Development of nanocrystalline silicon solar cells grown by LEPECVD: optimization of the intrinsic layer for PIN structures

Abstract

Low-energy plasma-enhanced chemical vapour deposition (LEPECVD) is a new technique for growth of hydrogenated microcrystalline silicon (μc-Si:H) at high growth rate. For all existing growing techniques, a silane dilution threshold exists below which the material s electronic and optical properties are crystalline-like and above which they are amorphous-like. As known in literature, optimized μc-Si:H materials for photovoltaics are always grown around this threshold. Our previous studies have demonstrated that up to 10% silane dilution (d = [SiH4] / ([SiH4] + [H2])), the grown material showed a rather crystalline behaviour. Therefore our present study focuses on higher dilution samples (30 and 50%) characterized electrically (dark and illuminated conductivity) and structurally (confocal Raman spectroscopy) in order to identify this dilution threshold for LEPECVD. The conductivity measured in a solar cell configuration allowed us to draw only a semi-quantitative picture. This was, however, sufficient to reveal that reducing the silane flow (from 20 to 12 sccm) leads to an increase of the structural homogeneity in the growth direction, of the surface crystallinity, and of the conductivity (dark and illuminated) while varying the silane dilution induces nearly no change of these properties. Then, from a conductivity criterion, the most suitable material seems to be obtained at a flow of 16 sccm of silane and a dilution d of 30%. However, this material shows an inhomogeneity of the microstructure in the growth direction which could be detrimental to the performance of the solar cell.