Yasuo Kishi

Interference-Free Determination of the Optical Absorption Coefficient and the Optical Gap of Amorphous Silicon Thin Films

A new method to determine the optical absorption coefficient (α) of thin films is presented. α of hydrogenated amorphous silicon (a-Si:H) based alloys can be accurately determined from transmittance (T) and reflectance (R) by using T/(1-R), which almost completely eliminates disturbance from the optical interference effect. The method is applicable to any thin films, as long as the film is a single layer. Based on the interference-free α, various methods to determine the optical gap (EOPT) of a-Si:H, a-SiC:H, and a-SiGe:H films are discussed. The (nαhν)1/3 plot and the (αhν)1/3 plot are most suitable for characterizing these films. The well-known (αhν)1/2 plot is less suited for detailed discussion of the EOPT than the cube root plot, because the plot includes a large ambiguity in the EOPT. The effect of the optical interference effect on the determination of the EOPT is also discussed.

Preparation of high-quality n-type poly-Si films by the solid phase crystallization (SPC) method

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/Vs 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.

More than 16% solar cells with a new 'HIT' (doped a-Si/nondoped a-Si/crystalline Si) structure

A HIT (heterojunction with intrinsic thin-layer) structure solar cell has been developed. In this structure, a nondoped a-Si thin layer was inserted between a p-type a-Si layer and an n-layer c-Si substrate. The open-circuit voltage and fill factor (FF) were significantly improved in these HIT structure solar cells compared with conventional p/n heterojunction solar cells. The improvement seems to originate in the reduction of backward current density. For higher efficiency, this HIT structure has been applied to textured substrates and achieved an efficiency of 18.1% (1 cm/sup 2/ cell). This efficiency is the highest value reported for a solar cell in which the junction was fabricated at a low temperature (120 degrees C). Application of this structure to the poly-Si thin film will yield a-Si/poly-Si thin-film solar cells of high efficiency.<<ETX>>

High Performance A–Si Solar Cells and Narrow Bandgap Materials

High performance a-Si solar cells were developed. A conversion efficiency of 11.5% was achieved for a textured TCO/p-SiC/in/Ag structure with a size of 1 cm 2 using the high quality i-layer fabricated by a new consecutive, separated reaction chamber apparatus. A conversion efficiency of 9.0% was obtained with a size of 10cm × 10cm. A high quality a-SiGe:H:F, which is a new narrow bandgap material for a-Si solar cells, was fabricated by a glow discharge decomposition of SiF 4 + GeF 4 + H 2 . A photo-CVD method was investigated in order to improve the interface properties of a–Si solar cells. A conversion efficiency of 11.0% was obtained with a solar cell in which the p-layer is fabricated by the photo-CVD method. a-SiGe:H films were fabricated by the photo-CVD method for the first time as a narrow bandgap material for multi-bandgap a-Si solar cells.

A Laser Welding and Scribing (LWS) Method for a High-Yield Integrated-Type a-Si Solar Cell

A new fabrication method for integrated-type amorphous-silicon (a-Si) solar cell submodules, called the laser welding and scribing (LWS) method, was investigated. The conditions for laser scribing and laser welding were calculated from a three-dimensional thermal analysis of a multilayer structure. Calculated results were confirmed by experiment, and the optimum laser power density range for scribing was found to be much wider than that for conventional laser patterning methods. Subsequent connection of adjacent cells in a series by laser welding was also easily performed. This fabrication method has led to a total area conversion efficiency of 10.3% for a 100 cm2 integrated-type a-Si solar cell.

Prevention of the light induced effect of a-Si:H films and solar cells

Abstract The light induced degradation of a-Si shows strong correlation with oxygen and nitrogen concentrations. It was found that simultaneous doping of oxygen and nitrogen can reduce the light induced degradation in the higher concentration region. It was also found that the ESR spin density and mechanical stress cannot represent the light induced effect well in lower oxygen concentration regions. A new analysis method for the depth profile of dynamic inner collection efficiency (DICE) was developed, and it was shown that the DICE method is helpful for clear understanding of the light induced effect in a-Si solar cells.

A unified relationship among the opto-electric properties of a-Si:H and approaches for further improvement

The optical, electric and structural properties of hydrogenated amorphous silicon (a-Si:H) films, deposited using the plasma-CVD method, are systematically investigated as functions of the substrate temperature (T s ) and plasma parameters such as the RF power, gas pressure, and the dimensions of the reactors in order to optimize their properties for solar cells. Those properties are successfully controlled over a wide range by the plasma parameters at a fixed T s . The decrease in the film deposition rate and the increase in the T s have practically identical effects on the properties of a-Si:H in this study. The experimental results suggest that the properties of a-Si:H films 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 during film deposition. Limitation in the controllability of ‘device-quality’ a-Si:H can be overcome by diluting SiH 4 with hydrogen or helium at low T s , or by hydrogen-plasma treatment of a-Si:H. The present results provide a guide for further improvement in the characteristics of a-Si:H for the photovoltaic layer of solar cells.

A New Type of Ultralight Flexible a-Si Solar Cell

A new type of ultralight flexible a-Si solar cell fabricated on a transparent polyimide film substrate, has been developed. High photoconductivity (σph) of 2×10-5 Ω-1 cm-1 and high photosensitivity (σph/σd) of 1×106 were obtained with a-Si films deposited by H2-dilution of SiH4 on a transparent polyimide film substrate at 180°C. Prebaking the transparent polyimide film substrate effectively reduced the impurity content of the a-Si film. A maximum output power of 550 mW and a power-to-weight ratio of 275 mW/g were obtained for an integrated-type a-Si solar cell with a size of 110×115 mm.

On p layers for high efficiency amorphous silicon solar cells

Abstract The p layer is one of the key factors for improving conversion efficiency of amorphous silicon solar cells. This paper summarizes various approaches for achieving high quality p layers which are used in window layers of solar cells. Theoretical considerations, preparation methods, doping gas, and doping methods are reviewed in conjunction with solar cell structures and characteristics.

High-Performance a-SiGe Solar Cells Using a Super Chamber Method

The influence of impurities on the a-SiGe film properties was investigated using a super chamber. It was found that a-SiGe films with a low impurity concentration can maintain a high photosensitivity of about 106 for Ge content up to 13%, and impurity incorporation deteriorates film rigidity, which was first deduced from the STM (scanning tunneling microscope) measurement. Using a super chamber method, the highest conversion efficiency of 3.34% was obtained for an a-SiGe single-junction cell under red light (long-wavelength light (>~650 nm) by filtering AM-1.5, 100 mW/cm2 light). A conversion efficiency of 11.9% was also achieved for a stacked cell of a-Si/a-Si/a-SiGe.