The role of copper during the growth of stoichiometric Cu2ZnSnS4 by successive ionic layer adsorption and reaction method

Abstract

The synthesis and characterization of Cu2ZnSnS4 (CZTS) thin films prepared on glass/CdS substrates by the successive ionic layer adsorption and reaction (SILAR) method are presented. The effect of copper concentration and atmosphere annealing on the properties of CZTS thin films were studied by means X-ray diffraction (XRD), Raman spectroscopy, Scanning Electron Microscopy (SEM), X-ray photoelectron spectroscopy (XPS), optical spectroscopy and current-voltage curves. The XPS depth profiling data revealed that a minima copper concentration in cationic solution is required to start the adsorption of zinc and tin ions at the CdS surface. The use of 0.001–0.002 M copper concentration allows the adsorption of zinc and tin ions in a suitable quantity inducing the formation of stoichiometric and polycrystalline CZTS thin films. Furthermore, it was found that above of this copper concentration, the segregation of Cu2S phase occurs, even when it is ten times smaller than Zn and Sn concentrations. Otherwise, the sulfurization process doesn't imply a significant change on the structural and chemical properties, as if it does in the optical and electrical properties. This suggests that the excess of Na2S used in the anionic solution is enough to compensate the loss of sulfur during the thermal annealing. The optical band gap energy and the electrical conductivity of these films were evaluated in the range of 1.41–1.53 eV and 10−1 Ω cm−1, respectively. All results suggested that this material is suitable for being applied as an absorber layer in photovoltaic solar cells.