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

Abstract

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, [(XC)unc/(XC)nc] > 1, average crystallite size ∼3.6 nm, number density ∼1.2 × 1013 cm−2 and Si–C bond density ∼1.2 × 1022 cm−3. The hetero-structure contributes a wide optical band gap, Eg ∼ 2.07 eV, high conductivity, σD ∼ 3 × 10−10 S cm−1, σPh ∼6 × 10−8 S cm−1, Ea ∼ 0.73 eV and a very low microstructure factor R ∼ 0.075. Imminent steps for doping will indeed lead to prospective window layers in nc-Si solar cells.