Electronic and chemical properties of non-vacuum deposited chalcopyrite solar cells

We have investigated the electronic and chemical surface properties of a Cu(In_1-xGa_x)Se₂ (CIGSe) thin-film solar cell absorber and a CdS/CIGSe interface sample taken from Nanosolar´s manufacturing line. Using soft x-ray and UV photoelectron spectroscopy, inverse photoemission, and soft x-ray emission spectroscopy employing high-brilliance synchrotron radiation, we have determined the chemical composition of the surface and near-surface bulk, as well as some of the relevant electronic structure parameters (e.g., the surface band gap of the absorber). We find that the (previously air-exposed) surfaces show a surprisingly low degree of carbon-containing surface adsorbates, the presence of sodium and selenium oxide species on the surface of both samples, a significant S/Se intermixing at the CdS/CIGSe interface, and, as is common for high-efficiency CIGSe absorbers after surface cleaning, an electronic surface band gap (1.45 ± 0.15 eV) that is noticeably larger than the optical bulk band gap.