A new type of a-Si/c-Si heterojunction solar cell, called the HIT (Heterojunction with Intrinsic Thin-layer) solar cell, has been developed based on ACJ (Artificially Constructed Junction) technology. A conversion efficiency of more than 18% has been achieved, which is the highest ever value for solar cells in which the junction was fabricated at a low temperature (<200°C).
Polycrystalline silicon (poly-Si) thin films prepared by the solid phase crystallization (SPC) method were investigated for application as photovoltaic materials. To improve the properties of the poly-Si thin film, two methods were developed to control crystallization. One is the partial doping method, in which starting material of a-Si consists of a doped layer and an undoped layer. We have succeeded in controlling nuclei generation using partial doping, and high mobility of 196 cm2/V·s was obtained at a carrier concentration of 1×1018 cm-3. SPC temperature can also be decreased to 500°C. The other is adoption, for the first time, of a textured substrate which exerted effects on the enlargement of grain size in poly-Si thin films prepared by the SPC method. By combining the partial doping method with the textured substrate, an n-type poly-Si thin-film with the grain size of 6 µm was fabricated which showed the Hall mobility of 623 cm2/V·s (n: 3.0\times1015 cm-3). In a solar cell (thickness: 12 µm) applying this film, a conversion efficiency of 6.2% was obtained and a collection efficiency of 50% was achieved at a wavelength of 900 nm.
For further improvement of conversion efficiency in a-Si solar cells, it is necessary to develop materials with high photosensitivity in the long-wavelength region. A new solid phase crystallization (SPC) method was developed to grow a Si crystal at temperatures as low as 600°C. Using this method, high-quality thin-film polycrystalline silicon (poly-Si) with a Hall mobility of 70 cm2/V·s was obtained. Quantum efficiency in the range of 800 nm ∼ 1000 nm was achieved up to 80% in an experimental solar cell using the n-type poly-Si with a grain size of about 1.5 µm. Therefore, it was found that our SPC method was suitable as a new technique to prepare high-quality solar cell materials.
A new solar cell structure named HIT (Heterojunction with Intrinsic Thin layer) has been developed based on new artificially constructed junction (ACJ) technology. In this structure a non‐doped a‐Si thin layer was inserted between the p(a‐Si)/n(c‐Si) heterojunction, improving the output characteristics and achieving a conversion efficiency of 18.1%. This structure was applied to cast polycrystalline silicon solar cells of a practical size. A high conversion efficeincy of 13.6% was obtained with a cell size of 10 cm × 10 cm using various technologies, including hydrogen plasma passivation.
High-quality p-type a SiC films can be fabricated by using a new type of doping gas, B(CH3)3, instead of B2H6 in a photo-CVD method and a glow discharge method. The photoconductivity and doping efficiency of a-SiC films fabricated by the photo-CVD method are improved by using B(CH3)3. A reduction of tail state density and an increase in photoluminescence are also observed. Furthermore, a bandgap narrowing in highly B-doped a-SiC films fabricated by the glow discharge method can be prevented by using B(CH3)3. A conversion efficiency of 10.0% (total area efficiency of 9.02%) is obtained for a 100 cm2 integrated-type a-Si solar cell whose p-layer was fabricated by the glow discharge method with B(CH3)3.
A mold coating material for a melt‐shaped crystal growth technique has been developed. After studying the contact angle and reaction between the molten silicon and other various high melting point materials, it was found that for shaped crystal growth with a lower impurity contamination level, a combination coating of
Si3N4
and
SiO2
is suitable for a mold coating material. A double layer coating was applied to the graphite mold base. The bottom layer in the mold, which had a 100–150 μm thickness, consisted of
Si3N4
that had been formed by nitriding a spray‐coated Si powder. The 200 μm thick top layer consisted of a
Si3N4‐SiO2
material. It had been produced by oxidizing a sprayed‐on coating consisting of
Si3N4
powder mixed in a silanol liquid. Using the mold prepared with these coatings, a melt‐shaped crystal growth technique was carried out by the spinning‐casting process. Polycrystalline silicon sheets, with a
10×10 normalcm
rectangular dimension and 0.5 mm thickness the size of a mold cavity, were obtained without adhesion to the mold. The sheet had an oxygen content of 1.7 ppm and a carbon content of 0.33 ppm. The mold was usable repeatedly up to 10 times.
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