Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
2001-10-31
2004-09-07
Diamond, Alan (Department: 1753)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S246000, C136S259000, C136S249000, C136S244000, C136S261000, C136S255000, C257S436000, C257S458000, C257S434000, C428S702000, C428S332000
Reexamination Certificate
active
06787692
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a solar cell substrate, thin-film solar cell, and multi-junction thin-film solar cell, which are capable of inexpensively providing solar cells having stable and high photo-electric conversion efficiency.
BACKGROUND OF THE INVENTION
Fossil fuel such as petroleum is expected to be in short of supply in the future, and has the problem of carbon dioxide emission, in addition to causing the global warming effect. In recent years, solar cells have been under the focus of attention as a replacement energy source for fossil fuel such as petroleum.
The solar cells incorporate semiconductors with the p-n junction in a photo-electric conversion layer which converts light energy into electricity. The most common semiconductor with the p-n junction is silicon. In view of photo-electric conversion efficiency, mono-crystalline silicon is preferably used for the semiconductor. However, semiconductors with mono-crystalline silicon have problems in that it is difficult to supply the raw material, to increase its area, and to reduce cost.
Meanwhile, a thin-film solar cell which incorporates amorphous silicon as the photo-electric conversion layer has been put in practical applications to increase area and reduce cost. Further, use of crystalline silicon as the photo-electric conversion layer has been investigated in an effort to realize a solar cell with high and stable photo-electric conversion efficiency which can match up against that of the mono-crystalline silicon solar cell, and with a large area and low cost comparative to that attained by the amorphous silicon solar cell. A particular interest has been on a thin-film solar cell with a crystalline silicon thin film (“crystalline silicon thin-film solar cell” hereinafter), which is formed by the thin-film deposition technique employing the chemical vapor deposition method (“CVD method” hereinafter) which is used in forming the amorphous silicon.
Japanese Unexamined Patent Publication No. 289173/1989 (Tokukaihei 1-289173) (published date: Nov. 21, 1989) discloses a multi-junction thin-film solar cell which is formed by depositing a photo-electric conversion element having an amorphous silicon active layer as an active layer, and another photo-electric conversion element having an active layer of crystalline silicon which has a comparatively smaller energy gap than the amorphous silicon. This multi-junction thin-film solar cell is structured so that sun light is incident on the side of the photo-electric conversion element having the active layer of amorphous silicon, which is advantageous in utilizing the solar energy more efficiently than the mono-junction type. Further advantages of this structure are high open-circuit voltage due to the series connection of the plurality of photo-electric conversion elements, and slow degradation rate of photo-electric conversion efficiency which degrades due to the Staebler-Wronski effect. This slow degradation rate of photo-electric conversion efficiency is made possible by the thin amorphous silicon layer as the active layer. Another advantage of this structure is that it allows the amorphous silicon layer and the crystalline silicon layer to be fabricated using the same device, which has made this structure a target of active research and development to attain high efficiency and low cost at the same time.
Note that, in the following description of the present invention, the term “crystalline” is meant to indicate not only a crystalline state of essentially pure crystals such as “mono-crystal” or “poly-crystal”, but also a mixed state of crystal component and amorphous component, which state is referred to as “microcrystal”.
One of the important techniques which is an important factor in realizing a thin-film solar cell with high photo-electric conversion efficiency is light-trapping. The light-trapping is the phenomenon in which the quantity of light absorbed in the photo-electric conversion layer is increased by increasing the optical path length, which is attained by forming irregularities on the surface of the transparent conductive film or metal layer in contact with the photo-electric conversion layer, so as to cause light to scatter at the interface.
For example, Japanese Patent No. 1681183 (published date: Apr. 6, 1983) and No. 2862174 (issued Feb. 24, 1999) disclose solar cell substrates which specify particle size or size of irregularities of the transparent conductive film which is formed on the glass substrate.
The improved photo-electric conversion efficiency by the light-trapping effect enables the photo-electric conversion layer to have a thinner thickness. This effect suppresses deterioration of light caused by the Staebler-Wronski effect, in the case of the amorphous silicon solar cell.
Further, due to its light absorbing characteristics, the crystalline silicon solar cell conventionally required a thickness on the order of several microns, which is several times to several ten times greater than that required for the amorphous silicon. However, even with the crystalline silicon solar cell, a deposit time can be made much shorter when the photo-electric conversion efficiency is improved by the effect of light-trapping.
That is, light-trapping is the essential technique for realizing high efficiency, high stability, and low cost at the same time, which are all required for practical applications of the thin-film solar cell.
However, despite active research and development to this date, the photo-electric conversion efficiency of the conventional crystalline silicon thin-film solar cells has only reached the level of the photo-electric conversion efficiency of the amorphous silicon.
Technical Digest of the International PVSEC-11, Sapporo, Hokkaido, Japan, 1999 (H. Yamamoto et al.) has the following report.
Microcrystalline silicon, when deposited by the plasma CVD method on the Asahi-U substrate, which is a glass substrate with tin oxide deposited thereon to have microscopic irregularities, causes crystal grains of the silicon to grow primarily in a vertical direction with respect to each surface of the microscopic irregularities of tin oxide. The crystal grains grown in this manner from each different surface of the irregularities have different crystal directions and they collide with one another. The result is mass defects. These defects need to be suppressed to a minimum because they become a recombination center of carriers (electrons and holes) to severely degrade photo-electric conversion efficiency.
H. Yamamoto et al. has the following report as well.
The size of the irregularities was made smaller by depositing zinc oxide to a thicker thickness on the tin oxide having surface irregularities. The result was the same as that obtained using only tin oxide, causing growth of crystal grains of the silicon in a vertical direction with respect to the surface of zinc oxide and thereby causing collision of crystal grains which grow from each different surface. However, the differences of directions were smaller in this case, and less defects were incurred.
It is therefore apparent that the size of irregularities on the substrate surface should be reduced as much as possible in order to reduce defects in the crystalline silicon thin-film. While this may be the case, as noted above, light-trapping is necessary for the thin-film solar cell, and it is not entirely preferable to eliminate or reduce the surface irregularities when practical applications are at hand.
Meanwhile, the solar cell substrate with the transparent conductive film having surface irregularities, as disclosed in the foregoing Japanese Patent No. 1681183 and No. 2862174, has the problem of cost, which is one factor that prevents wide-spread use of the thin-film solar cell. One approach to solve this problem is to use zinc oxide for the transparent conductive layer. Zinc oxide is comparatively cheaper than other materials such as tin oxide or ITO which are widely used as the material of the transparent conductive film. Further, the advantage of high plasma resistan
Kondo Michio
Matsuda Akihisa
Nasuno Yoshiyuki
Wada Kenji
Birch & Stewart Kolasch & Birch, LLP
Diamond Alan
National Institute of Advanced Industrial Science & Technology
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