Silicon Thin-Films from Nanoparticle dispersion: Tailoring Morphological, Electrical and Optical Characteristics
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Silicon Thin-Films from Nanoparticle dispersion: Tailoring Morphological, Electrical and Optical Characteristics. Etienne Drahi, Sylvain Blayac and Patrick Benaben Centre Microélectronique de Provence – Georges Charpak, Ecole Nationale Supérieure des Mines de Saint Etienne, 13541 Gardanne, France ABSTRACT Amorphous and microcrystalline silicon are currently used for electronic devices such as solar cells and thin-film transistors. This paper shows that silicon nanoparticle dispersion has the potential to be used as source material for polycrystalline silicon thin-film thus opening a route to solution processed silicon devices. After deposition, a classical thermal or microwave annealing step is used to induce the coalescence of the silicon nanoparticles. Both sintering techniques are studied in terms of morphology, electrical and optical properties. INTRODUCTION Oil exhaustion as well as growing needs in energy guarantee a bright future for renewable energies. Solar energy is very promising for heat and electricity generation. Nevertheless technological efficiency improvements are necessary in order to make solar energy competitive. This motivated the development of new generations of solar cells with the main objective to lower the cost of photovoltaic energy. Amorphous (a-Si) and microcrystalline (μc-Si) silicon allow much thinner and cheaper devices than crystalline silicon (c-Si) solar cells technology. Nevertheless, the use of vacuum processes raises the cost of large area devices. Solution processed technologies are already used to lower the cost of the processes for organic or chalcogenide solar cells. Recently, silicon nanoparticle dispersion has been used to lower the contact resistance between the contact layer and the metallic fingers [1] or to enhance the UV absorption [2] of a solar cell. Thus, the idea of an entire printed silicon solar cell is emerging. Annealing is mandatory for post deposition film properties as a morphological and functional properties restoring step. In an industrial point of view both thermal budget and processing time are key evaluation parameters. Laser annealing [3,4] or ALuminum Induced Layer Exchange (ALILE) [4] are two promising candidates. Nevertheless, laser annealing is not compatible with very large area processes, while ALILE process applied to nanoparticles lowers the process speed by two orders of magnitude. Microwave annealing shows a good potential for both time and temperature reduction [5]. This paper reports a comparison between thermal versus microwave annealing. EXPERIMENT This part explains the experimental settings from suspension fabrication from dry silicon nanoparticles, deposition of this suspension and annealing processes and the characterizations made on the samples. Spherical undoped silicon nanoparticles (99% metals basis) with BET (BrunauerEmmett-Teller) diameter of 32.19 nm and size dispersion between 20 and 200 nm were used.
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Native oxide shell thickness was estimated to about 0.3-0.5 nm by thermogravimetric analysis (TGA) coupled with diffe
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