Michitoshi Ohnishi

The Influence of the Si-H2 Bond on the Light-Induced Effect in a-Si Films and a-Si Solar Cells

The influence of the Si-H2 bond on light-induced degradation and the thermal recovery of a-Si films and a-Si solar cells were studied. The influence of the Si-H2 bond on light-induced degradation depends on the impurity content in a-Si films, and light-induced degradation can be reduced by decreasing the Si-H2 bond density in a-Si films with impurity content of 1018 cm-3. The activation energy of the thermal recovery process was about 1.0 eV, and it did not depend on the Si-H2 bond density. However, an irreversible phenomenon was observed in film properties and solar cell characteristics with high Si-H2 bond density. It is thought that the structural flexibility of the Si-H2 bond is related to this irreversible phenomenon.

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.

Development and application of see-through a-Si solar cells

Abstract A new type of translucent amorphous silicon (a-Si) solar cell, called the see-through a-Si solar cell, is developed. It has multiple microscopic holes within its effective area to transmit light and it generates electric power. A series of technical data on the fabrication processing with various patterning and photovoltaic performance are presented. Some examples of application systems such as car sunroof and home interior are introduced and discussed on a wide variety of new areas of PV applications. The see-through a-Si solar cell was mounted on a car sunroof to drive the cars ventilating system or to charge its battery. The ventilating system reduced the interior temperature of the car from 61 to 47°C during daytime parking.

Raman Study on the Silicon Network of Hydrogenated Amorphous Silicon Films Deposited by a Glow Discharge

A Raman study on the relation between the silicon bonds and the deposition conditions of amorphous silicon films is presented. Changes in the Raman spectra are investigated as functions of the deposition conditions. The Raman shift of the TO peak is found to become larger when the substrate temperature is increased. Numerical calculations on the local vibrations of the silicon network are carried out and it is shown that the change in the peak position is caused by the change in the stretching force constant of the silicon bonds.

Preparation and Properties of Amorphous Silicon Produced by a Consecutive, Separated Reaction Chamber Method

A new method of preparing a-Si film, in which each of the p, i, and n layers are deposited in consecutive, separated reaction chambers, is presented. It was confirmed experimentally that, in the conventional single reaction chamber method for a-Si film, the residual dopant gases which remain in the reaction chamber cause undesirable doping of the film, resulting in deterioration of the film quality. This undesirable doping can be avoided in the separated reaction chamber method. The photoconductivity of the i-layer prepared by this method was about 1×10-3 (Ω-1cm-1) in sunlight of AM–1 100 mW/cm-2. The best conversion efficiency of p-i-n a-Si solar cells prepared by this method was 6.91%.

Light-induced instability of amorphous silicon photovoltaic cells

Abstract Changes in the characteristics of amorphous silicon (a-Si) solar cells caused by light exposure were studied. The degradation ratio of the conversion efficiency of p-i-n a-Si solar cells caused by light exposure depends on the thickness of the i layer. A decrease in the fill factor was commonly observed, and in such cases the diode quality factor and shunt current density increased, which suggested a change in junction properties. It was shown that additional doping of the i layer with a small amount of boron prevents the decrease in conversion efficiency with light exposure. In a 1 year experiment on a 2 kW a-Si power generating system, a 10% decrease in conversion efficiency was observed (without additional boron doping).

Superlattice structure a-Si films fabricated by the photo-CVD method and their application to solar cells

Amorphous silicon superlattice structure films were fabricated by the photo-CVD method for the first time; also, the structural, optical and electrical properties of the films were investigated. A comparison of the photoluminescence intensities indicated that low damage to the interface was accomplished by using the photo-CVD method. A new type of solar cell was also developed using a superlattice structure as the p-layer of an a-Si solar cell. A conversion efficiency of 10.5% was obtained for a glass/TCO/p-superlattice structure/in/Metal a-Si solar cell.

Advanced photovoltaic technologies and residential applications

Expectations for solar cells have grown with deepening concern over global environmental and energy problems. This paper reviews the development history of solar cells and their current status. Applications of solar cells are also described, especially residential applications, such as a solar-powered air conditioner and grid connected photovoltaic power generating systems.

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.