organic solar cells depends critically on the morphology of the active layer. Both, electron-hole separation and charge carrier transport to the electrodes benefit from the formation of suitable aggregates or crystallites. Using a blend of the polymer PM6 and the non-fullerene acceptor Y6 as example, I shall demonstrate how we can use relatively simple absorption and emission spectroscopy, in combination with a careful Franck-Condon analysis, to identify the formation of aggregates in films made for solar cells. The talk will be based on two papers whose abstracts I give below.

(1)

In organic solar cells, the resulting device efficiency depends strongly on the local morphology and intermolecular interactions of the blend film. Optical spectroscopy was used to identify the spectral signatures of interacting chromophores in blend films of the donor polymer PM6 with two state-of-the- art nonfullerene acceptors, Y6 and N4, which differ merely in the branching point of the side chain. From temperature-dependent absorption and luminescence spectroscopy in solution, it is inferred that both acceptor materials form two types of aggregates that differ in their interaction energy. Y6 forms an aggregate with a predominant J-type character in solution, while for N4 molecules the interaction is predominantly in a H-like manner in solution and freshly spin-cast film, yet the molecules reorient with respect to each other with time or thermal annealing to adopt a more J-type interaction. The different aggregation behavior of the acceptor materials is also reflected in theblend films and accounts for the different solar cell efficiencies reported with
the two blends.

(2)

In an endeavor to understand why the dissociation of charge-transfer (CT) states in a PM6:Y6 solar-cell is not a thermally activated process, measurements of energy-resolved impedance as well as of intrinsic photoconduction are employed. This study determines the density of states distributions of the pertinent HOMO and LUMO states and obtains a Coulomb binding energy (Eb,CT ) of ≈150 meV. This is 250 meV lower than the value expected for a pair of localized charges with 1 nm separation. The reason is that the hole is delocalized in the polymer and the electron is shared among Y6 molecules forming a J-like aggregate. There are two key reasons why this binding energy of the CT state is not reflected in the temperature dependence of the photocurrent of PM6:Y6-diode: i) The e–h dissociation in a disordered system is a multi-step process whose activation energy is principally different from the binding energy of the CT state and can be substantially less than Eb,CT , and ii) since dissociation of the CT state competes with its intrinsic decay, the dissociation yield saturates once the rate of dissociation grossly exceeds the rate of intrinsic decay. This study argues that these conditions are met in a PM6:Y6-solar cell.

The development of new, high performance organic semiconductor materials has led to increases in power-conversion efficiency of organic solar cells to reach almost 20%. Some of the best performing systems appear to generate photocurrents without the need for a type II donor acceptor charge separating heterojunction.  These impressive advances have raised the question of whether the traditional ‘bulk heterojunction’ architecture is essential for photovoltaic work.

In this work, we use device physics, modelling and spectroscopy to explore the mechanisms of free charge generation in different types of organic semiconductor system. We investigate devices based on single organic components, controlled heterojunction architectures, and more complex macromolecular structures where the positions of donor and acceptor components are chemically constrained.  By combining experimental characterisation under varying conditions (field, temperature, excitation) with molecular and device-level calculations, we endeavour to relate exciton and charge dissociation efficiency in single-component devices to molecular parameters.

Session 2A1 - Advanced characterization and automation Chair introduction - Wiebke Albrecht

Halide perovskites have revolutionized the field of photovoltaics through their remarkable performance in single as well as tandem solar cell devices. One intriguing property of this material class is the wide tunability of the band gap which enables a fine-tuning of optical and electronic properties. In this talk, I will I revisit some of our earlier work regarding the analysis of the valence and conduction band positions of tin and lead based 3D perovskites. Combining photoelectron spectroscopy with density functional theory we were able to distinguish influences from the atomic level positions, the bond hybridization strength, as well as lattice distortions.

Recently, we also started working on 2D perovskites, which are gaining more and more attention as a strategy to tailor interfaces; they turned out to be a key factor to unlock high efficiencies in perovskite solar cells. Here, the effect of the choice of the bulky cation on the position of the charge transport layers is less clear and published measurements often seem contradicting. I will present a systematic study on alkyl-based organic cations with varying chain length, which are selected to form Ruddlesden Popper as well as Dion-Jacobson structures.

Since such 2D perovskites are most commonly used as a thin interlayer on top of 3D films, I will also present some of our results gathered at such modified surfaces. Notably, using reflection energy loss spectroscopy (REELS) we are able to determine the surface band gap of these samples which helps us to understand the formation of such 2D surface layers.

Metal halide perovskites have emerged as promising alternatives to traditional semiconductors, yet their practical implementation faces several challenges that limit their impact in society. In particularly, material stability and performance areas have been key aspects that affect perovskite photovoltaics as well as various semiconductor-based applications. Overcoming these obstacles necessitates precise tuning of the material, and strategies such as crystallization control or additive engineering become crucial.

Here, we introduce comprehensive approach to perovskite formation control that enable the optimization of the systems for specific applications. As a result, we design techniques such as an in-situ highly efficient nanocrystals fabrication approach through humidity-induced methods [1], additive adjustment for efficient and stable Pb-free perovskite solar cells, and a laser-based passivation of macrocrystal surfaces [2]. Through a thorough investigation into the physical properties of these monocrystals, we explore their impact on diverse applications, such as photovoltaic devices and memristors [3]. Leveraging the robustness of monocrystalline systems, we develop an impedance spectroscopy model to analyze the phase dispersion resulting from ionic modulation, providing valuable electrical insights applicable to polycrystalline thin-film devices. Our research contributes to advancing high-performance metal halide perovskite devices by elucidating crucial factors influencing their performance and proposing potential solutions to enhance their functionality.

The dynamic response of metal halide perovskite devices shows a variety of physical responses that need to be understood and classified for enhancing the performance and stability and for identifying physical behaviours that may lead to developing new applications. A multitude of chemical, biological, and material systems present an inductive behavior that is not electromagnetic in origin, termed a chemical inductor. We show that the structure of the chemical inductor consists of a two-dimensional system that couples a fast conduction mode and a slowing down element. Therefore, it is generally defined in dynamical terms rather than by a specific physicochemical mechanism. The impedance spectra announce the type of hysteresis, either regular for capacitive response or inverted hysteresis for inductive response.¹ We develop the methods to characterize time transient and photocurrent response to a voltage pulse. We can obtain important control of the time constant that determine hysteresis. Based on these measurements, it is possible to establish the shortest time to measure hysteresis-free stable power conversion efficiency of high performance solar cells.² We can apply these insights in kinetics processes to the development of memristors and neurons.

Halide perovskites display outstanding photoluminescence quantum yield, tunable emission and simple deposition, which make them attractive for optoelectronics. At the same time, their facile ion migration and transformation under optical, electrical and chemical stress are seen as a major limitation. Mixed halide perovskites are particularly problematic since optical excitation can cause changes in the bandgap that are detrimental for solar cell and light-emitting diode efficiency and stability. Instead of preventing such changes, in this work I will discuss how we are exploiting photo-induced halide segregation in perovskites to enable responsive, reconfigurable and self-optimizing materials. First, I will detail how we can train a mixed halide perovskite film to give directional light emission using a nanophotonic microlens: through a self-optimized process of halide photosegregation, the system mimics the training stimulus.[1] Longer training leads to more highly directional emission, while different halide migration kinetics in the light (fast training) and dark (slow forgetting) allow for material memory. This self-optimized material performs significantly better than lithographically aligned quantum dots, because it eliminates lens-emitter misalignment and automatically corrects for lens aberrations. The system shows a combination of mimicking, improving over time, and memory, which comprise the basic requirements for learning, and give the intriguing prospect of intelligent optoelectronic materials. Second, I will show how we are trying to use this phenomenon to make self-tracking solar concentrators that can utilize diffuse light, including optical and full device simulations benchmarked by absorption and luminescence measurements.[2] I will conclude by discussing some surprising results we find in the time-dependence of photoluminescence quantum yield, which vary dramatically with slight changes in material composition and interfacial passivation layer. 

Small organic molecules,¹ sometimes referred to as pseudohalides,² are an essential component of halide perovskite compositions today that enable passivation of defects and enhance their optoelectronic properties in solar cells and light emitting devices. However, the chemical reactivity of key groups of compounds identified in the literature as promising passivation agents has been largely unexplored. In particular, very little is known experimentally about the atomic-level interactions between small organic molecules and halide perovskite surfaces. I will show how we use solid-state magic angle spinning NMR and first-principles calculations to address these questions, determine binding modes, probe proximities and elucidate the reactivity of small organic molecules with a broad range of functional groups. I will also discuss experimental strategies that enhance the solid-state NMR signal of surfaces in solids, enabling unprecedented, atomic-level insight into their chemistry. The application of some of these techniques, such as Surface Enhanced NMR Spectroscopy (DNP SENS),^3,4 or MAS DNP, while mature for other classes of solids, is only now emerging for halide perovskites.⁵

Session 2B2 - Towards commercialization and applications Chair introduction - Narges Yaghoobi Nia

In the context of climate change mitigation, decarbonizing energy infrastructures underscores the pivotal role of transitioning to renewable energy sources. Important to notice, that approximately half of our energy demand is used for heating or cooling. Therefore effective harvesting and storage of thermal energy is the same as important as energy harvested with photovoltaic.

This presentation explores the synergies between photovoltaic and thermal energy harvesting, emphasizing their complementary nature in realizing the concept of energy-neutral buildings. Recognizing the importance of efficient energy storage, the focus extends to technologies dedicated to storing energy in the form of heat. The discussion delves into the benefits of thermal energy storage, with a particular emphasis on its alignment with the substantial demand for heating. The presentation further outlines a specific business case, illustrating the collection of electric energy through photovoltaics and its storage in the form of heat.

The overarching goal is to broaden the application scope of solar energy, showcasing the potential and perspectives of thermal energy harvesting and storage. By establishing robust connections between diverse energy technologies, the presentation aims to contribute to a holistic understanding of sustainable energy solutions.

Metal-halide perovskites (MHP) have gained prominence as highly promising and cost-effective optoelectronic materials, owing to their exceptional optoelectronic properties and versatile fabrication methods [1-6]. These materials find applications across diverse fields, including solar cells, light-emitting diodes, photodetectors, and quantum emitters. Quantum confinement, can bring to light unexpected and advantageous characteristics, fostering the development of high-performance devices.

An effective approach to induce quantum confinement involves creating layers of quantum-confined materials through the deposition of multiple thin films. Thermal evaporation stands out as a particularly promising technique for fabricating halide perovskite films. It provides precise control over layer thickness, enables fine-tuning of composition, ensures stress-free film deposition, and allows modification of surface properties. The utilization of thermal evaporation in perovskite fabrication has broadened the possibilities of thin film production, showcasing its capability to generate ultrathin perovskite films serving as the foundation for multi-quantum well structures.

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

References:

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

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

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

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

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

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

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

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

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

Perovskite-silicon tandem solar cells are a highly promising technology for the next generation of solar cells. Textures on the front and the rear side help maximize energy yield under field conditions [1]. To coat such textured surfaces uniformly we use a two-step hybrid route to deposit mixed cation, mixed halide perovskite absorber films. In a first step, the inorganic components are thermally evaporated to form a scaffold, allowing for conformal coating of random pyramid textured silicon. In a second step, the conversion into perovskite is accomplished using a solution containing the organic components. To enable the scaling and industrial use of perovskite-silicon solar cells it is crucial to understand and optimise the physical and chemical processes happening during the deposition of the perovskite films and adjacent contact layers. In this contribution we present our insights into the hybrid deposition route and our efforts to upscale perovskite-silicon tandem solar cells from small solar cells to full wafer area devices. We discuss the progress that has been made and the challenges that remain. Furthermore, we will give an outlook towards perovskite based triple junction solar cells.

Perovskite photovoltaics have been shown to recover, or heal, after radiation damage. Here, we deconvolve the effects of radiation based on different energy loss mechanisms from incident protons which induce defects or can promote efficiency recovery. We design a dual dose experiment first exposing devices to low-energy protons efficient in creating atomic displacements. Devices are then irradiated with high-energy protons that interact differently. Correlated with modeling, high-energy protons (with increased ionizing energy loss component) effectively anneal the initial radiation damage, and recover the device efficiency, thus directly detailing the different interactions of irradiation. We relate these differences to the energy loss (ionization or non-ionization) using simulation. Dual dose experiments provide insight into understanding the radiation response of perovskite solar cells and highlight that radiation-matter interactions in soft lattice materials are distinct from conventional semiconductors. These results present electronic ionization as a unique handle to remedying defects and trap states in perovskites.1

Of the potential vacuum techniques for perovskite synthesis, the most widely reported is thermal co-sublimation. This is done by subliming perovskite precursors synchronously, and controlling the stoichiometry by in-situ rate monitoring. Thermal evaporation is a scalable deposition method with demonstrations of high-efficiency on both small- and large- area devices as well as modules. A notable advantage of this technique is its additive nature in that there is no processing limit to film thickness, and thermal evaporation sources can be sequentially positioned in a process line to increase deposition speed and/or tune composition. We will report on the progress on vapor phase deposited perovskites, including novel low vacuum based deposition methods such as close space sublimation. Using substrate configuration we optimize the incoupling of sunlight which leads to current densities very close to the detailed balance limit. We have prepared semi-transparent with varying perovskite thickness allowing for their use in building integrated PV as well as for bi-facial solar cells.

Session 2B3 - Emerging materials Chair introduction - Eline Hutter

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

First, I will show that in 2D perovskites, the distortion imposed by the organic spacers governs the effective mass of the carriers. As a result, and unlike in any other semiconductor, the effective mass of the carriers in 2D perovskites can be easily tailored. Secondly, I will highlight controversy related to exciton fine structure in different perovskite compounds and demonstrate that the soft lattice can suppress relaxation of excitons to dark state making 2D perovskites great light emitters. Finally, I will demonstrate the first experimental evidences of polaron formation in the optical spectra of these materials

Organic-inorganic perovskites are hybrid materials with a three-dimensional (3D) structure which have recently been found to demonstrate excellent optical and electronic properties, making them highly suitable for applications such as solar cells, light-emitting diodes, and thin film field-effect transistors (FETs) [1]. While preliminary reports on perovskite FETs showed that the device performance was greatly affected by ion migration, a detailed understanding of how ion migration leads to the instabilities in the characteristics is yet unclear [2]. We systematically investigated the development of non-idealities in the transfer characteristics by turning the film stoichiometry. The FET devices were further investigated by photoluminescence mapping and elemental analysis using electron microscopy enabling the identification of the migration of ionic defects and electrochemical reactions with metal electrodes as the key aspects of the mechanism [3].

To manage these behaviors, we explored the use of two-dimensional (2D) Ruddlesden-Popper perovskites based on the spacer – isobutylammonium, by integrating them into 3D perovskite FETs by forming 3D/2D perovskite heterostructures on the surface of the film. Remarkably, in the 3D/2D heterostructure FETs, there is a reduction in the unwanted hysteresis. Our work presents an effective strategy to integrate 2D perovskites into 3D perovskite FETs, exploit the salient features of both components, and obtain improved device performance. Furthermore, it takes a step towards the realization of the predicted charge transport behavior of perovskite materials for various optoelectronic applications.

High energy demands of our society stimulate the development of sustainable technologies, such as those based on solar energy conversion or photovoltaics. Perovskite solar cells (PSCs) are an emerging photovoltaic technology that consist of the perovskite film positioned between the electron transport and hole transport layer, while the photovoltaic properties are determined by each component of the structure, as well as the corresponding interfaces. Since 2009, power conversion efficiency of solar cells has already achieved values above 23%. This rapid progress in perovskite solar cell research led to the increased performance and operational stability, which nevertheless remains a challenge towards the commercialization of PSC technologies. In study, new strategies in the design of perovskite solar cells were established based on molecular engineering by addressing perovskite layers with functional analogs. The research objectives focused on two key investigation domains, namely charge recombination in the perovskite grain boundaries or interfaces of perovskite. The investigation of crystal growth mechanism was performed to discover the methods to control the grain size distribution within perovskite films that affects their performance by employing organic functional analogs. The study revealed the crystal growth mechanisms and resulted in the achievement of improving device efficiency and stability.

Keywords: perovskite, crystallization, passivation, molecular engineering, additives, stability

References

[1] Sun Ju Kim, Ramesh Kumar Chitumalla, Jong‐Min Kim, Joonkyung Jang, Jin‐Woo Oh, Jovana V Milić*, Ji‐Youn Seo*, Helvetica Chimica Acta. (2023), 106, e202200193.

[2] Ji‐Youn Seo, Taisuke Matsui, Jingshan Luo, Juan‐Pablo Correa‐Baena, Fabrizio Giordano, Michael Saliba, Kurt Schenk, Amita Ummadisingu, Konrad Domanski, Mahboubeh Hadadian, Anders Hagfeldt, Shaik M Zakeeruddin, Ullrich Steiner, Michael Grätze*l, Antonio Abate*, Advanced Energy Materials (2016), 6, 1600767

Neuromorphic computation is a promising way to overcome the limits of the von Neumann chip architecture in modern computing. Memristors can play a key role in the realization of neuromorphic devices due to their inherent memory storage and ability to dynamically alter physical properties such as resistance in response to an incoming stimulus.¹ Organic memristive devices have the potential to generate new paradigms in the memristor device field due to the inherent ability to tune the electronic properties of organic molecules for greater electrical or memory/volatility performance, which is currently a bottleneck for solid-state transition metal oxide (TMO) memristors.^2,3 We have recently developed a new approach to fabricate high performance memristive devices based 2,4-bis[4-(diethylamino)-2-hydroxyphenyl]squaraine (SQ) nanowires (NWs).⁴ In our initial study, SQ nanowire memristors were fabricated using a very simple but effective approach whereby SQ NWs were grown on an interdigitated gold electrode to form a lateral Au/SQ/Au architecture. These prototype devices demonstrate the hallmarks of memristor operation such as current hysteresis loops and dynamic conductivity in response to multiple voltage sweeps. The volatility of the conductivity states written to the device is also shown to have long memory retention without voltage bias, demonstrating non-volatility. These results are on par with benchmark transition metal oxide devices (TMO). These results demonstrate a straightforward and very promising approach to fabricate robust and low-cost memristive devices. Furthermore, the fabrication method offers an excellent platform for device prototyping and high-throughput screening of potential memristive materials. More recently, advanced neuromorphic tests have been applied and devices demonstrate key neuromorphic properties, including state retention, cyclability, potentiation, depression, pulsed paired facilitation and Hebbian learning. In addition to our NW studies, a brief update on our progress towards more scalable thin-film based (opto)electronic memristors will be presented.

REFERENCES:

1. Zidan, M. A.; Strachan, J. P.; Lu, W. D., Nature Electron., 1 (1), 22 (2018)
2. van de Burgt, Y.; Melianas, A.; Keene, S. T.; Malliaras, G.; Salleo, A, Nature Electron., 1 (7), 386 (2018)
3. Sangwan, V. K.; Hersam, M. C., Nat. Nanotechnol., 15 (7), 517 (2020)
4. O’Kelly, C. J., Nakayama, T. & Ryan, J. W., ACS Appl. Electron. Mater., 2, 3088 (2020)

Tailored organic based semiconductors have great potential for applications in next generation energy conversion and storage technologies, potentially enabling a substitution of inorganic or scarce materials for a more sustainable infrastructure and economy.

Their versatile structural features, often related to porosity, can enable tailored functions and strong property modifications, especially in presence and context of ions and light.[1, 2] These range from intrinsic photocharging properties enabling next generation solar batteries and sensors,[3, 4, 5] over applications as light driven microrobots in biological context and for medical applications,[6, 7, 8] as well as tailored modifications of intrinsic photophysical properties – all being enabled by tailored interactions with their (ionic) environment.

In this presentation, we will exemplify such structure-function relationships on different organic based materials, including ionic carbon nitrides, organic framework materials and polymers. These include current viewpoints on related photophysical properties, how they can be modified, and exploited for applications related to light energy conversion and storage.

HOPV Rising Stars: Organic & Hybrid Photovoltaics - Chair introduction- Bruno Ehrler

Hybrid organic-inorganic perovskite materials have gained importance in global photovoltaic (PV) research, mainly due to their impressive power conversion efficiencies (PCEs) above 26% as well as their potential of becoming a candidate for low-cost mass production¹. Perovskite-based research today is mostly on thin layer solution based deposition techniques given their potential for integration into semitransparent. However, photovoltaic windows have not been demonstrated to a satisfactory level as efficiencies and aesthetic parameters have not been reached the market required values. The use of transparent electrodes is one of the main challenge to obtain high transmittance and in this work semitransparent gold electrodes and three-layered dielectric/metal/dielectric (DMD) thin film sequences have been successfully employed in perovskite-based solar cells resulting in comparable PCEs up to 15%², superior average visible transmittance (AVT), and good light utilization efficiency (LUE)³, defined as the product between AVT and PCE. Moreover the DMD electrode results in higher transmittance due to optical effects, allowing for a higher efficiencies at a critical angle of inclination of 45º. The approach is also universal, as we demonstrated, by making a screening upon the mixed halide perovskite, from pure bromide to pure iodide. This work proves the potential use of semi-transparent electrode to improve the light harvesting of the perovskite layer, by using optical methods to compensate chemical limitation, paving the way for application in building integration.

Long-term stability of perovskite solar cells (PSC) is the impending bottleneck for commercialization of the technology in the renewable energy sector. In this work, we critically assess effects of material stoichiometry and surface morphology to understand their impact on the long-term stability of caesium-formamidinium-based PSC. Key findings demonstrate that the variation in the perovskite precursor - lead iodide (PbI₂) to formamidinium iodide (FAI) ratio impacts the stability under various stress conditions (elevated temperature and light). A high molar ratio PbI₂/FAI >1.1 in the perovskite precursor contributes to a higher open-circuit voltage (V_OC) and hence better power conversion efficiency (PCE). However, the quenching techniques (anti-solvent and vacuum quenching) during the processing do not affect the long-term stability of PSCs. When tested under ISOS-D2 (dark, 85 °C, intermittent current density-voltage J-V characterization) condition, the degradation of the perovskite layer or interfaces between the perovskite layer and the charge transport layers lead to a decrease in performance for the devices implementing non-standard PbI₂/FAI ratio (>1.1 or <1.1) over a period of 500 h. Under ISOS-L1 (100 mW/cm², 25 °C, maximum power point tracking) condition, the devices with PbI₂/FAI ≤1.1 remain stable over 500 h whereas devices with PbI₂/FAI >1.1 show a drastic drop in J. Interestingly, we observe a contradictory trend in post-degradation analysis of devices stressed under ISOS-L1. The devices with PbI₂/FAI ≤1.1 are stable under stress but their PCEs begin to decrease during storage in dark as characterized by intermittent J-V. The presence of iodide vacancies (V_I-) in the absorber layer results in non-radiative recombination and migration of iodide ions (I^-) to the hole transport layer causes formation of shunts during storage in the dark. This work highlights the importance of reporting stability under different stress conditions as well as post-degradation and dark recovery analysis of PSCs to understand a process as complex as perovskite instability.

Controlling and designing the crystallization process [1,2,3] of halide perovskites is key to their higher efficiency and long-term stability. Therefore, to achieve the required stability of record efficiency perovskite solar cells for their industrialization, it is essential to understand the atomic level fundamentals details of nucleation and growth process of perovskites on interfaces. Experimental techniques are often limited by their temporal and spatial resolution to get the atomic details of the formation process. Moreover, there could be electron beam-induced degradation and phase transitions due to the soft ionic nature of halide perovskites, therefore limiting the use of powerful techniques such as transmission electron microscopy to even achieve imaging of full crystallization pathway. In this talk, I will show how alternate methodology of molecular dynamics simulations [4,5] reveal the molecular details of the nucleation and growth process of halide perovskites on widely used interfaces such as TiO2, SnO2, NiO and commonly employed SAMs. To obtain precise results, I use quantum accurate machine learning potentials to simulate the all-atom dynamics of these multi-species complex systems and provide fundamental insights for designing reproducible experiments on interfaces. Apart from the growth mechanism, I will also show the molecular details of the buried interface and the origin of most detrimental defects, such as stacking faults [6], and nanovoids [7], and possible ways to eliminate these defects.

Perovskite solar cells (PSCs) offer an efficient, inexpensive alternative to current photovoltaic technologies. However, solution-processing PSCs involves volatilising large volumes of toxic solvents, presenting significant challenges for industrial-scale production. Moreover, perovskite materials are compositionally unstable under operational conditions. Here we employ a novel low-toxicity, biorenewable solvent system to process a range of 2D Ruddlesden Popper perovskite (RPP) compositions.

We demonstrate that such 2D RPPs act as highly effective solid-state ‘templates’ for subsequent solution-processed conversion to α-formamidinium lead triiodide (α-FAPbI₃). By tracking the evolution of excitonic photoluminescence from the material progressively throughout conversion we observe the sequence of 2D perovskite intermediates via which this transformation proceeds. It is demonstrated that precise control of the 2D RPP template’s phase-composition is of paramount importance to obtain the favourable optoelectronic and material properties of 2D-templated α-FAPbI₃. From these findings we propose a ‘templated’ growth regime in which the inorganic haloplumbate scaffold of the 2D perovskite is retained during conversion and is effective in templating the subsequent growth of low-defect, large-grain 3D perovskite materials.

Optical-pump terahertz-probe spectroscopy reveals an electron-hole mobility in excess of 60 cm² V^-1 s^-1 for the optimised 3D perovskite, which is among the highest recorded for thin-film α-FAPbI₃. Concurrently, an order of magnitude improvement in charge carrier lifetime compared to conventionally processed α-FAPbI₃ leads to a carrier diffusion length of 5.3 ± 0.2 μm in our material. Moreover, incorporation of 2D-templated α-FAPbI₃ layers into PSCs yields power conversion efficiencies of >21 %.

2D-templated α-FAPbI₃ demonstrates substantially improved stability in comparison to other state-of-the-art perovskite compositions. We find remarkably improved thermal stability of FAPbI₃ made by our route, which we show is due to the combined novel solvent and templated growth mechanisms; retention of low-volatility solvents (e.g. DMF, DMSO) is avoided, while degradation of the FA⁺ cation into sym-triazine is substantially reduced. As a result, PSCs based on 2D-templated FAPbI₃ show no degradation after 2,000 hours at 85 °C and 85% relative humidity (ISOS-D-3). Moreover, the isolated FAPbI₃ thin film material demonstrates heat and light stability of over 3,500 hours and, once incorporated in PSCs, these devices show champion t₈₀ stability of >800 hours in 85 °C, 1 sun equivalent light (ISOS-L-2).

The phenomenon of mode coupling holds significance across various domains of solid-state physics. In the context of organic crystals, understanding the implications of coupled lattice modes is crucial for unraveling the mechanisms behind phase transformations and electron-phonon interactions. In my talk, I will explore how the coupling between vibrational modes is evident in inelastic light scattering. Through experimental case studies, I will illustrate how employing mode coupling models in analyzing Raman spectra enables us to shed light on critical phenomena, such as how a phase transition arises from the thermal population of vibrational states.

In the initial segment of my talk, I will demonstrate that the polarization dependence of Raman scattering in organic crystals at finite temperatures necessitates a description through a fourth-rank tensor formalism. This expansion beyond the conventional second-rank Raman tensor is represented by off-diagonal components in the crystal self-energy on the light scattering process. In the latter part, I will discuss the temperature-dependent evolution of vibrational modes in α α-glycine crystals. Despite its straightforward structure and the absence of known phase transitions, I will present evidence of a phase transition manifesting within the vibrational modes closely associated with the potential surface of its hydrogen bonds.

Hybrid halide-perovskites are considered a material with high potential in optoelectronics, especially for light-emitting diodes and solar cells. Perovskite-based solar cells (PSCs) are a promising substitute for commercially used silicon solar cells. Nowadays, the power conversion efficiency of PSCs reaches over 25 % [1]. However, further increase of the PSCs performance is limited by defects located at the layer interfaces and grain boundaries in the case of widely used polycrystalline thin films [2, 3]. Understanding and controlling the formation and evolution of perovskite phases is critical to reducing trap state density, thereby enhancing solar cell efficiency. The in situ studies have shown their significance in revealing the pathways and intermediary mechanisms that affect the resulting properties of the final PSCs.

In this work, we show simultaneous measurement of in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) and photoluminescence (PL) to study the structural and optoelectronic properties of perovskite films during their formation. We used these in situ techniques for perovskite deposited by various methods, such as spin-coating (in a nitrogen atmosphere), chemical vapor deposition, and vacuum deposition. Despite different formation conditions, all perovskite layers exhibit the non-monotonous character of the PL intensity with initial PL increase and subsequent quenching, indicating the defective state formation correlated with the crystalline structure changes observed by GIWAXS. These studies contributed to developing passivation and additive strategies to improve the films' structural, morphological, and optoelectronic properties and move beyond spin-coating with scalable techniques.

Interfaces properties in solar cells play a crucial role on the device’s performance and stability, hence the importance of investigating the chemical behavior at the buried interfaces in solar devices [1]. Nevertheless, a challenge remains: how to access these buried interfaces without modifying the initial chemical information. This work addresses this problematic and aims to develop an innovative methodology of coupling two in-depth profile characterizations, to better understand the chemical composition from the surface to the interfaces. For this purpose, Glow Discharge Optical Emission Spectroscopy (GD-OES) and X-Ray Photoelectron Spectroscopy (XPS) were applied consecutively on half-cells. First, an optimization of the operating conditions was carried out to minimize the degradation of the perovskite layers. In the case of GD-OES not only the Radio Frequency power and the plasma gas pressure are changed, but also the nature of this gas (Ar, Ar/O). The craters resulting from profiling by GD-OES were then chemically studied by XPS in order to determine the chemical composition at different levels of the layer as well as at the interface area. We observed that all the conditions employed for GD-OES profiling led to iodine loss, a systematic reduction or oxidation of lead as well as the degradation of the organic part, more or less pronounced depending on the plasma gas. A comparison of SEM (Scanning Electron Microscopy) images inside and outside the craters also showed a remarkable change in the surface morphology for a bombarded surface by Ar or Ar/O.

The application of GD-OES and XPS coupling was then tested on a full stack device, where GD-OES was applied for etching the Au electrode and stopped before reaching the interface. Indeed, the live acquisition of the profile enables us to precisely stop before the area of interest, making it possible to preserve the integrity of the chemical information registered through the modified residual overlayer. The in-depth profile was then proceeded by using XPS sputtering inside the crater created by GD-OES. Reduction of lead was detected all along the profile, and a new triiodine species emerged at the interface. The degradation of the organic compounds in perovskite after etching was also noticed, but this behavior was fully studied and understood in previous work [2].

Accordingly, to better understand the origin of the detected species and their behavior after a certain time, this coupling methodology must be first studied on a fresh reference device and then applied on aged solar devices for better comparison. It can also be carried out on tandem solar cells to reach different levels of the device.

Lead halide perovskite solar cells have shown promising efficiencies; however, their large-scale implementation has been hindered by their instability towards environmental impacts such as moisture or light, next to other challenges such as large-scale fabrication. To further develop lead halide perovskite solar cells, in depth understanding of the intrinsic properties of these absorber materials is necessary. This includes a need to understand the intrinsic electron and ion dynamics under light. So far, most studies on behavior under light exposure focused on perovskite thin films, and largely on the degradation under light. However, slight differences in the preparation of the thin films lead to different grain sizes, which affects the thin film’s properties. Studying single crystals instead of thin films excludes these variations and allows to study the intrinsic properties of different compositions.¹ Furthermore, especially the surface and interfaces of the material interact with the environment and possible other device layers. Using photoelectron spectroscopy (PES), the electronic structure and chemical composition of surfaces can be investigated.

Here I will present the laser light-induced electron and ion dynamics of different, clean lead halide perovskite single crystal surfaces, while avoiding X-ray induced changes in the samples. Furthermore, to ensure comparability within the solar cell community, the laser power was set to the equivalent of 1 sun. By using PES for characterizing these crystal surfaces, we were able to follow both compositional and electronic changes and we found distinct behaviors for the different crystal compositions, which give insight into how ion and electron movement are impacted by the perovskite composition.

In order to investigate the transient behaviour of charge carriers in semiconductors followingphotoexcitation, time-resolved photoluminescence (TRPL) is a powerful approach. Commonly, mono- orbiexponential models are used to analyse the TRPL data. These are simplistic models because they can onlybe used to extract one or two lifetimes from the data, neither of which necessarily have any physical meaning.Following a recent publication, we assess the TRPL data using Bayes' theorem instead.[1,2]
In this methodology, the PL response is calculated from reported physical models using a large number ofparameter combinations.[3,4] The different draws of each parameter combination are then given a probability,depending on how well the calculated overlaps with the experimental data. Depending on the number ofparameters, this can be a time-consuming approach.
In this work, we present a revised approach, expanding upon previous work by incorporating a Markov-ChainMonte-Carlo (MCMC) technique for exploring the n-dimensional parameter space. Computational time is savedby reducing the total number of draws needed to assess each parameter. With some additional optimizations,a typical run of 10'000 parameter combinations can be performed in < 2 hours. As a result, we obtain aprobability distribution for each parameter, which can give many important insights into the underlying physics.For instance, some parameters are non-identifiable based on the given, experimental data set and should bereported as such. 'Regular' fitting approaches would fail to grasp the non-identifiability and may thus lead toconfusing or even contradictory results.
To validate the approach, we use FAPbI
thin films and half-stacks and assess them with TRPL alone at first.We find a good agreement between extracted values for the different parameters and respective valuesreported in literature. In addition, correlations between these parameters can be probed and accuratelydescribed as well and may be useful to understand new physical processes. The extracted parameters arethen used to simulate the response of the sample during an intensity-dependent PLQE measurement and asurprisingly good agreement between the two is found. Based on this, the extracted parameters of the FAPbI
-SpiroOMeTAD half-stack are studied in more detail. Some novel insights into the processes governing PLquenching at the perovskite-HTL interface are gained and open new questions to why some transport layersresult in goo interfaces and other don't. Lastly, we compare the extracted parameters to other techniquesavailable in our group to find good agreements between all of them.
Overall, we show that this method of evaluating data can be extremely powerful, as it won't be limited by'overfitting' and in the case of TRPL allows the extraction of a variety of fundamental parameters from justasimple optical measurement.

Metal halide perovskites forms a very exciting material class for various opto-electronic applications. These materials form multi-crystalline films of exceptionally high quality by methods as simple as spin-coating. The soft and ionic nature of the polycrystalline perovskite films leads to highly complex material properties that can vary over time and from grain to grain. To include perovskites in commercial applications, we need to understand their optoelectronic and structural characteristics on the scale of single grains (eg nanometers). Cathodoluminescence microscopy has been used successfully in the past to study grain-to-grain variation in luminescence efficiency and has shown that surface traps not only dominate carrier trapping, but also are distributed unevenly amongst various grains.¹ These detailed optical studies are often correlated to bulk film orientation and only  few studies have correlated optoelectronic properties to structure at the nanoscale.^2,3 We combine nanoscale characterization of optical and structural properties by correlating cathodoluminescence and electron backscatter diffraction analysis on the same location in perovskite thin films. Electron backscatter diffraction is an electron microscopy technique that gives information on the local grain orientation, grain boundaries and strain.^2,4  By combining these electron microscopy-based techniques, we can learn if and how the existence of surface traps is related to specific grain orientations. On top of this, we can correlate photoluminescence quenching at grain boundaries to the relative orientation of the crystallites that make up the boundary. By pin-pointing the points of origin of processes such as electronic carrier trapping or halide segregation, we hope to further the understanding of perovskites at the nanoscale, and inform the efforts towards more effective film passivation.

Our group developed photoluminescence (PL) measurement solutions addressing defect identification of organic PV (OPV) structures and devices. The photo-response of the absorber materials was investigated with dynamic and static PL measurements aiming the industrial application of these techniques. 

The dynamics of the photo-response of the OPV samples were studied using Time Correlated Single Photon Counting (TCSPC) measurement technique. The unique feature of the TCSPC technique is to obtain the time-resolved fluorescence dynamics of molecular orbit transitions at extremely low injection conditions.

The luminescence decay characteristics featured very short, 100-500 ps decay component. Such fast dynamics might not be detected using a conventional photodiode-based measurement system. Although the signal amplitude depends on the structural properties, but the evaluated decay time relates only to the given molecular specie of the absorber layer.

The industrially more important and viable technique is the PL imaging to detect problems with layer deposition. Therefore, we present the key steps of the development of roll-to-roll imaging PL characterization of organic PV structures.

The proper choice of the excitation light source type, wavelength, power together with the very efficient light management made possible to build several experimental measuring setups with different cameras and filter sets to define the optimal configuration of the integration-ready PL imaging system.

The final PL configuration using line cameras and recording PL images from large area samples was finally proven to provide outstanding quality and resolution images from all relevant organic layers and clearly indicating its unique capability for the real-time revealing of homogeneity issues in the layer deposition processes. The system optimization resulted in a fast-enough image capturing ensuring the easy adoption of the imaging PL technique in organic PV production lines.

Full solution processing of perovskite solar cells (PSCs) has been one of the main enablers for commercialization of this photovoltaic technology. The utilization of versatile deposition techniques such as doctor blade and slot die facilitate uniform and large-scale fabrication, particularly suitable for roll-to-roll processing. This scalable approach not only ensures high throughput but also holds promise for reducing production costs. This can be possible if all the layers are solution processed and if the production method ensures fast processed of drying and crystallization. In the first case, the main limiting factor is the top electrode, however the alternative of having a carbon-based electrode has become a promising option, while for the second case, the use of flash infrared annealing (FIRA) allows the fabrication of PSCs within the second timescale. This talk will present the late results, including limitations and advantages of having direct and inverted PSCs manufactured by doctor blade and slot-die, aiming at having full solution processed devices.

In recent years, perovskite solar cells (PSCs) have emerged as a promising photovoltaic technology with significant potential to meet global energy needs. However, PSCs still need further development, especially in terms of scalability, in order to be commercialized.[1] Creating precise scribes on the perovskite solar modules (PSMs) is a very challenging task since it is crucial to ensure no damaging of the individual cells that form the modules, which is probable to occur when performing such thin scribes (~45 μm width each). Different methods such as shadow masks, mechanical scribing or laser ablation can be used to create those scribes, which are usually selected based on the deposition technique used.[2] Laser ablation offers numerous advantages, characterized by its versatility, precision and smoothness, making it an excellent method for ablating the scribes needed to create a well-organized module. Usually, the procedure used to make the scribes by laser ablation is the P1-P2-P3 process which consists of three scribings, each of which has a function in connecting the cells. P1 cuts the TCO on the substrate to pattern the sub-cell insulation. P2 scribing removes the HTL, perovskite, and ETL layers to form each sub-cell. In the P3 scribe, it is necessary to remove all layers, except for the bottom electrode (TCO), to separate each cell. This isolates the back-contact (electrode) of neighboring cells. The organization of a module can be quantified by the geometric fill factor (GFF), which is the ratio between the active area and the combined area of both active and dead regions within the module.[3]

This work focused on optimizing P1, P2 and P3 scribes for maximizing the GFF of inverted PSM (glass/FTO/PTAA/F-PEIA/Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃/F-PEIA/PCBM/BCP/Ag structure) with different number of individual cells. The laser ablation system used was a UV laser ablation system equipped with a UV laser source (355 nm) of 5 W power, with a pulse frequency range of 20 - 200 kHz and a minimum pulse width of 15 ns. Besides photovoltaic performance assessment, different techniques such as Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) were used to measure the scribe width and spacing between scribes, and to verify the presence of material residues on them. GFF of 99 % was reached for the described system with optimized P1, P2 and P3 scribes of 45 μm width each.

In recent years, perovskite single junction solar cells (PSCs) and perovskite-based tandem solar cells (TSCs) have demonstrated significant advancements in power conversion efficiency (PCE) on the laboratory scale. However, the commercialization of this promising technology faces challenges related to upscaling without compromise in performance, high process expenses, and the use of hazardous materials. Specifically, solvent-based material deposition techniques, such as coating and printing, pose difficulties when applied to larger areas due to a substantial increase in solvent consumption and the potential use of harmful solvents, raising safety and environmental concerns.            
In this study, we investigate the viability of employing non-hazardous and environmentally friendly solvents, commonly referred to as "green solvents”, for the commercialization of PSCs and TSCs. We delineate the specific criteria that such solvents must meet to facilitate cost-effective, non-hazardous upscaling routes while preserving solar cell PCE. Our research proposes two fundamental approaches to tackle the aforementioned challenges, focusing on inkjet printing technology as an example: A one-step perovskite deposition with an adapted solvent system and processing parameters, and a highly optimized hybrid two-step perovskite deposition. For the hybrid two-step deposition process, inkjet printing is leveraged to introduce organic cation precursor materials into evaporated lead iodide thin films. This approach promises to overcome upscaling hurdles by addressing issues related to homogeneous thin film formation and conformity, particularly important for the fabrication of efficient TSCs.
Presented here are our latest results, achieved through meticulous optimization of the hybrid inkjet printing process. These efforts have led to the fabrication of PSCs that exhibit high PCE exceeding 18% and remarkable reproducibility, along with high conformity even on textured surfaces. Importantly, both fundamental approaches eliminate the need for the use of toxic solvents such as dimethylformamide.
We are confident that our research contributes valuable insights and practical strategies for a safer and more environmentally friendly scalable production of efficient perovskite photovoltaics, that brings the technology closer to the realization of large-scale commercial applications.

Lightweight and flexible plastic-based perovskite solar cells (PSCs) are seen as strong emerging rivals to the rigid heavy-block photovoltaics composed of crystalline silicon. To boost further the competitiveness of these devices, the scientific community is now looking for compatible, effective and scalable manufacturing methods to attain efficiencies of greater than 20%, while their fabrication using cost-effective and greener materials is also increasingly studied. The present work sheds light on the development of high-efficiency and stable flexible-plastic PSCs under ambient atmospheric conditions through a comprehensive investigation on a series of primary and secondary monohydric alcohols usage as green-antisolvent alternatives. The findings revealed that the plastic substrates have much better manufacturing compatibility (studied in terms of nanomechanical testing) with alcohols compared to the reference case of chlorobenzene antisolvent. Simultaneously, high-quality perovskites and, therefore, photovoltaics are able to be obtained through the concurrent consideration of antisolvent properties (i.e., polarity, density, viscosity, flash point and water solubility) (thorough experimental evidence and interpretations are given to proof the concept, including time-dependent XRD, SEM, UV-VIS, FTIR, miscibility testing, nano-mechanical analysis, J-V curves, IPCE-APCE, EIS, stability assessment under ISOS-D-1 protocol conditions and bending fatigue). To this end, 2-butanol was found to aid in the development of mirror-like, pinhole-free and mechanically resilient ambient-air-processed perovskite structures, boosting the efficiency of scalable carbon-based flexible PSCs to a record of over 20% with decreased hysteresis. Notably, the unencapsulated devices also exhibited remarkable stability under ISOS-D-1 protocol conditions (T₈₅ >1000 h) and bending fatigue (T_{80 (5-mm-radius)} >5000 bending cycles).

Lead- based perovskite materials are a braking trough technology in the use of optoelectronics devices, which in particularly for solar cells have achieved an efficiency close to 26 % for lab scale devices. However, the toxicity of Pb to the environment and human health, hampers the commercialization of this technology. Therefore, other non-toxic elements have been employed to substitute the highly hazardous Pb in the perovskite structure. Among the candidates, tin-based perovskites (Sn-PS) are the promising alternatives for the development of highly efficient and low-cost photovoltaics based on low-toxicity materials. However, one of the main limitations for Sn-PS is the easy oxidation of Sn⁺² into Sn⁺⁴ upon the exposure to ambient conditions and even during the film and device preparation, which promotes degradation mechanisms and compromises the quality and long-term stability of the material.

Besides this intrinsic challenge, most studies have focused on spin coating as the main deposition method for the absorber layer, which is not compatible with large area manufacturing techniques like roll-to-roll or sheet-to-sheet. In this work, we have successfully deposited Sn-PS films trough bladecoating technic; ink engineering of the solvent system and additives were used as a strategy to control the crystallization dynamics and nucleation to obtain pinhole-free films on flexible substrates. This strategy allows us to deposit successfully uniform FASnI₃ films onto polyethylene terephthalate (PET) sheets up to 700 cm². Our findings demonstrate how to upscale Tin-based perovskites with a simple method; thinking on how to commercialize lead-free perovskites.

Perovskite solar cells (PSCs) are a promising candidate for next-generation photovoltaics, demonstrating remarkable advances in performance during the last decade, with record power conversion efficiencies (PCEs) exceeding 26%. Vacuum deposition techniques are widely used for fabrication of thin-films and have several advantages compared to solution-based fabrication methods. These include conformal deposition of high-quality layers, low material consumption and the ease of scalability to larger areas. However, PCEs of thermally evaporated PSCs have been lacking behind those of solution-processed PSCs for years. While originally most research regarding thermally evaporated PSCs was dedicated to co-evaporation processes, reaching maximum PCEs of 20.6%,^[1] recent record n-i-p PCEs > 21% were achieved via sequential (two-step) layer deposition approaches.^[2,3] Here, the individual perovskite precursors are deposited in two steps and converted to the final perovskite film during a subsequent annealing step.

In this work, we present a sequential evaporation process to fabricate all-vacuum-processed methylammonium-free PSCs in the p-i-n architecture. In the first process step, the inorganic layer is deposited onto the substrate, followed by the deposition of formamidinium iodide (FAI) in the second evaporation step. The conversion of these two layers into the final perovskite film is performed during an optimized annealing step under ambient atmosphere. We will present phase-pure formamidinium lead iodide (FAPI) PSCs using an all-vacuum-processed layer stack with PCEs above 16%, among the highest reported for evaporated FAPI PSCs in the p-i-n architecture.

Interestingly, we observe a significant difference in X-ray diffraction measurements of the final perovskite thin film when changing the underlying hole transport layer (HTL). Further experiments demonstrate variations in microstructure and morphology of the inorganic layer on various HTLs, which impact the interaction with FAI and the conversion into a perovskite film. Proving this substrate-dependent film growth and understanding how to manipulate the microstructure/morphology of the inorganic layer by adjusting process parameters during the evaporation marks a huge step forward in understanding the sequential evaporation process. We will discuss these findings at the conference.

Furthermore, addition of further inorganic precursor materials (lead bromide, caesium iodide, caesium bromide) in the first evaporation step allows adjusting the bandgap, facilitating fabrication of efficient wide-bandgap PSCs and perovskite-based tandem solar cells. Our work paves the way for efficient all-evaporated PSCs and their application to monolithic tandem solar cells with an up-scalable and industrially relevant deposition technique.

In the quest for optimizing both the stability and efficiency of perovskite-based devices, the composition of the perovskite layer plays a crucial role. Typically, compositions with mixed A-site cations and mixed halides are employed to obtain optimal band gap and structural stability. While halide perovskite films are typically prepared using solution processing, the deposition by thermal evaporation is gaining attention due its ability to produce uniform and high quality films with precise control over thickness. This enables the investigation of ultrathin layers and the fabrication of large area thin film devices. However, in the case of thermal evaporation, controlling the co-evaporation of two or more precursor sources present significantly more challenges, compared to the more conventional solvent-based approaches.

Using thermal evaporation, we studied the incorporation of bromide into the FAPbI₃ perovskite structure, which is necessary for fine-tuning the bandgap (1.48 - 2.3 eV), e.g. for the use in tandem solar cells. However, for evaporated FAPbI₁Br₂ perovskite films, we observed unexpected substrate effects which triggered the need for further investigations. In this work, we therefore systematically studied the influence of substrates, such as PTAA, NiO_x, PEDOT:PSS, and the self-assembled monolayer MeO-2PACz on the composition and electronic structure of the mixed halide perovskite. Characterization techniques include surface sensitive techniques such as X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and reflection electron energy loss spectroscopy (REELS) – all of which show that the surface halogen composition is strongly affected by the choice of substrate. This is confirmed by bulk sensitive techniques such as X-ray diffraction (XRD) and UV-vis spectroscopy, confirming the surprising role the substrates play during thermal evaporation.

This investigation not only deepens our understanding of the interactions of substrates with perovskite materials but also holds important insights for advancing the development of thermal evaporation technique on high-performance perovskite-based devices.

Halide perovskites are promising semiconductors for next-generation optoelectronic and spintronic applications. Yet, we still don’t fully understand what governs the charge and spin dynamics in these materials. This is especially true when studying device-relevant thin films of halide perovskites, which lack single-crystalline perfection.

In this talk, I will give an overview of our recent efforts to understand the spin-optoelectronic performance of these films better by using time-, space- and polarization-resolved spectroscopy and microscopy. We will find that the energetically heterogeneous energy landscape in mixed-halide perovskites can lead to the local accumulation of charges, with unexpected consequences for devices [1]; how despite strong differences in vertical diffusivity and across grains charge extraction can remain very efficient [2], and how locally varying degrees of symmetry-breaking drive spin domain formation [3,4] in this fascinating class of solution-processable semiconductors.

Time permitting, I will conclude with briefly outlining the fundamentals & artifacts involved in measuring circularly polarized luminescence (CPL) reliably [4,5], and show our most recent development of full Stokes-vector polarimetry with unprecedented time- and polarization resolution to track the emergence of chiral light emission.

[1] Nature Photonics 14, 123 (2020)

[2] Nature Materials 21, 1388 (2022)

[3] Nature Materials 22, 977 (2023)

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

[5] Advanced Materials 35, 2302279 (2023)

Among the various objectives required to achieve complete commercialization of perovskite solar cells (PSCs), the use of eco-friendly solvents is a priority in creating devices whose properties are increasingly less affected by the surrounding environment, ensuring greater stability of the initial efficiency. Lately, various eco-friendly antisolvents have emerged as potential alternatives to hazardous chlorobenzene [1,2]. We explored the potential substitution of toxic chlorobenzene (CB) with a safer alternative, anisole, in the fabrication of planar triple-cation perovskite solar cells using tin dioxide as an electron transport material and SpiroMeOTAD as hole transport material. Anisole (ANI) was effective in achieving functional perovskite solar cells comparable to those obtained with CB. Notably, the devices with anisole as an antisolvent exhibited lower hysteresis, maintaining 80% of the initial power conversion efficiency (PCE) value over 90 days storage [3]. The champion device, 10% CsI and anisole as antisolvent, showed a starting PCE of 20.2%. We also varied the cesium concentration in the triple-cation precursor, which is known to enhance the stability. Perovskite devices, even though manufactured in a N2-filled glove box, were influenced by environmental conditions when they were electrically characterized in ambient air with more than 40% of relative humidity. A higher Cs concentration yielded more stable unencapsulated PSCs, which were less susceptible to fluctuating temperature and humidity [4]. Moreover, we have stated that anisole can be used as an antisolvent because it is able to ensure a good growth of the perovskite film and is a sustainable alternative to chlorobenzene.

While silicon (Si) photovoltaics has almost reached its maximum theoretical efficiency, perovskite solar cells (PSC) have emerged in the last decade as a new generation of photovoltaics, with a record efficiency of 26.1%, almost matching that of Si cells [1]. Taking advantage of both silicium mature technology and perovskite versability (tunable bandgap, ease of fabrication and low cost), PSC/Si tandem based on a silicon sub-cell and a perovskite top-cell have the potential to harvest more than
40% of incoming photons [2]. However, if for the top-cell the inversed architecture (p-i-n) has been identify as substantially more stable than its n-i-p counterpart [3], it still suffers from poor stability compared to the silicon technology, which has so far hindered their commercialization both as single junction and as top-cells in tandems [4]. Recombination at interfaces having been identified as a bottleneck for both long term efficiency and stability, strategies based on the integration of low
dimensional perovskite (displaying higher intrinsic stability) at interfaces have been proposed [5]. Alternatively, the integration of Lithium fluoride (LiF) at the perovskite/ electron transport layer interface (typically C60/SnO2) enables for field effect passivation, however so far at the cost of higher defect densities [6]. We expect both strategies to be highly complementary, as low dimensional perovskite integrated at the perovskite/C60 interface not only passivates the surface but protects 3D perovskite from the subsequent evaporation steps (LiF and C60).

The low dimensional layer (quasi-2D perovskite) is formed at the top surface of 3D perovskite through  he spin coating of bulky ammonium cations on top of 3D perovskite. First, several promising candidates were identified, namely phenethylammonium iodide (PEAI), bromide and chloride (PEABr, PEACl), 4-Fluoro-Phenethylammonium iodide (4F-PEAI) and butylammonium bromide and iodide (BABr, BAI). Solvent engineering strategies as well as the introduction of additives (MASCN, MAI, ethylene diamine) were studied in order to induce the recrystallization of the top surface of 3D
perovskite upon 2D cations addition and control the dimensionality of the quasi-2D perovskite. The quasi-2D layer was systematically characterized by SEM, KPFM, XRD and photoluminescence (PL), revealing major differences depending on the 2D cation used and the deposition strategy. Relying on PL results and I-V characterization, we identified a mixture of BAI and BABr as the most promising candidate to passivate the perovskite surface and boost the efficiency of semi-transparent p-i-n perovskite top cells, with a +0.1V enhancement in VOC.
3D perovskite has been shown to be sensitive to LiF deposition [6]. We hypothesize that the thin layer of quasi-2D perovskite formed at the perovskite top surface could protect the active material from the subsequent evaporation steps and reduce trap density at the perovskite/C60 interface. An important stability study aiming at comparing reference cells (no quasi-2D perovskite, no LiF), cells with LiF and cells with the dual passivation (quasi-2D perovskite and LiF), is planned for the incoming months based on ISOS protocol tests. We intent to demonstrate that the integration of both materials is highly complementary and could enable to take fully advantage of LiF field effect passivation

Following the advances in increasing power conversion efficiencies of hybrid organic-inorganic metal-halide perovskite solar cells (PSCs), recent research efforts have focused more on enhancing their long-term stability. Among the various factors that impact the device’s lifetime, an important role is played by humidity-driven degradation, which, within hours, can significantly lower the starting performance of PSCs.
The influence of humidity has been attributed to hydrogen bonds forming between water molecules in moist air and the nitrogen atoms in widely used ammonium-based organic cations like methylammonium (MA) and formamidinium (FA). To mitigate this process, we introduce sulfonium- and sulfoxonium-based cations like trimethyl sulfonium (TMS) and trimethyl sulfoxonium (TMSO) at the top and buried interface of the perovskite active layer. By examining the interfaces using advanced spectroscopic and microscopic methods and evaluating the performance and stability of the devices, we demonstrate that this approach leads to significantly enhanced moisture stability. Importantly, the strategy can be applied to various perovskite compositions without altering the recipe or structure of the bulk perovskite.

Perovskite solar cells (PSCs) represent a highly promising emerging photovoltaic technology with an extremely high potential for large-scale practical application since these devices combine the processability and scalability of organic photovoltaics (OPVs) with the high efficiency of crystalline silicon solar cells. Unfortunately, the low operational stability of perovskite solar cells remains the major obstacle to the transition of PSCs from research labs to industrial-scale production and mass application.

The stability of complex lead halides can be tuned by using different types of additives or modifiers capable of specific interactions with the perovskite absorber material, e.g. forming coordination bonds with coordinationally unsaturated Pb²⁺ cations exposed on the film surface. There are hundreds of various molecular modifiers or passivation coatings tested directly in photovoltaic cells, whereas the information on their action mechanisms is very scarce and controversial. One of the reasons is that these molecular modifiers were mostly screened towards improving ambient stability of perovskite solar cells, which is probably not the best approach since efficient encapsulation should solve the extrinsic stability problem. Surprisingly, there are very few studies on the influence of such additives on the intrinsic photochemical and thermal stability of perovskite films. Obviously, lack of such fundamental information complicates drawing any reliable conclusions about action mechanisms of certain molecular modifiers.

Herein, we investigated the impact of a broad range of molecular modifiers (>30 compounds) on intrinsic photochemical and thermal stability of MAPbI₃, FAPbI₃, Cs_0.12FA_0.88PbI₃ and Cs_0.1MA_0.15FA_0.85PbI₃ thin films, where MA⁺ and FA⁺ are methylammonium and formamidinium cations, respectively. All experiments were performed under well-controlled anoxic conditions. The obtained results allowed us to identify the most promising additives and establish correlations between the molecular structures of the modifiers and their stabilization effects induced in perovskite films and draw some conclusions about the action mechanisms of the most promising additives.

In particular, we explored 4,6,10-trihydroxy-3,5,7-trimethyl-1,4,6,10-tetraazaadamantane hydrochloride (NAdCl) as a molecular modifier for MAPbI₃ perovskite films. It was shown for the first time that molecular modifier can slow down the decomposition of perovskite films in the absence of oxygen and moisture. [1] Furthermore, we presented a comparative study of two azaadamantane-based molecular modifiers NAdCl and MAdI (an iodide of N-methylated 1,3,5,7-tetraazaadamantane) as stabilizing additives for perovskite films. [2] The designed absorber materials based on Cs_0.10MA_0.15FA_0.75PbI₃ with NAdCl modifier and Cs_0.12FA_0.88PbI₃ with both NAdCl and MAdI modifiers could withstand 5000 h and 16000 h of continuous light exposure, respectively, which are among the record values of the intrinsic stability of lead halide perovskites.

More recently, we have introduced the antibacterial drug octenidine dihydroiodide (OctI₂) as a highly promising molecular modifier for designing complex lead halides with spectacularly enhanced intrinsic photostability. [3] The Oct(FA)_n−1Pb_nI_3n+1 and Oct(Cs_0.12FA_0.88)_n−1Pb_nI_3n+1No material formulations showed no signs of decomposition under white light exposure for 9000 h and 20000 h, respectively, which to the best of our knowledge represent the record lifetimes of perovskite films reported to date.

The performed studies featured a tremendous potential of rationally designed molecular modifiers to be used for blocking the main intrinsic degradation pathways in complex lead halides and boosting the operational stability of perovskite solar cells.

In recent years, self-assembled monolayers (SAMs) have revolutionized the design of perovskite solar cells (PSCs), particularly in p-i-n ("inverted") architectures, thanks to their low-temperature processing, minimal material usage, and excellent operational stability. This has made them the preferred material for hole-selective contacts on ITO. However, this preference doesn't extend to n-i-p (or "regular") architectures. Despite the widespread use of SAMs for hole-selective layers (HSL), it's remarkable that very few studies have explored electron-selective SAMs.

This study investigates a series of novel molecules for their potential as electron-selective SAMs on ITO. Charge selectivity and transfer rates were assessed using transient surface photovoltage (trSPV) across timescales ranging from 1 nanosecond to 1 second. The results demonstrate clear electron selectivity for certain molecular structures, indicating their strong electron injection capability into ITO and excellent hole-blocking properties.

This study highlights novel molecules with potential for efficient electron-selective SAMs, offering improved performance in perovskite solar cells.

Hybrid organic-inorganic metal halide perovskites have become one of the dominant semiconductors in photovoltaics due to their attractive properties. These materials are soft yet crystalline mixed ionic-electronic conductors that have unique optoelectronic and optoionic properties. However, they suffer from instability under device operating conditions due to ion migration that leads to device degradation. Several approaches have been established to overcome this issue, including those that incorporate functionalized organic cations that template lower dimensional perovskites with enhanced operational stability. These organic species are usually electronically insulating, limiting their charge transport, and resulting in lower efficiency of the solar cells. There is a need to enhance their functionalities by using (photo)electroactive organic cations that are responsive to external stimuli. We demonstrate the molecular design and synthesis of representative functional organic spacers followed by their implementation in the corresponding layered hybrid perovskites. They are characterized by a combination of techniques to assess their potential in photovoltaics.

Nowadays, polyurethane-based materials are massively exploited in a plethora of applications, from adhesives to foams, from building insulations to athletic tracks. Recently, the specifical use of aliphatic thermosetting polyurethanes (aPUs) has enormously increased in the industrial context for their versatile synthesis and tunable physicochemical properties. In the context of photovoltaics, and more specifically, the emerging field of Perovskite Solar Cells (PSCs), we proposed the use of thermosetting polyurethanes as low-cost but effective encapsulants on rigid devices. [1]

The advantage of thermosetting PUs over other polymeric encapsulants lies in their tunable flexibility. Indeed, a properly designed combination of precursors leads to a PU that could be coupled with PET in flexible PSCs, allowing PU-protected devices to outperform the non-encapsulated cells in both conventional and high-humidity (RH > 70%) environments. Another possibility with thermosetting PU is their application as both encapsulant and interlayer in tandem devices; more in detail, we exploited a specifically designed formulation (i.e., having a refractive index comparable to the one of glass and a transmittance higher than 90%) to glue together a NIR-Dye Sensitized Solar Cell and a UV-absorbing PSC. The final tandem device reached a total efficiency close to 10% with an Average Visible Transmittance (AVT) as high as 35%, leading to a Light Utilization Efficiency close to 3.5%. All the proposed formulations have been engineered to improve their sustainability by replacing fossil fuel precursors with bio-based or waste-derived ones [2], thus leading to high-performing but sustainable encapsulants and interlayers for emerging photovoltaics.

Impurities are of large concern in inorganic semiconductor technology as they strongly influence the electrical and optical characteristis of electronic devices. However, purity is often not discussed in the organic semiconductor community, although almost every group has eperiences with strong batch-to-batch variations on the performance on organic solar cells and some factors, e.g. the residual content of metal catalysts used during synthesis of the materials are know to dentrimental influences the electrical characteristics. Additional problems might arise due to contaminations of the processing solvents or the in-situ-formation of degradation products due to photoelectrical or photooxidative stress. Despite the intrinsic importance to know these impurities, the qualitiative and even more the quantitative analysis is extremely difficult due to 1) the similar nature of the impurities and the semiconductor, i.e. comprising the same elements (C, H, N, S, O, Cl, F...) and 2) due to their very low concentration in the active layer.

This work focuses on a systematic investigation of the impurities of bulk heterojunction organic solar cells by means of pyrolysis (Py) gas chromatography (GC) coupled to a high-resolution time-of-flight (HRTOF) mass spectrometer (MS). We thereby have chosen one of the currently standard materials combination PM6/Y6 in the device architecture ITO/PEDOT:PSS/PM6:Y6/PNDIT-F3N-Br/Ag. 1-chloronaphthalene was used as additve. We thereby analysed the single materials (PEDOT:PSS, PM6, Y6, PNDIT-F3N) as reference, but the main focus was set on the characterisation of the complete device, as additional impurities might be introduced during additves and solvents during device fabrications.

Thermogravimetric analysis (TGA) revealed that PM6 – which has the highest thermal stability - limits the thermal range for Py-GC/MS to temperatures above appr. 450 °C. Consequently, pre-investigations were performed between 500 and 750 °C. A pyrolysis temperature of 550 °C was found to the best compromise with respect to intensity and diversity of characteristic pyrolysis products. The pyrolsis of all materials of a solar cells at once leads to a complex mixture of volatile products and thermal fragementation species, which were separated to a large extent by GC and then analysed by MS. However, the main challenge was to identify impurities in the ppm range, which were hidden under the many fragmentation products of the semiconductors. It was possible to identify even the additive 1-chloronaphthalene in the ppm range and it was found, that its concentration is strongly dependent on the processing conditions. Additionally even impurities such as dichloronaphthalene or remains of the metal catalysts, organic phosphin oxides, could be detected.

A further interesting aspect is, that already in the early stages some degradation products related to defluorination of the organic semiconductor had been identified. An unexpected observation was the fact, that the change in aggregation due to an thermal annealing step leads to a significant change in the degradation pattern of the Y6 acceptor whciht might be related to differences in the thermal stability.

Organic photovoltaics is a promising emerging technology for solar energy conversion due to low production cost, large area roll-to-roll production possibilities, flexibility, lightweight, and semitransparency.[1-2] Most organic solar cells contain non-fullerene acceptors with large fused-ring systems. However, the production of these acceptors is due to their synthetic complexity costly and often involves toxic materials and solvents. Non-fused electron acceptors are an alternative, since they require fewer synthesis steps, are cheaper to produce, and still have the required planarity of the molecule.

In our work, we have successfully synthesized derivatives of the COTIC-4F acceptor comprising a cyclopentadithiophene core, thiophene linkers and fluorinated IC end groups. The molecules B1 and B2 differ from the COTIC-4F molecule in their side chains (hexyl instead of 2-ethylhexyloxy) attached to the thiophene linker. In B2, we additionally changed the position of the hexyl chain at the thiophene ring from 3 to 4. By this modification, we expected an increased photostability due to structural confinement mitigating photoisomerization and photooxidation of the molecule.[3] The NFAs B1 and B2 reveal higher optical band gaps (1.31 eV) than COTIC-4F (1.1 eV).[4] In bulk heterojunction organic solar cells in conventional architecture, power conversion efficiencies of 9.59% were obtained for solar cells with the PTB7-Th:B1 absorbers and 9.94% for PTB7-Th:B2 based absorber layers. The higher performance of the B2 based solar cells is most presumably due to a more balanced hole and electron mobility and higher exciton dissociation probabilities.

In addition, we investigated the stability of these materials and solar cells in detail. For instance, we studied the change of absorption properties of B1 and B2 films under continuous solar simulator illumination in ambient conditions and obtained a significantly decreased fading in absorption intensity of the B2 film. Moreover, PTB7-Th:B1 and PTB7-Th:B2 based solar cells were tested under various conditions (shelf life at room temperature and 65 °C, continuous illumination) and found that the PTB7-Th:B2 solar cells show a significantly higher photostability (t₈₀: ~600 h) as well as higher thermal stability (t₈₀: ~600 h) compared to the B1 based solar cells, which reveal t₈₀-times of only ~125 h under both testing conditions. This work reveals that the investigated modification in the NFA design does not only lead to improved photostability but also to increased stability at elevated temperatures, which we will investigate in more detail also for other non-fused NFA structures in the near future.

Solar cell analysis often relies on the assumption that the superposition principle[1,2] holds true, implying that the illuminated current-voltage curve can be obtained by subtracting the short-circuit current density from the dark current-voltage curve. However, in reality, this principle doesn't apply to any type of solar cell, including crystalline Si[1,2]. Instead, the illuminated current-voltage curve exhibits a distinct photoshunt, differing from the dark shunt resistance.[3,4] This apparent photoshunt is introduced due to the increase of charge carrier density under illumination, reflecting transport limitations in the absorber or transport layer of solar cells.[4,5] In this discussion, we focus on how understanding the photoshunt can enhance our comprehension of charge extraction and recombination in organic solar cells with the help of analytical expression for the photoshunt. Additionally, we explore the connection between the photoshunt and other measures of transport losses, such as fill-factor losses or recombination losses at short circuit.

Nowadays renewable energy technologies attracted a lot of attention due to the limitation of fossil fuel. One of the best renewable energy harvesting devices is organic solar cells (OSCs) as bulk heterojunction. The active layer is composed of a blend of donor usually a low bandgap conjugated polymer and acceptor like fullerene (PC71PM) or non-fullerene (Y6). Tremendous efforts have been devoted to increase the power conversion efficiencies (PCE) over 20%^[1]. Recently metallooligomers containing Pt(II) metal based on DPP (diketopyrrolopyrrole) are well used, since they display additional optical and opto-electronic features that leads to ultrafast photoinduced electron transfer and the increase of excitons population^[2].

The optoelectronic properties of metallooligomers containing OSCs can be enhanced by tuning the structure of DPP to enhance electronic transfer between donor and acceptor and improving π-π inter-chain aggregation by self-assembly to favor charge carrier’s mobility. To enhance the optoelectronic properties and solar cell performances (PCE>15%), my project consists in  enhancing the optoelectronic properties and solar cell performances by ① functionalization of the side chains from DPP core with an organizing group as triphenylene^[3] with distinct spacers (length, aliphatic, axially conjugated heterocycle) (T) to improve molecular organization , ② the increase  of the conjugated backbone planarity to decrease the bandgap (Eg)^[4] and finally ③ preparing a new donor whose the molecular design combines the advantages of the two first strategies.

Organic solar cells (OSCs) have attracted considerable attention for potential commercial applications because of their light weight, mechanical flexibility, semitransparency, and large-area manufacturing properties. Recent advancements in Y-series non-fullerene acceptors (Y-NFAs) and polymer donors have significantly improved the power conversion efficiency of OSCs. In light of these rapid efficiency improvements, it is essential to focus on the stability of photovoltaic materials. These materials are pivotal in determining the operational lifetime of OSCs under real-world conditions, ensuring they meet the necessary standards for future commercial viability. However, the relationship between the molecular structure and the outdoor stability of these devices remains unclear, and there is a deficiency in comprehensive studies that examine operational performance in combination with measurements of photostability and thermal stability.

Here, we explore the stability of various Y-NFAs alongside a common polymer donor and vice versa. Regarding Y-NFAs, we establish a connection between the molecular structure, specifically the endgroup and side-chain, and their photostability. Employing density functional theory (DFT) calculations on the energy barrier for photoisomerization, coupled with device photostability outcomes, we discern that inhibiting light-induced vinyl rotation effectively prolongs device lifetime. Shifting focus to polymer donors, we examine the significance of different building blocks in dictating device longevity. We propose a molecular descriptor to assess the possibility of side-chain breakage. The chemical alterations undergone by Y-NFAs and polymer donors under illumination ultimately lead to notably enhanced trap-assisted recombination, thus compromising device performance in outdoor conditions. Furthermore, we systematically compare the photostability, thermostability, and outdoor stability of devices based on state-of-the-art materials to obtain a comprehensive understanding of how the photoactive layers influence long-term performance in the hot and sunny climate of Saudi Arabia. Our findings offer valuable insights into designing and synthesizing photoactive materials with the goal of achieving high efficiency and long-term stability in outdoor organic solar cells (OSCs).

Two-dimensional transition metal dichalcogenides (2D-TMDCs) can be combined with organic semiconductors to form hybrid van der Waals heterostructures [1]. Specially, non-fullerene acceptors (NFAs) stand out by their excellent absorption and exciton diffusion properties [2]. Here, we couple monolayer tungsten diselenide (ML-WSe₂) with two well performing NFAs, ITIC, and IT-4F (fluorinated ITIC) to achieve hybrid architectures. Using steady state and time resolved spectroscopic techniques; we reveal sub-picosecond free charge generation in the bilayer of ML-WSe₂ with ITIC. However, bimolecular recombination of spin uncorrelated charge carriers in the bilayer causes rapid formation of low-lying triplet (T1) states in ITIC.  Importantly, this unwanted process is effectively suppressed in the bilayers with the fluorinated derivative of ITIC, IT-4F. We observe a similar scenario when replacing the ML-TMDC by copper thiocyanate (CuSCN) as the hole acceptor meaning that triplet state formation is not driven by the spin-orbit coupling of ML-WSe₂. From ab initio calculations using density functional theory, we interpret the high triplet formation in the ML-WSe₂/ITIC hybrid bilayer as due to changes in the nature and energies of the interfacial charge transfer (CT) levels. Our results highlight the delicate balance between excitons and charges in such inorganic/NFA heterostructures.

Special Session: Energy Policy and Diplomacy participation

To combat the massively detrimental effects of climate change, the world will have to stop emitting carbon dioxide and will even have to move to negative emissions in the next few decades. This will entail totally decarbonizing the world’s energy system and transforming how the world uses energy and other resources to produce everything that is needed to sustain society as we know it. Photovoltaics (PV) will play a major role in this newly decarbonized world that should be realized in a very short timeframe of only three decades [1]. It has been estimated that collectively, the world needs to install between 60 and 70 TW of PV in that timeframe which is a massive undertaking considering that currently there is approximately 1 TW installed. It also begs the question whether there are economically sustainable trajectories to reach this goal, whether there are ways to speed up this process, and whether this can be done in a sustainable fashion. In this presentation, we will explore whether there is an economically sustainable way to ramp up PV production capacity to produce the needed 60+ TW of installed capacity.[2] All the while still ensuring that manufacturers continue to make rational decisions towards investing in production capacity and avoid stranded assets after the relative short period of three decades is done. The modeling presented considers increased learning and continued improvements in design of PV and manufacturing as well as limits to learning having to do with embodied energy. The scale-up challenge is considerable as the massive and needed ramp up in production capacity of the next few decades will be followed by very modest demand for new PV driven only by needed replacement of existing panels and continued population increases. We demonstrate that this ramp up followed by ramp down is possible and calculate the base-line cost of this ramp up in production capacity and also calculate the cost of the PV needed as well as discuss the scale of natural resources needed and the challenges associated with what happens to the panels that need to be replaced.[3] We also consider what happens if a new technology such as high-efficiency tandems based on perovskites or other materials gets introduced during the carbonization period, and show that this can make a sustainable trajectory more likely and make it cost less than continuing along existing technology pathways. Such new technologies can also significantly reduce the needed capital to build the necessary manufacturing capacity, lower the needed installation area and resources, and decrease the total embodied energy.

Mind the Gap: what is still lacking? How can we break through the impasse we have faced in Europe to get our PV manufacturing going? Do we need an EU IRA, or can we design a better deal for Europe? How much time do we have?

Know your oranges and apples in the EU PV landscape. PV modules are not PV modules, and PV cells are not PV cells. Although often referred to as a commodity, PV is increasingly a high-tech product with manufacturing processes similar to those of the semiconductor sector. This is reflected in the new generation of cell technologies as well as their respective pricing. The narrative on building a PV manufacturing industry in Europe should be precise to avoid a wrong perception about the opportunity and competitiveness of European PV manufacturing.

What Next to Build a Resilient EU PV Industry? With the right speed of implementation and focused measures, Europe still has a good chance of building a globally competitive PV industry. Whether we achieve it is up to all of us.

Session 3A - Hybrid photovoltaics (HPV) Chair opening - Amita Ummadisingu

The use of a broad variety of additives (ranging from e.g. elemental ions to inorganic and organic molecular cations, neutral hydrophilic, hydrophobic or amphiphillic molecules and many more) that are supplemented during the growth of halide perovskite materials from solution or as a posteriori surface treatment, have often shown beneficial effects in improving both photoconversion efficiencies and long-term stability. Such additives can play a multitude of roles by e.g. leading to the formation of mixed 2D/3D perovskites, inducing a preferential phase stabilization (e.g. for the alpha perovskite phase of formamidinium lead halide over the delta-phase) or even catalyze phase conversion (e.g. from delta to alpha phase). They can also passivate surface point defects and grain boundaries and mediate or impede charge carrier transport between the perovskite material and the electron or the hole transporting layers .  Moreover, they can also directly influence the nucleation and crystal growth process. The underlying atomistic mechanisms are only just starting to emerge and without mechanistic rationalization, the search of the vast available chemical space for optimal additives has to rely on a trial and error strategy.

During the last years, we have used a combination of different computational techniques to elucidate the manifold roles of different types of additives with the aim of establishing a fundamental mechanistic and  possible universal structure-function relationships. In this talk, I will present some prototypical case studies that illustrate some of the possible atomistic impacts that additives can have and the corresponding descriptors that characterize their activities.

PST is EnliTech's special techonolgy of using single xenon lamp, you're not just getting a light source; you're receiving a beacon of innovation. Our high-power xenon lamps, enhanced by patented spectrum adjustment tech, sacrifice some brightness for A++ spectrum quality. With automatic spectrum fine-tuning, each beam becomes a crafted masterpiece, offering an unmatched visual experience. It's not just a lamp; it's a commitment to excellence, bringing brilliance of PV soulutio

Organic or perovskite photovoltaics poses a multi-objective optimization problem in a large multi-dimensional parameter space. Massive progress was achieved in developing methods to accelerate solving such complex optimization tasks. We have demonstrated for both types of semiconductors, that the combination of Gaussian Process Regression (GPR) and Bayesian Optimization (BO) are most efficient in predicting new materials, identify optimized processing conditions or invent alternative device architectures in larger parameter rooms. For a 4 dim space (solvent, donor-acceptor ratio, spin speed, concentration) with about 1000 variations in a 10 % grid space, 30 samples are sufficient to find the optimum. For 5 & 6 dimensional spaces, the possible variations go into the millions. Nevertheless, our automated lines, being operated in an autonomous optimization mode, were able to identify globalized optima within several hundred´s of experiments. That raises the question whether these large material spaces as well hold the promise for discoveries. We extended the BO concept towards the discovery of new molecules that can be integrated into the device optimization cycle. The research campaign found molecular semiconductors that had not been published before but yielded performance values bypassing the current state of the art materials.

Abstract of Hemamala Karunadasa TBD

Session 3B1 - Device physics and engineering Chair introduction - Jarla Thiesbrummel

Lead-tin (Pb-Sn) perovskites are a highly promising composition for single-junction and all-perovskite tandem solar cells due to their narrower bandgap and reduced toxicity. While the use of quasi-two-dimensional (quasi-2D) Ruddlesden-Popper phases has resulted in superior stability towards the environment and large improvement in the crystallization with respect to the 3D compositions, very little work has been done towards their deposition with scalable techniques. In my presentation, I will show that PEA2(FA0.5MA0.5)4(Pb0.5Sn0.5)5I16 (n=5) with a gradient structure is successfully prepared for the first time by a two-step blade coating. Perovskite films which are treated with tin (II) acetate (SnAc2) along with N, N-dimethylselenourea (DMS) exhibit a reduced number of surface traps and enhanced surface crystallization, owing to the in-situ formation of tin selenide (SnSe). Record devices with power conversion efficiency (PCE) of 15.06%, an open circuit voltage (VOC) of 0.855 V, and negligible hysteresis are obtained. More importantly, the hydrophobic SnSe significantly protects the active layer from the environment. These devices retain 91% of the original PCE after 10 days in ambient air (30%-40% humidity) without encapsulation, and nearly no-degradation of the PCE is detected after over a month of storage in inert atmosphere, and under continuous MPP tracking for 15 hours. At the end I will also show how it is possible to transform the fabrication process introducing less toxic solvents.

Solar cells have a great potential in replacing fossil fuels in electricity generation, if requirements of low production costs can be met. In the last years, lead halide perovskites have drastically changed the solar cell research field due to their ease of synthesis and high power conversion efficiencies, which now reach over 25%. The future success of these developments crucially depends on understanding the details charge separation, charge transport and charge recombination at the interfaces between the different layers in a solar cell as well as what parameters limit solar cell stability. X-ray based techniques such as photoelectron spectroscopy (PES) are powerful tools for obtaining electronic structure information of materials at an atomic level. By varying the photon energy from soft to hard X-rays, photoelectron spectroscopy can be used for non-destructive depth profiling of the solar cell interfaces giving information about the energy alignment and chemical structure and composition at the interface.

In this presentation, I will show how we have used photoelectron spectroscopy to gain insights into the surface properties and electronic structure of perovskite single crystals. Through in-vacuum cleaving it was possible to obtain clean crystalline perovskite surfaces [1]. Our investigations range from using resonance spectroscopy to map out the orbital contributions in the perovskite valence band, to studying degradation mechanism [2,3], to investigating interface formation with charge transport layers and metal contacts [4].

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

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

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

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

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

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

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

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

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

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

Transient photoluminescence allows us to quantify recombination by determining the characteristic decay time of an exponential decay. This decay time is frequently referred to as the charge-carrier lifetime and is often considered to be a single value for a certain perovskite film. However, we show that in lead-halide perovskites, the decay is primarily not following an exponential decay or a superposition of several exponential decays. This observation is somewhat counter to many previous findings on the topic, which we ascribe to the difficulties in distinguishing between multi-exponential and power-law decays if data is obtained with a limited dynamic range of 1 to 3 orders of magnitude (1). Here, I present a range of transient PL data obtained with larger dynamic range that thereby allows us to clearly distinguish decays that follow a power law from those that follow an exponential decay. Subsequently, I discuss the implications of power-law photoluminescence decays and, therefore, decay times that vary continuously as a function of time or injection level. The variation of the decay time by orders of magnitude implies that the decay time becomes a rather difficult to use figure of merit. One peculiar mathematical aspect of applying the concepts of exponential decays to power-law data is that the decay time follows the time after the pulse. Thus, if the time window of the measurement is increased (i.e. by reducing the repetition rate), the decay time taken towards the end of the decay also increases. As fitting biexponential functions to semilogarithmic plots of PL vs. linear time puts a strong emphasis on the end of the decay, the repetition rate also significantly influences exponential fits of power law decays. The peculiar finding here is that if decay time is presented without the associated carrier density at every point in the decay, it is meaningless as long as the decay is mathematically following a power law. A work-around for this problem is to either present the decay time as a function of carrier density or Fermi-level splitting or to directly determine the recombination coefficient of a bimolecular decay. I then explain possible physical interpretations of non-radiative bimolecular recombination coefficients and show how they are related to steady-state assays of recombination, such as the photoluminescence quantum efficiency.

Session 3B2 - Emerging concepts Chair introduction - Francesca Brunetti

Perovskite solar cells (PSCs) have been considered as a strong candidate for next generation photovoltaic technology, exhibiting a certified power conversion efficiency (PCE) of 26.1% in 2023. Intensive efforts have been consistently made to reach an extremely high PCE close to theoretical value on one hand and to ensure the long-term stability of PSCs on the other hand, where the residual tensile strain in perovskite lattice plays a detrimental role. Lattice strain engineering of perovskite layer was pursued by implementing the heteroepitaxial growth on the bottom. The heteroepitaxial growth of perovskite film enabled the residual lattice tensile strain to be released, affecting the interface recombination and the crystal phase stability as well. The effect of strain engineering was well reflected in both the photovoltaic parameters, particularly in open-circuit voltage and fill factor, and the long-term stability, which highlights the importance of the lattice strain engineering for halide perovskite-based electronic devices.

With the explosive development of the Internet of Things (IoT) technology, indoor photovoltaics (IPVs) are becoming a promising candidate to sustainably power billions of wireless sensors for secured and smart buildings. Among the various photovoltaics technologies available today, halide perovskite-based IPVs are most promising for integration with IoT because of their excellent optoelectronic properties, easy and cost-effective processability using solution-based methods such as roll-to-roll printing, high specific power, and earth-abundance. The low intensity of the indoor light sources means the absence of beneficial light-induced trap filling of the perovskite photoactive layer.  This demands stringent defect minimisation approaches at every functional layer to maximize the power conversion efficiency of IPVs and thereby reduce the efficiency gap (more than 20 % now) between the theoretically predicted and experimentally observed power conversion efficiency of IPVs [1].

In this talk I will discuss the effect of interfacial layer selection in maximizing efficiency and suppressing the hysteresis effects under indoor lighting [2]. Our study shows how the selection of hole extraction layers (HELs) (organic vs metal oxide) impacts the overall device performance of halide perovskite indoor photovoltaics. Two commonly used organic semiconductors (Poly-TPD and 2PACz) and metal oxide (NiO and CuOx) based hole extraction layers in the p-i-n device architecture were considered. Our results reveal that, though under 1 Sun illumination, the photovoltaic device performance is comparable, under low-intensity indoor illumination, the organic semiconductor HELs outperform consistently the metal oxides. In addition to the poor performance, the metal oxide HEL-based devices suffer severe light soaking effects and the bulk vs interface traps contribution to the detrimental light soaking effects were decoupled by studying the photovoltaic performance under different illumination (1 Sun vs 1000 lux) and the measurement sequence. Interface modification of metal oxide transport layers using 2PACz eliminated the light-soaking effects, passivated the defects, suppressed the leakage, and enhanced the indoor light harvesting efficiency. Thus, in addition to reporting the light-soaking effect of halide perovskites under ‘indoor’ lighting, our study put forward a useful design strategy to overcome the deleterious effect of metal oxide HELs.

Two‐dimensional (2D) organic-inorganic hybrid perovskites have recently become an attractive alternative to three‐dimensional (3D) perovskites due to their chemical and structural diversity and improved stability.  Beyond photovoltaics, 2D perovskites have found applications in several different applications. However, the vast majority of studies focuses on a very narrow set of organic ligands which provide little functionality and constrain the potential capabilities of these materials.[1] 

In this talk, we will describe our efforts to leverage the tools and diversity of organic chemistry to provide hybrid perovskites with properties or capabilities beyond their traditional ones and pushing the capabilities beyond conventional applications.  Our endeavors encompass diverse advancements, including chirality transfer [2], reversible and irreversible thermochromism [3], in situ polymerizations and incorporation of conductive polymers[4],[5], intra-material reactivity and reactivity with foreign molecules[6],[7].  Through these examples, we showcase the transformative potential of incorporating non-innocent organic molecules into hybrid perovskite frameworks, unlocking a new realm of functionalities that extend far beyond the conventional spectrum.

Replacing lead by less toxic elements remains a major challenge for the widespread uptake of perovskite-based technologies. Tin appears the only candidate to replace lead, due to the accidentally similar structural and electronic properties of these two elements. A major difference, however, is the stability of Sn(IV) phases, which are related to the lower oxidation potential of tin compared to lead. A related phenomenon is the stability of tin vacancies, which introduce significant p-doping in tin-halide perovskites (THPs), while their lead-based counterpart are essentially intrinsic semiconductors. Defect activity clearly controls doping and could also contribute to the instability towards Sn(IV) phases. Controlling doping and defect activity thus represents a pathway towards obtaining stable THPs with optimal optoelectronic properties. The different defect activity of tin- and lead-based materials is at the origin of their respective thermal and phot-induced degradation phenomena, including halide demixing and loss of I₂ in lead-halide perovskites. 

Here we present results of advanced modelling studies on the defect mediated degradation pathways of prototypical THPs. We show how Sn-vacancies are central in promoting both material p-doping and formation of Sn(IV) phases. Interestingly, while p-doping dominates in the bulk, Sn oxidation is only favoured at surfaces or grain boundaries. Thus achieving uniform thin films coupled with proper surface passivation strategies represent a pathway towards achieving more stable THP-based devices. Surprisingly, THPs have also received a large attention because of their superior stability in water environment compared to their lead counterparts. We further unveil the key factors determining the stability of mixed-halide THPs against photoinduced halide segregation phenomena. Molecular and ionic strategies to mitigate p-doping in THPs are also presented.

Photochromic solar cells are a new type of photovoltaic device with dynamic optical properties that could potentially be used in glazing and buildings in the future. These solar cells exhibit changes in colour, transparency and photovoltaic performance in response to sunlight conditions.

In 2020, we have shown that the design of push-pull photochromic photosensitizers of the diphenylnaphthopyran series and their use in a dye-sensitised solar cell configuration is an effective strategy for achieving solar cells with light-driven optical properties. This approach resulted in excellent reversibility of the colouring and decolouring process, as well as efficiencies of the order of 4% for the solar cells. We have also shown that these dyes can be used to produce semi-transparent mini-modules with a maximum power output of the order of 35mW ^[1].

However, the first generation of dyes, once incorporated into devices, resulted in semi-transparent solar cells with very slow decoration kinetics and poor colour rendering index, below 70 in the coloured state. To overcome these problems, we have sought to develop donor-acceptor photochromic dyes with faster decoloration kinetics and better optical properties. In this talk, we will outline the synthetic routes used to prepare these photochromic dyes and detail their optoelectronic properties and structure-property relationships ^[2-3].

In particular, we will show that by using an appropriate molecular design for the photochromic naphthopyran dyes, it is possible to obtain molecules that exhibit 6 times faster decolouration kinetics in devices ^[4]. We will also present the synthesis and characterisation of new dyes containing carbazole moieties as electron donors. These dyes allow a hypsochromic shift of the absorption of the coloured isomers in the visible region compared to the first generation of dyes, and a better tuning of their spectra to the photopic response of the human eye. Using one of these dyes (SF4) we have achieved a PCE of up to 3% in cells. Additionally, a semi-transparent mini-module has been produced with an average visible transmittance varying between 66% and 50% and a colour-rendering index of around 95 in both the uncoloured and coloured states.^[5]

Session 3B3 - Hybrid Session: Hybrid Photovoltaics Chair introduction -  Imme Schuringa & Ghewa AlSabeh

I will give a quick overview of recent progress and future prospects of perovskite tandem solar cells with the aim of overcoming the efficiency limit of single junction cells. I will then talk about some of the research activities in my group. In the first part, I will talk about our various perovskite-based monolithic tandem cell monolithic cell capabilities and recent results including how our integration design strategy has evolved and how our research activity also includes triple junction tandems. In the second part, I will talk about how diffusion plays a role in the performance and stability of our solar cells.

Perovskite Solar Cells have emerged only in the last decade and have experienced unprecedented interest and growth because of exceptional properties like high absorption coefficient, efficient charge carrier mobilities, solution processibility, low cost, and compatibility with roll-to-roll industrial processing. Amidst much anticipated potential and excitement around the fastest-ever-growing photovoltaic technology, challenges remain in understanding the fundamentals of charge carrier transport, material composition, and film growth dynamics. Moreover, degradation and instability remain one of the major issues impeding commercialization. Firstly, the work discusses some intriguing results that were yielded through the modifications made in the electron transfer layer (SnO₂) of the solar cell devices with the chlorinated salts (Li, Na, and K) made in the classical structure of perovskite CH₃NH₃PbI₃ [1]. Secondly, wide-bandgap compositions with CH₃NH₃PbBr₃ and the mixed iodide-bromide system (CH₃NH₃PbI_x Br_1-x) as photoactive layers were studied to assess their viability in tandem solar cells. A substantial research component focuses on tracing these devices' degradation profiles. The performance and stability experiments attempt to utilize advanced and novel Spectroscopic procedures [2]. Finally, a few anomalous degradation pathways in the perovskite solar cells and observations made by integrating piezoceramic material in the photovoltaic architecture resembling the memory effect will be briefly introduced [3]. The insights developed will potentially assist in resolving some of the critical challenges in the field.

Recombination of electrons and holes at the defect states of perovskite absorber limits the power conversion efficiency of the organic-inorganic halide perovskite solar cells. Recently, with the help of in-situ PL and GIWAXS measurements, we have shown that the defects are formed mainly at the grain boundaries and surface.^1,2 Therefore, it is necessary to selectively passivate surface and grain boundaries. It was shown that the removal of thin surface layers using mechanical,³ or laser⁴ polishing may improve the performance of perovskite thin films and PCEs.

In our work, we performed an initial study examining the effect of the DCSBD (the diffuse coplanar surface barrier discharge) plasma on perovskite thin films. The sample was placed in the vicinity of the plasma and exposed to the plasma for different duration. Using in-situ photoluminescence (PL) spectroscopy, we found that the exposure of perovskite surface to plasma in nitrogen atmosphere causes the removal of oxygen from perovskite film. We found that, at first, the PL intensity decreased significantly, which is caused by surface defects created by exposure to plasma. However,  after this abrupt change, the plasma lines of singly ionized oxygen appear in the spectrum of the plasma, and the PL intensity increases until the oxygen is completely removed. This means that the removal of oxygen has a positive impact on defect densities in perovskite film. However, when completely removed, further exposure to plasma has only a negative impact.

We believe this method could be an intermediate step before the deposition of a passivation layer on the perovskite. Additionally, the DCSBD plasma treatment is already used in different industrial applications and thus it is ready for use in large production lines.

Electrodeposition was investigated in this work as a substitute method to develop large area perovskite active layers for perovskite solar cells application. In order to make perovskite solar cells, a metal or metal oxide, usually a lead compound (Pb, PbO, PbO₂,…), is electroplated and used as the basis for the perovskite. The electrodeposition process of PbO₂ together with the following conversion steps used in this work are shown on the Figure.

This innovative perovskite deposition process could pave the way for the industrialization and marketing of perovskite solar cells architecture.  Indeed, it is a low-cost process, leading to smooth & uniform layers, over large areas. In addition, it is produced under ambient conditions (with no need of glove box), and is environmentally friendly in terms of solvent engineering, since no toxic solvents are needed to stabilize or improve the morphology of the layers obtained. The present study looks at the influence of 5-AVAI (AVAI = ammonium valeric acid iodide) in the fabrication of traditional MAPbI₃ perovskite. Several studies have already shown that the addition of this additive improves the stability and performance of perovskite solar cells produced by spin coating or drop casting. However, in the literature, the effect of 5-AVAI has never been studied on cells produced by electrodeposition. The present study therefore involved varying the ratios of MAI: AVAI in the conversion bath in order to study the effect of this additive on devices realized by electrodeposition. It was noticed that according to MAI:AVAI ratios infiltrated in the perovskite lattice, different microstructures, optical and chemical properties are obtained. The perovskites obtained thus have different bandgap energies, crystallinity rates and stabilities. A correlation will thus be established with the photovoltaic performance of electrodeposited photovoltaic devices varying with the amount of 5-AVAI. In addition, it was shown that for a certain quantity of 5-AVAI, the cells showed a 65% enhancement in PCE after 150h of post treatment at 40°C, under vacuum.

A photovoltiac (PV) device that can change its color state has the potential to open up new opportunites and markets in building integrated PV. The development of smart PV windows can improve the energy performance of buildings with large glass facades and provide unique opportunites for occupant comfort. However, designing such multi-functional devices remains a significant challenge. This work will cover some of the different approaches that researchers have used to attack this problem, including chromogenic organic dyes and halide perovskite semiconducotrs. For halide perovskites, both the applicability of the perovskite to non-perovskite phase change and solvatochromism (color change in response to solvent intercalation-deintercalation) will be discussed, including demonstrations of initial work centered on both of these color change mechanisms and steps taken to address the key hurdles for each. Specifically, someinitial investigations on the rate of the perovskite to non-perovskite phase change will be presented in the context of developing this mechanism into a workable device. Secondly, work will present the prospects on designing 2D halide perovskites for more robust solvatochromic materials. This work will provide an improved understanding of the opportunities and challenges for different mechanisms for color-changing photovoltaic window operation and provide pathways forward to make windows into multifunctional components of future buildings.

Session 3B4 - HOPV Industry Talks Chair introduction - Andrés Redondo

HOPV Industry Talks - Chair introduction - Andrés Soto

The scientific interest on perovskite solar cells (PSCs) has steadily grown over the last decade – together with their efficiencies – and this technology currently stands as the most promising solution to overcome the limitations of silicon solar-based cells, whether as standalone devices or in tandem with silicon. With PSCs industrialization and worldwide production bound to happen in the near future, it is fundamental to develop suitable recycling protocols that can recover any reusable component and safely handle the residual hazardous waste, thus reducing the overall life cycle impact of this technology.
Among all the materials comprising a PSC, the transparent conductive oxide (TCO) glass substrate is the most promising and suitable component for recovery and reuse, thanks to its high market value, strong physico-chemical stability and easy processability. In literature this task is commonly achieved via chemical dissolution of the PSC layers using DMF or other efficient yet hazardous solvents; in this work we report the use of acetone as an inexpensive, green solvent to recover the TCO glass substrate from different PSC architectures, resulting in a recycling process with a reduced economic and environmental impact. The quality of the TCO glass substrates thus recovered is assessed using different characterization techniques (UV-vis spectrophotometry, Raman spectroscopy, XPS, ToF-SIMS), to measure the key properties of the components after the treatment and to probe their surface for residuals and contaminants; by increasing the reaction time to compensate for its slower dissolution kinetics, acetone is able to achieve satisfactory levels of surface cleaning and contaminants removal, comparable to a DMF-based benchmark. Finally, the effectiveness of our recycling process is validated by fabricating new PSCs using the recovered TCO glass substrates: the photovoltaic performance of these "recycled" devices are very close to those of brand new devices, with a remaining factor above 96%. These results showcase not only the viability of an acetone-based PSC recycling process, but also the clear benefits provided by the use of a more sustainable solvent.

Over the last two decades the efficiency of perovskite cells has almost doubled, reaching current values of 26.1 %. However, since the emergence of this technology the phenomenon of hysteresis, that is the difference in the current-voltage (J-V) scans noticed, was observed. This was initially overlooked by the scientific community, thus focussing on the scan with the more favourable efficiency results [1]. Overtime it was realised that the changes taking place within the system, causing the hysteretic effect could not be ignored, as they led to confusion on the performance of the perovskite device.  This led to new measurement techniques to combat the phenomenon, by using slower rates of J-V scan during the studies, as well as by measuring the stabilized or steady-state photocurrent at the maximum power point (MPP) voltage. The hysteretic behaviour is still a controversial topic with respect to its origin, however it is important to continue studying it in order to unlock key information about this phenomenon.

During this study the perovskite mini-modules used were of the p-i-n architecture. The evolution of the hysteresis index was studied over a period of 2 years. Initially, it was noticed that the hysteresis index, increases with increasing temperature within a fixed irradiance range of 950-1050 W/m². Similarly, the hysteresis index also seems to increase with irradiance. However, it was noticed that the hysteresis index was larger in the morning than in in the afternoon for the same irradiance values. This is thought to be due to enhanced performance during the morning hours, due to overnight recovery. Furthermore, the diurnal degradation of the hysteresis index is being studied as well as its effect with temperature.

Hybrid organic-inorganic metal halide-perovskite semiconductors open up new possibilities for the production of cost-effective thin-film photovoltaics. With cell efficiencies on small areas above 25%, produced with a spin-coating process, perovskite photovoltaics have already surpassed established thin-film technologies. However, the large-area deposition of perovskite semiconductor layers processed from solution poses major challenges.

Slot die coating is a particularly suitable deposition method for upscaling. Unfortunately, the efficiency decreases significantly as the coating area increases, but so does the number of published works on slot die coated perovskites. In addition, in the literature often only one layer, the perovskite absorber layer, is deposited using slot die coating. In this work all six functional layers (hole transport layer, electron transfer layer, absorber layer and intermediate layers) for a p-i-n perovskite structure are deposited by slot die coating on flexible substrates. Only the current-carrying contacts are applied using vacuum-based processes.

High throughput rates are also necessary for an industry-relevant process, which is why we realized a deposition with a high speed of 1.2 m/min for all slot die coated layers. This fast processes makes a fast quenching of the wet film necessary for a crystallization with appropriate morphology of the perovskite layer. We used gas quenching for this purpose, which also allows an energy saving moderate temperature that enables the use of flexible substrates. Homogenous depositions could be achieved for substrate sizes of 5 cm x 18 cm with this process speed on flexible substrates, but still using toxic solvents. We cut out cells with active area of 0.24 cm² and could demonstrate efficiencies up to 14 %.

Most of the publications, even if they describe industry-relevant processes, use toxic solvents for the absorber precursor solution. Therefore, we employed nontoxic DMSO for our perovskite precursor formulation in a single solvent process. With this, we achieved efficiencies of up to 15 % PCE on flexible cells with an active area of 0.24 cm² using processes that can be directly transferred to roll-to-roll coating systems but at the expense of process speed and a narrower process window. The lowered speed was still quite high with a value of 0.45 m/min. The results are an important step towards roll to roll production of perovskite solar cells.

     In the landscape of photovoltaic technologies, perovskite-based solar cells (PSCs) have been showcasing rapid advancements in power conversion efficiency (PCE) achieving 26.1%, rivaling the conventional silicon-based counterparts [1]. Within the perovskite solar cells technology, the inverted (p–i–n) configuration has gathered significant attention due to their high stability and decent efficiency [2]. Recently, self-assembled monolayers (SAMs) have emerged in the inverted configuration, as hole transport layers (HTLs) or as a passivation. Indeed, when applied on top of Nickel Oxide (NiO_x), SAMs contribute to work function alignment, enhancing charge extraction while mitigating undesirable charge carrier recombination. Consequently, SAMs have evolved into indispensable components in all inverted perovskite batches.

     The predominant deposition technique of these molecules is the especially simple spin coating method. However, this technique is not compatible with large-scale production lines. Nevertheless, one of today’s challenges lies in its industrial feasibility. In this regard, in literature different upscale deposition techniques have been reported for SAMs deposition like evaporation, immersion, dip coating, spray coating [3].  In addition to the upscaling challenge, inverted PSCs face other problems such as the poor coverage of the perovskite over layer with the NiO_x  [4].

     In the current study, we aim to introduce a low-cost, rapid, and accessible process tailored for large-scale deposition using a solution-based method, specifically through the immersion technique. In comparison to the studies done in the PSC community [5, 6], the optimized immersion in this study is significantly faster, taking only one hour instead of 8 hours or more. This technique will be compared to the spin coating technique and to another large-scale deposition method optimized in this study: the co-deposition of SAMs within the perovskite. Initially conducted on cells, this study has demonstrated efficiencies of 16.7%FW, and 180h stability under constant illumination (ISOS-L1 test). More stability tests will be performed. This process is under optimization for module. The production of modules using only scalable techniques (slot die, evaporation, sputtering, ALD, immersion) has been achieved. To have a better understanding of our method various characterization techniques such as electrical measurements as well as in-depth analyses such as XPS, PL, contact angle, XRD, SEM, and AFM have been performed. A complementary objective of this work is to improve this layer and address perovskite coverage issues through chemical engineering strategies by utilizing molecules such as PFN-Br, 6dpa as additives to SAMs solution and mixing different SAMs molecules.

Perovskite solar cells promise to yield efficiencies beyond 30% by further improving the quality of the materials and devices. Electronic defect passivation and suppression of detrimental charge-carrier recombination at the different device interfaces has been used as a strategy to achieve high performance perovskite solar cells.[1] However, the mechanisms that allow for carriers to be transferred across these interfaces are still unknown. Through the contributions to better understand 2D and 3D defects the perovskite solar cell field has been able to improve device performance. Albeit the rapid improvements in performance, there is still a need to understand how these defects affect long term structural stability and thus optoelectronic performance over the long term. In this presentation, I will discuss the role of crystal surface structural defects on optoelectronic properties of lead halide perovskites through synchrotron-based techniques.[2] The importance of interfaces and their contribution to detrimental recombination will also be discussed. Finally, a discussion on the current state-of-the-art of performance and stability of perovskite solar cells will be presented.

References

[1]        C. A. R. Perini et al., “Interface Reconstruction from Ruddlesden–Popper Structures Impacts Stability in Lead Halide Perovskite Solar Cells,” Advanced Materials, p. 2204726, Nov. 2022.

[2]        J. Hidalgo et al., “Synergistic Role of Water and Oxygen Leads to Degradation in Formamidinium-Based Halide Perovskites,” J Am Chem Soc, vol. 145, pp. 24549–24557, 2023.

Hybrid lead halide perovskite materials exhibit promising potential to outperform silicon-based modules and dominate the photovoltaic market. Despite their outstanding properties, these materials suffer from several drawbacks such as stability issues and the presence of toxic Pb in their structure. Tin halide perovskite solar cells (THPSCs) have emerged as strong candidates to replace Pb-based counterparts due to their excellent optoelectronic properties and reduced toxicity. However, THPSCs still exhibit lower efficiency compared to their Pb-based counterparts, and the voltage losses remain significantly higher, surpassing 0.6 V, almost double that of the best-performing Pb-based PSCs (0.3–0.4 V).

On this basis, we designed and synthesized two novel fullerene derivatives, namely C₆₀-1 and C₆₀-2, functionalized with different fluorinated moieties, and incorporated them as interlayers between the perovskite and the C₆₀ electron transport layer. The LUMO levels of C₆₀-1 and C₆₀-2, at -3.98 eV and -4.01 eV respectively, exhibited better alignment with the conduction band of the tin perovskite layer (-3.92 eV) compared to C₆₀ (-4.05 eV). This enhanced alignment minimized the energy level mismatch, significantly improving the overall device performance. Consequently, the efficiency of the devices increased from 9.3% for the reference device to 10.45% and 11% for the C₆₀-1 and C₆₀-2 devices respectively. These results highlight the potential of functionalized fullerenes in mitigating voltage losses and improving the performance of THPSCs, paving the way for future advancements in their design and development.

A perovskite p-i-n architecture PV device generally adopts solution-processed organic hole transport layers (HTLs), like PTAA or self-assembled monolayers (SAMs). However, this approach can result in an inhomogeneous HTL surface coverage, especially when processed on textured substrates [1-2]. Recent reports have shown that the adoption of atomic layer deposited (ALD) nickel oxide (NiO) in combination with organic layers, such as PTAA or SAM, addresses the above-mentioned issue and leads to higher device yield, for both single junction [3] as well as tandem (in combination with c-Si or CIGS) devices [1-2]. Nevertheless, implementing NiO in devices without PTAA or SAM is seldom reported to lead to highly performing devices.

In the present contribution, we systematically assess the effect of key properties of NiO deemed relevant in literature, namely- resistivity and surface energy, on the device performance and compare the ALD NiO-based devices to those based on PTAA. To this purpose, (thermal) atomic layer deposited (ALD) NiO, Al-doped NiO, and plasma-assisted ALD (PA-ALD) NiO films are investigated as HTLs in a single junction mid-bandgap perovskite solar cell. The resistivity of Al-doped NiO and PA-ALD NiO films are 400 Ω∙cm and 80 Ω∙cm respectively and they are lower than that of thermal ALD NiO (10 kΩ∙cm). However, the devices implementing Al-doped and PA-ALD NiO HTLs exhibit only a modest V_OC gain of ~30 mV compared to thermal ALD NiO-based devices. Overall, the best-performing NiO-based devices (~14.8% PCE) still lag behind the PTAA-based devices (~17.5%) primarily due to a V_OC loss of ~100 mV. Moreover, we observe that the average grain size and the overall crystal quality of the perovskite absorber, which can impact the V_OC, is not affected by the surface energy of the different NiO HTLs. Further investigation based on the light intensity analysis of the V_OC and FF and the decrease in V_OC compared to the quasi-Fermi level splitting (QFLS), indicate that the V_OC is limited by trap-assisted recombination at the NiO/ perovskite interface and that a better charge extraction occurs when PTAA is adopted. Additionally, SCAPS simulations show that the V_OC of the NiO-based devices decreases when trap states are present at the NiO/ perovskite interface. Whilst tuning the resistivity of NiO has a negligible impact on the device performance, we also show that passivating the NiO/ perovskite interface with Me-4PACz SAM recovers this V_OC loss with an increase of ~200 mV. Our study shows that the potential positive effect of decreasing NiO bulk resistivity on the device performance is shadowed by the high recombination at the NiO/perovskite interface. Lastly, our work highlights the necessity of comparing devices based on emerging transport layers, such as NiO, with state-of-the-art transport layers-based devices, which is often neglected in the literature, in order to draw conclusion about the influence of specific material properties on the device performance.

Quantifying recombination in halide perovskites is essential for controlling and enhancing the performance of perovskite solar cells. Here, we present a comprehensive analysis of recombination dynamics in lead halide perovskites via transient and steady-state photoluminescence, focusing on the impact of the energetic position of electronic traps. We emphasize that the decay times extracted from transient photoluminescence measurements cannot generally be reduced to a single value but rather depend strongly on the charge carrier concentration. It follows that the magnitude of the decay time is reliant on the sensitivity of the measurement. We show photoluminescence decay curves with a large dynamic range of more than 10 orders of magnitude that demonstrate decay times from tens of nanoseconds up to hundreds of microseconds in perovskite films. Assuming, that these long decay times correspond to the lifetime of a deep trap in steady state, one would expect a material with perfect photoluminescence quantum yield, which does however not coincide with the experimental findings. Instead, the decay times in transient measurements are affected by charge trapping and detrapping. We quantitatively explain both the transient and steady-state photoluminescence with the presence of a high density of shallow traps without the influence of deep traps. In summary, this study offers a deeper understanding of recombination dynamics in halide perovskites through the analysis of transient and steady-state measurements. Our findings underscore the importance of considering the influence of shallow traps in interpreting decay times and optimizing device performance.

The performance of both single junction (SJ) perovskite solar cells and perovskite-silicon tandem cells have tremendously improved in recent years; respective certified efficiencies currently stand at 26.1% and 33.7%[1]. However, a considerable fraction of the remaining photovoltaic losses is attributed to carrier recombination at the perovskite interface. To mitigate such losses, interfacial extraction layers must be reviewed to determine whether substitutions or additional (mono)layers are required to further improve the photovoltage of the device. Addressing this requirement, this work presents a facile framework elucidating how time-resolved photoluminescence spectroscopy (TRPL) measurements can be utilised to locate points of recombination in perovskite solar cells. Drawing inspiration from well-established silicon PV analytical methods, we show how TRPL analysis can be extended to determine the bulk and surface lifetimes, surface recombination velocity (SRV), the recombination parameter, J0, and the implied open-circuit voltage (iVoc) of any perovskite device configuration[2,3]. Following this framework, we experimentally compare the extent of perovskite passivation on 18 contacts that are of interest in perovskite photovoltaics, discussing differences in electron transport layer (ETL) and hole transport layer (HTL) materials, their deposition methods and position in the stack. Furthermore, the iVoc calculated from the TRPL-based framework are directly compared to the determined iVoc from photoluminescence quantum yields, noting the benefits and caveats in both techniques. Finally, the hypothetical iVoc from full cell stacks based on the summation of the contact SRVs are compared to true solar cell Voc. We emphasise that this novel and simple technique serves as a practical guide for screening and selecting multifunctional, passivating perovskite contact layers in next generation SJ and tandem solar cells. Just as with more established silicon solar cells, most of the material and interface analysis can be done without making full devices or measuring power conversion efficiency. These purely optical measurements are actually preferable when studying the quality of bulk and interfacial passivation approaches, since they remove complicating effects from poor carrier extraction. 

Co-evaporation is an established technique for deposition of thin films and a promising method to create solvent-free perovskite solar cells with high performance. Among these perovskites is FACsPbIBrCl, which is a type of perovskite that is well suited for wide band gap applications such as tandem cells. To further increase their performance, we want to increase the open circuit voltage of these subcells. This is, among other things, affected by the halide composition of the perovskite. In our co-evaporated FACsPbIBrCl perovskite solar cell with organic transport layers, we tried to achieve this by varying the chloride content. However, adding an increasing amount of chloride to the perovskite results in a substantial drop in open-circuit voltage up to 0.2V. Here we show that the drop in open circuit voltage can only be explained by an increased step in band energies between the different layers in the device. We demonstrate this via a comprehensive and thorough JV curve fitting strategy using the drift-diffusion simulation software SIMsalabim[1]. We found that the introduction of an energy step between the valence bands of the perovskite and the electron transport layer is the only scenario that can explain the drop in open-circuit voltage. The energy step must be at least 0.3 eV. Our results provide an explanation for the decrease in performance when increasing the chloride content of the co-evaporated FACsPbIBrCl, in contrast to our initial expectations. We anticipate that, as we have shown numerically, that with an energetic re-optimization of the electron transport layer, the drop in open-circuit voltage can be avoided.

Metal halide perovskites are high-quality semiconductors with outstanding opto(electro)ionic properties originating in their mixed ionic-electronic conductivity. These characteristics broaden the range of their applications in optoelectronic devices, particularly solar cells, where most research efforts are focused due to their remarkable performance in solar energy harvesting. However, hysteresis in current-voltage characteristics is a feature of perovskite solar cells responsible for causing losses in performance¹, which has been associated with ion migration, charge trapping, and ferroelectricity as some of the contributing factors². Contrary to solar cells, hysteresis effects are desired traits for applying halide perovskites in resistive switching memories³. To this end, perovskite materials have been implemented for information storage, logic operations, artificial synapses, and crossbar arrays, with the advantage of their low-cost, low-temperature, and solution-processed fabrication, in contrast to the techniques required for conventional oxide-based memristors⁴. Devices with high ON/OFF ratios, fast switching speed, and good retention have been demonstrated⁵; however, the relatively low cycling endurance (~10⁴ cycles) limits their potential use for practical applications^6,7,8. In this work, we address these operational stability issues of halide perovskite-based resistive switching memories by assembling 2D/3D heterostructures based on perfluorinated spacer cations⁹. We compare the effect of Ruddlesden-Popper and Dion-Jacobson phases in the 2D/3D heterostructure on the performance of the halide perovskite memristive device. As a result, we show that devices with 2D/3D heterostructures outperform reference cells by extending their cycling endurance and retention, offering a versatile strategy for advancing halide perovskite-based memristors.

Since the pioneer work of Brian O’Regan and Michael Grätzel in 1991, dye-sensitized solar cells (DSSC) have attracted many attentions for their ease of fabrication, their performances and their stability. Despite efficiencies above 15% nowadays, DSSCs are not yet able to compete with more mature technologies such as silicon solar cells (efficiency around 20%). However, DSSCs are displaying unique advantages such as semi-transparency, high efficiency under low-light intensity and a pleasing aesthetic making them highly attractive for indoor application or building integrated photovoltaic.

The development of photochromic DSSCs, which can self-adapt their light transmission to the intensity of the ambient light, could be crucial for developing BIPV. We focus on the previously reported diphenyl-naphthopyran series [1], analyzing their optoelectronic behavior by adapting small-perturbation techniques to the inherent properties of the photochromic dye to unravel the electronic processes at the electrode-dye-electrolyte interfaces[2]. We use molecular engineering to develop different series of naphthopyran dyes with an identical pi-conjugated backbone and varying alkyl substituents reported to control the discoloration kinetics and reduce the recombination processes, achieving power conversion efficiencies of over 4.3% [3]. We also report a new family of photochromic dyes introducing a diphenylamine-type donor moiety and functionalized carbazoles, to tune their optical properties to match the photopic response of the human eyes[4].

Mixtures of bromide and iodide in halide perovskites provide solutions for tunable optoelectronic material design. However, such mixtures suffer from photo-induced phase segregation when exposed to light (photo de-mixing)^[1]. While the process is reversible (two phases remix back in the dark to the pristine state, dark re-mixing),^[2] it can potentially lead to unstable optoelectronic properties and device performance during operation, making its understanding essential to progress the field of halide perovskites. Since the observed light-induced phase separation involves significant ion transport, clarifying the ionic and electronic transport properties and therefore underlying defect chemical mechanisms involved in photo de-mixing is crucial.

Here, mixed bromide-iodide perovskites with different dimensionalities (2D, 3D) are investigated, in terms of phase and charge transport properties using a wide range of experimental techniques as well as theoretical calculations. We focus on 2D Dion-Jacobson mixed bromide-iodide perovskites (PDMA)Pb(Br_0.5I_0.5)₄ (PDMA: 1,4-phenylenedimethanammonium spacer) as a model system to study photo de-mixing, due to its established reversibility.^[2] Firstly, we track the compositional evolution in the films during de-mixing and re-mixing by analyzing their time-dependent in-situ optical absorption properties. We also simultaneously monitor the conductivity changes during de-mixing and re-mixing, which allows for a local probe of the ionic and electronic charge carriers concentration and ion transport through the de-mixed phases. The dependence of the phase behavior and charge transport properties of mixed halide perovskites by varying dimensionality and surface conditions is further revealed using similar techniques. Furthermore, we take advantage of SEM and TEM to investigate the morphological changes and the nature of the iodide-rich and bromide-rich phases resulting from phase segregation. Lastly, we propose a model that considers possible opto-ionic effects, which can contribute to the driving force of de-mixing^{[3, 4]} and should therefore be considered in the overall energy balance of the process, together with the electronic effects discussed in the literature.^[5]Our work sheds light on fundamental questions related to the phase behavior and the role of defects on the driving force of de-mixing. These findings will also aid compositional engineering related to halide mixtures, which will enable the optimization of optoelectronic devices as well as the development of other emerging systems exploiting photo de-mixing or in general photo-ionic effects.

Recently, two-dimensional (2D) perovskites have gained significant interest in perovskite solar cells and light-emitting diodes (LEDs) due to their combined chemical stability and tunable bandgap properties. The layer thickness and interlayer distance in 2D perovskite structures significantly influence their physical and chemical properties, such as a decreasing bandgap with increasing layer number. However, research on the dependence of ion transport properties on interlayer distance in 2D halide perovskites remains limited. This study investigates the crucial role of interlayer distance in ion transport properties of 2D perovskites. By incorporating cations of varying sizes (propylammonium iodide (PAI), hexylammonium iodide (HAI), and octylammonium iodide (OAI)), we demonstrate a strong dependence of both ionic and electronic resistivity on interlayer distance and crystallographic direction. For vertical transport (perpendicular to PbI6 layers), both resistivities increase markedly with increasing interlayer distance, with ionic resistivity exhibiting a more pronounced rise. Conversely, horizontal transport (parallel to PbI6 layers) shows decreasing ionic resistivity but increasing electronic resistivity with increasing interlayer distance. Notably, ionic resistivity and the time constant for ion migration decrease under illumination as the interlayer distance increases. This trend coincides with the observed acceleration of degradation by iodine extraction under illumination with increasing interlayer distance, suggesting faster degradation. These results provide valuable criteria for selecting 2D perovskites for surface passivation applications aimed at enhancing perovskite stability.

Empirical A-site cation engineering has significantly advanced the stability and efficiency of hybrid organic-inorganic lead halide perovskites solar cells and the functionality of X-ray detectors.
Yet, the fundamental mechanisms underpinning these improvements in this novel class of semiconductors, known for their outstanding optoelectronic properties, remain elusive.
Our comprehensive multi-modal study, designed to explore the link between microscopic structural dynamics and macroscopic properties in these materials, utilises X-ray diffuse scattering, inelastic neutron spectroscopy, and optical microscopy.
This approach uncovers the presence of dynamic, lower-symmetry local phases embedded within the higher-symmetry average structures in various perovskite compositions.
We find that local structure is tunable via the A-site cation selection: methylammonium induces anisotropic, planar domains of out-of-phase octahedral tilts, while formamidinium favours isotropic, spherical domains with in-phase tilting, even when crystallography reveals cubic symmetry on average.
The variations in local structure observed are in agreement with our simulations and are reflected in the differing macroscopic optoelectronic and ferroelastic behaviours of these compositions.
By demonstrating that the selection of the A-site cation dictates the local structure and, in turn, macroscopic properties, we establish a foundation for correlated order engineering in lead halide perovskite materials as a means of controlling their optoelectronic performance and stability.

Piezoelectricity is the phenomena of strain-induced electric polarization or vice versa; electric-field induced strain, with applications ranging from actuators, to vibrational energy harvesters. Organic-inorganic hybrid (OIH) ferro- and piezo-electric materials have recently attracted interest over traditional piezo-ceramics due to simpler processing methods, their light-weight nature, mechanical flexibility and lower toxicity compared to traditional ceramics [1,2,3]. Recent reports have shown that by incorporation of a ferroelectric OIH perovskite into a (FA,MA)Pb(I,Br)3 perovskite solar cell device, and poling it, a higher fill factor and VOC is achieved via suppression of interfacial recombination due to ferroelectricity-induced modification of the built-in field [4]. Further, molecular ferro- and piezo-electrics often have bandgaps within the optical range, and have thus been studied for their anomalous photovoltaic properties arising from their intrinsic non-centrosymmetry which allows for above-bandgap open-circuit voltage (VOC) to be measured [5]. Here, we design a novel, stable lead-free semiconducting hybrid halide ferroelectric OMX, which crystallizes in a polar space group at room temperature, and exhibits a band gap within the optical range (2.3 eV), in close agreement with that calculated from first-principles methods. Piezoresponse force microscopy (PFM), polarization hysteresis loops and pyroelectric measurements on both single crystals and thin films of OMX reveal its ferroelectric nature, indicating potential applications for flexible self-powered electronics or bio-sensors as well as for ferroelectricity-induced enhancements to the performance of perovskite solar cells.

The rapid evolution of technologies demands innovative approaches for electronic devices to enhance their performance and energy efficiency. Halide perovskite memristors have emerged as promising candidates due to their unique properties and potential applications in neuromorphic computing. In this presentation, we delve into the intricate mechanism governing the behavior of halide perovskite memristors, aiming to unravel the underlying principles that dictate their operation.

To evaluate the mechanism responsible for the resistance changes observed in halide perovskite memristors we conducted different optical and electrical measurements, including voltage-current transients, conductive atomic force microscopy (C-AFM), and photoluminescence mapping (PL‑mapping). By employing this combination of experimental techniques, we inspected the structural and electronic dynamics within the halide perovskite material, seeking to elucidate the fundamental processes driving the memristive behavior.

Furthermore, we have modeled this type of memristor theoretically by making simple considerations. This has allowed us to describe this system mathematically with simple equations. By developing these equations we were able to reproduce the electrical responses obtained experimentally. We will present this model in this talk emphasizing that its application can go beyond the memristors studied in this work. We believe that this model could be applied to other memristors based on halide perovskite and even other materials.

This presentation contributes to the growing and required knowledge of halide perovskite memristors, providing a deeper understanding of their operational principles and paving the way for the development of more efficient and versatile electronic devices.

Organic-inorganic halide perovskites are a versatile material class with excellent optoelectronic properties which hold promise for application in solar cells, photodetectors, lasers, and LEDs. Stability under device relevant conditions is a challenge for these materials. Thus, one of the current research directions is the stabilization of 3D halide perovskite thin films with 2D or quasi-2D layered perovskite phases. In this talk I will describe our joint efforts in stabilizing 3D perovskite films with 2D perovskites. I will first take a closer look at the 2D/3D interface formation[1] and second at interface diffusion. In situ photoluminescence measurements show that formation and diffusion significantly depend on the organic backbone of the 2D molecule, the nature of the halide, as well as on the 3D absorber composition which makes generalized statements about the 2D passivation strategy challenging. Another aspect I will briefly touch on is reproducibility challenges in the field. A robotic platform for the fabrication and characterization of thin films will be introduced to systematically screen the fabrication parameter space and thus improve reproducibility.[2]

Doping, controlled “electronic contamination” of semiconductors, allows to regulate the electronic behavior inside the semiconductor. Metal halide perovskite (HaP) materials, the functional materials of several types of optoelectronic devices, challenge our understanding of semiconductors. We show that, for HaPs, the control of doping type and density and properties derived from these is, to a first approximation, via their surfaces (some of which will become part of their interfaces with electrodes), i.e., defects with energy levels within the bandgap (E_G). These defects can impact and even dominate bulk electrical and electronic HaP properties and, ultimately, the device performance.

While such effect can be relevant to all semiconductors, it is dominant in HaPs because of their intrinsically low densities of electrically active bulk and surface defects. Even for most polycrystalline (< 1 mm grain diameter) thin HaP films, the volume carrier densities (cm^-3) deduced from experiments are below those that result if even < 0.1% of surface sites function as electrically active defects (for the bulk grain). A direct implication is that interface defects will control HaP-based devices, as those consist of multi-layered polycrystalline structures with two interfaces with the HaP layer, where the action is. While surface/interface passivation effects on bulk electrical properties are relevant to all semiconductors and have been crucial for developing each current semiconductor technology, they are even more important for HaPs because bulk doping at electronics-relevant densities has turned out to be so difficult, certainly by established methods.

The demand for eco-friendly and sustainable energy sources has prompted intensive research in the field of photovoltaics, leading to investigate the lead-free materials as alternatives to traditional perovskite solar cells (PSCs). Recently, Cs₂AgBiBr₆ based double perovskite materials have been explored in photovoltaic devices due to their superior optoelectronic properties and inherent non-toxicity. Here, high quality Cs₂AgBiBr₆ is synthesized with controlled cooling temperature to improve the crystal growth. Further, the spinel NiCO₂O₄ nanostructure was investigated to apply as efficient hole transport layer in PSCs. Various techniques were employed to characterize the synthesized materials, including UV–vis spectroscopy (UV), field emission scanning electron microscopy (FESEM), X-ray absorption spectroscopy (XAS), Fourier transform infrared spectroscopy, and transmission electron microscopy (TEM). The crystal structure, band gap, elemental mapping, surface morphology, and coordination of Ni in the Co₃O₄ matrix were examined using X-ray diffraction (XRD), UV, FESEM, and XAS, respectively. Finally, lead-free inorganic PSCs is fabricated with spinel NiCo₂O₄ HTL and exhibited enhanced solar cells parameters.

Third generation solar cells have the potential to provide a viable source of renewable energy to help decarbonize our economy and power wearable devices. With their exceptional performance in diffused light under indoor conditions, dye-sensitized solar cells (DSCs) remain competitive for powering the next digital revolution forming the internet of things.[1] When integrated with an energy storage system such as an electric double layer capacitor (EDLC), DSCs can offer reliable uninterrupted energy output as a photocapacitor. However, conventional DSCs use liquid electrolyte as the redox mediator, which limits the suitability of the technology for scale up and commercialization. Joining the quest for stable, reproducible solid-state hole transport materials (HTM), we are exploring nanostructured metal-organic frameworks (MOFs). These emerging materials possess the highly ordered structure of inorganic materials combined with the chemically tailorable properties and low cost of organics, and have outperformed their precursors in a wide range of applications.[2]

In my work, I will introduce copper benzenetetrathiol (CuBTT) system with highly efficient redox conductivity as a solid state HTM. We demonstrate that creation of highly interconnected networks of these polymeric nanowires improves the conductivity and when epitaxially assembled, these nanowire architectures infiltrate and form better contact with the dye molecules. We also show that modulator assisted bottom-up synthesis gives stacked 2D layers penetrated by 1D cylindrical channels for ion conduction.  This can act as a suitable interface when the DSC is coupled with an EDLC in a photocapacitor architecture. This work establishes MOF nanosheets as efficient interface materials for solar capture and storage devices.

In recent years, organic-inorganic hybrid perovskite solar cells have been attracting considerable attention due to their low cost and facile fabrication [1]. Since 2009 power conversion efficiency of perovskite solar cells devices has increased dramatically and currently exceeds 25% [2]. Although perovskite solar cells have achieved high efficiency, there are still several challenges limiting their commercialization. One of them is long-term stability. Under-coordinated defects are ubiquitous in hybrid organic-inorganic perovskite materials [3]. Such defects cause the electronic states of the perovskite to become discrete and form trap states within the band gap, which trap the free carriers to reduce the performance of perovskite solar cells, and particularly impact on larger-scale modules. To overcome the drawbacks, organic compounds with two N-carbazolyl based chromophores linked by aliphatic chains via two-step synthesis have been developed to merge the electronic states between defective and perfect perovskite sites. When the mentioned organic compounds were used in perovskite solar cells, it results in an improved fill factor (FF) of 84% and stability of 99.6% over 1000 h operation. Furthermore, the power conversion efficiency of a small cell reached 23.22%, and for a mini-module (6.5×7 cm², active area = 30.24 cm²) 21.71% was obtained. This work shows that merging the trap states to a continuous electronic state reduces defect induced recombination and is an effective way to improve the performance of perovskite solar cells.

Metal-halide perovskites have emerged as a promising class of next-generation solar cells. Here, we assess what lifetimes and efficiencies perovskite solar cells (PSCs) have to reach to lower the price of commercial residential photovoltaic (PV) further. We find that using light and flexible substrates, as opposed to heavy and rigid ones, reduces the total installed system cost of PSCs. The flexibility and lighter weight culminate in a lower balance of systems (BOS) cost, as it is possible to use different mounting methods. Concretely, we analyse the scenario when the modules are directly stuck onto a roof without requiring a racking. That reduces both labour and material costs. This effectively lowers the necessary efficiency or lifetime of PSCs (T80 value) required to achieve the same electricity cost as commercialised silicon. A rigid perovskite module with 17% efficiency would need at least 24 years to be competitive with residential-installed silicon. In comparison, a light, flexible module with the same efficiency would only need to last 19 years. We find that flexible PSCs present a most promising commercialisation route because it can enable low manufacturing and BOS deployment costs, which opens up commercial viability at lower efficiencies or lifetimes. [1]

The three-dimensional (3D) organic-inorganic hybrid perovskite (OIHP), with many salient optoelectronic properties, has gained enormous attention for various applications such as solar cells, LED, photodetector, laser, and sensors. Despite possessing excellent optoelectronic properties, the perovskite material still needs to overcome the challenge of long-term stability, which is the primary hurdle to obtaining stable optoelectronic devices. With focus on the current challenge, in this
presentation initially the discussion regarding the role of structural dimensionality in influencing the stability and optoelectronic properties of perovskite will be included. Based on this, the choice of novel chemical strategies to boost perovskite stability and optoelectronic properties will be discussed. Thus, the presentation would emphasize the role of interfacial and dimensional engineering in altering the structural and morphological properties, which thus impacts the stability and properties of the perovskites. Finally, the future perspectives on perovskite material engineering, offering an excellent opportunity to obtain the stable, efficient, and structurally versatile class of perovskite materials for desired optoelectronic applications will be summarized.

Special Session: Raising the New Generation of HOPV Scientists participation

Special Session: Raising the New Generation of HOPV Scientists participation

Special Session: Raising the New Generation of HOPV Scientists participation