Numerical modeling of thin-film solar cells with Cu(In,Ga)Se2 (CIGS) and CdTe absorbers

Abstract

This thesis analyses the world of thin-film solar cells focusing on two kind of technologies, the first based on copper-indium-gallium diselenide, also called CIGS, the second one on cadmium telluride. The aim of this work in particular is giving suggestions on how to increase the conversion efficiency of these devices by improving knowledge and understanding of their physical behavior. Thin-film solar cells based on CIGS or CdTe absorbers have been studied in research laboratories for at least 20 years, but only recently entered the phase of industrial production. While still lagging behind their Si-based competitors in terms of efficiency and dollars/Watt, these technologies are poised to conquer increasing market shares, and perhaps even market dominance. Thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) show record efficiencies among thin-film technologies, with manufacturers introducing mass production processes yielding cells with efficiencies in the 13-15% range. Lab specimens can provide power conversion efficiency as high as 20%, despite the poly-crystalline structure of the semiconductor thin film. Several papers were published on material growth, processing and characterization on one side, and on the performance of finished cells and modules on the other, but there is still a gap to fill in between: some of the specific features of CIGS cells, specifically those pertaining to the behavior of grain boundaries and hetero-junctions, are still under debate, and a complete understanding of the relationship between material characteristics and cell behavior is not available yet.