The quaternary sulfide Cu₂ZnSnS₄ (customarily called kesterite) is a non-oxide semiconductor, which has been anticipated for sound reasons to replace in photovoltaic cells the currently utilized silicon Si in various forms, cadmium telluride CdTe, cadmium indium gallium selenide CIGS, and a few others. Its electronic properties (especially, advantageous energy band gap of E_g = 1.4-1.5 eV) depend to some degree on non-intentional oxygen substitutions in the preparation stage whereas its long-term stability relies on the susceptibility to oxidation in air.

Herein [1], in stage 1, high energy ball milling of the mixture of metal precursors and sulfur (2Cu + Zn + Sn + 4S -> Cu₂ZnSnS₄) initially affords raw nanopowders of the cubic polytype (called by us pre-kesterite) with no semiconductor properties. In stage 2, upon heating at 500 °C in argon the pre-kesterite is converted to the tetragonal kesterite semiconductor. Both of these nanopowders were exposed to the ambient air for a period of time up to six months. The fresh powders and the resultant powders after 1, 3, and 6 month exposure times were characterized by powder XRD, FT-IR and UV-Vis spectroscopies, and direct oxygen O- and hydrogen H-content analyses.

The XRD patterns for the freshly made powders of pre-kesterite and kesterite show only one phase, i.e., cubic or tetragonal, respectively. The O- and H-content data indicate a relatively low level of oxygen in both powders in the order of a few wt% with a share of some adsorbed water vapor. A pronounced increase in the O-contents and, thus, in the proportion of oxygen-bearing compounds, mostly hydrated metal sulfates formed by the oxidation of the nanopowders, can be seen after the first month of exposure. In the course of time spent by the materials in the air still higher and higher O-contents and, consequently, larger amounts of the oxidation by-products are determined but at much slower pace. The formation of the metal sulfates is supported also by FT-IR spectroscopy. As the oxygen content increases, the energy band gap width in the tetragonal kesterite component appears to slightly decrease. In this case, too, the greatest change is observed in the nanopowder after 1 month in the air.

Acknowledgement: Study was funded by Polish NCN Grant No. 2020/37/B/ST5/00151.

References

[1] K. Kapusta, M. Drygas, J.F. Janik, P. Jelen, M.M. Bucko, Z. Olejniczak Journal of Alloys and Compounds 2019, 770, 981-988