Zinc Oxide Nanostructures for Flexible and Transparent Electronics

As a multifunctional material, ZnO possesses remarkable and unique properties and has attracted much research interest for use in a variety of applications. Especially, it has been regarded as a leading material for flexible and transparent electronics, which is a promising emerging technology in electronics. This dissertation studies doping behavior of Ga in ZnO for transparent electrode applications and presents new approaches to ZnO nanostructures for next-generation flexible and transparent electronics. These approaches include developing techniques that enable multiple stacked ZnO nanoflowers and thermal treatment processes at high temperature. Transparent conductive oxides have been extensively studied for the use as a transparent electrode, which is one of the most fundamental and essential parts in transparent electronic devices. In this study, Ga-doped ZnO nanorods were grown on glass substrates, and the effects of Ga doping concentration on the physical properties of ZnO nanorods were investigated using various characterization tools. ZnO nanoflower is a highly preferred nanostructure for solar cells, sensors, and photodetectors due to its high surface area to volume ratio. To-date, ZnO nanoflowers have mostly been synthesized in the form of nanopowders without a substrate, and ZnO nanoflowers grown on substrates have only been single-stacks. Atmospheric pressure plasma jet treatment was used to increase the surface area to volume ratio of ZnO nanoflowers. The plasma treatment induced a significant increase in the height and density of the ZnO nanoflowers/nanorods because the plasma effectively increased the surface energy and roughness of the seed layers while barely affecting the crystal shape and phase of the ZnO nanoflowers/nanorods. Flexible and transparent mica substrates were used for the growth of vertically well-aligned ZnO nanorods. The adoption of mica as a substrate material permitted high temperature annealing processes, which improved the structural and optical properties of ZnO nanorods with uniform surface coverage and excellent adhesion. A practical application for the synthesized ZnO nanorods is also presented in this dissertation. ZnO nanorod-based flexible and transparent dye-sensitized solar cells (DSSCs) and piezoelectric nanogenerators (NGs) were fabricated and the device performances were investigated. Although only two kinds of energy-harvesting devices (DSSCs and NGs) are presented as examples of applications in this dissertation, it is expected that this new approach will provide a breakthrough for overcoming the limited process temperature on plastic and cellulose nanopaper substrates because mica can be extensively used as a flexible and transparent substrate material for electronics, optoelectronics, energy/environmental, and biomedical applications where high temperature processes are required