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This paper discusses developments in the mitigation of light-induced degradation caused by boron-oxygen defects in boron-doped Czochralski grown silicon. Particular attention is paid to the fabrication of industrial silicon solar cells with treatments for sensitive materials using illuminated annealing. It highlights the importance and desirability of using hydrogen-containing dielectric layers and a subsequent firing process to inject hydrogen throughout the bulk of the silicon solar cell and subsequent illuminated annealing processes for the formation of the boron-oxygen defects and simultaneously manipulate the charge states of hydrogen to enable defect passivation. For the photovoltaic industry with a current capacity of approximately 100 GW peak, the mitigation of boron-oxygen related light-induced degradation is a necessity to use cost-effective B-doped silicon while benefitting from the high-efficiency potential of new solar cell concepts.

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The fast and slow boron-oxygen related degradation in p-type Czochralski silicon is often attributed to two separate defects due to the different time constants and the determination of different capture cross section ratios (k). However, a recent study using high lifetime samples demonstrated identical recombination properties for the fast and slow degradation and proposed an alternative hypothesis that these were in fact due to a single defect. The study presented in this article provides further experimental evidence to support the single defect hypothesis. Thermal annealing after light soaking is used to investigate the behaviour of subsequent boron-oxygen related degradation. Modifying the temperature and duration of dark annealing on pre-degraded samples is observed to alter the fraction of fast and slow degradation during subsequent illumination. Dark annealing at 173 °C for 60 s is shown to result in almost all degradation occurring during the fast time-scale, whereas annealing at 155 °C for 7 h causes all degradation to occur during the slow time-scale. This modulation occurs without changing the total extent of degradation or changing the capture cross-section ratio. The results are consistent with the fast decay being caused by defect formation from immediately available defect precursors after dark annealing, whereas the slow degradation is caused by the slow transformation of another species into the defect precursor species before the more rapid reaction of defect formation can proceed.

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