A comparison of the material and device properties of homogeneous and compositional-graded Cu(In,Ga)(Se,S)2 chalcopyrite thin films

In this study, the material properties of compositionally graded and homogeneous Cu(In1-xGax)(Se1-ySy)(2) (CIGSSe) alloys were compared. The chalcopyrite alloys were synthesized by the chemical reaction of sputtered copper - indium - gallium metallic alloys to a H2Se/H2S/Ar gaseous ambient. The temperature profiles during heating and cooling, gas concentrations, pressures and reaction periods were varied in order to produce two main classes of films, namely (i) compositional-graded absorbers with steep gallium and/or sulfur gradients and (ii) homogeneous absorbers with uniform elemental distributions through the entire depth of the films. The respective CIGSSe films were coated with standard buffer and window layers in order to evaluate the influence of the absorber quality on the device characteristics of completed photovoltaic devices. The findings from these studies clearly indicated that a better understanding of the CIGSSe film formation process led to absorber material with greatly improved material properties. It was also demonstrated that the conversion efficiencies of devices produced with an industrial-scale two-step growth technology is approaching that of multi-source co-evaporation processes.