Effect of Se Flux and Se Treatment on the Photovoltaic Performance of β–CIGS Solar Cells

Abstract

Cu(In,Ga)3Se5 (β-CIGS) has a band gap of 1.35 eV which is an optimum value for high solar-energy conversion efficiency. However, β-CIGS film was not well characterized yet due to lower efficiency compared to Cu(In,Ga)Se2 (α-CIGS). In this work, β-CIGS films were fabricated by a three-stage co-evaporation of elemental sources with various Se fluxes. As the Se flux increased, the crystallinity of β-CIGS phase was improved from the analysis of Raman spectroscopy and a deep-level defect was reduced from the analysis of photoluminescence spectroscopy. A Se treatment of the β-CIGS film at 200°C increased Ga content and decreased Cu content at the surface of the film. With the Se treatment at 200°C, the cell efficiency was greatly improved for the CIGS films prepared with low Se flux due to the increase of short-circuit current and fill factor. It was found that the main reason of performance mprovement was lower Cu content at the surface instead of higher Ga content.