Phase Segregation of Mixed-Halide Perovskites Revealed by Cryogenic Electron Microscopy

Mixed-halide perovskites, with tunable bandgaps and high photovoltaic efficiency, hold significant promise for tandem solar cells. However, phase segregation under illumination, forming iodine-rich phase that reduces the bandgap, remains a barrier to applications. We provide microscopic insights into light-induced phase segregation of mixed-halide perovskite using cryo-EM combined with electron energy loss spectroscopy (EELS) and four-dimensional scanning transmission electron microscopy (4D-STEM). To synthesize mixed-halide perovskite CsPbIBr₂, a CsBr/CsI precursor was evaporated onto a copper TEM grid, followed by the deposition of PbBr₂/PbI₂. Subsequently, the sample was annealed at 175°C for 15 minutes. The as-synthesized sample was then frozen in liquid nitrogen using plunge freezing for cryoEM imaging. Cryo-STEM annular dark field (ADF) image revealed that the as-synthesized CsPbIBr₂ were nanoparticles with a diameter of approximately 200 nm, with distinct boundaries between individual grains. EELS analysis revealed a uniform distribution of iodine within the as-synthesized CsPbIBr₂compound. However, upon exposure to light with a wavelength of 400 nm and a power of 0.39 W/cm² for 5 minutes, a notable increase in iodine concentration was observed near the centers of the grains, while the iodine concentration appeared to be comparatively lower near the grain boundaries. This phenomenon was corroborated by 4D-STEM analysis, which demonstrated that the lattice constant of the CsPbIBr₂ {100} planes at the grain centers exhibited an approximate 6% expansion compared to those near the grain boundaries. This expansion indicated a higher iodine concentration in the grain centers post-illumination. Subsequently, after allowing the illuminated sample to rest in a dark environment for 2 hours, both EELS and 4D-STEM analyses revealed a consistent and uniform distribution of iodine concentration across various grains, indicating the light induced phase segregation of mixed-halide perovskite CsPbIBr₂ was reversible. Our results provide insights into potential strategies for mitigating light-induced degradation and improving the light-stability of mixed-halide perovskite solar cells.