Photoinduced Phase Segregation in Mixed Halide Perovskites: Thermodynamic and Kinetic Aspects of Cl–Br Segregation

Abstract Suppression of halide ion mobility remains a key issue in defining the device performances and stability of perovskite solar cells. The halide ion migration is facilitated by the lattice‐distortion‐mediated halide vacancy and hence dictated by the polarizability (or rigidity) of the halide sublattice and framework. Photoinduced halide ion segregation and dark recovery of MAPb(Cl_0.5Br_0.5)₃ films are now probed. The temperature dependence of the rate constants of segregation and dark remixing allow to determine the activation energy barriers (E_a) for photosegregation (38 kJ mol⁻¹) and dark recovery (42 kJ mol⁻¹). A comparison of the segregation activation energy (E_a) and excitation intensity threshold between MAPb(Cl_0.5Br_0.5)₃ and MAPb(Br_0.5I_0.5)₃ films reflects that the presence of Cl stabilizes mixed halide composition. The thermodynamic stabilization is attributed to the formation of more rigid [PbX₆]⁴⁻ frameworks with an increased barrier for halide ion migration. The thermodynamic rationale behind the intriguing halide ion mobility offers a fundamental insight into the role of Cl and design principle of perovskite solar cells with improved efficiency and long‐term stability.