U.S. patent application number 13/216977 was filed with the patent office on 2012-02-23 for solar cell equipped with electrode having mesh structure, and process for manufacturing same.
This patent application is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Koji Asakawa, Akira Fujimoto, Ryota Kitagawa, Kumi Masunaga, Tsutomu Nakanishi, Hideyuki Nishizawa, Eishi Tsutsumi.
Application Number | 20120042946 13/216977 |
Document ID | / |
Family ID | 42739384 |
Filed Date | 2012-02-23 |
United States Patent
Application |
20120042946 |
Kind Code |
A1 |
Masunaga; Kumi ; et
al. |
February 23, 2012 |
SOLAR CELL EQUIPPED WITH ELECTRODE HAVING MESH STRUCTURE, AND
PROCESS FOR MANUFACTURING SAME
Abstract
The embodiment provides a solar cell and a manufacturing process
thereof. The solar cell is equipped with an electrode on the light
incident surface side; and the electrode has both low resistivity
and high transparency, can efficiently utilize solar light for
excitation of carriers, and can be made of inexpensive materials.
The solar cell comprises a photoelectric conversion layer, a first
electrode layer arranged on the light incident surface side, and a
second electrode layer arranged opposed to the first electrode
layer. The first electrode layer has a thickness in the range of 10
to 200 nm, and has plural penetrating openings. Each of the
individual openings occupies an area in the range of 80 nm.sup.2 to
0.8 .mu.m.sup.2, and the aperture ratio thereof is in the range 10
to 66%. The first electrode layer in the cell can be produced by
etching procedure using an etching mask obtained by use of a single
particle layer of fine particles, by use of a dot pattern formed by
self-assembly of a block copolymer, or by use of a stamper.
Inventors: |
Masunaga; Kumi;
(Kawasaki-Shi, JP) ; Fujimoto; Akira;
(Kawasaki-Shi, JP) ; Nakanishi; Tsutomu; (Tokyo,
JP) ; Tsutsumi; Eishi; (Kawasaki-Shi, JP) ;
Kitagawa; Ryota; (Tokyo, JP) ; Asakawa; Koji;
(Kawasaki-Shi, JP) ; Nishizawa; Hideyuki; (Tokyo,
JP) |
Assignee: |
Kabushiki Kaisha Toshiba
Minato-ku
JP
|
Family ID: |
42739384 |
Appl. No.: |
13/216977 |
Filed: |
August 24, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP09/71460 |
Dec 24, 2009 |
|
|
|
13216977 |
|
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Current U.S.
Class: |
136/256 ;
257/E31.124; 438/98 |
Current CPC
Class: |
H01L 31/0236 20130101;
H01L 31/022433 20130101; Y02E 10/50 20130101 |
Class at
Publication: |
136/256 ; 438/98;
257/E31.124 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; H01L 31/18 20060101 H01L031/18 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 18, 2009 |
JP |
2009-066146 |
Claims
1. A solar cell comprising a photoelectric conversion layer
containing at least a p-type semiconductor and an n-type
semiconductor, a first electrode layer made of metal and formed on
the light incident side surface of said photoelectric conversion
layer, and a second electrode layer formed on the surface opposite
to said light incident side surface; wherein said first electrode
layer has a thickness in the range of 10 nm to 200 nm, said first
electrode layer has plural penetrating openings each of which
occupies an area in the range of 80 nm.sup.2 to 0.8 .mu.m.sup.2,
and the aperture ratio, which is a ratio of the total area of said
openings based on the total area of said first electrode layer, is
in the range of 10% to 66%.
2. The solar cell according to claim 1, wherein said photoelectric
conversion layer comprises a depletion layer at least partly
positioned within a distance of 1 .mu.m from the interface between
said first electrode layer and said photoelectric conversion
layer.
3. The solar cell according to claim 1, wherein said openings are
so arranged that the average distance between adjacent openings is
in the range of 10 nm to 200 nm.
4. The solar cell according to claim 1, wherein said first
electrode layer is made of at least one metal selected from the
group consisting of aluminum, silver, gold, platinum, nickel,
cobalt, chromium, copper and titanium.
5. A process for manufacturing a solar cell, comprising the steps
of forming a photoelectric conversion layer, forming a first
electrode layer on the light incident side surface of said
photoelectric conversion layer, and forming a second electrode
layer on the side opposite to the light incident side surface of
said photoelectric conversion layer; wherein said step of forming a
first electrode layer comprises the sub-steps of forming a metal
thin layer, coating a resist composition on at least a part of said
metal thin layer, to form a resist layer, forming a single particle
layer of fine particles on the surface of said resist layer,
etching said resist layer by use of said single particle layer as
an etching mask, to form a resist pattern, filling openings in said
resist pattern with inorganic substance, to form a reverse pattern
mask, and etching said thin metal layer by use of said reverse
pattern mask as an etching mask, to form the first electrode layer
having fine openings.
6. The process according to claim 5, wherein said fine particles
are silica particles.
7. A process for manufacturing a solar cell, comprising the steps
of forming a photoelectric conversion layer, forming a first
electrode layer on the light incident side surface of said
photoelectric conversion layer, and forming a second electrode
layer on the side opposite to the light incident side surface of
said photoelectric conversion layer; wherein said step of forming a
first electrode layer comprises the sub-steps of forming a thin
metal layer, coating a block copolymer-containing composition on at
least a part of said thin metal layer, to form a block copolymer
layer, causing phase separation of said block copolymer, to form
microdomains in a dot pattern, and etching said thin metal layer by
use of said dot pattern of microdomains as an etching mask, to form
the first electrode layer having fine openings.
8. The process according to claim 7, wherein said block copolymer
is a diblock copolymer of polystyrene-polymethyl methacrylate.
9. A process for manufacturing a solar cell, comprising the steps
of forming a photoelectric conversion layer, forming a first
electrode layer on the light incident side surface of said
photoelectric conversion layer, and forming a second electrode
layer on the side opposite to the light incident side surface of
said photoelectric conversion layer; wherein said step of forming a
first electrode layer comprises the sub-steps of forming a thin
metal layer, preparing a stamper whose surface has a fine relief
pattern corresponding to the shape of the first electrode layer
intended to be formed, transferring a resist pattern onto at least
a part of said thin metal layer by use of said stamper, and etching
said thin metal layer by use of said resist pattern as an etching
mask, to form the first electrode layer having fine openings.
10. The process according to claim 9, wherein said stamper is
produced by use of electron beam exposure.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No.
2009-066146, filed on Mar. 18, 2009, the entire contents of which
are incorporated herein by reference.
FIELD
[0002] The present embodiment relates to a solar cell equipped with
an electrode having mesh structure, and also to a process for
manufacturing the same.
BACKGROUND
[0003] Solar cells directly convert inexhaustible and clean
pollution-free solar energy into electrical energy, and hence they
can be said to be important key devices in view of the
environmental and energy exhaustion problems.
[0004] In general, a solar cell comprises a light incident surface
side electrode arranged on the side where solar light enters, a
counter electrode, and a semiconductor photoelectric conversion
layer sandwiched between the electrodes. The photoelectric
conversion layer now industrially produced is commonly made of
silicon (Si), and the solar cell using Si normally includes a PN or
PIN junction of monocrystalline Si, polycrystalline Si or amorphous
Si (hereinafter, often referred to as "a-Si"). Besides that, there
are also practical solar cells using compound semiconductors such
as GaAs and chalcopyrite. The light incident-side electrodes
adopted in many solar cells are comb-shaped metal electrodes, which
are called "finger electrodes". However, solar cells using
semiconductors having large surface resistivity, such as solar
cells of a-Si type, are often equipped with not finger electrodes
but transparent electroconductive films as the light incident-side
electrodes.
[0005] At present, the largest problem of solar cells is to
increase the photoelectric conversion efficiency. The photoelectric
conversion efficiency of solar cells is generally in the range of
about 10 to 15%. In order to increase the conversion efficiency,
various improvements have been hitherto made. Those improvements
are, for example, in that an antireflection film is formed and/or
the light receiving surface is made to have a texture structure so
as to reduce the reflection loss and in that a getter layer or a
surface passivation film is provided so as to prevent the carrier
recombination in the bulk or on the surface. Further, the
improvements particularly for enhancing the light-receiving
efficiency are, for example, in that the semiconductor layer is
thickened and/or made of materials having large light-absorption
coefficients and in that the effective incident area is enlarged by
adopting an embedded electrode or a back electrode type solar
cell.
[0006] It is also studied to improve the electrode structure for
the sake of increasing the light transmittance and/or the
conversion efficiency.
[0007] Those prior improvements, which, for example, aim at
enlarging the effective incident area, are mainly for the purpose
of increasing the light transmittance, and hence they by no means
increase the conversion efficiency of the absorbed solar light for
carrier excitation. The conversion efficiency, therefore, is not
significantly improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 shows conceptual sketches of solar cells according to
one embodiment of the present invention.
[0009] FIG. 2 is a schematic sectional view of a solar cell
according to one embodiment of the present invention.
[0010] FIG. 3 is a schematic sectional view illustrating the
working principle of a solar cell according to one embodiment of
the present invention.
[0011] FIG. 4 shows conceptual drawings illustrating the working
principle of a solar cell according to one embodiment of the
present invention.
[0012] FIG. 5 shows simulation results of the electric
field-enhancement effect in a solar cell.
[0013] FIG. 6 shows simulation results of the electric
field-enhancement effect in a solar cell.
[0014] FIG. 7 shows simulation results of the electric
field-enhancement effect in a solar cell.
[0015] FIG. 8 shows schematic sectional views illustrating a
process for manufacturing a solar cell according to one embodiment
of the present invention.
[0016] FIG. 9 shows schematic sectional views illustrating another
process for manufacturing a solar cell according to one embodiment
of the present invention.
[0017] FIG. 10 shows schematic sectional views illustrating still
another process for manufacturing a solar cell according to one
embodiment of the present invention.
[0018] FIG. 11 shows schematic sectional views illustrating yet
another process for manufacturing a solar cell according to one
embodiment of the present invention.
DETAILED DESCRIPTION
[0019] Embodiments will now be explained with reference to the
accompanying drawings.
[0020] One embodiment resides in a solar cell comprising
[0021] a photoelectric conversion layer containing at least a
p-type semiconductor and an n-type semiconductor,
[0022] a first electrode layer made of metal and formed on the
light incident side surface of said photoelectric conversion layer,
and
[0023] a second electrode layer formed on the surface opposite to
said light incident side surface; wherein
[0024] said first electrode layer has a thickness in the range of
10 nm to 200 nm,
[0025] said first electrode layer has plural penetrating openings
each of which occupies an area in the range of 80 nm.sup.2 to 0.8
.mu.m.sup.2, and
[0026] the aperture ratio, which is a ratio of the total area of
said openings based on the total area of said first electrode
layer, is in the range of 10% to 66%.
[0027] First, the working principle of the embodiment is described
in detail. FIG. 1 illustrates structures of solar cells according
to one embodiment of the present invention. Each solar cell in FIG.
1 has a structure comprising a first electrode 101 arranged on the
light incident surface side, a second electrode 102 arranged
opposite to the first electrode, and a photoelectric conversion
layer 103 sandwiched between the electrodes. As the photoelectric
conversion layer usable for the solar cell, various types of
semiconductor layers are known and any of them can be selected to
use. Examples of them include pn-junction type, pin type and tandem
structure type layers made of monocrystalline Si, polycrystalline
Si, amorphous Si, compound semiconductors such as GaAs, and
chalcopyrite-type semiconductors. The solar cell according to one
embodiment of the present invention is partly characterized in that
the first electrode 101 formed on the light-receiving surface is a
thin metal film having openings 104 in a mesh structure. There is
no particular restriction on the arrangement of the openings, and
the openings may be positioned either regularly as shown in FIG.
1(a) or randomly as shown in FIG. 1(b).
[0028] FIG. 2 schematically shows a vertical sectional view of the
solar cell shown in FIG. 1. As illustrated in FIG. 2, since the
first electrode 101 is made of metal, the photoelectric conversion
layer in the areas covered with the metal does not transmit light
and the light coming to those areas is reflected. Consequently, the
light penetrates only through the openings and reaches the
photoelectric conversion layer in the areas not covered. This means
that the photoelectric conversion layer generally receives light in
an amount corresponding to the area ratio of the openings based on
the whole electrode surface. The photoelectric conversion layer is
therefore generally thought to generate electric current in
proportion to the amount of the received light.
[0029] Surprisingly, however, the present inventors have actually
found that the first electrode having a particular structure
enables to increase the electric current more than expected from
the amount of light received by the photoelectric conversion
layer.
[0030] This phenomenon can be presumed to be caused by the
following mechanism. It is already known that, when a thin metal
film having fine openings is exposed to light, surface plasmons are
excited under the condition that the openings have diameters
corresponding to the wavelength of the incident light. FIG. 3 shows
a conceptual drawing illustrating behavior of the plasmons. When a
thin metal film receives light, free electrons in the film are
induced to oscillate perpendicularly to the light propagation
direction. However, the oscillation of free electrons is not
uniform in the thickness direction. The nearer to the surface
irradiated with the light the free electrons are positioned, the
more easily they are oscillated. Accordingly, in the thin metal
film, the electron density on the upper side 301 differs from that
on the lower side 304 at the edges, so that alternating electric
fields 302 oscillating along the thickness are generated at the
edges. As a result, those electric fields extend into the
photoelectric conversion layer, so that the electric field as a
whole is enhanced at areas 303 right under the peripheries of the
openings. The electric field-enhancement effect thus given by the
particular electrode is explained below by referring to FIG. 4 in
comparison with a case where a conventional electrode is used. FIG.
4(a) conceptually illustrates an electric field and electron-hole
separation caused thereby in a solar cell comprising a conventional
electrode such as a comb-shaped electrode. In the solar cell
equipped with a conventional electrode, an electric field is
generated by incident light coming onto the light incident surface
but it becomes weaker according to the depth from the light
incident surface. On the other hand, FIG. 4(b) illustrates an
electric field in a solar cell according to the embodiment. As
described above, the electric field is enhanced at edges of the
thin metal film and hence extends deeply into the photoelectric
conversion layer. It is thought that this enhanced electric field
prevents recombination of carriers to improve the photoelectric
conversion efficiency.
[0031] The solar cell according to one embodiment of the present
invention comprises a mesh metal electrode as the first electrode
layer on the light incident surface side, so that light penetrating
through the openings can be photoelectrically converted and also so
that the electric field can be enhanced near the edges of the fine
openings. In this way, a great amount of carriers are presumed to
be excited to increase the power generation efficiency. In other
words, according to one embodiment of the present invention, the
photoelectric conversion is also promoted even by light coming to
the metal part of the first electrode, namely, even by light not
reaching the photoelectric conversion layer.
[0032] With respect to a solar cell according to one embodiment of
the present invention, the electric field strength was estimated on
the basis of a simulation performed by use of the finite difference
time domain method (hereinafter, referred to as "FDTD method"). The
simulation was carried out under the assumptions that the
photoelectric conversion layer was made of Si and that the first
electrode was a 30 nm-thick aluminum film provided with openings
(opening diameter: 140 nm, period (interval between centers of
adjacent openings): 200 nm). The results were shown in FIG. 5,
which verified that enhanced electric fields were generated at the
edges of the first electrode. Further, FIG. 6 shows the results of
another simulation performed under the assumption that 500
nm-light, which is included in the solar light spectrum, was
applied in air onto the first electrode side of a solar cell
comprising a Si photoelectric conversion layer and a 50 nm-thick
aluminum first electrode provided with periodically arranged
openings. The results indicate that the z-component of the electric
field is constant if the opening size (slit width) is more than a
certain value but that the electric field is enhanced at the edges
of the surface electrode if the openings have a particular
size.
[0033] Further, still another simulation was carried out to
estimate how the distance between two adjacent openings, namely,
the length of the minimum metal first electrode part between two
adjacent openings (hereinafter referred to as "electrode-width
between adjacent openings"), is related to the strength of local
electric fields at the edges of the electrode. The results are
shown in FIG. 7, which indicates that the electric field strength
has a peak in a particular range of the distance between two
adjacent openings. That is because, if the electrode-width between
adjacent openings is less than 10 nm on average, alternating
electric fields appearing along the thickness at both ends of each
electrode part are cancelled out by each other and hence are
incapable of enhancing the electric field. On the other hand, if
the electrode-width between adjacent openings is more than 200 nm
on average, the above alternating electric fields do not interact
with each other and hence the electric field has constant strength.
Further, in order that the electrode may have sufficient
electroconductivity, the electrode-width between adjacent openings
needs to be 10 nm or more.
[0034] Accordingly, in the first electrode proposed by the
embodiment, the length of the minimum metal electrode part between
two adjacent openings is preferably from 10 nm to 200 nm, more
preferably from 30 nm to 100 nm, on average.
[0035] In order that light may penetrate though the first
electrode, it is advantageous for the openings to occupy large
areas in the first electrode. On the other hand, however, for the
purpose of keeping high electroconductivity, the openings
preferably occupy small areas. Form those viewpoints, the ratio of
the total area of the openings based on that of the first electrode
layer, namely, the aperture ratio, is necessarily in the range of
10% to 66%, preferably in the range of 25% to 66%.
[0036] Under the condition that the distance between adjacent
openings is within the above range, the total length of the edges,
namely, the total peripheral length of the openings is preferably
long enough to further enhance the electric field per unit area.
Specifically, if the openings are in the form of circles having
predetermined diameters and are periodically placed, the number of
the openings can be increased and accordingly the total peripheral
length can be extended by shorting the distance between adjacent
openings, so as to strengthen the electric field-enhancement
effect. On the other hand, if the openings are in the form of
circles and are placed so periodically that the electrode-width
between adjacent openings may have a predetermined length, the
number of the openings can be increased and accordingly the total
peripheral length can be extended by reducing the opening
diameters, so as to strengthen the electric field-enhancement
effect.
[0037] The openings are, however, not necessarily placed
periodically, and may be positioned in any arrangement such as
periodical, pseudo-periodical or random arrangement. Since the
openings in any arrangement can give the effect of the embodiment,
there is no particular restriction on how periodically the openings
are arranged. The shapes of the openings are also not restricted to
circles. In view of the electric field-enhancement effect,
star-shaped or figure-C-shaped openings are advantageous rather
than circular ones because the total peripheral length of those
openings is longer than that of circular openings. On the other
hand, however, circular openings have the advantage of easily
producing the electrode.
[0038] As described above, the electric field-enhancement effect
depends both on the distance between adjacent openings and on the
shapes of openings. However, the results of the simulations
according to the FDTD method indicate that each individual opening
occupies necessarily an area in the range of 80 nm.sup.2 to 0.8
.mu.m.sup.2, preferably an area in the range of 1000 nm.sup.2 to
0.03 .mu.m.sup.2. If the openings are circles in shape, the opening
diameter (diameter of opening) is preferably in the range of 10 nm
to 1000 nm, more preferably in the range of 40 nm to 200 nm.
[0039] Further, the first electrode layer needs to have a thickness
of 10 nm to 200 nm. If the thickness is less than 10 nm, the metal
layer has too high a resistivity to keep sufficient
electroconductivity and accordingly the photoelectric conversion
efficiency is lowered. It is, therefore, unfavorable. On the other
hand, if the thickness is more than 200 nm, the photoelectric
conversion layer is often incapable of benefiting from the electric
field-enhancement effect sufficiently to improve the conversion
efficiency. It is, therefore, also unfavorable.
[0040] As described above, the particular structure of the first
electrode enhances the electric field at the edges of the first
electrode (namely, at the peripheral areas of the openings). This
electric field-enhancement effect works on a semiconductor layer
and a depletion layer in the photoelectric conversion layer, so as
to improve the photoelectric conversion efficiency. The depletion
layer is, therefore, necessarily placed within a short distance
from the first electrode. Accordingly, the depletion layer is at
least partly positioned within a distance of preferably 1 .mu.m or
less, more preferably 500 nm or less from the interface between the
first electrode and the photoelectric conversion layer.
[0041] In the above description, the solar cell according to one
embodiment of the present invention is explained from the viewpoint
of the structure. The structure of the solar cell can be made of
any materials freely selected from conventionally known ones.
[0042] The first electrode on the light incident surface side in
the embodiment can be made of any known metal, which can be freely
selected to use. Here, the "metal" means a material which is an
electroconductive simple substance, which has metallic gloss, which
has malleability, which consists of metal atoms and which is solid
in room temperature; or an alloy thereof. In the embodiment, the
electric field-enhancement effect is caused when the
electromagnetic wave penetrates into the metal electrode, and hence
the metal electrode in the embodiment is preferably made of a
material in which electrons can oscillate in accordance with
oscillation of the incident light electric field. This means that
the material in the form of a bulk body having a flat surface
preferably reflects light in the wavelength range of solar light.
Further, the material preferably less absorbs light in the
wavelength range intended to be used. Examples of the material
include aluminum, silver, gold, platinum, nickel, cobalt, chromium,
copper, and titanium. Among them, preferred are aluminum, silver,
platinum, nickel and cobalt. However, these examples by no means
restrict the material as long as the material is a metal having a
lower plasma frequency than the incident light. It is, therefore,
unnecessary to use rare metals such as indium and hence typical
metal materials are usable in the embodiment.
[0043] In a solar cell most popularly used at present, the
photoelectric conversion layer comprises p-type semiconductor and
n-type semiconductor. Accordingly, the conversion layer preferably
comprises p-type and n-type semiconductors so that it can be
produced easily at low cost. In view of availability, the
semiconductor is preferably silicon such as monocrystalline
silicon, polycrystalline silicon or amorphous silicon. For example,
layers of p-type crystal silicon and n-type crystal silicon are
laminated to form a pn-junction type photoelectric conversion
layer. The p-type/n-type crystal silicon may be in any form such as
single crystal, poly-crystal, fine crystallite or amorphous solid.
However, the single crystal silicon has the advantage of high
photoelectric conversion efficiency, while the poly-crystal silicon
has the advantage of low production cost. Further, it is also
possible to use a pin-junction type photoelectric conversion layer
in which layers of p-type amorphous silicon, i-type undoped
amorphous silicon and n-type amorphous silicon are laminated in
order. This photoelectric conversion layer has the advantages that
it can be produced at low cost and that the output power is hardly
lowered even at a high temperature.
[0044] The material of the photoelectric conversion layer is not
restricted to silicon, and may be a III-V group compound
semiconductor such as GaAs, a II-VI group compound semiconductor or
a chalcopyrite-type compound semiconductor. The structure of the
conversion layer is also not restricted to the laminate type
described above, and may be a hetero-junction type, a fine particle
type, a tandem type, a dot type or a junction type. In the
embodiment, there is no particular restriction on the structure of
the photoelectric conversion layer.
[0045] The second electrode opposite to the first one may be made
of any material as long as it can have an ohmic contact with the
contiguous semiconductor. For example, materials usable for the
first electrode are also usable for the second one.
[0046] Meanwhile, there are various studies for increasing the
photoelectric conversion efficiency of the solar cell. For example,
it has been studied to improve an antireflection layer or to modify
the bottom structure of the photoelectric conversion layer. Those
techniques can be combined with the solar cell according to one
embodiment of the present invention unless they impair the effect
of the embodiment.
[0047] As another embodiment of the present invention, the process
for producing the solar cell is described below.
[0048] The solar cell produced in the embodiment comprises a
photoelectric conversion layer, a first electrode layer formed on
the photoelectric conversion layer, and a second electrode layer
formed on the opposite surface. There is no particular restriction
on the order of forming those layers, which may be formed by either
of: [0049] (1) a method in which the photoelectric conversion layer
is formed, and then the first electrode is formed on one surface
thereof, and finally the second electrode is formed on the other
surface; and [0050] (2) a method in which semiconductor is
accumulated on the first electrode or on the second electrode to
form the photoelectric conversion layer, and then the second
electrode or the first electrode, respectively, is formed
thereon.
[0051] The photoelectric conversion layer can be formed by any
method according to the semiconductor intended to be used. For
example, a substrate of p-type or n-type semiconductor is partly
doped with impurities, or otherwise another semiconductor layer is
formed on the substrate by vapor-deposition. Further, the
photoelectric conversion layer can be also obtained by the steps of
forming an electrode layer on a transparent substrate by, for
example, vapor deposition, and then laminating a p-type, n-type or
i-type semiconductor layer thereon.
[0052] The solar cell according to an embodiment of the present
invention is characterized by the first electrode having openings.
The structure of the first electrode can be formed by the steps of:
first forming a thin metal layer on a surface of the conversion
layer, and then boring the openings. In a different way, a thin
metal film beforehand provided with openings may be laminated on
the photoelectric conversion layer.
[0053] For forming fine openings on the first electrode, any method
can be selected to use. For example, in a generally known method,
an etching procedure is carried out by use of an electron beam
exposure system capable of forming a super-fine structure. However,
if this method is adopted, there is a fear that the production cost
increases. In contrast, the fine openings can be formed at low cost
according to the following methods:
[0054] (A) a method comprising the steps of:
[0055] coating a resist on a thin metal film intended to be an
electrode, to form a resist layer;
[0056] forming a single particle layer of fine particles on the
resist layer,
[0057] etching the resist layer by use of the single particle layer
as an etching mask, to form a resist pattern having openings
corresponding to the aimed fine openings,
[0058] filling the openings in the resist pattern with inorganic
substance, to form a reverse pattern mask, and
[0059] etching the thin metal film by use of the reverse pattern
mask, to form fine openings;
[0060] (B) a method comprising the steps of:
[0061] coating a block copolymer-containing composition on a thin
metal film intended to be an electrode, to form a block copolymer
layer,
[0062] forming microdomains of the block copolymer in a dot
pattern, and
[0063] etching the thin metal film by use of the dot pattern of the
formed microdomains, to form fine openings; and
[0064] (C) a method comprising the steps of:
[0065] preparing a stamper whose surface has a fine relief pattern
corresponding to the shape of the first electrode intended to be
formed,
[0066] transferring a resist pattern onto a thin metal film
intended to be an electrode by use of the stamper, and
[0067] forming the pattern on the thin metal film by use of the
resist pattern.
[0068] Further, the first electrode can be also produced by another
process in which a pattern of resist or of inorganic substance is
directly formed on the photoelectric conversion layer before the
thin metal layer is formed, and then metal is accumulated on spaces
in the pattern by vapor deposition and the like.
[0069] The embodiment is further explained by the following
examples, which by no means restrict the embodiment.
Example 1
Solar Cell Using Monocrystalline Si
[0070] The manufacturing process and characteristics of a
monocrystalline Si type solar cell are explained in the following
example by referring to FIG. 8.
[Production Procedure of Photoelectric Conversion Layer]
[0071] First, the procedure for producing a photoelectric
conversion layer of monocrystalline Si is described below.
[0072] As shown in FIG. 8(a), a p-type silicon substrate 601 of
monocrystalline Si is prepared as a semiconductor substrate. In
Example 1, an ingot of silicon doped with boron as an impurity was
obtained according to the Czochralski pulling method, and then
sliced with a multi-wire saw to prepare a p-type silicon substrate
601 of monocrystalline Si having a thickness of 540 .mu.m and a
specific resistance of about 8 .OMEGA.cm. The silicon substrate 601
was then thinned down to 380 .mu.m by mechanical polishing. In the
embodiment, the semiconductor substrate may be made of
polycrystalline Si and the silicon may be doped with generally
known impurities other than boron.
[0073] Thereafter, an n.sup.+ layer 602 containing many n-type
impurity elements such as phosphorus is formed on one of the major
faces of the p-type semiconductor substrate 601. The n.sup.+ layer
602 can be formed by a thermal diffusion method in which the
semiconductor substrate 601 is placed in a high temperature gas
containing phosphorus oxychloride (POCl.sub.3) so that n-type
impurity elements such as phosphorus can be diffused into one of
the major faces of the substrate 601. As the result of the thermal
diffusion method, the n.sup.+ layer 602 may be formed on both faces
and ends of the p-type semiconductor substrate 601. In that case,
in order to remove the n.sup.+ layer 602 formed on the unwanted
surface, the p-type semiconductor substrate 601 may be immersed in
a fluoro-nitric acid solution after the layer 602 formed on the
aimed surface is covered with an acid-resistant resin. In Example
1, the n.sup.+ layer 602 was formed by a thermal diffusion method
in which the semiconductor substrate 601 was placed in POCl.sub.3
atmosphere at 1100.degree. C. for 15 minutes. The formed n.sup.+
layer 602 had a sheet resistivity of about 50 .OMEGA./square.
[0074] Subsequently, the n.sup.+ layer 602 on the aimed face was
covered with an acid-resistant resin, and then the p-type
semiconductor substrate 601 was immersed in a fluoro-nitric acid
solution for 15 seconds to remove the n.sup.+ layer 602 not covered
with the resin. After that, the acid-resistant resin was removed to
obtain the n.sup.+ layer 602 on only one of the major faces of the
p-type semiconductor substrate 601. The resultant n.sup.+ layer 602
had a thickness of 500 nm.
[0075] Although the n.sup.+ layer was thus formed on the p-type
semiconductor substrate 601 in the present example, any other
processes may be used to form a pn junction.
[0076] On the other surface of the p-type semiconductor substrate
601, Au/Zn was vapor-deposited in vacuum to form a second electrode
layer 604. This second electrode layer 604 of Au/Zn functions not
only as a second electrode but also as an anti-reflection layer
[0077] Thereafter, a first electrode 605A having fine openings is
formed on the sunlight-incident side surface of the n.sup.+ layer
602.
[Production of First Electrode Having Mesh Structure]
[0078] As the first electrode having fine openings, an aluminum
electrode having mesh structure was formed on the n.sup.+ layer
602. The present inventors have developed a process comprising the
steps of: forming a single particle layer in which fine particles
are aligned in a closest packing arrangement on a substrate; and
shaving the aligned nano-particles by etching to a desired size, so
as to form a dot pattern. The formed dot pattern is transferred
onto a thin metal layer 605, which can be used as the first
electrode 605A having fine openings. This method for forming a
first electrode is described below in detail.
[0079] First, on a major face of the n.sup.+ layer provided on the
silicon substrate, aluminum was vapor-deposited in vacuum to form a
thin metal layer 605 of 50 nm thickness (FIG. 8(a)).
[0080] Independently, an i-line positive thermosetting resist (THMR
IP3250 [trademark], manufactured by Tokyo Ohka Kogyou Co., Ltd.)
was diluted with ethyl lactate by 1:1. After filtrated through 0.2
.mu.m-mesh filter, the solution was spin-coated on the thin metal
layer 605 at 2000 rpm for 60 seconds and then heated on a hot-plate
at 110.degree. C. for 90 seconds, and further heated at 270.degree.
C. for 1 hour in an oxidation-free inert oven under nitrogen
gas-atmosphere to undergo a thermosetting reaction. The resist
layer 606 thus formed had a thickness of approx. 240 nm.
[0081] The resist layer 606 was then subjected to reactive etching
for 3 seconds under the conditions of O.sub.2: 30 sccm, 100 mTorr
and a RF power of 100 W by means of a reactive etching system
(RIE-200L [trademark], manufactured by SAMCO Inc.), and thereby the
surface was made hydrophilic (FIG. 8(b)). The hydrophilized surface
functioned in the following step as a trap layer for catching fine
silica particles. The trap layer may be formed, for example, by
coating the resist layer with an organic polymer.
[0082] Subsequently, a dispersion solution of fine silica particles
having a size of 200 nm (PL-13 [trademark], manufactured by Fuso
Chemical Co., Ltd.) was diluted with an acryl polymer-containing
composition to 5 wt %, and filtrated through a 1 .mu.m-mesh filter
to prepare a coating solution of fine silica particle dispersion
609. The solution was spin-coated at 2000 rpm for 60 seconds on the
above resist-coated substrate (FIG. 8(c)), and then the substrate
was annealed at 150.degree. C. for 1 hour in an oxidation-free
inert oven under nitrogen gas-atmosphere. Thereafter, the substrate
was cooled to room temperature, and thereby a single particle layer
of regularly arranged fine silica particles was formed on the
hydrophilized resist layer (FIG. 8(d)). Although fine silica
particles were adopted as the fine particles in the present
example, any organic or inorganic fine particles can be used as
long as they can be etched in a rate different from the resist
layer, as described later. The size of the fine particles depends
on the pattern of the first electrode, but is generally 60 to 700
nm.
[0083] The single particle layer of fine silica particles was
subjected to etching for 2 minutes under the conditions of
CF.sub.3: 30 sccm, 10 mTorr and a RF power of 100 W (FIG. 8(e)), to
reduce the size of the particles and accordingly to expand
intervals among the particles. The etching conditions were so
selected that the underlying resist layer might not undergo the
etching. Since the particles and the resist layer are etched in
different rates, it is possible to etch only the silica particles
so as to form intervals among them. After the above procedure, the
single particle layer was observed by electron microscopy to find
that the size of the fine silica particles 608A and the intervals
among them were about 120 nm and about 80 nm, respectively.
[0084] Thereafter, the remaining silica particles were used as a
mask while the underlying thermosetting resist layer was subjected
to etching for 270 seconds under the conditions of O.sub.2: 30
sccm, 2 mTorr and a RF power of 100 W.
[0085] As a result, columnar structures of high aspect ratios were
formed in the areas where the etched silica particles had been
previously positioned, to obtain a columnar resist pattern 606A of
high aspect ratios (FIG. 8(f)).
[0086] Independently, a spin-on-glass (hereinafter, referred to as
SOG) solution (SOG-14000 [trademark], manufactured by Tokyo Ohka
Kogyou Co., Ltd.) was filtrated through 0.3 .mu.m-mesh filter. The
SOG solution was then spin-coated at 2000 rpm for 40 seconds on the
obtained columnar resist pattern, so that the intervals among the
columns of the resist pattern were filled with SOG. After that, the
substrate was heated on a hot-plate at 110.degree. C. for 90
seconds and further heated at 250.degree. C. for 1 hour in an
oxidation-free inert oven under nitrogen gas-atmosphere.
[0087] Subsequently, the formed SOG layer and fine silica particles
included therein were etched for 11 minutes under the conditions of
CF.sub.3: 30 sccm, 10 mTorr and a RF power of 100 W, and thereby
the remaining silica particles and excess SOG covering the columnar
resist pattern were removed to form a columnar resist pattern
including SOG 609 filling the intervals among the columns 606A
(FIG. 8(g)).
[0088] The remaining columns 606A of thermosetting resist were then
etched for 150 seconds under the conditions of O.sub.2: 30 sccm, 10
mTorr and a RF power of 100 W, so that a SOG mask 609A having a
pattern structure in reverse to the above columnar resist pattern
was formed on the metal thin layer 605 (FIG. 8(h)).
[0089] After that, the metal thin layer 605 was etched through the
SOG mask 609A by means of ICP-RIE system (manufactured by SAMCO
Inc.). In general, when an aluminum film is exposed to air, a few
nanometer-thick Al.sub.2O.sub.3 layer is immediately formed
thereon. Therefore, the metal thin layer 605 was first subjected to
sputter-etching for 1 minute under the conditions of Ar: 25 sccm, 5
mTorr, an ICP power of 50 W and a Biass power of 150 W to remove
Al.sub.2O.sub.3, and was then etched for 50 seconds under the
conditions of Cl.sub.2/Ar mixed gas: 2.5/25 sccm, 5 mTorr, an ICP
power of 50 W and a Biass power of 150 W.
[0090] Thereafter, the remaining SOG mask 609A was removed by
etching for 150 seconds by means of a reactive etching system under
the conditions of CF.sub.3: 30 sccm, 10 mTorr and a RF power of 100
W (FIG. 8(i)).
[Shape of First Electrode Having Mesh Structure]
[0091] The above procedure gave a 50 nm-thick surface electrode
605A on the aforementioned n.sup.+ layer. The electrode 605A had a
mesh structure provided with openings having an average opening
area of 9.8.times.10.sup.-3 .mu.m.sup.2 (opening diameter: 112 nm)
and an average aperture ratio of 28.4%. The transmittance of the
produced first electrode was measured at an incident light
wavelength of 500 nm, and found to be about 39%. The resistivity
thereof was also found to be about 107.3 .mu..OMEGA.cm.
[Characteristics of First Electrode Having Mesh Structure]
[0092] The solar cell produced above in Example 1 was exposed to
simulated solar light of AM1.5, to evaluate the photoelectric
conversion efficiency at room temperature. As a result, the
efficiency was found to be as high as 6.1%. Further, it was also
verified that the effect of the embodiment was obtained even if the
first electrode was made of metals other than aluminum.
Comparative Example 1
[0093] In the same manner as in Example 1, a comparative solar cell
was produced. The comparative solar cell was equipped with a first
electrode whose thickness and average aperture ratio were the same
as those of the first electrode in Example 1 but whose openings had
an average diameter of 2 .mu.m (average opening area: 3.1
.mu.m.sup.2), namely, about twenty times as large an average
diameter as those in the first electrode in Example 1. Since having
large diameters, the openings were made by use of photolithographic
technology. The produced solar cell was evaluated in the same
manner as in Example 1, and was found to have a photoelectric
conversion efficiency of 3.6%.
Example 2
Solar Cell Using Polycrystalline Si
[0094] Example 2 explains the manufacturing process of a
polycrystalline Si type solar cell. The process for a
polycrystalline Si type solar cell is similar to that for a
monocrystalline Si type one described above in Example 1.
[0095] First, an ingot of silicon material was sliced with a
multi-wire saw to prepare a 400 .mu.m-thick p-type semiconductor
substrate of polycrystalline Si. Since mechanically damaged in the
slicing procedure, the substrate surface was washed by etching with
NaOH. The substrate was then placed in a diffusion furnace and
heated under oxychloride (POCl.sub.3) atmosphere at 1100.degree. C.
for 30 minutes, so that phosphorus atoms were diffused into the
surface of the semiconductor substrate to form an n-type
semiconductor area having a sheet resistivity of 60 .OMEGA./square.
Thus, a pn-junction was formed in the wafer.
[0096] Thereafter, the whole back surface of the substrate was
coated with aluminum paste and heated to form a p.sup.+ layer and a
second electrode. Subsequently, on the light incident side opposite
to the second electrode, a first electrode having mesh structure of
aluminum was formed in the same manner as in Example 1.
[0097] With respect to the polycrystalline Si type solar cell thus
produced, the photoelectric conversion efficiency was evaluated in
the same manner as in Example 1. As a result, the efficiency was
found to be as high as 5.8%. Further, it was also verified that the
effect of the embodiment was obtained even if the first electrode
was made of metals other than aluminum.
Comparative Example 2
[0098] In the same manner as in Example 2, a comparative solar cell
was produced. The comparative solar cell was equipped with a first
electrode whose thickness and average aperture ratio were the same
as those of the first electrode in Example 2 but whose openings had
an average diameter of 2 .mu.m (average opening area: 3.1
.mu.m.sup.2), namely, about twenty times as large an average
diameter as those in the first electrode in Example 2. Since having
large diameters, the openings were made by use of photolithographic
technology. The produced solar cell was evaluated in the same
manner as in Example 2, and was found to have a photoelectric
conversion efficiency of 3.7%.
Example 3
Solar Cell Using Amorphous Si
[0099] The manufacturing process of an amorphous Si type solar cell
is explained in this example by referring to FIG. 9.
[0100] In the first step, a 50 nm-thick thin metal layer of
aluminum was vapor-deposited on a transparent quartz substrate 701
and then processed by use of fine particles in the same manner as
in Example 1 to form a first electrode 702 having mesh structure of
aluminum (FIG. 9(a)).
[0101] Subsequently, the transparent substrate 701 was treated in a
plasma CVD apparatus, so that a p-layer 703 of p-type Si, an
i-layer 704 of i-type Si and an n-layer 705 of n-type Si were
successively formed and accumulated thereon in order from
PH.sub.3/SiH.sub.4 mixed gas, SiH.sub.4 gas and
B.sub.2H.sub.6/SiH.sub.4 mixed gas, respectively, to form a
pin-type photoelectric conversion layer 706 (FIG. 9(b)).
Independently, the procedure was repeated except for not forming
the i-layer, to form a pn-type photoelectric conversion layer.
Thereafter, a second electrode layer 707 was formed on the n-layer
in each conversion layer by means of sputtering apparatus from a
silver alloy containing aluminum (FIG. 9(c)).
[0102] With respect to the amorphous Si type solar cells thus
produced, the photoelectric conversion efficiency was evaluated in
the same manner as in Example 1. As a result, the efficiency was
found to be as high as 4.6% in the pin-type and 5.8% in the
pn-type. The reason why the pn-type layer gave a higher efficiency
than the pin type one is thought to be because a depletion layer,
in which carries were excited, in the pn-type layer was positioned
nearer to the first electrode than in the pin type one.
Example 4
Solar Cell Using Chalcopyrite-Type Compound Semiconductor
[0103] This example explains the manufacturing process of a
chalcopyrite-type compound semiconductor solar cell.
[0104] In the first step, a Mo electrode was formed on a substrate
of soda-lime glass by vacuum vapor deposition to produce a lower
electrode. The lower electrode may be made of metals other than
molybdenum, for example, titanium and tungsten.
[0105] Subsequently, copper (Cu), indium (In) and gallium (Ga) were
sputtered to form a layer called "precursor". The precursor was
then annealed at 400 to 600.degree. C. under hydrogen selenide
(H.sub.2Se) gas atmosphere in a furnace, and was thereby converted
into a CIGS layer.
[0106] As for formation of the photoelectric conversion layer, some
techniques are developed. For example, Cu, In, Ga and Se may be
vapor-deposited to form a layer, and then annealed. Accordingly,
the embodiment is not restricted to the above manner and any method
can be adopted to form the photoelectric conversion layer.
[0107] Thereafter, a first electrode having fine openings was
provided on the formed CIGS layer. The first electrode was formed
in the same manner as in Example 1.
[0108] With respect to the chalcopyrite-type compound semiconductor
solar cell thus manufactured, the photoelectric conversion
efficiency was evaluated in the same manner as in Example 1. As a
result, the efficiency was found to be as high as 7.3%.
Example 5
Solar Cell Using GaAs
[0109] This example explains a compound semiconductor type solar
cell using GaAs.
[0110] According to the MOCVD (metal organic chemical vapor
deposition) method, an n.sup.+-type layer was epitaxially grown on
a p-type GaAs wafer to produce a cell. Subsequently, a first
electrode having fine openings and a second electrode were formed
in the same manner as in Example 1. With respect to the
thus-produced compound semiconductor type solar cell using GaAs,
the photoelectric conversion efficiency was evaluated in the same
manner as in Example 1. As a result, the efficiency was found to be
as high as 6.3%.
Example 6
Manufacturing Process Using Block Copolymer
[0111] This example explains the manufacturing process of a
monocrystalline Si type solar cell equipped with a first electrode
having openings formed by use of phase-separation of block
copolymer. The photoelectric conversion layer of monocrystalline Si
was formed in the same manner as in Example 1.
[0112] The present inventors have developed a method in which a
pattern structure having dots arranged in a period of 50 to 70 nm
is formed by use of phase-separation of block copolymer and then is
used to form a first electrode having mesh structure. The method is
described below.
[0113] By referring to FIG. 10, the following description explains
the manufacturing process of an electrode having aluminum-made
nano-mesh structure formed according to the method using a block
copolymer.
[0114] First, an n.sup.+ layer 802 was formed on one surface of a
p-type silicon substrate 801 in the same manner as in Example 1, to
produce a photoelectric conversion layer 803. After that, Au/Zn was
vapor-deposited in vacuum to form a second electrode layer 804 on
the p-type layer.
[0115] Independently, an i-line positive thermosetting resist (THMR
IP3250 [trademark], manufactured by Tokyo Ohka Kogyou Co., Ltd.)
was diluted with ethyl lactate by 1:3, to prepare a resist
composition. The resist composition was spin-coated on the n.sup.+
layer of the photoelectric conversion layer 803 on the
light-receiving side, and then heated at 250.degree. C. for 1 hour
in an oxidation-free inert oven under nitrogen gas-atmosphere to
undergo thermosetting reaction and thereby to form a resist layer
805 (FIG. 10(a)).
[0116] Subsequently, an SOG (SOG-5500 [trademark], manufactured by
Tokyo Ohka Kogyou Co., Ltd.) was diluted with ethyl lactate. The
diluted SOG was spin-coated on the resist-coated substrate at 2000
rpm for 45 seconds, and then heated at 250.degree. C. for 1 hour in
an oxidation-free inert oven under nitrogen gas-atmosphere to form
a SOG layer 806 (FIG. 10(b)).
[0117] Thereafter, a diblock copolymer of polystyrene
(PS)-polymethyl methacrylate (PMMA) was mixed with PMMA (Mw: 1500)
in an weight ratio of 6:4. The mixed polymer was dissolved in
propylene glycol monomethyl ether acetate (PGMEA) in an amount of 3
wt %. The solution was spin-coated on the above substrate at 2000
rpm for 30 seconds, and then pre-baked at 110.degree. C. for 90
seconds to evaporate the solvent and thereby to form a layer of 120
nm thickness.
[0118] The formed layer was then annealed at 210.degree. C. for 4
hours under nitrogen gas-atmosphere, to cause phase separation
between PS and PMMA and thereby to form a block copolymer layer
807. The diblock copolymer had molecular weights of 78000 g/mol at
the PS part and 170000 g/mole at the PMMA part, and hence gave a
morphology in which PS forms a dot pattern of microdomains 807B
having diameters of about 50 to 90 nm in PMMA matrix 807A (FIG.
10(c)).
[0119] The block copolymer layer 807 was subjected to etching so as
to selectively remove the PMMA matrix. The etching conditions were
O.sub.2: 30 sccm, 100 mTorr and a RF power of 100 W. In this step,
the SOG layer in the area right under the matrix 807A was
completely bared (FIG. 10(d)). After that, the SOG layer was then
subjected to etching of CF.sub.4-RIE by use of the remaining PS as
a mask. As a result of this etching procedure, the dot pattern of
PS is transferred onto the SOG layer to form a SOG pattern
corresponding to the phase separation of block copolymer (FIG.
10(e)). Subsequently, the underlying thermosetting resist layer was
subjected to etching of O.sub.2-RIE by use of the SOG pattern as a
mask, to form a columnar pattern 805B which had high aspect ratio
columns at the positions previously occupied by PS in the former
step (FIG. 10(f)).
[0120] On the obtained columnar pattern, aluminum was
vapor-deposited in a thickness of 30 nm. Thereafter, the substrate
was subjected to ashing treatment with O.sub.2 plasma, and then
immersed in water. The substrate in water was subjected to
ultrasonic washing for lift-off treatment, so as to remove the
columnar pattern. As a result, a first electrode 808 having desired
openings was formed on the photoelectric conversion layer.
[0121] The surface electrode formed by the above procedure had a
thickness of 30 nm, an average opening area of 2.0.times.10.sup.3
nm.sup.2 (opening diameter: 50 nm) and an average aperture ratio of
52%. The transmittance of the produced aluminum nano-mesh electrode
was measured at an incident light wavelength of 500 nm. As a
result, it was found that the transmittance and the resistivity
were about 50% and about 30 .mu..OMEGA.cm, respectively.
[0122] With respect to the solar cell thus produced, the
photoelectric conversion efficiency was evaluated in the same
manner as in Example 1. As a result, the efficiency was found to be
as high as 6.9%. Further, it was also verified that the effect of
the embodiment was obtained even if the first electrode was made of
metals other than aluminum.
Example 7
Manufacturing Process by Nano-Imprint
[0123] By referring to FIG. 11, this example explains the
manufacturing process of monocrystalline Si type solar cell
equipped with an electrode having fine openings formed according to
a nano-imprint method. The photoelectric conversion layer of
monocrystalline Si was formed in the same manner as in Example
1.
[0124] First, an n.sup.+ layer 902 was formed on one surface of a
p-type silicon substrate 901 in the same manner as in Example 1, to
produce a photoelectric conversion layer 903. After that, Au/Zn was
vapor-deposited in vacuum to form a second electrode layer 904 on
the p-type layer of the photoelectric conversion layer. On the
n.sup.+ layer of the photoelectric conversion layer 903, aluminum
is vapor-deposited in vacuum to form a thin metal layer 905 having
50 nm thickness (FIG. 11(a)).
[0125] Independently, an i-line positive thermosetting resist (THMR
IP3250 [trademark], manufactured by Tokyo Ohka Kogyou Co., Ltd.)
was diluted with ethyl lactate by 1:2, to prepare a resist
composition. The resist composition was spin-coated on the thin
metal layer 905 at 3000 rpm for 35 seconds, and then heated at
110.degree. C. for 90 seconds on a hot-plate to undergo
thermosetting reaction and thereby to form a resist layer 906 (FIG.
11(b)). The resist layer had a thickness of about 150 nm.
[0126] Subsequently, a fine relief pattern corresponding to the
openings proposed by the embodiment is transferred onto the resist
layer 906 by use of a stamper 907 as a mold.
[0127] In this example, a quartz plate was subjected to an electron
beam lithographic process to prepare a stamper having a surface
structure in which holes of 120 nm depth and 130 nm diameter were
aligned in a closest packing arrangement of 200 nm period.
[0128] In the process for manufacturing a solar cell according to
the embodiment, there is no particular restriction on the materials
of the stamper and on how to make the fine relief structure on the
stamper surface. For example, the stamper can be formed according
to the above-described method employing fine particles or block
copolymer.
[0129] The stamper was then subjected to release treatment. The
surface of the stamper was coated with a fluorine-type release
agent such as perfluoropolyether so that the stamper might have
such a low surface energy as to improve the releasability.
[0130] Thereafter, the stamper was pressed onto the above-described
resist layer by means of a heater plate press (N4005-00 type
[trademark], manufactured by NPa SYSTEM Co., Ltd.) at a temperature
of 128.degree. C. under 60 kN, and then gradually cooled for 1 hour
to room temperature. After that, the stamper was vertically
released therefrom, so that a pattern in reverse to the relief
pattern of the stamper was transferred on the resist layer (FIG.
11(c)). Thus, the procedure gave a periodical opening resist
pattern having periodically arranged columns 906A of 130 nm
diameter (FIG. 11(d)).
[0131] The embodiment is not restricted to the thermal
nano-imprinting process described above. Various imprinting
techniques such as photo imprint and soft imprint can be used to
form the above pattern, and they by no mean impair the functions of
the solar cell according to the embodiment.
[0132] The above aluminum layer was etched through the formed
resist pattern as an etching mask by means of ICP-RIE system
(manufactured by SAMCO Inc.). In the same manner as in Example 1,
the aluminum layer was first subjected to sputter-etching for 1
minute under the conditions of Ar: 25 sccm, 5 mTorr, an ICP power
of 50 W and a Biass power of 150 W to remove Al.sub.2O.sub.3, and
was successively etched for 80 seconds under the conditions of
Cl.sub.2/Ar mixed gas: 2.5/25 sccm, 5 mTorr, an ICP power of 50 W
and a Biass power of 150 W.
[0133] The above procedure gave a first electrode 905A of aluminum
nano-mesh structure having a thickness of 50 nm, an average opening
area of 1.3.times.10.sup.-2 .mu.m.sup.2 (opening diameter: 130 nm)
and an average aperture ratio of 35.4%. The transmittance of the
produced first electrode was measured at an incident light
wavelength of 500 nm. As a result, it was found that the
transmittance and the resistivity were about 47% and about 30
.mu..OMEGA.cm, respectively.
[0134] With respect to the solar cell thus produced, the
photoelectric conversion efficiency was evaluated in the same
manner as in Example 1. As a result, the conversion efficiency was
found to be as high as 6.4%. Further, it was also verified that the
effect of the embodiment was obtained even if the first electrode
was made of metals other than aluminum.
[0135] Needless to say, the above examples by no means restrict the
embodiment and various modifications may be made and applied.
[0136] Specifically, the embodiment is not limited to the specific
details of the above examples, and the constituting elements of the
embodiment can be variously modified and used in practice unless
they depart from the spirit or scope of the general inventive
concept. The constituting elements disclosed in the above examples
may be properly combined to form various embodiments. For example,
it is possible to omit some of the elements described in the
examples. Further, the elements of different embodiments may be
properly combined.
[0137] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the novel
methods and systems described herein may be embodied in a variety
of other forms; furthermore, various omissions, substitutions and
changes in the form of the methods and systems described herein may
be made without departing from the spirit of the inventions. The
accompanying claims and their equivalents are intended to cover
such forms or modifications as would fail within the scope and
spirit of the invention.
* * * * *