Simplified surface cleaning for fabrication of silicon heterojunction solar cells

For the fabrication of silicon heterojunction (SHJ) solar cells, hydrogenated amorphous silicon (a-Si:H) layers are deposited on each side of textured c-Si wafers. The high conversion efficiency of these cells relies significantly on the excellent surface passivation provided by these layers. A perfectly clean surface, free from organic and metallic impurities, is therefore a very important aspect in the fabrication of SHJ cells. Moreover, also for future heterojunction structures, using for example metal oxides in passivating contacts, a good control of surface quality will be important. In this work, the wet-chemical cleaning steps after KOH random pyramid texturing are investigated. The current cleaning process at ECN, based on procedures described in literature, takes about 90 min and includes many steps. This is very elaborative and costly and forms a barrier for industrial acceptance of the process. The aim of this work is to simplify the cleaning process, while maintaining good surface passivation and therefore high solar cell efficiency. By systematic testing of different sequences and process times of wet-chemical oxidation and etching steps, we arrived at a good qualitative understanding of their purposes and interactions. Optimization resulted in a wet-chemical cleaning sequence that takes less than 30 min and results in high minority carrier lifetime values that are similar to the reference procedure.