Constructing a Surface Multi-cationic Heterojunction for CsPbI1.5Br1.5 Perovskite Solar Cells with Efficiency beyond 14%

All-inorganic CsPbI_1.5Br_1.5 perovskite solar cells are considered as top cell candidates for tandem cells as a result of their excellent thermal stability and photoelectric performance. However, their power conversion efficiencies (PCEs) are still low and far below the theoretical limit mainly as a result of the severe non-radiative recombination and optical loss. Herein, we introduce an versatile method to construct a surface multi-cationic heterojunction to achieve an efficient and stable CsPbI_1.5Br_1.5 perovskite solar cell. By precisely controlling the content of FA⁺ and MA⁺ on PbBr₂-rich perovskite films, a high-quality heterojunction layer is formed to help effectively passivate the surface defects and reduce the optical loss of the CsPbI_1.5Br_1.5 perovskite. In addition, the incorporation of a heterojunction layer can also improve energy-level alignment and reduce interfacial charge recombination loss. As a result, the champion device with the incorporation of SMH exhibits a PCE of 14.11%, which presents the highest reported efficiency for inorganic CsPbI_1.5Br_1.5 solar cells thus far while retaining 85% of the initial efficiency after 1000 h of storage without encapsulation.