Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Dr. Zhifeng Ding

Abstract

Solar energy is free and globally abundant and harnessed cost-effectively, it has the potential to be the world’s main source of energy. CuInGa(S,Se)2 solar cells have a very high efficiency of 20 %. However, gallium is a rare and expensive element. We have decided to work on its alternative CuInSe2 (CISe) and CuInS2 (CIS) by removing gallium or both gallium and selenium. The p-type CISe or CIS light-absorbing semiconducting layer is crucial for this type of solar cell. Preparation methods for CIS and CISe films were developed to reduce the costs and enhance the photoelectrochemical efficiency.

The first preparation method was a low-temperature, one-pot, solvothermal synthesis of CIS nanocrystals (NCs) using metal salts as precursors, which was optimized based on a photoelectrochemical (PEC) measurement. The resultant NCs were fully characterized in Chapter 2 by conventional analytical and physical methods such as XRD, EDX, SEM, XPS, and TEM. X-ray absorption near edge structure (XANES) was utilized to not only investigate the absence of secondary phases and oxidation states, but also to obtain information about the connectivity of the capping ligand, effects of composition on the electronic structure of the materials, and the position of the conduction band (Chapter 3).

A linker molecule (3-mercaptopropyl) trimethoxysilane (MPTMS) was utilized to link the back contact to the CIS NCs. The monolayer, when tested with PEC measurements, produced a similar photocurrent density to much thicker films of CIS NCs. Polarization XANES was carried out to assess the CIS NC orientation (Chapter 4).

NC films were cast in a variety of ways including spin-coating and drop-casting. The addition of the next layer, CdS, in the full device fabrication was also investigated. It was discovered that a low-temperature annealing process of the CIS/CdS bilayer followed by an additional CdS layer produced a significant photocurrent enhancement (Chapter 5).

Also explored was the successive electrochemical deposition of copper and indium followed by selenization to form CISe. All films formed by these methods were fully characterized with respect to structure, composition, photoreactions, morphology, and optical properties as well as extended X-ray absorption fine structure (EXAFS) (Chapter 6)

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