Importance of Low Humidity and Selection of Halide Ions of Octylammonium Halide in 2D-3D Perovskite Solar Cells Fabricated in Air

Air fabrication of efficient and stable perovskite solar cells with substantial reproducibility is of importance for commercialization of perovskite solar cells. There has been little consensus on which halide is the best selection for thin 2D perovskite layer on 3D perovskite when processing in ambient air. In this work, the influence of humidity and halide ions in alkylammonium halide (OAX)-treated perovskite layer on photovoltaic performance is investigated. The authors find that a combination of high humidity and the presence of chloride ions can induce the deprotonation of methylammonium (MA(+)) ions. With X-ray diffraction, cross-sectional scanning electron microscope, Fourier-transform infrared spectroscopy, the authors elucidate that the deprotonation reaction in the octylammonium chloride (OACl)-treated perovskite layer makes MA(+) ions (CH3NH3+) volatile MA (CH3NH2), which results in void volumes and defects in the bulk of perovskite layer as well as at the interface. The defective perovskite solar cells induced by the deprotonation reaction have the suppressed hole transfer at the interface between the perovskite layer and the hole transport layer, yielding the reduced internal quantum efficiency and J(SC). Space charge-limited current analysis reveals that the reduced V-OC is caused by the high trap density estimated from V-TFL of hole-only devices.