In the photovoltaic cellule industry, the development of thin silicon-based layers on ceramic, glass or metallic substrates is the alternative to the current shortage of single crystal or massive multi-crystal silicon. In fact technologies to develop thin films allow the quantities of silicon used to be considerably reduced.
Photovoltaic cellules with thin layers currently present two distinct development networks. The first network concerns thin layers of amorphous, polymorph, nanocrystal and microcrystalline silicon. The second network concerns thin layers of polycrystalline silicon.
In the first network, the thin layers of amorphous silicon are generally deposited at low temperatures (100° C. to 350° C.) by plasma techniques like for example the technique of Plasma Enhanced Chemical Vapour Deposition (PECVD) on glass substrates or on flexible low cost substrates like polymers and stainless steels.
This technology presents advantages of an economic nature but also two major weak points that are a conversion efficiency limited to 10% in industrial process and a deterioration of the efficiency under illumination known as Staebler-Wronski instability in the case of amorphous silicon.
The phenomenon of degradation can be limited by developing thin layers of polymorph silicon characterised by incorporating nano-crystallites of silicon within the amorphous silicon.
In the case of polycrystalline silicon, procedures for obtaining silicon require stages at high temperature. It is possible to deposit amorphous silicon at a low temperature but it is re-crystallised by annealing at a high temperature.
It is difficult to optimise the compromise between the conversion efficiency and the cost of manufacturing photovoltaic cells.
We know the document “Roedern, K. Zweibel and H S. Ullal, The role of polycrystalline thin-film PV technologies for achieving mid-term market competitive PV modules—B.—31st IEEE Photovoltaics Specialists Conference and Exhibition—NREL/CP-520-37353—Lake Buena Vista, Florida, Jan. 3-7, 2005”, methods allowing thin films of polycrystalline silicon (poly-Si) to be developed.
It is admitted today that to obtain a high efficiency, it is necessary to develop layers of polycrystalline silicon as described in the document “Bergmann and JH. Werner, The future of crystalline silicon films on foreign substrates—Thin Solid Films, 403-404, 162-169, 2002”.
The significant increase of the conversion efficiency of thin layers of crystallised silicon requires implementing development techniques offering a significant crystallised volumetric concentration and the biggest grains possible. Usual deposit procedures include an amorphous or partially crystallised silicon deposit phase. The degree of crystallisation depends on the deposit temperature.
Usual deposit procedures also include a crystallisation phase of the amorphous silicon via heat treatment between 600° C. and 1000° C. (e.g.: heat treatment under vacuum, laser heat treatment) or by introducing a thin film in a specific reactor (e.g.: plasma with hydrogen, microwave, etc).
So various deposit techniques at high temperatures (T>650° C.) of thin films of polycrystalline silicon, including for example vapour phase procedures and heat treatment procedures in a static furnace or by means of a laser were initially used.
Nonetheless, these crystallisation techniques lead to problems of unstable substrates at high temperature, or an interaction between the substrate and the thin films.
To obtain a deposit and a silicon crystallisation at low temperatures (T<600° C.), it is known from the document EP 0 571 632, to deposit amorphous silicon on glass by techniques of chemical deposit in vapour phase (CVD) or chemical deposit in vapour phase assisted by plasma (PECVD), at T<450° C., then to expose the thin film in an electric microwave field of 400 W in the presence of hydrogen. You obtain a thin film of textured polycrystalline silicon according to a preferred orientation {110}.
The document “T. Matsuyama, N. Bada, T. Sawada, S. Tsuge, K. Wakisaka, S. Tsuda, High-quality polycrystalline silicon thin film prepared by a solid phase crystallisation method, J. of non-Crystalline Solids, 198-200, 940-944, 1996”, divulges another solution consisting in depositing an initial film of silicon used as a nucleation layer on quartz at 600° C. by the PECVD deposit technique. The film obtained is made up of crystals of 0.1 μm of silicon placed within the amorphous phase.
A second amorphous film of silicon is then deposited by the PECVD deposit technique and crystallised by a thermal process at 600° C. for 10 hours.
You obtain a thin crystallised film with a columnar structure presenting a conversion efficiency of 9.2%.
Other solutions to obtain thin polycrystalline layers with a high conversion efficiency, consist in realising oriented or epitaxy silicon grain structures.
The document WO 96/17388 divulges a widely known procedure that is the use of silicon priming layers deposited in the amorphous state then crystallised to be used as epitaxic growth nuclei for the following thin layer. This procedure is a multi-layer procedure.
The document U.S. Pat. No. 5,340,410 divulges another technique consisting in choosing an orientation {111} of silicon grains by selective etching of a film of large grain polycrystalline silicon (40 μm to 50 μm, obtained by heat treatment), in a solution of potassium hydroxide. A second thin film of silicon presenting an orientation {111} is thus obtained by a deposit procedure in liquid phase (solution of liquid metal oversaturated in silicon).
Deposit methods from the prior art suggested above do not allow problems concerning the substrate to be solved. There are in fact two categories of substrates used according to the processing temperature.
For the deposit of thin silicon-based layers some substrates used are at a high melting temperature (T>1000° C.): silicon, quartz, graphite, ceramics, metals (for example titanium), alloys and steels.
Other substrates require processing at a low melting temperature (T<1000° C.): polymers and glass.
As previously seen, these substrates all present at least one major disadvantage to be used for the industrial manufacturing of photovoltaic cells.