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 Voc 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.