ZnO Nanostructure Synthesis & Laser Direct Writing Process for Optoelectronic Devices
Zinc oxide (ZnO) has a long history of usage in electronics. Recently, ZnO has been gathering great interest of researchers in nanoscience due to its diverse and versatile morphologies such as nanoparticles (NP), nanowires (NW), nanorods, nanotubes, nanohelixes, etc. This dissertation deals with studies covering from the synthesis of ZnO nanostructures to deposition & patterning methods and their applications for optoelectronic devices such as transparent electrodes, active layers for thin film transistor and photovoltaics.
A very well–dispersed, transparent and concentration–tunable ZnO NP solution was successfully synthesized with a new process. Highly transparent ZnO thin films were fabricated by spin coating and subsequent ultra short–pulsed UV laser annealing was performed to change the film properties. While as–deposited NP thin films were not electrically conductive, laser annealing imparted a substantial conductivity increase. Thus, selective annealing for conductive patterns directly on the NP thin film without a photolithographic process was achieved. The conductivity is by a factor of 105 higher than that of the previously reported furnace–annealed ZnO NP films and even comparable to that of vacuum–deposited, impurity–doped ZnO films within a factor of 10. The ZnO film obtained from the process developed in this work has been applied to the fabrication of a thin film transistor (TFT) showing enhanced performance compared with the TFT fabricated on furnace annealed ZnO film. The ZnO TFT performance test reveals that by just changing the laser annealing parameters the solution–deposited ZnO thin film properties can be tuned suitable for both transparent conductors and semiconductor active layers.
Two kinds of nanomaterial patterning methods via direct writing have been demonstrated. First, laser–assisted nanoimprinting of metal and semiconductor nanoparticles has been presented as a large area one step patterning method. With the method, submicron structures including mesh, line, nanopillar and nanowire arrays were fabricated on various kinds of wafer scale substrates. Using the rapid laser–based nanolithography, the prohibitive constraints of e–beam patterning could be overcome. Therefore, this method opens a way to the fabrication of electronic and energy devices with high throughput and ultra low–cost. Second, a drop on demand (DOD) inkjet printing of ZnO seed layers integrated with a CAD (computer aided design) system for a fully digital selective ZnO NW array growth has been discussed. Through proper natural convection suppression during the hydrothermal growth, successful ZnO nanowire local growth could be achieved. Without any need for the photolithographic process or stamp preparation, the NW growth location can be easily modified with high degree of freedom. These two methods are compatible with flexible plastic substrates.
As an application of ZnO nanostructures for high efficiency solar cells, ZnO dye–sensitized solar cells (DSSCs) with greatly enhanced surface area for higher dye loading and light harvesting were demonstrated. The selective growth of “nanoforests” composed of high density, long branched tree–like multi generation hierarchical ZnO nanowire photoanodes by utilizing seed particles and a capping polymer increased the energy conversion efficiency significantly. The overall light–conversion efficiency of the branched ZnO nanowire DSSCs was almost 5 times higher than the efficiency of DSSCs constructed by upstanding ZnO nanowires. A parametric study to determine the optimum hierarchical ZnO nanowire photoanode was performed through the combination of both length–wise and branched growth processes.