Unlocking Wavelength‐Selective Modulations of Open‐Circuit Voltage in Metal Halide Perovskite Solar Cells

The photovoltaic performance of metal halide perovskite solar cells often respond divergently to environmental conditions during storage. In particular, light exposure can either enhance or degrade device efficiency, yet the mechanisms underlying these antithetical behaviors are still under investigation. In this study, we explore the modulation of the open‐circuit voltage ( V oc) in triple‐cation mixed‐halide perovskite solar cells by systematically controlling storage environments. While light intensity exhibits minimal impact during storage, the spectral composition of illumination selectively enhances V oc comprising reversible and irreversible contributions. Structural characterization reveals that prolonged storage degrades the quality of perovskite crystals in the upper region of the perovskite layer, whereas light storage promotes the relaxation of microstrain at the buried interface with a p‐type organic layer. This structural reorganization at the interface, accompanied by lattice expansion, accounts for suppressed nonradiative recombination and a corresponding increase in quasi‐Fermi level splitting. Consequently, devices fabricated without chemical defect passivation achieve a power conversion efficiency of higher than 40% under indoor lighting conditions after preconditioned by continuous exposure to ambient light during storage. These findings highlight the critical role of controlled light exposure during storage not only in enhancing efficiency, but also in ensuring reproducibility of perovskite solar cell characterization.