Efficient and Stable β-CsPbI3 Solar Cells through Solvent Engineering with Methylamine Acetate Ionic Liquid

CsPbI₃, an all-inorganic perovskite material with suitable band gap and excellent thermal stability, has garnered significant attention for its potential in perovskite solar cells (PSCs). However, CsPbI₃ is susceptible to phase changes from photoactive to photoinactive in humid environments. Hence, it is crucial to achieve controllable growth of CsPbI₃ perovskite thin films with the desired β-crystal phase and compact morphology for efficient and stable PSCs. Herein, MAAc was used as a solvent for the CsPbI₃ precursor to fabricate β-CsPbI₃ perovskite. An intermediate compound of Cs_xMA_1-xPbI_xAc_3-x was initially formed in the MAAc solution, and during annealing, the MA⁺ and Ac^- ions were replaced by Cs⁺ and I^- ions, respectively. Furthermore, the incorporation of strong C═O···Pb coordination stabilized the black-phase β-CsPbI₃ and facilitated the growth of crystals with a narrow vertical orientation and large grain size. As a result, the PSCs with an efficiency of 18.9% and improved stability (less than 10% decay after 2000 h of storage in N₂ and less than 30% decay after 500 h of storage in humid air without any encapsulation) were achieved.

Taking up the quest for novel molecular solar thermal systems: Pros and cons of storing energy with cubane and cubadiene

Molecular solar thermal (MOST) systems are working their way as a possible technology to store solar light and release it when necessary. Such systems could, in principle, constitute a solution to the energy storage problem characteristic of solar cells and are conceived, at a first instance, as simple molecular photoswitches. Nevertheless, the optimization of their different required properties is presently limiting their technological scale up. From the chemical perspective, we need to design a novel MOST system based on unconventional photoswitches. Here, by applying multi-configurational quantum chemistry methods, we unravel the potentialities of ad hoc-designed molecular photoswitches, which aim to photoproduce cubane or cubadiene as high-energy isomers that can be thermally (or eventually catalytically) reverted to the initial structure, releasing their stored energy. Specifically, while cubane can be photoproduced via different paths depending on the reactant tricycle diene conformation, an undesired bicyclic by-product limits its application to MOST systems. An evolution of this starting design toward cubadiene formation is therefore proposed, avoiding conformational equilibria and by-products, considerably red shifting the absorption to reach the visible portion of the solar spectrum and maintaining an estimated storage density that is expected to overcome the current MOST reference system (norbornadiene/quadricyclane), although consistently increasing the photoisomerization energy barrier.

Synchronous Surface Reconstruction and Defect Passivation for High-Performance Inorganic Perovskite Solar Cells

The nonradiative charge recombination caused by surface defects and inferior crystalline quality are major roadblocks to further enhancing the performance of CsPbI_{3- x} Br_x perovskite solar cells (PSCs). Theoretical calculations indicate that sodium diethyldithiocarbamate (NaDDTC), a popular bacteriostatic benign material, can initiate multiple interactions with the CsPbI_{3- x} Br_x perovskite surface to effectively passivate the defects. The experimental results reveal that the NaDDTC can indeed passivate the electron trap states and lock active sites for charge traps and water adsorption. In addition, it is found that a solid-state reaction is triggered for perovskite crystal regrowth by the NaDDTC post-treatment, which not only enlarges grain size for reducing the density of grain boundary defects but also compensates some surface defects induced by the primary film growth. Consequently, the power conversion efficiency (PCE) of the CsPbI_{3- x} Br_x PSC is increased to as high as 20.40%, with significant improvement in fill factor and open-circuit voltage (V_OC ), making it one of the highest for this type of solar cell. Furthermore, the optimized devices exhibit better environmental stability. Overall, this robust synchronous strategy provides efficient surface reconstruction and defect passivation for achieving both high PCE and stable inorganic perovskite.

Vacuum-Assisted Thermal Annealing of CsPbI3 for Highly Stable and Efficient Inorganic Perovskite Solar Cells

Inorganic cesium lead iodide perovskite CsPbI₃ is attracting great attention as a light absorber for single or multi-junction photovoltaics due to its outstanding thermal stability and proper band gap. However, the device performance of CsPbI₃ -based perovskite solar cells (PSCs) is limited by the unsatisfactory crystal quality and thus severe non-radiative recombination. Here, vacuum-assisted thermal annealing (VATA) is demonstrated as an effective approach for controlling the morphology and crystallinity of the CsPbI₃ perovskite films formed from the precursors of PbI₂ , CsI, and dimethylammonium iodide (DMAI). By this method, a large-area and high-quality CsPbI₃ film is obtained, exhibiting a much reduced trap-state density with prolonged charge lifetime. Consequently, the solar cell efficiency is raised from 17.26 to 20.06 %, along with enhanced stability. The VATA would be an effective approach for fabricating high-performance thin-film CsPbI₃ perovskite optoelectronics.

Temperature-Reliable Low-Dimensional Perovskites Passivated Black-Phase CsPbI3 toward Stable and Efficient Photovoltaics

Low-dimensional (LD) perovskites can effectively passivate and stabilize 3D perovskites for high-performance perovskite solar cells (PSCs). Regards CsPbI₃ -based PSCs, the influence of high-temperature annealing on the LD perovskite passivation effect has to be taken into account due to fact the black-phase CsPbI₃ crystallization requires high-temperature treatment, however, which has been rarely concerned so far. Here, the thermal stability of LD perovskites based on three hydrophobic organic ammonium salts and their passivation effect toward CsPbI₃ and the whole device performance, have been investigated. It is found that, phenyltrimethylammonium iodide (PTAI) and its corresponding LD perovskites exhibit excellent thermal stability. Further investigation reveals that PTAI-based LD perovskites are mainly distributed at grain boundaries, which not only enhances the phase stability of CsPbI₃ but also effectively suppresses non-radiative recombination. As a consequence, the champion PSC device based on CsPbI₃ exhibits a record efficiency of 21.0 % with high stability.

Rational Surface-Defect Control via Designed Passivation for High-Efficiency Inorganic Perovskite Solar Cells

Iodine vacancies (V_I ) and undercoordinated Pb²⁺ on the surface of all-inorganic perovskite films are mainly responsible for nonradiative charge recombination. An environmentally benign material, histamine (HA), is used to passivate the V_I in perovskite films. A theoretical study shows that HA bonds to the V_I on the surface of the perovskite film via a Lewis base-acid interaction; an additional hydrogen bond (H-bond) strengthens such interaction owing to the favorable molecular configuration of HA. Undercoordinated Pb²⁺ and Pb clusters are passivated, leading to significantly reduced surface trap density and prolonged charge lifetime within the perovskite films. HA passivation also induces an upward shift of the energy band edge of the perovskite layer, facilitating interfacial hole transfer. The combination of the above raises the solar cell efficiency from 19.5 to 20.8 % under 100 mW cm^-2 illumination, the highest efficiency so far for inorganic metal halide perovskite solar cells (PSCs).

Organic Matrix Assisted Low-temperature Crystallization of Black Phase Inorganic Perovskites

All-inorganic perovskites have attracted increasing attention for applications in perovskite solar cells (PSCs) and optoelectronics, including light-emitting devices (LEDs). Cesium lead halide perovskites with tunable I/Br ratios and a band gap aligning with the sunlight region are promising candidates for PSCs. Although impressive progress has been made to improve device efficiency from the initial 2.9 % with low phase stability to over 20 % with high stability, there are still questions regarding the perovskite crystal growth mechanism, especially at low temperatures. In this Minireview, we summarize recent developments in using an organic matrix, including the addition and use of organic ions, polymers, and solvent molecules, for the crystallization of black phase inorganic perovskites at temperatures lower than the phase transition point. We also discuss possible mechanisms for this low-temperature crystallization and their effect on the stability of black phase perovskites. We conclude with an outlook and perspective for further fabrication of large-scale inorganic perovskites for optoelectronic applications.