Irreversible phase back conversion of α‐FAPbI₃ driven by lithium‐ion migration in perovskite solar cells

Abstract Typically n‐i‐p structured perovskite solar cells (PSCs) incorporate 2,2′,7,7′‐tetrakis (N,N‐di‐p‐methoxyphenyl amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) as the hole‐transporting material. Chemical doping of spiro‐OMeTAD involves a lithium bis(trifluoromethyl sulfonyl)imide dopant, causing complex side‐reactions that affect the device performance, which are not fully understood. Here, we investigate the aging‐dependent device performance of widely used formamidinium lead triiodide (FAPbI₃)‐based PSCs correlated with lithium‐ion (Li⁺) migration. Comprehensive analyses reveal that Li⁺ ions migrate from spiro‐OMeTAD to perovskite, SnO₂, and their interfaces to induce the phase‐back conversion of α‐FAPbI₃ to δ‐FAPbI₃, generation and migration of iodine defects, and de‐doping of spiro‐OMeTAD. The rapid performance drop of FAPbI₃‐based PSCs, even aging under dark conditions, is attributed to a series of these processes. This study identifies the hidden side effects of Li⁺ ion migration in FAPbI₃‐based PSCs that can guide further work to maximize the operational stability of PSCs. image