Mitigation of nonradiative recombination by enhanced anharmonic lattice relaxation in mixed-anion perovskites

Lead halide perovskite semiconductors have been revealed to exhibit high anharmonicity in their lattice dynamics, which has an important influence on associated carrier dynamic processes, including nonradiative carrier recombination. Anion vacancies in lead halide perovskites, which are one of the most common defects in halide perovskites, can create trap states and induce nonradiative recombination, impacting the photoelectric performance of perovskites. In this work, we demonstrate that the defect level of VX (X = Cl, Br, I) in the mixed-anion perovskites CsPb(ClxBr1-x)3 and CsPb(BrxI1-x)3 deepens with increasing composition x because of the elevated conduction band. Our results further show that VX in halide perovskites can easily capture electrons with a small energy barrier. In contrast, hole capture is much more difficult for VX due to pronounced lattice relaxations, which give rise to a high energy barrier. In the mixed-anion perovskites CsPb(ClxBr1-x)3 and CsPb(BrxI1-x)3, the resulting lattice softening further increases the holecapture barrier, thereby preventing VX defects from capturing holes. Therefore, through mixing anions in lead halide perovskites, nonradiative recombination caused by the halogen vacancy in perovskites can be significantly mitigated by enhanced anharmonic lattice relaxation and softening.