Tsugufumi Matsuoka

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.

Preparation and Properties of High-Quality a-Si Films with a Super Chamber (Separated Ultra-High Vacuum Reaction Chamber)

A separated ultra-high vacuum (UHV) reaction chamber system, called the super chamber, has been newly developed. A background pressure of 10-9 Torr was obtained, and the impurity concentrations of oxygen, nitrogen and carbon in an a-Si film fabricated in the super chamber were 2×1018 cm-3, 1×1017 cm-3, and 2×1018 cm-3, respectively. The space charge density and the ESR spin density of the a-Si film were 5×1014 cm-3 and 2×1015 cm-3, respectively. These values were much lower than those for films fabricated in a conventional chamber. The ratio of the light-induced degradation in the photoconductivity of the a-Si film was also small compared with that of conventional a-Si films. A conversion efficiency of 11.7% was obtained for a glass/textured TCO/pin/Ag a-Si solar cell, whose i-layer was fabricated in the super chamber.

Laser Patterning Method for Integrated Type a-Si Solar Cell Submodules

A laser patterning method was investigated as a fabrication method for integrated-type amorphous-silicon (a-Si) solar cell submodules. A three-dimensional thermal analysis of a multilayer structure was performed to determine the selective scribing conditions for each layer of an a-Si solar cell. The optimum laser power densities calculated from a three-dimensional thermal analysis were confirmed by the experiments. It was found that not only transparent conductive oxide and a-Si films, but also the metal electrodes of the integrated-type a-Si solar cell submodule were selectively scribed. The total output power of an a-Si solar cell submodule patterned by optimum laser-power densities was 9% higher than that achieved by a conventional patterning method.

A New Analytical Method of Amorphous Silicon Solar Cells

A new analytical method for amorphous silicon solar cells, called DICE (dynamic inner collection efficiency), has been developed. The depth profile of the photovoltaic characteristics of solar cells can be obtained by using the DICE method under any operating condition in a non-destructive manner for the first time. The DICE value is defined as the probability that an electron-hole pair generated at a certain depth in the generated region of an a-Si solar cell becomes an output current. In this paper the theory and the calculation method of DICE are described, and the results of applications to practical solar cells are reported. By using the DICE method it was found that carrier recombination at the p/i interface affects the open-circuit voltage.

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>>

Controlling hydrogen contents and bond configurations for high-quality and high-reliability a-Si films

Abstract We previously reported that a reduction of SiH 2 bond density in the i-layer effectively prevents light-induced degradation of a-Si solar cells. For further reduction of SiH 2 bond density, we fabricate a-Si films at higher substrate temperature (Ts:250 ∼ 400°C) with a glow discharge method by using super chamber. SiH 2 bond density can be reduced to about 10 20 cm −3 , and highly photoconductive and stable a-Si films are obtained. As a new attempt to reduce SiH 2 bond density, we have investigated an ion-gun CVD method. Hydrogen bond configurations are found to be strongly affected by the accelerating voltage of ions.

Approach from a-Ge Films for Development of High-Quality a-SiGe Films

a-Ge:H films fabricated by means of a separated ultrahigh-vacuum reaction chamber, called the super chamber, were systematically studied. In the conventional glow-discharge method, there is a very narrow substrate-temperature region for fabricating high-density a-Ge:H films; that is, a minimum deposition rate and a maximum refractive index were obtained at about 250°C. From the point of view of optoelectrical properties, it was clear that not only rigidity of the film network but also total hydrogen content are important. In order to satisfy the two above-mentioned factors simultaneously, a low substrate-temperature high hydrogen-dilution method was effective, and film properties of a-Ge:H were largely improved; δd~4.0×10-5 Ω-1 cm-1, δph~1.5×10-4 Ω-1 cm-1, B value ~803 (eVcm)-1/2, and the ESR spin density ~1.5×1017 cm-3.

A new solar cell roofing tile

Abstract A new solar cell roofing tile combining a building material (a glass Japanes-style roof tile) and an amorphous silicon (a-Si) solar cell has been developed. It was made possible by developing new technologies for uniformly fabricating a-Si film on complex curved surfaces such as Japanese tiles and for patterning integrated-type solar cells on large-area curved surfaces. The roofing tiles were installed on a model house and are presently undergoing exposure tests. Their performance as solar cells and as building materials has been satisfactory over the past 2 years.

High-Quality p-Type µc-Si Films Prepared by the Solid Phase Crystallization Method

A device-quality p-type microcrystalline silicon (µc-Si)film was prepared using a new solid phase crystallization (SPC) method from the p-type amorphous silicon (a-Si:H) deposited by plasma-CVD. The obtained properties of high conductivity (σd=2×103 (Ωcm)-1) and a high optical transmittance in the range of 2.0 eV~3.0 eV are better as a window material for a-Si solar cells than those of conventional p-type µc-Si:H and a-SiC:H.

Quality improvement in a-Si films and their application to a-Si solar cells

A theoretical analysis conducted in order to raise the efficiency of amorphous-silicon (a-Si) solar cells is discussed. Based on the analysis, the quality of the i and p layers was improved. A high-quality a-Si film with a lower impurity concentration and lower defect density than conventional films was fabricated using the super chamber. A reduction in damage to the transparent conductive oxide and an improvement in p and i interface properties was achieved by using a photo-CVD method. This same method was used to study superlattice structure films, to fabricate high-conductivity films with a wider optical bandgap than conventional a-SiC films, and to improve sensitivity in the short-wavelength region. B(CH/sub 3/)/sub 3/ was used as a p-type doping gas for a-Si film that had a higher quality than that fabricated using B/sub 2/H/sub 6/. These technologies made possible a conversion efficiency of 11.7% for 1-cm/sup 2/ single-junction solar cells and a total area conversion efficiency of 9.60% for 100-cm/sup 2/ single-junction integrated-type submodules. >