U.S. patent application number 13/859800 was filed with the patent office on 2014-02-20 for method of manufacturing solar cell electrode and conductive paste.
This patent application is currently assigned to E I DU PONT DE NEMOURS AND COMPANY. The applicant listed for this patent is E I DU PONT DE NEMOURS AND COMPANY. Invention is credited to HIDEKI AKIMOTO.
Application Number | 20140048752 13/859800 |
Document ID | / |
Family ID | 46801369 |
Filed Date | 2014-02-20 |
United States Patent
Application |
20140048752 |
Kind Code |
A1 |
AKIMOTO; HIDEKI |
February 20, 2014 |
METHOD OF MANUFACTURING SOLAR CELL ELECTRODE AND CONDUCTIVE
PASTE
Abstract
The present invention relates to a method of manufacturing a
solar cell electrode, comprising: preparing a semiconductor
substrate having a preformed electrode on a front side, a back
side, or both of the front and the back side of the semiconductor
substrate; applying a conductive paste onto the preformed
electrode, wherein the conductive paste comprises a conductive
powder, an amorphous saturated polyester resin with glass
transitional temperature (Tg) of 50.degree. C. or lower, and an
organic solvent; drying the applied conductive paste; putting a tab
electrode on the dried conductive paste; and soldering the tab
electrode.
Inventors: |
AKIMOTO; HIDEKI; (Kawaskaki
Kanagawa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
E I DU PONT DE NEMOURS AND COMPANY |
Wilmington |
DE |
US |
|
|
Assignee: |
E I DU PONT DE NEMOURS AND
COMPANY
Wilmington
DE
|
Family ID: |
46801369 |
Appl. No.: |
13/859800 |
Filed: |
April 10, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13585049 |
Aug 14, 2012 |
8502067 |
|
|
13859800 |
|
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Current U.S.
Class: |
252/514 ;
438/98 |
Current CPC
Class: |
H01B 1/22 20130101; Y02E
10/50 20130101; H01L 31/18 20130101; H01L 31/022425 20130101 |
Class at
Publication: |
252/514 ;
438/98 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; H01L 31/18 20060101 H01L031/18 |
Claims
1. A method of manufacturing a solar cell electrode comprising
steps of: preparing a semiconductor substrate having a preformed
electrode on a front side, a back side, or both of the front and
the back side of the semiconductor substrate; applying a conductive
paste onto the preformed electrode, wherein the conductive paste
comprises a conductive powder, an amorphous saturated polyester
resin with glass transitional temperature (Tg) of 50.degree. C. or
lower, and an organic solvent; drying the applied conductive paste;
putting a tab electrode on the dried conductive paste; and
soldering the tab electrode.
2. The method of manufacturing a solar cell electrode of claim 1,
wherein the conductive powder is from 40 to 90 weight percent, the
amorphous saturated polyester resin is from 3 to 20 weight percent,
and the organic solvent is 5 to 50 weight percent, based on the
total weight of the conductive paste.
3. The method of manufacturing a solar cell electrode of claim 1,
wherein the preformed electrode is a transparent electrode.
4. The method of manufacturing a solar cell electrode of claim 1,
wherein the preformed electrode is a collective electrode
comprising a bus bar and a finger line at the front side of the
semiconductor substrate.
5. The method of manufacturing a solar cell electrode of claim 1,
wherein the preformed electrode is a backside electrode on the back
side of the semiconductor substrate.
6. The method of manufacturing a solar cell electrode of claim 1,
wherein the conductive paste is dried at from 80 to 250.degree. C.
at the drying step.
7. A conductive paste for manufacturing a solar cell electrode,
comprising, based on the total weight of the conductive paste: 40
to 90 weight percent of a conductive powder; 5 to 50 weight percent
of a solvent; and 3 to 20 weight percent of an amorphous saturated
polyester resin with a Tg of 50.degree. C. or lower.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a solar cell electrode, more
specifically to a conductive paste to form a solar cell electrode
and a method of manufacturing a solar cell electrode.
TECHNICAL BACKGROUND OF THE INVENTION
[0002] There are two kinds of conductive pastes. One is a firing
type that contains glass frit to adhere to a substrate by melting
during firing at high temperature, for example 500.degree. C. or
higher. The other is a polymer type of conductive paste that
contains substantially no glass frit and adheres to a substrate by
a polymer resin itself in the conductive paste by being heated at
relatively low temperature, for example 300.degree. C. or lower. A
solar cell electrode manufactured by using a polymer type
conductive paste is required because it does not need a high
temperature in a firing step. However, a solar cell electrode made
of the polymer type conductive paste sometimes peels off by an
effect of soldering heat.
[0003] US20080196757 discloses a solar cell electrode formed by a
conductive composition including conductive particles, and a
thermosetting epoxy resin composition.
SUMMARY OF THE INVENTION
[0004] An objective of the present invention is to provide a
polymer type conductive paste to form a solar cell electrode with
sufficient adhesion between a substrate and the electrode as well
as between the electrode and a copper ribbon attached by
soldering.
[0005] An aspect of the invention relates to a method of
manufacturing a solar cell electrode, comprising: preparing a
semiconductor substrate having a preformed electrode on a front
side, a back side, or both of the front and the back side of the
semiconductor substrate; applying a conductive paste onto the
preformed electrode, wherein the conductive paste comprises a
conductive powder, an amorphous saturated polyester resin with
glass transitional temperature (Tg) of 50.degree. C. or lower, and
an organic solvent; drying the applied conductive paste at from 80
to 250.degree. C.; putting a tab electrode on the dried conductive
paste; and soldering the tab electrode.
[0006] Another aspect of the invention relates to a conductive
paste for manufacturing a solar cell electrode, comprising, based
on the total weight of the conductive paste; 40 to 90 weight
percent of a conductive powder; 5 to 50 weight percent of a
solvent; and 3 to 20 weight percent of an amorphous saturated
polyester resin with Tg of 50.degree. C. or lower.
[0007] A solar cell electrode by the present invention can obtained
sufficient adhesion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a cross-sectional schematic drawing of a hybrid
type solar cell.
[0009] FIGS. 2(a) to 2(d) are drawings for explaining a method of
forming a hybrid type solar cell electrode.
[0010] FIGS. 3(a) to 3(c) are drawings for explaining a method of
forming a crystal type solar cell electrode at the front side.
[0011] FIGS. 4(a) to 4(c) are drawings for explaining a method of
forming a crystal type solar cell electrode at the front side.
[0012] FIGS. 5(a) to 5(c) are drawings for explaining a method of
forming a crystal type solar cell electrode at the back side.
[0013] FIGS. 6(a) to 6(c) are drawings for explaining a method of
forming an organic thin film solar cell electrode.
DETAILED DESCRIPTION OF THE INVENTION
[0014] A method for manufacturing a solar cell electrode of the
present invention is described below along with figures as well as
a conductive paste used in the method.
[0015] (Method of Manufacturing a Solar Cell Electrode)
[0016] There are several types of a solar cell, such as a hybrid
type, a crystal type, an amorphous type, or a thin film type.
However, type of a solar cell is not limited.
[0017] In an embodiment, a solar cell can be a hybrid type solar
cell. In case of the hybrid type, a substrate can comprise a
semiconductor substrate, 101, and a transparent electrode, 102. The
transparent electrode, 102, can be formed on both a front side and
a back side of the semiconductor layer, 101, as shown FIG. 1. The
transparent electrode, 102, can be made from indium titanium oxide
(ITO). The preformed electrode is the transparent electrode, 102,
in this case.
[0018] The semiconductor layer, 101, of the hybrid type solar cell,
can be at least three layered of a mono-crystal semiconductor layer
and amorphous semiconductor layers on both front and back sides of
the mono-crystal semiconductor layer.
[0019] "Front side" is defined in this specification as a side
receiving sunlight when it is introduced to generate electricity
from the sunlight. "Back side" is defined as the opposite side of
the front side.
[0020] A collective electrode, 103, can be formed on the
transparent electrode, 102, at both of the front side and the back
side. The collective electrode, 103, on the front side can be
connected to the collective electrode on the back side of an
adjacent solar cell by using a tab electrode, 104. The tab
electrode, 104, can be covered by solder, 105, to adhere. The
conductive paste of the present invention can be used for the
collective electrode, 103. Consequently, the solar cell can secure
firm adhesion of the collective electrode and the tab electrode
even after the soldering.
[0021] A method of manufacturing a solar cell is explained
according to FIG. 2 to FIG. 4.
[0022] In an embodiment, a semiconductor substrate, 201, having the
transparent electrodes, 202, as a preformed electrode is prepared
as illustrated in FIG. 2(a). The transparent electrode, 202, can be
formed at least on the front side of the semiconductor substrate,
201. However, it is also possible to form the transparent
electrode, 202, on the both of the front side and the back side as
illustrated in FIG. 2(a). The transparent electrode, 202, can be
formed by, for example, chemical vapor deposition (CVD) with
ITO.
[0023] Applying the conductive paste, 203, onto the transparent
electrode layer, 202, at the front side is carried out to form the
collective electrode. Applying can be carried out by screen
printing the conductive paste, 203, especially when forming a fine
line pattern. The pattern of the applied conductive paste, 203, can
be a comb shape that comprises at least one line of bus bar,
203(a), and many fine finger lines, 203(b) as shown FIG. 2(b). The
bus bar, 203(a), crosses over the finger lines, 203(b), to transfer
electricity which the semiconductor layer, 101, generates through
the transparent electrode, 202, and the finger lines, 203(b).
[0024] Viscosity of the conductive paste can be from 50 to 300
Pascal second at 10 rpm from the view point of applying method. For
example, when applying it by screen printing, viscosity can be from
100 to 300 Pascal second at 10 rpm. The viscosity can be measured
with a viscometer Brookfield HBT SSA14/6R using a spindle #14 at 10
rpm at room temperature.
[0025] Drying is then carried out in an oven or on a hotplate. The
drying temperature can be from 50 to 250.degree. C. The drying time
can be from 1 to 30 minutes. Under the drying temperature and time,
organic solvent can sufficiently evaporate to dry up. The
conductive paste would be then hardened to become an electrode
after drying up.
[0026] The thickness of the collective electrode, 203, can be at
least 10 .mu.m in an embodiment, at least 15 .mu.m in another
embodiment, at least 20 .mu.m in another embodiment. With such
thickness, the adhesion of the electrode can be sufficient as shown
in Example below. There is no limitation of the maximum thickness,
because the thicker an electrode thickness is, the lower resistance
an electrode could obtain. However, in consideration that a fine
pattern is required on a solar cell electrode, the thickness of the
electrode can be not larger than 100 .mu.m in an embodiment, not
larger than 80 .mu.m in another embodiment, not larger than 70
.mu.m in another embodiment.
[0027] A tab electrode, 204, for interconnecting solar cells can be
put on the bus bar, 203(a), as shown in FIG. 2(c). The tab
electrode, 204, is then covered with solder, 205, to adhere as
shown in FIG. 2(d). The solder, 205, can be heated at 200 to
300.degree. C. to melt and cover the bus bar, 203(a). When
soldering, the conductive paste could be adequately melt by the
heat to firmly adhere to the substrate. An amorphous saturated
polyester resin which is a thermoplastic in the conductive paste
would have effect on the adhesion. In an embodiment, a solder
ribbon comprising a copper ribbon as a tab electrode coated with a
solder paste can be used to have the forming steps simpler.
[0028] FIG. 3 illustrates another embodiment of a crystal type
solar cell. In the crystal type solar cell, a semiconductor
substrate having a collective electrode comprising at least a bus
bar, 302(a), and a finger line, 302(b), at the front side of the
semiconductor substrate, 301, is prepared as shown in FIG. 3(a).
The collective electrode, 302, can be formed by applying a
conductive paste comprising a conductive powder, glass powder, and
an organic medium and then firing the conductive paste at from
400.degree. C. to 900.degree. C. The preformed electrode is the
collective electrode, 302, in this case.
[0029] The conductive paste, 303, is then applied on the preformed
bus bar, 302(a). The conductive paste, 303, can be applied at least
partially onto the bus electrode. It is not necessary to apply it
onto the entire surface of the bus electrode. However, when
desiring to increase adhesion of a solar cell electrode, the
conductive paste, 303, can be applied entirely onto the bus bar,
302(a). Method of applying and drying the conductive paste, 304,
can be same as explained above.
[0030] A tab electrode, 304, is then put on the dried conductive
paste, 303, and then covered with solder, 305, to adhere as shown
in FIG. 3(c). The soldering method can be the same as explained
above.
[0031] The conductive paste can be used to form the bus bar at the
front side of the semiconductor substrate in another embodiment as
illustrated in FIG. 4. A semiconductor substrate, 401, having
finger lines, 402, at least on the one side which is the front side
of the semiconductor substrate, 401, is prepared as shown in FIG.
4(a). The preformed electrode is the finger lines, 402, in this
case.
[0032] The conductive paste, 403, is applied on the preformed
finger lines, 402, formed on the front side of the semiconductor
substrate, 401, as illustrated in FIG. 4(b). At this time, the
conductive paste, 403, crosses over the finger lines, 402, and
partially attaches to the semiconductor substrate, 401, or a
passivation layer in case that the passivation layer, for example a
silicon nitride layer, is formed on the semiconductor substrate. It
is observed in Example below that the conductive paste, 403,
sufficiently adheres not only to the preformed electrode but also
the silicon nitride layer.
[0033] The drying step is then carried out as explained above. A
tab electrode, 404, is put on the dried conductive paste, 403, and
then covered with solder, 405, to adhere as shown in FIG. 4(c). The
soldering method can be the same as explained above.
[0034] The conductive paste can be used at the back side of the
semiconductor substrate as illustrated in FIG. 5 in an embodiment.
A semiconductor substrate, 501, having a back side electrode, 502,
at the back side of the semiconductor substrate, 501, is prepared
as shown in FIG. 5(a). The backside electrode, 502, can be formed
by applying a conductive paste comprising at least a conductive
powder and an organic medium, and firing it. The backside
electrode, 502, can be formed on the most surface of the
semiconductor substrate where the back side would hardly receive
the sunlight. The conductive powder used in the conductive paste
for the backside electrode, 502, can be an aluminum powder. The
preformed electrode is the backside electrode, 502, in this
case.
[0035] The conductive paste, 503, can be applied on the preformed
back side electrode, 502, as illustrated in FIG. 5(b). A tab
electrode, 504, is put on the dried conductive paste, 503, and then
covered with solder, 505, to adhere as shown in FIG. 5(c). The
drying and soldering step can be carried out as explained
above.
[0036] The conductive paste can be used in an organic thin film
solar cell in another embodiment as illustrated in FIG. 6. A
semiconductor substrate, 601, having a transparent electrode, 602,
at the front side and having a backside electrode, 604, at the back
side of the semiconductor substrate, 601, is prepared as shown in
FIG. 6(a). A glass substrate as a supportive base is equipped at
the side of the transparent electrode, 602, of the semiconductor
substrate, 601. In this embodiment, the sunlight comes into the
semiconductor substrate, 601, from the side of the glass substrate,
603. The semiconductor layer, 601, can be formed by chemical vapor
deposition (CVD) with silicon with thickness of 100 .mu.m or
thinner. The transparent electrode, 602, can be formed by
sputtering with ITO. The back side electrode, 604, can be generally
formed by sputtering a metal such as silver, nickel or
aluminum.
[0037] A conductive paste, 605(a), can be applied onto a part of
the transparent electrode, 602, as illustrated in FIG. 6(b). In
this case, the preformed electrode is the transparent electrode,
602. A tab electrode, 606(a), is put on the applied conductive
paste, 605(a), and then adheres with solder, 607(a), as shown in
FIG. 6(c).
[0038] In another embodiment, the conductive paste, 605(b), can be
applied onto a part of the back side electrode, 604. as illustrated
in FIG. 6(b). In this case, the preformed electrode is the back
side electrode, 604. A tab electrode, 606(b), is put on the applied
conductive paste, 605(b), and then covered with solder, 607(b), to
adhere as shown in FIG. 6(c).
[0039] A method of applying the conductive paste in any embodiments
above is not limited, however, it can be, for example, screen
printing or nozzle discharging, offset printing method, doctor
blade method or transfer method.
[0040] The conductive paste used above embodiments is explained in
detail below. The conductive paste is a polymer type and contains
substantially no glass frit. Even if the glass frit is contained,
the amount can be 0.5 wt % or lower in an embodiment, 0.1 wt % or
lower in another embodiment, based on the total weight of the
conductive paste.
[0041] (Conductive Powder)
[0042] A conductive powder is a powder to transport electrical
current in an electrode.
[0043] The conductive powder can be a metal powder with electrical
conductivity 1.00.times.10.sup.7 Siemens (S)/m or more at 293
Kelvin in an embodiment. Such conductive powder can be selected
from the group consisting of iron (Fe; 1.00.times.10.sup.7 S/m),
aluminum (Al; 3.64.times.10.sup.7 S/m), nickel (Ni;
1.45.times.10.sup.7 S/m), copper (Cu; 5.81.times.10.sup.7 S/m),
silver (Ag; 6.17.times.10.sup.7 S/m), gold (Au; 4.17.times.10.sup.7
S/m), molybdenum (Mo; 2.10.times.10.sup.7 S/m), magnesium (Mg;
2.30.times.10.sup.7 S/m), tungsten (W; 1.82.times.10.sup.7 S/m),
cobalt (Co; 1.46.times.10.sup.7 S/m), zinc (Zn; 1.64.times.10.sup.7
S/m) and a mixture thereof.
[0044] The electrical conductivity of the conductive powder can be
3.00.times.10.sup.7 S/m or more at 293 Kelvin in another
embodiment. Such conductive powder can be selected from the group
consisting of Al, Cu, Ag and a mixture thereof. Electrical property
of a solar cell could be further improved by using these conductive
powders with relatively high electrical conductivity. The
conductive powder can be selected from the group consisting of Al,
Cu, Ni, Fe and a mixture thereof in another embodiment. These metal
powders that can be easily oxidized by heated at high temperature
such as 400.degree. C. could be suitable as a conductive powder in
the conductive paste which is not fired or cured at such high
temperature.
[0045] There is no limitation on shape of the conductive powder.
However, a flake type conductive powder, spherical type conductive
powder or a mixture thereof are generally often used. In an
embodiment, the conductive powder shape can be the flake type
conductive powder. The flake type conductive powder can increase
contact area of the each other, consequently it obtains sufficient
conductivity. The conductive powder can be also a mixture of
spherical powder and flaky powder.
[0046] Particle diameter (D50) of the conductive powder can be 0.1
to 10.0 .mu.m in an embodiment, 0.5 to 8 .mu.m in another
embodiment, 1 to 5 82 m in another embodiment. The particle
diameter within the range can be dispersed well in an organic
medium. The conductive powder comprising a mixture of two or more
of conductive powders with different particle diameters can be used
in an embodiment. The smaller particles can fill interspaces of the
larger particles so as to enhance electrode's conductivity. For
example, the conductive powder can be a mixture of a conductive
powder with the particle diameter of 0.1 to 3 .mu.m and a
conductive powder with the particle diameter of 4 to 10 .mu.m.
[0047] The average diameter (D50) is obtained by measuring the
distribution of the powder diameters by using a laser diffraction
scattering method with Microtrac model X-100.
[0048] The conductive powder can be of ordinary high purity of 99%
or higher. However, depending on the electrical requirements of the
electrode pattern, less pure one can also be used. The purity of
the conductive powder can be 95% or higher in another embodiment,
90% or higher in another embodiment.
[0049] The conductive powder can be 40 to 90 weight percent (wt %)
in an embodiment, 50 to 80 wt % in another embodiment, 55 to 70 wt
% in another embodiment, based on the total weight of the
conductive paste. Within the range of conductive powder content,
conductivity of the solar cell electrode can be sufficient.
[0050] (Amorphous Saturated Polyester Resin)
[0051] An amorphous saturated polyester resin is a compound
synthetically-prepared by polyesterification reaction of an acid
component such as polycarboxylic acid and an alcohol component such
as polyalcohol. The amorphous saturated polyester resin comprises a
disorder structure, for example, a structure of a polymer
main-chain polymer and side-chains polymer which disorderly
attached to the main-chain polymer.
[0052] The amorphous saturated polyester resin is meant to
designate a polyester which shows substantially no crystallization
and which does not present a melting point as measured by
Differential Scanning calorimetry, for example EXSTAR6000 from
Seiko Instruments Inc., with a heating gradient of 10.degree. C.
per minute.
[0053] A solar cell electrode can firmly adhere to a substrate by
using the amorphous saturated polyester resin. The amorphous
saturated polyester resin has thermoplastic property.
"Thermoplastic" herein is defined in the specification as a nature
to be deformed by an external force when heated at a temperature of
Tg of the polyester resin.
[0054] The acid component can be selected from the group consisting
of aliphatic dicarboxylic acid, aromatic dicarboxylic acid,
alicyclic dicarboxylic acid and a mixture thereof, in an
embodiment.
[0055] The aliphatic carboxylic acid can be selected from the group
consisting of azelaic acid, succinic acid, dimer acid, sebacic
acid, adipic acid, dodecanedioic acid, and a mixture thereof in an
embodiment.
[0056] The aromatic carboxylic acid can be selected from the group
consisting of isophthalic acid, terephthalic acid, orthophthalic
acid, hexahydrophthalic acid, maleic acid, benzene tricarboxylic
acid, benzene tetracarboxylic acid and a mixture thereof, in an
embodiment.
[0057] The alicyclic carboxylic acid can be cyclohexanedicarboxylic
acid or decanedicarboxylic acid, in an embodiment.
[0058] The alcohol component can be selected from the group
consisting of ethylene glycol, neopentyl glycol, butyl glycol,
propylene glycol, 1,5-Pentanediol, 1,6-Hexanediol, orthoxylene
glycol, paraxylene glycol, 1,4-Phenylene glycol, and bisphenol-A.
These alcohol ethylene oxide adduct can be also used. The amorphous
saturated polyester resin can be made by a known method such as
dehydration and condensation reaction of the acid component and the
alcohol component.
[0059] Average molecular weight of the amorphous saturated
polyester resin can be from 2,000 to 50,000. When the average
molecular weight is in the range, the paste viscosity could be
proper.
[0060] Glass transition temperature (Tg) of the amorphous saturated
polyester resin is 50.degree. C. or less. An amorphous saturated
polyester resin with Tg within the range can obtain sufficient
adhesion even after soldering as shown in Example below.
[0061] In an embodiment, the amorphous saturated polyester resin
can be at least 3 wt %, 4.5 wt % in another embodiment, 5 wt % in
another embodiment, 6 wt % in another embodiment, based on the
total weight of the conductive paste. The amorphous saturated
polyester resin can be not more than 20 wt % in an embodiment, not
more than 18 wt % in another embodiment, not more than 15 wt % in
another embodiment, not more than 10 wt % in another embodiment,
based on the total weight of the conductive paste. With such amount
of the amorphous saturated polyester resin, conductive powder and
glass frit could sufficiently disperse.
[0062] (Organic Solvent)
[0063] An organic solvent is an organic compound dissolving the
amorphous saturated polyester resin.
[0064] The organic solvent can comprise ketone solvent, aromatic
solvent, ester solvent, glycol ether solvent, glycol ether acetate
solvent, and terpenoid solvent, in an embodiment.
[0065] The ketone solvent can be selected from the group consisting
of acetone, methyl ethyl ketone, 1-Nonanal, methyl isobutyl ketone,
cyclohexanone and a mixture thereof.
[0066] The aromatic solvent can be selected from the group
consisting of toluene, xylene, benzene, ethyl benzene, trimethyl
benzene, alkyl benzene and a mixture thereof.
[0067] The ester solvent can be selected from the group consisting
of methyl acetate, ethyl acetate, propyl acetate, isopropyl
acetate, butyl acetate, isobutyl acetate, amyl acetate, isoamyl
acetate, and a mixture thereof.
[0068] The glycol ether solvent can be selected from the group
consisting of ethylene glycol monomethyl ether, ethylene glycol
monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol
monoethyl ether, diethylene glycol monobutyl ether, propylene
glycol monomethyl ether, propylene glycol n-butyl ether, propylene
glycol n-propyl ether, propylene glycol phenyl ether, dipropylene
glycol methyl ether, dipropylene glycol n-butyl ether, dipropylene
glycol n-propyl ether, dipropylene glycol methyl ether,
tripropylene glycol methyl ether, tripropylene glycol n-butyl
ether, and a mixture thereof.
[0069] The glycol ether acetate solvent can be selected from the
group consisting of ethylene glycol monomethyl ether acetate,
ethylene glycol monoethyl ether acetate, ethylene glycol monobutyl
ether acetate, diethylene glycol monoethyl ether acetate,
diethylene glycol monobutyl ether acetate, propylene glycol
monomethyl ether acetate, dipropylene glycol methyl ether acetate,
and a mixture thereof.
[0070] The Terpene solvent can be selected from the group
consisting of terpineol, limonene, pinene, and a mixture
thereof.
[0071] The organic solvent can comprise a glycol ether, glycol
ether acetate, alkyl benzene, terpineol or a mixture thereof, in an
embodiment. These organic solvent evaporates quickly.
[0072] Any other solvent can be added to the forementioned
solvents. A solvent to add can be an aliphatic solvent, an alcohol
solvent or a mixture thereof that is compatible with the above
forementioned solvents.
[0073] Flash point of the solvent is adjustable depending on how to
apply the conductive paste. For example, the flash point can be at
least 70.degree. C. when the conductive paste is applied by screen
printing.
[0074] Evaporation rate is also adjustable depending on how to
apply the conductive paste. For example, the evaporation rate which
is expressed by a relative value when that of butyl acetate is 100
can be not more than 50 when the conductive paste is applied by
screen printing.
[0075] Content of the organic solvent can be adjusted by type or
amount of the amorphous saturated polyester resin or the conductive
powder. The organic solvent can be 5 to 50 wt % in another
embodiment, 10 to 40 wt % in another embodiment, 15 to 30 wt % in
an embodiment, based on the total weight of the conductive paste.
With such content, the amorphous saturated polyester resin can be
dissolved sufficiently or a conductive powder can be dispersed in a
mixture of the resin and the solvent sufficiently.
[0076] (Additives)
[0077] Thickener, stabilizer, viscosity modifier, oxidation
inhibitor, UV absorber, or surfactant as additives can be added to
a conductive paste. Amount of the additive depends on a desired
characteristic of the resulting electrically conducting paste and
can be chosen by people in the industry. The additives can also be
added in multiple types.
EXAMPLES
[0078] The present invention is illustrated by, but is not limited
to, the following examples.
[0079] (Conductive Paste Preparation)
[0080] The conductive paste was produced using the following
materials. The weight percent (wt %) herein means weight percent
based on the total weight of the conductive paste unless especially
defined.
[0081] Conductive Powder: 60 wt % of silver (Ag) powder was used.
The shape was flake. The particle diameter (D50) was 3.0 .mu.m as
determined with a laser scattering-type particle size distribution
measuring apparatus (Microtrac model X-100).
[0082] Organic solvent: 32 wt % of Propylene glycol phenyl ether
(Dowanol PPh, Dow Chemical Company)
[0083] Amorphous saturated polyester resin: 8 wt % of the amorphous
saturated polyester resin was used. The amorphous saturated
polyester resin and Tg of the resin in the each example are shown
in Table 1.
[0084] The Tg was measured by following procedure with a
differential scanning calorimeter (DSC), EXSTAR6000 from Seiko
Instruments Inc. An aluminum pan on which 15 mg of the amorphous
saturated polyester resin was put was put into a cell in the DSC as
well as an aluminum pan as a blank sample. The temperature in the
cell was gradually raised up to 150.degree. C. by 10.degree. C./min
and cooled down to the room temperature of 25.degree. C. The
temperature was raised up to 150.degree. C. by 10.degree. C. /min
again as a second run. The first transition temperature in the
second temperature raise was recorded as the Tg and shown in Table
1. The Tg below 30.degree. C. is shown as <30.degree. C. because
it was not measurable in this measuring environment.
[0085] Weight-average molecular weight (Mw) was measured by
following procedure with a High performance liquid chromatography,
Waters LC600 from SpectraLab Scientific Inc.). The measuring
conditions are as follows. Column: Shodex GPC K-806L.times.2 from
SHOWA DENKO K.K.; Solvent: tetrahydrofuran; Flow Rate: 1.0 ml/min;
Column Temperature: 35.degree. C.; Detector: reflective index;
Injection Amount: 100 .mu.l, Sample Concentration: 0.20%; Standard:
Polystyrene. The measured weight-average molecular weights (Mw) are
shown in Table 1.
[0086] The conductive paste was prepared with the following
procedure. The amorphous saturated polyester resin and the organic
solvent were mixed in a glass vial for 48 hours at 100.degree. C.
The Ag powder was added to the mixture of the resin and solvent and
mixed for 5 minutes by a planetary centrifugal mixer to be a
conductive paste.
[0087] (Manufacture of Test Pieces)
[0088] The conductive paste was applied by doctor blade method onto
an indium titanium oxide (ITO) layer coated glass substrate. The
applying pattern of the conductive paste was square shape (50 mm
wide.times.50 mm long.times.58 .mu.m thick). The conductive paste
on the glass substrate was dried at 150.degree. C. for 5 minutes
and then further dried at 200.degree. C. for 12 minutes in an oven
(KOYO IR belt furnace) to form an electrode. Thickness of the
electrode varied from 29 to 47 .mu.m depending on the conductive
pastes.
[0089] (Measurement of Adhesion)
[0090] Adhesion of the electrode was measured by the following
procedures. The glass substrate with the electrode was put on a
90.degree. C. hot plate. A copper ribbon coated with a Sn/Pb solder
(Cu--O-155-2-B, Marusho. Co., Ltd.) was dipped into a soldering
flux (Kester-955, Kester, Inc.) and then dried for five seconds in
air. Half of the solder coated copper ribbon was placed on the
electrode and soldering was done by a soldering system (Hakko-926,
Hakko Corporation). The soldering iron setting temperature was
400.degree. C. and the actual temperature of the soldering iron at
the tip was from 230 to 240.degree. C. measured by K-type
thermocouple. The glass substrate with the soldered electrode was
taken off the hotplate, and cooled down to the room
temperature.
[0091] The rest part of the copper ribbon which did not adhere to
the electrode was horizontally folded and pulled at 120 mm/min by a
machine (Peel Force 606, MOGRL Technology Co., Ltd.). The strength
(Newton, N) at which the copper ribbon was detached was recorded as
adhesion of the electrode.
[0092] (Results)
[0093] The adhesion of each example is shown in Table 1. In example
1 to 5, the solar cell electrodes obtained sufficient adhesion over
0.5 N while comparative example 1 to 3 obtained zero adhesion.
TABLE-US-00001 TABLE 1 Amorphous saturated Electrode polyester
resin thickness Tg (.degree. C.) Mw (.mu.m) Adhesion (N) Com. ex. 1
72.2.sup.1) 57900 34 0 Com. ex. 2 61.8.sup.2) 49400 38 0 Com. ex 3
51.3.sup.3) 13200 47 0 Ex 1 47.9.sup.4) 13200 28 0.8 Ex 2
43.8.sup.5) 40800 28 2.1 Ex 3 39.4.sup.6) 50100 39 1.2 Ex 4
<30.0.sup.7) 65300 29 1.4 Ex 5 <30.0.sup.8) 76000 40 1.0
[0094] In turn, the effect of the electrode thickness was examined.
A solar cell electrode was prepared in the same manner of Example 3
except the applying pattern thickness of the conductive paste and
the substrate. The thickness of the applied conductive paste was 6,
13, 26, 39, 52 .mu.m respectively. The substrate was a crystal type
silicon wafer coated with a silicon nitride layer or an aluminum
(Al) layer instead of the glass substrate coated with the ITO
layer. The conductive paste was applied on the silicon nitride
layer or the Al layer. The electrode adhesion on the silicon
nitride was tested by assuming the conductive paste was applied as
a bus bar on the finger lines as illustrated in FIG. 4(b).
[0095] The adhesion was measured in the same manner for each of the
electrode. The results are shown in Table 2. The adhesion over 1 N
was obtained in from Example 6 to Example 9 where the conductive
paste was applied with thickness of 13, 26, 39 and 52 .mu.m thick
on the silicon nitride layer.
[0096] When formed on the aluminum layer, the adhesion over 1N was
obtained in Example 10 and Example 11 where the conductive paste
was applied with thickness of 39 and 52 .mu.m thick.
TABLE-US-00002 TABLE 2 Thickness Adhesion Substrate surface (.mu.m)
(N) Comparative example 4 Silicon nitride 6 0.2 Example 6 Silicon
nitride 13 1.5 Example 7 Silicon nitride 26 2.5 Example 8 Silicon
nitride 39 2 Example 9 silicon nitride 52 2.5 Comparative example 5
Aluminum 13 0 Comparative example 6 Aluminum 26 0 Example 10
Aluminum 39 3 Example 11 Aluminum 52 3.5
* * * * *