Highly efficient inverted planar solar cell using formamidinium-based quasi-two dimensional perovskites

Formamidinium (FA)-based quasi-two dimensional (Q-2D) perovskite normally has better environmental stability than methylammonium (MA)-based one, however, it is difficult to form larger n (n > 2) phases of layered FA-based perovskite due to their larger formation energies, resulting in insufficient absorption of sunlight and then poor performance of its solar cell. Here we partially replace FA with MA to form (BA)2(FA1-xMAx)3Pb4I13 mixture, which contains enriched larger n (n = 3, 4, and larger) phases, where BA is butylammonium. Particularly, the (BA)2(FA0.8MA0.2)3Pb4I13 perovskite film show the best stability under ambient conditions. Furthermore, using inverted planar heterojunction structure, the photovoltaic device based on (BA)2(FA0.8MA0.2)3Pb4I13 demonstrates power conversion efficiency of 14.2 %, which is among the highest efficiencies for the FA-rich Q-2D perovskites with n ≤ 4. This work highlights that FA-rich Q-2D perovskites have great promise for optoelectronic devices.