The operational stability of organic solar cells is influenced by several factors including microstructure stability, the electrochemical stability of electrodes, and the photochemical degradation of the materials. Whilst some materials appear to perform better than others, there is as yet insufficient information on the material chemistry or device structure and degradation pathways to design better materials. We investigate the specific chemical structure – degradation relationship for a series of chemically similar materials. For the study we select isolated isomers of the bis-adduct of PCBM, given that the small differences in chemical structure between the different isomers are known exactly and the common degradation pathways for solar cells containing fullerenes are also known. We investigate degradation in the presence and absence of air and seek correlation between the chemical structure (i.e. side chain positions) and different mechanisms involving oxygen radicals and fullerene interactions. We consider more general criteria for improving the photochemical and microstructural stability.

The stability of perovskite solar cells is still an unresolved issue and must be addressed for making perovskites a valid option for industrial products, e.g. in the form of enhancing the performance of silicon solar cells in tandem cells. As an initial step for the systematic investigation of structural changes, it is therefore beneficial to enable us to track structural changes within hybrid perovskite thin films. Furthermore, we would like to have the ability to determine the dynamics of such changes as well as the responsible driving forces.

We are using in-situ time-resolved x-ray scattering to resolve structural changes that active layers of hybrid perovskite solar cells undergo during processing [1,2] and conditions necessary for operation. Systematically examining the resulting structural changes while deliberately accelerating the ageing process [3] can help on the route to discover options that help to suppress these structural changes in the end.

Organic solar cells (OSCs) are one of the most promising cost-effective options for utilizing solar energy in high energy-per-weight or semi-transparent applications. Recently, the OSC field has been revolutionized through synthesis and processing advances, primarily through the development of numerous novel non-fullerene small molecular acceptors (NFA) with efficiencies now reaching >19% when paired with suitable donor polymers. The device stability and mechanical durability of these  non-fullerene OSCs have received less attention and developing devices with high performance, long-term morphological stability, and mechanical robustness remains challenging, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and desirable ductility requirements. Yet, morphological and mechanical stability is a prerequisite for OSC commercialization. Here, we discuss our current understanding of the phase behavior of OSC donor:acceptor mixtures and the relation of phase behavior and the underlying hetero- and homo-molecule interactions to performance, processing needs (e.g., kinetic quenches), and morphological and mechanical stability. Characterization methods range from SIMS and DSC measurements to delineate phase diagrams and miscibility to x-ray scattering to determine critical morphology parameters and molecule packing and dynamic mechanical analysis (DMA) to assess specifically the hetero-interactions. The results presented and its ongoing evolution are intended to uncover fundamental molecular structure-function relationships that would allow predictive guidance on how desired properties can be targeted by specific chemical design. Comparative studies show that the molecular hetero-interactions between the donor and NFA are not always the geometric mean of the homo-interactions. This underscores the limited success often encountered when Hanson Solubility Parameters and surface energies are used to estimate molecular interactions.

Humankind is living in a climate emergency that requires immediate action. However, in the meantime, the world’s energy demand is continuously increasing; thus, making clean energy generation of paramount importance. Photovoltaics is at the forefront of this clean energy movement with organic photovoltaics emerging as a promising candidate to address this issue and meet this demand. Having said that, organic semiconductors came a long way from fullerene-based solar cells to state-of-the-art non-fullerene acceptor-based (NFAs) cells that show promise as next-generation photovoltaic devices. Organic solar cells employing such materials have already surpassed 20% in power conversion efficiencies (PCEs) after less than a decade of research, excluding initial attempts to outclass their fullerene counterparts in the 90s.[1] Much is done to realize high PCEs by the research community; nonetheless, the urgency to unravel the underlying mechanisms crippling the stability of the photovoltaic devices based on such material systems picked up only in recent years.

Even so, a thorough understanding or a guideline is still lacking to tackle this limitation once and for all. In this work, we implement a novel strategy, which we refer to as donor dilution, to improve the operational and outdoor stability of NFA-based systems in an archetypal donor:acceptor blend, namely, PTB7-Th:IEICO-4F.[2,3] Organic solar cells exploiting this material system with decreasing polymer content in the polymer:small molecule blend progressively perform superior both in operational and outdoor stability tests. Unencapsulated donor-diluted devices retain 85% of their initial PCE under continuous 1 sun equivalent metal halide lamp illumination at 40⁰C after 1000 hours. Furthermore, the in-house developed encapsulation procedure enables these devices to endure the harsh outdoor conditions (~23⁰C and ~64% relative humidity on average throughout winter) for an extended period in the Kingdom of Saudi Arabia.

These findings underpin the universal effort to improve the organic photovoltaic device stability, provide a practical strategy as a proof-of-concept to enhance the lifetime of organic photovoltaic devices, and bring this technology a step closer to commercialization. Future work consists of further examination of the universality of the donor dilution approach and is underway by testing various donor:acceptor blends with different classes of non-fullerene acceptors, such as state-of-the-art Y-series molecules.

With the advent of recent state-of-the-art blend materials, organic solar cells have become competitive to established technologies by achieving power conversion efficiencies of more than 18~\%. On the path to commercialization, device stability is the next challenge needing to be overcome. We researched the degradation of inverted devices of the high-performance material system PM6:Y6 and found two distinct degradation pathways. While the first one is marked by a reduction of short-circuit current and requires presence of both illumination and oxygen, we focused our research on the second one, which is induced thermally and shows losses of open-circuit voltage and fill factor. We find that, while bulk and interface properties remain overall stable, defect state formation is the primary cause for thermal degradation. The increased trap density reduces charge carrier mobility and leads to increased non-radiative recombination limiting the open-circuit voltage. In addition, we find an aging-induced transport resistance to be the cause for the reduced fill factor. Our findings show that device stability could be increased by suppressing trap formation.

Recently, organic photovoltaics (OPV) achieved efficiencies close to 20 % under AM1.5G. [1] In parallel OPV for harvesting artificial indoor light approaching efficiencies close to 30 % under 1000 lux warm white LED light. [2] In both cases the development of new donor and acceptor molecules as well as new interlayer materials was in the focus of interest, while the stability of OPV devices has drawn less attention. However, together with high performance the long-term stability of solar cells and modules is one of the key factors for a successful market entry. For indoor OPV the requirements are typically less harsh: much lower irradiation (UV light free), moderate and relatively constant temperature and moisture. Despite, the intrinsic stability of the materials, the stability of the full cell stack and the packaging have to be investigated.

For rigid devices a glass/glass encapsulation is probably the best choice, for flexible photovoltaics however, other solutions have to be found. Challenging are high barrier properties against moisture and oxygen while maintaining high transmission rates, UV resistance and mechanical flexibility (bendability without delamination).

We present stability data for different organic absorber materials in indium tin oxide (ITO)-based and ITO-free cell architectures on rigid and flexible substrates. [3] Our main focus is the long-term stability of devices for indoor usage, thus we performed accelerated (light) aging experiments under typical and elevated indoor light conditions.

For PV-X plus based ITO-free rigid cells with an efficiency of 17% under 500 lux, we could achieve remarkable stability with no losses observedduring more than 3000 h at 50.000 lux. This photon dose corresponds to almost 50 years at 500 lux which is much more than required for typical internet-of-things purposes. For flexible ITO-free devices promising preliminary data is obtained.

While elevated temperatures are typically not present in indoor PV, however for lamination processes temperatures of about 120 °C for shorter periods are required. We performed detailed analysis of the processes involved during thermal aging.

Organic solar cells (OSCs) have attracted great attention owing to their unique advantages such as lightweight, flexibility and solution processability for further commercial applications[1]. In recent years, the efficiency of OSC has grown rapidly with the help of burgeoning Y-series electron acceptors[2]. Benefits from structural modification, well-adjusted energy levels, absorption property, crystallinity and mobility have all attributed to the efficiency enhancement[2]. However, as another critical factor for commercialization, stability, especially photostability change generated from such structural modification remains elusive. To unveil the relationship, four Y-series acceptors with different terminal groups, internal and outer alkyl chains were selected to compare the photo-induced degradation from both device and structural perspectives. Significant increased trap-assisted recombination was observed after continuous illumination, which could be related to the structure change of the acceptors exhibited in NMR and Raman spectra. The energy barriers of happening the light-triggered reactions on acceptors are further evaluated through density functional theory (DFT) calculation. As a result, acceptors with fluorinated terminal groups and long internal alkyl chains will enhance the photostability of devices. This trend is further shown on additional two Y-series acceptors based devices. Finally, encapsulated devices with Y-series acceptors are tested under real-world conditions, showing a similar conclusion as photostability results. Our results provide an in-depth understanding of the light-soaking and outdoor stability of Y-series acceptors, and important guidelines for the designation acceptor materials to achieve stable OSCs.

With the increasing efficiency of organic solar cells, it is now imperative to focus investigation on the improvement of device stability and the understanding of the degradation mechanisms. It is known that the interface between the active layer and the charge transport materials, such as metal-oxides, can strongly influence the device stability, and many passivation strategies have been reported recently [1]. Herein, we have identified a major source of instability when using nanoparticles of SnO₂, which is determined by the presence of ionic species at the nanoparticle’s surface. The induced degradation mechanism is unveiled by means of photophysical and electrical measurements. Furthermore, we discover that with a very simple washing of the metal oxide nanoparticle layer we are able to reduce the device degradation. After the washing procedure, several non-fullerene acceptor based solar cells show a boost of the T80 device lifetime under illumination up to hundreds of hours.

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. Beyond the well-established characteristics of regular impedance arcs, we address the appearance of inductor effect at high voltage in perovskite solar cell. We present a physical model in terms of delayed recombination current that explains the evolution of impedance spectra and the evolution of current-voltage curves. A multitude of chemical, biological, and material systems present an inductive behavior that is not electromagnetic in origin. Here, it is 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. We apply the dynamic picture based on a few neuron-like equations to the characterization of halide perovskite memristors. It has a resistance that depends on the history of the system, and the states can be switched by applied voltage. Memristors show an intense hysteresis that can be characterized in terms of the emergence of inductive components.^1-5

(1) Bisquert, J.; Guerrero, A. Dynamic Instability and Time Domain Response of a Model Halide Perovskite Memristor for Artificial Neurons, Jounal of Physical Chemistry Letters 2022, 13, 3789-3795.

(2) Bisquert, J.; Guerrero, A. Chemical Inductor, J. Am. Chem. Soc. 2022, 10.1021/jacs.1022c00777.

(3) Berruet, M.; Pérez-Martínez, J. C.; Romero, B.; Gonzales, C.; Al-Mayouf, A. M.; Guerrero, A.; Bisquert, J. Physical model for the current-voltage hysteresis and impedance of halide perovskite memristors, ACS Energy Lett. 2022, 7, 1214–1222.

(4) Bisquert, J.; Guerrero, A.; Gonzales, C. Theory of Hysteresis in Halide Perovskites by Integration of the Equivalent Circuit, ACS Phys. Chem Au 2021, 1, 25-44.

(5) Guerrero, A.; Bisquert, J.; Garcia-Belmonte, G. Impedance spectroscopy of metal halide perovskite solar cells from the perspective of equivalent circuits, Chemical Reviews 2021, 121, 14430–14484.

The use of layered, 2D perovskites can greatly improve the stability of metal halide perovskite thin films and devices. However, the charge carrier transport properties in layered perovskites are still not fully understood. We investigated the sum of the electron and hole mobilities (Σμ) in thin films of the 2D perovskite PEA₂PbI₄, through transient electronically contacted nanosecond-to-millisecond photoconductivity measurements, which are sensitive to long-time, long-range (micrometer length scale) transport processes. After careful analysis, accounting for both early-time recombination and the evolution of the exciton-to-free-carrier population, a long-range mobility of 8.0 +/− 0.6 cm² (V s)^–1, which is ten times greater than the long-range mobility of a comparable 3D material FA_0.9Cs_0.1PbI₃ is determined. These values are compared to ultra-fast transient time-resolved THz photoconductivity measurements, which are sensitive to early-time, shorter-range (tens of nm length scale) mobilities. Mobilities of 8 and 45 cm² (V s)^–1 in the case of the PEA₂PbI₄ and FA_0.9Cs_0.1PbI₃, respectively, are obtained. This previously unreported concurrence between the long-range and short-range mobility in a 2D material indicates that the polycrystalline thin films already have single-crystal-like qualities. Hence, their fundamental charge carrier transport properties should aid device performance, while keeping the benefits for stability as well.

Adapted from [1]

Abstract: With the emergence of high-performance nonfullerene electron acceptors (NFA), organic solar cells (OSCs) are particularly appealing as their power conversion efficiencies (PCEs) approaching 20%. Identifying and eliminating the sources of degradation in photovoltaic devices is as crucial as improving the PCEs from the manufacturing and commercialization point of view. In this talk, I will show the strategies we developed to improve the thermal stability of NFA OSCs. Further, I will share the photo degradation behavior of the state-of-the-art NFA OSCs in both indoor and outdoor conditions with multimodal characterizations and in-operando transient electro-optical spectroscopy. Their outdoor operational stability will be assessed under real-world conditions (in a hot and sunny climate in Saudi Arabia). Finally, I present a comparison of indoor and outdoor performance of Y‑series OSCs to demonstrate the importance of materials stability and device longevity for balancing the efficiency, stability, and cost potential of organic photovoltaics in real-world outdoor applications.

OPV cells have a proven efficiency of over 18 % while OPV modules have a proven record efficiency of 13.5 %. Both values are still increasing towards > 20 % for small area cells and > 15 % for large scale modules. With these performance values, OPV is reaching out to applications that are going beyond the typical niche markets. The first generation of commercially available OPV modules shows lifetimes in the order of 5 years and more under outdoor conditions. Independent of the application, the lifetime limitations of organic solar cells are not fully understood. Few publications reported operational lifetimes of over 25000 hrs. Organic solar cell materials being stable against photooxidation were as well reported, and, most recently, we demonstrated solar cells that canbe operated in water and under 1 sun for hundreds of hours – unpackaged. However, all these “best you can do” lifetime values are reported for different combinations of materials and interface systems.

This talk will discuss a degradation mechanism that has been overlooked so far but probably is decisive to understand for longlived operation. The community is well aware of the impact of the light source on the degradation mechanism. Blocking the UV part of the solar spectrum till 380 nm became an unwritten law for outdoor as well as indoor testing. However, one has to critically ask whether a 380 nm cutoff is indeed sufficient to stabilize a solar cell. We have investigated degradation as a function of the spectral distribution, and found degradation mechanisms that have not been reported yet. Most specifically, the combination of the interface material, the semiconductor and the spectrum of the light source dominate degradation kinetics. The spectral excitation spectrum for degradation does not coincide with the EQE of the solar cell, indicating interface induced mechanisms. All these findings indicate that OPV has to overcome another hurdle before further improving the operational stability.

The modest stability of perovskite solar cells (PSCs) in ambient conditions represents a major impediment to their widespread commercialization. The interface between the hole-transport material (HTM) and perovskite is known to significantly affect the stability of PSCs.[1] Recently, the idea of combining the HTM function with the passivation of the perovskite surface via ad-hoc chemical interaction between the HTM and perovskite has been successfully exploited.[2] As a result, these HTMs anchored to the perovskite surface led to compact and ordered interfaces and to PSCs with a remarkable increase in both the efficiency and stability. In this talk, I will summarize key recent examples reported by us of low-cost HTM designs that can effectively interact with the surface of a triple cation (CsFAMA) perovskite promoting the interfacial charge transfer dynamics and the stability of PSCs. These examples include halogen-bonded[3], sulfonated[4], hydrogen-bonded, and fluorene-based HTM molecular designs.

Furthermore, I will highlight the doping engineering of traditional HTMs (e.g., Spiro-OMeTAD) as an additional valid strategy when aiming at competitive long-term stability. When the widely used hygroscopic dopants (e.g., LiFTSI) of Spiro-OMeTAD are replaced with a single molecular dopant, i.e., F4-TCNQ, a remarkable stability of mesoporous PSCs in air is achieved[5]. We demonstrate that the high uniformity of F4-TCNQ doping in Spiro-OMeTAD and reduced dopant aggregation and dopant migration towards the anode are the main reasons for the increased stability of PSCs in air, beyond the well-known hydrophobic protection of the perovskite induced by the F4-TCNQ dopant.

Quantifying the role of extrinsic (humidity and oxygen) and intrinsic (light, bias, and temperature) environmental stressors on the degradation of halide perovskites is key for the further development of stable optoelectronic devices. In this talk, I will present a suite of optical and electrical complementary tools that provide a detailed description of the dynamic responses within these materials, from the macro to the nanoscale. At the macroscopic scale, we unravel the influence of relative humidity on charge carrier radiative recombination in CsxFA1−xPb(IyBr1−y)3 perovskites through in situ PL, where we temporally and spectrally measure light emission within loops of critical relative humidity (rH) levels. Then, we apply machine learning algorithms to forecast the material optical response once exposed to distinct temperature and humidity levels. At the nanoscale, we realize state-of-the-art scanning probe microscopy methods to image and quantify ion motion within and between grains, as a function of illumination and relative humidity. Combined, the macro- and nanoscale environmental measurements performed provide an ample framework for tracking, in real-time, the relevant changes that can lead to degradation. Additionally, they exemplify trustworthy diagnosis tools that can be expanded to any perovskite chemical composition.      

Record efficiencies of perovskite solar cells (PSCs) are obtained with spiro-OMeTAD as the hole transport layer. Spiro-OMeTAD is conventionally doped by hygroscopic lithium salts with the assistance of volatile 4-tert-butylpyridine, which, however, brings a time-consuming doping process as well as poor device stability. We successfully develop an instantly efficient and clean doping strategy to replace the conventional spiro-OMeTAD doping. As demonstrated by experimental and theoretical investigations, the radical dopant leads to significant increase of the conductivity through efficient hole polarons generation. The ionic salts can further modulate the work function with negligible effects on the film conductivity, critical for reaching optimal V_OC values by a favorable energetic level alignment. Spiro-OMeTAD based on our new doping strategy enables PSCs with a high power conversion efficiency over 25% and an excellent stability against moisture, heat and illumination. Our findings pave the way for achieving PSCs with high efficiencies and excellent stability at the same time. In addition, our doping strategy goes beyond traditional organic semiconductor doping, providing new understanding of organic doping mechanisms which can inspire further optimizations of different optoelectronic devices.

Halide perovskites quickly overrun research activities in new materials for cost-effective and high-efficiency photovoltaic technologies. Since the first demonstration from Kojima and co-workers in 2009, several perovskite-based solar cells have been reported and certified with rapidly improving power conversion efficiency. Recent reports demonstrated that perovskites could compete with the most efficient photovoltaic materials. At the same time, they still allow processing from solutions as a potential advantage to delivering a cost-effective solar technology.

The most stable and efficient perovskite contains lead, among the most toxic elements on earth. The lead from perovskite is more dangerous than other lead contaminants since it is absorbed rapidly from plants, and thus passed to the food chain, if dispersed in the environment. Lead-free alternatives have been reported with impressive progress in power conversion efficiency for tin-based (lead-free) perovskites. However, the stability of tin-based perovskite solar cells is still unexplored. In the present talk, we will focus on the strength of tin-based (lead-free) perovskite solar cells.

Formamidinium lead iodide (FAPbI3) is the 3D lead perovskite with the highest theoretical efficiency due to its narrower band gap in comparison with Cs or methylammonium perovskites. Unfortunately, the black phase allowing this narrow bandgap is not the most stable one at room temperature. Here, we shown as the fabrication conditions and the interaction with semiconductor quantum dots can boost significantly the stability of FAPbI3. The interaction of halide perovskite and colloidal semiconductor nanostructures (quantum dots or nanoplatelets) can produce interesting synergistic interactions. We show that the interaction of PbS quantum dots and nanoplatelets can produce the stabilization of FAPbI3and FACsPbI3 perovskite black phase and also the increase of the efficiency, stability and reproducibility of the photovoltaic devices prepared with these halide perovskites. Incorporation of PbS QDs allows the dramatic decrease of the annealing temperature for the formation of black FAPbI3 phase perovskite thin film, from the 170ºC required without QDs to 85ºC when QDs are present. In addition, stability of these systems including embedded nanostructures is extended not just for samples prepared in the glove box but fabricated in ambient conditions. In addition, fabrication of devices under air condition can further increase the stability of FAPbI3. We have also verified the synergic combination of different additives for a significant increase of device stability. These result points the interest of Perovskite-Quantum Dot Nanocomposites, for further development of advanced optoelectronic devices. Stabilization of tin based perovskite solar cells will be analyzed. Eventually, stabilization of halide perovskite nanoparticles is a necessary step for the development of high performance Halide Perovskite LEDs. Control of post synthetic washing processes allows the preparation of LEDs with enhanced performance and stability.

Perovskite solar cells (PSCs) have created much excitement in the past years and attract spotlight attention. This talk will provide an overview of the reasons for this development highlighting the historic development as well as the specific material properties that make perovskites so attractive for the research community.[1-3]

The current challenges are exemplified using a high-performance model system for PSCs (multication Rb, Cs, methylammonium (MA), formamidinium (FA) perovskites).[2,3] The triple cation (Cs, MA, FA) achieves high performances due to suppressed phase impurities. This results in more robust materials enabling breakthrough reproducibility.

Through multication engineering, usually not-considered alakali metals, such as Rb, can be studied[5] resulting in one of the highest voltages compared to the bandgap. Polymer-coated cells maintained 95% of their initial performance at elevated temperature for 500 hours under working conditions, a crucial step towards industrialisation of PSCs.

To explore the theme of multicomponent perovskites further, molecular cations were re-evaluated using a globularity factor. With this, we calculated that ethylammonium (EA) has been misclassified as too large. Using the multication strategy, we studied an EA-containing compound that yielded a high open-circuit voltage of 1.59 V. Moreover, using EA, we demonstrate a continuous fine-tuning for perovskites in the "green gap" which is relevant for lasers and display technology. [6]

The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years, also in areas beyond photovoltaics, for example for medical scintillation detectors.[6,7]

Perovskite semiconductors differ from most inorganic and organic semiconductors due to the presence of mobile ions in the active layer. Although the phenomenon is intensively investigated, important questions remain, such as the dominant species and overall density of the mobile ions and their exact impact on the device stability and steady-state power conversion efficiency (PCE). In this talk, we propose a simple method to estimate the efficiency loss due to mobile ions via “fast-hysteresis” measurements by preventing the perturbation of mobile ions out of their equilibrium position at fast scan speeds (1000 V/s) and choosing a suitable prebias condition.[1,2] The ”ion-free” PCE of fresh pin-type cells with low levels of apparent hysteresis at typical scan speeds (100 mV/s) is between 1.5-3% higher than the steady-state PCE. However, with device aging, e.g. under elevated temperatures or continuous light illumination, the ionic losses increase substantially. This represents the dominant degradation loss of a triple cation based pin-type perovskite cell. The enhanced ionic losses with aging are also accompanied by a significant increase of the mobile ion density. Finally, we will discuss how the stoichiometry and in particular the PbI₂ excess influence the ionic losses and how these losses vary with the charge transport properties. Finally, an outlook is presented on how these losses affect the degradation of solution processed perovskite based tandem cells. Overall, the proposed methods to quantify the ion-induced field screening and PCE losses allow for a better understanding of several key phenomena in perovskite solar cells and shed light on the complex device degradation process.

References:

1 Le Corre, V. M. et al. Sol. RRL 2100772 (2022), DOI: 10.1002/solr.202100772

2 Thiesbrummel, J. et al. Adv. Energy Mater. 11, 2101447 (2021), DOI: 10.1002/aenm.202101447

Halide perovskite semiconductors are generating enormous excitement for next-generation photovoltaics in both single junction and tandem device forms. Their performance is striking given that these absorber layers are rife with heterogeneity on the nanoscale in their structural, chemical and optoelectronic properties. Here, I will describe how this heterogeneity impacts not just performance but also operational stability of halide perovskite solar cells. Nanoscale trap clusters are present in the absorber layers that act as unwanted non-radiative recombination sites [1] and sites of instability [2] -- and thus must be removed. These trap clusters relate to phase impurities [3] and form in regions that are not stabilised by octahedral tilt [4]. Furthermore, interfaces impact local recombination and instability pathways, both on a local scale and on a macroscopic scale. I will outline how appropriate passivation and compositional control can mitigate these issues, and provide a pathway to realise highly efficient and stable perovskite solar cells.

In recent years perovskite solar cells (PSCs) have attracted attention due to their promising electro-optical properties leading to high power conversion efficiencies of 25.7% in single-junction devices [1]. However, the stability of PSCs is still limited and thus the bottleneck on their way to commercialisation. One of the main causes of the poor stability is the presence of mobile ions in the perovskite absorber, facilitated by their soft ionic crystal structure. Interestingly, the density of mobile ions reported in literature spans multiple orders of magnitude reaching from 10¹⁵ - 10¹⁹ cm^-3 [2, 3]. One technique that has been previously applied to quantify the activation energy, diffusion coefficient, and density of mobile ions is transient ion drift (TID). In TID, the modulation of the device capacitance due to redistribution of mobile ions after applying a voltage pulse is measured. However, so far, the interpretation of TID was based on an analytical model that assumed a p-doped perovskite and did only account for the drift but not for the diffusion of mobile ions [3, 4]. In this work, we provide an updated interpretation of TID, also accounting for low doping densities in the perovskite, and show what it can teach us about the activation energy, diffusion coefficient, and density of mobile ions.

Achieving long-term stability of perovskite solar cells is arguably the most important challenge required to enable widespread commercialization. Understanding the perovskite crystallization process and its direct impact on device stability is critical to achieve this goal. Surprisingly, we find that 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 accurately 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)_yCs_1‑yPb(I_xBr_1-x)₃ perovskite system. A population of encapsulated devices showed significantly improved operational stability, with a median T₈₀ lifetime, for the steady-state PCE, of 1190 hours and a champion device showed a T₈₀ of 1410 hours, under simulated sun light at 65 °C in air, under open‑circuit conditions. Our work introduces an innovative 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 in order to achieve long-term operational stability of perovskite optoelectronic devices.

Here we report a detailed study at the interface for dipole molecule treated inorganic perovskite CsPbI3 based solar cells. An energy level band bending at the passivated CsPbI3 perovskite and hole selective contact interface was introduced by a dipole molecule, trioctylphosphine oxide. X-ray photoelectron Spectroscopy (XPS) study shows binding energy shifts towards lower value, which indicating the acceptance of electrons from perovskite to TOPO, similar to the role as a Lewis acid. The upward shifts in work function in TOPO-treated samples, as compared to the control sample, has been revealed by Ultraviolet Photoelectron Spectroscopy (UPS) and Kelvin Probe measurements independently. Steady-state Photoluminescence (PL) and Time-resolved Photoluminescence Spectroscopy (trPL) measurements show that the dipole molecule contributes marginally to defects passivation of perovskite films. Charge dynamics at the interface was characterized by using transient Surface Photovoltage Spectroscopy (trSPV) at varied photon energies. The charge selectivity at the interface is increased by over six times for TOPO treated samples. Finally, a champion efficiency of 18.7% has been achieved for CsPbI3 perovskite solar cells with an increase of 100 mV in average in open-circuit voltage. Influence of dipole treatment on long-term stability of these devices were characterized by maximum power point tracking of over 500 hours under 1 sun intensity at room temperature with cycled illumination. It has been seen that due to better energy alignment at the interface, dipole molecule treated samples shows a better stability as compared to control CsPbI3 based devices.

A striking difference between lead-halide perovskites and conventional semiconductors (e.g., silicon, III-V’s) is the dual ionic-covalent bond nature within the anionic inorganic framework. This weaker bond nature compared to the purely covalent bond of conventional semiconductors results in the mechanically soft and dynamically disordered perovskite lattice whose alteration affects the optoelectronic properties and the stability of these solids. Thus, metal-halide perovskites are particularly sensitive to variations in composition, fabrication and external stimuli that can induce strain in the material, interesting for developing next-generation memory devices, sensors, and energy-storage technologies. Layered Dion–Jacobson (DJ) and Ruddlesden–Popper (RP) hybrid perovskites are promising materials for optoelectronic applications due to their modular structure. To fully exploit their functionality, mechanical stimuli can be used to control their properties without changing the composition. However, the responsiveness of these systems to pressure compatible with practical applications and industrial fabrication methods (<1 GPa) remains unexploited. Hydrostatic pressure is used to investigate the structure–property relationships in representative iodide and bromide DJ and RP 2D perovskites based on 1,4-phenylenedimethylammonium (PDMA) and benzylammonium (BzA) spacers in the 0–0.35 GPa pressure range. Pressure-dependent X-ray scattering measurements reveal that lattices of these compositions monotonically shrink and density functional theory calculations provide insights into the structural changes within the organic spacer layer. These structural changes affect the optical properties; the most significant shift in the optical absorption is observed in (BzA)2PbBr4 under 0.35 GPa pressure, which is attributed to an isostructural phase transition. Surprisingly, the RP and DJ perovskites behave similarly under pressure, despite the different binding modes of the spacer molecules. This study provides important insights into how the manipulation of the crystal structure affects the optoelectronic properties of such materials, whereas the reversibility of their response expands the perspectives for future applications in e.g., sensors, actuators.