Understanding of chlorine incorporation in wide-bandgap perovskites for efficient and stable solar cells

Cl-based salts are magical additives to control the perovskite crystallization and enhance film morphology. Especially for the I/Br halide wide-bandgap (WBG) perovskites, alloying Cl to form triple halide perovskites can effectively enhance their optoelectronic characteristics. However, the alloying mechanism of Cl into the I/Br-based perovskite lattice remains unclear. Here, we conduct a systematic in-situ photoluminescence (PL) exploration on the crystallization processes of I/Br-based WBG with Cl-based additives including MACl and PbCl2. The results reveal that only the Cl from PbCl2 is easy to incorporate into the I/Br-based perovskite lattice structure at the initial stage of perovskite nucleation. However, PbCl2 incorporation results in the precipitation of excess PbI2, which leads to unfavorable charge transport and decreased photostability. With co-incorporation of MACl and CsCl, the transition of crystal orientation during the annealing process is effectively regulated, significantly eliminating the accumulation of excess PbI2. This improvement enhances phase homogeneity and reduces defect density. Consequently, the optimized WBG perovskite solar cell achieves a high efficiency of 21.58%, which is the highest value for 1.68 eV perovskite with bromine content lower than 10%. In addition, the operational stability is significantly enhanced, along with ameliorated burn-in aging behavior.