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

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.