Well-Passivated a-Si:H Back Contacts for Double-Heterojunction Silicon Solar Cells: Preprint

We have developed hydrogenated amorphous silicon (a Si:H) back contacts to both p- and n-type silicon wafers, and employed them in double-heterojunction solar cells. These contacts are deposited entirely at low temperature (<250..deg..C) and replace the standard diffused or alloyed back-surface-field contacts used in single-heterojunction (front-emitter only) cells. High-quality back contactsrequire excellent surface passivation, indicated by a low surface recombination velocity of minority-carriers (S) or a high open-circuit voltage (Voc). The back contact must also provide good conduction for majority carriers to the external circuit, as indicated by a high light I-V fill factor. We use hot-wire chemical vapor deposition (HWCVD) to grow a-Si:H layers for both the front emittersand back contacts. Our improved a-Si:H back contacts contribute to our recent achievement of a confirmed 18.2% efficiency in double-heterojunction silicon solar cells on p type textured silicon wafers.