Spectroscopic ellipsometry (SE) is used to study the dependence of the band-gap energy for Cu2ZnSnSe4 (CZTSe) on temperature ranging from 50 to 350 K. A CZTSe thin film prepared by the pseudo-bulk approach allows direct observation of the fundamental band-gap E0(A,B) in the SE data without need for multi-layer modeling. We obtain accurate energy values for E0(A,B) and its spin-orbit splitting component E0(C) from standard lineshape analysis of the secondenergy-derivative spectra. The E0(A,B) and E0(C) energies for CZTSe decrease with increasing temperature, as for many semiconductors, but their temperature dependencies are relatively weak. Our experimental observation can be explained in terms of relatively small change in bond-length and strong p-d states coupling at the valence band maximum.

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