Light absorption enhancement in ultra-thin Cu(In,Ga)Se₂ solar cells by substituting the back-contact with a transparent conducting oxide based reflector

Cu(In,Ga)Se₂ (CIGS) based solar cells including an ultra-thin absorber layer (< 500 nm) allow cost reduction but lack of sufficient generation of current. A solution would be to increase the optical path length within the CIGS absorber with a reflective back-contact. This study demonstrates that the standard Mo back-contact can be replaced by a Transparent Conducting Oxide back-contact coupled with a copper-reflector to enhance the short circuit current of cells. Simulation shows that the current density of 450 nm thick CIGS can theoretically be increased up to 29.8 mA/cm² with this structure. 450 and 300 nm thick coevaporated CIGS layer were deposited on ZnO:Al and SnO₂:F at high temperature with an optimized NaF post-deposition treatment. The cells with the SnO₂:F back contact coupled with the Cu-reflector reaches an efficiency of 11.4% for the 450 nm thick CIGS and 9.5% for 300 nm thick CIGS which are higher than reference cells with Mo (10.2% and 7.2% respectively). With the ZnO:Al substrate, the efficiency is lower (9.2% for 450 nm thick CIGS). This is caused by a more resistive ZnO:Al/CIGS interface than SnO₂:F/CIGS. Moreover, cells exhibit evidences of back surface recombination suggesting that a passivation of the back interface is required.