Perovskite Composition Research Articles

Rashba Effect and Spin-Dependent Excitonic Properties in Chiral Two-Dimensional/Three-Dimensional Composite Perovskite Films.

Among various chiral semiconductor materials, chiral two-dimensional (2D)/three-dimensional (3D) composite perovskites (CPs) offer the benefits of strong interface asymmetry and energy transfer between 2D and 3D phases, making the chiral CPs promising for spintronic devices. Therefore, understanding their spintronic properties will be greatly important for expanding their relevant applications. In this work, we synthesized one pair of chiral 2D/3D CP films. Their Rashba effect and spin relaxation process have been investigated by polarization-dependent femtosecond transient absorption spectroscopy. Interestingly, under left- and right-handed circularly polarized light (CPL) excitation, a two-photon emission intensity difference is observed in chiral 2D/3D CP films at 298 K. This work sheds light on the spin-dependent excitonic characteristics of chiral 2D/3D CPs and confirms the feasibility of their application in near-infrared CPL detection.

Carrier Trapping Deactivation by Halide Alloying in Formamidinium‐Based Lead Iodide Perovskites

AbstractFormamidinium lead iodide (FAPbI3) is the benchmark material for the most efficient near‐infrared perovskite light‐emitting diodes (LEDs) and a promising gain medium for perovskite‐based coherent light sources. Thus, it is crucial to understand and control its defect chemistry to harness the full potential of its exceptional radiative recombination properties. Here, this topic is addressed by tailoring the I− to Br− ratio in the perovskite composition. It is found that introducing small Br− quantities improves the yield of radiative recombination with a beneficial impact on both spontaneous and amplified spontaneous emission (ASE) and improves the semiconductor photostability leading to reduced luminescence efficiency roll‐off and enhanced radiance in LEDs. By employing photoemission electron microscopy (PEEM), this improvement in optoelectronic performance can be directly correlated to a reduced hole‐trapping activity achieved by replacing iodide with bromide, thus, providing a convenient yet powerful synthetic approach to control the defect chemistry of the material and fostering its implementation in advanced photonic platforms.

Oxide-Halide Perovskite Composites for Simultaneous Recycling of Lead Zirconate Titanate Piezoceramics and Methylammonium Lead Iodide Solar Cells.

Global concerns over energy availability and the environment impose an urgent requirement for sustainable manufacturing, usage, and disposal of electronic components. Piezoelectric and photovoltaic components are being extensively used. They contain the hazardous element, Pb (e.g., in widely used and researched Pb(Zr,Ti)O3 and halide perovskites), but they are not being properly recycled or reused. This work demonstrates the fabrication of upside-down composite sensor materials using crushed ceramic particles recycled from broken piezoceramics, polycrystalline halide perovskite powder collected from waste dye-sensitized solar cells, and crystal particles of a Cd-based perovskite composition, C6H5N(CH3)3CdBr3 xCl3(1- x ). The piezoceramic and halide perovskite particles are used as filler and binder, respectively, to show a proof of concept for the chemical and microstructural compatibility between the oxide and halide perovskite compounds while being recycled simultaneously. Production of the recycled and reusable materials requires only a marginal energy budget while achieving a very high material densification of >92%, as well as a 40% higher piezoelectric voltage coefficient, i.e., better sensing capability, than the pristine piezoceramics. This work thus offers an energy- and environmentally friendly approach to the recycling of hazardous elements as well as giving a second life to waste piezoelectric and photovoltaic components.

Large-Area Flexible Perovskite Light-Emitting Diodes Enabled by Inkjet Printing.

Metal halide perovskite light-emitting diodes (PeLEDs) are attracting increasing attention due to their potential applications in flat panel lighting and displays. The solution process, large-area fabrication, and flexibility are attractive properties of PeLEDs over traditional inorganic LEDs. However, it is still very challenging to deposit uniform perovskite films on flexible substrates using a blade or slot-die coating, as the flexible substrate is not perfectly flat. Here, the inkjet printing technique is adopted, and the key challenges are overcome step-by-step in preparing large-area films on flexible substrates. Double-hole transporting layers are first used and a wetting interfacial layer to improve the surface wettability so that the printed perovskite droplets can form a continuous wet film. The fluidic and evaporation dynamics of the perovskite wet layer is manipulated to suppress the coffee ring effect by solvent engineering. Uniform perovskite films are obtained finally on flexible substrates with different perovskite compositions. The peak external quantum efficiency of the inkjet-printed PeLEDs reaches 14.3%. Large-area flexible PeLEDs (4×7 cm2 ) also show very uniform emission. This work represents a significant step toward real applications of large-area PeLEDs in flexible flat-panel lighting.

Minimizing Interfacial Recombination in 1.8eVTriple-Halide Perovskites for 27.5% Efficient All-Perovskite Tandems.

All-perovskite tandem solar cells show great potential to enable the highest performance at reasonable costs for a viable market entry in the near future. In particular, wide-bandgap (WBG) perovskites with higher open-circuit voltage (VOC ) are essential to further improve the tandem solar cells' performance. Here, a new 1.8eV bandgap triple-halide perovskite composition in conjunction with a piperazinium iodide (PI) surface treatment is developed. With structural analysis, it is found that the PI modifies the surface through a reduction of excess lead iodide in the perovskite and additionally penetrates the bulk. Constant light-induced magneto-transport measurements are applied to separately resolve charge carrier properties of electrons and holes. These measurements reveal a reduced deep trap state density, and improved steady-state carrier lifetime (factor 2.6) and diffusion lengths (factor 1.6). As a result, WBG PSCs achieve 1.36V VOC , reaching 90% of the radiative limit. Combined with a 1.26eV narrow bandgap (NBG) perovskite with a rubidium iodide additive, this enables a tandem cell with a certified scan efficiency of 27.5%.

Cs2TlRhX6 (X = Cl, Br, I): promising halide double perovskites for efficient energy harvesting in photovoltaic and thermoelectric applications

The majority of halide double perovskites, particularly chlorides and bromides, possess large band gaps. However, we have identified a composition of halide double perovskites with a narrow band gap, making them ideal for energy harvesting purposes. First-principle methods are employed to compute the structural, electronic, mechanical, optical, and thermoelectric properties of Cs2TlRhX6 (X= Cl, Br, I). The stability of the cubic perovskite compounds is confirmed through the formation energy, the Goldschmidt tolerance factor, and the elastic constant. The optical bandgaps of all the compositions are determined through the TBmBJ potential. The ductile nature of the present compounds is verified by the values of Pugh’s ratio and Cauchy’s pressure. The optical properties are investigated to check the availability of the materials for harvesting solar energy. Temperature-dependent parameters including electrical conductivity, see-beck coefficient, power factor, and figure of merit also have been computed for thermoelectric applications. All the explored characteristics of the double perovskites under study have been discussed briefly on the basis of interesting and informative Physics behind the observed results.

Constructing tin oxides Interfacial Layer with Gradient Compositions for Efficient Perovskite/Silicon Tandem Solar Cells with Efficiency Exceeding 28.

Atomic layer deposition (ALD) growth of conformal thin SnOx films on perovskite absorbers offers a promising method to improve carrier-selective contacts, enable sputter processing, and prevent humidity ingress toward high-performance tandem perovskite solar cells. However, the interaction between perovskite materials and reactive ALD precursor limits the process parameters of ALD-SnOx film and requires an additional fullerene layer. Here, it demonstrates that reducing the water dose to deposit SnOx can reduce the degradation effect upon the perovskite underlayer while increasing the water dose to promote the oxidization can improve the electrical properties. Accordingly, a SnOx buffer layer with a gradient composition structure is designed, in which the compositionally varying are achieved by gradually increasing the oxygen source during the vapor deposition from the bottom to the top layer. In addition, the gradient SnOx structure with favorable energy funnels significantly enhances carrier extraction, further minimizing its dependence on the fullerene layer. Its broad applicability for different perovskite compositions and various textured morphology is demonstrated. Notably, the design boosts the efficiencies of perovskite/silicon tandem cells (1.0cm2) on industrially textured Czochralski (CZ) silicon to a certified efficiency of 28.0%.

Improved Performance of Air-Processed Perovskite Solar Cells via the Combination of Chlorine Precursors and Potassium Thiocyanate.

Due to their low cost and high efficiency, hybrid perovskite solar cells (PSCs) have shown the most outstanding competitiveness among third-generation photovoltaic (PV) devices. However, several challenges remain unresolved, among which the limited stability is arguably the main. Chlorine (Cl) has been widely employed to yield PV performances, but the Cl-doping mechanism and its role in mixed-halide PSCs are not entirely understood. Here, we investigate the effect of Cl-doping using different precursors such as formamidinium chloride (FACl), cesium chloride (CsCl), and lead chloride (PbCl2), which lead to the incorporation of Cl at different sites of the perovskite crystal. We demonstrate that the stability and efficiency of air-processed PSCs are strongly affected by Cl bonding into the cationic chloride precursor. Furthermore, adding potassium thiocyanate (KSCN) leads to the maximum efficiency of 18.1%, improving the operational stability with only 18% PCE loss after 520 h, stored under ambient conditions. Incorporating CsCl and KSCN presents an effective approach to further boost the performance and thermal stability of PSCs by tailoring the composition of the perovskite's composition. Finally, we used the slot-die method to demonstrate that our strategy is scalable for large-area devices that have shown similar performance. Our results show that fully air-processed and stable PSCs with high efficiency for large production and commercialization are achievable.

Perovskite/silicon tandem solar cells-compositions for improved stability and power conversion efficiency.

Perovskite/Silicon Tandem Solar Cells (PSTSCs) represent an emerging opportunity to compete with industry-standard single junction crystalline silicon (c-Si) solar cells. The maximum power conversion efficiency (PCE) of single junction cells is set by the Shockley-Queisser (SQ) limit (33.7%). However, tandem cells can expand this value to ~ 45% by utilising two stacked solar cells to harvest the solar spectrum more efficiently. 33.9% PCE has already been achieved with PSTSCs. This perspective analyses recent advances in PSTSC technology, with an emphasis on optimal perovskite composition, the problem and mitigation of light-induced halide phase segregation, self-assembled hole transporting monolayers and additives that can improve and stabilise the perovskite. Top-performing compositions show three cationic components (Cs+, FA+, Pb2+) and three anionic (I-, Br-, Cl-) with a bandgap between 1.55 and 1.77eV and a theoretical maximum of 1.73eV (717nm). Anionic additives such as (Br3)- and SCN- reduce trap states and segregation. 2D-perovskite grain boundary interfaces are created with cationic alkylammonium additives such as methyl-phenethylammonium (MPEA) and result in improved performance. 2-, 3- or 4-terminal devices with a (partly) textured silicon heterojunction (SHJ) bottom cell are ideal. An ultra-thin interfacial recombination layer (~ 5nm) of indium tin oxide (ITO) or indium zinc oxide (IZO) containing a carbazole-based hole transporting self-assembled monolayer (Me-4PACz) is used for optimal 2-terminal tandem devices.

Exciton Chirality Transfer Empowers Self-Triggered Spin-Polarized Amplified Spontaneous Emission from 1D-Anchoring-3D Perovskites.

Spin-polarized lasers, arising from stimulated emission of imbalanced spin populations, play a vital role in spin-optoelectronics.It is usually tackled by external spin injection, inevitably suffering from additional losses across the barriers from injection sources to gain materials. Herein, spin-polarized coherent light emission is self-triggered from the 1D-anchoring-3D perovskites, where the imbalanced populations in achiral 3D perovskites are endowed with the spin selectivity of exciton chirality (EC) underpinned by chiral 1D perovskites. Efficient transfer of EC is enabled by rapid energy transfer, thereby creating an imbalance of the spin population of excited states. Stimulated emission of such populations brings self-triggered spin-polarized amplified spontaneous emission in the composite perovskites, yielding a higher degree of polarization (DOP) than that based on optical spin injection into bare achiral 3D perovskites. Chemical diversity of composite perovskites not only enables to adjust band gap for broadband output of spin-polarized lightsignals but also promises to manipulate radiative decay and spin relaxation toward remarkably increased DOP. These results highlight the importance of EC transfer mechanism for spin-polarized lasing and represent a crucial step toward the development of chiral-spintronics.

Unusual Phase Behaviour for Organo-Halide Perovskite Nanoparticles Synthesized via Reverse Micelle Templating

Micelle templating has emerged as a powerful method to produce monodisperse nanoparticles. Herein, we explore unconventional phase transformations in the synthesis of organo-halide perovskite nanoparticles utilizing reverse micelle templates. We employ diblock-copolymer reverse micelles to fabricate these nanoparticles, which confines ions within micellar nanoreactors, retarding reaction kinetics and facilitating perovskite cage manipulation. The confined micellar environment exerts pressure on both precursors and perovskite crystals formed inside, enabling stable phases not typically observed at room temperature in conventional synthesis. This provides access to perovskite structures that are otherwise challenging to produce. The hydrophobic shell of the micelle also enhances perovskite stability, particularly when combined with anionic exchange approaches or large aromatic cations. This synergy results in long-lasting stable optical properties despite environmental exposure. Reverse micelle templates offer a versatile platform for modulating perovskite structure and behavior across a broad spectrum of perovskite compositions, yielding unique phases with diverse emission characteristics. By manipulating the composition and properties of the reverse micelle template, it is possible to tune the characteristics of the resulting nanoparticles, opening up exciting opportunities for customizing optical properties to suit various applications.

Ion migration mediated high Seebeck effect in halide perovskites and application in infrared detection

Thermoelectric materials play an important role in thermopower systems, thermal sensing and infrared photo-thermoelectric imaging. Ionic thermoelectric materials have recently attracted attentions considering their relatively high Seebeck coefficient. However, the ionic thermoelectric materials have to use liquid or hydrogels for ion conduction, and are susceptible to leakage, evaporation and portability issues. Metal halide perovskites possess low thermal conductivity and prominent ion migration characteristics, making them theoretically suitable candidates for thermoelectric conversion. Here we present the first-ever study of the ionic thermoelectric properties in typical solid perovskite composition MAPbI3, revealing an extremely high Seebeck coefficient of 35.7 mV/K. Furthermore, the assembled photo-thermoelectric detector of carbon/MAPbI3 single crystal/carbon demonstrated a high response (0.58 V/W) toward infrared light and the imaging test was well resolved. This research provides a novel avenue to investigate efficient thermoelectric devices based on ionic and electronic mixed conductors and develop photo-thermoelectric applications.

The investigation of co-assembled high-performance (Ba, Sm/Sr)(Co, Zr)O3-δ composite perovskite cathodes for intermediate-temperature solid oxide fuel cell

The cobalt-based perovskites are the most common solid oxide fuel cell (SOFC) cathode materials, which generally have the issues of thermal compatibility and performance at the reducing temperature. In this study, two Ba0.5(Sm/Sr)0.5Co0.8Zr0.2O3-δ (BSmCZ/BSrCZ) perovskite composites were synthesized using a smart co-assembled method, which were primarily composed of cubic doped-BaZrO3 and hexagonal doped-BaCoO3 perovskites. Both BSmCZ and BSrCZ composites exhibited more suited thermal expansion coefficient (12.8 × 10-6 and 13.9 × 10-6 K−1) to the electrolyte, nanostructure with extensive heterointerfaces and excellent cathode|electrolyte interfaces. Compared to BSrCZ composite, the BSmCZ composite possessed a relatively higher content (65 %) of activated oxygen species and higher electrical conductivity (70.7 S cm−1). The cells with two composite cathodes could achieve the remarkable maximum power density, especially for BSmCZ composite cathode of 2.00–1.11 W∙cm−2 at 800–650 °C. The electrochemical analysis revealed that the BSmCZ composite cathode demonstrated the enhanced surficial oxygen catalysis processes, which could benefit from the active oxygen species and heterointerface.

Stabilized hole-selective layer for high-performance inverted p-i-n perovskite solar cells.

P-i-n geometry perovskite solar cells (PSCs) offer simplified fabrication, greater amenability to charge extraction layers, and low-temperature processing over n-i-p counterparts. Self-assembled monolayers (SAMs) can enhance the performance of p-i-n PSCs but ultrathin SAMs can be thermally unstable. We report a thermally robust hole-selective layer comprised of nickel oxide (NiOx) nanoparticle film with a surface-anchored (4-(3,11-dimethoxy-7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (MeO-4PADBC) SAM that can improve and stabilize the NiOx/perovskite interface. The energetic alignment and favorable contact and binding between NiOx/MeO-4PADBC and perovskite reduced the voltage deficit of PSCs with various perovskite compositions and led to strong interface toughening effects under thermal stress. The resulting 1.53-electron-volt devices achieved 25.6% certified power conversion efficiency and maintained >90% of their initial efficiency after continuously operating at 65 degrees Celsius for 1200 hours under 1-sun illumination.

Anion-π interactions suppress phase impurities in FAPbI3 solar cells.

Achieving both high efficiency and long-term stability is the key to the commercialization of perovskite solar cells (PSCs)1,2. However, the diversity of perovskite (ABX3) compositions and phases makes it challenging to fabricate high-quality films3-5. Perovskite formation relies on the reaction between AX and BX2, whereas most conventional methods for film-growth regulation are based solely on the interaction with the BX2 component. Herein, we demonstrate an alternative approach to modulate reaction kinetics by anion-π interaction between AX and hexafluorobenzene (HFB). Notably, these two approaches are independent but work together to establish 'dual-site regulation', which achieves a delicate control over the reaction between AX and BX2 without unwanted intermediates. The resultant formamidinium lead halides (FAPbI3) films exhibit fewer defects, redshifted absorption and high phase purity without detectable nanoscale δ phase. Consequently, we achieved PSCs with power conversion efficiency (PCE) up to 26.07% for a 0.08-cm2 device (25.8% certified) and 24.63% for a 1-cm2 device. The device also kept 94% of its initial PCE after maximum power point (MPP) tracking for 1,258 h under full-spectrum AM 1.5 G sunlight at 50 ± 5 °C. This method expands the range of chemical interactions that occur in perovskite precursors by exploring anion-π interactions and highlights the importance of the AX component as a new and effective working site to improved photovoltaic devices with high quality and phase purity.

Synthesis of stable iodoplumbate and perovskite for efficient annealing‐free device and long‐term storage

AbstractAs a next‐generation photovoltaic device, perovskite solar cells are rapidly emerging. Nevertheless, both solution and device stability pose challenges for commercialization due to chemical degradation caused by internal and external factors. Especially, the decomposition of iodoplumbate in a perovskite solution hinders the long‐term use of perovskite solutions. Moreover, the synthesis of stable perovskites at low temperature is important for stable devices and wide applications (flexible devices and high reproducibility). Herein, the critical composition of perovskite is found to obtain high stabilities of both iodoplumbate and perovskite crystals by utilizing CsPbBr3 and FAPbI3, exhibiting high device performance and long‐term solution storage. The novel composition of CsPbBr3‐alloyed FAPbI3 not only crystallizes under annealing‐free conditions but also demonstrates excellent iodoplumbate stability for 100 days (∼3000 h) without any degradation. Furthermore, high device stabilities are achieved over 2000 and 3000 h under extreme conditions of A.M. 1.5 and 85°C/85% relative humidity, respectively. Overall, the device exhibited a high power conversion efficiency of 23.4%, and furthermore, CsPbBr3‐alloyed FAPbI3 was devoted to widen the applications in both flexible and carbon‐electrode devices, thereby addressing both scientific depths and potential commercial materials.

Ni-Doped La0.6Sr0.4CoO3 Perovskite as an Efficient Electrocatalyst for Oxygen Reduction and Evolution Reactions in Alkaline Media

The Co-based perovskite La0.6Sr0.4CoO3 has received significant attention as a potential electrocatalyst for its oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) due to its abundance, facile synthesis, and high oxygen kinetics. However, research on the catalytic performance of Ni-doped La0.6Sr0.4Co1−xNixO3 as a bifunctional cathode catalyst for Zn-air batteries (ZABs) is still scarce. In this work, lanthanum strontium cobalt-based perovskite catalysts with various Ni contents (La0.6Sr0.4Co1−xNixO3, x = 0, 0.2, 0.5, and 0.8) were synthesized using a simple combustion method. The effects of Ni doping on the morphology, structure, surface oxygen-related species, and valence states of the transition metals of the perovskite were characterized. The electrochemical behaviors of the perovskite catalysts in both ORR and OER were also assessed. The characterization results revealed that proper Ni doping can decrease particle size, increase surface oxygen vacancies, and create mixed valence states of the transition metal and, thus, lead to improvement of the electrocatalytic activity of perovskite catalysts. Among the different perovskite compositions, La0.6Sr0.4Co0.8Ni0.2O3 exhibited the best ORR/OER activity, with a higher limiting current density, smaller Tafel slope, higher half-wave potential, lower overpotential, and lower potential difference than the other compositions. When La0.6Sr0.4Co0.8Ni0.2O3 was applied as the cathodic catalyst in a primary ZAB, it delivered a peak power density of 81 mW cm−2. Additionally, in rechargeable ZABs, the La0.6Sr0.4Co0.8Ni0.2O3 catalyst exhibited a lower voltage gap (0.94 V) and higher stability during charge–discharge cycling than the commonly used catalyst Pt/C. These results indicate that Ni-doped La0.6Sr0.4Co0.8Ni0.2O3 is a promising bifunctional electrocatalyst for ZAB.

Machine‐Learning Accelerating the Development of Perovskite Photovoltaics

Perovskite solar cells (PSC) are a potential candidate for next‐generation photovoltaic technology. Despite the significant advancements in the field of PSCs, the ongoing development of stable and efficient metal halide perovskite materials, along with their successful integration into photovoltaic applications, remains challenges. These challenges originate from the diverse range of device structures and perovskite compositions, requiring meticulous consideration and optimization. Traditional trial‐and‐error methods are characterized by their sluggishness and labor‐intensive nature. Recently, the emergence of extensive datasets and advancements in computer hardware have facilitated the utilization of machine learning (ML) across multiple domains, including in various fields for material discovery and experimental optimization. Herein, the fundamental procedure of ML is briefly introduced, and latest progress of ML in the materials development and solar cell fabrication is comprehensively reviewed. The utilization of ML in PSCs at all stages of design can be categorized into four main areas: screening perovskite material, fabrication process optimization, device structure optimization, and understanding mechanism. The challenges and outlooks on the future development of ML are finally discussed. It is highly expected that this review can offer valuable guidance for the design and development of highly efficient and stable PSCs.

Improvement of metal–insulator transition and mechanical strength of RENiO3 by co-sintering

Rare-earth nickelates (RENiO3: RE≠La) exhibit metal–insulator transition (MIT) properties that enable potential applications, such as critical temperature resistance thermistors, optoelectronic switches, and correlated logical devices. Nevertheless, their abrupt structural distortion across MIT results in mechanical stresses and forms microcracks within the bulk RENiO3, and this irreversibly reduces their resistive change during MIT that further impedes their practical applications. Herein, we demonstrate a compositing strategy that simultaneously improves the MIT performances and mechanical strength of RENiO3 by introducing a secondary phase of perovskite oxides with similar lattice parameters and high resistivity. Despite its much higher resistivity compared to RENiO3 (e.g., RE = Sm or Pr), introducing the LaMnO3 compositing phase under high oxygen pressure surprisingly reduces the matrix resistivity. Furthermore, such a compositing process (e.g., 20% LaMnO3) also effectively improves the mechanical strength of RENiO3 by eight times. Such counterintuitive variations are attributed to the similar structure and lattice parameter between RENiO3 and the perovskite composites that modify the grain boundary. As a result, the resistive change is more abrupt across MIT owing to the reduction in the resistivity associated with the grain boundary, while the defect generation and propagation are also suppressed that improves the mechanical properties. This further pave the way to the application of bulk RENiO3 as discrete devices in correlated electronics.

Surface Cleaning and Passivation Strategy for Durable Inverted Formamidinium–Cesium Triiodide Perovskite Solar Cells

AbstractFormamidinium‐cesium triiodide (FAxCs1‐xPbI3) perovskite exhibits excellent phase stability, making it the most promising candidate for commercial perovskite solar cell (PSC) applications, particularly those with inverted structures present a promising contribution to the field of perovskite production. However, this composition often forms small grain sizes and has a large number of defects and PbI2 residues on its surface, which can damage device performance. In this study, a post‐surface engineering strategy called the “clean‐passivation” method is proposed to address the interfacial problem between the perovskite and the electron transport layer (ETL). This method significantly reduces surface and grain boundary defects and eliminates unreacted PbI2, resulting in suppressed iodine decomposition and ion migration during operation. As a result, an excellent power conversion efficiency of 24.27% with superior stability is achieved, as the unencapsulated device maintains 97.12% of its initial efficiency after 1500 h of continuous light soaking. Furthermore, this new surface clean‐passivate strategy can be universally applied to other typical perovskite compositions.