Date of Graduation

12-2013

Document Type

Thesis

Degree Name

Master of Science in Electrical Engineering (MSEE)

Degree Level

Graduate

Department

Electrical Engineering

Advisor/Mentor

Varadan, Vasundara V.

Committee Member

Naseem, Hameed A.

Second Committee Member

Yu, Shui-Qing "Fisher"

Keywords

Applied sciences; Nanostructures; Plasmonics; Thin film solar cells

Abstract

Incorporating plasmonic structures into the back spacer layer of thin film solar cells (TFSCs) is an efficient way to improve their performance. The fishnet structure; which is a tunable, plasmonic light scatterer is used to enhance light absorption. Unlike other plasmonic particles that have been previously suggested, the fishnet is an electrically connected wire mesh and does not result in electric field localization, hence it results in greater absorption in the intrinsic Si layer. Unlike other designs, the fishnet structure is placed in the back spacer layer of the TFSC, so it does not block any incident light. There is also the possibility of integrating back contacts with the fishnet for efficient carrier collection. In addition to its performance, the fishnet structure can be fabricated using low cost nano-imprinting techniques.

The fishnet structure has been studied theoretically in previous research, but this is the first time that the performance of the fishnet has been verified experimentally. The fishnet structure originally proposed in the theoretical study was made of gold, but keeping industrial viability in mind, the choice of metal was changed to silver for this study. The design for the silver fishnet was optimized using full wave electromagnetic simulations in the High Frequency Structure Simulator (HFSS) by Ansoft. The design geometry was tailored to resonate at the band gap of the absorber material (Si), where the absorption coefficient of Si is very low. The goal was to enhance absorption in this region, without causing any degradation of absorption in the other parts of the spectrum.

The silver fishnet structure was fabricated using Electron Beam Lithography (EBL) and thermal evaporation. The other layers of the thin film solar cell (TFSC) were deposited on top of this structure, so that the final fabricated structure optically resembled a TFSC. The absorption of the sample was measured using Spectroscopic Ellipsometry (SE) and was compared with the absorption of a control sample sans the fishnet. It was shown that light absorption is enhanced by a factor of 12.8 at the resonance frequency due to the presence of the fishnet structure. The short circuit current (JSC) increased by 30%. Not only was there no observed degradation at other wavelengths, but the overall absorption also increased as a result of using the fishnet. The theoretical model was also improved to provide better correlation between theory and experiment.

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