Beyond the Phase Segregation: Probing the Irreversible Phase Reconstruction of Mixed-Halide Perovskites.

Mixed-halide perovskites can undergo a photoinduced phase segregation. Even though many reports have claimed that such a phase segregation process is reversible, what happens after phase segregation and its impact on the performance of perovskite-based devices are still open questions. Here, the phase transformation of MAPb(I 1- x Br x ) 3 after phase segregation and probe an irreversible phase reconstruction of MAPbBr 3 is investigated. The photoluminescence imaging microscopy technique is introduced to in situ record the whole process. It is proposed that the type-I band alignment of segregated I-rich and Br-rich domains can enhance the emission of the I-rich domains by suppressing the nonradiative recombination channels. At the same time, the charge injection from Br-rich to I-rich domains drives the expulsion of iodide from the lattice, and thus triggers the reconstruction of MAPbBr 3 . The work highlights the significance of ion movements in mixed-halide perovskites and provides new perspectives to understand the property evolution.