Improving Performance of Lead-Free Formamidinium Tin Triiodide Perovskite Solar Cells by Tin Source Purification

Lead (Pb)‐free tin (Sn)‐based perovskite solar cells (PSCs) have been recognized as one of the solutions to the toxicity of Pb and drawn considerable attention. However, Sn 4+ caused by oxidation or incomplete reduction during synthesis severely deteriorates the device performance. Herein, the authors firstly reveal that the addition of Sn powder into the FASnI 3 (FANH 2 CHNH 2 + ) precursor solution prepared from SnI 2 with 99% purity leads to great improvement of the device performance with a maximum power conversion efficiency (PCE) of 6.75%, which is, to the best of their knowledge, the highest efficiency among those of the FASnI 3 ‐based PSCs with SnF 2 as the only additive, comparable to and even higher than the device fabricated from SnI 2 with a high purity of 99.999%.