Award Date

5-1-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry and Biochemistry

First Committee Member

Clemens Heske

Second Committee Member

Balakrishnan Naduvalath

Third Committee Member

David Hatchett

Fourth Committee Member

Shubhra Bansal

Number of Pages

138

Abstract

In recent years, various thin film solar devices have reached markedly high efficiencies on both the laboratory and large area scale. To further evaluate their potential, and help drive device optimization of efficient solar devices, a detailed understanding of the chemical and electronic structure of the surfaces and interfaces is required. It is these interfaces that play a pivotal role in dictating aspects of device performance. Chalcopyrite-based materials, such as Cu(In,Ga)S2 (CIGS) are regarded as one of the most promising absorber materials for use in highly efficient solar devices. In the context of photoelectrochemical (PEC) hydrogen generation, the tunability of the band gap, and possibly the band edges of these materials, relative to the water splitting potentials, make them interesting candidates for utilization in advanced, direct PEC water-splitting devices.

In this dissertation, the chemical and electronic structure of CIGS-based thin films, and their interfaces to cadmium sulfide (CdS) buffer layers, are investigated by a number of different soft x-ray and electron spectroscopic techniques, including: X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), Inverse photoemission spectroscopy (IPES) at UNLV, and X-ray emission spectroscopy at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. XPS and XES are utilized to determine surface and near-surface chemical composition, respectively. UPS and IPES are mutually complementary techniques used to experimentally derive the surface band gap by probing the valence band maximum (VBM) and conduction band minimum (CBM), respectively. The combination of XPS, UPS, and IPES allow for the full and direct determination of the band alignment at the interface. Other facets of this dissertation involve monitoring the formation of the buffer layer/absorber interface as a function of CdS buffer layer thickness, investigating the effects of an annealing treatment on the thickest buffer layer sample, and comparative analysis of the surface electronic properties between copper-poor and copper-rich chalcopyrite-based absorbers. For these purposes, specific sample sets were designated by UNLV and prepared by the Hawaii Natural Energy Institute (HNEI). These optimized samples allowed for the characterization of the electronic structure at the buffer/absorber interface, including the band alignment, band edges, and band gaps. In the last part of this dissertation, a summary of everything we know about the band alignments of chalcopyrite-based materials investigated hitherto by the Heske research group will be discussed, including band edges, surface band gaps, and work functions, as they relate to PEC water-spitting devices.

The goal of this dissertation is to perform advanced spectroscopic analysis of the electronic and chemical structure of chalcopyrite-based materials used for direct water-splitting. In addition to the composition and chemical bonding of the absorber and buffer interface formation, a comprehensive and direct determination of the band alignment at the interface is presented. These experimental results will provide valuable information about pertinent interfaces, helping PEC researchers identify promising materials candidates for cost-effective solar hydrogen production.

Keywords

Cu(In,Ga)S2; Hydrogen production; Interface formation; X-ray spectroscopy

Disciplines

Engineering Science and Materials | Materials Science and Engineering | Physical Chemistry

File Format

pdf

File Size

3.3 MB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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