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Embargoed until 2018-03-31
Copyright: Yan, Chang
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.