High-performance CsPbI2Br perovskite solar cells based on excess CsBr and ultra thin MgF2 layer

Cesium-based all-inorganic perovskite material CsPbI2Br has attracted much attention due to its excellent thermal stability and suitable bandgap in tandem solar cells. However, the poor performance of CsPbI2Br-based perovskite solar cell (PSC) hinders its development towards commercial application. In this study, excess CsBr is incorporated into CsPbI2Br precursors for high quality perovskite films. Meanwhile, an ultra-thin MgF2 layer is introduced as the buffer layer to improve the thermal and humidity stability of photovoltaic device. With the optimal thickness of MgF2 (1 nm) and 3 % excess CsBr, the power conversion efficiency (PCE) of the CsPbI2Br-based PSC reaches 15.6 %, which has a great improvement than that of PSC without buffer layer (13.1 %). More importantly, the extremely stable MgF2 buffer layer can effectively isolate the perovskite light absorber and the metal electrode, even under the stress of high temperature, thus leading to excellent long-term stability of the CsPbI2Br PSCs.