Solvothermal synthesis of Cu2ZnSnS4 nanocrystalline thin films for application of solar cells

In this study, a facile solvothermal method was proposed to grow Cu2ZnSnS4 (CZTS) thin films directly on transparent conductive fluorine-doped tin oxide (FTO) substrates. The hexadecyl trimethyl ammonium Bromide (CTAB) is used as surfactant which is expected to have great effect on the nucleation and growth of the final CZTS thin films on FTO substrate. The effects of CTAB and thiourea concentrations on the crystallographic structure, composition, morphology, optical properties and growth mechanism of CZTS thin films were investigated using scanning electronic microscope (SEM), X-ray diffraction (XRD), Raman spectroscopy, energy dispersive spectrometry (EDS) and UV-visible spectrophotometer, respectively. The solar cells with structure of glass/FTO/CZTS/CdS/i-ZnO/Al-ZnO/Ag were prepared, and the photovoltaic performance of solar cells was studied. The results indicated that Cu2ZnSnS4 thin films are in kesterite phase, composed of a large number of uniform sphere-like particles with an average diameter of about 450-500 nm. Every spherical particle contains many nanocrystals that are 11-12 nm in crystallite size. The concentrations of thiourea or CTAB in precursors do not obviously affect crystal structures, but markedly affect the Cu: Zn: Sn: S atom ratio, surface morphology and microstructure of CZTS thin films. The values of the optical band gap are from 1.37 to 1.54 eV, and the conversion efficiency of CZTS thin film solar cell is 0.16%. It is believed that the similar tetragonal structure and small lattice mismatch between the FTO substrate and Cu2ZnSnS4 both play a key role in promoting the epitaxial nucleation and growth of the Cu2ZnSnS4 thin films on FTO substrates. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.