Authors: Muhammad Bello Ladan
Source: University of the Western Cape Electronic Theses and Dissertations Repository (University of the Western Cape) (article, 2020-01-01)
Field: Materials Science — Materials Chemistry

Application of vertically aligned arrays of metal-oxide nanowires in heterojunction photovoltaics The commercial need to improve the performance of low-cost organic solar cells has led to the idea for this research.The study discusses the synthesis of one dimensional TiO 2 and ZnO nanowire arrays synthesised using a hydrothermal autoclave method and their application in bulk heterojunction inverted organic solar cells.Previous literature has shown that the precise manipulation, positioning and assembly of 1D nanostructures remain one of the greatest challenges in the field of nanotechnology, with much of the difficulty arising primarily from the lack of size and scale of the materials as well as the inability to visualise the nanostructures.In particular, one dimensional metal-oxides such as TiO 2 , ZnO and Fe 2 O 3 have emerged as attractive alternatives to traditional semiconductor structures such as Si and GaAs as they are simple and inexpensive to manufacture, with research showing that application of ZnO nanocones yield efficiencies of 8.4%, which is very attractive given the scope that exists in optimising the metal-oxide architecture.Much is still to be learned from the precise structural features of these materials and their influence on device performance.In this regard, this work largely focuses on this aspect of metal-oxide nanowires prior their application in organic solar cells.Understanding how these processes are influenced by the inherent opto-electronic properties potentially gives greater insights into the mechanisms that define device efficiency and performance.For example, although the 1D structure acts as atemplate for the manufacturing of radial, hetero-junction devices, the natural n-type character also allows metal-oxides to be used as charge-mediating material during operation, which leads to uncertainty in the exact role of the material during operation.This uncertainty, though, forms the basis of this study in that it offers the opportunity to investigate the performance of solar cells with natural n-type metal-oxide nanowires, as typically reported in literature, to co-doped nanowires to test the hypothesis of http://etd.uwc.ac.za/ xi charge transport in solar cells.A significant portion of the study is focused on understanding the doping mechanism and influence on the structure-property relationship prior to application in the well-researched regioregular poly(3-hexylthiophene-2,5-diyl) -[6,6]-Phenyl-C61-butyric acid methyl ester (P3HT-PCBM) polymer solar cells, or more commonly known as organic solar cells.Simple processing methods such as hydrothermal synthesis of the array of nanowires, spincoating of the polymer-blends, cost-effective thermal evaporation and other common, low-cost lab-scale methods are employed to demonstate that effective devices can be manufactured without the use of expensive humidity controlled processing equipment such as glove-boxes and clean rooms.A novel approach explored in this study is the use of nitrogen plasma in combination with standard metal-doping strategies to introduce a controlled level of nitrogen dopants in the metal-oxide lattices of TiO 2 and ZnO to induce p-type doping, which in turn allows for improved electron transport during device operation.To further investigate the performance of solar cells with natural n-type metal-oxide nanowires, as typically reported in literature, the co-doping of TiO 2 and ZnO nanowires with modifiers of Fe, V, Cr and Sn to test the hypothesis of charge transport in solar cells was also studied.Characterisation of the structural features of individual nanowires is achieved by electron microscopy and incident X-ray diffraction, with optical spectroscopy combined with X-ray photo-electron spectroscopy, electron energy loss spectroscopy and energy dispersive X-ray spectroscopy mapping give insights into the doping character, distribution of dopants and alteration of the electronic structure of the TiO2 and ZnO nanowires and their influence on the measured optical propertes.It will be shown that the use of plasma irradition morphologically causes a distortion of the vertical alignment of the nanowires due to a dissociation of the weak Van der Waals force clustering the nanowires, whereas optical spectroscopy studies show that plasma treatment increases the light transmission of TiO 2 arrays from 48% to 90%, with the ZnO arrays exhibiting an increase from 70% to 90% in the visible to UV range.The as-synthesised TiO 2 array has an indirect band gap of 3.13 eV, which reduces to 3.03 eV after N 2 treatment, with the ZnO equivalent decreasing from 3.20 to 3.17 eV post plasma exposure.A study of the 3d transition metal near edge fine structure of both Ti and Zn show that the N 2 plasma treatment of the nanowires results in nitrogen doping of both TiO 2 and ZnO lattices; this is confirmed by scanning transmission electron microscopy coupled with energy dispersive spectroscopy x-ray http://etd.uwc.ac.za/ xii maps collected of single nanowires, which show a clear distribution of nitrogen throughout the metal-oxide.Application of these structures in P3HT:PCBM polymer blends shows progressive improvement in the photoluminescence quenching of the photoactive layer when incorporating both undoped and nitrogen-doped nanowires.Inverted, hetero-junction organic solar cells are manufactured with as-prepared and nitrogen plasma irradiated nanowires, with device performance results showing that the plasma-assisted technique holding definite promise for improving the electron transport in organic solar cells.Photon-to-current conversion efficiency results show an increase from a poor performance of 0.167 for as-synthesised ZnO nanowirebased devices to a respectable value of 0.57% post plasma exposure.The TiO 2 based devices show overall better performance with an increase in efficiency of 15% from 1.7 for unmodified nanowire-based devices to 1.95% post irradiation. Photovoltaics ; Nanowire ; Heterojunction ; Materials science ; Optoelectronics ; Nanotechnology ; Oxide ; Engineering physics ; Photovoltaic system ; Metallurgy ; Electrical engineering ; Engineering Anodic Oxide Films and Nanostructures ; Nanowire Synthesis and Applications