Chemical interaction of Na with cleaved (011) surfaces of CuInSe2

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Abstract
To study the beneficial effect of sodium‐containing substrate material on the photovoltaic properties of thin‐film CuInSe2 solar cells the chemical interaction of Na with CuInSe2 has been investigated by synchrotron excited photoelectron spectroscopy. A clean CuInSe2 (011) surface was prepared by cleaving an oriented single crystal in UHV. The cleaved surface exhibits an electron affinity of χ≊4.6 eV. The Se 3d level shows a surface core level shift of −0.4 eV. Na was sequentially deposited in UHV in small steps from a commercial dispenser source. Initially ionized Na adsorbs on the surface leading to a shift of the surface Fermi level by 0.3 eV closer to the conduction band. Removal of Cu from the surface is observed. For higher Na deposition a chemical reaction of Na with the CuInSe2 surface occurs leading to the formation of metallic indium and Na2Se.