Unveiling the Crystal Formation of Cesium Lead Mixed-Halide Perovskites for Efficient and Stable Solar Cells

Limitations and Progresses in Carbon-Based Cesium Lead Halide Perovskite Solar Cells

Inorganic cesium lead halide perovskites (CsPbIxBr3-x, 0≤x≤3) are promising alternatives with great thermal stability. Additionally, the choice of moisture-resistive and dopant-free carbon as the electrode material can simultaneously solve the problems of stability and cost. Therefore, carbon electrode-based inorganic PSCs (C-IPSCs) represent a promising candidate for commercialization, yet both the efficiencies and stability of related devices demand further progress. This article reviews the recent advancement of C-IPSCs and then unravels the distinctive merits and limitations in this field. Subsequently, our perspective on various modification strategies is analyzed on a methodological level. Finally, this article outlooks the promising research contents and the remaining unresolved issues in this field. We believe that understanding and analyzing the related problems in this field are instructive to stimulate the future development of C-IPSCs.

Zirconium Doping to Enable High-Efficiency and Stable CsPbI2Br All-Inorganic Perovskite Solar Cells

All-inorganic wide-bandgap perovskite CsPbI2Br has attracted much attention because of its inherent thermal stability and ideal bandgap for the front subcell of tandem solar cells (TSCs). However, the low power conversion efficiency (PCE) and poor moisture stability of CsPbI2Br still restrict its future commercialization. Herein, zirconium tetrachloride (ZrCl4) was doped into CsPbI2Br films to modulate the crystal growth and improve the film quality. The partial substitution of the B-site Pb2+ of CsPbI2Br with Zr4+ suppresses the unwanted phase conversion from the crystallized black α-phase to the δ-phase, resulting in improved phase stability. Consequently, the humidity and thermal stability of the film are greatly improved. Additionally, the incorporation of ZrCl4 suppresses nonradiative recombination and forms a matched energy-level alignment with the hole-transport layer (Spiro-OMeTAD). Benefiting from these features, the ZrCl4-doped CsPbI2Br perovskite solar cell (PSC) shows an outstanding efficiency of 16.60% with a high open-circuit voltage of 1.29 V. The unencapsulated devices simultaneously show excellent humidity and thermal stability, retaining over 91% of PCEinitial after 1000 h of aging in ambient air conditions and 92% PCEinitial after 500 h of continuous heating at 85 °C in a nitrogen environment, respectively. Furthermore, ZrCl4-doped CsPbI2Br was employed as the front subcell of perovskite/organic TSCs and achieved a remarkable PCE of 19.42%, showing great potential for highly efficient and stable TSCs.

Solvent engineering of MAPbI3 perovskite thick film for a direct X-ray detector

The emergence of organic-inorganic hybrid perovskites with a high μτ product and a high absorption coefficient has made it possible to adopt an aerosol-liquid-solid technology for direct X-ray detectors. The film quality from the ALS process is often compromised, especially on the film surface, when deposited in ambient conditions with uncontrolled humidity. Herein we develop a solvent engineering strategy in the ALS process to obtain high-quality MAPbI₃ thick films. The key is the introduction of a molecular additive to intervene and regulate the perovskite crystallization process so that the negative effect of the ALS ambience is minimized. This strategy allows us to prepare direct X-ray detectors with much reduced dark current, enhanced response speed and improved overall performance.

In situ crystal reconstruction strategy-based highly efficient air-processed inorganic CsPbI2Br perovskite photovoltaics for indoor, outdoor, and switching applications

All-inorganic CsPbI₂Br (CPIB) perovskite has gained strong attention due to their favorable optoelectronic properties for photovoltaics. However, solution-processed CPIB films suffer from poor morphology due to the rapid crystallization process, which must be resolved for desirable photovoltaic performance. We introduced phenethylammonium iodide (PEAI) as an additive into a perovskite precursor that effectively controls the crystallization kinetics to construct the preferred quality α-CPIB film under ambient conditions. Various photophysical and structural characterization studies were performed to investigate the microstructural, morphological, and optoelectronic properties of the CPIB and PEAI-assisted perovskite films. We found that PEAI plays a vital role in decreasing pinholes, ensuring precise crystal growth, enhancing the crystallinity, improving the uniformity, and tailoring the film morphology by retarding the crystallization process, resulting in an improved device performance. The device based on the optimized PEAI additive (0.8 mg) achieved a respectably high power conversion efficiency (PCE) of 17.40% compared to the CPIB perovskite solar cell (PSC; 15.75%). Moreover, the CPIB + 0.8 mg PEAI PSC retained ∼87.25% of its original PCE, whereas the CPIB device retained ∼66.90% of the initial PCE after aging in a dry box at constant heating (85 °C) over 720 h, which revealed high thermal stability. Furthermore, the indoor photovoltaic performance under light-emitting diode (LED) lighting conditions (3200 K, 1000 lux) was investigated, and the CPIB + 0.8 mg PEAI PSC showed a promising PCE of 26.73% compared to the CPIB device (19.68%). In addition, we developed a switching function by employing the optimized PSC under LED lighting conditions, demonstrating the practical application of constructed indoor PSCs.

Novel C3N4-Assisted Bilateral Interface Engineering for Efficient and Stable Perovskite Solar Cells

g-C₃N₄-assisted interface engineering has been developed as an effective method to improve the efficiency and stability of perovskite solar cells (PSCs). However, most of the reported works used g-C₃N₄-induced single-interface modification, which is difficult to passivate the bilateral interfaces of the perovskite layer at the same time. In this paper, we fabricated two kinds of C₃N₄ materials simultaneously (w-CN and y-CN) after the twice calcination of melamine and used them in the bilateral interface modification toward all-inorganic PSCs. The two kinds of C₃N₄ play different roles in different interface engineering. On the front interface, w-CN could optimize band level arrangement and improve the perovskite film quality, which contributes to the efficiency of the device. On the back interface, y-CN could also improve the film quality of the perovskite layer, accelerating the extraction of charge carriers. The champion efficiency of the CsPbIBr₂-based device treated by the bilateral interface is significantly enhanced from 7.8 to 10.1%. Moreover, the modified perovskite film exhibits negligible degradation after 40 min of exposure in the ambient environment with a relative humidity of 70%, while the pristine perovskite film has a rapid degradation within 20 min.

One-Step Thermal Gradient- and Antisolvent-Free Crystallization of All-Inorganic Perovskites for Highly Efficient and Thermally Stable Solar Cells

All-inorganic perovskites have emerged as promising photovoltaic materials due to their superior thermal stability compared to their heat-sensitive hybrid organic-inorganic counterparts. In particular, CsPbI2 Br shows the highest potential for developing thermally-stable perovskite solar cells (PSCs) among all-inorganic compositions. However, controlling the crystallinity and morphology of all-inorganic compositions is a significant challenge. Here, a simple, thermal gradient- and antisolvent-free method is reported to control the crystallization of CsPbI2 Br films. Optical in situ characterization is used to investigate the dynamic film formation during spin-coating and annealing to understand and optimize the evolving film properties. This leads to high-quality perovskite films with micrometer-scale grain sizes with a noteworthy performance of 17% (≈16% stabilized), fill factor (FF) of 80.5%, and open-circuit voltage (VOC ) of 1.27 V. Moreover, excellent phase and thermal stability are demonstrated even after extreme thermal stressing at 300 °C.

A double perovskite participation for promoting stability and performance of Carbon-Based CsPbI2Br perovskite solar cells

All-inorganic perovskite materials (Typically: CsPbI₂Br) have attracted enormous attention due to their illustrious thermal stability and appropriate bandgap, and their use in perovskite solar cells (PSCs) has been extensively investigated. However, the inevitable defects of the perovskite layer, energy level mismatch between perovskite and carbon electrodes, and the phase instability of CsPbI₂Br limit the power conversion efficiency (PCE) and stability of carbon-based CsPbI₂Br PSCs. Herein, we demonstrate a simple and effective strategy for regulating energy level, inhibiting carrier recombination, and delaying the degradation of perovskite by modifying the surface of CsPbI₂Br with a new type of 2D perovskite Cs₂PtI₆. The carbon-based CsPbI₂Br PSCs achieve a higher PCE (13.69 %) than the control device (11.10 %). The excellent matching of the energy level and suppression of charge carrier recombination should be responsible for the improvement in efficiency. Furthermore, the excellent hydrophobic performance of Cs₂PtI₆ enhances the moisture resistance of the device. This study provides a potential strategy for improving the performance and stability of all-inorganic CsPbI₂Br PSCs.

A Universal Dopant-Free Polymeric Hole-Transporting Material for Efficient and Stable All-Inorganic and Organic-Inorganic Perovskite Solar Cells

Hole-transporting materials (HTMs) with desired properties play a crucial role in achieving efficient and stable perovskite solar cells (PSCs). However, most high-performance devices generally employ HTMs that require additional complicated doping treatments, which are harmful to the device stability. In this work, a fluorine-substituted polymer electron-donor material, PM6, is developed as a dopant-free HTM in regular all-inorganic CsPbI2Br PSCs. Benefiting from the matched energy-level alignment, high hole mobility, and effective defect passivation, a champion power conversion efficiency (PCE) of 16.06% with an ultrahigh fill factor of 82.54% is achieved for the PM6-based PSCs. Compared to doped Spiro-OMeTAD (PCE of 14.46%), PM6 significantly enhances the PCE of CsPbI2Br PSCs with negligible hysteresis owing to its more efficient charge transportation, suppressed recombination, and strong trap passivation effect. Moreover, remarkable improvements in long-term stability, thermal stability, and operational stability are all gained for the PM6-based PSCs. In addition, the successful application of PM6 as a dopant-free HTM in organic-inorganic hybrid PSCs enables an impressive PCE of 20.05% with superb device stability, manifesting the generality of the polymer donor material in various PSC systems.

Resistance Switching Effect of Memory Device Based on All-Inorganic Cspbbri2 Perovskite

In this study, the CsPbBrI₂ perovskite film was prepared by the preparation of the sol-gel and the spin-coating method, and the cubic lattice was stabilized by introducing Br⁺ into the CsPbI₃ film, which solved the problem of instability of the traditional perovskite phase. Based on the CsPbBrI₂ perovskite film, the Ag/CsPbBrI₂/ITO memory device with a resistance switching effect was prepared. The morphology and phase compositions of the film were analyzed by scanning electron microscope and X-ray diffraction. The non-volatile and repeatable resistance switching effect of the Ag/CsPbBrI₂/ITO memory device was measured under open-air conditions. The experimental results show that the surface of the CsPbBrI₂ perovskite film is uniform and dense, and the Ag/CsPbBrI₂/ITO memory device has an order of magnitude resistance-on-off ratio after 500 cycles of cyclic voltage. This study shows that Ag/CsPbBrI₂/ITO memory devices based on CsPbBrI₂ perovskite films have potential applications in the field of non-volatile memory devices. At the same time, the transient properties of the CsPbBrI₂ film that can quickly dissolve in deionized water make it potentially useful in short-period data storage units and implantable electronic devices with human or environmental sensors.

Defects in CsPbX3 Perovskite: From Understanding to Effective Manipulation for High-Performance Solar Cells

The rapid development of all inorganic metal perovskite (CsPbX₃ , X represents halogen) materials holds great promise for top-cells in tandem junctions due to their glorious thermal stability and continuous adjustable band gap in a wide range. Due to the presence of defects, the power conversion efficiency (PCE) of CsPbX₃ perovskite solar cells (PSCs) is still substantially below the Shockley-Queisser (SQ) limit. Therefore, it is imperative to have an in-depth understanding of the defects in PSCs, thus to evaluate their impact on device performances and to develop corresponding strategies to manipulate defects in PSCs for further promoting their photoelectric properties. In this review, the latest progress in defect passivation in the CsPbX₃ PSCs field is summarized. Starting from the effect of non-radiative recombination on open circuit voltage (V_oc ) losses, the defect physics, tolerance, self-healing, and the effect of defects on the photovoltaic properties are discussed. Some techniques to identify defects are compared based on quantitative and qualitative analysis. Then, passivation manipulation is discussed in detail, the defect passivation mechanisms are proposed, and the passivation agents in CsPbX₃ thin films are classified. Finally, directions for future research about defect manipulation that will push the field to progress forward are outlined.

Interfaces and Interfacial Layers in Inorganic Perovskite Solar Cells

Owing to their superior thermal stability, metal halide inorganic perovskite materials continue to attract interest for photovoltaics applications. The highest reported power conversion efficiency (PCE) for solar cells based on inorganic perovskites is over 20 %. As this PCE corresponds to 73 % of the theoretical limit, there remains more room for further improving the device PCEs than for improving organic-inorganic hybrid perovskite solar cells (PSCs). The main loss is in the photovoltage, which is limited by interfaces in terms of non-radiative recombination caused by traps and energy-level mismatch. Furthermore, inefficient charge extraction at interfacial contacts reduces the photocurrent and fill factor. This Minireview summarizes the recent developments in the fundamental understanding of how the interfaces and interfacial layers influence the performance of solar cells based on inorganic perovskite absorbers. An outlook for the development of highly efficient and stable inorganic PSCs from the interface point of view is also given.

Stiffening the Pb-X Framework through a π-Conjugated Small-Molecule Cross-Linker for High-Performance Inorganic CsPbI2Br Perovskite Solar Cells

Inorganic CsPbI₂Br perovskites have witnessed incredible advances as a promising representative for translucent and tandem solar cells, but unfortunately, they are still plagued by serious energy losses and undesired phase instability. Herein, a new type of π-conjugated small molecule of 4-guanidinobenzoic-acid-hydrochloride (4-GBACl) is demonstrated to effectively cross-link the Pb-X framework of perovskites. The strong coordination between 4-GBACl and the [PbX₆]^4- octahedron of perovskites effectively stiffens the Pb-X framework to suppress the ion migration, thus stabilizing the perovskite phase structure against light and thermal conditions. Apart from the physical barrier for phase instability resulting from the hydrophobic benzene ring at grain boundaries (GBs), guanidinium cations and -COOH and Cl^- groups can simultaneously afford the passivation of positively and negatively charged defects at the GBs and surface, including undercoordinated halide species and undercoordinated Pb²⁺ ions, thereby effectively inhibiting the charge trapping/recombination centers. Two-dimensional confocal-fluorescence mapping images provide a visualized sight into the significantly suppressed nonradiative recombination and the prolonged carrier lifetime. It is suggested that the 4-GBACl additive plays multiple roles in grain cross-linking to regulate crystallization, distinctly reducing the trap-state density, ion migration inhibition, and moisture barrier in CsPbI₂Br films. Consequently, the 4-GBACl-treated device exhibits a champion power conversion efficiency (PCE) of 15.59% accompanied with a considerably improved V_oc of 1.28 V and maintains 88% of the initial PCE value after 1200 h aging under 20% relative humidity.

Low photoactive phase temperature all-inorganic, tin-lead mixed perovskite solar cell

CsPbI₃ films have recently attracted significant attention as efficient absorbers for thermally stable photovoltaic devices. However, their large bandgap and photoactive black phase formation at high temperature impede their use for practical applications. Using the concept of lattice contraction, we demonstrate a low bandgap (≤1.44 eV) cesium-based inorganic perovskite CsPb_xSn_1−xI₃ that can be solution processed at low temperature for photovoltaic devices. The results from systematic measurements imply that the partial substitution of lead (Pb) with tin (Sn) results in crystal lattice contraction, which is essential for realizing photoactive phase formation at l00 °C and stabilizing photoactive phase at room temperature. These findings demonstrate the potential of using cesium-based inorganic perovskite as viable alternatives to MA- or FA-based perovskite photovoltaic materials.

In Cs-based all inorganic perovskite solar cells based, doping Sn can cause lattice shrinkage, which reduces annealing temperature of forming photoactive phase.

Single-emissive-layer all-perovskite white light-emitting diodes employing segregated mixed halide perovskite crystals

Metal halide perovskites have demonstrated impressive properties for achieving efficient monochromatic light-emitting diodes. However, the development of white perovskite light-emitting diodes (PeLEDs) remains a big challenge. Here, we demonstrate a single-emissive-layer all-perovskite white PeLED using a mixed halide perovskite film as the emissive layer. The perovskite film consists of separated mixed halide perovskite phases with blue and red emissions, which are beneficial for suppressing halide anion exchange and preventing charge transfer. As a result, the white PeLED shows balanced white light emission with Commission Internationale de L'Eclairage coordinates of (0.33, 0.33). In addition, we find that the achievement of white light emission from mixed halide perovskites strongly depends on effective modulation of the halide salt precursors, especially lead bromide and benzamidine hydrochloride in our case. Our work provides very useful guidelines for realizing single-emissive-layer all-perovskite white PeLEDs based on mixed halide perovskites, which will spur the development of high-performance white PeLEDs.

CsPbI2Br Perovskite Solar Cells Based on Carbon Black-Containing Counter Electrodes

CsPbI₂Br perovskite solar cells (PSCs) based on carbon electrodes (CEs) are considered to be low-cost and thermally stable devices. Nevertheless, the insufficient contact and energy level mismatch between the CsPbI₂Br layer and CE hinder the further enhancement of the cell efficiency. Herein, a carbon black (CB) interlayer was introduced between the perovskite layer and CE. The hole extraction was facilitated due to the larger contact area and suitable energy band alignment in the CsPbI₂Br/CB interface. Further investigation indicated the diffusion of CB nanoparticles from the CE or CB layer to the CsPbI₂Br film after a certain period of time. We disclosed the formation of a CB-CsPbI₂Br bulk heterojunction structure due to the carbon diffusion, which resulted in an efficiency enhancement. As a result, a record efficiency of 13.13% is achieved for carbon-based inorganic PSCs. This work also reveals that the diffusion of CB nanoparticles in CB-containing PSCs is universal and inevitable, although this kind of diffusion results in the enhancement of cell efficiency.

Versatile vapor phase deposition approach to cesium tin bromide materials CsSnBr3, CsSn2Br5 and Cs2SnBr6

We report on the successful application of RF-magnetron sputtering to deposit, by using a single type of target, three different materials in the form of thin films within the Cs-Sn-Br compositional range, namely, CsSnBr₃, CsSn₂Br₅ and Cs₂SnBr₆. It is shown that, by playing with the deposition parameters and post-deposition treatments, it is possible to stabilize these three perovskites or perovskite related compounds by exploiting the versatility of vapor phase deposition. Full characterization in terms of crystal structure, optical properties and morphology is reported. The power of vapor phase methods in growing all-inorganic materials of interest for photovoltaic and optoelectronic applications is demonstrated here, indicating the advantageous use of sputtering for these complex materials.

Vacuum-Controlled Growth of CsPbI2Br for Highly Efficient and Stable All-Inorganic Perovskite Solar Cells

A high-temperature annealing process (>250 °C) is always needed to obtain high-quality α-CsPbI₂Br perovskite films, which makes it a challenge in the manufacture and application of flexible photovoltaic devices. In this work, a vacuum-controlled growth (VCG) method that can effectively control the crystallization of perovskite and provide high-quality films with larger grain size and low defect density at a lower temperature is demonstrated. Besides, a facile introduction of polyethyleneimine ethoxylated (PEIE) interlayer improves the charge extraction and suppresses the carrier recombination. Therefore, the power conversion efficiency (PCE) of an all-inorganic CsPbI₂Br perovskite solar cell (PSC) reaches 12.32%. The unencapsulated PSCs with VCG treatment and PEIE modification show outstanding stabilities while retaining over 95% of the initial PCE after being stored in a N₂ glovebox for 1000 h. This low-temperature crystallization method and cheap interlayer introduction can drive the development of future commercialization of all-inorganic perovskite solar cells.

Black phosphorus quantum dots in inorganic perovskite thin films for efficient photovoltaic application

Black phosphorus quantum dots (BPQDs) are proposed as effective seed-like sites to modulate the nucleation and growth of CsPbI₂Br perovskite crystalline thin layers, allowing an enhanced crystallization and remarkable morphological improvement. We reveal that the lone-pair electrons of BPQDs can induce strong binding between molecules of the CsPbI₂Br precursor solution and phosphorus atoms stemming from the concomitant reduction in coulombic repulsion. The four-phase transition during the annealing process yields an α-phase CsPbI₂Br stabilized by BPQDs. The BPQDS/CsPbI₂Br core-shell structure concomitantly reinforces a stable CsPbI₂Br crystallite and suppresses the oxidation of BPQDs. Consequently, a power conversion efficiency of 15.47% can be achieved for 0.7 wt % BPQDs embedded in CsPbI₂Br film-based devices, with an enhanced cell stability, under ambient conditions. Our finding is a decisive step in the exploration of crystallization and phase stability that can lead to the realization of efficient and stable inorganic perovskite solar cells.

Suppression of Iodide Ion Migration via Sb2S3 Interfacial Modification for Stable Inorganic Perovskite Solar Cells

In mixed halide perovskite, the halide phase segregation is commonly observed due to halide ion migration, which causes severe stability issues in perovskite devices. Here, we directly revealed the iodide-migration process via potentiostatic treatment in CsPbIBr₂ perovskite. The absence of iodide ions was reduced significantly via a Sb₂S₃ interfacial modification. We further employed the density functional theory (DFT) calculation to optimize the geometry positions at the perovskite interface and radial distribution functions (RDF) to analyze the atom perturbation. The simulation yielded a slight distortion of the perovskite lattice at the Sb₂S₃-CsPbIBr₂ interface and iodide ion fluctuation was reduced due to the decrease of halide vacancies. In addition, the thermally stimulated current was calculated to evaluate the defect density in the modified perovskite device. Due to the Sb₂S₃ interaction with perovskite, the device became stable against humidity and maintained its photoactivity over 400 h. The champion efficiency of 9.31% with 26.31% improvement was obtained in modified CsPbIBr₂ perovskite solar cells.

Thermally Stable Inorganic CsPbI2Br Mesoscopic Metal Halide Perovskite Solar Submodules

Highly efficient and thermally stable inorganic CsPbI₂Br mesoscopic metal halide perovskite (MHP) solar cells with a poly-3-hexylthiophene (P3HT) hole transporting layer (HTL) are demonstrated by spin-washing of the P3HT HTL since the light harvesting efficiency is improved by minimizing the coabsorption of light by P3HT, and the open-circuit voltage is enhanced because of the increased valence band maximum position of the spin-washed P3HT HTL. The spin-washed CsPbI₂Br MHP solar cell exhibited 1.24 V open-circuit voltage (V_oc), 14.20 mA/cm² short-circuit current density (J_sc), 81.52% fill factor (FF), and 14.35% power conversion efficiency (PCE). The unencapsulated spin-washed CsPbI₂Br MHP solar cell went through 7.56% degradation after a 1000 h thermal stability test under 100 °C/25% relative humidity (RH) and simultaneous 1 sun light soaking conditions. In addition, the unencapsulated spin-washed CsPbI₂Br MHP solar submodule with 25 cm² of masked active area showed a 98% geometrical FF, 115.09 mA short-circuit current, 3.54 V V_oc, 71.09% FF, and 11.58% PCE while exhibiting 8.80% of degradation during a thermal stability test at 100 °C/25% RH and 1 sun light soaking for 1000 h.

Dual Interfacial Design for Efficient CsPbI2 Br Perovskite Solar Cells with Improved Photostability

A synergic interface design is demonstrated for photostable inorganic mixed-halide perovskite solar cells (PVSCs) by applying an amino-functionalized polymer (PN4N) as cathode interlayer and a dopant-free hole-transporting polymer poly[5,5'-bis(2-butyloctyl)-(2,2'-bithiophene)-4,4'-dicarboxylate-alt-5,5'-2,2'-bithiophene] (PDCBT) as anode interlayer. First, the interfacial dipole formed at the cathode interface reduces the workfunction of SnO₂ , while PDCBT with deeper-lying highest occupied molecular orbital (HOMO) level provides a better energy-level matching at the anode, leading to a significant enhancement in open-circuit voltage (V_oc ) of the PVSCs. Second, the PN4N layer can also tune the surface wetting property to promote the growth of high-quality all-inorganic perovskite films with larger grain size and higher crystallinity. Most importantly, both theoretical and experimental results reveal that PN4N and PDCBT can interact strongly with the perovskite crystal, which effectively passivates the electronic surface trap states and suppresses the photoinduced halide segregation of CsPbI₂ Br films. Therefore, the optimized CsPbI₂ Br PVSCs exhibit reduced interfacial recombination with efficiency over 16%, which is one of the highest efficiencies reported for all-inorganic PVSCs. A high photostability with a less than 10% efficiency drop is demonstrated for the CsPbI₂ Br PVSCs with dual interfacial modifications under continuous 1 sun equivalent illumination for 400 h.

Hysteresis-Less CsPbI2Br Mesoscopic Perovskite Solar Cells with a High Open-Circuit Voltage Exceeding 1.3 V and 14.86% of Power Conversion Efficiency

High-performance and hysteresis-less mesoscopic CsPbI₂Br perovskite solar cells (PSCs) are demonstrated by adapting hole-transporting materials (HTMs) with controlled highest occupied molecular orbital (HOMO) values. The used model HTMs are poly-3-hexylthiophene (P3HT), poly-triarylamine (P-TAA), poly-fluoren-8-triarylamine (PF8-TAA), and poly-indenofluoren-8-triarylamine (PIF8-TAA), and their HOMO energy levels position to -4.98, -5.09, -5.45, and -5.52 eV, respectively. By controlling the HOMO of the HTMs, the average open-circuit voltages of 25 mesoscopic CsPbI₂Br PSCs are controllable from 1.11 ± 0.030 V for a P3HT HTM-based device to 1.17 ± 0.023, 1.21 ± 0.027, and 1.27 ± 0.028 V for P-TAA, PF8-TAA, and PIF8-TAA HTM-based devices. As a result, the PIF8-TAA HTM-based mesoscopic PSC exhibits the highest open-circuit voltage of 1.31 V and power conversion efficiency (PCE) of 14.20% for the forward scan condition and 14.86% for the reverse scan condition under 1 sun illumination (100 mW/cm² AM 1.5G). In addition, the unencapsulated mesoscopic CsPbI₂Br PSCs exhibited 10-14% of PCE degradation compared to their initial efficiency in maximum power point tracking under continuous 1 sun light soaking at 85 °C for 1000 h.

Excess Cesium Iodide Induces Spinodal Decomposition of CsPbI2Br Perovskite Films

We report an exploratory study on the crystal formation behavior of CsPbI₂Br perovskite films by adding excess cesium iodide (CsI). Surprisingly, facile co-crystallization of CsI and CsPbI₂Br in the form of spinodal decomposition is observed. Significantly, the two phases spontaneously form morphing into a remarkably uniform bicontinuous nanoscale blend with high orientational correlation through the well-matched (110) plane of CsI and the (200) plane of CsPbI₂Br. The CsPbI₂Br films produced by the spinodal decomposition method not only enjoy a compact surface, low defect concentration, and long carrier lifetimes, they also retain their excellent charge transport property. By employing such a CsPbI₂Br film for carbon-based perovskite solar cells, power conversion efficiency exceeding 10% is achieved with remarkable thermal stability. Our results provide valuable insight into the role of CsI in perovskite crystallization and a promising approach for designing inorganic halide perovskite-based devices.

Low-Temperature Stable α-Phase Inorganic Perovskite Compounds via Crystal Cross-Linking

Inorganic perovskite compounds hold promise to improve the stability of perovskite solar cells (PSCs). However, the phase instability and high annealing temperature process of these metastable structural polymorphs such as α-phase CsPbI_{3- x}Br _x hinder their further development. Herein, we demonstrated the successful reduction of temperature from 250 to 100 °C during the growth of stable and high-crystallinity α-phase CsPbI₂Br via crystal cross-linking. 4-Aminobenzoic acid (ABA) was used as a nonvolatile additive with slight solubility in water to bridge the adjacent CsPbI₂Br to form a grain-interconnected film. The interaction between ABA and perovskite grains leads to the growth of highly crystalline perovskite films. The optimized PSC based on ABA-incorporated CsPbI₂Br using a TiO₂/Al₂O₃/NiO/carbon architecture exhibits an efficiency of 8.44%. More importantly, the additive enables significant improvement of phase stability against moisture. The good performance highlights the chemical modification via cross-linking to achieve efficient and stable inorganic perovskite devices.

Highly crystalline CsPbI2Br films for efficient perovskite solar cells via compositional engineering

All-inorganic CsPbI₂Br shows high thermal stability for promising application in perovskite solar cells (PSCs). The performance of PSCs is significantly affected by their morphology and crystallinity induced by compositional ratio, solvent/anti-solvent engineering and post thermal annealing. In this study, the compositional ratio effect of two precursors, PbI₂ and CsBr, on the power conversion efficiency (PCE) of a device with ITO/SnO₂/CsPbI₂Br/Spiro-MeOTAD/Au structure was investigated. With the assistance of anti-solvent chlorobenzene, perovskite with a PbI₂ : CsBr ratio of 1.05 : 1 showed a high quality thin film with higher crystallinity and larger grain size. In addition, the molar ratio of precursors PbI₂ and CsBr improved the PCE of the PSCs, and the PSCs fabricated using the perovskite with an optimal ratio of PbI₂ and CsBr exhibited a PCE of 13.34%.

All-inorganic CsPbI₂Br shows high thermal stability for promising application in perovskite solar cells (PSCs).

Effect of Structure Change on Luminescent Properties of CsPbBr2I Perovskite Nanocrystals after Heat Treatment

Lead halide perovskite nanocrystals (NCs) are still a popular subject in the field of luminescence because of their high photoluminescence quantum yield (PLQY) and colour tenability through anion exchange reactions or quantum confinement. We investigated the structure and luminescence properties of the mixed halide perovskites CsPbBr2I NCs with annealing temperatures. The results showed that the average grain size of CsPbBr2I increased with increasing the temperature from 300 to 440K, and the morphology changed above 380K. The normalised PL spectra of CsPbBr2I NCs had an obvious blue-shift, and then displayed a slight red-shift with increasing annealing temperature. In addition, the PL intensities, peak energies, and bandwidths of the NC films as a function of temperature were discussed in detail.

Iodine-Optimized Interface for Inorganic CsPbI2Br Perovskite Solar Cell to Attain High Stabilized Efficiency Exceeding 14

Recently, inorganic CsPbI2Br perovskite is attracting ever-increasing attention for its outstanding optoelectronic properties and ambient phase stability. Here, an efficient CsPbI2Br perovskite solar cell (PSC) is developed by: 1) using a dimension-grading heterojunction based on a quantum dots (QDs)/bulk film structure, and 2) post-treatment of the CsPbI2Br QDs/film with organic iodine salt to form an ultrathin iodine-ion-enriched perovskite layer on the top of the perovskite film. It is found that the above procedures generate proper band edge bending for improved carrier collection, resulting in effectively decreased recombination loss and improved hole extraction efficiency. Meanwhile, the organic capping layer from the iodine salt also surrounds the QDs and tunes the surface chemistry for further improved charge transport at the interface. As a result, the champion device achieves long-term stabilized power conversion efficiency beyond 14%.

CsBr-Induced Stable CsPbI3- xBr x ( x < 1) Perovskite Films at Low Temperature for Highly Efficient Planar Heterojunction Solar Cells

All-inorganic cesium lead perovskites have emerged as alternative absorbing layers in solar cells owing to their superb thermal stability compared with the organic-inorganic hybrid perovskites. However, the desired cubic CsPbI₃ phase forms at a high temperature and suffers from a phase transition to the orthorhombic yellow phase at room temperature. A developed nonstoichiometric method is applied to fabricate CsPbI_{3- x}Br _x ( x < 1) films by adding excess CsBr into the precursor solution. The excess CsBr in the precursor solution helps to produce a microstrain in the lattice to stabilize the cubic CsPbI₃ phase at low temperature and incorporate a small part of Br^- into the CsPbI₃ lattice. At the optimal CsBr concentration (0.5 M), the corresponding solar cell achieves a power conversion efficiency of 10.92%. This work provides an effective way to stabilize the cubic CsPbI_{3- x}Br _x ( x < 1) phase at low temperature to further improve the performance of all-inorganic perovskite solar cells.

Low Temperature Fabrication for High Performance Flexible CsPbI2Br Perovskite Solar Cells

All-inorganic CsPbX3-based perovskites, such as CsPbI2Br, show much better thermal and illumination stability than their organic-inorganic hybrid counterparts. However, fabrication of high-quality CsPbI2Br perovskite film normally requires annealing at a high temperature (>250 °C) that is not compatible with the plastic substrate. In this work, a Lewis base adduct-promoted growth process that makes it possible to fabricate high quality CsPbI2Br perovskite films at low temperature is promoted. The mechanism is attributed to synthesized dimethyl sulfoxide (DMSO) adducts which allow a low activation energy route to form CsPbI2Br perovskite films during the thermal annealing treatment. A power conversion efficiency (PCE) of 13.54% is achieved. As far as it is known, this is the highest efficiency for the CsPbI2Br solar cells fabricated at low temperature (120 °C). In addition, the method enables fabrication of flexible CsPbI2Br PSCs with PCE as high as 11.73%. Surprisingly, the bare devices without any encapsulation maintain 70% of their original PCEs after being stored in ambient air for 700 h. This work provides an approach for preparing other high performance CsPbX3-based perovskite solar cells (PSCs) at low temperature, particularly for flexible ones.

Room Temperature Processing of Inorganic Perovskite Films to Enable Flexible Solar Cells

Inorganic lead halide perovskite materials have attracted great attention recently due to their potential for greater thermal stability compared with hybrid organic perovskites. However, the high processing temperature to convert from the non-perovskite phase to the cubic perovskite phase in many of these systems has limited their application in flexible optoelectronic devices. Here, we report a room temperature processed inorganic perovskite solar cell (PSC) based on CsPbI2Br as the light harvesting layer. By combining this composition with key precursor solvents, we show that inorganic perovskite films can be prepared by the vacuum-assist method under room temperature conditions in air. Unencapsulated devices achieved power conversion efficiency up to 8.67% when measured under 1-sun irradiation. Exploiting this room temperature process, flexible inorganic PSCs based on an inorganic metal halide perovskite material are demonstrated.

Li dopant induces moisture sensitive phase degradation of an all-inorganic CsPbI2Br perovskite

All-inorganic halide perovskites without volatile components have great potential for long term thermal stability. However, the phase stability of all-inorganic perovskites is sensitive to moisture and has been seldom studied. In this work, the phase stability of α-CsPbI2Br was studied in different relative humidity conditions. The moisture resistance of α-CsPbI2Br can be significantly enhanced by using a polymer or organic molecule capping layer including Spiro-MeOTAD. Although an all-inorganic CsPbI2Br perovskite based device using the typical Li salt doped Spiro-MeOTAD layer could offer an efficiency up to 12.6%, it exhibited even worse moisture resistance than a bare perovskite film under the same ambient conditions. This unusual phenomenon is ascribed to the hygroscopic properties of Li-TFSI inducing moisture sensitive phase degradation. Therefore, it is desirable to decrease the amount of, or avoid the Li salt, as a dopant for stable all-inorganic perovskite solar cells with balanced stability and high performance.

Methodologies toward Efficient and Stable Cesium Lead Halide Perovskite-Based Solar Cells

In an attempt to replace thermally vulnerable organic perovskites, considerable research effort has recently been focused on the development of all-inorganic perovskites in the field of photovoltaics. The preceding studies demonstrated that cesium lead halide perovskites are promising candidates for thermally stable and efficient solar cell materials. Here, the recent progress in cesium lead halide perovskite-based solar cells is summarized. Whether organic cations are essential for the superiority of halide perovskites is controversial. However, more than 13% efficient solar cells have been successfully fabricated by employing cesium lead halide perovskites in a short amount of time. The state-of-the-art materials engineering techniques will help to achieve a remarkable photovoltaic performance comparable to that of organic perovskites. In addition, improved understanding of the intrinsic photophysical behaviors will provide new insights that will facilitate further improvements in solar cell applications.

High-Voltage-Efficiency Inorganic Perovskite Solar Cells in a Wide Solution-Processing Window

Inorganic halide perovskites exhibit significant photovoltaic performance due to their structural stability and high open-circuit voltage ( V_oc). Herein, a general strategy of solution engineering has been implemented to enable a wide solution-processing window for high V_oc (∼1.3 V) and power conversion efficiency (PCE, ∼12.5%). We introduce a nontoxic solvent of dimethyl sulfoxide (DMSO) and an assisted heating process in the fabrication of CsPbI₂Br (CPI2) to control the improved crystallization. A wide solution-processing window including a wide range of solvent components and solute concentrations has been realized. The CPI2-based inorganic perovskite solar cells (IPSCs) exhibit a high PCE up to 12.52%. More importantly, these devices demonstrate a remarkable V_oc of 1.315 V. The performance has possessed such a region with high V_oc and PCE in all Cs-based IPSCs, unveiling wide solution-processing windows with enhanced solution processability facilitating potential industrial application especially for tandem solar cells.

Interface Engineering for All-Inorganic CsPbI2 Br Perovskite Solar Cells with Efficiency over 14

In this work, a SnO2 /ZnO bilayered electron transporting layer (ETL) aimed to achieve low energy loss and large open-circuit voltage (Voc ) for high-efficiency all-inorganic CsPbI2 Br perovskite solar cells (PVSCs) is introduced. The high-quality CsPbI2 Br film with regular crystal grains and full coverage can be realized on the SnO2 /ZnO surface. The higher-lying conduction band minimum of ZnO facilitates desirable cascade energy level alignment between the perovskite and SnO2 /ZnO bilayered ETL with superior electron extraction capability, resulting in a suppressed interfacial trap-assisted recombination with lower charge recombination rate and greater charge extraction efficiency. The as-optimized all-inorganic PVSC delivers a high Voc of 1.23 V and power conversion efficiency (PCE) of 14.6%, which is one of the best efficiencies reported for the Cs-based all-inorganic PVSCs to date. More importantly, decent thermal stability with only 20% PCE loss is demonstrated for the SnO2 /ZnO-based CsPbI2 Br PVSCs after being heated at 85 °C for 300 h. These findings provide important interface design insights that will be crucial to further improve the efficiency of all-inorganic PVSCs in the future.

Polymer-Passivated Inorganic Cesium Lead Mixed-Halide Perovskites for Stable and Efficient Solar Cells with High Open-Circuit Voltage over 1.3 V

Cesium-based trihalide perovskites have been demonstrated as promising light absorbers for photovoltaic applications due to their superb composition stability. However, the large energy losses (E_loss ) observed in inorganic perovskite solar cells has become a major hindrance impairing the ultimate efficiency. Here, an effective and reproducible method of modifying the interface between a CsPbI₂ Br absorber and polythiophene hole-acceptor to minimize the E_loss is reported. It is demonstrated that polythiophene, deposited on the top of CsPbI₂ Br, can significantly reduce electron-hole recombination within the perovskite, which is due to the electronic passivation of surface defect states. In addition, the interfacial properties are improved by a simple annealing process, leading to significantly reduced energy disorder in polythiophene and enhanced hole-injection into the hole-acceptor. Consequently, one of the highest power conversion efficiency (PCE) of 12.02% from a reverse scan in inorganic mixed-halide perovskite solar cells is obtained. Modifying the perovskite films with annealing polythiophene enables an open-circuit voltage (V_OC ) of up to 1.32 V and E_loss of down to 0.5 eV, which both are the optimal values reported among cesium-lead mixed-halide perovskite solar cells to date. This method provides a new route to further improve the efficiency of perovskite solar cells by minimizing the E_loss .