Polymer‐Passivated Inorganic Cesium Lead Mixed‐Halide Perovskites for Stable and Efficient Solar Cells with High Open‐Circuit Voltage over 1.3 V

Cesium‐based trihalide perovskites have been demonstrated as promising light absorbers for photovoltaic applications due to their superb composition stability. However, the large energy losses (Eloss) observed in inorganic perovskite solar cells has become a major hindrance impairing the ultimate efficiency. Here, an effective and reproducible method of modifying the interface between a CsPbI2Br absorber and polythiophene hole‐acceptor to minimize the Eloss is reported. It is demonstrated that polythiophene, deposited on the top of CsPbI2Br, can significantly reduce electron‐hole recombination within the perovskite, which is due to the electronic passivation of surface defect states. In addition, the interfacial properties are improved by a simple annealing process, leading to significantly reduced energy disorder in polythiophene and enhanced hole‐injection into the hole‐acceptor. Consequently, one of the highest power conversion efficiency (PCE) of 12.02% from a reverse scan in inorganic mixed‐halide perovskite solar cells is obtained. Modifying the perovskite films with annealing polythiophene enables an open‐circuit voltage (VOC) of up to 1.32 V and Eloss of down to 0.5 eV, which both are the optimal values reported among cesium‐lead mixed‐halide perovskite solar cells to date. This method provides a new route to further improve the efficiency of perovskite solar cells by minimizing the Eloss. The interfacial properties between CsPbI2Br absorber and poly(3‐hexylthiophene) (P3HT) hole‐acceptor are improved by passivating the surface defects of CsPbI2Br and reducing the energy disorder of P3HT. Consequently, a stable inorganic perovskite solar cell with high power conversion efficiency of 12.02% and minimal energy loss of 0.50 eV is obtained.