Band gap determination of Cu2ZnSnSe4 thin films

We demonstrate experimental data to elucidate the reason for the discrepancies of reported band gap energy (E_g) of Cu₂ZnSnSe₄ (CZTSe) thin films, i.e., 1.0 or 1.5 eV. E_g of the co-evaporated CZTSe film synthesized at substrate temperature (T_sub) of 370 °C, which was apparently phase pure CZTSe confirmed by X-ray diffraction (XRD) and Raman spectroscopy, is found to be around 1 eV regardless of the measurement techniques. However, depth profile of the same sample reveals the formation of ZnSe at CZTSe/Mo interface. On the other hand, E_g of the co-evaporated films increases with T_sub due to the ZnSe formation, from which we suggest that the existence of ZnSe, which is hardly distinguishable from CZTSe by XRD, is the possible reason for the over-estimation of overall E_g.