Hidenori Nishiwaki

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.

Optimization of a-SiGe:H Alloy Composition for Stable Solar Cells

The film properties and solar cell performance of amorphous SiGe:H (a-SiGe:H) samples have been systematically investigated, using constant optical gap and various compositions of hydrogen and germanium. It was found that the hydrogen content and bonding configurations play important roles in determining both the initial properties and stability. The optimum compositions were clarified for the minimum Urbach tail characteristic energy and defect density in the as-deposited film, and for the maximum conversion efficiency of the solar cells. The stability of a-SiGe single and a-Si/a-SiGe tandem solar cells becomes higher as the hydrogen content of the photovoltaic layer becomes lower. As a result, the optimum composition after light soaking shifts to the region of lower hydrogen content. Applying the above findings to the design of devices, the highest stabilized conversion efficiencies of 3.3% (initial 3.7%) under red light (λ>650 nm) for an a-SiGe single-junction solar cell and 10.6% (initial 11.6%) for an a-Si/a-SiGe tandem solar cell have been achieved (area: 1 cm2).

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.

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).

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.

Influence of excess carriers on the Staebler and Wronski effect of a-Si solar cells

Abstract The degradations of the photovoltaic properties of a-Si solar cells were investigated. There was no large difference between light exposure and current injection in the degradation behavior, nor in the recovering behavior by annealing. The irradiation temperature dependence of the degradation in the collection efficiency was different between Ar + laser and HeNe laser irradiation. It was also found that the photovoltaic characteristics of the light soaked cell were recovered by IR irradiation. These results suggest that the Staebler-Wronski effect is not related to the light itself but related to the trapping of excess carriers.

Film Property Control of Hydrogenated Amorphous Silicon Germanium for Solar Cells

The optoelectric properties of a-SiGe:H alloys, deposited by the plasma chemical vapor deposition (plasma-CVD) method, were investigated with precise measurement of their germanium content (CGe) and hydrogen content (CH). This investigation revealed that the optical gap of a-SiGe:H alloys can be approximated by a linear function of CH and CGe and various combinations of CH and CGe resulted in identical optical gaps. For each optical gap, the optimum composition for the lowest defect density was derived by comparison with the subgap absorptions measured by the constant photocurrent method (CPM). Based on these, the highest conversion efficiency of 3.7% under red light illumination (>650 nm) for a 1 cm2 a-SiGe single-junction solar cell was achieved.

Preparation and Properties of a-Si Films Deposited at a High Deposition Rate under a Magnetic Field

An rf plasma decomposition of SiH4 under a magnetic field was investigated. It was confirmed by the optical emission spectra that a high-electron-density plasma can be produced under a magnetic field. High-quality a-Si films with a photosensitivity of σph/σd of 7×105 were obtained at a high deposition rate of 10 A/s under the magnetic field. The a-Si solar cells with i-layers deposited at a high deposition rate under a magnetic field have a higher open-circuit voltage and a higher conversion efficiency than those without the magnetic field; a conversion efficiency of 10.1% under AM1(100mW/cm2) illumination was obtained at a deposition rate of 10 A/s. The rf plasma decomposition of SiH4 under a magnetic field is thought to be very suitable for fabricating a-Si solar cells with a high conversion efficiency at a high deposition rate.

Preparation and properties of amorphous silicon produced by a consecutive, separated reaction chamber method

Abstract A new fabrication apparatus was developed from the consecutive, separated reaction chamber method in order to fabricate the multi-gap amorphous solar cell. In this fabrication process, the different amorphous materials are deposited in different reaction chambers. It was confirmed by IMA measurement that the intermixing of different amorphous materials was clearly avoided. The space charge density (Ni) of the films, into which a slight amount of boron is doped in this method, was measured. The minimum Ni was about 2 × 1014 cm−3 at the gas ratio B2H6/SiH4 of 2 × 10−6. The best conversion efficiency of p-i-n amorphous solar cells fabricated by this method was 10.0%.