Extended X-ray absorption fine structure (EXAFS) of FAPbI₃ for understanding local structure-stability relation in perovskite solar cells

Perovskite solar cells (PSCs) employing formamidinium lead iodide (FAPbI₃) have shown high efficiency. However, operational stability has been issued due to phase instability of α phase FAPbI₃ at ambient temperature. Excess precursors in the perovskite precursor solution has been proposed to improve not only power conversion efficiency (PCE) but also device stability. Nevertheless, there is a controversial issue on the beneficial effect on PCE and/or stability between excess FAI and excess PbI₂. We report here extended X-ray absorption fine structure (EXAFS) of FAPbI₃ to study local structural change and explain the effect of excess precursors on photovoltaic performance and stability. Perovskite films prepared from the precursor solution with excess PbI₂ shows better stability than those from the one with excess FAI, despite similar PCE. A rapid phase transition from α phase to non-perovskite δ phase is observed from the perovskite film formed by excess FAI. Furthermore, the (Pb-I) bond distance evaluated by the Pb L_III-edge EXAFS study is increased by excess FAI, which is responsible for the phase transition and poor device stability. This work can provide important insight into local structure-stability relation in the FAPbI₃-based PSCs.