Polycrystalline silicon wafers have been subjected to annealing (700 °C, 1 h) and to a hydrogen plasma (350 °C, 30 min) during the processing of solar cells. The annealing treatment enhances the bulk minority-carrier recombination lifetime by 19%, presumably by impurity gettering. The plasma treatment improves the lifetime by 26%; hydrogen passivation accounts for at least 2/3 of this improvement. Gettering and passivation are found to be complementary: application of both treatments results in a 43% increase in lifetime compared to standard.
Plasma-enhanced chemical vapor deposition (PECVD) is used for the deposition of a silicon nitride anti-reflection coating (ARC) onto 10 x 10 cm2 Wacker Silso polycrystalline silicon solar cells. It is found that the short-circuit current I,, is improved by 7 to 10% in comparison to reference cells with a standard screenprinted TazO5 coating. Part of the increase in Iscis because of a smaller reflectivity of the silicon nitride ARC. The other part of the improvement comes from an enhanced average minority-carrier diffusion length (Lmi"). The increase in Lmin results from hydrogen passivation, and is attributed to the generation of hydrogen ions during PECVD of Si3N4. Furthermore it is shown that the passivation effect by PECVD of Si3N4 is comparable to that obtained with a 1/2 hour hydrogen plasma treatment, and that it is stable during a 1 hour anneal at 700 "C. We did not observe a significant influence of the substrate temperature during Si3N4 deposition in the range of 350 to 450°C.
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