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Document Type

Thesis - University Access Only

Award Date

2013

Degree Name

Master of Science (MS)

Department

Electrical Engineering and Computer Science

First Advisor

Venkat Bommisetty

Abstract

Nanocrystalline silicon (nc-Si:H) has shown strong potential as a cost-competitive alternative for crystalline silicon due to higher absorption, cost-effective production techniques and improved stability. The microstructure and nanoscale defect dynamics of nanocrystalline silicon needs to be better understood to engineer the material’s properties for commercially viable solar cells. Despite significant studies on microstructure, optoelectronic properties and device performance of nanocrystalline silicon the nanoscale carrier dynamics, electronic band profile at grain/grain-boundary interfaces, and the processes leading to light induced degradation such as changes in microstructure and local electronic properties have not been investigated. Sputter deposited nanocrystalline silicon thin films were investigated by scanning probe microscopy to study the microstructure, surface charge density, grain-boundary potential barriers and grain-boundary trap density under dark and illuminated conditions at room temperature in an inert environment. Nanometer scale blisters all over the surface were observed for the first time, which were formed on electrostatic field excitation and were independent of light-induced degradation. The presence of these blisters, however, resulted in increased percent of active grain-boundaries. Photogenerated defects at grain-boundaries increased the percent of active grain-boundaries. This thesis is the first report measuring increased grain-boundary trap density (by 2 x 1011 cm-2), using scanning probe microscopy, to generate a grain-boundary band profile for nanocrystalline silicon. The band profile indicated both upward and downward band bending suggesting lower carrier recombination near grain-boundaries, which could be a potential reason for the improved stability of this material. After light soaking, the optical band gap increased from 1.17 eV to 1.2 eV due to increased density of grain-boundary defects. Nanocrystalline silicon with uniform grain size and the minimum density of large grain-boundaries, deposited using chemical vapor deposition process was proposed for the optimized charge transport hence improved device stability.

Library of Congress Subject Headings

Scanning probe microscopy
Solar cells
Nanostructured materials
Silicon crystals
Thin films

Description

Includes bibliographical references (pages 106-124).

Format

application/pdf

Number of Pages

139

Publisher

South Dakota State University

Rights

In Copyright - Educational Use Permitted
http://rightsstatements.org/vocab/InC-EDU/1.0/

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