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

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.