Effectively suppressing nonradiative recombination at the SnO₂/perovskite interface is imperative for perovskite solar cells. Although the capabilities of alkali salts at the SnO₂/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 (A₂CO₃; Li₂CO₃, Na₂CO₃, K₂CO₃, Rb₂CO₃, and Cs₂CO₃). Theoretical calculations demonstrate that A₂CO₃ coordinates with undercoordinated Sn and O on the surface, effectively mitigating oxygen vacancy (V_O) defects with increasing A-cation size, whereas Cs₂CO₃ exhibits diminished preferability owing to enhanced steric hindrance. The experimental results highlight the crucial role of Rb₂CO₃ in actively passivating V_O defects, forming a robust bond with SnO₂, 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 Rb₂CO₃ on the local structure of the perovskite film. Consequently, a Rb₂CO₃-treated device with aperture area of 0.14 cm² achieves a notable efficiency of 22.10%, showing improved stability compared to the 20.11% achieved for the control device.

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