Yoshihisa Tawada

Properties and structure of a‐SiC:H for high‐efficiency a‐Si solar cell

A series of experimental investigations on optical and optoelectronic properties of methane‐ and ethylene‐based a‐SiC:H films has been made. The chemical bonding structure of two kinds of a‐ SiC:H films has also been explored from infrared (IR) absorption structural analysis. An experimental verification for the wide gap window material in the amorphous silicon solar cell is shown on methane‐ and ethylene‐based a‐SiC:H. The methane‐based a‐SiC:H film shows one or two orders of magnitude larger photoconductivity recovery effect of doping than the ethylene‐ based one. IR absorption analysis shows that the methane‐based a‐SiC:H film is recognized as a rather ideal amorphous SiC alloy as compared with the ethylene‐based one. It has been found through these investigations that the methane‐based a‐SiC:H film is superior to the ethylene‐ based one as a window material. Utilizing the methane‐based a‐SiC:H, an 8% efficiency barrier has been broken through with an a‐SiC:H/a‐Si:H heterojunction structure.

a‐SiC:H/a‐Si:H heterojunction solar cell having more than 7.1% conversion efficiency

A clear valency electron controllability with substitutional impurity doping in the hydrogenated amorphous silicon carbide has been found. The amorphous silicon carbide is produced by the plasma decomposition of [SiH4(1−X)+CH4(X)] with the dopant gas of a B2H6 or PH3 system. Electrical and optical properties of doped amorphous SiC films are briefly demonstrated. Utilizing this a‐SiC:H as a wide‐band‐gap window material, we have developed a new type of a‐SiC:H/a‐Si:H heterojunction solar cell. A typical cell performance is Voc = 0.887 V, Jsc = 12.33 mA/cm2, fill factor = 0.653, and the conversion efficiency of 7.14% under AM‐1 illumination.

Hydrogenated amorphous silicon carbide as a window material for high efficiency a-Si solar cells

Properties and fabrication technical data on glow discharge produced hydrogenated amorphous silicon carbide have been described. A series of experimental studies of the effects of impurity doping on amorphous silicon carbide has also been carried out. It has been shown from the experiment that hydrogenated amorphous silicon carbide prepared by the plasma decomposition of [SiH4(1−x) + CH4(x)] gas mixture has a good valency electron controllability. Employing the property of the valency controlled a-SiC:H as a wide gap window junction, a-SiC:H/a-Si:H heterojunction solar cells have been fabricated. As a result, we have succeeded in breaking through an 8% efficiency barrier with this new material. A typical performance of a-SiC:H/a-Si:H heterojunction cell is Jsc of 15.21 mA/cm2, Voc of 0.88 V, FF of 60.1% and η of 8.04%.

Hydrogen Content in a-SiC:H Films Prepared by Plasma Decomposition of Silane and Methane or Ethylene

The hydrogen content in a-SiC:H films prepared by the plasma decomposition of gas mixtures of silane and methane or ethylene was measured by 1H(15N, αγ)12C nuclear reaction (NR) and infrared absorption spectroscopy (IR). The contents observed by both methods can be made to agree well for most of the films by putting the average value of the inverse absorption cross section for the CHn (n=1, 2 and 3) stretching mode over all values of n as As=1.0×1021 cm-2. In some films prepared from silane and ethylene mixtures, the hydrogen content given by NR was higher than that by IR. Hydrogen release from films with a hydrogen content of over 30 atm.% was observed during NR analysis. The mechanism of the hydrogen release is discussed by comparing the IR spectra before and after ion bombardment.

Valency Control of Glow Discharge Produced a-SiC:H and its Application to Heterojunction Solar Cells

A new type of amorphous silicon solar cell having a conversion efficiency of 8% level is introduced. The cell has a wide band gap window layer made of hydrogenated amorphous silicon carbide, (a-SiC:H), with a good valency control. Electrical, optical and optoelectronic properties of a-SiC:H have been investigated, together with their valency controllability. A design concept and some key technologies to improve solar cell performance with this new material are demonstrated. A series of technical data on material preparation and cell performance are presented. Clear improvements in cell performance, not only IDC but also VDC, have been obtained. The realistic limit of the conversion efficiency in a-Si solar cells is estimated and discussed.

Alignment of nematic liquid crystals by polyimide Langmuir-Blodgett films

Nematic liquid crystals with a high degree of alignment were prepared using polyimide (PI) Langmuir-Blodgett (LB) films. Polyimides with a linear primary structure are favorable for attaining highly oriented PI LB films. Furthermore, orientation of the PI LB films is essential in order to achieve well-aligned nematic liquid crystal molecules.

Stability of an amorphous SiC/Si tandem solar cell with blocking barriers

A very stable amorphous SiC/Si (a‐SiC/a‐Si) heterojunction solar cell has been developed. The cell structure is of multijunction type with blocking barriers for the dopants and metal diffusion. This new tandem cell exhibits excellent stability for both thermal and sunlight conditions. After 2000 h of light exposure, an observable photodegradation is not seen in the cell performance. The improvements are achieved by the low‐temperature deposition of first a p a‐SiC layer, and by the blocking barriers for dopants and metal diffusion.

Spectral Effects of a Single-Junction Amorphous Silicon Solar Cell on Outdoor Performance

The device current–voltage (I–V) parameters of single-junction amorphous Si (a-Si) solar cells in natural sunlight at various locations have been investigated over three years. The effective efficiency was compared among testing locations from the latitude of 12.8 degrees to 43.8 degrees. The effective efficiency was varied periodically and the most advantageous performance was obtained in summer at all locations. The increase of output current dominantly contributed to the excellent performance. The output current of single-junction a-Si modules depended not only on module temperature and the annealing effect but also the seasonal variation of the air mass and the atmospheric conditions, such as turbidity and water vapor at each location. The annual performance ratio to initial efficiency measured under standard test conditions reached 82% in Hamamatsu, Japan.

Physical properties and structure of carbon-rich a-SiC:H films prepared by r.f. glow discharge decomposition

Abstract Carbon-rich a-SiC:H films were prepared by glow discharge decomposition from SiH4 and CH4. The deposition rate, hydrogen content, optical band gap and microhardness were investigated for various carbon contents. In particular, carbon atom coordination was investigated by solid-state13C magic angle spinning nuclear magnetic resonance measurements of the sp2 and sp3 bonding sites. Unsaturated character (sp2 carbon) appears clearly in the film of composition a-Si0.25C0.75 at about 130 ppm. Film hardness and optical band gap correlate with both the hydrogen content and the fraction of tetrahedral(sp3) vs. graphitic(sp2) bonding. It was also observed from annealing treatment experiments that a change of network structure takes place, from high density to low density, at a carbon content of about 0.63.

Optimizations of the Film Deposition Parameters for the Hydrogenated Amorphous Silicon Solar Cell

Optimization of the photovoltaic performance in the plasma deposited a-Si:H heteroface p-i-n junction solar cells has been experimentally studied. A series of systematic experiments on the relation between photovoltaic performances and substrate-positive column distance with several power density have been carried out. As the results, a-Si:H solar cell fabrication technology realizing a conversion efficiency of higher than 5.5% under AM-1 sun light has been developed with a considerably good reproduceablity. Moreover, it has been demonstrated that a conversion efficiency exceeding 6.1% is obtained in designing of an optimized i-layer sandwitched with a wide band gap a-SiC:H front p-layer and a low resistivity n-type back electrode.