Cesium Halides-Assisted Crystal Growth of Perovskite Films for Efficient Planar Heterojunction Solar Cells

Efficient planar heterojunction perovskite solar cells were successfully demonstrated by using a two-step film fabrication method with postinterdiffusion of cesium halides and formamidinium iodide (FAI). It is found that the incorporation of a small fraction of cesium halides, such as CsI and CsBr, into FAPbI₃(Cl) lattice could effectively increase the grain size and film quality, leading to improved and balanced charge mobility, reduced carrier recombination, and long carrier lifetime. With these, the cesium halides-based perovskite films fabricated using our method result in a champion device with a power conversion efficiency (PCE) of 20.43% and a stabilized PCE of 19.60%, combined with a very notable fill factor (FF) of 81.58%, which could be mainly attributed to the increased grains size of perovskite film with reduced carrier recombination.