Stabilizing black-phase FAPbI3 in humid air with secondary ammoniums

Abstract

Stabilizing black-phase (α) FAPbI₃, the most promising candidate for perovskite solar cells (PSCs), has aroused great interest. Although many efforts have been made, such as adopting various primary ammoniums as additives, FAPbI₃ films and PSCs are still fragile in highly humid air. We herein report the use of secondary ammoniums (dimethylammonium (DMA), diethylammonium (DEA) and diisopropylammonium (DiPA)) to stabilize α-FAPbI₃ and elucidate their substituent-dependent distributions and functions in FAPbI₃ films. While the smallest DMA entering the FAPbI₃ lattices is detrimental to humidity stability, the linear DEA and bulky DiPA mainly locating on the surface and at the grain boundary of the FAPbI₃ film, respectively, block water invasion and lead to prominent phase stability under a high relative humidity of 60%∼80%. Moreover, the surface-enriched DEA can efficiently passivate the defects of the FAPbI₃ film, offering efficiencies of 23.38% for PSCs based on a doped hole transport layer (HTL) and 21.14% for those based on a dopant-free HTL with outstanding humidity stability. This work demonstrates that secondary ammoniums with an appropriate molecular configuration can be an effective solution to fortify the phase stability of FAPbI₃ for efficient PSCs.