In-Se surface treatment of Cu-rich grown CuInSe2

This work focuses on the chalcopyrite CuInSe2 as a model for the more complex but also more widely used thin-film material Cu(In,Ga)Se2. Both materials are characterized by a very broad existence region that allows Cu-poor as well as stoichiometric growth. Although Cu-poor solar cells are more studied and commercially available, Cu-rich CuInSe2 exhibits qualities that make it the superior material. But due to an inherently high doping and interface problems, it has not been possible to take advantage of these. On the other hand it has been shown in previous studies, that forming a Cu-poor surface layer on Cu-rich grown CuInSe2-absorbers can greatly improve the open-circuit voltage of these solar cells. Surface treatments will be discussed, that are comprised of an indium and selenium co-deposition stage with the goal to form the Cu-poor layer by copper migration. They were performed on a new Cu-rich material, which is characterized by a low Se environment during growth. Through this it was possible to reduce the doping level greatly, which results in reliably delivering devices with high currents. Making them excellent candidates for interface optimization, that mainly effects the open-circuit voltage. Thus it became possible to produce high efficiency Cu-rich devices. There is still room for improvement though, as the devices show absorption losses in a wavelength region in accordance with a remainder of InSe on top of the CIS surface. Optimization of the process is a straightforward approach to remove this layer and shows potential for even greater efficiencies. Still the striking point is, that the here presented solar cells, are already as efficient as the Cu-poor devices, that have been published by our group.