Reaction Time and Film Thickness Effects on Phase Formation and Optical Properties of Solution Processed Cu2ZnSnS4 Thin Films

Copper-zinc-tin-sulfide (Cu₂ZnSnS₄ or CZTS) is a promising p-type semiconductor material as absorber layer in thin film solar cells. The sulfides of copper and tin as well as zinc and sulfur powders were dissolved in hydrazine. The effect of chemical reaction between precursor species, at room temperature, was assessed for 6 to 22 h. For 22 h reaction time, the effect of spin coated film thickness on the resulting composition, after annealing under N₂ flow at 500 °C for 1 h, was investigated. The morphology, composition, and optical properties of the annealed films were determined by means of x-ray diffraction, scanning electron microscope, and spectrophotometer studies. It was found that, for less than optimal reaction time of 22 h or film thickness below 1.2 µm, other ternary phases namely Cu₄SnS₄, Cu₅Sn₂S₇, and ZnS co-exist in different proportions besides CZTS. Formation of phase-pure CZTS films also exhibited a tendency to minimize film cracking during annealing. Depending on the processing conditions, the band gap (E _g) values were determined to be in the range of 1.55 to 1.97 eV. For phase-pure annealed CZTS film, an increase in the E _g value may be attributed to quantum confinement effect due to small crystallite size.