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 Voc of these devices. A CuIn1-xGaxSe2 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 cm 2 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 Eg,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.