Award Date

12-1-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry and Biochemistry

First Committee Member

Clemens Heske

Second Committee Member

Kathleen Robins

Third Committee Member

Dong-Chan Lee

Fourth Committee Member

Rama Venkat

Number of Pages

107

Abstract

The surface and interface properties are of the upmost importance in the understanding, optimization, and application for photovoltaic devices. Often the chemical, electronic, and morphological properties of the films are empirically optimized, however when progress slows, a fundamental understanding of these properties can lead to breakthroughs. In this work, surfaces and interfaces of solar cell-relevant films are probed with a repertoire of X-ray analytical and microanalysis techniques including X-ray photoelectron (XPS), X-ray excited Auger electron (XAES), X-ray emission (XES) spectroscopies, and atomic force (AFM) and scanning electron (SEM) microscopies.

Silicon-based devices currently dominate the solar market, which is rather inflexible in application. Cadmium telluride (CdTe)-based technologies offer a cost-effective alternative with additional benefits including roll-to-roll production and high conversion efficiencies. This, like other next generation thin film solar cells, needs more optimization to replace Si. The charge transport across a heterojunction is of great importance to drive up the conversion efficiency of the device.

The interface of a CdS buffer layer and SnO2:F front contact was investigated as a function of CdCl2-treatment. In order to measure the fully formed interface, after subsequent layer deposition and heat treatments, mechanical stressing of the layer stack resulted in physical separation at the desired interface. By combining multiple spectroscopic and morphologic methods a complete picture has evolved.

CdS is often used as a buffer layer in CdTe based devices. This layer is empirically optimized to be very thin (~100 nm) due to the parasitic light absorption in and around the 2 eV range. By widening the band gap or replacing it with a more transparent material, more photons can be absorbed by the CdTe layer and significantly increase the overall conversion efficiency of the device. CdS:O and Zn(1-x)MgxO were studied as possible alternatives to CdS. The chemical composition of CdS:O was studied at the surface and bulk of the film with respect to oxygen content. The interfacial properties of SnO2/Zn(1-x)MgxO and Zn(1-x)MgxO/CdTe were also investigated with particular emphasis on energy level alignments at the interfaces.

Keywords

Buffer layers; CdS; CdTe; Photovoltaic; XPS; Zinc Magnesium Oxide

Disciplines

Chemistry | Engineering Science and Materials | Materials Science and Engineering | Oil, Gas, and Energy

Language

English