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Title: The growth of silicon nanowires for solar cells
Author: Ball, Jeremy
Awarding Body: London South Bank University
Current Institution: London South Bank University
Date of Award: 2013
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At present silicon wafer technologies dominate the market place with cost driven by the technological requirement for optically thick and electronically pure silicon. A solution to the high cost of wafer based panels is a thin film approach where micron thick layers of silicon replace the ~250 micron thick silicon wafers. Thin film silicon has gone to market in the form of amorphous and microcrystalline Si where performance is an issue as well as stability due to the hydrogenated amorphous Si structure. This project involves the growth of three dimensional wire structures based on crystalline silicon and their integration into solar cell devices. Nanowire solar cells featuring a radial junction have the potential to reduce the required volume and purity of the silicon used in cell fabrication compared to wafer based technologies. The vapour-liquid-solid effect (VLS) has been used to grow Si nanowires using Au and Sn as catalyst metals together with electron cyclotron resonance chemical vapour deposition (ECRCVD) for silicon deposition. Experimental results include the formation of seed particles from both gold and tin catalyst layers leading to the growth of silicon nanowires on both silicon wafer and glass substrates. The study of nanowire growth parameters from both gold and tin catalysts has led to the proposal of a model for growth in a low pressure environment as found in ECRCVD. Structural characterisation of the wires has taken place. Wires grown from gold catalyst layers on silicon are single crystal where the growth direction has a clear dependence on the orientation of the substrate. Those from tin exhibit a nanocrystalline shell over a single crystal core and have a tapered morphology. Furthermore, the growth direction is independent of substrate orientation. Reasons for this are discussed. Optical characterisation of the nanowire arrays has revealed high levels of broadband absorption. These results have been compared with finite difference time domain modelling (FDTD) and conclusions on the effects of wire density and catalyst layer thickness, along with that of the underlying silicon layer, have been drawn. The implication of these results for the design of solar cells based on these wires is discussed. Simple solar cell devices have been fabricated demonstrating photovoltaic response but with limited performance. The reasons for this are discussed with areas for improvement.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available