Date of Award

10-5-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Tar-pin Chen

Second Advisor

Jingbiao Cui

Abstract

Cu2ZnSn(S1-xSex)4 (CZTSSe) is a kesterite material with similar properties to CuInxGa1-xS2 (CIGS) but less expensive element contained. A significant progress has been made in terms of CZTSSe in solar cell applications. Despite its energy conversion efficiency of 12.6% has been achieved by IBM using solution processes, further research and understanding of the materials is still needed in order to improve the solar cell performance of CZTSSe. In this study, we use a facile chemical route to fabricating CZTSSe thin-film solar cells by spin-coating followed by selenization process. An environmentally friendly non-toxic solution of highly soluble, inexpensive, and commercially available precursors were used in this study. To obtain uniform high crystal quality crystals, two strategies including isotropic and anisotropic growth for CZTSSe active materials were investigated. In the anisotropic growth study – that is selenization under vacuum – in situ nucleation and anisotropic growth of CZTSe crystals was observed in this process. Selenium amount, annealing time and temperature all had positive impacts on grain size. High selenium vapor pressure also appeared to help sustain tin in CZTSSe film at a high selenization temperature of up to 630 oC. However, pin-holes are inevitably formed due to anisotropic growth which may lead to electric leaks through the solar cell. The highest efficiency for devices obtained from the anisotropic growth process was 2.60%. To overcome the pin-hole issue and realize large-area solar cells, isotropic crystal growth was investigated. The selenization process was carried out in an argon-filled tube at atmospheric pressure. A bi-layer structure, resulting from the nucleation and isotropic growth on the surface of film, was observed. With the assistance of high Se vapor pressure, cations diffused to the surface to form large crystals with Se, and the isotropic growth of crystals could cover the surface and fix the pin-hole problem. The effect of Se amount, selenization time and temperature were studied and the best device obtained from isotropic growth had an efficiency of 8.91% which is better than the best record for DMSO solution (8.32%).

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