U.S. patent application number 12/719285 was filed with the patent office on 2010-07-01 for metal inks.
This patent application is currently assigned to Alliance for Sustainable Energy, LL. Invention is credited to Calvin J. Curtis, David S. Ginley, Marinus Franciscus Antonius Maria van Hest, Tatiana Kaydanova, Alex Miedaner.
Application Number | 20100163810 12/719285 |
Document ID | / |
Family ID | 38876984 |
Filed Date | 2010-07-01 |
United States Patent
Application |
20100163810 |
Kind Code |
A1 |
Ginley; David S. ; et
al. |
July 1, 2010 |
METAL INKS
Abstract
Self-reducing metal inks and systems and methods for producing
and using the same are disclosed. In an exemplary embodiment, a
method may comprise selecting a metal-organic (MO) precursor,
selecting a reducing agent, and dissolving the MO precursor and the
reducing agent in an organic solvent to produce a metal ink that
remains in a liquid phase at room temperature. Metal inks,
including self-reducing and fire-through metal inks, are also
disclosed, as are various applications of the metal inks.
Inventors: |
Ginley; David S.;
(Evergreen, CO) ; Curtis; Calvin J.; (Lakewood,
CO) ; Miedaner; Alex; (Boulder, CO) ; Hest;
Marinus Franciscus Antonius Maria van; (Lakewood, CO)
; Kaydanova; Tatiana; (Lakewood, CO) |
Correspondence
Address: |
PAUL J WHITE, PATENT COUNSEL;NATIONAL RENEWABLE ENERGY LABORATORY (NREL)
1617 COLE BOULEVARD, MS 1734
GOLDEN
CO
80401-3393
US
|
Assignee: |
Alliance for Sustainable Energy,
LL
|
Family ID: |
38876984 |
Appl. No.: |
12/719285 |
Filed: |
March 8, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11427270 |
Jun 28, 2006 |
|
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12719285 |
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Current U.S.
Class: |
252/519.21 |
Current CPC
Class: |
C09D 11/30 20130101;
C23C 18/31 20130101; H01B 1/12 20130101 |
Class at
Publication: |
252/519.21 |
International
Class: |
H01B 1/02 20060101
H01B001/02 |
Goverment Interests
[0001] The United States Government has rights in this invention
under Contract No. DE-AC36-99G010337 between the United States
Department of Energy and the National Renewable Energy Laboratory,
a Division of the Midwest Research Institute.
Claims
1-20. (canceled)
21. A fire-through metal ink for a solar cell, the metal ink
comprising: a soluble metal complex; a soluble organo-metallic
reagent; a solution, the solution containing a particulate metal or
metal organic precursor at room temperature, the solution combined
with the soluble organo-metallic reagent and the soluble metal
complex to produce the metal ink.
22. The fire-through metal ink of claim 21, further comprising
dissolving the metal organic precursor and a reducing agent in an
organic solvent to produce the fire-through metal ink.
23. The fire-through metal ink of claim 21, wherein the metal ink
remains in a liquid phase at room temperature.
24. The fire-through metal ink of claim 21, wherein the metal ink
has a ratio of precursor to organic solvent providing viscosity for
inkjet printing.
25. The fire-through metal ink of claim 24, wherein the ratio of
precursor to organic solvent is in a range of about 1:10 and about
1:1.
26. The fire-through metal ink of claim 21, further comprising a
ratio of precursor to organic solvent providing viscosity and
wetting properties for inkjet printing, the ratio being
approximately within the range of about 1:10 and about 1:1
27. The fire-through metal ink of claim 26, wherein the ratio of
precursor to organic solvent is in a range of about 1:10 and about
1:1.
28. The fire-through metal ink of claim 26, further comprising a
substantially uncontaminated metal deposit on a substrate.
29. The fire-through metal ink of claim 26, further comprising
metal deposits formed at elevated temperatures.
30. The fire-through metal ink of claim 29, further comprising
metal deposits selected from the group consisting essentially of:
copper (Cu), gold (Au), silver (Ag), lead (Pb), palladium (Pd),
platinum (Pt), cobalt (Co), iron (Fe), Tin (Sn), and metal
alloys.
31. A fire-through metal ink comprising lead acetate in ethylene
glycol, wherein when deposited on an AR coating 34 the lead acetate
is reduced to PbO forming continuous layers and patterns of
controlled thickness.
32. The fire-through metal ink of claim 31, wherein the PbO
completely etches the AR coating at temperatures as low as about
500 C after only 10 minutes.
33. The fire-through metal ink of claim 32, wherein increasing the
temperature increases the etching rate.
34. The fire-through metal ink of claim 33, wherein increasing the
temperature to about 750 C for 10 min produces a 1 micrometer deep
etch pits.
35. The fire-through metal ink of claim 31, wherein both mixed and
single-oxide liquid chemical precursors facilitate a "burn-through"
process on solar cell substrates having an AR coating.
36. The fire-through metal ink of claim 35, wherein the mixed-oxide
liquid chemical precursors consist essentially of lead (lead
acetate, lead formate), boron (boric acid) and silicon
(silane).
37. The fire-through metal ink of claim 31, wherein functional
components are added to the fire-through metal ink to control
chemical and/or physical properties of the fire-through metal
ink.
38. A fire-through metal ink comprising lead acetate in ethylene
glycol, wherein when deposited on an AR coating 34 the lead acetate
is reduced to a metal oxide forming continuous layers and patterns
of controlled thickness.
39. The fire-through metal ink of claim 38, wherein the metal oxide
is at least one of the following, PbO, SnO, SnO2 and ZnO.
Description
SUMMARY
[0002] The following embodiments and aspects thereof are described
and illustrated in conjunction with systems, tools and methods that
are meant to be exemplary and illustrative, not limiting in scope.
In various embodiments, one or more of the above-described problems
have been reduced or eliminated, while other embodiments are
directed to other improvements.
[0003] An exemplary method comprises the steps of selecting a
metal-organic (MO) precursor which is non-reacting at room
temperature the MO precursors are configured or arranged to have a
metal ion that is reduced to a pure metallic state at a potential
positive of a reduction potential of the reducing agent, which
includes formate ions; electing a reducing agent which is
non-reacting at room temperature; reacting the MO precursor and the
reducing agent at an activating temperature in the range of about
150-250.degree. C. wherein reacting the MO precursor and the
reducing agent produces substantially pure metal deposits on a
substrate; and dissolving the MO precursor and the reducing agent
in an organic solvent to produce a metal ink that remains in a
liquid phase at room temperature; the organic solvent is selected
from a group of organic solvents that have a sufficiently high
boiling point so as to remain in a liquid phase at room temperature
and provide viscosity and wetting properties for inkjet printing.
The exemplary method further comprises elevating the temperature of
the metal ink; reacting the reducing agent with the MO precursor;
and producing a substantially uncontaminated metal deposit on a
substrate.
[0004] Another exemplary method comprises the steps of providing a
metal ink in a liquid phase at room temperature; and applying the
metal ink to a substrate at an elevated temperature by at least one
of the following processes: spraying, dipping, spinning,
direct-write deposition, and/or inkjet printing, wherein the metal
ink reacts in a single step at the elevated temperature to produce
substantially pure metal deposits on the substrate.
[0005] A further exemplary method comprises the steps of providing
a fire-through metal ink in a liquid phase at room temperature; and
applying the fire-through metal ink to a coated surface or
substrate of a solar cell by at least one of the following
processes: spraying, dipping, spinning, direct-write deposition,
and/or inkjet printing, wherein the fire-through metal ink reacts
with the coated surface of the solar cell to produce electrical
contacts with a p-n layer beneath the coated surface of the solar
cell.
[0006] An exemplary metal ink may be produced in accordance with
the methods described above wherein the metal ink forms metal
deposits at elevated temperatures and wherein the metal deposits
consist of copper (Cu), gold (Au), silver (Ag), lead (Pb),
palladium (Pd), platinum (Pt), cobalt (Co), iron (Fe), Tin (Sn),
and other metal alloys. Additionally self-reducing and fire-through
metal inks may be produced according to the exemplary methods
described above. The fire-through metal ink further comprises a
soluble metal complex and soluble organo-metallic reagent in a
solution containing a particulate metal or metal organic precursor
at room temperature.
[0007] In addition to the exemplary aspects and embodiments
described above, further aspects and embodiments will become
apparent by reference to the drawings and by study of the following
descriptions.
BRIEF DESCRIPTION OF THE DETAILED DRAWINGS
[0008] Exemplary embodiments are illustrated in referenced figures
of the drawings. It is intended that the embodiments and figures
disclosed herein are to be considered illustrative rather than
limiting.
[0009] FIG. 1 is a block diagram illustrating an exemplary system
which may be implemented for inkjet printing of metal inks on
various substrates.
[0010] FIG. 2 is a conceptual view of an exemplary metal ink.
[0011] FIGS. 3a and 3b illustrate an exemplary application of
fire-through metal inks in the production of solar cells.
[0012] FIG. 4 is a plot showing X-ray diffraction (XRD) analysis of
exemplary nickel deposits.
[0013] FIG. 5 are plots showing X-ray diffraction (XRD) analysis of
exemplary silver, copper, and nickel deposits.
[0014] FIG. 6 is a digital photograph showing exemplary copper
lines that were inkjet printed on a printed circuit board in a
nitrogen environment.
[0015] FIG. 7 is a digital photograph showing an exemplary silver
grid that was inkjet printed on a silicon solar cell in air.
[0016] FIG. 8 is a three-dimensional illustration of a pattern
etched in an anti-reflection coating for a silicon solar cell using
an exemplary fire-through metal ink.
DESCRIPTION
[0017] Deposition techniques, such as, e.g., vacuum deposition,
screen printing and electroplating, are used for depositing a
variety of inorganic and organic electronic materials on a
substrate. As disclosed herein, inkjet printing is a viable,
low-cost alternative to these deposition approaches. Inkjet
printing is capable of producing high-resolution deposits on a
substrate without the need for masking or templates. Inkjet
printing is also a "non-contact" deposition technique (i.e., there
is no contact between the print head and the substrate), making it
well suited for producing metal deposits on thin and/or fragile
polycrystalline substrates which could otherwise break using
"contact" deposition techniques. In addition, inkjet printing may
be used to produce three-dimensional (3-D) metal deposits on
substrates.
[0018] Briefly, metal inks disclosed herein are well suited for
inkjet printing on any of a wide variety of substrates. For
example, the printed inks adhere well to glass, silicon (Si),
printed circuit boards (PCB), and many other substrates. The metal
inks undergo a reducing reaction wherein pure, highly conductive
metal deposits on the substrates at relatively low temperatures.
Inkjet printing the metal inks enables the controlled deposition of
metal lines and grids, in addition to multi-layer and
multi-component metal features on the substrates.
[0019] The metal inks may also be readily tailored for a variety of
different uses, e.g., by addition of adhesion promoters, doping
compounds, and/or other additives that further enhance the
mechanical and/or electronic properties of the resulting metal
deposit. For example, the metal inks may include etching agents for
use in the production of high efficiency solar cells.
[0020] Exemplary metal inks, and systems and methods for production
and use of the metal inks, may be better understood with reference
to the figures and following discussion.
[0021] FIG. 1 is a block diagram illustrating an exemplary system
10 which may be implemented for inkjet printing of metal inks on
various substrates. The exemplary system 10 may include an ink jet
12 and a translation stage 14 for positioning a substrate 16 (e.g.,
in the X or Y directions) adjacent the ink jet 12. A heating
element 18 may also be provided, e.g., on the translation stage 14
directly adjacent the substrate 16. One or more optional gas
supplies (illustrated in FIG. 1 by gas cylinder 20 and attached
supply lines) may also be provided for controlling atmospheric
conditions for printing the metal inks on the substrate 16. For
example, the gas supplies may be used to provide an inert (e.g.,
nitrogen or argon) atmosphere during the inkjet printing
process.
[0022] Exemplary system 10 may also include a controller 22.
Controller 22 may be implemented to control various process
parameters, such as, e.g., operation of the translation stage 14 to
position the substrate 16 adjacent the ink jet 12, application rate
of the metal inks by the ink jet 12 onto the substrate 16,
increasing/decreasing temperature of the heating element 18, and/or
application of one or more gasses from the gas supplies.
[0023] In an exemplary embodiment, the controller 22 may be
implemented as a desktop or laptop personal computer (PC) executing
control software developed using LabVIEW. LabVIEW is a graphical
programming language available from National Instruments (NI) for
data acquisition and instrumentation control. Such control software
may be readily developed by one having ordinary skill in the art
after becoming familiar with the teachings herein. Therefore,
further discussion is not needed to fully enable the controller 22.
It is noted, however, that any suitable controller 22 may be
implemented and is not limited to a PC executing software developed
with LabVIEW.
[0024] In an exemplary embodiment, the ink jet 12 may be
implemented as a piezoelectric print head (commercially available
from MicroFab Technologies, Inc., Plano, Tex., 75074). The print
head may have a 30-50 .mu.m orifice and may be operated using a
piezoelectric actuator for ejecting the ink, e.g., at a drop
generation rate of up to about 2000 Hertz (Hz).
[0025] Inkjet printing of metal inks enables direct production of
patterned deposits on the substrate 16 using a non-vacuum process
that produces results comparable to those obtained using
traditional vacuum-based techniques. In addition, inkjet printing
is a non-contact process which enables production of patterned
deposits on thin and/or fragile substrates 16 that could not
otherwise be accomplished using traditional contact-based
techniques. It is noted, however, that other techniques for
applying the metal inks to the substrate are also contemplated.
Other direct application techniques may include, but are not
limited to spraying, dipping, and/or spinning techniques.
[0026] FIG. 2 is a conceptual view of an exemplary metal ink.
Exemplary metal inks may comprise one or more metal-organic (MO)
precursors (illustrated by molecular structure 26) and a reducing
agent (illustrated by the formate ion 28) dissolved in water or
other solvent 24. The metal ion (M) may be a chelated metal with a
set of ligands that readily decompose to impart solubility to
solution. Such an embodiment is well-suited for processing by
inkjet printing or other direct application techniques.
[0027] The solvent 24 may be selected based on a number of design
considerations. For example, the polarity of the solvent 24 may be
selected based on the solubility of the MO precursor 26. The
viscosity of the solvent 24 may be selected based on the stability
of the metal ink. Other factors may include, but are not limited to
desired boiling point, evaporation rate, viscosity, and surface
tension. For example, the solvent 24 may have a high boiling point
so that it does not change to a gas phase during the inkjet
printing process. The solvent 24 may also be selected such that it
provides the desired viscosity and wetting properties suitable for
inkjet printing. Exemplary solvents include, but are not limited
to, polyethers (di-glymes, triglymes, tetraglymes), ethylene glycol
mono- and bis alkyl ethers, ethylene glycol, alcohols, aldehydes,
water, surfactants, and/or combinations thereof.
[0028] A wide variety of MO precursors 26 may be used for producing
the metal inks. In an exemplary embodiment, the MO precursor
includes a metal ion (M) that is reduced to its pure metallic state
at a potential positive of the reduction potential of the reducing
agent (e.g., -0.20 V for formate). Accordingly, metal inks may be
produced for copper (Cu), gold (Au), silver (Ag), lead (Pb),
palladium (Pd), platinum (Pt), cobalt (Co), iron (Fe), tin (Sn),
and metal alloys.
[0029] The reducing agent (e.g., formate ion 28) provides a
counter-ion for reaction with the MO precursor 26. At room
temperature, the reducing agent does not react with the MO
precursor 26. Accordingly, the MO precursor 26 may remain soluble,
e.g., as a metal ink solution 24 that is suitable for inkjet
printing. During the inkjet printing process, however, the
temperature of the metal ink solution 24 is elevated to an
activating temperature (e.g., in the range of about 150-250.degree.
C.). This activating temperature causes the reducing agent to react
with the MO precursor 26 and form pure metal deposits on the
substrate.
[0030] In an exemplary embodiment, formate (HCO.sub.2.sup.-) 28 may
be used as the reducing agent. During the inkjet printing process,
the temperature of the metal ink is elevated, causing the formate
ions to react with the MO precursor 26. This reaction produces an
uncontaminated metal deposit on the substrate, along with a carbon
dioxide (CO.sub.2) byproduct. It is noted, however, that formate is
merely an example of a reducing agent that may be used. Other
reducing agents may also be used, such as, e.g., halide or nitrate
salts, alcohols, aldehydes, acetals, ethylene glycol, ethylene
glycol diformate, benzaldehyde, acetealdehyde, etc.
[0031] In alternative embodiments, reducing conditions may be
applied in a second step following application of the metal ink.
For example, the MO precursor 26 may be deposited on the substrate
at temperatures below about 100.degree. C., and then in a second
step, annealed with a forming gas or vapors of organic reducing
agents (e.g., ethylene, glycol, formaldehyde, acetaldehyde,
hydrazine, etc.) to reduce the MO precursor to a pure metal deposit
on the substrate.
[0032] Before continuing, it should be noted that the metal ink may
also comprise other components, e.g., to enhance the inkjet
printing process in air or an inert atmosphere at relatively low
temperatures. The specific composition of the metal ink may depend
at least to some extent on various design considerations. Exemplary
design considerations include, but are not limited to, the MO
precursor, the substrate that the metal ink is being applied to,
and the application that the printed substrate will be used
for.
[0033] For purposes of illustration, the metal ink may include
components, such as, e.g., dispersants, binders, and/or
surfactants, for enhancing deposition, resolution, and/or adhesion
of the metal inks to the substrate. For example, the surface
properties of the ink may be adjusted for higher printing
resolution by adding surfactants such as, Triton X-100, alkyl
sulfonate, alkyl phosphate and phosphonate, alkyl amine and
ammonium, etc.
[0034] In addition, one or more process parameters may be adjusted
for the particular metal ink being used to optimize the inkjet
printing process and/or properties of the printed features. For
example, the substrate temperature, gas flow rate, and/or
application rate of the metal inks may be adjusted to optimize
deposition rate of the metal ink, purity/phase of the deposited
metal, and/or adhesion to the substrate. Or for example, the
substrate temperature, gas flow rate, and/or application rate of
the metal inks may be adjusted to optimize resolution, quality,
thickness, conductivity and other electrical properties of the
printed features.
[0035] The metal inks may be used for coating a substrate with
metal (e.g., by spraying, dipping, and/or spinning techniques)
and/or for producing metal features on a substrate (e.g., as lines,
grids, or patterns) by inkjet printing or other direct-write
deposition techniques. In addition, the metal inks may be used in a
wide variety of different applications. It is readily appreciated
that applications of this technology may include, but are not
limited to, printed circuit boards (PCBs), touch-screen display
devices, organic light emitting diodes (OLEDs), organic solar
cells, cell phone displays, photo-voltaic devices (e.g., solar
cells), catalysts, decorative coatings, structural materials,
optical devices, flexible electronics, and other electronic and
micro-electronic devices, to name only a few examples.
[0036] In an exemplary embodiment, the metal inks may be inkjet
printed on a substrate in air or inert environment (e.g., nitrogen
or argon) by heating the substrate to about 180.degree. C.
(100-200.degree. C.), and then applying the metal ink using a drop
generation rate of about 50 Hz (25-100 Hz). This embodiment results
in a deposition rate of about 1 .mu.m per pass. Thicker deposits
may be obtained by inkjet printing multiple layers.
[0037] Inkjet printing multiple layers of metal inks also enables
deposition of a contact formation layer, followed by a separate
metal forming layer. Accordingly, the contact formation process can
be better controlled, and also results in conductor lines having
higher conductivity.
[0038] FIGS. 3a and 3b illustrate an exemplary application of metal
inks (i.e., so called "fire-through" metal inks) in the production
of solar cells 30. In traditional silicon solar cell fabrication, a
paste containing silver and glass frits is applied to a
silicon-nitride anti-reflection (AR) coating over a silicon p-n
junction. This paste helps the silver metal adhere to the AR
coating, and enables the silver metal to "burn through" the AR
coating and establish electrical contact with the silicon p-n
junction. During this process, however, molten glass frits may flow
between the silver and the silicon p-n junction, increasing
resistance of the contacts and decreasing performance of the solar
cells.
[0039] The metal inks disclosed herein may be implemented as
fire-through metal inks (in place of the glass frits paste) for
fabricating solar cells. Use of fire-through metal inks overcome
the disadvantages of pastes containing glass frits, while at the
same time offering other advantages (e.g., smaller, high resolution
contacts resulting in a smaller shadow area). By eliminating glass
frits, the fire-through metal inks also eliminate the need for
other constituents needed in the pastes for controlling physical
properties of the glass frits (e.g., softening agents). Removing
these and other constituents of the pastes also helps to improve
conductivity and reduce contact resistance in the finished solar
cell 30.
[0040] In an exemplary embodiment, the fire-through metal inks may
comprise soluble chemical precursors (metal complexes and
organo-metallic reagents) to various metal oxides. These precursors
are provided in a solution containing a particulate metal or metal
organic precursor at room temperature. The fire-through metal inks
may be applied to the anti-reflection (AR) coating 34 of the solar
cell (e.g., by inkjet printing or other deposition techniques). For
purposes of illustration, fire-through metal inks are shown by the
deposits 32 in FIG. 3a after application on the AR coating 34 by
inkjet printing.
[0041] The fire-through metal inks may be used at relatively low
process temperatures. For example, process temperatures may be in
the range of about 200-500.degree. C., well below temperatures
considered to be detrimental to solar cells. In addition, the
fire-through metal inks may be applied by inkjet printing,
spin-coating, or other highly-controlled deposition processes,
thereby enabling the production of patterns having uniform and
controlled thickness, patterns, and other features.
[0042] During and/or following application, the printed ink reacts
with the AR coating 34. During this reaction, the oxide precursor
and metal precursor decompose (or "burn through") the AR coating 34
and establish electrical contact with the p-n junction 36. For
example, the metal deposits 38 are shown following "burn through"
in contact with the p-n layer 36 in FIG. 3b. It is noted that the
precursor reacts directly at the interface between the metal
deposits 38 and the p-n junction, thereby controlling properties of
the interface (e.g., size, shape) without adversely affecting
conductivity of the metal deposits 38.
[0043] In an exemplary embodiment, the metal inks may comprise lead
acetate in ethylene glycol. When deposited on the AR coating 34
(e.g., by inkjet printing), the lead acetate is reduced to PbO,
forming continuous layers and patterns of controlled thickness, as
illustrated in FIG. 3a. The PbO completely etches the AR coating at
temperatures as low as about 500.degree. C. after only 10 minutes,
as illustrated in FIG. 3b. Increasing the temperature increases the
etching rates. For example, increasing the temperature to about
750.degree. C. for 10 min produced 1 .mu.m deep etch pits, which is
comparable to rates achieved using commercial glass frits at
similar temperatures. In other exemplary embodiments, tin or zinc
may be used in place of lead.
[0044] It is noted that the fire-through metal inks are not limited
to any particular composition or formulation. For example, both
mixed and single-oxide liquid chemical precursors may be used to
facilitate the "burn-through" process on solar cell substrates
having an AR coating. Mixed component solutions may include, but
are not limited to, lead (lead acetate, lead formate), boron (such
as boric acid) and silicon (silane). Still other chemical
precursors may be used. Likewise, other functional components, such
as, e.g., reducing agents, binders, and solvents may also be added
to the fire-through metal inks to control chemical and/or physical
properties of the fire-through metal inks and/or product.
Example 1
Metal Inks
[0045] The following examples are provided to illustrate production
and use of various metal inks that may be inkjet printed to produce
metallic features, such as, e.g., layers and patterns on a
substrate. The inkjet printing system was operated at 10-60 V,
50-1000 Hz, and using an x-y translation speed of 1-30 mm/sec.
[0046] In one example, nickel inks were produced by adding 0.5 gram
(g) nickel(II) formate dihydrate [Ni(HCO.sub.2).sub.2.2H.sub.2O] to
0.2 miliLiters (mL) ethylene diamine and 2.0 mL ethylene glycol. In
this example, the ethylene diamine complexed Ni(II) to make the
metal soluble, and the formate served as the reducing agent.
[0047] The nickel inks were spray printed on glass substrates in an
air environment at temperatures of about 250.degree. C. to produce
metallic nickel deposits on the glass. The nickel deposits had
electrical properties comparable to bulk nickel. Specifically, a 4
micron layer of Ni metal deposited by spray deposition exhibited a
resistivity of about 100 .mu..OMEGA.cm (compared to bulk Ni which
has a resistivity of about 7 .mu..OMEGA.cm). X-ray diffraction
(XRD) analysis (plot 40 shown in FIG. 4) of the nickel deposits
confirmed that the deposit was a pure nickel metal, and other
phases of nickel (e.g., NiO) were not present.
[0048] In another example, silver (Ag) inks were produced by adding
1.0 g silver trifluoroacetate [Ag(CF.sub.3CO.sub.2)] to 2.0 mL of
ethylene glycol and 0.1 mL Triton X-100. Silver inks were also
produced by adding 2.0 g silver trifluoroacetate to 1.5 mL H.sub.2O
and 0.2 mL ethylene glycol. In this example, the ethylene glycol
and Triton X-100 served as the reducing agents and also helped
lower the processing temperature for silver formation to
180.degree. C. The Triton X-100 also served as a surfactant for
better resolution printing.
[0049] In yet another example, copper (Cu) inks were produced by
adding 0.2 g copper(II) formate tetrahydrate
[Cu(HCO.sub.2).sub.2.4H2O] to 0.5 mL ethanolamine and 2.0 mL
ethylene glycol. In this example, the ethanolamine increased the
solubility of the copper(II) formate tetrahydrate. The formate
served as the reducing agent.
[0050] The Ag, Cu and Ni inks were printed on substrates heated to
about 200-250.degree. C. in air. Although better results were
obtained by inkjet printing metal inks containing Cu in an inert
atmosphere (N.sub.2 or Ar), copper deposition was also possible in
air when followed by rapid thermal processing. In all cases,
results of X-ray diffraction analysis (plot 50 shown in FIG. 5)
showed that the metal deposits were pure metal, without detectable
traces of carbon or oxides.
[0051] Thick (up to 15 .mu.m), highly conducting lines of Ag and Cu
were printed on a variety of substrates demonstrating good adhesion
to glass, silicon, and printed circuit boards. For purposes of
illustration, FIG. 6 is a photograph 60 showing 5 .mu.m thick and
300 .mu.m wide copper "lines" 62 that were inkjet printed on a
printed circuit board 64 in a nitrogen environment at about
200.degree. C. FIG. 7 is a photograph 70 showing a 10 .mu.l m thick
and 250 .mu.m wide silver "grid" 72 that was inkjet printed on a
silicon solar cell 74 in air at about 200.degree. C.
[0052] The metal deposits exhibited conductivities comparable to
those of the bulk metals. For example, the silver deposits
exhibited conductivities of about 2 .mu..OMEGA.cm, the copper
deposits exhibited conductivities of about 10 .mu..OMEGA.cm, and
the nickel deposits exhibited conductivities of about 100
.mu..OMEGA.cm. It is noted that the conductivity for the silver
deposits is essentially the same as bulk silver metal, and although
the resistivity for the copper and nickel deposits is approximately
one order of magnitude higher than the bulk metals, these results
are comparable to conductivities achieved for copper and nickel
deposits using conventional screen printing techniques.
Example 2
Fire-Through Metal Inks
[0053] The following examples are provided to illustrate the
production and use of so-called fire-through metal inks that may be
inkjet or spray printed on a SiN anti-reflection (AR) coating in
the fabrication of solar cells. In this example, the fire-through
metal inks were produced by adding 1.0 g lead acetate hydrate
[Pb(CH.sub.3CO.sub.2)2.H.sub.2O] to 3.0 mL ethylene glycol.
[0054] The fire-through metal inks were inkjet printed on the AR
coating to produce 15 .mu.m thick, 300 .mu.m wide contacts. Ohmic
contacts were achieved at low temperatures (650-750.degree. C.)
with a very short annealing cycle (less than one minute). A 1 m
deep, 70 .mu.m wide etch pattern 80 obtained by inkjet printing and
consequent thermal processing at 750.degree. C. for 10 min of the
AR coating 82 on a silicon p-n junction is shown in FIG. 8.
Complete burn-through of the AR coating was observed at
temperatures as low as 500 C. Layered printing decreased the
processing temperature for contact formation to as low as
650.degree. C., and improved performance of the printed cell.
[0055] It is noted that the Examples discussed above are provided
for purposes of illustration and are not intended to be limiting.
Still other embodiments of processes for the production and use of
metal inks will be readily appreciated by those having ordinary
skill in the art after understanding the teachings herein.
[0056] While a number of exemplary aspects and embodiments have
been discussed above, those of skill in the art will recognize
certain modifications, permutations, additions and sub combinations
thereof. It is therefore intended that the following appended
claims and claims hereafter introduced are interpreted to include
all such modifications, permutations, additions and
sub-combinations as are within their true spirit and scope.
* * * * *