A Closer Look into Two-Step Perovskite Conversion with X-ray Scattering

Microstrain and Crystal Orientation Variation within Naked Triple-Cation Mixed Halide Perovskites under Heat, UV, and Visible Light Exposure

The instability of perovskite absorbers under various environmental stressors is the most significant obstacle to widespread commercialization of perovskite solar cells. Herein, we study the evolution of crystal structure and microstrain present in naked triple-cation mixed CsMAFA-based perovskite films under heat, UV, and visible light (1 Sun) conditions by grazing-incidence wide-angle X-ray scattering (GIWAXS). We find that the microstrain is gradient distributed along the surface normal of the films, decreasing from the upper surface to regions deeper within the film. Moreover, heat, UV, and visible light treatments do not interfere with the crystalline orientations within annealed polycrystalline films. However, when subjected to heat, the naked perovskite films exhibit a rapid component decomposition, induced by phase separation and ion migration. Conversely, under exposure to UV and 1 Sun light soaking, the naked perovskite films undergo a self-optimization structure evolution during degradation and develop into smoother films with reduced surface potential fluctuations.

Morphological Insights into the Degradation of Perovskite Solar Cells under Light and Humidity

Perovskite solar cells (PSCs) have achieved competitive power conversion efficiencies compared with established solar cell technologies. However, their operational stability under different external stimuli is limited, and the underlying mechanisms are not fully understood. In particular, an understanding of degradation mechanisms from a morphology perspective during device operation is missing. Herein, we investigate the operational stability of PSCs with CsI bulk modification and a CsI-modified buried interface under AM 1.5G illumination and 75 ± 5% relative humidity, respectively, and concomitantly probe the morphology evolution with grazing-incidence small-angle X-ray scattering. We find that volume expansion within perovskite grains, induced by water incorporation, initiates the degradation of PSCs under light and humidity and leads to the degradation of device performance, in particular, the fill factor and short-circuit current. However, PSCs with modified buried interface degrade faster, which is ascribed to grain fragmentation and increased grain boundaries. In addition, we reveal a slight lattice expansion and PL redshifts in both PSCs after exposure to light and humidity. Our detailed insights from a buried microstructure perspective on the degradation mechanisms under light and humidity are essential for extending the operational stability of PSCs.

Water-Regulated Lead Halide Perovskites Precursor Solution: Perovskite Structure Making and Breaking

Identifying the impact of water on iodoplumbate complexes in various solutions is essential for linking the coordination environment of the perovskite precursor to its final perovskite solar cell (PSC) properties. In this study, we propose a digital twin approach based on X-ray absorption fine structure and molecular dynamic simulation to investigate the structure evolution of iodoplumbate complexes in precursor solutions as a function of storage time under a constant humidity environment. A full picture about what water does in the perovskite formation process is brought out, and the "making and breaking" role of water molecules is uncovered to link the structure of iodoplumbate complexes to its final properties. This study sheds light on a full picture about what water does in the perovskite formation process and the role of water, which will lead to developing water-involved strategies for consistent PSC fabrication under ambient conditions.

Mapping structure heterogeneities and visualizing moisture degradation of perovskite films with nano-focus WAXS

Extensive attention has focused on the structure optimization of perovskites, whereas rare research has mapped the structure heterogeneity within mixed hybrid perovskite films. Overlooked aspects include material and structure variations as a function of depth. These depth-dependent local structure heterogeneities dictate their long-term stabilities and efficiencies. Here, we use a nano-focused wide-angle X-ray scattering method for the mapping of film heterogeneities over several micrometers across lateral and vertical directions. The relative variations of characteristic perovskite peak positions show that the top film region bears the tensile strain. Through a texture orientation map of the perovskite (100) peak, we find that the perovskite grains deposited by sequential spray-coating grow along the vertical direction. Moreover, we investigate the moisture-induced degradation products in the perovskite film, and the underlying mechanism for its structure-dependent degradation. The moisture degradation along the lateral direction primarily initiates at the perovskite-air interface and grain boundaries. The tensile strain on the top surface has a profound influence on the moisture degradation.

Understanding the correlation between moisture degradation and structural features of perovskite films is essential to improve their stability. Here, the authors apply nano-focused wide-angle X-ray scattering to map the heterogeneities over several micrometers across lateral and vertical directions.

Growth and Degradation Kinetics of Organic-Inorganic Hybrid Perovskite Films Determined by In Situ Grazing-Incidence X-Ray Scattering Techniques

Organic-inorganic halide perovskite (OIHP) solar cells hold a great promise for commercial breakthrough since their power conversion efficiency has been pushed beyond the mark of 25%, making them capable of competing with traditional crystalline silicon solar cells. The key to achieve efficient and stable perovskite solar cells is inherently related to the film morphology. The understanding of the kinetic processes of film formation and degradation opens up possibilities to tailor the film morphology via the regulation of precursor and processing parameters. In situ grazing-incidence X-ray scattering (GIXS) techniques allow for tracking the morphology evolution of thin films at different length scales and with high temporal resolution. In this review, the selected examples for application of in situ grazing-incidence wide-angle X-ray scattering and grazing-incidence small-angle X-ray scattering techniques to the growth and stability of OIHPs are summarized after a brief introduction to both techniques, highlighting particularly the morphological evolution of perovskite films over time. Then the correlated mathematical models are reviewed to give a toolbox for analyzing the mechanisms of film formation and degradation. Thus, an overview on the in situ GIXS methods is linked to the research of OIHP kinetics.

Controlled Growth of Porous InBr3: PbBr2 Film for Preparation of CsPbBr3 in Carbon-Based Planar Perovskite Solar Cells

Due to the low solubility of CsBr in organic solvents, the CsPbBr₃ film prepared by the multi-step method has holes and insufficient thickness, and the light absorption capacity and current density of the perovskite film hinder the further improvement in the power conversion efficiency (PCE) of CsPbBr₃ solar cells. In this study, we introduced InBr₃ into the PbBr₂ precursor solution and adjusted the concentration of PbBr₂, successfully prepared PbBr₂ with a porous structure on the compact TiO₂ (c-TiO₂) substrate to ensure that it fully reacted with CsBr, and obtained the planar carbon-based CsPbBr₃ solar cells with high-quality perovskite film. The results reveal that the porous PbBr₂ structure and the increasing PbBr₂ concentration are beneficial to increase the thickness of the CsPbBr₃ films, optimize the surface morphology, and significantly enhance the light absorption capacity. Finally, the PCE of the CsPbBr₃ solar cells obtained after conditions optimization was 5.76%.

Flexible and Filter-Free Color-Imaging Sensors with Multicomponent Perovskites Deposited Using Enhanced Vapor Technology

Halide perovskites are promising photoactive materials for filter-free color-imaging sensors owing to their outstanding optoelectronic properties, tunable bandgaps, and suitability for large-scale fabrication. However, producing patterned perovskite films of sufficiently high quality for such applications poses a challenge for existing fabrication methods: using solution processes to prepare patterned perovskite films is complicated, while evaporation methods often result in perovskite photodetectors with limited performance. In this paper, the authors report the development of an improved evaporation method in which substrates are treated with a brominated (3-aminopropyl) triethoxysilane self-assembled monolayer to improve the properties of the patterned perovskite films. The resulting perovskite photodetectors exhibit significantly enhanced photosensitivity and long-term stability (exceeding 100 days). Additionally, the polymer substrates facilitate device flexibility. Finally, perovskites comprising three different halide components, each with a different bandgap, are integrated into a device array using the developed evaporation technology, yielding sensors that enable the discrimination of red, green, and blue colors. Thus, the flexible photosensor arrays can generate colorful images closely resembling perceived patterns, demonstrating reliable color imaging. Therefore, this study successfully demonstrates filter-free color-imaging by integrating high-performance patterned and multicomponent perovskite photodetectors, highlighting the potential of such detectors for advanced optoelectronic applications, including hyperspectral imaging.

Curing the fundamental issue of impurity phases in two-step solution-processed CsPbBr3 perovskite films

Inorganic lead halide perovskite CsPbBr₃ offers attractive photophysical properties and phase stability for high-performance optoelectronic devices. However, CsPbBr₃ films produced by the classic solution-based two-step method are always accompanied with impurity phases of CsPb₂Br₅ and Cs₄PbBr₆, which represents a major efficiency-limiting factor for future advances of CsPbBr₃-based devices. The challenge lies in the complexity of the Cs-Pb-Br phase system, requiring both spatially and temporally precise control of the precursor stoichiometry during solution-phase growth of CsPbBr₃ films. By adopting 2-methoxyethanol as the solution conversion medium instead of commonly applied methanol, the reaction between CsBr and PbBr₂ can be finely controlled to yield single phase CsPbBr₃ films within a few minutes; extending the solution-conversion step to 24 h does not alter the phase purity of resulting CsPbBr₃ films. The present work paves the way to regulate the crystal growth behaviors of two-step solution-processed CsPbBr₃ films by simple solvent engineering.

Nanoscale crystallization of a low band gap polymer in printed titania mesopores

The crystallization behavior of the low band gap polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3'''-di(2-octyldodecyl)2,2';5',2'';5'',2'''-quaterthiophen-5,5'''-diyl)] (PffBT4T-2OD) induced in printed mesoporous titania films with different pore sizes is studied to optimize the crystal orientation for an application in hybrid solar cells. The correlation between the crystal structure of PffBT4T-2OD and the titania pore size is investigated with a combination of grazing incidence wide-angle X-ray scattering (GIWAXS) and grazing incidence small-angle X-ray scattering (GISAXS). For comparison, poly(3-hexylthiophene) (P3HT) is also backfilled into the same four types of printed titania mesoporous scaffolds. Both, lattice constants and crystal sizes of edge-on oriented P3HT crystals decrease with increasing the titania pore size. Similarly and irrespective of the crystal orientation, a denser stacking of PffBT4T-2OD chains is found for larger pore sizes of the titania matrix. For an edge-on orientation, also bigger PffBT4T-2OD crystals are favorably formed in smaller pores, whereas for a face-on orientation, PffBT4T-2OD crystals increase with increasing size of the titania pores. Thus, the best ratio of face-on to edge-on crystals for PffBT4T-2OD is obtained through infiltration into large titania pores.

A Review of the Role of Solvents in Formation of High-Quality Solution-Processed Perovskite Films

Recently, perovskite solar cells have attracted great attention because of their outstanding photovoltaic performance and ease of fabrication. High-quality perovskite films hold a key in getting highly efficient perovskite solar cells. Solution-processed fabrication technique is the most widely adopted for preparing perovskite films because of its low cost. In the solution-proceed perovskite films, solvents not only play the role of dissolving the solute but also participate in the crystallization of perovskite. In the one-step method, solvents play key roles in controlling morphology, widening process window, and achieving room-temperature crystallization of perovskite films. In addition, the solvents play important roles in controlling the nuclei/growth, suppressing volume expansion during the two-step method. Especially, the solvent can induce grain coarsening during the annealing process. A deep understanding of the multiplicity of roles during the formation of perovskite films will help understand the formation mechanism of perovskite films. Here, a systematic review on the progress in fabrication of high-quality perovskite films by making use of solvent to control the crystallization is presented. Meanwhile, we elucidate the key roles of solvent in the fabrication of high-quality perovskite films.

Influence of Cl Incorporation in Perovskite Precursor on the Crystal Growth and Storage Stability of Perovskite Solar Cells

Solar cells based on organic-inorganic hybrid lead-halide perovskites are very promising because of their high performance and solution process feasibility. Elemental engineering on perovskite composition is a facile path to obtain high-quality crystals for efficient and stable solar cells. It was found that partially substituting I^- with Cl^- in the perovskite precursor promoted crystal growth, with the grain size larger than the layer thickness, and facilitated the generation of a self-passivation layer of PbI₂. Whereas the residual Cl^- ions were suspected to diffuse to the hole-transport layer consisting of ubiquitously spiro-OMeTAD, the formation of highly bounded ionic pairing of Cl^- with the oxidized state of spiro-OMeTAD led to insufficient charge extraction and severely reversible performance degradation. This issue was effectively alleviated upon Br^- doping owing to the generation of Pb-Br bonds in the lattice that strengthened the phase stability by improving the binding energy between each unit. The binary halide (Br^-/Cl^-)-doped perovskites resulted in a champion power conversion efficiency of 20.2% with improved long-term storage stability.

Luminescent Intermediates and Humidity-Dependent Room-Temperature Conversion of the MAPbI3 Perovskite Precursor

Preparation of metal-halide perovskites under room temperature attracts attention because of energy saving by removing thermal annealing. Room-temperature transformation of spin-cast wet films consisting of methylammonium (MA) iodide, PbI₂, and dimethylformamide toward solid MAPbI₃ perovskite proceeds via several intermediate crystalline states and is strongly dependent on ambient humidity. Light transmission and photoluminescence (PL) microscopy and spectroscopy were used to monitor the growth of crystals and transformation of their properties in time under nitrogen atmosphere at room temperature. Under low humidity, a highly luminescent intermediate phase with low absorption in the visible range appears, with the PL spectra composed of several bands in the range from 600 to 760 nm. We assign these bands to low-dimensional (nanocrystals and two-dimensional inclusions) MAPbI₃ intermediates, where the exciton confinement shifts the spectrum to higher energies in comparison with the bulk MAPbI₃. The intermediate levels of ambient humidity (10-50%) appear to catalyze the conversion of the intermediate phase to MAPbI₃. At a high ambient humidity (>80%), the initially formed MAPbI₃ is quickly transformed to the transparent hydrate phase of MAPbI₃. The role of ambient water catalyzing the material transformation by competing for Pb coordination with the solvent molecules is discussed.

In Situ Monitoring the Uptake of Moisture into Hybrid Perovskite Thin Films

Solution-processed hybrid perovskites are of great interest for use in photovoltaics. However, polycrystalline perovskite thin films show strong degradation in humid atmospheres, which poses an important challenge for large-scale market introduction. With in situ grazing incidence neutron scattering (GISANS) we analyzed water content, degradation products, and morphological changes during prolonged exposure to several humidity levels. In high humidity, the formation of metastable hydrate phases is accompanied by domain swelling, which transforms the faceted crystals to a round-washed, pebble-like form. The films incorporate much more water than is integrated into the hydrates, with smaller crystals being more affected, making the degradation strongly dependent on film morphology. Even at low humidity, water is adsorbed on the crystal surfaces without the formation of crystalline degradation products. Thus, although production in an ambient atmosphere is of interest for industrial production it might lead to long-term degradation without appropriate countermeasures like postproduction drying below 30% RH.

Top-Down Approaches Towards Single Crystal Perovskite Solar Cells

Solar cells employing hybrid perovskites have proven to be a serious contender versus established thin-film photovoltaic technologies. Typically, current photovoltaic devices are built up layer by layer from a transparent substrate (bottom-up approach), while the deposition of the perovskite layer itself comes with many challenges including the control of crystal size, nucleation density and growth rate. On the other hand, single crystals have been used with great success for studying the fundamental properties of this new class of optoelectronic materials. However, optoelectronic devices fabricated from single crystals often employ different materials than in their thin film counterparts. Here, we demonstrate various top-down approaches for low-temperature processed organic-inorganic metal halide perovskite single crystal devices. Our approach uses common and well-established material combinations that are often used in polycrystalline thin film devices. The use of a polymer bezel allows easier processing of small crystals and the fabrication of solution-processed, free-standing perovskite single crystal devices. All in all these approaches can supplement other measurements of more fundamental material properties often requiring perovskite single crystals by rendering a photovoltaic characterization possible on the very same crystal with comparable material combinations as in thin film devices.

Grain Boundaries Act as Solid Walls for Charge Carrier Diffusion in Large Crystal MAPI Thin Films

Micro- and nanocrystalline methylammonium lead iodide (MAPI)-based thin-film solar cells today reach power conversion efficiencies of over 20%. We investigate the impact of grain boundaries on charge carrier transport in large crystal MAPI thin films using time-resolved photoluminescence (PL) microscopy and numerical model calculations. Crystal sizes in the range of several tens of micrometers allow for the spatially and time resolved study of boundary effects. Whereas long-ranged diffusive charge carrier transport is observed within single crystals, no detectable diffusive transport occurs across grain boundaries. The observed PL transients are found to crucially depend on the microscopic geometry of the crystal and the point of observation. In particular, spatially restricted diffusion of charge carriers leads to slower PL decay near crystal edges as compared to the crystal center. In contrast to many reports in the literature, our experimental results show no quenching or additional loss channels due to grain boundaries for the studied material, which thus do not negatively affect the performance of the derived thin-film devices.

Distinguishing crystallization stages and their influence on quantum efficiency during perovskite solar cell formation in real-time

Relating crystallization of the absorber layer in a perovskite solar cell (PSC) to the device performance is a key challenge for the process development and in-depth understanding of these types of high efficient solar cells. A novel approach that enables real-time photo-physical and electrical characterization using a graphite-based PSC is introduced in this work. In our graphite-based PSC, the device architecture of porous monolithic contact layers creates the possibility to perform photovoltaic measurements while the perovskite crystallizes within this scaffold. The kinetics of crystallization in a solution based 2-step formation process has been analyzed by real-time measurement of the external photon to electron quantum efficiency as well as the photoluminescence emission spectra of the solar cell. With this method it was in particular possible to identify a previously overlooked crystallization stage during the formation of the perovskite absorber layer. This stage has significant influence on the development of the photocurrent, which is attributed to the formation of electrical pathways between the electron and hole contact, enabling efficient charge carrier extraction. We observe that in contrast to previously suggested models, the perovskite layer formation is indeed not complete with the end of crystal growth.

A PbI2-xClx seed layer for obtaining efficient planar-heterojunction perovskite solar cells via an interdiffusion process

Despite the previous reports on the fabrication of CH₃NH₃PbI_3-xCl_x films via sequential deposition, the positioning and formation of PbI₂ in MAPbI_3-xCl_x perovskite films made from the seed layer containing PbI₂ and PbCl₂ in different ratios have not yet been addressed. In this study, the PbI₂ content in a perovskite absorber layer is controlled by changing the PbCl₂ ratio in a PbI_2-xCl_x seed layer. The addition of PbCl₂ in the seed layer facilitates PbI₂ generation and affects the morphology of the perovskite film. By integrating a perovskite absorber via the PbI_2-xCl_x seed-layer into a solar cell, we investigated the effects of the correlation between the chlorine and PbI₂ contents on the device performance through intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy. Elemental depth profiling analyses confirm that not only was the formed PbI₂ preferentially located near the metal-oxide layer, but residual chlorine was adsorbed at the TiO₂ layer. Our findings demonstrate that the geometric features of the formed PbI₂ affected the perovskite solar cells according to the chlorine content, likely because of the elemental gradient induced by annealing. The PbI_2-xCl_x-derived planar-heterojunction perovskite solar cells exhibited maximum power-conversion efficiencies of 17.56% at reverse scan and 17.21% at forward scan, suppressed current density-voltage hysteresis, and good performance distributions.

In situ dynamic observations of perovskite crystallisation and microstructure evolution intermediated from [PbI6]4- cage nanoparticles

Hybrid lead halide perovskites have emerged as high-performance photovoltaic materials with their extraordinary optoelectronic properties. In particular, the remarkable device efficiency is strongly influenced by the perovskite crystallinity and the film morphology. Here, we investigate the perovskites crystallisation kinetics and growth mechanism in real time from liquid precursor continually to the final uniform film. We utilize some advanced in situ characterisation techniques including synchrotron-based grazing incident X-ray diffraction to observe crystal structure and chemical transition of perovskites. The nano-assemble model from perovskite intermediated [PbI₆]⁴⁻ cage nanoparticles to bulk polycrystals is proposed to understand perovskites formation at a molecular- or nano-level. A crystallisation-depletion mechanism is developed to elucidate the periodic crystallisation and the kinetically trapped morphology at a mesoscopic level. Based on these in situ dynamics studies, the whole process of the perovskites formation and transformation from the molecular to the microstructure over relevant temperature and time scales is successfully demonstrated.

The photovoltaic performances of perovskite materials are strongly influenced by their crystallinity and film morphology. Here, the authors investigate the formation and morphology evolution mechanisms of lead halide perovskites and reveal that bulk polycrystals grow from intermediate [PbI6]⁴⁻ cage nanoparticles.

Effect of the Microstructure of the Functional Layers on the Efficiency of Perovskite Solar Cells

The efficiencies of the hybrid organic-inorganic perovskite solar cells have been rapidly approaching the benchmarks held by the leading thin-film photovoltaic technologies. Arguably, one of the most important factors leading to this rapid advancement is the ability to manipulate the microstructure of the perovskite layer and the adjacent functional layers within the device. Here, an analysis of the nucleation and growth models relevant to the formation of perovskite films is provided, along with the effect of the perovskite microstructure (grain sizes and voids) on device performance. In addition, the effect of a compact or mesoporous electron-transport-layer (ETL) microstructure on the perovskite film formation and the optical/photoelectric properties at the ETL/perovskite interface are overviewed. Insight into the formation of the functional layers within a perovskite solar cell is provided, and potential avenues for further development of the perovskite microstructure are identified.

Gold and iodine diffusion in large area perovskite solar cells under illumination

Operational stability is the main issue hindering the commercialisation of perovskite solar cells. Here, a long term light soaking test was performed on large area hybrid halide perovskite solar cells to investigate the morphological and chemical changes associated with the degradation of photovoltaic performance occurring within the devices. Using Scanning Transmission Electron Microscopy (STEM) in conjunction with EDX analysis on device cross sections, we observe the formation of gold clusters in the perovskite active layer as well as in the TiO₂ mesoporous layer, and a severe degradation of the perovskite due to iodine migration into the hole transporter. All these phenomena are associated with a drastic drop of all the photovoltaic parameters. The use of advanced electron microscopy techniques and data processing provides new insights on the degradation pathways, directly correlating the nanoscale structure and chemistry to the macroscopic properties of hybrid perovskite devices.

Crystal Growth and Dissolution of Methylammonium Lead Iodide Perovskite in Sequential Deposition: Correlation between Morphology Evolution and Photovoltaic Performance

Crystal morphology and structure are important for improving the organic-inorganic lead halide perovskite semiconductor property in optoelectronic, electronic, and photovoltaic devices. In particular, crystal growth and dissolution are two major phenomena in determining the morphology of methylammonium lead iodide perovskite in the sequential deposition method for fabricating a perovskite solar cell. In this report, the effect of immersion time in the second step, i.e., methlyammonium iodide immersion in the morphological, structural, optical, and photovoltaic evolution, is extensively investigated. Supported by experimental evidence, a five-staged, time-dependent evolution of the morphology of methylammonium lead iodide perovskite crystals is established and is well connected to the photovoltaic performance. This result is beneficial for engineering optimal time for methylammonium iodide immersion and converging the solar cell performance in the sequential deposition route. Meanwhile, our result suggests that large, well-faceted methylammonium lead iodide perovskite single crystal may be incubated by solution process. This offers a low cost route for synthesizing perovskite single crystal.

Facet-Dependent Property of Sequentially Deposited Perovskite Thin Films: Chemical Origin and Self-Annihilation

Quantification of intergrain length scale properties of CH₃NH₃PbI₃ (MAPbI₃) can provide further understanding of material physics, leading to improved device performance. In this work, we noticed that two typical types of facets appear in sequential deposited perovskite (SDP) films: smooth and steplike morphologies. By mapping the surface potential as well as the photoluminescence (PL) peak position, we revealed the heterogeneity of SDP thin films that smooth facets are almost intrinsic with a PL peak at 775 nm, while the steplike facets are p-type-doped with 5-nm blue-shifted PL peak. Considering the reaction process, we propose that the smooth facets have well-defined crystal lattices that resulted from the interfacial reaction between MAI and PbI₂ domains containing low trap states density. The steplike facets are MAI-rich originated from the grain boundaries of PbI₂ film and own more trap states. Conversion of steplike facets to smooth facets can be controlled by increasing the reaction time through Ostwald ripening. The improved stability, photoresponsivity up to 0.3 A/W, on/off ratio up to 3900, and decreased photo response time to ∼160 μs show that the trap states can be annihilated effectively to improve the photoelectrical conversion with prolonged reaction time and elimination of steplike facets. Our findings demonstrate the relationship between the facet heterogeneity of SDP films and crystal growth process for the first time, and imply that the systematic control of crystal grain modification will enable amelioration of crystallinity for more-efficient perovskite photoelectrical applications.

Elemental Mapping of Perovskite Solar Cells by Using Multivariate Analysis: An Insight into Degradation Processes

The technology of perovskite-based solar cells is evolving rapidly, reaching certified power conversion efficiency values now exceeding 20 %. One of the main drawbacks hindering progress in the field is the long-term stability of the cells: the mixed halide perovskites used in most devices are sensitive to humidity and degrade on a timescale varying from hours to weeks. The degradation mechanisms are poorly understood, but likely arise from combined physical and chemical modifications at the nanometer scale. The characterization of pristine and degraded materials is difficult owing to their complex chemical and physical structure and their relatively poor stability. In this work, we investigated the changes in local composition and morphology of a standard device after 2 months of air exposure in the dark, using scanning transmission electron microscopy (STEM) with nanometer resolution for imaging and analysis. Because of a state-of-the-art technique that combines STEM and energy dispersive X-ray spectroscopy (EDX), and the use of different decomposition algorithms for multivariate analysis, we highlighted the migration of elements across the interfaces between the layers comprising the device. We also noticed a morphological degradation of the hole-transporting layer (HTL), representing one of the main factors enabling the infiltration of moisture in the device, which results in reduced performance.

Towards Optimum Solution-processed Planar Heterojunction Perovskite Solar Cells

Recently, organic–inorganic hybrid perovskites have been proven to be excellent photovoltaic materials, exhibiting outstanding light absorption, high carrier mobility and facile solution processability. Besides the low-cost manufacturing of perovskite thin-films, the power conversion efficiencies demonstrated for this class of materials are already at the same level as those of poly-crystalline silicon. The pursuit of efficiency in the field of metal halide perovskite solar cells has been achieved mainly through the improvement to perovskite deposition processing and optimization of the contact materials. In this chapter, we review the commonly employed perovskite deposition techniques, with special emphasis on the morphological quality of the prepared perovskite films. Films which exhibit the largest grains and highest orientation also achieve the highest performance, as long as full surface coverage is ensured. Here, it is also important to tune the energy levels of the electron and hole acceptors, and several strategies have led to champion devices with open circuit voltages between 1.1 and 1.15 V for state-of-the-art systems. However, most of the organic materials used currently are synthesized using expensive cross-coupling reactions that require stringent reaction conditions and extensive product purification, so that they cannot be produced at a low-cost at present. For perovskite solar cells to be able to enter the photovoltaic market, their cost and stability need to be competitive with current established technologies. The development of new chemistries resulting in simple compound purification, such as those based on azomethine bonds, will be an essential part of future molecular design for perovskite solar cells.

Insight into the CH3NH3PbI3/C interface in hole-conductor-free mesoscopic perovskite solar cells

Perovskite solar cells (PSCs) with hole-conductor-free mesoscopic architecture have shown superb stability and great potential in practical application. The printable carbon counter electrodes take full responsibility of extracting holes from the active CH3NH3PbI3 absorbers. However, an in depth study of the CH3NH3PbI3/C interface properties, such as the structural formation process and the effect of interfacial conditions on hole extraction, is still lacking. Herein, we present, for the first time, an insight into the spatial confinement induced CH3NH3PbI3/C interface formation by in situ photoluminescence observations during the crystallization process of CH3NH3PbI3. The derived reaction kinetics allows a quantitative description of the perovskite formation process. In addition, we found that the interfacial contact between carbon and perovskite was dominant for hole extraction efficiency and associated with the photovoltaic parameter of short circuit current density (JSC). Consequently, we conducted a solvent vapor assisted process of PbI2 diffusion to carefully control the CH3NH3PbI3/C interface with less unreacted PbI2 barrier. The improvement of interface conditions thereby contributes to a high hole extraction proved by the charge extraction resistance and PL lifetime change, resulting in the increased JSC valve.

Facile fabrication of large-grain CH3NH3PbI3-xBrx films for high-efficiency solar cells via CH3NH3Br-selective Ostwald ripening

Organometallic halide perovskite solar cells (PSCs) have shown great promise as a low-cost, high-efficiency photovoltaic technology. Structural and electro-optical properties of the perovskite absorber layer are most critical to device operation characteristics. Here we present a facile fabrication of high-efficiency PSCs based on compact, large-grain, pinhole-free CH₃NH₃PbI_3−xBr_x (MAPbI_3−xBr_x) thin films with high reproducibility. A simple methylammonium bromide (MABr) treatment via spin-coating with a proper MABr concentration converts MAPbI₃ thin films with different initial film qualities (for example, grain size and pinholes) to high-quality MAPbI_3−xBr_x thin films following an Ostwald ripening process, which is strongly affected by MABr concentration and is ineffective when replacing MABr with methylammonium iodide. A higher MABr concentration enhances I–Br anion exchange reaction, yielding poorer device performance. This MABr-selective Ostwald ripening process improves cell efficiency but also enhances device stability and thus represents a simple, promising strategy for further improving PSC performance with higher reproducibility and reliability.

Organolead halide perovskite optoelectronic devices require high material quality. Here, Yang et al. show that methylammonium lead iodide films can undergo Ostwald ripening and become mixed-halide films using methylammonium bromide treatment. This processing increases both the device efficiency and stability.

Improving the Photoluminescence Properties of Perovskite CH3NH3PbBr3-xClx Films by Modulating Organic Cation and Chlorine Concentrations

The photoluminescence (PL) properties of inorganic-organic perovskites can be drastically changed by tuning the halogen composition, especially the Cl content. However, our research demonstrated that in addition to the influence of Cl concentration, the PL emission intensity of CH3NH3PbBr3 strongly depends on the content of CH3NH3Br in the coating solution. The effects of CH3NH3Br and Cl concentrations on the PL properties of CH3NH3PbBr3-xClx were investigated. We found that a strong PL emission intensity of CH3NH3PbBr3 can be obtained from solutions with a high CH3NH3Br concentration. The PL emission intensities of CH3NH3PbBr3-xClx films were enhanced by adjusting the molar ratio of PbBr to PbCl2 only in a highly concentrated CH3NH3Br environment. Moreover, it was found that an optimum CH3NH3Br/PbBr2/PbCl2 ratio in the precursor solutions can be used to obtain the strongest PL emission intensity of CH3NH3PbBr3-xClx films. Further studies revealed that both CH3NH3Br and Cl concentrations significantly influence the CH3NH3PbBr3-xClx films evolution, which affects their PL properties.

Intercalation crystallization of phase-pure α-HC(NH₂)₂PbI₃ upon microstructurally engineered PbI₂ thin films for planar perovskite solar cells

The microstructure of the solid-PbI2 precursor thin film plays an important role in the intercalation crystallization of the formamidinium lead triiodide perovskite (α-HC(NH2)2PbI3). It is shown that microstructurally engineered PbI2 thin films with porosity and low crystallinity are the most favorable for conversion into uniform-coverage, phase-pure α-HC(NH2)2PbI3 perovskite thin films. Planar perovskite solar cells fabricated using these thin films deliver power conversion efficiency (PCE) up to 13.8%.

Effect of relative humidity on crystal growth, device performance and hysteresis in planar heterojunction perovskite solar cells

Due to the hygroscopic nature of organolead halide perovskites, humidity is one of the most important factors affecting the efficiency and longevity of perovskite solar cells. Although humidity has a long term detrimental effect on device performance, it also plays a key role during the initial growth of perovskite crystals. Here we demonstrate that atmospheric relative humidity (RH) plays a key role during the formation of perovskite thin films via the sequential deposition technique. Our results indicate that the RH has a substantial impact on the crystallization process, and hence on device performance. SEM and pXRD analysis show an increase in crystallite size with increasing humidity. At low RH, the formation of small cubic crystallites with large gaps between them is observed. The presence of these voids adversely affects device performance and leads to substantial hysteresis in the device. At higher RH, the perovskite crystals are larger in size, with better connectivity between the crystallites. This produced efficient planar heterojunction solar cells with low hysteresis. By careful control of the RH during the cell fabrication process, efficiencies of up to 12.2% are reached using P3HT as the hole-transport material.

A Long-Term View on Perovskite Optoelectronics

Recently, metal halide perovskite materials have become an exciting topic of research for scientists of a wide variety of backgrounds. Perovskites have found application in many fields, starting from photovoltaics and now also making an impact in light-emitting applications. This new class of materials has proven so interesting since it can be easily solution processed while exhibiting materials properties approaching the best inorganic optoelectronic materials such as GaAs and Si. In photovoltaics, in only 3 years, efficiencies have rapidly increased from an initial value of 3.8% to over 20% in recent reports for the commonly employed methylammonium lead iodide (MAPI) perovskite. The first light emitting diodes and light-emitting electrochemical cells have been developed already exhibiting internal quantum efficiencies exceeding 15% for the former and tunable light emission spectra. Despite their processing advantages, perovskite optoelectronic materials suffer from several drawbacks that need to be overcome before the technology becomes industrially relevant and hence achieve long-term application. Chief among these are the sensitivity of the structure toward moisture and crystal phase transitions in the device operation regime, unreliable device performance dictated by the operation history of the device, that is, hysteresis, the inherent toxicity of the structure, and the high cost of the employed charge selective contacts. In this Account, we highlight recent advances toward the long-term viability of perovskite photovoltaics. We identify material decomposition routes and suggest strategies to prevent damage to the structure. In particular, we focus on the effect of moisture upon the structure and stabilization of the material to avoid phase transitions in the solar cell operating range. Furthermore, we show strategies to achieve low-cost chemistries for the development of hole transporters for perovskite solar cells, necessary to be able to compete with other established technologies. Additionally, we explore the application of perovskite materials in optoelectronic applications. We show that perovskite materials can function efficiently both as a film in light-emitting diodes and also in the form of nanoparticles in light-emitting electrochemical cells. Perovskite materials have indeed a very bright future.

Photoluminescence characterisations of a dynamic aging process of organic-inorganic CH3NH3PbBr3 perovskite

After unprecedented development of organic-inorganic lead halide perovskite solar cells over the past few years, one of the biggest barriers towards their commercialization is the stability of the perovskite material. It is thus important to understand the interaction between the perovskite material and oxygen and/or humidity and the associated degradation process in order to improve device and encapsulation design for better durability. Here we characterize the dynamic aging process in vapour-assisted deposited (VASP) CH3NH3PbBr3 perovskite thin films using advanced optical techniques, such as time-resolved photoluminescence and fluorescence lifetime imaging microscopy (FLIM). Our investigation reveals that the perovskite grains grow spontaneously and the larger grains are formed at room temperature in the presence of moisture and oxygen. This crystallization process leads to a higher density of defects and a shorter carrier lifetime, specifically in the larger grains. Excitation-intensity-dependent steady-state photoluminescence shows both N2 stored and aged perovskite exhibit a super-linear increase of photoluminescence intensity with increasing excitation intensity; and the larger slope in aged sample suggests a larger density of defects is generated, consistent with time-resolved PL measurements.

A facile way to prepare nanoporous PbI2 films and their application in fast conversion to CH3NH3PbI3

Nanoporous PbI₂ films, prepared in a facile way, are applied to accelerate the reaction in the two-step deposition of CH₃NH₃PbI₃.

Optimizing Photovoltaic Response by Tuning Light-Harvesting Nanocrystal Shape Synthesized Using a Quick Liquid-Gas Phase Reaction

The electron recombination lifetime in a sensitized semiconductor assembly is greatly influenced by the crystal structure and geometric form of the light-harvesting semiconductor nanocrystal. When such light harvesters with varying structural characteristics are configured in a photoanode, its interface with the electrolyte becomes equally important and directly influences the photovoltaic efficiency. We have systematically probed here the influence of nanocrystal crystallographic structure and shape on the electron recombination lifetime and its eventual influence on the light to electricity conversion efficiency of a liquid junction semiconductor sensitized solar cell. The light-harvesting cadmium sulfide (CdS) nanocrystals of distinctly different and controlled shapes are obtained using a novel and simple liquid-gas phase synthesis method performed at different temperatures involving very short reaction times. High-resolution synchrotron X-ray diffraction and spectroscopic studies respectively exhibit different crystallographic phase content and optical properties. When assembled on a mesoscopic TiO2 film by a linker molecule, they exhibit remarkable variation in electron recombination lifetime by 1 order of magnitude, as determined by ac-impedance spectroscopy. This also drastically affects the photovoltaic efficiency of the differently shaped nanocrystal sensitized solar cells.

Microstructures of Organometal Trihalide Perovskites for Solar Cells: Their Evolution from Solutions and Characterization

The use of organometal trihalide perovskites (OTPs) in perovskite solar cells (PSCs) is revolutionizing the field of photovoltaics, which is being led by advances in solution processing of OTP thin films. First, we look at fundamental phenomena pertaining to nucleation/growth, coarsening, and microstructural evolution involved in the solution-processing of OTP thin films for PSCs from a materials-science perspective. Established scientific principles that govern some of these phenomena are invoked in the context of specific literature examples of solution-processed OTP thin films. Second, the nature and the unique characteristics of OTP thin-film microstructures themselves are discussed from a materials-science perspective. Finally, we discuss the challenges and opportunities in the characterization of OTP thin films for not only gaining a deep understanding of defects and microstructures but also elucidating classical and nonclassical phenomena pertaining to nucleation/growth, coarsening, and microstructural evolution in these films. The overall goal is to have deterministic control over the solution-processing of tailored OTP thin films with desired morphologies and microstructures.