Electrochemical and Spectro-Microscopic Analyses of Charge Accumulation and Ion Migration in Dry Processed Perovskite Solar Cells under Electrical Biasing

We study the influence of the electrical biasing on the modification of the chemical composition and the electrical performance of perovskite solar cells (PSCs) by coupling Electrochemical impedance spectroscopy (EIS) and scanning transmission X-ray microscopy (STXM) techniques. EIS reveals the formation of charge accumulation at the interfaces and changes in resistive and capacitive properties. STXM study on PSCs after applying strong electric field for a long biasing time indicates the break-down of methylammonium (MA+) cation promoting iodide ions to migrate and create defects at the interface. This complementary EIS and STXM study allows to suggest a degradation mechanism that includes the migration of iodide ions that leads to interface defects and subsequent degradation of solar cell performance. In addition, we study the evolution of the performance of PSCs under air. We observe an increased hysteresis index on current-voltage curves and fill factor reduction of the perovskite solar cells with ageing in air. EIS measurements show the formation of a capacitive layer resulting from accumulation of iodide ions through modification of the mobile ion concentration and ion mobility.