Observation of Negative Photoconductivity in (CH₃NH₃)₃Bi₂(Br_xCl_1–x)₉: Correlating Ion Migration, Stability, and Efficiency in Mixed Halide Perovskite Solar Cell

Electric field and light induced ion migration resulting in poor stability is a key hindrance for the development of perovskite solar cells and devices. Methods to identify the pathways of ion migration and subsequently to develop strategies to suppress them are prerequisites to develop perovskite solar cells with improved stability and high efficiency. In this report, we probe ion migration through the observation of negative photocurrent (I) measured by temperature (100–283 K) dependent transient photocurrent response characteristics (I–t) as well as current–voltage (I–V) characteristics under dark and light conditions. Controlled experiments are carried out on lead (Pb)-free hybrid perovskite (CH₃NH₃)₃Bi₂(Br_xCl_1–x)₉ (where x = 0, 0.33, 0.44, 0.55, 0.66) samples with varying Br/Cl ratio. The mixed halide system is found to have the lowest bandgap of 2.557 eV corresponding to bromine content x = 0.44. Incorporation of bromine has a visible effect on the flake-like microstructures of the perovskite materials. While, the I–V characteristics confirmed the ion migration and phase segregation of halide ions and their irreversibility in the system, the I–t profiles showed the existence of an induced electric field acting in the direction opposite to the applied field due to the migration of halide ions and creation of halide vacancies, resulting in a negative photocurrent. The I–t profiles are found to be reversible and strongly dependent on temperature as well as the Br/Cl ratio. I–V characteristics under light illumination also display hysteresis behavior, the effect of which gets reduced with the increase in bromine content. The photocurrent hysteresis and its irreversibility upon change in scanning direction of the applied electric field is believed to be arising out of ion migration dominant in perovskite materials. A comprehensive analysis has been made to infer the charge transfer process and ion migration in the mixed halide system contributing to the stability and efficiency of perovskite solar cells and devices.