Influence of high growth rates on evaporated Cu(In,Ga)Se2 layers and solar cells

Thin film solar cells based on polycrystalline Cu(In,Ga)Se2 were prepared by elemental co-evaporation using modified three-stage processes on soda lime glass substrates at a low substrate temperature of 450 degrees C intended for application on polyimide foils. The growth rates in the different stages of the growth process were varied, and it was observed that the final composition profile and structural quality of the film are mainly determined by the growth rate in the third stage. Application of high growth rates in the second stage was found to have no significant impact on layer morphology and gallium grading profile, which was confirmed by scanning electron microscopy, secondary ion mass spectroscopy, and x-ray diffraction measurements. On the other hand, scanning electron microscopy cross sections revealed that high growth rates in the third stage lead to a fine-grained structure toward the surface as well as smaller grains toward the back contact. Secondary ion mass spectroscopy and x-ray diffraction measurements of such layers revealed a pronounced gallium grading profile, while Raman spectroscopy showed strong occurrence of group III-rich phases in the near-surface region. The final device performance was found to deteriorate by about 10% relative to the baseline process efficiency when growth rates of up to 500?nm?min-1 were applied in the second stage or 600?nm?min-1 in the third stage. Copyright (c) 2011 John Wiley & Sons, Ltd.