Preferred orientation control of Cu(In1-xGax)Se2 (x ≈ 0.28) thin films and its influence on solar cell characteristics

The crystal structure of polycrystalline Cu(InGa)Se-2 (CIGS) films deposited by physical vapor deposition using a 3-stage growth process onto Mo/soda-lime glass substrates were examined. It was found that the [Se]/[In + Ga] beam flux ratio during deposition of In-Ga-Se precursors at the first stage was the most critical parameter for control of the crystal orientation. (220)/(204)-oriented films were obtained at high beam flux ratios while (112)-oriented films were obtained at lower [Se]/[In + Gal beam flux ratios. The growth parameters at the second stage of Cu-Se coevaporation had a smaller effect on the preferred orientation, but could influence the surface texture, surface morphology and grain size. The difference in Cd diffusion behavior between the (112)- and (220)/(204)-oriented CIGS thin films was examined by a chemical bath process. It was found that Cd atoms were more easily doped into the (220)/(204) -oriented films than into the (112)-oriented films. The influences of the absorber texture on the I-V parameters of ZnO/CdS/CIGS solar cells have also been investigated. By optimizing the Cd buffer layer growth condition, a high efficiency of 17.6% with high FF value of 0.79 has been achieved with a (220)/(204)oriented absorber.