Local Atomic Structure and Analysis of Secondary Phases in Non-Stoichiometric Cu2ZnSnS4 Using X-ray Absorption Fine Structure Spectroscopy

The local atomic structure in non-stoichiometric Cu2ZnSnS4 (CZTS) thin films was investigated using X-ray absorption fine structure spectroscopy (XAFS). From the XAFS spectra we infer the presence of disorder in the Cu-Zn plane suggesting that this characteristic might be intrinsic of non-stoichiometric CZTS. This disorder increases with the Cu/(Zn+Sn) ratio. Structural parameters derived from fits to the data show a contraction in the Cu-S bond length and an expansion in the Sn-S bond length, compared to distances obtained from diffraction, for all samples. X-ray absorption near edge spectroscopy (XANES) indicates presence of secondary phases not observed by X-ray diffraction. In Cu-poor samples, ZnS and SnS were identified as secondary phases. The amount of those phases correlates with the Cu deficit and were quantified by a linear combination fit of the XANES of standard materials. This work provides direct evidence of intrinsic disorder in CZTS, which must be considered to generate a realistic atomic structural model, in addition to lattice relaxations and secondary phases.