Unveiling the Effect of Potassium Treatment on the Mesoporous TiO₂/ Perovskite Interface in Perovskite Solar Cells

The morphological bulk defects and trap sites between TiO₂ and the perovskite layer are critical issues in charge separation and electron transport. In this study, an effective method for improving the mesoporous TiO₂/perovskite interfacial characteristics in perovskite solar cells (PSCs) is demonstrated by modifying mesoporous TiO₂ with potassium (K) treatment. It is found that the modification of mesoporous TiO₂ with the K treatment enhances the perovskite crystallization process, producing a perovskite film with higher crystallinity and larger grain sizes. It is also found that the K treatment not only effectively passivates the trap sites in mesoporous TiO₂ but also reduces the sub-band-gap deep defect states at the interface of the perovskite, as revealed by photothermal deflection spectroscopy, thereby suppressing the nonradiative recombination and improving the V_oc of the PSCs. Moreover, stronger photoluminescence quenching and shorter carrier lifetime are observed for the perovskite on K-treated mesoporous TiO₂, indicating more efficient charge collection across the interface. As a result of these advancements, the PSC based on K-treated mesoporous TiO₂ shows a high power conversion efficiency of 20.60% compared to the PSC without the K treatment (18.17%) and also significantly improves the environmental stability in ambient air.