B. Späth

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.

Effect of chlorine activation treatment on electron beam induced current signal distribution of cadmium telluride thin film solar cells

We have investigated CdTe thin film solar cells without activation treatment and with CdCl2 activation treatment at temperatures between 370 and 430 °C using a constant activation time of 25 min. For this purpose, CdS/CdTe layers were deposited by closed-space-sublimation on FTO coated float glass. The solar cells were characterized by measurements of the JV characteristics and quantum efficiencies. In addition, ion polished cross sections of the solar cells were prepared for high-resolution FE-SEM imaging of the microstructure and the simultaneous registration of electron beam induced current (EBIC) signal distribution. By measurement of the EBIC signal distribution, it can be shown that without activation treatment the CdTe grain boundaries itself and grain boundary near regions exhibit no EBIC signal, whereas centres of some singular grains already show a distinct EBIC signal. In contrast, after the chlorine activation treatment, the grain boundary near regions exhibit a significant higher EBIC signal ...

p-ZnTe for Back Contacts to CdTe Thin Film Solar Cells

CdTe thin film solar cells still suffer from problems related to back contacts which provide stable ohmic contacts without electrical losses. In previous studies metal/CdTe contacts have shown significant limitations. A promising option for the realization of ohmic back contacts is the use of a highly p-doped ZnTe interlayer on top of the CdTe absorber, in particular since metal/ZnTe contacts have shown very good electrical properties. In this work we studied the electronic und chemical properties of nitrogen doped p-ZnTe films. p-ZnTe:N films were prepared by using reactive RF magnetron sputtering with N2/Ar gas mixtures or by thermal evaporation with an additional nitrogen plasma source. Samples and their contacts have been prepared in DAISY-SOL (DArmstadt Integrated SYstem for SOLar energy research) which combines a full vacuum production with an in-situ photoelectron spectroscopy (XPS/UPS) analysis. The results of XPS/UPS investigations and electrical measurements will be discussed in comparison to previous results on metal/CdTe contacts. As ZnTe forms an interlayer in the CdTe thin film solar cell, the ZnTe/CdTe interface properties must be also taken into consideration. Our experiments have shown in agreement to previous studies that the valence band offset is nearly ideal for hole transport across the interface. Also electrical measurements have been carried out to investigate the metal/ZnTe/CdTe layer sequences in their contact properties.

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.