Organic and inorganic hybrid perovskites (e.g., CH₃NH₃PbI₃), with advantages of facile processing, tunable bandgaps, and superior charge-transfer properties, have emerged as a new class of revolutionary optoelectronic semiconductors promising for various applications. Perovskite solar cells constructed with a variety of configurations have demonstrated unprecedented progress in efficiency, reaching about 20% from multiple groups after only several years of active research. A key to this success is the development of various solution-synthesis and film-deposition techniques for controlling the morphology and composition of hybrid perovskites. The rapid progress in material synthesis and device fabrication has also promoted the development of other optoelectronic applications including light-emitting diodes, photodetectors, and transistors. Both experimental and theoretical investigations on organic–inorganic hybrid perovskites have enabled some critical fundamental understandings of this material system. Recent studies have also demonstrated progress in addressing the potential stability issue, which has been identified as a main challenge for future research on halide perovskites. Here, we review recent progress on hybrid perovskites including basic chemical and crystal structures, chemical synthesis of bulk/nanocrystals and thin films with their chemical and physical properties, device configurations, operation principles for various optoelectronic applications (with a focus on solar cells), and photophysics of charge-carrier dynamics. We also discuss the importance of further understanding of the fundamental properties of hybrid perovskites, especially those related to chemical and structural stabilities.

Tetragonal phases for perovskite solar cells: The power conversion efficiencies (PCEs) of all-inorganic perovskites are lower than those of materials with organic cations. This is in part because these materials have larger bandgaps. The cubic crystal phases of these materials also exhibit poor stability. Wang et al. synthesized the tetragonal β-phase of CsPbI ₃ from HPbI ₃ and CsI. The material exhibited higher stability and a more favorable bandgap, which allowed for PCEs of 15%. Passivation of the surface trap state with choline iodide boosted PCEs to 18%. Science , this issue p. 591

Insights into the chemical stability and instability of inorganic halide perovskites are provided.

Among various all-inorganic halide perovskites exhibiting better stability than organic-inorganic halide perovskites, α-CsPbI₃ with the most suitable band gap for tandem solar cell application faces an issue of phase instability under ambient conditions. We discovered that a small amount of two-dimensional (2D) EDAPbI₄ perovskite containing the ethylenediamine (EDA) cation stabilizes the α-CsPbI₃ to avoid the undesirable formation of the nonperovskite δ phase. Moreover, not only the 2D perovskite of EDAPbI₄ facilitate the formation of α-CsPbI₃ perovskite films exhibiting high phase stability at room temperature for months and at 100°C for >150 hours but also the α-CsPbI₃ perovskite solar cells (PSCs) display highly reproducible efficiency of 11.8%, a record for all-inorganic lead halide PSCs. Therefore, using the bication EDA presents a novel and promising strategy to design all-inorganic lead halide PSCs with high performance and reliability.