Defects Healing in Two-Step Deposited Perovskite Solar Cells via Formamidinium Iodide Compensation

Photovoltaics based on metal halide perovskites have recently achieved a certificated efficiency of 25.2%. One of the factors that limit further development of these devices comes from the defective boundaries between crystalline domains in perovskite solar cells (PSCs). Such boundaries represent a significant loss channel causing nonradiative recombination, but systematic optimization procedures have not been developed yet to control their properties. Herein, we propose a facile but effective defect healing method to passivate the defects along the grain boundaries in PSCs by post-treatment of formamidinium iodide (FAI) solution in isopropyl alcohol (IPA). We use a combination of methods including space-charge-limited current, steady-state and time-resolved photoluminescence, confocal laser scanning microscopy, and transient absorption spectroscopy to show the reduction of density of defect states in perovskite films processed with 1 mg/mL FAI solution. The resultant FAI healed PSCs achieve an average power conversion efficiency of 19.26% (with a champion efficiency of 20.62%), higher than that of 16.45% in the control cell. FAI healed devices without encapsulation maintain nearly 95% of the initial efficiency after 60-day storage under N₂ environment and nearly 78% of the initial efficiency after 30-day storage under the ambient condition with varied humidity. Our results demonstrate that FAI healing is an effective way to passivate the defect states along grain boundaries for high-efficiency and stable PSCs.