Interaction between process technology and material quality during the processing of multicrystalline silicon solar cells
Multicrystalline silicon is the most used material for the production of silicon solar cells. The quality of the as grown material depends on the quality of the feedstock and the crystallization process. Bulk impurities, crystal defects like dislocations and of course the grain boundaries determine the material quality and thus the solar cell conversion efficiency. Therefore minority carrier lifetime measurements are often done to characterize the material quality. But the measured values are from limited use because it is known that the solar cell process itself can dramatically change the minority carrier lifetime and the solar cell efficiency. In order to obtain more detailed information of the behaviour of different defect types additionally high-resolution LBIC (light beam induced current)-measurements have been done. Since LBIC needs a pn-junction for photocurrent generation the LBIC technique has been combined with the a-Si/c-Si heterojunction cell process, which makes it possible to manufacture solar cells even from as cut wafers without changing the material quality. With this combination of measurement and preparation techniques it was possible to analyze the influence of the diffusion process and the firing process on the behaviour of the three different defect types: grain boundaries, dislocation networks and bulk impurities.