Deconstructing the dynamic loop of phase segregation in mixed-halide perovskites

Mixed-halide perovskites (MHPs) stand at the forefront of next-generation optoelectronics, their tunable bandgaps rendering them ideal for high-efficiency tandem photovoltaics and spectrally pure light-emitting diodes. However, the commercialization of this promising technology is critically impeded by an intrinsic instability: light- and bias-induced phase segregation. This phenomenon, wherein a homogeneous mixed-halide film separates into iodide- and bromide-rich domains, leads to erratic bandgap fluctuations and rapid device degradation. While historically attributed to ion migration, recent advanced characterization and theoretical study reveal phase segregation to be a dynamic, multi-scale loop. Thermodynamic instabilities inherent to the mixed-halide lattice are activated by kinetic pathways facilitated by defects, local strain, and interfacial energetics, creating a self-amplifying cycle of degradation. Here, we deconstruct this loop across atomic, nano-, and micro-scales by examining the core mechanisms—halide redox chemistry, defect-mediated ion transport, and dimensional confinement—and present an evolving landscape of stabilization strategies, from additives to interfacial engineering. We find that achieving long-term stability requires an integrated approach that simultaneously disrupts multiple points within the degradation cascade. By framing the problem through this lens, we provide a coherent roadmap for guiding the rational design of phase-stable MHP systems, outlining future directions rooted in operando analysis and multi-functional material discovery.