U.S. patent application number 11/246756 was filed with the patent office on 2006-03-02 for transparent conductive film and its manufacturing method, and photoelectric conversion device comprising it.
This patent application is currently assigned to NIPPON SHEET GLASS CO., LTD.. Invention is credited to Hiroki Ando, Takahiro Asai, Akira Fujisawa, Masahiro Hirata, Kiyotaka Ichiki, Koichiro Kiyohara, Masatoshi Nara, Yukio Sueyoshi.
Application Number | 20060046026 11/246756 |
Document ID | / |
Family ID | 27531718 |
Filed Date | 2006-03-02 |
United States Patent
Application |
20060046026 |
Kind Code |
A1 |
Fujisawa; Akira ; et
al. |
March 2, 2006 |
Transparent conductive film and its manufacturing method, and
photoelectric conversion device comprising it
Abstract
A transparent conductive film wherein the height number
distribution of projections present on the surface is expressed by
a distribution function of X.sup.2 type having a degree of freedom
of 3.5 to 15 when the unit of the horizontal axis is a nanometer,
the height/width ratio number distribution is expressed by a
distribution function of X.sup.2 type having a degree of freedom of
10-35X.sup.2, the projections having a height of 50-350 nm account
for 70% of more, and the projections having a height/width ratio of
0.25-1.02 account for 90% or more.
Inventors: |
Fujisawa; Akira; (Osaka,
JP) ; Nara; Masatoshi; (Osaka-shi, JP) ; Asai;
Takahiro; (Osaka-Shi, JP) ; Sueyoshi; Yukio;
(Osaka-Shi, JP) ; Ichiki; Kiyotaka; (Osaka-Shi,
JP) ; Kiyohara; Koichiro; (Osaka-Shi, JP) ;
Hirata; Masahiro; (Osaka-Shi, JP) ; Ando; Hiroki;
(Osaka-Shi, JP) |
Correspondence
Address: |
HAMRE, SCHUMANN, MUELLER & LARSON, P.C.
P.O. BOX 2902-0902
MINNEAPOLIS
MN
55402
US
|
Assignee: |
NIPPON SHEET GLASS CO.,
LTD.
OSAKA-SHI
JP
KANEKA CORPORATION
OSAKA-SHI
JP
|
Family ID: |
27531718 |
Appl. No.: |
11/246756 |
Filed: |
October 7, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10432505 |
Aug 22, 2003 |
|
|
|
PCT/JP01/10149 |
Nov 20, 2001 |
|
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11246756 |
Oct 7, 2005 |
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Current U.S.
Class: |
428/142 ;
257/E31.042; 257/E31.126; 257/E31.13; 427/226; 427/248.1;
428/432 |
Current CPC
Class: |
C23C 16/407 20130101;
C03C 2217/77 20130101; H01L 31/1884 20130101; C03C 17/3464
20130101; Y02E 10/50 20130101; H01L 31/0236 20130101; H01L 31/02366
20130101; H01L 31/022483 20130101; H01L 31/022466 20130101; H01L
31/02363 20130101; Y10T 428/24364 20150115; H01L 31/03921 20130101;
C03C 17/3482 20130101 |
Class at
Publication: |
428/142 ;
427/226; 427/248.1; 428/432 |
International
Class: |
B05D 3/02 20060101
B05D003/02; D06N 7/00 20060101 D06N007/00; B32B 17/06 20060101
B32B017/06 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 21, 2000 |
JP |
2000-355068 |
Nov 21, 2000 |
JP |
2000-355079 |
Nov 21, 2000 |
JP |
2000-354956 |
Dec 19, 2000 |
JP |
2000-385163 |
Nov 15, 2001 |
JP |
2001-350207 |
Claims
1-10. (canceled)
11. A manufacturing method for a transparent conductive film,
wherein a transparent conductive film is formed on a surface of a
substrate at 550.degree. C. to 650.degree. C. through a thermal
decomposition method, and wherein a mixed gas used in the thermal
decomposition method includes at least 10 mole percent of a gas
whose density in a standard state is not more than 1 g/L.
12. The manufacturing method for a transparent conductive film
according to claim 11, wherein the gas is an inert gas.
13. The manufacturing method for a transparent conductive film
according to claim 12, wherein the inert gas is helium.
14. A manufacturing method for a transparent conductive film,
wherein a transparent conductive film is formed through a thermal
decomposition method, and wherein a mixed gas used in the thermal
decomposition method includes a chlorine atom containing gas at a
concentration of not more than 10 mole percent and tin
compounds.
15. The manufacturing method for a transparent conductive film
according to claim 14, wherein the chlorine atom containing gas is
hydrogen chloride.
16. The manufacturing method for a transparent conductive film
according to claim 14, wherein the chlorine atom containing gas is
chlorine.
17-20. (canceled)
Description
[0001] This application is a divisional of application Ser. No.
10/432,505, filed May 21, 2003, which application(s) are
incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to transparent conductive
films that can be used as thin film electrodes of photoelectric
conversion devices, of which a representative example is a solar
cell, methods for manufacturing them, substrates provided with
them, and photoelectric conversion devices using these
substrates.
BACKGROUND ART
[0003] Recent energy and environmental issues have drawn attention
toward solar cells. From the standpoint of conserving resources,
thin-film type solar cells will likely become mainstream. In
general, devices in which a transparent conductive film, made of
tin oxide (SnO.sub.2) for example, and an amorphous semiconductor
such as amorphous silicon or amorphous silicon germanium serving as
a photoelectric conversion layer are formed on a transparent
substrate such as a glass sheet are used as thin-film solar
cells.
[0004] Thermal decomposition methods often are used to form such
transparent thin films, and transparent conductive films formed by
such methods are polycrystalline. In the case of polycrystalline
bodies of tin oxide, crystal growth proceeds as the transparent
conductive film grows in thickness, resulting in the surface of the
transparent conductive film becoming uneven. Thin-film solar cells
are designed to exploit this unevenness in the surface of the
transparent thin film and scatter incident light, and thereby
increase the length of the optical path in the photoelectric
conversion layer, exhibiting a so-called "light trapping effect."
Exhibiting this light trapping effect results in an improvement in
the conversion efficiency of the photoelectric conversion
device.
[0005] For example, JP S62-7716B discloses an amorphous silicon
solar cell in which the average grain diameter in the surface of
the transparent electrode is not less than 0.1 .mu.m and not more
than 2.5 .mu.m, and mentions that the correlation between the size
of the crystal grain diameter of the transparent electrode and the
efficiency of the amorphous silicon solar cell was examined.
However, the grain diameter generally exhibits the above-mentioned
average grain diameter if a transparent electrode such as an indium
oxide film or a tin oxide film is formed through a spray method or
CVD (chemical vapor deposition), which are mentioned in this
publication. The publication does not mention the ideal height of
the crystal grain diameter or the distribution of the diameter or
the height of the grains.
[0006] JP H2-503615A discloses a solar cell substrate that includes
a conductive film having projections with diameters of 0.1 to 0.3
.mu.m and a height/diameter ratio of 0.7 to 1.2 in its surface.
However, this publication does not mention the distribution of the
height of the projections or of the height/diameter ratio.
[0007] JP H4-133360A discloses a photovoltaic device that includes
a tin oxide film whose surface has projections in the shape of
truncated pyramids with heights of 100 to 300 nm and in which the
angles between them and the normal of the principle surface of the
substrate are 30.degree. to 60.degree.. However, this publication
does not mention the distribution of the height of the projections
or of the angles between the normal lines and the main surface of
the substrate.
[0008] JP H3-28073B discloses a photovoltaic device having a
translucent conductive oxide provided with an uneven surface with
an average grain diameter of approximately 50 to 200 nm, a height
difference of approximately 100 to 500 nm, and a spacing between
projections of approximately 200 to 1000 nm. However, it does not
mention the distribution of the grain diameters or of the height
difference.
[0009] All of the photoelectric conversion devices disclosed in
these laid-open publications are under the premise that amorphous
silicon is used for the photoelectric conversion layer. Amorphous
silicon as a material is well suited for the photoelectric
conversion layer of photoelectric conversion devices because even
at a low film thickness it exhibits high photoelectric conversion
efficiency. The film thickness of photoelectric conversion layers
made of intrinsic amorphous silicon is about 50 to 700 nm, whereas
in the case of p-type and/or n-type conductive amorphous silicon
layers in contact with the transparent conductive film, the film
thickness is about 3 to 100 nm.
[0010] With a photoelectric conversion layer made of amorphous
silicon, however, although the absorption coefficient of light at
wavelengths shorter than its energy gap is large, the absorption
coefficient of light on the long wavelength side thereof
progressively diminishes as the wavelength increases. For this
reason, most light on the long wavelength side is discharged
outside the system of the photoelectric conversion device without
being absorbed by the photoelectric conversion layer. Increasing
the length of the optical path in the photoelectric conversion
layer is the most effective way to increase the amount of light on
the long wavelength side that is absorbed. Although the light
trapping effect is exhibited somewhat even with conventional
transparent conductive films, because there is such a strong
relationship between the shape of the unevenness in the surface and
the wavelength of the light, a shape that effectively scatters
light in the absorption region of amorphous silicon is not
necessarily also effective for light at long wavelengths. In other
words, there is a separate shape that is suited for dispersing
light at long wavelengths for the unevenness in the surface of a
transparent conductive film.
[0011] In recent years, photoelectric conversion layers constituted
by amorphous silicon-germanium, thin film polycrystalline silicon,
or microcrystalline silicon, for example, in place of amorphous
silicon have started to come into use. These absorb much of the
light at wavelengths longer than the visible spectrum region, which
is the absorption region of amorphous silicon.
[0012] Consequently, future increases the conversion efficiency of
solar cells will require transparent conductive films that can
effectively scatter light of wavelengths longer than the absorption
region of amorphous silicon.
[0013] Conventionally, due to inadequate research on the
relationship between the surface shape of transparent conductive
films and the wavelength of the scattered light, it was thought
that it was important to scatter light of wavelengths below the
absorption region of amorphous silicon. This ultimately resulted in
a tendency toward greater uniformity in the surface unevenness of
transparent conductive films. When the surface unevenness of a
transparent conductive film is made uniform, the reflectance of the
transparent conductive film is increased, and if the transparent
conductive film has a high reflectance, then light cannot
sufficiently enter the photoelectric conversion layer because
incident light that has been transmitted through the transparent
substrate is reflected by the transparent conductive film. This
resulted in the problem of a drop in the conversion efficiency of
the photoelectric conversion device.
[0014] The present invention has an aspect that was arrived at in
light of the above problems. It is an object of this aspect, at
least in a preferable embodiment of the invention, to provide a
transparent conductive film with which light of wavelengths longer
than the absorption region of amorphous silicon can be scattered
effectively and which itself has a low reflectance. It is a further
object to provide a photoelectric conversion device that is
furnished with this transparent conductive film and that has
improved photoelectric conversion efficiency. Hereinafter, the
invention having this aspect is referred to as the "invention of
the first embodiment."
[0015] The above-mentioned publications frequently mention the
shape of the surface of the transparent conductive film, however,
they do not touch upon a case in which the projections are of
different shapes or mention their size. A transparent conductive
film having many projections with trapezoid-shaped cross sections
has increased reflectance, lowering the amount of incident light on
the photoelectric conversion layer. When the trapezoid-shaped
projections are large, parts of the p-type amorphous silicon layer
of the photoelectric conversion layer in contact with the
transparent conductive film become thin, leading to instances in
which shorts occur between the transparent conductive film and the
i-type layer formed on the p-type layer. Also, the thickness
distribution of the p-type layer becomes nonuniform, which at times
also leads to nonuniformity at the p-i junction or the i-n
junction. As a result, there is a drop in the characteristics of
the photoelectric conversion device.
[0016] The present invention has another aspect that was arrived at
in light of these problems. It is an object of this aspect, in at
least a preferable embodiment of the invention, to provide a high
performance photoelectric conversion device, and a transparent
conductive film used in that photoelectric conversion device, in
which the reflectance of the transparent conductive film is kept
low by reducing the proportion of projections regarded as having
trapezoid-shaped cross sections, and in which shorts are prevented
by reducing the number of large trapezoid-shaped projections.
Hereinafter, this is referred to as the "invention of the second
embodiment."
[0017] When there are defects such as pinholes in even one of the
p-type, i-type, or n-type layers in the photoelectric conversion
layer, shorts may occur and the photoelectric conversion efficiency
drops noticeably. When the p-type and/or n-type layers are
increased in thickness in order to prevent such defects from
occurring, the amount of light that is absorbed in those layers is
increased, reducing the amount of light that is incident on the
i-type layer, which is the photoelectric conversion layer. Also, it
leads to nonuniformity at the p-i junction or the i-n junction,
which lowers the photoelectric conversion efficiency. Consequently,
in light of this problem, the transparent conductive film should be
given a level surface. On the other hand, if the light trapping
effects are to be exploited, then it is conceivably preferable that
the surface has numerous large protrusions in order to raise the
photoelectric conversion efficiency and in particular to increase
the effect of scattering long wavelength light.
[0018] After intense research regarding the shape of a transparent
conductive film surface that adequately achieves these conflicting
desirable characteristics, the present inventors found that a state
in which there are both few very large projections that protrude
out and also numerous relatively large projections is ideal.
[0019] In other words, in yet another aspect of the invention, in
at least a preferable embodiment thereof, it is an object to
provide a transparent conductive film having tin oxide at its main
component and an ideal surface shape to serve as the thin film
electrode of a photoelectric transducer element. Hereinafter, this
invention is referred to as the "invention of the third
embodiment."
DISCLOSURE OF INVENTION
[0020] The present invention comprises the above-mentioned three
embodiments, and the object shared by these three embodiments is to
increase the photoelectric conversion efficiency of a photoelectric
conversion device.
[0021] According to the invention of the first embodiment, there is
provided a transparent conductive film
[0022] wherein, with respect to projections present in its
surface,
[0023] the average value of a number distribution of height is
larger than its mode, and when the number distribution of height is
regarded as following an X.sup.2 type distribution function in
which a horizontal axis is displayed in nanometer units, then the
degree of freedom with which it can best be approximated is 3.5 to
15;
[0024] the average value of a number distribution of the
height/width ratio is larger than its mode, and when the number
distribution of the height/width ratio is regarded as following an
X.sup.2 type distribution function, then the degree of freedom with
which it can best be approximated is 10 to 35;
[0025] the number of projections having a height of 50 to 350 nm is
at least 70%; and
[0026] the number of projections having a height/width ratio of
0.25 to 1.05 is at least 90%.
[0027] According to the invention of the second embodiment, there
is provided a transparent conductive film whose area occupied by
projections that, when their cross section shape is regarded as a
trapezoid, have a slope of an upper side with respect to a lower
side of the trapezoid of not more than 20.degree. and a ratio of a
length of the upper side to the lower side of at least 0.8,
corresponds to not more than 10% of an area of a surface on which
the transparent conductive film is formed.
[0028] According to the invention of the third embodiment, there is
provided a transparent conductive film that is a thin film whose
main component is tin oxide and whose thickness is at least 400 nm,
and a variance in diameter of the projections in its surface is not
more than 0.01 .mu.m.sup.2.
[0029] The invention also provides a method suited for the
production of these transparent conductive films. In one embodiment
of this method, a transparent conductive film is formed on a
surface of a substrate at 550.degree. C. to 650.degree. C. through
a thermal decomposition method and a mixed gas used in the thermal
decomposition method includes at least 10 mole percent of a gas
whose density in a standard state is not more than 1 g/L. In
another embodiment of this method, a transparent conductive film is
formed through a thermal decomposition method and a mixed gas used
in the thermal decomposition method includes a chlorine atom
containing gas at a concentration of not more than 10 mole percent
and tin compounds.
[0030] The invention further encompasses a substrate for a
photoelectric conversion device including the above transparent
conductive film and a transparent substrate, wherein the
transparent conductive film is formed on the substrate. The
invention also includes a photoelectric conversion device including
the above substrate for photoelectric conversion device and at
least one photoelectric conversion layer.
BRIEF DESCRIPTION OF DRAWINGS
[0031] FIG. 1 is a cross-sectional diagram that schematically shows
an embodiment of a photoelectric conversion element of the
invention.
[0032] FIG. 2 is a diagram that schematically shows a device for
use in an on-line CVD method.
[0033] FIG. 3 shows the appearance of the transparent conductive
film produced in Working Example 2 observed using a scanning
electron microscope (SEM).
[0034] FIG. 4 is a diagram showing the number distribution function
of the height of the projections of the transparent conductive film
produced in Working Example 2.
[0035] FIG. 5 is a diagram showing the number distribution function
of height/width of the projections of the transparent conductive
film produced in Working Example 2.
[0036] FIG. 6 is a diagram showing the number distribution
function, measured using the method disclosed in JP H2-503615A, for
the height/diameter of the projections of the transparent
conductive film produced in Working Example 2.
[0037] FIG. 7 is a diagram showing the measured data on the
reflectance of the transparent conductive film produced in Working
Example 2.
[0038] FIG. 8 is a conceptual diagram showing the cross-sectional
shape of the transparent conductive film according to the invention
of the second embodiment.
[0039] FIG. 9 is a diagram showing the distribution of the
diameters of the projections of the transparent conductive film
produced in Working Example 3 calculated from the maximum cross
section area of the projections.
[0040] FIG. 10 shows the appearance of the surface of the
transparent conductive film of Working Example 8 observed by SEM,
where FIGS. 10A and 10B correspond to magnifications of 20,000 and
45,000 times, respectively.
[0041] FIG. 11 shows the appearance of the surface of the
transparent conductive film of Working Example 9 observed by SEM,
where FIGS. 11A and 11B correspond to magnifications of 20,000 and
45,000 times, respectively.
EMBODIMENTS OF THE INVENTION
[0042] Embodiments of the invention are explained in detail below,
however, the invention is not limited to the following
embodiments.
[0043] First, the first embodiment of the invention, that is, the
invention relating to a transparent conductive film that
effectively scatters light of wavelengths longer than the
absorption region of amorphous silicon and that has a low
reflectance, is described.
[0044] As mentioned above, the size of the unevenness in the
surface of the transparent conductive film and the wavelength of
the light that is scattered by the surface are closely related, and
it is necessary to increase the size of the surface unevenness over
that seen conventionally in order to scatter light of wavelengths
longer than the absorption region of amorphous silicon. However,
when each projection of the transparent conductive film is grown
large, the scattering of light near the absorption region of
amorphous silicon is reduced, which conversely poses the risk of
lowering the light trapping effects in all wavelength regions and
leading to shorts due to large projections.
[0045] Accordingly, after intense research on the relationship
between the surface shape of the transparent conductive film and
the wavelength of the scattered light, the present inventors found
that if the transparent conductive film is to be used in a thin
film solar cell, then a condition in which a certain degree of
homogeneity is maintained in the surface shape of the transparent
conductive film and at the same time the surface is provided with
appropriate dispersion properties is preferable.
[0046] A surface shape that is appropriately provided with both
homogeneity and dispersion properties is provided with the
following characteristics with regard to the projections in its
surface. With respect to the number distribution of the height of
the projections, the average value is larger than its mode, and
when the distribution is regarded as following an X.sup.2-type
distribution function (X.sup.2 distribution) in which the
horizontal axis is displayed in nanometer units, the degree of
freedom with which it can be best approximated is from 3.5 to 15.
Also, with respect to the number distribution of the height/width
ratio distribution of the projections, the average value is larger
than its mode and when the distribution is regarded as following an
X.sup.2 distribution the degree of freedom with which it can best
be approximated is from 10 to 35. At least 70% of the projections
have a height of 50 to 350 nm and at least 90% of the projections
have a height/width ratio of 0.25 to 1.05. These conditions were
obtained recursively based on the results of a large number of
experiments performed by the present inventors. With this
transparent conductive film, the reflectivity of the film itself is
reduced, allowing interference by the reflectance spectrum to be
suppressed, and a large amount of light can be incorporated into
the photoelectric conversion layer of the solar cell, even with the
amount of light in the wavelength regions remaining the same.
[0047] Here, the method for measuring the height of the projections
and the width of the projections is described. The height of the
projections is calculated based on an SEM photograph captured from
above at an elevation angle of 30.degree. and a magnification of
45,000 times. More specifically, for projections entirely captured
in the SEM photograph, a vertical line is drawn from the vertex of
each projection to its base line, and from the length of this
vertical line the height is calculated. The width of the
projections is found for projections entirely captured in the SEM
photograph by calculating the distance between the vertex of a
certain projection and the vertex of the nearest projection based
on the SEM photograph.
[0048] If the number distribution of the height of the projections
follows an X.sup.2-type distribution function, then there are
hardly any large projections that protrude outward and at the same
time a certain degree of variation is maintained in the height of
the projections. To effectively exhibit the light trapping effects,
it is necessary that the average grain diameter is large to a
certain extent, and variation in the grain diameter is necessary in
order to cause light scattering at various wavelengths. However,
although a certain extent is necessary, too many large projections
leads to poorer photoelectric conversion properties, in that the
film thickness of the p-type amorphous silicon layer formed on the
transparent conductive film becomes thin in parts, leading to
shorts between the transparent conductive film and the i-type layer
formed after the p-type layer, and the thickness distribution of
the p-type layer becomes nonuniform, leading to heterogeneity in
the later-formed p-i or i-n junctions.
[0049] One example of a representative distribution function is a
normal distribution function, however, in a normal distribution, in
principle the value at which the frequency is a maximum (mode) is
the average value, and grains smaller and larger than that are
symmetrical. When the distribution of the number of projections
follows a normal distribution, the proportion of large grains
increases if the average grain diameter demonstrates a certain
degree of size. For this reason, variation according to an
X.sup.2-type distribution function, in which the percentage of
large grains is small, is preferable. With respect to the degree of
freedom of an X.sup.2-type distribution function, it is desirable
that when the horizontal axis represents the height of the
projections in nanometer units and grades are set every 50 nm and
the frequency is displayed for each grade, then the degree of
freedom can be best approximated at 3.5 to 15. When the degree of
freedom is less than 3.5, there are too many tiny grains, and thus
the light trapping effects cannot be adequately exhibited. When the
degree of freedom is greater than 15, there are too many large
grains.
[0050] If the number distribution representing the ratio of the
height of the projections with respect to the width of the
projections follows an X.sup.2-type distribution function, then a
certain extent of variation is maintained in the width of the
projections and at the same time there are not many projections
that are either too steep or too flat. The degree of freedom of the
X.sup.2-type distribution function at this time can be best
approximated at 10 to 35. When the degree of freedom is less than
10, there are too many projections with gentle slopes, and thus the
effect of reducing the reflectance is inhibited. On the other hand,
when the degree of freedom is greater than 35, there are too many
steep projections, and thus the p-type amorphous silicon layer
formed on the transparent conductive film becomes thin in parts,
leading to deterioration of the photoelectric conversion properties
in the same manner as described above.
[0051] Here, an X.sup.2-type distribution function (X.sup.2
distribution) will be explained. In a normal distribution function,
in principle the mode is the average value, whereas in an
X.sup.2-type distribution function, a value that is slightly larger
than the mode becomes the average value. For this reason, in an X-Y
correlation diagram, there is relatively large expansion in the
positive direction of the x-axis in an X.sup.2-type distribution
function. Also, with respect to the height of the projections and
the ratio of the height of the projections to the width of the
projections, if those measured results are plotted in an
X.sup.2-type distribution function and the most appropriate degree
of freedom falls within the above-mentioned range, then the effects
of the invention of the first embodiment are achieved reliably.
[0052] The light trapping effects cannot be attained adequately
when there is a large number of projections that are less than 50
nm in height, and the effects of reducing the reflectance cannot be
adequately obtained if there are too many projections in which the
height to width ratio (height/width) of the projections is less
than 0.25. On the other hand, if there are too many projections
with heights over 350 nm and if there are too many projections with
a height to width ratio in excess of 1.05, then the p-type
amorphous silicon layer formed on the transparent conductive film
becomes thin in parts, leading to shorts between the later-formed
i-type layer and the transparent conductive film, and the thickness
distribution of the p-type layer becomes nonuniform. This leads to
heterogeneity in the later-formed p-i junction and i-n junction,
deteriorating the photoelectric conversion properties. In other
words, with respect to the projections in the surface of the
transparent conductive film, it is preferable that the number of
projections that have a height of 50 to 350 nm is not less than 70%
and that the number of projections with a height/width ratio of
0.25 to 1.05 is not less than 90%. More preferably, the number with
a height of 100 to 350 nm is preferably not less than 30% and the
number with a height to width ratio of 0.35 to 0.95 is not less
than 80%. When there are too many projections that are less than
100 nm in height, it becomes difficult to exhibit the light
trapping effects, and the effects of reducing the reflectance
cannot be adequately attained either. To adequately obtain the
effect of reducing reflectance, it is preferable that the number of
projections in which the height to width ratio is less than 0.35
also is reduced.
[0053] To scatter light in long wavelength regions and obtain the
effect of trapping light in these regions, it is preferable that
the number of projections that are at least 250 nm high is in the
range of 0.2 to 20%. When less than 0.2%, the effects of trapping
light in long wavelength regions is not adequately exhibited, and
when greater than 20%, the p-type amorphous silicon layer formed on
the transparent conductive film becomes thin in parts.
[0054] If the surface unevenness of the transparent conductive film
is in the above state, then the average reflectance of a
transparent substrate provided with the transparent conductive film
can be kept to 10% or less for wavelengths of 300 to 1200 nm. Here,
the average reflectance is the value measured at the opposite
surface to the surface on which the transparent conductive film is
formed. The average reflectance is the total amount of light that
is directly reflected by the surface of the transparent substrate
and the light that is reflected by the transparent conductive film.
It should be noted that to accurately carry out measurement, it is
necessary that the directionality of the measuring device is
adjusted suitably so that the device does not pick up stray light
that has been subjected to multiple scattering and/or multiple
reflecting.
[0055] With this transparent conductive film, the conversion
efficiency of the photoelectric conversion device can be improved
not only if the photoelectric conversion layer is amorphous
silicon, of course, but also in a case in which it is thin film
polycrystalline silicon, for example, which promises to become
mainstream in the future.
[0056] A configuration in which this transparent conductive film is
used in a thin film solar cell, that is, a photoelectric conversion
device, is described with reference to FIG. 1. In the photoelectric
conversion device shown in FIG. 1, undercoating films 1 and 2, a
transparent conductive film 3, a photoelectric conversion layer 7,
and a rear surface electrode 8 are formed in that order on a
transparent substrate 5 disposed on the light incidence side.
[0057] As long as the transparent substrate 5 is a transparent
insulating body, there are no particular limitations with respect
to its type. However, it is preferably provided with sufficient
heat resistance to withstand thermal decomposition methods.
Consequently, a glass sheet is ideal as the transparent
substrate.
[0058] If a glass sheet is used as the transparent substrate, then
it is preferable that the undercoating films 1 and 2 are provided
in advance before the transparent conductive film is formed. Glass
sheets ordinarily contain large quantities of alkaline components.
Thus when a transparent conductive film is formed directly on its
surface the alkaline components diffuse into the transparent
conductive film, deteriorating performance by raising the
electrical resistance or lowering the transmittance, for example.
In addition, undercoating films also perform the functions of
adjusting the surface shape of the transparent conductive film to a
specific shape and increasing the adhesion strength between the
transparent substrate and the transparent conductive film, for
example. The film thickness of the undercoating films is preferably
5 to 150 nm, and it is preferable that their main component is
silicon oxide and/or aluminum oxide. The undercoating layer can be
constituted by one layer, or two or more layers.
[0059] If the undercoating layer is constituted by two or more
layers, then it is preferable that the first undercoating layer 1,
which is formed directly on the glass substrate, has a film
thickness of 5 to 100 nm and has a refractive index of 1.6 to 2.5.
The first undercoating layer 1 preferably includes as its main
component at least one species selected from the group consisting
of tin oxide, titanium oxide, and aluminum oxide. The second
undercoating layer 2, which is nearer the transparent conductive
film 3, preferably has a film thickness of 5 to 100 nm and a
refractive index of 1.4 to 2.0. The second undercoating layer 2
preferably includes as its main component at least one selected
from the group consisting of silicon oxide, aluminum oxide, and tin
oxide. The refractive index of the first undercoating layer is
preferably higher than the refractive index of the second
undercoating layer.
[0060] The transparent conductive film 3 is ideally a thin film
whose main component is tin oxide, a thin film whose main component
is indium oxide, or a thin film whose main component is zinc oxide.
In this specification, "main component," as it is usually defined,
refers to a component that constitutes at least 50 atom percent of
the composition, and by definition it does not exclude the presence
of trace components. For example, a thin film whose main component
is tin oxide may include tin oxide doped with fluorine or antimony,
for example. Tin oxide doped with a predetermined concentration of
fluorine (SnO.sub.2:F) is particularly preferable. If it is doped
with fluorine, it is preferable that the concentration of the
fluorine is not more than 0.15 weight percent and that the
refractive index at that time is approximately 1.9. Other examples
of components include silicon, aluminum, zinc, copper, indium,
bismuth, gallium, boron, vanadium, manganese, and zirconium.
However, it is preferable that the concentration of these
components is not more than 0.02 weight percent. The sheet
resistance of the transparent conductive film is preferably 5 to 30
.OMEGA./.quadrature. (square). In consideration of this value, the
film thickness of the transparent conductive film on the light
incidence side is preferably 500 to 2000 nm.
[0061] The photoelectric conversion layer 7 is made of amorphous
silicon, for example, and after the transparent conductive film 3
has been formed, it is formed on the transparent conductive film 3
through publicly known means, such as vapor deposition, thermal
CVD, or plasma CVD using glow discharge in which monosilane diluted
by hydrogen gas serves as the raw material. The thin film of
amorphous silicon can be formed by suitably adding methane,
aluminum, diborane, or phosphine, for example, to a thin film of
silicon and then forming a p-type layer, an i-type layer, and an
n-type layer in that order from the transparent conductive film
side in order to form an ordinary p-i-n junction. Of course, in
place of amorphous silicon it is also possible to form a compound
semiconductor thin film such as amorphous silicon germanium,
microcrystalline silicon, microcrystalline silicon carbide, thin
film polycrystalline silicon, crystalline silicon, CdTe, or
CuInSe.sub.2, for example, as the photoelectric conversion layer.
The photoelectric conversion layer can be made of a single thin
film layer of amorphous silicon or can be made of a layered thin
film in which amorphous silicon and microcrystalline silicon are on
top of one another. The film thickness of conductive-type (p-type,
n-type) microcrystalline silicon is preferably 3 to 100 nm and more
preferably 5 to 50 nm. In a photoelectric conversion device
provided with a crystalline photoelectric conversion layer such as
microcrystalline silicon, the transmittance of the transparent
conductive film has a greater effect on the photoelectric
conversion efficiency of the film than the sheet resistance does.
It should be noted that before the rear surface electrode is formed
it is also possible to form a thin film such as a zinc oxide film
beforehand for the purpose of improving the reflectance and
preventing the diffusion of impurities.
[0062] The shape of the surface of the photoelectric conversion
layer 7 depends on the surface shape of the transparent conductive
film 3. For that reason, by processing the transparent conductive
film beforehand through etching, blasting, or stamping, for
example, the shape of the unevenness in the surface of the
photoelectric conversion layer can be controlled.
[0063] The intrinsic amorphous silicon layer (i-type layer) is
preferably formed through plasma CVD with the temperature of the
transparent substrate at not more than 450.degree. C. This layer
can be formed as a substantially intrinsic semiconductor thin film
in which the density of the conduction-type determining impurity
atoms is not more than 1.times.10.sup.18cm.sup.-3. The film
thickness of the intrinsic amorphous silicon layer is preferably
0.05 to 0.5 .mu.m.
[0064] The rear surface electrode 8 is formed on the photoelectric
conversion layer. This rear surface electrode, like the transparent
conductive film, can be a thin film whose main component is tin
oxide, or it may be at least one layer of a metallic thin film of
at least one material selected from aluminum (Al), silver (Ag),
gold (Au), copper (Cu), platinum (Pt), and chromium (Cr). This
metallic thin film can be formed through sputtering or vapor
deposition. If a metallic thin film is used as the rear surface
electrode, then it is also possible to provide a layer made of a
conductive oxide such as indium-doped tin oxide (ITO), tin oxide
(SnO.sub.2), or zinc oxide (ZnO) between the photoelectric
conversion layer and the rear surface electrode.
[0065] It should be noted that the present inventors measured the
surface unevenness of the photoelectric conversion layer and
analyzed these results in combination with the results of a
measurement of the surface unevenness of the transparent conductive
film, and found that the reflectance of the rear surface electrode
is low if the ratio of the average projection height divided by the
average width to adjacent projections at the interface of the rear
surface electrode is smaller than that of the transparent
conductive film.
[0066] There are no particular limitations to the method for
forming the transparent conductive film, although it is preferably
formed through a thermal decomposition method. Publicly known
examples of thermal decomposition methods include spray methods and
CVD, and CVD is preferable because the surface shape can be
adjusted through controlling the type or flow of the raw material
for the tin, the raw material for oxidation, and the raw material
for the additives. If undercoating films are provided, CVD is
preferably used in the same fashion as it is for the transparent
conductive film. The surface shape of the undercoating films has a
large impact on the surface shape of the transparent conductive
film, and thus selecting the type, number, and thickness, for
example, of the undercoating films is important. If CVD is used,
the surface shape of the undercoating films can be adjusted easily
and moreover the undercoating films and the transparent conductive
film can be formed contiguously in a series of film-formation
steps.
[0067] An example of a thermal decomposition method is so-called
"on-line CVD," which is a CVD method in which the thermal
decomposition reaction of the raw gas is allowed to proceed on the
surface of a molten glass ribbon on the line in which the glass
sheet is manufactured through a float method. Using on-line CVD
allows thin films to be formed on the surface of the glass ribbon,
which is hotter than the softening point of glass, and thus a thin
film that is higher quality than those formed through other thermal
decomposition reactions is formed. Localized temperature
irregularities in the surface temperature of the glass ribbon are
small, and thus variations in the thickness of the film, for
example, are small and the formation of massive projections and
pinholes is prevented. Also, because the heat that is generated
from the glass ribbon expedites the thermal decomposition
reactions, the films are formed quickly and energy costs can be
kept down because the addition of heat is not necessary. On-line
CVD is suited for forming films with large areas, and it is
particularly useful in applications in which the films can be
applied to large-area photoelectric conversion devices, such as the
windowpanes of buildings and the like or as roof material, for
example.
[0068] FIG. 2 shows an example of a device used in on-line CVD.
With this device, a glass ribbon 10 flows from a melting furnace
(float crucible) 11 into a tin float tank (float bath) 12 and is
moved in a belt-like manner over a tin bath 15. A predetermined
number of coaters 16 (in the configuration of the drawing, there
are three coaters 16a, 16b, and 16c) are disposed at a
predetermined distance from the surface of the glass ribbon 10. Raw
material in a gaseous state is supplied from these coaters, forming
a contiguous thin film on the glass ribbon 10. Employing a
plurality of coaters allows an undercoating film and a conductive
film to be formed contiguously on the glass ribbon 10. The glass
ribbon 10 on which the thin films, including the transparent
conductive film, have been formed is lifted up by rollers 17 and
delivered into a cooling furnace 13. It should be noted that the
glass sheet cooled in the cooling furnace 13 is cut into glass
sheets of a predetermined size by a float method general-purpose
cutting device not shown in the drawing.
[0069] When using CVD to form undercoating films and/or a
transparent conductive film whose main component is tin oxide,
examples of the raw material for the tin include tin tetrachloride,
dimethyltin dichloride, dibutyltin dichloride, tetramethyltin,
tetrabutyltin, dioctyltin dichloride, and monobutyltin trichloride.
Organic tin halides such as dimethyltin dichloride and monobutyltin
trichloride are particularly preferable. Oxygen, water vapor, or
dry air, for example, can be employed as the oxidizing agent for
forming tin oxide from the raw material for tin. Examples of the
raw material for fluorine include hydrogen fluoride,
trifluoroacetate, bromotrifluoromethane, and chlorodifluoromethane,
for example. If antimony is used as the dopant, then antimony
pentachloride or antimony trichloride, for example, can be
used.
[0070] If a thin film preferably made of silicon oxide is formed
through CVD as an undercoating film, then examples of the raw
material for the silicon include monosilane, disilane, trisilane,
monochlorosilane, dichlorosilane, 1,2-dimethylsilane,
1,1,2-trimethyldisilane, 1,1,2,2-tetramethyldisilane, tetramethyl
orthosilicate, or tetraethyl orthosilicate, for example. Examples
of the oxidizing agent in this case include oxygen, water vapor,
dry air, carbon dioxide, carbon monoxide, nitrogen dioxide, and
ozone, for example. It should be noted that if silane is used, then
an unsaturated hydrocarbon gas such as ethylene, acetylene, or
toluene can be used in concert therewith for the purpose of
preventing reaction of the silane before it arrives on the glass
surface.
[0071] Similarly, if a thin film preferably with aluminum oxide as
its main component is formed through CVD as an undercoating film,
then examples of the raw material for the aluminum include
trimethylaluminum, aluminum triisopropoxide, diethylaluminum
chloride, aluminum acetylacetonate, and aluminum chloride. In this
case, examples of the oxidizing agent include oxygen, water vapor,
and dry air.
[0072] If a transparent conductive film whose main component is tin
oxide is formed through CVD, then it is preferable that in addition
to the raw material for the tin, a chlorine atom containing gas is
mixed in with the raw gas. By mixing in the chlorine atom
containing gas, the projections of the transparent conductive film
can be kept from becoming massive projections and their tip
portions can be kept from becoming sharp angles. The technical
basis for this is not completely clear, although the present
inventors believe the following to be a likely explanation. When
the chlorine atom containing gas reacts in a gaseous state with the
tin raw material in the raw gas or hydrogen included in other mixed
gases, it yields hydrogen chloride (HCl) gas. This hydrogen
chloride gas exhibits an etching function when crystals of a
transparent conductive film whose main component is tin oxide are
formed, corroding the surface of the projections. For this reason,
localized growth of the projections is not possible and the tip
portion of the projections cannot maintain a sharp angle. As a
result, the surface becomes relatively smooth, lacking prominent
projections, and a transparent conductive film with an ideal
surface shape that can contribute to the light-trapping effects
expected of the electrode of a photoelectric conversion element is
obtained. If the tip portion of the projections of the transparent
conductive film is not sharp, then it becomes difficult for defects
to occur as the films of the transparent conductive layer are
formed, and inconveniences such as shorts and diminished
photoelectric conversion efficiency in the photoelectric conversion
device can be inhibited.
[0073] Here, it is preferable that hydrogen chloride, chlorine, or
chloroform, for example, serves as the chlorine atom containing
gas. In particular, hydrogen chloride naturally exhibits an etching
action, and thus is useful if in the raw gas there is little of the
component providing hydrogen or if the reaction system temperature
is low.
[0074] It is preferable that the concentration of the chlorine atom
containing gas is not more than 10 mole percent of the raw gas.
When it exceeds 10 mole percent, there is a drop in the speed at
which a transparent conductive film whose main component is tin
oxide is formed. Although the technical basis behind why the
film-formation speed drops is not clear, conceivable causes are
that the etching action becomes too strong and that the addition of
the chlorine atom containing gas interferes with the reaction that
yields tin oxide through the reaction of a tin chloride and an
oxidizing agent. On the other hand, when it is less than 0.1 mole
percent, hardly any etching action is exhibited, although this also
depends on the temperature of the film formation reaction
system.
[0075] It should be noted that SEM photography can be used to
confirm that the tip portions of the projections are not sharp
angles. However, when the SEM photograph is low magnification, the
shape of the projections may make the tip portions appear sharp,
and thus the magnification is set to about 45,000 times or higher.
If the transparent conductive film is formed through CVD, then the
surface temperature of the transparent substrate, such as a glass
sheet, is preferably at least 590.degree. C. and even more
preferably at least 615.degree. C. There are no particular
limitations with regard to the type of the substrate as long as it
is a transparent insulating body. However, a glass sheet is ideal
because chemical stability sufficient to withstand the hydrogen
chloride gas is required.
[0076] The invention of the second embodiment, that is, a
transparent conductive film in which the reflectance of the
transparent conductive film is kept low by reducing the proportion
of projections that can be seen to have trapezoid-shaped cross
sections, and in which defects such as shorts are prevented by
reducing the number of large trapezoid-shaped projections, is
described next.
[0077] The transparent conductive film is formed on a transparent
substrate and is provided with projections of various sizes. Also,
the area occupied by projections with a trapezoid-shaped cross
section in which the slope of the upper side with respect to the
lower side is not more than 20.degree. and the ratio of the length
of the upper side with respect to the lower side (upper side
length/lower side length) is 0.8 or more, is adjusted to a
predetermined percentage. This percentage is not more than 10% of
the surface area of the surface on which the transparent conductive
film is formed, for example, the surface area of the surface of the
substrate. After intense research into the relationship between the
shape of the surface unevenness of the transparent conductive film
and its reflectance, the present inventors found that if the above
conditions are met, then the average reflectance from the
transparent substrate side drops in the wavelength range of 300 to
1200 nm. When the percentage of the area occupied by the above
projections is greater than 10%, the reflectance of the transparent
conductive film increases. Therefore, there is a reduction in the
amount of incident light on the photoelectric conversion layer and
a drop in the photoelectric conversion rate.
[0078] Here, as shown in FIG. 8, the lower side more precisely
refers to a base line 20 of the projections in the surface of the
transparent conductive film, and it is not necessarily in the
surface of the transparent conductive film that is in contact with
the transparent substrate or an undercoating film. The upper side
is an upper edge 22 of the projections when the cross section shape
is approximated to a trapezoid shape. If a line 21 is drawn
parallel to the lower side, then the slope of the upper side to the
lower side can be displayed by the angle .theta. formed between the
parallel line 21 and the upper side. The ratio of the length of the
upper side to the lower side is (upper side length)/(lower side
length), and strictly speaking, the length of the upper side is
defined as the length L2 in which the upper side is projected
perpendicularly onto the parallel line 21. The ratio of the length
of the upper side to the lower side is defined as L2/L1. The area
occupied by each projection can be calculated as the area of the
circle whose diameter is the lower side of the projection. The
total area of this circle becomes the surface area occupied by that
projection.
[0079] If the slope .theta. of the upper side to the lower side is
greater than 20.degree., then the cross-sectional shape of the
projection becomes near triangular, and can no longer be regarded
as a trapezoid. The cross-sectional shape also becomes nearly
triangular if the ratio of the length of the upper side to the
lower side is less than 0.8. These conditions generally are for
determining whether a projection can be regarded as
trapezoid-shaped. To inhibit the reflectance, it is preferable that
there are few projections that can be regarded as trapezoid-shaped,
and it is desirable for the percentage occupied by these
projections to be 1 to 5% of the surface area of the surface on
which the transparent conductive film is formed.
[0080] Of the projections that are regarded as trapezoid-shaped,
the area occupied by projections in which the lower side L1 is at
least 450 nm is preferably not more than 5% of the area of the
surface on which the transparent conductive film is formed. When
this percentage is too high, the p-type amorphous silicon layer
that is formed on the transparent conductive film becomes thin and
shorts occur between the i-type layer formed thereon and the
transparent conductive film, or the film thickness of the p-type
layer becomes nonuniform, leading to heterogeneity in the p-i
junction or the i-n junction. These result in a drop in the
properties of the photoelectric conversion device.
[0081] The transparent conductive film is preferably formed through
a thermal decomposition method in this embodiment as well. This is
because a homogenous transparent conductive film having a desired
surface shape can be formed through simple devices and means.
[0082] Of the thermal decomposition methods, CVD and particularly
on-line CVD are preferable. Incidentally, to provide a uniform size
in the surface unevenness and the projections, it is effective to
control the glass temperature to within the range of 670.degree. C.
to 620.degree. C. before the transparent conductive film is formed
and to use helium or hydrogen or the like, which have a low gas
density, as the carrier gas that supplies the raw gas that onto the
glass. Gases with small densities easily create laminar flows
because the Reynold's number of the gas is small, and can reduce
the percentage of surface that is occupied by projections regarded
as having trapezoid-shaped cross sections. Although helium is
preferable as the low-density gas, it is also possible to use a
density-adjusted mixed gas of helium and nitrogen, a mixed gas of
helium and neon, or a mixed gas of helium and hydrogen. In the same
manner as in the invention of the first embodiment, it is also
possible to add a chlorine atom containing gas to the mixed
gas.
[0083] The transparent conductive film according to the invention
of the second embodiment is processed into a photoelectric
conversion device through the same means as the transparent
conductive film according to the invention of the first
embodiment.
[0084] The invention of the third embodiment, that is, a
transparent conductive film whose main component is tin oxide,
which lacks massive projections that are protruding, and which has
numerous relatively large projections, is described next.
[0085] The properties of the transparent conductive film are most
effectively exhibited when it is utilized as the electrode of a
photoelectric conversion device. In photoelectric conversion
devices, low-resistance electrodes are necessary in order to
extract the electrical energy generated at the photoelectric
conversion layer without attenuation. Due to the structure of
photoelectric conversion devices, the electrode is a thin-film type
transparent conductive film. The electrical resistance of the thin
film conductive film is inversely proportional to its thickness.
However, when the conductive film is made too thick in an attempt
to lower the resistance, the amount of light that is absorbed there
increases and the conversion efficiency of the photoelectric
conversion device drops. Taking into consideration these
conflicting properties, at present the standard film thickness of
transparent conductive films is 400 to 1000 nm. Accordingly, the
present inventors conducted research on transparent conductive
films that were at least 400 nm thick.
[0086] The transparent conductive film has tin oxide as its main
component, and for example includes indium-doped tin oxide (ITO) or
tin oxide (SnO.sub.2) doped with either chlorine (Cl) or fluorine
(F). Because each of these is a crystalline metallic oxide, tiny
recessions and protrusions are formed in its surface when it is
manufactured in mass quantities using a thermal composition method,
a representative example of which is CVD. For example, when
crystals of a fluorine-doped tin oxide (SnO.sub.2:F) thin film are
allowed to grow up to 400 nm thick, massive projections with
heights of 400 nm or more can be seen scattered throughout its
surface. However, as long as the unevenness in its surface is
comprehensively determined using average values, as in conventional
techniques, it is not possible to solve the problem of shorts
occurring due to heterogeneity in the film thickness or pinholes
and the problem of a drop in photoelectric conversion
efficiency.
[0087] Accordingly, the present inventors intensively researched
the production conductions for transparent conductive films
produced using a thermal decomposition method and whose main
component is tin oxide, and came upon a method that allowed
relatively large projections to be arranged substantially uniformly
across the surface of the transparent conductive film. In other
words, in a case where the transparent conductive film is formed
through thermal decomposition, it is a method for controlling the
crystal growth conditions of the transparent conductive film, whose
main component is tin oxide, by keeping the substrate surface at
550.degree. C. to 650.degree. C. and mixing into the raw gas (the
gas for forming the film) suitable quantities of a gas that does
not contribute to the thermal decomposition reaction.
[0088] When the temperature of the substrate surface exceeds
650.degree. C., the speed of the thermal decomposition reaction
becomes too fast, facilitating the localized growth of large
crystal grains. This is because when relatively large crystal
grains accidentally form in the thermal decomposition reaction
system, the temperature around those crystal grains drops and they
become the only spots where the reaction temperature is suitable,
resulting in extremely fast crystal growth there. Thus, very large
projections can be seen scattered about. On the other hand, when
the substrate surface temperature is less than 550.degree. C., the
rate at which the crystals grow drops and the directionality of the
crystals becomes weak and they grow radially, and as a result
relatively small projections are formed uniformly. Thus, the
transparent conductive film does not effectively exhibit light
trapping effects. Also, the productivity of the transparent
conductive film becomes extremely poor. Moreover, if the substrate
surface temperature is 400.degree. C. or less, then the tin oxide
easily becomes noncrystalline and it becomes difficult for
sufficiently large grains to form.
[0089] As the gas that is included in the carrier gas and that does
not contribute to the thermal decomposition reaction, a gas whose
density in a standard state is not more than 1 g/L is preferable,
and the concentration of this gas in the raw gas is preferably at
least 10 mole percent. The smaller the Reynold's number of the raw
gas, the more easily it creates a laminar flow, and the gas does
not easily aggregate disproportionately in parts of the substrate
surface. As a result, the surface shape of the transparent
conductive film becomes more uniform. Here, the Reynold's number
refers to a dimensionless number R=.rho.LU/.eta.=LU/v created from
the length L, velocity U, density .rho., viscosity coefficient
.eta., and kinematic viscosity v=.eta./.rho. representative of
objects within a flow.
[0090] From the results of numerous experiments, the present
inventors found that a gaseous substance whose density in a
standard state is not more than 1 g/L is suitable as a gas that
does not contribute to the thermal decomposition reaction. Although
there are no particular limitations as to the type of this gas, it
is preferably an inert gas, and examples thereof include the inert
gases helium (He), neon (Ne), and argon (Ar). In particular, when
the density is taken into account, helium (He) is most suitable. It
should be noted that tin tetrachloride or a chlorine atom
containing gas, for example, which were the most suitable gases in
the first and the second embodiments, can be similarly used as this
raw gas, and likewise, it is preferable that a glass sheet is used
as the transparent substrate.
[0091] When a transparent conductive film whose main component is
tin oxide is formed through this method, very uniform unevenness,
in which the variance in the diameters of the projections is not
more than 0.01 .mu.m.sup.2, is formed in its surface. With this
transparent conductive film, the film thickness distribution of the
p-type and/or n-type amorphous silicon layers becomes uniform, and
nonuniformity at the p-i junction or n-i junction can be
inhibited.
[0092] It is preferable that in the surface of the transparent
conductive film there are not more than three projections having
diameters 400 nm or more per 4 .mu.m.sup.2 and that there is at
least one projection having a diameter of 350 nm or more per 4
.mu.m.sup.2. Such surface unevenness can be formed by suitably
selecting the substrate temperature, the composition and flow
amount of the raw gas, and the concentration of the carrier gas in
the thermal decomposition method. If large projections with
diameters 400 nm or more exist in greater quantities than 3 per 4
.mu.m.sup.2, then the film thickness of the amorphous silicon
p-type and/or n-type layers becomes thin, and shorts occur between
the i-type layer and the transparent conductive film. Also, the
distribution of the film thickness of the p-type and/or n-type
layers becomes nonuniform, leading to heterogeneity at the p-i or
n-i junctions and resulting in a drop in the conversion efficiency
of the photoelectric conversion element. On the other hand, if
there are absolutely no projections with diameters at least 350 nm,
then all the projections are small, and the light trapping effects
are not adequately exhibited at the transparent conductive film
surface.
[0093] The diameter and the distribution density of the projections
is determined based on an SEM photograph (at about 50,000 times
magnification) capturing the surface of the transparent conductive
film. It should be noted that with respect to the diameter of the
projections, the maximum cross section area of each projection can
be found based on the SEM photograph using an image analysis device
made by Nexus, and that area can be calculated under the assumption
that it is a circle.
[0094] If a fluorine-doped tin oxide thin film (SnO.sub.2:F) is
formed, then doping can be carried out using hydrogen fluoride,
trifluoroacetate, bromotrifluoromethane, or chlorodifluoromethane,
for example, the raw material for the fluorine in the raw gas. If
an ITO thin film is formed, then it is possible to use antimony
pentachloride or antimony trichloride, for example. If a
chlorine-doped tin oxide thin film (SnO.sub.2:Cl) is formed, then
it is possible to add hydrogen chloride, chlorine, or chloroform,
for example, as the chlorine atom containing gas. In particular,
hydrogen chloride itself exhibits etching action, and thus it is
useful in inhibiting the formation of massive projections.
[0095] The transparent conductive film according to the invention
of the third embodiment is processed into a photoelectric
conversion device through the same means as the transparent
conductive film according to the inventions of the first embodiment
and the second embodiment. The conditions mentioned above in the
embodiments are closely related to one another, and can be attained
simultaneously in a single transparent conductive film.
Consequently, it is possible to produce a transparent conductive
film provided with a plurality, or in certain cases, all, of the
characteristics of the first through third embodiments.
WORKING EXAMPLES
[0096] Hereinafter, the invention will be described in detail
through working examples, however, it is not limited to the
following working examples.
[0097] Using the device shown in FIG. 2, thin films, including the
transparent conductive film, were formed on a glass ribbon through
on-line CVD. It should be noted that in order to keep the inside of
the tin float bath at a slightly higher pressure than outside the
bath, nitrogen at 98 volume percent and hydrogen at 2 volume
percent were supplied into the space of the tin float bath to
maintain a non-oxidizing atmosphere inside the bath.
Working Example 1
[0098] A mixed gas made of dimethyltin dichloride (vapor), oxygen,
nitrogen, water vapor, and helium was supplied from the first
coater 16a, which is located furthest upstream, onto a glass ribbon
to form a 35 nm thick tin oxide thin film (first undercoating
layer). Next, a mixed gas made of monosilane, ethylene, oxygen, and
nitrogen was supplied from the second coater 16b to form a 25 nm
thick silicon oxide thin film (second undercoating layer) on the
tin oxide film. Then, a mixed gas made of 2.3 mole percent
dimethyltin dichloride (vapor), 43 mole percent oxygen, 32 mole
percent water vapor, 23 mole percent nitrogen, and hydrogen
fluoride was supplied from a plurality of coaters (only 16c is
shown) to form a 770 nm thick transparent conductive film made of
tin oxide containing fluorine (SnO.sub.2:F) on the silicon oxide
thin film. It should be noted that the temperature of the glass
ribbon immediately before the transparent conductive film is formed
was 680.degree. C. The raw gases used to form the undercoating
films and the transparent conductive film and the film-formation
conditions are shown together in Table 1.
[0099] The following points were confirmed from an SEM photograph
of the surface of the transparent conductive film at an elevation
angle of 30.degree. and a magnification of 45,000 times. The number
distribution of the height of the projections when applied to an
X.sup.2-type distribution function can be best approximated at a
degree of freedom of 6.5. The number of projections with a height
in the range of 50 to 350 nm was 97% of the total. The number
distribution of the ratio of the height of the projections to the
width of the projections when applied to an X.sup.2-type
distribution function can be best approximated at a degree of
freedom of 21. The number of projections whose ratio was 0.25 to
1.05 was 96%. Moreover, the number of projections with a height in
the range of 100 to 350 nm was 57%, and the number of projections
whose height to width ratio was 0.35 to 0.95 was 85%. The number of
projections with a height of 250 nm or more was 3.3% of the
total.
[0100] Next, the average reflectance of the glass substrate
provided with the transparent conductive film was found to be 9.5%
by measuring the reflectance spectrum at wavelengths of 300 to 1200
nm using a spectrophotometer and then averaging the reflectance at
10 nm wavelength intervals. The average reflectance was measured
with the glass sheet serving as the light incidence side.
[0101] Using the above SEM photograph, the slope of the upper side
with respect to the lower side and the ratio of the length of the
upper side to the lower side of each projection in the surface of
the transparent conductive film were found. The area occupied by
projections with trapezoid-shaped cross-sections having an upper
side sloped not more than 20.degree. with respect to the lower side
and the ratio of the length of the upper side to the lower side at
least 0.8, corresponded to 14% of the area of the surface on which
the transparent conductive film was formed. Of these projections,
the area occupied by projections with lower sides at least 450 nm
long was 7.5% of the area of the surface on which the transparent
conductive film was formed.
[0102] A 300 nm thick amorphous silicon thin film (photoelectric
conversion layer) was formed on the glass sheet, on which the
transparent conductive film was formed, through plasma CVD with
monosilane serving as the raw material. A 300 nm thick silver layer
(rear surface electrode) was then formed through electron beam
deposition.
[0103] The properties of the photoelectric conversion device
produced in this fashion were evaluated. First, the average
reflectance of the photoelectric conversion device was calculated
by measuring the reflectance spectrum at wavelengths of 300 to 1200
nm using a spectrophotometer and then averaging the reflectance at
10 nm wavelength intervals. The average reflectance of the
photoelectric conversion device was 28%. Next, the conversion
efficiency of the photoelectric conversion device was measured
through publicly-known means. At this time, there were several
locations for which measured data could not be obtained. This is
likely because the projections of the transparent conductive film
reached up to the i-type layer of the photoelectric conversion
layer. Measurable locations were located and data on the
photoelectric conversion efficiency were measured, which was found
to be 8.8%. The properties of the above transparent conductive film
and the properties of the photoelectric conversion device are shown
collectively in Table 2.
Working Example 2
[0104] Aside from changing the raw gases and the film-formation
conditions as shown in Table 1, undercoating films and a
transparent conductive film were formed on a glass substrate in the
same fashion as in Working Example 1. The properties thereof were
evaluated based on an SEM photograph of the surface of the
transparent conductive film at an elevation angle of 30.degree. and
a magnification of 45,000 times. The results showed that the number
distribution of the height of the projections could be best
approximated by an X.sup.2 type distribution function at a degree
of freedom of 6.5, the number of projections that have heights of
50 to 350 nm was 96%, and the number distribution of the ratio of
the height to the width of the projections could be best
approximated by an X.sup.2 type distribution function at a degree
of freedom of 20. The number of projections with a height to width
ratio of 0.25 to 1.05 was 97%. The number of projections with a
height of 100 to 350 nm was 59%, and the number of projections with
a height to width ratio of 0.35 to 0.95 was 90%. The percentage of
projections at least 250 nm high was 1.6%.
[0105] Next, in the same manner as in Working Example 1, the
average reflectance of the glass sheet provided with the
transparent conductive film was measured, and the calculated value
was 8.5%. The reflectance spectrum of the transparent conductive
film that was obtained in this measurement is shown in FIG. 7. From
FIG. 7, it is clear that little interference is seen in this
transparent conductive film.
[0106] FIGS. 4 and 5 show the number distribution function of the
height of the projections of the transparent conductive film and
the number distribution function of height/width of the projections
of the transparent conductive film produced in Working Example 2,
respectively.
[0107] Next, the number distribution indicating the height to width
ratio of the projections was found according to the method
described in JP H2-503615A. Following the method disclosed in this
publication, the height of the projections was measured as a film
thickness of t0 by a tracer-type film thickness meter, after which
0.5 .mu.m diamond paste was used for polishing and a film thickness
t1 at the point that the haze value is less than or equal to 2% was
found, and t0-t1 was regarded as the height of the projection. For
the diameter of the projections, a surface photograph was taken
with an electron microscope and the area of each grain was found
using an image analysis device produced by Nexus, and the diameter
when the projections are taken as circular was found. The
distribution diagram that was obtained is shown in FIG. 6. However,
height to diameter ratios not only larger than the range of 0.7 to
1.2, which is set forth in the scope of the claims in the
publication, but in excess of 2.0 were found. The presence of
grains at this size is considered necessary to scatter light at
long wavelengths.
[0108] The surface of the transparent conductive film was captured
photographically using an SEM at an elevation angle of 30.degree.
and a magnification of 45,000 times. The slope of the upper side
with respect to the lower side and the ratio of the length of the
upper side to the lower side of each projection of the tin oxide
film surface were found. The area occupied by projections with
trapezoid-shaped cross-sections having an upper side sloped not
more than 20.degree. with respect to the lower side and the ratio
of the length of the upper side with respect to the lower side at
least 0.8 was 5% of the area of the surface on which the
transparent conductive film was formed. Also, there were no
projections with trapezoid-shaped cross sections and lower sides of
450 nm or more. This SEM photograph is shown in FIG. 3.
[0109] Moreover, a photoelectric conversion layer and a rear
surface electrode were formed in the same fashion as in Working
Example 1, producing a photoelectric conversion device. The average
reflectance of this photoelectric conversion device was 24% and the
photoelectric conversion efficiency was 9.7%. The properties of the
transparent conductive film and the properties of the photoelectric
conversion device are shown together in Table 2.
Working Examples 3 to 7
[0110] Conditions other than those shown in Table 1 were set in the
same fashion as in Working Example 2 and undercoating films and a
transparent conductive film were formed on a glass sheet, producing
a glass sheet provided with a transparent conductive film.
Moreover, for Working Examples 3, 4, and 6, a photoelectric
conversion device was produced in the same manner as in Working
Example 2. The properties of these transparent conductive films and
the properties of the photoelectric conversion devices are shown
collectively in Table 2.
Working Example 8
[0111] As shown in Table 1, hydrogen chloride is added to the raw
gas for the transparent conductive film up to 5 mole percent, and
the film-formation conditions that are not shown are set in the
same manner as in Working Example 2 and undercoating films and a
transparent conductive film were formed on a glass sheet. A portion
of this glass sheet was cut away and the surface of the transparent
conductive film as captured photographically at 45,000 times
magnification using an EM. This photograph is shown in FIG. 10.
Working Example 9
[0112] Hydrogen chloride was not added to the raw gas for the
transparent conductive film as in Working Example 8, and all other
conditions that are not shown are set in the same manner as in
Working Example 2 and a glass sheet provided with undercoating
films and a transparent conductive film was produced. A portion of
this glass sheet provided with the transparent conductive film was
cut away and the surface of the transparent conductive film was
captured photographically at 45,000 times magnification using an
SEM. This photograph is shown in FIG. 11.
[0113] As is clear from the SEM photographs of FIG. 10 and FIG. 11,
there are neither massive projections nor projections having tip
portions with sharp angles in the surface of the transparent
conductive film produced in Working Example 8. On the other hand,
there are a plurality of projections with pointed tip portions in
the surface of the transparent conductive film produced in Working
Example 9.
Comparative Example 1
[0114] A transparent conductive film was formed through ordinary
CVD (off-line CVD) instead of on-line CVD. A 4 mm thick soda-lime
glass sheet with the same composition as in Working Example 1 and
cut in advance to a size of 450.times.450 mm is placed on a mesh
belt and sent through a heating furnace, and was heated up to
approximately 600.degree. C. As this heated glass sheet was
delivered further, a mixed gas made of monosilane, oxygen, and
nitrogen was supplied from coaters provided above the glass
delivery route, forming a 25 nm thick silicon oxide film on the
glass sheet. After the glass sheet was cooled it was once again
placed on the mesh belt and sent through the heating furnace, and
was heated to approximately 600.degree. C. This heated glass was
then further delivered and at the same time a mixed gas made of
monobutyltin trichloride (vapor), oxygen, water vapor, nitrogen,
and trifluroacetate was supplied from coaters positioned above the
glass delivery route, forming an 800 nm thick transparent
conductive film made of fluorine-doped tin oxide on the silicon
oxide film.
[0115] From an SEM photograph of the surface of the transparent
conductive film taken at 45,000 times magnification, it was clear
that the number distribution of the height of the projections can
be best approximated by an X.sup.2-type distribution function at a
degree of freedom of 6, the number of projections with heights of
50 to 350 nm was 95%, the number distribution of the ratio of the
height to the width of the projections could be best approximated
by an X.sup.2-type distribution function at a degree of freedom of
23, and the number of projections with a height to width ratio of
0.25 to 1.05 was 89% of the total. The number of projections with a
height of 100 to 350 nm was 55%, and the number of projections with
a height to width ratio of 0.35 to 0.95 was 79%. The percentage of
projections at least 250 nm high was 17%, and the average
reflectance of the glass sheet provided with the transparent
conductive film was 12.5%.
[0116] Then, a photoelectric conversion layer and a rear surface
substrate were formed on the glass sheet provided with the
transparent conductive film in the same manner as in Working
Example 1, producing a photoelectric conversion device. The average
reflectance of this photoelectric conversion device was 12.5% and
the photoelectric conversion efficiency was 7.1%. The properties of
the transparent conductive film and the properties of the
photoelectric conversion device are shown collectively in Table
2.
Comparative Example 2
[0117] As shown in Table 1, a mixed gas made of dimethyltin
dichloride (vapor), oxygen, nitrogen, and helium is supplied onto a
glass sheet in the same manner as in Working Example 2, forming a
35 nm thick tin oxide film (first undercoating layer), a 25 nm
thick silicon oxide film (second undercoating layer) is formed on
the first undercoating layer, and a mixed gas made of dimethyltin
dichloride (vapor), oxygen, water vapor, nitrogen, and
trifluoroacetate was supplied to form a 950 nm thick transparent
conductive film made of tin oxide doped with fluorine on the
silicon oxide film. Then, the transparent conductive film was
polished and made somewhat smooth, after which unevenness was
formed in the surface of the transparent conductive film through a
blasting technique in which sand is blown from ejection nozzles as
polishing particles. The thickness of the film at this time was
approximately 770 nm, and a spot in which the unevenness in the
surface was relatively uniform was selected and the following
measurements were performed.
[0118] The surface of the transparent conductive film was captured
photographically using an SEM at 45,000 times magnification, and
the shape of the unevenness in its surface was confirmed. As a
result, it was confirmed that the number distribution of the height
of the projections demonstrated a normal distribution centered
substantially around 200 nm. The number of projections with heights
in the range of 50 to 350 nm was 100%. It was also found that the
number distribution of the height to width ratio of the projections
followed a normal distribution centered substantially at 0.3. The
number of projections with height to width ratios of 0.25 to 1.05
was 95%. Also, the number of projections with heights in the range
of 100 to 350 nm was 98% of the total and the number of projections
with height to width ratios of 0.35 to 0.95 was 10%. Also, the
number of projections with heights at least 250 nm was 1% and the
average reflectance of the transparent conductive film was 21.7%.
The properties of the transparent conductive film are also shown in
Table 2. TABLE-US-00001 TABLE 1 Comparative Working Example Example
Items 1 2 3 4 5 6 7 8 9 1 2 First Undercoating layer Dimethyl
dichloride .smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. SnO.sub.2 Thin Film Oxygen
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. Nitrogen .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle. Water Vapor
.smallcircle. x x x x x x x x x Helium .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle. Film
Thickness 35 40 40 40 40 40 40 35 35 35 nm nm nm nm nm nm nm nm nm
nm Second Undercoating layer Monosilane .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. SiO.sub.2 Thin Film Ethylene .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle. x
.smallcircle. Oxygen .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle. Nitrogen
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. Film Thickness 25 25 25
25 25 25 25 25 25 25 25 nm nm nm nm nm nm nm nm nm nm nm
Transparent Conductive Film Dimethyl dichloride 2.3 2.5 2.5 2.5 3 3
3 3 3 .smallcircle.(*1) 2.5 SnO.sub.2:F Thin Film Oxygen 43 40 40
40 42 42 42 40 42 .smallcircle. 40 Water Vapor 32 30 30 30 31 31 31
30 31 .smallcircle. 30 Nitrogen 23 1.5 6.5 10.5 2 23 23 1 2
.smallcircle. 1.5(*2) Hydrogen Fluoride 1 1 1 1 1 .smallcircle.(*2)
Helium x 26 21 17 x x x 20 21 x 26 Hydrogen Chloride x x x x x x x
5 x x x Film Thickness 770 890 950 790 800 780 630 800 800 800 770
nm nm nm nm nm nm nm nm nm nm nm Glass Temperature 680 640 610 630
670 670 670 630 670 600 640 .degree. C. .degree. C. .degree. C.
.degree. C. .degree. C. .degree. C. .degree. C. .degree. C.
.degree. C. .degree. C. .degree. C. Photoelectric Conversion Layer
Monosilane .smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. Amorphous Silicon Thin
Film Film Thickness 300 300 300 300 300 300 300 nm nm nm nm nm nm
nm Rear Surface Electrode Type Ag Ag Ag Ag Ag Ag Ag Film Thickness
300 300 300 300 300 300 300 nm nm nm nm nm nm nm Remarks -- -- --
-- -- -- -- -- -- Off Sand Line Blast (*1) Use monobutyltin
trichloride in place of dimethyltin dichioride (*2) Use
trifluroacetate in place of hydrogen fluoride Note: An
.smallcircle. indicates that raw material is included, x indicates
that it is not included, and values without units are percentages
at which a component is present
[0119] TABLE-US-00002 TABLE 2 Comparative Working Example Example
Items Favorable Range 1 2 3 4 5 6 7 1 2 X.sup.2 50 to 350 nm height
(%) 70% or more 97 96 75 94 77 74 73 95 100 Height/Width 0.25 to
1.05 (%) 90% or more 96 97 95 96 92 95 91 89 95 100 to 350 nm
height (%) 30% or more 57 59 35 51 40 30 30 55 98 Height/Width 0.35
to 0.95 (%) 80% or more 85 90 83 87 81 83 82 79 10 250 nm or more
height (%) 0.2 to 20% 3.3 1.6 0.3 1.2 0.6 0.5 0.2 17 1 Height
distribution follows X.sup.2 type with .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. x degree of freedom of 3.5 to 15 Ratio
of height/width follows X.sup.2 type with .smallcircle.
.smallcircle. .smallcircle. .smallcircle. .smallcircle.
.smallcircle. .smallcircle. .smallcircle. x degree of freedom of
3.5 to 15 Trapezoid Area occupied by trapezoid 1 (*1) 10% or less
14 5 4 5 7 5 4 Area occupied by trapezoid 2 (*2) 5% or less 7.5 0 0
0 3 1 0 Density Transparent conductive 400 nm or more 770 890 950
790 800 780 630 800 770 film thickness (nm) Diameter distribution
variance 0.01 or less 0.008 0.008 0.009 0.011 0.012 0.012 Density
of diameter 400 nm or more 3 per 4 .mu.m.sup.2 or less 2 3 2 4 4 0
Density of diameter 350 nm or more 1 per 4 .mu.m.sup.2 or more 7 6
4 6 5 0 Effect Average reflectance of transparent 10% or less 9.5
8.5 8.2 8.3 8.1 8.1 8.3 12.5 21.7 conductive film (%) Average
reflectance of rear surface 28 24 22 23 22 36 electrode (%) Film
defects in photoelectric No pinholes or Yes No No No Yes Yes No Yes
conversion layer shorts Photoelectric conversion Higher the better
8.8 9.7 9.6 9.5 9.1 7.1 efficiency (%) (*1) Slope of upper side to
lower side is 20.degree.or less and ratio of length of upper side
to lower side is at least 0.8. (*2) Above *1 + lower side is at
least 450 nm. Note: An .smallcircle. indicates that number
distribution for projections follows X.sup.2 type, and x indicates
that it does not.
[0120] The following aspects become clear by comparing and
evaluating these working examples and comparative examples.
[0121] By comparing Working Examples 1 to 7 to Comparative Examples
1 and 2, it is clear that if the shape of the surface of the
transparent conductive film meets the criteria of the invention of
the first embodiment, then the reflectivity of the transparent
conductive film drops noticeably and as a result the photoelectric
conversion efficiency is improved.
[0122] By comparing Working Example 1 to Working Examples 2 to 4,
it is clear that if the projections of the transparent conductive
film meet the criteria of the invention of the second embodiment,
then shorts do not occur in the photoelectric conversion device and
the reflectivity of the transparent conductive film also drops,
resulting in improved photoelectric conversion efficiency. It
should be noted that shorts occurred in Working Example 1
conceivably because water vapor was used in the formation of the
first undercoating film, thus increasing the size of the recessions
and protrusions in the surface of the first undercoating layer and
leading to localized growth of crystals of the transparent
conductive film originating at the projections of the undercoating
film. Another conceivable reason for the progression of localized
growth which forming the transparent conductive film is that the
raw gas did not include helium.
[0123] By comparing Working Examples 2 to 4 and Working Examples 5
to 7, it is clear that by mixing helium into the raw gas when
forming the transparent conductive film, a transparent conductive
film that meets the criteria of the invention of the third
embodiment is obtained. In other words, it was found that if the
transparent conductive film is formed by a thermal decomposition
method, then by suitably mixing in low-density gas with a density
of not more than 1 g/L into the raw gas it is possible to inhibit
the formation of massive projections. Large projections did not
form in the surface of the transparent conductive film in Working
Example 7, and this is likely because the thickness of the
transparent conductive film was thinner than in Working Examples 5
and 6.
[0124] By comparing Working Example 8 to Working Example 9, it is
clear that the surface of the projections of the transparent
conductive film is etched if a chlorine atom containing gas is
added to the raw gas for the transparent conductive film,
eliminating the sharp top portions of the projections. Accordingly,
it can be presumed that a photoelectric conversion layer with few
defects will be formed if the photoelectric conversion layer is
formed on this transparent conductive film.
[0125] The invention is configured as set forth above, and thus
exhibits the following effects.
[0126] According to the invention of the first embodiment, in the
transparent conductive film, light at wavelengths longer than the
absorption region of amorphous silicon can be scattered effectively
and the average reflectance can be lowered. Also, it is possible to
provide a photoelectric conversion device with improved
photoelectric conversion efficiency by providing the transparent
conductive film on a transparent substrate.
[0127] According to the invention of the second embodiment, the
average reflectance can be lowered and defects such as shorts can
be kept from occurring by reducing the proportion of the
transparent conductive film occupied by massive projections with
trapezoid-shaped cross sections. Also, it is possible to provide a
photoelectric conversion device with high photoelectric conversion
efficiency and no defects such as shorts or pinholes by providing
the transparent conductive film on a transparent substrate.
[0128] According to the invention of the third embodiment, there
are no protruding massive projections and numerous relatively large
projections in the transparent conductive film, and thus the light
trapping effects can be exhibited effectively in the photoelectric
conversion layer without the occurrence of defects such as shorts.
Also, it is possible to provide a photoelectric conversion device
with which a rated output can be reliably obtained by providing
this transparent conductive film on a transparent substrate.
* * * * *