Irreversible Performance Degradation Driven by Lithium-Ion Migration in FAPbI₃ Perovskite Solar Cells

The typical n-i-p structured perovskite solar cells (PSCs) incorporate 2,2′,7,7′-Tetrakis (N, N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) as a hole transporting material. The chemical doping process of the spiro-OMeTAD involves a redox reaction with bis(trifluoromethane) sulfonimide lithium salt (Li-TFSI) accompanying complex side reactions of which impact on device performance is not fully understood. Here, we investigate correlation between aging dependent device performance of widely used formamidinium lead tri-iodides (FAPbI₃) based PSCs and migration of lithium-ion (Li⁺) generated from Li-TFSI. The cross-sectional Kelvin probe force microscopy unraveled aging time dependent change in hetero-interface energetics in PSCs, which was correlated with progressive migration of Li⁺ ions generated from Li-TFSI confirmed by Time-of-flight secondary ion mass spectroscopy. Comprehensive analysis revealed that the Li⁺ migrate from spiro-OMeTAD to perovskite, SnO_2, and their carrier extraction interfaces to induce phase back conversion of α-FAPbI₃ to δ-FAPbI₃ with local microstrain, and consequent generation and migration of iodine defects and dedoping of the spiro-OMeTAD. The rapid performance drop of PSCs even aging under dark conditions was attributed to a series of these processes. Our work unraveled the hidden side effect of Li⁺ ion migration in FAPbI₃ based PSCs that can guide further work to maximize the operational stability of the PSCs.