Effect of post-deposition annealing on the growth of Cu2ZnSnSe4 thin films for a solar cell absorber layer

The effect of substrate temperature and post-deposition annealing on the growth and properties of Cu(2)ZnSnSe(4) thin films, a potential candidate for a solar cell absorber layer, is investigated. The substrate temperature (T(s)) is chosen to be in the range 523-673 K and the annealing temperature (T(pa)) is kept at 723 K. Powder x-ray diffraction (XRD) patterns of as-deposited films revealed that the films deposited at T(s) = 523 K and 573 K contain Cu(2-x)Se as a secondary phase. Single phase, polycrystalline Cu(2)ZnSnSe(4) films are obtained at T(s) = 623 K and films deposited at T(s) = 673 K have ZnSe as a secondary phase along with Cu(2)ZnSnSe(4). Direct band gap of as-deposited CZTSe films is found to lie between 1.40 eV and 1.65 eV depending on T(s). XRD patterns of post-deposition annealed films revealed that the films deposited at T(s) = 523-623 K are single phase CZTSe and films deposited at T(s) = 673 K still contain ZnSe secondary phase. CZTSe films are found to exhibit kesterite structure with the lattice parameters a = 0.568 nm and c = 1.136 nm. Optical absorption studies of post-deposition annealed films show that there is a slight increase in the band gap on annealing, due to decrease in the Cu content. Electrical resistivity of the films is found to lie in the range 0.02-2.6 Omega cm depending on T(s).