Dependence of the recombination in thin-film Si solar cells grown by ion-assisted deposition on the crystallographic orientation of the substrate

• Published in: Solar energy materials and solar cells 2002-10, Vol.74 (1), p.225-232
• Main Authors: Neuhaus, D.H, Harder, N.-P, Oelting, S, Bardos, R, Sproul, A.B, Widenborg, P, Aberle, A.G
• Format: Article
• Language: English
• Subjects: Applied sciences; Crystal orientation; Crystallographic orientation; Crystallography; Electronic properties; Energy; Epitaxial growth; Exact sciences and technology; Hydrogenation; Ion-assisted deposition; Natural energy; Photocurrents; Photovoltaic conversion; Polysilicon; Semiconducting glass; Semiconductor diodes; Silicon wafers; Solar cells. Photoelectrochemical cells; Solar energy; Thin films; Thin-film silicon; Twin defects
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/S0927-0248(02)00078-8

One promising strategy for achieving high-quality polycrystalline silicon thin-film solar cells on glass is based on low-temperature ion-assisted deposition for epitaxial thickening of a thin, large-grained seeding layer on glass. The crystal growth on the seeding layer is influenced by various factors, amongst which the crystal orientation of the grains plays a substantial role. In this paper we investigate how the electronic properties of solar cells grown on “ideal” seeding layers (Si wafers) are influenced by the crystallographic orientation of the substrate. The Si wafers are heavily doped p-type, ensuring that their contribution to the photogenerated current is small. The films grown on (1 0 0)-oriented Si substrates have a very low density of structural defects, while the films grown on (1 1 1)-oriented Si substrates display a high density of twin defects. The electronic properties of the thin-film solar cells were investigated by means of open-circuit voltage measurements as a function of the incident light intensity. The (1 0 0)-oriented diodes consistently exhibit a higher V oc than the (1 1 1)-oriented diodes throughout the entire illumination range from 10 −3 to 10 3 Suns. We determine 7 μm as the bulk minority carrier diffusion length of the as-grown (1 0 0)-oriented Si film. A lower bound of 3 μm was found for the bulk minority carrier diffusion length in the as-grown (1 1 1)-oriented Si film. The performances of both types of solar cells were improved by hydrogenation in an ammonia plasma. At voltages around the 1-Sun maximum power point the improvement is due to a reduction of non-ideal current mechanisms. The diffusion length of the (1 0 0) diode remains unaffected by hydrogenation while the lower bound of the diffusion length of the (1 1 1) diode improves to 10 μm.