T. Mayer

Band energy diagram of CdTe thin film solar cells

Abstract The knowledge of band energy diagrams of solar cells is essential for a fundamental understanding of their function. We have used photoelectron spectroscopy (PES) as a powerful tool for a systematic study of the formation of interfaces of CdTe solar cells in which the different layers CdS/SnO2, CdTe/CdS and Te/CdTe are deposited step by step by thermal evaporation in model experiments. The results of these studies show that in contrast to other investigations the energy converting heterojunction is not a simple n-CdS/p-CdTe contact. Although depth profiling reveals a homogeneously intrinsic CdTe bulk layer, contact formation and CdCl2-activation are assumed to form an n–i–p CdTe absorber. Such non-ideal conditions may strongly affect optimization processes of conversion efficiencies. The main limitations are evidently due to back-contact formation. Our results do not confirm that the electrochemically formed Te layer produces a good ohmic contact between the CdTe layer and the metallic back contact. From model experiments, we assume the formation of a tunneling contact, instead.

Surface analysis of CdTe thin film solar cells

The surface properties of CdTe thin film solar cells prepared by ANTEC using the close-space sublimation — (CSS) — technique have been analyzed by X-ray diffraction (XRD), atomic force microscopy (AFM), photoelectron emission microscopy (PEEM), high-resolution scanning electron microscopy (HRSEM) and photoelectron spectroscopy (XPS) after different pretreatment conditions. Exposure of the CdTe films to air leads to surface oxidation with the formation of TeO2 and CdO. The amount of surface oxides depends on the CdCl2 activation process. Activated surfaces are less oxidized than non-activated surfaces. Due to that surface oxidation, the surface is more n-type, indicating the formation of a surface barrier. The surface oxide can be removed by mild sputtering. The results suggest that no extra surface defects are introduced by this procedure. Before sputtering, Cl is found on the surface of the activated material, although no such contamination is found in the stoichiometric bulk material using XPS. A variation in the Fermi level position is observed for the non-activated to the activated CdTe material from weakly to higher p-doped levels. This type of conversion is evidently restricted to the near surface area as further in the bulk, weakly p-doped CdTe is found again. The results indicate that, besides the surface composition, the electronic properties of the film also depend on the different pretreatment steps.

Cs adsorption on oxide films (Al2O3, MgO, SiO2)

Abstract Films of alumina, magnesia, and silica were exposed to cesium atoms at room temperature. Cesium adsorption as a function of exposure time was studied with metastable impact electron spectroscopy (MIES) and photoelectron spectroscopy (UPS; He I). On silica, cesium atoms readily chemisorb in the initial stages of exposure. The bonding is apparently due to the interaction with active surface oxygen such as non-bridging oxygen atoms. We suggest that on alumina and magnesia, the chemisorption of cesium, in the form of an ionic state, takes place at surface defects, presumably edge sites such as steps, kinks, corners, etc. In all cases studied, prolonged exposure leads to additional Cs adsorption. Apparently, patches with metallic properties (but no uniform adlayer) develop on the surface; they disappear after the cesium supply is interrupted.

Black spots on carbon steel after contact to lubricating oil with extreme pressure additives: an XPS study

Unrevealed synergistic effects of certain additive composition of lubricating oil cause the formation of black spots on guiding planes of cast iron. We performed an X-ray photoemission study comparing samples with black spots to samples without. Sputter depth profiles of the elements Fe, O, C, Si, Ca, Mn, S, P and N were recorded. While the shapes of the iron and oxygen spectra do not differ markedly, the thickness of the iron oxide is much larger on black spots. While Si and Mn were found in detectable concentrations in the oxide layer of samples with and without black spots, Ca, S and P were detected on black spot samples only. Since the detected iron oxides are of dark colour and Mn is known to form dark compounds with O, S and P, we conclude that a thick iron oxide layer containing Mn compounds is responsible for the colouring of the black spots.

Study of the electronic structure of Si(100)2 × 1 and CsSi(100)2 × 1 with MIES and UPS (HeI)

Abstract We report the MIES (He∗1s2s) and UPS (HeI) spectra for clean Si(100)(2 × 1) and Cs Si (100)(2 × 1) . The cesium coverage is varied at room temperature between zero and saturation. The MIES results are compared with corresponding ones for Cs Al (111) and Cs W (110) (also presented here). Simulations of the MIES spectra for clean and cesiated Si(100)(2 × 1), based on presently available information on the electronic structure of the clean and cesiated surfaces, facilitate the interpretation of the spectra. The results obtained for small Cs coverages are compatible with ionic adsorption of Cs whereby the Cs 6s electron is transferred into the Si dangling bonds. For larger coverages the surface displays metallic character. We cannot decide however whether this is caused by the formation of a metallic-like Cs adlayer or by the filling of the Si dangling bonds leading to metallization of the Si substrate.

Growth of ultrathin alumina films on W(110)

Abstract The interaction of oxygen with Al(111) and Al adlayers ( Al W(110) suface requires an additional heat treatment to become transformed into alumina.

The study of surface adsorbed C60 molecules with metastable impact electron spectroscopy and UPS (He I)

Metastable impact electron spectroscopy (MIES) in combination with UPS (He I) is applied to study the electronic properties of surface adsorbed C60 molecules. The growth of C60 layers on clean, thermally oxidized and cesiated Si(100) is monitored by recording the spectra of the electrons ejected during the layer growth at room temperature. The various mechanisms for electron ejection during the interaction of photons and He* metastables with C60 are discussed. The results furnish also information on the interaction between the studied Si(100)-based substrates and the C60 adsorbates.

H2O adsorption on the layered chalcogenide semiconductors WSe2, InSe and GaSe

Abstract The interaction of n- and p-type WSe 2 and n-InSe and p-GaSe van der Waals planes (0001) with adsorbed H 2 O is investigated by photoelectron spectroscopy (UPS, XPS) in comparison to the semiconductor/electrolyte interface. H 2 O is molecularly adsorbed on all compounds without any detected surface reaction. It forms three-dimensional clusters on the (0001) planes. As expected for electron donors, H 2 O induces strong band bending on p-WSe 2 (0.8 eV) and a decrease of electron affinity by 0.65 eV; on n-WSe 2 band bending is considerably smaller (0.2 eV), but the decrease of electron affinity is similar (0.7 eV). In correspondence we observe stronger band bending (0.3 eV) on p-GaSe compared to n-InSe (0.2 eV). The electron affinities are decreased by 0.6 to 0.7 eV, respectively. The considerably stronger band bending observed for p-WSe 2 is related to the stronger electronic coupling of H 2 O molecular orbitals to the metal d-based band edges which are absent in InSe or GaSe.

Interface Engineering of Chalcogenide Semiconductors in Thin Film Solar Cells: CdTe as an Example

In this paper the electronic properties of the different interfaces of CdTe thin film solar cells will be analysed by using a surface science approach. Experimental basis for the experiments is an integrated UHV systems which allows to prepare and analyse real solar cells as well as appropriate model interfaces. Recently obtained data on the ITO surface, the ITO/SnO2/CdS front contact, the CdS/CdTe heterojunction and the CdTe/Te back contact will be presented. In addition, bulk properties as doping and lateral inhomogeneities will be addressed. For all these interfaces experimentally determined band energy diagrams will be given and discussed in relation to solar cell performance. Finally, the sum of the results will be used to propose a modified band energy diagram of the complete CdTe thin film solar cell and its implication for further cell improvement will be presented.

Coadsorption of Na and Br2 on WSe2 (0001). Creating a surface redox couple

Abstract The interface between UHV-cleaved n-WSe2 (0001) and coadsorbed Br2 and Na has been investigated as a model of a semi-conductor/electrolyte interface. Br2 is molecularly adsorbed and leads to a strong inversion layer of n-WSe2. Na coadsorption forms Br− on the surface with no pronounced change of semiconductor band bending for Na coverage