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 (<i>Z</i>/<i>T</i>) electrodes. The <i>Z</i>/<i>T</i> electrodes were sensitized with either the Ru complex dye N719 or the organic indoline dye D149. For each dye, the DSSCs with the <i>Z</i>/<i>T</i> photoelectrodes showed the highest open-circuit voltage (<i>V</i>_oc), short circuit current (<i>J</i>_sc), and power conversion efficiency compared to those with ZnO, TiO₂, or TiO₂-coated TiO₂ (<i>T</i>/<i>T</i>) electrodes. To study the effects of the TiCl₄ treatment, the relationships between the electron lifetime (τ), cell voltage, and electron density (<i>n</i>) 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, τ increased with the use of the organic D149 and in the absence of a dye. The DSSC with a D149-sensitized <i>Z</i>/<i>T</i> layer showed the highest efficiency of 4.89% under 100 mW cm^–2 irradiation