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

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.