Gettering approaches for n-type multicrystalline silicon solar cells

Abstract

The inherent resistance of n-type silicon towards metal contamination can potentially offset the higher metal content of multicrystalline silicon, allowing n-type multicrystalline silicon solar cells to reach high lifetime. Nevertheless, n-type multicrystalline silicon is still affected by metal contaminations and can be further improved by gettering. In terms of the high temperature steps involved, n-type cells require either an additional boron diffusion step or a longer aluminium annealing step to form the p+ emitter region. If not managed carefully, the additional high temperature steps can degrade the lifetime of n-type multicrystalline silicon cells. On the other hand, these additional steps can also be potentially engineered to provide additional gettering effects. This thesis seek to investigate the gettering options available for n-type multicrystalline silicon solar cells. First, the gettering effectiveness of standard phosphorus diffusion, boron diffusion, and aluminium annealing are compared. Results show that all 3 methods considered can be very effective, but the gettering effectiveness of boron diffusion depends on the Boron-Rich Layer (BRL) and is hence very sensitive to the processing conditions. Next, the gettering effect of boron diffusions is investigated in more detail, taking into consideration the trade-off between surface passivation, bulk degradation and gettering effectiveness of BRL. The results show that the gettering effect of the BRL is reversed when oxidised thermally, but is maintained if oxidised chemically using boiling nitric acid. However, the J0e values are lower when the BRL is oxidised thermally instead of chemically, partly due to the lower surface concentration resulting from thermal oxidation. The gettering effectiveness of light phosphorus diffusion overlapping with a deeper boron diffusion, and also the dependence of the gettering effect on the phosphorus concentration were also studied. The results show that the light phosphorus diffusion is effective at gettering, even if only 50nm of the wafer is converted to net n-type doping. In addition, the effect boron diffusion gettering and phosphorus diffusion gettering on sister wafers from n-type multicrystalline silicon ingots were compared. Both diffusion gettering methods resulted in improved average lifetime, with intra-grain lifetime reaching up to 1ms after gettering. However, the gettering on the grain boundaries are less effective, and limits the overall average lifetime. Lastly, we propose a method based on the 2-D continuity equation to reverse the smearing effect caused by lateral carrier diffusion in photoluminescence (PL) images. Since the de-smearing method is very sensitive to the measurement noise, noise filtering methods using smoothing spline, wiener filter, and diffusion smoothing are compared. Furthermore, a method for automatic selection of the noise filter parameter based on the convergence of multiple noisy images is implemented and demonstrated.