Optical properties of Cu₂ZnSnSe₄ thin films and identification of secondary phases by spectroscopic ellipsometry

We apply spectroscopic ellipsometry (SE) to identify secondary phases in Cu₂ZnSnSe₄ (CZTSe) absorbers and to investigate the optical properties of CZTSe. A detailed optical model is used to extract the optical parameters, such as refractive index and extinction coefficient in order to extrapolate the band gap values of CZTSe samples, and to obtain information about the presence of secondary phases at the front and back sides of the samples. We show that SE can be used as a non-destructive method for detection of the secondary phases ZnSe and MoSe₂ and to extrapolate the band gap values of CZTSe phase.