Low-temperature gettering in multicrystalline silicon materials for photovoltaics

This thesis presents results on the effects of low-temperature gettering processes on minority carrier lifetime in multicrystalline silicon. Wafers are sourced from different height positions of a commercially-grown ingot. The distribution of different key material properties including bulk lifetime, interstitial iron concentration, and dislocation density are characterised and are found to vary widely with ingot height position. Lifetimes are measured by using temporary liquid iodine-ethanol passivation at room temperature or silicon nitride films deposited by plasma-enhanced chemical vapour deposition. Lifetimes are lower in samples from the extrema of ingot than the centre parts. Interstitial iron concentrations are found to be highest in the bottom samples and lowest at the centre of the ingot. Dislocation density is lowest at the bottom of the ingot and increases with ingot height position. In as-grown wafers, low-temperature gettering can improve lifetime substantially in relatively poor samples from the extrema of the ingot. Iodine-ethanol passivation is used to separate thermal effects of annealing from any bulk passivation which may occur during surface passivation from lifetime measurement. The largest relative lifetime improvement (from 5.5 μs to 38.7 μs) is achieved in material from the bottom of the ingot with annealing at 400°C for 35 h. The benefit of low-temperature annealing is marginal for middle samples. Bulk interstitial iron concentrations decrease by up to 2.1 order of magnitude in the bottom samples. The reduction in interstitial iron concentration is not found to be systematically dependent on annealing temperature. For bottom samples a good correlation between the changes in lifetime and interstitial iron concentration is found. The effects of different passivation schemes on low-temperature gettering is also investigated. The results show that starting lifetime and interstitial iron concentration strongly depends on the choice of passivation scheme. The effect of different surface passivation schemes is more pronounced in relatively high lifetime samples. In samples from the bottom of the middle of the wafer, lifetime improves from 113 μs to 171 μs with silicon nitride passivation upon annealing at 400 °C for 25 h. Supporting results from secondary ion mass spectrometry show that substantial concentrations of iron exist in the silicon nitride film after low-temperature annealing. This suggests silicon nitride layer might be an additional gettering centre for interstitial iron. This thesis also studies the effects of low-temperature annealing combined with a standard phosphorus diffusion process to form an emitter. Lifetime in samples from the top and bottom of the ingot can be improved by annealing at 300°C and 400°C even after the phosphorus diffusion process. The largest improvement is from 54 μs to 78 μs upon post-diffusion annealing of bottom samples at 300°C, and the results suggest gettering of impurities other than interstitial iron is likely. The phosphorus diffused emitter layers do not act as effective additional gettering sites for interstitial iron upon low-temperature annealing. The lifetime improvement upon pre-diffusion annealing is retained after the diffusion process. In summary, low-temperature annealing has the potential to improve the lifetime in as-grown multicrystalline silicon and after a phosphorus diffusion gettering under some conditions. Low-temperature annealing thus provides a potential low cost route to improve multicrystalline solar cell efficiencies.