Synthesis of Stable Cs-Rich FA-Cs Perovskite Solar Cells by Assistance of a Lewis Base Additive

Cs-rich FA-Cs perovskites theoretically have good humidity stability, photothermal stability, and suitable band gap for top cells of tandem solar cells. However, the phase separation of Cs-rich FA-Cs perovskite films occurs when the Cs content exceeds 30%, so it is difficult to synthesize pure Cs-rich FA-Cs perovskite films. Here, pure Cs-rich FA-Cs perovskite Cs1–xFAxPbI2Br(0 ≤ x ≤ 0.6) films without any impurity phase are first synthesized by reducing its phase formation energy barrier through assistance of PbI2(DMSO) and PbBr2(DMSO) admixtures. Compared with the pristine CsPbI2Br films, the humidity stability of those films is significantly improved and a remarkably enhanced power conversion efficiency (PCE) of 14.55% in α-Cs0.7FA0.3PbI2Br solar cells is achieved. Meanwhile, after annealing at 140 °C for 1 h or irradiating with ultraviolet light in N2 or dry air for 12 h, no phase separation and band gap change are found in Cs0.7FA0.3PbI2Br. The distributions of the PCE are similar, and the champion PCE of the corresponding Cs0.7FA0.3PbI2Br solar cells remains within 14.24–14.55%. This shows that α-Cs0.7FA0.3PbI2Br has good thermal and ultraviolet radiation stability.