A novel approach to the deposition of ZnO nanostructures for use in solar cells

• Main Author: Lockett, Alexander Michael
• Other Authors: O'Brien, Paul
• Published: University of Manchester 2012
• Subjects: 620.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.558049

IIn a world where natural resources are not abundant and our dependence for them is ever growing, greatest interest has always been shown to find new technologies which can sustain our needs without the use of non-renewable resources. One of the most promising areas is that of photovoltaic devices - solar cells. Zinc Oxide is a II-VI group semiconductor with a wide, direct band gap (3.37 eV) and a large exciton binding energy (60 meV). Nanocrystalline ZnO materials have been widely studied for high-technology applications such as photovoltaic devices, light-emitting diodes, photodetectors and gas sensors. The electrical and optical properties of nanocrystalline ZnO make it a viable alternative to TiO2 in Grätzel type photovoltaic devices. Chemical bath deposition - CBD, is a solution analogue of chemical vapour deposition (CVD), which utilises controlled chemical reaction in a solution (usually aqueous) to effect the deposition of a thin film by precipitation. Liquid-Liquid Interface deposition - LIF occurs at the region in which two immiscible liquids come into contact. In many ways, the LIF is a unique environment because of the discontinuity in physical properties, these properties provide a facile environment for growing materials at interface. In a typical water-toluene interfacial system, metal precursor dissolved in toluene is held in contact with an aqueous layer containing a reducing or oxidizing agent. The reaction proceeds at the interface of the liquids and results in deposits suspended in the interfacial region. In both instances the processes is simple, convenient and inexpensive route to semiconducting thin films and nanocrystals. Both don’t require harsh conditions and can be deposited at low temperatures, on a wide array of substrates as well as the ability to use commonly available, cheap precursors. Selectivity over physical and chemical properties easily tunable through control of reaction parameters. All these factors twinned with overall low productions cost mean both are environmentally "green". The influence template seeding layers and polymers have on the growth of ZnO crystallites by chemical bath deposition has been studied. The methods by which the SLs are deposited, the crystallite size and also the thickness have a significant effect on the morphology, density as well as the critical dimensions of the ZnO nanostructures formed. Physical deposition methods have been used to obtain highly ordered, dense nanorod arrays with both narrow and broad size distributions. Chemicals methods using spin coated nanoparticles have produced rods with plus 1 m lengths, with the uncommon orientation along (1010) with growth along the a- axis. The reduction of thickness of the sputtered seeding layers also resulted in overlayers changing from dense nanorod arrays to 3D clusters. Depositions involving polymers yield crystallite with high surface area, high energy (0001) facets. The size of the crystallites can be tuned by the variation of polymer functionality, molecular weight and concentration. In combining both SL and polymers, highly density nanoplatelets arrays with extremely high surface area of (0001) facets are available, There combination also gives rise to unusual phase selective growth and facilitates crystalline growth using polymers which were unsuccessful previously. Interfacial growth of crystalline ZnO has also been investigated using several metal precursors. Using highly basic conditions 3D nanorods clusters where achieved in cases involving dithiocarbamate and ketoacidoximate precursors, however systems using basic conditions and thiobiuret precursors gave rise to a unique nanotulip morphology. The influences of reaction temperature, concentration and deposition time have been investigated. The possibility of polymer-mediated deposition of materials at the interface has been studied. The use of polyethyleneimine in place of NaOH gave rise to globular structures comprised of agglomerated nanocrystals. The nature and characteristics of the thin films, overlayers and nanocrystals obtained by both CBD and LIF were studied using powder X-ray diffraction (p-XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDAX) and UV/Visible spectroscopy.