Power conversion efficiencies of the perovskite solar cells (PSCs) are drastically increased in this 5 years. PSCs without a mesoporous TiO₂ layer, planar-type cells, commonly obtained poorer cell performance as compared to cells 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 edge (CBE) levels of perovskite and the compact TiO₂ layer is thus essential for enhancing solar cell performance. In this work, we discussed about the interface between perovskite and electron transport layer (ETL) to improve the performance of the PSCs with adjustment of the CBE levels for reducing the energy loss at the material interfaces in the PSCs. The CBE levels were evaluated with the electrochemical methods.

The chemical bath treatment with TiCl₄ (TiCl₄ treatment) for the compact TiO₂ layer to increase the performance of PSC has been reported. The effect of the TiCl₄ treatment was considered as the cause of the wettability increases of the precursor solution of the perovskite on TiO₂ compact layer and the efficient charge transport from perovskite layer to TiO₂ layer. However, TiCl₄ treatment may change the CBE potentials. The adjustment of the CBE potentials will be important issue to obtain the low barrier potentials for electron transfer from perovskite to TiO₂ compact layers. To study the effect of the TiCl₄ treatment on the CBE potentials, we prepared the electrochemical cells with the iodide and tri-iodide redox electrolyte and treated substrate with TiCl₄ aqueous solution and the relation between open circuit voltages ₍V_oc) and the electron density in TiO₂ layer of the cells were measured with the charge extraction methods. The V_oc as the quasi CBE potential was negatively shifted with the TiCl₄ treatment but positively shifted with heating after TiCl₄ treatment. We compared the PSCs performance as the different heating temperature at 150 °C, 300 °C, and 450 °C of the TiCl₄ treated compact TiO₂ layers. The Voc was decreased and the short circuit current was increased with increasing the temperature. The highest PCE was obtained with the heating at 300 °C.