Tsuyoshi Takahama

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.

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).

Polycrystalline Si thin-film solar cell prepared by solid phase crystallization (SPC) method

Abstract The solid phase crystallization (SPC) method has been studied for fabricating polycrystalline (poly) Si thin films for solar cells. The approach was to optimize the “partial doping structure” (nondoped a-Si/phosphorus(P)-doped a-Si) which we proposed as a starting structure before SPC. A conversion efficiency of 6.3% was obtained by using nondoped a-Si with a large structural disorder. This cell showed a collection efficiency of 51% at a wavelength of 900 nm. In order to significantly reduce the incubation time which is the important factor for the enlargement of the grain size, P doping of more than 1020 cm−3 was required for the P-doped layer.

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.

Dependence of Open Circuit Voltage of Amorphous Silicon Solar Cells on Thickness and Doping Level of the p-Layer

We have focused on the thickness and the boron-doping concentration of the p-layer of amorphous silicon solar cells and systematically obtained data for open circuit voltage (Voc) and the built-in potential to reveal the mechanism causing a high Voc. A highly doped p-layer gives a higher built-in potential in the entire thickness range, but Voc is limited by the carrier recombination caused by the doping-induced defects. A low-doped p-layer causes a higher Voc in a sufficiently thick film because less carrier recombination occurs due to the lower density of the doping-induced defects. After light-soaking, significant Voc degradation occurs with the low-doped p-layer. The light-induced defects are not negligible compared with the initial defects in the low-doped p-layer and thus more carrier recombination occurs. Moreover, some of the acceptors are compensated by light-induced defects. The highly doped p-layer, however, does not cause much Voc degradation because the light-induced defects are negligible compared with the large number of doping-induced defects and acceptors. The experimental data show that the midgap defects induced by doping or light-soaking near the p/i interface cause the Voc limitation.

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.

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%.

High efficiency a-Si solar cells with a superlattice structure p-layer and stable a-Si solar cells with reduced SiH2 bond density

Abstract In order to improve the conversion efficiency of a-Si solar cells, a superlattice structure p-layer and the use of B(CH 3 ) 3 as a dopant gas have been investigated for the first time. The collection efficiency spectrum of a glass/TCO/p-superlattice structure (a-SiC/a-Si)/in/Metal cell shows a remarkable increase in the short wavelength region. This result suggests that the superlattice structure p-layer is an active photovoltaic layer. It is also found that the photoconductivity of p-type a-Si:H films increases by using B(CH 3 ) 3 as a dopant gas. A conversion efficiency of 10.0 % (module efficiency 9.02 %) is obtained for 10 cm × 10 cm integrated single junction a-Si solar cell submodule using B(CH 3 ) 3 . As for reliability it is found that the light induced degradation of a-Si solar cells can be reduced by simultaneous reduction of the impurity concentrations in a-Si with the super chamber (separated UHV reaction chamber) and the SiH 2 bond density.

a-Si technologies for high efficiency solar cells

Abstract Conversion efficiencies of a-Si solar cells have been significantly improved in recent years, and 12.0% efficiency was achieved for a 10 cm × 10 cm a-Si solar cell submodule. As new trials, a-Si technologies are also applied to the fabrication of crystalline silicon solar cells. This paper summarizes our latest developments in a-Si technologies for high efficiency solar cells, and discusses our view of the future.