R Wendt

Telekom Germany GmbH, Bonn, North Rhine-Westphalia, Germany

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Publications (7)10.28 Total impact

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    ABSTRACT: Recent developments in the following areas are briefly reviewed: a) the electrical structure of grain boundaries in CdTe absorbers, b) impurities and non-stoichiometry in CdTe solar cells and c) use of Sb2Te3 in contacts to CdTe. Nominally identical solar cells fabricated using 99.999% pure CdTe feedstock from two different suppliers were compared. Differences in the photovoltaic response and absorber grain size were correlated with the purity of the feedstock, the purer material giving the higher Voc, FF and efficiency, and larger grain size. Quantum efficiency and C-V measurements indicated that the performance differences are most likely to result from reduced doping at the back contact surface in the less pure sample. A quantitative SIMS study of Sb-Te contacts to CdTe reveals that annealing in air at 400 °C causes an influx of Sb and O into the absorber layer. Free energy calculations indicate that this is driven by the preferential reaction of O with Sb compared to CdTe oxidation.
    physica status solidi (b) 01/2002; 229(2). DOI:10.1002/1521-3951(200201)229:2<1055::AID-PSSB1055>3.0.CO;2-W · 1.49 Impact Factor
  • G Friesen · E.D Dunlop · R Wendt ·
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    ABSTRACT: Two CdTe solar cells with very different IV-characteristics were analysed using impedance spectroscopy (IS) and capacitance–voltage (CV) measurements at different temperatures and under dark and illuminated conditions. The two samples were distinguished by different durations of the nitric–phosphoric (NP) etching process applied before back contact deposition. The cell with a short etching time (30 s) shows a strong roll over in the forward bias region, and the cell with the surface that was etched for a longer time (3 min) showed a very strong decrease of the roll-over and a better fill factor. The results show that the etching process dramatically changes the CV characteristics, the resulting CdTe effective doping concentration and that this change can be correlated to different equivalent circuits obtained by the fitting of the impedance spectra of the two samples.
    Thin Solid Films 05/2001; 387(s 1–2):239–242. DOI:10.1016/S0040-6090(00)01832-0 · 1.76 Impact Factor
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    ABSTRACT: 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.
    Thin Solid Films 05/2001; 387(1-387):161-164. DOI:10.1016/S0040-6090(00)01851-4 · 1.76 Impact Factor
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    D.L Bätzner · A Romeo · H Zogg · R Wendt · A.N Tiwari ·
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    ABSTRACT: To make CdTe/CdS solar cells highly efficient, a Cu containing back contact (BC) is generally used. These cells degrade due to Cu diffusion to the front contact which causes shunting; this is shown with secondary ion mass spectroscopy (SIMS) depth profiling. To get a stable but still highly efficient cell, different BC materials and etching treatments were investigated. Chemical etching creates a back surface field (BSF) due to a p+-doped Te-rich CdTe surface. To overcome the naturally existing Schottky barrier between p-CdTe and any metal, a thin buffer layer was evaporated prior to the metallization. Amongst the many investigated BC materials, the most suitable are Sb or Sb2Te3 as a buffer and Mo for metallization. These cells showed high stability under accelerated tests corresponding to 70 years.
    Thin Solid Films 05/2001; 387(1-2-387):151-154. DOI:10.1016/S0040-6090(01)00792-1 · 1.76 Impact Factor
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    ABSTRACT: This paper discusses the characterisation of different CdTe modules and the main issues raised by such technology compared to conventional Si devices, from indoor STC (1000 W/m2, AM1.5G spectrum, 25°C) performance to the actual outdoor field data. The choice of the reference detector, indoor to outdoor calibration, hysteresis, transient effects and stability are variables that make the intercomparison of data difficult, and this work aimed to contribute to their classification and systematisation. Indoor calibration values reasonably agree with outdoor performance when a spectral mismatch factor correction is applied to short circuit current. However, the fill factor is underestimated. Temperature coefficients show different trends and classic approaches to translation equation for temperature and irradiance fail. Finally, the importance of STC performance with respect to actual field performance was analysed over a period covering 3 months of outdoor testing.
    Thin Solid Films 05/2001; 387(1):172-174. DOI:10.1016/S0040-6090(00)01735-1 · 1.76 Impact Factor
  • D. L. Batzner · A. Romeo · H. Zogg · R. Wendt · A. N. Tiwari ·

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    D.L. Bätzner · A Romeo · H Zogg · A N Tiwari · R Wendt ·
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    ABSTRACT: Conventional back contacts on CdTe/CdS solar cells are commonly made with Cu/Au or Cu/graphite. Often the contact limit the solar cells efficiency and the performance degrades because of Cu diffusion to the junction. In order to get stable and "non-rectifying" back contacts Sb has been applied. Pre-depo-sition etching treatments, post-deposition annealing, influence of Sb layer thickness and stability issues have been studied. Different etchants not only clean the surface but they also produce a conducting Te layer on the grain boundaries. Using a mixture of nitric and phosphoric acid and Sb/Au as a back con-tact, 12.5% efficiency cells are obtained. The stability of solar cells depends on the etching solution. Stability tests under continues 1 sun illumination suggest that under optimum condition stable cells with Sb/Au contact can be developed. A comparative analysis of the photovoltaic properties of solar cells with different back contacts will be presented.
    Thin Solid Films 02/2000; 361. DOI:10.1016/S0040-6090(99)00842-1 · 1.76 Impact Factor