New results using a low temperature anneal in processing of multicrystalline solar cells

It was previously shown, that an additional low temperature (LT) anneal directly after phosphorus diffusion improves the efficiency of multicrystalline solar cells made of border brick of a silicon ingot. We now investigated different materials. The effect was not seen in the middle of the ingots, even not for upgraded metallurgical silicon or for material with high densities of dislocations and grain boundaries. However, the positive effect of the LT-anneal was again found, when the wafers were deteriorated from the crucible walls or its coating. To find an optimum temperature, the LT-anneal was done on many neighbouring wafers using fine temperature steps of 25 °C between 300 °C and 800 °C. The best results were obtained at 575 °C. Detailed LBIC-measurements were performed. Furthermore, the origin of the positive effect of the LT-anneal was investigated by removing the emitter from some cells before the anneal. The LBIC-topograms as well as lifetime topograms show that this positive effect is more likely due to phosphorus gettering than internal gettering.