FAPbI₃ (FA⁺ = formamidinium) perovskite, one of the most efficient basic components in nowadays perovskite solar cells (PCSs), undergoes spontaneous phase transition while molecular-level investigations are still lacking. Here, we report a structural study on the phase transition process of the FA-based perovskites based on isolation and identification of a series of single crystal intermediates. A key intermediate of 8H phase with unique structure is discovered, which helps elucidate the complete phase evolution and stabilization mechanism of FA-based perovskites. Our new insight based on the Pauling’s rules not only deepens the understanding of the phase stabilization roles of cations and anions in maximizing the concentrations of corner-sharing halogens and cubic cages but also forms suggestion for fabrication of high-performance and long-term stable PCSs by controlling the factors such as composition, precursor solution, stoichiometry, and additives.

FAPbI ₃ (FA ⁺ = 甲脒) 钙钛矿是当今钙钛矿太阳能电池 (PCS) 中最有效的基本成分之一，经历了自发相变，但仍缺乏分子水平的研究。在这里，我们报告了基于一系列单晶中间体的分离和鉴定的 FA 基钙钛矿相变过程的结构研究。发现了具有独特结构的 8H 相的关键中间体，这有助于阐明 FA 基钙钛矿的完整相演化和稳定机制。