Colloidal CZTS nanoparticles and films: Preparation and characterization

Abstract

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 ∼8–16nm) with the band gap of ∼1.5eV 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°C. The CZTS films on glass substrates were prepared by drop-casting, from the colloidal 10wt% CZTS–toluene solution where the CZTS colloids were synthesized at 260°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 (∼5μm in thickness) could be obtained by drying and sintering in vacuum. The CZTS films have the band-gap around 1.6–2.0eV, 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.