Hierarchically Structured ZnO Film for Dye-Sensitized Solar Cells with Enhanced Energy Conversion Efficiency

The interest in dye-sensitized solar cells has increased due to reduced energy sources and higher energy production costs. For the most part, titania (TiO2) has been the material of choice for dye-sensitized solar cells and so far have shown to exhibit the highest overall light conversion efficiency ∼ 11 %. However, zinc oxide (ZnO) has recently been explored as an alternative material in dye-sensitized solar cells with great potential. The main reasons for this increase in research surrounding ZnO material include: 1) ZnO having a bandgap similar to that for TiO2 at 3.2 eV, and 2) ZnO having a much higher electron mobility ∼ 115–155 cm V s than that for anatase titania (TiO2), which is reported to be ∼ 10 cm V s. In addition, ZnO has a few advantages as the semiconductor electrode when compared to TiO2, including 1) simpler tailoring of the nanostructure as compared to TiO2, and 2) easier modification of the surface structure. These advantages are thought to provide a promising means for improving the solar cell performance of the working electrode in dye-sensitized solar cells. It was reported that the surface structure, the particle size and shape, and the porosity are all important factors for optimizing the solar cell performance of dye-sensitized solar cells. With ZnO, these factors can easily be tailored through the modification of solution growth and wet-chemical methods to fabricate various nanostructures. In addition, the surface structure and crystallinity of ZnO for dye-sensitized solar cells can be easily modified through the use of aqueous solution methods to increase the surface area. For example, aligned ZnO nanowires can be prepared by electrochemical deposition, VLS or nucleation growth, or thermal evaporation; whereas, the growth of TiO2 nanowires are less likely to occur and much more difficult to obtain using such solution growth methods. So far, the highest overall light conversion efficiency obtained for ZnO nanoparticle film has been ∼ 5 % by utilizing additional compression methods for better particle packing. With typical nanoparticle film processing techniques, the highest overall light conversion efficiency obtained for ZnO has been ∼ 1.5 %. Here, we describe solar cells consisting of ZnO films with primary nanoparticles and secondary colloidal spheres, fabricated by way of solvothermal processing, and compare the overall light conversion efficiency to that of ZnO films fabricated from commercially-available nanoparticles. Figure 1 depicts the hierarchical structure of the ZnO film. It is thought that hierarchically-structured ZnO particles would promote light scattering through the presence of secondary colloidal spheres, thus, enhancing photon absorption to improve the short-circuit current density and the overall light conversion efficiency. There has been numerous research groups who have achieved higher short-circuit current densities and higher overall light conversion efficiencies by promoting light C O M M U N IC A TI O N