Title of Invention: SOLAR CELL AND SOLAR CELL DEVICE
Technical Field
[0001]
The present invention relates to a solar cell using
carbon nanotubes and a solar cell device using the solar
cell.
Background Art
[0002]
The most common solar cells are monocrystalline,
polycrystalline, and amorphous silicon solar cells, which
have been spreading into homes and businesses. However,
these silicon solar cells are disadvantageously low in
energy conversion efficiency. One of the factors in the
disadvantage is that the common solar cell has only one
band gap. Long-wavelength light having energy below the
bad gap cannot undergo photoelectric conversion, but
conversely, short-wavelength light having energy above
the band gap can undergo photoelectric conversion for
only the amount of energy equivalent to the band gap.
[0003]
In order to deal with such a diseidvantage, a solar
cell having at least two different band gaps has been
proposed (for example, see Japanese Patent Application
Laid-Open Publication No. 2003-197930).
Summary of Invention
Technical Problem
[0004]
In order to increase the energy conversion efficiency
as described above, as shown in the Japanese Publication,

it is considered that a solar cell is provided with, at
least two band gaps, in other words, multiple band gaps.
[0005]
However, it is difficult for a semiconductor using
crystal such as silicon to obtain any band gaps, since
band gaps are fixed by the selection of elements
including compound semiconductors. Thus, the energy
conversion efficiency cannot be increased. Further, in
tandem or superposed solar cells, the solar cell in the
upper layer absorbs and scatters the sunlight, thereby
attenuating light required for the lower solar cells.
[0006]
Hence, an object of the present invention is to
provide a solar cell and a solar cell device which can
increase the energy conversion efficiency.
Solution to Problem
[0007]
In order to solve the above problems, a first aspect
of the present invention is a solar cell including: a
transparent electrode; a plurality of carbon nanotube
groups placed in parallel to each other on and
perpendicularly to the surface of the transparent
electrode, wherein each group contains a number of carbon
nanotubes,- and metal electrodes placed on the carbon
nanotube groups opposite to the transparent electrode,
wherein the diameters of the carbon nanotubes of the
carbon nanotube groups in parallel to each other are
varied stepwise from one side of the transparent
electrode to another side thereof.
[0008]
A second aspect of the present invention is the solar
cell according to the first aspect, wherein the
transparent electrode is formed as an n-type
semiconductor, and the group III atoms of the periodic

table are doped into the carbon nanotube groups to make
the carbon nanotube groups p-type semiconductors.
[0009]
A third aspect of the present invention is the solar
cell according to the first aspect, wherein the group V
atoms of the periodic table are doped into the
transparent-electrode-side portions of the carbon
nanotube groups to form n-type semiconductors, and the
group III atoms of the periodic table are doped into the
metal-electrode-side portions of the carbon nanotube
groups to form p-type semiconductors.
[0010]
A fourth aspect of the present invention is the solar
cell according to the first aspect, wherein the group V
atoms of the periodic table are doped into the carbon
nanotube groups to make the carbon nanotube groups n-type
semiconductors, and a p-type semiconductor layer is
placed between the metal electrodes and the carbon
nanotube groups.
[0011]
A fifth aspect of the present invention is a solar
cell device using the solar cell according to the first
aspect, the solar cell device including: a spectroscope
for splitting the sunlight being placed on the surface of
the transparent electrode of the solar cell; and a
voltage regulator for regulating electric power obtained
by the carbon nanotube groups of the solar cell to a
predetermined voltage.
Advantageous Effects of Invention
[0012]
In the above-described solar cell and solar cell
device, the carbon nanotubes are placed between the
transparent electrode and the metal electrodes, and the
diameters of the carbon nanotubes are varied stepwise.

Thus, it is possible to form the carbon nanotubes in
accordance with the respective wavelengths of, for
example, the split light beams of the sun light. That is
the carbon nanotubes have any band gaps. Accordingly,
since photoelectric conversion can be performed over the
wide wavelength range of the sunlight, it is possible to
provide a solar cell and a solar cell device having
excellent energy conversion efficiency, that is,
excellent photoelectric conversion efficiency.
Brief Description of Drawings
[0013]
[FIG. 1] FIG. 1 is a perspective view showing the
schematic configurations of a solar cell and a solar cell
device according to an embodiment of the present
invention.
[FIG. 2A] FIG. 2A is a perspective view illustrating a
method for manufacturing the solar cell.
[FIG. 2B] FIG. 2B is a perspective view illustrating the
method for manufacturing the solar cell.
[FIG. 2C] FIG. 2C is a perspective view illustrating the
method for manufacturing the solar cell.
[FIG. 2D] FIG. 2D is a perspective view illustrating the
method for manufacturing the solar cell.
[FIG. 3A] FIG. 3A is a graph illustrating photoelectric
conversion efficiency in the solar cell according to the
embodiment of the present invention.
[FIG. 3B] FIG. 3B is a graph illustrating photoelectric
conversion efficiency in a common tandem solar cell.
[FIG. 4] FIG. 4 is a perspective view showing the
schematic configuration of a solar cell according to a
first embodiment of the present invention.
[FIG. 5] FIG. 5 is a perspective view showing the
schematic configuration of a solar cell according to a
second embodiment of the present invention.

[FIG. 6] FIG. 6 is a perspective view showing the
schematic configuration of a solar cell according to a
third embodiment of the present invention.
[FIG. 7] FIG. 7 is a perspective view showing the
schematic configuration of a solar cell according to a
fourth embodiment of the present invention.
[FIG. 8] FIG. 8 is a perspective view showing the
schematic configuration of a modified example of the
solar cell according to the fourth embodiment.
Description of Embodiments
[0014]
The following will describe a solar cell and a solar
cell device according to an embodiment of the present
invention (corresponding to claims 1 and 5).
[0015]
First, the basic configurations of the solar cell and
the solar cell device using the solar cell will be
described.
[0016]
The solar cell basically includes: a transparent
electrode; a plurality of carbon nanotube groups placed
in parallel to each other on and perpendicularly to the
surface of the transparent electrode, wherein each group
contains a number of carbon nanotubes; and metal
electrodes as opposite electrodes placed on a side of the
carbon nanotube group opposite to the transparent
electrode. The diameters of the carbon nanotubes of the
carbon nanotube groups are varied stepwise from one side
of the transparent electrode to the other side thereof.
[0017]
In more detail, an n-type semiconductor and a p-type
semiconductor are arranged between a pair of electrodes,
and at least one of the semiconductors are composed of
carbon nanotubes (CNTs). Multiple (at least three) carbon

nanotube groups are provided in parallel to each other
perpendicularly to the electrode surface (so-called
perpendicular orientation). For example, the carbon
nanotube groups are separated into multiple areas and
lines, and the diameters of the carbon nanotubes are
sequentially varied in the respective groups.
[0018]
Specifically, five areas (may be at least three areas,
or at least three lines) containing multiple carbon
nanotubes (hereinafter, may be also referred to as tube
lines) are located in parallel to each other (side by
side), and the diameters of the carbon nanotubes are
varied stepwise in the respective tube lines. For example,
the carbon nanotubes are placed in order of decreasing
diameter.
[0019]
The electrodes are formed on the upper and lower
surfaces of the carbon nanotube groups in the respective
tube lines.
[0020]
As a matter of course, the perpendicular orientation
in which the above-described carbon nanotube groups are
formed has a tolerance. A line connecting the base to tip
of the carbon nanotube may be within the range of, for
example, 90° ± 10° relative to the electrode surface. In
other words, the carbon nanotube groups may be formed
substantially perpendicular to the electrode surface.
[0021]
Further, as described above, at least one of the
semiconductors are composed of carbon nanotubes. This
indicates that the electrodes on one side are formed as
semiconductors or a semiconductor layer is formed on one
electrode side, and the carbon nanotubes are formed as
semiconductors, or alternatively, an n-type semiconductor

and a p-type semiconductor are formed in the carbon
nanotube groups.
[0022]
The following will describe a method for
manufacturing the solar cell having the above basic
configuration with reference to FIGS. 1 and 2A to 2D.
[0023]
First, the specific configuration of the solar cell
will be described on the basis of FIG. 1.
[0024]
A solar cell 1 includes: a rectangular plate-like
transparent electrode 2 formed as an n-type
semiconductor; multiple carbon nanotube groups 3 arranged
in parallel to each other perpendicularly to the lower
surface of the transparent electrode 2; and metal
electrodes 4 placed as opposite electrodes on the lower
end surfaces (may be also referred to as the surfaces) of
the carbon nanotube groups 3. (The upper and lower
surfaces of the transparent electrode 2 are specified on
the basis of FIG. 1, and specifically, the incidence side
of the sunlight is set as the upper side. As a matter of
course, the upper and lower surfaces may be reversed, and
thus, the lower surface may be simply referred to as the
surface of the transparent electrode.) Specifically, the
carbon nanotube groups are arranged in at least three
(multiple) lines, herein, in five lines.
[0025]
A spectroscope (may be also referired to as a
spectroscopic element) 12 such as a prism for guiding the
light beams of the sunlight split in five wavelength
ranges to the solar cell 1, and a voltage regulator
(serving also as a voltage output circuit) 14 for
receiving electric power obtained by the carbon nanotube
groups 3 of the solar cell 1 arranged in lines via
electric wires 13 and regulating the electric power to a

predetermined voltage are provided to configure a solar
cell device 11, which will be specifically described
later. Further, a condensing lens unit 15 for condensing
the sunlight is placed in front of the spectroscope 12.
[0026]
For example, cylindrical lenses halving different
sizes are used in the condensing lens unit 15.
Specifically, the condensing lens unit 15 includes a
first cylindrical lens 15a having a large diameter and a
second cylindrical lens 15b having a small diameter. An
interval L between the cylindrical lenses is set to a
distance obtained by adding a focal length fl of the
first cylindrical lens 15a to a focal length f2 of the
second cylindrical lens 15b (L = fl + f2). Thus, the
sunlight having entered parallel to the first cylindrical
lens 15a enters the second cylindrical lens 15b after
being focused once, and then exits as parallel light. At
this point, the width of the outgoing parallel light is
reduced to f2 / fl. Further, the width of the outgoing
parallel light should be as small as possible.
[0027]
The spectroscope 12 allows the parallel light as a
split light beam to be incident on the solar cell 1
(exactly on the transparent electrode 2). Desirably, a
distance between the spectroscope 12 and the solar cell 1
is set such that the solar cell 1 is smaller in size than
the first cylindrical lens 15a. The condensing lens unit
15 having a plurality of cylindrical lenses can guide the
sunlight to the solar cell 1 without any waste. The shape
of the lens is not limited to this, and may be circular.
[0028]
Next, the method for manufacturing the solar cell 1
will be schematically described with reference to FIGS.
2A to 2D.
[0029]

First, as shown in FIG. 2A, a metal [for example,
iron (Fe)] thin layer serving as a catalyst is formed on
the surface of the transparent electrode 2 (in which
fluorine-doped tin oxide, zinc oxide, indium tin oxide,
fluorine-doped tin oxide / indium tin oxide, gallium-
doped zinc oxide, aluminum-doped zinc oxide or the like
is used) of the n-type semiconductor by, for example,
sputtering. After that, incisions are made across the
length and breadth of the transparent electrode 2 by an
electron beam with intervals adjusted therebetween, to
form catalytic nanoparticles 10. The iron catalytic
nanoparticles 10 are dimensioned in accordance with the
diameters of the carbon nanotubes 3 in the respective
tube lines. For example, the catalytic nanoparticles are
arranged from the left to right of the drawing in order
of decreasing diameter size. In other words, the
diameters of catalytic nanoparticles 10A on the left are
large while the diameters of catalytic nanoparticles 10B
to 10E are reduced stepwise toward the right.
[0030]
Further, in order to dimension the catalytic
nanoparticles 10 in accordance with the diameters of the
carbon nanotubes 3, the thickness of the thin layer
formed by, for example, sputtering may be gradually
reduced from the left to the right of the drawing. In the
case of sputtering, the thickness of the thin layer can
be changed depending on sputtering conditions (sputtering
time and a distance between a sputtering source and a
thin layer formation surface). Moreover, since a
continuous change of the distance between the sputtering
source and the thin layer formation surface can change
the thickness of the thin layer continuously, the
diameters of the carbon nanotubes 3 can be continuously
changed.
[0031]

' Next, as shown in FIG. 2B, the carbon nanotubes 3 are
formed on the iron catalytic nanoparticles 10 by the
thermal chemical vapor deposition method. At this point,
the diameters, that is, the thicknesses of the carbon
nanotubes 3 formed on the upper surfaces of the catalytic
nanoparticles 10 are determined depending on the sizes of
the catalytic nanoparticles 10.
[0032]
As shown in FIG. 2C, the group III atoms of the
periodic table are doped into the carbon nanotubes 3 (the
thermal chemical vapor deposition method may be
implemented with trace gas containing the group III
atoms).
[0033]
As shown in FIG. 2D, the metal electrodes 4 are
formed by the physical vapor deposition method on the
upper surfaces of the respective tube lines with masking
on the electrode boundary portion.
[0034]
The thermal chemical vapor deposition method will be
specifically described.
[0035]
The iron catalytic nanoparticles are formed on the
board of the transparent electrode, and the carbon
nanotubes are deposited by supplying source gas to the
catalytic nanoparticles as a nucleus in high temperature
environment. The catalytic nanoparticles may be made of
nickel or cobalt instead of iron.
[0036]
Specifically, after these metals or the solutions of
the compounds such as the complexes of these metals are
applied to the board of the transparent electrode by a
spray or brush or are beat on the board of the
transparent electrode by a cluster gun, drying and, as
required, heating are performed to form a layer.

The layer with an excessively large thickness is
hardly particulated by heating. Thus, it is preferable
that the thickness of the layer fall within the range of
1 ran to 100 nm.
[0038]
Next, when the layer is heated to the range of 650°C
to 800°C, preferably, under reduced pressure or non-
oxidizing atmosphere, iron catalytic nanoparticles having
diameters of about 0.1 nm to 5 0 nm are formed. Further,
the catalytic nanoparticles may be formed by sputtering
as described above. The source gas of the carbon
nanotubes may be aliphatic hydrocarbon such as acethylene,
methane, and ethylene, and acethylene is particularly
preferable. In acethylene, carbon nanotubes with
thicknesses of 0.4 nm to 38 nm are formed brush-like on
the transparent electrode with the iron catalytic
particles as a nucleus. The carbon nanotubes are
preferably formed at 6 50°C to 800°C, and are formed by
the thermal chemical vapor deposition method in 1 minute
to 30 minutes (hereinafter, will be referred to as CVD
time).
[0039]
The physical vapor deposition method for forming the
metal electrodes 4 is a vacuum deposition method or a
sputtering method.
[0040]
The following will describe the solar cell device 11
using the above-described solar cell 1.
[0041]
The spectroscope 12 is placed such that the split
light beams of the sunlight are irradiated on tube lines
3A to 3E of the solar cell 1. Thus, the split light beams
are guided onto the transparent electrode 2 in the tube

lines 3A to 3E for the respective wavelengths of the
split light beams.
[0042]
The voltage regulator 14 is connected to the metal
electrodes 4 via the electric wires 13, so that a
predetermined voltage can be obtained. Further, the
voltage regulator 14 includes DC-to-DC converters 16
connected to the tube lines 3 via the electric wires 13,
and an electricity adder 18 connected to the DC-to-DC
converters 16 via electric wires 17, so that electric
power at the predetermined voltage is outputted. The DC-
to-DC converters regulate (convert) voltages from the
tube lines 3A to 3E to be the same (the predetermined
voltage).
[0043]
Photoelectric conversion capability, that is, energy
band gap in the case that the carbon nanotubes of the
carbon nanotube groups 3 have different diameters will be
described.
[0044]
The carbon nanotubes having different diameters are
different in band gap. Thus, a carbon nanotube having a
band gap equal to energy hv of the split light beam of
the sunlight may be formed as a p-type or n-type
semiconductor.
[0045]
That is, in the case where there are n carbon
nanotubes having different band gaps (the band gaps are
represented by Ega to Egn, but Egn-! < Egn) , light having
energy which is smaller than Eg2 but not smaller than Egx
is received by the solar cell having the band gap Egi, so
that photoelectric conversion is performed. Further,
light having energy which is smaller than Eg3 but not
smaller than Eg2 is received by the solar cell having the
band gap Eg2, so that photoelectric conversion is

performed. Similarly, light having maximum energy which
exceeds the band gap Egn but does not exceed ultraviolet
light is received by the solar cell having the band gap
Egn/ so that photoelectric conversion is performed.
[0046]
FIG. 3A is a graph showing the amounts of energy
which can be photoelectrically converted by using the
solar cell having the above configuration. FIG. 3B shows,
as a comparative example, the case of a common tandem
solar cell of the related art. It can be noted from these
graphs that the amounts of energy obtained by varying the
diameters of carbon nanotubes stepwise, that is, the
amounts of photoelectrically convertible energy are
remarkably large.
[0047]
That is, in the configuration of the solar cell, the
carbon nanotubes are placed between the transparent
electrode and the metal electrodes, and the diameters of
the carbon nanotubes are varied stepwise. Thus, it is
possible to form carbon nanotubes having band gaps for
the respective wavelengths of, for example, the split
light beams of the sunlight. Accordingly, since
photoelectric conversion can be performed over the wide
wavelength range of the sunlight, it is possible to
provide a solar cell having excellent energy conversion
efficiency, that is, excellent photoelectric conversion
efficiency.
[0048]
The above-described solar cell device uses a single
solar cell. As a matter of course, multiple solar cells
can be provided to obtain a large amount of electric
power. In this case, the voltage regulators of the solar
cells can be integrated into a single unit.
[0049]

In the above description, the diameters of the carbon
nanotubes are adjusted in accordance with the sizes of
the catalytic nanoparticles. The diameters can be
controlled even by, for example, adjusting the CVD time.
[0050]
Specific examples, that is, embodiments of the solar
cell will be described below.
First Embodiment
[0051]
The following will describe a solcir cell according to
a first embodiment (corresponding to claims 1 and 2) of
the present invention.
[0052]
As shown in FIG. 4, a solar cell 21 includes: a
transparent electrode (for example, a fluorine-doped tin
oxide electrode) 22 formed as an n-type semiconductor; a
plurality of carbon nanotube groups 23 placed in parallel
to each other on and perpendicularly to the lower surface
(surface) of the transparent electrode 22; and metal
electrodes 24 placed as opposite electrodes on the lower
surfaces (surfaces) of the carbon nanotube groups 23
opposite to the transparent electrode 22. The diameters
of the carbon nanotubes of the side-by-side carbon
nanotube groups 23 are varied stepwise from one side of
the electrode 22 to the other side of the electrode 22,
and the group III atoms of the periodic table are doped
into the carbon nanotube groups 23 to form p-type
semiconductors.
[0053]
A method for manufacturing the solar cell 21 will be
described.
[0054]
Iron (Fe) catalytic nanoparticles (may be platinum or
cobalt nanoparticles instead) having different sizes are

formed on the surface of the transparent electrode 22 of
the n-type semiconductor. Further, the sizes are divided
into five stages, that is, the nanoparticles are provided
in five lines as described above.
[0055]
The sizes of the catalytic nanoparticles are varied
by the following two methods.
(1) After a metal thin layer as a catalyst is formed
on the transparent electrode, incisions are made across
the length and breadth of the transparent electrode by an
electron beam with intervals adjusted therebetween, to
form catalytic nanoparticles. In adjusting the intervals,
the sizes and shapes of the nanoparticles are adjusted by
heating.
(2) A metal thin layer as a catalyst is formed on the
surface of the transparent electrode by sputtering. In
forming the thin layer, a distance between the metal
source and the transparent electrode is varied, so that
catalytic nanoparticles having different diameters are
formed.
[0056]
Next, the carbon nanotube groups 2 3 are formed by the
thermal chemical vapor deposition method on the catalytic
nanoparticles formed on the surface of the transparent
electrode 22. In other words, the carbon nanotubes of the
carbon nanotube groups 2 3 are deposited.
[0057]
For example, when the transparent electrode 22 is
heated and supplied with pyrolytic hydrocarbon gas such
as pyrolytic C2H2 gas and CH4 gas, the carbon nanotubes of
the carbon nanotube groups 2 3 are deposited from the
catalytic nanoparticles. As a matter of course, the
diameters (thicknesses) of the deposited carbon nanotubes
of the carbon nanotube groups 23 depend on the sizes of
the catalytic nanoparticles.

As described above, the diameters of the carbon
nanotubes may be varied by changing the CVD time instead
of the sizes of the catalytic nanoparticles. Specifically,
when the CVD time is short, the carbon nanotubes are thin
with a low number of layers, and when the CVD time is
long, the carbon nanotubes are thick with multiple layers.
Thus, the thicknesses of the carbon nanotubes can be
adjusted by the CVD time. Further, the thick carbon
nanotubes have a multi-layered structure, but the band
gap of the carbon nanotube of -the outermost layer is used.
[0059]
Next, the group III atoms of the periodic table (such
as boron, aluminum, gallium, indium, and titanium) are
doped into the carbon nanotube groups 23 to form p-type
semiconductors. In detail, gas containing the group III
atoms such as diborane (B2H6) gas is thermally decomposed
and sprayed to the carbon nanotube gruops 23. In the step
of forming the carbon nanotube groups 23, a trace of the
gas containing the group III atoms may be mixed in the
hydrocarbon gas.
[0060]
Next, when the metal electrodes 24 are formed for the
respective carbon nanotube groups 2 3 having carbon
nanotubes with different diameters, that is, the
respective tube lines, the solar cell 21 can be obtained.
[0061]
Specifically, after masking is applied to separate
the respective tube lines, metal such as copper, gold,
silver, and aluminum is attached to the upper ends of the
carbon nanotube groups 23, to which masking cannot be
applied, by the physical vapor deposition method. Instead
of the physical vapor deposition method, thermal vacuum
deposition, deposition with an electron beam, and
sputtering may be adopted.

Second Example
[0062]
The following will describe a solar cell according to
a second embodiment of the present invention
(corresponding to claim 2).
[0063]
As shown in FIG. 5, a solar cell 31 includes: a
transparent board 32 formed of silicon dioxide (Si02) ; a
transparent electrode (for example, a fluorine-doped tin
oxide electrode) 3 3 formed as an n-type semiconductor on
the lower surface (surface) of the transparent board 32;
a plurality of carbon nanotube groups 34 placed in
parallel to each other on and perpendicularly to the
lower surface (surface) of the transparent electrode 33;
and metal electrodes 35 placed as opposite electrodes on
the lower surfaces (surfaces) of the carbon nanotube
groups 34 opposite to the transparent electrode 33. The
diameters of the carbon nanotubes of the carbon nanotube
groups 34 in parallel to each other are varied, for
example, are reduced stepwise from one side toward the
other side, and the group III atoms of the periodic table
are doped into the carbon nanotube groups 34 to form p-
type semiconductors.
[0064]
A method for manufacturing the solar cell 31 will be
described.
[0065]
The manufacturing method will be schematically
described since it is basically the same as that in the
first embodiment.
[0066]
First, the transparent electrode 33 of the n-type
semiconductor is formed on the surface of the transparent
board 32 of silicon dioxide.

[0067]
Next, catalytic nanoparticles having different sizes
such as iron, platinum, and cobalt are formed on the
surface of the transparent electrode 33. The sizes of the
catalytic nanoparticles are divided into five stages as
described above, that is, the catalytic nanoparticles
having different sizes are formed in five lines.
[0068]
The carbon nanotube groups 34 are formed, by the
thermal chemical vapor deposition method, on the
catalytic nanoparticles formed on the surface of the
transparent electrode 3 3 by the same method as in the
first embodiment.
[0069]
The group III atoms such as boron, aluminum, gallium,
indium, and titanium are doped into the carbon nanotube
groups 34 to form p-type semiconductors.
[0070]
The metal electrodes 35 are formed for the respective
tube lines having different diameters by, for example,
sputtering to obtain the solar cell 31.
[0071]
A brief description of the manufacturing method is
given as follows:
[0072]
In the method for manufacturing the solar cell, a
plurality of carbon nanotube groups are formed in
parallel to each other on and perpendicularly to the
surface of the transparent electrode of the n-type
semiconductor such that the diameters of the carbon
nanotubes of the carbon nanotube groups are varied
stepwise from one side of the electrode to the other side
of the electrode, the group III atoms of the periodic
table are doped into the carbon nanotube groups to form
p-type semiconductors, and metal electrodes are formed on

the end surfaces of the carbon nanotube groups. Contrary
to the above description, a transparent electrode may be
formed by sputtering on the end surfaces of carbon
nanotubes formed beforehand.
Third Embodiment
[0073]
The following will describe a solar cell according to
a third embodiment of the present invention
(corresponding to claim 3).
[0074]
As shown in FIG. 6, a solar cell 41 includes: a
transparent board 42 formed of silicon dioxide (Si02) ; a
transparent electrode (for example, a fluorine-doped tin
oxide electrode) 4 3 formed as an n-type semiconductor on
the lower surface (surface) of the transparent board 42;
a plurality of carbon nanotube groups 44 placed in
parallel to each other on and perpendicularly to the
lower surface (surface) of the transparent electrode 43;
and metal electrodes 45 placed as opposite electrodes on
the lower surfaces (surfaces) of the carbon nanotube
groups 44 opposite to the transparent electrode 43. The
group V atoms of the periodic table are doped into
transparent-electrode-side portions 44a of the carbon
nanotube groups 44 to form n-type semiconductors, and the
group III atoms of the periodic table are doped into
metal-electrode-side portions 44b of the carbon nanotube
groups 44 to form p-type semiconductors. Further, the
diameters of the carbon nanotubes of the carbon nanotube
groups 44 in parallel to each other are varied stepwise
from one side of the electrode toward the other side of
the electrode.
[0075]
The following will describe a method for
manufacturing the solar cell 41.

[0076]
First, the transparent electrode 43 is formed by-
sputtering on the surface of the transparent board 42 of
silicon dioxide.
[0077]
Next, iron catalytic nanoparticles are formed on the
surface of the transparent electrode 4 3 by sputtering.
[0078]
The carbon nanotube groups 44 are then formed on the
catalytic nanoparticles by, for example, the thermal
chemical vapor deposition method. At this point, a trace
of phosphine (PH3) is added to form the transparent-
electrode-side portions 44a of the carbon nanotube groups
44 as the n-type semiconductors.
[0079]
Further, the p-type semiconductor portions 44b are
formed on the end surfaces of the n-type semiconductor
portions 44a. At this point, a trace of diborane (B2H6)
is added to form p-type semiconductors. In other words,
the metal-electrode-side portions 44b of the carbon
nanotube groups 44 are formed as the p-type
semiconductors.
[0080]
The metal electrodes 4 5 are formed of, for example,
aluminum for the respective carbon nanotube groups 44
having carbon nanotubes with different diameters, that is,
the respective tube lines.
[0081]
In the third embodiment, the transparent electrode 43
is placed on the surface of the transparent board 42, but
as a matter of course, only the transparent electrode may
be provided.
[0082]
A brief description of the manufacturing method is
given as follows:

[0083]
In the method for manufacturing the solar cell, a
plurality of carbon nanotube groups are formed in
parallel to each other between the transparent electrode
on one side and the metal electrodes on the other side
and perpendicularly to the surfaces of the electrodes
such that the diameters of the carbon nanotubes of the
carbon nanotube groups are varied stepwise from one side
to the other side. The group V atoms of the periodic
table are doped into the transparent-electrode-side
portions of the carbon nanotube groups to form n-type
semiconductors, and the group III atoms of the periodic
table are doped into the metal-electrode-side portions of
the carbon nanotube groups to form p-type semiconductors.
Fourth Embodiment
[0084]
The following will describe a solar cell according to
a fourth embodiment of the present invention
(corresponding to claim 4).
[0085]
As shown in FIG. 7, a solar cell 51 includes: a metal
electrode 52 made of SUS (JIS code of stainless steel) or
the like,- a p-type semiconductor board (p-type
semiconductor layer) 53 placed on the surface of the
metal electrode 52 and doped with, for example, boron (B)
atoms of the group III of the periodic table; a plurality
of carbon nanotube groups 54 placed in parallel to each
other on and perpendicularly to the surface of the p-type
semiconductor board 53; and transparent electrodes (such
as fluorine-doped tin oxide electrodes) 55 formed as
opposite electrodes on the surfaces of the carbon
nanotube groups 54 opposite to the metal electrode 52.
The diameters of the carbon nanotubes of the carbon
nanotube groups 54 in parallel to each other are varied

' stepwise from one side to the other side, and the carbon
nanotubes 53 are doped with, for example, phosphorous (P)
atoms of the group V of the periodic table to form n-type
semiconductors.
[0086]
A method for manufacturing the solar cell 51 will be
described.
[0087]
The manufacturing method will be schematically-
described since it is basically the same as that in the
first embodiment.
[0088]
For example, on the surface of the metal electrode 52
formed of a rectangular stainless steel (SUS) plate or
the like, the p-type semiconductor board 53 is formed by
doping, for example, boron (B) atoms of the group III of
the periodic table into a board of silicon or the like.
[0089]
Next, iron catalytic nanoparticles (may be platinum
or cobalt instead) having different diameters are formed
on the surface of the p-type semiconductor board 53. As
described above, the sizes of the catcilytic nanoparticles
are divided into five stages, that is, the catalytic
nanoparticles having different sizes eire provided in five
lines.
[0090]
The carbon nanotube groups 54 are formed, by the
thermal chemical vapor deposition method, on the
catalytic nanoparticles formed on the surface of the p-
type semiconductor board 53 by the same method as in the
first embodiment.
[0091]
The group V atoms of the periodic table such as
phosphorus atoms are doped into the carbon nanotube
groups 54 to form the n-type semiconductors.

[0092]
The transparent electrodes 55 are then formed on the
respective tube lines having different diameters by, for
example, sputtering. Thus, the solar cell 51 can be
obtained.
[0093]
In the fourth embodiment, as described above, since
the transparent electrodes 55 are formed after the carbon
nanotube groups 54 are formed, it is possible to use
indium tin oxide which is difficult to apply at high
temperature in the thermal chemical vapor deposition
method.
[0094]
A brief description of the manufacturing method is
given as follows:
[0095]
In the method for manufacturing the solar cell, a p-
type semiconductor layer is formed on the surface of the
metal electrode, a plurality of carbon nanotube groups
are formed in parallel to each other on and
perpendicularly to the surface of the p-type
semiconductor layer such that the diameters of the carbon
nanotubes of the carbon nanotube groups are varied
stepwise from one side of the metal electrode to the
other side of the metal electrode, the group V atoms of
the periodic table are doped into the carbon nanotube
groups to form n-type semiconductors, and transparent
electrodes are formed on the end surfaces of the carbon
nanotube groups.
[0096]
Instead of the metal electrode 52 and the p-type
semiconductor board 53 of the solar cell according to the
fourth embodiment, as shown in FIG. 8, a metal electrode
62 formed of copper or the like and a metal board 63

formed of molybdenum or the like can be used to obtain a
solar cell 61.
[0097]
In the above-described embodiments, the groups of
carbon nanotubes having different band gaps (that is,
different diameters) are sequentially formed on the
surface of the electrode. However, the groups of carbon
nanotubes formed beforehand for the respective different
band gaps on the surface of the electrode may be
sequentially combined to manufacture a solar cell.
Industrial Applicability
[0098]
According to the present invention, since the carbon
nanotube groups are placed between the transparent
electrode and the metal electrodes and the diameters of
the carbon nanotubes are varied stepwise, it is possible
to form carbon nanotubes having any band gaps in
accordance with the respective wavelengths of, for
example, split light beams of the sunlight. Thus,
photoelectric conversion can be performed over the wide
wavelength range of the sunlight, so that it is possible
to provide a solar cell excellent in energy conversion
efficiency.

[Claim 1]
A solar cell comprising:
a transparent electrode;
a plurality of carbon nanotube groups placed in
parallel to each other on and perpendicularly to a
surface of the transparent electrode, wherein each group
contains a number of carbon nanotubes; and
metal electrodes placed on the carbon nanotube group
opposite to the transparent electrode, wherein
diameters of the carbon nanotubes of the carbon
nanotube groups in parallel to each other are varied
stepwise from one side of the transparent electrode to
another side thereof.
[Claim 2]
The solar cell according to claim 1, wherein the
transparent electrode is formed as an n-type
semiconductor, and group III atoms of a periodic table
are doped into the carbon nanotube groups to form p-type
semiconductors.
[Claim 3]
The solar cell according to claim 1, wherein group V
atoms of a periodic table are doped into transparent-
electrode-side portions of the carbon nanotube groups to
form n-type semiconductors, and group III atoms of the
periodic table are doped into metal-electrode-side
portions of the carbon nanotube groups to form p-type
semiconductors.
[Claim 4]
The solar cell according to claim 1, wherein group V
atoms of a periodic table are doped into the carbon
nanotube groups to form n-type semiconductors, and a p-
type semiconductor layer is placed between the metal
electrodes and the carbon nanotube groups.
[Claim 5]

A solar cell device using the solar cell according to
any one of claims 1 to 4, the solar cell device
comprising: a spectroscope for splitting sunlight being
placed on the surface of the transparent electrode of the
solar cell; and a voltage regulator for regulating
electric power obtained by the carbon nanotube groups of
the solar cell to a predetermined voltage.

ABSTRACT

Provided is a solar cell including: a transparent
electrode (2) formed as an n-type semiconductor; a
plurality of carbon nanotube groups (3) placed in
parallel to each other on and perpendicularly to the
lower surface of the transparent electrode (2); and metal
electrodes (4) placed on the lower surfaces of the carbon
nanotube groups (3) opposite to the electrode (2). The
diameters of the carbon nanotubes of the carbon nanotube
groups (3) in parallel to each other are varied from one
side of the electrode (2) to the other side thereof, and
the group III atoms are doped into the carbon nanotube
groups to form p-type semiconductors.