Wide-Gap Cu(In,Ga)Se2 Solar Cells with Zn(O,S) Buffer Layers Prepared by Atomic Layer Deposition

Wide-gap Cu(In0.4,Ga0.6)Se2 solar cells with Zn(O,S) buffer layers deposited by atomic layer deposition (ALD) technique have been investigated. The band-gap energy (Eg) of the Zn(O,S) layer estimated by optical transmission and reflection measurements was varied from 3.2 to 3.6 eV. The solar cells with sulfur (S)-poor Zn(O,S) buffer layers showed a low open-circuit voltage (VOC) owing to the cliff nature of the conduction band offset (CBO). In contrast, the solar cells with S-rich Zn(O,S) buffer layers showed a low short-circuit current density (JSC) owing to the spike nature of CBO. Even if the CBO values were adequate, the best solar cell efficiencies were considerably low. These results suggest that the main cause for the low efficiencies is not interface recombination at the Zn(O,S)/Cu(In,Ga)Se2 interface, but mainly bulk recombination in the Cu(In,Ga)Se2 (CIGS) absorber layer.