Unraveling Ion\nMigration Mechanisms under Operating\nConditions in Perovskite Solar Cells by Variable-Load Transient Photoelectric\nTechnique

Ion migration in perovskite solar cells (PSCs) significantly\nimpacts\ntheir photoelectric performance and physicochemical stability. Existing\nresearch has predominantly focused on inhibiting ion migration through\nchemical strategies or observing it under open-circuit or short-circuit\nconditions. This focus has led to a limited theoretical understanding\nand control of ion migration under practical conditions, constraining\nadvances in long-term stability. In this study, we introduce a novel\nvariable-load transient photoelectric technique (VL-TPT) to investigate\nion migration dynamics in PSCs under practical operating conditions.\nResults show that ion accumulation correlates with photogenerated\ncarrier concentration under open-circuit conditions. During operation,\nion accumulation decreases with reduced load, because charge is transferred\nto the external circuit, leading to a reduction in carrier concentration\nwithin the device. An unusual increase in interface ions is observed\nat low loads due to interactions between charges accumulated in the\npotential well and ions. Introducing FA<sup>+</sup> in MA<sub>0.75</sub>FA<sub>0.25</sub>PbI<sub>3</sub> devices suppresses ion migration\nin the open-circuit state but accelerates interface ion buildup under\noperating conditions. These findings provide valuable insights for\nenhancing device stability and performance.