Fabrication of a smooth, large-grained Cu(In,Ga) Se-2 thin film using a Cu/(In, Ga)(2)Se-3 stacked precursor at low temperature for CIGS solar cells

Abstract

Cu(In,Ga)Se-2 (CIGS) thin films used for commercial CIGS solar cells were fabricated from Cu-Ga-In metal precursors; they contained large voids at the CIGS/Mo interface and their surface was rather rough due to large volume expansion. To solve these problems, we employed a Cu/(In,Ga)(2)Se-3 stacked precursor, in which the pre-existing Se could reduce the volume expansion during CIGS formation. With the pre-contained Se in the precursor, a uniform and void-free CIGS film with good adhesion was formed. SEM morphology revealed that a liquid phase was generated at 400 degrees C by the reaction of Cu and (In,Ga)(2)Se-3 under Se deficient conditions even though the melting point of Cu and (In,Ga)(2)Se-3 are much higher. A large-grained CIGS film can be formed as low as 450 degrees C with the help of liquid phase formation and we proposed the reaction mechanism. The film was applied to CIGS solar cells to achieve 13.5% efficiency without AR coating.