Structural, hydrogen bonding and in situ studies of the effect of hydrogen dilution on the passivation by amorphous silicon of n-type crystalline (100) silicon surfaces

Abstract

Hydrogenated amorphous silicon (a-Si : H) layers deposited by chemical vapour deposition provide an attractive route to achieve high-performance crystalline silicon (c-Si) solar cells due to their deposition at low temperatures and their superior passivation quality. Hydrogen certainly plays an additional crucial role by passivating the dangling bonds, and thus improving the electrical and optical properties. In this work, we present the variation of the effective lifetime with the hydrogen dilution ratio R = (H-2/SiH4). We find that at lower hydrogen dilution rates (R < 2), the polymerization reaction of silane molecules in the plasma bulk as well as the excess of dihydride (Si-H-2) incorporation in as-deposited layers result in higher microvoid density and worse passivation quality. In contrast, the deposition at higher hydrogen dilution rates (R > 5) leads to a degradation in the film quality with very low hydrogen content and crystalline epitaxial growth at the a-Si/c-Si interface. Thus, the best material quality is obtained just below the onset of amorphous-to crystalline transition (R = 3), which favours further improvement of the passivation during a post-deposition annealing.