Developing high efficiency Cu2ZnSnS4 (CZTS) thin film solar cells by sputtering

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Embargoed until 2018-03-31
Copyright: Yan, Chang
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Abstract
Kesterite Cu2ZnSnS4 (CZTS) has demonstrated high potential and value as a new promising absorber material for thin film photovoltaics recently. It possesses the merits of earth abundant element constituent, facilitating its low-cost and large-scale production. Moreover, as a high bandgap material (Eg~1.5eV), CZTS is able to be combined with other photovoltaic cells with lower bandgap (like Si) for tandem solar cells, enabling higher power conversion efficiency beyond the single junction solar cell Shockley-Queisser efficiency limit. To be widely deployed in either case, the development of high efficiency single junction CZTS solar cells is the first priority. This thesis aims to investigate, understand and address the present efficiency-limiting issues for the sake of boosting the efficiency of CZTS solar cells. Firstly, the effect of CZTS composition has been carefully studied. The Cu content can greatly affect the microstructure of CZTS grains and, the electrical property of CZTS film such as carrier concentration and minority carrier lifetime, thereby influencing the CZTS solar cell efficiency. Secondly, the effects of sulfurization annealing atmosphere and post-heat treatment have been studied. The “epitaxial” CdS/CZTS interface and moderate Cd diffusion effects were discovered, found to play critical roles in improving the cell efficiency. Thirdly, the conduction band alignments of different buffer materials with CZTS have been carefully investigated. Conduction band offset (CBO) at the CdS/CZTS hetero-interface was found to be unfavourably “cliff-like” whereas CBO of In2S3/CZTS was confirmed to be “spike-like”. Based on these results, the CdS/In2S3 hybrid buffers with different stacking sequences were studied, among which CdS/In2S3/CZTS shows the best cell performance. Last but not least, Cd alloying with CZTS has been studied. Cd alloyed CZTS (i.e. CZCTS (Cu2ZnxCd1-xSnS4)) can effectively boost energy conversion efficiency to over 10% (active area). The improved efficiency is believed to arise from better quality of the CZCTS absorber, i.e. larger grains, longer minority carrier lifetime and reduced band tailing issues. Based on above combined processing strategies, a full-sized 7.6% efficient CZTS solar cell (with total area over 1.0 cm2) has been achieved (certified by NREL), setting a new world record for such a standard CZTS solar cell.
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Author(s)
Yan, Chang
Supervisor(s)
Green, Martin
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Publication Year
2016
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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