Conducting wide band gap nc-Si/a-SiC:H films for window layers in nc-Si solar cells

Progressive development towards all-silicon solar cells insists on the top cell of the tandem structure having a wide band gap p-layer in the superstrate configuration. With the advent of nc-Si solar cells having improved stability, the efficient growth of an i-nc-Si layer of the top cell prefers a nc-p-layer as its substrate. Accordingly, a conducting crystalline silicon alloy material with a wide band gap is a basic requirement at the p-layer. The present investigation deals with the development of a nc-Si/a-SiC:H hetero-structure wherein Si-C bonds in an amorphous matrix widen the optical band gap and the embedded high density tiny Si ultranano-crystallites of mostly < 220 > crystallographic orientation provide high electrical conductivity as well as an enhanced optical band gap due to a quantum size effect. A typical nc-Si/a-SiC:H network is obtained that contains 47% crystallinity with a prevailing ultranano-crystalline component, [(X-C)(unc)/(X-C)(nc)] > 1, average crystallite size similar to 3.6 nm, number density similar to 1.2 x 10(13) cm(-2) and Si-C bond density similar to 1.2 x 10(22) cm(-3). The hetero-structure contributes a wide optical band gap, E-g similar to 2.07 eV, high conductivity, sigma(D) similar to 3 x 10(-10) S cm(-1), sigma(Ph) similar to 6 x 10(-8) S cm(-1), Ea similar to 0.73 eV and a very low microstructure factor R similar to 0.075. Imminent steps for doping will indeed lead to prospective window layers in nc-Si solar cells.