Adjustment of Conduction Band Edge of Compact TiO 2 Layer in Perovskite Solar Cells Through TiCl 4 Treatment.

Perovskite solar cells (PSCs) without a mesoporous TiO ₂ layer, that is, planar-type PSCs exhibit poorer cell performance as compared to PSCs with a porous TiO ₂ layer, owing to inefficient electron transfer from the perovskite layer to the compact TiO ₂ layer in the former case. The matching of the conduction band levels of perovskite and the compact TiO ₂ layer is thus essential for enhancing PSC performance. In this study, we demonstrate the shifting of the conduction band edge (CBE) of the compact TiO ₂ layer through a TiCl ₄ treatment, with the aim of improving PSC performance. The CBE of the compact TiO ₂ layer was shifted to a higher level through the TiCl ₄ 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 TiCl ₄ -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.