Surface treatment of CIS solar cells grown under Cu-excess

High-efficiency devices based on Cu(In, Ga)Se₂ (CIGS) use an overall Cu-poor absorber. However, all the electronic properties (defect densities, transport properties, bulk recombination) are better in material that was grown under Cu-excess. Therefore the objective of this work is to make even better solar cells from "Cu-rich" absorbers. In all Cu-poor chalcopyrite based solar cells, the limiting factor for the open circuit voltage is the recombination in the space charge region whereas for the ones based on "Cu-rich" absorbers it is dominated by recombination at the interface. In this work, pure CuInSe₂ (CIS) absorbers without Ga are grown under Cu-excess by coevaporation. After removal of the Cu_xSe secondary phase we obtain single phase material. In order to achieve interfaces that do not dominate the recombination, the surface was made Cu-poor by deposition of indium and Se and annealing in selenium vapor. The cell performance was remarkably improved by using this surface treatment and a 13% efficient solar cell was achieved using "Cu-rich" absorbers compared to 14% achieved with Cu-poor absorbers in a standard 3-stage process. In addition to classical characterization of the cell (IV and IVT, QE), the electronic structure of the surfaces and interfaces are investigated by photoluminescence, Auger electron spectroscopy and capacitance-voltage measurements, which show that it is indeed possible to keep a stoichiometric absorber while making the surface Cu-poor.