First principles simulation of amorphous silicon bulk, interfaces, and nanowires for photovoltaics

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dc.contributor.advisor Nolan, Michael en Belayneh, Merid Legesse 2015-11-11T11:26:59Z 2015 2015
dc.identifier.citation Belayneh, M. L. 2015. First principles simulation of amorphous silicon bulk, interfaces, and nanowires for photovoltaics. PhD Thesis, University College Cork. en
dc.identifier.endpage 198
dc.description.abstract Amorphous silicon has become the material of choice for many technologies, with major applications in large area electronics: displays, image sensing and thin film photovoltaic cells. This technology development has occurred because amorphous silicon is a thin film semiconductor that can be deposited on large, low cost substrates using low temperature. In this thesis, classical molecular dynamics and first principles DFT calculations have been performed to generate structural models of amorphous and hydrogenated amorphous silicon and interfaces of amorphous and crystalline silicon, with the ultimate aim of understanding the photovoltaic properties of core-shell crystalline amorphous Si nanowire structures. We have shown, unexpectedly, from the simulations, that our understanding of hydrogenated bulk a-Si needs to be revisited, with our robust finding that when fully saturated with hydrogen, bulk a-Si exhibits a constant optical energy gap, irrespective of the hydrogen concentration in the sample. Unsaturated a-Si:H, with a lower than optimum hydrogen content, shows a smaller optical gap, that increases with hydrogen content until saturation is reached. The mobility gaps obtained from an analysis of the electronic states show similar behavior. We also obtained that the optical and mobility gaps show a volcano curve as the H content is varied from 7% (undersaturation) to 18% (mild oversaturation). In the case of mild over saturation, the mid-gap states arise exclusively from an increase in the density of strained Si-Si bonds. Analysis of our structures shows the extra H atoms in this case form a bridge between neighboring silicon atoms which increases the corresponding Si-Si distance and promotes bond length disorder in the sample. That has the potential to enhance the Staebler-Wronski effect. Planar interface models of amorphous-crystalline silicon have been generated in Si (100), (110) and (111) surfaces. The interface models are characterized by structure, RDF, electronic density of states and optical absorption spectrum. We find that the least stable (100) surface will result in the formation of the thickest amorphous silicon layer, while the most stable (110) surface forms the smallest amorphous region. We calculated for the first time band offsets of a-Si:H/c-Si heterojunctions from first principles and examined the influence of different surface orientations and amorphous layer thickness on the offsets and implications for device performance. The band offsets depend on the amorphous layer thickness and increase with thickness. By controlling the amorphous layer thickness we can potentially optimise the solar cell parameters. Finally, we have successfully generated different amorphous layer thickness of the a-Si/c-Si and a-Si:H/c-Si 5 nm nanowires from heat and quench. We perform structural analysis of the a-Si-/c-Si nanowires. The RDF, Si-Si bond length distributions, and the coordination number distributions of amorphous regions of the nanowires reproduce similar behaviour compared to bulk amorphous silicon. In the final part of this thesis we examine different surface terminating chemical groups, -H, - OH and –NH2 in (001) GeNW. Our work shows that the diameter of Ge nanowires and the nature of surface terminating groups both play a significant role in both the magnitude and the nature of the nanowire band gaps, allowing tuning of the band gap by up to 1.1 eV. We also show for the first time how the nanowire diameter and surface termination shifts the absorption edge in the Ge nanowires to longer wavelengths. Thus, the combination of nanowire diameter and surface chemistry can be effectively utilised to tune the band gaps and thus light absorption properties of small diameter Ge nanowires. en
dc.description.sponsorship European Commission (ICT-FET-Proactive Initiative: SiNAPS Project (257856)); Science Foundation Ireland (Project “EMOIN” (09/SIRG/I1620)) en
dc.format.mimetype application/pdf en
dc.language.iso en en
dc.publisher University College Cork en
dc.rights © 2015, Merid L. Belayneh. en
dc.rights.uri en
dc.subject Interfaces en
dc.subject Amorphous silicon en
dc.subject Optical and mobility gap en
dc.subject Core-shell nanowires en
dc.subject Band offsets en
dc.title First principles simulation of amorphous silicon bulk, interfaces, and nanowires for photovoltaics en
dc.type Doctoral thesis en
dc.type.qualificationlevel Doctoral en
dc.type.qualificationname PhD (Science) en
dc.internal.availability Full text available en Please note that Chapters 6-8 (pp.103-198) are unavailable due to a restriction requested by the author. en 2018-11-11
dc.description.version Accepted Version
dc.contributor.funder European Commission en
dc.contributor.funder Science Foundation Ireland en
dc.description.status Not peer reviewed en Chemistry en
dc.check.reason This thesis is due for publication or the author is actively seeking to publish this material en
dc.check.opt-out Not applicable en
dc.thesis.opt-out false
dc.check.chapterOfThesis 6,7,8
dc.check.embargoformat Not applicable en
dc.internal.conferring Summer Conferring 2015

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© 2015, Merid L. Belayneh. Except where otherwise noted, this item's license is described as © 2015, Merid L. Belayneh.
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