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Resumo(s)
Copper zinc tin sulfide (CZTS) is a promising Earthabundant
thin-film solar cell material; it has an appropriate
band gap of ~1.45 eV and a high absorption coefficient.
The most efficient CZTS cells tend to be slightly Zn-rich
and Cu-poor. However, growing Zn-rich CZTS films can
sometimes result in phase decomposition of CZTS into
ZnS and Cu2SnS3, which is generally deleterious to solar
cell performance. Cubic ZnS is difficult to detect by XRD,
due to a similar diffraction pattern. We hypothesize that
synchrotron-based extended X-ray absorption fine
structure (EXAFS), which is sensitive to local chemical
environment, may be able to determine the quantity of
ZnS phase in CZTS films by detecting differences in the
second-nearest neighbor shell of the Zn atoms. Films of
varying stoichiometries, from Zn-rich to Cu-rich (Zn-poor)
were examined using the EXAFS technique. Differences in
the spectra as a function of Cu/Zn ratio are detected.
Linear combination analysis suggests increasing ZnS
signal as the CZTS films become more Zn-rich. We
demonstrate that the sensitive technique of EXAFS could
be used to quantify the amount of ZnS present and
provide a guide to crystal growth of highly phase pure
films.