October 17, 2011
Speaker: Professor B. Drévillon, Head of Thin Film Laboratory (LPICM), Paris
Hosted by: Prof. John Kymissis
The precise control of the plasma parameters of the silane (SiH4) plasma discharge allow the growth of a variety of silicon thin films and nanomaterials, ranging from the fully disordered hydrogenated amorphous silicon (a-Si:H) to crystalline films through partially ordered materials (polymorphous-nano-micro-polycrystalline. The typical size of the crystalline phase ranges from 1 nm (polymorphous silicon, pm-Si) to 10 nm (microcrystalline, μc-Si) and 100 nm (polycrystalline silicon). For practical applications like solar energy conversion, the Plasma Enhanced Chemical Vapor Deposition (PECVD) takes advantage of the capability to grow films on large area (a few meter squares) at low temperature (~ 200 °C).
The PECVD growth of silicon nanowires, vertically or horizontally oriented, was recently demonstrated. Nanowires can be very useful to increase the solar light trapping.
The rather weak efficiency and instability of a-SiH thin film solar cells can be partially overcome by combining pm-Si and μc-Si junctions in micromorphous cells, leading to 12-14 % efficiency. The combination of pm-Si films and bulk c-Si in heterojunctions allows efficiencies > 20%. The optimization of these heterojunctions needs a precise knowledge of the interface formation provided by in situ diagnostics such spectroellipsometry.
The capability to grow crystalline silicon thin films by PECVD appears very promising for future photovoltaic applications. In particular, some Fe-Ni alloys (FeNi42) can display lattice parameters close to silicon which favors the growth of crystalline silicon. On the other hand, the epitaxial growth of silicon on III-V substrates, like GaAs, can lead to the preparation of high efficiency solar cells (> 30 %).