Andreas Thissen

Characterization of tellurium layers for back contact formation on close to technology treated CdTe surfaces

We have studied the contact formation on CdTe surfaces following the technologically applied procedure. The electronic properties of wet chemically etched CdTe surfaces has been investigated with photoelectron spectroscopy. For the characterization of the morphology, structure, and elemental distribution in the etched layer atomic force microscopy, scanning electron microscopy, grazing incidence x-ray diffraction, and secondary ion mass spectroscopy have been used. Etching of the samples has been performed in air and in an electrochemistry chamber directly attached to the UHV system. In both cases the formation of an elemental polycrystalline Te layer with a thickness of about 80 A is detected. For comparison, a thin Te layer has been deposited by physical vapor deposition onto a CdTe substrate. We determine a valence-band offset of ΔEVB=0.5±0.1 eV, independent of the preparation of the interface.

In situ preparation and interface characterization of TiO2/Cu2S heterointerface

The electronic structures and interface properties of the TiO2/Cu2S interface have been in situ studied after each growth step by x-ray and ultraviolet photoelectron spectroscopy. The p-doped Cu2S films (BEVBM=0.1 eV) were grown on the highly n-doped chemical vapor deposition prepared TiO2 (BEVBM=3.4 eV) films by thermal evaporation in a multistep growth procedure. The conduction band offset (0.7 eV), valence band offset (2.9 eV) and interface dipole (0.5 eV) were determined based on the quantitative examination of band bending occurring in the Cu2S films at higher coverage, leading to a staggered energy level configuration.

Alternative back contacts for CdTe solar cells: a photoemission study of the VSe2/CdTe and TiSe2/CdTe interface formation

Abstract Low resistance ohmic and stable back contacts are still one of the major issues in improving CdTe based solar cells. According to band alignment arguments chemically inactive and conductive materials of high work functions are most promising for CdTe back contacts. In this study VSe 2 /CdTe and TiSe 2 /CdTe interfaces have been prepared by PVD under vacuum conditions and subsequently characterized by photoelectron spectroscopy in order to determine their electronic properties. Despite the high work function of the contact materials (5.7–5.8 eV) no ohmic contacts have been realized. Obviously, a thin Cd interlayer is formed before stoichiometric CdTe growth occurs leading to an interface dipole that compensates the high work function of VSe 2 and TiSe 2 . The resulting band energy diagrams are presented in this study.

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.

Photoemission Spectroscopy and Electronic Structures of LiMn2O4

We study the electronic structures of LiMn2O4 by x-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) and resonant photoelectron spectroscopy (RPES). XPS data suggest that the average oxidation state of Mn ions is 3.55, probably due to the small amount of lithium oxides on the surface. UPS and RPES data imply that Mn ions are in a high spin state, and RPES results show strong Mn3d–O2p hybridization in the LiMn2O4 valence band.

Electronic passivation of Si(111) by Ga–Se half-sheet termination

A Si(111):GaSe van der Waals surface is prepared using sequential deposition of Ga and Se at elevated temperature on a Si(111)-7×7 surface. Surface properties were investigated by soft x-ray photoelectron spectroscopy and low-energy electron diffraction. The Si(111)-1×1:GaSe surface remains with electronic surface potentials near flatband condition.