New Approaches for the Contact Theory and new Doping Profile Suggestion for BLM Metallization

The traditional contract theory of solar cell does not consider metal ion diffusion effect of paste during the firing-through (FT) metallization process. In this paper, we have designed 3 pastes that contain the simple glass composition to study the direct and indirect electron transfer path and the specially designed Beyond Metallization Limit (BML) paste to investigate the metal ion diffusion effect on the crystalline silicon solar cell. The glass layer masking and etching ability for the direct path and silver dissolution ability into glass for indirect path were thoroughly examined with 3 pastes. From the observation, we have revealed that lead oxide has higher etching ability while tellurium dioxide has higher silver dissolution ability in the glass composition. The specially designed BLM paste has recorded efficiency over 18% at a firing temperature of 100°C lower than the normal firing temperature. The BLM paste damaged the P-N junction stability at high firing temperature. But the first contact resistance value between two first patterns in conventional TLM method had the lower value than that of baseline. Based on this research, we have concluded the contact resistance has to be consisted of direct path, indirect path and the doping concentration compensation effect by the metal ion diffusion on crystalline silicon solar cell. This postulation could have the possibility to break down the limitation the conventional contact theory. Finally, we have suggested the ultra-low surface doping concentration and ultra-long junction depth to study the metal ion diffusion effect further.