Efficiency Enhancement of ZnO-Based Dye-Sensitized Solar Cells by Low-Temperature TiCl₄ Treatment and Dye Optimization

ZnO is a promising candidate as a low-cost porous semiconductor material for photoelectrodes in dye-sensitized solar cells (DSSCs). However, ZnO-based DSSCs tend to exhibit lower energy conversion efficiencies than do those based on TiO 2 . In this study, the performance of ZnO porous electrodes was enhanced using a surface treatment carried out by immersion in cold aqueous TiCl 4 solution that resulted in TiO 2 -coated ZnO ( Z / T ) electrodes. The Z / T electrodes were sensitized with either the Ru complex dye N719 or the organic indoline dye D149. For each dye, the DSSCs with the Z / T photoelectrodes showed the highest open-circuit voltage ( V oc ), short circuit current ( J sc ), and power conversion efficiency compared to those with ZnO, TiO 2, or TiO 2 -coated TiO 2 ( T / T ) electrodes. To study the effects of the TiCl 4 treatment, the relationships between the electron lifetime (τ), cell voltage, and electron density ( n ) of the cells prepared with each electrode, with each of the two dyes, or without either dye were assessed. It was found that the TiCl 4 treatment negatively shifted the conduction band edge (CBE) potential of the ZnO electrodes by more than 100 mV for both dyes and also in the absence of a dye. In addition, τ increased with the use of the organic D149 and in the absence of a dye. The DSSC with a D149-sensitized Z / T layer showed the highest efficiency of 4.89% under 100 mW cm –2 irradiation.