U.S. patent application number 13/894908 was filed with the patent office on 2014-09-25 for electrically conductive paste for front electrode of solar cell and preparation method thereof.
This patent application is currently assigned to Soltrium Technology, LTD. Shenzhen. The applicant listed for this patent is Ran Guo, Delin Li, Xiaoli Liu. Invention is credited to Ran Guo, Delin Li, Xiaoli Liu.
Application Number | 20140287583 13/894908 |
Document ID | / |
Family ID | 50253409 |
Filed Date | 2014-09-25 |
United States Patent
Application |
20140287583 |
Kind Code |
A1 |
Liu; Xiaoli ; et
al. |
September 25, 2014 |
ELECTRICALLY CONDUCTIVE PASTE FOR FRONT ELECTRODE OF SOLAR CELL AND
PREPARATION METHOD THEREOF
Abstract
The present invention provides an electrically conductive paste
for a front electrode of a solar cell and a preparation method
thereof. The electrically conductive paste is composed of a
corrosion binder, a metallic powder and an organic carrier. The
corrosion binder is one or more glass-free Pb--Te based crystalline
compounds having a fixed melting temperature in a range of
440.degree. C. to 760.degree. C. During a sintering process of the
electrically conductive paste for forming an electrode, the
corrosion binder is converted into a liquid for easily corroding
and penetrating an antireflective insulating layer on a front side
of the solar cell, so that a good ohmic contact is formed. At the
same time, the electrically conductive metallic powder is wetted,
and the combination of the metallic powder is promoted. As a
result, a high-conductivity front electrode of a crystalline
silicon solar cell is formed.
Inventors: |
Liu; Xiaoli; (ShenZhen,
CN) ; Guo; Ran; (ShenZhen, CN) ; Li;
Delin; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Liu; Xiaoli
Guo; Ran
Li; Delin |
ShenZhen
ShenZhen
San Jose |
CA |
CN
CN
US |
|
|
Assignee: |
Soltrium Technology, LTD.
Shenzhen
Shenzhen
CN
|
Family ID: |
50253409 |
Appl. No.: |
13/894908 |
Filed: |
June 21, 2013 |
Current U.S.
Class: |
438/660 ;
252/512; 252/513; 252/514 |
Current CPC
Class: |
H01L 31/1864 20130101;
H01L 29/456 20130101; H01B 1/22 20130101; H01L 31/1804 20130101;
H01L 31/02168 20130101; H01L 31/068 20130101; Y02E 10/50 20130101;
Y02E 10/547 20130101; H01L 31/022425 20130101; C09D 5/24 20130101;
H01L 31/028 20130101; H01B 1/16 20130101 |
Class at
Publication: |
438/660 ;
252/512; 252/513; 252/514 |
International
Class: |
H01L 21/288 20060101
H01L021/288 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 22, 2013 |
CN |
PCT/CN2013/073098 |
Claims
1. An electrically conductive paste for a front electrode of a
solar cell, the electrically conductive paste comprising: a
metallic powder having a weight ratio of 70 wt % to 95 wt % based
on a given total weight; a corrosion binder having a weight ratio
of 0.5 wt % to 12 wt % based on the given total weight; and an
organic carrier having a weight ratio of 5 wt % to 25 wt % based on
the given total weight; wherein the corrosion binder is a
glass-free Pb--Te--O crystal compound having a fixed melting
temperature between 440.degree. C. and 760.degree. C.; and the
metallic powder and the corrosion binder are randomly dispersed in
the organic carrier.
2. The electrically conductive paste of claim 1 wherein the
corrosion binder comprises one compound selected from
PbTe.sub.4O.sub.9, PbTeO.sub.3.0.33H.sub.2O, PbTeO.sub.3,
PbTeO.sub.4, PbTe.sub.3O.sub.7, PbTe.sub.5O.sub.11,
Pb.sub.2TeO.sub.4, Pb.sub.2Te.sub.3O.sub.7,
Pb.sub.2Te.sub.3O.sub.8, Pb.sub.3TeO.sub.5, Pb.sub.3TeO.sub.6,
Pb.sub.3Te.sub.2O.sub.8.H.sub.2O, Pb.sub.4Te.sub.1.5O.sub.7,
Pb.sub.5TeO.sub.7, Pb.sub.5TeO.sub.7,
Pb.sub.6Te.sub.5O.sub.18.5H.sub.2O, PbTe.sub.2O.sub.5,
PbH.sub.4TeO.sub.6, PbTeCO.sub.5 and Pb.sub.3TeN.sub.2O.sub.8
crystal compounds or any combination of above compounds.
3. The electrically conductive paste of claim 1 wherein the
corrosion binder is a glass-free crystalline compound having a
fixed melting point between 460.degree. C. and 650.degree. C.,
which is converted from a solid state into a liquid state when
temperature reaches or exceeds the melting point substantially
without any glass-softening transition.
4. The electrically conductive paste of claim 1 wherein the
corrosion binder comprises a plurality of dispersed glass-free
crystal particles having shapes selected from sphere, droplet,
aciculate, dendritic-shape, massive, spherical-shape, flake,
granular-shape, and colloidal-particle-shape and sizes ranging from
0.1 .mu.m to 15.0 .mu.m.
5. The electrically conductive paste of claim 1 wherein the
metallic powder comprises one or more metal materials selected from
silver, gold, platinum, copper, iron, nickel, zinc, titanium,
cobalt, chromium, manganese, palladium, and rhodium.
6. The electrically conductive paste of claim 1 wherein the
metallic powder comprises a plurality of particles made from one or
more metals selected from copper, iron, nickel, zinc, titanium,
cobalt, chromium, aluminum and manganese, each particle being
associated with a thickness of silver coating ranging from 10 nm to
2,000 nm.
7. The electrically conductive paste of claim 1 wherein the
metallic powder is a mixture of a first powder without silver
coating and a second powder with silver coating; wherein the first
powder without silver coating comprises a first plurality of
particles made from one or more metals selected from silver, gold,
platinum, copper, iron, nickel, zinc, titanium, cobalt, chromium,
manganese, palladium and rhodium, and the second powder with silver
coating comprises a second plurality of particles made from one or
more metals selected from copper, iron, nickel, zinc, titanium,
cobalt, chromium, aluminum and manganese with each particle being
coated with a silver layer ranging from 10 nm to 2,000 nm; wherein
a weight ratio of the first powder without silver coating to the
second powder with silver coating is in a range of 5:95 to
95:5.
8. The electrically conductive paste of claim 1 wherein the
metallic powder comprises a plurality of particles having sizes
ranging from 0.1 .mu.m to 5.0 .mu.m.
9. A method for forming a conductive paste comprising: providing a
plurality of metal particles with a weight composition ranging from
70 wt % to 95 wt % based on a predetermined total weight; providing
an organic carrier with a weight composition ranging from 5 wt % to
25 wt % based on the predetermined total weight; providing a
corrosion binder made from a plurality of glass-free
Pb--Te--O-based crystalline particles with a weight composition
ranging from 0.5 to 12 wt % based on the predetermined total
weight; mixing the plurality of metal particles, the corrosion
binder, and the organic carrier to form a mixture material; and
grinding the mixture materials to obtain a conductive paste.
10. The method of claim 9 wherein the corrosion binder comprises
one or a combination of two or more selected from the following
glass-free Pb--Te--O based crystalline compounds:
PbTe.sub.4O.sub.9, PbTeO.sub.3.0.33H.sub.2O, PbTeO.sub.3,
PbTeO.sub.4, PbTe.sub.3O.sub.7, PbTe.sub.5O.sub.11,
Pb.sub.2TeO.sub.4, Pb.sub.2Te.sub.3O.sub.7,
Pb.sub.2Te.sub.3O.sub.8, Pb.sub.3TeO.sub.5, Pb.sub.3TeO.sub.6,
Pb.sub.3Te.sub.2O.sub.8.H.sub.2O, Pb.sub.4Te.sub.1.5O.sub.7,
Pb.sub.5TeO.sub.7, Pb.sub.5TeO.sub.7,
Pb.sub.6Te.sub.5O.sub.18.5H.sub.2O, PbTe.sub.2O.sub.5,
PbH.sub.4TeO.sub.6, PbTeCO.sub.5 and Pb.sub.3TeN.sub.2O.sub.8,
characterized by a fixed melting point between 440.degree. C. and
760.degree. C.
11. The method of claim 9 wherein providing a corrosion binder
comprises: mixing a telluric acid solution and a lead acetate
solution to form a mixed solution, wherein the molar ratio of Te to
Pb in the mixed solution is in a range of 0.1:10 to 10:0.1;
stirring the mixed solution at temperature between 80.degree. C.
and 120.degree. C. using a stirring speed ranging from 1,000 to
1,500 r/min for 2 to 5 hours, to generate a precipitate; collecting
the precipitate as a solid material through solid-liquid
separation; washing the solid material using filtered water, till
that a pH value of the filtrated water is in a range of 5 to 7;
drying the solid material at about 150.degree. C. for 2 to 3 hours,
to obtain the Pb--Te based crystalline compound; and pulverizing
the Pb--Te based crystalline compound to obtain the plurality of
glass-free Pb--Te--O-based crystalline particles.
12. The method of claim 9 wherein providing a corrosion binder
comprises: introducing Pb.sub.xTe.sub.y alloy vapor into a reaction
chamber filled with oxygen atmosphere; reacting the
Pb.sub.xTe.sub.y alloy vapor with oxygen at a temperature ranging
from 1,000.degree. C. to 1,400.degree. C. for 1 to 4 hours to form
a reaction product; cooling the reaction product naturally to
25.degree. C. to obtain a glass-free Pb--Te--O based crystalline
compound; and pulverizing the glass-free Pb--Te based crystalline
compound to obtain the plurality of glass-free Pb--Te--O-based
crystalline particles.
13. The method of claim 9, wherein providing a corrosion binder
comprises: heating a tellurium oxide and a lead oxide in a
non-reducing atmosphere comprising oxygen, air, nitrogen, and argon
gas, to a temperature between 700.degree. C. and 1,000.degree. C.
to form a reaction product; cooling the reaction product naturally
in air to 25.degree. C. to obtain glass-free Pb--Te--O-based
crystal compounds; pulverizing the glass-free Pb--Te--O-based
crystal compounds to small chunks; and grinding the small chunks to
obtain the plurality of glass-free Pb--Te--O-based crystalline
particles.
14. The method of claim 9, wherein providing a corrosion binder
comprises: melting a tellurium oxide and a lead oxide in a vacuum
atmosphere, at a temperature between 700.degree. C. and
1,000.degree. C. to from a product material; cooling the product
material naturally to 25.degree. C.; and pulverizing and grinding
the product material to obtain the plurality of glass-free
Pb--Te--O based crystalline particles.
15. The method of claim 9 wherein the plurality of glass-free
Pb--Te--O based crystalline particles has particle sizes ranging
from 0.1 .mu.m to 15.0 .mu.m.
16. The method of claim 9 wherein the plurality of metal particles
comprises one or more metals selected from silver, gold, platinum,
copper, iron, nickel, zinc, titanium, cobalt, chromium, aluminum,
manganese, palladium and rhodium.
17. The method of claim 9 wherein the plurality of metal particles
comprises one or more metals selected from copper, iron, nickel,
zinc, titanium, cobalt, chromium, aluminum and manganese and
respectively coated with a thickness of silver ranging from 10 nm
to 2,000 nm.
18. The method of claim 9 wherein the plurality of metal particles
is a mixture of a first powder without silver coating and a second
powder with silver coating; wherein the first powder without silver
coating comprises a first plurality of particles made from one or
more metals selected from silver, gold, platinum, copper, iron,
nickel, zinc, titanium, cobalt, chromium, manganese, palladium and
rhodium, and the second powder with silver coating comprises a
second plurality of particles made from one or more metals selected
from copper, iron, nickel, zinc, titanium, cobalt, chromium,
aluminum and manganese with each particle being coated with a
silver layer ranging from 10 nm to 2,000 nm; wherein a weight ratio
of the first powder without silver coating to the second powder
with silver coating is in a range of 5:95 to 95:5.
19. The method of claim 9 wherein the plurality of metal particles
has particle sizes ranging from 0.1 .mu.m to 5.0 .mu.m.
20. A method for manufacturing a front electrode of a semiconductor
device, the method comprising: providing a semiconductor device
including an insulation surface coating; printing an electrically
conductive paste overlying a patterned contact region of the
insulation surface coating, the electrically conductive paste
comprising: a metallic powder with a weight composition ranging
from 70 to 95 wt % based on a given total weight of the
electrically conductive paste; a corrosion binder made from a
plurality of glass-free Pb--Te--O-based crystalline particles with
a weight composition ranging from 0.5 to 12 wt % based on the given
total weight; an organic carrier with a weight composition ranging
from 4.5 to 25 wt % based on the given total weight; wherein the
corrosion binder is one or a combination of two or more glass-free
Pb--Te--O based crystalline compounds, having a fixed melting
temperature in a range of 440.degree. C. to 760.degree. C.;
sintering the electrically conductive paste overlying the patterned
contact region of the insulation surface coating, wherein the
sintering comprises: drying the electrically conductive paste at a
first temperature range from 180.degree. C. to 260.degree. C. for
30 s up to 70 s; heating up to a second temperature range from
720.degree. C. to 950.degree. C. for 20 s up to 50 s; and cooling
back to 25.degree. C. to form an electrode; wherein the drying and
heating from the first temperature range to the second temperature
range are associated with releasing of the organic carrier, melting
of the corrosion binder at the fixed melting temperature after the
releasing of the organic carrier, and forming of a metallic bulk
from the metallic powder wet by molten corrosion binder; wherein
the molten corrosion binder induces etch-removing of the insulation
surface coating at the patterned contact region to form an ohmic
contact between the metallic bulk and the crystalline silicon solar
cell.
21. The method of claim 20 wherein the corrosion binder comprises
one or a combination of two or more selected from the following
glass-free Pb--Te--O based crystalline compounds:
PbTe.sub.4O.sub.9, PbTeO.sub.3.0.33H.sub.2O, PbTeO.sub.3,
PbTeO.sub.4, PbTe.sub.3O.sub.7, PbTe.sub.5O.sub.11,
Pb.sub.2TeO.sub.4, Pb.sub.2Te.sub.3O.sub.7,
Pb.sub.2Te.sub.3O.sub.8, Pb.sub.3TeO.sub.5, Pb.sub.3TeO.sub.6,
Pb.sub.3Te.sub.2O.sub.8.H.sub.2O, Pb.sub.4Te.sub.1.5O.sub.7,
Pb.sub.5TeO.sub.7, Pb.sub.5TeO.sub.7,
Pb.sub.6Te.sub.5O.sub.18.5H.sub.2O, PbTe.sub.2O.sub.5,
PbH.sub.4TeO.sub.6, PbTeCO.sub.5 and Pb.sub.3TeN.sub.2O.sub.8.
22. The method of claim 20 wherein the metallic powder comprises
one or more metals selected from silver, gold, platinum, copper,
iron, nickel, zinc, titanium, cobalt, aluminum, chromium,
manganese, palladium and rhodium.
23. The method of claim 20 wherein the metallic powder comprises a
plurality of particles made from one or more metals selected from
copper, iron, nickel, zinc, titanium, cobalt, chromium, aluminum
and manganese and respectively coated with silver.
24. The method of claim 20 wherein the metallic powder is a mixture
of a first powder without silver coating and a second powder with
silver coating; wherein the first powder without silver coating
comprises a first plurality of particles made from one or more
metals selected from silver, gold, platinum, copper, iron, nickel,
zinc, titanium, cobalt, chromium, manganese, palladium and rhodium,
and the second powder with silver coating comprises a second
plurality of particles made from one or more metals selected from
copper, iron, nickel, zinc, titanium, cobalt, chromium, aluminum
and manganese with each particle being coated with a silver layer
ranging from 10 nm to 2,000 nm; wherein a weight ratio of the first
powder without silver coating to the second powder with silver
coating is in a range of 5:95 to 95:5.
25. The method of claim 20 wherein the metallic powder comprises a
plurality of particles having particle ranging from 0.1 .mu.m to
5.0 .mu.m.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority to a PCT Application No.
PCT/CN2013/073098, filed on Mar. 22, 2013, commonly assigned and
incorporated as reference for all purposes.
[0002] The subject matter of the present application is related to
China Patent Application No. 201210360864.5, filed on Sep. 25,
2012, by Ran Guo, U.S. patent application Ser. No. 13/730,939,
filed on Dec. 28, 2012, by Ran Guo, and U.S. patent application
Ser. No. 13/787,997, filed on Mar. 7, 2013, by Xiaoli Liu et al.,
commonly assigned and incorporated by reference herein to their
entireties for all purposes.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to the technical field of
solar cells, and more particularly to an electrically conductive
paste for a front electrode of a solar cell and a preparation
method thereof.
[0004] Solar energy is an inexhaustible source of clean energy.
With the increasing depletion of coal, oil and other non-renewable
energy, development and use of solar energy have become a big
trend. Use of solar cells is a typical means of using solar energy,
and crystalline silicon solar cells for which industrial production
has been achieved are one type of solar cells.
[0005] As the most important core part of crystalline silicon solar
cells, a cell sheet needs to collect and export a current generated
under light irradiation, so two electrodes need to be fabricated on
a front side and a back side of the cell sheet. Many methods can be
used to fabricate the electrodes, among which screen printing and
co-sintering are the most commonly used production processes
currently.
[0006] In a crystalline silicon solar cell, an electrically
conductive paste for a front electrode, an electrically conductive
paste for a back electrode, and a paste for an aluminum
back-surface field are coated on a silicon chip by adopting screen
printing, and a front electrode is formed on the front side of the
silicon chip through co-sintering.
[0007] The co-sintered electrode of a crystalline silicon solar
cell is required to have strong adhesion, have no ash falling and
no deformation of the silicon chip, and be easy to weld and
convenient to collect and export the current generated under light
irradiation by means of a wire. Compared with the electrically
conductive paste for a back electrode of the silicon solar cell,
the electrically conductive paste for a front electrode of a
silicon solar cell is required to have the ability to penetrate the
silicon nitride antireflective layer.
[0008] In the prior art, the electrically conductive paste for the
front electrode of a crystalline silicon solar cell is composed of
a silver powder, a glass frit, an additive and an organic carrier.
The glass frit, as an inorganic adhesive, binds the
high-conductivity silver powder and the silicon substrate together,
and during co-sintering, the molten glass frit etches and
penetrates the silicon nitride antireflective layer, so that a good
contact is formed between the silver powder and the silicon
substrate. Generally, the glass frit in the paste has the following
effects: (1) wetting the metallic powder to promote the sintering
of the metallic powder; and (2) etching the antireflective layer to
promote the contact of the metal and the silicon surface and ensure
the binding effect between the metal and the silicon surface. In
order to achieve a good ohmic contact of the metallic powder and
the silicon surface, the antireflective layer is required to be
etched through but not penetrate into a P--N junction region. In
the selection of the glass frit, the composition, softening point,
thermal expansion coefficient, wetting properties and amount will
affect the physical and chemical changes in the sintering process,
thereby affecting the performance of the solar cell. In the
sintering process, the glass frit is gradually softened, and within
a short process cycle, usually 1 to 2 minutes, part of the softened
glass frit remains around the metallic powder and flows, and the
other part of the softened glass frit flows to reach the
antireflective layer at the bottom and induces a reaction. If the
content of the glass frit is low, full contact and complete
reaction of the glass frit and the antireflective layer cannot be
ensured. If it is ensured that the antireflective layer is
completely penetrated, a sufficient amount of the glass frit needs
to be added. The higher the amount of the glass frit is, the lower
the relative content of the electrically conductive metallic phase
is, and the lower the probability of contact of metallic particles
is, resulting in serious deterioration of conductivity. If a glass
frit with a low softening point such as a softening point of lower
than 400.degree. C. is selected to ensure that a sufficient amount
of glass frit is deposited on the surface of the antireflective
layer in the entire process, and react with the antireflective
layer completely. But excessively-early softening of the glass frit
can clog the communicating pores in the metallic powder, thereby
hindering the effective discharge of the organic carrier.
[0009] Presently, a Pb--Si based glass material is widely used as
the glass frit in the front electrode paste. At the same time, Pb
oxide, Te oxide and other oxides or fluorides are used to go
through a series of processes of melting, mixing and quenching, to
prepare a Pb--Te--O glass material. However, regardless the use of
various glass frit materials, due to restrictions of the physical
properties of the glass frit, the above technical problems still
exist, resulting in process difficulties with narrow windows in
preparation of a suitable glass frit and a subsequent conductive
paste. Therefore, improved techniques are desired for the
manufacture of an electrically conductive paste for forming front
electrodes of semiconductor devices.
BRIEF SUMMARY OF THE INVENTION
[0010] The objective of the present invention is to improve an
electrically conductive paste for the manufacture of an electrode
on a semiconductor surface. The electrically conductive paste is
characterized by a strong adhesion property by adding a
full-crystal-based corrosion binder free of any glass frit for
facilitating a formation of an electrode with excellent
metal-semiconductor electrical contact. In particular, the
electrically conductive paste can be applied for forming a front
electrode of a silicon-based solar cell with overall enhanced light
conversion efficiency. An alternative objective of the present
invention is to provide a method of making the electrically
conductive paste free-from any glass frit in additives using a
simple process with easy-controlled conditions and reduced
production cost.
[0011] In a specific embodiment, the present invention provides an
electrically conductive paste for a front electrode of a solar
cell. The electrically conductive paste includes a metallic powder
having a weight ratio of 70 wt % to 95 wt % based on a given total
weight. Additionally, the electrically conductive paste includes a
corrosion binder having a weight ratio of 0.5 wt % to 12 wt % based
on the given total weight. Moreover, the electrically conductive
paste includes an organic carrier having a weight ratio of 5 wt %
to 25 wt % based on the given total weight. The corrosion binder is
a glass-free Pb--Te--O crystal compound having a fixed melting
temperature between 440.degree. C. and 760.degree. C. The metallic
powder and the corrosion binder are randomly dispersed in the
organic carrier.
[0012] In another specific embodiment, the present invention
provides a method for forming a conductive paste. The method
includes providing a plurality of metal particles with a weight
composition ranging from 70 wt % to 95 wt % based on a
predetermined total weight. The method further includes providing
an organic carrier with a weight composition ranging from 5 wt % to
25 wt % based on the predetermined total weight. Additionally, the
method includes providing a corrosion binder made from a plurality
of glass-free Pb--Te--O-based crystalline particles with a weight
composition ranging from 0.5 to 12 wt % based on the predetermined
total weight. The method further includes mixing the plurality of
metal particles, the corrosion binder, and the organic carrier to
form a mixture material. Furthermore, the method includes grinding
the mixture materials to obtain a conductive paste.
[0013] In an alternative embodiment, the present invention provides
methods for making the electrically conductive paste, which is to
mix the metallic powder, corrosion binder, and organic carrier
followed by grinding so that the metallic powder and the corrosion
binder are uniformly randomly dispersed in the organic carrier. The
corrosion binder of the present invention is a glass-free Pb--Te--O
based crystalline compound, which is made by using one of the
following methods: chemical reaction method, chemical vapor phase
method, high-temperature melting reaction method, wet method and
vacuum melting method. The corrosion binder of the present
invention includes one or a combination of two or more selected
from the following glass-free Pb--Te--O based crystalline
compounds: PbTe.sub.4O.sub.9, PbTeO.sub.3.0.33H.sub.2O,
PbTeO.sub.3, PbTeO.sub.4, PbTe.sub.3O.sub.7, PbTe.sub.5O.sub.11,
Pb.sub.2TeO.sub.4, Pb.sub.2Te.sub.3O.sub.7,
Pb.sub.2Te.sub.3O.sub.8, Pb.sub.3TeO.sub.5, Pb.sub.3TeO.sub.6,
Pb.sub.3Te.sub.2O.sub.8.H.sub.2O, Pb.sub.4Te.sub.1.5O.sub.7,
Pb.sub.5TeO.sub.7, Pb.sub.5TeO.sub.7, Pb.sub.6Te.sub.5O.sub.18.
5H.sub.2O, PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5 and
Pb.sub.3TeN.sub.2O.sub.8. The corrosion binder is a power with
particles at least in one shape selected from sphere, droplet,
aciculate, dendritic-shape, massive, spherical-shape, flake,
granular-shape, and colloidal-particle-shape. The particles have
sizes in a range of 0.1 to 5.0 .mu.m in most applications, but has
sizes in a range of 0.1 to 15.0 .mu.m in some applications.
[0014] In another alternative embodiment, the present invention
provides a method for manufacturing a front electrode of a
semiconductor device. The method includes providing a semiconductor
device including an insulation surface coating and printing an
electrically conductive paste overlying a patterned contact region
of the insulation surface coating. The electrically conductive
paste includes a metallic powder with a weight composition ranging
from 70 to 95 wt % based on a given total weight of the
electrically conductive paste, a corrosion binder made from a
plurality of glass-free Pb--Te--O-based crystalline particles with
a weight composition ranging from 0.5 to 12 wt % based on the given
total weight, and an organic carrier with a weight composition
ranging from 4.5 to 25 wt % based on the given total weight. The
corrosion binder is one or a combination of two or more glass-free
Pb--Te--O based crystalline compounds, having a fixed melting
temperature in a range of 440.degree. C. to 760.degree. C. The
method further includes sintering the electrically conductive paste
overlying the patterned contact region of the insulation surface
coating. The sintering process includes a step of drying the
electrically conductive paste at a first temperature range from
180.degree. C. to 260.degree. C. for 30 s up to 70 s. The sintering
process further includes a step of heating up to a second
temperature range from 720.degree. C. to 950.degree. C. for 20 s up
to 50 s and a step of cooling back to 25.degree. C. to form an
electrode. The drying and heating from the first temperature range
to the second temperature range are associated with releasing of
the organic carrier, melting of the corrosion binder at the fixed
melting temperature after the releasing of the organic carrier, and
forming of a metallic bulk from the metallic powder wet by molten
corrosion binder. The molten corrosion binder induces etch-removing
of the insulation surface coating at the patterned contact region
to form an ohmic contact between the metallic bulk and the
crystalline silicon solar cell.
[0015] The electrically conductive paste used for the manufacture
of front side electrodes on solar cell light receiving surface
comprises full-crystal corrosion binder and free from any glass
material as a binding additive in the paste. By controlling proper
metal-oxide materials with selective weight ratio among several
ingredients and crystal particle sizes during the preparation of
the full-crystal corrosion binder, and further by controlling the
way of mixing with metallic powder and organic carrier, the
conductive paste bearing this full-crystal corrosion binder can be
subjected to a broader range of sintering process conditions to
form electrodes on solar cells with greatly reduced series
resistance and enhanced photovoltaic conversion efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The present disclosure will become more fully understood
from the detailed description given herein below for illustration
only, and thus are not limitative of the present disclosure, and
wherein:
[0017] FIG. 1 is a schematic view of a process of preparing a front
electrode by using an electrically conductive paste of the present
invention.
[0018] FIG. 2 is a partial structural view of a crystalline silicon
solar cell printed with an electrically conductive paste before
sintering.
[0019] FIG. 3 is a partial structural view of a crystalline silicon
solar cell after an electrically conductive paste is sintered.
[0020] FIG. 4 shows a cooling curve in preparing a glass-free
Pb--Te--O crystalline corrosion binder.
[0021] FIG. 5 is an exemplary diagram of XRD measured from a
Pb--Te--O-based crystalline compound corrosion binder, in which
sharp diffraction characteristic peaks exist in a small angle
range.
[0022] FIG. 6 is an exemplary diagram of XRD measured from a
Pb--Te--O-based glass frit, in which a bump with a wide
distribution and low intensity exists in a small angel range, and
no sharp diffraction characteristic peaks exist.
[0023] FIG. 7 is a schematic view of a process of preparing an
electrically conductive paste of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The present invention provides an electrically conductive
paste for a front electrode of a solar cell and a preparation
method thereof, and a method for preparing a front electrode of a
crystalline silicon solar cell by using the electrically conductive
paste.
[0025] The electrically conductive paste for a front electrode of a
solar cell according to the present invention includes the
following components: [0026] a metallic powder, having a weight
ratio of 70 wt % to 95 wt % in the electrically conductive paste
for a front electrode of a solar cell; [0027] a corrosion binder,
having a weight ratio of 0.5 wt % to 12 wt % in the electrically
conductive paste for a front electrode of a solar cell; and [0028]
an organic carrier, having a weight ratio of 5 wt % to 25 wt % in
the electrically conductive paste for a front electrode of a solar
cell.
[0029] The corrosion binder is a glass-free Pb--Te--O based
crystalline compound having a melting temperature in a range of
440.degree. C. to 760.degree. C.; and the metallic powder and the
corrosion binder are randomly dispersed in the organic carrier. The
corrosion binder of the present invention includes one or a
combination of two or more selected from the following glass-free
Pb--Te--O based crystalline compounds: PbTe.sub.4O.sub.9,
PbTeO.sub.3.0.33H.sub.2O, PbTeO.sub.3, PbTeO.sub.4,
PbTe.sub.3O.sub.7, PbTe.sub.5O.sub.11, Pb.sub.2TeO.sub.4,
Pb.sub.2Te.sub.3O.sub.7, Pb.sub.2Te.sub.3O.sub.8,
Pb.sub.3TeO.sub.5, Pb.sub.3TeO.sub.6,
Pb.sub.3Te.sub.2O.sub.8.H.sub.2O, Pb.sub.4Te.sub.1.5O.sub.7,
Pb.sub.5TeO.sub.7, Pb.sub.5TeO.sub.7,
Pb.sub.6Te.sub.5O.sub.18.5H.sub.2O, PbTe.sub.2O.sub.5,
PbH.sub.4TeO.sub.6, PbTeCO.sub.5 and Pb.sub.3TeN.sub.2O.sub.8.
[0030] Furthermore, a method for preparing the corrosion binder
includes: mixing a hot telluric acid solution (tellurous acid
solution, tellurate solution or tellurite solution) and a lead
acetate solution, where the molar ratio of Te to Pb in the solution
is in a range of 0.1:10 to 10:0.1; stirring the mixed solution at a
temperature in a range of 80.degree. C. to 120.degree. C. at a
stirring speed in a range of 1,000 to 1,500 r/min for reaction for
2 to 5 hours, to generate a precipitate, and collecting a solid
through solid-liquid separation and washing, till the pH value of
the filtrated water is in a range of 5 to 7, and drying the solid
at 150.degree. C. for 2 to 3 hours, to obtain the Pb--Te based
crystalline compound, which is then pulverized and ground, to
obtain a Pb--Te based crystalline compound particle.
[0031] Furthermore, a method for preparing the corrosion binder
includes: continuously introducing Pb.sub.xTe.sub.y alloy vapor
into a reaction chamber filled with oxygen atmosphere; reacting at
a temperature in a range of 1,000.degree. C. to 1,400.degree. C.
for 1 to 4 hours, and naturally cooling the resulting product to
25.degree. C. to obtain the Pb--Te--O based crystalline compound,
which is then pulverized and ground, to obtain a Pb--Te--O based
crystalline compound particle.
[0032] Furthermore, a method for preparing the corrosion binder
includes: in a non-reducing atmosphere (including oxygen
atmosphere, air atmosphere, nitrogen atmosphere, and argon gas
atmosphere), heating a tellurium oxide and a lead oxide to a
temperature in a range of 700.degree. C. to 1,000.degree. C.,
melting the tellurium oxide and the lead oxide for reaction,
naturally cooling the resulting product to 25.degree. C., and then
pulverizing and grinding, to obtain a Pb--Te--O based crystalline
compound particle.
[0033] Furthermore, a method for preparing the corrosion binder
includes: in a vacuum atmosphere, melting a tellurium oxide and a
lead oxide at a temperature in a range of 700.degree. C. to
1,000.degree. C., naturally cooling the resulting product to
25.degree. C., and then pulverizing and grinding, to obtain a
Pb--Te--O based crystalline compound particle.
[0034] FIG. 1 is a schematic view of a process of preparing a front
electrode of a crystalline silicon solar cell by using an
electrically conductive paste of the present invention. The method
for preparing a front electrode of a high-performance crystalline
silicon solar cell according to the present invention includes
providing a crystalline silicon semiconductor device having an
insulating film on an upper surface. The insulating film is an
overlaying layer of one or a combination of two or more selected
from silicon nitride, titanium oxide, aluminum oxide and silicon
oxide. The method further includes printing an electrically
conductive paste for a front electrode of a solar cell on the
insulating film of the crystalline silicon semiconductor device.
The electrically conductive paste for a front electrode of a solar
cell contains formulation components of the following parts by
weight, based on the total weight of 100 parts: 1) a metallic
powder 70 to 95 parts; 2) an organic carrier 5 to 25 parts; and 3)
a sum of a corrosion binder and a glass frit 0.5 to 12 parts. The
corrosion binder is one or a combination of two or more glass-free
Pb--Te--O based crystalline compounds, having a melting temperature
in a range of 440.degree. C. to 760.degree. C. and a particle size
in a range of 0.1 to 15.0 .mu.m. Additionally, the method includes
sintering.
[0035] The sintering process includes: first, drying the
electrically conductive paste printed on the insulating film of the
crystalline silicon semiconductor device at a temperature in a
range of 180.degree. C. to 260.degree. C.; next, sintering by
heating to 720.degree. C. to 950.degree. C.; and then cooling to
obtain the electrically conductive electrode. In the process of
sintering the electrically conductive paste for an electrode, the
organic carrier is removed through evaporation and the corrosion
binder is converted into a liquid and easily flows, corrodes, and
penetrates an antireflective insulating layer on a front side of a
crystalline silicon solar cell, and wets the electrically
conductive metallic powder, thereby promoting the combination of
the metallic powder. As a result, a good ohmic contact is formed
between the electrically conductive metallic powder and the
crystalline silicon solar cell, and a high-conductivity front
electrode of a crystalline silicon solar cell is formed.
[0036] FIG. 2 is a partial structural view of a crystalline silicon
solar cell printed with an electrically conductive paste before
sintering. It should be noted that FIG. 2 only shows an example,
which should not be used to limit the scope of the present
invention. As shown in FIG. 2, the crystalline silicon solar cell
is composed of a semiconductor substrate 100, an emitter 102 and an
insulating layer 110, a P--N junction region exists between the
semiconductor substrate 100 and the emitter 102, the electrically
conductive paste 120 for an electrode is selectively printed on a
partial surface of the insulating layer 110 by adopting screen
printing. The electrically conductive paste for an electrode
includes a metallic powder 122, a corrosion binder 126 and an
organic carrier 128. According to the design and application of a
solar cell, the printing width of the electrically conductive paste
120 for an electrode may be in a range of 20 .mu.m to 3 mm, and the
printing width of a thin electrode may be in a range of 20 to 70
.mu.m, and the printing width of the main electrode may be in a
range of 1 to 3 mm.
[0037] FIG. 3 is a partial structural view of a crystalline silicon
solar cell after an electrically conductive paste is sintered. It
should be noted that FIG. 3 only shows an example, which should not
be used to limit the scope of the present invention. As shown in
FIG. 3, after sintering, the electrically conductive paste is
converted into an electrode 200. The sintering process includes
drying the electrically conductive paste by heating from 25.degree.
C. to a temperature in a range of 180.degree. C. to 260.degree. C.,
and then heating to a temperature in a range of 720.degree. C. to
950.degree. C. for sintering, and cooling, to form the electrode
200. In the sintering process, with the raise of the temperature,
the organic carrier in the electrically conductive paste is
removed, and the corrosion binder is melted into a liquid 210 and
flows towards a surface of the insulating layer 110, etches and
penetrates the insulating film 110 on a surface of the crystalline
silicon semiconductor device, so that the metallic powder directly
contacts the substrate of the crystalline silicon semiconductor
device, and an ohmic layer 240 is formed. The molten corrosion
binder promotes the combination of the metallic powder, and a
high-conductivity metallic powder combination 220 is formed, which
has a good ohmic contact with the semiconductor 102 through the
ohmic layer 240. As a result, a high-conductivity front electrode
200 of a crystalline silicon solar cell is formed. In an
embodiment, the front electrode of the present invention includes a
metallic powder uncoated with silver, which includes one or a
combination of two or more selected from silver, gold, platinum,
copper, iron, nickel, zinc, titanium, cobalt, chromium, aluminum,
manganese, palladium and rhodium. In another embodiment, the front
electrode of the present invention includes one or a combination of
two or more selected from copper, iron, nickel, aluminum, zinc,
titanium, cobalt, chromium and manganese coated with silver. In
another embodiment, the front electrode of the present invention
includes a mixture of a metallic powder uncoated with silver and a
metallic powder coated with silver, and the weight ratio of the
metallic powder uncoated with silver to the metallic powder coated
with silver is 5:95 to 95:5.
[0038] The components, the preparation method and the use of the
electrically conductive paste for a front electrode of a
crystalline silicon solar cell according to the present invention
are described in detail below.
[0039] I. Corrosion Binder
[0040] Conventional electrically conductive paste for a front
electrode of a crystalline silicon solar cell composes a silver
powder, a glass frit, an additive and an organic carrier. The
electrically conductive paste is generally printed on a front side
or a light irradiation side of the crystalline silicon solar cell,
and then is sintered to form a front electrode. In the sintering
process, the glass frit in the electrically conductive paste etches
and penetrates an antireflective insulating layer on the front side
or the light irradiation side of the crystalline silicon solar
cell, so that the silver powder contacts the substrate of the
crystalline silicon solar cell to form a front electrode, where the
antireflective insulating layer is made of, for example, silicon
nitride, titanium oxide, aluminum oxide and silicon oxide or
silicon oxide/titanium oxide. The glass frit used in the
conventional electrically conductive paste for a front electrode of
a crystalline silicon solar cell is an amorphous structure
material. In a heating process, the glass frit gets soft first,
which is referred to as a softening temperature. The different
types of glass frits have different softening temperature. Atoms in
the glass frit are arranged disorderly, and X-ray diffraction (XRD)
measurement shows that a bump with wide distribution and low
intensity exists and no sharp diffraction characteristic peaks
exist, which is different from the situation of a crystalline
compound.
[0041] The glass frit is generally prepared by heating oxide
material or materials to a melting state followed by quenching the
molten. For example, US Patent No. US2011/0308595 discloses an
electrically conductive paste for a front electrode of a
crystalline silicon solar cell, in which a glass frit is made from
a lead tellurium oxide
[0042] (Pb--Te--O) material, the method for preparing of the glass
frit includes: mixing a lead oxide and a tellurium oxide, heating
the mixture to a molten state in an air atmosphere or oxygen
atmosphere; then, quenching the molten mixture, and grinding, to
obtain the led tellurium oxide (Pb--Te--O)-based glass frit. In
another example, PCT Patent No. WO2012/129554 discloses an
electrically conductive paste for a front electrode of a
crystalline silicon solar cell, in which a Pb--Te--O-based glass
frit is used. A method for preparing the glass frit is a
conventional glass preparation method, and includes: mixing a lead
oxide and a tellurium oxide, heating the mixture to a molten state;
then quenching the molten mixture, and grinding, to obtain the
Pb--Te--O-based glass frit. In another example, PCT Patent No.
WO2012/129554 discloses that atoms in glass frit are arranged
disorderly, and XRD measurement shows that a bump with wide
distribution and low intensity exists and no sharp diffraction
characteristic peaks exist, which is different from that for a
crystalline compound. The softening temperature of the glass frit
per WO2012/129554 is in a range of 300.degree. C. to 800.degree. C.
For another example, US Patent No. US2011/0232747 discloses an
electrically conductive paste for a front electrode of a
crystalline silicon solar cell, in which a Pb--Te--O-based glass
frit is used, and the method for preparing the glass frit includes:
mixing TeO.sub.2, PbO and Li.sub.2CO.sub.3, heating the mixture to
900.degree. C. to melt and keeping at 900.degree. C. for one hour,
and then quenching the molten mixture to obtain the Pb--Te--O-based
glass frit. For another example, US Patent No. US2011/0232746
discloses an electrically conductive paste for a front electrode of
a crystalline silicon solar cell, in which a Pb--Te--B--O-based
glass frit is used, and a method for preparing the glass frit
includes: heating a lead, tellurium and boron mixture to a
temperature in a range of 800.degree. C. to 1,200.degree. C. to
melt, and quenching the molten mixture to obtain the
Pb--Te--B--O-based glass frit. The glass frit is an amorphous
material and does not have a melting temperature.
[0043] The corrosion binder of the present invention is a
glass-free crystalline compound, has a melting point, and is
different from the glass frit. The corrosion binder of the present
invention is one or a combination of two or more selected from the
following Pb--Te--O based crystalline compounds: PbTe.sub.4O.sub.9,
PbTeO.sub.3.0.33H.sub.2O, PbTeO.sub.3, PbTeO.sub.4,
PbTe.sub.3O.sub.7, PbTe.sub.5O.sub.11, Pb.sub.2TeO.sub.4,
Pb.sub.2Te.sub.3O.sub.7, Pb.sub.2Te.sub.3O.sub.8,
Pb.sub.3TeO.sub.5, Pb.sub.3TeO.sub.6,
Pb.sub.3Te.sub.2O.sub.8.H.sub.2O, Pb.sub.4Te.sub.1.5O.sub.7,
Pb.sub.5TeO.sub.7, Pb.sub.6Te.sub.5O.sub.18.5H.sub.2O,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5 and
Pb.sub.3TeN.sub.2O.sub.8, and these crystalline compounds have a
melting temperature in a range of 440.degree. C. to 760.degree. C.
The corrosion binder is crystalline compounds, has a typical
crystal characteristics, and when being heated to the melting
temperature, the crystal begins to melt into a liquid and has no
softening temperature, and the melting point is related to the
material composition.
[0044] The crystal corrosion binder is distinctly different from
the conventional glass frit in terms of its internal atomic
structure. Each particle in the prepared corrosion binder is a
crystalline compound having ordered atomic structure while each
glass frit particle is in an amorphous structure with a random
atomic network. Significant difference can be shown by X-ray
diffraction (XRD) measurement. XRD scan of a corrosion binder
sample yields several sharp peaks at certain diffraction angles
that are specifically associated with the corresponding crystalline
compound. While XRD scan of a glass frit material shows a mostly
flat distribution and only a small bump of low strength near the
small diffraction angles region. Additionally, the structural
difference between the crystalline corrosion binder and the glass
frit can be directly revealed by transmission electron microscopy
(TEM) image. When a TEM image of the crystalline compound of any
one of the corrosion binder added into the conductive paste of the
present invention is taken, it demonstrates a surface with ordered
atomic arrangement. But TEM image of any glass frit exhibits a
surface with disordered atomic arrangement.
[0045] Additional difference between crystalline corrosion binder
and glass frit may resulted from their preparation methods, even
though they may be started from substantially same oxide material
with same or very similar compositions. Conventional glass frit is
typically formed by first heating the oxide materials till melt
followed by quenching the melt in certain processes, which are
different with the methods of making the corrosion binders as
described above.
[0046] The corrosion binder of the present invention is prepared by
one of the following methods: liquid phase chemical reaction
method, gas-phase chemical reaction method, melting controlled
cooling method and vacuum melting controlled cooling method. The
corrosion binder has a shape of one or a combination of two or more
selected from sphere, sphere-like shape, sheet, particle and
colloidal particle. The size of the corrosion binder of the present
invention is not particularly limited, and in an embodiment, the
size is less than 15 .mu.m; in an embodiment, the size is less than
5 .mu.m; in another embodiment, the size is less than 3 .mu.m; in
another embodiment, the size is in a range of 0.1 to 2.0 .mu.m; in
another embodiment, the size is in a range of 0.3 to 1.7 .mu.m.
[0047] In an exemplary embodiment, the corrosion binder of the
present invention may be prepared by using a chemical reaction
method as: mixing a hot telluric acid solution (tellurous acid
solution, tellurate solution or tellurite solution) and a lead
acetate solution, where the molar ratio of Te to Pb in the solution
is in a range of 0.1:10 to 10:0.1; stirring the mixed solution at a
temperature in a range of 80.degree. C. to 120.degree. C. at a
stirring speed in a range of 1,000 to 1,500 r/min for reaction for
2 to 5 hours, to generate a precipitate, and collecting a solid
through solid-liquid separation and washing, till the pH value of
the filtrated water is in a range of 5 to 7, and drying the solid
at 150.degree. C. for 2 to 3 hours, to obtain the Pb--Te based
crystalline compound, which is then pulverized and ground, to
obtain a Pb--Te based crystalline compound particle. As well known
to persons of ordinary skill in the art, by changing the chemical
reaction conditions in the exemplary embodiment, including changing
the chemical components or the reaction temperature or time, Pb--Te
based crystalline compounds with similar properties can be
obtained. In another exemplary embodiment, the corrosion binder of
the present invention may be prepared by using a gas-phase chemical
reaction method, and the preparation process includes: continuously
introducing Pb.sub.xTe.sub.y alloy vapor into a reaction chamber
filled with oxygen atmosphere; reacting at a temperature in a range
of 1,000.degree. C. to 1,400.degree. C. for 1 to 4 hours, and
naturally cooling the resulting product to 25.degree. C. to obtain
the Pb--Te based crystalline compound, which is then pulverized and
ground, to obtain a Pb--Te based crystalline compound particle. As
well known to persons of ordinary skill in the art, by changing the
chemical reaction conditions in the exemplary embodiment, Pb--Te
based crystalline compounds with similar properties can be
obtained. In another exemplary embodiment, the corrosion binder of
the present invention may be prepared by using a solid-phase
reaction method, and the preparation process includes: in a
non-reducing atmosphere (including oxygen atmosphere, air
atmosphere, nitrogen atmosphere and argon gas atmosphere), heating
a tellurium oxide and a lead oxide to a temperature in a range of
700.degree. C. to 1,000.degree. C., melting the tellurium oxide and
the lead oxide for reaction, naturally cooling the resulting
product to 25.degree. C., and then pulverizing and grinding, to
obtain a Pb--Te based crystalline compound particle. As well known
to persons of ordinary skill in the art, by changing the chemical
reaction conditions in the exemplary embodiment, Pb--Te based
crystalline compounds with similar properties can be obtained. For
example, a tellurium oxide and a lead oxide may be melted at a
temperature of 700.degree. C. or less or a temperature of
1,000.degree. C. and more, a flowing protection gas (such as
N.sub.2, CO.sub.2 and Ar) that is not heated flows through the
surface of the molten to accelerate the cooling rate; or a
protection gas (such as N.sub.2, CO.sub.2 and Ar) that is heated
flow through the surface of the molten to decrease the cooling
rate, to obtain a Pb--Te--O-based crystalline compound. In another
exemplary embodiment, the corrosion binder of the present invention
may be prepared by using a vacuum melting controlled cooling
method, and the preparation process includes: in a vacuum
atmosphere, melting a tellurium oxide and a lead oxide at a
temperature in a range of 700.degree. C. to 1,000.degree. C.,
naturally cooling the resulting product to 25.degree. C., and then
pulverizing and grinding, to obtain a Pb--Te based crystalline
compound particle. As well known to persons of ordinary skill in
the art, by changing the chemical reaction conditions in the
exemplary embodiment, Pb--Te--O based crystalline compounds with
similar properties can be obtained. For example, a tellurium oxide
and a lead oxide may be melted at a temperature of 700.degree. C.
or less or a temperature of 1,000.degree. C. and more; a gas (such
as N.sub.2, CO.sub.2 and Ar) may be used to flows through the
surface of the molten to accelerate the cooling rate; or a hot gas
(such as N.sub.2, CO.sub.2 and Ar) may be used to flow through the
surface of the molten to decrease the cooling rate, to obtain a
Pb--Te--O-based crystalline compound.
[0048] In one or more embodiments, the differences in structures
and preparation methods between a Pb--Te--O based glass material
and crystalline compounds are illustrated by following
examples.
[0049] In an example according to an embodiment of the present
invention, chemical compound TeO.sub.2 and PbO in powder forms are
mixed with a mole ratio of 1:1. After mixing, the mixture is heated
in air environment to about 900.degree. C. (which is above the
melting point of either chemical compound) and is further held at
the temperature for about 30 minutes to form a molten mixture. Then
the molten mixture is removed from furnace and cooled naturally in
a room-temperature atmosphere to form a bulk material. Upon the
removal from the furnace, the temperature of the molten mixture is
first cooled from 900.degree. C. in the furnace to 732.degree. C.
in about 3 seconds. FIG. 4 shows a plot of temperature drop after
removal of the molten mixture from furnace versus its cooling time
in a process for preparing the corrosion binder according to an
embodiment of the present invention. As shown, the recording starts
from 732.degree. C. and the temperature drops to 593.degree. C. in
the first 1 minute. The average cooling rate is about 139.degree.
C./min. In a second minute, the temperature drops further to
504.degree. C. with an average cooling rate of 89.degree. C./min.
In a third minute, the temperature drops to 449.degree. C. with an
average cooling rate of 55.degree. C./min. Furthermore, in a fourth
minute, it drops to 416.8.degree. C. with an average cooling rate
of 32.8.degree. C./min. At this stage, the molten mixture has
become a bulk material. The bulk material is crushed into small
particles and further ball-milled into fine powders with
substantially round shape having D.sub.50 sizes ranging from 0.1 to
15 .mu.m. Using XRD to exam samples of the fine powders, the
resulted diffraction pattern (marked as M1) is shown in FIG. 5,
plotted as the diffraction intensity versus 2.theta. (.theta. is
X-ray incident angle) values across a range from 10 degrees to 80
degrees. As shown, the plot yields many sharp peaks at certain
2.theta. values corresponding to characteristic peaks specific for
crystal compounds PbTeO.sub.3, indicating that the corrosion binder
in powered form obtained via above preparation method shows a
substantially PbTeO.sub.3 crystalline characteristic.
[0050] In another example, same chemical compounds TeO.sub.2 and
PbO in powders were mixed with the same mole ratio of 1:1. The
powder mixture was placed in a crucible and heated in air
atmosphere to form a melt, and maintained at 900.degree. C. for 30
min. Then the melt was cooled quickly by a quenching method. In an
implementation, the melt was quenched by pouring the melt directly
on a stainless steel platen to obtain a bulk platelet according to
methods presented in U.S. patent Ser. No. 13/100,550 and other
related references. In another implementation, the melt was
quenched by pouring into deionized water to form a bulk material.
The bulk platelet material was crunched by grinding into small
particles which are further ball-milled into fine powders having
D.sub.50 sizes of 0.115 microns. Using XRD to exam samples of the
fine powders, the resulted diffraction pattern (marked as M2) is
shown in FIG. 6. As shown, the plotted diffraction intensity versus
2.theta. (.theta. is X-ray incident angle) values across a range
from 10 degrees to 80 degrees yield a wide range of low intensity
curve with only a small bump near the small angular regions. This
is a clear indication that no crystalline structure exists in these
fine powders, instead, the powders obtained by following
conventional preparation method is predominantly glass material
with an amorphous structure.
[0051] In the above-mentioned two examples, although the use of the
same composition and proportion of TeO.sub.2 and PbO oxide powders,
different preparation method yields different material with
different atomic structure and physical property. The material made
using the method of the present invention shows glass-free
Pb--Te--O-based crystalline particles while another method
following prior art references yields only glass particles.
Consequently, the obtained different particles have different
physical property, which shows different performance during a
sintering/firing process in associated with the application of the
conductive paste. Specifically, for the electrically conductive
paste with the above two different particles, as temperature
increases during the sintering/firing process, the glass-free
Pb--Te--O-based crystalline particles go through a direct physical
phase transition from a solid phase to a liquid phase while the
particles with glass structure particles in the same conductive
paste go through a phase transition from a solid state to a
glass-softening state and stay in the softening condition in a
range of the temperature before finally transforming into a liquid
state.
[0052] The electrolytic conductive paste of the present invention
contains a corrosion binder which is a glass-free Pb--Te--O-based
crystalline compound. During the process of sintering, the
Pb--Te--O based crystalline compound corrosion binder changes
liquid from solid when the temperature reaches the melting point.
Before the melting point, the corrosion binder is a solid, and will
not fill the pores in the metallic powder and hinder the discharge
of the organic components, thereby solving the problem that the
glass frit clogs the pores when being softened at the early stage;
after being melted, the corrosion binder is in a liquid state and
has a low viscosity, can rapidly flows to the bottom through voids
among the metallic powder, and can effectively etch and penetrate
the antireflective insulating layer on the front side of the
crystalline silicon solar cell, so that a good ohmic contact is
formed between the electrically conductive metallic powder and the
crystalline silicon solar cell, and the electrically conductive
metallic powder can be effectively wet, thereby promoting the
combination of the metallic powder. As a result, a
high-conductivity front electrode of a crystalline silicon solar
cell is formed. Therefore, compared with the glass frit, the
corrosion is faster and fuller, and the amount of the
Pb--Te--O-based crystalline compound corrosion binder is less.
Further, since the low-viscosity Pb--Te--O-based compound melt
easily spreads, many interfaces are provided, so that electrically
conductive contact points are increased, and the tunneling effect
is improved, and the resistance is reduced. The inventors find in
the study that, if the content of the corrosion binder in the
electrically conductive paste for a front electrode of a
crystalline silicon solar cell in the embodiments is greater than
12 parts by weight, the P--N junction region may be penetrated,
resulting in a short circuit; if the content of the corrosion
binder is less than 0.5 part by weight, the antireflective layer
may be not removed completely, resulting in deterioration of the
performance of the crystalline silicon solar cell. Therefore, the
weight ratio of the corrosion binder in the electrically conductive
paste for a front electrode of a solar cell is in a range of 0.5 wt
% to 12 wt %.
[0053] II. Metallic Powder
[0054] The electrically conductive paste for a front electrode of a
crystalline silicon solar cell of the present invention contains a
metallic powder. In an embodiment, the metallic powder is a
metallic powder uncoated with silver, including at least one
selected from silver, gold, platinum, copper, iron, nickel, zinc,
titanium, cobalt, aluminum, chromium, palladium and rhodium or an
alloy thereof. In another embodiment, the metallic powder is any
one or a combination of two or more selected from copper, iron,
nickel, aluminum, zinc, titanium, cobalt, chromium and, manganese
coated with silver, the thickness of the silver coating layer is in
a range of 10 to 2,000 nm, and the size of the metallic powder
coated with silver is in a range of 0.1 to 5.0 .mu.m. In another
embodiment, the metallic powder is a mixture of a metallic powder
uncoated with silver and a metallic powder coated with silver, and
the weight ratio of the metallic powder uncoated with silver to the
metallic powder coated with silver is in a range of 5:95 to 95:5.
Specifically, the metallic powder is used for exerting an
electrically conductive effect in the embodiments of the present
invention, and is a component for forming an electrode. In a
preferred embodiment, the melting point of the metallic powder is
preferably in a rang of 350.degree. C. to 2,000.degree. C., further
preferably in a range of 450.degree. C. to 1,800.degree. C., and
more preferably in a range of 600.degree. C. to 1,450.degree. C.
The inventors find in the study that, if the melting point of the
metallic powder is lower than 350.degree. C., during sintering, the
metal particles will be melted excessively early, and hinder the
discharge of the organic carrier, and flow during sintering, so
that the aspect ratio of gate lines is reduced; if the melting
point of the metallic powder is higher than 2,000.degree. C., the
metallic powder cannot be sintered effectively during the sintering
process, too many voids exist in the electrically conductive
metallic block, resulting in a high resistance of channels and
deterioration of performance.
[0055] In a further preferred embodiment, the metallic powder is at
least one selected from silver, gold, platinum, palladium and
rhodium, or at least selected from silver, gold, platinum,
palladium and rhodium doped with copper, iron, nickel, zinc,
titanium, cobalt, aluminum, chromium and manganese, or an alloy
thereof, such as manganese-copper alloy, constantan alloy and
nickel-chromium alloy.
[0056] In yet a further preferred embodiment, the metallic powder
is any one selected from copper, iron, nickel, aluminum, zinc,
titanium, cobalt, chromium and manganese coated with silver, that
is, any one metallic particle of copper, iron, nickel, aluminum,
zinc, titanium, cobalt, chromium and manganese having a layer of
silver continuously coated on the outer surface.
[0057] In an embodiment of a metallic powder having a silver
coating structure, the thickness of the sliver coating layer is
preferably in a range of 1 to 2,000 nm, and more preferably in a
range of 2 to 1,000 nm. The inventors find in the study that, if
the thickness of the Ag layer is less than 1 nm, the Ag content is
excessively low, and the contact resistance or the drain current of
the electrode is significantly increased; if the thickness of the
Ag layer is greater than 10.sup.4 nm, the particle diameter of the
electrically conductive metallic powder is excessively large, since
Ag is noble metal, the cost of the metallic powder is increased,
thereby increasing the cost of the crystalline silicon solar cell.
Definitely, the silver layer of the metallic powder having a silver
coating structure may be replaced by other noble metals such as
gold and platinum. The metallic particle coated with silver may be
an alloy of metals selected from copper, iron, nickel, aluminum,
zinc, titanium, cobalt, chromium and manganese, such as
manganese-copper alloy, constantan alloy and nickel-chromium alloy.
The metallic powder coated with silver is formed by plating a layer
of silver on a metallic powder of copper, iron, nickel, aluminum,
zinc, titanium, cobalt, chromium or manganese. In an exemplary
embodiment, one or more types of metallic powder of copper, iron,
nickel, titanium, aluminum, cobalt, chromium, zinc or manganese
having a particle diameter in a range of 0.1 to 5.0 .mu.m or an
alloy thereof are placed in a dilute weak acid and immersed for 10
to 300 s to remove the oxide layer on the surface of the metal, and
then silver is plated on the metallic powder to a thickness of
about 10 to 2,000 nm by using a chemical plating method. In an
exemplary embodiment, the composition of the solution and the
process conditions for chemical plating are: AgNO.sub.3: 2.4 to
14.2 g/L, ammonia: 0.8 g/L, formaldehyde: 1 to 3 g/L, hydrazine
hydrate: 1 to 4 g/L, a composite dispersant: 1.0 g/L, pH value: 11,
bath temperature: 60.degree. C., stirring speed: 1,000 r/min,
drying: 50.degree. C., 30 min. In another exemplary embodiment, one
or more types of metallic powder of copper, iron, nickel, titanium,
cobalt, aluminum, chromium, zinc or manganese having a particle
diameter in a range of 0.1 to 5.0 .mu.m or an alloy thereof are
placed in a dilute weak acid and immersed for 10 to 300 s to remove
the oxide layer on the surface of the metal. The metal powder is
washed with deionized water to remove residual acid. The wet powder
is dried in a vacuum oven, and then the dry metallic particles free
of oxide layer are placed in a vacuum deposition device for vacuum
deposition, to obtain the metallic powder coated with silver.
[0058] In the embodiments of the metallic powder, the size of the
metallic powder particle is first required to meet requirements for
printing, for example, not clogging the printing stencil.
Therefore, preferably, the particle diameter of the metallic powder
is distributed in a range of 0.1 to 5.0 .mu.m, and if the particle
diameter of the metallic powder is greater than 5 .mu.m, problems
of clogging the printing stencil and disconnection of the electrode
easily occur; if the particle diameter of the metallic powder is
less than 0.1 .mu.m, the viscosity of the paste is greatly
improved, resulting in failures in normal printing. In addition,
the metallic powder having the preferred particle diameter can also
reduce the area occupied by the electrode, thereby improving the
light conversion efficiency of the solar cell, and at the same
time, effectively reducing the thickness of the electrode, reducing
the amount of materials and reducing the production cost.
[0059] III. Organic Carrier
[0060] The electrically conductive paste for a front electrode of a
crystalline silicon solar cell of the present invention contains an
organic carrier. The organic carrier includes an organic solvent, a
binder, a wetting and dispersing agent, a thixotropic agent and
other functional additives. The weight ratio of the organic carrier
in the electrically conductive paste for an electrode is 5 to 25.
Based on the total weight of 100 parts of the organic carrier, the
organic solvent accounts for 50 to 95 parts by weight, the binder
accounts for 1 to 40 parts by weight, the wetting and dispersing
agent accounts for 0.1 to 10 parts by weight, and the thixotropic
agent and other functional additives account for 1 to 20 parts by
weight. In an exemplary embodiment, the organic solvent may be at
least one with a medium or high boiling temperature, such as
alcohol (such as terpineol, butyl carbitol), alcohol ester (such as
alcohol ester-12), terpene and the like. The binder may be at least
one selected from ethyl cellulose, polymethacrylate, alkyd resin,
and the like. The wetting and dispersing agent is used to help
dispersing inorganic powders in the organic carrier, and is not
particularly limited. The thixotropic agent is used to increase the
thixotropy of the paste in the printing process, so as to ensure
the resolution of electrode pattern and better aspect ratio. The
thixotropic agent may be an organic thixotropic agent selected from
hydrogenated castor oil derivatives or polyamide wax. The other
functional agents may be added as required, such as
microcrystalline wax may be added for reducing the surface tension,
DBP may be added for improving the flexibility of the paste, and
PVB may be added for improving the adhesion.
[0061] IV. Preparation Method of the Electrically Conductive
Paste
[0062] The present invention provides a method for preparing an
electrically conductive paste for a front electrode of a solar cell
by using a simple process with easy-controlled conditions and
reduced production cost. The method includes, based on the total
weight of 100 parts, weighing materials of the following
formulation: 1) a metallic powder 70 to 95 parts; 2) an organic
carrier 5 to 25 parts; and 3) a corrosion binder 0.5 to 12 parts.
The corrosion binder is a glass-free Pb--Te--O based crystalline
compound, having a melting temperature in a range of 440.degree. C.
to 760.degree. C. Additionally, the method includes mixing and
grinding the weighed corrosion binder, metallic powder and organic
carrier, to obtain the electrically conductive paste for a front
electrode of a solar cell.
[0063] An exemplary preparation process of an electrically
conductive paste for an electrode is shown in FIG. 7. First, a
metallic powder including a metallic powder uncoated with silver
and a metallic powder coated with silver is weighed; next, a lead
oxide and a tellurium oxide are weighed, and a corrosion binder is
prepared, where the method for preparing the corrosion binder is as
described above; the corrosion binder and the metallic powder are
premixed. Then, an organic compound is weighed, and an organic
carrier is prepared. Finally, the premixed corrosion binder and
metallic powder is mixed with the organic carrier, and the
resulting mixture is ground to obtain the electrically conductive
paste for an electrode. It should be noted that FIG. 7 only shows
an example, which should not be used to limit the scope of the
present invention.
[0064] Several exemplary embodiments of preparation of an
electrically conductive paste for an electrode are described below.
In an embodiment, first, a weighed corrosion binder and a weighed
metallic powder are premixed, the mixture is mixed with a weighed
organic carrier, and the resulting mixture is ground to obtain the
electrically conductive paste for a front electrode of a
crystalline silicon solar cell. In another embodiment, first, a
weighed corrosion binder and a weighed organic carrier are
premixed, a weighed metallic powder is added to the mixture for
further mixing, and then the resulting mixture is ground to obtain
the electrically conductive paste for a front electrode of a
crystalline silicon solar cell. In another embodiment, first, a
weighed metallic powder and a weighed organic carrier are premixed,
a corrosion binder is added to the mixture, and then the resulting
mixture is ground to obtain the electrically conductive paste for a
front electrode of a crystalline silicon solar cell. In another
embodiment, first, a weighed metallic powder and part of a weighed
organic carrier are premixed, a corrosion binder and the rest
weighed organic carrier are premixed, the tow premixed mixtures are
mixed, and then the resulting mixture is ground to obtain the
electrically conductive paste for a front electrode of a
crystalline silicon solar cell.
[0065] The electrically conductive paste for a front electrode of a
solar cell of the present invention contains a glass-free
crystalline corrosion binder. The amount of the crystal-based
corrosion binder as the functional additive in the conductive paste
is controlled between 0.5 to 12 parts by weight, such as 1 part by
weight, 4 parts by weight, 8 parts by weight and 10 parts by
weight. Of course, there are many variations, alternatives, and
modifications. For example, in a preferred embodiment, the
corrosion binder is controlled within a range of 1 to 10 wt %. In
another preferred embodiment, the corrosion binder is controlled
within a range of 3 to 8 wt %. The corrosion binder of the present
invention is one or a combination of two or more selected from the
following Pb--Te--O based crystalline compounds: PbTe.sub.4O.sub.9,
PbTeO.sub.3.0.33H.sub.2O, PbTeO.sub.3, PbTeO.sub.4,
PbTe.sub.3O.sub.7, PbTe.sub.5O.sub.11, Pb.sub.2TeO.sub.4,
Pb.sub.2Te.sub.3O.sub.7, Pb.sub.2Te.sub.3O.sub.8,
Pb.sub.3TeO.sub.5, Pb.sub.3TeO.sub.6,
Pb.sub.3Te.sub.2O.sub.8.H.sub.2O.sub.5, Pb.sub.4Te.sub.1.5O.sub.7,
Pb.sub.5TeO.sub.7, Pb.sub.6Te.sub.5O.sub.18.5H.sub.2O,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5 and
Pb.sub.3TeN.sub.2O.sub.8, and these Pb--Te--O based crystalline
compounds. Selection of the Pb--Te--O-based crystalline compound
for the corrosion binder is partially based on that its melting
temperature usually is around 500.degree. C. or higher which is
high enough to let substantially all organic carrier to release
without being clogged in the paste structure (hence to degrade the
sintering process). When several Pb--Te--O crystal compounds are
mixed, the melting point of the mixture may be lower from the phase
diagram analysis. In certain embodiments, the selected
Pb--Te--O-based crystalline corrosion binder has a melting
temperature in a preferred range of 440-760.degree. C. as a
functional additive in the conductive paste for manufacturing
electrode on semiconductor device.
[0066] In any conductive paste for forming electrodes of
semiconductor devices, metallic powder is one major component
designated as electrical conductive medium of the electrode. In an
embodiment, the metallic powder of the present invention is a
metallic powder uncoated with silver, including at least one or a
combination of two or more selected from silver, gold, platinum,
copper, iron, nickel, zinc, titanium, cobalt, aluminum, chromium,
manganese, palladium and rhodium. In another embodiment, the
metallic powder of the present invention is one or a combination of
two or more selected from copper, iron, nickel, aluminum, zinc,
titanium, cobalt, chromium and manganese coated with silver, where
the thickness of the silver coating layer is in a range of 2 to
2,000 nm. The size of the metallic powder coated with silver is in
a range of 0.1-5.0 .mu.m. In another embodiment, the metallic
powder of the present invention is a mixture of a metallic powder
uncoated with silver and a metallic powder coated with silver, and
the weight ratio of the metallic powder uncoated with silver to the
metallic powder coated with silver is in a range of 5:95 to
95:5.
[0067] In the electrically conductive paste for a front electrode
of a solar cell of the present invention, a corrosion binder having
a melting temperature in a range of 440.degree. C. to 760.degree.
C. is used. During use, when the sintering temperature of the
electrically conductive paste reaches the melting temperature of
the corrosion binder, the corrosion binder is rapidly melted and
converted from a solid state of crystal into a liquid state, and
effectively corrodes and penetrates the antireflective insulating
layer on the front side of the crystalline silicon solar cell, so
that a good ohmic contact is formed between the electrically
conductive metallic powder and the crystalline silicon solar cell,
and the electrically conductive metallic powder is effectively wet,
thereby promoting the combination of the metallic powder. As a
result, a high-conductivity front electrode of a crystalline
silicon solar cell is formed.
[0068] The electrically conductive paste for a front electrode of a
crystalline silicon solar cell of the present invention contains a
corrosion binder. During the process of sintering the electrically
conductive paste, when the temperature reaches the melting point of
the corrosion binder, the corrosion binder is converted from a
solid state into a liquid state, rapidly deposits on the surface of
the antireflective layer and reacts with the antireflective layer
fully in a short period of time. Through the rapid conversion of
the physical form of the corrosion binder from the solid state into
the liquid state, the clogging problem caused by the glass frit in
softening state is solved, and a space for discharging the organic
carrier is solved. By means of this type of solid state-liquid
state combination, not only the antireflective insulating layer on
the front side of the crystalline silicon solar cell can be
effectively corroded and penetrated, but also the organic carrier
can be easily discharged, and at the same time, the metallic powder
is effectively sintered to have a more compact structure, thereby
improving the soldering strength and the bulk conductivity.
[0069] In order to make the technical problems to be solved, the
technical solutions and the beneficial effects of the present
invention more clear and comprehensive, the present invention is
further described in detail below with reference to exemplary
embodiments. It should be noted that the specific embodiment
described herein are merely used for illustrate the present
invention, but not intended to limit the present invention.
Embodiment 1
[0070] Based on the total weight of 100 parts, the electrically
conductive paste for a front electrode of a solar cell contains:
8.3 parts of a corrosion binder, 81.7 parts of a metallic powder
and 10 parts of an organic carrier. The corrosion binder is a
PbTeO.sub.3 compound; the metallic powder is a silver powder, and
the organic carrier includes, based on the total weight of 100
parts, 70 parts of a terpineol organic solvent, 14 parts of an
ethyl cellulose binder, 10 parts of a wetting and dispersing agent,
5.5 parts of a thixotropic agent and 0.5 part of microcrystalline
wax. Preparation of the corrosion binder includes: preparing a hot
tellurous acid solution having a concentration of 0.1 mol/L and
heating to 90.degree. C., adding a lead acetate solution having a
concentration of 0.1 mol/L in proportion (the molar ratio of
tellurous acid to lead acetate being 1:1), heating the mixture
solution for reaction, to obtain a precipitate of the PbTeO.sub.3
compound. The metallic powder is a silver powder, and has a
particle size in a range of 1 to 3 .mu.m. After weighing the
corrosion binder, the metallic powder and the organic carrier
according to the above formulation, the corrosion binder and the
organic carrier were mixed uniformly, and then the metallic powder
was added and mixed uniformly, and finally the mixture was ground
to a particle diameter of less than 5 .mu.m by using a three-roll
mill, to obtain the electrically conductive paste for a front
electrode of a crystalline silicon solar cell.
Embodiment 2
[0071] Based on the total weight of 100 parts, the electrically
conductive paste for a front electrode of a solar cell contains:
7.3 parts of a corrosion binder, 81.7 parts of a metallic powder
and 11 parts of an organic carrier. The corrosion binder is a
Pb.sub.3TeO.sub.5 compound, the metallic powder is a silver-coated
nickel powder, the organic carrier includes, based on the total
weight of 100 parts, 50 parts of an alcohol ester-12 organic
solvent, 40 parts of a polymethacrylate binder, 5 parts of a
wetting and dispersing agent, 4 parts of a thixotropic agent and 1
part of DBP. Preparation of the corrosion binder includes: at a
proportion of a molar ratio of PbO and TeO.sub.2 being 3:1,
continuously introducing Pb.sub.3Te alloy vapor into a reaction
chamber filled with oxygen atmosphere of 1,300.degree. C., and
inducing a chemical reaction, to obtain a powder of a
Pb--Te--O-based crystalline compound deposited at the bottom of the
chamber, and collecting a Pb.sub.3TeO.sub.5 compound. The metallic
powder is prepared by using a chemical plating method, including:
plating silver on a nickel powder to about 200 nm, where the
particle size of the nickel powder is in a range of 0.5 to 3 .mu.m.
After weighing the corrosion binder, the metallic powder and the
organic carrier according to the above formulation, the metallic
powder and the organic carrier were mixed uniformly, and then the
corrosion binder was added and mixed uniformly, and finally the
mixture was ground to a particle diameter of less than 5 .mu.m by
using a three-roll mill, to obtain the electrically conductive
paste for a front electrode of a crystalline silicon solar
cell.
Embodiment 3
[0072] Based on the total weight of 100 parts, the electrically
conductive paste for a front electrode of a solar cell contains:
8.8 parts of a corrosion binder, 80 parts of a metallic powder and
11.2 parts of an organic carrier. The corrosion binder is a
PbTeCO.sub.5 compound, the metallic powder is silver-coated copper,
and the organic carrier includes, based on the total weight of 100
parts, 65 parts of a terpineol organic solvent, 20 parts of an
alkyd resin binder, 0.1 part of a wetting and dispersing agent, 2.5
parts of a thixotropic agent1 and 2.4 parts of PVB. Preparation of
the corrosion binder includes: in CO.sub.2 atmosphere, placing PbO
and TeO.sub.2 at a proportion of a molar ratio of being 1:1 in a
high-temperature (1,200.degree. C.) furnace, and reacting for 1
hour, naturally cooling the resulting product, and pulverizing and
grinding, to obtain the PbTeCO.sub.5 compound. The metallic powder
is a silver-coated copper powder, and is prepared by using a
chemical plating method, including: plating silver on a copper
powder to about 200 nm, where the particle size of the copper
powder is in a range of 0.5 to 3 .mu.m. After weighing the
corrosion binder, the metallic powder and the organic carrier
according to the above formulation, the metallic powder and the
corrosion binder were mixed uniformly, and then the organic carrier
was added and mixed uniformly, and finally the mixture was ground
to a particle diameter of less than 5 .mu.m by using a three-roll
mill, to obtain the electrically conductive paste for a front
electrode of a crystalline silicon solar cell.
Embodiment 4
[0073] Based on the total weight of 100 parts, the electrically
conductive paste for a front electrode of a solar cell contains:
3.4 parts of a corrosion binder, 86.6 parts of a metallic powder
and 10 parts of an organic carrier. The corrosion binder is a
PbTeO.sub.3 compound, the metallic powder is a titanium powder, and
the organic carrier includes, based on the total weight of 100
parts, 60 parts of a butyl carbitol organic solvent, 15 parts of an
ethyl cellulose binder, 5 parts of a wetting and dispersing agent,
15 parts of a thixotropic agent and 5 parts of PVB. Preparation of
the corrosion binder includes: placing PbO and TeO.sub.2 at a molar
ratio of 1:1 in a high-temperature (500.degree. C. to 900.degree.
C.) furnace, and reacting for 1 hour, naturally cooling the
resulting product, and pulverizing and grinding, to obtain the
PbTeO.sub.3 compound. The metallic powder is a titanium powder
having a particle size in a range of 0.5 to 10 .mu.m. After
weighing the corrosion binder, the metallic powder and the organic
carrier according to the above formulation, the metallic powder and
the corrosion binder were added into the organic carrier, and mixed
uniformly, and then the mixture is ground to a particle diameter of
less than 5 .mu.m by using a three-roll mill, to obtain the
electrically conductive paste for a front electrode of a
crystalline silicon solar cell.
Embodiment 5
[0074] Based on the total weight of 100 parts, the electrically
conductive paste for a front electrode of a solar cell contains:
9.8 parts of a corrosion binder, 72 parts of a metallic powder and
18.2 parts of an organic carrier. The corrosion binder is a
PbTeN.sub.2O.sub.8 compound, the metallic powder is a cobalt
powder, and the organic carrier includes, based on the total weight
of 100 parts, 70 parts of an alcohol ester-12 organic solvent, 15
parts of an ethyl cellulose binder, 2 parts of a wetting and
dispersing agent, 8 parts of a thixotropic agent and 5 parts of
PVB. Preparation of the corrosion binder includes: placing PbO and
TeO.sub.2 at a molar ratio of 1:1 in a high-temperature reactor in
NO.sub.2 atmosphere and melting the mixture at 950.degree. C. for 1
hour, and then naturally cooling the resulting product, and
pulverizing and grinding, to obtain the PbTeN.sub.2O.sub.8
compound. The metallic powder is a cobalt powder, and has a
particle size in a range of 0.5 to 3 .mu.m. After weighing the
corrosion binder, the metallic powder and the organic carrier
according to the above formulation, the metallic powder and the
corrosion binder were added into the organic carrier, and mixed
uniformly, and then the mixture was ground to a particle diameter
of less than 5 .mu.m by using a three-roll mill, to obtain the
electrically conductive paste for a front electrode of a
crystalline silicon solar cell.
Embodiment 6
[0075] Based on the total weight of 100 parts, the electrically
conductive paste for a front electrode of a solar cell contains:
8.9 parts of a corrosion binder, 85.6 parts of a metallic powder
and 5.5 parts of an organic carrier. The corrosion binder is a
Pb.sub.2Te.sub.3O.sub.8 compound, the metallic powder is platinum,
and the organic carrier includes, based on the total weight of 100
parts, 95 parts of an alcohol ester-12 organic solvent, 1 part of
an ethyl cellulose binder, 3 parts of a wetting and dispersing
agent and 1 part of a thixotropic agent. Preparation of the
corrosion binder includes: placing PbO and TeO.sub.2 at a molar
ratio of 2:3 in a vacuum high-temperature reactor and melting the
mixture at 900.degree. C. for 1 hour, and then naturally cooling
the resulting product, and pulverizing and grinding, to obtain the
Pb.sub.2Te.sub.3O.sub.8 compound. The metallic powder is prepared
by plating silver on an aluminum powder to about 200 nm through a
chemical plating method, where the particle size of the aluminum
powder is in a range of 0.5 to 3 .mu.m. After weighing the
corrosion binder, the metallic powder and the organic carrier
according to the above formulation, the metallic powder and the
corrosion binder were added into the organic carrier, and mixed
uniformly, and then the mixture was ground to a particle diameter
of less than 5 .mu.m by using a three-roll mill, to obtain the
electrically conductive paste for a front electrode of a
crystalline silicon solar cell.
Embodiment 7
[0076] Based on the total weight of 100 parts, the electrically
conductive paste for a front electrode of a crystalline silicon
solar cell contains: 5 parts of a corrosion binder, 80 parts of a
metallic powder and 15 parts of an organic carrier. The corrosion
binder is a PbTeO.sub.3 compound, the metallic powder is
silver-coated chromium, and the organic carrier includes, based on
the total weight of 100 parts, 70 parts of an alcohol ester-12
organic solvent, 15 parts of an ethyl cellulose binder, 2 parts of
a wetting and dispersing agent, 10 parts of a thixotropic agent and
3 parts of PVB. Preparation of the corrosion binder includes:
placing PbO and TeO.sub.2 at a molar ratio of 1:1 in a vacuum
high-temperature reactor and melting the mixture at 900.degree. C.
for 1 hour, and then naturally cooling the resulting product, and
pulverizing and grinding, to obtain the PbTeO3. The metallic powder
is prepared by plating silver on a chromium powder to about 200 nm
through a chemical plating method, where the particle size of the
chromium powder is in a range of 0.5 to 3 .mu.m. After weighing the
corrosion binder, the metallic powder and the organic carrier
according to the above formulation, the metallic powder and the
corrosion binder were added into the organic carrier, and mixed
uniformly, and then the mixture was ground to a particle diameter
of less than 5 .mu.m by using a three-roll mill, to obtain the
electrically conductive paste for a front electrode of a
crystalline silicon solar cell.
[0077] The above descriptions are merely preferred embodiments of
the present invention, and are not used to limit the present
invention. Any modifications, equivalent substitutions and
improvements made without departing from the spirit and principle
of the present invention shall fall within the protection scope of
the present invention.
* * * * *