DOI

10.17077/etd.i3dikkj8

Document Type

Thesis

Date of Degree

Spring 2017

Degree Name

MS (Master of Science)

Degree In

Electrical and Computer Engineering

First Advisor

Fatima Toor

First Committee Member

Anton Kruger

Second Committee Member

David Andersen

Abstract

The fabrication of low cost, high efficiency solar cells is imperative in competing with existing energy technologies. Many research groups have explored using III-V materials and thin-film technologies to create high efficiency cells; however, the materials and manufacturing processes are very costly as compared to monocrystalline silicon (Si) solar cells. Since commercial Si solar cells typically have efficiencies in the range of 17-19%, techniques such as surface texturing, depositing a surface-passivating film, and creating multi-junction Si cells are used to improve the efficiency without significantly increasing the manufacturing costs. This research focused on two of these techniques: (1) a tandem junction solar cell comprised of a thin-film perovskite top cell and a wafer-based Si bottom cell, and (2) Si solar cells with single- and double-layer silicon nitride (SiNx) anti-reflection coatings (ARC).

The perovskite/Si tandem junction cell was modeled using a Matlab analytical program. The model took in material properties such as doping concentrations, diffusion coefficients, and band gap energy and calculated the photocurrents, voltages, and efficiencies of the cells individually and in the tandem configuration. A planar Si bottom cell, a cell with a SiNx coating, or a nanostructured black silicon (bSi) cell can be modeled in either an n-terminal or series-connected configuration with the perovskite top cell. By optimizing the bottom and top cell parameters, a tandem cell with an efficiency of 31.78% was reached.

Next, planar Si solar cells were fabricated, and the effects of single- and double-layer SiNx films deposited on the cells were explored. Silicon nitride was sputtered onto planar Si samples, and the refractive index and thicknesses of the films were measured using ellipsometry. A range of refractive indices can be reached by adjusting the gas flow rate ratios of nitrogen (N2) and argon (Ar) in the system. The refractive index and thickness of the film affect where the minimum of the reflection curve is located. For Si, the optimum refractive index of a single-layer passivation film is 1.85 with a thickness of 80nm so that the minimum reflection is at 600nm, which is where the photon flux is maximized. However, using a double-layer film of SiNx, the Si solar cell performance is further improved due to surface passivation and lowered surface reflectivity. A bottom layer film with a higher refractive index passivates the Si cell and reduces surface reflectivity, while the top layer film with a smaller refractive index further reduces the surface reflectivity. The refractive indices and thicknesses of the double-layer films were varied, and current-voltage (IV) and external quantum efficiency (EQE) measurements were taken. The double-layer films resulted in an absolute value increase in efficiency of up to 1.8%.

Public Abstract

Commercial monocrystalline silicon solar cells have efficiencies in the range of 17-19%, prompting the research of different manufacturing techniques and processes to increase the efficiency of these cells without significantly increasing the cost. Multi-junction solar cells and the use of textured surfaces and anti-reflection coatings are two of the techniques used to create high efficiency silicon solar cells.

A tandem junction solar cell comprised of a perovskite top cell and silicon bottom cell was modeled using Matlab. By adjusting the material properties and using a nanostructured black silicon bottom cell, the tandem cell reached a theoretical efficiency of greater than 30%. The analytical model results support the shift in solar research towards tandem cells and the use of organic-inorganic materials.

Next, planar silicon solar cells were fabricated, and the effects of silicon nitride anti-reflection coatings were studied. The silicon nitride films act as both an anti- reflection coating and a surface-passivating coating, improving the performance of the cell significantly. By adjusting the refractive indices and thicknesses of the layers, the silicon nitride films can be optimized so that the lowest reflectivity is in the range of wavelengths where the number of incident photons on the cell is maximized. The refractive indices were varied by changing the gas flow rate ratio of nitrogen and argon in the deposition system, resulting in cells that had efficiency increases of up to 1.8%. The optimization of the two techniques presented in this thesis is imperative in the progress towards cost competitive, high efficiency solar cells.

Keywords

anti-reflection coatings, black silicon, Matlab model, silicon nitride, silicon solar cells, tandem junction solar cells

Pages

ix, 59 pages

Bibliography

Includes bibliographical references (pages 58-59).

Copyright

Copyright © 2017 Lauren Michel Davidson

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