Abstract
The power conversion efficiency of hybrid perovskite (PSK) solar cells already exceeds 22%. Unfortunatley, perovskite materials are unstable in ambient operating condition. In this study, we first reported the fabrication of triple-cation, mixed halide PSK materials that contain methylammonium (MA), formamidinium (FA), and cesium with the expected improvement of stability. The stoichiometry of the best film was optimized to be Cs0.1(FA0.83MA0.17)0.9Pb(I0.83Br0.17)3 based on the solar cell performance that reached 15.3%. The comparative stability test was performed with the PSK films irradiated with AM1.5 light while exposed to three types of gaseous ambient for 6 h: dry O2, moist N2, and moist O2, respectively. The as-treated PSK films were then analyzed with XPS, XRD, UV-vis, and SEM in order to derive at the degradation pathway. The degradation products include amines after PSK reaction with O2 and H2O. The relative stabillity of three cations increases from MA to FA and then Cs. The formation of PbO can be identified. Furthermore, the rate of perovskite degradation is accelerated by the superoxide formed from oxygen molecules with electrons generated from light absorption of perovskite. The gaseous HI and HBr are formed after a series of successive reactions. The PSK degradation becomes most severe under the condition where water and oxygen are simultaneously present.