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)Se-2 (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/cm(2) 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 (102% 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. (C) 2016 Elsevier B.V. All rights reserved.