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/perovskite interface is imperative for perovskite solar cells. Although the capabilities of alkali salts at the SnO/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 (ACO; LiCO, NaCO, KCO, RbCO, and CsCO). Theoretical calculations demonstrate that ACO coordinates with undercoordinated Sn and O on the surface, effectively mitigating oxygen vacancy (V) defects with increasing A-cation size, whereas CsCO exhibits diminished preferability owing to enhanced steric hindrance. The experimental results highlight the crucial role of RbCO in actively passivating V defects, forming a robust bond with SnO, 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 RbCO on the local structure of the perovskite film. Consequently, a RbCO-treated device with aperture area of 0.14 cm achieves a notable efficiency of 22.10%, showing improved stability compared to the 20.11% achieved for the control device.