Wide-bandgap (WBG) perovskites are one of the most promising semiconducting materials for high-efficiency tandem solar cells and emerging applications such as indoor photovoltaics and building-integrated photovoltaics. However, mixed-halide WBG perovskites suffer from photo-induced phase segregation, which induces bandgap inhomogeneity, enhances non-radiative recombination, and limits device stability. Here, we present a concise and integrated understanding of phase segregation by linking fundamental mechanisms with experimental characterization. We analyze the interplay among thermodynamic instability, strain-coupled carrier-lattice interactions, and electric-field-driven ion migration, and summarize key techniques for probing phase segregation across multiple length scales. Building on this mechanistic analysis, we review state-of-the-art strategies for suppressing phase segregation, including composition design, crystallization control, interface/passivation engineering, ion-migration regulation, strain modulation, and redox-mediated self-healing approaches. Finally, we outline key challenges and future directions toward achieving intrinsically stable, high-efficiency WBG perovskite solar cells. This review provides both fundamental insights and practical guidance for overcoming one of the most critical barriers to the deployment of WBG perovskite photovoltaics.

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.