U.S. patent application number 09/980512 was filed with the patent office on 2003-01-09 for photoelectric device.
Invention is credited to Hirata, Masahiro, Otani, Tsuyoshi, Tawada, Yuko.
Application Number | 20030005956 09/980512 |
Document ID | / |
Family ID | 27278771 |
Filed Date | 2003-01-09 |
United States Patent
Application |
20030005956 |
Kind Code |
A1 |
Hirata, Masahiro ; et
al. |
January 9, 2003 |
Photoelectric device
Abstract
The present invention provides a photoelectric conversion device
that improves photoelectric conversion efficiency with the
interaction between a transparent substrate with a transparent
conductive film, an antireflection film, and a photoelectric
conversion unit. The antireflection film contains fine particles
having an average particle diameter of 0.01 to 1.0 .mu.m and has an
uneven surface derived from the fine particles. The glass sheet
with a transparent conductive film has a light transmittance of 75%
or more in the wavelength region of 800 nm to 900 nm. The
photoelectric conversion unit includes at least a photoelectric
conversion unit including a photoelectric conversion layer having a
band gap of 1.85 eV or less.
Inventors: |
Hirata, Masahiro;
(Osaka-shi, JP) ; Otani, Tsuyoshi; (Osaka-shi,
JP) ; Tawada, Yuko; (Settsu-shi, JP) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Family ID: |
27278771 |
Appl. No.: |
09/980512 |
Filed: |
October 31, 2001 |
PCT Filed: |
March 2, 2001 |
PCT NO: |
PCT/JP01/01604 |
Current U.S.
Class: |
136/256 ;
136/255; 257/E31.12; 257/E31.13 |
Current CPC
Class: |
H01L 31/02161 20130101;
G02B 1/02 20130101; G02B 1/116 20130101; G02B 1/111 20130101 |
Class at
Publication: |
136/256 ;
136/255 |
International
Class: |
H01L 031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 2, 2000 |
JP |
2000-057771 |
Apr 18, 2000 |
JP |
2000-116688 |
Claims
1. A photoelectric conversion device comprising: a transparent
substrate having a first principal surface and a second principal
surface that are parallel to each other; an antireflection film
formed on the first principal surface; a transparent conductive
film formed on the second principal surface; at least one
photoelectric conversion unit formed on the transparent conductive
film; and a back electrode formed on the photoelectric conversion
unit, wherein the antireflection film contains fine particles with
an average particle diameter in a range of 0.01 .mu.m to 1.0 .mu.m
and has an uneven surface derived from the fine particles, the
transparent substrate has a light transmittance of 75% or more in a
wavelength region in a range of 800 nm to 900 nm when measured with
the transparent conductive film formed thereon, and at least one of
the photoelectric conversion units includes a thin film of
semiconductor material having a band gap of 1.85 eV or less as a
photoelectric conversion layer.
2. The photoelectric conversion device according to claim 1,
wherein an external quantum efficiency at a wavelength of 700 nm is
0.2 or more.
3. The photoelectric conversion device according to claim 1,
wherein the thin film of semiconductor material is a crystalline
silicon based thin film, and the photoelectric conversion unit
including the crystalline silicon based thin film as a
photoelectric conversion layer has a thickness of 10 .mu.m or
less.
4. The photoelectric conversion device according to claim 1,
wherein the at least one photoelectric conversion unit includes an
amorphous silicon based photoelectric conversion unit having an
amorphous silicon based thin film as the photoelectric conversion
layer and a crystalline silicon based photoelectric conversion unit
having a crystalline silicon based thin film as the photoelectric
conversion layer, the two photoelectric conversion units being
stacked in this order from a side of the transparent conductive
film.
5. The photoelectric conversion device according to claim 1,
wherein unevenness derived from the fine particles is formed in an
area of at least 60% of the first principal surface of the
transparent substrate.
6. The photoelectric conversion device according to claim 1,
wherein the fine particles are made of a material having a
refractive index of 2.0 or less.
7. The photoelectric conversion device according to claim 1,
wherein the fine particles have an average particle diameter in a
range of 0.05 .mu.m to 0.8 .mu.m.
8. The photoelectric conversion device according to claim 1,
further comprising an underlying film formed between the
transparent substrate and the transparent conductive film.
Description
TECHNICAL FIELD
[0001] The present invention relates to a photoelectric conversion
device. More specifically, the present invention relates to a
photoelectric conversion device including a photoelectric
conversion layer of semiconductor material that has a relatively
small band gap and photosensitivity even in a longer wavelength
region.
BACKGROUND ART
[0002] In a thin film-type photoelectric conversion device that
uses a glass sheet as a substrate, a transparent conductive film,
acting as a transparent electrode, is formed on the glass sheet,
and a thin film photoelectric conversion unit including a
photoelectric conversion layer is formed on the transparent
conductive film. A tin oxide film often is employed as the
transparent conductive film. The unevenness generated on the
surface of the transparent conductive film with the growth of
crystal grains has the effect of improving photoelectric conversion
efficiency by trapping incident light in the photoelectric
conversion layer or in the vicinity of the layer (i.e., light
trapping effect). Thus, as for transparent conductive films,
various surface shapes to improve the photoelectric conversion
efficiency have been proposed.
[0003] Examples of the known thin film photoelectric conversion
units are as follows: a unit including a photoelectric conversion
layer formed of an amorphous silicon thin film, a unit including a
photoelectric conversion layer formed of an amorphous silicon
germanium thin film, and a unit including a photoelectric
conversion layer formed of a crystalline silicon based thin film,
such as microcrystalline silicon. Moreover, a tandem-type
photoelectric conversion device has been under active development
because of its ability to utilize light in a broad wavelength
region. The tandem-type photoelectric conversion device includes
two thin film photoelectric conversion units formed on a
transparent conductive film, each unit having a different
photoelectric conversion layer.
[0004] To increase the photoelectric conversion efficiency, a
photoelectric conversion device requires some adaptation according
to a photoelectric conversion layer to be used. For example, the
thin film photoelectric conversion unit including the photoelectric
conversion layer formed of a crystalline silicon based thin film
(i.e., a crystalline silicon based thin film photoelectric
conversion unit) has a smaller absorption coefficient than that of
the amorphous silicon based unit. However, when the film thickness
is increased simply to increase optical absorption, the
manufacturing cost becomes higher. Therefore, the improvement in
photoelectric conversion efficiency by taking advantage of the
light trapping effect is important particularly to the
photoelectric conversion device employing the crystalline silicon
based thin film photoelectric conversion unit.
[0005] In general, the unit including the photoelectric conversion
layer formed of an amorphous silicon germanium thin film or
crystalline silicon based thin film has high sensitivity in the
long wavelength region, compared with the unit including the
photoelectric conversion layer formed of a general amorphous
silicon thin film. Even with the amorphous silicon thin film,
however, the spectral response becomes high in the long wavelength
region as the film thickness increases. Therefore, when these thin
films are used as a photoelectric conversion layer, it is necessary
to pay considerable attention to the photoelectric conversion
efficiency also in a relatively long wavelength region, e.g., at a
wavelength of 650 nm or more.
[0006] However, a conventional photoelectric conversion device is
not always provided with a structure suitable for the
characteristics of its photoelectric conversion layer. For example,
the photoelectric conversion device employing a crystalline silicon
based thin film photoelectric conversion unit can have a large
light trapping effect when the slope of concave and convex portions
in the surface of a transparent conductive film is made sharp.
However, this may degrade the quality of a crystalline silicon
based thin film to be formed on the transparent conductive film.
Even if the crystalline silicon based thin film is formed via other
thin films, as in the tandem structure, the surface unevenness with
a large degree of slope causes degradation in the crystallinity of
the crystalline silicon. Thus, it is desired that the crystalline
silicon based thin film-type photoelectric conversion unit should
achieve the light trapping effect, which is important to improve
the photoelectric conversion efficiency, without relying only on
the surface unevenness of the transparent conductive film.
[0007] Even when using a photoelectric conversion layer that
renders the photoelectric conversion efficiency in a relatively
long wavelength region important, the conventional photoelectric
conversion unit does not always adjust properly the characteristics
of other members and thin films to be used with the photoelectric
conversion layer, particularly their transmittance and
contributions to the light trapping effect in that wavelength
region.
DISCLOSURE OF INVENTION
[0008] Therefore, with the foregoing in mind, it is an object of
the present invention to provide a photoelectric conversion device
that includes a photoelectric conversion layer having high
photoelectric conversion efficiency even in a relatively long
wavelength region, i.e., a photoelectric conversion layer having a
relatively small band gap, and that is provided with a structure
capable of improving the photoelectric conversion efficiency of the
photoelectric conversion layer. In particular, it is an object of
the present invention to provide a photoelectric conversion device
that improves the photoelectric conversion efficiency of a
crystalline silicon based thin film photoelectric conversion unit
without relying only on the light trapping effect obtained by a
transparent conductive film.
[0009] To achieve the above objects, a photoelectric conversion
device of the present invention includes the following: a
transparent substrate having a first principal surface and a second
principal surface that are parallel to each other; an
antireflection film formed on the first principal surface; a
transparent conductive film formed on the second principal surface;
at least one photoelectric conversion unit formed on the
transparent conductive film; and a back electrode formed on the
photoelectric conversion unit. The antireflection film contains
fine particles with a particle diameter in the range of 0.01 .mu.m
to 1.0 .mu.m and has an uneven surface derived from the fine
particles. The transparent substrate has a light transmittance of
75% or more in a wavelength region in the range of 800 nm to 900 nm
when measured with the transparent conductive film formed thereon.
At least one of the photoelectric conversion units includes a thin
film of semiconductor material having a band gap of 1.85 eV or less
as a photoelectric conversion layer.
[0010] It is preferable that the above photoelectric conversion
device has photosensitivity even in a long wavelength region so
that an external quantum efficiency at a wavelength of 700 nm is
0.2 or more. It is more preferable that the external quantum
efficiency at the same wavelength is 0.3 or more. In the above
photoelectric conversion device, when the thin film, of
semiconductor material is a crystalline silicon based thin film, it
is preferable that the photoelectric conversion unit including the
crystalline silicon based thin film as a photoelectric conversion
layer has a thickness of 10 .mu.m or less.
BRIEF DESCRIPTION OF DRAWINGS
[0011] FIG. 1 is a cross-sectional view of an embodiment of a
photoelectric conversion device of the present invention.
[0012] FIG. 2 shows the results of scanning electron microscope
observation of the surface of a glass sheet on which an example of
an antireflection film is formed.
[0013] FIG. 3 shows the results of scanning electron microscope
observation of the surface of a glass sheet on which another
example of an antireflection film is formed.
[0014] FIG. 4 shows the results of scanning electron microscope
observation of the surface of a glass sheet on which yet another
example of an antireflection film is formed.
[0015] FIG. 5 shows an example of the wavelength dependence of
external quantum efficiency of a photoelectric conversion device
(tandem-type) of the present invention, including the case where
the device has no antireflection film.
[0016] FIG. 6 shows the spectral reflection characteristics of a
glass sheet with an antireflection film.
[0017] FIG. 7 shows an example of the wavelength dependence of
external quantum efficiency of a photoelectric conversion device
(having the photoelectric conversion layer formed of an amorphous
silicon layer with an increased thickness) of the present
invention, including the case where the device has no
antireflection film.
[0018] FIG. 8 shows an example of the wavelength dependence of
external quantum efficiency of a photoelectric conversion device
(having the photoelectric conversion layer formed of an amorphous
silicon germanium layer) of the present invention, including the
case where the device has no antireflection film.
DETAILED DESCRIPTION OF THE INVENTION
[0019] Hereinafter, the preferred embodiments of the present
invention will be described.
[0020] A photoelectric conversion device of the present invention
can use not only a crystalline silicon based thin film but also an
amorphous silicon based thin film as a photoelectric conversion
layer. Examples of the amorphous silicon based thin film include an
amorphous silicon thin film, an amorphous silicon germanium thin
film, and an amorphous silicon tin thin film. The amorphous silicon
thin film generally has its peak sensitivity in the wavelength
region of 500 to 600 nm, and thus it is not so sensitive to light
in a long wavelength region. Therefore, when the amorphous silicon
thin film is used as a photoelectric conversion layer, it is
preferable that the external quantum efficiency at a wavelength of
700 nm is 0.2 or more. This can be achieved by increasing the film
thickness or reducing the band gap with addition of germanium, tin,
or the like. However, a tandem structure including a plurality of
photoelectric conversion layers, which will be described later, can
achieve the effects of the invention sufficiently without
increasing the external quantum efficiency of the amorphous silicon
thin film, as long as it can provide the external quantum
efficiency substantially equal to that described above as a whole.
The photoelectric conversion device of the present invention is
suitable particularly for using a thin film of semiconductor
material having a band gap of 1.75 eV or less as a photoelectric
conversion layer.
[0021] The band gap of a crystalline silicon thin film is smaller
than that of the amorphous silicon thin film, generally about 1.1
eV. When the crystalline silicon thin film is used as a
photoelectric conversion layer, it is preferable that the thin film
photoelectric conversion unit has a thickness of 10 .mu.m or less,
more preferably 5 .mu.m or less, in order to prevent an excessive
increase in the manufacturing cost. There is no particular
limitation to the above thickness, and it is preferably 0.1 .mu.m
or more.
[0022] To improve the photoelectric conversion efficiency of a
photoelectric conversion unit, the present invention allows an
antireflection film containing fine particles to be formed on the
light incident side of a transparent substrate. A thin film having
an appropriate refractive index, which is calculated optically
based on the refractive index of the substrate, often is used to
suppress reflection from the transparent substrate. A typical
antireflection film for a glass sheet is a magnesium fluoride thin
film with a flat surface formed by vacuum evaporation or the
like.
[0023] However, the present inventors have found out that an
antireflection film whose surface is made uneven by fine particles
is more suitable for the photoelectric conversion layer formed of
material with a relatively small band gap, particularly for the
photoelectric conversion layer formed of a crystalline silicon thin
film with low absorption. In an embodiment of the present
invention, the antireflection film contributes to the enhancement
of the light trapping effect. In another embodiment of the present
invention, the antireflection film has the effect of reducing
reflection in a broad wavelength region, particularly in a long
wavelength region, thereby improving the characteristics of a
photoelectric conversion device.
[0024] When a thin film photoelectric conversion unit including the
above photoelectric conversion layer is used with the
antireflection film having an uneven surface, the transmittance of
a transparent substrate on which a transparent conductive film is
formed exerts a greater influence on the efficiency. Specifically,
it is preferable that the transparent substrate with a transparent
conductive film has a light transmittance of 75% or more in the
wavelength region of 800 to 900 nm. Such high light transmittance
in the above wavelength region combines the effects of reducing
reflection and trapping light by the antireflection film,
particularly the light trapping effect, with the improvement in
photoelectric conversion efficiency.
[0025] The light trapping effect resulting from the surface shape
of the antireflection film is produced in the area including the
transparent substrate with a transparent conductive film (i.e., the
area from the surface of the antireflection film to a back
electrode) in a photoelectric conversion device. Therefore, the
amount of light passing through the transparent substrate with a
transparent conductive film is increased with the light trapping
effect. Thus, the increased amount of light through the transparent
substrate with a transparent conductive film reinforces the light
trapping effect obtained by the antireflection film.
[0026] A combination of the above thin film photoelectric
conversion unit, antireflection film, and glass sheet with a
transparent conductive film can improve the photoelectric
conversion efficiency rationally while preventing unnecessary
increase in the manufacturing cost.
[0027] FIG. 1 is a cross-sectional view of a preferred embodiment
of a photoelectric conversion device of the present invention. The
photoelectric conversion device has a tandem structure, in which an
antireflection film 1, a glass sheet 2, an underlying film 3, a
transparent conductive film 4, an amorphous silicon based thin film
photoelectric conversion unit 5, a crystalline silicon based thin
film photoelectric conversion unit 6, and a back electrode 7 are
stacked in this order from the light incident side. The
antireflection film 1 of the photoelectric conversion device has an
uneven surface derived from the shapes of fine particles.
[0028] It is preferable that the antireflection film 1 contains
fine particles and a binder. As the fine particles, oxide fine
particles are preferable, and fine particles made of material
having a refractive index of 2.0 or less, particularly 1.6 or less,
are preferable. The particularly suitable oxide fine particles are
silica fine particles. Here, fine particles including silicon oxide
as the main component are referred to as silica fine particles,
even if they include other minor components. Examples of silica
fine particles include the following: silica fine particles
synthesized by the reaction of silicon alkoxide in the presence of
a basic catalyst, such as ammonia, with a sol-gel process,
colloidal silica of sodium silicate or the like, and fumed silica
synthesized in a vapor phase.
[0029] As long as unevenness derived from fine particles is present
at the surface of the antireflection film 1, it is not necessary to
expose the fine particles directly on the surface. For example, a
group of fine particles may be covered with a film-like coating of
binder, as long as the film surface is made uneven by the shapes of
the fine particles under the coating. However, the antireflection
film having the exposed fine particles on the surface, as the film
prepared in the example to be described later, has a remarkable
effect of improving the photoelectric conversion efficiency.
[0030] In many cases, the antireflection film formed of fine
particles has gaps between the fine particles. The inclusion of
gaps reduces a substantial refractive index of the film and
improves the antireflection effect. Actually, the structure of the
antireflection film changes depending on the particle size of the
fine particles. The gaps between particles decrease as the particle
size of the fine particles is diminished. Consequently, capillary
force is increased to allow moisture and organic matters in the air
to enter the gaps gradually. When the gaps disappear, a refractive
index of the film is increased. On the other hand, the adhesion to
a glass sheet is lowered with increasing particle size of the fine
particles. In view of this, it is preferable that the fine
particles have an average particle diameter of 0.01 .mu.m to 1.0
.mu.m.
[0031] The particle size of the fine particles also can be used to
adjust the optical characteristics of the antireflection film. The
particle diameter suitable for the enhancement of antireflection
effect is in the range of 0.05 .mu.m to 0.15 .mu.m. For example,
when the surface of a glass sheet is coated with silica fine
particles having a particle size within the above range, an
antireflection effect is obtained that exceeds a magnesium fluoride
film in a broad wavelength region, the magnesium fluoride film with
an appropriate thickness being formed by vacuum evaporation.
[0032] Surprisingly, the fine particles with a particle diameter of
0.2 .mu.m to 0.8 .mu.m have been found to improve the light
trapping effect significantly as well. For example, when a part of
the surface of a glass sheet is coated with silica fine particles
having a particle size within the above range, that alone leads to
the improvement in the photoelectric conversion efficiency of a
unit including a photoelectric conversion layer whose peak spectral
response is shifted relatively to the long wavelength region, such
as a crystalline silicon thin film. The fine particles described
above may interact strongly with light in the wavelength region of
600 to 1000 nm and contribute largely to trapping light in the
wavelength region where the spectral response of a crystalline
silicon based photoelectric conversion layer or the like is
high.
[0033] As described above, it is preferable that the fine particles
have an average particle diameter of about 0.05 .mu.m to 0.8 .mu.m.
Moreover, it is preferable that unevenness derived from the fine
particles is formed in the area of at least 60% of the principal
surface of a glass sheet. In particular, a large antireflection
effect can be obtained when the fine particles having a particle
diameter of 0.05 .mu.m to 0.15 .mu.m occupy at least 50% of the
uneven area derived from the fine particles. Also, a large light
trapping effect in the wavelength region where the spectral
response of a crystalline silicon based photoelectric conversion
layer is high can be obtained when the fine particles having a
particle diameter of 0.2 .mu.m to 0.8 .mu.m occupy at least 30% of
the uneven area derived from the fine particles. The presence of
both fine particles together on the surface with the above
percentage provides especially preferable results.
[0034] Here, the particle size of fine particles can be evaluated
roughly with a scanning electron microscope. For precise
measurements, a transmission electron microscope can be used. In
the case where the fine particles are aggregated, the particle size
of individual particles (primary particles) instead of aggregated
particles (e.g., secondary particles linked in a chain form) is
employed.
[0035] It is preferable that the antireflection film includes a
binder as well as fine particles. The binder serves to bind the
fine particles together and provide adhesion between the fine
particles and a glass sheet. It is preferable that the binder is at
least one metal oxide selected from the group consisting of silicon
oxide, aluminum oxide, titanium oxide, zirconium oxide, and
tantalum oxide. Considering film strength and chemical stability,
it is preferable that the binder material is at least one alkoxide
of metal selected from Si, Al, Ti, Zr, and Ta. For the film having
a relatively large content of binder components, the refractive
index of the binder components affects reflectance. Therefore,
silicon alkoxide having a small refractive index, particularly
silicon tetraalkoxide or its oligomer, is preferable.
[0036] The method for manufacturing an antireflection film
containing silica fine particles and a binder is described in more
detail below. The antireflection film can be formed with a coating
solution that is prepared using silica fine particles and a metal
compound such as metal alkoxide. The coating solution may be
prepared by mixing a hydrolyzate of the metal compound and the
silica fine particles. However, it is preferable that the coating
solution is prepared by hydrolyzing a hydrolyzable metal compound
in the presence of the silica fine particles. The hydrolysis of
metal alkoxide in the presence of the silica fine particles
accelerates a condensation reaction between a silanol group on the
silica fine particle surface and the metal alkoxide in the coating
solution. The condensation reaction enhances the adhesion between
the silica fine particles. In addition, it reinforces the adhesion
of the fine particles to a glass substrate by increasing reactivity
of the silica fine particle surface. The coating solution thus
prepared is applied to a glass sheet and then heated, resulting in
an antireflection film formed on the glass sheet.
[0037] An antireflection film with a high area ratio of unevenness
derived from silica fine particles can be formed in the following
manner: the hydrolysis of a metal compound that acts as a binder is
promoted in the presence of the silica fine particles or the
content of binder is set to be within an appropriate range (e.g.,
the weight ratio of the binder is not more than that of the silica
fine particles). There is no particular limitation to the amount of
binder, as long as the antireflection film has an uneven surface
derived from the shapes of the fine particles.
[0038] Next, the glass sheet 2 will be described. The glass sheet
is not limited particularly by composition and thickness, as long
as it has a light transmittance of 75% or more in the wavelength
region of 800 to 900 nm with a transparent conductive film formed
thereon. For the most typical soda-lime glass, the above light
transmittance can be obtained easily when the glass has such a
composition that the total amount of iron oxide, expressed by
weight percentage in terms of Fe.sub.2O.sub.3, is 0.1% or less,
preferably 0.08% or less. In forming an amorphous silicon based
thin film photoelectric conversion unit, it is preferable that the
glass sheet with a transparent conductive film has a light
transmittance of 80% or more in the wavelength region of 500 to 600
nm.
[0039] When a float glass is used as the glass sheet, it is
preferable that an antireflection film is formed on the bottom
surface of the glass sheet (i.e., the surface in contact with the
molten tin in a float bath). Since the bottom surface is superior
to the opposite top surface in flatness, it is suitable for the
application of a coating solution containing fine particles.
[0040] The following is an explanation for the underlying film 3
and the transparent conductive film 4.
[0041] As the transparent conductive film 4, an ITO film or zinc
oxide film may be used. However, it is preferable to use a film
including tin oxide as the main component, specifically a tin oxide
film doped with impurities such as fluorine to increase
conductivity. The thickness of the transparent conductive film can
be determined properly by conductivity required in accordance with
a photoelectric conversion unit to be used and its desired
photoelectric conversion efficiency. Considering the necessity to
set the light transmittance of the glass sheet with a transparent
conductive film within the above range, a suitable thickness of the
transparent conductive film ranges from about 400 nm to 1000
nm.
[0042] The surface unevenness of the transparent conductive film
formed by the growth of crystal grains or the like further improves
the light trapping effect. However, the formation of unevenness in
the transparent conductive film surface is not essential in this
embodiment. If the unevenness of this film is excessively sharp,
the quality of a crystalline silicon based thin film may be
degraded. Thus, it is preferable that the transparent conductive
film has a haze ratio of 20% or less, particularly when a
crystalline silicon based thin film is used.
[0043] The underlying film 3 is often provided to prevent the
diffusion of alkaline component from a glass sheet and to adjust
the optical characteristics of the glass sheet with a transparent
conductive film. The underlying film is formed so that the glass
sheet with a transparent conductive film has a light transmittance
of 75% or more in the wavelength region of 800 to 900 nm with the
underlying film interposed between the glass sheet and the
transparent conductive film.
[0044] The underlying film may be composed of a single layer or
plurality of layers. A preferred example of the underlying film is
a film having a two-layer structure that includes a high refractive
index film and a low refractive index film in this order from the
glass sheet side. It is preferable that the high refractive index
film is formed of at least one selected from tin oxide, titanium
oxide, zinc oxide, tantalum oxide, niobium oxide, cerium oxide,
zirconium oxide, silicon nitride, silicon oxynitride (SiON), and a
mixture of these substances. It is preferable that this film has a
thickness of 5 nm to 100 nm and a refractive index of 1.7 to
2.7.
[0045] It is preferable that the low refractive index film, having
a refractive index relatively lower than that of the high
refractive index film, is formed of at least one selected from
silicon oxide, aluminum oxide, silicon oxide containing carbon
(SiOC), and a mixture of these substances. It is preferable that
this film has a thickness of 1 nm to 60 nm and a refractive index
of 1.4 to 1.8.
[0046] There is no particular limitation on a method for forming
the underlying film and the transparent conductive film, and a
method that includes thermal decomposition of a material for
producing a film on a glass sheet or glass ribbon in the glass
sheet production line, particularly a CVD method, is
preferable.
[0047] Next, a photoelectric conversion unit will be described. The
photoelectric conversion unit preferably includes a plurality of
layers, though a single layer may be used. The photoelectric
conversion device in FIG. 1 has a so-called tandem structure, in
which the unit including the photoelectric conversion layer formed
of an amorphous silicon based thin film (i.e., an amorphous silicon
based thin film photoelectric conversion unit) 5 and the unit
including the photoelectric conversion layer formed of a
crystalline silicon based thin film (i.e., a crystalline silicon
based thin film photoelectric conversion unit) 6 are stacked in
this order from the glass sheet side.
[0048] The photoelectric conversion device in FIG. 1 can achieve
effective utilization of light in a broad wavelength region by
using the photoelectric conversion layer formed of a crystalline
silicon based thin film having a relatively small band gap (about
1.1 eV) with that formed of an amorphous silicon based thin film
having a relatively large band gap. Here, the photoelectric
conversion device of the present invention is not limited to the
structure shown in FIG. 1. For example, crystalline silicon is used
as a material with a small band gap (.ltoreq.1.85 eV) in this
structure. However, a semiconductor material having a band gap of
1.85 eV or less is not limited to the crystalline silicon.
[0049] In general, the amorphous silicon based thin film
photoelectric conversion unit is formed by depositing each of the
p-type, i-type, and n-type semiconductor layers in this order by
plasma CVD. Specifically, e.g., the semiconductor layers may be
deposited in the following order: a p-type microcrystalline silicon
based layer doped with at least 0.01 at % boron that is an impurity
atom for determining the conduction type, an intrinsic amorphous
silicon layer that acts as a photoelectric conversion portion, and
an n-type microcrystalline silicon based layer doped with at least
0.01 at % phosphorus that is an impurity atom for determining the
conduction type. However, each of the semiconductor layers is not
limited to that described above. For example, the impurity atom in
the p-type microcrystalline silicon based layer may be aluminum or
the like, and an amorphous silicon based layer may be used as the
p-layer. In addition, the p-layer can be formed of alloy material
such as amorphous or microcrystalline silicon carbide and silicon
germanium. The suitable thickness of the amorphous silicon based
thin film photoelectric conversion unit is 0.5 .mu.m or less.
[0050] It is preferable that the conduction type (p-type and
n-type) microcrystalline silicon based layers each have a thickness
of 3 nm to 100 nm, more preferably 5 nm to 50 nm.
[0051] It is preferable that the intrinsic amorphous silicon layer
is formed by plasma CVD with the temperature of an underlying layer
maintained at 450.degree. C. or less. This layer is formed as a
substantially intrinsic semiconductor thin film that includes
impurity atoms for determining the conduction type with a density
of 1.times.10.sup.18 cm.sup.-3 or less. The generally preferred
thickness of the intrinsic amorphous silicon layer, though it
depends on the configuration of a photoelectric conversion device,
ranges from 0.05 .mu.m to 0.5 .mu.m. However, the amorphous silicon
based thin film photoelectric conversion unit can use a layer of
alloy material instead of the intrinsic amorphous silicon layer.
Examples of such a layer include an amorphous silicon carbide layer
(e.g., the amorphous silicon carbide layer of amorphous silicon
containing not more than 10 at % carbon) and an amorphous silicon
germanium layer (e.g., the amorphous silicon germanium layer of
amorphous silicon containing not more than 30 at % germanium).
[0052] When a crystalline silicon based thin film photoelectric
conversion unit to be described below is not provided (i.e., the
device is formed as a single cell), it is preferable that the
intrinsic amorphous silicon layer whose thickness is increased to
the extent that the external quantum efficiency at a wavelength of
700 nm is 0.2 or more is used as a photoelectric conversion layer.
Alternatively, the photoelectric conversion layer may be formed of
an amorphous silicon alloy based material to which germanium or the
like is added so as to increase the external quantum efficiency to
the above extent.
[0053] The crystalline silicon based thin film photoelectric
conversion unit also is formed by depositing each of the pin-type
semiconductor layers in this order by plasma CVD, following the
same procedure as that for the amorphous silicon based thin film
photoelectric conversion unit. For example, it is preferable that a
crystalline silicon based photoelectric conversion layer, acting as
the photoelectric conversion layer (i-layer) in the crystalline
silicon based thin film photoelectric conversion unit, is formed by
plasma CVD with the temperature of an underlying layer maintained
at 450.degree. C. or less.
[0054] As the crystalline silicon based photoelectric conversion
layer, a non-doped intrinsic silicon polycrystalline thin film, a
microcrystalline silicon thin film with a crystalline volume
fraction of at least 80%, a weak p-type or n-type silicon based
thin film containing a trace amount of impurities but having a
sufficient photoelectric conversion function, or the like can be
used. Moreover, layers of alloy material such as silicon carbide
and silicon germanium may be used.
[0055] It is preferable that the crystalline silicon based
photoelectric conversion layer has a thickness of 0.1 .mu.m to 10
.mu.m, more preferably 5 .mu.m or less. The photoelectric
conversion layer formed at low temperatures of 450.degree. C. or
less contains a relatively large amount of hydrogen atoms to
terminate or deactivate defects in and between crystal grains. It
is preferable that the hydrogen content in the layer ranges from
0.5 to 30 at %, more preferably 1 to 20 at %.
[0056] Most crystal grains in the crystalline silicon based
photoelectric conversion layer grow in the thickness direction from
an underlying layer to form columns. It is preferable that most
crystal grains have (110) preferred orientation planes parallel to
the film surface.
[0057] Compared with the amorphous silicon based thin film
photoelectric conversion unit, the crystalline silicon based thin
film photoelectric conversion unit generates a low open-circuit
voltage and has a high short-circuit current density. Therefore,
light transmittance contributes more to the photoelectric
conversion efficiency than the sheet resistance of a conductive
film on a glass sheet. Accordingly, providing the antireflection
film having an uneven surface derived from fine particles and the
glass sheet with a transparent conductive film having an improved
light transmittance increases the amount of incident light and
enables effective trapping of the incident light, which leads to a
significant improvement in the photoelectric conversion
efficiency.
[0058] The amorphous silicon photoelectric conversion layer has its
maximum spectral response in the wavelength region of about 500 to
600 nm. On the other hand, the crystalline silicon photoelectric
conversion layer has a maximum in the range of about 700 to 900 nm.
Thus, the antireflection film containing fine particles with a
particle diameter of 0.05 .mu.m to 0.15 .mu.m and those with a
particle diameter of 0.2 .mu.m to 0.8 .mu.m, both fine particles
being exposed on the surface, is suitable particularly for a
tandem-type photoelectric conversion device including the above two
photoelectric conversion layers.
[0059] Besides a polycrystalline substance, a "crystalline"
material in this embodiment includes a material having a
crystalline volume fraction of at least 50%, even if the material
is locally amorphous. A "silicon based" material includes a
semiconductor material containing at least 50 at % silicon, such as
amorphous silicon germanium, in addition to amorphous or
crystalline silicon.
[0060] The foregoing discussed an example of the photoelectric
conversion device employing the silicon based thin film. However,
the present invention can be applied to a photoelectric conversion
device including a thin film of compound semiconductor (e.g., CdTe,
CuIn(S, Se).sub.2, and Cu(In, Ga)(S, Se).sub.2) as the
photoelectric conversion layer.
EXAMPLES
[0061] Hereinafter, the present invention will be described more
specifically by way of example, but is not limited thereto.
[0062] Manufacture of Glass Sheet with Transparent Conductive
Film
[0063] Sample 1
[0064] In a production line for float glass, a tin oxide film
(SnO.sub.2 film), a silicon oxide film (SiO.sub.2 film), and a
fluorine-containing tin oxide film (SnO.sub.2:F film) were formed
in this order on a glass ribbon using a plurality of coaters
arranged in a float bath. The glass ribbon is formed to have a
thickness of 4 mm and the total amount of iron oxide of 0.01 wt %
in terms of Fe.sub.2O.sub.3.
[0065] Specifically, the glass ribbon had a temperature of about
650.degree. C. immediately before reaching a coater located at the
furthest upstream position, and a mixed gas of dimethyltin
dichloride (vapor), oxygen and nitrogen was supplied from the
coater, so that a SnO.sub.2 film having a thickness of 25 nm was
formed on the glass ribbon. Then, a mixed gas of monosilane,
ethylene, oxygen and nitrogen was supplied from a coater located
downstream from said coater, and a SiO.sub.2 film having a
thickness of 25 nm was formed on the SnO.sub.2 film. Then, a mixed
gas of dimethyltin dichloride (vapor), oxygen, nitrogen and
hydrogen fluoride (vapor) was supplied from a coater located
further downstream from said coater, and a SnO.sub.2:F film having
a thickness of 500 nm was formed on the SiO.sub.2 film. The glass
ribbon was cut into a predetermined size, resulting in a glass
sheet with a transparent conductive film (hereinafter, referred to
as "Sample 1").
[0066] Sample 2
[0067] Using a continuous atmospheric-type CVD apparatus, a
SiO.sub.2 film and a SnO.sub.2:F film were formed in this order on
a borosilicate glass sheet having a thickness of 0.7 mm.
[0068] Specifically, the glass sheet that was cut into a
predetermined size beforehand was heated to about 600.degree. C.,
and a mixed gas of monosilane, oxygen and nitrogen was supplied to
the surface of the glass sheet to form a SiO.sub.2 film having a
thickness of 25 nm. Then, a mixed gas of monobutyltin trichloride
(vapor), oxygen, water vapor, nitrogen and trifluoroacetic acid
(vapor) was supplied, and a SnO.sub.2:F film having a thickness of
600 nm was formed on the SiO.sub.2 film. Thus, a glass sheet with a
transparent conductive film (hereinafter, referred to as "Sample
2") was obtained.
[0069] Sample 3
[0070] A glass sheet with a transparent conductive film
(hereinafter, referred to as "Sample 3") was obtained in the same
manner as that for Sample 1, except that the total amount of iron
oxide in the glass ribbon was 0.11 wt % in terms of Fe.sub.2O.sub.3
and the thickness of the SnO.sub.2:F film was 800 nm.
[0071] After washing and drying each of Samples 1 to 3, the
spectral transmission characteristics in the wavelength region of
400 to 1000 nm were measured with an integrating-sphere photometer.
Table 1 shows an average transmittance in the wavelength regions of
500 to 600 nm and 800 to 900 nm, together with a sheet resistance
of the film surface.
1 TABLE 1 Sample 1 Sample 2 Sample 3 Transmittance (%) 500-600 nm
87 86 80 800-900 nm 80 85 70 Sheet resistance
(.OMEGA./.quadrature.) 19 11 8 SnO.sub.2:F thickness (nm) 500 600
800
[0072] Formation of Antireflection Film
[0073] Samples 4 to 11
[0074] The glass sheet with a transparent conductive film obtained
in the above was washed, dried, and then an antireflection film was
formed on the opposite surface of the glass sheet to the
transparent conductive film.
[0075] Specifically, to a dispersion of silica fine particles
having a predetermined average primary particle size (manufactured
by Nippon Shokubai Co., Ltd.) was added ethanol, tetraethoxysilane,
and concentrated hydrochloric acid successively while stirring the
dispersion, which then was stirred further to cause a reaction. The
mixture was diluted with hexylene glycol to give a coating
solution. The coating solution was applied to the surface of the
glass sheet by spin coating, and the glass sheet was placed in an
electric furnace at 700.degree. C. for 2 minutes to form an
antireflection film.
[0076] Depending on samples, the silica fine particle dispersions
with different average particle sizes were mixed at a predetermined
ratio of the solid matter.
[0077] Sample 12
[0078] Similarly, the glass sheet with a transparent conductive
film obtained in the above was washed, dried, and then a magnesium
fluoride film having a thickness of 100 nm was formed on the
opposite surface of the glass sheet to the transparent conductive
film by EB evaporation, which results in an antireflection film.
Here, the film deposition temperature was set to a room temperature
and the film deposition rate was 1 nm/sec.
[0079] The surfaces of the antireflection films of Samples 4 to 11
were observed with a scanning electron microscope to measure the
ratio of the area of the glass surface coated with silica fine
particles (i.e., a coating ratio). When those samples included
silica fine particles having different average particle sizes,
further measurement was made to determine the ratio of the area
occupied by each of the fine particles to the area coated with the
fine particles. Table 2 shows the results.
2 TABLE 2 Sample 4 Sample 5 Sample 6 Sample 7 Sample 8 Sample 9
Sample 10 Sample 11 Sample 12 Glass sheet with Sample 2 Sample 2
Sample 2 Sample 2 Sample 1 Sample 3 Sample 1 Sample 2 Sample 2
transparent conductive film Fine particle coating ratio 95 72 61 98
95 95 67 45 (MgF.sub.2 film) (%) Fine particles A Average particle
diameter 0.1 0.1 0.1 0.07 0.1 0.1 0.25 -- -- (.mu.m) Occupancy (%)
100 52 61 100 100 100 58 -- -- Fine particles B Average particle
diameter -- 0.3 0.5 -- -- -- 0.9 0.5 -- (.mu.m) Occupancy (%) -- 48
39 -- -- -- 42 100 --
[0080] FIGS. 2 to 4 show the results of scanning electron
microscope observation of Samples 4 to 6, respectively. These
results indicated that the fine particles to form the respective
antireflection films were substantially spherical in shape and the
variation in particle diameter was rather small. In each sample,
substantially all the fine particles A have a particle diameter of
0.05 to 0.15 .mu.m and substantially all the fine particles B have
a particle diameter of 0.2 to 0.8 .mu.m.
[0081] Formation of Photoelectric Conversion Unit and Back
Electrode (Tandem Type)
[0082] Samples 13 to 24
[0083] Using the glass sheet with a transparent conductive film
having an antireflection film (Samples 4 to 12) and the glass sheet
with a transparent conductive film having no antireflection film
(Samples 1 and 2), an amorphous silicon thin film photoelectric
conversion unit and a crystalline silicon thin film photoelectric
conversion unit were formed in this order on the transparent
conductive film by plasma CVD. The amorphous silicon photoelectric
conversion unit included a pin junction where a p-type amorphous
silicon carbide layer had a thickness of 15 nm and an n-type
amorphous silicon layer had a thickness of 30 nm. An intrinsic
amorphous silicon layer was formed by RF plasma CVD. The film
deposition conditions were such that silane (SiH.sub.4) was used as
a reaction gas, a pressure in the reaction chamber was about 40 Pa,
an RF power density was 15 mW/cm.sup.2, and a film deposition
temperature was 150.degree. C. An intrinsic amorphous silicon film,
which was deposited directly on the glass substrate to have a
thickness of 300 nm under the same conditions as those described
above, had a dark conductivity of 5.times.10.sup.-10 S/cm. The
intrinsic amorphous silicon layer had a thickness of 150 nm. In
addition, the intrinsic amorphous silicon layer prepared in the
same manner as that described above had a band gap of about 1.75
eV.
[0084] The crystalline silicon thin film photoelectric conversion
unit was prepared by depositing a boron-doped p-type
microcrystalline silicon based layer, a non-doped intrinsic
crystalline silicon layer, and a phosphorus-doped n-type
microcrystalline silicon based layer in this order on the n-type
amorphous silicon layer. The crystalline silicon thin film
photoelectric conversion unit had a thickness of 1.5 .mu.m. The
thickness of the p-type microcrystalline silicon based layer was 15
nm and that of the n-type microcrystalline silicon based layer was
30 nm. The intrinsic crystalline silicon layer prepared in the same
manner as that described above had a band gap of about 1.1 eV.
[0085] The intrinsic crystalline silicon layer was formed by plasma
CVD under the condition that silane was used as a reaction gas, a
pressure in the reaction chamber was about 670 Pa, an RF power
density was 150 mW/cm.sup.2, and a film deposition temperature was
350.degree. C. The measurement with a secondary ion mass
spectrometry showed that the intrinsic crystalline silicon layer
contained 2 at % hydrogen. As a result of the peak intensity ratio
based on an X-ray diffraction method, it was confirmed that the
crystal grains in this layer had (110) planes as preferred
orientation planes in the direction parallel to the film
surface.
[0086] Moreover, a back electrode was formed by depositing an ITO
layer (with a thickness of 80 nm) and a silver layer (with a
thickness of 300 nm) in this order by sputtering, thus providing a
photoelectric conversion device. The wavelength dependence of
external quantum efficiency in each photoelectric conversion device
was measured. The resultant efficiency for each wavelength was
multiplied by the amount of incident light to give an external
quantum efficiency, which then was integrated over the entire
wavelength, so that the total current was calculated. Table 3 shows
the results.
3 TABLE 3 Sample 13 Sample 14 Sample 15 Sample 16 Sample 17 Sample
18 Sample 19 Sample 20 Sample 21 Sample 22 Sample 23 Glass sheet
with Sample 2 Sample 2 Sample 2 Sample 2 Sample 1 Sample 3 Sample 1
Sample 2 Sample 2 Sample 2 Sample 1 transparent conductive film
Glass sheet with Sample 4 Sample 5 Sample 6 Sample 7 Sample 8
Sample 9 Sample 10 Sample 11 Sample 12 -- -- transparent (MgF.sub.2
conductive film film) and antireflection film Total current 21.1
21.9 20.9 20.7 19.8 17.2 18.1 20.4 20.3 20.0 17.8 (mA/cm.sup.2)
[0087] As shown in Table 3, Samples 13 to 16 and 20 had more
favorable characteristics than Samples 22 and 21. Each of Samples
13 to 16 and 20 used Sample 2 on which an antireflection film was
formed with fine particles exposed on the film surface, while
Sample 22 used Sample 2 without forming the antireflection film and
Sample 21 used Sample 2 on which a magnesium fluoride film was
formed. Samples 17 and 19, each using Sample 1 on which an
antireflection film was formed with fine particles exposed on the
film surface, also had more favorable characteristics than Sample
23, using Sample 1 without forming the antireflection film. On the
other hand, Sample 18, which included the glass sheet with a
transparent conductive film of low transmittance, was degraded
significantly in its characteristics.
[0088] FIG. 5 shows the wavelength dependence of external quantum
efficiency of Sample 13 (fine particle film), Sample 14 (fine
particle mixed film) and Sample 22 (without an antireflection
film), respectively. For the samples having the antireflection
films formed of fine particles, the external quantum efficiency was
increased in a broad wavelength region. In particular, using Sample
14 (fine particle mixed film) with the addition of fine particles B
improved the external quantum efficiency in the wavelength region
of 800 to 900 nm.
[0089] Sample 21, using magnesium fluoride as the antireflection
film, had increased external quantum efficiency in the wavelength
region near 500 to 600 nm. However, the degree of increase is small
compared with the antireflection film containing fine particles,
and the external quantum efficiency was hardly increased in the
wavelength region near 800 to 900 nm.
[0090] To investigate the reason for this, samples were prepared in
such a manner that the same film as the antireflection film used in
Samples 13, 14, and 21 was formed on a borosilicate glass sheet
having a thickness of 0.7 mm. In each of the samples, the opposite
surface of the glass sheet to the film was ground with a grindstone
to be in a frosted glass form, which then was coated with a black
paint to remove the luster. Consequently, reflection from this
surface was suppressed to the extent that it was substantially
negligible. The spectral reflection characteristics of the
antireflection film surface of each sample were measured. FIG. 6
shows the results.
[0091] As shown in FIG. 6, the fine particle film with an average
particle diameter of 0.1 .mu.m was superior to the magnesium
fluoride film in antireflection effect in a broad wavelength
region. The fine particle mixed film; in which the fine particles
having an average particle diameter of 0.1 .mu.m were mixed with
those having an average particle diameter of 0.3 .mu.m, was
inferior to the fine particle film in antireflection effect.
However, the use of the fine particle mixed film improved the
external quantum efficiency in the wavelength region near 800 to
900 nm, as shown in FIG. 5. Such improvement in the efficiency was
due to the light trapping effect in that wavelength region.
[0092] Formation of Photoelectric Conversion Unit and Back
Electrode (Only an Amorphous Silicon Based Unit)
[0093] Instead of the above tandem-type unit, a photoelectric
conversion device was prepared, including an amorphous silicon
based thin film photoelectric conversion unit having the
photoelectric conversion layer formed of an amorphous silicon layer
with an increased thickness or amorphous silicon germanium layer.
Here, Sample 4, 5, or 2 was used as the glass sheet with a
transparent conductive film so as to correspond to the samples
whose external quantum efficiencies were shown in FIG. 5 (i.e.,
Sample 13 (fine particle film), Sample 14 (fine particle mixed
film), and Sample 22 (without an antireflection film)).
[0094] The photoelectric conversion unit including an amorphous
silicon layer as the photoelectric conversion layer was prepared in
the same manner as that for the amorphous silicon photoelectric
conversion unit in a tandem type. However, the thickness of an
intrinsic amorphous silicon layer was 320 nm. The intrinsic
amorphous silicon layer prepared in the same manner as that
described above had a band gap of 1.75 eV.
[0095] In the photoelectric conversion unit including an amorphous
silicon germanium layer as the photoelectric conversion layer,
p-type and n-type layers were formed in the same manner as that for
the amorphous silicon photoelectric conversion unit in a tandem
type. However, the photoelectric conversion layer was formed by RF
plasma CVD under the condition that SiH.sub.4 and GeH.sub.4 were
used as a reaction gas, a pressure in the reaction chamber was
about 40 Pa, an RF power density was 15 mW/cm.sup.2, and a film
deposition temperature was 150.degree. C. The ratio of GeH.sub.4 to
the entire film deposition gas was about 5 mol %. The thickness of
an intrinsic silicon germanium layer was 200 nm. The intrinsic
silicon germanium layer prepared in the same manner as that
described above had a band gap of 1.55 eV.
[0096] Moreover, a back electrode was formed on each of the
photoelectric conversion units in the same manner as that described
above to complete a photoelectric conversion device. The wavelength
dependence of external quantum efficiency of each photoelectric
conversion device was measured in the same manner as that described
above.
[0097] FIG. 7 shows the wavelength dependence of external quantum
efficiency in using the photoelectric conversion unit having the
photoelectric conversion layer formed of an amorphous silicon layer
with an increased thickness. FIG. 8 shows the wavelength dependence
of external quantum efficiency in using the photoelectric
conversion unit having the photoelectric conversion layer formed of
an amorphous silicon germanium layer. As shown in FIG. 7, even if
an intrinsic amorphous silicon thin film was used as the
photoelectric conversion layer, the sufficient effect of improving
photoelectric conversion efficiency was able to be obtained by
increasing the film thickness to provide high photosensitivity in a
long wavelength region. In view of this, the intrinsic amorphous
silicon thin film was formed to have a thickness of at least about
320 nm, though the degree of increase in the thickness depended on
a manufacturing method or the like.
[0098] In either case shown in FIGS. 7 and 8, the external quantum
efficiency was improved in a relatively broad wavelength region,
including a long wavelength region, by the antireflection films
containing fine particles. Such antireflection films were suitable
for a photoelectric conversion device whose external quantum
efficiency was 0.2 or more at a wavelength of 700 nm when including
no antireflection film, particularly 0.3 or more.
[0099] As described in detail above, the present invention can
provide a photoelectric conversion device that improves the
photoelectric conversion efficiency with the interaction between a
transparent substrate with a transparent conductive film, an
antireflection film formed of fine particles, and a photoelectric
conversion unit (in particular, a crystalline silicon based
photoelectric conversion unit). The photoelectric conversion device
of the present invention, in which the characteristics of each
member are mutually adjusted properly, can achieve an extremely
rational improvement in the photoelectric conversion efficiency
without unnecessary increase in the manufacturing cost.
[0100] The invention may be embodied in other forms without
departing from the spirit or essential characteristics thereof. The
embodiments disclosed in this application are to be considered in
all respects as illustrative and not limiting. The scope of the
invention is indicated by the appended claims rather than by the
foregoing description, and all changes which come within the
meaning and range of equivalency of the claims are intended to be
embraced therein.
* * * * *