Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
1998-07-21
2003-09-02
Pyon, Harold (Department: 1773)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C136S252000, C136S258000, C136S263000, C438S609000, C427S204000, C427S205000, C205S183000, C205S184000, C205S186000, C205S220000
Reexamination Certificate
active
06613603
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photovoltaic device composed of a non-single-crystalline semiconductor material of a silicon type, and a process for producing the photovoltaic device. In particular, the present invention relates to an inexpensive solar cell exhibiting a high photoelectric conversion efficiency and a process for producing it. The present invention relates also to a zinc oxide thin film, and a process for producing it.
2. Related Background Art
In recent years, the demand for solar cells is growing as power generation equipment. In particular, photoelectric power generation is attracting attention which is conducted by solar cell modules of about 3 kW mounted on a roof of a private house and which is connected to a commercial power supplying system, whereby electric power is soled to or brought from the commercial system. A governmental subsidy system has already been enforced. Under such circumstances, the photoelectric power generation has disadvantages that the power generation cost is too high to compete the commercial power generation, and that the generation capacity is low to meet the power demand. To offset the above disadvantages, the solar cell is required to have a high photoelectric conversion efficiency, stable power generation ability for a long term of about 20 years or more, a high power generation capacity, and a low cost/power performance. At present, the material for the solar cell includes crystalline silicon (c-Si), polycrystalline silicon (poly-Si), amorphous silicon (a-Si), GaAs, and CdS. Of these, the amorphous silicon solar cell is advantageous in production performance and the cost/power performance. After the Kobe-Osaka-Awaji Great Earthquake Disaster, weight reduction of the roof material comes to be demanded. In this respect, light-weight amorphous silicon solar cell is advantageous. Further, the amorphous silicon solar cell is advantageous in installation on a curved surface. However, the amorphous solar cell does not achieve the high photoelectric conversion efficiency of the crystalline silicon solar cell at the moment, and is naturally deteriorated in photoelectric conversion efficiency by exposure to intense light.
Therefore, improvements of non-single-crystalline silicon type solar cells are widely investigated as below. For example, for amorphous silicon type materials, trials are being made for improving light collection efficiency by narrowing the bandgap by addition of Ge or Sn to the i-type layer in an amount ranging from 1% to 50%, or for raising the open circuit voltage by broadening the bandgap by addition of C, N, O, or the like in an amount ranging from 0.1% to 10%. Other trials are being made for collecting broader range of light by stacking an element having a higher open circuit voltage at a light introducing side and an element having a lower open circuit voltage at a back side. With the stack type solar cell, the thickness of the i-type layer is tried to be made smaller to retard the photo-deterioration. For example, a photoelectric conversion efficiency of 9.5% after photo-deterioration was achieved with a solar cell having constitution of a-Si/a-SiGe/a-SGe, or a-Si/a-Si/a-SiGe by S. Guha, J. Yang: Technical Digest of 7th International Photovoltaic Science and Engineering Conference Nov. 1993, NAGOYA JAPAN, p43 “Progress in Multijunction Amorphous Silicon Alloy-Based Solar Cells and Modules”. A photoelectric conversion efficiency 10.2% after photo-deterioration of was achieved with a solar cell having constitution of a-SiC/a-SiGe/a-SiGe by K. Nomoto, Y. Yamamoto: Technical Digest of 7th International Photovoltaic Science and Engineering Conference Nov. 1993, NAGOYA JAPAN, p43 “Progress in Multijunction Amorphous Silicon Alloy-Based Solar Cells and Modules”. “a-Si Alloy Three-Stacked Solar Cells with High Stabilized-Efficiency”.
An attempt was made to lower the power cost by forming an a-Si layer and a-SiGe layer by microwave plasma CVD at a higher deposition rate by K. Saito, I. Kajita: Journal of Non-Crystalline Solids 146-166 (1993) p689-692 “High efficiency a-Si:H alloy cell deposited at high deposition rate”. According to this report, a photoelectric conversion efficiency of 11.6% was achieved with a constitution of a-Si/a-SiGe/a-SiGe by use of a-Si formed at a deposition rate of 75 A/sec and a-SiGe formed at a deposition rate of 100 A/sec.
A plasma CVD apparatus is disclosed which forms continuously semiconductor layers of different conduction types by a roll-to-roll system in Japanese Patent Application Laid-Open No. 05-121331. This apparatus has a plurality of deposition chambers, and a belt-like flexible substrate is arranged along the path so as to pass through the deposition chambers successively. The substrate is delivered in its length direction while a semiconductor layer of a desired conduction type is formed in each of the respective deposition chambers, thereby continuously producing a photovoltaic device having a p-i-n junction. The above disclosed apparatus employs a gas gate which prevents diffusion of the source gas for introducing valence electron controlling agent into the semiconductor layer to another deposition chamber and thereby prevents contamination of another semiconductor layer. Specifically, the deposition chambers are separated by a slit-shaped separation path where a sweeping gas such as Ar, H
2
and He is introduced to prevent mutual diffusion of the source gases, whereby a desired p-i-n junction is formed. This roll-to-roll system for thin film formation improves remarkably the productivity of photovoltaic devices having a stacked structure.
A transparent electroconductive layer having a surface of a projection-recess structure (texture structure) is known to improve light collection efficiency. For example, Preprint of 51th Applied Physics Society Meeting p747 (1990 Autumn) 29p-MF-2 “Optical Confinement Effect in a-SiGe Solar Cell on Stainless Steel Substrate”; and Sannomiya et al., Technical Digest of the International PVSEC-5, Kyoto, Japan, p387, 1987 disclose improvement of short-circuiting photoelectric current by forming a back reflection layer composed of Ag and a transparent layer composed of zinc oxide in a suitable surface texture structure. T. Tiedje, et al.: Proc. 16th IEEE Photovoltaic Specialist Conf. (1982) p1423, and H. Deckman, et al.: Proc. 16th IEEE Photovoltaic Specialist Conf. (1982) p1425 disclose improvement of photoelectric conversion efficiency by forming a back electrode into a projection-recess shape (texture structure) having a size approximate to light wavelength for scattering light to scatter long wavelength light which has not been absorbed in the semiconductor layer and lengthen the optical path in the semiconductor layer, thereby raising sensitivity of the photovoltaic device to respect with the long wavelength light to increase short-circuit photoelectric current.
Zinc oxide is more resistant to plasma than tin oxide and indium oxide, and when zinc oxide is exposed to plasma containing hydrogen, it is not reduced by hydrogen. When a semiconductor layer composed of amorphous silicon is formed on zinc oxide by plasma CVD, zinc oxide is positively used as a transparent electroconductive layer.
Japanese Patent Application Laid-Open No. 60-84888 (Energy Conversion Devices) discloses a technique for decrease of electric current passing through defective regions in a semiconductor layer by interposition of a transparent electroconductive layer between a back electrode and a semiconductor layer.
Japanese Patent Application Laid-Open No. 7-23775, and Masanobu Izaki, Takasi Omi: Journal of Electrochemical Soc. Vol.143, No.3 “Electrolyte Optimization for Cathodic Growth of Zinc Oxide Films” discloses electrochemical deposition of a transparent zinc oxide thin film by applying electric current between counter electrodes immersed in an aqueous zinc nitrate solution. This method requires neither an expensive vacuum apparatus nor an expensive target, thereby remarkably reducing the production cost of
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Miggins Michael C.
Pyon Harold
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