Preparation of diverse flower-like ZnO nanoaggregates for dye-sensitized solar cells

Three-dimensional (3D) ZnO nanoaggregates with different morphologies and sizes were fabricated by the hydrothermal method, including cauliflower-like microspheres with an average diameter of 1-2 mu m, nano-sheet aggregated safflower-like microspheres with 3-4 mu m and ixora-like nano-structures with 500-600 nm. We found that their morphology formation was dependent on the concentration of OH- and construction agent (glutamic acid) during the synthesis process, based on which we proposed the mechanism for the formation of ZnO nanoaggregates. The studies showed that, the photo to current conversion efficiencies (PCEs) of the dye-sensitized solar cells (DSSCs) in which the photoanodes were fabricated using the prepared 3D ZnO nanoaggregates were all higher than those obtained employing the ZnO nanoparticles (NPs). In particular, the PCE of the DSSC based on the cauliflower-like ZnO photoanode (4.52%) was about 21% higher than that fabricated with the ZnO NP-based photoanode. This can be attributed to the higher specific surface area of the cauliflower-like ZnO photoanode leading to a greater amount of dye adsorption, more suitable size for light scattering and better inner connection for the transportation of electrons. Moreover, when these 3D ZnO nanoaggregates were used as the scattering layers in the P25-based photoanode in DSSCs, higher PCE of up to 6.74% was achieved, compared to 5.37% obtained for the DSSC without a scattering layer.