Effect of the crucible and its coating on the red zone in multicrystalline silicon - analysis on solar cell level

Abstract

The purpose of this work is to investigate the influence of the crucible and its coating on the deteriorated edge zone (also called “red zone”) during directional solidification of multicrystalline silicon (mc-Si). Since the material quality is changed by the processing of solar cells due to, e.g., gettering and hydrogenation, this study was performed on material after having processed it into lab-type solar cells. The use of a standard industrial quartz/SiO2 crucible coated with a standard Si3N4 powder leads to a clear impact of the “red zone” on solar cells. This effect decreases over the block height. A thick very highly purified coating powder seems to considerably reduce the in-diffusion of the impurities from this standard crucible. Using a purified industrial crucible with a coating powder of very high purity results in a notable reduction of the “red zone” effect on solar cell level. Moreover, the use of a purified crucible shows no effect of the “red zone” on solar cell until to a distance of ~12 mm from the crucible wall. This is probably due to the reduction of metallic contamination from the crucible during crystallization as shown by Schubert et al. [5]. It is shown that the main source of impurities for contamination of the “red zone” is the crucible whereas the coating has solely an insignificant contribution. Efficiencies of up to 19.0% were achieved on 2 x 2 cm2 solar cells using mc-Si material from a block crystallized with the highly purified crucible and coating. For the standard crucible, efficiencies of up to 18.9% were realised.