The Importance of Sodium Control in CIGSe Superstrate Solar Cells

In this paper, the importance of sodium control in ZnO/Cu(In,Ga)Se₂ superstrate devices is studied. The superstrate devices were fabricated by the deposition of the Cu(In,Ga)Se₂ (CIGSe) absorber material directly onto intrinsic ZnO. Sodium is added to the CIGSe layer as a precursor prior to the absorber deposition or via postdeposition. Capacitance measurements combined with device simulations are presented, which indicate that sodium, if present at the heterointerface, catalyzes the interface reaction between ZnO and CIGSe and induces a high density of deep acceptor states at the heterointerface. This limits the efficiency of the photovoltaic devices. It is shown that only by a very controlled deposition of sodium after the CIGSe deposition, it is possible to achieve devices that allow efficient photocurrent transport across the interfacial GaO_x layer. By employing a 10-nm-thick molybdenum buffer layer on top of the absorber´s back surface, the diffusion of sodium during the posttreatment can be well controlled in order to achieve efficient and long-time stable devices.