U.S. patent application number 14/621681 was filed with the patent office on 2015-06-11 for conductive paste for front electrode of semiconductor device and method of manufacturing thereof.
The applicant listed for this patent is Soltrium Technology, LTD. Shenzhen. Invention is credited to Delin Li, Xiaoli Liu.
Application Number | 20150162481 14/621681 |
Document ID | / |
Family ID | 50086358 |
Filed Date | 2015-06-11 |
United States Patent
Application |
20150162481 |
Kind Code |
A1 |
Liu; Xiaoli ; et
al. |
June 11, 2015 |
CONDUCTIVE PASTE FOR FRONT ELECTRODE OF SEMICONDUCTOR DEVICE AND
METHOD OF MANUFACTURING THEREOF
Abstract
The present invention provides a conductive paste characterized
by a crystal-based corrosion binder being combined with a Pb-free
glass frit and mixed with a metallic powder and an organic carrier.
Methods for preparing each components of the conductive paste are
disclosed including several embodiments of preparing
Pb--Te--O-based crystallized corrosion binder characterized by
melting temperatures in a range of 440.degree. C. to 760.degree. C.
and substantially free of any glass softening transition upon
increasing temperature. Method for preparing the conductive paste
includes mixture of the components and a grinding process to ensure
all particle sizes in a range of 0.1 to 5.0 microns. Method of
applying the conductive paste for the formation of a front
electrode of a semiconductor device is presented to illustrate the
effectiveness of the crystal-based corrosion binder in transforming
the conductive paste to a metallic electrode with good ohmic
contact with semiconductor surface.
Inventors: |
Liu; Xiaoli; (ShenZhen,
CN) ; Li; Delin; (San Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Soltrium Technology, LTD. Shenzhen |
|
|
|
|
|
Family ID: |
50086358 |
Appl. No.: |
14/621681 |
Filed: |
February 13, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13787997 |
Mar 7, 2013 |
|
|
|
14621681 |
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Current U.S.
Class: |
438/98 |
Current CPC
Class: |
H01B 1/16 20130101; Y02E
10/50 20130101; C09D 5/24 20130101; H01L 31/022425 20130101; H01L
31/18 20130101 |
International
Class: |
H01L 31/18 20060101
H01L031/18; H01L 31/0224 20060101 H01L031/0224 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 4, 2013 |
CN |
PCT/CN2013/071352 |
Claims
1. A method for manufacturing a front electrode of a semiconductor
device, the method comprising: providing a semiconductor device
including an insulation surface coating; printing a conductive
paste material overlying a patterned region of the insulation
surface coating, the conductive paste material comprising, a
plurality of metal particles with a weight composition ranging from
70 to 93 wt % based on a given total weight of the conductive
paste; a corrosion binder made from a plurality of Pb--Te--O-based
crystalline particles and a glass frit made from a plurality of
glass particles free from Pb element with a combined weight
composition ranging from 2 to 15 wt % based on the given total
weight; an organic carrier with a weight composition ranging from 5
to 25 wt % based on the given total weight, wherein the organic
carrier dispersedly holds the plurality of metal particles, the
plurality of Pb--Te--O-based crystalline particles, and the
plurality of glass particles, all particles having sizes in a range
of 0.1 to 5.0 microns; subjecting at least the conductive paste
material and the patterned region in contact with the insulation
surface coating to a sintering process with temperatures ramped up
to about 900.degree. C. followed by a cooling back, wherein the
temperature rise causes releasing of the organic carrier, melting
of the corrosion binder after the releasing of the organic carrier
along with softening of the glass frit, and sintering of the
plurality of metal particles into a metallic bulk assisted by
wetting effects from the molten corrosion binder and soften glass
frit. The molten corrosion binder and softened glass frit induce an
etch-removing of the insulation surface coating at the patterned
region to form a direct bonding between the sintered metallic bulk
with the semiconductor device.
2. The method of claim 1 wherein the corrosion binder made from a
plurality of Pb--Te--O-based crystalline particles comprises one
crystal 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.4O.sub.9,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5, and
Pb.sub.3TeN.sub.2O.sub.8 or a mixture of two or more these crystal
compounds characterized by a melting temperature ranging from
440.degree. C. to 760.degree. C.
3. The method of claim 1 wherein the glass frit comprises one or a
combination of two or more selected from Bi--Si--B--Zn--O,
Zn--B--P--Li--O, Bi--V--Ba--P--O, B--Al--Li--O, Bi--Si--O,
Bi--Te--O, Bi--B--O, P--Zn--Na--O, Na--Al--B--O, B--Zn--Ba--O, and
V--P--Ba--O-based glass materials characterized by a glass
softening transition temperature in a range of 500.degree. C. to
650.degree. C.
4. The method of claim 1 wherein the corrosion binder and the glass
frit comprise a weight composition ratio ranging from 5:95 to 95:5
per any fixed amount of the conductive paste material.
5. The method of claim 1 wherein the etch-removing of the
insulation surface coating is accelerated as the corrosion binder
is quickly melted into a liquid phase accumulated at the patterned
region under the printed conductive paste material to allow
substantially complete penetration of the metallic bulk through the
insulation surface coating to form an electrode having a conductive
contact with the semiconductor device at the patterned region.
6. The method of claim 1 wherein the plurality of metal particles
comprises one metal material selected from silver, gold, platinum,
copper, iron, nickel, zinc, titanium, cobalt, chromium, manganese,
palladium, and rhodium or a metal alloy of two or more of them.
7. The method of claim 1 wherein the plurality of metal particles
comprises one metal material selected from copper, iron, nickel,
zinc, titanium, cobalt, chromium, manganese, or a metal alloy of
two or more of them, and at least partially being coated by a
thickness of silver layer in a range of 10.about.500 nm.
8. The method of claim 1 wherein the plurality of metal particles
comprises a first plurality of silver particles mixed with a second
plurality of silver-coated copper, iron, nickel, zinc, titanium,
cobalt, chromium, manganese particles with a weight composition
ratio between the first plurality of silver particles and the
second plurality of silver-coated copper, iron, nickel, zinc,
titanium, cobalt, chromium, manganese particles in a range from
5:95 to 95:5 per any fixed amount of the plurality of metal
particles.
9. The method of claim 1 wherein the plurality of metal particles
comprises particle sizes substantially in a range from 0.1 to 5.0
microns.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] The present application is a divisional application of U.S.
patent application Ser. No. 13/787,997, filed on Mar. 7, 2013. This
application claims priority of a PCT application No.
PCT/CN2013/071352 filed on Feb. 4, 2013, and incorporated by
reference herein to its entirety for all purposes.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a solar cell manufacture
technique. More particularly, the invention is objected to provide
an electrically conductive paste designated for forming front
electrode of solar cells and methods for manufacturing the
conductive paste.
[0003] 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
of exploring renewable energy. The use of solar cells is a typical
means of collecting solar energy, wherein the solar cell made by
crystalline silicon is currently a major solar cell technology and
will be in the market for substantially long time even though new
generation thin-film solar cell has also been developed.
[0004] Crystalline silicon solar cells are in general composed of a
front side electrode, an anti-reflective coating, an emitter, a P-N
junction, a base, an aluminum back surface field, and a back
electrode. The front side electrode collects photon-generated
charge carriers near the front side electrode and supplies
current.
[0005] The front electrode in crystalline silicon solar cells of
the prior art is made from a conductive paste composed of silver
powder, glass frit, one or more additives, and an organic carrier.
Usually a glass frit in the electrically conductive paste has the
following effects: a) wetting the metal powder to promote the
sintering of the metal powder; b) etching the antireflective
coating layer which is an insulating layer (e.g., silicon nitride)
to promote the contact between the sintered metal and the
silicon-based material. In order to achieve a good ohmic contact,
the antireflective coating layer must be etched through but free
from penetrating into the PN junction region of the silicon-based
material.
[0006] The choice of the glass frit, its composition, softening
point, thermal expansion coefficient, wetting properties, and
amount (within the conductive paste), etc. will affect the physical
and chemical changes of the conductive paste in the sintering
process to form the electrode, thereby affecting the solar cell
performance. In the sintering process, the glass frit material is
gradually softened. Within a short process cycle, part of the glass
frit wetting the metal powders while another part of the glass frit
material flows to reach the antireflective coating layer at bottom
and induce an etching reaction there. On one aspect, the amount of
the glass frit is an important factor affecting quality of the
electrode, which also causes many physical constraints to the
manufacture process. If the amount of the glass frit is not enough,
there is no sufficient contact formed between the glass frit
material and the antireflective coating layer to ensure that the
antireflective layer is completely penetrated. If the glass frit is
added too much, the probability of mutual contact between the
conductive silver powders is too low so that the conductive phase
among the as-formed electrode material becomes too scarce, causing
the conduction performance of the front electrode of solar cell
severely deteriorated. On another aspect, the glass frit is
engineered through the selection of glass materials in order to
ensure that a sufficient amount of glass frit is deposited on the
surface of the antireflective layer in the entire process, and
therewith complete the etch reaction to remove the antireflective
coating. But premature softening of the glass frit, can clog the
communicating channels between the metal powders, and hinder the
effective discharge of the organic carrier.
[0007] Pb--Si based glass materials usually are chosen for the
manufacture of the front electrode paste. More recently, Pb--Te
oxide and other oxide materials or fluoride materials are chosen to
go through a series of processes of melting, mixing, and quenching
the molten mixture to form a glass material before milling the
glass material into the glass frit. However, regardless the use of
various alternative materials, the nature of the glass material by
itself sets many physical performance and chemical reaction
constraints such as a narrow sintering process window for
transforming the printed conductive paste to the electrode with
desirable electrical conductivity while preventing from the emitter
being penetrated. Presently, most of the conductive pastes on sale
for manufacturing the front side electrodes of crystalline silicon
based solar cells have these technical limitations.
BRIEF SUMMARY OF THE INVENTION
[0008] 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 corrosion
binder to form good ohmic contact with the semiconductor surface.
In particular, the electrically conductive paste can be applied for
forming a front electrode of a solar cell to improve overall light
conversion efficiency. An alternative objective of the present
invention is to provide a method of making the electrically
conductive paste using a simple process with easy-controlled
conditions and reduced production cost.
[0009] In a specific embodiment, the present invention provides an
electrically conductive paste characterized by compositions
including 70-93 wt % of an electrically conductive powder, 0.5-3.0
wt % of a glass frit, 0.5-15 wt % of a corrosion binder, and 5-25
wt % of an organic carrier, the corrosion binder being
characterized by a plurality of particles with crystallized
structure having 0.1.about.5.0 .mu.m sizes and a fixed melting
point.
[0010] In an alternative embodiment, the present invention provides
a method for making the electrically conductive paste. The method
includes providing a plurality of metal particles with a weight
composition ranging from 70 to 93 wt % based on a given total
weight of the conductive paste. Additionally, the method includes
providing an organic carrier with a weight composition ranging from
5 to 25 wt % based on the given total weight. The method further
includes providing a corrosion binder made from a plurality of
Pb--Te--O-based crystalline particles and a glass frit made from a
plurality of glass particles with a combined weight composition
ranging from 1 to 15 wt % based on the given total weight.
Furthermore, the method includes mixing the plurality of metal
particles, the corrosion binder, the glass frit, and the organic
carrier to form a mixture material. Moreover, the method includes
grinding the mixture material to keep sizes of all the plurality of
metal particles, the plurality of Pb--Te--O-based crystalline
particles, and the plurality of glass particles substantially in a
range of 0.1 to 5.0 .mu.m.
[0011] 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 having an insulation surface coating. Additionally, the
method includes printing a conductive paste material overlying a
patterned region of the insulation surface coating. The conductive
paste material includes a plurality of metal particles with a
weight composition ranging from 70 to 93 wt % based on a given
total weight of the conductive paste. The conductive paste material
further includes a corrosion binder made from a plurality of
Pb--Te--O-based crystalline particles and a glass frit made from a
plurality of glass particles with a combined weight composition
ranging from 1 to 15 wt % based on the given total weight.
Furthermore, the conductive paste material includes an organic
carrier with a weight composition ranging from 5 to 25 wt % based
on the given total weight. The organic carrier dispersedly holds
the plurality of metal particles, the plurality of Pb--Te--O-based
crystalline particles, and the plurality of glass particles within
the conductive paste material. All particles have sizes controlled
in a range of 0.1 to 5.0 microns. Moreover, the method includes
subjecting at least the conductive paste material and the patterned
region in contact with the insulation surface coating to a
sintering process with temperatures ramped up to about 800.degree.
C. followed by a cooling back. The temperature rise causes
releasing of the organic carrier and melting of the corrosion
binder after the releasing of the organic carrier along with
softening of the glass frit and further causes sintering of the
plurality of metal particles into a metallic bulk assisted by
wetting effects from the molten corrosion binder and soften glass
frit. Furthermore, etch-removing of the insulation surface coating
at the patterned region is induced by the molten corrosion binder
and softened glass frit and form an ohmic contact between the
sintered metallic bulk and the semiconductor device.
[0012] Many advantages are provided when crystalline corrosion
binders are added along with the glass frit as functional additives
of the conductive paste for the manufacture of front electrodes of
semiconductor devices. Specifically, the added crystalline
corrosion binder is made from a Pb--Te--O-based crystal compound
bearing ordered atomic structure and fixed melting point in
contrary to the amorphous structure and wide range of softening
temperatures associated with the glass frit made from even the same
Pb--Te--O material with the same composition. These structural and
physical property differences lead to different physical state
transitions during the thermal process. According to embodiments of
the present invention, the corrosion binder exits in the conductive
paste as solid particles having dispersed space from metal
particles and will not clog the intermediate regions between
particles to prevent organic carrier's release. After melt, the
liquid corrosion binder has very low viscosity and can flow down
much faster than soften glass frit towards the insulation interface
region for inducing an etching reaction to remove the insulation
layer sufficiently within a shortened process cycle, resulting in
better metal-semiconductor contact with reduced contact resistance.
At the same time, the glass frit within the conductive paste
according to embodiments of the present invention can have higher
softening temperature ranges to reduce chances of clogging the
release channel of organic carrier due to premature glass
softening. Embodiments of the present invention also shows certain
amount of glass frit plays important role in assisting the
sintering of metal particles together to form a denser metallic
bulk material of the electrode with enhanced weldability. These and
other benefits will be described in more detailed throughout the
present specification and particularly below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Many aspects of the present invention can be better
understood with reference to the following drawings. The components
in the drawings are not necessarily drawn to scale, the emphasis
instead being placed upon clearly illustrating the principles of
the present apparatus. Moreover, in the drawings, like reference
numerals designate corresponding parts throughout the several
views.
[0014] FIG. 1 is a cross sectional view of an electrically
conductive paste printed on antireflection coating surface of a
semiconductor substrate according to an embodiment of the present
invention.
[0015] FIG. 2 is a cross sectional view of an electrode transformed
from the electrically conductive paste on the semiconductor surface
via a sintering process according to an embodiment of the present
invention.
[0016] FIG. 3 is an exemplary diagram showing a process temperature
profile for preparing crystalline corrosion binder according to an
embodiment of the present invention.
[0017] FIG. 4 is an exemplary diagram of XRD measured from a
corrosion binder, as an ingredient in the electrically conductive
paste, showing crystalline characteristics according to a specific
embodiment of the present invention.
[0018] FIG. 5 an exemplary diagram of XRD measured from a glass
frit, as another ingredient in the electrically conductive paste,
showing no crystalline characteristics according to a specific
embodiment of the present invention.
[0019] FIG. 6 is a chart diagram showing a method for forming an
electrically conductive paste according to an embodiment of the
present invention.
[0020] FIG. 7 is a chart diagram showing a method for forming an
electrode on semiconductor device surface from an electrically
conductive paste according to an alternative embodiment of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention relates to a solar cell manufacture
technique. More particularly, the invention is objected to provide
an electrically conductive paste with novel ingredients and
corresponding composition and also provide methods for
manufacturing the electrically conductive paste. Merely by way of
examples, the present invention provides methods for manufacturing
the electrically conductive paste and applying the conductive paste
for forming front electrode of silicon-based solar cells with
improved electrical performance.
[0022] In an embodiment, the electrically conductive paste is made
by, based on total weight of all materials thereof, 70-93 wt % of
an electrically conductive powder, 0.5-3.0 wt % of a glass frit,
0.5-15 wt % of a corrosion binder, and 5-25 wt % of an organic
carrier, wherein the corrosion binder is characterized by a
plurality of particles with crystallized structure having
0.1.about.5.0 .mu.m sizes and a fixed melting point. As defined,
the organic carrier is counted as part of the total materials in
the conductive paste composition.
[0023] FIG. 1 is a cross sectional view of an electrically
conductive paste printed on antireflection coating surface of a
semiconductor substrate according to an embodiment of the present
invention. This diagram is merely an example, which should not
unduly limit the scope of the claims herein. One of ordinary skill
in the art would recognize other variations, modifications, and
alternatives. As shown, a cross sectional view of an electrical
conductive paste 120 is applied on a surface 110 of a semiconductor
device, which comprises semiconductor substrate 100, emitter 102,
and insulating layer 110, by screen printing technique. The
electrical conductive paste 120 includes several solid ingredients
provided in powder form including a metallic powder 122, a glass
frit 124, and a corrosion binder 126, mixed with an organic carrier
128. The semiconductor device can be made by semiconductor
substrate 100 having an emitter 102 and insulating layer 110 coated
over the emitter surface. The conductive paste 120 is only disposed
on a patterned region 121 (for example, a pre-patterned line) with
substantially all the inorganic powder materials (122, 124, and
126) carried by the organic media 128 to penetrate through the
printing screen and form a contact at its bottom region 121 with
the insulating layer 110. In a specific embodiment, the
semiconductor device is a silicon-based solar cell and the
insulating layer 110 is an anti-reflective coating (e.g., SiN.sub.x
material).
[0024] FIG. 2 is a cross sectional view of an electrode transformed
from the electrically conductive paste on the semiconductor surface
via a sintering process according to an embodiment of the present
invention. This diagram is merely an example, which should not
unduly limit the scope of the claims herein. One of ordinary skill
in the art would recognize other variations, modifications, and
alternatives. As shown, the conductive paste 120 is subjected to a
thermal treatment process with a baking at about 250.degree. C.
followed by ramping to temperatures around 800.degree. C. for the
formation of a front electrode 200 on the emitter 102 of the
semiconductor device. The thermal treatment process is a sintering
process to bond the originally dispersed metallic powder 122 into a
metallic bulk material 220 and to cause an etching reaction,
assisted by the liquid or molten corrosion binder 210 and soften
glass phase 230, to form an ohmic contact layer 240 between the
bulk material 220 and emitter 102 of the semiconductor device. The
etching reaction leads to a substantially complete removal of all
insulating layer 110 within the interface region to form an ohmic
contact layer 240 between the sintered metallic bulk 220 and the
emitter 102 of the semiconductor device. The ohmic contact layer
240 comprises bulk material 200 and residue of the etching reaction
of the anti-reflective coating. At the interface region, a
conduction path is formed as temperature further cooled down from a
peak value (around 800.degree. C.) via metal precipitate colloids
and crystal grains structures to connect the metallic bulk material
220 and the emitter 102 of the semiconductor device. As a result,
the metallic bulk 220 is transformed into an electrode having a
good ohmic contact with the emitter 102 of the semiconductor
device. As disclosed throughout the specification, the
transformation process of the conductive paste to the electrode is
highly depended on the physical and chemical nature of the
conductive paste and specially depended on the individual
ingredient therein. Upon the illustrations of each ingredient
provided according to embodiments of the present invention in
following sections of the specification, the improvement in the new
conductive paste and manifestations of the electrodes formed from
the conductive paste on the performance of associated silicon-based
solar cells will be revealed.
[0025] In an embodiment of the present invention, the electrically
conductive paste for forming a front electrode of crystalline
silicon based solar cells uses a glass frit with relatively high
glass softening temperatures ranging from 500.degree. C. to
650.degree. C. mixed with a corrosion binder with crystalline
structure, so that the front electrode formed from corresponding
conductive paste can substantially improve integrated performance
of the crystalline silicon solar cells. When a conventional
conductive paste uses only glass frit as additive in addition to
the electrically conductive powder, the glass frit usually is made
by materials having glass softening temperatures starting at
300.degree. C. for effectively etching an insulating layer on top
of the semiconductor surface and facilitating a formation of
sintered conductive bulk from the powders. However, this approach
requires relative larger amount of the glass frit based on 100%
total weight of the conductive paste. A consequence of more glass
frit in the conductive paste is high contact resistance and high
series resistance through the as-formed electrode. Also it is
difficult to control how fast the softened glass frit sinks towards
the insulating layer region and corresponding etching rate of the
insulating layer during the sintering process unless the sintering
process window is delicately fine-tuned, thereby increasing
production cost.
[0026] When adding crystalline corrosion binder material with a
melting point about 440-760.degree. C. (depending on specific
chemical compound used) into the glass frit, glass materials with
relatively high softening temperatures that are closer to the
melting point can be chosen to make the glass frit so that the
organic carrier can be substantially released before the glass frit
softens or the corrosion binder crystal melts to avoid the clog at
inter-particle spacing without block the releasing channel of the
organic carrier. Once the sintering temperature reaches the melting
point of crystalline corrosion binder, it quickly turns into liquid
phase and sink through the conductive powders with very low
viscosity so that the liquid phase can wet the particles of the
conductive powders and also sink down to reach the insulating layer
on the semiconductor device to induce an etching reaction to remove
the insulating layer. The softened glass frit, if their softening
temperatures are around the melting point above, also starts their
role in wetting the conductive powders and slowly flowing downward
to reach the insulating layer and to participate in the etching
reaction for removing the insulating layer. As a result of adding
crystalline corrosion binder, the etching reaction at the
insulating layer occurs faster and more complete while the etching
depth can still be controlled to avoid over-etching into the
emitter 102 of the semiconductor device by limiting the amount of
corrosion binder as well as the glass frit. Another advantage of
adding crystalline corrosion binder, the conductive powders can be
wetted easier and quicker because of existence of low viscosity
liquid phase in addition to the relative high viscosity glass
phase. The wetted particles promote the sintering process to
transform a plurality of conductive powders into a bulk material
and more effective sintering leads to a formation of a denser bulk
conductor material (used as electrode), yielding a lower electrical
resistance and stronger soldering strength for connecting external
electrical leads. Therefore, by adding crystalline corrosion binder
as an alternative additive along with traditional glass frit for
the formation of a conductive paste many advantages can be realized
for the manufacture of solar cells in terms of higher photovoltaic
conversion efficiency, higher open circuit voltage, higher
short-circuit current, lower series resistance, and higher filling
factor. More details on each composition components within the
conductive paste according to embodiments of the present invention
are given below.
Glass Frit
[0027] In an embodiment of the present invention, the glass frit as
an additive of the conductive paste is formed from materials that
have the softening temperature ranged from 500.degree. C. to
650.degree. C. Specifically, the glass frit is selected from one of
the group of Pb--Si--O, Pb--B--O, Pb--Te--O, Bi--Si--O, Bi--B--O,
Bi--Te--O, P--Zn--Na--O, B--Al--Na--O, B--Zn--Ba--O, and
V--P--Ba--O based glass materials. The dimension of the glass frits
is between 0.1 .mu.m and 5.0 .mu.m.
[0028] In a specific embodiment, the glass frit is Pb--Si--Al--B--O
based glass wherein the weight percentage of its ingredients (based
on total solids of the glass frit) can be:
PbO 65.about.85 wt %,
SiO.sub.2 10.about.20 wt %,
Al.sub.2O.sub.3 0.1.about.10 wt %,
B.sub.2O.sub.3 0.1.about.10 wt %, and
[0029] additional oxide 0.about.5 wt %, which include a metal oxide
of one of following metal elements: Li, Na, K, Mg, Ca, Sr, Ba, Ti,
Zr, Sc, Zn, and Bi or other chemicals that can decompose into the
metal oxide just mentioned.
[0030] In another specific embodiment, the glass frit is
Bi--Si--B--Zn--O based glass wherein the weight percentage of its
ingredients (based on total solids of the glass frit) can be:
Bi.sub.2O.sub.3 65.about.85 wt %,
SiO.sub.2 10.about.20 wt %,
B.sub.2O.sub.3 0.1.about.10 wt %,
ZnO 0.1.about.10 wt %, and
[0031] additional oxide 0.about.5 wt %, which include a metal oxide
of one of following metal elements: Li, Na, K, Mg, Ca, Sr, Ba, Ti,
Al, Zr, Sc, W, Co, Cu, Fe, Ni, Sn, Mn, and Ag or other chemicals
that can decompose into the metal oxide just mentioned.
[0032] In yet another specific embodiment, the glass frit is
Zn--B--P--Li--O based glass wherein the weight percentage of its
ingredients (based on total solids of the glass frit) can be:
B.sub.2O.sub.3 0.1.about.10 wt %,
ZnO 30.about.50 wt %,
P.sub.2O.sub.5 30.about.50 wt %,
Li.sub.2O 0.1.about.10 wt %, and
[0033] additional oxide 0.about.5 wt %, which include a metal oxide
of one of following metal elements: Na, K, Mg, Ca, Sr, Ba, Ti, Al,
Zr, Sc, Ni, Co, Cu, Fe, Sn, Mn, Ag, Co, Bi, and Ga or other
chemicals that can decompose into the metal oxide just
mentioned.
[0034] In still another specific embodiment, the glass frit is
B--Al--Li--O based glass wherein the weight percentage of its
ingredients (based on total solids of the glass frit) can be:
B.sub.2O.sub.3 60.about.85 wt %,
Al.sub.2O.sub.3 3.about.30 wt %,
Li.sub.2O 1.about.10 wt %, and
[0035] additional oxide 0.about.5 wt %, which include a metal oxide
of one of following metal elements: Na, K, Mg, Ca, Sr, Ba, Ti, Zr,
Sc, Zn, Co, Cu, Fe, Sn, Mn, and Ag or other chemicals that can
decompose into the metal oxide just mentioned.
[0036] In still yet another specific embodiment, the glass frit is
Pb--Te--Li--O based glass wherein the weight percentage of its
ingredients (based on total solids of the glass frit) can be:
PbO 20.about.50 wt %,
TeO.sub.2 40.about.70 wt %,
Li.sub.2O 0.1.about.10 wt %, and
[0037] additional oxide 3.about.15 wt %, which include a metal
oxide of one of following metal elements: Na, K, Mg, Ca, Sr, Ba,
Ti, Si, B, Al, Zr, Sc, Zn, Co, Cu, Fe, Sn, Mn, and Ag or other
chemicals that can decompose into the metal oxide just
mentioned.
[0038] In still further another specific embodiment, the glass frit
is Na--Al--B--O based glass wherein the weight percentage of its
ingredients (based on total solids of the glass frit) can be:
Na.sub.2O 5.about.20 wt %,
Al.sub.2O.sub.3 5.about.40 wt %
B.sub.2O.sub.3 35.about.75 wt %, and
[0039] additional oxide 0.about.5 wt %, which include a metal oxide
of one of following metal elements: Li, K, Mg, Ca, Zn, Sr, Ba, Ti,
Zr, Sc, Si, Ni, Co, Cu, Fe, Sn, Mn, and Ag or other chemicals that
can decompose into the metal oxide just mentioned.
[0040] In furthermore another specific embodiment, the glass frit
is Bi--V--Ba--P--O based glass wherein the weight percentage of its
ingredients (based on total solids of the glass frit) can be:
V.sub.2O.sub.5 45.about.60 wt %
Bi.sub.2O.sub.3 5.about.25 wt %
BaO 5.about.25 wt %
P.sub.2O.sub.5 15.about.30 wt %, and
[0041] additional oxide 0.about.5 wt %, which include a metal oxide
of one of following metal elements: Li, K, Mg, Ca, Si, Zn, Sr, Ba,
Ti, Zr, Sc, Cr, Co, Cu, Fe, Sn, Mn, Sb, and Ag or other chemicals
that can decompose into the metal oxide just mentioned.
Corrosion Binder
[0042] The corrosion binder added into the conductive paste
according to embodiments of the present invention is a crystalline
compound, having a fixed melting point substantially free from any
glass softening transition. In certain embodiments, the corrosion
binder is at least one selected from the following
lead-tellurium-based crystalline oxide 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.4O.sub.9,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5 and
Pb.sub.3TeN.sub.2O.sub.8 or a mixture with two or more of them,
having a melting temperature between 440.degree. C. and 760.degree.
C. The melting temperature value for any actual supplied corrosion
binder, of course, is depended on the composition of the compounds.
The corrosion binder is prepared as a plurality of particles,
having typical crystalline characteristics, unlike the convention
glass frit additive being provided as a plurality of glass
particles without a fixed melting point instead a wide softening
temperature range.
[0043] In a specific embodiment, the crystal-based corrosion binder
is provided with at least one shape selected from sphere, droplet,
aciculate, dendritic-shape, massive, spherical-shape, flake,
granular-shape, colloidal-particle-shape or a combination of any
two or more of them and each particle size is controlled to be in a
range of 0.1.about.5.0 .mu.m. When the corrosion binder particle
diameter is less than 0.1 .mu.m, the corrosion binder particles do
not match with the metal powders (more details are given below) in
the conductive paste, which do not facilitate the sintering process
by causing a dispersing effect. If the corrosion binder particle
diameter is more than 5.0 .mu.m, the as-formed paste is difficult
for screen printing during its application onto semiconductor
device surfaces as the large particles are difficult to pass
through screening. The corrosion binder can be prepared by one of
the following methods: liquid phase chemical reaction; gas-phase
chemical reaction; molten reaction and controlled cooling method;
vacuum melting and controlled cooling method. More details on the
corrosion binder preparation methods are described below.
[0044] In a specific embodiment, the crystal corrosion binder
selected for the conductive paste is a Pb--Te--O-based crystal
material is prepared using a liquid phase chemical reaction method.
In particular, a Tellurium-based compound solution is mixed with
Lead Acetate solution. The mixed solution is stirred at
80.about.150.degree. C. while maintaining the stirring speed in a
range of 800.about.2500 r/min. to cause a chemical reaction. The
reaction, after 2.about.4 hours, produces solid precipitate which
can be collected by a solid-liquid separation. The solid
precipitate is washed until PH value of the filtrate is about
5.about.7, then the solid precipitate is accumulated and dried at
about 150.degree. C. for 2 hours. Afterward, the dried solid
includes one or more types of Pb--Te--O-based crystal compounds
characterized by melting temperatures within a range of 440 to
760.degree. C. Fine powders (having particle size ranging between
0.1 and 5.0 microns) of these Pb--Te--O-based crystal compounds are
used as the corrosion binder additive of the conductive paste. Of
course, many variations, alternatives, and modifications in the
process conditions may be applied depending on embodiments to form
similar Pb--Te--O-based crystal compounds with their melting
temperatures and particle sizes in the above desired range.
[0045] In the above reaction process, the Tellurium-based compound
solution is a telluric acid solution with or without oxygen
deficit, or a Tellurite solution with or without oxygen deficit.
The solution concentration is in a range of 0.1.about.6 mol/L.
Correspondingly the Lead Acetate solution concentration is in a
range of 0.1.about.10.sup.8 .mu.mol/L. The proportion of the two
reactants, tellurium-based compound and Lead Acetate, can be added
in accordance with the molar relationship of the chemical reaction
equation, such as the molar ratio of 1:1. Of course, in order to
drive the chemical reaction to a forward direction for improving a
formation rate of the Pb--Te--O-based crystalline compound
precipitate, the added amount of one reactant may be an appropriate
excess.
[0046] In another specific embodiment, the crystal corrosion binder
selected for the conductive paste is a Pb--Te--O based crystal
material is prepared using a gas-phase chemical reaction method.
The method includes continuously introducing Pb.sub.xTe.sub.y alloy
vapor into a reaction chamber under oxygen atmosphere and inducing
chemical reaction for forming the Pb--Te--O-based crystal compound.
In the embodiment, the molar ratio of Pb and Te is set to 2:3, or
1:1, or 1:4 or 3:1. The reaction chamber is set at about
1000.about.1400.degree. C. for about 2.about.4 hours. Then heating
is stopped in the chamber to allow a powder-like material obtained
from the reaction to accumulate at a bottom region of the chamber.
As it is cooled naturally the Pb--Te--O-based crystalline compound
is collected as a powder material. Of course, many variations,
alternatives, and modifications in the process conditions may be
applied depending on embodiments to form Pb--Te--O-based
crystalline powders with similar properties.
[0047] In still another specific embodiment, the crystal corrosion
binder selected for the conductive paste is a Pb--Te--O-based
crystal material is prepared using a molten reaction and controlled
cooling method. In this method, the Tellurium Oxide and Lead Oxide
in their solid phases are provided in a furnace to start a solid
phase reaction at 500.about.900.degree. C. The reaction product is
pulverized after natural cooling and further milled to form the
Pb--Te--O-based crystal compound in powder form. In certain
implementations, Tellurium Oxide and Lead Oxide are provided with
one of the following molar ratios: 2:3, 1:1, 1:4 or 3:1. In
particular, the Tellurium Oxide is TeO.sub.2, Lead Oxide is PbO or
other oxides. The reaction can be carried out in a high temperature
furnace. The Pb--Te--O-based crystal compound is generated by
crushing, grinding, milling the solid phase reaction product,
preferably forming a powder material with particle diameters within
a range of 0.1.about.5.0 .mu.m. Of course, many variations,
alternatives, and modifications in the process conditions may be
applied depending on embodiments to form Pb--Te--O-based
crystalline powders with similar properties. For example, after
Tellurium Oxide and Lead Oxide are molten and mixed, heated air
flow or inert gas flow (such as N.sub.2, Argon, etc.) may be
introduced to flow through the surface of the melt for reducing the
cooling rate to obtain the Pb--Te--O-based crystal compound.
[0048] In yet still another specific embodiment, the crystal
corrosion binder in the conductive paste is a Pb--Te--O-based
crystal material prepared using a vacuum melting and controlled
cooling method. In a vacuum furnace, a mixture of Tellurium Oxide
and Lead Oxide with a molar ratio of 2:3, or 1:1, or 1:4 or 3:1, is
melted and further mixed at 500.about.900.degree. C. After the
process, the melt is cooled naturally within the furnace, or cooled
in air outside the furnace, or slowly cooled following a
predetermined cooling temperature profile to form a bulk material.
The Pb--Te--O-based crystal compound is generated as a powder
material having particle size substantially in a range of 0.1 to
5.0 .mu.m by crushing, grinding, milling the bulk material. Of
course, many variations, alternatives, and modifications in the
process conditions may be applied depending on embodiments to form
Pb--Te--O-based crystalline powders with similar properties. For
example, after Tellurium Oxide and Lead Oxide are molten and mixed,
heated air flow or inert gas flow (such as N.sub.2, Argon, etc.)
may be introduced to flow through the surface of the melt for
reducing the cooling rate to obtain the Pb--Te--O-based crystal
compound.
[0049] 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.
[0050] 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 very similar compositions. Conventional glass frit is
typically formed by first heating the oxide material till melt
followed by quenching the melt in certain processes. For example,
U.S. Patent Publication No. US2011/0308595 discloses a conductive
paste for front electrode of crystalline silicon solar cell. The
conductive paste includes a glass frit made from lead tellurium
oxide (Pb--Te--O) material. A method of glass frit preparation is
shown as follows: lead oxide and tellurium oxide are mixed first,
and heated to a molten state in an atmosphere of air or under an
oxygen atmosphere; then, the molten material is subjected to a
quenching process to form a solid material; the solid material is
ground or milled to form the lead tellurium oxide (Pb--Te--O)-based
glass frit. In another example, PCT Patent Pub. No. WO2012/129544
discloses a similar melting-and-quenching method for preparing a
glass frit using several different lead oxides and tellurium oxides
as raw materials. The glass frit obtained has a softening
temperature zone of 300-800.degree. C. In another example, US
Patent Pub. No. US2011/0232747 discloses a method of preparing
glass frit by mixing TeO.sub.2, PbO, and Li.sub.2CO.sub.3 row
materials, heating up to 900.degree. C. to melt and holding for
molten state mixture for one hour, then quenching the molten
mixture to form Pb--Te--O-based glass frit. In yet another example,
US Patent Pub. No. US2011/0232746 also discloses a method for
preparing the glass frit. This glass frit is non-crystal and does
not have a fixed melting temperature. From a dynamics perspective,
the crystallization process must overcome a certain energy barrier,
including interfacial energy for nucleation and activation energy
for new nuclei to grow up via atomic diffusion. If the energy
barrier is relatively high and the cooling rate of the material
from melt is too fast to cause viscosity increasing greatly so that
no sufficient movement of atoms exists for overcoming the barrier
for cause a regular atomic arrangement. Therefore, the quenching
process proposed in the prior art, which is essentially a
super-cooling process, substantially results in formation of a
glass state material with amorphous structure instead of leading to
formation of crystalline nuclei and growth of a crystal structure.
Alternatively, by changing the cooling process to reduce the
cooling rate substantially, the molten mixture formed from same raw
material can be transformed into crystalline solid compound. Of
course, there are many other methods for forming crystalline
compounds from certain raw materials other than using a
melting-and-cooling process.
[0051] 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.
[0052] 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 800.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 800.degree. C. in the furnace to 732.degree. C.
in 3 seconds with an average cooling rate of 1360.degree. C./min.
FIG. 3 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 5 .mu.m. Using XRD
to exam samples of the fine powders, the resulted diffraction
pattern (marked as M1) is shown in FIG. 4, 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 20 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.
[0053] In another example, same chemical compounds TeO.sub.2 and
PbO in powders are mixed with the same mole ratio of 1:1. The
powder mixture is placed in a crucible and heated in air atmosphere
to form a melt. Then the melt is cooled quickly by a quenching
method. In an implementation, the melt is quenched by pouring the
melt directly on a stainless steel platen or using metal roller 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 is quenched by pouring into deionized
water to form a bulk material. The bulk platelet material is
crunched by grinding into small particles which are further
ball-milled into fine powders having D.sub.50 sizes of 0.1.about.5
microns. Using XRD to exam samples of the fine powders, the
resulted diffraction pattern (marked as M2) is shown in FIG. 5. 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.
[0054] In the above-mentioned two examples, although the use of the
same composition and proportion of TeO.sub.2 and PbO oxide powder
material, different preparation process yields different material
with different atomic structure and physical property. A method
according to the present invention leads to powders of
Pb--Te--O-based crystalline particles while another method
following prior art references yields only powders of glass
particles. Consequently, the obtained different powders behave
differently in their physical state transformation during a
sintering/firing process in associated with the application of the
conductive paste. Specifically, as temperature increases during the
sintering/firing process the particles with crystalline structure
added as additive in the conductive paste go through a direct
physical phase transition from a solid phase to a liquid phase
while the particles with glass structure in the same conductive
paste go through a phase transition from a solid state to a
glass-softening state before finally transforming into a liquid
state.
[0055] In a specific embodiment, the crystal-based corrosion binder
is controlled in a weight composition range from 0.1 wt % to 15 wt
% based on total weight of conductive paste formed for the
formation of front electrode on top of a PN junction of a
crystalline silicon solar cell. If the amount of crystal-based
corrosion binder is greater than 15 wt %, the conductive paste may
burn through the PN junction, causing a short circuit. If the
crystal-based corrosion binder is less than 0.1 parts by weight, it
may be difficult to completely remove an antireflective layer
between the conductive paste and the semiconductor PN junction,
resulting in the performance deterioration of the crystalline
silicon solar cell. Therefore, the amount of the crystal-based
corrosion binder used in the conductive paste is controlled between
0.1 to 15 parts by weight, such as 1 part by weight, 5 parts by
weight, 10 parts by weight and 12 parts by weight.
Glass Frit Combined with Corrosion Binder
[0056] In one or more embodiments, the present invention provides a
conductive paste includes a glass frit having a relative high glass
softening zone ranged from 500 to 650.degree. C. combined with the
Pb--Te--O-based crystalline corrosion binder with a melting point
within 440-760.degree. C. as additives with metal powders and
organic carrier. The combined weight of the crystalline corrosion
binder and the glass frit is of 1% to 15% of a total weight of the
conductive paste. The weight ratio among the crystalline corrosion
binder and the glass frit combination may be a 5/95-95/5.
[0057] According to embodiments of the present invention, the glass
frit material combined with corrosion binder for preparation of the
conductive paste comprises a lead-based glass frit and a lead-free
glass frit. The lead-based glass frit includes Pb--Si--Al--B--O,
Pb--Te--B--O, Pb--Te--O, and Pb--Te--Li--O glass material. The
lead-free glass frit includes Bi--Si--B--Zn--O, Zn--B--P--Li--O,
B--Al--Li--O, Na--Al--B--O, and Bi--V--Ba--P--O glass material.
[0058] In a specific embodiment, the Pb--Si--Al--B--O glass frit is
formed from lead oxide, silicon oxide, aluminum oxide, and boron
oxide mixing in proportion and heated to a molten state. Then the
molten mixture is quenched followed by a grinding process to obtain
the glass frit in powdered form. The glass frit is not a
crystalline structure and is characterized by a softening
temperature zone from 500.degree. C. to 650.degree. C. In an
example, the combined weight of the Pb--Si--Al--B--O glass frit and
the crystalline corrosion binder is about 1% to 15% of a total
weight of the conductive paste. The weight ratio among the
crystalline corrosion binder and the glass frit combination may be
a 5/95-95/5. The crystalline corrosion binder is at least one or a
mixture of two or more following crystal 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.4O.sub.9,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5, and
Pb.sub.3TeN.sub.2O.sub.8, have a fixed melting temperature falling
in the range of 440.degree. C. to 760.degree. C. During an
application of the conductive paste on the formation of electrodes
of silicon solar cell, the crystalline corrosion binder quickly
turns its crystal solid state into a molten state as the
temperature reaches the melting point. Assisted by the
Pb--Si--Al--B--O glass frit, the molten state corrosion binder can
effectively etch and penetrate an antireflection layer on the front
surface of the silicon solar cell and cause a formation of good
ohmic contact between the metal material in the conductive paste
and the semiconductor solar cell. The combination of softened glass
frit and the molten corrosion binder also effectively wet the metal
powders for facilitating their sintering into conductive bulk to
form a solar cell front electrode with excellent electrical
conductance performance.
[0059] In another specific embodiment, the Pb--Te--O glass frit is
formed from lead oxide and tellurium oxide mixing in proportion and
heated to a molten state. Then the molten mixture is quenched
followed by a grinding process to obtain the glass frit in powdered
form. The glass frit is not a crystalline structure and is
characterized by a softening temperature zone from 500.degree. C.
to 650.degree. C. In an example, the combined weight of the
Pb--Te--O glass frit and the crystalline corrosion binder is about
1% to 15% of a total weight of the conductive paste. The weight
ratio among the crystalline corrosion binder and the glass frit
combination may be a 5/95-95/5. The crystalline corrosion binder is
at least one or a mixture of two or more following crystal
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.4O.sub.9,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5, and
Pb.sub.3TeN.sub.2O.sub.8, have a fixed melting temperature falling
in the range of 440.degree. C. to 760.degree. C. During an
application of the conductive paste on the formation of electrodes
of silicon solar cell, the crystalline corrosion binder quickly
turns its crystal solid state into a molten state as the
temperature reaches the melting point. Assisted by the Pb--Te--O
glass frit, the molten state corrosion binder can effectively etch
and penetrate an antireflection layer on the front surface of the
silicon solar cell and cause a formation of good ohmic contact
between the metal material in the conductive paste and the
semiconductor solar cell. The combination of softened glass frit
and the molten corrosion binder also effectively wet the metal
powders for facilitating their sintering into conductive bulk to
form a solar cell front electrode with excellent electrical
conductance performance.
[0060] In yet another specific embodiment, the Pb--Te--B--O glass
frit is formed from lead oxide, tellurium oxide, and boron oxide
mixing in proportion and heated to a molten state. Then the molten
mixture is quenched followed by a grinding process to obtain the
glass frit in powdered form. The glass frit is not a crystalline
structure and is characterized by a softening temperature zone from
500.degree. C. to 650.degree. C. In an example, the combined weight
of the Pb--Te--B--O glass frit and the crystalline corrosion binder
is about 1% to 15% of a total weight of the conductive paste. The
weight ratio among the crystalline corrosion binder and the glass
frit combination may be a 5/95-95/5. The crystalline corrosion
binder is at least one or a mixture of two or more following
crystal 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.4O.sub.9,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5, and
Pb.sub.3TeN.sub.2O.sub.8, have a fixed melting temperature falling
in the range of 440.degree. C. to 760.degree. C. During an
application of the conductive paste on the formation of electrodes
of silicon solar cell, the crystalline corrosion binder quickly
turns its crystal solid state into a molten state as the
temperature reaches the melting point. Assisted by the Pb--Te--B--O
glass frit, the molten state corrosion binder can effectively etch
and penetrate an antireflection layer on the front surface of the
silicon solar cell and cause a formation of good ohmic contact
between the metal material in the conductive paste and the
semiconductor solar cell. The combination of softened glass frit
and the molten corrosion binder also effectively wet the metal
powders for facilitating their sintering into conductive bulk to
form a solar cell front electrode with excellent electrical
conductance performance.
[0061] In still another specific embodiment, the Pb--Te--Li--O
glass frit is formed from lead oxide, tellurium oxide, and lithium
oxide mixing in proportion and heated to a molten state. Then the
molten mixture is quenched followed by a grinding process to obtain
the glass frit in powdered form. The glass frit is not a
crystalline structure and is characterized by a softening
temperature zone from 500.degree. C. to 650.degree. C. In an
example, the combined weight of the Pb--Te--Li--O glass frit and
the crystalline corrosion binder is about 1% to 15% of a total
weight of the conductive paste. The weight ratio among the
crystalline corrosion binder and the glass frit combination may be
a 5/95-95/5. The crystalline corrosion binder is at least one or a
mixture of two or more following crystal 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.4O.sub.9,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5, and
Pb.sub.3TeN.sub.2O.sub.8, have a fixed melting temperature falling
in the range of 440.degree. C. to 760.degree. C. During an
application of the conductive paste on the formation of electrodes
of silicon solar cell, the crystalline corrosion binder quickly
turns its crystal solid state into a molten state as the
temperature reaches the melting point. Assisted by the
Pb--Te--Li--O glass frit, the molten state corrosion binder can
effectively etch and penetrate an antireflection layer on the front
surface of the silicon solar cell and cause a formation of good
ohmic contact between the metal material in the conductive paste
and the semiconductor solar cell. The combination of softened glass
frit and the molten corrosion binder also effectively wet the metal
powders for facilitating their sintering into conductive bulk to
form a solar cell front electrode with excellent electrical
conductance performance.
[0062] In yet still another specific embodiment, the
Bi--Si--B--Zn--O glass frit is formed from bismuth oxide, silicon
oxide, boron oxide, and zinc oxide mixing in proportion and heated
to a molten state. Then the molten mixture is quenched followed by
a grinding process to obtain the glass frit in powdered form. The
glass frit is not a crystalline structure and is characterized by a
softening temperature zone from 500.degree. C. to 650.degree. C. In
an example, the combined weight of the Bi--Si--B--Zn--O glass frit
and the crystalline corrosion binder is about 1% to 15% of a total
weight of the conductive paste. The weight ratio among the
crystalline corrosion binder and the glass frit combination may be
a 5/95-95/5. The crystalline corrosion binder is at least one or a
mixture of two or more following crystal 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.4O.sub.9,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5, and
Pb.sub.3TeN.sub.2O.sub.8, have a fixed melting temperature falling
in the range of 440.degree. C. to 760.degree. C. During an
application of the conductive paste on the formation of electrodes
of silicon solar cell, the crystalline corrosion binder quickly
turns its crystal solid state into a molten state as the
temperature reaches the melting point. Assisted by the
Bi--Si--B--Zn--O glass frit, the molten state corrosion binder can
effectively etch and penetrate an antireflection layer on the front
surface of the silicon solar cell and cause a formation of good
ohmic contact between the metal material in the conductive paste
and the semiconductor solar cell. The combination of softened glass
frit and the molten corrosion binder also effectively wet the metal
powders for facilitating their sintering into conductive bulk to
form a solar cell front electrode with excellent electrical
conductance performance.
[0063] In an alternative embodiment, the Zn--B--P--Li--O glass frit
is formed from zinc oxide, boron oxide, phosphorus oxide, and
lithium oxide mixing in proportion and heated to a molten state.
Then the molten mixture is quenched followed by a grinding process
to obtain the glass frit in powdered form. The glass frit is not a
crystalline structure and is characterized by a softening
temperature zone from 500.degree. C. to 650.degree. C. In an
example, the combined weight of the Zn--B--P--Li--O glass frit and
the crystalline corrosion binder is about 1% to 15% of a total
weight of the conductive paste. The weight ratio among the
crystalline corrosion binder and the glass frit combination may be
a 5/95-95/5. The crystalline corrosion binder is at least one or a
mixture of two or more following crystal 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.4O.sub.9,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5, and
Pb.sub.3TeN.sub.2O.sub.8, have a fixed melting temperature falling
in the range of 440.degree. C. to 760.degree. C. During an
application of the conductive paste on the formation of electrodes
of silicon solar cell, the crystalline corrosion binder quickly
turns its crystal solid state into a molten state as the
temperature reaches the melting point. Assisted by the
Zn--B--P--Li--O glass frit, the molten state corrosion binder can
effectively etch and penetrate an antireflection layer on the front
surface of the silicon solar cell and cause a formation of good
ohmic contact between the metal material in the conductive paste
and the semiconductor solar cell. The combination of softened glass
frit and the molten corrosion binder also effectively wet the metal
powders for facilitating their sintering into conductive bulk to
form a solar cell front electrode with excellent electrical
conductance performance.
[0064] In another alternative embodiment, the B--Al--Li--O glass
frit is formed from boron oxide, aluminum oxide, and lithium oxide
mixing in proportion and heated to a molten state. Then the molten
mixture is quenched followed by a grinding process to obtain the
glass frit in powdered form. The glass frit is not a crystalline
structure and is characterized by a softening temperature zone from
500.degree. C. to 650.degree. C. In an example, the combined weight
of the B--Al--Li--O glass frit and the crystalline corrosion binder
is about 1% to 15% of a total weight of the conductive paste. The
weight ratio among the crystalline corrosion binder and the glass
frit combination may be a 5/95-95/5. The crystalline corrosion
binder is at least one or a mixture of two or more following
crystal 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.4O.sub.9,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5, and
Pb.sub.3TeN.sub.2O.sub.8, have a fixed melting temperature falling
in the range of 440.degree. C. to 760.degree. C. During an
application of the conductive paste on the formation of electrodes
of silicon solar cell, the crystalline corrosion binder quickly
turns its crystal solid state into a molten state as the
temperature reaches the melting point. Assisted by the B--Al--Li--O
glass frit, the molten state corrosion binder can effectively etch
and penetrate an antireflection layer on the front surface of the
silicon solar cell and cause a formation of good ohmic contact
between the metal material in the conductive paste and the
semiconductor solar cell. The combination of softened glass frit
and the molten corrosion binder also effectively wet the metal
powders for facilitating their sintering into conductive bulk to
form a solar cell front electrode with excellent electrical
conductance performance.
[0065] In yet another alternative embodiment, the Na--Al--B--O
glass frit is formed from sodium oxide, aluminum oxide, and boron
oxide mixing in proportion and heated to a molten state. Then the
molten mixture is quenched followed by a grinding process to obtain
the glass frit in powdered form. The glass frit is not a
crystalline structure and is characterized by a softening
temperature zone from 500.degree. C. to 650.degree. C. In an
example, the combined weight of the Na--Al--B--O glass frit and the
crystalline corrosion binder is about 1% to 15% of a total weight
of the conductive paste. The weight ratio among the crystalline
corrosion binder and the glass frit combination may be a 5/95-95/5.
The crystalline corrosion binder is at least one or a mixture of
two or more following crystal 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.4O.sub.9,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5, and
Pb.sub.3TeN.sub.2O.sub.8, have a fixed melting temperature falling
in the range of 440.degree. C. to 760.degree. C. During an
application of the conductive paste on the formation of electrodes
of silicon solar cell, the crystalline corrosion binder quickly
turns its crystal solid state into a molten state as the
temperature reaches the melting point. Assisted by the Na--Al--B--O
glass frit, the molten state corrosion binder can effectively etch
and penetrate an antireflection layer on the front surface of the
silicon solar cell and cause a formation of good ohmic contact
between the metal material in the conductive paste and the
semiconductor solar cell. The combination of softened glass frit
and the molten corrosion binder also effectively wet the metal
powders for facilitating their sintering into conductive bulk to
form a solar cell front electrode with excellent electrical
conductance performance.
[0066] In still another alternative embodiment, the Bi--V--Ba--P--O
glass frit is formed from bismuth oxide, vanadium oxide, barium
oxide, and phosphorus oxide mixing in proportion and heated to a
molten state. Then the molten mixture is quenched followed by a
grinding process to obtain the glass frit in powdered form. The
glass frit is not a crystalline structure and is characterized by a
softening temperature zone from 500.degree. C. to 650.degree. C. In
an example, the combined weight of the Bi--V--Ba--P--O glass frit
and the crystalline corrosion binder is about 1% to 15% of a total
weight of the conductive paste. The weight ratio among the
crystalline corrosion binder and the glass frit combination may be
a 5/95-95/5. The crystalline corrosion binder is at least one or a
mixture of two or more following crystal 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.4O.sub.9,
PbTe.sub.2O.sub.5, PbH.sub.4TeO.sub.6, PbTeCO.sub.5 and
Pb.sub.3TeN.sub.2O.sub.8, have a fixed melting temperature falling
in the range of 440.degree. C. to 760.degree. C. During an
application of the conductive paste on the formation of electrodes
of silicon solar cell, the crystalline corrosion binder quickly
turns its crystal solid state into a molten state as the
temperature reaches the melting point. Assisted by the
Bi--V--Ba--P--O glass frit, the molten state corrosion binder can
effectively etch and penetrate an antireflection layer on the front
surface of the silicon solar cell and cause a formation of good
ohmic contact between the metal material in the conductive paste
and the semiconductor solar cell. The combination of softened glass
frit and the molten corrosion binder also effectively wet the metal
powders for facilitating their sintering into conductive bulk to
form a solar cell front electrode with excellent electrical
conductance performance.
[0067] Since both the Pb--Te--O-based crystalline corrosion binder
and a glass frit made from similar raw oxide materials are mixed
into the conductive paste according to one or more embodiments of
the present invention, it is better to point out their individual
roles played during the application of the conductive paste. When
applying a sintering process to transform the conductive paste to a
metallic electrode on semiconductor device, both types of additives
experiences different physical state transition thereby causing
different chemical/physical effects in the process and different
end-results that are not anticipated from prior art references.
Before the temperature increases to a value close to the melting
point of the crystalline corrosion binder, the corrosion binder is
embedded in the paste as solid particles dispersed from the metal
powder particles, leaving gaps between particles to allow the
organic carrier to release with the increasing temperature. Since
glass frit in the conductive paste has a reduced proportion and a
raised softening zone, the potential effect of clogging of those
gaps between the solid particles by premature glass softening, as
maybe seen with convention paste in use, is suppressed to avoid
problems related to incomplete releasing of organic carrier. As the
temperature reaches the melting point of the Pb--Te--O-based
crystal, the corrosion binder particles change their physical form
from a solid state to liquid phase almost instantly. It has very
low viscosity and can quickly flow through the gap between the
metal particles into a bottom surface where the paste is in contact
with an insulation layer overlying the semiconductor device.
Because of the low viscosity of the Pb--Te--O-based crystal melt,
the liquid phase corrosion binder easily spread to reach entire
insulation layer within the bottom surface to cause an etching
reaction of the corresponding insulation layer for substantially
remove entire insulation layer and form metal-semiconductor
interface regions with increased conductive contact and enhanced
tunnel effect. Consequently, a metal-semiconductor contact with low
contact resistance is formed. This process also occurs in a shorter
cycle than that with a conventional paste without adding
crystal-based corrosion binder and total amount of Pb--Te--O
material added in the conductive paste in order for sufficiently
removing the insulation layer is also reduced. In addition, the
glass frit retained with reduced amount in the present invention
has relative high softening temperatures so that premature glass
softening in the sintering process is greatly suppressed to prevent
blocking the discharge of organic components. At the same time, the
softened glass material still effectively aid sintering of metal
powders into a bulk metal with enhanced compactness and increased
weldability. Using crystalline silicon solar cell as an example, as
the results of applying the conductive paste including crystalline
corrosion binder combined with a reduced amount of glass frit,
performance of the solar cell is substantially improved in terms of
the open-circuit voltage, the short-circuit current, the series
resistance, the fill factor, and the photoelectric conversion
efficiency.
Metallic Powder
[0068] In the conductive paste for forming electrode of
semiconductor device, metallic powder is one major component
designated as electrical conductive medium of the electrode. In an
embodiment, the metallic powder is made from materials having
melting temperatures at least above 350.degree. C. In a preferred
embodiment, the melting temperature of the metallic powder is above
600.degree. C. If its melting point is too low, during the
sintering process, a premature melting of the metallic powders
occurs to hinder the discharge of the organic carrier.
Additionally, the premature melting produces significant sideward
flows reducing height aspect ratio of the electrode lines across
the semiconductor device. Of course, if its melting point is too
high, e.g., above 2000.degree. C., it is difficult to carry an
effective sintering process to complete transform the powder form
into bulk material so that the as-formed electrode contains many
metallic blocks with bulk-sized voids, resulting in a large passage
resistance and deterioration of device performance.
[0069] In a specific embodiment, the metallic powder is at least
one material selected from the group consisting of silver, gold,
platinum, palladium, and rhodium, or an alloy material containing
either silver, gold, platinum, palladium or rhodium doped with at
least one metal of copper, iron, nickel, zinc, titanium, cobalt,
chromium, and manganese, such as manganese copper, constantan,
nickel-chromium alloy. The metallic powder is made as a plurality
of small particles from grinding of bulk metal material and further
ball-milling into substantially round shaped fine powders.
[0070] In another specific embodiment, the metallic powder is a
plurality of silver-coated metal particles wherein the metal is at
least one selected from copper, iron, nickel, zinc, titanium,
cobalt, chromium, manganese, copper, iron, nickel, zinc, and
titanium. The silver coating is formed by either electroless Ag
plating or electroplating methods or vapor deposition In an
embodiment, the silver coating method includes the following
process: 1) placing metal particles with diameters 0.1-5.0 .mu.m of
copper, iron, nickel, titanium, cobalt, chromium, zinc, or
manganese, or an alloy thereof in a dilute weak acid and soaking
for 10-300 seconds to strip out any surface oxide layer; 2) washing
with deionized water repeatedly to remove residual acid; 3)
electroless Ag on the metal particles in a solution comprising:
2.4.about.14.2 g/L AgNO.sub.3, 0.8 g/L NH.sub.3, 1.about.3 g/L
HCHO, and 1.about.4 g/L N.sub.2H.sub.4.H.sub.2O, 1.0 g/L. The PH of
the solution is between 10 and 11, the temperature is between
55-65.degree. C., stirring rate is 1000 r/min. In another
embodiment, the silver coating method includes the following
process: 1) placing metal particles with diameters 0.1-5.0 .mu.m of
copper, iron, nickel, titanium, cobalt, chromium, zinc, or
manganese, or an alloy thereof in a dilute weak acid and soaking
for 10-300 seconds to strip out any surface oxide layer; 2) washing
with deionized water repeatedly to remove residual acid; 3) drying
the wet powder in a vacuum oven of <10.sup.-3 pA with the drying
temperature set below the melting point of the metal; 4) disposing
dried particles free of surface oxide layer in an evaporation
equipment to ensure effective dispersion between particles. 5)
vaporizing a silver target by resistance heating, electron beam or
laser beam heating, controlling the heating power, the deposition
time and other process conditions to form silver coating over the
metal particles. The method results in the silver-coated metal
particles of copper, iron, nickel, titanium, cobalt, chromium,
zinc, or manganese, or an alloy thereof. The thickness of the
silver coating is related to the process time and other plating
parameters. The silver coating preferably is controlled within a
thickness range of 1.about.10.sup.4 nm, and more preferably is 10
to 10.sup.2 nm. If the thickness of the silver coat layer is less
than 1 nm, silver content is too small and contact resistance of
the electrode or the drain current will be significantly increased.
If the thickness of the silver coating layer is more than 10.sup.4
nm, it will make the particle diameter of the conductive metal
powder too large and also increase manufacturing cost of the
metallic powder and subsequently the semiconductor device. Of
course, silver coating can be replaced by coating of gold or
platinum.
[0071] In yet another specific embodiment, the particle sizes of
the metallic powders are selected to be able to meet the paste
printing requirements, e.g., the size should not clog the printing
stencil. Preferably, the particle size distribution of the metallic
powder is ranged from 0.1 to 5.0 .mu.m. If the size is more than
5.0 .mu.m, it is likely to cause clogging of printing stencil and
correspondingly the electrode disconnection. If the size is less
than 0.1 .mu.m, the viscosity of the paste will substantially
increase, making it difficult for normal printing. Additionally,
when the conductive paste is used for making electrode on solar
cell, the well selected particle sizes in the metallic powders can
effectively reduce contact area of the front electrode and allow
more area for active solar conversion, resulting in higher solar
cell conversion efficiency. At the same time, the smaller electrode
has less height due to enhance conductivity so as to save cost by
saving the expensive materials.
Organic Carrier
[0072] The organic carrier in the conductive paste for the
formation of electrode on semiconductor device includes organic
solvent, organic binder, wetting dispersant reagents, thixotropic
agents and other functional additives. In an embodiment, based on
100 parts by weight of the organic carrier, the organic solvent
constitutes 50 to 95 parts by weight; organic binder accounts for 1
to 40 parts by weight; wetting dispersant reagents accounts for 0.1
to 10 parts by weight; thixotropic agents and other functional
additives comprises 1 to 20 parts by weight. In particular, optimum
amount of the organic solvent within the organic carrier should be
greater than 50 wt % to prevent viscosity of the paste becoming too
large to affect screen printing quality. But if the organic solvent
amount is over 95 wt %, the paste formed by such organic carrier
may lack of bonding phase so that when the paste is printed on a
semiconductor surface there will be incomplete printed pattern. The
paste may have poor adhesion characteristic to cause easy
separation between the organic carrier and the metallic powder or
glass powder therein. Similarly, the organic binder composition is
also optimized to provide good quality of bonding the powders in
the paste while having proper viscosity for easy printing.
[0073] In a specific embodiment, the organic solvent can be at
least one with a medium or high boiling temperature such as alcohol
(e.g., terpineol, butyl carbitol), alcohol ester (e.g., Alcohol
ester-12), terpene and others. Suitable organic binder includes
ethyl cellulose, polymethacrylate, alkyd resin, and derivatives
thereof. The wetting dispersant reagent helps to disperse inorganic
powders in the organic carrier. The thixotropic agent is used to
increase the thixotropy of the conductive paste in the printing
process to ensure the resolution of electrode pattern and better
aspect ratio. Suitable thixotropic agent includes an organic
thixotropic agent selected from hydrogenated castor oil derivatives
or polyamide wax, etc. Other organic functional agents may be
added, such as microcrystalline wax may be added for reducing the
surface tension, DBP is added for improving the flexibility of the
paste, and PVB is added for improving the adhesion.
Preparation Method of the Conductive Paste
[0074] FIG. 6 is a chart diagram showing a method for forming an
electrically conductive paste according to one or more embodiments
of the present invention. The method includes providing a certain
amount of each component based on a total weight including a
metallic powder, a glass frit, a corrosion binder, and an organic
carrier. In an embodiment, the corrosion binder and the glass frit
can be supplied with a combined weight composition and a
corresponding weight ratio between themselves. The method further
includes individual processes for preparing each type of component
respectively to a desired physical form in terms of particle sizes
and viscosity control. Additionally, the method includes mixing all
individually prepared components within desired composition ranges
to form the substantially uniform mixture material. Moreover, the
method includes finally grinding the mixture material to ensure all
particles therein with desired sizes and dispersions. Several
embodiments are disclosed below.
[0075] In an embodiment, after weighing the corrosion binder and
glass frit, they are uniformly mixed with an organic carrier. Then
a metal powder is weighed and mixed in before a final grinding to
ensure a desired particle size range.
[0076] In another embodiment, the weighed metal powder and organic
vehicle are uniformly mixed before further adding a weighed
corrosion binder and a glass frit. A final grinding is applied to
ensure a desired particle size range.
[0077] In yet another embodiment, after mixing the weighed metal
powder, glass frit and corrosion binder to a solid mixture. An
organic carrier is added to the solid mixture before a final
grinding process to ensure a desired particle size range.
[0078] In still another embodiment, the weighed metal powder,
corrosion binder, glass frit and an organic vehicle are mixed. Then
the mixture is milled further to ensure a desired particle size
range.
[0079] The final grinding process mentioned above is part of the
mixing processing to form the conductive paste with desired
property. The process can be performed using a three-roll mill. Of
course, other grinding apparatus can also be used. After grinding,
the particle sizes of the crystalline corrosion binder and the
metal powder are controlled in a desired range for facilitating
paste screen print and effectiveness in a process for forming an
electrode on semiconductor surface. In an example, all particle
sizes are controlled substantially within a range from 0.1 to 5.0
.mu.m.
[0080] In one or more embodiments, the method for forming an
electrically conductive paste provides one or more processes for
preparing a metallic powder. In an embodiment, the metallic powder
is made from at least one metal material selected from silver,
gold, platinum, copper, iron, nickel, zinc, titanium, cobalt,
chromium, manganese, palladium, and rhodium or a metal alloy of two
or more of them. In another embodiment, the metallic powder
includes a plurality of metal particles made by one metal material
selected from copper, iron, nickel, zinc, titanium, cobalt,
chromium, and manganese or a metal alloy of two or more of them,
and at least 5% or more partially coated by a thickness of silver
layer in a range of 1.about.2000 nm. The silver coating can be
performed using electroless plating or electroplate technique. Each
metal particle can have its size being limited to certain desired
level by controlling one or more grinding or milling process. In
yet another embodiment, the metallic powder includes a first
plurality of silver particles mixed with a second plurality of
silver-coated nickel particles with a weight composition ratio
between the first plurality of silver particles and the second
plurality of silver-coated nickel particles in a range from 5:95 to
95:5 per any fixed amount of the metallic powder.
[0081] In an alternative embodiment, the present invention provides
a method for making an electrode on a semiconductor surface from an
electrically conductive paste provided in the same invention. FIG.
7 shows a chart diagram illustrating the method for making an
electrode on a semiconductor surface from an electrically
conductive paste. As shown, the method includes providing a
semiconductor device having an insulation surface coating.
Additionally, the method includes printing a conductive paste
material overlying a patterned region of the insulation surface
coating on the semiconductor device. This can be referred to FIG. 1
where a sectional view of a conductive paste being applied
overlying an insulation layer overlying a semiconductor device. The
conductive paste material printed in the method is substantially
the one provided in one or more embodiments shown in FIG. 6 and
described in corresponding specifications above. In particular, the
conductive paste material includes a plurality of metal particles
with a weight composition ranging from 70 to 93 wt % based on a
given total weight of the conductive paste material. Furthermore,
the conductive paste material includes a corrosion binder made from
a plurality of Pb--Te--O-based crystalline particles and a glass
frit made from a plurality of glass particles with a combined
weight composition ranging from 1 to 15 wt % based on the given
total weight. Moreover, the conductive paste material includes an
organic carrier with a weight composition ranging from 5 to 25 wt %
based on the given total weight. The organic carrier dispersedly
holds the plurality of metal particles, the plurality of
Pb--Te--O-based crystalline particles, and the plurality of glass
particles, wherein all particles have sizes in a range of 0.1 to
5.0 microns.
[0082] Referring to FIG. 7, the method for making an electrode on a
semiconductor surface further includes a sintering process in which
at least the conductive paste material and the patterned region in
contact with the insulation surface coating is subjected to
elevating temperatures up to about 900.degree. C. In one or more
embodiments, the elevating temperatures cause releasing of the
organic carrier from the printed conductive paste through a
plurality of intermediate regions between the dispersed particles,
leaving corresponding intermediate regions as empty spaces or
channels. As the sintering process temperature increases above the
melting point of corresponding Pb--Te--O-based crystalline
particles, which is at about 440.degree. C. to 760.degree. C.,
these crystal particles instantly melt into a liquid phase
corrosion binder, starting to flow downward through the empty
intermediate regions or channels to the patterned region between
the applied conductive paste and the insulation surface coating.
Along the way of flowing down, the liquid or molten corrosion
binder also wets the plurality of metal particles at least
partially. As the sintering process temperature rises into a stage
over the softening temperatures (500.degree. C. to 650.degree. C.)
associated with the glass frit made by the plurality glass
particles proposed in the present invention, these glass particles
become soften and also can wet the metal particle at least
partially. As the temperature rises more, the plurality of metal
particles are moved from dispersed positions to start sintering
together to become a metallic bulk, assisted by wetting effects
from the molten corrosion binder and soften glass frit. At the same
time, the molten corrosion binder that reaches the insulation
surface coating can induce an etching reaction with the coating
material which is a redox reaction participated also by the nearby
metal particles. The product of the etching reaction is
precipitation of metal crystals and colloids at the expense of the
insulation material (e.g., SiN.sub.x anti-reflective layer on
emitter surface of a Si-based solar cell) in the surface coating.
In a specific embodiment, the amount and weight composition of the
corrosion binder and glass frit as well as their weight ratio
provided according to the present invention substantially ensure a
complete removal of the insulation surface coating to allow metal
particles to form crystallized interface directly bonding with the
semiconductor device as well as some metal colloids precipitation
resulted from the redox reaction. The sintered metallic bulk forms
an electrode having a good electrical contact with the
semiconductor device.
[0083] Referring back to FIG. 2, as sintering process temperature
increases from below 300.degree. C., the organic carrier 128 first
is released (or vaporized) through intermediate regions (see FIG.
1) between the dispersed particles (of the metal powder 122, glass
frit 124, and corrosion binder 126), leaving behind a lots of void
spacing. As the temperature reaches above 440.degree. C. or higher,
the crystal-based corrosion binder particles 126 start to melt.
Depending on specific crystal compounds for making the corrosion
binder, the melting temperature may be higher, up to 760.degree. C.
But whenever the temperature reaches the corresponding melting
points, the corrosion binder 126 instantly melts into a liquid form
210 without any glass soften transition process. The liquid/molten
corrosion binder 210 flows downward to reach the bottom region of
the applied conductive paste 120. At the bottom region, the liquid
corrosion binder 210 is in contact with the insulating layer 110 to
induce an etching reaction. Since the corrosion binder is made of
crystalline particles so that the above process occurs much faster
than conventional paste with only glass frit material as etching
additive, which must go through a much slower glass softening
transition and become a relative low viscosity material at much
higher temperature or later time. Therefore, the corresponding
sintering process is accelerated in terms of delivering the
corrosion binder towards the insulating layer 110 to start the
etching reaction there.
[0084] At the same time, along the way of the liquid corrosion
binder 210 flowing down, it also wets a plurality of metal
particles 122 (see FIG. 1) while the metal particles 122 start to
bond together becoming a bigger metallic bulk 220. During
substantially the same time or subsequently later, the elevating
sintering temperature may reach to a stage of the glass softening
temperatures associated with the glass frit 124 in the conductive
paste. For example, the glass softening temperatures are in a range
of 500.degree. C. to 650.degree. C., for the glass frit made from
Pb--Si--O, Bi--Si--O, Pb--B--O, Bi--B--O, Pb--Te--O, Bi--Te--O,
P--Zn--Na--O, B--Al--Na--O, B--Zn--Ba--O, and V--P--Ba--O-based
glass materials according to the present invention. The glass frits
124 become soft and gradually fuse into glass phases 230 and also
wet (at least partially) nearby metal particles 122, which promotes
sintering of the metal particles 122 together to form a metallic
bulk 220 and at the same time help metal particles to transfer
toward a reactive interface region where the etching reaction is
induced by the liquid corrosion binder 210. The etching reaction is
a redox reaction in which part of metal oxide within the metallic
bulk 220 is dissolved into the liquid corrosion binder 210 and the
glass phases 230. As the sintering temperature further rises beyond
the glass softening temperature range (over 650.degree. C. up to
900.degree. C.), the glass phases 230 become substantially liquid
state and also sink towards the interface between the paste and the
insulation surface coating 110 to further assist the etching
reaction. Now the insulation material 110 at the interface region
is substantially dissolved to expose the semiconductor device 102
and form a ohmic contact layer 240. In the embodiment, the combined
amount of the corrosion binder and glass frit within the applied
conductive paste 120 should be controlled in an optimized range so
that no over-etching effect occurs to cause unwanted etching into
the semiconductor device 100.
[0085] Near the end of the sintering process, temperature falls
back to the softening temperature range (between 500.degree. C. and
650.degree. C.) before further cooling, a plurality of metallic
colloids precipitates in the mixture of liquid corrosion binder 210
and glass phases 230 near the just-formed metal crystal grains 220,
forming the ohmic contact layer 240 which connects the metallic
bulk 220 with the emitter 102 of the semiconductor device 100. In a
specific embodiment, the electrical resistance between the metallic
bulk 220 and the semiconductor device 100 contributes to the
overall series resistance of the device. In the implementation of
forming a front electrode of crystalline silicon solar cell, the
lower the overall series resistance, the better in solar cell
performance. In particular, the electrode line can be formed
narrower and lower in height to enhance active area and reduce cost
by saving expensive conductor material to provide cost-effective
solar cell with higher photovoltaic conversion efficiency.
EXAMPLES
[0086] Illustrative preparations and evaluations of conductive
paste compositions for the formation of front electrode of
crystalline silicon-based solar cell are described below.
Example 1
[0087] The conductive paste for forming a front electrode of
crystalline silicon-based solar cell includes, according to the
total weight based on 100 parts, 85 wt % of metallic powder, 3 wt %
of crystal corrosion binder, 2 wt % of glass frit, and 10 wt % of
organic carrier. The crystal corrosion binder is substantially
PbTe.sub.4O.sub.9 compound. The metallic powder is silver powder.
The glass frit is made from Pb--Si--Al--B--O-based glass material
containing 81 wt % of PbO, 16 wt % of SiO.sub.2, 2 wt % of
Al.sub.2O.sub.3, and 1 wt % of B.sub.2O.sub.3. The organic carrier
includes 70 wt % of Terpineol organic solvent, 14 wt % of Ethyl
Cellulose binder, 10 wt % of wetting and dispersing agent, 5.5 wt %
of thixotropic agent, and 0.5 wt % of microcrystalline wax.
[0088] A method of preparation of the conductive paste for forming
a front electrode of crystalline silicon-based solar cell includes:
[0089] 1) Preparation of Corrosion Binder: Preparing a telluric
acid solution having a concentration of 0.1 mol/L and heating to
about 90.degree. C.; mixing the telluric acid solution in
proportion of molar ratio of 1:1 with a lead acetate solution
having a concentration of 0.1 mol/L; heating the mixture solution
to cause a chemical reaction to produce precipitate of
PbTe.sub.4O.sub.9 compound; filtering to obtain the
PbTe.sub.4O.sub.9 compound, and drying it for further usage. [0090]
2) Preparation of Metallic Powder: the metallic powder is silver
powder having particle sizes less than 5.0 .mu.m. [0091] 3)
Preparation of Glass Frit: Mixing each corresponding component of
the Pb--Si--Al--B--O-based glass material through a tapered
blender; disposing the mixture into a container loaded in a muffle
furnace; heating the mixture up to a peak temperature set to
1000.degree. C. and maintain at the peak temperature for 60 minutes
to completely melt the mixture into a uniform liquid; pouring the
melt into a water-cooled quench system to obtain a glass material;
grinding and milling the glass material into powders with particle
sizes being controlled to be less than 5.0 .mu.m. [0092] 4)
Preparation of Conductive Paste: Selecting individually prepared
glass frit, crystal corrosion binder, and organic carrier in weight
compositions to mix uniformly; adding separately prepared metallic
powder (silver powder) in corresponding weight composition with the
above mixture and further mixing uniformly; milling the final
mixture material through a three-roll mill to obtain the conductive
paste with all particle sizes less than 5.0 .mu.m.
Example 2
[0093] The conductive paste for forming a front electrode of
crystalline silicon-based solar cell includes, according to the
total weight based on 100 parts, 83 wt % of metallic powder, 4 wt %
of crystal corrosion binder, 1.6 wt % of glass frit, and 11.4 wt %
of organic carrier. The crystal corrosion binder is substantially
Pb.sub.3TeO.sub.5 compound. The metallic powder is silver-coated
nickel powder. The glass frit is made from Bi--Si--B--Zn--O-based
glass material containing 70 wt % of Bi.sub.2O.sub.3, 15 wt % of
SiO.sub.2, 3 wt % of B.sub.2O.sub.3, 7 wt % of ZnO, and some other
oxides including 3.5 wt % of BaO and 1.5 wt % of CuO. The organic
carrier includes 50 wt % of Lauryl Alcohol Ester organic solvent,
40 wt % of Polymethacrylates binder, 5 wt % of wetting and
dispersing agent, 4 wt % of thixotropic agent, and 0.5 wt % of
DBP.
[0094] A method of preparation of the conductive paste for forming
a front electrode of crystalline silicon-based solar cell includes:
[0095] 1) Preparation of Corrosion Binder: Selecting PbO and
TeO.sub.2 with molar ratio of 3:1 ratio to form a mixture;
vaporizing the mixture material to generate Pb.sub.3Te alloy vapor
and introducing Pb.sub.3Te alloy vapor into a reaction chamber
filled with oxygen at temperature of about 1300.degree. C.;
producing a crystalline powder of Pb--Te--O compound which is
deposited at a bottom of the chamber; collecting the crystalline
powder after natural cooling to obtain substantially
Pb.sub.3TeO.sub.5 compound in powder form. [0096] 2) Preparation of
Metallic Powder: the metallic powder is silver-coated nickel powder
having particle sizes less than 5.0 .mu.m. [0097] 3) Preparation of
Glass Frit: Mixing each corresponding component of the
Bi--Si--B--Zn--O-based glass material through a blade mixer;
disposing the mixture into a container loaded in a muffle furnace;
heating the mixture up to a peak temperature set to 1200.degree. C.
and maintain at the peak temperature for 60 minutes to completely
melt the mixture into a uniform liquid; pouring the melt into a
water-cooled quench system to obtain a glass material; grinding and
milling the glass material into powders with particle sizes being
controlled to be less than 5.0 .mu.m. [0098] 4) Preparation of
Conductive Paste: Selecting individually prepared silver-coated
nickel powder and organic carrier in weight compositions to mix
uniformly; adding separately prepared powder of glass frit and
crystal corrosion binder in corresponding weight compositions with
the above mixture and further mixing uniformly; milling the final
mixture material through a three-roll mill to obtain the conductive
paste with all particle sizes less than 5.0 .mu.m.
Example 3
[0099] The conductive paste for forming a front electrode of
crystalline silicon-based solar cell includes, according to the
total weight based on 100 parts, 80 wt % of metallic powder, 6 wt %
of crystal corrosion binder, 2.5 wt % of glass frit, and 11.5 wt %
of organic carrier. The crystal corrosion binder is substantially
PbTeCO.sub.5 compound. The metallic powder is silver-coated copper
powder. The glass frit is made from Zn--B--P--Li--O-based glass
material containing 36 wt % of B.sub.2O.sub.3, 22 wt % of ZnO, 36
wt % of P.sub.2O.sub.5, 1 wt % of Li.sub.2O, 4 wt % of MgO, and 1
wt % of Mn.sub.2O. The organic carrier includes 65 wt % of
Terpineol organic solvent, 20 wt % of Alkyd resin binder, 0.1 wt %
of wetting and dispersing agent, 12.5 wt % of thixotropic agent,
and 2.4 wt % of PVB.
[0100] A method of preparation of the conductive paste for forming
a front electrode of crystalline silicon-based solar cell includes:
[0101] 1) Preparation of Corrosion Binder: Selecting PbO and
TeO.sub.2 with molar ratio of 1:1 ratio to form a mixture;
disposing the mixture material in a furnace with temperatures
controlled between 700.about.900.degree. C.; thermally inducing
solid state reaction for .about.1 hour to produce a crystalline
Pb--Te--O compound; milling the crystalline Pb--Te--O compound
after natural cooling to obtain substantially PbTeO.sub.3 compound
in powder form. [0102] 2) Preparation of Metallic Powder: the
metallic powder is silver-coated copper powder having particle
sizes less than 5.0 .mu.m. [0103] 3) Preparation of Glass Frit:
Mixing each corresponding component of the Zn--B--P--Li--O-based
glass material through a gravity mixer; disposing the mixture into
a container loaded in a muffle furnace; heating the mixture up to a
peak temperature set to 900.degree. C. and maintain at the peak
temperature for 60 minutes to completely melt the mixture into a
uniform liquid; pouring the melt into a water-cooled quench system
to obtain a glass material; grinding and milling the glass material
into powders with particle sizes being controlled to be less than
5.0 .mu.m. [0104] 4) Preparation of Conductive Paste: Selecting
individually prepared silver-coated copper powder, corrosion binder
powder, and glass frit powder in weight compositions to mix
uniformly; adding separately prepared organic carrier in
corresponding weight composition with the above mixture and further
mixing uniformly; milling the final mixture material through a
three-roll mill to obtain the conductive paste with all particle
sizes less than 5.0 .mu.m.
Example 4
[0105] The conductive paste for forming a front electrode of
crystalline silicon-based solar cell includes, according to the
total weight based on 100 parts, 70 wt % of metallic powder, 2 wt %
of crystal corrosion binder, 3 wt % of glass frit, and 25 wt % of
organic carrier. The crystal corrosion binder is substantially
PbTeO.sub.3 compound. The metallic powder is silver-coated titanium
powder. The glass frit is made from B--Al--Li--O-based glass
material containing 56 wt % of B.sub.2O.sub.3, 35 wt % of
Al.sub.2O.sub.3, 4 wt % of Li.sub.2O, 4 wt % of MgO, and 5 wt % of
Na.sub.2O. The organic carrier includes 60 wt % of Amethocaine
organic solvent, 15 wt % of Ethyl Cellulose binder, 5 wt % of
wetting and dispersing agent, 15 wt % of thixotropic agent, and 5
wt % of PVB.
[0106] A method of preparation of the conductive paste for forming
a front electrode of crystalline silicon-based solar cell includes:
[0107] 1) Preparation of Corrosion Binder: Selecting PbO and
TeO.sub.2 with molar ratio of 2:3 ratio to form a mixture;
disposing the mixture material in a vacuum furnace at temperature
900.degree. C.; thermally inducing solid state reaction for
.about.1 hour to produce a crystalline Pb--Te--O compound; milling
the crystalline Pb--Te--O compound after natural cooling to obtain
substantially Pb.sub.2Te.sub.3O.sub.8 compound in powder form.
[0108] 2) Preparation of Metallic Powder: the metallic powder is
silver-coated titanium powder having particle sizes less than 5.0
.mu.m. [0109] 3) Preparation of Glass Frit: Mixing each
corresponding component of the B--Al--Li--O-based glass material
through a gravity mixer; disposing the mixture into a container
loaded in a muffle furnace; heating the mixture up to a peak
temperature set to 900.degree. C. and maintain at the peak
temperature for 60 minutes to completely melt the mixture into a
uniform liquid; pouring the melt into a water-cooled quench system
to obtain a glass material; grinding and milling the glass material
into powders with particle sizes being controlled to be less than
5.0 .mu.m. [0110] 4) Preparation of Conductive Paste: Selecting
individually prepared silver-coated titanium powder, corrosion
binder powder, and glass frit powder in weight compositions; mixing
all above powder materials with separately prepared organic carrier
in corresponding weight composition to form a uniform mixture;
milling the mixture material through a three-roll mill to obtain
the conductive paste with all particle sizes less than 5.0
.mu.m.
Example 5
[0111] The conductive paste for forming a front electrode of
crystalline silicon-based solar cell includes, according to the
total weight based on 100 parts, 72 wt % of metallic powder, 15 wt
% of crystal corrosion binder, 2.8 wt % of glass frit, and 10.2 wt
% of organic carrier. The crystal corrosion binder is substantially
PbTeN.sub.2O.sub.8 compound. The metallic powder is silver-coated
cobalt powder. The glass frit is made from Na--Al--B--O-based glass
material containing 8.4 wt % of Na.sub.2O, 14.2 wt % of
Al.sub.2O.sub.3, 72.4 wt % of B.sub.2O.sub.3, and 5 wt % of BaO.
The organic carrier includes 70 wt % of Lauryl Alcohol Ester
organic solvent, 15 wt % of Ethyl Cellulose binder, 2 wt % of
wetting and dispersing agent, 8 wt % of thixotropic agent, and 5 wt
% of PVB.
[0112] A method of preparation of the conductive paste for forming
a front electrode of crystalline silicon-based solar cell includes:
[0113] 1) Preparation of Corrosion Binder: Selecting PbO and
TeO.sub.2 with molar ratio of 3:1 ratio to form a mixture;
disposing the mixture material in a vacuum furnace at temperature
950.degree. C.; thermally inducing solid state reaction for
.about.1 hour to produce a crystalline Pb--Te--O compound; milling
the crystalline Pb--Te--O compound after natural cooling to obtain
substantially Pb.sub.3TeO.sub.5 compound in powder form. [0114] 2)
Preparation of Metallic Powder: the metallic powder is
silver-coated cobalt powder having particle sizes less than 5.0
.mu.m. [0115] 3) Preparation of Glass Frit: Mixing each
corresponding component of the Na--Al--B--O-based glass material
through a gravity mixer; disposing the mixture into a container
loaded in a muffle furnace; heating the mixture up to a peak
temperature set to 1000.degree. C. and maintain at the peak
temperature for 60 minutes to completely melt the mixture to a
uniform liquid; pouring the melt onto a stainless steel platen
quench system to obtain a glass material; grinding and milling the
glass material into powders with particle sizes being controlled to
be less than 5.0 .mu.m. [0116] 4) Preparation of Conductive Paste:
Selecting individually prepared silver-coated cobalt powder,
corrosion binder powder, and glass frit powder in weight
compositions; mixing all above powder materials with separately
prepared organic carrier in corresponding weight composition to
form a uniform mixture; milling the mixture material through a
three-roll mill to obtain the conductive paste with all particle
sizes less than 5.0 .mu.m.
Example 6
[0117] The conductive paste for forming a front electrode of
crystalline silicon-based solar cell includes, according to the
total weight based on 100 parts, 90 wt % of metallic powder, 4.5 wt
% of crystal corrosion binder, 0.5 wt % of glass frit, and 5 wt %
of organic carrier. The crystal corrosion binder is substantially
Pb.sub.2Te.sub.3O.sub.8 compound. The metallic powder is platinum
powder. The glass frit is made from Bi--V--Ba--P--O-based glass
material containing 5 wt % of Bi.sub.2O.sub.3, 45 wt % of
V.sub.2O.sub.5, 30 wt % of P.sub.2O.sub.5, and 20 wt % of BaO. The
organic carrier includes 95 wt % of Lauryl Alcohol Ester organic
solvent, 1 wt % of Ethyl Cellulose binder, 3 wt % of wetting and
dispersing agent, and 1 wt % of thixotropic agent.
[0118] A method of preparation of the conductive paste for forming
a front electrode of crystalline silicon-based solar cell includes:
[0119] 1) Preparation of Corrosion Binder: Selecting PbO and
TeO.sub.2 with molar ratio of 1:4 ratio to form a mixture;
disposing the mixture material in a vacuum furnace at temperature
of 900.degree. C.; thermally inducing solid state reaction for
.about.1 hour to produce a crystalline Pb--Te--O compound; milling
the crystalline Pb--Te--O compound after natural cooling to obtain
substantially PbTe.sub.4O.sub.9 compound in powder form. [0120] 2)
Preparation of Metallic Powder: the metallic powder is platinum
powder having particle sizes less than 5.0 .mu.m. [0121] 3)
Preparation of Glass Frit: Mixing each corresponding component of
the Bi--V--Ba--P--O-based glass material through a gravity mixer;
disposing the mixture into a container loaded in a muffle furnace;
heating the mixture up to a peak temperature set to 1150.degree. C.
and maintain at the peak temperature for 60 minutes to completely
melt the mixture to a uniform liquid; pouring the melt onto a
stainless steel platen quench system to obtain a glass material;
grinding and milling the glass material into powders with particle
sizes being controlled to be less than 5.0 .mu.m. [0122] 4)
Preparation of Conductive Paste: Selecting individually prepared
platinum powder, corrosion binder powder, and glass frit powder in
weight compositions; mixing all above powder materials with
separately prepared organic carrier in corresponding weight
composition to form a uniform mixture; milling the mixture material
through a three-roll mill to obtain the conductive paste with all
particle sizes less than 5.0 .mu.m.
Example 7
[0123] The conductive paste for forming a front electrode of
crystalline silicon-based solar cell includes, according to the
total weight based on 100 parts, 80 wt % of metallic powder, 0.5 wt
% of crystal corrosion binder, 3 wt % of glass frit, and 16.5 wt %
of organic carrier. The crystal corrosion binder is substantially
PbTeO.sub.3 compound. The metallic powder is silver-coated chromium
powder. The glass frit is made from Pb--Te--Li--O-based glass
material containing 21 wt % of PbO, 65.5 wt % of TeO.sub.2, 0.5 wt
% of Li.sub.2O, 8 wt % of SiO.sub.2, and 5 wt % of ZnO. The organic
carrier includes 70 wt % of Lauryl Alcohol Ester organic solvent,
15 wt % of Ethyl Cellulose binder, 2 wt % of wetting and dispersing
agent, 10 wt % of thixotropic agent, and 3 wt % of PVB.
[0124] A method of preparation of the conductive paste for forming
a front electrode of crystalline silicon-based solar cell includes:
[0125] 1) Preparation of Corrosion Binder: Selecting PbO and
TeO.sub.2 with molar ratio of 1:1 ratio to form a mixture;
disposing the mixture material in a vacuum furnace at temperature
900.degree. C.; thermally inducing solid state reaction for
.about.1 hour to produce a crystalline Pb--Te--O compound; milling
the crystalline Pb--Te--O compound after natural cooling to obtain
substantially PbTeO.sub.3 compound in powder form. [0126] 2)
Preparation of Metallic Powder: the metallic powder is
silver-coated chromium powder having particle sizes less than 5.0
.mu.m. [0127] 3) Preparation of Glass Frit: Mixing each
corresponding component of the Pb--Te--Li--O-based glass material
through a gravity mixer; disposing the mixture into a container
loaded in a muffle furnace; heating the mixture up to a peak
temperature set to 900.degree. C. and maintain at the peak
temperature for 60 minutes to completely melt the mixture to a
uniform liquid; pouring the melt onto a stainless steel platen
quench system to obtain a glass material; grinding and milling the
glass material into powders with particle sizes being controlled to
be less than 5.0 .mu.m. [0128] 4) Preparation of Conductive Paste:
Selecting individually prepared silver-coated copper powder,
corrosion binder powder, and glass frit powder in weight
compositions; mixing all above powder materials with separately
prepared organic carrier in corresponding weight composition to
form a uniform mixture; milling the mixture material through a
three-roll mill to obtain the conductive paste with all particle
sizes less than 5.0 .mu.m.
[0129] Further, the conductive paste of the present invention has a
functionality to control the dissolving and precipitating of both
the silver colloids and the silver crystal grains during the
sintering process. This is achieved by controlling the ratio
between glass network former, glass network intermediate, and
fluxing agent as well as the ratio between the three components of
glass network intermediate. As a result, uniform and dense silver
colloids and silver crystal grains in a range of about 20 nm to
about 150 nm are formed. Oversize or undersize effects of both of
the silver colloids and the silver crystal grains are overcome.
Controlling the size of the silver crystal grains during the
sintering process is important. If the silver crystal grains are
oversized, they might penetrate the emitter to the P-N junction,
cause a short circuit and solar cell failure. Especially for
crystalline silicon solar cells with shallow doped high sheet
resistance (sheet resistance >75 .OMEGA./sq.), its emitter is
thinner and easily be broken. The conductive paste of the present
invention has a functionality to control the silver crystal grain
size in an optimum range of 20-150 nm during the sintering process.
If the silver crystal grains are undersized or non-uniformly
distributed, photon-generated charge carriers are consumed before
arriving at the silver crystal grains, causing not only lower
short-circuit current but also lower filling factor. If less silver
colloids are produced during the sintering process, the resistance
between the silver bulk and the emitters is high, which causes
higher series resistance and lower filling factor of the solar
cell. If the silver colloids precipitate too close to the reaction
interface, the silver colloids cannot conduct the photon-generated
charge carriers from the emitter to the silver bulk to obtain
suitable series resistance and filling factor.
[0130] Finally, the above-discussion of the embodiments,
comparatives, and reference are intended to be mere illustrations
of the present invention and should not be construed as limiting
the appended claims to any particular embodiment or group of
embodiments. Thus, while the invention has been described with
reference to exemplary embodiments, it should also be appreciated
that numerous modifications and alternative embodiments may be
devised by those having ordinary skill in the art without departing
from the broader and intended spirit and scope of the disclosure as
set forth in the claims that follow. In addition, the section
headings included herein are intended to facilitate a review but
are not intended to limit the scope of the present system.
Accordingly, the specification and drawings are to be regarded in
an illustrative manner and are not intended to limit the scope of
the appended claims.
* * * * *