Adjustment of Conduction Band Edge of Compact TiO2 Layer in Perovskite Solar Cells Through TiCl4 Treatment.

Perovskite solar cells (PSCs) without a mesoporous TiO2 layer, that is, planar-type PSCs exhibit poorer cell performance as compared to PSCs with a porous TiO2 layer, owing to inefficient electron transfer from the perovskite layer to the compact TiO2 layer in the former case. The matching of the conduction band levels of perovskite and the compact TiO2 layer is thus essential for enhancing PSC performance. In this study, we demonstrate the shifting of the conduction band edge (CBE) of the compact TiO2 layer through a TiCl4 treatment, with the aim of improving PSC performance. The CBE of the compact TiO2 layer was shifted to a higher level through the TiCl4 treatment and then shifted in the opposite direction, that is, to a lower level, through a subsequent heat treatment. These shifts in the CBE were reflected in the PSC performance. The TiCl4-treated PSC showed an increase in the open-circuit voltage of more than 150 mV, as well as a decrease of 100 mV after being heated at 450 °C. On the other hand, the short-circuit current decreased after the treatment but increased after heating at temperatures higher than 300 °C. The treated PSC subjected to subsequent heating at 300 °C exhibited the best performance, with the power conversion efficiency of the PSC being 17% under optimized conditions.