U.S. patent application number 16/931809 was filed with the patent office on 2020-11-05 for methods to incorporate silver nanowire-based transparent conductors in electronic devices.
The applicant listed for this patent is Cambrios Film Solutions Corporation. Invention is credited to Pierre-Marc Allemand, Paul Mansky.
Application Number | 20200350091 16/931809 |
Document ID | / |
Family ID | 1000004959992 |
Filed Date | 2020-11-05 |
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United States Patent
Application |
20200350091 |
Kind Code |
A1 |
Allemand; Pierre-Marc ; et
al. |
November 5, 2020 |
METHODS TO INCORPORATE SILVER NANOWIRE-BASED TRANSPARENT CONDUCTORS
IN ELECTRONIC DEVICES
Abstract
Disclosed herein are optical stacks that are stable to light
exposure by incorporating light-stabilizers.
Inventors: |
Allemand; Pierre-Marc; (San
Jose, CA) ; Mansky; Paul; (San Francisco,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cambrios Film Solutions Corporation |
Tortola |
|
VG |
|
|
Family ID: |
1000004959992 |
Appl. No.: |
16/931809 |
Filed: |
July 17, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13840864 |
Mar 15, 2013 |
10720257 |
|
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16931809 |
|
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61765420 |
Feb 15, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01B 1/02 20130101; G06F
2203/04112 20130101; Y10T 428/2804 20150115; G06F 3/045
20130101 |
International
Class: |
H01B 1/02 20060101
H01B001/02; G06F 3/045 20060101 G06F003/045 |
Claims
1. An optical stack comprising: a first substrate; a conductor
layer disposed over the first substrate, wherein the conductor
layer comprises a conductive network of nanostructures and a
polymer in which the nanostructures are disposed; and an optically
clear adhesive (OCA) layer abutting the conductor layer, wherein:
an interface is defined where the OCA layer abuts the conductor
layer, the polymer is only present at least one of at or below the
interface, and the OCA layer comprises one or more
photo-stabilizers.
2. The optical stack of claim 1, wherein a total weight of the one
or more photo-stabilizers in the OCA layer is at least 10% of a
total weight of the OCA layer.
3. The optical stack of claim 1, wherein the nanostructures are
entirely disposed below the interface.
4. The optical stack of claim 1, wherein the nanostructures
comprise silver nanostructures.
5. The optical stack of claim 1, wherein the OCA layer comprises
one or more photo-sensitive species.
6. The optical stack of claim 5, wherein the one or more
photo-stabilizers suppress an oxidation of the nanostructures
induced by the one or more photo-sensitive species.
7. The optical stack of claim 1, wherein the one or more
photo-stabilizers comprise at least one of an alkene, a hindered
phenol, a tetrazole, a triazole, a phosphine, a thioether, or a
metallic photo-desensitizer.
8. The optical stack of claim 1, wherein the OCA layer is between
the first substrate and the conductor layer.
9. The optical stack of claim 1, wherein the conductor layer is
between the first substrate and the OCA layer.
10. The optical stack of claim 1, comprising: a second substrate,
wherein: the conductor layer is between the first substrate and the
OCA layer, and the OCA layer is between the conductor layer and the
second substrate.
11. The optical stack of claim 10, wherein: the conductor layer
abuts the first substrate, and the OCA layer abuts the second
substrate.
12. An optical stack, comprising: a first substrate; a conductor
layer disposed over the first substrate, wherein the conductor
layer comprises a conductive network of nanostructures and a binder
in which the nanostructures are disposed, wherein the binder has a
first composition; and an optically clear adhesive (OCA) layer
disposed over the conductor layer, wherein: the OCA layer has an
uppermost surface and a bottommost surface, an interface is defined
where the bottommost surface of the OCA layer abuts the conductor
layer, the OCA layer has a homogenous composition extending from
the uppermost surface of the OCA layer to the bottommost surface of
the OCA layer and different than the first composition, and the OCA
layer comprises one or more photo-stabilizers.
13. The optical stack of claim 12, wherein the nanostructures are
entirely disposed below the interface.
14. The optical stack of claim 12, wherein the OCA layer comprises
one or more photo-sensitive species.
15. The optical stack of claim 14, wherein the one or more
photo-stabilizers suppress an oxidation of the nanostructures
induced by the photo-sensitive species.
16. A method of forming an optical stack, comprising: forming an
optically clear adhesive (OCA) layer on a substrate; exposing the
OCA layer to a liquid comprising one or more photo-stabilizers,
wherein at least some of the one or more photo-stabilizers
infiltrate into the OCA layer during the exposing; removing excess
liquid on the OCA layer after the exposing; and forming a conductor
layer on the OCA layer after the removing, wherein the conductor
layer comprises a conductive network of nanostructures and a binder
in which the nanostructures are disposed.
17. The method of claim 16, wherein the removing comprises:
spinning the OCA layer and the substrate; and baking the OCA layer
and the substrate.
18. The method of claim 16, wherein the removing comprises:
spinning the OCA layer and the substrate; and exposing the OCA
layer and the substrate to a nitrogen atmosphere.
19. The method of claim 16, wherein the one or more
photo-stabilizers comprise at least one of terpineol, limonene, or
cyclohexanol.
20. The method of claim 16, wherein the exposing comprises spraying
the liquid comprising the one or more photo-stabilizers onto the
OCA layer.
Description
RELATED APPLICATIONS
[0001] The instant application is a continuation of U.S.
Non-Provisional patent application Ser. No. 13/840,864, titled
"METHODS TO INCORPORATE SILVER NANOWIRE-BASED TRANSPARENT
CONDUCTORS IN ELECTRONIC DEVICES" and filed on Mar. 15, 2013, which
claims priority to U.S. Provisional Patent Application 61/765,420,
titled "METHODS TO INCORPORATE SILVER NANOWIRE-BASED TRANSPARENT
CONDUCTORS IN ELECTRONIC DEVICES" and filed on Feb. 15, 2013. U.S.
Non-Provisional patent application Ser. No. 13/840,864 and U.S.
Provisional Patent Application 61/765,420 are incorporated herein
by reference.
BACKGROUND
Technical Field
[0002] The disclosure relates to processing methods for making
stable and reliable optical stacks comprising transparent
conductive films of silver nanowires and optically clear adhesives
(OCA).
Description of the Related Art
[0003] Transparent conductors refer to thin conductive films coated
on high-transmittance surfaces or substrates. Transparent
conductors may be manufactured to have surface conductivity while
maintaining reasonable optical transparency. Such surface
conducting transparent conductors are widely used as transparent
electrodes in flat liquid crystal displays, touch panels,
electroluminescent devices, and thin film photovoltaic cells; as
anti-static layers; and as electromagnetic wave shielding
layers.
[0004] Currently, vacuum deposited metal oxides, such as indium tin
oxide (ITO), are the industry standard materials for providing
optical transparency and electrical conductivity to dielectric
surfaces such as glass and polymeric films. However, metal oxide
films are fragile and prone to damage during bending or other
physical stresses. They also require elevated deposition
temperatures and/or high annealing temperatures to achieve high
conductivity levels. For certain substrates that are prone to
adsorbing moisture, such as plastic and organic substrates (e.g.,
polycarbonates), it becomes problematic for a metal oxide film to
adhere properly. Applications of metal oxide films on flexible
substrates are therefore severely limited. In addition, vacuum
deposition is a costly process and requires specialized equipment.
Moreover, the process of vacuum deposition is not conducive to
forming patterns and circuits. This typically results in the need
for expensive patterning processes such as photolithography.
[0005] In recent years there is a trend to replace current industry
standard transparent conductive ITO films in flat panel displays
with a composite material of metal nanostructures (e.g., silver
nanowires) embedded in an insulating matrix. Typically, a
transparent conductive film is formed by first coating on a
substrate an ink composition including silver nanowires and a
binder. The binder provides the insulating matrix. The resulting
transparent conductive film has a sheet resistance comparable or
superior to that of the ITO films.
[0006] Nanostructure-based coating technologies are particularly
suited for printed electronics. Using a solution-based format,
printed electronic technology makes it possible to produce robust
electronics on large-area, flexible substrates. See U.S. Pat. No.
8,049,333, in the name of Cambrios Technologies Corporation, which
is hereby incorporated by reference in its entirety. The
solution-based format for forming nanostructure-based thin film is
also compatible with existing coating and lamination techniques.
Thus, additional thin films of overcoat, undercoat, adhesive layer,
and/or protective layer can be integrated into a high through-put
process for forming optical stacks that include nanostructure-based
transparent conductors.
[0007] Although generally considered as a noble metal, silver can
be sensitive to corrosion under specific circumstances. One result
of silver corrosion is a loss of conductivity either locally or
uniformly, which manifests as drifts in sheet resistance of the
transparent conductive film, leading to an unreliable performance.
Accordingly, there remains a need in the art to provide reliable
and stable optical stacks incorporating nanostructure-based
transparent conductor.
BRIEF SUMMARY
[0008] Disclosed are optical stacks including silver
nanostructure-based transparent conductors or thin films that are
stable to prolonged heat and light exposure.
[0009] One embodiment provides an optical stack comprising: a first
substrate; a conductive layer of metal nanostructures on the first
substrate; an optically clear adhesive (OCA) layer; wherein at
least one of the conductive network layer or the OCA layer further
comprises one or more photo-stabilizers.
[0010] In various embodiment, the metal nanostructures are
interconnecting metal nanowires, more specifically, silver
nanowires.
[0011] In a further embodiment, the metal nanostructures are in
contact with the OCA layer.
[0012] In various embodiments, the photo-stabilizer is an alkene, a
terpene (e.g.,) limonene or terpineol, a tetrazole, a triazole, a
hindered phenol, a phosphin, a thioether, or a metallic
photo-desensitizer.
[0013] In one embodiment, the photo-stabilizer is incorporated in
the OCA layer.
[0014] In another embodiment, the photo-stabilizer is incorporated
in the conductive network layer of silver nanostructures.
[0015] In a further embodiment, a drift in sheet resistance of the
conductive layer is less than 10% after exposing the optical stack
to accelerated light of at least 200 mW/cm2 measured at 365 nm for
at least 200 hours.
[0016] In yet another embodiment, a drift in sheet resistance of
the conductive layer is less than 30% after exposing the optical
stack to light of at least 200 mW/cm2 measured at 365 nm for at
least 800 hours.
[0017] In various embodiment as set forth above, the sheet
resistance of the conductive layer is less than 500 .OMEGA./sq.
prior to exposing the optical stack to accelerated light.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0018] In the drawings, identical reference numbers identify
similar elements or acts. The sizes and relative positions of
elements in the drawings are not necessarily drawn to scale. For
example, the shapes of various elements and angles are not drawn to
scale, and some of these elements are arbitrarily enlarged and
positioned to improve drawing legibility. Further, the particular
shapes of the elements as drawn are not intended to convey any
information regarding the actual shape of the particular elements,
and have been selected solely for ease of recognition in the
drawings.
[0019] FIG. 1 shows an optical stack including a metal
nanostructure-based transparent conductor.
[0020] FIG. 2 shows the impacts of various photo-stabilizers on the
% drift of sheet resistance of various optical stacks under an
accelerated light condition.
[0021] FIGS. 3-6 show the percentage drifts of sheet resistance of
various optical stacks treated with various photo-stabilizers
according to several embodiments.
DETAILED DESCRIPTION
[0022] Transparent conductive films are essential components in
flat panel display devices such as a touch screen or a liquid
crystal display (LCD). The reliability of these devices is dictated
in part by the stability of the transparent conductive films, which
are exposed to light and heat under the devices' normal operating
conditions. As discussed herein in more detail, it is discovered
that prolonged light exposure may induce corrosion of silver
nanostructures, causing localized or uniform increase in sheet
resistance of the transparent conductor.
[0023] Thus, disclosed are optical stacks including silver
nanostructure-based transparent conductors or thin films that are
stable to prolonged heat and light exposure and methods for
preparing the same.
[0024] As used herein, "optical stack" refers to a multi-layer
structure or panel that is generally placed in a light path of an
electronic device (e.g., a flat panel display). The optical stack
includes at least one layer of a metal nanostructure-based
transparent conductive film (or transparent conductor). Other
layers of the optical stack may include, for example, a substrate,
an overcoat, an undercoat, an adhesive layer, a protective layer
(e.g., cover glass) or other performance-enhancing layers such as
anti-reflective or anti-glare films. Preferably, the optical stack
includes at least one layer of optically clear adhesive.
[0025] FIG. 1 shows an optical stack (10) including a first
substrate (12), a silver nanostructure-based transparent conductor
(14), an OCA layer (16) and a second substrate (18). The optical
stack (10) may be formed by first forming a basic transparent
conductor (20) by depositing on the first substrate (12) a coating
solution of silver nanostructures, a binder and a volatile solvent.
After drying and/or curing, the silver nanostructures are
immobilized on the first substrate (12). The first substrate (12)
may be a flexible substrate (e.g., PET). An example of the basic
transparent conductor (20) is commercially available under the
trade name ClearOhm.RTM. by Cambrios Technologies Corporation, the
assignee of the present application. The basic transparent
conductor (20) may be laminated to the second substrate (18) via
the OCA layer (16).
[0026] The propensity for corrosion of the silver nanostructures
upon light exposure may be attributed to a complex set of factors.
It is discovered that a close proximity to the OCA in some cases
appears to induce and aggravate the corrosion of the silver
nanostructures.
[0027] Optically clear adhesives (OCA) are thin adhesive films
often used to assemble or bond several functional layers (e.g.,
cover glass and transparent conductors) into an optical stack or
panel (FIG. 1). Such a panel can serve as, for example, a
capacitive touch panel. An OCA often contains mixtures of alkyl
acrylates formed by free radical polymerization. As a result, the
OCA may contain unreacted initiators or photo-initiators, residual
monomers, solvents, and free radicals. Some of these species are
photo-sensitive and can be harmful to the silver nanostructures in
close proximity to the OCA. As used herein, the OCA may be pre-made
(including commercial forms) and laminated onto a substrate, or
coated directly onto a substrate from a liquid form.
[0028] Photo-sensitive species readily absorb photos and undergo or
induce complex photochemical activities. One type of photochemical
activity involves excitation of a compound from a ground state to a
higher energy level, i.e., an excited state. The excited state is
transient and generally would decay back to the ground state with
release of heat. Yet it is also possible for the transient excited
state to cause complex, cascading reactions with other species.
[0029] Regardless of the mechanisms, it is discovered that certain
photochemical activities cause corrosion of the silver
nanostructures through an oxidation reaction:
Ag.sup.0.fwdarw.Ag.sup.++e.sup.-
In certain embodiments, the corrosions are inhibited by suppressing
the photochemical activities of the excited state or facilitating a
rapid return to the ground state.
[0030] Thus, various embodiments provide stable optical stacks in
which one or more photo-stabilizers are combined with any of the
layers. As used herein, a photo-stabilizer generally refers to a
compound or additive that may act on any mechanism to suppress
photochemical activities, particularly with respect to
photo-induced oxidation of the silver nanostructures. For instance,
the photo-stabilizer may act as a hole trap to scavenge holes
generated from the photo-sensitive species that are most likely
associated with the OCA layer. The photo-stabilizer may also act as
a desensitizer which forestalls the hole creation. The
photo-stabilizer may also act an anti-oxidant that undergoes a
sacrificial oxidation reaction to destroy oxidants before they can
interact with the silver nanostructures.
Photo-Stabilizer
[0031] The photo-stabilizer may be any one of the following classes
of compounds. Generally speaking, they are non-volatile (having a
boiling point of at least 150.degree. C.) and may be a liquid or
solid. They may be a small organic molecule with molecular weight
of no more than 500, or may be an oligomer that has 2-100 monomers
or a polymer of more than 100 monomers.
[0032] 1. Alkenes
[0033] Alkenes are hydrocarbons that contain at least one
carbon-carbon double bond. The double bond makes an alkene a
candidate for sacrificial oxidation reaction. Alkenes may have a
linear, cyclic or a combination of linear and cyclic carbon
framework. On the carbon framework, alkene may be further
substituted with hydroxy, alkoxy, thiol, halogen, phenyl, or amine
groups.
[0034] In one embodiment, a suitable alkene has an alternate double
bond and single bond arrangement to provide an extended conjugated
structure. Such a conjugated structure allows for delocalization of
radicals, thus stabilizing the same. Examples of conjugated alkenes
include, without limitation, carotenes or carotenoids, certain
terpenes or terpenoids.
[0035] In other embodiments, alkenes may have multiple, but
un-conjugated double bonds. Examples of unconjugated alkenes
include certain terpenes, rosins, polybutadiene, and the like.
[0036] In addition to being a photo-stabilizer, certain alkenes are
also tackifiers and can be incorporated directly into an OCA.
[0037] i. Terpenes
[0038] Terpenes are a subset of alkenes. They are derived from
resin produced by a variety of plants, particularly conifers.
Although terpenes include a large diverse class of hydrocarbons,
they all contain at least one isoprene unit. Terpenes may have
cyclic as well as acyclic carbon framework. As used herein,
terpenes also include terpenoids, which are derivatives of terpenes
through oxidation or rearrangement of the carbon framework.
[0039] Due to their shared structures of isoprene, terpenes also
have at least one carbon-carbon double bond, which may participate
in a sacrificial oxidation reaction.
[0040] In certain embodiments, the photo-stabilizer is limonene.
Limonene is a cyclic terpene containing two isoprene units. The
ring double bond readily undergoes an oxidation reaction to form an
epoxide:
##STR00001##
[0041] Other suitable terpenes include humulene, squalene,
farnesene, and the like. Suitable terpenoids include, without
limitation, terpineol, genaniol and the like. Like limonene, these
terpenes similarly undergo a sacrificial oxidation reaction.
[0042] ii. Resin Tackifiers
[0043] Resin tackifiers are alkenes derived from plant source or
petroleum source. Resin tackifiers are excellent adhesives and may
be incorporated directly into OCA, where they participate in
sacrificial oxidation reactions to prevent the photo-sensitive
species in the OCA from corroding the silver nanostructures.
[0044] Resin tackifiers may include rosins and polyterpenes, which
are solid residues of plant-derived resin after the removal of the
terpenes (which have lower boiling points). Suitable rosin or
polyterpenes are commercially available from Pinova, Inc.
(Brunswick, Ga.), or Eastman (Kingsport, Tenn.). Pertroleum-based
resins may also be obtained from Eastman.
[0045] 2. Hindered Phenols
[0046] Hindered phenols refer to phenol derivatives that have bulky
substituents in the proximity of the hydroxyl group. The steric
hindrance as well as the delocalization afforded by the phenyl
group stabilize a hydroxyl radical, making hindered phenols
suitable as a photo-stabilizer.
[0047] In one embodiment, the photo-stabilizer is a butylated
hydroxy toluene (BHT). BHT (below) has two t-butyl groups adjacent
to the hydroxyl group, making it a powerful anti-oxidant as the
hydroxyl radical is stabilized by the adjacent t-butyl groups and
the phenyl group.
##STR00002##
[0048] Other suitable hindered phenols include, without limitation,
butylated hydroxy anisole (BHA), alkyl gallate (e.g., methyl
gallate, propyl gallate), tertiary-butylatedhydroquinone (TBHQ),
vitamin E (alpha tocopherol), and the like.
[0049] 3. Tetrazoles and Triazoles
[0050] Tetrazoles are organic compounds that contain a five-member
ring of four nitrogens and one carbon. Triazoles are organic
compounds that contain a five-member ring of three nitrogens and
two carbons. Both tetrazoles and triazoles are photo-desensitizers.
They also tend to bind to silver to form a protective coating that
may further prevent corrosion.
[0051] In addition to the ring structures, as used herein,
tetrazoles and triazoles may contain further substituents including
thiol (SH), alkyl, phenyl, thio (.dbd.S), azo group and the like.
They may be also further fused with other rings such as phenyl,
pyridine or pyrimidine, etc.
[0052] In one embodiment, the photo-stabilizer is
1-phenyl-1H-tetrazole-5-thiol (PTZT). In another embodiment, the
photo-stabilizer is benzotriazole (BTA)
[0053] In further various embodiments, a suitable photo-stabilizer
may be any one of the photo-desensitizer compounds (including all
the tetrazole and triazole compounds) disclosed in U.S. Pat. Nos.
2,453,087, 2,588,538, 3,579,333, 3,630,744, 3,888,677, 3,925,086,
4,666,827, 4,719,174, 5,667,953, and European Patent No. 0933677.
All of these patents are incorporated herein by reference in their
entireties.
[0054] 4. Phosphines
[0055] Phosphines are organophosphorous compounds having three
substituents attached to a phosphor (III). Phosphine undergoes
oxidation reaction in which phosphor (III) is oxidized to phosphor
(V). The substituents may the same or different and are typically
aryl (e.g., substituted or unsubstituted phenyl) or alkyl
(substituted or unsubstituted).
[0056] In one embodiment, the photo-stabilizer is triphenyl
phosphine, which may be oxidized as follows:
##STR00003##
[0057] 5. Thioethers
[0058] Thioethers or sulfides are organosulfurous compounds having
two substituents attached to sulfur group. The central sulfur group
may be oxidized to a sulfoxide (S.dbd.O), which may be further
oxidized to sulfone (S(.dbd.O).sub.2). The substituents may be the
same or different and are typically aryl (e.g., substituted or
unsubstituted phenyl) or alkyl (substituted or unsubstituted).
[0059] In one embodiment, the photo-stabilizer is thioether, which
may be oxidized as follows:
##STR00004##
[0060] 6. Metallic Photo-Desensitizers
[0061] Certain metals may serve as inorganic photo-stabilizers as
they can desensitize photochemical activities. Examples include
rhodium salt (see U.S. Pat. No. 4,666,827) and zinc or cadmium
salts (see U.S. Pat. No. 2,839,405). All of these patents are
incorporated herein by reference in their entireties.
Incorporating the Photo-Stabilizer
[0062] The photo-stabilizer may be incorporated into any one of the
layers of a given optical stack. In particular, because most of the
functional layers of the optical stack may be formed by
solution-based coating methods, the photo-stabilizer can be
combined with the coating solution prior to coating. For instance,
the photo-stabilizer may be incorporated into the nanostructure
layer, an overcoat, an undercoat, the substrate, or an adhesive
layer (e.g., OCA) through co-deposition.
[0063] While all of the layers, once incorporated with one or more
photo-stabilizers, may contribute to stabilizing the silver
nanostructures, the photo-stabilizers in the OCA layer can have a
significant impact. Because the OCA layers are generally the
thickest layers in an optical stack, they allow for a higher total
content (e.g. in mg/m.sup.2) of the photo-stabilizers. For example,
the nanostructure layers considered herein typically have total
thickness of 100-200 nm, whereas OCA layers have thicknesses
ranging from 25 .mu.m to 250 .mu.m. Hence, even with a very low
total concentration of a light stabilizing additive, a large total
amount of the additive can be included in the OCA. This is
especially beneficial if the additive is consumed while carrying
out its protective function.
[0064] In certain embodiments, the photo-stabilizers, such as
terpenes and certain resin tackifiers, are non-volatile liquid or
semi-solid. Thus, the liquid photo-stabilizer can be combined
directly with a pre-made OCA (e.g., in its commercial form). The
pre-made OCA may be sprayed with, dipped in or otherwise contact
the liquid photo-stabilizer. After a period of allowing the liquid
to infiltrate, the residue liquid on the surface of the OCA layer
can be wiped off or spun off. Examples of OCAs that may be used
include those that are commercially available from 3M Company under
product numbers 8146-2, 8142KCL, 8172CL, 8262N; Nitto Denko
Corporation under the product number CS9662LS; and Hitachi Chemical
Corporation under the product number TE7070. However, the above
technique is not limited to commercial forms of OCA. Any adhesive
layers may be similarly incorporated with one or more
photo-stabilizers as described herein.
[0065] A transparent conductor (silver nanostructures conductive
network formed on a substrate) may also be treated with a
photo-stabilizer in the same manner as the OCA. For instance, the
photo-stabilizer may contact (spray or dip) the transparent
conductor for a period of time to allow for the diffusion of the
photo-stabilizer into the transparent conductor.
[0066] The photo-stabilizers may also be first in the form of a
dispersion that contains a volatile solvent (e.g., alcohols,
acetone, water and the like). The dispersion is then combined with
the coating solution prior to coating. Alternatively, the
dispersion may be coated in a separate step independently from the
other coating steps for forming the optical stack. The volatile
solvent is thereafter removed along with other volatile solvent(s)
in the coating solution. Thirdly, the dispersion may contact a
layer (OCA or transparent conductor) for a period of time to allow
for the diffusion of the photo-stabilizer into the layer.
[0067] Regardless of the form of the photo-stabilizer, it can also
be combined directly with any film-forming coating solution prior
to coating. For instance, the photo-stabilizer may be combined with
a coating solution of the silver nanostructures, or a coating
solution of overcoat or undercoat, or a coating solution for
forming an adhesive layer.
[0068] Thus, one embodiment provides an optical stack comprising a
substrate, a transparent conductor including a plurality of
interconnecting silver nanostructures; an optically clear adhesive
layer, wherein at least one of the transparent conductor and the
optically clear adhesive layer incorporates one or more
photo-stabilizers. In various embodiments, the photo-stabilizer may
be an alkene (e.g., a terpene), a hindered phenol, a tetrazole or
triazole, a phosphine, a thioether or metals as described
herein.
[0069] Typically, the photo-stabilizer is present in a given layer
(e.g., OCA) at a concentration (by weight) of at least 0.02%, or at
least 0.05%, or at least 0.1%, or at least 2%, or at least 5%, or
at least 10%.
Testing Photo-Stability
[0070] To test the photo-stability of the optical stack, sheet
resistance of the optical stack under light exposure is measured as
a function of time to detect any drift. Because the normal service
or operating life of a display device can be years, an "accelerated
light condition" may be designed to simulate the total light
exposure within a normal operating life in a compressed time frame.
Thus, "accelerated light condition" refers to an artificial or
testing condition that exposes the optical stack to continuous and
intense simulated light. Often, the accelerated light condition can
be controlled to simulate the amount of light exposure that the
optical stack is subjected to during a normal service life of a
given device. Under the accelerated light condition, the light
intensity is typically significantly elevated compared to the
operating light intensity of the given device; the duration of the
light exposure for testing the reliability of the conductive films
can therefore be significantly compressed compared to the normal
service life of the same device. Typically, light intensity is
measured in Lumens, which is a unit of luminous flux. Under an
accelerated light condition, the light is about 30 to 100 times
more intense than the light condition of a device.
[0071] FIG. 2 shows accelerated light tests of the various optical
stacks with or without any additives in the OCA layer. Some of the
additives acted as photo-stabilizers (terpineol and limonene), as
evidenced by the significantly lower drifts in sheet resistance
over time as compared to the control (optical stack with no
additive in the OCA layer). The other additive (cyclohexanol) in
fact accelerated the sheet resistance drift.
[0072] Thus, the accelerated light tests can be used to evaluate
the effectiveness of a photo-stabilizer.
[0073] Certain other features of the present disclosure are further
discussed in more detail below.
Metal Nanostructures
[0074] As used herein, "metal nanostructures" generally refer to
electrically conductive nano-sized structures, at least one
dimension of which (i.e., width or diameter) is less than 500 nm;
more typically, less than 100 nm or 50 nm. In various embodiments,
the width or diameter of the nanostructures are in the range of 10
to 40 nm, 20 to 40 nm, 5 to 20 nm, 10 to 30 nm, 40 to 60 nm, 50 to
70 nm.
[0075] The nanostructures can be of any shape or geometry. One way
for defining the geometry of a given nanostructure is by its
"aspect ratio," which refers to the ratio of the length and the
width (or diameter) of the nanostructure. In certain embodiments,
the nanostructures are isotropically shaped (i.e., aspect ratio=1).
Typical isotropic or substantially isotropic nanostructures include
nanoparticles. In preferred embodiments, the nanostructures are
anisotropically shaped (i.e. aspect ratio #1). The anisotropic
nanostructure typically has a longitudinal axis along its length.
Exemplary anisotropic nanostructures include nanowires (solid
nanostructures having aspect ratio of at least 10, and more
typically, at least 50), nanorod (solid nanostructures having
aspect ratio of less than 10) and nanotubes (hollow
nanostructures).
[0076] Lengthwise, anisotropic nanostructures (e.g., nanowires) are
more than 500 nm, or more than 1 .mu.m, or more than 10 .mu.m in
length. In various embodiments, the lengths of the nanostructures
are in the range of 5 to 30 .mu.m, or in the range of 15 to 50
.mu.m, 25 to 75 .mu.m, 30 to 60 .mu.m, 40 to 80 .mu.m, or 50 to 100
.mu.m.
[0077] Metal nanostructures are typically a metallic material,
including elemental metal (e.g., transition metals) or a metal
compound (e.g., metal oxide). The metallic material can also be a
bimetallic material or a metal alloy, which comprises two or more
types of metal. Suitable metals include, but are not limited to,
silver, gold, copper, nickel, gold-plated silver, platinum and
palladium. It should be noted that although the present disclosure
describes primarily nanowires (e.g., silver nanowires), any
nanostructures within the above definition can be equally
employed.
[0078] Typically, metal nanostructures are metal nanowires that
have aspect ratios in the range of 10 to 100,000. Larger aspect
ratios can be favored for obtaining a transparent conductor layer
since they may enable more efficient conductive networks to be
formed while permitting lower overall density of wires for a high
transparency. In other words, when conductive nanowires with high
aspect ratios are used, the density of the nanowires that achieves
a conductive network can be low enough that the conductive network
is substantially transparent.
[0079] Metal nanowires can be prepared by known methods in the art.
In particular, silver nanowires can be synthesized through
solution-phase reduction of a silver salt (e.g., silver nitrate) in
the presence of a polyol (e.g., ethylene glycol) and poly(vinyl
pyrrolidone). Large-scale production of silver nanowires of uniform
size can be prepared and purified according to the methods
described in U.S. Published Application Nos. 2008/0210052,
2011/0024159, 2011/0045272, and 2011/0048170, all in the name of
Cambrios Technologies Corporation, the assignee of the present
disclosure.
Conductive Network
[0080] A conductive network refers to a layer of interconnecting
metal nanostructures (e.g., nanowire) that provide the electrically
conductive media of a transparent conductor. Since electrical
conductivity is achieved by electrical charge percolating from one
metal nanostructure to another, sufficient metal nanowires must be
present in the conductive network to reach an electrical
percolation threshold and become conductive. The surface
conductivity of the conductive network is inversely proportional to
its surface resistivity, sometimes referred to as sheet resistance,
which can be measured by known methods in the art. As used herein,
"electrically conductive" or simply "conductive" corresponds to a
surface resistivity of no more than 10.sup.4.OMEGA./.quadrature.,
or more typically, no more than 1,000.OMEGA./.quadrature., or more
typically no more than 500.OMEGA./.quadrature., or more typically
no more than 200.OMEGA./.quadrature.. The surface resistivity
depends on factors such as the aspect ratio, the degree of
alignment, degree of agglomeration and the resistivity of the
interconnecting metal nanostructures.
[0081] In certain embodiments, the metal nanostructures may form a
conductive network on a substrate without a binder. In other
embodiments, a binder may be present that facilitates adhesion of
the nanostructures to the substrate. Suitable binders include
optically clear polymers including, without limitation:
polyacrylics such as polymethacrylates (e.g., poly(methyl
methacrylate)), polyacrylates and polyacrylonitriles, polyvinyl
alcohols, polyesters (e.g., polyethylene terephthalate (PET),
polyester naphthalate, and polycarbonates), polymers with a high
degree of aromaticity such as phenolics or cresol-formaldehyde
(Novolacs.RTM.), polystyrenes, polyvinyltoluene, polyvinylxylene,
polyimides, polyamides, polyamideimides, polyetherimides,
polysulfides, polysulfones, polyphenylenes, and polyphenyl ethers,
polyurethane (PU), epoxy, polyolefins (e.g. polypropylene,
polymethylpentene, and cyclic olefins),
acrylonitrile-butadiene-styrene copolymer (ABS), cellulosics,
silicones and other silicon-containing polymers (e.g.
polysilsesquioxanes and polysilanes), polyvinylchloride (PVC),
polyacetates, polynorbornenes, synthetic rubbers (e.g., EPR, SBR,
EPDM), and fluoropolymers (e.g., polyvinylidene fluoride,
polytetrafluoroethylene (TFE) or polyhexafluoropropylene),
copolymers of fluoro-olefin and hydrocarbon olefin (e.g.,
Lumiflon.RTM.), and amorphous fluorocarbon polymers or copolymers
(e.g., CYTOP.RTM. by Asahi Glass Co., or Teflon.RTM. AF by Du
Pont).
[0082] "Substrate" refers to a non-conductive material onto which
the metal nanostructure is coated or laminated. The substrate can
be rigid or flexible. The substrate can be clear or opaque.
Suitable rigid substrates include, for example, glass,
polycarbonates, acrylics, and the like. Suitable flexible
substrates include, but are not limited to: polyesters (e.g.,
polyethylene terephthalate (PET), polyester naphthalate, and
polycarbonate), polyolefins (e.g., linear, branched, and cyclic
polyolefins), polyvinyls (e.g., polyvinyl chloride, polyvinylidene
chloride, polyvinyl acetals, polystyrene, polyacrylates, and the
like), cellulose ester bases (e.g., cellulose triacetate, cellulose
acetate), polysulphones such as polyethersulphone, polyimides,
silicones and other conventional polymeric films. Additional
examples of suitable substrates can be found in, e.g., U.S. Pat.
No. 6,975,067.
[0083] Typically, the optical transparence or clarity of the
transparent conductor (i.e., a conductive network on a
non-conductive substrate) can be quantitatively defined by
parameters including light transmission and haze. "Light
transmission" (or "light transmissivity") refers to the percentage
of an incident light transmitted through a medium. In various
embodiments, the light transmission of the conductive layer is at
least 80% and can be as high as 98%. Performance-enhancing layers,
such as an adhesive layer, anti-reflective layer, or anti-glare
layer, may further contribute to reducing the overall light
transmission of the transparent conductor. In various embodiments,
the light transmission (T %) of the transparent conductors can be
at least 50%, at least 60%, at least 70%, or at least 80% and may
be as high as at least 91% to 92%, or at least 95%.
[0084] Haze (H %) is a measure of light scattering. It refers to
the percentage of the quantity of light separated from the incident
light and scattered during transmission. Unlike light transmission,
which is largely a property of the medium, haze is often a
production concern and is typically caused by surface roughness and
embedded particles or compositional heterogeneities in the medium.
Typically, haze of a conductive film can be significantly impacted
by the diameters of the nanostructures. Nanostructures of larger
diameters (e.g., thicker nanowires) are typically associated with a
higher haze. In various embodiments, the haze of the transparent
conductor is no more than 10%, no more than 8%, or no more than 5%
and may be as low as no more than 2%, no more than 1%, or no more
than 0.5%, or no more than 0.25%.
Coating Composition
[0085] The patterned transparent conductors according to the
present disclosure are prepared by coating a
nanostructure-containing coating composition on a non-conductive
substrate. To form a coating composition, the metal nanowires are
typically dispersed in a volatile liquid to facilitate the coating
process. It is understood that, as used herein, any non-corrosive
volatile liquid in which the metal nanowires can form a stable
dispersion can be used. Preferably, the metal nanowires are
dispersed in water, an alcohol, a ketone, ethers, hydrocarbons or
an aromatic solvent (benzene, toluene, xylene, etc.). More
preferably, the liquid is volatile, having a boiling point of no
more than 200.degree. C., no more than 150.degree. C., or no more
than 100.degree. C.
[0086] In addition, the metal nanowire dispersion may contain
additives and binders to control viscosity, corrosion, adhesion,
and nanowire dispersion. Examples of suitable additives and binders
include, but are not limited to, carboxy methyl cellulose (CMC),
2-hydroxy ethyl cellulose (HEC), hydroxy propyl methyl cellulose
(HPMC), methyl cellulose (MC), poly vinyl alcohol (PVA),
tripropylene glycol (TPG), and xanthan gum (XG), and surfactants
such as ethoxylates, alkoxylates, ethylene oxide and propylene
oxide and their copolymers, sulfonates, sulfates, disulfonate
salts, sulfosuccinates, phosphate esters, and fluorosurfactants
(e.g., Zonyl.RTM. by DuPont).
[0087] In one example, a nanowire dispersion, or "ink" includes, by
weight, from 0.0025% to 0.1% surfactant (e.g., a preferred range is
from 0.0025% to 0.05% for Zonyl.RTM. FSO-100), from 0.02% to 4%
viscosity modifier (e.g., a preferred range is 0.02% to 0.5% for
HPMC), from 94.5% to 99.0% solvent and from 0.05% to 1.4% metal
nanowires. Representative examples of suitable surfactants include
Zonyl.RTM. FSN, Zonyl.RTM. FSO, Zonyl.RTM. FSH, Triton (x100, x114,
x45), Dynol (604, 607), n-Dodecyl b-D-maltoside and Novek. Examples
of suitable viscosity modifiers include hydroxypropyl methyl
cellulose (HPMC), methyl cellulose, xanthan gum, polyvinyl alcohol,
carboxy methyl cellulose, and hydroxy ethyl cellulose. Examples of
suitable solvents include water and isopropanol.
[0088] The nanowire concentration in the dispersion can affect or
determine parameters such as thickness, conductivity (including
surface conductivity), optical transparency, and mechanical
properties of the nanowire network layer. The percentage of the
solvent can be adjusted to provide a desired concentration of the
nanowires in the dispersion. In preferred embodiments the relative
ratios of the other ingredients, however, can remain the same. In
particular, the ratio of the surfactant to the viscosity modifier
is preferably in the range of about 80 to about 0.01; the ratio of
the viscosity modifier to the metal nanowires is preferably in the
range of about 5 to about 0.000625; and the ratio of the metal
nanowires to the surfactant is preferably in the range of about 560
to about 5. The ratios of components of the dispersion may be
modified depending on the substrate and the method of application
used. The preferred viscosity range for the nanowire dispersion is
between about 1 and 100 cP.
[0089] Following the coating, the volatile liquid is removed by
evaporation. The evaporation can be accelerated by heating (e.g.,
baking). The resulting nanowire network layer may require
post-treatment to render it electrically conductive. This
post-treatment can be a process step involving exposure to heat,
plasma, corona discharge, UV-ozone, or pressure as described
below.
[0090] Examples of suitable coating compositions are described in
U.S. Published Application Nos. 2007/0074316, 2009/0283304,
2009/0223703, and 2012/0104374, all in the name of Cambrios
Technologies Corporation, the assignee of the present
disclosure.
[0091] The coating composition is coated on a substrate by, for
example, sheet coating, web-coating, printing, and lamination, to
provide a transparent conductor. Additional information for
fabricating transparent conductors from conductive nanostructures
is disclosed in, for example, U.S. Published Patent Application No.
2008/0143906, and 2007/0074316, in the name of Cambrios
Technologies Corporation.
[0092] The transparent conductor structures, their electrical and
optical properties, and the methods of patterning are illustrated
in more detail by the following non-limiting examples.
EXAMPLES
Example 1
Synthesis of Silver Nanowires
[0093] Silver nanowires were synthesized by the reduction of silver
nitrate dissolved in ethylene glycol in the presence of poly(vinyl
pyrrolidone) (PVP) following the "polyol" method described in,
e.g., Y. Sun, B. Gates, B. Mayers, & Y. Xia, "Crystalline
silver nanowires by soft solution processing", Nanoletters, (2002),
2(2) 165-168. A modified polyol method, described in U.S. Published
Application Nos. 2008/0210052 and 2011/0174190, in the name of
Cambrios Technologies Corporation, produces more uniform silver
nanowires at higher yields than does the conventional "polyol"
method. These applications are incorporated by reference herein in
its entirety.
Example 2
Control Stack
[0094] A control stack was made by (1) preparing a transparent
conductor of silver nanostructure conductive network deposited on a
PET film (e.g., ClearOhm.RTM.); (2) laminating an OCA on glass, and
(3) laminating the transparent conductor on the OCA/glass, the
silver nanostructures being in contact with the OCA.
[0095] The optical stack was exposed to an accelerated light test
with the PET film facing the light source. The lighting condition
was 200 mW/cm.sup.2 measured at 365 nm. The sheet resistance was
measured as a function of time with a non-contact method with a
Delcom resistance measurement instrument. The resistivity drifts
were shown in Table 1. As shown, the sheet resistance steadily
drifts upwards, and the optical stack became essentially
non-conductive after 181 hours.
TABLE-US-00001 TABLE 1 OCA control % R drift at 26 hrs 23% % R
drift at 72 hrs 35% % R drift at 121 hrs 67% % R drift at 181 hrs
Open
Example 3
UV Exposure
[0096] An optical stack was prepared in the same manner as in
Example 1. It was then exposed to UV radiation using a Fusion
system equipped with an H-bulb, for 3 passes at 3 ft/min on one
side of the stack, then 3 passes at 3 ft/min on the other side.
[0097] This stack was thereafter exposed to an accelerated light
test (200 mW/cm.sup.2 measured at 365 nm) and the sheet resistance
was measured as a function of time with a non-contact method. As
shown in Table 2, compared to the control of Example 2, the initial
(first 100 hrs) resistance drift was greatly suppressed when the
stack was first exposed to UV radiation.
TABLE-US-00002 TABLE 2 OCA UV exposure % R drift at 22 hrs 0% % R
drift at 70 hrs 6% % R drift at 113 hrs 19% % R drift at 181 hrs
66%
Example 4
OCA Treated with Photo-Stabilizers
[0098] An optical stack was prepared by first laminating an OCA
layer on a glass substrate, exposing the OCA to a puddle of
terpineol then spinning excess off, followed by baking at
80.degree. C. for 60 seconds in an oven.
[0099] Thereafter, a transparent conductor of silver nanostructures
on PET substrate was laminated on the OCA, the silver
nanostructures being in contact with the OCA treated with
terpineol. The stack was exposed to an accelerated light test (200
mW/cm.sup.2 measured at 365 nm) and the sheet resistance was
measured as a function of time with a non-contact method. It was
found that, as shown in Table 3, the long term (up to 449 hrs)
resistance drift was greatly suppressed when the OCA was treated
with terpineol.
TABLE-US-00003 TABLE 3 OCA treated with terpineol % R drift at 26
hrs 18% % R drift at 72 hrs 22% % R drift at 12 hrs 25% % R drift
at 181 hrs 30% % R drift at 449 hrs 41%
[0100] In making another optical stack, the OCA layer was first
treated with limonene a similar manner as terpineol. The liquid
limonene was allowed to puddle on the OCA layer for about 60
seconds. Limonene was then spun off and dried in a nitrogen
atmosphere. Thereafter, the OCA/glass was laminated on a silver
nanostructure-based transparent conductor, the OCA layer being in
contact with the silver nanostructure. The starting resistance of
the transparent conductor was less than 500 .OMEGA./sq. The film
stack was then exposed to an accelerated light test (200
mW/cm.sup.2 measured at 365 nm).
[0101] This procedure was repeated for cyclohexanol.
[0102] As shown in FIG. 2, the film stack exposed to limonene had a
resistance drift (as measured using a non-contact method) below 30%
out to 800 hours and below 40% out to almost 1000 hours, indicating
that limonene is an effective photo-stabilizer that is capable of
forestalling silver corrosion upon light exposure.
Example 5
Incorporating Photo-Stabilizer into Transparent Conductor
[0103] A transparent conductor of silver nanostructures was first
formed on PET ("NW film on PET"). A 1% dispersion of
1-phenyl-1H-tetrazole-5-thiol (PTZT) in methanol was prepared. The
NW film on PET was then soaked in the PTZT solution before drying
with nitrogen and wiping off the excess. The treated transparent
conductor was then laminated to a glass substrate using an OCA The
starting resistance of the transparent conductor was less than 500
.OMEGA./sq. The accelerated light test (200 mW/cm.sup.2 measured at
365 nm) is shown in FIG. 3. As shown, the transparent conductor
treated with PTZT shows less than 10% drift after 200 hours,
whereas the untreated transparent conductor became non-conductive
after 150 hours. The result indicates that PTZT was effective in
preventing photo corrosion of the silver nanostructures.
[0104] FIG. 4 shows the accelerated light tests of PTZT-treated NW
film and limonene-treated OCA film as compared to the untreated
optical stack. As shown, both photo-stabilizers were comparably
effective in reducing or preventing photo corrosion.
[0105] FIG. 5 shows the accelerated light tests of PTZT-treated NW
film and PTZT-treated OCA film. As shown, PTZT was comparably
effective at different locations of an optical stack (e.g., the NW
film or OCA film).
Example 6
Incorporating Photo-Stabilizer into Transparent Conductor
[0106] Another transparent conductor was treated with benzothiazole
(BTA) in a similar manner as described above. A control, untreated
transparent conductor was also prepared, as was a control,
methanol-treated transparent conductor. FIG. 6 shows that the
BTA-treated transparent conductors are more stable than transparent
conductors untreated by BTA under the accelerated light
condition.
[0107] All of the above U.S. patents, U.S. patent application
publications, U.S. patent applications, foreign patents, foreign
patent applications and non-patent publications referred to in this
specification and/or listed in the Application Data Sheet, are
incorporated herein by reference, in their entirety.
[0108] From the foregoing it will be appreciated that, although
specific embodiments of the invention have been described herein
for purposes of illustration, various modifications may be made
without deviating from the spirit and scope of the invention.
* * * * *