Intrinsic point defects in off-stoichiometric Cu₂ZnSnSe₄: A neutron diffraction study

This work is an experimental study of intrinsic point defects in off-stoichiometric kesterite type CZTSe by means of neutron powder diffraction. We revealed the existence of copper vacancies (V_Cu), various cation anti site defects (Cu_Zn, Zn_Cu, Zn_Sn, Sn_Zn, and Cu_Zn), as well as interstitials (Cu_i, Zn_i) in a wide range of off-stoichiometric polycrystalline powder samples synthesized by the solid state reaction. The results show that the point defects present in off-stoichiometric CZTSe agree with the off-stoichiometry type model, assuming certain cation substitutions accounting for charge balance. In addition to the known off-stoichiometry types A–H, new types (I–L) have been introduced. For the very first time, a correlation between the chemical composition of the CZTSe kesterite type phase and the occurring intrinsic point defects is presented. In addition to the off-stoichiometry type specific defects, the Cu/Zn disorder is always present in the CZTSe phase. In Cu-poor/Zn-rich CZTSe, a composition considered as the one that delivers the best photovoltaic performance, mainly copper vacancies, Zn_Cu and Zn_Sn anti sites are present. Also, this compositional region shows the lowest degree of Cu/Zn disorder.