United States Patent
`Niwa
`[45] Date of Patent: May 30, 1995
`
`[191
`
`[11] Patent Number:
`
`5,420,043
`
`USOOS420043A
`
`[54] METHOD OF MANUFACTURING A SOLAR
`CELL
`
`[75]
`
`Inventor:
`
`Mitsuyuki Niwa, Nagahama, Japan
`
`[73] Assignee:
`
`Canon Kabushiki Kaisha, Tokyo,
`Japan
`
`[21] Appl. No.:
`
`197,875
`
`[22] Filed:
`
`Feb. 17,1994
`
`Related U.S. Application Data
`
`[62] Division of Ser. No. 948,317, 'Sep. 22, 1992, Pat. No.
`5,324,365.
`Foreign Application Priority Data
`[30]
`Sep.24, 1991 [JP]
`Japan .................................. 3-272092
`Sep.24, 1991 [JP]
`Japan .................................. 3272093
`
`[51]
`
`Int. 01.6 ..................... H01L 31/18; HOIL 31/20;
`HOIL 31/0224
`[52] U.S. c1. ................................... 437/4; 204/192.29;
`427/576; 427/528; 427/529; 427/2552;
`427/255.3; 437/131
`[58] Field of Search .................... 437/4, 181; 136/256,
`136/258 AM; 204/19229; 427/74, 576,
`'
`528—529, 255.2, 255.3
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`..... 257/54
`
`4,064,521 12/1977 Carlson .......
`
`437/ 165
`4,226,897 10/1980 Coleman .
`..... 252/501.1
`5,057,244 10/1991 Nitta et a1.
`..
`
`........ 136/256
`5,064,477 11/1991 Delahoy ......
`
`5,078,803
`1/1992 Pier et a1.
`136/256
`............................ 257/53
`5,101,260 3/1992 Nath et al.
`
`FOREIGN PATENT DOCUMENTS
`
`55-108780 8/1980 Japan ........................... H01L 31/04
`55-125681
`9/1980 Japan ..
`H01L 031/04
`
`6/1981 Japan ........... HOIL 31/04
`56-69875
`OTHER PUBLICATIONS
`
`Response of Top a—Si:H Solar Cells for Tadnem Struc-
`mes),
`Technical Digest, 3rd International Photovoltaic Science
`and Engineering Conference, Nov. 1987, pp. 171—174,
`Yutaka Hattori et a1., “High Efficiency Amorphous
`Heterojunction Solar Cell Employing ECR—CVD Pro-
`duced p—type Microcrystalline SiC Film”.
`'
`Solid State Communications, vol. 17, pp. 1193—1196,
`1975, W. E. Spear et a1., “Substitutional Doping of
`Amorphous Silicon”.
`Solar Energy Materials, vol. 13, pp. 75—84, 1986, C. X.
`Qiu et al., “Tin-and Indium—Doped Zinc Oxide Films
`Prepared by RF Magnetron Sputtering”.
`Japanese Journal ofApplied Physics, vol. 20 (1981), Sup-
`plement 20—2, pp. 219-225, Yoshihisa Tawada et al.,
`“Optimizations of the Film Deposition Parameters for
`the Hydrogenated Amorphous Silicon Solar Ce ”.
`Igasaki &. Shimaoka, Research Report of Shizuoka Uni-
`versity, Electronic Engineering Lab., vol. 21, No. l, 1986;
`pp. 23—35.
`Koinuma et a1., Transactions of the Society of Japanese
`Ceramics, vol. 97, No. 10, 1989, pp. 1160—1163.
`Optical Materials, H. C. Pan et a1.; “Nitrogen Doping of
`ZnO Prepared by Organometallic Chemical Vapor
`Deposition” pp. 215—219; Processing and Science Sym-
`posium, San Diego, Calif., USA. 24—26 Apr. 1989.
`
`Primary Examiner—Aaron Weisstuch
`Attorney, Agent, or Finn—Fitzpatrick, Cella, Harper &
`Scinto
`
`[57]
`
`ABSTRACT
`
`A solar cell has a semiconductor layer sandwiched
`between first and second electrodes, wherein a zinc
`oxide layer containing carbon atoms, nitrogen atoms, or
`carbon and nitrogen atoms is located between the semi-
`conductor layer and at least one of the first and second
`electrodes. The density of carbon atoms, nitrogen
`atoms, or carbon and nitrogen atoms in the zinc oxide
`layeris constant or continuously changed within the
`range of 5 atm % or less.
`
`Nineteenth IEEE Photovoltaic Specialists Conference
`(1987), W. Kusian et al. pp. 599—603, “Enhanced Blue
`
`36 Claims, 9 Drawing Sheets
`
`199
`
`\
`
`106 ////////////
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`U.S. Patent
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`May 30, 1995
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`Sheet 1 of 9
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`5,420,043 _
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`H5.
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`U.S. Patent
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`I May 30, 1995
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`Sheet 2 of 9
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`5,420,043
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`FIG. 3
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`1'9.
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`305
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` 307
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`U.S. Patent
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`May 30, 1995
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`Sheet 3 of 9
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`5,420,043
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`FIG. 5
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`U.S. Patent
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`May-30, 1995
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`Sheet 4‘ of 9
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`0
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`5,420,043
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`606
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`6:.
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`613 _
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`605
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`609
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`608
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`U.S. Patent
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`May 30, 1995
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`Sheet 5 of 9
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`5,420,043
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`FIG. 7 .
`
`705 K 701.
`a.
`
`701
`
`I —_ 703
`M
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`702
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`707
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`709
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`U.S. Patent
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`May 30, 1995
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`Sheet 6 of 9
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`5,420,043
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`FIG. 8
`
`.0
`
`9m
`PHOTOELECTRICCONVERSION go
`
`
`
`STAMJARUZEDEFFICIENCYOF
`
`10"
`10"
`10”
`10" 10"
`10"
`10"
`DENSITY CF CARBCN ATOHS N ZNC OXIDE
`IATOH NUMBER RATIO)
`
`FIG. 9
`
`
`
`
`
`c9:-‘oo
`STANDARDIZEDRATEOFDEGRADATION °.0ONI
`
`
`
`
`
`
`
`
`0U!
`
`10"
`1o"
`10"
`10"
`10's
`10"
`10"
`DENSITY OF CARBCN ATOHS N ZNC OXIDE
`IATOH NUMBER RATIO)
`
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`US. Patent
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`May 30, 1995
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`Sheet 7 of 9
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`5,420,043
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`FIG. 70
`
`
`
`10"
`19'
`10"
`10"
`10" 10"
`10"
`DENSITY or CARBON nous IN quc OXIDE
`(ATOM NUMBER RATIO)
`
`FIG.
`
`77
`
`I
`'
`I
`I
`
`I I
`
`l
`I
`
`WI]
`
`I
`I
`I
`~1—
`I
`
`III IIII
`
`m
`
`10'1
`10"
`10-5 10“ 10°3
`10"
`10"
`DENSITY OF CARBW ATOHS m ZINC OXIDE
`(ATOM NUMBER RATIOI
`
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`U.S. Patent
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`May 30, 1995
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`Sheet 8 of 9
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`5,420,043
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`Fl 6. 72
`
`
`IZEDEFFICIENCYOFTRIC
`
`CONVERSION p
`
`
`pN]
`
`10"
`
`10“
`
`10"
`
`10" 10"
`
`‘10"
`
`10"
`
`DENSITY OF NITRMEN ATmS IN ZINC OXIDE
`I ATOM NIMBER RATIO)
`
`FIG. 73
`
`
`
`STANDSRDIZEDRATEOFDEBRAA" o m
`
`I17
`
`.0o
`
`107
`
`10“
`
`‘IO‘5
`
`10“
`
`10" 10‘
`
`10‘1
`
`KNSITY OF NITROGEN ATOMS IN ZNC OXIDE
`IATOI‘I NUMBER RATIO)
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`IMay 30, 1995
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`Sheet 9 Of 9
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`5,420,043
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`FIG. 74
`
`Or‘30::
`
`.°
`
`STANDARDIZEDEFFICIEKYOFPHOTOELECTRICCONVERSION O NI
`
`
`
`
`
`
`.0
`
`3.
`
`1o“
`10'2
`10‘5 1o" 10"
`1o"
`10"
`DENSITY 0F NITROGEN ATOMS IN zINc OXIDE
`(ATOM men RATIO)
`
`FIG. 75 »
`
`.
`
`o,0o....'4comb
`
`.°0
`
`
`
`STANDARDIZEDRATEOF
`
`DEGRADATION
`
`.° m
`
`10" 10" 10's 10“ 10"
`
`10°2
`
`10"
`
`DENSITY OF NITROGEN ATCMS IN ZINC OXIDE
`(ATOM NUMBER RATIO)
`
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`1
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`5,420,043
`
`METHOD OF MANUFACTURING A SOLAR CELL
`
`This application is a division of application Ser. No.
`07/948,317, filed Sep. 22, 1992, now U.S. Pat. No.
`5,324,365, issued Jun. 28, 1994.
`
`5
`
`BACKGROUND OF THE INVENTION
`
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`1. Field of the Invention
`The present invention relates to a solar cell and, more
`particularly, to a solar cell in which output characteris-
`tics are stably improved and degradation is prevented.
`2. Related Background Art
`In recent years, strong worldwide demand has arisen
`for increased power supply, and active power genera-
`tion to answer this demand now poses a serious problem
`of environmental pollution.
`Under
`these circumstances, a power generation
`scheme of solar cells using sunlight has received a great
`deal of attention as a clean power generation scheme in
`which problems posed by terrestrial warming caused by
`radioactive contamination and emission of gases having
`a greenhouse effect can be solved. Moreover, energy
`sources are less localized because sunlight is radiated on
`the entire area of the earth, and relatively high power
`generation efficiency can be obtained without requiring
`complicated bulky equipment. Thus, this power genera»
`tion scheme is expected to cope with increased future
`power supply demand without destroying the environ-
`ment, and various studies have been made for practical
`applications of the scheme.
`'
`In order to establish a power generation scheme
`which uses solar cells, and which can satisfy power
`supply needs, the solar cell must have sufficiently high
`efficiency of photoelectric conversion and stable char—
`acteristics and must allow mass-production as the basic
`requirements.
`For this reason, the following solar cell and manufac-
`turing method have received a great deal of attention.
`That is, an easily accessible source gas such as silane is
`used and decomposed by glow discharge to deposit a
`semiconductor thin film such as nonomonocrystalline
`silicon on a relatively inexpensive substrate such as a
`glass substrate or a metal sheet, thereby mass-producing
`solar cells. These solar cells can be manufactured with a
`small amount of energy at a low cost as compared with
`a monocrystalline silicon solar cell or the like.
`The study of applications of non-monocrystalline
`silicon to photovoltaic elements such as solar cells was
`started with the invention of a solar cell by D. E. Carl—
`son (U.S. Pat. No. 4,064,521) based on the success of
`doping such materials by W. E. Spear and P. G. Le-
`Comber (Solid State Communications, Vol. 17, pp.
`1193—1196, 1975). Although the field of solar cells has a
`short history of research and development, a variety of 55
`fruitful studies have been reported.
`Semiconductor layers as important constituent ele-
`ments of a solar cell form semiconductor junctions such
`as the so—called p-n and p-i-n junctions. These semicon-
`ductor junctions can be formed by sequentially stacking
`semiconductor layers having different conductivity
`types, or incorporating a dopant having a conductivity
`type different from that of a given semiconductor layer
`into said semiconductor layer in accordance with ion
`implantation or the like, or by diffusion of a dopant by
`thermal diffusion. A solar cell obtained using a thin film
`semiconductor such as amorphous silicon as the non-
`monocrystalline silicon described above has been stud-
`
`65
`
`2
`ied. In the manufacture of the solar cell according to
`known techniques, a source gas containing an element
`serving as a dopant such as phosphine (PH3 for n-type
`semiconductor) or diborane (32H6 for p-type semicon-
`ductor) is mixed in silane serving as a main source gas,
`and the gas mixture is decomposed by glow discharge
`or the like to obtain a semiconductor film having a
`desired conductivity type. Such semiconductor films
`are sequentially stacked on a desired substrate in a p—i—n
`or n-i-p structure, thereby easily obtaining a semicon-
`ductor junction.
`As a result of these studies, solar cells using non-
`monocrystalline silicon have already been used in a
`variety of power generation applications, e.g. compact
`devices such as a wristwatch/clock, a compact calcula-
`tor, and a street light. When non-monocrystalline sili-
`con is to be applied to a large device for power genera-
`tion, many problems left unsolved (e.g., lower conver-
`sion efficiency than that of a monocrystalline or com-
`pound semiconductor solar cell, and degradation) must
`be solved. These problems are posed as disadvantages
`of the non-monocrystalline silicon solar cells. Numer-
`ous attempts have been made to solve these problems.
`For example, these attempts include the use of p-type
`nonomonocrystalline silicon carbide having a large for-
`hidden band width as an incident-side window layer (Y.
`Uchida, US-Japan Joint Seminar, Technological Appli-
`cations of Tetrahedral Amorphous Solids, Palo Alto,
`Calif. (1982)), use of p-type silicon carbide having fine
`crystal grains as a window layer (Y. Uchida et al., Tech-
`nical Digest of the International PVSEC-3, Tokyo,
`Japan (1987) A-IIa-3 pp. 171—174), and the like.
`In the use of non-monocrystalline silicon carbide
`having a large forbidden band width as a window layer,
`there is attempted a method of forming a sovcalled
`graded buffer layer (i.e., the forbidden band width is
`continuously changed) in which an energy band step
`formed at the p-i interface is eliminated to prevent the
`efficiency of photoelectric conversion from being de~
`graded, in a short-wavelength range, the degradation
`being caused by redistribution or recombination of the
`carriers (R. R. Arya et al., Technical'Digest of the
`International PVSEC-S, Tokyo, Japan 1987 A-IIIa-4).
`According to another attempt, phosphorus atoms (P)
`or boron atoms (B) are doped in an i-type layer in a very
`small amount of 10 ppm or less to increase the carrier
`mobility in the i-type layer (W. Kuwano et al., The
`Conference Record of the Nineteenth IEEE Photovol-
`taic Specialists Conference-1987, P. 599; M. Kondo et
`al., The Conference Record of the Nineteenth IEEE
`Photovoltaic Specialists Conferenceol987, p. 604).
`According to still another attempt, p- and n-type
`dopants are diffused in a semiconductor layer having
`another conductivity type to weaken the semiconduc-
`tor junctions at the p-n, p—i, and n-i interfaces, thereby
`preventing degradation in efficiency of photoelectric
`conversion of a photovoltaic element. An example of
`such an attempt is disclosed in Japanese Laid-Open
`Patent Application No. 55-12568] (applicant: Sanyo
`Electric Co., Ltd.). This prior art discloses a method of
`forming a solar cell by arranging a partition door in a
`plasma reaction chamber through which a glass sub-
`strate on a conveyor passes. U.S. Pat. No. 4,226,897
`(applicant: Plasma Physics Inc.) discloses a method in
`which spaces for forming respective semiconductor
`layers are separated from each other by gas gates (i.e., a
`mechanism in which a gas which does not contribute to
`film formation forcibly and strongly flows so as to serve
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`5,420,043
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`3
`as partition walls for the film formation gases) in an
`apparatus for continuously forming solar cell on an
`elongated substrate of belt-like shape, thereby prevent-
`ing a dopant from entering into the wrong film forma-
`tion space.
`Japanese Laid-Open Patent Application No. 56-69875
`(applicant: Fuji Electric Co., Ltd.) discloses a method in
`which a transparent conductive layer is formed be-
`tween a conductive substrate and a semiconductor layer
`in a solar cell having the semiconductor layer on the
`conductive substrate to increase the adhesion strength
`between the semiconductor layer and the substrate, or
`to improve the surface smoothness of the substrate,
`thereby improving the characteristics of the solar cell.
`Japanese Laid-Open Patent Application No. 55-108780
`(applicant: Sharp Corporation) discloses a method in
`which a decrease in reflectance at an interface caused
`by formation of an alloy between the semiconductor
`atoms and a diffused metal element constituting the
`lower electrode located on the surface of a semiconduc-
`tor layer at a side opposite to the light-receiving surface
`can be prevented by forming a transparent conductive
`layer such as a zinc oxide layer between the lower elec-
`trode and the semiconductor layer.
`According to still another attempt, tin or indium is
`doped in a zinc oxide layer to decrease the resistivity of
`the zinc oxide layer (C. X. Qiu, I. Shin, Solar Energy
`Materials, Vol. 13, No. 2, pp. 75—84, 1986). An example
`of aluminum doping in a zinc oxide layer is also re-
`ported (Igasaki & Shimaoka, Research Report of Shizu-
`oka University, Electronic Engineering Lab., Vol. 21,
`No. 1, 1986), and an example of fluorine doping is also
`reported (Koinuma et al., Transactions of the Society of
`Japanese Ceramics, Vol. 97, No. 10, pp. 1160—1163,
`1989).
`As a result of efforts made by a number of engineers
`in addition to the above attempts, the disadvantages
`(e.g., low efficiency of photoelectric conversion and
`degradation) of non-monocrystalline silicon solar cells
`have been improved. However, the following problems
`are still left unsolved.
`
`When a solar cell is formed by forming a semiconduc-
`tor layer on a conductive substrate through a zinc oxide
`layer, the adhesion strengths between the zinc oxide
`layer and the conductive substrate and between the zinc
`oxide layer and the semiconductor layer are not suffi-
`cient. Slight peeling may occur due to temperature
`shocks and vibrations in formation of the semiconduc-
`tor layer and the subsequent step. This peeling poses a
`problem of initial characteristics,
`i.e., degradation in
`efficiency of photoelectric conversion of the solar cell.
`In addition, since the resistivity of the zinc oxide
`layer cannot be reduced to a negligible degree, an in-
`crease in series resistance of the solar cell can be ex-
`pected, thus posing a problem of initial characteristics,
`i.e., degradation in efficiency of photoelectric conver-
`sion of the solar cell.
`This also applies not only to formation of the semi-
`conductor layer on the conductive substrate but also to
`a solar cell in which a transparent conductive layer is
`formed on a transparent insulating substrate and a semi-
`conductor layer is formed on the transparent conduc-
`tive layer. More specifically, slight peeling occurs be-
`tween the transparent conductive layer and the semi-
`conductor layer even during the manufacture due to
`insufficient adhesion strength between the transparent
`conductive layer and the semiconductor layer, thereby
`posing a problem of initial characteristics, i.e., degrada-
`
`4
`tion in efficiency of photoelectric conversion of solar
`cell.
`
`Even if slight peeling does not occur in the manufac-
`ture of a solar cell and efficiency of photoelectric con-
`version in the initial manufacturing period of the solar
`cell is rather high, slight peeling may occur between the
`conductive substrate and the transparent conductive
`layer and between the transparent conductive layer and
`the semiconductor layer in practical application states
`under various climatic and installation conditions,
`thereby degrading reliability due to gradual degrada-
`tion in efficiency of photoelectric conversion of the
`solar cell.
`
`SUMMARY AND OBJECTS OF THE
`INVENTION
`
`The present invention has been made in consideration
`of the above, and has as an object to provide a solar cell
`having a semiconductor layer sandwiched between first
`and second electrodes, wherein adhesion strengths be-
`tween the respective layers are increased to effectively
`prevent the characteristics from being degraded by
`slight film peeling occurring during the manufacture,
`thereby improving the initial characteristics.
`It is another object of the present invention to pro-
`vide a solar cell in which adhesion strengths between
`the respective layers are increased to effectively pre-
`vent the characteristics from being degraded by the
`slight film peeling occurring under practical application
`conditions, thereby improving the reliability.
`It is still another object of the present invention to
`provide a solar cell in which the resistance of a zinc
`oxide layer is decreased to decrease the series resistance
`of the solar cell,
`thereby improving the initial effi-
`ciency.
`The present invention has been achieved based upon
`extensive studies of the present inventor to achieve the
`above objects of the present invention, and there is
`provided a solar cell having a semiconductor layer
`sandwiched between first and second electrodes,
`wherein a zinc oxide layer containing carbon atoms is
`located between the semiconductor layer and at least
`one of the first and second electrodes.
`The density of carbon atoms in the zinc oxide layer
`may be constant within the range of 5 atm % or less or
`may be continuously changed within the range of 5 atm
`% or less.
`According to the present invention, there is also pro-
`vided a solar cell having a semiconductor layer sand-
`wiched between first and second electrodes, wherein a
`zinc oxide layer containing nitrogen atoms is located
`between the semiconductor layer and at least one of the
`first and secdnd electrodes.
`The density of nitrogen atoms in the zinc oxide layer
`may be constant within the range of 5 atm % or less or
`may be continuously changed within the range of 5 atm
`% or less.
`According to the present invention, there is also pro-
`vided a solar cell having a semiconductor layer sand—
`wiched between first and second electrodes, wherein a
`zinc oxide layer containing carbon and nitrogen atoms
`is located between the semiconductor layer and at least
`one of the first and second electrodes.
`'
`The density of carbon and nitrogen atoms in the zinc
`oxide layer may be constant within the range of 5 atm
`% or less or may be continuously changed within the
`range of 5 atm % or less.
`
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`5
`A solar cell manufactured by the technique of the
`present invention using a zinc oxide film containing
`carbon atoms, nitrogen atoms, or carbon and nitrogen
`atoms has higher adhesion strengths between the re-
`spective layers and hence better initial characteristics
`and higher reliability than those of a solar cell manufac-
`tured by a conventional technique using a zinc oxide
`layer not containing carbon atoms, nitrogen atoms, or
`carbon and nitrogen atoms.
`The reason why the adhesion strength is increased by
`containing carbon atoms, nitrogen atoms, or carbon and
`nitrogen atoms in the zinc oxide layer is not yet clari-
`fied. However, it is assumed that the carbon atoms,
`nitrogen atoms, or carbon and nitrogen atoms in the
`zinc oxide layer cause a change in a bond or crystalline
`state to effectively relax various mechanical and ther-
`mal stresses which may cause the slight peeling during
`the manufacture or during practical use in conventional
`solar cells.
`According to the present invention, the density of
`carbon atoms in the zinc oxide layer is preferably con-
`stant within the range of 5 atm % or less. The adhesion
`strength of the film can be increased within the above
`range, and the initial characteristics including efficiency
`of photoelectric conversion can be improved. The
`“constant density” in the present invention includes
`i10% variations with respect to the average density.
`When the carbon density in the zinc oxide layer is
`continuously changed within the range of 5 atm % or
`less, the adhesion strength of the layer can be further
`increased, and the initial characteristics and reliability
`of the solar cell are also further improved due to the
`following reason. When the density of carbon atoms in
`the zinc oxide layer has a given distribution, the struc-
`ture in the zinc oxide layer is changed to relax the stress
`caused by the formation of different types of deposition
`films on two sides of the zinc oxide layer and various
`external stresses,
`thereby effectively preventing the
`slight film peeling occurring during the manufacture
`and practical use.
`According to the present invention, the density of
`nitrogen atoms in the zinc oxide layer is preferably
`constant within the range of 5 atm % or less. The adhe-
`sion strength of the film can be increased within the
`above range, and the initial characteristics including
`efficiency of photoelectric conversion can be improved.
`The “constant density” in the present invention in-
`cludes 110% variations with respect to the average
`density.
`When the nitrogen density in the zinc oxide layer is
`continuously changed within the range of 5 atm % or
`less, the adhesion strength of the layer can be further
`increased, and the initial characteristics and reliability
`of the solar cell also further improved due to the follow-
`ing reason. When the density of nitrogen atoms in the
`zinc oxide layer has a given distribution, the structure in
`the zinc oxide layer is changed to relax the stress caused
`by formation of different types of deposition films on
`two sides of the zinc oxide layer and various external
`stresses, thereby effectively preventing the slight film
`peeling occurring during the manufacture and practical
`use.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`55
`
`According to the present invention, the total density
`of carbon and nitrogen atoms in the zinc oxide layer is
`preferably constant within the range of 5 atm % or less.
`, The adhesion strength of the film can be increased
`within the above range, and the initial characteristics
`including the efficiency of photoelectric conversion can
`
`65
`
`5,420,043
`
`6
`be improved. The “constant density” in the present
`invention includes i 10% variations with respect to the
`average density.
`When the total density of carbon and nitrogen atoms
`in the zinc oxide layer is continuously changed within
`the range of 5 atm % or less, the adhesion strength of
`the layer can be further increased, and the initial charac-
`teristics and reliability of the solar cell are also further
`improved due to the following reason. When the mixing
`density of carbon and nitrogen atoms in the zinc oxide
`layer has a given distribution, the structure in the zinc
`oxide layer is changed to relax the stress caused by the
`formation of different types of deposition films on two
`sides of the zinc oxide layer and various external
`stresses, thereby effectively preventing the slight film
`peeling occurring during the manufacture and practical
`use.
`
`The series resistance characteristics of the solar cell
`manufactured by the technique of a present invention
`are better than those of a solar cell manufactured by the
`conventional technique. As a result, the fill factor can
`be increased, and hence the efficiency of photoelectric
`conversion can be improved because it is assumed that
`the carbon and nitrogen atoms contained in the zinc
`oxide layer effectively serve as doping materials to
`effectively decrease the resistivity of the zinc oxide
`layer.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a conceptual view for explaining a solar cell
`according to an embodiment of the present invention;
`FIG. 2 is a conceptual, view for explaining a solar cell
`according to another embodiment of the present inven-
`tion;
`FIG. 3 is a conceptual view for explaining a solar cell
`according to still another embodiment of the present
`invention;
`FIG. 4 is a conceptual view for explaining a solar cell
`according to still another embodiment of the present
`invention;
`FIG. 5 is a conceptual view illustrating the arrange-
`ment of a planar DC magnetron sputtering apparatus as
`a means for realizing the present invention;
`FIG. 6 is a conceptual view illustrating the arrange-
`ment of a pW plasma CVD apparatus as another means
`for realizing the present invention;
`FIG. 7 is a conceptual view illustrating the arrange-
`ment of an RF plasma CVD apparatus as still another
`means for realizing the present invention;
`FIG. 8 is a graph showing the relationship between
`the density of carbon atoms contained in a zinc oxide
`layer of a solar cell manufactured by the method of
`Example 1 and the efficiency of photoelectric conver-
`sion thereof;
`.
`FIG. 9 is a graph showing the relationship between
`the density of carbon atoms contained in a zinc oxide
`layer of a solar cell manufactured by the method of
`Example 1 and the standardized rate of degradation
`thereof;
`FIG. 10 is a graph showing the relationship between
`the density of carbon atoms contained in a zinc oxide
`layer of a solar cell manufactured by the method of
`Example 5 and the efficiency of photoelectric conver-
`sion thereof;
`FIG. 11 is a graph showing the relationship between
`the density of carbon atoms contained in a zinc oxide
`layer of a solar cell manufactured by the method of
`
`TSMC1013
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`IPR of U.S. Pat. No. 7,335,996
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`TSMC1013
`IPR of U.S. Pat. No. 7,335,996
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`

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`5,420,043
`
`7
`Example 5 and the standardized rate of degradation
`thereof;
`FIG. 12 is a graph showing the relationship between
`the density of carbon atoms contained in a zinc oxide
`layer of a solar cell manufactured by the method of 5
`Example 9 and the efficiency of photoelectric conver-
`sion thereof;
`FIG. 13 is a graph showing the relationship between
`the density of carbon atoms contained in a zinc oxide
`layer of a solar cell manufactured by the method of 10
`Example 9 and the standardized rate of degradation
`thereof;
`FIG. 14 is a graph showing the relationship between
`the density of carbon atoms contained in a zinc oxide
`layer of a solar cell manufactured by the method of 15
`Example 13 and the efficiency of photoelectric conver-
`sion thereof; and
`FIG. 15 is a graph showing the relationship between
`the density of carbon atoms contained in a zinc oxide
`layer of a solar cell manufactured by the method of 20
`Example 13 and the standardized rate of degradation
`thereof.
`
`8
`electrode 302. A pin type solar cell element 307 consist-
`ing of a p-type semiconductor layer 304, an i-type semi-
`conductor layer 305, and an n-type semiconductor layer
`306 is formed on the zinc oxide layer 303. A zinc oxide
`layer 308 containing carbon atoms, nitrogen atoms, or
`carbon and nitrogen atoms, and a lower or rear elec-
`trode 309 serving as the second electrode are sequen-
`tially formed on the pin type solar cell element 307.
`Light is incident through the transparent substrate 301.
`FIG. 4 is a view illustrating still another typical ex-
`ample of a solar cell according to the present invention.
`A solar cell 400 of this example is a so-called tandem
`cell in which pin type solar cell elements including two
`different semiconductor layers having different band
`gaps or film thicknesses as i-type layers are stacked to
`obtain a two-element structure. In the solar cell 400
`
`shown in FIG. 4, a zinc oxide layer 402 containing
`carbon atoms, nitrogen atoms, or carbon and nitrogen
`atoms is formed on a conductive substrate 401 serving
`as the first electrode. A first pin type solar cell element
`406 consisting of a first n-type semiconductor layer 403,
`a first i-type semiconductor layer 404, and a first p-type
`semiconductor layer 405 is formed on the zinc oxide
`layer 402. A second pin type solar cell element 410
`consisting of a first second n-type semiconductor layer
`407, a second i-type semiconductor layer 408, and a
`second p-type semiconductor layer 409 is formed on the
`first pin type solar cell element 406. A zinc oxide layer
`411 containing carbon atoms, nitrogen atoms, or carbon
`and nitrogen atoms, a transparent electrode 412 serving
`as the second electrode, and a collector electrode 413
`are sequentially formed on the second p-type semicon-
`ductor layer 409. Light is incident through the transpar-
`ent electrode 412.
`In any solar cell example described above, the stack-
`ing order of the n~ and p-type semiconductor layers can
`be reversed in accordance with an application purpose.
`However, the p-type semiconductor layer is preferably
`formed close to the light-incident side because the gen-
`erated carriers can then be effectively utilized.
`The constituent elements of these solar cells will be
`described below.
`Substrate
`
`Applicable materials for the conductive substrate 101
`can be exemplified by a plate or film consisting of a
`material selected from molybdenum, tungsten, titanium,
`cobalt, chromium,
`iron, copper,
`tantalum, niobium,
`zirconium, metal aluminum, and alloys thereof. In addi-
`tion, stainless steel, a nickel-chromium alloy, nickel,
`tantalum, niobium, zirconium, metal titanium, and/or
`alloys thereof can be preferably used in view of corro-
`sion resistance. A film or sheet of a synthetic resin (e.g.,
`polyester, polyethylene, polycarbonate, cellulose ace-
`tate, polypropylene, polyvinyl chloride, polyvinylidene
`chloride, polystyrene, or polyamide), glass, or ceramic
`on which one of the above metals and/or alloys thereof
`is formed by various methods such as deposition and
`sputtering can be used.
`The conductive substrate 101 can be used singly, but
`a layer (referred to as a reflective conductive layer)
`having reflection properties with respect to visible light
`and electrical conductivity is preferably formed on the
`conductive substrate 101 to utilize light having passed
`through the semiconductor layer without being ab-
`sorbed or to reduce the series resistance of the solar cell.
`Examples of applicable materials for the reflective con-
`ductive layer are silver, silicon, aluminum, iron, copper,
`nickel, chromium, molybdenum, and alloys thereof. Of
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`IPR of U.S. Pat. No. 7,335,996
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`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`30
`
`45
`
`Structures for realizing the present invention will be
`described with reference to the accompanying draw-
`mgs.
`FIG. 1 is a view illustrating a typical structure of a
`solar cell according to the present invention. In a solar
`cell 100 (FIG. 1) as a typical example of the present
`invention, a zinc oxide layer 102 containing carbon
`atoms, nitrogen atoms, or carbon and nitrogen atoms is
`formed on a conductive substrate 101 serving as the first
`electrode, a pin type solar cell element 106 consisting of 35
`an n-type semiconductor layer 103, an i-type semicon-
`ductor layer 104, and a p-type semiconductor layer 105
`is formed on the zinc oxide layer 102, and a transparent
`electrode 107 serving as the second electrode and a
`collector electrode 108 are sequentially formed on the
`pin type solar cell element 106. Light
`is incident
`through the transparent electrode 107.
`The solar cell according to the present invention is
`greatly different from the conventional solar cell in that
`a zinc oxide layer 102 containing carbon atoms, nitro-
`gen atoms, or carbon and nitrogen atoms is formed.
`FIG. 2 is a vi