Kesterite Cu₂ZnSnS₄ is a chemically complex semiconductor material with p-type conductivity and an energy band gap in the optimal range for photovoltaic applications: E_g = 1.4-1.5 eV. However, any changes in the structure and composition such as disorders, defects, non-stoichiometry as well as the presence of secondary phases will strongly affect its optoelectronic properties. Furthermore, the use of photovoltaic cells containing kesterite is likely to involve a prolonged exposure to air which may affect the material’s basic chemical integrity. Our previous studies [1,2] show that this material in the form of nanopowders is sensitive to oxygen conditions, namely, it is quite easily oxidized in the air being partially decomposed with time to specific hydrated metal sulfates and oxides. On overall, this may cause changes in the width of its band gap.

In this work, the relation of the directly determined oxygen O-content with the kesterite‘s energy band gap is analyzed. The nanopowders of semiconductor kesterite are prepared via the mechanochemical synthesis method from three different precursor systems that we mastered, namely, from the mixtures of: (1) respective metals and sulfur, (2) in-situ mechanochemically made copper alloys and sulfur, and (3) selected metal sulfides and sulfur. Herein, in stage (1) high-energy reactive milling of substrates in a planetary ball mill took place followed by stage (2) of the thermal treatment under argon of the milled raw products at the optimum temperatures of 500-550 °C. Both types of materials in the form of nanopowders, i.e., raw after milling and heat-treated after annealing, were examined by powder XRD, direct O- and H-content analysis, and UV-Vis spectroscopy as the freshly-made materials and after 1, 3, and 6 months of exposure to ambient air.

In all cases, after milling the cubic, sphalerite-like phase was obtained with no semiconductor properties. The annealing caused structure transformation from cubic to tetragonal, the latter with the band gap in the range E_g = 1.3-1.4 eV. With the increase of time the nanopowders spent in the air, the O-content clearly increased as well while the band gap appeared to slightly decrease. For all three initial kesterite nanopowders, this trend was the fastest at the beginning of the experimental periods of time (1 month) and, then, visibly slowed down (3 and 6 months).

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

References

[1] K. Lejda, M. Drygaś, J.F. Janik, J. Szczytko, A. Twardowski, Z. Olejniczak Materials, 2020, 13, 3487;

[2] K. Lejda, J.F. Janik, M. Perzanowski, S. Stelmakh, B. Pałosz International Journal of Molecular Sciences, 2023, 24, 3159.