Influence of morphology on the performance of ZnO-based dye-sensitized solar cells

ZnO nanomaterials with different morphologies, obtained by a sonochemical synthesis method at pH values of 5.5, 8, 10 and 12, have been used as starting materials for the fabrication of dye-sensitized solar cells. The morphology of the nanomaterials and the texture of the films deposited using screen printing depend on the synthesis pH, and the various exposed surface facets interact in a different manner with the dye and electrolyte solutions. The best cell performance was obtained with the morphology that resulted from the synthesis at pH 10, where the {100} and {110} crystal forms are predominant, and where dye coverage was largest. Interestingly, the BET total surface area was lowest for this nanomaterial illustrating the importance of morphology. The influence of the synthesis pH was also evident in the energetics and recombination kinetics of the solar cells. For the ZnO material synthesized at pH 5.5, the band edges appear to be shifted to more negative potentials, which could have resulted in a larger open circuit potential based on thermodynamic considerations. However, the electron life time for the pH 5.5 ZnO material is significantly smaller than for the other three synthesis pH values, indicating that the recombination kinetics are significantly faster for these cells as well, resulting in a smaller open circuit potential based on kinetics arguments. The balance between these two effects determines the experimentally observed open circuit potential. Overall, the results indicate that the dependence of the dye adsorption characteristics on ZnO nanomaterial morphology and film texture are the dominating factors that determine the solar cell performance.