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 (FAPbI(3)) have shown high efficiency. However, operational stability has been issued due to phase instability of alpha phase FAPbI(3) at ambient tem-perature. 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 FAPbI(3) to study local structural change and explain the effect of excess precursors on photovoltaic performance and stability. Perovskite films prepared from the pre-cursor solution with excess PbI2 shows better stability than those from the one with excess FAI, despite similar PCE. A rapid phase transition from alpha phase to non-perovskite delta 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 FAPbI(3)-based PSCs. (C) 2021 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press.

Automated summary

In this study, EXAFS was used to investigate the local structural changes of FAPbI(3) and found that perovskite films prepared from precursor solution with excess PbI2 showed better stability compared to those from excess FAI, despite similar power conversion efficiency. Excess FAI resulted in a rapid phase transition and poor device stability, as confirmed by the increased (Pb-I) bond distance evaluated by the Pb L-III-edge EXAFS study. This work provides important insights into the local structure-stability relation in FAPbI(3)-based PSCs.