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 Cs 1– x FA x PbI 2 Br(0 ≤ x ≤ 0.6) films without any impurity phase are first synthesized by reducing its phase formation energy barrier through assistance of PbI 2 (DMSO) and PbBr 2 (DMSO) admixtures. Compared with the pristine CsPbI 2 Br films, the humidity stability of those films is significantly improved and a remarkably enhanced power conversion efficiency (PCE) of 14.55% in α-Cs 0.7 FA 0.3 PbI 2 Br solar cells is achieved. Meanwhile, after annealing at 140 °C for 1 h or irradiating with ultraviolet light in N 2 or dry air for 12 h, no phase separation and band gap change are found in Cs 0.7 FA 0.3 PbI 2 Br. The distributions of the PCE are similar, and the champion PCE of the corresponding Cs 0.7 FA 0.3 PbI 2 Br solar cells remains within 14.24–14.55%. This shows that α-Cs 0.7 FA 0.3 PbI 2 Br has good thermal and ultraviolet radiation stability.