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 TiO2. In this study, the performance of ZnO porous electrodes was enhanced using a surface treatment carried out by immersion in cold aqueous TiCl4 solution that resulted in TiO2-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, TiO2, or TiO2-coated TiO2 (T/T) electrodes. To study the effects of the TiCl4 treatment, the relationships between the electron lifetime (tau), 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 TiCl4 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, tau 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.