Halide Phase Segregation in Wide‐Bandgap Perovskite Photovoltaics: Mechanisms, Advanced Characterization, and Rational Mitigation Strategies Toward Stable Solar Cells

ABSTRACT Mixed‐halide (I/Br) wide‐bandgap perovskites have emerged as promising top‐cell candidates for tandem photovoltaics due to their tunable bandgap and excellent optoelectronic properties. However, halide phase segregation poses a critical challenge to their commercialization, as initially homogeneous perovskite films spontaneously demix into iodide‐rich and bromide‐rich domains under illumination or electrical bias. This phenomenon leads to severe open‐circuit voltage ( V OC ) losses, efficiency degradation, and compromised device stability. This comprehensive review systematically examines the fundamental origins of halide phase segregation from thermodynamic, kinetic, and defect chemistry perspectives, with particular emphasis on the oxidation‐driven irreversible degradation pathways. We survey advanced characterization techniques including transmission electron microscopy (TEM), Kelvin probe force microscopy (KPFM), conductive atomic force microscopy (c‐AFM), photoluminescence (PL), and cathodoluminescence (CL) that have provided unprecedented insights into the spatiotemporal dynamics of phase segregation. Furthermore, we critically evaluate multidimensional mitigation strategies encompassing compositional engineering, grain boundary passivation, and interface optimization. This review aims to provide a holistic understanding of halide phase segregation and guide the development of next‐generation stable perovskite photovoltaics.