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.