All-inorganic CsPbI1.5Br1.5 perovskite solar cells are considered as top cell candidates for tandem cells as a result of their excellent thermal stability and photoelectric performance. However, their power conversion efficiencies (PCEs) are still low and far below the theoretical limit mainly as a result of the severe non-radiative recombination and optical loss. Herein, we introduce an versatile method to construct a surface multi-cationic heterojunction to achieve an efficient and stable CsPbI1.5Br1.5 perovskite solar cell. By precisely controlling the content of FA+ and MA+ on PbBr2-rich perovskite films, a high-quality heterojunction layer is formed to help effectively passivate the surface defects and reduce the optical loss of the CsPbI1.5Br1.5 perovskite. In addition, the incorporation of a heterojunction layer can also improve energy-level alignment and reduce interfacial charge recombination loss. As a result, the champion device with the incorporation of SMH exhibits a PCE of 14.11%, which presents the highest reported efficiency for inorganic CsPbI1.5Br1.5 solar cells thus far while retaining 85% of the initial efficiency after 1000 h of storage without encapsulation.

Over the past decade, organic–inorganic hybrid perovskite solar cells (PVSCs) have shown unprecedented growth in power conversion efficiency (PCE) from 3.8% to 25.7%. However, intrinsic thermal instability and lead toxicity are obstacles limiting its large–scale commercialization. Thus, all-inorganic CsSnI3 perovskite has drawn remarkable interest owing to its nontoxicity, excellent thermal stability, low-cost fabrication, and spectacular photoelectric characteristics, including ideal bandgap range, long carrier lifetime, and large absorption coefficient. Many studies have shown that the device performances are closely related to the morphology and crystallinity of perovskite films. In this review, the physical properties of CsSnI3 perovskite are summarized. Furthermore, this review primarily narrates the recent progress in optimizing the morphology by various strategies such as additive engineering, composition regulation, and deposition techniques, emphasizing their effects on grain sizes, film uniformity, grain boundary, and defect passivation.