Photoemission investigation of the electronic structure at polycrystalline CuInSe2 thin-film interfaces

The surface versus bulk composition and electronic structure of polycrystalline CuInSe2 thin-film interfaces were studied by synchrotron radiation soft-x-ray photoemission spectroscopy. An n-type In2Se3/CuIn3Se5 surface layer forms on enhanced-grain polycrystalline thin-film p-type CuInSe2 during fabrication. Enhanced-grain CuInSe2 films were sputter etched (500 V Ar) and analyzed in situ to determine core-level binding energies and Fermi-level positions for the n-type surface and the p-type CuInSe2 bulk within ±0.1 eV. The transition between the n-type surface and the p-type bulk was experimentally observed by noting the change in the position of the valence-band maximum relative to the Fermi level EF. From these measurements, the valence-band offset ΔEv between the layers was determined to be 0.50 eV. Measurement of the work functions φ was also completed and reveals φ=4.75 eV for the In2Se3 (CuIn3Se5) surface layer and φ=4.04 eV for the bulk CuInSe2. Combining these results allows construction of a surface band diagram for this device configuration as well as determination of the relationship between composition, electronic structure, and device performance.