Stabilizing black-phase FAPbI3 in humid air with secondary ammoniums

Stabilizing black-phase (alpha) FAPbI(3), 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, FAPbI(3) 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 alpha-FAPbI(3) and elucidate their substituent-dependent distributions and functions in FAPbI(3) films. While the smallest DMA entering the FAPbI(3) lattices is detrimental to humidity stability, the linear DEA and bulky DiPA mainly locating on the surface and at the grain boundary of the FAPbI(3) film, respectively, block water invasion and lead to prominent phase stability under a high relative humidity of 60%similar to 80%. Moreover, the surface-enriched DEA can efficiently passivate the defects of the FAPbI(3) 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 FAPbI(3) for efficient PSCs.