Wolfram Calvet

Experimental indication for band gap widening of chalcopyrite solar cell absorbers after potassium fluoride treatment

The implementation of potassium fluoride treatments as a doping and surface modification procedure in chalcopyrite absorber preparation has recently gained much interest since it led to new record efficiencies for this kind of solar cells. In the present work, Cu(In,Ga)Se2 absorbers have been evaporated on alkali containing Mo/soda-lime glass substrates. We report on compositional and electronic changes of the Cu(In,Ga)Se2 absorber surface as a result of a post deposition treatment with KF (KF PDT). In particular, by comparing standard X-ray photoelectron spectroscopy and synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), we are able to confirm a strong Cu depletion in the absorbers after the KF PDT which is limited to the very near surface region. As a result of the Cu depletion, we find a change of the valence band structure and a shift of the valence band onset by approximately 0.4 eV to lower binding energies which is tentatively explained by a band gap widening as expected for Cu defic...

Formation of an electron hole doped film in the α-Fe2O3 photoanode upon electrochemical oxidation

Solar hydrogen generation by water splitting in photoelectrochemical cells (PEC) is an appealing technology for a future hydrogen economy. Hematite is a prospective photoanode material in this respect because of its visible light conjugated band gap, its corrosion stability, its environmentally benign nature and its low cost. Its bulk and surface electronic structure has been under scrutiny for many decades and is considered critical for improvement of efficiency. In the present study, hematite films of nominally 500 nm thickness were obtained by dip-coating on fluorine doped tin oxide (FTO) glass slides and then anodised in 1 molar KOH at 500, 600, and 700 mV for 1, 10, 120 and 1440 minutes under dark conditions. X-ray photoelectron spectra recorded at the Fe 3p resonant absorption threshold show that the e(g) transition before the Fermi energy, which is well developed in the pristine hematite film, becomes depleted upon anodisation. The spectral weight of the e(g) peak decreases with the square-root of the anodisation time, pointing to a diffusion controlled process. The speed of this process increases with the anodisation potential, pointing to Arrhenius behaviour. Concomitantly, the weakly developed t(2g) peak intensity becomes enhanced in the same manner. This suggests that the surface of the photoanode contains Fe(2+) species which become oxidized toward Fe(3+) during anodisation. The kinetic behaviour derived from the experimental data suggests that the anodisation forms an electron hole doped film on and below the hematite surface.

Surface vs. volume stoichiometry of MBE grown CuInS2 films on Si

Molecular beam epitaxy CuInS 2 layers were grown on Si single crystals of different orientation. The stoichiometry was varied between 0.5 and 1.5 for the nominal Cu/In ratio and the film thickness was limited to 150 nm. Composition analysis was done by in situ photoelectron spectroscopy (PES), Rutherford backscattering (RBS) and X-ray diffraction (XRD). PES was used to determine the surface composition, RBS monitored the volume contribution and XRD probed phase formation. It was found that the surface composition differed significantly from that of the volume: instead of the expected variation along the Cu 2 S-In 2 S 3 binary, the actual composition varied along the Cu 2 S-InS line at the surface (S-deficiency). The role of sulfur incorporation into the growing film is discussed.

Silicon based tandem cells: novel photocathodes for hydrogen production

A photovoltaic tandem cell made of amorphous silicon (a-Si) and microcrystalline silicon (μc-Si) was investigated as a photocathode for hydrogen evolution in a photoelectrochemical device. The electronic and electrochemical properties of the samples were characterized using X-ray photoemission spectroscopy (XPS) and cyclic voltammetry (CV), whereas the morphology of the surface in contact with the electrolyte was investigated by scanning electron microscopy (SEM). The electric efficiency of the tandem cell was determined to be 5.2% in a photoelectrochemical (PEC) setup in acidic solution which is only about half of the photovoltaic efficiency of the tandem cell. A significant improvement in efficiency was achieved with platinum as a catalyst which was deposited by physical vapour deposition (PVD) under ultra-high vacuum (UHV) conditions.

Initial phases of CuInS2–Si heteroepitaxy

The heterojunction formation between silicon Si(111) and copper indium disulfide CuInS2 is investigated using ultraviolet photoelectron spectroscopy (UPS)/x-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Thin layers of CuInS2 films were deposited on hydrogen terminated Si(111) surfaces by molecular beam epitaxy (MBE). The film growth process starts from a partially sulfurized Si surface with a nominal composition of SiS0.8±0.1 and a thickness of 2.5±0.8 A. The dependence of XPS measured substrate and film peak intensities and LEED patterns on deposition time indicates a disturbed layer-by-layer growth mode. In addition, a change in film stoichiometry from Cu rich to In rich is observed at the beginning of the film growth. The analysis of the Si 2p core level shows that the Si substrate is disturbed to a depth of about 7 A beyond the initial hydrogenated surface. The hydrogenated surface exhibits a band bending of 0.4 eV. Upon S exposure, the band bending increases to 0.64...