T. Schulmeyer

Interface properties and band alignment of Cu2S/CdS thin film solar cells

The stoichiometry and electronic properties of bulk Cu2S thin films obtained by vacuum evaporation were investigated by optical spectroscopy, X-ray diffraction and photoemission spectroscopy. The Cu2S/CdS heterojunction interface has been prepared in situ and characterized by photoelectron spectroscopy (X-ray photoemission spectroscopy and ultraviolet photoelectron spectroscopy) after each growth step under ultra high vacuum conditions. The XPS core level spectra as well as valence band spectra of the substrate Cu2S and overlayer CdS were acquired after each step. From these measurements, a large overall band bending of 0.9 eV is observed. The valence band offset is determined to be ΔEVB=1.2 eV and the conduction band offset is ΔECB=0±0.1 eV.

Photoemission study and band alignment of the CuInSe2(001)/CdS heterojunction

The contact formation of thin-film epitaxial CuInSe2(001) with a physical-vapor-deposited CdS layer is presented in this work. Synchrotron-excited photoelectron spectroscopy was used for this investigation. The epitaxial CuInSe2 films contain a surface layer of reduced Cu stoichiometry similar to the ordered defect compound CuIn3Se5. A valence band offset of 0.79±0.15 eV has been determined for this heterojunction. The comparison to literature data indicates that neither surface orientation nor surface copper content have a major impact on the valence band offset of CuIn3Se5, respectively, CuInSe2 with CdS.

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.

Utilization of sputter depth profiling for the determination of band alignment at polycrystalline CdTe/CdS heterointerfaces

The band alignment at polycrystalline CdS/CdTe heterointerfaces for thin-film solar cells is determined by photoelectron spectroscopy from stepwise CdTe deposition on polycrystalline CdS substrates and from subsequent sputter depth profiling. Identical values of 0.94±0.05 eV for the valence band offset are obtained.

Interface modification of CdTe thin film solar cells by CdCl2-activation

Abstract In thin film solar cell production several materials are subsequently deposited onto a glass substrate. The interface properties between the different layers are important for the opto-electrical performance of the solar cell device. CdTe thin film solar cells are currently produced using a layer sequence of CdTe/CdS/SnO 2 /ITO/glass. In order to reach reasonable conversion efficiencies the device has to be activated in a CdCl 2 atmosphere. Finally, the back contact is prepared. The influence of the activation step on the solar cell is still not understood in detail. Therefore in this study model experiments have been carried out in which CdS and CdTe thin films with a thickness of 100 nm have been deposited by thermal evaporation onto ITO/SnO 2 -coated glass substrates in an UHV system. The influence of the CdCl 2 -activation step on the morphology, chemistry and band alignment of the interfaces has been investigated with atomic force microscopy and sputter depth profiling using X-ray photoelectron spectroscopy. A change in surface morphology due to the CdCl 2 -activation has only been found for the CdTe layer, while the SnO 2 and CdS films are unaffected. It can be shown that the activation step leads to diffusion processes at both interfaces. For the CdS/CdTe interface an interdiffusion of CdS and CdTe takes place. At the SnO 2 /CdS interface Cd diffuses into the SnO 2 layer with a constant amount of approximately 5% Cd in the whole SnO 2 layer. The diffusion and interdiffusion changes the electronic properties of the interfaces. A strong n-type doping for all semiconductor films is observed after the activation.

Effect of in situ UHV CdCl2-activation on the electronic properties of CdTe thin film solar cells

Abstract To reach reasonable conversion efficiencies of approximately 10% and above with CdTe thin film solar cells an activation step involving chlorine at elevated temperatures seems to be necessary before back contact formation. This activation process has been simulated in an ultrahigh-vacuum (UHV) system. Solar cells with a maximum efficiency of 9.1% have been prepared using this process. In addition the effect of the CdCl 2 activation process on the electronic properties of each solar cell layer, SnO 2 , CdS and CdTe has been investigated in situ using photoelectron spectroscopy. The effects of the activation on the Fermi level position of all investigated layers is presented and discussed.

Band offset at the CuGaSe2∕In2S3 heterointerface

We have investigated the electronic properties of the CuGaSe2∕In2S3 heterointerface by photoelectron spectroscopy. In2S3 was evaporated by physical vapor deposition onto contamination free polycrystalline CuGaSe2 surface prepared by the selenium decapping process. A valence band offset ΔEVB=0.78±0.1 has been determined.

Interfaces in thin film solar cells

Interfaces are important for the efficiencies of thin film solar cells. In particular, for polycrystalline chalcogenide semiconductors as CdTe and Cu(In,Ga)(S,Se)/sub 2/ (CIGS) the existing physical concepts, which describe the electronic properties of elemental or III-V compound semiconductor interfaces quite well, are not sufficient. The increased complexity is mostly due to the non-abruptness of the interfaces and the strong tendency for the formation of defects in the more polar bonded II-VI compounds. Photoelectron spectroscopy has significantly contributed to the understanding of the mechanisms governing the properties of semiconductor interfaces in thin film solar cells. The experimental approach using integrated surface analysis and thin film deposition systems and selected results will be presented.