An aperture-area conversion efficiency of 20.0% (intrinsic efficiency: 21 .O%) has been achieved for a 1 .Ocm2 CZ n-type single crystalline silicon (c-Si) solar cell, by using the "HIT (Heterojunction with Intrinsic Thinlayer)" structure on both sides of the cell. This is the world's highest value for a c-Si solar cell in which the junction is fabricated at a low temperature of below 200 'C.In this paper, the junction fabrication technologies and features of the HIT structure are reviewed. The stability under light and thermal exposure, and the temperature dependence on performance of a highefficiency HIT solar cell are also reported.

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The optical, electrical, and structural properties of hydrogenated amorphous silicon (a-Si:H) films are systematically investigated as functions of the substrate temperature (Ts) and plasma parameters, such as the rf power, gas pressure, and electrode dimensions. The films are deposited by the plasma chemical vapor deposition method. The properties of a-Si:H can be controlled over a wide range by varying the plasma parameters at fixed Ts. Reducing the film deposition rate and raising Ts have the same effect on the properties of a-Si:H. A unified relationship is found to exist among those properties of a-Si:H in the range of deposition conditions in this study, which includes ‘‘device-quality’’ conditions. No apparent effects of gas-phase polymerization or ion bombardment are observed. The experimental results suggest that during device-quality a-Si:H film deposition under conventional plasma conditions, the film properties are governed by a competition between the rate of film growth and the rate of thermally activated surface reactions at or near the film-growing surface. The limitations on the controllability of plasma-deposited a-Si:H, especially at low Ts, can be surmounted by adding hydrogen or helium to the plasma, and by treating a-Si:H with the hydrogen plasma.

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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.

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