Impact of Zn1-xMgxO:Al transparent electrode for buffer-less Cu(In, Ga)Se2 solar cells

Abstract

Buffer layers such as CdS and ZnS are used in high efficiency Cu(In,Ga)Se2 (CIGS) thin film solar cells. Eliminating buffer layer is attractive to realize low-cost thin film solar cells by reducing fabrication process. However, the elimination of the buffer layers leads to shunting due to the interface recombination between transparent conductive oxide (TCO) and CIGS layers. To reduce the interface recombination, the control of conduction band offset (CBO) is effective. In this study, we fabricated Zn1−xMgxO:Al (ZMO:Al) as the TCO for the CBO control. ZMO:Al was prepared by co-sputtering of ZnO:Al2O3 (ZnO:Al) and MgO:Al2O3 targets. ZMO:Al shows high transmittance in visible region and the band gap energy widen with the addition of Mg to ZnO:Al. Buffer-less CIGS solar cells with an Al/NiCr/TCO/CIGS/Mo/soda-lime glass structure using ZMO:Al and ZnO:Al were fabricated. For comparison, ZnO/CdS buffered cell was also fabricated. Current density-voltage characteristics of the devices showed the cell with ZMO:Al film achieved higher efficiency compared to the buffer-less cell with ZnO:Al. This result suggested that the control of CBO is important to reduce interface recombination between TCO layer and CIGS absorber.