Impact of Precursor Compositions on the Structural and Photovoltaic Properties of Spray-Deposited Cu2ZnSnS4 Thin Films

Pure sulfide Cu2ZnSnS4 thin films were fabricated on Mo-coated glass substrates by facile spray deposition of aqueous precursor solutions containing Cu(NO3)(2), Zn(NO3)(2), Sn(CH3SO3)(2), and thiourea followed by annealing at 600 degrees C. When a precursor solution containing a stoichiometric composition of Cu, Zn, and Sn was used, the resulting Cu2ZnSnS4 thin film contained a Cu2-xS impurity phase owing to the evaporation of Sn components during the annealing process. The Cu2-xS impurity in the Cu2ZnSnS4 thin film was removed by reducing the concentration of Cu in the precursor solution. This resulted in an improvement of the structural features (i.e., grain sizes and compactness) as well as the electric properties such as acceptor densities, the nature of the acceptor defects, and carrier lifetimes. A solar cell based on the Cu2ZnSnS4 film with an empirically optimal composition showed conversion efficiency of 8.1%. The value achieved was one of the best efficiencies of Cu2ZnSnS4-based cells derived from a non-vacuum process.