Neutron Diffraction and X-ray Absorption Fine Structure Evidence for Local Lattice Distortions and Aperiodic Antisite Substitution in Cu₂ZnSnS₄ Nanoparticles

Abstract

A thorough structure determination has been performed on Cu2ZnSnS4 nanoparticles, a popular photovoltaic material, using neutron diffraction—to characterize the long-range average crystal structure—and X-ray absorption fine structure (XAFS) spectroscopy at the Cu, Zn, and Sn K-edges to elucidate the element-specific local structure. This is the first combined multiscale approach on nanoparticles of this material. The results indicate the presence of aperiodic disorder on the cation sites that is diminished by annealing. This disorder involves local lattice distortions around the crystallographic sites rather than the presence of interstitial atoms. It is most consistent with the known antisite substitutions that are integral to CZTS (referring to the ordering of the Cu, Zn, and Sn between planes). However, instead of being confined within single unit cells so as to maintain the crystallographic symmetry, periodicity, and homogeneity, the substitutional disorder appears to extend over larger regions consisting of multiple unit cells but still smaller than the physical dimensions of the nanoparticles. These results therefore imply the presence of nanoscale domains characterized by local fluctuations in composition that cause the individual domains to be enriched in certain metal ions and depleted in others. These will be mirrored by domains with the opposite fluctuations at other locations in the crystal so that the overall composition remains close to the stoichiometric Cu2ZnSnS4. This disorder is likely pronounced in these samples due to the relatively low temperature reaction (300 °C) and annealing (350 °C) conditions and can be expected to have a significant effect on the resulting physical properties of the material and its photovoltaic performance.