Electronically Stimulated Degradation of Crystalline Silicon Solar Cells
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Electronically Stimulated Degradation of Crystalline Silicon Solar Cells J. Schmidt1, K. Bothe1, D. Macdonald2, J. Adey3, R. Jones3, and D. W. Palmer3 1 Institute of Solar Energy Research Hameln/Emmerthal (ISFH), Am Ohrberg 1, D-31860 Emmerthal, Germany 2 Department of Engineering, Australian National University, Canberra ACT 0200, Australia 3 School of Physics, University of Exeter, Exeter, EX4 4QL, United Kingdom ABSTRACT Carrier lifetime degradation in crystalline silicon solar cells under illumination with white light is a frequently observed phenomenon. Two main causes of such degradation effects have been identified in the past, both of them being electronically driven and both related to the most common acceptor element, boron, in silicon: (i) the dissociation of iron-boron pairs and (ii) the formation of recombination-active boron-oxygen complexes. While the first mechanism is particularly relevant in metal-contaminated solar-grade multicrystalline silicon materials, the latter process is important in monocrystalline Czochralski-grown silicon, rich in oxygen. This paper starts with a short review of the characteristic features of the two processes. We then briefly address the effect of iron-boron dissociation on solar cell parameters. Regarding the boron-oxygen-related degradation, the current status of the physical understanding of the defect formation process and the defect structure are presented. Finally, we discuss different strategies for effectively avoiding the degradation. INTRODUCTION Two main causes have been identified for the illumination-induced degradation of solar cells fabricated on boron-doped mono- and multicrystalline silicon materials. Both of them are electronically driven defect reactions involving substitutional boron (Bs), leading to a pronounced decrease in the carrier recombination lifetime under solar cell operating conditions. In multicrystalline silicon (mc-Si), dissociation of interstitial iron-substitutional boron (FeiBs) pairs into isolated Fei and Bs has been identified as the most relevant process [1]. This wellknown process is linked to the degree of iron contamination in the material. It can also be observed in single-crystalline iron-contaminated B-doped float-zone (FZ) and Czochralski (Cz) silicon and is not restricted to mc-Si. Another carrier lifetime degradation effect has been observed in metal-impurity-free B-doped Cz-Si [2]. This effect has only recently been attributed to the simultaneous presence of Bs and interstitial oxygen (Oi) [3,4]. Interestingly, as for the FeiBs dissociation, this degradation effect occurs also in the dark when minority-carriers are injected (e.g., by a forward-biased pn junction), leading to the conclusion that the degradation is caused by the presence of minority-carriers and photons are not directly involved [5]. However, in contrast to the FeiBs-related lifetime degradation, which also occurs during annealing above ~100°C, the latter degradation effect is fully reversible by annealing above ~200°C, i.e., the degraded lifetime
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