Award Date

May 2023

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

Paul Forster

Fourth Committee Member

Arya Udry

Number of Pages

213

Abstract

In this dissertation, silicon-based materials for photovoltaics and chalcopyrite-based materials for photoelectrochemical water splitting are investigated using various spectroscopic and microscopic techniques. Although silicon dominates the photovoltaic market, further improvement can be made by using an alternative low temperature passivation approach. Currently, thermally grown SiO2 passivation is commonly used for silicon solar cells. However, this technique requires high processing temperatures (>800 °C), which increases the thermal budget, potentially decreases the bulk quality of Si, and can lead to difficulties in implementing in production lines. Here, a S-based passivation approach is studied that require lower processing temperatures of ~550 °C. Although this passivation approach is promising, the passivation quality decreases significantly with exposure to air. Hence, an additional SiNx capping layer is needed to protect the passivation layer from degradation, especially in subsequent manufacturing processes. Using surface-sensitive x-ray photoelectron spectroscopy and bulk-sensitive x-ray emission spectroscopy, surfaces and interfaces of these novel S-passivated silicon materials are investigated. In these studies, the changes in the chemical structure of S-passivated silicon with exposure to air, addition of a SiNx capping layer, and exposure to high temperatures are identified in view of the passivation quality as measured by the minority carrier lifetime (and other recombination parameters). Photoelectrochemical (PEC) water-splitting is a promising renewable technology as these devices produce hydrogen sustainably. However, for these devices to be “market viable”, materials need to be optimized to fabricate efficient, durable, and cost-effective PEC devices. In the second part of this dissertation, solution-based CuIn(S,Se)2 (CISSe) will be explored using photoelectron spectroscopy and x-ray emission spectroscopy. Differences in chemical and electronic structure between processing environments (air vs. inert) will be presented in context of the differences in surface morphology of the samples. Although solution-processing has the potential to lower fabrication costs, the nominal bulk band gap of CISSe (Eg = 1.0 – 1.6 eV) is too small for effective water-splitting. To widen the band gap, aluminum is incorporated into the solution process of CISSe to form Cu(In,Al)Se2 (CIASe). Using ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy, surface band gaps and interfacial band alignments between CdS and CISSe (Al-free and Al-incorporated) are experimentally derived. Overall, using a unique set of spectroscopic and microscopic techniques, the surfaces and interfaces are investigated, providing crucial insights into optimizing these applied materials. For the application of sulfur-passivated silicon, S-Si and Si-O bonds are formed after S-passivation. With exposure to air, a loss of sulfur and further formation of Si-O bonds at the surface are observed likely due the interaction between the S-passivated surface with the moisture in the air. With application of the SiNx capping layer, sulfur bonds are maintained. After subsequent high temperature exposure, an increase in sulfur is found at the surface, which is attributed to the diffusion of sulfur and/or the modification of the SiNx layer. For the application of photoelectrochemical water splitting, solution-processed CISSe layers are discussed. Differences in air-processed vs. glovebox-processed (inert environment) samples were found. A reduced amount of excess residues were observed in the air-processed precursor samples relative to glovebox-processed samples likely leading to the formation of larger grains after selenization. Studies at the interface between CdS and CISSe (Al-free and Al-incorporated) were also performed. The resulting surface band gap of the Al-free CISSe is 1.17 ± 0.15 eV and a conduction band “spike” at the interface with CdS is found. With the incorporation of aluminum, the band gap widens to 1.84 ± 0.15 eV and a favorable, “flat” conduction band offset with CdS is observed.

Keywords

chalcopyrite; PEC; photovoltaics; silicon; spectroscopy; surface science

Disciplines

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

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/

Download

Available for download on Wednesday, May 15, 2024


Share

COinS