ANU College ofEngineering & Computer Science

Removing metal impurities in silicon solar cellsBy Sieu-Pheng Phang, ANU CECS / CSES, pheng.phang@anu.edu.au

Supervisor: Dr. Daniel Macdonald

IntroductionMulticrystalline silicon (mc-Si) solar cells account for about 50% of the world silicon solar cell produced currently; the main advantage of mc-Si is the lower material cost, but mc-Si also contains greater amount of metal impurities, that can reduce the efficiency of the solar cells. One method that has been widely applied to counteract the high metal content is phosphorus diffusion gettering, which attracts and traps the metal impurities close to the surface of the solar cell.

We will demonstrate the application of boron diffusion gettering as an alternative method for removing the metal impurities. In particular, we investigate the properties of the thin Boron Rich Layer (BRL) growing on the silicon surface during boron diffusion as an effective gettering layer.

A model for the results

The results from the experiment suggests that the BRL is a very effective gettering layer. However, the Fe that is trapped inside the BRL is released back into the silicon when it is oxidised. Once released, the Fe impurities are free to diffuse back into the wafers, potentially degrading the solar cell performance again.

Conclusion

The Boron Rich Layer (BRL) is a very effective gettering layer, comparable to the well established phosphorus diffusion gettering. By controlling the thickness of the BRL, bulk degradation can beavoided. Results suggest that the BRL should be removed by usinglow temperature oxidation processes, such as chemical oxidation,to avoid undoing the BRL gettering effect.

Photo of multicrystalline silicon wafers, showing grains with different crystal orientations.

(From http://pveducation.org/pvcdrom)

Gettering effectivenessBoron diffusion gettering is more effective than phosphorus diffusion gettering, removing more than 99.9% of the Fe contamination, even without an additional low temperature annealing.

Diffusion furnace used for phosphorus or boron diffusion gettering. The diffusion process is normally done at temperatures between 800°C to 950°C. The furnace tube is filled with N2 with low concentration of O2 and BBr3 or POCl3 that is continuously replenished.

Bulk degradationIt has been reported that boron diffusions can generate dislocations and degrade the wafer bulk when a thick BRL (>10nm) is present. The bulk degradation have to be balanced with the gettering effect, to provide a net improvement in efficiency.

The experiment results show that good gettering can be achieved without significant bulk degradation, likely because the BRL deposited is thin.

Effect of oxidationAlthough the BRL is an effective gettering layer, it has to be removed after the gettering process, because it is also a recombination active surface that will decrease the cell efficiency. The most common method of removing the BRL is by oxidising it inO2 ambient at high temperature after the diffusion process.

With BRL Present

BRL Oxidised

(1)Fe Gettered to BRL

(2)Fe released to Si bulk when oxidised

(3)Small amount of Fe remains trapped in emitter