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 LIII-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.