The two-step solution-based process has demonstrated substantial success in fabricating high-efficiency perovskite solar cells in recent years. Despite the high performance, the underlying mechanisms that govern the formation of perovskite films and corresponding device performance are yet to be fully understood. Particularly, organic cation composition used in the two-step solution processing of mixed-cation lead halide perovskite solar cells plays a critical role in the perovskite film formation and the resultant device performance. However, little is understood about the impacts of organic cation composition on the current density-voltage (J-V) hysteretic behavior and stability of perovskite solar cells. To address this need, here, we study the effect of mixed organic cations, that is, the fraction of formamidinium (FA) and methylammonium (MA) contents, used for the two-step solution-processed perovskite thin films on solar cell performance, including efficiency, J-V hysteresis, and stability. In addition to the efficiency variations, we find that perovskite solar cells based on FA-rich and MA-rich stoichiometries show distinct characteristics in J-V hysteresis and stability. The origins of such a discrepancy are attributed to the thermodynamically driven conversion from lead iodide to perovskites, which is determined by the combination of organic cations. The perovskite solar cells based on the mixed cation FA0.6MA0.4PbI3 composition show a champion power conversion efficiency of over 21% and robust stability (retaining more than 90% of initial efficiency) under maximum power-point tracking in dry nitrogen for more than 500 h. Our work provides insights on understanding the formation of perovskite films in the two-step process, which may benefit further investigation on perovskite solar cells.
Keywords: J−V hysteresis; organic cation composition; perovskite solar cells; stability; two-step process.