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 (CsPbI 3 ) 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 Cs 2 PbI 2 Cl 2 . The 2D RP Cs 2 PbI 2 Cl 2 predominantly forms at the buried interface due to Cl segregation, improving the electron transfer and phase stability of the CsPbI 3 thin films. Perovskite solar cells based on CsPbI 3 /Cs 2 PbI 2 Cl 2 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.