Light-Independent Ionic Transport in Inorganic Perovskite and Ultrastable Cs-Based Perovskite Solar Cells

Due to light-induced effects in CH₃NH₃-based perovskites, such as ion migration, defects formation, and halide segregation, the degradation of CH₃NH₃-based perovskite solar cells under maximum power point is generally implicated. Here we demonstrated that the effect of light-enhanced ion migration in CH₃NH₃PbI₃ can be eliminated by inorganic Cs substitution, leading to an ultrastable perovskite solar cell. Quantitatively, the ion migration barrier for CH₃NH₃PbI₃ is 0.62 eV under dark conditions, larger than that of CsPbI₂Br (0.45 eV); however, it reduces to 0.07 eV for CH₃NH₃PbI₃ under illumination, smaller than that for CsPbI₂Br (0.43 eV). Meanwhile, photoinduced halide segregation is also suppressed in Cs-based perovskites. Cs-based perovskite solar cells retained >99% of the initial efficiency (10.3%) after 1500 h of maximum power point tracking under AM1.5G illumination, while CH₃NH₃PbI₃ solar cells degraded severely after 50 h of operation. Our work reveals an uncovered mechanism for stability improvement by inorganic cation substitution in perovskite-based optoelectronic devices.