Synthesis of simple, low cost and benign sol–gel Cu₂ZnSnS₄ thin films: influence of different annealing atmospheres

This study reports the preparation and characterization of CuZnSnS₄ (CZTS) thin films by a non-toxic sol–gel technique. The CZTS thin films were prepared on Molybdenum substrates and annealed in various atmospheres viz. N₂ gas, Sulphur (S) powder and H₂S gas. The effects of various annealing atmospheres on the morphological, structural, compositional and optical properties of the CZTS thin films were investigated. The field emission scanning electron microscopy studies on the as-deposited CZTS thin film showed that the film consisted nanocrystalline CZTS grains while all the annealed films were highly compact, uniform, and that the thickness varied from 1,884 to 832 nm. The X-ray diffraction and Raman studies on the CZTS thin films showed formation of the kesterite structure without any secondary phases. All the CZTS thin films were nearly stoichiometric with slightly Zn rich composition which is favorable for a high photovoltaic performance of solar cells. Photoluminescence (PL) spectroscopy study on all the CZTS thin films showed an asymmetric broad band emission. The direct band gap energy of the CZTS thin films was found to be between 1.3 and 1.6 eV, as confirmed by PL study. The S powder-annealed CZTS thin film was further used for the fabrication of a solar cell of structure SLG/Mo/CZTS/CdS/i-ZnO/AZO/Al grid. The best solar cell showed a short-circuit current density of 7.19 mA/cm², open-circuit voltage of 270 mV, fill factor of 39 % and power conversion efficiency of 0.77 % under air mass 1.5 (100 mW/cm²) illumination.