Elucidating the Role of Alkali Metal Carbonates in Impact on Oxygen Vacancies for Efficient and Stable Perovskite Solar Cells

Abstract Effectively suppressing nonradiative recombination at the SnO ₂ /perovskite interface is imperative for perovskite solar cells. Although the capabilities of alkali salts at the SnO ₂ /perovskite interface have been acknowledged, the effects and optimal selection of alkali metal cations remain poorly understood. Herein, a novel approach for obtaining the optimal alkali metal cation (A‐cation) at the interface is investigated by comparatively analyzing different alkali carbonates (A ₂ CO ₃ ; Li ₂ CO ₃ , Na ₂ CO ₃ , K ₂ CO ₃ , Rb ₂ CO ₃ , and Cs ₂ CO ₃ ). Theoretical calculations demonstrate that A ₂ CO ₃ coordinates with undercoordinated Sn and O on the surface, effectively mitigating oxygen vacancy (V _O ) defects with increasing A‐cation size, whereas Cs ₂ CO ₃ exhibits diminished preferability owing to enhanced steric hindrance. The experimental results highlight the crucial role of Rb ₂ CO ₃ in actively passivating V _O defects, forming a robust bond with SnO ₂ , and facilitating Rb ⁺ diffusion into the perovskite layer, thereby enhancing charge extraction, alleviating deep‐level trap states and structural distortion in the perovskite film, and significantly suppressing nonradiative recombination. X‐ray absorption spectroscopy analyses further reveal the effect of Rb ₂ CO ₃ on the local structure of the perovskite film. Consequently, a Rb ₂ CO ₃ ‐treated device with aperture area of 0.14 cm ² achieves a notable efficiency of 22.10%, showing improved stability compared to the 20.11% achieved for the control device.