U.S. patent application number 11/547656 was filed with the patent office on 2008-05-08 for solar cell.
This patent application is currently assigned to Mitsubishi Electric Corporation. Invention is credited to Shoichi Karakida, Hiroaki Morikawa, Mitsunori Nakatani, Takahiko Nishida.
Application Number | 20080105297 11/547656 |
Document ID | / |
Family ID | 38066985 |
Filed Date | 2008-05-08 |
United States Patent
Application |
20080105297 |
Kind Code |
A1 |
Nishida; Takahiko ; et
al. |
May 8, 2008 |
Solar Cell
Abstract
A solar cell includes a photoelectric conversion layer, a first
electrode on one surface of the photoelectric conversion layer, a
second electrode on other surface of the photoelectric conversion
layer, and a third electrode on the other surface of the
photoelectric conversion layer. The third electrode is
substantially rectangular with its corners rounded off in the
in-plane direction of the photoelectric conversion layer, and
overlaps the second electrode at the periphery thereof.
Inventors: |
Nishida; Takahiko; (Tokyo,
JP) ; Nakatani; Mitsunori; (Tokyo, JP) ;
Morikawa; Hiroaki; (Tokyo, JP) ; Karakida;
Shoichi; (Tokyo, JP) |
Correspondence
Address: |
BUCHANAN, INGERSOLL & ROONEY PC
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
Mitsubishi Electric
Corporation
Tokyo
JP
|
Family ID: |
38066985 |
Appl. No.: |
11/547656 |
Filed: |
November 28, 2005 |
PCT Filed: |
November 28, 2005 |
PCT NO: |
PCT/JP05/21782 |
371 Date: |
October 5, 2006 |
Current U.S.
Class: |
136/256 |
Current CPC
Class: |
Y02E 10/50 20130101;
H01L 31/022425 20130101; H01L 31/0504 20130101 |
Class at
Publication: |
136/256 |
International
Class: |
H01L 31/04 20060101
H01L031/04; H01L 31/02 20060101 H01L031/02; H01L 31/0224 20060101
H01L031/0224 |
Claims
1-4. (canceled)
5. A solar cell comprising: a photoelectric conversion layer having
a first surface and a second surface; a first electrode arranged on
the first surface; a second electrode arranged on the second
surface; and a third electrode that extracts electric power from
the second electrode and arranged on the second surface, wherein
the third electrode is substantially rectangular with at least one
corner removed in an in-plane direction of the photoelectric
conversion layer, and includes a periphery portion that overlaps
the second electrode.
6. The solar cell according to claim 5, wherein the corner of the
third electrode is rounded off.
7. The solar cell according to claim 5, wherein the corner of the
third electrode is chamfered.
8. The solar cell according to claim 5, wherein the second
electrode is made of aluminum, and the third electrode is made of
silver.
Description
TECHNICAL FIELD
[0001] The present invention relates to a solar cell, and more
specifically relates to a solar cell on which separation of
electrodes is prevented.
BACKGROUND ART
[0002] Photovoltaic power generation is a clean method of
generating electric power using inexhaustible light energy without
discharging toxic substances. A solar cell is used for the
photovoltaic power generation, which is a photoelectric converter
that generates electric power by converting light energy from the
sun into electric energy.
[0003] Conventionally, an electrode on the back of a light
receiving surface of a generally produced solar cell is formed by
screen-printing silver paste and aluminum paste on the back surface
of a silicon substrate, then drying and firing the pastes. The
aluminum formed substantially all over the back surface of the
silicon substrate serves as a positive electrode. However, in the
process of producing a solar cell module, a lead tab for extracting
electric power cannot be soldered directly to the aluminum
electrode formed of aluminum. Therefore, a silver electrode is
formed, as an electrode for extracting electric power, in such a
manner as to partially overlap the aluminum electrode on the back
surface of the silicon substrate (see Patent Documents 1 and
2).
[0004] As just described, on the back surface of the substrate of
the solar cell, an aluminum electrode for higher electric power
output and a silver electrode for extracting the electric power are
partially overlapped. In the area where the aluminum electrode and
the silver electrode are overlapped, three metals of silicon in the
silicon substrate, aluminum in the aluminum electrode, and silver
in the silver electrode are partially alloyed.
[0005] Patent Document 1: Japanese Patent Publication No.
2003-273378
[0006] Patent Document 2: Japanese Patent Publication No.
HEI10-335267
DISCLOSURE OF INVENTION
Problem to be Solved by the Invention
[0007] However, the overlapped area is very fragile due to stress
assumedly caused by different rate of thermal expansion of each
material that occurs during rapid heating and cooling in firing.
Therefore, after the firing, for example, when the silver electrode
is present on the aluminum electrode, the silver electrode can
separate from the aluminum electrode at a corner of the overlapped
area.
[0008] To reduce the cost of the solar cell, it is necessary to
make the silicon, substrate thinner, which occupies a high rate of
the material cost. However, if the thickness of the silicon
substrate is simply reduced, the warpage of the silicon substrate
caused by different rates of thermal shrinkage between silicon and
aluminum is larger compared with a thicker silicon substrate.
[0009] When the silicon substrate warps to a large extent, there is
a problem that the silicon substrate cracks in production process
after the firing, resulting in a low production yield, or the
production becomes impossible due to the crack in the silicon
substrate.
[0010] One approach to the problem could be to prevent the silicon
substrate from warping by, for example, reviewing materials of the
aluminum paste to improve the rate of thermal shrinkage of the
electrode material. However, if the materials of the aluminum paste
are simply modified, there is still a risk of the silver electrode
partially separating due to the different rates of thermal
shrinkage between aluminum and silver depending on the materials
combination.
[0011] In this case, when the silver electrode separates to a large
extent, there is also a problem that the solar cell cracks due to
stacking of solar cells or the characteristics of the solar cell
deteriorate, and thus the production yield decreases.
[0012] The present invention was made in view of the problems
described above, and it is an object of the present invention to
provide a solar cell on which the separation of electrodes is
effectively prevented.
Means for Solving Problem
[0013] To solve the problems described above and achieve the
object, the solar cell according to an aspect of the present
invention includes a photoelectric conversion layer; a first
electrode provided on one surface of the photoelectric conversion
layer; a second electrode provided on other surface of the
photoelectric conversion layer; and a third electrode for
extracting electric power from the second electrode, in which the
third electrode is provided on the other surface with periphery
thereof overlapping above the second electrode, and is
substantially square with substantially round corners.
EFFECT OF THE INVENTION
[0014] As described above, the solar cell according to an aspect of
the present invention includes a photoelectric conversion layer, a
first electrode formed on one side of the photoelectric conversion
layer, a second electrode formed on the other side of the
photoelectric conversion layer, and a third electrode to extract
electric power from the second electrode. The third electrode is
substantially square with corners rounded off, and overlaps the
second electrode at the periphery. Thus, the third electrode can be
reliably bonded with the second electrode even at the corners
thereof, whereby a solar cell on which the separation of the third
electrode is effectively prevented is realized.
[0015] Further, the electrodes are strongly adhered to the
substrate, and there is not a fragile area subjected to
concentrated stress, which reduces possibilities that the substrate
can wrap or crack. Therefore, the solar cell according to an aspect
of the present invention does not cause many cracks on the silicon
substrate differently from the conventional one even when the
thickness of the silicon substrate is reduced to lower the cost of
the solar cell, and is applicable enough, whereby the usable
materials can be selected from wider options.
[0016] Because the solar cell according to an aspect of the present
invention has the third electrode rounded off at the corners, the
area of the third electrode is smaller, and the amount of the
material used for the electrode can be reduced. This reduces the
material cost, and thus realizes an inexpensive solar cell.
BRIEF DESCRIPTION OF DRAWINGS
[0017] FIG. 1-1 is a cross section for explaining a general
configuration of a solar cell according to a first embodiment of
the present invention;
[0018] FIG. 1-2 is a plan view for explaining a general
configuration of a front surface (a light receiving surface) of the
solar cell according to the first embodiment of the present
invention;
[0019] FIG. 1-3 is a plan view for explaining a general
configuration of a back surface (a surface opposite to the light
receiving surface) of the solar cell according to the first
embodiment of the present invention;
[0020] FIG. 1-4 is an enlarged schematic of an alloyed area where
three metals of silicon, aluminum, and silver are partially alloyed
on the solar cell according to the first embodiment of the present
invention;
[0021] FIG. 1-5 is an enlarged cross section of a surrounding area
of a region B and a region C, where an aluminum electrode and a
back-surface silver electrode are partially overlapped on the back
surface of the solar cell according to the first embodiment of the
present invention;
[0022] FIG. 2 is an enlarged cross section of a surrounding area of
a region B' and a region C', where the aluminum electrode and the
back-surface silver electrode are partially overlapped on the back
surface of a conventional solar batter cell;
[0023] FIG. 3-1 is a cross section for explaining a method for
producing the solar cell according to the first embodiment of the
present invention;
[0024] FIG. 3-2 is a cross section for explaining the method for
producing the solar cell according to the first embodiment of the
present invention;
[0025] FIG. 3-3 is a cross section for explaining the method for
producing the solar cell according to the first embodiment of the
present invention;
[0026] FIG. 3-4 is a cross section for explaining the method for
producing the solar cell according to the first embodiment of the
present invention;
[0027] FIG. 3-5 is a cross section for explaining the method for
producing the solar cell according to the first embodiment of the
present invention;
[0028] FIG. 3-6 is a cross section for explaining the method for
producing the solar cell according to the first embodiment of the
present invention;
[0029] FIG. 3-7 is a plan view for explaining an example of a
screen mask used for printing with a silver paste in the production
of the solar cell according to the first embodiment of the present
invention;
[0030] FIG. 3-8 is a cross section for explaining an example of a
screen mask used for printing with a silver paste in the production
of the solar cell according to the first embodiment of the present
invention;
[0031] FIG. 3-9 is a cross section for explaining the method for
producing the solar cell according to the first embodiment of the
present invention;
[0032] FIG. 3-10 is a cross section for explaining the method for
producing the solar cell according to the first embodiment of the
present invention;
[0033] FIG. 4-1 is a plan view for explaining dimensions on the
back surface (a surface opposite to the light receiving surface) of
an actual solar cell that adopts the first embodiment of the
present invention;
[0034] FIG. 4-2 is a plan view for explaining a shape and
dimensions of a back-side silver electrode on the actual solar cell
that adopts the first embodiment of the present invention;
[0035] FIG. 5-1 is a plan view for explaining a general
configuration of the back surface (a surface opposite to the light
receiving surface) of a solar cell according to a second embodiment
of the present invention;
[0036] FIG. 5-2 is an enlarged schematic of an alloyed area where
three metals of silicon, aluminum, and silver are partially alloyed
on the solar cell according to the second embodiment of the present
invention;
[0037] FIG. 5-3 is an enlarged cross section of a surrounding area
of a region D and a region E, where an aluminum electrode and a
back-surface silver electrode are partially overlapped on the back
surface of the solar cell according to the second embodiment of the
present invention;
[0038] FIG. 6-1 is a plan view for explaining dimensions on the
back surface (a surface opposite to the light receiving surface) of
an actual solar cell that adopts the second embodiment of the
present invention; and
[0039] FIG. 6-2 is a plan view for explaining a shape and
dimensions of a back-side silver electrode on the actual solar cell
that adopts the second embodiment of the present invention.
EXPLANATIONS OF LETTERS OR NUMERALS
[0040] 10 Semiconductor layer [0041] 11 Silicon substrate [0042] 13
n-type diffused layer [0043] 13a n-type diffused layer [0044] 14 p+
layer [0045] 15 Antireflective coating [0046] 17 Aluminum electrode
[0047] 17a Aluminum paste layer [0048] 19 Back-surface silver
electrode [0049] 19a Silver paste layer [0050] 21 Front-surface
silver electrode [0051] 21a Silver paste layer [0052] 23 Alloyed
area [0053] 25 Mesh [0054] 27 Emulsion [0055] 31 Back-surface
silver electrode [0056] 33 Alloyed area
BEST MODE(S) FOR CARRYING OUT THE INVENTION
[0057] Exemplary embodiments of a solar cell according to the
present invention are explained below in detail while referring to
the accompanying drawings. It should be noted that the present
invention is not limited by the following description, but can be
changed in various manners within the scope of the present
invention. In the accompanying drawings, scale sizes may vary among
the drawings and among members depicted therein for better
understanding.
FIRST EMBODIMENT
[0058] FIGS. 1-1 to 1-3 are drawings for explaining a solar cell
according to a first embodiment of the present invention, and FIG.
1-1 is a cross section for explaining a general configuration of
the solar cell according to the first embodiment. FIG. 1-2 is a
plan view for explaining a general configuration of a front surface
(a light receiving surface) of the solar cell according to the
first embodiment, and FIG. 1-3 is a plan view for explaining a
general configuration of a back surface (a surface opposite to the
light receiving surface) of the solar cell according to the first
embodiment. Incidentally, FIG. 1-1 is a cross section taken along
the line A-A of FIG. 1-3.
[0059] The solar cell according to the embodiment includes, as
shown in FIGS. 1-1 to 1-3, a semiconductor layer 10 that is a
photoelectric conversion layer including a p-type layer 11 that is
a p-type silicon substrate as a semiconductor substrate, an n-type
diffused layer 13 with a conductivity type inverse to that of the
surface of the p-type layer 11, and a p+ layer (back surface field
(BSF) layer) 14 containing a high concentration of impurity; an
antireflective coating 15 provided on a light receiving surface of
the semiconductor layer 10 to prevent the reflection of incident
light; a front-surface silver electrode 21 that is a light
receiving surface electrode provided on the light receiving surface
of the semiconductor layer 10 substantially in the shape of a
stick; an aluminum electrode 17 that is a back-surface electrode
provided substantially all over the back surface of the
semiconductor layer 10 to extract electric power and reflect the
incident light; and a back-surface silver electrode 19 that is an
extracting electrode that extracts the electric power from the
aluminum electrode 17.
[0060] In the solar cell according to the embodiment configured as
above, when sunlight irradiates the side of the light receiving
surface (the side of the antireflective coating 15) of the solar
cell and reaches a p-n junction surface (a junction surface of the
p-type layer 11 and the n-type diffused layer 13) inside, a hole
and electron pair on the p-n junction surface is separated. The
separated electron moves toward the n-type diffused layer 13. On
the other hand, the separated hole moves toward the p+ layer 14.
This produces an electrical potential difference between the n-type
diffused layer 13 and the p+ layer 14 so that the p+ layer 14 has a
higher potential. This makes the front-surface silver electrode 21
connected to the n-type diffused layer 13 a positive electrode and
the aluminum electrode 17 connected to the n-type diffused layer 13
a negative electrode so that the electricity flows through an
external circuit (not shown).
[0061] Next, features of the solar cell according to the embodiment
are explained. As shown in FIGS. 1-3 and 1-4, in the solar cell
according to the embodiment, the aluminum electrode 17 and the
back-surface silver electrode 19 are partially overlapped on the p+
layer 14. FIG. 1-4 is a schematic that depicts an enlargement of an
area surrounding the back-surface silver electrode 19 shown in FIG.
1-3, i.e., a schematic that depicts an enlargement of the partially
overlapped area of the aluminum electrode 17 and the back-surface
silver electrode 19 provided on the back surface of the solar cell.
FIG. 1-5 is a schematic that depicts an enlargement of the area
surrounding the back-surface silver electrode 19 shown in the cross
section in FIG. 1-1, i.e., a cross section of the surrounding area
of a region B and a region C, where the aluminum electrode 17 and
the back-surface silver electrode 19 are partially overlapped on
the back surface of the solar cell.
[0062] In the region B and the region C where the aluminum
electrode 17 and the back-surface silver electrode 19 are partially
overlapped, three metals of silicon in the p+ layer 14 of the
silicon substrate, aluminum in the aluminum electrode 17, and
silver in the back-surface silver electrode 19 are partially
alloyed to form an alloyed area 23 as shown in FIGS. 1-4 and 1-5.
While borders of the metals (the silicon, the aluminum, and the
silver) in the region B and the region C are clearly defined in
FIGS. 1-1 and 1-5, it is needless to say that the regions are
partially alloyed and actually not clear.
[0063] In the solar cell according to the embodiment, as shown in
FIGS. 1-3 and 1-4, the back-surface silver electrode 19 presents a
substantially square (rectangular) shape in the in-plane direction
of the silicon substrate. The corners of the substantially square
(rectangular) back-surface silver electrode 19 are curved.
Specifically, the back-surface silver electrode 19 is rounded off
at the corners of the substantial square (rectangular) shape.
[0064] As a result of this, in the solar cell according to the
embodiment, as shown in FIG. 1-5, the alloyed area 23 is reliably
formed in the region B and the region C where the aluminum
electrode 17 and the back-surface silver electrode 19 are partially
overlapped, and also, the back-surface silver electrode 19 and the
aluminum electrode 17 are reliably bonded even at the periphery of
the back-surface silver electrode 19.
[0065] In a conventional solar cell, the back-surface silver
electrode 19 is of a substantially square (rectangular) shape in
the in-plane direction of the silicon substrate, and the angle of
each corner of the substantial square (rectangular) shape is
substantially 90 degrees. Similarly to the solar cell according to
the embodiment, the conventional solar cell also includes a region
B' and a region C', where the aluminum electrode 17 and the
back-surface silver electrode 19 are partially overlapped, as shown
in FIG. 2.
[0066] In such a solar cell, the overlapped area is very fragile
due to stress assumedly caused by difference in rate of thermal
expansion between materials that occurs during rapid heating and
cooling in firing of production process. Therefore, after the
firing, in the region B' and the region C' where the aluminum
electrode 17 and the back-surface silver electrode 19 are partially
overlapped, the back-surface silver electrode 19 can separate from
the aluminum electrode 17 at the corners of the back-surface silver
electrode 19. The stress tends to concentrate at sharp corners of
the back-surface silver electrode 19. Namely, the alloyed area 23
is not properly formed at the sharp corners of the back-surface
silver electrode 19, and the separation of the back-surface silver
electrode 19 tends to start from the 90-degree corner thereof.
[0067] In the solar cell according to the embodiment, the corners
are rounded off to remove sharp corners from the back-surface
silver electrode 19 so that the stress does not concentrate on the
corners of the back-surface silver electrode 19. By doing so, in
the solar cell according to the embodiment, the stress concentrated
on the corners of the back-surface silver electrode 19 is eased,
and the alloyed area 23 is reliably formed in the region B and the
region C where the aluminum electrode 17 and the back-surface
silver electrode 19 are partially overlapped as shown in FIG. 1-5,
which improves the bonding force between the aluminum electrode 17
and the back-surface silver electrode 19 and the substrate bonding
force of the aluminum electrode 17 and the back-surface silver
electrode 19. Therefore, according to the embodiment, a solar cell
can be realized on which the back-surface silver electrode 19 is
effectively prevented from separating from the aluminum electrode
17 by reliably bonding the back-surface silver electrode 19 and the
aluminum electrode 17 even at the corners of the back-surface
silver electrode 19.
[0068] When a round-off dimension R is larger than the dimension of
the alloyed area 23, the alloyed area cannot be partially formed
from the aluminum electrode 17 and the back-surface silver
electrode 19, which is undesirable for the back-surface silver
electrode 19. Therefore, as shown in FIG. 1-4, values of dimensions
L1 and L3 of the overlapping area where the aluminum electrode 17
and the back-surface silver electrode 19 overlap in the
longitudinal direction of the back-surface silver electrode 19 and
dimensions L5 and L7 of the overlapping area where the aluminum
electrode 17 and the back-surface silver electrode 19 overlap in
the lateral direction of the back-surface silver electrode 19 that
determine the dimension of the alloyed area 23 need to be
determined so that the alloyed area 23 can be reliably formed. In
addition, while the aluminum electrode 17 and the back-surface
silver electrode 19 are formed by screen-printing as described
later, the dimensions should be determined in consideration of
misalignment in printing aluminum paste and silver paste.
[0069] When the thickness of the silicon substrate is reduced in
the conventional solar cell to reduce the cost of the solar cell,
the warpage of the silicon substrate caused by different rates of
thermal shrinkage between silicon and aluminum is larger compared
with a thicker silicon substrate. When the silicon substrate warps
to a large extent, there is a problem that the silicon substrate
cracks in the production process after the firing, resulting in a
low production yield, or the production becomes impossible due to
the crack in the silicon substrate.
[0070] Even in an effort of preventing the silicon substrate from
warping by changing the material of the aluminum electrode to
improve the rate of thermal shrinkage of the electrode material as
a countermeasure to the problem, the back-surface silver electrode
partially separates due to the different rates of thermal shrinkage
between aluminum and silver depending on the materials combination.
When the back-surface silver electrode separates to a large extent,
there is also a problem that the solar cell cracks due to stacking
of the solar cells or the characteristics of the solar cell
deteriorate, and thus the production yield decreases.
[0071] However, in the solar cell according to the embodiment, as
described above, it is possible to improve the bonding force
between the aluminum electrode 17 and the back-surface silver
electrode 19 and the bonding force of the aluminum electrode 17 and
the back-surface silver electrode 19 with the silicon substrate,
and thus the separation of the back-surface silver electrode 19 or
the separation of the aluminum electrode 17 can be effectively
prevented. This can ensure the bonding between the aluminum
electrode 17 and the back-surface silver electrode 19 and the
bonding of the aluminum electrode 17 and the back-surface silver
electrode 19 with the substrate.
[0072] Therefore, the solar cell according to the embodiment does
not cause many cracks on the silicon substrate differently from the
conventional solar cell even when the thickness of the silicon
substrate is reduced to lower the cost of the solar cell, and is
applicable enough, whereby wider options and various types are
available for the silver paste.
[0073] Furthermore, because the solar cell according to the
embodiment has the back-surface silver electrode 19 rounded off at
the sharp corners which exist in the back-surface silver electrode
of the conventional solar cell, the area of the back-surface silver
electrode 19 is smaller, and the amount of the silver paste used
for the back-surface silver electrode 19 is reduced. Therefore,
according to the embodiment, material costs can be reduced, and
thus an inexpensive solar cell can be realized. Specific effects of
the reduction of the silver paste will be described later.
[0074] Next, a method for producing the solar cell according to the
embodiment configured as above is explained. To produce the solar
cell according to the embodiment, as shown in FIG. 3-1, a p-type
silicon substrate 11' is sliced out of, for example, a p-type
monocrystalline silicon ingot produced by the pulling method or a
polycrystalline silicon ingot produced by the casting method. The
silicon substrate 11' is etched by a thickness of about 10 to 20
micrometers using, for example, a few to 20 wt % of sodium
hydroxide or sodium carbonate, and a damaged layer and
contamination produced on the silicon surface during slicing are
removed.
[0075] Further, depending on necessity, the silicon substrate 11'
is washed using a mixed solution of hydrochloric acid and hydrogen
peroxide to remove heavy metals such as iron attached on the
surface of the substrate. An anisotropic etching is then performed
using a solution made by adding isopropyl alcohol (IPA) to a
similar low-concentrated alkaline solution to form a texture so
that, for example, the surface of silicon (111) is exposed.
[0076] Next, an n-type diffused layer 13a is formed to form a p-n
junction. In the process of forming the n-type diffused layer 13a,
for example, phosphorus oxychloride (POCl.sub.3) is used; a
diffusion process is performed in a mixture gas atmosphere of
nitrogen and oxygen at 800 to 900 degrees Celsius, and phosphorus
is thermally diffused as shown in FIG. 3-2 to form the n-type
diffused layer 13a with the inverse conductivity type all over the
surface of the silicon substrate 11'. The sheet resistance of the
n-type diffused layer 13a is, for example, several tens of (30 to
80) ohm/square, and the depth of the n-type diffused layer 13a is,
for example, about 0.3 to 0.5 micrometer.
[0077] To protect the n-type diffused layer 13a on the light
receiving surface, polymer resistive paste is printed by screen
printing and dried to form resist. The n-type diffused layer 13a
formed on the back and side of the silicon substrate 11' is removed
by soaking the silicon substrate 11' in a solution of, for example,
20 wt % potassium hydroxide for a few minutes. The resist is then
removed by an organic solvent to obtain the silicon substrate 11'
with the n-type diffused layer 13 formed all over the surface
(light receiving surface) thereof as shown in FIG. 3-3.
[0078] As shown in FIG. 3-4, the antireflective coating 15 made of
a silicon oxide film, a silicon nitride film, or titanium oxide
film is formed on the n-type diffused layer 13 in a uniform
thickness. In the case of the silicon oxide film, for example, the
antireflective coating 15 is formed by plasma CVD using silane
(SiH.sub.4) gas and ammonia (NH.sub.3) gas as raw materials at a
heating temperature equal to or higher than 300 degrees Celsius
under reduced pressure. For example, the refractive index is about
2.0 to 2.2, and the optimal thickness of the antireflective coating
15 is about 70 to 90 nanometers.
[0079] Next, the aluminum paste including glass is printed all over
the back surface (the surface opposite to the light receiving
surface) of the silicon substrate 11' using screen printing and
dried as shown in FIG. 3-5 so that an aluminum paste layer 17a is
formed all over the back surface of the silicon substrate 11'. The
aluminum paste layer 17a has openings corresponding to the
locations where the back-surface silver electrodes 19 are formed.
The thickness of the applied aluminum paste can be adjusted
according to the wire diameter that forms a screen mask, the
thickness of emulsion, and the like.
[0080] Subsequently, using screen printing, the silver paste for
the back-surface silver electrodes 19 is printed on the back
surface (the surface opposite to the light receiving surface) of
the silicon substrate 11' on which the aluminum electrode 17 is
formed, and dried as shown in FIG. 3-6 so that a silver paste layer
19a is formed. At this point, the form of the silver paste layer
19a is a substantial square (rectangle) with the corners rounded
off as shown in FIG. 1-3. Here, the silver paste can be printed
using a screen mask with a pattern formed by an emulsion 27 on a
mesh 25 extended on a mask frame 29 as shown in FIG. 3-7 and FIG.
3-8.
[0081] Further, using screen printing, the silver paste for the
front-surface silver electrode 21 is printed on the front surface
(the light receiving surface) of the silicon substrate 11' on which
the antireflective coating 15 is formed, and dried so that a silver
paste layer 21a is formed as shown in FIG. 3-9. The thickness of
the applied silver paste can also be adjusted according to the wire
diameter of the mesh that forms a screen mask, the thickness of
emulsion, and the like.
[0082] Next, in the firing process for forming the electrode, the
paste layers for the front and back electrodes are fired at the
same time at 600 to 900 degrees Celsius for a few to a dozen
minutes. On the front surface (the light receiving surface) of the
silicon substrate 11', the silver paste layer is fired to become
the front-surface silver electrode 21 as shown in FIG. 3-10; at the
same time, the silver material contacts the silicon in the silicon
substrate 11' through the glass material included in the silver
paste while the antireflective coating 15 is melting, and the
antireflective coating 15 is solidified again. This secures the
conductivity between the front-surface silver electrode 21 and the
silicon. The process is generally called a fire-through
process.
[0083] On the other hand, on the back surface (the surface opposite
to the light receiving surface) of the silicon substrate 11', the
aluminum paste layer is fired to become the aluminum electrode 17
as shown in FIG. 3-10, and the silver paste layer is burned to
become the back-surface silver electrode 19 as shown in FIG. 3-10.
Here, the aluminum in the aluminum paste reacts with the silicon in
the silicon substrate 11' so that the p+ layer 14 is formed
immediately below the aluminum electrode 17. The layer is generally
called a back-surface field (BSF) layer, which contributes to the
improvement of the energy conversion efficiency of the solar
battery. In the silicon substrate 11', an area between the n-type
diffused layer 13 and the p+ layer 14 is made into the p-type layer
11.
[0084] The silver paste reacts directly with the silicon in the
silicon substrate 11' where the silver paste directly contacts the
silicon substrate 11', and three metals including the silicon in
the silicon substrate 11', the aluminum in the aluminum paste (the
aluminum electrode 17), and the silver in the back-surface silver
electrode 19 are partially alloyed where the silver paste contacts
the aluminum paste. After the process described above, the cell is
completed based on the method for producing the solar cell. In a
module manufacturing process after the cell fabrication, a copper
lead tab is provided on the back-surface silver electrode 19 to
extract output power to the outside.
[0085] The solar cell described above can be realized by changing
only the shape of the back-surface silver electrode, that is, can
be realized by changing only the shape of the mask to screen-print
the silver paste for the back-surface silver electrode without
changing existing equipment.
[0086] Next, reduced area of the back-surface silver electrode and
reduced amount of the silver paste are explained with a specific
example. As shown in FIG. 4-1 and FIG. 4-2, the explanation is
given of a case of configuring the solar cell including adjacent
back-surface silver electrodes 19 arranged in two rows in the
longitudinal direction under the following conditions.
[0087] Length L1 of the long side of the back-surface silver
electrode 19=9.8 millimeters
[0088] Length L5 of the short side of the back-surface silver
electrode 19=7.8 millimeters
[0089] Distance L9 between back-surface silver electrode arrays=75
millimeters
[0090] Distance L11 between the back-surface silver electrodes 19
at both ends of the back-surface silver electrode array=135
millimeters
[0091] Distance L13 between the adjacent back-surface silver
electrodes 19 in the back-surface silver electrode array=22.5
millimeters
[0092] Table 1 shows the reduced area of the back-surface silver
electrode 19 and the reduction rate of the silver paste when the
curvature radius R of the portion rounded off of the back-surface
silver electrode 19 is changed from 1.0 millimeters to 3.0
millimeters in increments of 0.5 millimeters in the solar cell with
the dimensions as described above.
[0093] [Table 1]
TABLE-US-00001 TABLE 1 Reduced area of back-surface Reduction rate
of R(mm) silver electrode (mm.sup.2) silver paste (%) 3.0 7.7 10.1
2.5 5.4 7.0 2.0 3.4 4.5 1.5 1.9 2.5 1.0 0.9 1.1
[0094] As shown in table 1, with the curvature radius R of the
portion rounded off of the back-surface silver electrode 19
increased from 1.0 millimeters to 3.0 millimeters, the reduced area
of the back-surface silver electrode 19 increases from 0.9 square
millimeters to 7.7 square millimeters. The reduction rate of the
silver paste, namely the effect on reduction in the silver paste,
is increased from 1.1% to 10.1%. This proves that the embodiment of
the present invention can reduce the amount of the silver paste for
the back-surface silver electrode 19. Thereby, the cost of the
materials can be reduced in the solar cell according to the
embodiment, and an inexpensive solar cell can be realized.
SECOND EMBODIMENT
[0095] A solar cell according to another embodiment of the present
invention is explained in the chapter of a second embodiment. The
solar cell according to the second embodiment is configured
basically in the same manner as the solar cell according to the
first embodiment. Therefore, what are different between the solar
cell according to the first embodiment and the solar cell according
to the second embodiment are explained below. In the drawings
referenced below, the components identical to those in the solar
cell according to the first embodiment are denoted with the same
reference numerals.
[0096] FIGS. 5-1 to 5-3 are schematics for explaining a general
configuration of the solar cell according to the second embodiment.
FIG. 5-1 corresponds to FIG. 1-3, and is a plan view of the general
configuration of the back surface (a surface opposite to the light
receiving surface) of the solar cell according to the second
embodiment. FIG. 5-2 corresponds to FIG. 1-4, and is an enlarged
schematic of the surrounding area of a back-surface silver
electrode 31 shown in FIG. 5-1, depicting the area where the
aluminum electrode 17 provided on the back-surface of the solar
cell and the back-surface silver electrode 31 are partially
overlapped.
[0097] FIG. 5-3 corresponds to FIG. 1-5, and is an enlargement of
the surrounding area of the back-surface silver electrode 31,
depicting the cross section of the surrounding area of a region D
and a region E, where the aluminum electrode 17 provided on the
back surface of the solar cell and the back-surface silver
electrode 31 are partially overlapped. Because the cross section
and the light receiving surface (front surface) of the solar cell
are configured identically to those in the first embodiment, FIGS.
1-1 and 1-2 are referenced here.
[0098] The back-surface silver electrode 31 according to the second
embodiment corresponds to the back-surface silver electrode 19
according to the first embodiment; however, as shown in FIGS. 5-1
and 5-2, the back-surface silver electrode 31 is different in that
the corners are chamfered instead of being rounded off.
[0099] On the solar cell according to the second embodiment, the
back-surface silver electrode 31 presents a substantially square
(rectangular) shape in the in-plane direction of the silicon
substrate. The back-surface silver electrode 31 is chamfered at the
corners of the substantially square (rectangular) shape.
[0100] Although the shape of the corners of the back-surface silver
electrode is different from that in the first embodiment, in the
region D and the region E where the aluminum electrode 17 and the
back-surface silver electrode 31 are partially overlapped, three
metals of silicon in the p+ layer 14 of the silicon substrate,
aluminum in the aluminum electrode 17, and silver in the
back-surface silver electrode 31 are partially alloyed so that an
alloyed area 33 is formed as shown in FIGS. 5-2 and 5-3. While
borders of the metals (the silicon, the aluminum, and the silver)
in the region D and the region E are clearly defined in FIG. 5-3,
it is needless to say that the regions are partially alloyed and
actually not clear.
[0101] This ensures that, on the solar cell according to the
embodiment, the alloyed area 33 is reliably formed in the region D
and the region E where the aluminum electrode 17 and the
back-surface silver electrode 31 are partially overlapped as shown
in FIG. 5-3, and the back-surface silver electrode 31 and the
aluminum electrode 17 are reliably bonded at the edges of the
back-surface silver electrode 31.
[0102] With the solar cell according to the embodiment, the corners
of the back-surface silver electrode 31 are chamfered to remove any
sharp edges so that a stress is not concentrated on the corners of
the back-surface silver electrode 31. This can relieve the stress
concentrated at the corners of the back-surface silver electrode
31, ensures that the alloyed area 33 is formed in the region D and
the region E where the aluminum electrode 17 and the back-surface
silver electrode 31 are partially overlapped as shown in FIG. 5-3,
and thus increases the bonding force between the aluminum electrode
17 and the back-surface silver electrode 31 and the substrate
bonding force of the aluminum electrode 17 and the back-surface
silver electrode 31. Therefore, according to the embodiment, a
solar cell can be realized that reliably bonds the back-surface
silver electrode 31 and the aluminum electrode 17 at the corners of
the back-surface silver electrode 31 to effectively prevent the
separation of the back-surface silver electrode 31 from the
aluminum electrode 17.
[0103] When a chamfered dimension C is larger than the alloyed area
33, a part of the alloyed area of the aluminum electrode 17 and the
back-surface silver electrode 31 cannot be formed. Such an
electrode is not suitable as the back-surface silver electrode 31.
As shown in FIG. 5-2, dimensions L21 and L23 where the aluminum
electrode 17 and the back-surface silver electrode 31 are
overlapped in the longitudinal direction of the back-surface silver
electrode 31 and dimensions L25 and L27 where the aluminum
electrode 17 and the back-surface silver electrode 31 are
overlapped in the short-side direction of the back-surface silver
electrode 31 that determine the dimension of the alloyed area 33
need to be determined so as to reliably form the alloyed area 33.
Additionally, because the aluminum electrode 17 and the
back-surface silver electrode 31 are formed by screen printing as
described later, the dimensions should be determined with
consideration of the pattern misalignment of the aluminum paste and
the silver paste at the time of screen printing.
[0104] The solar cell according to the embodiment can also ensure
the bonding between the aluminum electrode 17 and the back-surface
silver electrode 31 and the bonding between the substrate and the
aluminum electrode 17 as well as the back-surface silver electrode
31 as described above. Therefore, in the solar cell according to
the embodiment, even when the thickness of the silicon substrate is
reduced to lower the cost the solar cell, the silicon substrate
will not have many cracks differently from the conventional solar
cell, and wider options and various types are available for the
silver paste.
[0105] Furthermore, because the solar cell according to the
embodiment has the sharp corners of the back-surface silver
electrode 31 chamfered, the area of the back-surface silver
electrode 31 is smaller, which reduces the amount of the silver
paste used for the back-surface silver electrode 31 compared to the
conventional solar cell where such sharp corners exist in the
back-surface silver electrode. As a result, according to the second
embodiment, it is also possible to reduce material costs, and thus
realize an inexpensive solar cell.
[0106] The solar cell according to the embodiment can be produced
in the same process as in the first embodiment except that the
silver paste is screen-printed in the substantially square
(rectangular) shape with chamfered corners as shown in FIG. 5-1.
The solar cell according to the embodiment also can be realized by
changing the shape of the back-surface silver electrode, namely by
changing only the shape of the mask for screen-printing the
back-surface silver electrode with the silver paste without
modifying the existing facilities.
[0107] Next, reduced area of the back-surface silver electrode and
reduced amount of the silver paste are explained with a specific
example. As shown in FIGS. 6-1 and 6-2, the explanation is given of
a case of configuring the solar cell including adjacent
back-surface silver electrodes 31 arranged in two rows in the
longitudinal direction under the following conditions.
[0108] Length L21 of the long side of the back-surface silver
electrode 31=9.8 millimeters
[0109] Length L25 of the short side of the back-surface silver
electrode 31=7.8 millimeters
[0110] Distance L9 between back-surface silver electrode arrays=75
millimeters
[0111] Distance L11 between the back-surface silver electrodes 31
at both ends of the back-surface silver electrode array=135
millimeters
[0112] Distance L13 between the adjacent back-surface silver
electrodes 31 in the back-surface silver electrode array=22.5
millimeters
[0113] Table 2 depicts the reduced area of the back-surface silver
electrode 31 and the reduction rate of the silver paste when the
chamfered dimension C of the chamfered portion of the back-surface
silver electrode 31 is changed from 1.0 millimeters to 3.0
millimeters in increments of 0.5 millimeters in the solar cell with
the dimensions as described above.
[0114] Table 2
TABLE-US-00002 TABLE 2 Reduced area of back-surface Reduction rate
of c(mm) silver electrode (mm.sup.2) silver paste (%) 3.0 18.0 23.5
2.5 12.5 16.4 2.0 8.0 10.5 1.5 4.5 5.9 1.0 2.0 2.6
[0115] As shown in table 2, with the chamfered dimension C of the
portion chamfered off of the back-surface silver electrode 31
increased from 1.0 millimeters to 3.0 millimeters, the reduced area
of the back-surface silver electrode 31 increases from 2.0 square
millimeters to 18.0 square millimeters. The reduction rate of the
silver paste, namely the effect on reduction in the silver paste,
is increased from 2.6% to 23.5%. This proves that the embodiment of
the present invention can reduce the amount of the silver paste for
the back-surface silver electrode 31. Thereby, the cost of the
material can be reduced in the solar cell according to the
embodiment, and an inexpensive solar cell can be realized.
[0116] In either case of the first embodiment and the second
embodiment, the curvature radius or the chamfered dimension needs
to be large to reduce more amount of the silver paste; however, too
large curvature radius or chamfered dimension prevents forming the
alloyed area of aluminum and silver. To determine the actual value
of the curvature radius or the chamfered dimension, pattern
misalignment that occurs on screen-printing the pastes for the
aluminum electrode and the silver electrode should be considered so
that the alloyed area is formed reliably.
[0117] The solar cells according to the first embodiment and the
second embodiment are examples of the present invention. The
present invention is not limited by the embodiments, but is
susceptible to various changes and modifications without departing
from the scope of the present invention.
INDUSTRIAL APPLICABILITY
[0118] As described above, the solar cell according to the present
invention is useful as a solar cell configured with an aluminum
electrode and a silver electrode for extracting electric power
partially overlapped with each other.
* * * * *