Doctor of Philosophy
Solar energy has very high potential for ensuring the world’s energy requirements for the long-term future. Earth-abundant materials like Cu2ZnSnS4 (CZTS) are especially desirable, with a non-toxic, low-cost nature, though the quaternary nature allows for a lot of crystal structure variability, and thus underwhelming performance. This Ph.D. thesis is devoted to deepening the understanding of the CZTS material formation, and the processes that can be used to control it, to construct a low-cost, high efficiency CZTS-based solar cell. The layer-by-layer approach presented within this thesis shows great potential for rectifying the problem. CZTS nanocrystal (NC) stoichiometric control was achieved, and led to reproducible structure formation within the films (Chapter 2). Structural correlations to photoresponse for these films were established by means of synchrotron spectroscopies, and increased charge-carrier flux out of the NC film (Chapter 3). Refinement of the NC stoichiometry (Chapter 4) enhanced these results, and extended the structural correlations. CdS addition to the CZTS film to form the p-n junction was investigated, and confirmed water intercalations in the film arising during CdS deposition. Mild thermal treatments were found to purify the films, and lead to further amplification of the charge-carrier flux (Chapter 5).
The CZTS/CdS films were found to not have the desired enhancement to the overall photoresponse due to surface oxides, and poor alignment in the valence/conduction bands of the materials interface. It was discovered that acetic acid etching of the CZTS layer prior to CdS addition removed the oxides, and drastically improved the charge-carrier flux (Chapter 6). In fact, the band structure was aligned favorably to create an ideal p-n junction. The band structure diagram was well established, and the electron flow in the conduction band overlap was confirmed to be favored. The full device was built by combining all refinement processes, and adding ZnO and Al-doped ZnO window layers with atomic layer deposition (Chapter 7). A high open-circuit potential of 0.85 V, and competitive device efficiency of 8.5% were achieved. The layer-by-layer approach is thus proven throughout this thesis to be a highly effective strategy and anticipated to guide intelligent solar cell designs and fabrications.
Turnbull, Matthew J., "Layer-by-Layer Construction Strategies Towards Efficient CZTS Solar Cells" (2018). Electronic Thesis and Dissertation Repository. 5735.