(Invited) Gallium Doped Silicon for High Efficiency Solar Cells

Abstract

In the past year or so, gallium doped silicon wafers have become a mainstream substrate for solar cell production in China [1], and hence for the world. They offer intrinsically better carrier lifetime stability than boron doped substrates [2] without requiring post-cell production stabilisation processes while requiring only minimal changes to established cell lines. Gallium doped monocrystalline silicon is likely to account for the majority of passivated emitter and rear cell (PERC) production in the coming years.High purity gallium doped silicon has been the subject of much less study than some other silicon material types, and the limits of its performance are not yet well established. In terms of effective carrier lifetime – one of the most important properties for solar cells – the highest reported values are typically around 1 ms. We have conducted a study to evaluate the performance limit of modern Ga doped silicon materials with high quality surface passivation. We find effective lifetime to vary with resistivity and measure effective lifetimes > 9 ms in 11.2 Ωcm material, which is much higher than reported previously. Furthermore, the injection dependence is less strong than in the boron case which may result in a higher fill factor and favourable cell performance in operation.One of Ga doped silicon’s perceived advantages is its stability under illumination relative to B doped silicon which suffers heavily from boron-oxygen light induced degradation if not mitigated. We have monitored the stability of commercial gallium doped PERC cells under illumination (> 3000 h in some cases) using a photoluminescence imaging proxy method. We have shown however that dark annealed Ga doped PERC cells can degrade when exposed to light, albeit on a much smaller scale than B doped PERC, and this degradation occurs due to a reduction in bulk lifetime [3]. We will now present our latest results which show inconsistencies in degradation behaviour between solar cell batches. Surprisingly, cells from one ingot exhibit degradation, then recovery behaviour when annealed at 300 °C, but near stability when not annealed, but, for another ingot, the opposite is observed. We have also investigated the effect of stabilisation processes of the kind typically used to mitigate boron-oxygen degradation. We find these processes make essentially no difference to the degradation behaviour in Ga doped PERC cells. Our investigations using secondary ion mass spectrometry (SIMS) find that commercial Ga doped PERC cells contain detectable levels of boron, and this could play a role in the degradation processes observed.In summary, our work generally confirms the positive effects of a transition from B to Ga doping for solar cells. Any degradation in cell performance under illumination is fairly small, but understanding the reasons for this is one of the most important questions in high purity silicon research at present. We will highlight urgent research in this area necessary to mitigate instability issues which may arise in newly manufactured modules featuring Ga doped silicon solar cells.