U.S. patent application number 14/229570 was filed with the patent office on 2014-07-31 for solar cell module and photovoltaic power generator using this.
This patent application is currently assigned to KYOCERA CORPORATION. The applicant listed for this patent is KYOCERA CORPORATION. Invention is credited to Shuichi FUJII, Yuko FUKAWA, Kenji FUKUI, Yosuke INOMATA, Hiroshi MORITA, Koichiro NIIRA, Koji NISHI, Tomonari SAKAMOTO, Mitsuo YAMASHITA, Tatsuya YASHIKI.
Application Number | 20140209152 14/229570 |
Document ID | / |
Family ID | 35056489 |
Filed Date | 2014-07-31 |
United States Patent
Application |
20140209152 |
Kind Code |
A1 |
FUJII; Shuichi ; et
al. |
July 31, 2014 |
Solar Cell Module and Photovoltaic Power Generator Using This
Abstract
A surface electrode (5) is installed on the light receiving
surface of a solar cell element, the surface electrode (5)
comprises three bus bar electrodes (5a) for extracting
light-produced at the solar cell element to the outside and
collecting finger electrodes (5b) connected to these bus bar
electrodes (5a), and the bus bar electrodes (5a) are not less than
0.5 mm and not more than 2 mm in width and the finger electrodes
(5b) are not less than 0.05 mm and not more than 0.1 mm in width. A
high-efficient solar cell module can be obtained with substantially
lowered resistance by increasing the number of bus bar electrode
(5a) and thereby decreasing the lengths of the finger electrodes
(5b).
Inventors: |
FUJII; Shuichi;
(Higashiomi-shi, JP) ; INOMATA; Yosuke;
(Higashiomi-shi, JP) ; SAKAMOTO; Tomonari;
(Higashiomi-shi, JP) ; NIIRA; Koichiro;
(Higashiomi-shi, JP) ; FUKAWA; Yuko; (Ise-shi,
JP) ; MORITA; Hiroshi; (Ise-shi, JP) ; NISHI;
Koji; (Ise-shi, JP) ; YASHIKI; Tatsuya;
(Ise-shi, JP) ; YAMASHITA; Mitsuo; (Ise-shi,
JP) ; FUKUI; Kenji; (Higashiomi-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KYOCERA CORPORATION |
Kyoto-shi |
|
JP |
|
|
Assignee: |
KYOCERA CORPORATION
Kyoto-shi
JP
|
Family ID: |
35056489 |
Appl. No.: |
14/229570 |
Filed: |
March 28, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10599539 |
Aug 9, 2007 |
|
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PCT/JP2005/006548 |
Mar 29, 2005 |
|
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14229570 |
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Current U.S.
Class: |
136/251 |
Current CPC
Class: |
H01L 31/048 20130101;
H01L 31/0508 20130101; H01L 31/049 20141201; H01L 31/02013
20130101; H01L 31/0504 20130101; H01L 31/0201 20130101; H01L
31/022433 20130101; Y02E 10/50 20130101 |
Class at
Publication: |
136/251 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2004 |
JP |
2004096809 |
Jun 10, 2004 |
JP |
2004172852 |
Jun 10, 2004 |
JP |
2004173178 |
Claims
1-20. (canceled)
21. A solar cell module comprising a plurality of solar cell
elements having a plate shape, and arranged between a translucent
panel and a back surface protective member such that the solar cell
elements are electrically connected to each other by innerleads and
spaces between these are filled with a filler member, wherein the
plurality of solar cell elements each comprises: a semiconductor
substrate; light-receiving surface electrodes on a light receiving
surface side of the semiconductor substrate comprising: three front
surface bus bar electrodes for providing output; and front surface
finger electrodes perpendicular to the front surface bus bar
electrodes; and three back surface bus bar electrodes for providing
output on a non-light receiving surface side of the semiconductor
substrate, wherein, in the plurality of solar cell elements, each
of the three front surface bus bar electrodes of one solar cell
element is connected by the innerleads to each of the three back
surface bus bar electrodes of another solar cell element, wherein
the front surface bus bar electrodes have widths of not less than
0.5 mm and not more than 2 mm, and the front surface finger
electrodes have widths of not less than 0.05 mm and not more than
0.1 mm, wherein each of the back surface bus bar electrodes is
arranged to be directly below each of the front surface bus bar
electrodes with respect to the semiconductor substrate
therebetween, wherein one of the three front surface bus bar
electrodes is disposed on a center line of the semiconductor
substrate, and wherein neighboring front surface bus bar electrodes
are electrically connected to each other via the front surface
finger electrodes.
22. The solar cell module according to claim 21, wherein the front
surface finger electrodes are directly connected to the filler
member without having solder therebetween.
23. The solar cell module according to claim 21, wherein the front
surface finger electrodes each have widths of not less than 0.06 mm
and not more than 0.09 mm.
24. The solar cell module according to claim 21, wherein an
opposite conductivity-type diffusion layer having a sheet
resistance of not less than 60 .OMEGA./square and not more than 300
.OMEGA./square is formed on a light-receiving surface of each of
the solar cell elements.
Description
TECHNICAL FIELD
[0001] The present invention relates to a solar cell module using
solar cell elements including a surface electrode on the light
receiving surface thereof and a photovoltaic power generator using
this.
BACKGROUND ART
[0002] A solar cell is a device for converting energy of incident
light into electrical energy.
[0003] The major types of solar cells are classified into
crystalline, amorphous and compound types. Most of the solar cells
that are currently distributed in the market are crystalline
silicon solar cells. The crystalline silicon solar cells are
further classified into monocrystalline type and multicrystalline
type. An advantage of monocrystalline silicon solar cells is that
improvement of the conversion efficiency is easy because of the
high quality of the substrates, while a disadvantage thereof is
high production cost of the substrates.
[0004] On the other hand, multicrystalline silicon solar cells have
a drawback that improvement of the conversion efficiency is
difficult due to inferior quality of the substrates, and an
advantage that they can be produced at low cost. In addition, with
the recent improvement in substrate quality and progress in cell
fabrication technology of multicrystalline silicon substrates, a
conversion efficiency of about 18% has been achieved at research
level.
[0005] Meanwhile, since multicrystalline silicon solar cells are
mass-produced at low cost, they have conventionally been
distributed in the market, and are today's mainstream solar
cells.
[0006] In recent years, solar cells have been required to have even
higher conversion efficiencies. Therefore, various approaches have
been devised for the surface electrode (bus bar electrode, finger
electrode) arranged on the light receiving surface.
[0007] Generally, for example, means such as decreasing optical
loss (reflectional loss) by fine wiring, and forming finger
electrodes and bus bar electrodes so as to cross orthogonal to each
other so that electrons collected in the finger electrodes are
carried to bus bar electrodes with minimum loss have been used.
[0008] Decreasing the electrode area of the surface electrode
thereby to increase the light receiving area is one approach to
meet the requirement of further improving the conversion efficiency
of solar cells.
[0009] However, a problem is that, in particular, when finger
electrodes are thinned to decrease the electrode area, the
resistance within the electrode increases, resulting in loss.
[0010] To solve this problem, increasing the thickness of finger
electrode thereby to increase the cross-section area within the
electrode and reduce the resistance is considered. However, in
reality, there is a limit to the thickness of electrodes when
electrodes are formed by screen printing, and the desired thickness
can only be obtained through a process including a plural times of
printing, and by using expensive equipment, namely, that for
sputtering or vapor deposition. This leads to the problem of
increase in solar cell production cost.
[0011] It is therefore an object of the present invention to
provide a solar cell module with a high conversion efficiency and a
photovoltaic power generator using this, which is realized by
reducing the substantial resistance by increasing the number of bus
bar electrodes to reduce the lengths of finger electrodes.
[0012] A single solar cell element is seldom used alone, and
usually, a plural number of them are connected together to be used
as a solar cell module. This is because even the silicon solar cell
element that is dominant in the market produces only a low voltage
on the order of 600 V when used as a single element and is not
practical, the cells therefore need to be series connected to
increase the voltage.
[0013] While there are various uses of this solar cell module, the
most typical use at present is installation of a plural number of
solar cell modules on the roofs of general houses. For this use,
the solar cell module is required to have a high conversion
efficiency for efficient power generation within a limited
installation area, and to have excellent design quality and
beautiful appearance because the external appearance of a house
depends on it.
[0014] In order to obtain a solar cell module with high efficiency,
apart from using solar cell elements with high efficiency, there
have been proposals such as forming irregularities on the glass on
the surface of the solar cell module and forming an antireflective
film on the surface of the glass so as to effectively introduce
light into the solar cell module (refer to Patent Document 1, for
example).
[0015] Also, a method in which the light diffusion/reflection
effect of a protective member on the backside of a solar cell
module is enhanced has been proposed (refer to Patent Document 2,
for example).
[0016] Moreover, enhancing the light diffusion/reflection effect by
using white color for a filler member 10 or a back surface
protective member 11 is also generally practiced.
[0017] In addition, in order to obtain a solar cell module with
high design quality, it is effective to form irregularities on the
glass on the surface of the module and to form an antireflective
film on the surface of the glass as mentioned above (refer to
Patent Document 1, for example).
[0018] Furthermore, providing an anti-glare film inside the solar
cell module so as to prevent reflection on the solar cell module
and light pollution and suppress the gloss to a low level has been
proposed (refer to Patent Document 3, for example). [0019] [1]
Japanese Unexamined Patent Publication No. 2003-124491 [0020] [2]
Japanese Unexamined Patent Publication No. 2003-234484 [0021] [3]
Japanese Unexamined Patent Publication No. 2001-203378
[0022] FIG. 16 illustrates an end portion of the light receiving
surface of a conventional solar cell module, and FIG. 17
illustrates an end portion of the back surface of the same.
[0023] FIGS. 18 and 19 are cross-sectional views of the
conventional solar cell module.
[0024] FIG. 18 is a cross-sectional view taken along the line G-G
of FIGS. 16 and 17, and FIG. 19 is a cross-sectional view taken
along the line H-H of FIGS. 16 and 17.
[0025] In each of the drawings, a solar cell element is denoted by
X, a wiring member by 8, a connecting member by 6, a terminal box
by 7 and a filler member by 10, respectively.
[0026] To connect solar cell elements together, electrodes on the
surface are connected to electrodes on the back surface of another
solar cell element by the wiring members.
[0027] To connect these wiring members 8 to the solar cell,
usually, bus bar electrodes are formed in the regions of the solar
cell elements where the wiring members 8 pass. In addition, a great
number of narrow finger electrodes to be connected to the bus bar
electrodes are formed to efficiently collect electric current from
the surfaces of the solar cell elements.
[0028] A copper foil coated with solder is generally used for the
wiring members 8, and they are fused to the bus bar electrodes on
the surfaces of the solar cell elements. Since the connecting
members 6 are also formed using a solder-coated copper foil as the
wiring members 8, as shown in FIG. 16, the wiring members 8 and
connecting members 6 with metallic gloss of solder are visible when
the solar cell module is viewed from the light receiving surface
side.
[0029] Meanwhile, the surfaces of the solar cell elements are
roughened to improve the efficiency, and an antireflective film is
formed to reduce the reflectance so as to effectively introduce the
sun light. For this reason, the surfaces of the solar cell elements
have a tone of color that is something between blue and dark blue
near black.
[0030] Moreover, as mentioned above, in order to improve the
properties of the solar cell module, the filler member 10 and the
back surface protecting member 11 located on the back surface side
of the solar cell elements are formed to have white color so as to
enhance light diffusion/reflection effect, which is also a
generally practiced method.
[0031] Accordingly, when the solar cell module is viewed from the
light receiving side, the gaps between the solar cell elements have
white color in many cases. This difference in color is one factor
to deteriorate the design quality of the solar cell module.
[0032] In order to solve this problem, there have been proposals
including coating the surfaces of the wiring members 8 and
connecting members 6 with a colored resin layer (refer to Patent
Document 4, for example), and providing a reflected light
controlling film over the wiring members 8 to which the solar cell
elements are connected (refer to Patent Document 5, for example) so
as to make the wiring members 8 and the connecting members 6 less
noticeable.
[0033] Also proposed is coloring the translucent panel 9 excluding
regions that are opposed to the solar cell elements so as to
prevent the wiring members 8, connecting members 6 and back surface
material among the solar cell elements from being visible (refer to
Patent Document 6, for example).
[0034] Furthermore, a technique for covering the connecting members
6 with a white sheet so that the connecting members 6 have the same
color as that of the back surface material among the solar cell
elements has been also devised. [0035] [4] Japanese Unexamined
Patent Application No. 2001-339089 [0036] [5] Japanese Unexamined
Patent Application No. 10-323344 [0037] [6] Japanese Unexamined
Patent Application No. 7-326789
[0038] When irregularities are formed in the glass on the surface
of a solar cell module or an antireflective film is formed on the
surface of the glass, the sun light is effectively introduced, and
light pollution can be prevented. However, in such a case, the
following problems arise: the cost for the glass material
increases; large scale equipment is necessary for forming an
antireflective film on the surface of the glass; and the production
cost increases because the number of processes increases. In
addition, when irregularities are formed on the surface of the
glass, dirt and dust tend to adhere to the solar cell module that
is set outside due to exposure to the elements, which intercept the
sunlight before it enters the solar cell module, causing the solar
cell module to have degraded output characteristics.
[0039] Similarly, in the case of a solar cell module provided with
an anti-glare film inside thereof, although the problem of light
pollution can be prevented, additional materials are required and
the production cost increases. In addition, the effect to enhance
light diffusion/reflection obtained by using white color for the
filler member 10 or a back surface protective member 11 located on
the back surface side of the solar cell elements of a solar cell
module cannot be expected, which hinders improvement of the
properties of the solar cell module.
[0040] Using the techniques such as covering the surfaces of the
wiring members 8 and connecting members 6 with a colored resin
layer, and providing a reflected light controlling film over the
wiring members 8 connecting the solar cell elements makes it
possible to make the wiring members 8 and connection members 6 less
noticeable. However, since covering the surfaces of the wiring
members 8 and connecting members 6 causes the problem of increase
in material cost and the number of steps, and large scale equipment
is required for forming a film on all of the solar cell elements
connected through the wiring members 8, the production cost
increases.
[0041] Using the technique of coloring the translucent panel 9
excluding regions that are opposed to the solar cell elements can
prevent the wiring members 8, connecting members 6 and the back
surface material seen among the solar cell elements from being
visible. However, since this requires an additional step of
coloring the translucent panel 9 and positioning between the
translucent panel 9 that has predetermined, preliminarily colored
regions and the solar cell elements connected through wiring
members 8, the process becomes complicated. In addition, the effect
to enhance light diffusion/reflection obtained by using white color
for the filler member 10 or the back surface material 11 located on
the back surface side of the solar cell elements cannot be
expected, hindering improvement of the properties of the solar cell
module.
[0042] As described so far, despite the high market demand, it has
been difficult to realize the production of a solar cell module
with high efficiency and high design quality at low cost.
[0043] The present invention has been made in consideration of the
problems above, and an object of the present invention is to
provide a solar cell module with high efficiency, high design
quality, which is excellent in external appearance and can be
produced at low cost.
DISCLOSURE OF THE INVENTION
[0044] A solar cell module according to the present invention
comprises a translucent panel, a back surface protective member, a
plurality of sheet-like solar cell elements that are arranged
between the translucent panel and the back surface protective
member and electrically connected to one another, and a filler
member filling spaces among the solar cell elements, wherein a
surface electrode is provided on light receiving surfaces of the
solar cell elements, comprising three bus bar electrodes for
retrieving light-produced electric current generated at the solar
cell elements to the outside and finger electrodes for collecting
electricity that are connected to the bus bar electrodes, and the
bus bar electrodes have widths of not less than 0.5 mm and not more
than 2 mm, and the finger electrodes have widths of not less than
0.05 mm and not more than 0.1 mm.
[0045] With this structure, while in the case of two bus bar
electrodes, when the widths of the finger electrodes are narrowed
for preventing light energy loss at the light receiving surfaces of
the solar cell elements, the fill factor FF tends to deteriorate
due to the series resistance component in the finger electrodes,
providing three bus bar electrodes allows the lengths of the finger
electrodes to be shortened, so that deterioration of the fill
factor FF due to the series resistance component of the finger
electrodes can be suppressed. A solar cell module with high output
characteristics and high efficiency can therefore be obtained.
[0046] It is preferred that the foregoing solar cell elements each
have a rectangular shape whose one side is not less than 100 mm and
not more than 350 mm in length, and another side is not less than
100 mm and not more than 350 mm in length.
[0047] More preferably, the widths of the finger electrodes are not
less than 0.05 mm and not more than 0.1 mm.
[0048] In addition, in the solar cell module according to the
present invention, the finger electrodes are preferably in contact
with the filler member. This prevents the finger electrodes from
being exposed to moisture and oxygen, by which the long term
reliability as well as the appearance of the solar cell module are
improved.
[0049] Moreover, it is preferred that the solar cell module
according to the present invention includes a diffusion layer of an
opposite conductivity-type having a sheet resistance of at least
60.OMEGA./.quadrature. and not more than 300.OMEGA./.quadrature.
formed on the light receiving surface side of the solar cell
elements. When the sheet resistance is less than
60.OMEGA./.quadrature., the short circuit current Isc is not
improved, and when it exceeds 300.OMEGA./.quadrature., it becomes
difficult to form the opposite conductivity-type diffusion layer
uniformly over the entire surfaces of the solar cell elements.
[0050] Furthermore, in the solar cell module according to the
present invention, it is preferred that a great number of fine
irregularities having widths and heights of not more than 2 .mu.m
and an aspect ratio of 0.1-2 are formed on the light receiving
surface side of the solar cell elements. By providing such
irregularities on the light receiving surface, the reflectivity can
be reduced and the short circuit current in the solar cell elements
can be improved.
[0051] According to the present invention, it is preferred that a
trajectories drawn by moving edge lines of a contact surface
between the bus bar electrodes and/or the finger electrodes
(referred to as "surface electrode") and the semiconductor region
continuously in the direction of the current flowing through the
surface electrode include at least in a part thereof a region where
the direction of a tangent line of the trajectories is not
coincident with the electric current flowing direction.
[0052] More specifically, the edge lines of the contact surface
between the surface electrode and the semiconductor region include
a rugged contour.
[0053] According to this structure, a trajectory formed after a
point at which a surface that is generally perpendicular to the
direction of the current flowing in the surface electrode and an
edge line of the surface electrode cross is moved continuously in
the direction of the current includes at least in a part thereof a
region where the direction of a tangent line of the trajectory is
not coincident with the direction of the current. By this
arrangement, as compared with when the edge line (an edge line of a
contact area between the electrode and the semiconductor region)
has a shape of a straight line, the length of the edge line is
substantially increased. Accordingly, the substantial contact area
between the surface electrode and the semiconductor region is
increased, so that the contact resistance of the contact area
between the both can be reduced effectively. The solar cell using
such solar cell elements can therefore be highly efficient.
[0054] When the contact surface is formed by contact between the
finger electrodes of the surface electrode and the semiconductor
region, and an area of the contact surface between the finger
electrodes and the semiconductor region is represented by S.sub.1,
an average value of distances between the edge lines of the contact
surface within each cut surface formed by cutting at a plurality of
cut planes that are generally perpendicular to the direction of the
electric current flowing through the finger electrodes is
represented by d.sub.1, and an entire length of the edge line is
represented by L.sub.1, the solar cell elements preferably include
at least one finger electrode where these S.sub.1, d.sub.1, and
L.sub.1 satisfy the following relationship:
0.5L.sub.1(S.sub.1d.sub.1.sup.-1+d.sub.1).sup.-1>1.2
[0055] As described above, by the arrangement such that the
foregoing formula is satisfied at the contact surface between the
finger electrodes that mainly execute a power collecting function
and the semiconductor substrate, the effective area of the contact
surface can be distinctly increased to reduce the contact
resistance. As a result, the output characteristics of the
photoelectric elements can be improved.
[0056] It is preferable that the profile of the edge lines of the
contact surface includes at least a part where the edge lines are
asymmetric with respect to a center line of the finger electrode
forming the contact surface that runs in the same direction as the
direction of the current flowing through the finger electrode.
Since this can eliminate parts where the width of the finger
electrode is particularly small, the line resistance of the finger
electrode will not be increased.
[0057] It is preferred that when the contact surface is formed by
contact between the bus bar electrodes of the surface electrode and
the semiconductor region, and with the contact surface being
planarly viewed from a direction vertical to the light receiving
surface, when an entire length of the edge lines is represented by
L.sub.2, an area of the contact surface is represented by S.sub.2,
and an area of the entire light receiving surface when planarly
viewed from a direction vertical to the light receiving surface is
represented by S.sub.3, these L.sub.2, S.sub.2, and S.sub.3 satisfy
the following relationships:
L.sub.2>5S.sub.3.sup.1/2
0.015<S.sub.2/S.sub.3<0.050
[0058] With the contact surfaces being planarly viewed, when the
length of the edge lines L.sub.2 is long, the regions (areas) into
which electron currents of the bus bar electrodes flow in a
concentrated manner can be extended (increased) and the contact
resistance between the bus bar electrodes and the semiconductor
region can be reduced, so that the conversion efficiency of the
solar cell elements can be improved.
[0059] However, only increasing the sum L.sub.2 of the entire
lengths of the edge lines causes the bus bar electrodes to shield
the light incident surface, which rather decreases the amount of
incident light. Accordingly, it is determined that the proportion
of the area S.sub.2 defined when the edge lines of the contact
surface are planarly viewed from a vertical direction to the area
S.sub.3 when the light receiving surface is planarly viewed is less
than 0.050 (5%).
[0060] Meanwhile, the purpose of adding the term "planarly viewed"
to the descriptions above is to ignore the irregularities and waves
on the surfaces.
[0061] It is preferred that when an area of the bus bar electrodes
and/or the finger electrodes planarly viewed from a direction
vertical to the light receiving surface side is represented by Sa,
and a surface area of a region of the light receiving surface of
the solar cell element in which the surface electrode is provided
is represented by Sb, the following relationship is satisfied:
1.10.ltoreq.Sb/Sa.ltoreq.2.10
[0062] This formula indicates that the ratio of the surface area of
the light receiving surface of the solar cell element in the region
where the surface electrode is provided to the area of the surface
electrode is 1.10 or more and 2.10 or less. This allows the
substantial contact area between the surface electrode and the
solar cell elements to be increased, and reduces adverse effect due
to electrical loss caused by the series resistance component, so
that the fill factor FF will not be deteriorated.
[0063] As a result of this, the solar cell element according to the
present invention allows to reduce optical loss due to the surface
electrode and improve the short circuit current density and fill
factor in a properly balanced manner, so that good conversion
efficiency can be achieved.
[0064] In addition, a solar cell module according to the present
invention comprises a translucent panel, a back surface protective
member, a plurality of sheet-like solar cell elements that are
arranged between the translucent panel and the back surface
protective member and electrically connected to one another, a
plurality of wiring members for electrically interconnecting
adjacent solar cell elements of the plurality of solar cell
elements, and connecting members for electrically interconnecting
the plurality of wiring members, wherein the connecting members are
disposed between the back surfaces of the solar cell elements and
the back surface protective member.
[0065] As described above, the connecting members interconnect the
wiring members at locations between the solar cell elements and the
back surface protective member, that is, at non-light-receiving
locations. This structure makes it possible to reduce the area of
the entire solar cell module. At the same time, it allows to
prevent the formation of lines with different lengths between the
wiring members and the solar cell elements, so that the design
quality of the solar cell module can be further improved.
[0066] As described so far, the present invention makes it possible
to obtain a solar cell module with high efficiency and high design
quality through a simple process without causing the parts and
steps to be increased. Thus, a solar cell module with high
efficiency and high design quality exhibiting excellent appearance
that can be produced at low cost can be realized.
[0067] Meanwhile, since the present invention exerts its
advantageous effect particularly on solar cell modules in which the
system's impression is determined by the external appearance, the
present invention is particularly effective for large scale solar
cell modules whose one side is about lm long or more. When the
present invention is applied to such a module having a long side,
not only high power generation efficiency can be achieved, but also
the impression of the lines penetrating the solar cell module that
are formed by spaces between the solar cell elements and the wiring
members can be improved. As a result, a solar cell module with high
design quality can be realized.
[0068] It is preferred that a sum of the areas of the plurality of
solar cell elements accounts for not less than 91.9% and not more
than 97.7% of an area on the light receiving surface side of the
solar cell module. By determining the range as above, the packing
density of the solar cell elements within the solar cell module can
be increased with electrical connection between the solar cell
elements being secured. It is therefore possible to render the
impression of the color of the solar cell elements to the entire
solar cell module, allowing the design quality of the solar cell
elements to be improved, as well as to improve the power generation
efficiency of the solar cell module (amount of power
generation/area of solar cell module).
[0069] Preferably, the shorter distance selected from distances
including the shortest distance between an end side of a solar cell
element located at the outermost periphery of the plurality of
arranged solar cell elements and an end of the perimeter of the
solar cell module and the shortest distance between the wiring
members or the connecting members and the end of the perimeter of
the solar cell module is not less than 5 mm and not more than 11
mm.
[0070] This structure makes it possible to reduce the ratio of the
outer peripheral region of the solar cell module that has a
different color from that of the solar cell elements, and thereby
to render a dark tone of the color of the surfaces of the solar
cell elements, which is something between blue and dark blue near
black, to the solar cell module. As a result, the design quality of
the solar cell module can be further improved, as well as the power
generation efficiency (amount of power generation/area of solar
cell module) of the solar cell module can be improved owing to the
high proportion of the solar cell elements.
[0071] The spacing between the plurality of solar cell elements is
preferably not less than 70% and not more than 143% of the widths
of the wiring members. This allows the spacings between the solar
cell elements and the widths of the wiring members to be generally
equal, which makes a plurality of lines in the same direction
appear to be penetrating the solar cell module, as an overall
impression of the solar cell module. Thus, the design quality of
the solar cell module can be further improved.
[0072] By designing all the widths of the plurality of wiring
members visible from the light receiving surface side to be
generally identical, it is possible to prevent the wiring members
from appearing to be uneven, thereby further improving the design
quality of the solar cell module.
[0073] In addition, by determining the widths of the wiring members
to be not less than 0.8 mm and not more than 2.0 mm, the wiring
members can be prevented from being noticeable.
[0074] Moreover, a photovoltaic power generator according to the
present invention is a device for extracting electric power by
connecting one or a plurality of the solar cell modules.
BRIEF DESCRIPTION OF THE DRAWINGS
[0075] FIG. 1(a) illustrates a solar cell element used for a solar
cell module according to the present invention viewed from the back
surface thereof.
[0076] FIG. 1(b) illustrates the solar cell element used for a
solar cell module according to the present invention viewed from
the surface thereof.
[0077] FIG. 2 illustrates a cross section of a solar cell element
according to the present invention.
[0078] FIG. 3(a) illustrates a cross section of a solar cell module
according to the present invention.
[0079] FIG. 3(b) is an enlarged cross-sectional view of FIG. 3
(a).
[0080] FIG. 4(a) shows an example of the configuration of
electrodes on the light receiving side (surface side) of a solar
cell element.
[0081] FIG. 4(b) is a partially enlarged view showing a cross
section of the part C of FIG. 4 (a) cut along the line B-B of FIG.
2.
[0082] FIG. 5 schematically illustrates electric current flowing
paths in a finger electrode (particularly its edge line part).
[0083] FIGS. 6(a), 6(b) illustrate embodiments of a contact surface
according to the present invention.
[0084] FIGS. 7(a), 7 (b) and 7 (c) illustrate other embodiments of
a contact surface according to the present invention.
[0085] FIG. 8 is a diagram for illustrating a preferred mode of
dimensional design of a finger electrode according to the present
invention.
[0086] FIG. 9(a) is a diagram for illustrating a preferred mode of
dimensional design of bus bar electrodes according to the present
invention.
[0087] FIG. 9(b) is an enlarged view of the part D of FIG.
9(a).
[0088] FIGS. 10(a), 10(b) illustrate bus bar electrodes according
to the present invention in a solar cell element with an irregular
shape.
[0089] FIG. 11(a) is an enlarged view of the part E of FIG. 4 (a)
showing one example of the configuration of electrodes on the light
receiving surface side (surface side) of a solar cell element
according to the present invention.
[0090] FIG. 11(b) is a partially enlarged view showing a cross
section cut along the line F-F of FIG. 11(a).
[0091] FIG. 12 is a partially enlarged view of an end portion on
the light receiving surface side of a solar cell module according
to the present invention.
[0092] FIG. 13 is a partially enlarged view of an end portion on
the non-light-receiving surface side of a solar cell module
according to the present invention.
[0093] FIG. 14 is a view on arrow of a cross section of the solar
cell module according to the present invention shown in FIGS. 12
and 13 cut along the line G-G.
[0094] FIG. 15 is a view on arrow of a cross section of the solar
cell module according to the present invention shown in FIGS. 12
and 13 cut along the line H-H.
[0095] FIG. 16 is a partially enlarged view showing an end portion
on the light receiving surface side of a conventional solar cell
module.
[0096] FIG. 17 is a partially enlarged view showing an end portion
on the non-light-receiving surface side of the conventional solar
cell module.
[0097] FIG. 18 is a view on arrow showing a cross section of the
conventional solar cell module shown in FIGS. 16 and 17 cut along
the line G-G.
[0098] FIG. 19 is a view on arrow showing a cross section of the
conventional solar cell module shown in FIGS. 16 and 17 cut along
the line H-H.
BEST MODE FOR CARRYING OUT THE INVENTION
[0099] The solar cell module according to the present invention
will be hereinafter described in detail with reference to the
accompanying drawings.
[0100] FIGS. 1(a) and 1(b) are cross-sectional views showing one
example of the configuration of electrodes of a solar cell element.
FIG. 1(a) shows the non-light-receiving surface (back surface)
side, and FIG. 1(b) shows the light receiving surface (surface)
side. FIG. 2 is a view of an A-A cross section showing a
cross-sectional structure of the solar cell element.
[0101] A p-type silicon semiconductor substrate is denoted by 1, an
n-type diffusion layer by 1a, an antireflective film by 2, a
semiconductor junction region by 3, back surface bus bar electrodes
by 4a, back surface power collecting electrode by 4b, surface
finger electrodes by 5b, and surface bus bar electrodes by 5a.
[0102] The surface bus bar electrodes 5a and the surface finger
electrodes 5b are collectively referred to as the "surface
electrode". The back surface bus bar electrodes 4a and the back
surface power collecting electrode 4b are collectively referred to
as the "back surface electrode".
[0103] Now, a product ion process of the solar cell elements is
described. First, a p-type semiconductor silicon substrate 1
comprising monocrystalline silicon, multicrystalline silicon, and
the like is prepared. This silicon substrate 1 includes a
semiconductor impurity such as boron (B) or the like in an amount
of about 1.times.10.sup.16-1.times.10.sup.18 atoms/cm.sup.3 and has
a specific resistivity of about 1.00-2.00 .OMEGA.cm.
[0104] When a monocrystalline silicon substrate is prepared, it is
formed by a pulling method or the like, and by a casting method or
the like in the case of a multicrystalline silicon substrate.
Multicrystalline silicon substrates can be mass-produced and
advantageous in terms of production cost over monocrystalline
silicon substrates. An ingot produced by the pulling, or the
casting method is sliced into about 300 .mu.m thick wafers, which
are then cut to obtain a substrate 1 of 15 cm 15 cm in size.
[0105] Thereafter, the surfaces are etched to a small extent using
hydrofluoric acid or hydrofluoric nitrate for cleaning the cut
surfaces of the substrate.
[0106] Then, an irregular structure having a reflectance reducing
function is formed on the surface side of the substrate, which is
the surface where light is incident.
[0107] This reflectance reduction by means of irregularities is a
very effective technique for increasing the short circuit current
of the solar cell element. At this stage, if a great number of fine
irregularities having widths and heights of not more than 2 .mu.m
where the aspect ratio is 0.1-2 are formed, in particular, the
reflectance can be effectively reduced, and the conversion
efficiency of the solar cell element can be improved.
[0108] While an anisotropic wet etching by means of an alkaline
liquid such as NaOH, KOH used for removing a surface region of the
substrate mentioned above may be adopted for the formation of this
irregularity structure, since in the case of a multicrystalline
silicon substrate formed by a casting method or the like, the
crystal plane orientations randomly scatter from grain to grain
within the substrate plane, it is very difficult to uniformly form
a good irregularity structure capable of effectively reducing the
light reflectance allover the substrate. In such a case, the
formation of a good irregularity structure allover the substrate
may be accomplished relatively easily by adopting gas etching by
means of an RIE (Reactive Ion Etching) method or the like.
[0109] Subsequently, with the silicon substrate 1 situated in a
diffusion furnace, a heat treatment is carried out in a gas
including an impurity element such as phosphorous oxychloride
(POCl.sub.3). By this process, phosphorus atoms are diffused into a
surface region of the silicon substrate 1 so that an n-type
diffusion layer 1a with a sheet resistance of about
60-300.OMEGA./.quadrature. is formed as an opposite
conductivity-type region. The interface between the silicon
semiconductor substrate 1 and the n-type diffusion layer 1a
constitutes a semiconductor junction region 3. While the diffusion
layer is formed to have a thickness of about 0.2-0.5 .mu.m, it may
be formed to have a desired thickness by controlling the diffusion
temperature and time.
[0110] When the sheet resistance is less than
60.OMEGA./.quadrature., the diffusion layer is too deep to improve
the short circuit current sufficiently. On the other hand, when it
is greater than 300.OMEGA./.quadrature., the diffusion layer is so
shallow that destruction of pn junctions may occur during the
formation of electrodes in a later process, or adequate adhesion
strength cannot be achieved between the substrate and the
electrodes.
[0111] The method for forming the opposite conductivity-type region
1a is not limited to a thermal diffusion method, but for example,
thin film deposition techniques and conditions thereof may be used
to form a hydrogenated amorphous silicon film or a crystalline
silicon film including a microcrystalline silicon layer at a
substrate temperature of about 400.degree. C. or below. However,
when such films are formed using thin film deposition techniques,
it is necessary to determine the order of the formation steps so
that the process temperature becomes lower as the process proceeds,
taking the respective process temperatures described below into
consideration.
[0112] When the opposite conductivity-type region 1a is formed
using a hydrogenated amorphous silicon film, it is formed to a
thickness of 50 nm or less, preferably 20 nm or less, and when it
is formed using a crystalline silicon film, the thickness is 500 nm
or less, preferably 200 nm or less.
[0113] Incidentally, when the opposite conductivity-type region 1a
is formed by the foregoing thin film deposition techniques, forming
an i-type silicon region (not shown) between the p-type bulk region
5 and the opposite conductivity-type region 1a to a thickness of 20
nm or less is effective to improve the properties.
[0114] Then, with the n-type diffusion layer 1a remaining only on
the surface side of the silicon substrate 1, the other portions are
removed, and thereafter it is cleaned with pure water. The removal
of the n-type diffusion layer 1a formed on regions other than the
surface side of the silicon substrate 1 is carried out by applying
a resist film on the surface side of the substrate 1, and then
etching with a liquid mixture of hydrofluoric acid and nitric acid,
and finally removing the resist film.
[0115] Subsequently, the antireflective film 2 is formed. As the
antireflective film 2, Si.sub.3N.sub.4 film, TiO.sub.2 film,
SiO.sub.2 film, MgO film, ITO film, SnO.sub.2 film, ZnO film and
the like may be employed. The thickness may be selected depending
on the material used so as to satisfy a non-reflection condition to
incident light (Let the index of refraction of the material be
represented by n, and the wavelength in the spectrum range where
non-reflection is desired be represented by .lamda., the optimum
thickness of the antireflective film is expressed as
(.lamda./n)/4=d.). For example, in the case of a generally used
Si.sub.3N.sub.4 film (n=about 2), if the wavelength for
non-reflection is 600 nm, the film thickness may be determined to
be about 75 nm.
[0116] The antireflective film 2 is formed by a PECVD method, vapor
deposition, sputtering or the like under a temperature of about
400-500.degree. C. Meanwhile, the antireflective film 2 is
patterned using a predetermined pattern for forming the surface
electrode. The patterning process may be etching (wet or dry) using
a resist as a mask, or forming a mask during the formation of the
antireflective film 2 and removing the mask after the formation of
the antireflective film 2.
[0117] As an alternative process, the so-called fire through
process is also generally used, in which the electrode material is
applied directly on the antireflective film 2 and baked thereby to
bring the surface electrode into contact with the opposite
conductivity-type region 1a. In this case, the foregoing patterning
is not necessary. This Si.sub.3N.sub.4 film can be provided with a
surface passivation effect during its formation process, and a bulk
passivation effect during the following heat treatment process,
which are combined with the antireflective function to contribute
to improving the electrical characteristics of the solar cell
element.
[0118] Then, with silver paste applied on the surface, and an
aluminum paste applied on the back surface, the silicon substrate 1
is baked to form the surface electrode 5 and the back surface
electrode 4.
[0119] As shown in FIG. 1(a), the back surface electrode 4
comprises back surface bus bar electrodes 4a and the back surface
power collecting electrode 4b.
[0120] As shown in FIG. 1(b), the surface electrode 5 comprises the
surface bus bar electrodes 5a for extracting outputs from the
surface, and surface finger electrodes 5b for collecting power
provided so as to perpendicularly cross the surface bus bar
electrodes.
[0121] The back surface power collecting electrode 4b is deposited
such that an organic vehicle and a glass frit are mixed with
aluminum powder at a ratio of 10-30 parts by weight, and 0.1-5
parts by weight, respectively, per 100 parts by weight of aluminum
to produce an aluminum paste, which is then printed, for example,
by a screen printing method and dried. Thereafter, it is baked at a
temperature of 600-800.degree. C. for about 1-30 minutes. During
this process, the aluminum diffuses into the silicon substrate 1 to
form a back surface field layer that prevents carriers generated at
the back surface from being recombined.
[0122] The back surface bus bar electrodes 4a, the surface bus bar
electrodes 5a, and the surface finger electrodes 5b are deposited
such that an organic vehicle and a glass frit are mixed with silver
powder at a ratio of 10-30 parts by weight, and 0.1-5 parts by
weight, respectively, per 100 parts by weight of silver to produce
a silver paste, which is then printed, for example, by a screen
printing method and dried. Thereafter, they are baked at a
temperature of 600-800.degree. C. for about 1-30 minutes at
once.
[0123] Incidentally, the surface electrode 5 may be formed after
the region of the antireflective film 2 that corresponds to the
electrodes are removed by etching, or may be directly formed over
the antireflective film 2 by a technique called fire through
process.
[0124] Meanwhile, the solar cell element according to the present
invention comprises three bus bar electrodes 5a. While in the case
of two bus bar electrodes 5a, when the widths of the finger
electrodes 5b are narrowed for preventing light energy loss on the
light receiving surface of the solar cell element, the fill factor
FF tends to deteriorate due to the series resistance component in
the finger electrodes 5b, providing three bus bar electrodes 5a
allows the lengths of the finger electrodes 5b to be shortened, so
that deterioration of the fill factor FF due to the series
resistance component of the finger electrodes 5b can be
suppressed.
[0125] The foregoing bus bar electrodes 5a preferably have widths
of not less than 0.5 mm and not more than 2 mm, and it is more
preferable that the finger electrodes 5b have widths of not less
than 0.05 mm and not more than 0.1 mm.
[0126] Widths of less than 0.5 mm are unpreferable for the bus bar
electrodes 5a, because with such widths increases the resistance of
the bus bar electrodes 5a, and the resistance of the wiring members
8 that are connected to the bus bar electrodes during the later
process also increases. On the other hand, in the case of widths of
more than 2 mm, although the resistance of the bus bar electrodes
5a can be decreased adequately, the electrodes become excessively
thick that the electrode area of the surface electrode increases
causing the light receiving surface area to be reduced, by which
the conversion efficiency of the solar cell element may also
unpreferably drop.
[0127] Widths of less than 0.05 mm are unpreferable for the finger
electrodes 5b, because with such widths increases the resistance of
the finger electrodes 5b. On the other hand, in the case of width
of more than 0.1 mm, although the resistance of the finger
electrodes 5b can be decreased adequately, the electrodes become
excessively thick that the electrode area of the surface electrode
increases causing the light receiving surface area to be reduced,
by which the conversion efficiency of the solar cell element may
also unpreferably drop.
[0128] While in the solar cell module according to the present
invention described above, embodiments in which the electrode
surface of the solar cell element is not covered with solder have
been described, the electrode surface may be covered with
solder.
[0129] The single solar cell element fabricated through the
foregoing process can generate only a small amount of electric
power. Therefore, it is necessary that a plurality of the solar
cell elements are series and parallel connected to be assembled
into a solar cell module so as to generate practical electric
power.
[0130] As one example of the solar cell module, a cross-sectional
view of a solar cell module Y constructed by combining the solar
cell element X of FIG. 2 is shown in FIG. 3(a).
[0131] As shown in FIG. 3(a), a plurality of solar cell elements X
are electrically interconnected through the wiring members 8, and
disposed between the translucent panel 9 and the back surface
protective member 11.
[0132] For the translucent panel 9, glass, a polycarbonate resin or
the like is used. While clear glass, tempered glass,
double-tempered glass, infrared-ray reflecting glass or the like
may be used as the glass, generally, clear tempered glass with a
thickness on the order of 3 mm-5 mm is commonly used. When
polycarbonate resin is employed, those having a thickness on the
order of 5 mm are commonly used.
[0133] For the filler member 10, materials with light
transmittance, heat resistance, and electrical insulation are
preferably used. Materials including ethylene vinyl acetate
copolymer (EVA) containing 20-40% vinyl acetate or those including
polyvinyl butyral (PVB) as the main component in sheet-like forms,
whose thicknesses are on the order of 0.4-1 mm are used. In many
cases of solar cell module production, the filler member 10 is
provided in both the surface side and back surface side of solar
cell elements, and during the laminating process under decreased
pressure, these are thermally crosslinked and fused to be
integrated with other members.
[0134] For the back surface protective member 11, a fluorine-based
resin sheet with weatherability including an aluminum foil held
therein so as not to allow moisture to permeate, or a polyethylene
terephthalate (PET) sheet including alumina or silica vapor
deposited thereon or the like is used.
[0135] For the wiring members 8, a conductive material, for
example, a material comprising a copper foil as the main body whose
surface is coated with solder is used. This is cut to predetermined
lengths, which are soldered to the bus bar electrodes 5a that
extract power from the solar cell elements and the back side output
electrodes 4a on the back side.
[0136] FIG. 3 (b) is a partially enlarged view of the internal
structure of the solar cell module Y in FIG. 3 (a).
[0137] As shown in FIG. 3 (b), a surface bus bar electrode 5a of a
solar cell element X1 and a back surface bus bar electrode 4a of a
solar cell element X2 adjacent thereto are connected to each other
by three wiring members 8 (since this is a cross-sectional view,
only one is shown in the Figure) so that a plurality of solar cell
elements X are electrically connected to each other.
[0138] For the wiring members 8, for example, a copper foil with a
thickness of about 100-300 .mu.m whose surface is coated with about
20-70 .mu.m thick solder is cut to predetermined lengths and
used.
[0139] When the wiring members 8 are actually installed, first, one
end of the wiring member 8 is soldered to a bus bar electrode 5a of
a solar cell element by means of hot air or hot plate.
Subsequently, the other end of the wiring member 8 is soldered in
the same way to a back surface bus bar electrode 4a of a solar cell
element that is adjacent to the foregoing one when arranged in a
module. This procedure is repeatedly carried out to produce a group
of solar cell elements including a plurality of solar cell elements
connected to one another.
[0140] In the present invention, the bus bar electrodes 4a, 5a of
the solar cell elements X are not preliminarily coated with solder,
and the solar cell elements X and wiring members 8 are connected by
fusing the solder that covers the wiring members 8.
[0141] Meanwhile, the effect of the present invention is exerted
when at least one solar cell element according to the present
invention is included in the group of solar cell elements. However,
in order to exert the effect of the invention advantageously, it is
more preferable that all the solar cell elements constituting the
solar cell element group are the solar cell elements having the
structure of the present invention.
[0142] Connection of the output wiring for extracting electric
current collected by the wiring members 8 to the outside will be
later described in detail with reference to FIG. 12-FIG. 15.
[0143] At this stage, a laminate including the foregoing
translucent panel 9, surface side filler member 10, the group of
solar cell elements including a plurality of solar cell elements to
which the wiring members 8 and output wiring lines are connected,
back surface side filler member 10 and the back surface protective
member 11 are bonded and integrated.
[0144] That is, the laminate including each component is set in an
apparatus called laminator for applying pressure to the laminate
under decreased pressure while heating the same. Then, with the
pressure being reduced to about 50-150 Pa to remove air inside the
solar cell module, pressure is applied to the laminate at a
temperature of 100-200.degree. C. for 15 minutes to 1 hour while
heating is maintained. By this process, the filler member 10
provided on both the surface side and the back surface side is
softened, crosslinked and fused, so that the components can be
bonded and integrated to produce a panel section of the solar cell
module.
[0145] In addition, a terminal box is attached by an adhesive agent
to the back surface of the panel section of the solar cell module
that has been produced by the foregoing method. The structure of
the terminal box will also be later described referring to FIGS.
12-15.
[0146] Usually, a module frame (not diagramed) is provided for the
respective side regions of the panel section of a solar cell
module. The module frame is fabricated by aluminum extrusion
molding in many cases, and the surface is subjected to anodization.
This module frame is fit to the respective sides of the panel
section of the solar module, and the respective corner portions are
fixed by means of screws or the like. Providing such a module frame
gives mechanical strength and weatherability and furthermore,
facilitates handling during the installation of the solar cell
module and the like.
[0147] In the foregoing manner, a solar cell module according to
the present invention is produced.
[0148] Now, the structure of the surface electrode of the solar
cell element according to the present invention will be described
in detail referring to the drawings.
[0149] FIG. 4(a) illustrates one example of the configuration of
electrodes on the light incident surface side (light receiving
surface side, surface side) of a solar cell element according to
the present invention.
[0150] FIG. 4(b) is a partially enlarged view showing a cross
section of the part C of FIG. 4 (a) cut along the line B-B in FIG.
2.
[0151] As shown in FIG. 4(b), the area at which a finger electrode
5b and the semiconductor substrate 1 are in contact with each other
is referred to as "contact surface 22a". Suppose the direction of a
current flowing through the finger electrode 5b is denoted by "I",
and the surface perpendicular to the current direction I is denoted
by "J". At least a part of the trajectory (which corresponds to the
edge line 22b in this example) of a cross point P at which the
perpendicular surface J crosses an edge line 22b of the contact
surface 22a that is formed when the point P is continuously moved
along the current direction I has a rugged contour.
[0152] By providing a rugged contour in the edge lines 22b of the
contact surface 22a as described above, the contact resistance
between the finger electrode 5b and the semiconductor substrate 1
can be reduced.
[0153] Now, the reason for this is discussed.
[0154] FIG. 5 is a cross-sectional view at the perpendicular
surface J schematically illustrating current paths at an edge line
region of a finger electrode 5b. In FIGS. 5, 5b denotes a finger
electrode, 1 denotes a p-type bulk semiconductor substrate, 1a
denotes an opposite conductivity-type region, and the numeral 2
denotes an antireflective film.
[0155] In FIG. 5, electrons and holes generated mainly at the
p-type bulk semiconductor substrate 1 are separated at a pn
junction, so that electron carriers are collected in the opposite
conductivity-type region 1a (hole carriers are collected on the p+
region side of the back surface of the p-type bulk semiconductor
substrate 1, which is not shown), and these collected electron
carriers flow laterally (in the direction horizontal to the
substrate plane) into the finger electrode 5b as electron currents,
the manner of which is indicated by arrows.
[0156] As shown in FIG. 5, the electron currents tend to flow into
the vicinity of the edge line of the finger electrode 5b in a
concentrated manner. The degree of this concentration is determined
depending on a magnitude relationship between the sheet resistance
of the opposite conductivity-type region 1a and the contact
resistance between the surface electrode and the opposite
conductivity-type region 1a.
[0157] That is, suppose arbitrary electric current paths that
differ from each other in the position at which electrons flow from
the opposite conductivity-type region 1a into the finger electrode
5b (the position at which electrons transverse the interface
between the both), and the total resistance of the current paths is
discussed.
[0158] If a resistance originated from the contact resistance is
sufficiently small as compared with a resistance originated from
the sheet resistance (normally, this condition is satisfied), an
electric current flows selectively through a path with the smallest
resistance loss. For this reason, electron currents flow into an
edge line region of the finger electrode 5b as shown in FIG. 5. If
the proportion of a resistance originated from the contact
resistance to the total resistance of the paths is extremely large
(e.g. in a rare case where a cell is defective and has poor contact
characteristics), the degree of concentration of electron currents
flowing into an edge line region is lowered, and electron currents
flow over a wider contact range (shown by dotted lines in FIG.
5).
[0159] In the present invention, at least a part of the foregoing
trajectory (edge line 22b) comprises a rugged contour as shown in
FIG. 4 (b). Owing to this structure, it is possible to
substantially expand the region of the edge line of the finger
electrode 5b into which electron currents flow in a concentrated
manner. As a result, the contact resistance between the surface
electrode and the semiconductor substrate 1 can be reduced.
[0160] Referring to the case related to the present invention where
normal contact characteristics are achieved, contact resistance Rc
[.OMEGA.] can be expressed using surface contact resistance Rcs
[.OMEGA.cm.sup.2] and contact area Sc [cm.sup.2] as follows:
Rc=Rcs/Sc
[0161] Here, if the contact area Sc is expressed using contact
width Wc (the direction perpendicular to the sheet surface in FIG.
5) and contact depth Dc (the direction from the contact edge line
toward the inside of the finger electrode 5b, and horizontal to the
sheet surface in FIG. 5) as Sc=Wc.times.Dc, the equation above can
be expressed as follows:
Rc=Rcs/(Wc.times.Dc) [0162] where Dc corresponds to the effective
contact width in FIG. 5.
[0163] Therefore, when Wc is increased, Rc can be reduced, and this
Wc can be effectively increased by the present invention.
[0164] Meanwhile, generally, finding the value of Dc is very
difficult. In this case, it is convenient to discuss this using
Rc.times.Dc instead of Rc on the basis of the unit [.OMEGA.cm].
This is because in this way, a magnitude comparison between values
proportional to Rc can be discussed based only upon the measurable
Rcs and We (Rcs can be easily measured by the four-point probe
method).
[0165] According to this, in a conventional structure in which the
foregoing trajectory (edge line 22b) is not provided with a rugged
contour, the value of RcxDc can be estimated to be about
2-4.OMEGA.cm. When this value is converted into conversion
efficiency with respect to a solar cell element using a
multicrystalline silicon substrate with a conversion efficiency on
the order of 15% that is used for crystalline silicon based modules
currently available in the market, this can be assumed to be a loss
of about 0.2-0.3%.
[0166] Contrary to this, in the structure according to the present
invention where the foregoing trajectory (edge line 22b) includes a
rugged contour, the resistance of the contact surface can be
relatively easily reduced by around 50% in this Rc.times.Dc, which
corresponds to an improvement of 0.1-0.15% over the conventional
structure when converted into conversion efficiency.
[0167] In the solar cell element according to the present
invention, the pattern of the surface electrode is arranged, as
already shown in FIG. 4 (b), such that a point P at which an
opposite conductivity-type region 1a and an edge line 22b of a
contact surface 22a of a power collecting electrode cross a surface
J that is generally perpendicular to a current direction I forms a
trajectory extending in I direction that comprises at least in a
part thereof a rugged contour.
[0168] In order to realize such an edge line 22b, specifically,
when a printing and baking process using paste is employed, it can
be deposited by screen printing with use of a screen having a
predetermined aperture pattern designed so that the edge lines 22b
of the contact surface 22a have zigzags as shown in FIG. 4 (b)
followed by baking as described above. By this, the effective
length of the edge lines 22b of the contact surface 22a of the
collecting electrode is increased, and the substantial contact area
with the opposite conductivity-type region is also increased, so
that the contact resistance can be reduced effectively.
[0169] Lastly, a solder region (not shown) is formed on the surface
electrode and the back surface electrode by a solder dipping
process. Meanwhile, when electrodes are formed as solderless
electrodes without using solder, the solder dipping process is
omitted.
[0170] Incidentally, while in the embodiment shown in FIG. 4 (b),
the configuration of the rugged contour provided in the edge lines
22b of the contact surface 22a between the surface electrode and
semiconductor substrate 1 is in zigzags as that of triangular waves
formed like a continuous row of triangles, it may be, for example
as shown in FIG. 6(a), formed like triangles appearing
intermittently in the edge lines 22b, or may be formed by curves as
shown in FIG. 6(b). In this way, the edge lines 22b can be formed
by polygons, rectangles, curves or combinations thereof.
[0171] In addition, it is more preferable that the configuration of
the edge lines of the contact surface 22a is asymmetric with
respect to the center line of the surface electrode forming the
contact surface 22a (the center line in the same direction as the
current direction I of the surface electrode).
[0172] FIGS. 7(a)-7(c) illustrate configurations of the contact
surface 22a between the finger electrode 5b and the semiconductor
substrate 1 and its edge lines 22b.
[0173] In FIG. 7(a), the edge lines 22b opposed to each other with
respect to a center line K are formed in zigzags with a phase
difference. In other words, the lines have an asymmetric positional
relationship. Since this arrangement enables to eliminate parts
where the width of the finger electrode 5b is particularly small,
the line resistance of the finger electrode 5b is not increased,
which is very effective.
[0174] As shown in FIGS. 7(b) and 7(c), the configuration of the
edge lines 22b is not limited to a zigzag form, but may include
polygons, rectangles, curves or combinations thereof, which are
arranged so that the edge lines are asymmetric with respect to the
electric current flowing direction.
[0175] While FIGS. 7 (a)-7 (c) show embodiments in which the phase
difference between the contours of the edge lines that are
asymmetric with respect to the center line of the finger electrode
5b is a half cycle (.pi.), the phase difference is not limited to a
half cycle. It may be any other form as long as narrow portions of
the finger electrode 5b can be reduced.
[0176] As shown in FIG. 8, when the area surrounded by the edge
lines 22b of the contact surface 22a formed by the finger electrode
5b and the semiconductor substrate 1 being in contact with each
other is represented by S1, an average value of distances between
the edge lines within each cut surface formed by cutting at a
plurality of cut planes that are generally perpendicular to the
direction of electric current flowing through the finger electrode
I is represented by d.sub.1, and the perimeter of the edge lines
22b is represented by L1, it is preferable that these
S.sub.1,d.sub.1,L.sub.1 satisfy the following relationship:
L.sub.1/2(S.sub.1d.sub.1.sup.-1+d.sub.1)>1.2 (1)
[0177] Now, what the formula (1) indicates will be described. The
area S.sub.1 surrounded by the edge lines 2b of the contact surface
22a formed by the finger electrode 5b and the semiconductor
substrate 1 being in contact with each other can be also described
as the area observed when the edge lines of the contact area 22a
are planarly viewed from a vertical direction.
[0178] Accordingly, suppose that the shape of the finger electrode
5b is rectangular, 2(S.sub.1d.sub.1.sup.-1+d.sub.1) is equal to the
perimeter of the rectangular shape. Therefore, L.sub.1 divided by
this gives:
R=0.5L.sub.1(S.sub.1d.sub.1.sup.-1d.sub.1).sup.-1 [0179] which
represents the ratio between the perimeter of the edge lines 22b
(with a rugged contour) of the contact surface 22a and the
perimeter without a rugged contour (in the case of a rectangular
shape). This can therefore be an index indicating the degree of the
rugged contour of the edge lines 22b.
[0180] As described above, by defining the perimeter L.sub.1 of the
edge lines 22b of the contact surface 22a between the finger
electrode 5b, which, in particular, functions mainly as a
collecting electrode in the surface electrode and the semiconductor
substrate 1 to be 1.2 or more times as large as the perimeter of
edge lines forming a rectangular shape with the same area, it is
possible to distinctly increase the effective area of the contact
surface 22a and thereby to reduce the contact resistance. The
output characteristics of the photoelectric conversion element can
therefore be improved.
[0181] Meanwhile, the upper limit of the foregoing proportion is
preferably 3-5, or more preferably 3.
R<3-5
[0182] That is, in cases where the rugged structure of the surface
of the semiconductor region is negligibly small and the edge lines
22b form a two-dimensional structure, if the foregoing proportion
exceeds the upper limit, the line width at recessed portions of the
rugged contour is bound to be too small that problems such as line
breakage arise.
[0183] Also, in cases where the edge lines 22b are formed to have a
three-dimensional structure reflecting the rugged structure of the
surface of the semiconductor region under the surface electrode, if
the foregoing proportion exceeds the upper limit, the aspect ratio
(height/pitch of rugged portion) of the rugged structure of the
surface of the semiconductor is too great that leak tends to occur
at the projected portions of the rugged structure.
[0184] In the above stated range of 1.2-3-5, these are well
balanced, so that the effect of the invention will be
advantageously exerted.
[0185] The measurement of the width of the finger electrode 5b may
be carried out by dividing the finger electrode 5b into m
(m.gtoreq.6) equal parts along the longitudinal direction and
determining the average thereof. For example, in the case of FIG.
8, the finger electrode 5b is equally divided into 6 parts at five
points including d.sub.11-d.sub.15. The average of values at these
five locations may be determined as d.sub.1.
d.sub.1=.SIGMA.d1i/n(i=1,2, . . . , n)
[0186] Meanwhile, the foregoing description is given by referring
to a structure including the finger electrode 5b as surface
electrode that forms the contact surfaces 22a by being in contact
with the semiconductor substrate 1, and the bus bar electrode 5a
for extracting power that is connected to at least one end of the
finger electrode 5b and has a larger line width than the finger
electrode 5b.
[0187] However, the element that forms a contact surface by being
in contact with the semiconductor region as surface electrode is
not limited to finger electrodes. It is preferable that bus bar
electrodes are also arranged so as to form the contact surfaces
according to the present invention by being in contact with the
semiconductor region, which exerts more advantageous effect.
[0188] Hereinafter, preferred structures of bus bar electrode 5b
according to the present invention will be described with reference
to FIGS. 9(a), 9(b).
[0189] FIG. 9(a) illustrates a surface electrode viewed from the
light incident surface side of a solar cell element according to
the present invention. FIG. 9(b) is an enlarged view of a part D of
FIG. 9(a).
[0190] As shown in FIG. 9(b), referring to a contact surface 32a
that is formed by a bus bar electrode 5a and a semiconductor
substrate 1 (a cross-sectional structure thereof is shown in FIG.
2) being in contact with each other, when the sum of the entire
lengths of the edge lines 32b is represented by L.sub.2, the area
of the contact surface 32a when the edge lines 32b are planarly
viewed from a direction vertical to the light incident surface is
represented by S.sub.2, and the area of the entire light incident
surface planarly viewed from a direction vertical to the light
incident surface is represented by S.sub.3, it is preferable that
these satisfy the following formulae:
L.sub.2>5 S.sub.3 (2)
0.015<S.sub.2/S.sub.3<0.050 (3)
[0191] First, a description will be given to formula (2).
[0192] This indicates, as shown in FIG. 9(b), that the sum of the
entire lengths L.sub.2 of the edge lines 32b when the contact
surface 32a is planarly viewed is 5 times or more the square root
(1/2th power) of the area S.sub.3 of the entire light incident
surface of the solar cell element when planarly viewed.
[0193] The example shown in FIG. 9 (b) is an enlarged view of apart
of the bus bar electrode 5a. The actual entire length of the edge
line 32b covers the entire length of the bus bar electrode 5a, and
to obtain the sum L.sub.2 of the entire lengths of the edge lines
32b, as many additions as the number of the bus bar electrodes 32b
are necessary.
[0194] In the present invention, the length of the edge lines 32b
is long when the contact surface 32a is planarly viewed, which
corresponds to substantially expanding (increasing) the region
(area) of the edge line portions of the bus bar electrode 5a into
which electron currents flow in a concentrated manner. This is the
same as already speculated referring to FIG. 5. As a result of
this, the contact resistance between the bus bar electrodes 5a and
semiconductor substrate 1 can be reduced, so that the conversion
efficiency of the solar cell element can be increased.
[0195] The sum L.sub.2 of the entire lengths of the edge lines 32b
is preferably 5 times or more the square root (1/2th power) of the
area S.sub.3 of the entire light incident surface of the solar cell
element. This condition can be satisfied as long as the number of
the bus bar electrodes 5a is three or more when the solar cell
element has a generally square shape and includes bus bar
electrodes 5a each of which has a length slightly shorter than the
length of one side of the solar cell element as shown in FIG.
9(a).
[0196] However, only increasing the sum L.sub.2 of the entire
lengths of the edge lines 32b causes the bus bar electrodes 5a to
shield the light incident surface, leading to decrease in the
amount of incident light.
[0197] Accordingly, it is preferred that, as the formula (3)
indicates, the proportion of the area S.sub.2 defined when the edge
lines 32b of the contact surface 32a are planarly viewed from a
vertical direction to the area S.sub.3 of the light incident
surface is less than 0.050 (5%).
[0198] By the way, the purpose for adding the limitation as
"planarly viewed" to the descriptions above is to exclude the
irregularities and waves on the surfaces.
[0199] The foregoing area S.sub.2 corresponds to the area of the
bus bar electrodes 5a. By suppressing the proportion of this area
to be smaller than a prescribed range with respect to the entire
area of the light incident surface, decrease of the conversion
efficiency can be suppressed. Meanwhile, if the proportion is 0.015
(1.5%) or less, the widths of the bus bar electrodes 5a become
narrower, and for this and other causes, the conduction resistance
increases, which is therefore unpreferable.
[0200] The operational effect of the foregoing formulae are
advantageously exerted even when the shape of the solar cell
element is irregular.
[0201] FIGS. 10(a), 10(b) illustrate examples of solar cell
elements with irregular shapes. FIG. 10(a) shows a horizontally
long solar cell element including finger electrodes 5b in the
longitudinal direction, and five bus bar electrodes 5a in the
shorter axis direction, and FIG. 10(b) shows a vertically long
solar cell element including one bus bar electrode 5a in the
longitudinal direction, and finger electrodes 5b in the shorter
axis direction.
[0202] For example, in the case of the example of FIG. 10(a),
although the length of each bus bar electrode 5a is short, the
number of the bus bar electrodes 5a is increased in this structure
based on formula (2). In such a horizontally long solar cell
element, the lengths of the finger electrodes 5b tend to be long,
causing the resistance to increase, which adversely affects the
characteristics of the solar cell element. However, by increasing
the number of the bus bar electrodes 5a as indicated by formula
(2), the distance from the finger electrodes 5b to the bus bar
electrodes 5a can be shortened, so that it is possible to avoid
adverse influences due to increased resistance.
[0203] In the case of the example of FIG. 10 (b), since the length
of each bus bar electrode 5a is long, the number of bus bar
electrodes 5a may be decreased according to formula (2) in this
structure. In such a vertically long solar cell element, since the
length of the finger electrodes 5b are shortened, even when the
number of the bus bar electrodes 5a is small, the possibility of
adverse influences due to the resistance of the finger electrodes
5b is small.
[0204] As described so far, the lengths and areas of the bus bar
electrodes according to the present invention can be optimized by
arrangements based on formulae (2) and (3). The solar cell element
according to the present invention can therefore achieve a good
conversion efficiency.
[0205] Meanwhile, when the semiconductor substrate 1 is provided
with numerous fine irregularities by gas etching such as the RIE
method, actual lengths of the edge lines 32b are substantially
longer than the lengths of the edge lines 32b observed when the
contact surface 32a is planarly viewed, so that a higher effect can
be obtained.
[0206] While the foregoing description has been made referring to
the examples where end portions of finger electrodes 5b are
connected to bus bar electrodes 5a for extracting power generally
perpendicularly thereto, the connection may be accomplished in
other ways than orthogonal crossing, and the structure may be such
that both end portions of one finger electrode are connected to a
bus bar electrode 5a to form a closed configuration. In addition,
while in the foregoing examples, as the edge lines 22b on the both
sides of the contact surface 22a of finger electrode 5b, edge lines
that are homotheically similar contours are described, however, the
contours may not be necessarily homotheic to each other.
[0207] Furthermore, the surface electrodes have generally linear
shapes are described in the foregoing description, they may have
generally curved shapes. While examples where the semiconductor
substrate is flat (when the structure of edge lines 22b is
two-dimensional) have been described above, the configuration is
not limited to this, but the surface of the semiconductor substrate
may have irregularities (such as a pyramidal structure formed by
alkaline etching, or a finely roughened structure formed by the RIE
process), or may have a curved (e.g. spherical) configuration (in
other words, when the edge lines 22b have a three dimensional
structure). In any case, it is needless to say that the same effect
can be obtained according to the principles and structure of the
present invention. In these cases, the current flowing direction
and the contact surface 22a themselves are in the form of a
generally curved line or a curved surface according to the shape of
the electrodes. A direction generally perpendicular to the current
flowing direction may be determined to be the direction
perpendicular to the normal line at the desired portion in a curved
line indicating the current flowing direction.
[0208] In addition, while the foregoing description has been made
referring to examples where the solar cell element according to the
present invention includes a trajectory (which corresponds to the
edge line 32b in the foregoing example) that includes a rugged
contour in at least a part thereof, also when the structure is
arranged such that at least a part of the trajectory includes a
region where the direction of the tangent line thereto is not
coincident with the current direction, the effect of the present
invention is exerted. The region where the direction of the tangent
line and the current direction are not coincident includes, for
example, in the case of a configuration with a rugged contour,
transition regions from a recess to a projection, or a projection
to a recess.
[0209] Now, the structure of the contact surface between the
surface electrode and semiconductor substrate will be described
referring to FIGS. 11(a) and 11(b).
[0210] FIG. 11 (a) is an enlarged view of the part E of the surface
electrode shown in FIG. 4(a), and FIG. 11(b) is a partially
enlarged cross-sectional view of the surface electrode in FIG.
11(a) cut along the line F-F.
[0211] As FIG. 11 (a) shows, an area defined when bus bar
electrodes 5a and finger electrodes 5b constituting the surface
electrode are planarly viewed from a vertical direction on the
light receiving surface side is denoted by Sa.
[0212] Meanwhile, the area Sa of the surface electrode can be
determined such that the solar cell element is photographed from a
vertical direction on the light receiving surface side, and the
surface image is digitized, which is thereafter converted into a
binary form using a threshold value for separating the surface
electrode from areas other than this by a known image processing
method, thereby separating the area of the surface electrode from
areas other than it, so that the area can be determined.
[0213] In addition, as shown in FIG. 11(a), the invisible surface
area of regions that are covered with the surface electrode, in
other words, the surface area of regions immediately under the
regions in which the surface electrode (bus bar electrodes 5a and
finger electrodes 5b) is provided is denoted by Sb.
[0214] The surface area Sb in the light receiving surface of this
solar cell element may be measured by removing the surface
electrode using a predetermined acid (for example, aqua regia for
the electrodes mainly including silver) that is selected according
to the kind of the material for the surface electrode, and
thereafter measuring the surface area of the region in which the
surface electrode has been provided. For measurements of the
surface area, either of contact type or noncontact type may be
used, an AFM (atomic force microscope) may preferably be used in
view of accuracy. Meanwhile, when AFM is used, since its observable
area is limited, it is preferred to measure at a plurality of
locations in the predetermined regions in which the surface
electrode is provided, and to process data statistically.
[0215] Here, the relationship between Sa and Sb satisfies the
following formula (4):
1.10.ltoreq.Sb/Sa.ltoreq.2.10 (4)
[0216] Now, an explanation of formula (4) will be given. As
described above, since the denominator Sa represents the area of
surface electrode, and numerator Sb represents the surface area of
the light receiving surface of a solar cell element in the region
in which the surface electrode is provided, Sa/Sb in formula (4)
serves as an index of the proportion of the surface electrode in
contact with the light receiving surface of a solar cell
element.
[0217] To help understanding of this, a cross-sectional view is
shown in FIG. 11(b). Since Sa is the area of the surface electrode
when viewed planarly, it is expressed as a one-dimensional linear
form in FIG. 11(b) as indicated by a double-headed arrow Ca, and Sb
is expressed as a two-dimensional irregular form enclosed by a
region Cb. However, actually, these are extended also in the depth
direction of FIG. 11(b), Sa is a two-dimensional (virtual) planar
surface, and Sb is a surface with three-dimensional
irregularities.
[0218] Here, by determining the surface area of the light receiving
surface of the solar cell element so that the proportion in this
formula is in the range of not less than 1.10 and not more than
2.10, it is possible to increase substantial contact area between
the surface electrode and the solar cell element.
[0219] Meanwhile, when Sb/Sa is less than 1.10, a problem that
hinders improvement in conversion efficiency of the solar cell
element may arise, that is, increasing the contact area for
improving an FF (Fill Factor) leads to an decrease of the light
receiving area to decrease the short circuit current, and reducing
the electrode area for improving the short circuit current leads to
a decrease of the FF. When Sb/Sa is greater than 2.10, it becomes
hard to fill over a surface of a silicon substrate with an
electrode material by the screen printing method.
[0220] As described so far, with the structure of the solar cell
element according to the present invention, the substantial contact
area between the surface electrode and the solar cell element is
increased to be an appropriate value, which enables to reduce
adverse influences due to the series resistance component. For this
reason, the fill factor FF is not be deteriorated.
[0221] As a result of this, the solar cell element according to the
present invention is capable of reducing optical loss due to the
surface electrode, and improving the short circuit current density
and the fill factor while keeping a proper balance therebetween, so
that good conversion efficiency can be achieved.
[0222] Incidentally, in order to reduce the value of Sb/Sa in
formula (4), measures such as making the surface of the
semiconductor substrate under the electrodes near flat or
interposing an insulating film between the electrodes and the
semiconductor substrate may be taken. On the other hand, in order
to increase it, the surface of the semiconductor substrate under
the electrodes may be roughened, or formed with recesses.
[0223] In particular, to realize the proper range of the present
invention, it is preferable that the surface of the semiconductor
substrate under the electrodes has a roughened form. In addition,
when the electrodes are formed by screen printing, it is preferable
to form fine irregularities whose heights are 2 .mu.m or less
uniformly. This makes it possible to fill along the profile of the
semiconductor substrate with the electrode material, so that the
series resistance can be more effectively reduced.
[0224] The foregoing area Sa of the surface electrode according to
the present invention viewed planarly is preferably 4%-7% with
respect to the light receiving surface of the solar cell element,
because at such proportions, light energy loss can be optimally
suppressed and influences of the resistance component of the
electrodes can be less likely exerted.
[0225] In addition, in the solar cell element according to the
present invention, it is preferred that a short cut current density
Jsc, determined by dividing short circuit current Isc defined by
JIS C 8913 (1998) by the area of the substrate, is 35.5 mA/cm2 or
more, and an FF defined by JIS C 8913 (1998) is 0.75 or more.
Designing the electrodes for a solar cell element with such high
short circuit current density and FF requires a severer control.
Therefore, the effect of the solar cell element according to the
present invention can be sufficiently exerted.
[0226] The short circuit current Isc is preferably 8000 mA or more.
The value of the short circuit current Isc can be controlled by the
size of the solar cell element, and when the short circuit current
density Jsc in the solar cell element according to the present
invention is 35.5 mA/cm2, the area of the light receiving surface
is required to be not less than the area of 15 cm.times.15 cm size
rectangular element. By designing the size of the solar cell
element so that the short circuit current Isc is in this range, it
is possible to obtain an advantageous effect that the production
cost can be reduced.
[0227] This irregular structure is closely related to the foregoing
formula (4). In order to decrease the value of Sb/Sa, the direction
that the depth of the irregularities decreases and the size of the
irregularities in the horizontal direction increases, that is, the
aspect ratio (vertical/horizontal) of the irregularities can be
decreased. On the other hand, in order to increase the value of
Sb/Sa, the direction that the depth of irregularities increases and
the size of the irregularities in the horizontal direction
decreases, that is, the aspect ratio (vertical/horizontal) of the
irregularities can be increased. In order to increase the aspect
ratio of the irregularities, the reaction pressure during the
etching process can be decreased, while in order to decrease, the
reaction pressure can be increased.
[0228] Meanwhile, in the case of wet etching, since the crystal
plane orientations randomly scatter from grain to grain within the
substrate plane as described above, it is difficult to uniformly
form an irregularity structure, and therefore, as compared to a gas
etching method such as the RIE process, it is difficult to freely
control Sb/Sa with good reproducibility.
[0229] While in the foregoing description, roughening the surface
of a multicrystalline silicon substrate by reactive ion etching in
order to satisfy the formula 1.10.ltoreq.Sb/Sa.ltoreq.2.10 is
described as an example, the present invention is not limited to
this example.
[0230] For example, it is also possible to form grooves in the
region to be provided with the electrodes by means of laser or
dicing, and then so as to embed the electrode material into the
grooves. Also, the configuration and number of these grooves are
not limited to specific ones, but may be a linear form, a dotted
form, or combinations thereof.
[0231] Moreover, it is possible to preliminarily form projected
portions in the region to be provided with the electrodes so as to
satisfy the formula 1.10.ltoreq.Sb/Sa.ltoreq.2.10 according to the
present invention. The projected portions can be formed by etching
the surface leaving the region to be provided with the electrodes.
Also, in this case, the configuration and the number of projected
portions are not limited to specific ones, but may be a linear
form, dotted form, or combinations thereof. In addition, this
method can be applied to a so-called selective emitter in which the
surface concentration in the diffusion layer under the electrodes
is increased and the layer is deepened, and the surface
concentration of the diffusion layer in other regions is decreased
and the layer is shallowed.
[0232] Furthermore, needless to say, it is possible to satisfy the
formula 1.10.ltoreq.Sb/Sa.ltoreq.2.10 also by carrying out reactive
ion etching with the surface under the electrodes having been
formed with grooves or projected portions.
[0233] Hereinafter, interconnection of the elements in a solar cell
module according to the present invention will be described in
detail referring to the drawings.
[0234] The basic structure of the solar cell module is the same as
the cross-sectional structure of the typical solar cell module
shown in FIGS. 3(a) and 3(b).
[0235] When the solar cell elements are series connected, the
surface side of a solar cell element X1 and the back surface side
of an adjacent solar cell element X2 are electrically connected to
each other by wiring members 8, and the back surface side is
further electrically connected to the surface side of another
adjacent solar cell element X3. By the repetition of this, a
plurality of solar cell elements are electrically series
connected.
[0236] The wiring members 8 of the solar cell elements at the ends
are electrically interconnected by connecting members 6 described
later to be connected a terminal box 7 (FIG. 13) disposed on the
back surface of the solar cell module so that output power can be
extracted outside.
[0237] A plurality of solar cell elements electrically
interconnected by the wiring members 8 in this way are in a
condition where they are two-dimensionally aligned in vertical and
horizontal directions at predetermined intervals.
[0238] FIG. 12 shows a partially enlarged view of an end region of
the light receiving surface of a solar cell module according to the
present invention.
[0239] A solar cell module according to the present invention is
arranged such that the proportion of the sum of the areas of a
plurality of solar cell elements packed in the solar cell module to
the area of the light receiving surface side of the entire solar
cell module is not less than 91.9% and not more than 97.7%.
[0240] In order to obtain such a high area proportion in this
range, it is necessary to increase the area of each solar cell
element as well as to narrow the intervals between the solar cell
elements, that is, the area of a peripheral region of the solar
cell module where solar cell elements are not present needs to be
as small as possible.
[0241] As described above, in the solar cell module according to
the present invention, while electrical connection between the
solar cell elements being secured, the packing density of the solar
cell elements within the solar cell module is increased. This not
only makes it possible to improve the power generation efficiency
of the solar cell module (amount of power generation/area of solar
cell module), but also to give the entire solar cell module an
impression of the same color as that of the solar cell elements, so
that a beautiful exterior appearance can be given to the solar cell
module to improve design quality.
[0242] In recent years, there are a wide variety of applications
and types of usage of solar cell modules. There are not only
rectangular solar cell modules, but also modules with pyramidal and
trapezoidal shapes.
[0243] A rigid frame formed to have a hollow cross section formed
by aluminum extrusion molding or the like is fit to the perimeter
of a solar cell module so as to ensure the strength of the solar
cell module, which is fixed to a stage preliminarily placed on a
roof or the like by securing the frame with screws and used.
[0244] Also cases where a frame made of simple metal and a resin
for protecting the perimeter from impact applied during the
installation is fit to a solar cell module, and solar cell modules
without frames (frameless module) are used as the roofing are
increasing.
[0245] The present invention relates to all of the foregoing solar
cell modules, and even when it is used with a frame fit thereto, a
complete unit comprising a plurality of solar cell elements
interconnected through wiring members 8 that are packed between a
transparent panel 9 in the frame detached condition and a back
surface protective member 11 is defined as a solar cell module, and
the area of the light receiving surface side of the solar cell
module refers to the area of regions located inside the perimeter
of the complete unit.
[0246] It is preferred that when comparing the minimum distance
between an end side of a solar cell element located at the outer
most periphery and an end of the perimeter of the solar cell module
with the minimum distance between the wiring members 8
interconnecting the solar cell elements or the connecting member 6
interconnecting the wiring members 8 and an end of the perimeter of
the solar cell module, the shorter distance is not less than 5 mm
and not more than 11 mm.
[0247] With this arrangement, it is possible to reduce the
proportion of the perimeter region with a different color from that
of the solar cell elements, thereby to provide the impression of
the whole solar cell module with a shade of the dark color, which
is between blue and dark blue near black, of the solar cell
elements, by which the design quality of the solar cell module is
further improved. At the same time, the high proportion of the area
of the solar cell elements allows the power generation efficiency
(amount of power generation/area of solar cell module) of the solar
cell module to be improved.
[0248] In addition, the foregoing structure is preferred also for
the following reason: while low cost, high efficiency and high
design quality are required for solar cell modules, prior to these,
it is needless to say that safety is necessary. In addition, since
they are installed on the roofs of houses and the like, long-term
reliability for use outdoors is also required. Accordingly, it is
necessary to ensure insulation between the solar cell elements
packed within the solar cell module and the outside so as to
prevent air and water penetration. For this reason, in conventional
modules, a large area where no solar cell elements, wiring members
8 or connecting members 6 are secured in the perimeter region. In
addition, since most of the conventional solar cell modules include
a rigid frame fit to the solar cell module for ensuring its
strength, a fitting margin of about 1 cm is necessary. Because
there has been no need for arranging the solar cell elements
immediately under the frame, and because of the use of a rigid and
thick frame, there are areas inside the frame, although not
immediately under the frame, which are shaded with the frame
depending on the angle of incidence of the sunlight. Therefore,
there has been no need for arranging the solar cell elements closer
to the perimeter of the solar cell module.
[0249] However, as describe above, because of the advent of
simplified module frames and use of frameless modules, the fitting
margin is reduced or not provided in increasing cases. For this
reason, solar cell modules in which solar cell elements are
disposed also in the perimeter region are required.
[0250] Meanwhile, to obtain the above described structure, measures
such as thickening the filler member 10 to 1.0 mm or more,
increasing the temperature of heat for the perimeter region of the
solar cell module during the lamination process, or increasing the
pressure for pressing the perimeter region during the lamination
process may be taken.
[0251] In the case of a thicknesses of 5 mm or less, outside air or
water may penetrate, as a result, insulation and long-term
reliability cannot be ensured. Or, in the case of a thickness of 11
mm or more, in a module with a high packing density of solar cell
elements, the perimeter region where no solar cell elements are
present is so wide that the solar cell module appears to be a
module with an edging, which is unpreferable in view of design.
[0252] Here, it is preferred to process the surfaces of the solar
cell elements with an antireflective treatment for improving the
efficiency, because its effect is particularly well exerted in
solar cell modules in which the light diffusion/reflection effect
is enhanced by the use of a color with a dark tone between blue and
dark blue near black for the solar cell elements and the use of a
white color for the filler member 10 or the back surface protective
member 11 located on the back surface side of the solar cell
elements within the solar cell module. This is because in this way,
contrast in color tone between the solar cell elements and other
areas is distinct, and the effect in design becomes remarkable.
[0253] Furthermore, in each of the foregoing embodiments, it is
preferred that the spacing between the plurality of solar cell
elements is not less than 70% and not more than 143% of the width
of the wiring member 8.
[0254] This makes the spacing between the solar cell elements and
the width of the wiring member 8 generally equal to each other so
that the impression of the entire solar cell module appears to be
penetrated with a plurality of lines in the same direction, which
further improves the design quality of the solar cell module.
[0255] In addition, since narrowing the spacing between the solar
cell elements to the level of the width of the wiring member 8
increases the proportion of the area of solar cell elements to the
area of the solar cell module, it is possible to further improve
the power generation efficiency (amount of power generation/area of
solar cell module).
[0256] Meanwhile, in order to maximize the power generation
efficiency of the solar cell module, the packing density of solar
cell elements in a solar cell module reaches 100%. However, as
described above, from viewpoints of safety (insulation) and
long-term reliability, realizing the 100% packing density is
impossible. Although there is a method so-called imbrication for
connecting solar cell elements in which solar cell elements are
overlapped each other in a part thereof, in this method, cracking
is prone to occur in the solar cell elements when heated and
pressed by the laminator, and optical loss is caused in the
overlapping portions, which is therefore inefficient, causing the
cost for modules to increase.
[0257] While the problems of cracking and optical loss may be
eliminated if solar cell elements are arranged without spacing,
when solar cell elements are series connected, due to the presence
of wiring members 8 between the solar cell elements, which are each
connected from the surface of one solar cell element to the back
surface of the adjacent solar cell element as shown in FIG. 26,
arranging them without spacing is substantially impossible. When
the spacing between the solar cell elements is too narrow, end
portions of the solar cell elements are pressed with an oblique
force while being heated and pressed by a laminator, causing
cracking to occur. This phenomenon often occurs particularly when
the thickness of the wiring members 8 is large, or when a leadless
Sn--Ag--Cu-based solder is used for coating the copper foil of the
wiring members 8. This is because the Sn--Ag--Cu-based solders are
hard.
[0258] In order to reduce the spacing between the solar cell
elements to the level of the width of the wiring members 8, it is
preferable that the wiring members 8 are preliminarily bent to
conform with the configuration thereof for connecting solar cell
elements adjacent to each other, so that occurrence of the
foregoing problems can be restricted.
[0259] Meanwhile, a spacing between the solar cell elements of less
than 70% of the width of the wiring members 8 is unpreferable,
because in such a case, a stress applied to the connection portions
between the solar cell elements and the wiring members 8 is
increased, causing cracking to increase. When the spacing exceeds
143%, the spacing between the solar cell elements is too wide as
compared with the width of the wiring members 8, which fails to
make the impression of the solar cell module as a whole appear to
be penetrated by a plurality of lines in the same direction. The
design quality is therefore degraded and the power generation
efficiency of the solar cell module is lowered.
[0260] Moreover, it is preferred that each of the foregoing
embodiments is arranged such that all the widths of the wiring
members 8 viewed from the light receiving surface side are
generally identical.
[0261] With this structure, all the wiring members 8 have a uniform
width, so that uniformity is enhanced while preventing imbalance,
by which design quality is further improved. In particular, when
this feature is combined with the foregoing embodiments of the
present invention, because all the spacings between the solar cell
elements and all the widths of the wring members 8 are viewed to be
uniform, the solar cell module is bound to have high design
quality. Here, the width of the wiring members 8 is preferably not
less than 0.8 mm and not more than 2.0 mm, by which the wiring
members 8 are made less noticeable. When it is smaller than this
range, the cross section area is reduced to increase the
resistance, which causes the characteristics to deteriorate. On the
other hand, increasing the thickness to increase the cross section
area causes the solar cell elements to crack in the vicinity of the
spaces between the solar cell elements as mentioned above, which is
therefore unpreferable. On the other hand, when it is greater than
this range, the impression of lines that are formed by the wiring
members 8, which seems to penetrate the solar cell module becomes
too strong. That fails to suppress the impression of the whole
solar cell elements with a shade of the dark color to deteriorate
the design quality of the solar cell module. In addition, since the
wiring members 8 reduce the light receiving area of the solar cell
module, it is unpreferable that the output properties of the solar
cell elements to deteriorate the output properties of the solar
cell module.
[0262] It is preferred that the solar cell module according to the
present invention is arranged, in each of the foregoing
embodiments, such that connecting members 6 for electrically
interconnecting wiring members 8 connect the wiring members 8 to
each other at locations between the solar cell elements and the
back surface protecting member 11, that is, at non-light-receiving
locations.
[0263] This will be further described with reference to FIGS.
12-15.
[0264] FIG. 12 is a partially enlarged view of an end region of the
light receiving surface side of a solar cell module, and FIG. 13 is
a partially enlarged view of an end region of the
non-light-receiving surface side.
[0265] In addition, FIG. 14 is a cross-sectional view on arrow of a
solar cell module cut along the line G-G, and FIG. 15 is a
cross-sectional view on arrow cut along the line H-H.
[0266] In each of the drawings, 6 denotes connecting members, 17
denotes a thermoflexible sheet and 18 denotes an insulation sheet.
The same elements as those in FIGS. 1-11 are represented by the
same reference numerals.
[0267] The connecting members 6 are members that transfer electric
power from the group of solar cell elements interconnected through
the wiring members 8 to the terminals of the terminal box 7, which
are usually made by coating solder of about 20-70 .mu.m thick over
the entire surface of a copper foil having a thickness of about 0.1
mm-0.5 mm and a width of about 6 mm, and cutting it into
predetermined lengths.
[0268] The terminal box 7 is a member for the connection of cables
(not shown) for connecting output wiring lines from the solar cell
elements to an circuit outside, and is made of a modified PPE resin
or the like that is usually colored black, taking resistance to
light such as ultraviolet rays into consideration. In many cases,
the approximate size of the terminal box 7 is on the order of
100.times.60.times.20 mm for typical solar cell modules with output
power of about 160 W.
[0269] According to the present invention, in order to electrically
interconnect wiring members 8 by the connecting members 6 disposed
at locations between the solar cell elements and the back surface
protective member 11, as shown in FIGS. 14 and 15, wiring members 8
connected to the surface side or back surface side of the solar
cell elements are bent toward the back surface protective member 11
and connected through the connecting members 6 by soldering or the
like.
[0270] At this stage, in order to prevent short circuit to
electrodes provided on the back surface side of the solar cell
elements, it is preferable to interpose, for example, an insulation
sheet 18 made of polyethyleneterephtalate (PET) between the wiring
members 8 and the connecting members 16 that are turned to the
solar cell elements and the back surface side of the solar cell
elements.
[0271] In addition, due to the bent wiring members 8, the thickness
is locally increased, which may cause the solar cell elements to
crack upon application of heat and pressure during the laminating
step in the solar cell module production process. To prevent this,
it is also possible to interpose, for example, a thermoflexible
sheet 17 made of an ethylene vinyl acetate copolymer (EVA) so as to
absorb the stress.
[0272] While conventionally, as shown in FIGS. 16 and 17, the
connecting members 6 are disposed in a perimeter region of the
solar cell module without overlapping the solar cell elements,
according to the present invention, connecting members 6 for
interconnecting the wiring members 8 are provided on the back
surface side of the solar cell elements.
[0273] With the foregoing structure according to the present
invention, the packing density of the solar cell elements within
the solar cell module can be further improved, and the conversion
efficiency of the solar cell module can be improved.
[0274] In a conventional structure as shown in FIG. 16, connecting
members 6 present in the perimeter region of the solar cell module
are viewed as lines disturbing the uniformity of a plurality of
lines formed by the wiring members 8 and lines between the solar
cell elements. However, in the solar cell module arrangement
according to this embodiment, since the connecting members 6 are
located at regions invisible from the light receiving surface side,
the appearance of the solar cell module can be further improved to
have a higher design quality.
[0275] In addition, since the distance between the frame section
and the solar cell elements can be narrowed to enable to reduce the
area of the entire solar cell module, power generation efficiency
per unit area of the solar cell module can be improved.
[0276] In the foregoing manner, a solar cell module according to
the present invention can be obtained.
[0277] The solar cell module according to the present invention
allows the production of a solar cell module with high design
quality and high efficiency by a simple process without requiring
additional members and steps. Therefore, a solar cell module with
high efficiency and high design quality that is excellent in beauty
of appearance can be realized. Since this module particularly
exhibits its advantageous effect in solar cell modules whose
exterior appearance is the key to determine the impression of the
system, this is advantageously applied particularly to large scale
solar modules whose one side length is on the order of 1 m or more.
When this is applied to such a module having a long side, not only
high power generation efficiency can be obtained, but also
impression of the lines penetrating the solar cell module that are
formed by the spaces between solar cell elements and wiring members
8 can be improved, so that the solar cell module can be a module
with high design quality.
[0278] Meanwhile, implementation of the present invention is not
limited to the foregoing embodiments, but various modifications may
be made without departing from the spirit and scope of the present
invention.
[0279] For example, while the description above is given to solar
cells using p-type silicon substrates, also in cases where n-type
silicon substrates are used, only by reversing the polarity in the
description, the effect of the present invention can be achieved
using the same process.
[0280] In addition, while in the foregoing description, a single
junction type solar cell module is described, the present invention
is applicable also to multijunction stacked type solar cell modules
formed by laminating thin film bonding layers comprising a
semiconductor multilayer film on a junction device using a bulk
substrate.
[0281] The foregoing description is given to a solar cell element
provided with two kinds of surface electrode including a surface
electrode and a back surface electrode. However, the present
invention is not limited to this type, but may be of a type
including all the electrodes provided on non-light-receiving
surface (back surface) side.
[0282] In addition, while surface electrode is described referring
to those including bus bar electrodes with a generally linear shape
and a plurality of finger electrodes whose one ends are connected
thereto, it is not limited to this type.
[0283] Furthermore, while the foregoing description takes a
multicrystalline silicon substrate fabricated by a casting method
as an example, it is not necessary to limit the method to the
casting method and to multicrystalline silicon. Also, the substrate
is not limited to semiconductor substrates, but may be
semiconductor thin films. The material is not necessarily a silicon
material, but may be applicable to a semiconductor in general. That
is, the present invention is applicable to compound solar cells and
organic solar cells.
[0284] In addition, while bulk-type silicon solar cells are taken
as example in the foregoing description, the present invention is
not limited to these, but any desired mode may be included without
departing from the principles and objects of the invention. That
is, as long as it includes solar cell elements each comprising a
semiconductor area having a light incident surface, and surface
electrodes of generally linear shape provided on the light incident
surface that collect light-produced carriers generated at the
semiconductor area by irradiation of light on the light incident
surface, it may be applied to a solar cell (element) in general
such as optical sensors other than solar cells.
Example
[0285] Hereinafter, the results of experiments conducted on solar
cell elements fabricated according to the foregoing embodiments
will be shown.
[0286] As the substrate, a flat plate p-type multicrystalline
silicon substrate of 150 mm.times.150 mm in size fabricated by a
casing method having a specific resistivity of 2.OMEGA.cm was
used.
[0287] A paste including silver as a main component was printed and
baked to form a surface electrode. The pattern for the surface
electrode as a whole was formed by disposing three lines including
one vertical line at the center of the substrate, and two lines
axisymmetrically thereto. The bus bar electrodes 5a were made to
have a length of 148.8 mm.
[0288] The widths of the bus bar electrodes 5a were varied to eight
different values as 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 mm.
[0289] The distance between the center lines of the bus bar
electrodes 5a was 49.3 mm, the length of finger electrodes from one
end to the other end of the substrate (including the widths of bus
bar electrodes 5a crossing therebetween) that are arranged
perpendicular to the bus bar electrodes and axisymmetrically to the
vertical center line of the substrate was 149 mm, and the average
distance between the center lines of adjacent finger electrodes was
2.4 mm.
[0290] Solar cell elements were fabricated by varying the width of
the finger electrodes 2 between 10-200 .mu.m.
[0291] Thereafter, 48 samples of the solar cell elements were
connected together by wiring members having the same width as the
bus bar electrodes to produce a solar cell module, and output
characteristics were measured.
[0292] Tables 1, 2, 3 show short circuit current (Isc; unit A),
Fill Factor (FF), conversion efficiency (Eff.; unit %) per cell,
respectively, converted from the output characteristics of the
solar cell module.
TABLE-US-00001 TABLE 1 WIDTH OF FINGER ELECTRODE WIDTH OF BUS BAR
ELECTRODE (mm) (mm) 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0.01 8.410 8.437
8.473 8.503 8.528 8.547 8.561 8.569 0.02 8.421 8.446 8.478 8.505
8.526 8.542 8.552 8.556 0.03 8.429 8.451 8.480 8.504 8.521 8.534
8.540 8.541 0.04 8.435 8.455 8.480 8.500 8.514 8.523 8.526 8.523
0.05 8.438 8.455 8.478 8.494 8.505 8.510 8.509 8.503 0.06 8.440
8.455 8.473 8.486 8.493 8.494 8.490 8.480 0.07 8.438 8.451 8.466
8.475 8.478 8.476 8.468 8.455 0.08 8.435 8.445 8.456 8.462 8.461
8.456 8.444 8.427 0.09 8.429 8.437 8.444 8.446 8.442 8.433 8.418
8.397 0.1 8.420 8.426 8.430 8.428 8.421 8.407 8.389 8.364 0.11
8.410 8.413 8.413 8.408 8.396 8.380 8.357 8.329 0.12 8.396 8.397
8.394 8.385 8.370 8.350 8.324 8.292 0.13 8.381 8.379 8.372 8.359
8.341 8.317 8.287 8.252 0.14 8.363 8.359 8.348 8.332 8.310 8.282
8.248 8.209 0.15 8.342 8.336 8.321 8.301 8.276 8.244 8.207 8.164
0.16 8.319 8.311 8.292 8.269 8.239 8.204 8.163 8.117 0.17 8.294
8.283 8.261 8.233 8.200 8.162 8.117 8.067 0.18 8.266 8.252 8.227
8.196 8.159 8.116 8.068 8.014 0.19 8.236 8.220 8.190 8.156 8.115
8.069 8.017 7.959 0.2 8.203 8.184 8.151 8.113 8.069 8.019 7.963
7.901
TABLE-US-00002 TABLE 2 WIDTH OF FINGER ELECTRODE WIDTH OF BUS BAR
ELECTRODE (mm) (mm) 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0.01 0.668 0.668
0.667 0.667 0.666 0.666 0.666 0.666 0.02 0.701 0.701 0.701 0.700
0.700 0.700 0.700 0.700 0.03 0.713 0.712 0.712 0.712 0.712 0.712
0.712 0.712 0.04 0.718 0.718 0.718 0.718 0.718 0.718 0.718 0.718
0.05 0.722 0.724 0.722 0.722 0.721 0.721 0.721 0.721 0.06 0.724
0.724 0.724 0.724 0.724 0.724 0.724 0.724 0.07 0.726 0.726 0.726
0.726 0.726 0.726 0.726 0.726 0.08 0.728 0.728 0.727 0.727 0.727
0.727 0.727 0.728 0.09 0.729 0.729 0.729 0.729 0.729 0.729 0.729
0.729 0.1 0.730 0.730 0.730 0.730 0.730 0.730 0.730 0.730 0.11
0.731 0.731 0.731 0.731 0.731 0.731 0.731 0.731 0.12 0.731 0.731
0.731 0.732 0.732 0.732 0.732 0.732 0.13 0.732 0.732 0.732 0.732
0.732 0.733 0.733 0.733 0.14 0.733 0.733 0.733 0.733 0.733 0.733
0.733 0.734 0.15 0.733 0.734 0.734 0.734 0.734 0.734 0.734 0.734
0.16 0.734 0.734 0.734 0.734 0.734 0.735 0.735 0.735 0.17 0.735
0.735 0.735 0.735 0.735 0.735 0.736 0.736 0.18 0.735 0.735 0.735
0.736 0.736 0.736 0.736 0.737 0.19 0.736 0.736 0.736 0.736 0.736
0.737 0.737 0.737 0.2 0.736 0.736 0.737 0.737 0.737 0.737 0.738
0.738
TABLE-US-00003 TABLE 3 WIDTH OF FINGER ELECTRODE WIDTH OF BUS BAR
ELECTRODE (mm) (mm) 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0.01 14.979 15.014
15.059 15.103 15.120 15.140 15.150 15.151 0.02 15.732 15.768 15.814
15.848 15.870 15.890 15.900 15.893 0.03 15.991 16.022 16.065 16.096
16.120 16.130 16.130 16.119 0.04 16.118 16.148 16.184 16.208 16.220
16.230 16.220 16.207 0.05 16.192 16.219 16.249 16.268 16.280 16.280
16.260 16.241 0.06 16.238 16.259 16.283 16.295 16.300 16.290 16.270
16.244 0.07 16.264 16.282 16.299 16.305 16.300 16.290 16.260 16.227
0.08 16.227 16.290 16.301 16.300 16.290 16.270 16.240 16.196 0.09
16.280 16.289 16.293 16.286 16.270 16.240 16.200 16.155 0.1 16.274
16.278 16.276 16.262 16.240 16.200 16.160 16.104 0.11 16.260 16.260
16.251 16.231 16.200 16.160 16.110 16.046 0.12 16.241 16.236 16.221
16.194 16.160 16.110 16.050 15.982 0.13 16.214 16.205 16.183 16.149
16.110 16.050 15.990 15.910 0.14 16.182 16.169 16.140 16.099 16.050
15.990 15.910 15.832 0.15 16.143 16.126 16.090 16.042 15.990 15.920
15.840 15.748 0.16 16.100 16.077 16.035 15.980 15.920 15.840 15.760
15.658 0.17 16.051 16.024 15.975 15.194 15.840 15.760 15.670 15.563
0.18 15.997 15.966 15.910 15.841 15.760 15.670 15.570 15.463 0.19
15.938 15.902 15.839 15.763 15.680 15.580 15.470 15.357 0.2 15.873
15.832 15.762 15.680 15.590 15.480 15.370 15.245
[0293] As is apparent from these Tables, under the condition where
the width of bus bar electrodes 5a is 1.4-2 mm, as the width of
finger electrodes narrows, the short cut current increases, and the
FF value decreases. Under the condition where the width of bus bar
electrodes is 0.6-1.2 mm, while as the width of finger electrodes
increases, the FF values increase, the short circuit current values
peak at widths between 0.02 and 0.06 mm of finger electrodes.
[0294] However, the conversion efficiencies were as high as more
than 16% at widths of 0.04-0.11 mm of finger electrodes
irrespective of the width of bus bar electrodes. In particular, the
conversion efficiencies peak at widths of 0.06-0.09 mm of finger
electrodes.
Example 2
[0295] The relationship between the configuration of the surface
electrode and the characteristic was investigated on a bulk-type
crystalline silicon solar cell as the solar cell element fabricated
according to the foregoing embodiment.
[0296] As the substrate, a flat plate p-type multicrystalline
silicon substrate of 150 mm.times.155 mm in size fabricated by a
casting method was used, and solar cell elements with the structure
shown in FIG. 4(a) were formed.
[0297] A paste including silver as a main component was printed and
baked to form the surface electrode according to the solar cell
element of the present invention.
[0298] The pattern for the surface electrode as a whole was formed,
such that when the substrate was oriented to have a length of 150
mm in the vertical direction and 155 mm in the horizontal direction
in FIG. 4(a), the length of bus bars 5a disposed axisymmetrically
to the vertical center line of the substrate was 147.5 mm, the
width of the bus bar electrodes 5a is 2 mm, and the distance
between the center lines of two bus bar electrodes 5a was 77.5 mm,
the length of finger electrodes 5b from one end to the other end of
the substrate (including the widths of bus bar electrodes 5a
crossing therebetween) that are arranged perpendicular (vertical
direction of the substrate) to the bus bar electrodes 5a and
axisymmetrically to the vertical center line of the substrate was
152.8 mm, the average width of the finger electrodes 5b was 165
.mu.m, and the average distance between the center lines of
adjacent finger electrodes 5b was 2.38 mm.
[0299] Meanwhile, the average width of finger electrodes 5b was
determined, as described above, by dividing the length between one
end and the other end of connected to a bus bar electrode 5a into
ten equal parts, and taking the average of widths measured at the
respective points (9 points).
[0300] With this whole pattern being a common condition,
experiments were conducted to examine the effects of the
configurations of finger electrodes shown in FIGS. 4(b), 6 and
7.
[0301] First, an experiment was conducted in the case where the
trajectory according to the solar cell element of the present
invention as shown in FIG. 4(b), that is, the edge lines 22b of the
contact surface 22a between the finger electrode 5b and the
semiconductor substrate 1 include a rugged contour.
[0302] The results of the experiment are shown in Table 4. Here,
the proportion of the area of the rugged contour portion to the
area of the light incident surface is set to be approximately the
same under any formation condition.
[0303] In addition, in Table 4, the proportions of perimeter
corresponding to the degrees of the rugged contour when the
perimeter in the case of edge line 22b of the contact surface 22a
of the finger electrode 5b that does not include a rugged contour
(i.e., generally linear shape) is standardized to 1 are shown. This
corresponds to the value R in the following formula:
R=0.5L.sub.1(s.sub.1d.sub.1.sup.-1+d.sub.1).sup.-1
[0304] Here, the distance of one cycle of the rugged contour was
about 10-20 .mu.m.
TABLE-US-00004 TABLE 4 STANDARDIZED PERIMETER LENGTH R EFFICIENCY
Isc [A] Voc [V] FF 1.0 16.03 7.970 0.6143 0.761 1.2 16.06 7.971
0.6147 0.762 1.4 16.11 7.971 0.6149 0.764 1.6 16.13 7.971 0.6151
0.765
[0305] It is apparent from Table 4 that as the standardized
perimeter increases, the efficiency increases, and the
characteristics are improved. The reason for this is speculated
that as a result of an increase of the substantial contact area by
providing the edge lines 22b of the contact area 22a of finger
electrode 5b with a rugged contour, the contact resistance
decreased, by which the characteristics were improved.
[0306] Subsequently, an experiment for grasping the effect of the
finger configuration shown in FIG. 7 was conducted.
[0307] In Table 5, the results of the experiment for the case where
the standardized perimeter R in Table 4 is 1.4 are shown, in which,
as shown in FIG. 7, the phase of an edge line with a rugged contour
symmetric to the other one with respect to the center line of the
finger electrode 5b in the same direction as the current flowing
direction was shifted by a half cycle to form an asymmetric
configuration.
TABLE-US-00005 TABLE 5 PHASE DIFFERENCE STANDARDIZED BETWEEN
PERIMETER RUGGED EFFI- Isc LENGTH R CONTOURS CIENCY [A] Voc [V] FF
1.4 NONE 16.11 7.971 0.6149 0.764 1.4 HALF CYCLE 16.14 7.970 0.6148
0.766
[0308] Table 5 shows that the efficiency increases by shifting the
phase of the rugged contour by a half cycle, the effect of
improving characteristics is thus apparent. That is, it is
speculated that by shifting the phase of the rugged contour on one
side by a half cycle to form an asymmetric configuration,
constricted portions of the finger electrodes 5b were effectively
resolved and the liner resistance was effectively reduced, thereby
the characteristics of the finger electrodes 5b were improved.
[0309] Then, an experiment for grabbing the effect of the
configuration of bus bar electrode shown in FIG. 9 was
conducted.
[0310] A flat plate p-type multicrystalline silicon substrate of
150 mm.times.150 mm in size fabricated by a casing method was used
to form a solar cell element with the structure shown in FIG. 1
(a).
[0311] A paste including silver as a main component was printed and
baked to form the surface electrode according to the present
invention. The basic pattern for the surface electrode was formed
to include three bus bar electrodes having the following
dimensions.
[0312] The length of three bus bar electrodes 5a including one as
the vertical center line of the substrate, and two disposed
axisymmetrically thereto was 148.8 mm. The width of the bus bar
electrodes 5a was 1.3 mm, the distance between the center lines of
two bus bar electrodes 5a was 50 mm, the length of finger
electrodes 5b from one end to the other end of the substrate
(including the widths of bus bar electrodes 5a crossing
therebetween) that are arranged perpendicular (vertical direction
of the substrate) to the bus bar electrodes 5a and axisymmetrically
to the vertical center line of the substrate was 149 mm, the width
of the finger electrodes 5b was 80 .mu.m, and the average distance
between the center lines of adjacent finger electrodes 5b was 2.4
mm.
[0313] In addition, the lengths of the edge lines 32b when the
contact surface 32a is planarly viewed from a vertical direction
and the area S.sub.2 were determined such that the solar cell
element was photographed from a vertical direction and the surface
image was digitized, which was thereafter converted into a binary
form using a threshold value for separating the electrodes from
areas other than these, thereby separating the area of the surface
electrode from other areas.
[0314] Meanwhile, regarding the determination of the area S.sub.2
and the length of the edge line 32b, the respective measured values
was t-tested as a rejection test of outliers with a significance
level of 0.05, and the validity thereof was confirmed.
[0315] The results of measurements of various characteristics of
the solar cell element are shown in Table 3. Incidentally, the
measurements of short circuit current (Isc) and fill factor (FF) as
the characteristics of the solar cell element defined in JIS C 8913
(1998) were carried out based upon this standards.
[0316] In addition, the same experiment was conducted also on the
horizontally long and vertically long multicrystalline silicon
substrates (whose areas are the same as that of the foregoing
substrate of 150.times.150 in size) shown in FIGS. 10(a) and 10(b).
As for the bus bar electrodes 5a, two kinds that three and four
were prepared for the horizontally long substrate, and two kinds
that two and three were prepared for the vertically long
substrate.
TABLE-US-00006 TABLE 6 S.sub.3 S.sub.2 L.sub.2 Isc Jsc No cm.sup.2
cm.sup.2 cm 5 S.sub.3.sup.1/2 S.sub.2/S.sub.3 mA mA/cm.sup.2 FF
.eta. % REMARKS 1 225 2.9 93.2 75.0 0.013 8273 36.76 0.743 16.80 2
225 3.6 92.5 75.0 0.016 8280 36.80 0.751 17.19 3 225 5.6 90.8 75.0
0.025 8255 36.69 0.756 17.20 4 225 7.9 89.8 75.0 0.035 8264 36.73
0.757 17.24 5 225 9.5 92.6 75.0 0.042 8241 36.63 0.753 17.10 6 225
12.4 91.4 75.0 0.055 8260 36.71 0.752 17.11 7 225 14.2 91.6 75.0
0.063 8253 36.70 0.740 16.88 8 225 5.2 70.4 75.0 0.023 7878 35.01
0.712 15.38 HORIZONTALLY LONG BSB .times. 3 9 225 5.0 86.6 75.0
0.022 7938 35.28 0.731 15.62 HORIZONTALLY LONG BSB .times. 4 10 225
5.9 80.3 75.0 0.026 7946 35.32 0.748 16.01 VERTICALLY LONG BSB
.times. 3 11 225 6.1 65.4 75.0 0.027 7942 35.30 0.748 16.01
VERTICALLY LONG BSB .times. 2
[0317] It is apparent from Table 6 that the effect of improving the
characteristics was noticeable in samples Nos. 2-6 and Nos. 9 and
10 that satisfied formulae (2) and (3) of the present
invention.
Example 3
[0318] Subsequently, the relationship between the roughness of the
contact surface between the surface electrode and the semiconductor
substrate and the solar cell element characteristics was
examined.
[0319] A flat plate p-type multicrystalline silicon substrate of
150 mm.times.150 mm in size fabricated by casting was used as the
substrate to form a solar cell element with the structure shown in
FIG. 1(a).
[0320] A paste including silver as a main component was printed and
baked to form the surface electrode according to the present
invention. The pattern for the surface electrode as a whole was
formed according to the following dimensions. The length of two bus
bar electrodes 5a disposed axisymmetrically to the vertical center
line of the substrate was 148.8 mm. The width of the bus bar
electrodes 5a was 2 mm, the distance between the center lines of
two bus bar electrodes 5a was 75 mm, the length of finger
electrodes 5b from one end to the other end of the substrate
(including the widths of bus bar electrodes 5a crossing
therebetween) that are arranged perpendicular (vertical direction
of the substrate) to the bus bar electrodes 5a and axisymmetrically
to the vertical center line of the substrate was 149 mm, the width
of the finger electrodes 5b was 160 .mu.m, and the average distance
between the center lines of adjacent finger electrodes 5b was 2.4
mm. The results of measurements of various characteristics of this
solar cell element are shown in Table 7.
[0321] In another case, the length of three bus bar electrodes 5a
in total including one as the vertical center line of the substrate
and two disposed axisymmetrically thereto was 148.8 mm. The width
of the bus bar electrodes 5a was 1.3 mm, the distance between the
center lines of the two bus bar electrodes 5a was 50 mm, the length
of finger electrodes 5b from one end to the other end of the
substrate (including the widths of bus bar electrodes 5a crossing
therebetween) that are arranged perpendicular (vertical direction
of the substrate) to the bus bar electrodes 5a and axisymmetrically
to the vertical center line of the substrate was 149 mm, the width
of the finger electrodes 5b was 80 .mu.m, and the average distance
between the center lines of adjacent finger electrodes 5b was 2.4
mm. Meanwhile, the average width of finger electrodes 5b was
determined, as described above, by dividing the length between one
end and the other end connected to bus bar electrodes 5a into ten
equal parts, and taking the simple average of widths measured at
the respective points (9 points). The results of measurements of
various characteristics of this solar cell element are shown in
Table 8.
[0322] The value of Sb was varied by roughening the portions that
correspond to the areas under the electrodes by reaction ion
etching. During this process, under Cl.sub.2 gas flow of 0.1 slm,
O.sub.2 gas flow of 0.6 slm and SF.sub.6 gas flow of 0.4 slm, an RF
power of 5 kW was applied. In addition, to vary the value of Sb,
the reactive gas pressure was varied as appropriate.
[0323] The values of Sb were measured after measurements of output
characteristics of the solar cell elements by dipping them into
aqua regia to remove the surface electrode, and the surface areas
were measured on the region at which these electrodes was provided.
For the measurements of surface areas, an AFM (atomic force
microscope, Nanoscope IIIa produced by Digital Instruments, Inc.)
was used to measure a 1 .mu.m square sample at 512.times.1024
points at 0.2 Hz with use of a cantilever with a tip diameter of 5
nm. The measurements were carried out at 9 points, which was the
same as measurements of the widths of finger electrodes, and then
the simple average was calculated to determine the values.
[0324] The values of Sb/Sa were rounded off the number to the third
decimal point, and comparisons were made to determine whether or
not the values were within the range of the present invention.
[0325] Meanwhile, regarding the determination of the foregoing
width of the finger electrodes and the area of the surface
electrode, the respective measured values was t-tested as a
rejection test of outliers with a significance level of 0.05, and
the validity thereof was confirmed.
[0326] Incidentally, the measurements of short circuit current
(Isc) and fill factor (FF) as the characteristics of the solar cell
element defined in JIS C 8913 (1998) were carried out based upon
this standards.
[0327] Furthermore, as a comparative example, the results of
measurements on a solar cell element produced with conventional
conditions are also listed as sample No. 32. Although the reactive
ion etching process was used in this method, the sample was
produced with conditions before the conditions with which the Sb/Sa
values were within the range of the present invention was
found.
TABLE-US-00007 TABLE 7 LIGHT RECEIVING SURFACE Jsc AREA Sa Sb Isc
(mA/ No (cm.sup.2) (cm.sup.2) (cm.sup.2) Sb/Sa (mA) cm.sup.2) FF
.eta. (%) 21* 225 20.6 20.6 1.00 7951 35.34 0.73 15.61 22* 225 20.6
21.6 1.05 7949 35.33 0.731 15.62 23 225 20.6 22.6 1.10 7946 35.32
0.748 16.01 24 225 20.6 24.7 1.20 7952 35.34 0.753 16.15 25 225
20.6 28.8 1.40 7963 35.39 0.756 16.19 26 225 20.6 32.9 1.60 7945
35.31 0.757 16.20 27 225 20.6 37.0 1.80 7968 35.41 0.759 16.26 28
225 20.6 39.1 1.90 7953 35.35 0.757 16.19 29 225 20.6 41.1 2.00
7944 35.31 0.747 16.01 30 225 20.6 43.2 2.10 7952 35.34 0.752 16.08
31* 225 20.6 45.2 2.20 7942 35.30 0.732 15.63 32* 225 20.6 22.0
1.07 7950 35.33 0.732 15.61 *Samples out of the range of the
invention
TABLE-US-00008 TABLE 8 LIGHT RECEIVING SURFACE Jsc AREA Sa Sb Isc
(mA/ No (cm.sup.2) (cm.sup.2) (cm.sup.2) Sb/Sa (mA) cm.sup.2) FF
.eta. (%) 21* 225 13.1 13.1 1.00 8267 36.74 0.738 16.81 22* 225
13.1 13.8 1.05 8272 36.76 0.743 16.80 23 225 13.1 14.4 1.10 8281
36.80 0.751 17.19 24 225 13.1 15.7 1.20 8241 36.63 0.753 17.10 25
225 13.1 18.4 1.40 8255 36.69 0.756 17.20 26 225 13.1 21.0 1.60
8298 36.88 0.757 17.31 27 225 13.1 23.6 1.80 8278 36.79 0.759 17.31
28 225 13.1 24.9 1.90 8264 36.73 0.757 17.24 29 225 13.1 26.2 2.00
8271 36.76 0.754 17.18 30 225 13.1 27.5 2.10 8259 36.71 0.752 17.11
31* 225 13.1 28.9 2.20 8258 36.70 0.74 16.76 32* 225 13.1 13.9 1.06
8270 36.76 0.741 16.79 *Samples out of the range of the
invention
[0328] As Table 7 shows, the values of short circuit current are
more than 35.3 mA/cm.sup.2 when 1.10.ltoreq.Sb/Sa.ltoreq.2.10, that
is, for samples Nos. 23-30, and FF values are as high as more than
0.747. On the other hand, although samples Nos. 21 and 22 whose
Sb/Sa values are less than 1.10 exhibit short circuit current
values almost at the same level, FF values are so low as 0.731 or
less. Also, in the case of sample No. 31 whose Sb/Sa value exceeds
2.10, the FF value dropped to 0.732. This shows that solar cell
elements having high conversion efficiencies exceeding 16% can be
obtained in the condition of 1.10.ltoreq.Sb/sa.ltoreq.2.10.
[0329] Also in the case of the solar cell element provided with
three bus bar electrodes having a width of 1.3 mm shown in Table 8,
the short circuit current density values areas high as more than
36.6 mA/cm.sup.2 in the range of 1.10.ltoreq.Sb/sa.ltoreq.2.10,
that is, for samples Nos. 23-30, and also the FF values thereof are
all as high as more than 0.75. As a result, solar cell elements
with high conversion efficiencies of more than 17% can be obtained
in the range of 1.10.ltoreq.Sb/sa.ltoreq.2.10.
[0330] By the way, in either case of FIG. 7 or FIG. 8, the FF
values tend to decrease in solar cell elements of sample No. 32
produced with the conventional conditions as compared with solar
cell elements according to the present invention.
[0331] When the solar cell modules shown in FIGS. 12-14 were
produced using the solar cell elements of the present invention
fabricated in the foregoing way, good results were obtained.
* * * * *