U.S. patent application number 15/155192 was filed with the patent office on 2016-09-08 for electrically conductive paste for front electrode of solar cell and preparation method thereof.
The applicant listed for this patent is Delin Li, Xiaoli Liu. Invention is credited to Delin Li, Xiaoli Liu.
Application Number | 20160260850 15/155192 |
Document ID | / |
Family ID | 50253409 |
Filed Date | 2016-09-08 |
United States Patent
Application |
20160260850 |
Kind Code |
A1 |
Liu; Xiaoli ; et
al. |
September 8, 2016 |
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
glass-free corrosion binder, a metallic powder and an organic
carrier. The corrosion binder is one or more 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
glass-free 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) ; Li; Delin; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Liu; Xiaoli
Li; Delin |
ShenZhen
San Jose |
CA |
CN
US |
|
|
Family ID: |
50253409 |
Appl. No.: |
15/155192 |
Filed: |
May 16, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13894908 |
Jun 21, 2013 |
|
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15155192 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y02E 10/547 20130101;
Y02E 10/50 20130101; H01L 29/456 20130101; H01B 1/16 20130101; H01L
31/1804 20130101; H01L 31/1864 20130101; H01L 31/068 20130101; C09D
5/24 20130101; H01L 31/022425 20130101; H01L 31/028 20130101; H01B
1/22 20130101; H01L 31/02168 20130101 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; C09D 5/24 20060101 C09D005/24; H01B 1/22 20060101
H01B001/22 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 22, 2013 |
CN |
PCT/CN2013/073098 |
Claims
1. 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
plurality of Pb--Te--O-based crystalline particles as a
substantially glass-free corrosion binder with a weight composition
ranging from 0.5 to 12 wt % based on the predetermined total
weight; mixing the plurality of metal particles, the glass-free
corrosion binder, and the organic carrier to form a mixture
material; and grinding the mixture materials to obtain a conductive
paste.
2. The method of claim 1 wherein the glass-free corrosion binder
comprises 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.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, the melting point being one
value within a range from 440.degree. C. to 760.degree. C.
depending on variation of said combination.
3. The method of claim 1 wherein providing a plurality of
Pb--Te--O-based crystalline particles 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 Pb--Te--O-based
crystalline particles.
4. The method of claim 1 wherein providing a plurality of
Pb--Te--O-based crystalline particles alternatively comprises:
introducing PbTe alloy vapor into a reaction chamber filled with
oxygen atmosphere; reacting the PbTe 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
to 25.degree. C. by natural convection to obtain a Pb--Te--O based
crystalline compound; and pulverizing the Pb--Te based crystalline
compound to obtain the plurality of Pb--Te--O-based crystalline
particles.
5. The method of claim 1 wherein providing a plurality of
Pb--Te--O-based crystalline particles alternatively 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 to 25.degree. C. by
natural convection in air to obtain Pb--Te--O-based crystal
compounds; pulverizing the Pb--Te--O-based crystal compounds to
small chunks; and grinding the small chunks to obtain the plurality
of Pb--Te--O-based crystalline particles.
6. The method of claim 1 wherein providing a plurality of
Pb--Te--O-based crystalline particles alternatively 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 by natural
convection to 25.degree. C.; and pulverizing and grinding the
product material to obtain the plurality of Pb--Te--O based
crystalline particles.
7. The method of claim 1 wherein the plurality of Pb--Te--O based
crystalline particles has particle sizes ranging from 0.1 .mu.m to
15.0 .mu.m.
8. The method of claim 1 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.
9. The method of claim 1 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.
10. The method of claim 1 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.
11. The method of claim 1 wherein the plurality of metal particles
has particle sizes ranging from 0.1 .mu.m to 5.0 .mu.m.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 13/894,908 filed on Jun. 21, 2013 and 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
glass-free 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
glass-free corrosion binder is a Pb--Te--O crystal compound having
a fixed melting temperature between 440.degree. C. and 760.degree.
C. The metallic powder and the glass-free 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 glass-free corrosion binder made from a
plurality of 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 glass-free 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, glass-free corrosion binder, and organic
carrier followed by grinding so that the metallic powder and the
glass-free corrosion binder are uniformly randomly dispersed in the
organic carrier. The glass-free 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
glass-free 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
glass-free 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 glass-free corrosion binder made
from a plurality of 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
glass-free corrosion binder is one or a combination of two or more
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 glass-free 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 glass-free corrosion binder. The molten glass-free
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 glass-free corrosion binder, and further by
controlling the way of mixing with metallic powder and organic
carrier, the conductive paste bearing this full-crystal glass-free
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 Pb--Te--O
crystalline glass-free corrosion binder.
[0021] FIG. 5 is an exemplary diagram of XRD measured from a
Pb--Te--O-based crystalline compound glass-free 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 glass-free
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 glass-free corrosion binder is a 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
glass-free corrosion binder are randomly dispersed in the organic
carrier. The glass-free corrosion binder of the present invention
includes 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.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 glass-free 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 glass-free 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 glass-free 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 glass-free 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 glass-free corrosion binder and a glass frit 0.5 to 12
parts. The glass-free corrosion binder is one or a combination of
two or more 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. 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 glass-free 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.
[0035] 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 glass-free 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.
[0036] 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 glass-free 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
glass-free 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.
[0037] 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.
[0038] I. Glass-Free Corrosion Binder
[0039] 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.
[0040] 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 (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.
[0041] The glass-free corrosion binder of the present invention is
a crystalline compound, has a melting point, and is different from
the glass frit. The glass-free 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 glass-free 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.
[0042] The crystal glass-free corrosion binder is distinctly
different from the conventional glass frit in terms of its internal
atomic structure. Each particle in the prepared glass-free
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
glass-free 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
glass-free 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 glass-free
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.
[0043] Additional difference between crystalline glass-free
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 glass-free
corrosion binders as described above.
[0044] The glass-free 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 glass-free 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 glass-free
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.
[0045] In an exemplary embodiment, the glass-free 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 glass-free 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 glass-free 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 glass-free 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.
[0046] 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.
[0047] 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 glass-free 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
glass-free corrosion binder in powered form obtained via above
preparation method shows a substantially PbTeO.sub.3 crystalline
characteristic.
[0048] 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 0.1.about.15 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.
[0049] 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.
[0050] The electrolytic conductive paste of the present invention
contains a glass-free corrosion binder which is a Pb--Te--O-based
crystalline compound. During the process of sintering, the
Pb--Te--O based crystalline compound glass-free corrosion binder
changes liquid from solid when the temperature reaches the melting
point. Before the melting point, the glass-free 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 glass-free 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 glass-free 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 glass-free
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
glass-free 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 glass-free 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 %.
[0051] II. Metallic Powder
[0052] 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 range 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.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] III. Organic Carrier
[0058] 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.
[0059] IV. Preparation Method of the Electrically Conductive
Paste
[0060] 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 glass-free corrosion binder 0.5 to
12 parts. The glass-free corrosion binder is a 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 glass-free corrosion binder,
metallic powder and organic carrier, to obtain the electrically
conductive paste for a front electrode of a solar cell.
[0061] 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 glass-free corrosion
binder is prepared, where the method for preparing the glass-free
corrosion binder is as described above; the glass-free corrosion
binder and the metallic powder are premixed. Then, an organic
compound is weighed, and an organic carrier is prepared. Finally,
the premixed glass-free 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.
[0062] Several exemplary embodiments of preparation of an
electrically conductive paste for an electrode are described below.
In an embodiment, first, a weighed glass-free 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 glass-free 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 glass-free 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 glass-free 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.
[0063] 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
glass-free 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 glass-free corrosion binder is controlled within a
range of 1 to 10 wt %. In another preferred embodiment, the
glass-free corrosion binder is controlled within a range of 3 to 8
wt %. The glass-free 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 Pb--Te--O based crystalline
compounds. Selection of the Pb--Te--O-based crystalline compound
for the glass-free 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 glass-free 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.
[0064] 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.
[0065] In the electrically conductive paste for a front electrode
of a solar cell of the present invention, a glass-free 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 glass-free corrosion binder, the glass-free
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.
[0066] The electrically conductive paste for a front electrode of a
crystalline silicon solar cell of the present invention contains a
glass-free corrosion binder. During the process of sintering the
electrically conductive paste, when the temperature reaches the
melting point of the glass-free corrosion binder, the glass-free
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
glass-free 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.
[0067] 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
[0068] 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 glass-free corrosion binder, 81.7 parts of a
metallic powder and 10 parts of an organic carrier. The glass-free
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 glass-free 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 glass-free corrosion binder, the metallic powder
and the organic carrier according to the above formulation, the
glass-free 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
[0069] 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 glass-free corrosion binder, 81.7 parts of a
metallic powder and 11 parts of an organic carrier. The glass-free
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 glass-free
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 glass-free 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 glass-free 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
[0070] 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 glass-free corrosion binder, 80 parts of a metallic
powder and 11.2 parts of an organic carrier. The glass-free
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 agent 1 and 2.4
parts of PVB. Preparation of the glass-free 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 glass-free corrosion
binder, the metallic powder and the organic carrier according to
the above formulation, the metallic powder and the glass-free
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
[0071] 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 glass-free corrosion binder, 86.6 parts of a
metallic powder and 10 parts of an organic carrier. The glass-free
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 glass-free 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 glass-free corrosion
binder, the metallic powder and the organic carrier according to
the above formulation, the metallic powder and the glass-free
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
[0072] 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 glass-free corrosion binder, 72 parts of a metallic
powder and 18.2 parts of an organic carrier. The glass-free
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 glass-free 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 glass-free corrosion binder, the metallic powder and
the organic carrier according to the above formulation, the
metallic powder and the glass-free 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
[0073] 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 glass-free corrosion binder, 85.6 parts of a
metallic powder and 5.5 parts of an organic carrier. The glass-free
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 glass-free 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 glass-free corrosion binder, the
metallic powder and the organic carrier according to the above
formulation, the metallic powder and the glass-free 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
[0074] 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 glass-free corrosion binder, 80
parts of a metallic powder and 15 parts of an organic carrier. The
glass-free 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 glass-free 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 glass-free corrosion binder, the metallic
powder and the organic carrier according to the above formulation,
the metallic powder and the glass-free 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.
[0075] 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.
* * * * *