Kaneo Watanabe

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.

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).

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.

Variations of the silicon network of amorphous silicon studied by Raman spectroscopy

Abstract The structure of the silicon network of ‘device quality’ a-Si:H films is studied by means of Raman spectroscopy. An increase in the substrate temperature results in shift of the TO peak in the Raman spectra toward a high wavenumber and an increase in the compressive mechanical stress. A quasi-interstitial hydrogen model is proposed for the silicon network of a-Si:H. The model suggests that the shift of the TO peak in the Raman spectra originates from the change in the force constant of SiSi bonds induced by hydrogen atoms in a monohydride configuration.

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.

Properties and Photovoltaic Characteristics of a-SiC: H Film

a-SiC: H films was prepared from a glow discharge in a mixture of SiH4 and CH4 in a capacitive coupled system with parallel plane electrodes. The fundamental properties of the a-SiC: H films were investigated. Using the a-SiC: H film for window material, Glass/SnO2/p(SiC)-i-n/Al cells were fabricated by the consecutive, separated reaction chamber method. The best conversion efficiency of the cell with a size of 4 mm2 was 8.15%, which is much larger than that prepared by the single reaction chamber method in our laboratory. It seems that one of the main reasons for the difference in the conversion efficiency is the undesirable mixing of carbon in the non-doped a-Si: H layer. An integrated type Glass/SnO2/p(SiC)-i-n/Al cell with a size of 10 cm×10 cm was also fabricated. The best conversion efficiency was 6.35%.

Preparation and Properties of a-SiGe:H:F Films by a Glow Discharge Decomposition

As a high quality narrow bandgap material, hydrogenated and fluorinated amorphous silicon germanium (a-SiGe:H:F) by a glow discharge decomposition of SiF4+GeF4+H2 was systematically investigated. Microcrystalline (µc) Ge was found to be obtained when the GeF4 gas flow ratio is high, and µc-Si can be observed when the substrate temperature is high. These results indicate a strong influence of the formation of microcrystalline structures on film properties. More than 107 rectification ratio was obtained for a-SiGe:H:F Schottky diodes at a bias voltage of 1 V. The collection efficiency at the wavelength of 800 nm was improved to 32% with p-SiC:H (hydrogenated amorphous silicon carbide)/i-SiGe:H:F/n-Si:H(hydrogenated amorphous silicon) diodes.

Difference in the influence of oxygen and nitrogen on the light induced effect

The influences of impurities such as oxygen and nitrogen on the light induced effect were studied. Although both oxygen and nitrogen had a strong influence on the light induced effect, there were large differences in their influences. The oxygen doped film had a longer time constant for the light induced change in the conductivity and gap state density than the nitrogen doped film. It was also found that oxygen was associated with the degradation of a‐Si solar cells. It is considered that the light induced effect is associated with the hole traps from the results of the degradation in n‐i‐n and p‐i‐p diodes. The degradation in a 2‐kW a‐Si power generating system was also analyzed.