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Stable Czochralski silicon solar cells using gallium-doped base material

: Glunz, S.W.; Rein, S.; Knobloch, J.

Scheer, H. ; European Commission:
Sixteenth European Photovoltaic Solar Energy Conference 2000 : Proceedings of the international conference held in Glasgow, United Kingdom, 1 - 5 May 2000
London: James & James, 2000
ISBN: 1-902916-18-2
European Photovoltaic Solar Energy Conference <16, 2000, Glasgow>
Fraunhofer ISE ()

The lifetime of boron-doped oxygen-contaminated Czochralski (Cz) silicon is strongly reduced under illumination or forward bias. This process can be fully reversed by a 200°C anneal step. In a couple of recent studies it was shown that boron and oxygen are the major components of the underlying metastable Cz-specific defect. The energy level of the defect in its active state, A, was determined to be between E(ind v)+0.35 eV and E(ind c)-0.45 eV (Schmidt et al.[1], 1999). In this work we show that the energy of the defect in its passive state, P, is very shallow. The kinetic of the carrier-induced transformation from state P to A was analyzed in more detail. The measured defect generation data can be described using recombination-enhanced defect reaction (REDR) theory. On the base of this analytical facts different solutions for the reduction or elimination of the metastable defect are suggested. Two very interesting solutions are discussed in more detail: the usage of gallium-doped Cz silicon and the introduction of high-temperature anneals into the solar cell process. Gallium-doped Cz silicon shows no degradation and excellent lifetimes over a wide resistivity range although the concentration of interstitial oxygen is in the same range as in standard Cz silicon. Stable efficiencies comparable to FZ silicon have been achieved. If standard boron-doped Cz silicon is used the defect reduction can be reduced permanently by a high-temperature anneal. In this work we show that already an optimized anneal step at 750°C for only 10 min can reduce the defect concentration by a factor of 2-3. Nevertheless, it is necessary to use an optimized set of process parameters because otherwise the lifetime of all oxygen-contaminated materials (including gallium-doped Cz silicon) is severely reduced.