Surface passivation properties of boron-doped plasma-enhanced chemical vapor deposited hydrogenated amorphous silicon films on p-type crystalline Si substrates

Heterostructures, such as the crystalline silicon (c-Si)/plasma-enhanced chemical vapor deposited (PECVD) hydrogenated amorphous silicon (a-Si:H) structure, form a possibility in the development of a low recombination rear contact for photovoltaic devices fabricated from p-type c-Si(p) substrates. To find a good compromise between limited charge carrier recombination at the surface and a limited resistivity of the contact, a sandwich structure, such as c-Si(p)/a-Si:H(i)/a-Si:H(p⁺) has been proposed in the past. However, in this letter, we report that whereas a very thin intrinsic a-Si:H layer (∼3 nm) may still yield very low values for the surface recombination velocity of low resistivity (0.5–1.5 Ω cm) c-Si(p) wafers, the surface passivation properties are lost when this intrinsic film is subsequently covered by a PECVD a-Si:H(p⁺) layer. This phenomenon suggests that surface recombination does not take place at the c-Si(p)/a-Si:H(i) interface, but more likely in the defect-rich PECVD a-Si:H(p⁺) material, by tunneling of minority carriers through the thin a-Si:H(i) layer.