Effect of sulfur and copper amounts in sol–gel precursor solution on the growth, crystal properties, and optical properties of Cu₂ZnSnS₄ films

Cu₂ZnSnS₄ (CZTS) thin films were deposited by sol–gel spin coating using precursor solutions prepared with copper acetate, zinc acetate, tin chloride, and thiourea in methanol and ethylenediamine followed by sulfurization. Sol–gel precursor solutions were prepared with different amounts of sulfur and copper, and their effects on film growth, crystal properties, and optical properties of CZTS films were investigated. CZTS film thickness increased with the amount of metal salt in the precursor solution. This is attributed to an increase in solution viscosity and a decrease in the solution density/viscosity ratio. All CZTS thin films exhibited kesterite structures with absorption coefficients larger than 10⁴ cm⁻¹ in the visible region. Band gap energy increased with increasing amounts of sulfur and decreasing amounts of copper. The blue shift of the band gap is attributed to changes in the degree of p–d hybridization related to Cu d- and S p-levels. The role of sulfur and copper on Hall mobility and carrier concentration was investigated. By optimizing the metal salt ratio in the precursor, CZTS film with a resistivity of 5.3 × 10⁻² Ωcm were prepared.