Efficient and stable CsPbI3 perovskite solar cells with spontaneously formed 2D-Cs2PbI2Cl2 at the buried interface

Despite its nature of superior thermal and photostability compared to the mixed cation or halide counterparts, cesium lead triiodide (CsPbI3) suffers from the undesired phase transition from β phase to δ phase, which is often initiated at the buried interface. In this study, we demonstrate that the addition of Cl induces the spontaneous formation of the two-dimensional (2D) Ruddlesden-Popper (RP) phase of Cs2PbI2Cl2. The 2D RP Cs2PbI2Cl2 predominantly forms at the buried interface due to Cl segregation, improving the electron transfer and phase stability of the CsPbI3 thin films. Perovskite solar cells based on CsPbI3/Cs2PbI2Cl2 light absorbers exhibited a power conversion efficiency (PCE) of 20.6% (with a stabilized efficiency of 20.1%) under simulated solar illumination. Furthermore, the unencapsulated devices retained approximately 80% of their initial PCEs after continuous light exposure for 1,000 h.