Effects of dye loading conditions on the energy conversion efficiency of ZnO and TiO2 dye-sensitized solar cells

In this paper, we report the significant effects of dye loading conditions on the overall light conversion efficiency of zinc oxide (ZnO) film electrodes in dye-sensitized solar cells. A comparison of the ZnO film electrodes was also made with TiO2 film electrodes prepared with similar dye loading conditions. It was found that using a higher and lower dye concentration requires a shorter and longer immersion time, respectively, for optimal sensitization of ZnO to obtain maximum efficiencies. A similar trend was found for the TiO2 film electrode as well; however, smaller differences in the overall light conversion efficiencies were observed with varying dye concentration and immersion time. It was found that the chemical stability was an issue for the ZnO film electrodes but was not pertinent for the TiO2 film electrodes. The film quality and structure of the ZnO film differed after prolonged immersion in high dye concentration, where the formation of N3 dye and Zn2+ aggregates and/or the deterioration of the ZnO colloidal spheres and nanoparticles on the surface may have occurred. On the other hand, the film quality and structure of the TiO2 film was not appreciably affected by prolonged immersion in high dye concentration, where the nanoparticle structure was not affected.