Growth of Cu2ZnSnS4 thin films by a two-stage process-Effect of incorporation of sulfur at the precursor stage

The effect of sulfur incorporation at the precursor stage on the growth and properties of Cu2ZnSnS4 (CZTS) thin films prepared by a two-stage process is investigated. Two stacks, one with the precursor sequence ZnS/Sn/Cu and the other with an additional sulfur layer in the stack, were deposited sequentially by thermal evaporation onto soda-lime glass substrates held at 300 °C and annealed at two different temperatures, 550 °C and 580 °C, to understand the effect of inclusion of sulfur precursor layer at the precursor stage. These films were analyzed by studying their elemental composition, structural, microstructural and optical properties. CZTS films exhibiting kesterite structure with (112) preferred orientation are obtained. The lattice parameters are found to be a=0.542 nm and c=1.089 nm. The crystallinity and the grain size are found to increase with increase in annealing temperature. With the inclusion of sulfur precursor layer in the stack, CZTS growth occurred at the precursor stage itself. On annealing this stack at 550 °C for 30 min, CZTS films that consist of distinct and compact grains with size in the range 400-800 nm are obtained. Compared to this, the grain size of CZTS films obtained from the stack without sulfur layer in the precursor stage is found to be in the range 200-500 nm. The direct optical band gap of these films is found to be between 1.44 eV and 1.56 eV depending on the annealing temperature and duration.