Local Structure Studies of As-Made Cu2ZnSnS4 Nanoparticles

Abstract

Though Cu2ZnSnS4 (CZTS) is a promising material for thin film solar cells, a significant challenge remains in understanding the structures being formed, particularly in non-stoichiometric materials. We use the extended x-ray absorption fine structure (EXAFS) technique to study the local structure and stoichiometry of as-made, Cu-deficient, CZTS nanoparticles and present K edge data and fits about each of the cations (Cu, Zn, Sn). The data show that all the metal-S (M-S) pairs have the bond lengths of the kesterite structure within 0.02Å, and the pair distribution function is very narrow (σ ∼ 0.07Å). This precludes significant fractions of other phases with different M-S bondlengths. The data also reveal some Sn second neighbors about 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 sub-lattices. The largest distortions are found about 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 go onto Cu sites but rather onto Sn sites. The samples are best described as a kesterite structure with significant antisite disorder.

Resolving the effects of compositional change on structures in Cu2ZnSnS4 nanocrystals by X-ray absorption fine structure

“…In each case, the slightly longer distances between atoms in Sample 3 suggests a more loosely-bound structure, which is more in line with the bond lengths of the pure CZTS kesterite structure. 10,45 This is likely the reason for the comparable photoresponses, despite the reported changes. While the increase in ideal substitutions would contribute favourably to the photoresponse, the above-noted changes in the bond lengths would create unfavourable size constraints, allowing less vibrational freedom in the crystal structure, and additional disorder in crystal alignment due to the smaller size.…”

EXAFS Study of the Local Order in Cu2ZnSn(SxSe1–x)4 Alloys

“…Only the kesterite phase peaks have been found, however we realize that the detection limit of the secondary phases is in most cases about 5% . Also in the EXAFS Fourier transformed spectra we have not noticed features corresponding to Cu–S(Se) binary phases as they have shorter bond lengths than in kesterite structure …”

“…X‐ray absorption fine structure (XAFS) technique has been used to study parent compound of the series CZTS . For the other end member of the series − CZTSe as well as for CZTSSe alloys, only a few XAFS papers have been published .…”