Well-grown low-defect MAPbI3–xClx films for perovskite solar cells with over 20% efficiency fabricated under controlled ambient humidity conditions

The conventional [(CH3NH3I (MAI)):PbI2:PbCl2 = 3 : 0: 1 [abbreviated as (3:0:1)]] precursor solution is known to result in CH3NH3PbI3–xClx films with large grain sizes when processed in an inert atmosphere, but it gives non-uniform perovskite films containing lots of voids and cracks when processed in ambient air. Furthermore, a dramatically longer annealing time (usually 100 min) is required for these films (3:0:1) due to the slow formation of the MAPbI3 phase via MACl loss, which is not conducive to perovskite film formation under ambient conditions due to perovskite degradation upon long exposure to moisture. Pure MAPbI3 films can be formed very rapidly from (1:1:0) (MAI:PbI2:PbCl2 = 1 : 1: 0) solution within a short annealing time, but they show small grain sizes and poor film quality. This work demonstrated that a fractional substitution of PbI2 with PbCl2 in the ([MAI]:[PbI2] = 1 : 1) precursor solution has a significant influence on film morphology and quality in terms of crystallization rate, grain size, crystallinity, and trap density of the formed perovskite film. Perovskite films can be formed with 5-min annealing at 100 °C from the precursor (MAI: PbI2:PbCl2 = 1: 0.8 : 0.2) processed in ambient air (humidity, 20% RH), exhibiting more uniform, increased grain size and higher film quality with reduced trap densities compared to film (1:1:0), thus leading to significantly improved power conversion efficiency (PCE), from 16.7% for perovskite solar cells (PrSCs) based on film (1:1:0) to 20.04% for the cell based on film (1:0.8:0.2). Further, the effects of R (R = [MAI]/[PbI2+PbCl2]) on morphology, hole mobility, carrier lifetime and efficiency of PrSCs were systematically and thoroughly investigated. This study found that MAPbI3–xClx at R = 1 can enable the highest hole mobility and longest carrier lifetime, thus giving the best performance at R = 1.