Emerging photovoltaic technologies, such as dye-sensitized solar cells and perovskite solar cells, offer huge potential for providing large-scale and affordable renewable energy to meet our growing power requirements. Central to the success of these technologies is the development of low cost production techniques and materials, often with control of morphological features at the nanoscale. Electrochemical anodization is one example of a technique that can meet these criteria. The aim of this PhD project is to develop ZnO nanostructures using electrochemical anodization and apply them as electron transport materials within dye-sensitized and perovskite solar cells.Aligned arrays of ZnO nanowires were produced by the anodization of zinc foil under mild reaction conditions. A systematic study of the influence of various reaction parameters on the growth of nanowires was conducted and subsequently used to optimise nanowire growth rates. Extremely high growth rates of over 3 μm min-1 were achieved, allowing high aspect ratio nanowires to be produced with lengths in excess of 100 μm. Annealing the nanowire arrays led to the production of polycrystalline ZnO nanowires with an average diameter of 160 nm and a radial slit-type pore structure along their length. Further synthetic modification of these nanowires led to the production of high surface area hierarchical structures.Direct application of the nanowire arrays in back-illuminated dye-sensitized solar cells was found to be unsuccessful due to issues with cracking. However, through the preparation of various pastes using these nanowires, it was possible to produce a range of front-illuminated dye-sensitized solar cell architectures. Whilst the anodic nanowires were found to reduce the efficiency of cells when incorporated within mesoporous ZnO films, they were found to increase the power conversion efficiencies from 1.4 % to 1.9 % when applied as light scattering layers above the mesoporous films. Furthermore, hierarchical core-shell nanostructures, derived from the anodic nanowires, were found to greatly increase the efficiency of quasi-solid state dye-sensitized solar cells with TiO2 photoanodes. Maximum power conversion efficiencies of 7.5 % were achieved through the incorporation of small quantities of these nanostructures. These are amongst the highest reported efficiencies for cells featuring ZnO and quasi-solid state cells in general.The use of ZnO nanostructures, including anodic nanowires, was compared with the use of TiO2 nanoparticles as electron transport materials within perovskite solar cells. Maximum power conversion efficiencies were 50 % lower for cells featuring ZnO rather than TiO2 mesoporous layers. Furthermore, no obvious advantage of using nanowires rather than nanoparticles could be observed. The lower performance of the cells based on ZnO was related to the detrimental thermal degradation of the perovskite material in contact with ZnO. This casts doubt over the use of ZnO nanostructures within perovskite solar cells.
|Date of Award||28 Apr 2016|
|Supervisor||Davide Mattia (Supervisor) & Petra Cameron (Supervisor)|
Anodized ZnO Nanostructures for Next-Generation Photovoltaics
Miles, D. (Author). 28 Apr 2016
Student thesis: Doctoral Thesis › PhD