In situ TEM unveils the role of residual local strain on light-induced phase segregation in halide perovskites

Deciphering the mechanisms governing photoinduced phase segregation in mixed halide perovskites is essential to unlock their full potential in stable, high-performance optoelectronic applications. We uncover the mechanism by which residual local strain acts as a key driving force of light-induced phase segregation. By combining in situ transmission electron microscopy with photoluminescence spectroscopy, we observe structural evolution and photocarrier behavior during phase segregation and after re-mixing. Although halide segregation is compositionally reversible, the perovskite lattice retains residual local strain, a "memory" of previously segregated halide domains, which evolves spatiotemporally with each phase segregation cycle. Residual local strain subsequently serves as a driver for successive phase segregation by trapping photocarriers and acts as the nucleation sites for iodide-rich domains. Our findings identify local strain as an intrinsic, evolving driving force of phase segregation, which offers a paradigm for improving the long-term stability of halide perovskites through strain management and compositional engineering.