Why do we make Cu(In,Ga)Se2 solar cells non-stoichiometric?

The superior transport and lifetime properties of Cu-rich chalcopyrite semiconductors are discussed. The reason why solar cells are made from Cu-poor absorbers in spite of their inferior properties is the CdS/absorber interface which leads to high recombination in the case of Cu-rich absorbers. We report on Cu-rich solar cells with a Cu-poor surface, which have reached 13.1% efficiency.