Peculiar Steric Hindrance Assists Monoclinic Phase Formation toward High-Quality All-Inorganic Perovskites

Bromine-containing metal halide all-inorganic perovskite CsPbI2Br exhibits excellent photoelectric performance and supreme thermal and structural stabilities; it is thus attractive for use as photoabsorbing layers in perovskite solar cells (PSCs). However, when steric hindrance molecules are introduced, the complicated phase transition mechanism and the difficult-to-control crystallization process in CsPbI2Br are not well understood. Here, we introduce a class of sterically hindered cesium naphthenate small molecules to control the crystallization process of CsPbI2Br films. Of interest, a new intermediate monoclinic phase has been discovered which leads to formation of dense and nonporous polycrystalline perovskite films. This phenomenon was also explained by density functional theory. The residues of steric hindrance molecules inside the CsPbI2Br film also improve its stability. We further show that as the ring number of cycloalkanes increases, the hindrance for the crystallization becomes more significant. Thus, by choosing the suitable steric hindrance, the optimal photovoltaic efficiency is 15.45%.

含溴金属卤化物全无机钙钛矿CsPbI2Br具备优异的光电性能，同时拥有极佳的热稳定性与结构稳定性，因而在钙钛矿太阳能电池（perovskite solar cells, PSCs）的光吸收层应用中极具潜力。然而，当引入空间位阻分子时，CsPbI2Br体系中复杂的相变机制与难以精准调控的结晶过程尚未得到充分解析。本文引入一类空间位阻型环烷酸铯小分子，用以调控CsPbI2Br薄膜的结晶过程。值得关注的是，本研究发现了一种全新的中间单斜晶相，该相可促成致密无孔的多晶钙钛矿薄膜的形成。这一现象同时通过密度泛函理论（Density Functional Theory）得到了合理解释。CsPbI2Br薄膜内部残留的空间位阻分子亦可进一步提升薄膜的稳定性。进一步研究表明，随着环烷烃的环数增加，其对结晶过程的位阻效应愈发显著。据此，通过选择适配的空间位阻分子，最优光电效率可达15.45%。