Structural trends in off stoichiometric kesterite type compound Cu2ZnSnSe4 semiconductor

Quaternary Cu2ZnSnSe4 is a promising low cost alternative absorber material for thin film solar cells. The current record conversion efficiency of 12.6 [1] for a Cu2ZnSn S,Se 4 based thin film solar cell was reached when the polycrystalline CZTSSe absorber layer exhibits an off stoichiometric composition. Deviations from stoichiometry in the absorber layer causes intrinsic point defects vacancies, anti site, interstitials [2, 3] which determine significantly the electronic properties of the semiconductor, and in the efficiency of the solar cell device. Unfortunately, the current number of reported literature on intrinsic point defects of kesterites is very limited and mainly theoretical work. This work focuses on the synthesis and characterization of off stoichiometric CZTSe. In order to perform experimental analysis of defect types and concentrations, polycrystalline powder samples have been synthesized by solid state reaction within different off stoichiometric compositions suggested by [4, 5]. Chemical and structural characterization by WDX spectroscopy and XRD has been performed. The main results show, that all synthesized samples present kesterite type CZTSe with an off stoichiometric composition as the main phase. Moreover, the formation of single phase off stoichiometric kesterite type CZTSe has been observed. Further structural analyses by neutron powder diffraction experiments were carried out. As main outcome the cation distribution were determined, from which the intrinsic cation point defects and their concentrations have been evaluated. The occurrence of the corresponding off stoichiometry type specific point defects could be proven. The results show, that it is possible to deduce the occurring point defect types from the chemical composition cation ratios of the kesterite phase. Additionally, the Cu Zn disorder, which causes ZnCu and CuZn anti site defects, has been determined within all off stoichiometric CZTSe phase