Temperature dependence of the band gap of Cu$_2$ZnSnS$_4$ single crystals

The transmittance spectra of Cu$_2$ZnSnS$_4$ single crystals grown by the technique of chemical gastransport reactions are studied in the region of the fundamental absorption edge in the temperature range $T$ = 20 to 300 K. From the experimental spectra, the band gap of the compound is determined and the temperature dependence of the band gap is established. It is found that the band gap increases, as temperature is increased. It is shown that the temperature dependence is adequately described by the corresponding theoretical expression.