The role of halide oxidation in perovskite halide phase separation

Halide perovskites display full solid solubility for Br:I and Cl:Br compositions at equilibrium, yet initially homogeneous distributions often partition into Br- and I-rich (Cl- and Br-) regions under non-equilibrium conditions (e.g., illumination), imposing a major obstacle to operationally stable bandgap tunability critical for applications such as tandem photovoltaics and light emitting diodes. Halide oxidation plays a major role in the physical demixing of alloyed halide compositions in perovskite films; this step appears to initiate halide segregation, ultimately directed by several underlying thermodynamic and kinetic driving forces operating in both concert and competition. Notably, the potential energy needed to oxidize the halide, determined by their relative oxidation potentials (I− < Br− < Cl−), can be supplied by applied voltage or illumination biases; segregation will be dictated by the most easily oxidized species leading to concentration gradients in oxidized halide products. In this perspective, we critically analyze the behaviors of halide segregation predicted by such a model capable of rationalizing a wide variety of reported observations with only photoelectrochemical halide oxidation as a commonality. The fundamental and interdisciplinary concepts invoked here clarify the role of photoelectrochemistry in halide segregation, linking illumination- and voltage-induced phenomena, and suggest that these instabilities are rooted at an atomic orbital level because of coordination environment. The path to overcome these instabilities should be paved through highly collaborative efforts, especially between material engineers and inorganic chemists to carefully manipulate dynamic disorder, macro-, and microstrains.