Double‐Sword Role of Water in Phase Segregation of Mixed Halide Perovskites

ABSTRACT Mixed halide perovskites (MHPs) are promising semiconductors for optoelectronic devices requiring tunable bandgaps, yet they suffer from operational instability caused by phase segregation. Phase segregation is strongly mediated by water, an inevitable and even intentionally introduced molecule during the fabrication of perovskites. However, the role of water in phase segregation remains unclear, as it is often coupled with the coexisting oxygen in air. Herein, we resolve this issue by utilizing vacuum to induce sequential desorption of oxygen and water from MHP, and in situ monitoring of the phase segregation evolution. Our results show that following the initial oxygen desorption, water at the perovskite surface and subsurface desorbs at two different stages. The former activates the defects and enhances the phase segregation, while the latter removes the water‐induced lattice expansion and suppresses the phase segregation. These effects can be correlated with the photovoltaic performance, where the surface water removal raises the hysteresis by 67% and lowers the efficiency by 6.6%, while the further removal of subsurface water reduces hysteresis by 60% and increases efficiency slightly by 0.9%. This work unveils the double‐sword role of water in the phase segregation of MHP, providing inspiring insights for the optimization of relevant optoelectronic device performance.