Analysis of the back contact properties of Cu(In,Ga)Se2 solar cells employing the thermionic emission model

Despite 20 years of research on Cu(In,Ga)Se2 (CIGSe) solar cells there is still no conclusive model to explain the electronic properties of the back contact between the CIGSe absorber and the molybdenum electrode. For this interface, Schottky-type as well as ohmic behavior has been reported previously. In particular, the intermediate MoSe2 layer which forms between the absorber and the metal during growth of the CIGSe layer determines the contact characteristics and might be critical for the device performance. In this study two types of samples were prepared from complete solar cells: MoSe2/Mo samples by lift-off of the CIGSe layer and CIGSe/MoSe2/Mo samples either via etch-removal of ZnO/CdS or etching including thickness reduction of the CIGSe layer. Au contacts were deposited on top of the CIGSe layer. To study a potential barrier-induced current limitation we performed temperature-dependent current-voltage measurements between 80 K and 300 K on both samples. We observed a limitation of the injection current following the thermionic emission model only for the latter sample, indicating the presence of a contact barrier with a barrier height between 0.21 eV and 0.24 eV at the CIGSe/MoSe2 interface. On the basis of a qualitative simulation a band diagram for the CIGSe/MoSe2 interface is proposed.