J. Fritsche

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.

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.

Oriented growth and band alignment at the CdTe/CdS interface

The CdTe/CdS interface has been investigated by photoemission and low energy electron diffraction. The growth of CdS on single crystalline CdTe substrates at elevated temperatures proceeds with the conservation of rotational symmetry. Initial results for the dependence of the band alignment on the crystallographic orientation based on calibrated core level binding energies are presented.

Properties of sputtered ZnO films and its interfaces with CdS

The surfaces of sputtered zinc oxide (ZnO) films and its interfaces with CdS are investigated with X-ray photoelectron spectroscopy and soft X-ray photoelectron spectroscopy measured at the synchrotron radiation facility BESSY II. Cadmium sulfide (CdS) films are prepared by thermal evaporation from the compound and the zinc oxide films by DC magnetron sputtering from an undoped zinc oxide target at low power densities with and without addition of oxygen to the sputtering gas. ZnO films deposited at room temperatures show an additional oxygen component located at the surface that can be mostly removed by heat treatment. The electronic interface properties are studied in dependence to ZnO deposition conditions.

Fermi level-dependent defect formation at Cu(In,Ga)Se2 interfaces

Abstract A removal of Cu from the surface is observed when the Fermi level moves upwards in the bandgap of Cu(In,Ga)Se 2 semiconductors during contact formation. A model based on a comparison of band edge energies and electrochemical redox energies is proposed, which qualitatively explains the observations and might be used as a simple rule for predicting similar defect formation processes.

Nitrogen doping of ZnTe and its influence on CdTe∕ZnTe interfaces

The properties of nitrogen doped ZnTe films and in situ formed heterointerfaces to CdTe were investigated using photoelectron spectroscopy and electrical measurements. The p doping of ZnTe with nitrogen is controlled during physical vapor deposition with an additional nitrogen plasma source. The resistivity was determined by four-point measurements and a minimum resistivity of ρ=0.04Ωcm was found. The valence band offset of the CdTe∕ZnTe interface is EVBO=0.05eV.

CdTe Thin Film Solar Cells: The CdS/SnO 2 Front Contact

Tin dioxide (SnO 2 ) coated glass is the commonly used substrate for thin film solar cells based on CdTe absorbers. We have investigated the properties of the CdS/SnO 2 interface by X-ray and ultraviolet photoelectron spectroscopy. SnO 2 coated glass substrates as used for solar cell preparation were cleaned by different procedures such as derinsing, sputtering, heating and annealing in oxygen atmosphere. Different surface properties with a strongly dependent number of defects in the SnO 2 band gap are identified. CdS films were deposited stepwise by thermal evaporation to determine the electronic interface properties for different surface preparation conditions. Comparative barrier heights at the CdSSnO 2 contact are found for most surface pretreatments. The Fermi level position in these cases is situated in the SnO 2 band gap. A different interface behaviour is determined for sputter cleaned SnO 2 surfaces, which is attributed to the formation of oxygen vacancies during sputtering and subsequent formation of an interfacial SnO x S y compound.