Incorporation of Ga in CIGS Absorber Layers Formed by RF-Magnetron Sputtering in Se Vapours

Abstract

Cu(In,Ga)Se2 (CIGS) thin film semiconductors are among the most attractive materials for thin film solar cell applications. Conversion efficiency exceeding 19% has been achieved for CIGS absorber layers deposited by three-stage co-evaporation technique. From a technological point of view the sputtering deposition process is more attractive than thermal co-evaporation, however, solar cell parameters obtained so far are worse. The highest efficiency value reported for co-sputtered CIS thin films is less than 8% and there is no data found for CIGS layers produced by a similar technique. We have developed a hybrid RF-magnetron sputtering/evaporation method for the deposition of the CIGS absorber layer. In this method Cu and In are sequentially sputtered from metallic targets in the presence of Se vapour. Ga depth profiling leads to a band gap grading which is known to play an important role in cell performance. Here, we report the results of our work on three different ways of Ga incorporation into the CIGS thin films. They consisted of sputtering from In-GaSe, Cu-GaSe composite targets and Ga evaporation. The Ga content and distribution across the layer thickness was investigated by AES measurements. The CIGS formation kinetics, structural and compositional studies were performed by SEM, XRD and AES measurements.