*Biwas Subedi, Chongwen Li, Maxwell M. Junda, Yue Yu, Lei Guan, Kiran Ghimire, Zhaoning Song, Yanfa Yan, Nikolas J. Podraza, University of Toledo, Toledo, OH
Highly efficient solution-processed organic-inorganic halide ABX₃ (A: methylammonium—MA, formamidinium—FA, Cs; B: Pb, Sn; X: I, Br) perovskite solar cells have taken the center stage of photovoltaics research. However, these perovskites exhibit degradation upon exposure to humidity, oxygen, and heat. Preparation of low defect density films that are stable against atmospheric exposure, and development of measurement strategies that account for the atmospheric sensitivity of the perovskites are the present challenges. A combination of photothermal deflection spectroscopy, spectroscopic ellipsometry, and unpolarized transmittance measurements are used to characterize solution processed FA_1-xCs_xPbI₃ and (FASnI₃)_x(MAPbI₃)_1-x films and solar cells. The impact of mixed cationic composition and atmospheric exposure on the defect-induced sub-gap absorption in films, quantified by Urbach energy, are probed and linked to the device performances. In general, FA_1-xCs_xPbI₃ and (FASnI₃)_x(MAPbI₃)_1-x films with the lowest sub-gap absorption correspond to the highest efficiency devices. However, the handling/processing of perovskites can affect the device performances. FA1-xCsxPbI3 device performance indicates that the absorbers in devices are better protected than films against atmospheric degradation. For high Sn-content (FASnI₃)_x(MAPbI₃)_1-x, lower device performance is obtained than expected considering the low values of the Urbach energy, indicating more perovskite degradation occurs during solar cell fabrication.