Resolving kinetics of halide separation in perovskites by polaron-based lattice model and in situ X-ray diffraction

Light-induced phase separation (LIPS) in mixed lead-halide perovskites is one of the major obstacles to high-efficiency perovskite-based tandem solar cells, yet the mechanisms that govern it remain unclear. Using in situ X-ray diffraction on MAPbBr 1.8 I 1.2 , we track coexisting phases through their lattice parameters and observe the same kinetics across all samples: phase separation sets in only after a delay, with an iodide-rich phase appearing abruptly at near-pure composition while the residual mixed phase drifts gradually towards bromide. To capture this behaviour, we extend the Kawasaki Hamiltonian, adding separate polaron-induced stress terms for electrons and holes, and evolve the halide lattice by Monte Carlo simulation. The model reproduces both the delay and the asymmetry, neither of which follows from standard Cahn-Hilliard kinetics. Within the model, the stronger coupling of electron polarons in bromide-rich regions dominates the driving force, pointing to electron-phonon coupling as a lever for suppressing LIPS.