Investigating the Growth of CH3NH3PbI3 Thin Films on RF‐Sputtered NiOx for Inverted Planar Perovskite Solar Cells: Effect of CH3NH3+ Halide Additives versus CH3NH3+ Halide Vapor Annealing

Abstract Investigating the low efficiency issue of radio frequency‐sputtered nickel oxide (sp‐NiO x )‐based perovskite solar cells (PSCs) due to a limited understanding of the correlation between perovskite growth and sp‐NiO x on the optoelectronic properties and photovoltaic device performance is critical. Herein, the crystallization of methylammonium (MA) lead iodide (MAPbI 3 ) thin film (obtained from stoichiometric precursor ratio) on sp‐NiO x is shown, resulting in appearance of residual PbI 2 grains. This is in contrast to perovskite growth on solution‐processed NiO x . The amount of residual PbI 2 is suppressed by 1) adding excess MACl/MAI additives and 2) annealing the perovskite film in MACl/MAI vapor atmosphere. Structural and morphological results reveal significant reduction in the amount of residual PbI 2 and enhanced grain size for all the cases while photophysical measurements reveal mitigation of trap/defect sites (within the bulk and at the interfaces) only for MACl/MAI vapor annealing case. As a result, photovoltaic devices exhibit improved performance only for the vapor annealing case. These results elucidate the critical role of maintaining stoichiometric ratio in perovskite and its crystallization on sp‐NiO x by eliminating the associated defects (influenced by sp‐NiO x ) in rendering improved performance, which can be insightful to further enhance the performance of PSCs.