U.S. patent application number 09/340464 was filed with the patent office on 2002-01-10 for photovoltaic element.
Invention is credited to KARIYA, TOSHIMITSU, NISHIO, YUTAKA, SANO, MASAFUMI.
Application Number | 20020002992 09/340464 |
Document ID | / |
Family ID | 16143519 |
Filed Date | 2002-01-10 |
United States Patent
Application |
20020002992 |
Kind Code |
A1 |
KARIYA, TOSHIMITSU ; et
al. |
January 10, 2002 |
PHOTOVOLTAIC ELEMENT
Abstract
Provided is a photovoltaic element comprising a p-type
semiconductor layer and a transparent conductive layer comprised of
indium tin oxide bonded to each other at a surface, wherein the sum
of tin oxide content and tin content of the transparent conductive
layer varies in the layer thickness direction and is minimum at the
bonding surface of the p-type semiconductor layer and the
transparent conductive layer. Thus provided is a photovoltaic
element which has a high photoelectric conversion efficiency with
less lowering even when exposed to an intense light for a long
term.
Inventors: |
KARIYA, TOSHIMITSU;
(SORAKU-GUN, JP) ; SANO, MASAFUMI; (SORAKU-GUN,
JP) ; NISHIO, YUTAKA; (KYOTANABE-SHI, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
16143519 |
Appl. No.: |
09/340464 |
Filed: |
June 28, 1999 |
Current U.S.
Class: |
136/255 ;
136/252; 136/256; 136/257; 257/432; 257/436; 257/E31.126 |
Current CPC
Class: |
H01L 31/202 20130101;
Y02P 70/521 20151101; Y02E 10/548 20130101; H01L 31/022483
20130101; H01L 31/1884 20130101; H01L 31/075 20130101; Y02P 70/50
20151101; H01L 31/022466 20130101; H01L 31/076 20130101; H01L
31/1824 20130101; Y02E 10/545 20130101 |
Class at
Publication: |
136/255 ;
136/252; 136/256; 136/257; 257/432; 257/436 |
International
Class: |
H02N 006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 1998 |
JP |
10-183884 |
Claims
What is claimed is:
1. A photovoltaic element comprising a p-type semiconductor layer
and a transparent conductive layer comprised of indium tin oxide
bonded to each other at a surface, wherein the transparent
conductive layer comprises a plurality of layers, and wherein the
sum of tin oxide content and tin content of a layer of the
plurality of layers closest to the bonding surface of the p-type
semiconductor layer and the transparent conductive layer is less
than the sum of tin oxide content and tin content of any of the
other layers.
2. The photovoltaic element according to claim 1, wherein the sum
of tin oxide content and tin content of the layer closest to the
bonding surface is not more than 10 mole %.
3. The photovoltaic element according to claim 1, wherein a layer
having the maximum of the sum of tin oxide content and tin content
of the other layers of the plurality of layers has the sum of tin
oxide content and tin content of not less than 12 mole % and not
more than 30 mole %.
4. The photovoltaic element according to claim 1, wherein the
thickness of the layer closest to the bonding surface is not more
than a half of the whole thickness of the transparent conductive
layer.
5. The photovoltaic element according to claim 1, wherein the
thickness of a layer having the maximum of the sum of tin oxide
content and tin content of the other layers of the plurality of
layers is not less than a half of the whole thickness of the
transparent conductive layer.
6. A photovoltaic element comprising a p-type semiconductor layer
and a transparent conductive layer comprised of indium tin oxide
bonded to each other at a surface, wherein the sum of tin oxide
content and tin content of the transparent conductive layer varies
successively in the layer thickness direction and is minimum at the
bonding surface of the p-type semiconductor layer and the
transparent conductive layer.
7. The photovoltaic element according to claim 6, wherein the sum
of tin oxide content and tin content of the transparent conductive
layer at the bonding surface is not more than 10 mole %.
8. The photovoltaic element according to claim 6, wherein a region
having the maximum of the sum of tin oxide content and tin content
of the transparent conductive layer has the sum of tin oxide
content and tin content of not less than 12 mole % and not more
than 30 mole %.
9. The photovoltaic element according to claim 6, wherein the
thickness of a region having the sum of tin oxide content and tin
content of not more than 10 mole % of the transparent conductive
layer is not more than a half of the whole thickness of the
transparent conductive layer.
10. The photovoltaic element according to claim 6, wherein the
thickness of a region having the sum of tin oxide content and tin
content of not less than 12 mole % of the transparent conductive
layer is not less than a half of the whole thickness of the
transparent conductive layer.
11. A photovoltaic element comprising a p-type semiconductor layer
and a transparent conductive layer comprised of indium tin oxide
bonded to each other at a surface in a thermal equilibrium state,
wherein the lower edge energy of the conduction band of the
transparent conductive layer varies in the layer thickness
direction, and wherein the difference between the lower edge energy
of the conduction band of the transparent conductive layer and the
Fermi level in the vicinity of the bonding surface of the p-type
semiconductor layer and the transparent conductive layer is larger
than an average of the difference therebetween of the whole of the
transparent conductive layer.
Description
BACKGROUND OF THE INVENTION
[0001] Field of the Invention
[0002] The present invention relates to a transparent conductive
layer on the light incident side of a solar cell, a photosensor, or
other photovoltaic element. The present invention relates in
particular to those technologies which enhance the photoelectric
conversion efficiency of photovoltaic elements and those
technologies which increases long-term stability by inhibiting
light degradation and heat degradation. The present invention
relates also to improvements of cost- efficiency of photovoltaic
elements.
[0003] Recently, many houses are mounted with solar cells on their
roof and have them connected to the general power system so as to
meet power requirement as much as possible. However, the power
generation cost with solar cells is still high, which prohibits
large-scale pervasion.
[0004] Although the employment of amorphous silicon-based
thin-films as photovoltaic layers is said to be effective in
improvement of the cost efficiency of solar cells, those thin-films
have a low photoelectric conversion efficiency (conversion
efficiency) as compared to crystalline solar cells and also suffer
from decreases in the conversion efficiency when exposed to light
irradiation, i.e. light degradation. With this, most of the past
researches so far pronounced on the solar cells employing amorphous
silicon-based thin-films, i.e. amorphous solar cells, have been
concerned about two respects, i.e. "how to increase the conversion
efficiency" and "how to decrease the light degradation".
[0005] So far, reportedly, transparent conductive layers could have
been improved to obtain high-efficiency photovoltaic elements, i.e.
solar cells. For example, according to Japanese Patent Application
Laid-Open No. 8-77845, indium tin oxide (ITO) is formed and then
exposed to a corpuscular beam of an inert gas to promote its own
crystallization, thus obtaining low-resistivity, high-transmittance
ITO Thin-films. Also, according to another Japanese Patent
Publication No. 7- 84651, the crystallinity of ITO is controlled
such that the <111> axis orientates perpendicularly to the
substrate surface, thus making a trigonometric cone of the ITO
surface in geometry to decrease reflection loss and improve
short-circuit current and also to improve the conversion
efficiency. Also, according to another Japanese Patent Application
Laid-Open No. 9-78236, ITO thin-films are formed with xenon gas
instead of argon gas to enhance a carrier density, thereby
obtaining low-resistance ITO thin-films at a relatively low
substrate temperature.
[0006] Besides, an attempt has been made to form transparent
thin-films into a stack structure. For example, according to
another Japanese Patent Publication No. 7-111482, transparent
thin-films having different refractive indexes are stacked to
obtain better reflection-preventing thin-films in a visible ray
region of 450 to 650 nm. Those thin-films, however, are formed into
a stack structure comprising non-conductive thin-films.
[0007] According to another Japanese Patent Application Laid-Open
No. 8-43840, a plurality of high-carrier-density Thin-films
(ITO:SnO.sub.2, 10% by weight (wt %) and high-carrier-mobility
thin-films (ITO:SnO.sub.2, 0.3 wt %) are stacked and annealed, to
obtain transparent conductive layers for LCD having a sheet
resistivity of 5.4 .OMEGA./.quadrature..
[0008] Recently on the other hand, such solar cells comprising
single cells using uc-Si:H thin-films as the i-type layer are
reported that have a high conversion efficiency and no light
degradation. This solar cell is attracting the world attention as
an alternative to such a type of a solar cell that employs an
a-SiGe:H thin-film as the i-type layer. The uc-Si:H thin-film
eliminates light degradation inherent to amorphous silicon-based
thin-films such as a-SiGe:H thin-films and, moreover, need not use
costly material gases such as German gas (GeH.sub.4). In addition,
this pc-Si:H thin-film does not have a high absorption coefficient
as compared to a-SiGe:H thin-films but has a possibility to gain a
short-circuit current (Jsc) almost like a-SiGe:H single cells by
providing 2 .mu.m or larger thin-film thickness of the i-type
layer. One example of reports to that effect is MRS Symposium
Proceeding Vol. 420, Amorphous Silicon Technology, 1996, pp. 3-13,
"On the Way Towards High Efficiency Thin-Film Silicon Solar Cells
by the Micromorph Concept", J. Meier et al., where solar cells are
reported that have the i-type layer comprised of microcrystalline
silicon. This solar cell is made by the VHF plasma enhanced CVD
method using a frequency of 110 MHz, achieving a conversion
efficiency of 7.7% for single cells having one pin junction. This
single cell has a great advantage of having no light degradation.
Also, by further stacking thereon another pin junction containing
amorphous silicon-based thin-film as an i-type layer to make a
stack cell, a conversion efficiency of 13.1% is attained. However,
its light degradation rate is still high, almost the same as that
for conventional amorphous silicon-based solar cells.
[0009] The inventors have made sure of such a time-wise phenomenon
that indium tin oxide (ITO) thin-films have a higher resistivity
with a rising temperature in the air. The inventors have also found
that as light irradiation continues, a photovoltaic element using
such ITO thin-films as transparent conductive layers has a higher
temperature of itself and a higher resistivity of the transparent
conductive layer and also a lower fill factor, short-circuit light
current, and conversion efficiency. For example, in an embodiment
of Japanese Patent Application Laid-Open No. 8-56004, the vacuum
evaporation method by use of electron beam is used to form
transparent conductive layers comprised of ITO on a substrate.
Also, in an embodiment of Japanese Patent Application Laid-Open No.
7-297428, the evaporation method is used to form transparent
conductive layers comprised of ITO on a photovoltaic layer. Those
photovoltaic elements having ITO thin-films formed by the vacuum
evaporation method have a high initial conversion efficiency but,
when exposed to intense light irradiation (e.g., 100 mW/cm.sup.2),
has a higher resistivity of the ITO thin-films and a lower
conversion efficiency as time passes. Also, according to Japanese
Patent Application Laid-Open No. 8-43840, a plurality of
high-carrier-concentration thin-films (ITO:SnO.sub.2, 10 wt %) and
high-carrier-mobility thin-films (ITO:SnO.sub.2, 0.3 wt %) are
stacked and then annealed, thereby obtaining low-resistivity
transparent conductive layers for liquid crystal displays. However,
when the above-mentioned thin-films are stacked on a photovoltaic
layer and annealed, dopants such as phosphorus or boron are
mutually diffused, thereby posing a problem of lowering in open
circuit voltage. Moreover, the light transmittancy (short-circuit
current) was not high enough as required for photovoltaic
elements.
[0010] In an example of Japanese Patent Application Laid-Open No.
6-5893, ITO is formed on a photovoltaic layer (pin layer) by the
sputtering method. However, it suffers from a respect that the
short-circuit current Jsc is small for single cells employing an
a-SiGe:H thin-film as the i-type layer. As against this, researches
by the inventors have shown that a certain type of ITO thin-film
formed by the sputtering method has a very high thermal stability,
having a change rate of resistivity of approximately 1.1 even over
a time lapse of 3000 hours at a temperature of 120.degree. C. The
research by the inventors have shown also that a photovoltaic
element having ITO thin-films formed on its photovoltaic layer by
the sputtering method suffers from a respect that its short-circuit
current is smaller than that of a photovoltaic element having ITO
thin-films formed on its photovoltaic layer by the vacuum
evaporation method. In addition, it has another problem that by the
sputtering method, plasma comes in a high energy state to damage
photovoltaic elements, thereby increasing the leakage current and
decreasing the open circuit voltage. In an even worse case, it
suffers from a respect that photovoltaic elements may be
short-circuited. However, the photovoltaic element having ITO
thin-films formed by the sputtering method has also advantages such
as a high fill factor and a very excellent heat resistance.
SUMMARY OF THE INVENTION
[0011] One object of the present invention is to provide a
photovoltaic element that has a high conversion efficiency and, the
conversion efficiency of which does not decrease so much even when
exposed to intense light for a long time. Another object of the
present invention is to eliminate the decrease in the conversion
efficiency due to a rise in the temperature of a photovoltaic
element caused by the irradiation of intense light.
[0012] A still another object of the present invention is to
improve the thermal stability of the conversion efficiency of
photovoltaic elements having ITO thin-films formed on their
photovoltaic layers.
[0013] As a means to solve the above-mentioned problems, the
present invention provides a photovoltaic element having a p-type
semiconductor layer and a transparent conductive layer comprised of
indium tin oxide (ITO) junctioned face-to-face, wherein the
transparent conductive layer comprises a plurality of layers and a
sum of the content of tin oxide of the layer and that of tin which
is the closest to the junction surface between the above-mentioned
p-type semiconductor layer and transparent conductive layer of the
plurality of layers is smaller than the sum of that for the other
layers.
[0014] Indium tin oxide (ITO) contains mainly indium atoms, tin
atoms, and oxygen atoms. Those indium atoms and tin atoms exist
respectively in the state of indium oxide or another state such as
an indium simple substance and the state of tin oxide or another
state of a tin simple substance. The "sum of the content of tin
oxide and that of tin" as referred to in the present invention
means, in terms of oxide, a sum of the mole concentration of tin
oxide and that of tin present in the state of tin oxide such as a
tin simple substance. In other words, the "sum of the tin oxide
content and the tin content" means the value of a content of tin
oxide calculated from an amount of tin atoms on the assumption that
all of the tin exists in the state of tin oxide and all of the
indium exists in indium oxide.
[0015] Such a value can be obtained by, for example, determining
the concentration of tin atoms by the inductive coupling plasma
emission (ICP) method etc. and converting it in terms as oxide. If
in this case the amount of tin present in a state other than tin
oxide in the ITO is negligible, the "sum of tin oxide content and
tin content" may be taken to be "tin oxide content".
[0016] In this case, it is preferable that the sum of content of
tin oxide and that of tin of a layer which is the closest to the
above-mentioned junction surface be 10 mole % or less.
[0017] It is also preferable that the sum of tin oxide content and
tin content for such a layer that has the largest sum of contents
of tin oxide and tin of the above-mentioned plurality of layers be
between 12 mole % and 30 mole %, both inclusive.
[0018] It is also preferable that the thickness of the layer
closest to the above-mentioned junction surface be a half or less
of the thickness of the entire transparent conductive layer.
[0019] It is also preferable that the thickness of the layer that
has the largest sum of tin oxide content and tin content of the
above-mentioned plurality of layers be a half or more of the
thickness of the entire transparent conductive layer.
[0020] As another means to solve the above-mentioned problems, the
present invention provides a photovoltaic element having a p-type
semiconductor layer and a transparent conductive layer comprised of
indium tin oxide (ITO) junctioned face-to-face, wherein the sum of
the content of tin oxide and that of tin within the transparent
conductive layer continuously changes in the film-thickness
direction, coming in a minimum at the junction surface between the
above-mentioned p-type semiconductor layer and transparent
conductive layer.
[0021] In this case, it is preferable that the sum of a tin oxide
content and a tin content within the transparent conductive layer
on the above-mentioned junction surface be 10 mole % or less.
[0022] It is also preferable that the sum of a tin oxide content
and a tin content of the region that has the largest
above-mentioned sum of the above-mentioned plurality of layers be
between 12 mole % and 30 mole %, both inclusive.
[0023] It is also preferable that the region whose sum of a tin
oxide content and tin content in the above-mentioned transparent
conductive layer is 10 mole % or less occupy a half or less of the
entire transparent conductive layer.
[0024] It is also preferable that the region whose sum of tin oxide
content and tin content in the above-mentioned transparent
conductive layer is 12 mole % or larger occupy a half or more of
the entire transparent conductive layer.
[0025] Another means to solve the above-mentioned problems, the
present invention provides a photovoltaic element in a thermal
equilibrium state having a p-type semiconductor layer and a
transparent conductive layer comprised of indium tin oxide (ITO)
junctioned face-to-face, wherein the lower end of the conduction
band of the transparent conductive layer changes in the
layer-thickness direction and also the difference between the level
of the lower end of the conduction band of the transparent
conductive layer in the vicinity of the junction surface between
the p-type semiconductor layer and the transparent conductive layer
and the Fermi level is larger than an average difference over the
entire area of the transparent conductive layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 shows a schematic partial cross sectional view of a
configuration of an photovoltaic element according to the present
invention and a layer-thickness directional distribution of a sum
of tin oxide content and a tin content;
[0027] FIG. 2 shows a schematic partial cross sectional view of
another configuration of an photovoltaic element according to the
present invention and a layer-thickness directional distribution of
a sum of tin oxide content and a tin content;
[0028] FIG. 3 shows a schematic partial cross sectional view of a
configuration of an conventional photovoltaic element and a
layer-thickness directional distribution of a sum of tin oxide
content and a tin content;
[0029] FIG. 4 shows a schematic partial cross sectional view of
another configuration of an photovoltaic element according to the
present invention and a layer-thickness directional distribution of
a sum of tin oxide content and a tin content;
[0030] FIG. 5 shows a schematic partial cross sectional view of
another configuration of an photovoltaic element according to the
present invention and a layer-thickness directional distribution of
a sum of tin oxide content and a tin content;
[0031] FIG. 6 is a diagram of a band of a photovoltaic element
according to the present invention prior to junction;
[0032] FIG. 7 is a diagram of a band of a photovoltaic element
according to the present invention;
[0033] FIG. 8 is a diagram of a band of a conventional photovoltaic
element;
[0034] FIG. 9 is a diagram of a band of another conventional
photovoltaic element;
[0035] FIG. 10 is a diagram of a band of another conventional
photovoltaic element;
[0036] FIG. 11 is an outer appearance view of a sample solar cell
used in experimental examples and embodiments;
[0037] FIG. 12 is a schematic cross sectional view of an embodiment
of a photovoltaic element according to the present invention;
[0038] FIG. 13 is a schematic cross-sectional view of another
embodiment of a photovoltaic element according to the present
invention;
[0039] FIG. 14 is a schematic cross-sectional view of another
embodiment of a photovoltaic element according to the present
invention; and
[0040] FIG. 15 is a schematic cross-sectional view of another
embodiment of a photovoltaic element according to the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] To improve the conversion efficiency of photovoltaic
elements, it is critical to decrease the specific resistance of and
increase the transmittance of the transparent conductive layer.
However, the inventors have found as a result of researches "when
the transparent conductive layer and the photovoltaic layer have
reached a relatively high level in quality, improvements in the
characteristics of the simple substances of the transparent
conductive layer such as specific resistance and transmittance do
not necessarily lead to improvement in the characteristics of the
photovoltaic element". The inventors have concluded that when a
semiconductor layer in contact with the transparent conductive
layer is in particular of a p-type, its junction state is very
important. The inventors have also found that when the transparent
conductive layer comprised of ITO in junction with semiconductor
layers is exposed to high temperatures such as 60.degree. C., the
specific resistance may sometimes rise as time passes. And the
extent of a rise in temperature is found to be correlated with the
sum of a tin oxide content and a tin content of the transparent
conductive layer comprised of ITO, in such manner that with smaller
contents of about 5 mole % the specific resistance rises as time
passes and with larger contents of about 15 mole %, the specific
resistance comes to have an excellent thermal stability. It has
been found however that if a highly-heat-stable transparent
conductive layer having a large sum (which may sometimes be
abbreviated simply as tin concentration hereinafter) of a tin oxide
content and tin content is junctioned as it is with a photovoltaic
layer, the conversion efficiency degrades a little. Discussion on
the conditions for forming the transparent conductive layer has
concluded that the conversion efficiency in this case extremely
rarely becomes higher than that with the case of junction with a
transparent conductive layer having a lower tin concentration.
[0042] The present invention has been worked out based on those
experimental results, resulting in a photovoltaic element having an
improved junction between transparent conductive layers and
photovoltaic layers as well as a high conversion efficiency and a
good thermal stability.
[0043] FIG. 3 is a partial cross-sectional view of a conventional
photovoltaic, which has in it a photovoltaic layer comprised of pn
junctions or pin junctions (not shown) and a transparent conductive
layer 302 comprised of ITO in contact with a p-type semiconductor
layer 301. This transparent conductive layer has a constant
concentration (C.sub.31) of tin. In this case, the concerned
junction may well be shown in the band diagram in FIGS. 8 and 9.
ITO, having its Fermi level above the lower end of conduction band,
is so called a degenerated semiconductor. The transparent
conductive layer shown in FIG. 3 is considered to have a smaller
work function .phi. with a smaller concentration of tin and a
larger work function with a larger concentration of tin (Report of
Research Laboratory of Engineering Shibaura Institute of
Technology, Vol. 3, 1988, pp. 35-55). If the electron affinity x is
supposed to be the same as each other, the absolute value of a
difference between the level (Ec) of the lower end of the
conduction band and the Fermi level (Ef) is smaller in the case of
a larger concentration of tin (FIG. 8, Eh2) than in the case of a
smaller concentration of tin (FIG. 9, Eh1), i.e. Eh1>Eh2.
[0044] Also, the tunnel current is considered to be larger with a
smaller band gap (Et) concerned with the tunnel current and smaller
with a larger value of Et. The above consideration leads to a
supposition of the following relationship:
Et1<Et2
[0045] Therefore, a band gap concerned with tunnel current is
smaller in the case of FIG. 9 with a smaller concentration of tin,
thus resulting in a larger flow tunnel current. If a tunnel current
is small, the photovoltaic effect generated at the transparent
conductive layer and the p-type semiconductor layer cannot be
neglected. Therefore, it is considered that such a photovoltaic
element is preferable to that of a type shown in FIG. 9, which has
a larger tunnel current, i.e. having a small or zero concentration
of tin. Actually, measurement of the solar cell characteristics of
an amorphous silicon solar cell having pin junctions has shown that
the type shown in FIG. 9 is preferable in terms of open circuit
voltage (Voc), short-circuit current (Jsc), and photo-electric
conversion efficiency (.eta.).
[0046] However, the photovoltaic element having a transparent
conductive layer shown in FIG. 9 suffers from, when exposed to a
high temperature such as about 90.degree. C. for a long time,
increases in the specific resistance and sheet resistance as well
as degradations in the solar cell characteristics, particularly
fill factor (FF). However, the photovoltaic element shown in FIG. 8
has, as compared to that in FIG. 9, slightly degraded open circuit
voltage (Voc), short-circuit current (Jsc), and photoelectric
conversion efficiency (.eta.), but its transparent conductive layer
has an excellent thermal stability, so that even if exposed to a
high temperature such as about 120.degree. C., its specific
resistance and sheet resistance are very stable and also that
open-circuit voltage (Voc), short-circuit current (Jsc), and
photoelectric conversion efficiency (.eta.) are also thermally
stable.
[0047] The present invention makes use of advantages of respective
types shown in FIGS. 8 and 9 and, at the same time, eliminates
their disadvantages. FIG. 1 is schematic partial cross-sectional
view of one embodiment of the photovoltaic element according to the
present invention, in which example, a transparent conductive layer
102 consists of a stack of two layers. That is, in the case of FIG.
1, a first transparent conductive layer 103 and a second
transparent conductive layer 104 are sequentially stacked on a
p-type semiconductor layer 101, with the first transparent
conductive layer 103 having a concentration smaller than that of
the second transparent conductive layer 104. FIGS. 6 and 7 show
band diagrams of the photovoltaic element shown in FIG. 1. In a
first transparent conductive layer, which has a smaller
concentration of tin, the energy gap concerned with the tunnel
current Et1 is relatively small. Therefore, the tunnel current
becomes larger to inhibit undesired photovoltaic effect from being
generated, thereby providing better open-circuit voltage (Voc),
short-circuit current (Jsc), and photoelectric conversion
efficiency (i). FIG. 2 is a schematic partial cross-sectional view
of one embodiment of the photovoltaic element according to the
present invention, in which on a p-type semiconductor layer 201 is
formed a transparent conductive layer 202 comprised of a first
transparent layer 203, a second transparent conductive layer 204,
and a third transparent conductive layer 205.
[0048] In the conventional photovoltaic elements, it is considered
that if a great deal of tin oxide or tin is present near the
interface, the concerned region has a larger carrier concentration,
so that caused by mirror image force, the energy band is made to
curve as FIG. 10. With this, essentially the energy gap Et3
concerning the tunnel current becomes even larger, decreasing the
tunnel current with loss which is as much as photovoltaic effect.
It is considered that photovoltaic elements according to the
present invention have a relatively small concentration of tin
concentration in the first transparent conductive layer near the
junction surface and so have a smaller carrier concentration,
eliminating undesired curves of the band near the junction surface
of the p-type semiconductor layer, as shown in FIG. 10. Therefore,
the tunnel current becomes larger, thus giving excellent
open-circuit voltage, short-circuit current, and photoelectric
conversion efficiency of the photovoltaic elements according to the
present invention.
[0049] Moreover, since the second transparent conductive layer
having a larger concentration of tin is stacked on the first
transparent conductive layer, an excellent thermal stability can be
obtained. Moreover, since the refractive index n2 of the second
transparent conductive layer is larger than the refractive index n1
of the first transparent conductive index, when the refractive
index of a material of the photovoltaic layer is larger than n2,
the light trapping effect increases to further increase the
short-circuit current. Also, since the first transparent conductive
layer functions as a buffer layer against the internal stress, the
transparent conductive layers of a photovoltaic element according
to the present invention is very difficult to be peeled off. Since
the tin concentration particularly near the junction surface is
small to promote stress relaxation, the interface state is very
low, thus providing excellent photoelectric characteristics. Also,
according to the present invention, the specific resistance is
smaller at the transparent conductive layers (the second
transparent conductive layer 124 in FIG. 1 and the third
transparent conductive layer 205 in FIG. 2) distant from the
junction surface than at those (the first transparent conductive
layers 103 and 203 in FIGS. 1 and 2) near the junction surface, so
that light carriers generated from the photovoltaic layer move in
the layer-thickness direction inside the transparent conductive
layer near the junction surface and in the into-the-surface
direction inside the transparent conductive layer distant from the
junction surface. Therefore, the interaction due to the movement of
carriers near the junction surface or inside the p-type
semiconductor layer is decreased, thus increasing the tunnel
current.
[0050] The above-mentioned effects can be obtained mostly the same
with a photovoltaic element shown in FIG. 2 that has a three-layer
stacked transparent conductive layer and also that the tin
concentration starts to increase at the junction surface. Also, the
above-mentioned effects can be exerted further more for
photovoltaic elements shown in FIGS. 4 and 5 that the tin
concentration starts to increase continuously at the junction
surface. That is, by continuously changing the concentration of tin
oxide or tin, the interface between the first and second
transparent conductive layers can be eliminated, thus providing an
advantage in the case where an interface state is present at the
concerned interface. Since there is no interface state, power loss
due to increases in resistance is eliminated. Note here that FIGS.
4 and 5 are both schematic partial cross-sectional views showing
the embodiments of the photovoltaic element according to the
present invention.
[0051] Also, according to the present invention, transparent
conductive layers may be stacked on p-type semiconductor layers or
vice versa. In addition, the light incidence direction may be from
the side of the transparent conductive layer or the photovoltaic
layer, to exert the effects by the present invention.
[0052] One preferred embodiment of the present invention features
that the concentration of tin of the layer closest to the junction
surface is 10 mole % or less. Another embodiment wherein the tin
concentration changes continuously features that the concerned
concentration becomes a minimum at the junction surface, i.e. 10
mole % or less. With this, an energy gap related to the tunnel
current becomes even smaller to further increase it, thus exerting
the above-mentioned effects even more conspicuously.
[0053] Another preferred embodiment of the present invention
features that the tin maximum concentration of any layer is between
12 mole % and 30 mole %, both inclusive. Also, another preferred
embodiment in which the tin concentration changes continuously
features that the tin concentration in its maximum region is
between 2 mole % and 30 mole %, both inclusive. With this, the
thermal stability of photovoltaic elements when exposed to a high
temperature can be further improved. This improvement exerts the
above-mentioned effects even more conspicuously.
[0054] Another preferred embodiment of the present invention
features that the layer thickness of a region where the tin
concentration near the junction surface is 10 mole % or less is a
half or less of the entire thickness of the ITO layer. With this,
the thermal stability of photovoltaic elements when exposed to a
high temperature can be further improved. This improvement exerts
the above-mentioned effects even more conspicuously.
[0055] Another preferred embodiment of the present invention
features that the layer thickness in a region where the tin
concentration is 12 mole % or higher is a half or larger of the
entire thickness of the ITO layer. With this, the thermal stability
of photovoltaic elements when exposed to a high temperature can be
further improved. This improvement exerts the above-mentioned
effects even more conspicuously.
[0056] Another preferred embodiment of the present invention
features that the film thickness of transparent conductive layers
is adjusted so that the intensity of reflected light may be a
minimum at the wavelength at which the intensity of illumination
light becomes a maximum. With this, the short-circuit current for
photovoltaic elements is increased.
[0057] (Configuration of Transparent Conductive Layers)
[0058] The transparent conductive layers employed in the present
invention are comprised of indium tin oxide (ITO) and may be in a
polycrystalline, microcrystalline, or amorphous state. Of these
states, the crystalline state makes it possible to obtain a lower
specific resistance and a higher transmittance. A forming
temperature of 100.degree. C. or higher is preferable in order to
form crystalline indium tin oxide layers. To further improve the
transmittance, the forming temperature should preferably be
150.degree. C. or higher. When the underlying layer is not a p-type
semiconductor layer when forming transparent conductive layers, the
forming temperature should preferably be 300.degree. C. or higher.
If the underlying layer is a p-type semiconductor layer, it is
necessary to optimize the forming temperature for the concerned
layers in order to obtain even a little higher conversion
efficiency. Generally, however, an appropriate forming temperature
for the underlying p-type semiconductor layers is 400.degree. C. or
lower. This is because that temperature is important to prohibit
the dopants etc. in the p-type semiconductor layer from being
mutually diffused into other layers in order to increase the
open-circuit voltage. It is also preferable to form this layer at a
highest possible temperature to make its surface uneven, thus
making the best use of the light trapping effect. It is also
preferable to adjust the film thickness so that the reflection
becomes a minimum at a wavelength at which the illumination light
intensity becomes a maximum, thereby absorbing the illumination
light into the photovoltaic layer as much as possible. When
transparent conductive layers according to the present invention
are used in a crystalline state, their orientation should
preferably be a (100) or (111) surface in order to grow larger
crystal particles even with a lower specific resistance and a
higher transmittance. With this, it is also easy to form uneven
structures in the surface.
[0059] (How to Form Transparent Conductive Layers)
[0060] The methods of forming transparent conductive layers in the
photovoltaic element according to the present invention includes
the spray method, the CVD method, the application method, the
resistance heating vacuum evaporation method, and the sputtering
method, of which the resistance heating vacuum evaporation method,
the electron-beam vacuum evaporation method, or the sputtering
method is the most appropriate one because it enables forming good
junction surfaces with semiconductor layers. When using the
resistance heating vacuum evaporation method or the electron-beam
vacuum evaporation method, it is preferable to evaporate a metal
evaporation source containing indium and tin at a pressure of
7.times.10.sup.-2 Pa as heating the source subjected to oxygen gas.
If compared at the same temperature, however, the vapor pressure of
tin is much lower than that of indium, so that in order to provide
an approximately 1 mole % of tin concentration in transparent
conductive layers, their weight ratio needs to be In:Sn=1:1 or so,
while in order to provide an approximately 10 mole % of tin
concentration, their weight ratio needs to be In:Sn=1:10 or so. The
sputtering method, particularly the DC magnetron sputtering method
is optimal because it provides a higher deposit rate and better
junction surfaces. Preferably, the forming temperature is
150.degree. C. or higher and the target voltage, -200 V to -500 V
or so. Also, as a sputtering gas, a mixture of argon and oxygen as
well as neon, helium, and other light gases is preferable to reduce
the plasma-damages at the junction surface. In addition, the
substrate may be electrically floated or a bias may be adjusted so
as to control electron current or ion current flowing into the
substrate. A voltage of -50 V or higher is preferable and
especially preferable is a voltage of +200 V or so in order to
reduce ion current and plasma-damages.
[0061] In order to change the tin concentration in the
layer-thickness direction, electric power applied to the target or
the evaporation source may be controlled independently. When the
sputtering method is employed, by preparing both a target comprised
of indium oxide and another target comprised of tin oxide and tin
beforehand, electric power onto the tin oxide target should be
increased in order to provide a higher content of tin oxide. In
order to provide a higher content of tin, on the other hand,
electric power onto the tin target should be increased. When the
resistance heating vacuum evaporation is employed, by preparing
both an indium evaporation source and a tin evaporation source, the
temperature for the tin evaporation source should be increased in
order to provide a higher concentration of tin.
[0062] (Photovoltaic Layer)
[0063] The photovoltaic layer may be of any junction aspect as long
as its configuration will generate photovoltaic effect when exposed
to light irradiation. The junction aspects preferred include the pn
junction, pin junction, p.sup.+ p.sup.- n.sup.+ junction, and
p.sup.+ n.sup.- n.sup.+ junction. The materials for photovoltaic
layers include Si, Ge, SiGe, SiC, CdS, CdTe, CdSe,
Cd.sub.xZn.sub.1-xS, GaAs, Ga.sub.xAl.sub.1-xAs, GaInP.sub.2, InP,
CuInSe.sub.2, CuIn.sub.xGa.sub.1-xSe.sub.2, Cu.sub.2S, ZnO,
Zn.sub.3P.sub.2, and Se. The junction may be formed in a
homostructure using the same kind of materials or a heterostructure
using different kinds of materials. The state of the materials also
may be monocrystal, polycrystal, microcrystalline, or amorphous,
sometimes coming in a hetero-junction of polycrystal and amorphous
states. The photovoltaic layers of the present invention may have a
plurality of junctions. Those junctions may also be formed in
series or have insulating layers inserted between themselves. They
also may have different kinds of junctions such as a pn junction
and a pin junction.
[0064] The methods of forming photovoltaic layers include the
plasma-enhanced CVD method, the photo-assisted CVD method, the
thermal CVD method, MOCVD method, the MBE method, the gas diffusion
method, the solid-phase diffusion method, the liquid-phase growth
method, the ion implantation method, the resistance heating vacuum
evaporation method, the spray method, the sputtering method, and
the electro-deposition method, of which however the most
appropriate method must be selected depending on materials and
junction aspects to be employed. Also a plurality of methods may be
used when forming junctions for the photovoltaic layer.
[0065] In order to form junctions by depositing thin-films of Si,
Ge, SiGe, or SiC, the plasma enhanced CVD method, the
photo-assisted CVD method, and the thermal CVD method are
preferable. In order to form junctions within the substrate, the
gas diffusion method, the solid-phase diffusion method, and the ion
implantation method are preferable. In order to form the i-layer in
particular, the RF plasma-enhanced CVD method using a frequency of
1 to 30 MHz, the VHF plasma-enhanced CVD method using a frequency
of 30 MHz to 0.5 GHz, and the microwave plasma-enhanced CVD method
using a frequency of 0.5 GHz to 10 GHz are preferable. In order to
form junctions by depositing thin-films of CdS, CdTe, CdSe, or
Cd.sub.xZn.sub.1-xS, the resistance heating vacuum evaporation
method, the MBE method, the sputtering method, the
electro-deposition method, the plasma-enhanced CVD method, and the
photo-assisted CVD method are preferable. In order to form
junctions by depositing thin-films of GaAs, Ga.sub.xAl.sub.1-xAs,
GaInP.sub.2, or InP, the MBE method, the MOCVD method, and the
liquid-phase growth method are preferable, while in order to form
junctions within the substrate, the gas diffusion method, the
solid-phase diffusion method, and the ion implantation method are
preferable. In order to form junctions by depositing thin-films of
CuInSe.sub.2, CuIn.sub.xGa.sub.1-xSe.sub.2, Cu.sub.2S, or ZnO, the
resistance heating vacuum evaporation method, the MBE method, and
the sputtering method are preferable. In order to form junctions by
depositing Zn.sub.3P.sub.2 thin-films, the ICB method and the MOCVD
method are preferable. In order to form junctions by depositing Se
thin-films, the resistance heating vacuum evaporation method and
the sputtering method are preferable.
[0066] After the junctions are formed by above-mentioned methods,
annealing may be carried out for re-crystallization or improvement
of the junction quality.
[0067] (p-type Semiconductor Layer)
[0068] The p-type semiconductor layer is junctioned with
transparent conductive layers, being comprised of a material having
the p-type conductivity of many photovoltaic materials used for the
photovoltaic layer. The material may come in Si, Ge, SiGe, SiC,
CdTe, GaAs, Ga.sub.xAl.sub.1-xAs, GaInP.sub.2, InP, CuInSe.sub.2,
CuIn.sub.xGa.sub.1-xSe.sub.2, Cu.sub.2S, Zn.sub.3P.sub.2, or Se.
The material may also come in such a state as a monocrystalline,
polycrystalline, microcrystalline, or amorphous state, so that
different states of for example polycrystalline and amorphous
states may be used to stack a plurality of layers.
[0069] The methods of forming p-type semiconductor layers include
the plasma-enhanced CVD method, the photo-assisted CVD method, the
thermal CVD method, the MOCVD method, the gas diffusion method, the
solid-phase diffusion method, the liquid-phase diffusion method,
the ion implantation method, the resistance heating vacuum
evaporation method, the spray method, the sputtering method, and
the electrodeposition method, of which however the most appropriate
method must be selected depending on the material and the junction
aspect to be employed. This method of forming p-type semiconductor
layers is to be in accordance with the above-mentioned method of
forming photovoltaic layers.
[0070] (Photovoltaic Element)
[0071] FIG. 12 shows a schematic cross-sectional view of the
specific configuration of one example of the photovoltaic element
according to the present invention. FIG. 12 illustrates a pin-type
solar cell employing a non-monocrystalline silicon-based
semiconductor. In FIG. 12, reference numeral 1201 indicates a
support structure which has a function to support a photovoltaic
element, coming in glass, stainless steel, heat-resistant resin
sheet, etc. in material. Reference numeral 1202 indicates a
reflecting layer which has functions as a back-surface electrode
and also a back-surface reflecting function, coming in Ag, Al, Au,
Cu, CuMg etc. in material. Reference numeral 1203 indicates a lower
transparent conductive layer which has functions to prevent
short-circuiting of photovoltaic layers and also a light trapping
function, coming in zinc oxide, tin oxide, indium oxide, ITO etc.
in material. Reference numerals 1204, 1205, and 1206 respectively
indicate an n-type semiconductor layer, an i-type semiconductor
layer, and a p-type semiconductor layer; and by junctioning these
layers the pin junctions can be formed to generate photovoltaic
effect. The stack structure comprised of these three types of
layers is called a photovoltaic layer. The materials used for that
are mentioned earlier. Reference numeral 1207 indicates a
transparent conductive layer comprised of indium tin oxide, coming
in a stack structure consisting of two layers or more in which the
tin concentration changes in the layer-thickness direction. This
transparent conductive layer has a function to lead a larger amount
of light to the photovoltaic layer and another function to lead
light carriers to the collecting electrode effectively, i.e. with a
smaller power loss. Reference numeral 1208 indicates the collecting
electrode which has a function to effectively lead light carriers
outside and another function to lead light to the photovoltaic
layer effectively. The materials to be employed include Ag, Al, Au,
Cu etc.
[0072] FIG. 13 is a schematic cross-sectional view of a
photovoltaic element, which is a pinpin-type tandem
non-monocrystalline silicon-based solar cell. The materials and
functions of each layer are almost the same as those in FIG.
12.
[0073] In FIG. 13, reference numeral 1301 indicates a support
structure, reference numeral 1302 indicates a reflecting layer,
reference numeral 1303 indicates a lower transparent conductive
layer, reference numeral 1304 indicates a first n-type
semiconductor layer, reference numeral 1305 indicates a first
i-type semiconductor layer, reference numeral 1306 indicates a
first p-type semiconductor layer, reference numeral 1307 indicates
a transparent conductive layer, reference numeral 1308 indicates a
collecting electrode, reference numeral 1309 indicates a second
n-type semiconductor layer, reference numeral 1310 indicates a
second i-type semiconductor layer, and reference numeral 1311
indicates a second p-type semiconductor layer.
[0074] FIG. 14 is a schematic cross-sectional view of another
example of a photovoltaic element, which is a p.sup.+ n.sup.-
n.sup.+-type monocrystalline silicon-based solar cell. Reference
numeral 1405 indicates an n.sup.--type semiconductor layer
consisting of an n.sup.--type monocrystalline silicon substrate
which is prepared by the pull method and then into which
phosphorous (P) is doped. Reference numeral 1404 indicates an
n.sup.+-type semiconductor layer which is prepared by implanting
phosphorous ions into the back surface of the concerned
n.sup.--type monocrystalline silicon substrate. Reference numeral
1406 indicates a p.sup.+-type semiconductor layer which is prepared
by implanting boron ions into the surface of the concerned
n.sup.--type monocrystalline silicon substrate. These layers
indicated by reference numerals 1404, 1405, and 1406 will in
combination generate photovoltaic effect and their stack structure
is called a photovoltaic layer. Reference numeral 1402 indicates a
reflecting layer which serves also as a back-surface electrode,
having a function as an electrode and also as a layer to reflect
light, being comprised of Al etc. Reference numerals 1407 and 1408
respectively function the same as the transparent electrode layer
1207 and the collecting electrode 1208.
[0075] FIG. 5 is a schematic cross-sectional view of another
example of a photovoltaic element, which causes the light to enter
from the side of a support structure 150. The support structure
1501, therefore, needs to be transparent. On the support structure
1501 are sequentially stacked a transparent conductive layer 1507,
a p-type semiconductor layer 1506, an i-type semiconductor layer
1505, an n-type semiconductor 1504, and a reflecting layer
1502.
[0076] [Experiments]
[0077] To investigate the effects by the present invention, a
sample of such a stack structure was prepared that consists of a
transparent conductive layer, a p-type semiconductor layer, and a
substrate in this order.
[0078] (Experiment 1)
[0079] First, as a substrate, a stainless plate (50 mm.times.50
mm.times.1 mm (thickness)) which had undergone specular polishing
was provided and on it, as a p-type semiconductor layer, a p-type
microcrystalline silicon (.mu.c-Si:H:B) was formed by the RF
plasma-enhanced CVD method. On it was formed a transparent
conductive layer 102 using a DC magnetron sputtering apparatus
having in it four targets, as shown in FIG. 1. Specifically, a mask
having in it 25 openings with a 6-mm diameter was placed on a
p-type semiconductor layer 101 and then placed in the DC magnetron
sputtering apparatus, to sequentially form a first transparent
conductive layer 103 and a second transparent conductive layer 104
under such conditions as listed in Table 1. Then, the heater was
turned off and, when the temperature reached approximately the room
temperature, a mask having in it cross-shaped openings was placed
on, to form collecting electrodes comprised of Al on the second
transparent conductive layer 104, thereby completely preparing a
sample shown in FIG. 11 as viewed from the top.
[0080] When the voltage-current characteristic of this sample under
the dark conditions was checked, it was found to be ohmic. Also,
when its voltage-current characteristic was checked by applying a
light of AM 1.5 and 100 mW/cm.sup.2, no photovoltaic effect was
detected.
[0081] Next, In much the same manner as mentioned above, a p-type
semiconductor layer 101, a first transparent conductive layer 103,
and a second transparent conductive layer 104 were stacked on a
glass-made substrate. When measured by the four-probe method, the
sheet resistance of the transparent conductive layer 102 was 48.0
.OMEGA./.quadrature.. Then, the sample was placed in a
high-temperature environmental testing machine held at 120.degree.
C. and allowed to stand in it for 1000 hours, whereupon the sheet
resistance of the transparent conductive layer 102 was measured to
be 50.8 (.OMEGA./.quadrature.). The layer forming conditions and
the results are summarized in Table 1.
[0082] (Comparative Experiment 1-1)
[0083] Next, the first transparent conductive layer was checked in
terms of its single-film characteristics. Like in Experiment 1, on
a stainless-steel substrate was formed a p-type semiconductor
layer, on which in turn was formed a transparent conductive layer
with a film thickness of 60 nm, on which in turn was formed
cross-shaped collecting electrodes. The voltage-current
characteristic of this sample under the dark conditions was checked
and found to be ohmic. When the voltage-current characteristic was
checked by applying a light of AM 1.5 and 100 mW/cm.sup.2, no
photovoltaic effect was detected. Next, on a glass-made substrate
was formed a p-type semiconductor layer, on which in turn was
formed a first transparent conductive layer with a film-thickness
of 60 nm, and then the resulting sample was measured for its sum of
indium content and tin content (tin concentration) by the ICP
method to be found to be 5.1 mole %. Also, the initial sheet
resistance and the sheet resistance after a time lapse of 1000
hours at a temperature of 120.degree. C. were measured like in
Experiment 1, to be found to be 70.4 (.OMEGA./.quadrature.) and
85.1 (.OMEGA./.quadrature.), respectively. The layer forming
conditions and the results are summarized in Table 2.
[0084] (Comparative Experiment 1-2)
[0085] Next, the second transparent conductive layer was checked
for its single-film characteristics. Like in Experiment 1, on a
stainless-steel substrate was formed a p-type semiconductor layer,
on which in turn was formed a second transparent conductive layer
with a film-thickness of 60 nm, on which in turn was formed
cross-shaped collecting electrodes. The voltage-current
characteristic of this sample under the dark conditions was to be a
little of rectifying. Also, when checked by applying a light of AM
1.5, 100 mW/cm.sup.2, the voltage-current characteristic was found
to be generating photovoltaic effect a little. Next, on a
glass-made substrate was formed a p-type semiconductor layer, on
which in turn was formed a second transparent conductive layer with
a film-thickness of 60 nm; and the resultant tin concentration was
measured by the ICP method to be 10.3 mole %. Also, like in
Experiment 1, the initial sheet resistance and the sheet resistance
after a time lapse of 1000 hours at a temperature of 120.degree. C.
were measured to be 42.1 (.OMEGA./.quadrature.) and 44.3
(.OMEGA./.quadrature.) respectively The layer forming conditions
and the results are summarized in Table 3.
[0086] (Experiment 2)
[0087] The same sample as that in Experiment 1 was prepared and
evaluated similarly except that the first transparent conductive
layer was formed by the resistance heating vacuum evaporation
method and also that such a target for forming the second
transparent conductive layer was employed that has a tin oxide
content of 15 mole %. The conditions and the results are summarized
in Table 4.
[0088] (Comparative Experiment 2-1)
[0089] Next, the first transparent conductive layer formed in
Experiment 2 was checked for its single-film characteristics. Like
in Comparative Experiment 1-1, on a stainless-steel substrate were
formed a p-type semiconductor layer and a first transparent
conductive layer, and a collecting electrode in this order to give
a sample, while on a glass-made substrate were formed a p-type
semiconductor layer and a first transparent conductive layer in
this order to give another sample; then they were checked for their
current-voltage characteristic, tin concentration, and initial
sheet resistance and then underwent heat-run tests. The layer
forming conditions and the results are summarized in Table 5.
[0090] (Comparative Experiment 2-2)
[0091] Next, the second transparent conductive layer formed in
Experiment 2 was checked for its single-film characteristics. Like
in Comparative Experiment 1-2, on a stainless-steel substrate were
formed a p-type semiconductor layer, a second transparent
conductive layer, and collecting electrodes to prepare a sample,
while on a glass-made substrate were formed a p-type semiconductor
layer and second transparent conductive layer to prepare another
sample, so that these two sample were tested to determine their
current-voltage characteristic and initial sheet resistance and
also underwent heat run tests. The layer forming conditions and the
results are summarized in Table 6.
[0092] As can be seen from these experimental examples, the
transparent conductive layer having a configuration shown in FIG. 1
of the present invention has found to form ohmic junctions with
p-type semiconductor layers, to generate no photovoltaic effect,
and to be excellent in thermal stability.
[0093] (Experiment 3)
[0094] The single-film characteristics were summarized for each of
various values of the concentrations of transparent conductive
layers. The results are summarized in Table 7. The same evaluation
method was taken as Experiments 1 and 2. In this case, the layers
were formed by the DC magnetron sputtering method, to change the
content of tin oxide contained in a target, thereby varying the tin
concentration in the transparent conductive layers. As can be seen
from this table, when the tin concentration is in a range of 12
mole % to 30 mole %, both inclusive, the sheet resistance is low
and the thermal stability is high.
[0095] (Experiment 4)
[0096] This experiment was conducted using a sputtering target
containing metal tin. By changing the metal tin content in the
target, the tin concentration in transparent conductive layers was
varied. The results have shown that when the tin concentration is
in a range of 12 mole % to 30 mole %, both inclusive, the sheet
resistance is low and the thermal stability is high.
[0097] (Experiment 5)
[0098] This experiment was conducted on various values of the tin
concentration in transparent conductive layers using the resistance
heating vacuum evaporation method a:=and the electron-beam vacuum
evaporation method. This experiment was carried out in the same
manner as Experiment 3, except for the method of forming
transparent conductive layers. The results have shown that when the
tin concentration is in a range of 12 mole % to 30 mole %, both
inclusive, the sheet resistance is low and the thermal stability is
high irrespective of the forming method employed.
[0099] (Experiment 6)
[0100] The experiment was conducted on, as a p-type semiconductor
layer, the polycrystalline CuInSe.sub.2 with a film-thickness of 50
nm formed by the seleniding method. In this experiment, a stack
layer comprised of Cu/In formed by the vacuum evaporation method
was selenided by undergoing heat treatment in a Se vapor. First, as
a substrate, a glass-made substrate on which Mo was sputtered was
used; it then was heated to 450.degree. C. at a rate of 200.degree.
C./minute, underwent the irradiation for 60 minutes by selenium
vapor from a Se vapor source to form a sample, and was cooled
slowly at a rate of 1.degree. C./minute down to 300.degree. C.,
whereupon the irradiation by selenium vapor was stopped. It was
further cooled slowly at a rate of 1.degree. C./minute to
sequentially form 25 first transparent conductive layers with a
6-mm diameter, another 25 second transparent conductive layers, and
cross-shaped Al-made collecting electrodes.
[0101] When its voltage-current characteristic under the dark
conditions was checked, the sample was found to be ohmic. Also,
when a light of AM 1.5, 100 mW/cm.sup.2 was applied to check its
voltage-current characteristic, the sample was found to generate no
photovoltaic effect.
[0102] Next, likewise, a p-type semiconductor layer, a first
transparent conductive layer, and a second transparent conductive
layer were stacked on a glass-made substrate in much the same
manner as above. The sheet resistance was measured by the
four-probe method and found to be 45.2 (.OMEGA./.quadrature.).
Then, the sample was placed in a high-temperature environmental
testing machine held at 120.degree. C., allowed to stand there for
1000 hours, and then measured for its sheet resistance to be found
to be 47.8 (.OMEGA./.quadrature.).
[0103] (Comparative Experiment 6-1)
[0104] Next, the first transparent conductive layer was checked for
its single-film characteristics. Like in Experiment 6, on a
Mo/glass-made substrate was formed a p-type semiconductor layer
comprised of CuInSe.sub.2 in the same manner as Experiment 6, on
which in turn was formed a first transparent conductive layer with
a 60-nm thickness in the same manner as Comparative Experiment 1-1,
on which in turn was formed cross-shaped collecting electrodes. The
sample was checked for its voltage-current characteristics under
the dark conditions and found to be ohmic. When checked also for
its voltage-current characteristics after the irradiation by a
light of AM 1.5, 100 mW/cm.sup.2, the sample was found to generate
no photovoltaic effect. Moreover, on a glass-made substrate was
formed a p-type semiconductor layer used in Experiment 6, on which
in turn was formed a first transparent conductive layer in the same
manner as Comparative Experiment 1-1. The initial sheet resistance
and the sheet resistance after a time lapse of 1000 hours at a
temperature of 120.degree. C. were measured and found to be 69.5
(.OMEGA./.quadrature.) and 83.2 (.OMEGA./.quadrature.)
respectively.
[0105] (Comparative Experiment 6-2)
[0106] Next, the second transparent conductive layer was checked
for its single-film characteristics. As in Experiment 6, on a
Mo/glass-made substrate was formed a p-type semiconductor layer
comprised of CuInSe.sub.2, on which in turn was formed a second
transparent conductor layer with a film-thickness of 60 nm in the
same manner as in Comparative Experiment 1-2, on which in turn was
formed cross-shaped collecting electrodes. When its voltage-current
characteristic under the dark conditions was checked, the sample
was found to be of rectification. When its voltage-current
characteristic was checked after light irradiation of AM 1.5, 100
mW/cm.sup.2, the sample was found to have a little photovoltaic
effect. Then, on a glass-made substrate was formed a p-type
semiconductor layer used in Experiment 6, on which a second
transparent conductive layer in the same manner as in Comparative
Experiment 1-2. The initial sheet resistance and the sheet
resistance after a time lapse of 1000 hours at a temperature of
120.degree. C. were measured and found to be 41.8
(.OMEGA./.quadrature.) and 43.9 (.OMEGA./.quadrature.).
[0107] Although p-type microcrystalline silicon (.mu.c-Si:H:B) and
polycrystalline CuInSe.sub.2 were used as p-type semiconductor
layers above, the effects by the present invention are not
restricted to the above-mentioned cases and can be obtained as long
as semiconductors that have a p-type conductivity are employed.
EXAMPLES
[0108] The effects by the present invention are detailed below with
reference to examples, but not restricted to them.
Example 1
[0109] A non-monocrystalline silicon-made solar cell was prepared
that has p-type microcrystalline silicon as p-type semiconductor
layers and one pin junction as photovoltaic layers. As a support
structure, a stainless-steel plate (SUS430BA) with a dimension of
50.times.50.times.0.15 (mm) was used, and the DC magnetron
sputtering method was used to form reflecting layers comprised of
Ag and lower transparent conductive layers comprised of ZnO. These
lower transparent conductive layers comprised of ZnO had unevenness
in the surface with an average center-line roughness Ra of 0.12
(.mu.m). The RF plasma-enhanced CVD method with a frequency of
13.56 MHz was used to form n-type semiconductor layers comprised of
a-Si:H:P, while the VHF plasma-enhanced CVD method with a frequency
of 13.56 MHz was used to form p-type semiconductor layers comprised
of .mu.c-Si:H:B. Next, like in Experiment 1, a mask was used that
has 25 openings with a 6-mm diameter was used to form a first
transparent conductive layer and a second transparent conductive
layer, then a cross-shaped mask was used to form collecting
electrodes. The detailed forming conditions are listed in Table 8.
Such solar cells were prepared as many as four. Those solar cells
are to be called here 1A, 1B, 1C, and 1D.
[0110] The solar cell characteristics of those four solar cells are
measured under light irradiation of AM 1.5, 100 mW/cm.sup.2. The
average conversion efficiency of 4.times.25 sub-cells was 7.01
(%).
[0111] Next, thermal tests were conducted on the solar cells. Like
in Experiment 1, a solar cell 1B was placed in a high-temperature
environmental testing machine held at a temperature of 120.degree.
C. and was measured for its solar cell characteristics after a time
lapse of 1000 hours and found to have a conversion efficiency of
6.99 (%)
[0112] Next, light irradiation tests were conducted on the solar
cells. The above-mentioned light was continuously applied to a
solar cell 1C held at a temperature of 50.degree. C. and was
measured for its solar cell characteristics after a time lapse of
1000 hours and found to have a conversion efficiency of 6.93
(%).
[0113] Next, torsion tests were conducted. In a state where it is
fixed at its three corners, a solar cell 1D was distorted upward by
as much as 9.0 (mm) and then downward by the same displacement,
which is 6 times the following displacement set forth in the Solar
Cell JIS C8917:
h=0.021.times.(0.05.sup.2+0.05.sup.2).sup.0.5=0.0015 (m)=1.5
(mm)
[0114] This operation was repeated 100 times and then also to the
other three corners in the same manner. No changes were to be
observed in the outer appearances and, the conversion efficiency
was found to be 6.97 (%) when the solar cell characteristics were
measured. The solar cell characteristics of this solar cell
including the others are summarized in Table 9. As can be seen from
this table, the photovoltaic element according to the present
invention was found to be excellent in all of the initial average
characteristics (initial characteristics), average characteristics
after thermal test (post-thermal-test characteristics), average
characteristics after light irradiation (post-light-irradiation
characteristics), and average characteristics after torsion test
(post-torsion-test characteristics).
Comparative Example 1-1
[0115] Almost the same solar cells as those in Experiment 1 were
prepared and similar tests were conducted except that a single
layer of the first transparent conductive layer with a thickness of
60 (nm) was used as a transparent conductive layer like in
Comparative Experiment 1-1. The results are summarized in Table 9.
The results have shown that as compared to those prepared in
Example 1, the solar cells of Comparative Example 1-1 are similarly
excellent in terms of the initial solar cell characteristics but
inferior in terms of thermal tests, light irradiation tests, and
torsion tests. After the torsion tests, no changes were to be
observed in the outer appearances, but short-circuiting was
observed at some sub-cells.
Comparative Example 1-2
[0116] Almost the same solar cells as in Example 1 were prepared
and similar tests were conducted except that a single layer of the
second transparent conductive layer with a film-thickness of 60
(nm) was used as the transparent conductive layer like in
Comparative Experiment 1-2. The results are listed in Table 9. The
results have shown that the solar cells used in Comparative Example
1-2 are inferior in terms of the initial solar cell characteristics
and in torsion tests. After the torsion tests, no changes were to
be observed in the outer appearances but short-circuiting was
observed at some sub-cells.
[0117] As can be seen from the above description, the photovoltaic
element according to the present invention was found to be superior
to the conventional photovoltaic elements in all of the initial
characteristics, post-thermal-test characteristics,
post-light-irradiation characteristics, and post-torsion-test
characteristics.
Example 2
[0118] In this example, non-monocrystalline silicon solar cells
having a configuration shown in FIG. 15 were prepared that have
p-type microcrystalline silicon carbide (.mu.c-SiC:H:B) as p-type
semiconductor layers and one pin junction as photovoltaic layers. A
support structure was comprised of glass with a dimension of
100.times.10.times.1.0 (mm), and like in Example 1, a second
transparent conductive layer and a first transparent conductive
layer were stacked sequentially except that the second transparent
conductive later had a tin oxide content of 15 mole %. Next, on the
first transparent conductive layer was formed a p-type
semiconductor layer, by the microwave plasma-enhanced CVD method,
which is comprised of microcrystalline silicon carbide, on which
were also formed an i-type semiconductor layer and an n-type
semiconductor layer like in Example 1. Next, the DC magnetron
sputtering method was used to sequentially stack a lower
transparent conductive layer comprised of ZnO and a reflecting
layer comprised of Ag. This lower transparent conductive layer
comprised of ZnO had unevenness in its surface and an average
center-line roughness Ra of 0.10 (.mu.m). Such solar cells were
prepared as many as four. Those solar cells are to be called 2A,
2B, 2C, and 2D. The detailed forming conditions are listed in Table
10.
[0119] Those four solar cells were measured for their solar cell
characteristics under a light irradiation of AM 1.5, 100
mW/cm.sup.2. The average conversion efficiency of these four solar
cells was found to be 7.25 (%).
[0120] Next, like in Example 1, thermal tests were conducted on
those solar cells, after which the conversion efficiency of a solar
cell 2B was 7.24 (%).
[0121] Next, light irradiation tests were conducted on those solar
cells. After the tests, the conversion efficiency of a solar cell
2C was 7.17 (%).
[0122] Next, hailing tests were conducted. Under the conditions
with a hail-ball diameter of 25 (mm) and a final speed of 23
(m/sec) taken from the Solar Cell JIS C8917, a total of 100 balls
struck the glass surface evenly. Then, the solar cell
characteristics were measured, to find the conversion efficiency to
be 7.19 (%). This item and the other solar cell characteristics are
listed in Table 11. As can be seen from this table, the
photovoltaic element according to the present invention is superior
in all of the initial characteristics, the post-thermal-test
characteristics, the post-light-irradiation characteristics, and
the post-torsion-test characteristics.
[0123] (Experiment 2-1)
[0124] Almost the same solar cells as those in Example 2 were
prepared and similar tests were conducted except that a single
layer of a fist transparent conductive layer with a film-thickness
of 60 (nm) was used as the transparent conductive layer likely in
Comparative Example 1-1. The results are indicated in Table 11. The
results have shown that the solar cells of Comparative Example 2-1
are excellent in the initial solar cell characteristics like those
in Example 2 but inferior to them in the thermal test, the light
irradiation test, and the hailing test. Also, the surface was
observed after the hailing test and found to have small peeling
with a diameter of 5 pm or so at a few positions. The XMA analysis
found that those delaminations occurred at the interface between
the transparent conductive layer and the p-type semiconductor
layer.
Comparative Example 2-2
[0125] Almost the same solar cells as those in Example 2 were
prepared and similar tests were conducted except that a single
layer of a second transparent conductive layer was used as the
transparent conductive layer like in Comparative Example 1-2. The
results are indicated in Table 11. The results have shown that the
solar cells in Comparative Example 2-2 are inferior in terms of the
initial solar cell characteristics and also in the hailing test.
Also, the surface was observed after the hailing test to have small
delaminations with a diameter of 10 .mu.m or so at a few positions.
The XMA analysis has shown that those delaminations occurred at the
interface between the transparent conductive layer and the p-type
semiconductor layer.
[0126] As can be seen from the above description, the photovoltaic
element according to the present invention is superior to the
conventional photovoltaic elements in all of the initial
characteristics, the post-thermal-test characteristics, the
post-light-irradiation-test characteristics, and the
post-torsion-test characteristics.
Example 3
[0127] In this example, solar cells were prepared that employs
transparent conductive layers in which the tin concentration
changes linearly in the film-thickness direction. Almost the same
solar cells were prepared as many as four shown in FIG. 12 as those
in Example 1, except that the transparent conductive layer in
Example 1 was made to have such a tin-concentration distribution as
shown in FIG. 4. In this example, the target came in indium oxide
and tin oxide and the tin oxide target power was changed as time
passes so as to provide C.sub.41=5 (mole %) and C.sub.42=10 (mole
%). On these solar cells, almost the same measurement and
evaluation as Example 1 were conducted and came up with a result
that the solar cells in Example 3 are a little superior to those in
Example 1 in all of the four items of characteristics, i.e.
open-circuit voltage, short-circuit current, fill factor, and
conversion efficiency. Those solar cells were also found to have
almost the same change rate in the thermal test but, in the light
irradiation test and the torsion test, to have a better change rate
than those in Example 1.
Example 4
[0128] In this example, such solar cells were prepared as many as
four that uses transparent conductive layers shown in FIG. 5 in
which the tin concentration steeply changes near the p-type
semiconductor layer. They were so prepared so as to provide
C.sub.51=1 (mole %) and C.sub.52=15 (mole %) in FIG. 5. Here, as
the target, the indium oxide and tin oxide were provided and argon
and oxygen were used as a sputtering gas, so that the tin target
power was changed as time passes. Almost the same measurement and
evaluation as Example 1 were conducted on those solar cells and
found that they are a little superior to those solar cells in
Example 1 in all of the four items of characteristics, i.e.
open-circuit voltage, short-circuit current, fill factor, and
conversion efficiency. Also, the change rate in the thermal test,
the light irradiation, and the torsion test was found to be better
than those in Example 1.
Example 5
[0129] In this example, such solar cells were prepared as many as
four that use a layer configuration shown in FIG. 2. They were so
prepared as to provide C.sub.21=3 (mole %), C.sub.22=10 (mole %),
and C.sub.23=15 (mole %). Also, the fist, second, and third
transparent conductive layers had the film thickness of 10 (nm), 10
(nm), and 40 (nm) respectively. Almost the same measurement and
evaluation as Example 1 were conducted on those solar cells and
found that they are a little superior to those in Example 1 in all
four items of characteristics, i.e. open-circuit voltage,
short-circuit current, fill factor, and conversion efficiency.
Also, they were found to have a better change rate in the thermal
test, the light irradiation test, and the torsion test than those
in Example 1.
Example 6
[0130] In this example, four solar cells were prepared that have a
pinpin junction with such a configuration as shown in FIG. 13. As
the second i-type semiconductor layer, a-Si:H was used that is
formed by the RF plasma-enhanced CVD method with a frequency of
13.56 MHz. The forming conditions for each layer are indicated in
Table 12. Also, almost the same tests as those in Example 1 were
conducted to measure the solar cell characteristics. The results
are indicated in Table 13. As can be seen from this table, the
photovoltaic element according to the present invention is superior
to the conventional ones in all of the initial characteristics, the
post-thermal-test characteristics, the post-light-irradiation
characteristics, and the post-torsion-test characteristics.
Comparative Example 6-1
[0131] Almost the same solar cells as those in Example 6 were
prepared and similar tests were conducted, except that a single
layer of the first transparent conductive layer was used as the
transparent conductive layer as in Comparative Example 1-1. The
results are indicated in Table 13. As can be seen from it, the
solar cells in Comparative Example 6-1 are excellent likely in
Example 6 but inferior to them in the thermal test, the light
irradiation test, and the torsion test. After the torsion test, no
changes were to be observed in the outer appearances but
short-circuiting was observed at some sub-cells.
Comparative Example 6-2
[0132] Almost the same solar cells as those in Example 6 were
prepared and similar tests were conducted, except that a single
layer of the second transparent conductive layer with a diameter of
film-thickness of 60 (nm) was used as the transparent conductive
layer as in Comparative Example 1-2. The results are indicated in
Table 13. As can be seen from it, the solar cells in Comparative
Example 6-2 are inferior in terms of the initial solar cell
characteristics and also in the torsion test. After the torsion
test, no changes were to be observed in the outer appearances but
short-circuiting was observed at some sub-cells.
[0133] As mentioned above, the photovoltaic element according to
the present invention was found to superior to the conventional
photovoltaic elements in all of the initial characteristics, the
post-thermal-test characteristics, the post-light-irradiation
characteristics, and the post-torsion-test characteristics.
Example 7
[0134] In this example, four solar cells using n-type
monocrystalline silicon substrates formed by the CZ method were
prepared. First, on one side of the n-type monocrystalline silicon
substrate, phosphorous ions were implanted under the conditions of
30 (keV) and 1.times.10.sup.15 (particles/cm.sup.2), to form
n.sup.+-type semiconductor layers. On the other side also, boron
ions were implanted under the conditions of 100 (keV) and
8.times.10.sup.15 (particles/cm.sup.2), to form p.sup.+-type
semiconductor layers. Next, they underwent annealing for 30 minutes
at a temperature of 950 degrees Celsius in an atmosphere of
nitrogen. Next, first and second transparent conductive layers
having a 6-mm diameter were formed-as ?many as 25 each under the
forming conditions indicated in Table 14, to finally form
collecting electrodes consisting of cross-shaped Al's similar to
those in Example 1. On these four solar cells, almost the same
tests as those in Example 2 were conducted. The solar cell
characteristics are indicated in Table 15. As can be seen from it,
the photovoltaic element according to the present invention is
superior in all of the initial characteristics, the
post-thermal-test characteristics, the post-light-irradiation
characteristics, and the post-hailing-test characteristics. After
the hailing test, no changes were to be observed in the outer
appearances.
Comparative Example 7-1
[0135] Almost the same solar cells as those in Example 7 were
prepared and similar tests were conducted, except that a single
layer of a transparent conductive layer with a film-thickness of 70
(nm) was used as the transparent conductive layer was used like in
Comparative Example 2-1. The results are indicated in Table 15. The
results have shown that the solar cells prepared in Comparative
Example 7-1 are excellent in the initial solar cell characteristics
likely in Example 7 but inferior to them in the thermal test, the
light irradiation test, and the hailing test. After the hailing
test, the surface was observed to have small delaminations having a
diameter of 1 .mu.m at a few positions. The XMA analysis has shown
that those delaminations occurred at the interface between the
transparent conductive layer and the p-type semiconductor
layer.
Comparative Example 7-2
[0136] Almost the same solar cells as those in Example 7 were
prepared and similar tests were conducted, except that a single
layer of a second transparent conductive layer with a film
thickness of 70 (nm) was used as the transparent conductive layer
likely in Comparative Example 2-2. The results are indicated in
Table 15. The results have shown that the solar cells prepared in
Comparative Example 7-2 are inferior in terms of the initial solar
cell characteristics and also in the torsion test. Also, after the
hailing test, the surface was observed to have small delaminations
with a diameter of 10 um or so at a few positions. The XMA analysis
has shown that those delaminations occurred at the interface
between the transparent conductive layer and the p-type
semiconductor layer.
[0137] As mentioned above, the photovoltaic element according to
the present invention is superior to the conventional photovoltaic
elements in all of the initial characteristics, the
post-thermal-test characteristics, the post-light-irradiation
characteristics, the post-torsion-test characteristics, and the
post-hailing-test characteristics.
[0138] According to the present invention, there can be provided a
photovoltaic element having a high open circuit voltage, short
circuit current and conversion efficiency. Further, it is possible
to improve the thermal stability, stability to continuous light
irradiation and mechanical strength of a photovoltaic element. In
addition, since the resistance of the transparent conductive layer
can be made small, it is possible to broaden the distance between
the collecting electrodes, so that the module efficiency can be
increased.
1TABLE 1 (Experiment 1) Substrate (stainless steel) Tempe- Layer
Forming Main SiH.sub.4 H.sub.2 BF.sub.3 Power Pressure rature
thickness method component Dopant (sccm) (sccm) (sccm) (W) (Pa)
(.degree. C.) (nm) P-type semi- RE plasma .mu.c-Si:H B 1 50 0.1 40
200 180 50 conductor layer CVD SnO.sub.2 concentration Tempe- Layer
Forming Main of target Ar He Power Pressure rature thickness method
component Dopant (mole %) (sccm) (sccm) (W) (Pa) (.degree. C.) (nm)
first transparent DC magnetron In.sub.2O.sub.3 SnO.sub.2 5 30 0 200
0.532 200 10 conductive layer sputtering second transparent DC
magnetron In.sub.2O.sub.3 SnO.sub.2 10 30 0 200 0.532 200 50
conductive layer sputtering Collecting DC magnetron Al -- -- 30 0
500 0.532 30 200 electrode sputtering Dark state voltage-current
characteristics: ohmic Photovoltaic effect: none P-type
semiconductor layer and first and second transparent conductive
layers stacked on glass substrate Initial sheet resistance Sheet
resistance after heat resistance test (.OMEGA./.quadrature.)
(.OMEGA./.quadrature.) 48.0 50.8
[0139]
2TABLE 2 (Comparative Experiment 1-1) Substrate (stainless steel)
Tempe- Layer Forming Main SiH.sub.4 H.sub.2 BF.sub.3 Power Pressure
rature thickness method component Dopant (sccm) (sccm) (sccm) (W)
(Pa) (.degree. C.) (nm) P-type semi- RF plasma .mu.c-Si:H B 1 50
0.1 40 200 180 50 conductor layer CVD SnO.sub.2 concentration
Tempe- Layer Forming Main of target Ar He Power Pressure rature
thickness method component Dopant (mole %) (sccm) (sccm) (W) (Pa)
(.degree. C.) (nm) first transparent DC magnetron In.sub.2O.sub.3
SnO.sub.2 5 30 0 200 0.532 200 60 conductive layer sputtering
Collecting DC magnetron Al -- -- 30 0 500 0.532 30 200 electrode
sputtering Dark state voltage-current characteristics: ohmic
Photovoltaic effect: none P-type semiconductor layer and first
transparent conductive layer stacked on glass substrate Initial
sheet Sheet resistance after Tin resistance heat resistance test
concentration (.OMEGA./.quadrature.) (.OMEGA./.quadrature.) (mole
%) 70.4 85.1 5.1
[0140]
3TABLE 3 (Comparative Experiment 1-2) Substrate (stainless steel)
Tempe- Layer Forming Main SiH.sub.4 H.sub.2 BF.sub.3 Power Pressure
rature thickness method component Dopant (sccm) (sccm) (sccm) (W)
(Pa) (.degree. C.) (nm) P-type semi- RF plasma .mu.c-Si:H B 1 50
0.1 40 200 180 50 conductor layer CVD SnO.sub.2 concentration
Tempe- Layer Forming Main of target Ar He Power Pressure rature
thickness method component Dopant (mole %) (sccm) (sccm) (W) (Pa)
(.degree. C.) (nm) second DC magnetron In.sub.2O.sub.3 SnO.sub.2 10
30 0 200 0.532 200 60 transparent sputtering conductive layer
Collecting DC magnetron Al -- -- 30 0 500 0.532 30 200 electrode
sputtering Dark state voltage-current characteristics: ohmic
Photovoltaic effect: none P-type semiconductor layer and second
transparent conductive layer stacked on glass substrate Initial
sheet Sheet resistance after Tin resistance heat resistance test
concentration (.OMEGA./.quadrature.) (.OMEGA./.quadrature.) (mole
%) 70.4 85.1 5.1
[0141]
4TABLE 4 (Experiment 2) Substrate (stainless steel) Tempe- Layer
Forming Main SiH.sub.4 H.sub.2 BF.sub.3 Power Pressure rature
thickness method component Dopant (sccm) (sccm) (sccm) (W) (Pa)
(.degree. C.) (nm) P-type semi- RF plasma .mu.c-Si:H B 1 50 0.1 40
200 180 50 conductor layer CVD Weight ratio Tempe- Layer Forming
Main of (metal Sn)/ O.sub.2 Pressure rature thickness method
component Dopant (metal In) (sccm) (mPa) (.degree. C.) (nm) first
transparent Resistance In.sub.2O.sub.3 SnO.sub.2 1 30 66.5 200 60
conductive layer heating vacuum evaporation SnO.sub.2 concentration
Tempe- Layer Forming Main of target Ar He Power Pressure rature
thickness method component Dopant (mole %) (sccm) (sccm) (W) (Pa)
(.degree. C.) (nm) second transparent DC magnetron In.sub.2O.sub.3
SnO.sub.2 15 30 0 200 0.532 200 50 conductive layer sputtering
Collecting DC magnetron Al -- -- 30 0 500 0.532 30 200 electrode
sputtering Dark state voltage-current characteristics: ohmic
Photovoltaic effect: none P-type semiconductor layer and first and
second transparent conductive layers stacked on glass substrate
Initial sheet resistance Sheet resistance after heat resistance
test (.OMEGA./.quadrature.) (.OMEGA./.quadrature.) 50.3 52.3
[0142]
5TABLE 5 (Comparative Experiment 2-1) Substrate (stainless steel)
Tempe- Layer Forming Main SiH.sub.4 H.sub.2 BF.sub.3 Power Pressure
rature thickness method component Dopant (sccm) (sccm) (sccm) (W)
(Pa) (.degree. C.) (nm) P-type semi- RE plasma .mu.c-Si:H B 1 50
0.1 40 200 180 50 conductor layer CVD Weight ratio of (metal Sn)/
Tempe- Layer Forming Main (metal In) O.sub.2 Pressure rature
thickness method component Dopant (mole %) (sccm) (Pa) (.degree.
C.) (nm) first transparent Resistance In.sub.2O.sub.3 SnO.sub.2 1
30 0.0665 200 60 conductive layer heating vacuum evaporation
Collecting DC magnetron Al -- -- 30 0 500 0.532 30 200 electrode
sputtering Dark state voltage-current characteristics: ohmic
Photovoltaic effect: none P-type semiconductor layer and first
transparent conductive layer stacked on glass substrate Initial
sheet Sheet resistance after Tin resistance heat resistance test
concentration (.OMEGA./.quadrature.) (.OMEGA./.quadrature.) (mole
%) 205 730 0.9
[0143]
6TABLE 6 (Comparative Experiment 2-2) Substrate (stainless steel)
Tempe- Layer Forming Main SiH.sub.4 H.sub.2 BF.sub.3 Power Pressure
rature thickness method component Dopant (sccm) (sccm) (sccm) (W)
(Pa) (.degree. C.) (nm) P-type semi- RF plasma .mu.c-Si:H B 1 50
0.1 40 200 180 50 conductor layer CVD SnO.sub.2 concentration
Tempe- Layer Forming Main of target Ar He Power Pressure rature
thickness method component Dopant (mole %) (sccm) (sccm) (W) (Pa)
(.degree. C.) (nm) second DC magnetron In.sub.2O.sub.3 SnO.sub.2 10
30 0 200 0.532 200 60 transparent sputtering conductive layer
Collecting DC magnetron Al -- -- 30 0 500 0.532 30 200 electrode
sputtering Dark state voltage-current characteristics: ohmic
Photovoltaic effect: none P-type semiconductor layer and first
transparent conductive layer stacked on glass substrate Initial
sheet Sheet resistance after Tin resistance heat resistance test
concentration (.OMEGA./.quadrature.) (.OMEGA./.quadrature.) (mole
%) 37.7 38.5 15.5
[0144]
7TABLE 7 Experiment 3 Sheet resistance Initial after heat Tin sheet
resistance concentration resistance test Rate of (mole %)
(.OMEGA./.quadrature.) (.OMEGA./.quadrature.) change 1.1 185 320
1.73 2.3 105 156 1.49 5.1 70.4 85.1 1.21 8.5 56.2 61.2 1.09 10.3
42.1 44.3 1.05 12.1 39.5 40.6 1.03 15.5 37.7 38.5 1.02 19.5 35.6
36.0 1.01 23.5 41.1 42.0 1.02 26.7 53.0 54.6 1.03 29.2 57.3 59.6
1.04 33.2 60.2 63.2 1.05
[0145]
8TABLE 8 (Example 1) Substrate (stainless steel) Tempe- Layer
Forming Main Ar He Power Pressure rature thickness method component
Dopant Target (sccm) (sccm) (W) (Pa) (.degree. C.) (nm) Reflecting
DC magnetron Ag Ag 25 400 0.665 300 800 layer sputtering Lower
trans- DC magnetron ZnO ZnO 25 400 0.665 300 1300 parent con-
sputtering ductive layer Tempe- Layer Forming Main SiH.sub.4
H.sub.2 BF.sub.3 PH.sub.3 Power Pressure rature thickness method
component Dopant (sccm) (sccm) (sccm) (sccm) (W) (Pa) (.degree. C.)
(nm) N-type semi- RF Plasma a-Si:H P 1 50 0.1 2 172.9 200 10
conductor layer CVD I-type semi- VHF plasma .mu.c-Si:H 100 3000 160
26.6 350 850 conductor layer CVD P-type semi- RF plasma .mu.c-Si:H
B 1 .dbd.50 0.1 40 200 180 5 conductor layer CVD SnO.sub.2 Tempe-
Layer Forming Main concentration Ar He Power Pressure rature
thickness method component Dopant of target(mole%) (sccm) (sccm)
(W) (Pa) (.degree. C.) (nm) first transparent DC magnetron
In.sub.2O.sub.3 SnO.sub.2 5 30 0 200 0.532 200 10 conductive layer
sputtering second DC magnetron In.sub.2O.sub.3 SnO.sub.2 10 30 0
200 0.532 200 50 transparent sputtering conductive layer Collecting
DC magnetron Al -- -- 30 0 500 0.532 30 200 electrode
sputtering
[0146]
9 TABLE 9 Initial average characteristics Average characteristics
after thermal test Short Short Open circuit Open circuit circuit
current Conversion circuit current Conversion voltage density Fill
efficiency voltage density Fill efficiency (V) (mA/cm.sup.2) factor
(%) (V) (mA/cm.sup.2) factor (%) Example 1 0.455 24.5 0.629 7.01
0.454 24.5 0.628 6.99 Rate after test 0.998 1.000 0.998 0.996
Comparative 0.455 24.6 0.625 7.00 0.454 24.4 0.586 6.49 Example 1-1
Rate after test 0.998 0.992 0.938 0.928 Comparative 0.432 23.7
0.622 6.37 0.432 23.7 0.622 6.37 Example 1-2 Rate after test 1.000
1.000 1.000 1.000 Average characteristics after light irradiation
test Average characteristics after torsion test Short Short Open
circuit Open circuit circuit current Conversion circuit current
Conversion voltage density Fill efficiency voltage density Fill
efficiency (V) (mA/cm.sup.2) factor (%) (V) (mA/cm.sup.2) factor
(%) Example 1 0.453 24.4 0.627 6.93 0.453 24.5 0.628 6.97 Rate
after test 0.996 0.996 0.997 0.988 0.996 1.000 0.998 0.994
Comparative 0.451 24.4 0.596 6.56 0.442 24.3 0.576 6.19 Example 1-1
Rate after test 0.991 0.992 0.954 0.938 0.971 0.988 0.922 0.884
Comparative 0.429 23.6 0.619 6.27 0.428 23.7 0.617 6.26 Example 1-2
Rate after test 0.993 0.996 0.995 0.984 0.991 1.000 0.992 0.983
Rate after test = (Characteristics after test)/(Initial
characteristics)
[0147]
10TABLE 10 (Example 2) Substrate (stainless steel) SnO.sub.2 con-
Tempe- Layer Forming Main centration of Ar He Power Pressure rature
thickness method component Dopant target (mole %) (sccm) (sccm) (W)
(Pa) (.degree. C.) (nm) second DC In.sub.2O.sub.3 SnO.sub.2 15 30 0
200 0.532 200 50 transparent magnetron conductive sputtering layer
first transparent DC In.sub.2O.sub.3 SnO.sub.2 5 30 0 200 0.532 200
10 conductive magnetron layer sputtering Tempe- Layer Forming Main
SiH.sub.4 H.sub.2 BF.sub.3 PH.sub.3 CH.sub.4 Power Pressure rature
thickness method component Dopant (sccm) (sccm) (sccm) (sccm)
(sccm) (W) (Pa) (.degree. C.) (nm) P-type MW plasma .mu.c-SiC:H B
10 50 0.1 2.5 400 1.33 350 5 semiconductor CVD layer I-type VHF
plasma .mu.c-Si:H 100 3000 160 26.6 350 850 semiconductor CVD layer
N-type RF plasma a-Si:H P 1 50 0.1 2 172.9 200 10 semiconductor CVD
layer Tempe- Layer Forming Main Target Ar Power Pressure rature
thickness method component Dopant (sccm) (W) (Pa) (.degree. C.)
(nm) Lower DC ZnO ZnO 25 400 0.665 300 1300 transparent magnetron
conductive sputtering layer Reflecting DC Ag Ag 25 400 0.665 30 100
layer magnetron sputtering
[0148]
11 TABLE 11 Initial average characteristics Average characteristics
after thermal test Short Short Open circuit Open circuit circuit
current Conversion circuit current Conversion voltage density Fill
efficiency voltage density Fill efficiency (V) (mA/cm.sup.2) factor
(%) (V) (mA/cm.sup.2) factor (%) Example 2 0.468 24.8 0.625 7.25
0.467 24.8 0.625 7.24 Rate after test 0.998 1.000 1.000 0.998
Comparative 0.467 24.9 0.623 7.24 0.465 24.7 0.589 6.76 Example 2-1
Rate after test 0.996 0.992 0.945 0.934 Comparative 0.452 24.1
0.620 6.75 0.451 24.0 0.619 6.70 Example 2-2 Rate after test 0.998
0.996 0.998 0.992 Average characteristics after light irradiation
test Average characteristics after torsion test Short Short Open
circuit Open circuit circuit current Conversion circuit current
Conversion voltage density Fill efficiency voltage density Fill
efficiency (V) (mA/cm.sup.2) factor (%) (V) (mA/cm.sup.2) factor
(%) Example 2 0.467 24.6 0.624 7.17 0.466 24.7 0.625 7.19 Rate
after test 0.998 0.992 0.998 0.988 0.996 0.996 1.000 0.992
Comparative 0.465 24.8 0.572 6.60 0.458 24.6 0.612 6.90 Example 2-2
Rate after test 0.996 0.996 0.918 0.911 0.981 0.988 0.982 0.952
Comparative 0.450 24.0 0.619 6.69 0.445 24.0 0.617 6.59 Example 1-2
Rate after test 0.996 0.996 0.998 0.990 0.985 0.996 0.995 0.976
Rate after test = (Characteristics after test)/(Initial
characteristics)
[0149]
12TABLE 12 (Example 6) Substrate (stainless steel) Temp- Layer
Forming Main Ar He Power Pressure erature thickness method
component Dopant Target (sccm) (sccm) (W) (Pa) (.degree. C.) (nm)
Reflecting DC magnetron Ag Ag 25 400 0.665 300 800 layer sputtering
Lower trans- DC ZnO ZnO 25 400 0.665 300 1300 parent con- magnetron
ductive layer sputtering Temp- Layer Forming Main SiH.sub.4 H.sub.2
BF.sub.3 PH.sub.3 Power Pressure erature thickness method component
Dopant (sccm) (sccm) (sccm) (sccm) (W) (Pa) (.degree. C.) (nm)
first n-type semi- RE Plasma a-Si:H P 1 50 0.1 2 172.9 200 10
conductor layer CVD first i-type semi- VHF plasma .mu.c-Si:H 100
3000 160 26.6 350 850 conductor layer CVD first p-type semi- RF
plasma .mu.c-Si:H B 1 50 0.1 40 200 180 5 conductor layer CVD
second n-type RF plasma a-Si:H P 1 50 0.1 2 172.9 200 10 semi- CVD
conductor layer second i-type RF plasma a-Si:H 2 50 2 172.9 200 350
semi- CVD conductor layer second p-type RF plasma .mu.c-Si:H B 1 50
0.1 40 200 180 5 semi- CVD conductor layer SnO.sub.2 concentration
Temp- Layer Forming Main of target Ar He Power Pressure erature
thickness method component Dopant (mole %) (sccm) (sccm) (W) (Pa)
(.degree. C.) (nm) first transparent DC magnetron In.sub.2O.sub.3
SnO.sub.2 3 30 5 200 0.532 180 20 conductive layer sputtering
second transparent DC magnetron In.sub.2O.sub.3 SnO.sub.2 15 30 5
200 0.532 180 40 conductive layer sputtering Collecting DC
magnetron Al -- -- 30 0 500 0.532 30 200 electrode sputtering
[0150]
13 TABLE 13 Average characteristics after Average characteristics
after Average characteristics after Initial average characteristics
thermal test light irradiation test torsion test Open Short Open
Short Open Short Open Short cir- circuit Conver- cir- circuit
Conver- cir- circuit Conver- cir- circuit Conver- cuit current sion
cuit current sion cuit current sion cuit current sion vol- density
efficien- vol- density efficien- vol- density efficien- vol-
density eficien- tage (mA/ Fill cy tage (mA/ Fill cy tage (mA/ Fill
cy tage (mA/ Fill cy (V) cm.sup.2) factor (%) (V) cm.sup.2) factor
(%) (V) cm.sup.2) factor (%) (V) cm.sup.2) factor (%) Example 6
1.396 13.0 0.712 12.9 1.396 13.0 0.711 12.9 1.382 12.9 0.674 12.0
1.395 13.0 0.710 12.9 Rate after test 1.000 1.000 0.999 0.999 0.990
0.992 0.947 0.930 0.999 1.000 0.997 0.996 Comparative 1.397 13.0
0.709 12.9 1.396 12.9 0.687 12.4 1.381 12.8 0.649 11.5 1.376 12.9
0.695 12.3 Example 6-1 Rate after test 0.999 0.992 0.969 0.961
0.989 0.985 0.915 0.891 0.985 0.992 0.980 0.958 Comparative 1.372
12.7 0.712 12.4 1.371 12.7 0.711 12.4 1.360 12.6 0.672 11.5 1.361
12.7 0.692 12.0 Example 6-2 Rate after test 0.999 1.000 0.999 0.998
0.991 0.992 0.944 0.928 0.992 1.000 0.972 0.964 Rate after test =
(Characteristics after test)/(Initial characteristics)
[0151]
14TABLE 14 (Example 7) Substrate (stainless steel) Forming method
Main component Dopant Temperature (.degree. C.) Layer thickness
(nm) N.sup.+-type semi-conductor layer Ion implantation CZ-Si P 25
500 N.sup.--type semi-conductor layer CZ-Si P P.sup.+-type
semi-conductor layer Ion implantation CZ-Si B 25 300 Heat treatment
in N.sub.2 atmosphere SnO.sub.2 950 concentration Temp- Layer
Annealing Forming Main of target Ar He Power Pressure erature
thickness treatment Method component Dopant (mole %) (sccm) (sccm)
(W) (Pa) (.degree. C.) (nm) first transparent DC In.sub.2O.sub.3
SnO.sub.2 3 30 10 200 0.532 300 20 conductive layer magnetron
sputtering second DC In.sub.2O.sub.3 SnO.sub.2 15 30 10 200 0.532
300 50 transparent magnetron conductive layer sputtering Collecting
DC Al -- -- 30 0 500 0.532 30 200 electrode magnetron
sputtering
[0152]
15 TABLE 15 Average characteristics after Average characteristics
after Average characteristics after Initial average characteristics
thermal test light irradiation test hailing test Open Short Open
Short Open Short Open Short cir- circuit Conver- cir- circuit
Conver- cir- circuit Conver- cir- circuit Conver- cuit current sion
cuit current sion cuit current sion cuit current sion vol- density
efficien- vol- density efficien- vol- density efficien- vol-
density efficien- tage (mA/ Fill cy tage (mA/ Fill cy tage (mA/
Fill cy tage (mA/ Fill cy (V) cm.sup.2) factor (%) (V) cm.sup.2)
factor (%) (V) cm.sup.2) factor (%) (V) cm.sup.2) factor (%)
Example 7 0.604 29.2 0.771 13.6 0.604 29.2 0.771 13.6 0.604 29.2
0.771 13.6 0.601 29.2 0.768 13.5 Rate after test 1.000 1.000 1.000
1.000 1.000 1.000 1.000 1.000 0.995 1.000 0.996 0.991 Comparative
0.604 29.3 0.771 13.6 0.604 29.2 0.758 13.4 0.604 29.3 0.760 13.4
0.597 29.2 0.721 12.6 Example 7-1 Rate after test 1.000 0.997 0.983
0.980 1.000 1.000 0.986 0.986 0.988 0.997 0.935 0.921 Comparative
0.591 28.3 0.772 12.9 0.591 28.3 0.757 12.7 0.591 28.3 0.767 12.8
0.581 28.3 0.754 12.4 Example 7-2 Rate after test 1.000 1.000 0.981
0.981 1.000 1.000 0.994 0.994 0.983 1.000 0.977 0.960 Rate after
test = (Characteristics after test)/(Initial characteristics)
* * * * *