Co-incorporation effects of O and Na with CulnS2 thin films

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Abstract
By using x-ray photoelectron spectroscopy we have examined changes in the electronic structure of CuInS2 thin films with co-incorporation of O and Na that raised up the photovoltaic performance of CdS/CuInS2 solar cells. The electron binding energies of both the core levels and valence band for the film with the co-incorporation were smaller by 0.9 eV than those for the film without the co-incorporation. For the co-incorporated film an increase in the spectral intensity of the valence band was observed below 1–6 eV from the Cu 3d10 nonbonding states, and it was due to both the Na s, p-S p hybridization and the In s-Op hybridization. The surface of the co-incorporated film can be expressed as (Cu, Na)In(S, O)2. The (Cu, Na)In(S, O)2 phase is related to the enlarged open-circuit voltage (0.75 eV) of n-CdS/p-CuInS2 solar cells.