Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Chemistry

Supervisor

Ding, Zhifeng

Abstract

In this thesis, Cu2ZnSnS4 (CZTS), Cu2ZnSn(S,Se)4 (CZTSSe) and CuIn(S,Se)2 (CISSe) thin-films have been optimized to use as the key light-absorbing and conversion layer for solar cells. CZTS nanocrystals (NCs) were solvothermally synthesized, etched with acetic acid and structurally analyzed using synchrotron spectroscopy. Electrodeposited CZTSSe films showed a non-ideal increase in sulfur with lower selenization temperature and post-process etching. Compositional studies of electrodeposited CISSe films confirmed the decrease in selenium after the acetic acid etching. Through PECMs and other conventional characterization techniques, it was determined that non-etched CZTSSe and CISSe solar devices performed better than their etched counterparts, achieving efficiencies of 5.3% and 2.1%, respectively. In contrast, the results of the CZTS NCs achieved a higher efficiency for the etched device at 6.5%. In the end, electrodeposition proved to be a cheaper, more replicable technique, while CZTSSe demonstrated to be the most cost-effective light-absorber-layer for efficient solar cells.

Summary for Lay Audience

Solar energy presents itself as an optimistic renewable source to meet increasing global energy demands. Sunlight is one of the cleanest and most abundant sources of energy available and can be harvested into thermal or electrical energy. The most common technology to convert the light into electricity are silicon-intensive photovoltaic (PV) devices. As a result, PV alternatives with lower material costs are being pursued.

Research has been devoted to developing thin-film solar cells to reduce manufacturing costs as they need less material. This type of device typically focuses on and employs p-type semiconducting materials to act as the light-absorbing and conversion layer. Nonetheless, thin-film solar cells are not yet widely commercialized due to their low power efficiencies and use of rare and expensive elements. The three light-absorbing layers that were studied in this thesis are Cu2ZnSnS4 (CZTS), Cu2ZnSn(S,Se)4 (CZTSSe) and CuIn(S,Se)2 (CISSe) as they consist of earth-abundant elements.

On all light-absorber-layers, post process etching with acetic acid was employed and its effects were studied by photoelectrochemical measurements (PECMs). This technique involves a three-electrode electrochemical cell and an oxidizing agent in solution. The charge transfer from the film to the solution, upon illumination of the cell, defines the quality and performance of the film. Post-process etching has been shown to improve film performance and device efficiency as it removes surface impurities that inhibit current. Yet this was only true for the films comprised of CZTS nanocrystals (NCs).

In order to improve film reproducibility and lower production costs, electrodeposition was used to fabricate the light-absorber-layers. Selenium was introduced into the CZTSSe and CISSe films in hopes to further enhance the photoresponse. Using conventional techniques along with synchrotron spectroscopy, all three materials were characterized before and after etching to determine how this post-process structurally affected the film and why only some materials reap the benefits. This thesis is split into three parts, one for each of the light-absorbing layers studied. Full solar cells are fabricated and resulting device efficiencies are compared to determine the most cost-effective light-absorber-layer for future focus.

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