U.S. patent application number 14/439344 was filed with the patent office on 2015-10-08 for solar cell and solar cell module.
This patent application is currently assigned to Shin-Etsu Chemical Co., Ltd.. The applicant listed for this patent is Shin-Etsu Chemical Co., Ltd.. Invention is credited to Yoko Endo, Hiroyuki Otsuka.
Application Number | 20150287851 14/439344 |
Document ID | / |
Family ID | 50627034 |
Filed Date | 2015-10-08 |
United States Patent
Application |
20150287851 |
Kind Code |
A1 |
Endo; Yoko ; et al. |
October 8, 2015 |
SOLAR CELL AND SOLAR CELL MODULE
Abstract
A solar cell includes: a semiconductor substrate on which at
least pn junctions are formed; a multiplicity of finger electrodes
that are formed in a comb-like shape on at least one surface of the
semiconductor substrate; and a plurality of bus bar electrodes that
are arranged so as to be orthogonal to the lengthwise direction of
the finger electrodes and are connected with the finger electrodes.
This solar cell is configured so that the finger electrodes
connected with one of the bus bar electrodes are separated from the
finger electrodes connected with another bus bar electrode that is
arranged so as to be parallel to this one of the bus bar
electrodes, and ends in the lengthwise direction of adjacent two or
more of the finger electrodes connected with each bus bar electrode
are electrically connected with one another by auxiliary
electrodes.
Inventors: |
Endo; Yoko; (Annaka-shi,
JP) ; Otsuka; Hiroyuki; (Annaka-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Shin-Etsu Chemical Co., Ltd. |
Tokyo |
|
JP |
|
|
Assignee: |
Shin-Etsu Chemical Co.,
Ltd.
Tokyo
JP
|
Family ID: |
50627034 |
Appl. No.: |
14/439344 |
Filed: |
September 17, 2013 |
PCT Filed: |
September 17, 2013 |
PCT NO: |
PCT/JP2013/074979 |
371 Date: |
April 29, 2015 |
Current U.S.
Class: |
136/244 ;
136/256 |
Current CPC
Class: |
H01L 31/068 20130101;
H01L 31/0504 20130101; H01L 31/048 20130101; Y02E 10/547 20130101;
H01L 31/022433 20130101 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; H01L 31/068 20060101 H01L031/068; H01L 31/0216
20060101 H01L031/0216; H01L 31/0236 20060101 H01L031/0236; H01L
31/05 20060101 H01L031/05; H01L 31/028 20060101 H01L031/028 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 1, 2012 |
JP |
2012-241498 |
Aug 21, 2013 |
JP |
2013-171202 |
Claims
1. A solar cell comprising a semiconductor substrate having at
least a pn junction formed therein, a multiplicity of finger
electrodes which are formed in comb shape on at least one surface
of the semiconductor substrate, and a plurality of bus bar
electrodes which extend orthogonal to the longitudinal direction of
the finger electrodes and are connected to the finger electrodes,
wherein first finger electrodes which are connected to a first bus
bar electrode are spaced apart from second finger electrodes which
are connected to a second bus bar electrode extending parallel with
the first bus bar electrode, and longitudinal ends of adjacent two
or more of the finger electrodes connected to each bus bar
electrode are electrically connected together by an auxiliary
electrode.
2. The solar cell of claim 1 wherein longitudinal ends of adjacent
two to four of the finger electrodes connected to each bus bar
electrode are connected together by an auxiliary electrode.
3. The solar cell of claim 1 wherein all adjacent longitudinal ends
of the finger electrodes connected to each bus bar electrode are
connected together by an auxiliary electrode.
4. The solar cell of claim 1 wherein the finger electrodes project
from the bus bar electrode to which they are connected, in opposite
directions orthogonal to the bus bar electrode, and at each of the
opposite ends of the finger electrodes projecting from the bus bar
electrode, longitudinal ends of adjacent finger electrodes are
electrically connected together by an auxiliary electrode.
5. The solar cell of claim 1 wherein at a position other than the
longitudinal end of the finger electrodes, adjacent finger
electrodes connected to a common bus bar electrode are electrically
connected by another auxiliary electrode extending orthogonal to
the longitudinal direction of the finger electrodes.
6. The solar cell of claim 5 wherein 1 to 10 auxiliary electrodes
are provided at a position other than the longitudinal end of the
finger electrodes.
7. The solar cell of claim 1 wherein the finger electrode has a
width of 30 to 120 .mu.m.
8. The solar cell of claim 1 wherein the auxiliary electrode has a
width of 30 to 500 .mu.m.
9. A solar cell module comprising a plurality of solar cells as set
forth in claim 1 wherein their bus bar electrodes are connected in
series.
Description
TECHNICAL FIELD
[0001] This invention relates to a solar cell which has long-term
reliability and high conversion efficiency and maintains high
output, and a solar cell module comprising solar cells arranged in
series.
BACKGROUND ART
[0002] Using a prior art technique, a solar cell is generally
manufactured as shown in FIGS. 1 to 3. In a p-type semiconductor
substrate 100b of silicon or the like, an n-type dopant is diffused
to form an n-type diffusion layer 101 to define a pn junction. On
the n-type diffusion layer 101, an antireflection coating layer 102
such as SiNx film is formed. On the rear side (lower side in FIG.
1) of the p-type semiconductor substrate 100b, aluminum paste is
printed over substantially the entire surface and alloyed into
silicon to form a back surface field (BSF) layer 103 and an
aluminum electrode 104 by firing. Further, for current collection
on the rear side, a conductive paste containing silver or the like
is printed and fired to form a thick electrode 106 which is known
as bus bar electrode. On the light-receiving surface side (upper
side in FIG. 1, on the antireflection coating layer 102), finger
electrodes 107 for current collection and thick electrodes formed
for collecting current from the finger electrodes known as bus bar
electrodes 105 are arranged in comb-grid shape so that they
intersect substantially at right angles.
[0003] The contact resistance between the front finger electrodes
107 and the semiconductor substrate 100b and the line resistance of
electrodes have a large impact on the conversion efficiency of the
solar cell. To gain high efficiency (low cell series resistance and
high curve fill factor (FF)), it is required that the contact
resistance and the line resistance of finger electrodes 107 are of
fully low values.
[0004] Meanwhile, screen printing method is often employed as the
electrode forming method for solar cells. The screen printing
method has advantages including ease of formation of printed
pattern, an ability to adjust printing pressure to minimize
substrate damage, a high working speed per cell, low cost and high
productivity. If a conductive paste which is so thixotropic that
the profile may be maintained even after transfer is used,
electrodes having a high aspect ratio can be formed.
[0005] Nevertheless, there are some problems. In general, silicon
(Si) is used for the solar cell substrate while Al, Ag and the like
are used for the electrode material. When conductive paste is
printed and sintered onto the solar cell substrate, warpage occurs
due to a difference of linear expansion coefficiency between the Si
substrate and the electrode material such as Al or Ag. Also, ends
of finger electrodes may be peeled off due to shortage of adhesive
strength, whereby solar cell performance is degraded.
[0006] As a countermeasure, as shown in FIG. 4, finger electrodes
107, 107 connected to bus bar electrodes 105, 105 are shortened
such that distal ends of finger electrodes 107, 107 are spaced
apart, thereby reducing warpage. This, however, induce
disconnection of the finger electrodes 107 between bus bar
electrodes 105, 105. Since the number of distal ends of finger
electrodes is doubled as compared with the finger electrodes in
FIG. 1, a peeling failure rate of distal ends may increase. It is
also proposed in JP-A 2006-324504 to enlarge the width (or area) of
the distal portions of finger electrode to increase the adhesive
area, for thereby increasing the adhesive strength. Surely the
adhesive strength of distal portions of the finger electrode is
increased. This method succeeded in mitigating the warpage of solar
cell substrate and increasing the adhesive strength of finger
electrodes, but failed to avoid the increase of line resistance due
to local breaks in finger electrodes and the problem of reduced
fill factor.
[0007] Specifically, the finger electrodes are typically designed
to a line width of about 60 to 120 .mu.m. They are printed and
formed by the screen printing method as mentioned above. Such
reduced line width may induce problems like skipping, further
narrowing of line width, and breakage. Such failure, if any, may
lead to the problem of interfering with electric conduction from
the finger electrode at the failed site to the bus bar
electrode.
CITATION LIST
Patent Document
[0008] Patent Document 1: JP-A 2006-324504
SUMMARY OF INVENTION
Technical Problem
[0009] An object of the invention, which has been made under the
above-mentioned circumstances, is to provide a solar cell and a
solar cell module, featuring mitigated warpage of a solar cell
substrate against high adhesive strength of distal portions of
finger electrodes and possible electric conduction of a finger
electrode to a bus bar electrode even if the electrodes are locally
cut or broken, eventually having long-term reliability and high
conversion efficiency, and maintaining high output.
Solution to Problem
[0010] To attain the above object, the invention provides a solar
cell and solar cell module as defined below.
[1] A solar cell comprising a semiconductor substrate having at
least a pn junction formed therein, a multiplicity of finger
electrodes which are formed in comb shape on at least one surface
of the semiconductor substrate, and a plurality of bus bar
electrodes which extend orthogonal to the longitudinal direction of
the finger electrodes and are connected to the finger electrodes,
wherein
[0011] first finger electrodes which are connected to a first bus
bar electrode are spaced apart from second finger electrodes which
are connected to a second bus bar electrode extending parallel with
the first bus bar electrode, and longitudinal ends of adjacent two
or more of the finger electrodes connected to each bus bar
electrode are electrically connected together by an auxiliary
electrode.
[2] The solar cell of [1] wherein longitudinal ends of adjacent two
to four of the finger electrodes connected to each bus bar
electrode are connected together by an auxiliary electrode. [3] The
solar cell of [1] wherein all adjacent longitudinal ends of the
finger electrodes connected to each bus bar electrode are connected
together by an auxiliary electrode. [4] The solar cell of any one
of [1] to [3] wherein the finger electrodes project from the bus
bar electrode to which they are connected, in opposite directions
orthogonal to the bus bar electrode, and at each of the opposite
ends of the finger electrodes projecting from the bus bar
electrode, longitudinal ends of adjacent finger electrodes are
electrically connected together by an auxiliary electrode. [5] The
solar cell of any one of [1] to [4] wherein at a position other
than the longitudinal end of the finger electrodes, adjacent finger
electrodes connected to a common bus bar electrode are electrically
connected by another auxiliary electrode extending orthogonal to
the longitudinal direction of the finger electrodes. [6] The solar
cell of [5] wherein 1 to 10 auxiliary electrodes are provided at a
position other than the longitudinal end of the finger electrodes.
[7] The solar cell of any one of [1] to [6] wherein the finger
electrode has a width of 30 to 120 .mu.m. [8] The solar cell of any
one of [1] to [7] wherein the auxiliary electrode has a width of 30
to 500 .mu.m. [9] A solar cell module comprising a plurality of
solar cells as set forth in any one of [1] to [8] wherein their bus
bar electrodes are connected in series.
Advantageous Effects of Invention
[0012] The solar cell of the invention eliminates drawbacks
associated with breaks of finger electrodes and offers a high fill
factor, high conversion efficiency, mitigated cell warpage,
improved manufacture yield, least cost increase, and long-term
reliability. The solar cell module comprising such solar cells has
a high percent output retention.
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIG. 1 is a cross-sectional view showing the construction of
a conventional solar cell.
[0014] FIG. 2 is a plane view showing an electrode pattern on the
front surface of the conventional solar cell.
[0015] FIG. 3 is a plane view showing an electrode pattern on the
back surface of the conventional solar cell.
[0016] FIG. 4 is a plane view showing an electrode pattern on the
front surface of a prior art solar cell.
[0017] FIG. 5 is a plane view showing an electrode pattern on the
front surface of a solar cell in one embodiment of the
invention.
[0018] FIG. 6 is a plane view showing an electrode pattern on the
front surface of a solar cell in another embodiment of the
invention.
[0019] FIG. 7 is a plane view showing a modified example (1) of the
electrode pattern on the front surface of the solar cell of FIG.
5.
[0020] FIG. 8 is a plane view showing a modified example (2) of the
electrode pattern on the front surface of the solar cell of FIG.
5.
[0021] FIG. 9 is a schematic cross-sectional view showing the basic
construction of a conventional solar cell module.
[0022] FIG. 10 illustrates how to evaluate warpage of a solar
cell.
[0023] FIG. 11 is a diagram showing short circuit current density
and fill factor in Example 3.
[0024] FIG. 12 is a diagram showing conversion efficiency in
Example 3.
DESCRIPTION OF EMBODIMENTS
[0025] Below, the solar cell and solar cell module according to the
invention are described. Understandably, the invention is not
limited to the solar cells of the illustrated embodiments.
[0026] Referring to FIGS. 5 to 8, the solar cell of the invention
is illustrated as comprising a semiconductor substrate having at
least a pn junction formed therein, a multiplicity of finger
electrodes 107a, 107b which are formed in comb shape on at least
one surface of the semiconductor substrate, and a plurality of (two
in FIGS. 5 to 8) bus bar electrodes 105a, 105b which extend
orthogonal to the longitudinal direction of the finger electrodes
107a, 107b and are connected to the finger electrodes 107a, 107b.
Herein, first finger electrodes 107a which are connected to a first
bus bar electrode 105a are spaced apart from second finger
electrodes 107b which are connected to a second bus bar electrode
105b extending parallel with the first bus bar electrode 105a, and
longitudinal ends (also referred to as distal portions,
hereinafter) of adjacent two or more electrodes of the finger
electrodes connected to each bus bar electrode are electrically
connected together by an auxiliary electrode 108. It is noted that
the solar cell of the invention is characterized by the front
surface electrode pattern while the remaining construction may be
as shown in FIG. 1, for example.
[0027] In the embodiment shown in FIG. 5, all adjacent distal
portions of the finger electrodes 107a, 107b connected to each of
the bus bar electrodes 105a, 105b are connected together by the
auxiliary electrode 108. In the embodiment shown in FIG. 6, two
adjacent distal portions are connected by the auxiliary electrode
108. The mode of connecting adjacent distal portions by the
auxiliary electrode 108 is not limited to these embodiments. Also,
as shown in FIGS. 5 and 6, the finger electrodes 107a, 107b project
from the bus bar electrodes 105a, 105b to which they are connected,
in opposite directions orthogonal to the bus bar electrodes 105a,
105b. Preferably at each of opposite ends of finger electrodes 107a
(or 107b) projecting from the bus bar electrode 105a (or 105b),
longitudinal ends of adjacent finger electrodes 107a (or 107b) are
electrically connected together by the auxiliary electrode 108.
[0028] It is further preferred in the solar cell of the invention
that at a position other than the longitudinal end of the finger
electrodes 107a (or 107b), adjacent finger electrodes 107a (or
107b) connected to a common bus bar electrode 105a (or 105b) are
electrically connected by another auxiliary electrode 108 which
extends orthogonal to the longitudinal direction of the finger
electrodes 107a (or 107b). In this embodiment wherein the
additional auxiliary electrode 108 is provided at the position
other than the longitudinal end of the finger electrodes (107a or
107b) connected to each bus bar electrode, the number of additional
auxiliary electrodes 108 is preferably 1 to 10. Even when only one
additional auxiliary electrode 108 is provided, this ensures to
suppress a lowering of solar cell fill factor in the event of
finger electrodes being broken. Also, if the number of additional
auxiliary electrodes 108 exceeds 10, the light receiving area is
accordingly reduced, resulting in the reduction in short circuit
current and conversion efficiency.
[0029] FIGS. 7 and 8 shows the embodiments wherein additional
auxiliary electrodes 108 are provided at positions other than the
longitudinal end (or distal portion) of the finger electrodes 107a,
107b, in the electrode pattern of FIG. 5.
[0030] In the embodiment of FIG. 7, in addition to the auxiliary
electrodes 108 provided at the distal portions of finger electrodes
107a, 107b, three auxiliary electrodes 108 extending orthogonal to
the longitudinal direction of finger electrodes 107a are provided
on each side such that they are equally spaced between the distal
portions of finger electrodes 107a and the bus bar electrode 105a,
for connecting the finger electrodes 107a all together; and three
auxiliary electrodes 108 extending orthogonal to the longitudinal
direction of finger electrodes 107b are provided on each side such
that they are equally spaced between the distal portions of finger
electrodes 107b and the bus bar electrode 105b, for connecting the
finger electrodes 107b all together.
[0031] This embodiment reduces the conductive paste necessary to
form auxiliary electrodes to the requisite minimum and is effective
for suppressing any reduction of the solar cell fill factor even if
by any chance finger electrodes are broken. The embodiment of FIG.
7 also has the advantage that if a finger electrode is broken at
any position, the distance from the broken site to the auxiliary
electrode is so short that the reduction of fill factor is
minimized.
[0032] In the embodiment of FIG. 8, in addition to the auxiliary
electrodes 108 provided at the distal portions of finger electrodes
107a, 107b, two auxiliary electrodes 108 extending orthogonal to
the longitudinal direction of finger electrodes 107a, 107b are
provided in proximity to the distal portions of finger electrodes
107a, 107b, for connecting the finger electrodes 107a all together
and connecting the finger electrodes 107b all together. The
proximity to the distal portions of finger electrodes 107a, 107b
indicates a region that is deviated from an intermediate point
between the distal portion of finger electrode and the bus bar
electrode toward the distal portion of finger electrode, preferably
a region that extends within a distance of L/3 from the distal
portion of finger electrode wherein L is the distance between the
distal portion and the bus bar electrode, and more preferably a
region that extends within a distance of L/4 from the distal
portion of finger electrode wherein L is the distance defined
above.
[0033] This embodiment reduces the conductive paste necessary to
form auxiliary electrodes to the requisite minimum and is effective
for suppressing any reduction of the solar cell fill factor even if
by any chance finger electrodes are broken. The invention relies on
an electrode printing method of forming bus bar electrodes, finger
electrodes, and auxiliary electrodes at a time by screen printing
technique. At this point, the printing direction is generally set
parallel to the finger electrodes and orthogonal to the bus bar
electrodes and auxiliary electrodes. This is because the screen
printing technique is difficult to print lines orthogonal to the
printing direction, with the risk that such lines may be thinned or
broken. When auxiliary lines are closely arranged as shown in FIG.
8, the resulting auxiliary electrodes can compensate for breakage
or thinning. Thickened auxiliary electrodes, though possible to
print, are undesirable because the light receiving area is
reduced.
[0034] Although the electrode patterns of FIGS. 7 and 8 are shown
as modified examples of FIG. 5, they may be applied to the
electrode pattern of FIG. 6.
[0035] Also, when adjacent distal portions are connected by an
auxiliary electrode, the auxiliary electrode is not limited to the
linear connection orthogonal to the longitudinal direction of
finger electrodes as shown in FIGS. 5 to 8. A connection method
using a connector of a curved shape which is convex outward of the
longitudinal direction of finger electrodes between two finger
electrodes to be connected is acceptable. That is, the auxiliary
electrode may be a connector of an arc shape (arch shape) or a
mountain-type protrusion (pseudo-arcuate shape) consisting of short
linear segments connecting between the ends of two adjacent or
close finger electrodes, so that the connection angle between the
auxiliary electrode and the finger electrode (angle formed between
auxiliary electrode and finger electrode) is not orthogonal.
[0036] It is noted that the bus bar electrode is preferably formed
to a line width of 0.5 to 3.0 mm, more preferably 1.0 to 1.5 mm.
The finger electrode is preferably formed to a line width of 30 to
120 .mu.m, more preferably 60 to 120 .mu.m, and most preferably 70
to 100 .mu.m. The auxiliary electrode is preferably formed to a
line width of 30 to 500 .mu.m, more preferably 60 to 500 .mu.m,
even more preferably 60 to 360 .mu.m, and most preferably 70 to 240
.mu.m. The spacing between bus bar electrodes is preferably 20 to
100 mm, more preferably 39 to 78 mm. The spacing between finger
electrodes is preferably 0.5 to 4.0 mm, more preferably 1.5 to 2.5
mm.
[0037] The ratio of the line width of auxiliary electrode to the
line width of finger electrode is preferably from 0.5 to 8.0, more
preferably from 0.5 to 2.5. A ratio of less than 0.5 may invite
difficult fabrication of a screen printing plate and a likelihood
of breakage whereas a ratio in excess of 8.0 may cause a reduction
in light receiving area and hence, a lowering of conversion
efficiency.
[0038] The solar cell of the invention as shown in FIG. 1 may be
manufactured by any well-known methods. Herein, bus bar electrodes,
finger electrodes and auxiliary electrodes may be formed by the
screen printing method. Desirably these electrodes are
simultaneously formed by the screen printing method. This gives
advantages of reducing the manufacturing cost by a single printing
step and improving the manufacture yield by the reduction in
cracking or fissure due to the reduced number of steps capable of
applying stress to the semiconductor substrate. The invention is
also applicable to a solar cell wherein finger electrodes and bus
bar electrodes are formed on the rear side, that is, bifacial solar
cell.
[0039] When the electrode pattern according to the invention is
used, not only the warpage of the substrate is minimized, but the
following advantages are also achieved.
[0040] First, even when a certain finger electrode is broken during
thermal history as by a thermal cycling test, because the end of
that broken finger electrode is connected to the end of another
finger electrode by the auxiliary electrode, current flow can be
taken out via the other finger electrode, avoiding any power loss.
Second, since the auxiliary electrode is connected to the ends of
finger electrodes, the contact area with the semiconductor
substrate at the finger electrode end is enlarged, whereby the
adhesive strength of the finger electrode end is improved to
prevent the finger electrode from peeling during long-term service.
Third, the same prevents the finger electrode end from peeling from
the semiconductor substrate upon thermal shrinkage after firing.
Fourth, the auxiliary electrodes serve to reduce the line
resistance, leading to increased fill factor and improved
conversion efficiency. Fifth, the provision of auxiliary electrodes
maintains a high conversion efficiency in that a loss of short
circuit current density (Jsc) associated with a reduction of light
receiving area is offset by an increase of fill factor.
[0041] The advantages of the invention are also available from a
solar cell module.
[0042] When the solar cell is exposed to an outdoor environment,
the current-collection electrode is damaged by the impact of
temperature, humidity, pressure or the like, resulting in the
decrease in the conversion efficiency. When dust and foreign
particles which are not transmissive to light deposit on the
light-receiving surface, they interfere with entry of sunlight,
inviting the substantial decrease in conversion efficiency. Thus,
in the prior art, a laminate of the order of transparent front-side
cover (e.g., colorless strengthened glass plate)/fill (e.g.,
ethylene-vinyl acetate=EVA)/solar cell/fill (e.g., EVA)/weather
resistant back-side cover of resin film (e.g., polyethylene
terephthalate=PET) is bonded under heat and pressure, yielding a
solar cell module which is constructed so as to minimize the loss
of conversion efficiency. Even the solar cell module constructed as
above, however, when exposed to a severe outdoor environment for
many years, tends to decrease its conversion efficiency gradually.
Among others, the electrodes are corroded with moisture and allow
metal particles to be leached out in moisture, in some cases, and
if so, they weaken their bond to the semiconductor substrate and
eventually peel off.
[0043] The above and other problems are solved using the solar cell
of the invention because the adhesive strength of finger electrode
ends is increased.
[0044] The solar cell module of the invention is constructed using
the solar cells of the invention. As shown in FIG. 9, a plurality
of solar cells are electrically connected by soldering conductors
or interconnectors 201 to their bus bar electrodes. In the
illustrated embodiment, interconnectors 201 are connected to front
bus bar electrode 105 and rear bus bar electrode 106 on the solar
cell 100 via solders 202.
[0045] The solar cell module of the invention is obtained by
arranging a plurality of solar cells 100 of the illustrated
construction along the longitudinal direction of bus bar electrodes
105, with their light-receiving surfaces faced in an identical
direction, and connecting a front bus bar electrode 105 of one
solar cell 100 to a rear bus bar electrode 106 of an adjacent solar
cell 100 via an interconnector 201. The number of solar cells
connected is typically 2 to 60.
[0046] In general, the front and back surfaces of solar cells in
the solar cell module should be protected. A solar cell module
product is thus constructed such that a plurality of solar cells
with interconnectors 201 as illustrated above are sandwiched
between a transparent substrate such as glass plate and a back
cover such as a back-sheet. In this case, a super-straight system
is generally employed, for example, wherein a plurality of solar
cells 100 with interconnectors 201 are sandwiched between the
transparent substrate and the back cover, with their
light-receiving surfaces facing the transparent substrate, and
encapsulated with a transparent fill material such as polyvinyl
butyrol (PVB) having a minimal loss of light transmittance or
ethylene vinyl acetate (EVA) having improved moisture resistance,
and external terminals are connected thereto. Herein, an external
extracting interconnector connected to the rear bus bar electrodes
106 of solar cells 100 is connected to one external terminal, and
an external extracting interconnector connected to the front bus
bar electrodes 105 of solar cells 100 is connected to the other
external terminal.
EXAMPLES
[0047] Examples and Comparative Examples are given below by way of
illustration of the invention, but not by way of limitation.
Examples 1 and 2 and Comparative Examples 1 and 2
[0048] To confirm the effectiveness of the invention, solar cells
100 as shown in FIGS. 2, 4, 5 and 6 were fabricated by processing
400 semiconductor substrates through the following steps.
[0049] First, there were furnished boron-doped {100} p-type silicon
substrates 100b as sliced of 15 cm squares and 250 .mu.m thick,
having a resistivity of 2.0 .OMEGA.cm. The substrate was treated
with a conc. potassium hydroxide aqueous solution to remove the
damaged layer, textured, heat treated in a phosphorus oxychloride
atmosphere at 850.degree. C. to form an n-type diffusion layer 101,
treated with hydrofluoric acid to remove phosphorus glass, cleaned,
and dried. Next, using a plasma-enhanced CVD system, SiNx was
deposited as an antireflection coating layer 102. On the rear side,
a paste obtained by mixing silver powder and glass frit with an
organic binder was screen-printed in a bus bar pattern for rear bus
bar electrode 106. Thereafter, a paste obtained by mixing aluminum
powder with an organic binder was screen-printed in a region except
on the previously printed bus bar pattern, for aluminum electrode
104. The organic solvent was dried off, yielding the semiconductor
substrate having rear electrodes formed thereon.
[0050] Next, on the semiconductor substrate, a conductive paste
containing silver powder, glass frit, organic vehicle and organic
solvent as main components and metal oxide as additive was printed
on the antireflection coating layer 102 on the semiconductor
substrate through a screen under such conditions as squeezer rubber
hardness 70.degree., squeezer angle 70.degree., printing pressure
0.3 MPa, and printing speed 50 mm/sec. After printing, the paste
was dried in a clean oven at 150.degree. C. to remove the organic
solvent, and fired at 800.degree. C. in an air atmosphere, yielding
the solar cell 100.
[0051] Two bus bar electrodes were formed with a spacing of 78 mm
(in case of three bus bar electrodes, 52 mm) and a line width of
1.5 mm. Finger electrodes had a line width of 90 .mu.m and a mutual
spacing of 2.0 mm. Auxiliary electrodes had a line width of 120
.mu.m.
[0052] Four hundred (400) solar cells thus fabricated were
evaluated by the following tests.
(1) Electric Properties
[0053] Solar cell electric properties were evaluated by using a
solar simulator (model YSS-160A by Yamashita Denso Corp.). I-V
characteristics were measured by irradiating simulated sun light
from the simulator to a solar cell sample (substrate temperature
25.degree. C., irradiance 1 kW/m.sup.2, spectrum AM 1.5 global).
From the data, fill factor, short-circuit current density and
conversion efficiency were computed. The measurement was reported
as an average of 100 solar cell samples for each Example.
(2) Warpage of Solar Cell
[0054] Warpage of a solar cell was evaluated as shown in FIG. 10.
In the measurement, a solar cell sample 100 was set on a platform
300, and the distance "d" from the topmost of the sample to the
platform 300 was measured.
[0055] The results are shown in Table 1.
TABLE-US-00001 TABLE 1 Comparative Comparative Example 1 Example 2
Example 1 Example 2 Electrode pattern FIG. 2 FIG. 4 FIG. 5 FIG. 6
Electrode area ratio 1.00 0.97 1.03 1.00 Electrode cost ratio 1.00
0.97 1.03 1.00 Short-circuit current 35.9 36.2 35.8 35.9 density
(mA/cm.sup.2) Fill factor (%) 75.0 73.0 78.0 77.9 Conversion 16.7
16.4 17.5 17.5 efficiency (%) Cell warpage (mm) 3.0 0.5 0.5 0.5
[0056] In Comparative Examples, short-circuit current increased
because the electrode area on the light-receiving surface was
reduced which means that the amount of sunlight entering the
substrate was accordingly increased. However, the line resistance
of solar cell increased due to breakage of finger electrodes and
peeling of finger electrode ends, resulting in a reduction in fill
factor.
[0057] By contrast, the electrode patterns of Examples 1 and 2
demonstrate that even when the electrode area is increased as in
Example 1, the reduction in short circuit current is less and the
fill factor increases remarkably. This leads to a 0.8% increase in
conversion efficiency over Comparative Example 1.
[0058] Next, using the solar cells fabricated in Examples 1 and 2
and Comparative Examples 1 and 2, modules were manufactured by the
following procedure.
[0059] A linear interconnector 201 of 2 mm wide and 0.2 mm thick
was used. As shown in FIG. 9, flux was previously applied to the
region for connection between interconnector 201 and bus bar
electrode 105, and the interconnector 201 was solder-connected to
the bus bar electrode 105 on the light-receiving surface of the
solar cell. Likewise, an interconnector 201 was soldered to the
rear bus bar electrode 106 of the solar cell. Next, components were
stacked in the order of colorless strengthened glass/ethylene vinyl
acetate (EVA)/interconnected solar cell 100/EVA/polyethylene
terephthalate (PET). By vacuuming the ambient atmosphere,
heat/pressure bonding at a temperature of 150.degree. C. for 10
minutes to make them a module. Furthermore, this module was
post-annealed at 150.degree. C. for 1 hour to completely cure.
Herein 60 solar cells were connected to each other by
interconnectors 201 and encapsulated.
[0060] By the foregoing procedure, the solar cell modules were
manufactured.
[0061] A thermal cycling test (JIS C8917) was performed on each of
the solar cell modules manufactured using the solar cells of
Examples 1 and 2 and Comparative Examples 1 and 2, comparing the
output of the solar cell module before and after the test. The
thermal cycling test included 400 cycles under the conditions
according to JIS C8917 standards. Specifically, the test includes
heating from room temperature (25.degree. C.) to 90.degree. C. at a
rate of up to 87.degree. C./hr, holding at the temperature
(90.degree. C.) for 10 minutes, then cooling down to -40.degree. C.
at a rate of up to 87.degree. C./hr, holding at the temperature
(-40.degree. C.) for 10 minutes, and heating up to 25.degree. C. at
a rate of up to 87.degree. C./hr. Provided that these steps are one
cycle (3 hours 20 minutes), the test repeated 400 cycles. The
output of the solar cell module was measured by the aforementioned
solar simulator under light exposure at AM 1.5 and 100 mW/cm.sup.2,
whereupon a percent output retention=(output after test)/(output
before test).times.100% was computed. The results are shown in
Table 2.
TABLE-US-00002 TABLE 2 Comparative Comparative Example 1 Example 2
Example 1 Example 2 Output retention 77% 54% 99% 97%
[0062] As seen from the results, after 400 cycles of the thermal
cycling test, the solar cell module using the solar cells of
Comparative Example 1 showed an output drop to 77%. In the solar
cell module using the solar cells of Comparative Example 2, the
output dropped to 54%.
[0063] The solar cell module using the solar cells of Example 1
showed an output retention of 99%. The solar cell module using the
solar cells of Example 2 showed an output retention of 97%. No
significant output drops could be found.
Example 3
[0064] Properties of the solar cell of Example 1 (structure of FIG.
5) were measured while the line width of auxiliary electrodes was
changed. Specifically, solar cells were fabricated under the same
conditions as in Example 1 except that the line width of auxiliary
electrodes was changed to 0 (no auxiliary electrode), 60, 120, 240,
360, 480, and 600 .mu.m. Electric properties of the solar cells
were measured under the same measurement conditions as in Example
1. The measurements of short-circuit current density and fill
factor are shown in FIG. 11 and the measurements of conversion
efficiency are shown in FIG. 12.
[0065] In the absence of auxiliary electrodes (line width 0 .mu.m),
a high short-circuit current and remarkably low fill factor were
observed, while the fill factor and conversion efficiency were
substantially improved by the provision of auxiliary electrodes. In
the region where the line width of auxiliary electrodes is 60 to
500 .mu.m, as the line width increases, the short-circuit current
decreases and the fill factor increases. A tradeoff relationship
between short-circuit current and fill factor is observed (FIG. 11)
and a certain high conversion efficiency is maintained (FIG. 12).
When the line width of auxiliary electrodes exceeds 500 .mu.m, the
reduction in short-circuit current becomes substantial due to the
shadow loss of auxiliary electrodes (FIG. 11) and the conversion
efficiency drops (FIG. 12) while the fill factor little
increases.
[0066] These results show that an optimum line width of auxiliary
electrodes is 60 to 500 .mu.m in the inventive solar cell.
REFERENCE SIGNS LIST
[0067] 100 solar cell [0068] 100b p-type semiconductor substrate
[0069] 101 n-type diffusion layer [0070] 102 antireflection coating
layer [0071] 103 BSF layer [0072] 104 aluminum electrode [0073]
105, 105a, 105b bus bar electrode [0074] 106 rear bus bar electrode
[0075] 107, 107a, 107b finger electrode [0076] 108 auxiliary
electrode [0077] 201 interconnector [0078] 202 solder
* * * * *