Colloidal CZTS nanoparticles and films: Preparation and characterization

Cu2ZnSnS4 (CZTS) compound semiconductor has the advantage of good matching with solar radiation in optical band-gap, large absorption coefficient, non-toxic and especially large abundance ratios of elements, so that CZTS has been considered as a good absorber layer used for the thin-film solar cells with most industrialization promising and environment friendly. In the present work, colloidal CZTS nanocrystals (average size similar to 8-16 nm) with the band gap of similar to 1.5 eV were synthesized via wet-chemical processing, using oleylamine (OLA) as solvent and capping molecules. The colloids were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and UV-Vis-NIR spectroscopy. The structure and morphology of nanocrystals were influenced with the reaction temperature. The resulting nanocrystals were kesterite-phase CZTS when the reaction temperature was lower, but were wurtzite-phase CZTS when the reaction temperature above 275 degrees C. The CZTS films on glass substrates were prepared by drop-casting, from the colloidal 10 wt% CZTS-toluene solution where the CZTS colloids were synthesized at 260 degrees C with three different recipes. The resulting films with different heat-treatments were investigated by XRD, SEM and energy dispersive spectroscopy (EDS). Densified CZTS films (similar to 5 mu m in thickness) could be obtained by drying and sintering in vacuum. The CZTS films have the band-gap around 1.6-2.0 eV, due to Zn rich and S poor in the films. The dark conductivity and photoconductivity under AM 1.5 irradiation of the CZTS films on ITO glass substrates with different heat-treatments were measured by the AC impedance method. (C) 2013 Elsevier B.V. All rights reserved.