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Hydrogenated Microcrystalline Silicon: From Material to Solar Cells N. Wyrsch, C. Droz, L. Feitknecht, M. Goerlitzer, U. Kroll, J. Meier, P. Torres, E. VallatSauvain, A. Shah, M. Vanecek1 Institut de Microtechnique, Université de Neuchâtel, Breguet 2, CH-2000 Neuchâtel, Switzerland. 1 Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague 6, CZ-16200 Czech Republic ABSTRACT Undoped hydrogenated microcrystalline silicon (µc-Si:H) layers and solar cells have been deposited by plasma-enhanced chemical vapour at low temperature and at different values of VHF plasma power and silane to hydrogen dilution ratios. Transport and defect density measurements on layers suggest that structural properties (e.g. crystallite shape and size) only marginally influence the electronic transport properties. The latter are influenced strongly by the Fermi level, which depends on the oxygen impurity content. Furthermore, they are best described by the quality parameter µ0τ0 (deduced from photoconductivity and ambipolar diffusion length). Cell efficiency correlates better with µ0τ0 than with the defect density as determined from subbandgap absorption. Anisotropy of the transport properties in some µc-Si:H is also demonstrated but does not seem to play a major role in µc-Si:H cells deposited at high rates under VHF glow discharge conditions. INTRODUCTION Microcrystalline hydrogenated silicon (µc-Si:H) deposited by plasma Chemical Vapour Deposition (CVD) was introduced as early as 1969 by Veprek et Marecek [1]. Although Veprek and co-workers very early studied the effect of oxygen contamination and were able to produce material with a relatively low defect density [2, 3, 4], it is only recently that these questions have been addressed systematically. Such recent work allowed for a rapid development of µc-Si:H as a photovoltaically active material for solar cell applications. Efficiencies in excess of 8% have been demonstrated on entirely microcrystalline p-i-n cells with no sign of light-induced degradation [5, 6]. Despite a multitude of activities in the field of µc-Si:H layers and cells, and more precisely on the growth of this material [7, 8, 9], knowledge on some more fundamental aspects, such as electronic transport or defects in the material, is still limited. Among critical properties for the cells performance, optical properties are nowadays the best understood [10, 11]. On the opposite side, the factors limiting transport [12, 13, 14] and the exact nature of the defects in this heterogeneous material [15, 16] remain yet quite a mystery. Improved knowledge in these topics would help optimising solar cells. µc-Si:H is usual deposited by CVD using a strong dilution of silane in hydrogen. µc-Si:H is not a unique and well defined material, but can exhibit various forms of microstructure [17], which depend critically on the deposition conditions (i.e. mostly on silane to hydrogen dilution ratio) and substrate [7]. Such variation in material morphology is expected to affect the layer transport properties,