Mechanism of light-induced halide segregation in mixed-halide perovskites
In plain words
Under light, mixed iodide-bromide perovskites un-mix into iodine-rich and bromine-rich patches, which lowers solar-cell voltage; in the dark they mix again. The driving force is debated.
Precise statement
In MAPb(I_xBr_1-x)3 with Br fraction above about 0.2, illumination produces I-rich domains emitting near 1.68 eV within minutes, reversibly in the dark (Hoke et al. 2015). Determine the driving force (free-energy lowering from carriers collecting in low-gap I-rich regions, polaron-induced strain, or field gradients from trapped charge) and predict the threshold intensity and steady-state composition versus intensity and temperature. An answer is a mechanism with a quantitative steady-state phase diagram.
What would settle it
Intensity- and temperature-resolved steady-state composition maps that match one model's predicted threshold and phase diagram while excluding the others.