Comparison of lift-off processes and rear-surface characterization of Cu(In,Ga)Se2 thin films for solar cells

Thick glasses are commonly used as substrates of Cu(In,Ga)Se2 (CIGS) solar cells because it is needed to deposit CIGS layers at high temperature to realize high efficiency solar cells. In this study, we attempted to separate 2 μm-thick CIGS layers from the primary substrates using a lift-off process and subsequently to transfer them onto the alternative substrates to be free the choice of substrates. In the lift-off process, the CIGS layers were bonded to alternative substrates with adhesives and separated from Mo-coated soda lime glasses (SLG) by tensile strain. As the alterative substrates with an adhesive, a silicone adhesive on polyimide (PI) film, an Al double-faced adhesive tape on SLG, and an epoxy on SLG and PI film were used. Among them, the epoxy on SLG and PI tape were suitable to detach the CIGS layer from the primary substrate with a low crack density. Also, the rear-surface of the detached CIGS film was very flat. CIGS solar cells fabricated based on the detached CIGS layers by the lift-off process using the epoxy on SLG showed a conversion efficiency of 3%.