Local Structure Studies of As-Made Cu2ZnSnS4 Nanoparticles
AbstractThough Cu2ZnSnS4 (CZTS) is a promising material for thin-film solar cells, a significant challenge remains in understanding the structures being formed, particularly in nonstoichiometric materials. We use the extended x-ray absorption fine-structure technique to study the local structure and stoichiometry of as-made, Cu-deficient CZTS nanoparticles and present K-edge data and fits for each of the cations (Cu, Zn, and Sn). The data show that all of the metal-S (M-S) pairs have the bond lengths of the kesterite structure within 0.02 Å, and the pair distribution function is very narrow (σ∼0.07 Å). These results preclude significant fractions of other phases with different M-S bond lengths. The data also reveal some Sn second neighbors around Sn, whereas there are none in the stoichiometric kesterite (or stannite) structure. Consequently, Sn antisite defects must be present on Cu or Zn sites; this is not surprising since there is some excess of Sn. More importantly, the second-neighbor Sn-Sn distance is significantly longer than other M-M distances, and the antisite Sn defects must therefore introduce significant disorder within the Cu and Zn sublattices. The largest distortions are found around Cu and are modeled using a strongly broadened (or split) peak distribution for the Cu-Cu/Zn pairs. We also find that excess Zn does not occupy Cu sites but instead, goes onto Sn sites. The samples are best described as a kesterite structure with significant antisite disorder.