U.S. patent application number 12/630898 was filed with the patent office on 2010-04-01 for paste for solar cell electrode and solar cell.
This patent application is currently assigned to E. I. DU PONT DE NEMOURS AND COMPANY. Invention is credited to Takashi Kitagaki, Hiroki Kojo, Takuya Konno.
Application Number | 20100078603 12/630898 |
Document ID | / |
Family ID | 37438773 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100078603 |
Kind Code |
A1 |
Konno; Takuya ; et
al. |
April 1, 2010 |
PASTE FOR SOLAR CELL ELECTRODE AND SOLAR CELL
Abstract
In a paste for a solar cell light-receiving surface electrode
including silver particles, glass frit, resin binder, and thinner,
silver particles with a specific surface of 0.20-0.60 m.sup.2/g are
used as the silver particles. The silver particles are preferably
included at 80 mass % or more to the total amount of silver
particles being included in the paste.
Inventors: |
Konno; Takuya; (Tochigi-Ken,
JP) ; Kitagaki; Takashi; (Utsunomiya-Shi, JP)
; Kojo; Hiroki; (Tochigi, JP) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1122B, 4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Assignee: |
E. I. DU PONT DE NEMOURS AND
COMPANY
Wilmington
DE
|
Family ID: |
37438773 |
Appl. No.: |
12/630898 |
Filed: |
December 4, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12547814 |
Aug 26, 2009 |
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12630898 |
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12162509 |
Jul 29, 2008 |
7648730 |
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12547814 |
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Current U.S.
Class: |
252/514 |
Current CPC
Class: |
H05K 1/092 20130101;
H01B 1/22 20130101; Y02E 10/50 20130101; H01L 31/022425
20130101 |
Class at
Publication: |
252/514 |
International
Class: |
H01B 1/22 20060101
H01B001/22 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 2, 2006 |
JP |
2006-026340 |
Claims
1. A conductive paste for a solar cell light-receiving surface
electrode, comprising 70-90 wt % silver particles wherein 80 mass %
or more of the silver particles have specific surface area of
0.20-0.60 m.sup.2/g, 0.5-10.0 wt % of glass frit and resin binder,
all based upon the weight of the paste.
2. The paste for a solar cell light-receiving electrode of claim 1
wherein the average particle diameter of the silver particles is
0.1-14 um.
3. The paste for a solar cell light-receiving electrode of claim 1
wherein softening point of the glass frit is 450-550 C.
Description
[0001] This application is a division of U.S. patent application
Ser. Nos. 12/162,509 filed on Aug. 1, 2006 and 12/547,814 filed on
Aug. 26, 2009.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention pertains to a solar cell. More
specifically, the present invention pertains to an electrode paste
for a solar cell light-receiving surface being used in the
manufacture of an electrode in a solar cell and a solar cell being
obtained by using said paste.
[0004] 2. Description of Related Art
[0005] In manufacturing an electrode for a solar cell, the
electrode is formed at the side in which an anti-reflection film is
formed. The generally used method for manufacturing the electrode
is a method that spreads a paste containing glass frit, resin
binder, thinner, and if necessary, additives on an anti-reflection
film and [then] sinters it.
[0006] In order to raise the power generation characteristics in
the solar cell, the characteristics of the electrode are important.
For example, the power generation efficiency is raised by lowering
the resistance value of the electrode. In order to achieve this
purpose, various methods are proposed.
[0007] Silver particles are generally known as the
electroconductive metal in the electrode of the solar cell.
However, the relationship between the specific surface of the
silver particles being used and the power generation
characteristics is not deeply reviewed. In paragraph [0009] of
Japanese Kokai Patent Application No. 2003-257243, "BET diameter"
is defined as "the particle diameter attained, assuming that the
particles have a spherical shape, based on the specific surface
(m.sup.2/g) of the particles measured in the BET method." According
to said publication, the BET diameter of the silver particles being
used is 0.10-0.50 .mu.m. As application examples and comparative
examples, silver powders with a BET diameter of 0.02-0.50 .mu.m are
presented.
[0008] Here, if the silver density=10.5
g/cm.sup.3=10.5.times.10.sup.6 g/m.sup.3, the radius=R
(.mu.m)=R.times.10.sup.-6 m, and the specific surface=S
(m.sup.2/g), the following equation is established.
[0009] Weight of one silver
particle=4.pi.(R.times.10.sup.-6).sup.2/S=(4.pi.(R.times.10.sup.-6).sup.3-
/3).times.10.5.times.10.sup.6
If this equation is solved with respect to S,
S=3/(10.5.times.R).
[0010] Since the BET diameter is a diameter, the radius R is its
half. Considering this, if the specific surface of the silver
powder with a BET diameter of 0.02-0.50 .mu.m, it [specific
surface] is 1.1-28.6 m.sup.2/g.
[0011] BET diameter of 0.02 .mu.m=radius of 0.01 .mu.m=specific
surface of 28.6 m.sup.2/g
[0012] BET diameter of 0.10 .mu.m=radius of 0.05 .mu.m=specific
surface of 5.7 m.sup.2/g
[0013] BET diameter of 0.50 .mu.m=radius of 0.25 .mu.m=specific
surface of 1.1 m.sup.2/g
BRIEF SUMMARY OF THE INVENTION
[0014] The purpose of the invention is to provide an
electroconductive paste contributing to the improvement of the
power generation efficiency of a solar cell.
[0015] An embodiment of the present invention is a paste for a
solar cell light-receiving surface electrode including silver
particles with a specific surface of 0.20-0.60 m.sup.2/g, glass
frit, resin binder, and thinner. The silver particles are
preferably included at 80 mass % or more to the total amount of
silver particles being included in an electrode.
[0016] An embodiment of the present invention is a solar cell light
having an electrode surface that consists of a paste for a solar
cell-light receiving surface electrode including silver particles
with a specific surface of 0.20-0.60 m.sup.2/g, glass frit, resin
binder, and thinner. The silver particles, optimally includes 80
mass % or more to the total amount of silver particles being
included in the electrode.
[0017] One embodiment herein is a conductive paste for a solar cell
light-receiving surface electrode, comprising
70-90 wt % silver particles wherein 80 mass % or more of the silver
particles have specific surface area of 0.20-0.60 m.sup.2/g,
0.5-10.0 wt % of glass frit and resin binder, all based upon the
weight of the paste.
[0018] The solar cell having an electrode prepared using the paste
for a solar cell electrode of the present invention has excellent
power generation efficiency.
BRIEF DESCRIPTION OF THE DRAWING(S)
[0019] FIG. 1 explains the manufacturing processes in manufacturing
a solar cell using the electroconductive paste of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0020] It has been clarified that the power generation
characteristics of the solar cell being obtained are improved by
using silver particles with a small specific surface as the silver
being included as an electroconductive metal in the paste. The
present invention is based on this knowledge.
[0021] The present invention pertains to a paste for a solar cell
light-receiving surface electrode including silver particles with a
specific surface of 0.20-0.60 m.sup.2/g, glass frit, resin binder,
and thinner. The electroconductive paste of the present invention
is used in forming a light-receiving surface electrode (surface
side electrode) of a solar cell.
[0022] Next, each component of the electroconductive paste of the
present invention is explained.
[0023] 1. Electroconductive Metal
[0024] In the paste of the present invention, silver (Ag) particles
are used as an electroconductive metal. The silver particles of the
present invention have a specific surface smaller that of general
silver particles. Specifically, the specific surface of the silver
particles is 0.20-0.60 m.sup.2/g, preferably 0.25-0.50 m.sup.2/g.
The power generation characteristics tend to be improved with the
decrease of the specific surface, however if the specific surface
is too small, the adhesive strength is likely to be lowered. With
the use of silver particles with a small specific surface, the
power generation characteristics of the obtained solar cell are
improved.
[0025] The specific surface [measured] can be attained by a
BET-point method (JIS-Z-8830). An apparatus on the market may used,
and for example, it can be measured using NOVA3000 made by
Quantachrome Co. In the present invention, if a meaningful
difference is made in the specific surface by the measuring
apparatus, the value measured by NOVA3000 made by Quantachrome Co.
is adopted.
[0026] The content of the silver particles is not particularly
limited. One or two kinds of silver particles with a specific
surface [requirement] that is regulated in the present invention
can be used. Sometimes, other silver particles may also be used. In
case silver particles which deviate from the specific surface
[requirement] that is regulated in the present invention are used,
the amount being used is preferably 20 wt % or less [compared] to
the total amount of silver particles. In other words, the content
of the silver particles that meet the specific surface
[requirement] that is regulated in the present invention is 80 mass
% or more to the total amount of silver particles being included in
the paste. The content is preferably 90 mass % or more, more
preferably 95 mass % or more, and especially preferably 100 mass %.
The obtained power generation characteristics tend to be improved
with the increase of the content of the silver particles with a
specific surface [requirement] that is regulated in the present
invention.
[0027] The silver particles that meet the specific surface
[requirement] that is regulated in the present invention can be
manufactured by the atomizing method, wet-reducing method, etc. The
specific surface can be controlled by controlling the conditions in
each manufacturing method. In general, the specific surface can be
reduced by setting conditions in which the particle diameter is
increased.
[0028] It has been considered that it is preferable for the silver
particles used in the light-receiving surface of the electrode of
the solar cell to be fine and to have a large specific surface. The
reason for this is considered to be that fine silver particles with
a large specific surface are easily introduced into the depressions
and projections of the wafer surface to prevent the reflection. On
the other hand, according to these inventors, it was clarified that
the power generation characteristics of the solar cell being
obtained were improved by using silver particles with a small
specific surface. Specifically, the characteristics such as Eff:
conversion efficiency (%), Rs: serial resistance (.OMEGA.cm.sup.2),
FF: fill factor (%), Voc: open voltage (mV), Jsc: short circuit
current (mAcm.sup.2), and Rsh: shunt resistance (.OMEGA.cm.sup.2)
are improved. The present invention is based on this knowledge.
[0029] The particle diameter and the specific surface of the silver
particles have a fixed relation, and if the particle diameter of
the silver particles is large, the specific surface tends to be
decreased. For this reason, in obtaining the silver particles with
a small specific surface of the present invention, the particle
diameter is apt to be increased. Also, in case the silver particles
are used for a general electroconductive paste, the particle
diameter of the silver particles is not particularly limited in
terms of technical effects, however the particle diameter has an
influence on the sintering characteristics of the silver (for
example, the silver particles with a large particle diameter are
sintered at a speed slower than that of the silver particles with a
small particle diameter). Furthermore, it is necessary for the
silver particles to have a particle diameter suitable for a method
for spreading an electroconductive paste (for example, screen
printing).
[0030] In consideration of these requirements, the average particle
diameter of the silver particles is preferably 0.1-14 .mu.m, more
preferably 2.0-8.0 .mu.m. With the use of the silver particles with
such a particle diameter, a paste suitable for spreading of the
electroconductive paste can be formed. Also, silver particles with
excellent sintering characteristics are easily obtained. For
example, the average particle diameter is calculated as an average
particle diameter (50% point) by the measured value of LA-920 made
by Horiba Seisakusho K.K.
[0031] Usually, it is preferable for the silver to have a high
purity (99%+). However, a substance with low purity can also be
used by an electrical requirement of an electrode pattern.
[0032] The content of the silver particles in the paste is not
particularly limited, however the content is preferably 70-90 wt %
based on the weight of the paste.
[0033] 2. Glass Frit
[0034] It is preferable for the electroconductive paste of the
present invention to include a glass frit as an inorganic binder.
The glass binder usable in the present invention is a glass frit
with a softening point of 450-550.degree. C. so that the
electroconductive paste may be sintered at 600-800.degree. C.,
appropriately wetted, and appropriately adhered to a silicon
substrate. If the softening point is lower than 450.degree. C., the
sintering is advanced, and the effects of the present invention
cannot be sufficiently obtained. If the softening point is higher
than 550.degree. C., since a sufficient melting flow is not caused
during sintering, a sufficient adhesive strength is not exerted,
and the liquid-phase sintering of the silver cannot be accelerated
in some cases.
[0035] Here, the "softening point" is a softening point obtained by
the fiber stretch method (fiber elongation method) of ASTM
C338-57.
[0036] Since the chemical composition of the glass frit is not
important in the present invention, any glass frit being used in
electroconductive paste for an electronic material can be used. For
example, silver borosilicate glass, etc., can be appropriately
used. Silver silicate glass and the lead borosilicate glass are
excellent materials in the present invention in terms of both the
range of the softening point and the glass fusibility. In addition,
a leadless glass such as zinc borosilicate can also be used.
[0037] The content of the glass frit is not particularly limited as
long as it is an amount that can achieve the purpose of the present
invention, however the content is preferably 0.5-10.0 wt %, more
preferably 1.0-3.0 wt % based on the weight of the
electroconductive paste.
[0038] If the amount of glass frit is smaller than 0.5 wt %, the
adhesive strength is sometimes not sufficient [sic; sufficient]. If
the amount of glass frit is more than 10.0 wt %, a trouble is
sometimes caused in the soldering postprocess by glass rise,
etc.
[0039] 3. Resin Binder
[0040] The electroconductive paste of the present invention
includes a resin binder. In this application example, the "resin
binder" is a concept including a mixture of a polymer and a
thinner. Therefore, an organic liquid (also called a thinner) may
also be included in the resin binder. In the present invention, the
resin binder in which organic liquid is included is preferable, and
if the viscosity is high, if necessary, the organic liquid can be
separately added as a viscosity regulator.
[0041] In the present invention, an optional resin binder can be
used. In the present invention, a pine oil solution or ethyl
cellulose or ethylene glycol monobutyl ether monoacetate solution
of resin (polymethacrylate, etc.), terpineol solution of ethyl
cellulose, etc., can be mentioned. In the present invention, a
terpineol solution of ethyl cellulose (ethyl cellulose content=5-50
wt %) is preferably used. Also, in the present invention, a solvent
containing no polymers, such as water or organic liquid, can be
used as a viscosity regulator. As the usable organic liquid, for
example, alcohol, ester of alcohol (for example, acetate or
propionate), and terpene (for example, pine oil, terpineol, etc.)
can be mentioned.
[0042] The content of the resin binder is preferably 5-50 wt %
based on the weight of the electroconductive paste.
[0043] 4. Additives
[0044] In the electroconductive paste of the present invention, a
thickener and/or a stabilizer and/or other general additives may
also be added or may not be added. In case the additives are added,
tackifier (thickener), stabilizer, etc., can be added. Also, as
other general additives, dispersant, viscosity regulator, etc., can
be added. The amount of tackifier, etc., being added is changed in
accordance with the viscosity of the electroconductive paste,
however it can be appropriately determined by the concerned
party.
[0045] Also, several kinds of additives may be added.
[0046] As explained below, it is preferable for the
electroconductive paste of the present invention to have a
prescribed range of viscosity. In order to render an appropriate
viscosity to the electroconductive paste, if necessary, a tackifier
(thickener) can be added. As examples of the tackifier, the
above-mentioned substances can be mentioned. The amount of
tackifier, etc., being added is changed in accordance with the
viscosity of the final electroconductive paste and can be
appropriately determined by the party concerned.
[0047] The electroconductive paste of the present invention is
favorably manufactured by mixing each above-mentioned component
with a three-roller kneader. The electroconductive paste of the
present invention is preferably spread on a desired part of the
light-receiving surface of the solar cell by screen printing,
however in spreading it in such a printing, it is preferable to
have a prescribed range of viscosity. The viscosity of the
electroconductive paste of the present invention is preferably
50-300 PaS when it is measured at 10 rpm and 25.degree. C. by a
utility cup using a #14 spindle as a Brookfield HBT viscometer.
[0048] As mentioned above, the electroconductive paste of the
present invention is used to form an electrode mainly composed of
silver on the light-receiving surface of the solar cell. In other
words, the paste of the present invention is printed and dried on
the light-receiving surface of the solar cell. Separately, a back
face electrode composed of aluminum or silver is also formed on the
back face of the solar cell. These electrodes are preferably
simultaneously sintered.
[0049] Then, the solar cell of the present invention is explained.
The solar cell of the present invention has a light-receiving
surface electrode formed of sliver particles with a specific
surface of 0.20-0.60 m.sup.2/g, glass frit, resin binder, and
thinner. Preferably, the silver particles are included at 80 mass %
or more [compared] to the total amount of silver particles being
included in the electrode. The silver particles and the glass frit
are the same as those mentioned above, their explanation is
omitted.
[0050] An example for preparing the solar cell using the
electroconductive paste of the present invention is explained
referring to FIG. 1.
[0051] First, a Si substrate 102 is prepared. An electroconductive
paste 104 for a solder connection is spread on the back face of the
substrate by screen printing and dried (FIG. 1(a)). As the
electroconductive paste, a conventional silver conductive paste
containing silver particles, glass particles, and resin binder can
be used. Next, aluminum paste for the back face electrode for the
solar cell (there is no particular limitation as long as it is used
for the solar cell, however for example, PV333, PV322 (applicant of
the patent of this case)) 106, etc., are spread by screen printing,
etc., and dried (FIG. 1(b)). The drying temperature of each paste
is preferably 180.degree. C. or lower. Also, the film thickness of
each electrode of the back face is the thickness after drying, and
the thickness is preferably 20-40 .mu.m in the aluminum paste and
15-30 .mu.m in the electroconductive silver paste. Also, the
overlapping part of the aluminum paste and the electroconductive
silver paste is about 0.5-2.5 mm.
[0052] Next, an electroconductive paste 108 of the present
invention is spread by screen printing, etc., and dried (FIG.
1(c)). For the substrate obtained, the aluminum paste and the
electroconductive silver paste are simultaneously sintered at a
temperature of about 600-900.degree. C. for about 2-15 min in an
infrared sintering furnace, so that an intended solar cell can be
obtained (FIG. 1(d)).
[0053] The solar cell being used by using the electroconductive
paste of the present invention, as shown in FIG. 1(d), has an
electrode 110 formed of the electroconductive paste of the present
invention on the light-receiving surface (surface) of the substrate
(for example, Si substrate) 102 and has an Al electrode (first
electrode) 112 mainly composed of Al and a silver electrode (second
electrode) mainly composed of Ag on the back face.
EXAMPLES
1. Preparation of Electroconductive Paste
Application Example 1
[0054] A mixture containing silver particles with a specific
surface of 0.25 m.sup.2/g, Si.B.Pb.O system glass frit, and
sintering auxiliary material was prepared. A terpineol solution
containing 20% ethyl cellulose was added as an organic vehicle to
the mixture. Furthermore, terpineol was added as a thinner to
adjust the viscosity. The content of each component is shown in
Table I. The silver particles were 83.4 wt %, the glass frit was
1.6 wt %, the organic vehicle was 10.0 wt %, the sintering
auxiliary material was 3.5 wt %, and the terpineol added as a
thinner to adjust the viscosity was 0.9 wt %.
[0055] This mixture was premixed in a universal mixer and kneaded
by a three-roll kneader, so that a paste for a solar cell electrode
was obtained. The particle diameters, the contents, and the
characteristics of the materials used were shown in Table I.
Application Examples 2-4 and Comparative Examples 1-3
[0056] Pastes for a solar cell electrode were obtained similarly to
Application Example 1 except for changing the kind of silver
particles being used and the amount being used to the amounts shown
in Table I.
TABLE-US-00001 TABLE I Application Application Application
Application Comparative Comparative Comparative Example 1 Example 2
Example 3 Example 4 Example 1 Example 2 Example 3 Silver specific
0.25 0.30 0.51 0.59 0.62 0.84 .084 particles surface A (m.sup.2/g)
wt % 83.4 83.4 71.0 71.0 71.0 83.4 71.0 wt % vs 100.0 100.0 85.1
85.1 85.1 100.0 85.1 Ag total Silver Specific -- -- 1.9 1.9 1.9 --
1.9 particles surface B (m.sup.2/g) wt % -- -- 12.4 12.4 12.4 --
12.4 wt % vs -- -- 14.9 14.9 14.9 -- 14.9 Ag total Glass wt % 1.6
1.6 1.6 1.6 1.6 1.6 1.6 frit Organic wt % 10.0 10.0 10.0 10.0 10.0
10.0 10.0 vehicle Additive wt % 3.5 3.5 3.5 3.5 3.5 3.5 3.5 Thinner
wt % 0.9 0.9 0.9 0.9 0.9 0.9 0.9
3. Preparation of Solar Cell
[0057] Using four kinds of obtained pastes, solar cells were
manufactured. First, a Si substrate was prepared. An
electroconductive paste (silver paste) for a soldering connection
was spread on the back face of the Si substrate by screen printing
and dried. Next, an aluminum paste (PV333 (made by E.I. du Pont de
Nemours and Company)) for a back face electrode was spread in a
partially overlapping fashion with the dried silver paste by screen
printing and dried. The drying temperature of each paste was set to
120.degree. C. Also, the film thickness of each electrode of the
back face being spread [upon] was 35 .mu.m of the aluminum paste
and 20 .mu.m of the silver paste as a film thickness after
drying.
[0058] Furthermore, the paste of the present invention was spread
on the light-receiving side surface (surface) by screen printing
and dried. A printer made by Price Co., and a mask having an 8 inch
and 10 inch frame and a stainless wire with a mesh of 250 were
used. A pattern for evaluation for 1.5 in consisting of finger
lines with a width of 100 p and bus bars with a width of 2 mm was
employed, and the film thickness was 13 .mu.m after sintering.
[0059] Next, for the obtained substrate, the spread pastes were
simultaneously sintered under the conditions of a peak temperature
of about 730.degree. C. and IN-OUT for about 5 min in an infrared
sintering furnace, so that intended solar cells were obtained.
[0060] The solar cells being obtained using the electroconductive
pastes of the present invention, as shown in FIG. 1, have the Ag
electrode 110 on the light-receiving surface (surface) of the
substrate 102 (for example, Si substrate) and the Al electrode
(first electrode) 112 mainly composed of Al on the back face and
the silver electrode (second electrode) 114 mainly composed of
Ag.
4. Evaluation of Cells
[0061] The electric properties (I-V characteristic) of the solar
cell substrates obtained were evaluated by a cell tester. As the
cell tester, an equipment (NCT-M-150AA) made by NPC Co., was
used.
[0062] Eff: conversion efficiency (%) and Rs: serial resistance
(.OMEGA.cm.sup.2) were measured. The higher Eff, the more excellent
power generation performances as a solar cell. The lower Rs, the
more excellent power generation performances as a solar cell. The
results are shown in Table II. The numerical values of each
electric property shown in Table II are the average of measured
values of 5 sheets of solar cell substrate samples and are relative
values of the case where each numerical value of Comparative
Example 1 was assumed as 1.
TABLE-US-00002 TABLE II Application Application Application
Application Comparative Comparative Comparative Example 1 Example 2
Example 3 Example 4 Example 1 Example 2 Example 3 Eff 1.123 1.221
1.034 1.048 1 0.641 0.464 Rs 0.552 0.432 0.817 0.799 1 2.492
5.611
[0063] As mentioned above, the characteristics of the obtained
solar cells are improved using the silver particles with a small
specific surface.
* * * * *