U.S. patent application number 14/217826 was filed with the patent office on 2014-07-17 for process for making silver powder particles with small size crystallites.
This patent application is currently assigned to E I DU PONT DE NEMOURS AND COMPANY. The applicant listed for this patent is E I DU PONT DE NEMOURS AND COMPANY. Invention is credited to ROBERTO IRIZARRY-RIVERA, Haixin Yang.
Application Number | 20140196772 14/217826 |
Document ID | / |
Family ID | 45888483 |
Filed Date | 2014-07-17 |
United States Patent
Application |
20140196772 |
Kind Code |
A1 |
IRIZARRY-RIVERA; ROBERTO ;
et al. |
July 17, 2014 |
PROCESS FOR MAKING SILVER POWDER PARTICLES WITH SMALL SIZE
CRYSTALLITES
Abstract
The process for making silver powder particles with small size
crystallites uses a combination of gum arabic and maleic acid with
the reduction of a silver salt with ascorbic acid, Silver thick
film paste containing these silver powder particles can be used in
electronic applications to form electrodes for semiconductor
devices and, in particular, solar cells.
Inventors: |
IRIZARRY-RIVERA; ROBERTO;
(Raleigh, NC) ; Yang; Haixin; (Chapel Hill,
NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
E I DU PONT DE NEMOURS AND COMPANY |
Wilmington |
DE |
US |
|
|
Assignee: |
E I DU PONT DE NEMOURS AND
COMPANY
Wilmington
DE
|
Family ID: |
45888483 |
Appl. No.: |
14/217826 |
Filed: |
March 18, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13406900 |
Feb 28, 2012 |
8715387 |
|
|
14217826 |
|
|
|
|
61450259 |
Mar 8, 2011 |
|
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Current U.S.
Class: |
136/252 ;
252/514; 420/501; 75/370 |
Current CPC
Class: |
B22F 9/24 20130101; B22F
2301/255 20130101; H01B 1/02 20130101; B22F 1/0011 20130101; Y02E
10/50 20130101; C23C 24/087 20130101; C22C 5/06 20130101; B22F
2304/10 20130101; B22F 1/0044 20130101; H01L 31/022425
20130101 |
Class at
Publication: |
136/252 ; 75/370;
420/501; 252/514 |
International
Class: |
B22F 9/24 20060101
B22F009/24; H01L 31/0224 20060101 H01L031/0224; H01B 1/02 20060101
H01B001/02 |
Claims
1. Silver powder particles made by a process comprising: (a)
preparing an acidic aqueous silver salt solution comprising a water
soluble silver salt dissolved in deionized water; (b) preparing an
acidic aqueous reducing and particle modifier solution comprising:
(i) a reducing agent selected from the group consisting of ascorbic
acid, ascorbates and mixtures thereof dissolved in deionized water;
(ii) nitric acid; (iii) gum arabic; (iv) maleic acid; and (v)
deionized water; (c) maintaining said acidic aqueous silver salt
solution and said acidic reducing and particle modifier solution at
the same temperature, wherein said temperature is in the range of
10.degree. C. to 65.degree. C., while stirring each said solution
during and after preparation; (d) adding said acidic aqueous silver
salt solution to said acidic aqueous reducing and particle modifier
solution while stirring to make a reaction mixture and maintaining
the temperature of said reaction mixture at said temperature of
(c), wherein said silver powder particles precipitate and are
contained within the resulting final aqueous solution; and (e)
increasing the temperature of said final aqueous solution to a
temperature in the range of 65.degree. C. to 80.degree. C. while
continuing stirring; said silver powder particles comprising
crystallites between 30 and 47 nm in size as determined by X-ray
diffraction and the Scherrer formula; and said silver powder
particles having d.sub.50 in the range of 0.5 to 3.5 .mu.m.
2. The silver particles of claim 1, said process further
comprising: (f) separating said silver powder particles from said
final aqueous solution; (g) washing said silver powder particles
with deionized water; and (h) drying said silver powder
particles.
3. The silver particles of claim 1, wherein said water soluble
silver salt is silver nitrate and said reducing agent is ascorbic
acid.
4. The silver particles of claim 1, said acidic aqueous reducing
and particle modifier solution further comprising a metal colloid
selected from the group consisting of gold colloid and silver
colloid.
5. The silver particles of claim 1, wherein the temperature in
process step (c) is in the range of 10.degree. C. to 35.degree. C.
and the temperature in process step (e) is in the range of
65.degree. C. to 75.degree. C.
6. A silver thick film paste comprising said silver powder
particles of any of claims 1-5.
7. The silver thick film paste of claim 6, said silver thick film
paste comprising 65 to 90 wt % said silver powder particles and
further comprising 0.1 to 8 wt % glass frit and 5 to 30 wt %
organic medium, wherein the wt % are based on the total weight of
the silver thick film paste.
8. A semiconductor device comprising an electrode, said electrode,
prior to firing, comprising said silver thick film paste of claim
7.
9. The semiconductor device of claim 8, wherein said semiconductor
device is a solar cell.
Description
FIELD OF THE INVENTION
[0001] The invention is directed to a process for making silver
powder particles with small size crystallites. These silver powder
particles are particularly useful in electronic applications.
BACKGROUND OF THE INVENTION
[0002] Silver powder is used in the electronics industry for the
manufacture of conductor thick film pastes. The thick film pastes
are screen printed onto substrates forming conductive circuit
patterns. These circuits are then dried and fired to volatilize the
liquid organic vehicle and sinter the silver particles.
[0003] Many processes currently used to manufacture metal powders
can be applied to the production of silver powders. For example,
thermal decomposition processes, electrochemical processes,
physical processes such as atomization or milling and chemical
reduction process can be used. Thermal decomposition processes tend
to produce powders that are spongy, agglomerated, and very porous
whereas electrochemical processes produce powders that are
crystalline in shape and very large. Physical processes are
generally used to make flaked materials or very large spherical
particles. Chemical precipitation processes produce silver powders
with a range of sizes and shapes.
[0004] Silver powders used in electronic applications are generally
manufactured using chemical precipitation processes. Silver powder
is produced by chemical reduction in which an aqueous solution of a
soluble salt of silver is reacted with an appropriate reducing
agent under conditions such that silver powder can be precipitated.
Inorganic reducing agents including hydrazine, sulfite salts and
formate salts can produce powders which are very coarse in size,
are irregularly shaped and have a large particle size distribution
due to aggregation. Organic reducing agents such as alcohols,
sugars or aldehydes are used with alkali hydroxides to reduce
silver nitrate. The reduction reaction is very fast;
[0005] hard to control and produces a powder contaminated with
residual alkali ions. Although small in size (<1 .mu.m), these
powders tend to have an irregular shape with a wide distribution of
particle sizes that do not pack well. It is difficult to control
the sintering of these types of silver powders and they do not
provide adequate line resolution in thick film conductor
circuits.
[0006] Therefore, there is a need for a process to produce silver
powder spherical particles comprising small size crystallites and
having a d.sub.50 particle size in the range of 0.5 to 3.5 .mu.m to
provide silver thick film paste with improved sintering
properties.
SUMMARY OF THE INVENTION
[0007] This invention provides a process for making silver powder
particles, the process comprising: [0008] (a) preparing an acidic
aqueous silver salt solution comprising a water soluble silver salt
dissolved in deionized water; [0009] (b) preparing an acidic
aqueous reducing and particle modifier solution comprising: [0010]
(i) a reducing agent selected from the group consisting of ascorbic
acid, ascorbates and mixtures; [0011] (ii) nitric acid; [0012]
(iii) gum arabic; [0013] (iv) maleic acid; and [0014] (v) deionized
water; [0015] (c) maintaining the acidic aqueous silver salt
solution and the acidic reducing and particle modifier solution at
the same temperature, wherein the temperature is in the range of
10.degree. C. to 65.degree. C., while stirring each the solution
during and after preparation; [0016] (d) adding the acidic aqueous
silver salt solution to the acidic aqueous reducing and particle
modifier solution while stirring to make a reaction mixture and
maintaining the temperature of the reaction mixture at the
temperature of (c), wherein the silver powder particles precipitate
and are contained within the resulting final aqueous solution; and
[0017] (e) increasing the temperature of the final aqueous solution
to a temperature in the range of 65.degree. C. to 80.degree. C.
while continuing stirring; the silver powder particles comprising
crystallites of between 30 and 47 nm in size as determined by X-ray
diffraction and the Scherrer formula; and the silver powder
particles having d.sub.50 in the range of 0.5 to 3.5 .mu.m.
[0018] Also provided is the above process further comprising:
[0019] (f) separating the silver powder particles from the final
aqueo solution; [0020] (g) washing the silver powder particles with
deionized water; and [0021] (h) drying the silver powder
particles.
[0022] Also provided are the silver powder particles made by the
above process, silver thick film paste made with the silver powder
particles and a semiconductor device, e.g., a solar cell,
comprising an electrode that prior to firing comprises the silver
thick film paste.
BRIEF DESCRIPTION OF THE FIGURES
[0023] FIG. 1 is a scanning electron microscope image at a
magnification of 5,000 of the silver powder particles made in
Example 1. The d.sub.50 particle size is 2.8 .mu.m.
[0024] FIG. 2 is a scanning electron microscope image at a
magnification of 10,000 of the silver powder particles made in
Example 2. The d.sub.50 particle size is 2.2 .mu.m.
[0025] FIG. 3 is a scanning electron microscope image at a
magnification of 10,000 of the silver powder particles made in
Example 3. The d.sub.50 particle size is 1.6 .mu.m.
[0026] FIG. 4 is a scanning electron microscope image at a
magnification of 10,000 of the silver powder particles made in
Comparative Experiment 1. The d.sub.50 particle size is 10.1
.mu.m.
[0027] FIG. 5 is a scanning electron microscope image at a
magnification of 10,000 of the silver powder particles made in
Comparative Experiment 2. The d.sub.50particle size is 8.7
.mu.m.
DETAILED DESCRIPTION OF THE INVENTION
[0028] This invention provides a process for making
spherically-shaped silver powder particles with small size
crystallites. A combination of gum arabic and maleic acid is used
with the reduction of a silver salt with ascorbic acid to control
the crystallite size. The process is a reductive process in which
silver particles comprising crystallites between 30 and 47 nm in
size as determined by X-ray diffraction and the Scherrer formula
are precipitated by adding together an acidic aqueous solution of a
water soluble silver salt and an acidic aqueous reducing and
particle modifier solution containing a reducing agent, nitric
acid, gum arabic and maleic acid.
[0029] The acidic aqueous silver salt solution is prepared by
adding a water soluble silver salt to deionized water. Any water
soluble silver salt, e.g., silver nitrate, silver phosphate, and
silver sulfate, can be used. Silver nitrate is preferred. No
complexing agents are used which could provide side reactions that
affect the reduction and type of particles produced. Nitric acid
can be added to increase the acidity.
[0030] The process can be run at concentrations up to 1.2 moles of
silver per liter of final aqueous solution. It is preferred to run
the process at concentrations of 0.47 to 0.8 moles of silver per
liter of final aqueous solution. It is especially preferred to run
the process at concentrations of 0.47 to 1.0 moles of silver per
liter of final aqueous solution. These relatively high
concentrations of silver make the manufacturing process cost
effective.
[0031] The acidic aqueous reducing and particle modifier solution
is prepared by adding the various components, i.e., a reducing
agent, gum arabic, maleic acid and nitric acid, to deionized water.
Suitable reducing agents for the process are ascorbic acids such
L-ascorbic acid and D-ascorbic acid and related ascorbates such as
sodium ascorbate. Ascorbic acid is preferred.
[0032] In one embodiment, the acidic aqueous reducing and particle
modifier solution also contains a metal colloid selected from the
group consisting of gold colloid and silver colloid. A suitable
metal colloid is gold colloid or silver colloid. Gold colloid is
preferred.
[0033] The components can be added to the deionized water in
various orders. In one embodiment, gum arabic is the first
component added to the deionized water. The gum arabic is added to
the deionized water and then stirred for a period of time before
adding the other components. In the Examples, 14 g of gum arabic
was added to 375 g or 750 g of deionized water and the mixture was
stirred for an hour before adding the other components. Times for
stirring smaller or larger quantities could be adjusted
accordingly. Nitric acid, maleic acid, ascorbic acid and gold
colloid solution were then added in that order. Stirring for a time
as indicated above should occur following addition of the gum
arabic no matter the order of adding the components.
[0034] The order of preparing the acidic aqueous silver salt
solution and the acidic reducing and particle modifier solution is
not important, The acidic aqueous silver salt solution can be
prepared before, after, or contemporaneously with the acidic
reducing and particle modifier solution. The acidic aqueous silver
salt solution is added to the acidic reducing and particle modifier
solution. The addition is carried out slowly. For example, with the
quantities used in the Examples, i.e., 125 and 250 g of deionized
water for the acidic aqueous silver salt solution and 375 and 750 g
of deionized water for the acidic aqueous reducing and particle
modifier solution, the acidic aqueous silver salt solution was
added to the acidic reducing and particle modifier solution over a
period of one hour, Smaller quantities could be mixed over a
shorter time interval and larger quantities over a longer time
period. The reaction mixture is stirred during the addition.
[0035] In this process the acidic aqueous silver salt solution and
the acidic reducing and particle modifier solution are both
maintained at the same temperature, i.e., a temperature in the
range of 10.degree. C. to 65.degree. C. and each solution is
stirred until they are mixed. In some embodiments the solutions are
maintained at a temperature in the range of 10.degree. C. to
35.degree. C. In one such embodiment the solutions are maintained
at 25.degree. C. When the two solutions are mixed to form the
reaction mixture, the reaction mixture is maintained at that same
temperature.
[0036] The mixture of the acidic aqueous silver salt solution and
the acidic reducing and particle modifier solution results in the
precipitation of the silver particles that are contained within the
resulting final aqueous solution. Following precipitation, the
temperature of the final aqueous solution is heated to a
temperature in the range of 65.degree. C. to 80.degree. C. In some
embodiments the temperature is in the range of 65.degree. C. to
75.degree. C. In one such embodiment the temperature is 70.degree.
C. The final aqueous solution containing the silver particles is
maintained at this temperature for about an hour and is stirred
during that time.
[0037] The silver particles are then separated from the final
aqueous solution by filtration or other suitable liquid-solid
separation operation and the solids are washed with deionized water
until the conductivity of the wash water is 100 microSiemans or
less. The silver particles are then dried.
[0038] The silver powder particles made by the process of the
invention have a particular set of physical characteristics. The
silver particles are described herein as spherically-shaped. It can
be seen from the scanning electron microscope (SEM) images of FIGS.
1-3 that the particles are generally spherical in shape but are not
perfect spheres. The average particle size determined from the SEM
images is in the range of 1 to 2.5 .mu.m. The silver powder
particles are comprised of crystallites between 30 and 47 nm in
size as determined by X-ray diffraction and the Scherrer formula.
In one group of embodiments the crystallites are between 30 and 39
nm in size. The silver powder particles have a d.sub.50 in the
range of 0.5 to 3.5 .mu.m and a size distribution such that
(d.sub.90-d.sub.10)/d.sub.50<2.2. The quantity
(d.sub.90-d.sub.10)/d.sub.50 is a measure of the size distribution
of the particles. Since (d.sub.90-d.sub.10)/d.sub.50<2.2, it is
an indication of the narrow size distribution of the particles,
i.e., the uniformity of particle size and the corresponding highly
dispersible nature of the silver powders comprising these
particles. The silver particles have a solids content of in excess
of 99% as determined by thermogravimetric analysis.
Thermomechanical analysis (TMA) is used herein as an indicator of
sinterabilty. A pellet was formed from the silver particles and
subjected to a temperature regime. The observed per cent shrinkage,
i.e., the dimensional change of the pellet, is a measure of the
sinterability of the particles. The results are presented herein as
TMA equal to the percent dimensional change, a negative per cent
change since it represents a shrinkage. The silver particles of the
invention showed TMA dimensional changes, i.e., shrinkages, of at
least -10%.
[0039] The silver powder particles made by the process of this
invention can be used in thick film paste applications, including
thick films for front side metallization of photovoltaic solar
cells and thick films for other semiconductor devices.
[0040] This invention also provides the silver powder particles
made by the process of this invention. These silver powder
particles can be used in thick film paste. The structure of these
silver particles will lend them to be more readily sintered and
provide improved thick film conductors. Also provided is a
semiconductor device, e.g., a solar cell, comprising an electrode
that prior to firing comprises the silver thick film paste.
Silver Thick Film Paste
[0041] This invention also provides a silver thick film paste
comprised of the silver powder particles made by the process of
this invention and glass frit dispersed in an organic medium. As
used herein, "thick film paste" refers to a composition which after
being deposited on a substrate and fired has a thickness of 1 to
100 .mu.m.
[0042] The glass frit compositions are described herein as
including percentages of certain components. The percentages are
the percentages of the components used in the starting material
that was subsequently processed as described herein to form a glass
composition. The composition contains certain components and the
percentages of those components are expressed as a percentage of
the corresponding oxide or fluoride form. The weight percentages of
the glass frit components are based on the total weight of the
glass composition. A certain portion of volatile species may be
released during the process of making the glass. An example of a
volatile species is oxygen.
[0043] Various glass frit compositions are useful in the silver
thick film pastes of the invention. The glass frit used has a
softening point of 300 to 600.degree. C. The glass frit
compositions described herein are not limiting. Minor substitutions
of additional ingredients can be made without substantially
changing the desired properties of the glass composition. For
example, substitutions of glass formers such as 0-3 wt %
P.sub.2O.sub.5, 0-3 wt % GeO.sub.2 and 0-3 wt % V.sub.2O.sub.5 can
be used either individually or in combination to achieve similar
performance.
[0044] The glass frit compositions can also contain one or more
fluorine-containing components such as salts of fluorine, fluorides
and metal oxyfluoride compounds. Such fluorine-containing
components include, but are not limited to BiF.sub.3, AlF.sub.3,
NaF, LiF, KF, CsF, PbF.sub.2, ZrF.sub.4, TiF.sub.4 and
ZnF.sub.2.
[0045] Exemplary lead free glass compositions contain one or more
of SiO.sub.2, B.sub.2O.sub.3, Al.sub.2O.sub.3, Bi.sub.2O.sub.3,
BiF.sub.3, ZnO, ZrO.sub.2, CuO, Na.sub.2O, NaF, Li.sub.2O, LiF,
K.sub.2O, and KF. In various embodiments the compositions comprise
the following oxide constituents in the compositional ranges, the
SiO.sub.2 is 17 to 26 wt %, 19 to 24 wt %, or 20 to 22 wt %; the
B.sub.2O.sub.3 is 2 to 9 wt %, 3 to 7 wt %; or 3 to 4 wt %; the
Al.sub.2O.sub.3 is 0.1 to 5 wt %, 0.2 to 2.5 wt %, or 0.2 to 0.3 wt
%; the Bi.sub.2O.sub.3 is 0 to 65 wt %, 25 to 64 wt %, or 46 to 64
wt %; the BiF.sub.3 is 0 to 67 wt %, 0 to 43 wt %, or 0 to 19 wt %;
the ZrO.sub.2 is 0 to 5 wt %, 2 to 5 wt %, or 4 to 5 wt %; the
TiO.sub.2 is 1 to 7 wt %, 1 to 5 wt %, or 1 to 3 wt %; CuO is 0 to
3 wt % or 2 to 3 wt %; Na.sub.2O is 0 to 2 wt % or 1 to 2 wt %; NaF
is 0 to 3 wt % or 2 to 3 wt %; Li.sub.2O is 0 to 2 wt % or 1 to 2
wt %; and LiF is 0 to 3 wt % or 2 to 3 wt %. Some or all of the
Na.sub.2O or Li.sub.2O can be replaced with K.sub.2O and some or
all of the NaF or LiF can be replaced with KF to create a glass
with properties similar to the compositions listed above.
[0046] In other embodiments, the glass frit compositions can
include one or more of a third set of components: CeO.sub.2,
SnO.sub.2, Ga.sub.2O.sub.3, In.sub.2O.sub.3, NiO, MoO.sub.3,
WO.sub.3, Y.sub.2O.sub.3, La.sub.2O.sub.3, Nd.sub.2O.sub.3, FeO,
HfO.sub.2, Cr.sub.2O.sub.3, CdO, Nb.sub.2O.sub.5, Ag.sub.2O,
Sb.sub.2O.sub.3, and metal halides (e.g. NaCl, KBr, NaI).
[0047] Exemplary lead containing glass compositions comprise the
following oxide constituents in the compositional range of 0-36 wt
% SiO.sub.2, 0-9 wt % Al.sub.2O.sub.3, 0-19 wt % B.sub.2O.sub.3,
16-84 wt % PbO, 0-4 wt % CuO, 0-24 wt % ZnO, 0-52 wt %
Bi.sub.2O.sub.3, 0-8 wt % ZrO.sub.2, 0-20 wt % TiO.sub.2, 0-5 wt %
P.sub.2O.sub.5, and 3-34 wt % PbF.sub.2. In other embodiments
relating to glasses containing bismuth oxide, the glass frit
composition contains 4-26 wt % SiO.sub.2, 0-1 wt % Al.sub.2O.sub.3,
0-8 wt % B.sub.2O.sub.3, 20-52 wt % PbO, 0-4 wt % ZnO, 6-52 wt %
Bi.sub.2O.sub.3, 2-7 wt % TiO.sub.2, 5-29 wt % PbF.sub.2, 0-1 wt %
Na.sub.2O and 0-1 wt % Li.sub.2O. In still other embodiments
relating to glasses containing 15-25 wt % ZnO, the glass frit
comprises 5-36 wt % SiO.sub.2, 0-9 wt % Al.sub.2O.sub.3, 0-19 wt %
B.sub.2O.sub.3, 17-64 wt % PbO, 0-39 wt % Bi.sub.2O.sub.3, 0-6 wt %
TiO.sub.2, 0-5 wt % P.sub.2O.sub.5 and 6-29 wt % PbF.sub.2. In
various of these embodiments containing ZnO, the glass frit
compositions comprises 5-15 wt % SiO.sub.2 and/or 20-29 wt %
PbF.sub.2 and/or 0-3 wt % ZrO.sub.2 or 0.1-2.5 wt % ZrO.sub.2.
Embodiments containing copper oxide and/or alkali modifiers
comprise 25-35 wt % SiO.sub.2, 0-4 wt % Al.sub.2O.sub.3, 3-19 wt %
B.sub.2O.sub.3, 17-52 wt % PbO, 0-12 wt % ZnO, 0-7 wt %
Bi.sub.2O.sub.3, 0-5 wt % TiO.sub.2, 7-22 wt % PbF.sub.2, 0-3 wt %
CuO, 0-4 wt % Na.sub.2O and 0-1 wt % Li.sub.2O.
[0048] An exemplary method for producing the glass frits described
herein is by conventional glass making techniques. Ingredients are
weighed then mixed in the desired proportions and heated in a
furnace to form a melt in platinum alloy crucibles or other
suitable metal or ceramic crucibles. As indicated above, oxides as
well as fluoride or oxyfluoride salts can be used as raw materials.
Alternatively, salts, such as nitrate, nitrites, carbonate, or
hydrates, which decompose into oxide, fluorides, or oxyfluorides at
temperature below the glass melting temperature can be used as raw
materials. Heating is conducted to a peak temperature of typically
800-1400.degree. C. and for a time such that the melt becomes
entirely liquid, homogeneous, and free of any residual
decomposition products of the raw materials. The molten glass is
then quenched between counter rotating stainless steel rollers to
form a 10-15 mil thick platelet of glass. The resulting glass
platelet was then milled to form a glass frit powder with its 50%
volume distribution set between to a desired target (e.g. 0.8-1.5
.mu.m). Alternative synthesis techniques such as water quenching,
sol-gel, spray pyrolysis, or others appropriate for making powder
forms of glass can be employed.
[0049] The organic medium used in the silver thick film paste is a
solution of a polymer in a solvent. The organic medium can also
contain thickeners, stabilizers, surfactants and/or other common
additives. In one embodiment, the polymer is ethyl cellulose. Other
exemplary polymers include ethylhydroxyethyl cellulose, wood rosin,
mixtures of ethyl cellulose and phenolic resins, polymethacrylates
of lower alcohols, and monobutyl ether of ethylene glycol
monoacetate, or mixtures thereof. The solvents useful in the
organic medium of the silver thick film paste compositions include
ester alcohols and terpenes such as alpha- or beta-terpineol or
mixtures thereof with other solvents such as kerosene,
dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexylene
glycol and high boiling alcohols and alcohol esters. The organic
medium can also contain volatile liquids for promoting rapid
hardening after application on the substrate.
[0050] The thick film silver composition is adjusted to a
predetermined, screen-printable viscosity with the organic
medium.
[0051] The inorganic components, i.e., the silver powder particles
and the glass frit are typically mixed with the organic medium by
mechanical mixing to form a viscous paste composition.
[0052] The ratio of organic medium in the silver thick film paste
composition to the inorganic components in the dispersion is
dependent on the method of applying the paste and the kind of
organic medium used, and it can vary. The dispersion will typically
contain 70 to 95 wt % of inorganic components and 5 to 30 wt % of
organic medium in order to obtain good wetting. The weight percents
(wt %) used herein are based on the total weight of the silver
thick film paste composition. Typically, the polymer present in the
organic medium is in the range of 8 wt % to 11 wt % of the weight
of the total composition.
[0053] In one embodiment, the silver thick film paste contains 65
to 90 wt % silver powder, 0.1 to 8 wt % glass frit and 5 to 30 wt %
organic medium. In another embodiment the silver thick film paste
contains 70 to 85 wt % silver powder, 1 to 6 wt % glass frit and 10
to 25 wt % organic medium. In still another embodiment the silver
thick film paste contains 78 to 83 wt % silver powder, 2 to 5 wt %
glass frit and 13 to 20 wt % organic medium.
Semiconductor Device; Solar Cell
[0054] The invention also provides a method of making a
semiconductor device, e.g., a solar cell or a photodiode. The
semiconductor device has an electrode, e.g., a front side electrode
of a solar cell or a photodiode, wherein prior to firing the
electrode is comprised of a silver thick film paste of the
invention.
[0055] The method of manufacturing a semiconductor device,
comprises the steps of: [0056] (a) providing a semiconductor
substrate, one or more insulating films, and the silver thick film
paste of the invention; [0057] (b) applying the insulating film to
the semiconductor substrate, [0058] (c) applying the silver thick
film paste to the insulating film on the semiconductor substrate,
and [0059] (d) firing the semiconductor substrate, the insulating
film and the silver thick film paste composition.
[0060] Exemplary semiconductor substrates useful in the methods and
devices described herein include, but are not limited to,
single-crystal silicon, multicrystalline silicon, and ribbon
silicon. The semiconductor substrate may be doped with phosphorus
and boron to form a pin junction. The semiconductor substrates can
vary in size (length x width) and thickness. As an example, the
thickness of the semiconductor substrate is 50 to 500 .mu.m; 100 to
300 .mu.m; or 140 to 200 .mu.m. The length and width of the
semiconductor substrate are each 100 to 250 mm; 125 to 200 mm; or
125 to 156 mm.
[0061] Typically an anti-reflection coating is formed on the front
side of a solar cell. Exemplary anti-refection coating materials
useful in the methods and devices described herein include, but are
not limited to: silicon nitride, silicon oxide, titanium oxide,
SiN.sub.x:H, hydrogenated amorphous silicon nitride, and silicon
oxide/titanium oxide film. The coating can be formed by plasma
enhanced chemical vapor deposition (PECVD), CVD, and/or other known
techniques known. In an embodiment in which the coating is silicon
nitride, the silicon nitride film can be formed by PECVD, thermal
CVD, or physical vapor deposition (PVD). In an embodiment in which
the insulating film is silicon oxide, the silicon oxide film can be
formed by thermal oxidation, thermal CVD, plasma CVD, or PVD.
[0062] The silver thick film paste of the invention can be applied
to the anti-reflective coated semiconductor substrate by a variety
of methods such as screen-printing, ink-jet printing, coextrusion,
syringe dispensing, direct writing, and aerosol ink jet printing.
The paste can be applied in a pattern and in a predetermined shape
and at a predetermined position. In one embodiment, the paste is
used to form both the conductive fingers and busbars of the
front-side electrode. In such an embodiment, the width of the lines
of the conductive fingers are 20 to 200 .mu.m, 40 to 150 .mu.m, or
60 to 100 .mu.m and the thickness of the lines of the conductive
fingers are 5 to 50 .mu.m, 10 to 35 .mu.m, or 15 to 30 .mu.m.
[0063] The paste coated on the ARC-coated semiconductor substrate
can be dried, for example, for 0.5 to 10 minutes during which time
the volatile solvents and organics of the organic medium are
removed.
[0064] The dried paste is fired by heating to a maximum temperature
of between 500 and 940.degree. C. for a duration of 1 second to 2
minutes. In one embodiment, the maximum silicon wafer temperature
reached during firing ranges from 650 to 80.degree. C. for a
duration of 1 to 10 seconds. In a further embodiment, the electrode
formed from the silver thick film paste is fired in an atmosphere
composed of a mixed gas of oxygen and nitrogen. In another
embodiment, the electrode formed from the conductive thick film
paste is fired above the organic medium removal temperature in an
inert atmosphere not containing oxygen, This firing process removes
any remaining organic medium and sinters the glass frit with the
silver powder and any metal oxide present to form an electrode.
Typically, the burnout and firing is carried out in a belt furnace.
The temperature range in the burnout zone, during which time the
remaining organic medium is removed, is between 500 and 700.degree.
C. The temperature in the firing zone is between 860 and
940.degree. C.
[0065] During firing, the fired electrode, preferably the fingers,
reacts with and penetrates the anti-reflective coating, thereby
making electrical contact with the silicon substrate.
[0066] In a further embodiment, prior to firing, other conductive
and device enhancing materials are applied to the back side of the
semiconductor device and cofired or sequentially fired with the
paste compositions of the invention. The materials serve as
electrical contacts, passivating layers, and solderable tabbing
areas.
[0067] In one embodiment, the back side conductive material
contains aluminum or aluminum and silver.
[0068] In a still further embodiment the materials applied to the
opposite type region of the device are adjacent to the materials
described herein due to the p and n region being formed side by
side. Such devices place all metal contact materials on the non
illuminated back side of the device to maximize incident light on
the illuminated front side.
Measurements
[0069] The particle size distribution numbers (d.sub.10, d.sub.50,
d.sub.90) used herein are based on a volume (mass) distribution.
The particle sizes were measured using a Microtrac.RTM. Particle
Size Analyzer from Leeds and Northrup. The d.sub.10, d.sub.50 and
d.sub.90 are the equivalent diameters that represent the 10th
percentile, the median or 50th percentile and the 90th percentile
of the particle sizes, respectively, as measured by volume. That
is, the d.sub.50 (d.sub.10, d.sub.90) is a value on the
distribution such that 50 volume % (10 volume %, 90 volume %) of
the particles have an equivalent diameter of this value or
less.
[0070] The solids content of the silver particles was determined by
thermogravimetric analysis, i.e., by a weight loss method after
heating at 850.degree. C. for 10 minutes.
[0071] The size of the crystallites making up the silver powder
particles was determined by X-ray diffraction and the Scherrer
formula. The X-ray diffractometer used was a Rigaku Rint RAD-rb.
The Cu target provided a wavelength of 0.15405 nm. The Bragg plane
was the (111). In the Scherrer formula:
L(111)=0.94.lamda./(.beta. cos .theta.),
L is the crystallite size, .lamda. is the wavelength, .beta. is the
line broadening at half the peak maximum intensity (full width-half
maximum) in radians and .theta. is the Bragg angle.
[0072] The TMA was carried out as follows. A pellet was formed by
pouring 0.9-1.1 g of silver powder into a die and onto a
cylindrical metal insert serving as the bottom. A second
cylindrical metal is inserted into the die on top of the silver
powder so that the powder is sandwiched between the cylindrical
metal inserts. The top insert is then firmly tapped 6 times with a
mallet. The pellet is placed in a Bargal TMA:Q400 analyzer which
measured the dimensional change as a function of temperature. The
temperature was raised from room temperature at a rate of
10.degree. C./min to 650.degree. C. and the pellet is maintained at
that temperature for 10 minutes. The final per cent dimensional
change is reported as the TMA for that powder pellet. A negative
value indicates shrinkage and a positive value expansion of the
pellet.
EXAMPLES
[0073] The following Examples and discussion are offered to further
illustrate, but not limit the process of this invention.
Example 1
[0074] The acidic aqueous silver salt solution was prepared by
dissolving 80 g of silver nitrate in 250 g of deionized water. This
solution was kept at 25.degree. C. while continuously stirring.
[0075] The acidic reducing and particle modifier solution was
prepared by adding 14 g of gum Arabic to 750 g of deionized water
in a separate container from the silver nitrate solution. This
solution was kept at 25.degree. C. while continuously stirring for
an hour. 20 g of nitric acid (70% w/w), 5 g maleic acid, 45 g
ascorbic acid and 1 g of gold colloid solution was then added to
the solution still maintained at 25.degree. C. and stirred.
[0076] The acidic aqueous silver salt solution was slowly added to
the acidic reducing and particle modifier solution over a period of
one hour to form a reaction mixture that was intensely stirred
during the addition. The reaction mixture was maintained at
25.degree. C. Silver particles precipitated and were contained
within the final aqueous solution. The final aqueous solution was
then heated to 70.degree. C. for one hour and stirring continued
during the heating.
[0077] As shown in the scanning electron microscope image of FIG.
1, the silver powder was comprised of spherically-shaped silver
particles. The size L of the crystallites in the particles was 37.4
nm. d.sub.10, d.sub.50, and d.sub.90 were 1.8 .mu.m, 2.8 .mu.m and
4.8 .mu.m, respectively. The silver particles were 99.2% solids.
The TMA=-12%.
[0078] Example 2
[0079] Example 2 was carried out using the process described in
Example 1 except that 125 g of deionized water was used in
preparing the acidic aqueous silver salt solution and 375 g of
deionized water was used in preparing the acidic reducing and
particle modifier solution
[0080] As shown in the scanning electron microscope image of FIG.
2, the silver powder was comprised of spherically-shaped silver
particles. The size L of the crystallites in the particles was 38.2
nm. d.sub.10, d.sub.50, and d.sub.90 were 1.2 .mu.m, 2.2 .mu.m and
3.9 .mu.m, respectively. The silver particles were 99.8% solids.
The TMA=-16%.
[0081] Example 3
[0082] Example 3 was carried out using the process described in
Example 2 except that 3 g of nitric acid (70% w/w), 1 g maleic acid
and 5 g of gold colloid solution was used in preparing the acidic
reducing and particle modifier solution.
[0083] As shown in the scanning electron microscope image of FIG.
3, the silver powder was comprised of spherically-shaped silver
particles. The size L of the crystallites in the particles was 35.2
nm. d.sub.10, d.sub.50, and d.sub.90 were 0.8 .mu.m, 1.6 .mu.m and
3.4 .mu.m, respectively. The silver particles were 99.7% solids.
The TMA=-14.
Comparative Experiment 1
[0084] Comparative Experiment 1 was carried out using the process
described in Example 1 except that no gum arabic was added in
preparing an acidic reducing and particle modifier solution.
[0085] As shown in the scanning electron microscope image of FIG.
4, the silver powder was comprised of irregular-shaped large silver
particles that appear to be highly crystalline. The size L of the
crystallites in the particles was 72.0 nm. d.sub.10, d.sub.50, and
d.sub.90 were 4.6 .mu.m, 10.1 .mu.m and 15.1 .mu.m, respectively.
The TMA=-9.8%.
Comparative Experiment 2
[0086] An acidic aqueous silver salt solution was prepared by
adding 14 g of gum arabic to 250 g of deionized water. This
solution was kept at 25.degree. C. while continuously stirring for
an hour. Then 80 g of silver nitrate was added, This solution was
kept at 25.degree. C. while continuously stirring. An acidic
reducing and particle modifier solution was prepared by adding 5 g
of nitric acid (70% w/w), 5 g maleic acid, 45 g ascorbic acid and 1
g of gold colloid solution to 750 g of deionized water in a
separate container from the silver nitrate solution. This solution
was kept at 25.degree. C. while continuously stirring.
[0087] The acidic aqueous silver salt solution was slowly added to
the acidic reducing and particle modifier solution over a period of
one hour to form a reaction mixture that was intensely stirred
during the addition. The reaction mixture was maintained at
25.degree. C. Silver particles precipitated and were contained
within the final aqueous solution. The final aqueous solution was
then heated to 70.degree. C. for one hour and stirring continued
during the heating.
[0088] The silver particles were recovered as described in Example
1.
[0089] As shown in the scanning electron microscope image of FIG.
5, the silver powder was comprised of very irregular-shaped
relatively large silver particles. The size L of the crystallites
in the particles was 47.6 nm. d.sub.10, d.sub.50, and d.sub.90 were
5.4 8.7 .mu.m and 14.0 respectively. The TMA=-7.5%.
Comparative Experiment 3
[0090] An acidic aqueous silver salt solution was prepared by
dissolving 80 g of silver nitrate in 250 g of deionized water. This
solution was kept at 25.degree. C. while continuously stirring.
[0091] An acidic reducing and particle modifier solution was
prepared by adding 12 g of gum arabic to 750 g of deionized water
in a separate container from the silver nitrate solution. This
solution was kept at 25.degree. C. while continuously stirring for
an hour. 20 g of nitric acid (70% w/w), 3 g K.sub.2SO.sub.4, 45 g
ascorbic acid and 0.1 g of gold colloid solution was then added to
the solution still maintained at 25.degree. C. and stirred,
[0092] The acidic aqueous silver salt solution was slowly added to
the acidic reducing and particle modifier solution over a period of
one hour to form a reaction mixture that was intensely stirred
during the addition. The reaction mixture was maintained at
25.degree. C. Silver particles precipitated and were contained
within the final aqueous solution. The final aqueous solution was
then heated to 70.degree. C. for one hour and stirring continued
during the heating.
[0093] The silver particles were recovered as described in Example
1.
[0094] The size L of the crystallites in the particles was 60.0 nm.
d.sub.10, d.sub.50, and d.sub.90 were 2.0 .mu.m, 3.5 .mu.m and 6.3
.mu.m, respectively. The TMA=-6%.
Comparative Experiment 4
[0095] Comparative Experiment 1 was carried out using the process
described in Comparative Experiment 3 except that 5 g of gold
colloid solution was used in preparing the acidic reducing and
particle modifier solution.
[0096] The size L of the crystallites in the particles was 56.2 nm.
d.sub.10, d.sub.50, and d.sub.90 were 0.9 .mu.m, 1.6 .mu.m and 2.9
.mu.m, respectively. The TMA=-6%.
* * * * *