Chemical Reduction of Iodine Impurities and Defects with Potassium Formate for Efficient and Stable Perovskite Solar Cells

The performance of perovskite solar cells (PSCs) is negatively affected by iodine (I-2) impurities generated from the oxidation of iodide ions in the perovskite precursor powder, solution, and perovskite films. In this study, the use of potassium formate (HCOOK) as a reductant to minimize the presence of detrimental I-2 impurities is presented. It is demonstrated that HCOOK can effectively reduce I-2 back to I- in the precursor solution as well as in the devices under external conditions. Furthermore, the introduced formate anion (HCOO-) and alkali metal cation (K+) can reduce the defect density within the perovskite film by modulating perovskite growth and passivating electronic defects, significantly prolonging the carrier lifetime and reducing the J-V hysteresis. Consequently, the maximum efficiency of the HCOOK-doped planar n-i-p PSCs reaches 23.8%. After 1000 h of operation at maximum power point tracking under continuous 1 sun illumination, the corresponding encapsulated devices retain 94% of their initial efficiency.