Fundamental study of the fabrication of zinc oxide nanowires and its dye-sensitized solar cell applications
McCune, Mallarie DeShea
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Because of its excellent and unique physical properties, ZnO nanowires have been widely used in numerous scientific fields such as sensors, solar cells, nanogenerators, etc. Although it is believed that single crystal ZnO has a much higher electron transfer rate than TiO₂, it was found that ZnO nanowire-based dye-sensitized solar cells (DSSCs) have lower efficiencies than TiO₂ nanoparticle-based DSSCs because the density and surface area of ZnO nanowires are usually lower than that of TiO₂ nanoparticles, limiting the cell's light absorption, and because the open-root structure of ZnO nanowires results in electron back transfer that causes charge shortage of the cell. Here, experimental studies were performed that utilize strategic manipulations of the design of the ZnO nanowire based DSSCs in efforts to address and solve its key challenges. It was shown that by incorporating various blocking layers into the design of the cell, the performance of the DSSC can be improved. Specifically, by placing a hybrid blocking layer of TiO₂-P4VP polymer between the substrate and the ZnO nanowires, the conversion efficiency of the cell was 43 times higher than that of a cell without this blocking layer due to the reduction of electron back transfer. Furthermore, in efforts to improve the surface area of the ZnO nanowire array, unique three dimensional structures of ZnO nanowires were fabricated. It was found that by significantly improving the overall density and surface area of the ZnO nanowire array through distinctive hierarchal nanowire structures, the light harvesting efficiency and electron transport were enhanced allowing the DSSC to reach 5.20%, the highest reported value for 3D ZnO NW based DSSCs. Additionally, the development of a theoretical model was explored in efforts to investigate how the geometry of ZnO nanowires affects the incident photon-to-current conversion efficiency of 1D ZnO nanowire-based N719-sensitized solar cells at the maximum absorption wavelength of 543 nm.