Irreversible phase back conversion of -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 (FAPbI(3))-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 alpha-FAPbI(3) to delta-FAPbI(3), generation and migration of iodine defects, and de-doping of spiro-OMeTAD. The rapid performance drop of FAPbI(3)-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 FAPbI(3)-based PSCs that can guide further work to maximize the operational stability of PSCs.