MoSe2 layer formation at Cu(In,Ga)Se2/Mo interfaces in high efficiency Cu(In1-xGax)Se2 solar cells

MoSe2 layers formed at the interface between Cu-In-Ga-Se and Mo layers were studied by X-ray diffraction and high resolution transmission electron microscopy, Various composition Cu-In-Ga-Se films such as Se, Cu-Se, In-Ga-Se and Cu-rich Cu-In-Ga-Se were deposited on Mo coated glass substrates by physical vapor deposition. For the case of the Se/Mo interface, a MoSe2 layer of about 100 Angstrom thickness was observed. The c-axis of the MoSe2 grains were found to be oriented normal to the surface of Mo layer. For the Cu-Se/Mo and Cu-rich Cu-In-Ga-Se/Mo structures, the thickness of the MoSe2 layers found at the interface was thin. For the case of the In-Ga-Se/Mo structure, a 0.1 mu m thick interfacial MoSe2 layer was observed whose c-axis was oriented parallel to the Mo surface. The microstructure of the In-Ga-Se/Mo film was similar To that of the device quality CIGS/Mo structure deposited by the "3-stage" process. A formation mechanism for the MoSe2 layers occurring during the "3-stage" process is proposed.