Efficiency limitations of chalcopyrite and kesterite solar cells

We consider the efficiency limitations of Cu(In, Ga)(S, Se)₂ as well as Cu₂ZnSn(S, Se)₄ champion solar cells based on published data and discuss the dominant recombination mechanism which limits the V_oc of these devices. A CuIn_1-xGa_xSe₂ solar cell with x≈0.3 is limited by recombination in the quasi-neutral-region including the back contact; we calculate a diffusion constant of electrons of about 0.5 cm² s^-1. Cells with x=1 (Cu-poor CuGaSe₂) appear to be at the edge between recombination in the space-charge-region with very low bulk carrier lifetime and interface recombination. Solar cells based on Cu-rich Cu(In, Ga)(S, Se)₂ as well as on kesterite Cu₂ZnSn(S, Se)₄ appear to be limited by interface recombination as the activation energy of the saturation current is smaller than E_{g, a}. Here, we discuss the impact of a type II band line-up both at the absorber/buffer interface and at the buffer/window interface.