U.S. patent application number 13/906309 was filed with the patent office on 2013-12-05 for low haze transparent conductive electrodes and method of making the same.
The applicant listed for this patent is Nuovo Film Inc.. Invention is credited to Hakfei Poon.
Application Number | 20130319729 13/906309 |
Document ID | / |
Family ID | 49462746 |
Filed Date | 2013-12-05 |
United States Patent
Application |
20130319729 |
Kind Code |
A1 |
Poon; Hakfei |
December 5, 2013 |
Low Haze Transparent Conductive Electrodes and Method of Making the
Same
Abstract
A transparent conductive electrode comprising metal nanowires
and method of making is described, wherein the transparent
conductive electrode has a pencil hardness more than 1 H,
nanoporous surface having pore sizes less than 25 nm and surface
roughness less than 50 nm. The transparent conductive electrode
further comprises an index matching layer, having a refractive
index between 1.1-1.5 and a thickness between 100-200 nm.
Inventors: |
Poon; Hakfei; (Mountain
View, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nuovo Film Inc. |
Suzhou |
|
CN |
|
|
Family ID: |
49462746 |
Appl. No.: |
13/906309 |
Filed: |
May 30, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61654408 |
Jun 1, 2012 |
|
|
|
61654098 |
Jun 1, 2012 |
|
|
|
61802496 |
Mar 16, 2013 |
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Current U.S.
Class: |
174/250 ;
977/932 |
Current CPC
Class: |
H05K 3/12 20130101; B82Y
30/00 20130101; H05K 1/0296 20130101; Y02E 60/10 20130101; H01B
1/22 20130101; H05K 3/1283 20130101; B82Y 99/00 20130101; H01M 4/38
20130101; B32B 7/00 20130101; B82Y 40/00 20130101; Y10T 428/24917
20150115; H05K 2203/125 20130101; Y10T 428/24942 20150115; H05K
3/125 20130101; H05K 1/09 20130101; H05K 2201/0108 20130101; H05K
2201/026 20130101; H05K 2203/0315 20130101 |
Class at
Publication: |
174/250 ;
977/932 |
International
Class: |
H05K 1/02 20060101
H05K001/02 |
Claims
1. A transparent conductive film, comprising: a substrate; a
transparent conductive layer, comprising a network of metal
nanowires of more than one distinctive sizes in diameters between
10 nm to 1 um; and an isotropic layer comprising diffused material
formed in contact with the metal nanowire network; wherein the
transparent conductive electrode film has a pencil hardness more
than 1 H.
2. The transparent conductive film of claim 1, wherein the
isotropic layer and the transparent conductive layer is
substantially the same layer, having no distinctive layer
interface.
3. The transparent conductive film of claim 1, wherein the
isotropic layer and the transparent conductive layer have at least
one distinctive layer interface.
4. The transparent conductive film of claim 1, wherein the
isotropic layer comprises a refractive index matching material.
5. The transparent conductive film of claim 4, wherein the
refractive index matching material has a refractive index between
1.1-1.5.
6. The transparent conductive film of claim 1, wherein the
conductive electrode has a thickness less than 200 nm.
7. The transparent conductive film of claim 4, wherein the
conductive electrode has a thickness more than 100 nm.
8. The transparent conductive film of claim 1, having a haze less
than 2%.
9. The transparent conductive film of claim 1, having a haze less
than 1%.
10. The transparent conductive film of claim 1, having a light
transmittance more than 80% in the wavelength between 400-1000
nm.
11. The transparent conductive film of claim 10, having a light
transmittance more than 90% in the wavelength between 400-800
nm.
12. The transparent conductive film of claim 1, having a sheet
resistance less than 100 ohms/square.
13. The transparent conductive film of claim 1, having a surface
roughness of less than 50 nm.
14. The transparent conductive film of claim 9, wherein the
nanowires have at least an aspect ratio of 500:1.
15. The transparent conductive film of claim 9, wherein the
nanowires have at least at least 50-100.mu. in length.
16. The transparent conductive film of claim 1, wherein the
substrate has a refractive index between 1.2-2.25.
17. The transparent conductive film of claim 1, having pencil
hardness more than 3 H.
18. The transparent conductive film of claim 4, wherein the index
matching layer comprises conductive materials, semi-conductive
materials or nonconductive materials.
19. A touch screen device, comprising a transparent conductive
electrode, comprising a substrate; a transparent conductive layer,
comprising metal nanowires, deposited above of the substrate; and
an isotropic layer, comprising organic or inorganic materials,
situated above the substrate, wherein the transparent conductive
electrode film has a pencil hardness more than 1 H.
20. The touch screen device of claim 19, wherein the conductive
electrode has a haze of less than 2%
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit under 35 U.S.C.
.sctn.119(e) of U.S. Provisional Patent Applications 61,654,408,
61,654,098 and 61,802,496. The subject matter as set forth in each
one of the following US utility patent applications are
incorporated herein by reference in its entirety:
[0002] Ser. No. 61,654,408, filed Jun. 1, 2012;
[0003] Ser. No. 61,654,098, filed Jun. 1, 2012; and
[0004] Ser. No. 61,802,496, filed Mar. 16, 2013.
BACKGROUND
[0005] 1.Technical Field
[0006] This disclosure is related to low-haze transparent
conductive electrodes (Herein after "TCE"), and method for making
the same.
[0007] 2. Description of the Related Art
[0008] Transparent conductive electrodes are optically clear and
electrically conductive films, comprising substrates and
transparent conductive materials deposited on top of the
substrates. The substrates can either be glass or plastic. The
transparent conductive materials currently used in the art, are
indium tin-oxide (ITO), aluminum doped zinc oxide (AZO), fluorine
doped tin oxide (FTO), carbon nanotubes, graphenes, or metal
nanowires.
[0009] Many commercial applications of transparent conductive
electrodes, such as display applications, typically require both
excellent optical properties, for example, high optical
transmittance and low haze, and superior electrical properties,
i.e. high conductivities or low sheet resistance. Most of the
research effort in the field has been working towards how to
balance the optical properties and electrical properties, as most
of development has been found to improve one property at the
sacrifice of the other.
[0010] In addition to electrical and optical properties of
transparent conductive electrodes, physical or mechanical
properties of the film, such as film hardness, are also important.
It is well known that film hardness of a transparent conductive
film is directly related to the production yield of the electronic
devices. For example, when using metal nanowire based transparent
conductive electrode to make touch screen device in a manufacturing
scale, one of the significant factors contributing to the yield
loss is the scratching or denting of the film in the manufacturing
processes. However, most of the metal nanowire based transparent
conductive electrodes in the prior art, for example, the
transparent conductive electrode taught in U.S. Pat. No. 8,018,568,
have metal nanowires embedded in a polymer matrix, such as
polyurethane or acrylate polymers. The polymer matrix generally
does not offer enough scratch, scuff or abrasion resistance.
[0011] In view of the foregoing, a harder metal nanowire based TCE
film is needed. Here we present a low haze metal nanowire based
conductive film that not only has excellent electrical properties
but an optimized portfolio of optical and mechanical properties as
well.
SUMMARY OF THE INVENTION
[0012] The present invention discloses low haze transparent
conductive electrodes as films having excellent electrical, optical
and mechanical properties, suitable for touch screen device
manufacturing.
[0013] Described herein are transparent conductive electrodes
having a haze of less than 2%, more typically, less than 1%, while
maintaining high electrical conductivities (e.g., less than 100
ohms/square), or more preferably less than 60 ohms/square.
[0014] In one embodiment a transparent conductive electrode having
a haze of less than 2%, more typically, less than 1%, while
maintain a light transmission of over 90% in the visible light
range.
[0015] In another embodiment a transparent conductive electrode
having a pencil hardness of more than 1 H, more typically 3 H,
while maintaining high electrical conductivities (e.g., less than
100 ohms/square), or more preferably less than 60 ohms/square.
[0016] In another embodiment a transparent conductive electrode
having a pencil hardness of more than 1 H, more typically 3 H,
where maintaining a light transmittance of more than 90%.
[0017] In another embodiment a transparent conductive electrode
having a pencil hardness of more than 1 H, more typically 3 H,
while maintaining a haze of less than 2%, more typically, less than
1%, and methods of making the same.
[0018] In another embodiment a transparent conductive electrode
having a surface roughness <2 Ra or 50 nm, high electrical
conductivities (e.g., less than 100 ohms/square), or more
preferably less than 60 ohms/square.
[0019] In another embodiment a transparent conductive electrode
having a surface roughness <2 Ra or 50 nm, where maintaining a
light transmittance of more than 90%.
[0020] In another embodiment a transparent conductive electrode
having a surface roughness <2 Ra or 50 nm, while maintaining a
haze of less than 2%, more typically, less than 1%, and methods of
making the same.
[0021] In another embodiment a transparent conductive electrode
having nanoporous surface having pore size less than 25 nm, high
electrical conductivities (e.g., less than 100 ohms/square), or
more preferably less than 60 ohms/square.
[0022] In another embodiment a transparent conductive electrode
having nanoporous surface having pore size less than 25 nm, where
maintaining a light transmittance of more than 90%.
[0023] In another embodiment a transparent conductive electrode
having nanoporous surface having pore size less than 25 nm, while
maintaining a haze of less than 2%, more typically, less than 1%,
and methods of making the same.
[0024] In a further embodiment a transparent conductive electrode
having a haze of less than 1.5%, more typically, less than 0.5%,
comprises a substrate and a index matching layer, wherein the
refractive index of the material in the index matching layer is
between 1.1 to 1.5.
[0025] In a further embodiment a transparent conductive electrode
having a haze of less than 2%, more typically, less than 1%,
comprises a substrate and a index matching layer, wherein the
thickness of the materials in the index matching layer is about
100-200 nm.
[0026] In still a further embodiment of the present invention, a
method of making a conductive electrode film, comprising: [0027]
synthesizing and purifying metal nanowires; [0028] suspending
purified metal nanowires in a binder free solvent to form
dispersion; [0029] coating a thin layer of metal nanowire above a
substrate; [0030] heating the substrate with coated layer of
nanowire at a temperature 55-150.degree. C. to remove the solvent;
[0031] applying a layer of sol-gel solution comprising an index
matching material above the substrate; and [0032] annealing the
film at a temperature between 55-150 .degree. C. and releasing the
residual solvent and stress in the film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] Exemplary embodiments of the disclosure will be more clearly
understood from the following detailed description taken in
conjunction with the accompanying drawings in which:
[0034] FIG. 1 diagrammatically illustrates a cross-section view of
the transparent electrode in the prior art;
[0035] FIG. 2 diagrammatically illustrates a cross-sectional view
of an exemplary transparent conductive electrode in the present
invention;
[0036] FIG. 3 diagrammatically illustrates a cross-sectional view
of an exemplary transparent conductive electrode in the present
invention;
[0037] FIG. 4 diagrammatically illustrates a cross-sectional view
of another exemplary transparent conductive electrode in the
present invention.
[0038] FIG. 5 is a SEM picture showing nanowires embedded in
nanoporous index matching layer;
[0039] FIG. 6 is process flow diagram of an exemplary method to
make transparent conductive electrode.
DETAILED DESCRIPTION OF SELECTED EXAMPLES
[0040] Hereinafter, selected examples of a transparent conductive
electrode will be discussed in the following with reference to the
accompanying drawings. It will be appreciated by those skilled in
the art that the following discussion is for demonstration
purposes, and should not be interpreted as a limitation. Other
variances within the scope of this disclosure are also
applicable.
[0041] "Optional" or "optionally" means that the subsequently
described circumstance may or may not occur, so that the
description includes instances where the circumstance occurs and
instances where it does not.
[0042] "Above" means one layer is located on top of another layer.
In one example, it means one layer is situated directly on top of
another layer. In another example, it means one layer is situated
over the second layer with more layers or spacers in between.
[0043] It is understood that by those skilled in the art when a
number of metal nanowires intersecting with each other, they form a
network structure. Terms like "metal nanowire network", " metal
nano mesh" or "metal nano web" can be used interchangeably and
refer to the same object.
[0044] FIG. 1 diagrammatically illustrates a cross-section view of
the transparent electrode in the prior art. The electrode in the
prior art (98) comprises a substrate 102, an isotropic matrix layer
(104) on top of the substrate 102 and a layer of metal nanowire
(106). The isotropic matrix layer (104) usually acts as a
protective blanket on top of the metal nanowire layer.
[0045] The present invention discloses low haze transparent
conductive electrodes with both excellent electrical properties,
optical properties and mechanical properties. Optical properties
include haze and transmittance of the film, which can be measured
directly from the samples. Optical properties also include film
properties that indirectly impact the optical appearance of the
film, such as the surface roughness, film or layer thickness and
other surface feature of the film. Mechanical properties include
physical properties of the film such as hardness, adhesion
strength.
[0046] The present invention discloses, in one aspect, a
transparent conductive electrode film, having a sheet resistance
less than 350 .OMEGA./.quadrature.. In one example, the transparent
conductive electrode has a sheet resistance less than 100
.OMEGA./.quadrature.. In another example, the transparent
conductive electrode has a sheet resistance less than 50
.OMEGA./.quadrature..
[0047] The present invention discloses, in another aspect, a
transparent conductive electrode film, having an optical
transmittance more than 80% in the wavelength between 400-1000 nm.
In one example, the transparent conductive electrode has an optical
transmittance more than 80% in the wavelength between 400-800 nm.
In a more preferred example, the transparent conductive electrode
has an optical transmittance more than 90% in the wavelength
between 400-800 nm.
[0048] The present invention discloses, in another aspect, a
transparent conductive electrode film, having a haze value less
than 2%. In one example, the transparent conductive electrode has a
haze value less than 1%. In another example, the transparent
conductive electrode has a haze value less than 0.5%.
[0049] The present invention discloses, in a particular aspect, a
transparent conductive electrode film having a pencil hardness of
more than 1 H. In a more preferred example, transparent conductive
electrode film has a pencil hardness of more than 3 H.
[0050] The present invention discloses, in one aspect, a
transparent conductive electrode film having a surface roughness
less than 25 nm.
[0051] The present invention discloses, in a special aspect, a
transparent conductive electrode film having a nanoporous surface
with pore size less than 25 nm.
[0052] The present invention discloses a low haze transparent
conductive electrode film comprising an index matching layer, is
comprised of materials with refractive indexes between 1.1-1.5.
[0053] The present invention discloses a low haze transparent
conductive electrode film comprising an index matching layer having
a thickness of between 100-200 nm.
[0054] The present invention discloses a low haze transparent
conductive electrode film comprises metal nanowires at least
50-100.mu. in length and less than 50 nm in diameters.
Index Matching Multilayer
[0055] Referring to FIG. 2, in one embodiment, the low haze
transparent electrode (100) in the present invention comprises a
substrate (102), and a substantially a single layer of conductive
material deposited on top of the substrate. The single layer
conductive material comprises one or more metal nanowires (106).
Optionally, the single layer conductive material further comprises
an anisotropic layer of metal oxide. The anisotropic layer of metal
oxide can be metal oxide material 104, like ITO or can be a
different conductive material, such as FTO, AZO. The anisotropic
layer can also be a semi-conductive layer based on metal oxides,
conductive polymers, carbon nanotubes or graphenes.
[0056] In one example of the present invention, the low haze
transparent conductive electrode comprises an index-matching layer
(108). The index-matching layer can be an isotropic layer with a
refractive index in between the refractive index of the substrate
and that of the working environment of the transparent conductive
electrode. In one instance of the present invention, glass is used
as the substrate, having a refractive index of n_glass, and air as
the working environment of the electrode (i.e. incoming media of
the light), having a refractive index of n_air, the ideal
refractive index of the matching layer should be a square root of
(n_glass*n_air).
[0057] Optionally, the layer thickness of the index-matching layer
(108) is preferably optimized to be around one quarter of
wavelength of light. Said wavelength of light is preferably to be a
specific target wavelength. In one preferred example of the present
invention, the thickness of the index-matching layer is designed
that the overall appearance of the color of the electrode viewing
from the bottom substrate matching to the substrate and the
contrast between the patterned areas covered by metal nanowires
(106) and unpatented area is minimal. By doing so, the index
matching layer (108) reduce the reflection/light scattering from
metal nanowire (106) surface, which tend to give a hazy appearance.
Optionally, additional layers of index-matching layer could be
coated on top to further reduce the hazy appearance.
[0058] The material in the index-matching layer can be conductive
materials, or semi-conductive materials, or nonconductive
materials. The conductive materials includes metal oxides such as
ITO, FTO, AZO, and polymers such as PEDOT; semi-conductive
materials includes zinc oxides, titanium oxides, tin oxides; and
nonconductive materials includes silicon dioxides, silicates,
polyurethane, PMMA, PVA and silicones.
[0059] In the index-matching layer (108) is situated on top of the
nanowire layer (106) as shown in FIG. 2. But in another example of
the present invention, as shown in FIG. 3, the index-matching layer
is situated on the bottom surface of the substrate glass.
Index Match Single Layer
[0060] Referring to FIG. 4, in another embodiment of the present
invention, the low haze transparent electrode (100) in the present
invention comprises a substrate (102), and a substantially a single
layer of conductive material deposited on top of the substrate. The
single layer conductive material comprises one or more metal
nanowires (106) and an index matching material layer, wherein the
index material layer is substantially the same layer of the layer
comprising metal wires and no interface between the metal nanowires
and the index matching materials.
[0061] The substantial single layer conductive film is made by
first coating a thin layer of metal nanowire dispersion onto a
surface of a substrate. After heating to remove solvent, a layer of
sol-gel solution comprising index matching material was applied on
top of baked sample by a wet coating process. The resulted sample
was further baked to remove solvent and anneal. In the finished
transparent conductive film, made by the process described herein,
comprises a substrate with a connected metal nanowire network
buried or embedded in closely packed particles of the index
matching material.
[0062] Table 1 in the example section summarizes a portfolio
optical properties of samples as substrate, nanowire bearing
substrate and nanowire bearing substrate along with index match
layer. The comparison between nanowire bearing substrate and
nanowire bearing substrate along with index match layer has clearly
set forth the additional index matching layer or material
significantly improved overall optical performance of the
conductive film.
[0063] In one example of the present invention, the low haze
transparent conductive electrode comprises an index-matching layer
(108). The index-matching layer can be an isotropic layer with a
refractive index in between the refractive index of the substrate
and that of the working environment of the transparent conductive
electrode. In one instance of the present invention, glass is used
as the substrate, having a refractive index of n_glass, and air as
the working environment of the electrode (i.e. incoming media of
the light), having a refractive index of n_air, the ideal
refractive index of the matching layer should be a square root of
(n_glass*n_air).
[0064] Optionally, the layer thickness of the index-matching layer
(108) is preferably optimized to be around one quarter of
wavelength of light. Said wavelength of light is preferably to be a
specific target wavelength. In one preferred example of the present
invention, the thickness of the index-matching layer is designed
that the overall appearance of the color of the electrode viewing
from the bottom substrate matching to the substrate and the
contrast between the patterned areas covered by metal nanowires
(106) and unpatterned area is minimal. By doing so, the index
matching layer (108) reduce the reflection/light scattering from
metal nanowire (106) surface, which tend to give a hazy appearance.
Optionally, additional layers of index-matching layer could be
coated on top to further reduce the hazy appearance.
[0065] The material in the index-matching layer can be conductive
materials, or semi-conductive materials, or non-conductive
materials. The conductive materials includes metal oxides such as
ITO, FTO, AZO, and polymers such as PEDOT; semi-conductive
materials includes zinc oxides, titanium oxides, tin oxides; and
nonconductive materials includes silicon dioxides, silicates,
polyurethane, PMMA, PVA and silicones.
[0066] In one example, the metal nanowire is silver metal nanowire.
And the index matching material is silica. The silica sol-gel
solution is spun coated on to a PET substrate.
[0067] It must be noted that spin coating is used as depositing
technique for silver nanowire and the silica index matching layer
in a lab. However, other coating technique such as dip coating,
gravure coating, slot die coating may also be used in either small
scale fabrication or large scale manufacturing.
[0068] In addition, other than the wet coating method to place the
index matching material on top of or above the substrate,
convention sputtering technique can be used when it is
appropriate.
[0069] It must be noted that the silica oxide can also be replaced
by other materials, such as ITO, FTO, and these metal oxides can be
introduced to the surface of the substrate or metal nanowire either
by a sol-gel process or film by sputtering.
[0070] In one preferred example, the thickness of index matching
layer is about 100-200 nm. In one instance, the final thickness of
the layer is determined or controlled by the concentration of the
solution comprising index matching materials, such as silica
sol-gel solution. In another instance, the wet film thickness of
the index matching layer is determined by the coating process. For
example, the coating process including spin speed etc. if index
matching layer is deposited using spin coating.
[0071] It must be further noted that the refractive index of the
index matching layer can be tuned by silica colloidal particle
size. Preferred refractive index should be in the range of 1.1 to
1.5 which is governed by square root of n_air*n_substrate. For
example, when PET is used, n_sub is 1.58, the refractive index for
the index matching layer is 1.25. In another example, the substrate
is a glass, the refractive index of the index matching layer is
1.22.
Surface Roughness
[0072] In particular aspect of the present invention, the surface
roughness of the low haze index matched TCE is less than 1 Ra.
Conventionally prepared metal nanowire transparent conductive
electrode film usually comprise two or more wires overlapping each
other; and the surface roughness is usually in the order of the
nanowire diameter, which are typically more than 2 Ra. As a result,
rougher surface led to higher haze due to more light scattering on
the surface.
Nanoporous
[0073] In another particular aspect of the present invention, a
novel feature in the low haze transparent conductive electrode
described herein, is the nanoporous surface nature. FIG. 4 is a SEM
picture showing nanowires embedded in nanoporous index matching
layer. Nanoporous surface offers significant advantages. One
benefit associated with this is extremely low electrical contact
resistance due to high surface contact area. Another benefit is
improved adhesion to adhesive and other bonding materials. One
benefit associated with this is extremely low electrical contact
resistance due to high surface contact area. Another benefit is
improved adhesion to adhesive and other bonding materials.
TABLE-US-00001 Contact Resistance Commercial TCE (ITO) 24.6 Our TCE
9.7
[0074] In a preferred example, the transparent conductive electrode
in the present invention, has a nanoporous surface with pore size
less than 25 nm.
Hardness
[0075] In a further aspect of the present invention, the
transparent conductive electrode has a pencil hardness of more than
1 H, which is critical for touch screen manufacturing.
[0076] Table 3 summarizes a comparison of pencil hardness among
different samples. A set of pencils ranging from 2 B to 4 H are
used for pencil hardness testing. Conventional transparent
conductive electrode which do not have index matching layer, are
failed at 2 H or blew 2 H. The samples having index matching layer
or index matching materials all survived 4 H pencil test.
TABLE-US-00002 TABLE 3 A comparison of pencil hardness among
different samples S: bare substrate; S + N: bare substrate having
metal nanowire network; S + N + I: bare substrate having metal
nanowire network, with additional index matching layer) Hardness S
B S + N <2H S + N + I >4H
Synthesis of Nanowires of High Aspect Ratios
[0077] The present invention discloses low haze transparent
conductive electrodes with both excellent optical properties,
electrical properties and mechanical properties. The low haze
transparent conductive electrodes disclosed herein have an optical
transmittance higher than 90%, a haze value less than 2%, and
typically less than 1%, while maintaining the sheet resistance
lower than 100 Ohms/sq and typically less than 50 Ohms/sq.
[0078] The low haze transparent electrode in the present invention
comprises a substrate, and a substantially single layer of
conductive material deposited on top of the substrate. The single
layer conductive material comprises one or more metal nanowires.
The haze value less than 2% is achieved by utilizing metal
nanowires which are longer and thinner than conventional metal
nanowires.
[0079] Optionally, the single layer conductive material further
comprises an anisotropic layer of metal oxide, metal nitrides, or
semiconductive oxides.
Surface Roughness
[0080] In one embodiment of the present invention, the single layer
of conductive material with anisotropic layer of metal oxide or
non-metallic oxide have average surface roughness <10 nm.
Current commercially available metal nanowire based conductive
layer have roughness >20 nm, and typically over 50 nm. The
rougher the surface, the higher the haze due to light scattering
from uneven surface textures.
[0081] In one embodiment of the present invention, the low haze
transparent conductive electrode comprises nanowires having a
longer than usual length and thinner than usual diameters. Said
nanowires are at least 20-100.mu. in length and less than 50 nm in
diameters. The longer the nanowire in length, the better the
conductivity due to fewer contact junctions. The smaller the
nanowire diameters, the less pronounced light scattering/reflection
observed from the nanowires, leading to an appearance with reduced
haze.
[0082] In another embodiment of the present invention, a method of
making of longer than usual length and thinner than usual diameter
nanowires is disclosed. The method comprises a proceeding step,
forming most of the nanowire nucleation seeds and a growing step
where the nucleation seeds grow preferentially in one dimension in
a controlled manner.
Super Long and Thin Nanowires Details
[0083] In one embodiment of the present invention, the low haze
transparent conductive electrode comprises nanowires having a
longer than usual length and thinner than usual diameters. The
usual length is defined as about 10-30 micrometers in length and
the usual diameters are defined as about 80-100 nm in diameters in
the scope of the present invention.
[0084] Transparent conductive electrodes in the art typically
employing metal nanowires having diameters around 80-100 nm, with
20-30 micrometers in length. The low haze transparent conductive
electrodes disclosed in the present invention, comprises metal
nanowires are less than 50 nm in diameters and longer than about
20-100 micrometers. In one preferred example of the present
invention, the low haze transparent conductive electrode comprises
nanowires having less than 30 nm in diameters. The longer and
thinner metal nanowires significantly reduces the amount of light
scattering and the contrast between the areas having nanowires and
the areas without, leading to a conductive electrode with lower
haze. In addition, the longer than usual nanowires further
facilitates the electron transport within the electrode, leading to
improved electrical conductivity and reduced sheet resistance. In
the examples of the present invention, the low haze transparent
electrode comprising metal nanowires having less than 50 nm in
diameters and 20-100 micrometers in length, have an optical
transmittance higher than 90%, a haze value less than 0.6%, while
maintaining the sheet resistance lower than 50 Ohms/sq.
[0085] In another embodiment of the present invention, a method to
make the unusual long and unusual thin metal nanowires is disclosed
herein. Apart from the conventional one step process of making
metal nanowires in the art, which typically lead to metal nanowires
80-100 nm in diameters and 20-30 micrometers in length, the method
disclosed in the present invention comprises a two-step process. A
first step is a proceeding step, forming most of the nanowire
nucleation seeds. A second step is a growing step, where the
nucleation seeds grow preferentially in one dimension in a
controlled manner. Further, the nucleation seeds can be purified
before being used in the growing step. Subsequently, the metal
nanowires collected after the growing step can be further purified
to have nanowires with even narrower distribution in length and
diameters. Comparing the conventional one step method, the two-step
process has two advantages. First, the proceeding step incubates
the formation of the nucleation seed, which significantly reduces
the concentration of the nanowire "growing centers". Second, the
growing step is conditioned that the precursors are continuously
growing in one predetermined direction in length, thus reducing the
formation of junctions and branches in the metal nanowire
network.
[0086] Once the desired metal nanowires have been prepared, a
transparent conductive electrode can be made through a printing,
coating process or electro-spinning process to lie down into a
nanowire film.
[0087] In still another embodiment of the present invention, the
low haze transparent conductive electrode further comprises an
index matching film between the substrate and nanowire layer. The
index matching film comprises conductive material, semi-conductive
materials or nonconductive materials as described herein in other
embodiments and/or examples.
[0088] In still another embodiment of the present invention, the
low haze conductive electrode further comprises a protective index
matching film on top of the conductive layer and the substrate. The
protective film has a refractive index between the substrate and
air. The protective film comprises conductive material,
semi-conductive materials or nonconductive materials.
[0089] The present invention is directed to a method of
manufacturing metal nanowires, which are especially the unusually
long and thin metal nanowires, having aspect ratio at least
500:1.
[0090] As used herein, the phrase "aspect ratio" designates that
ratio which characterizes the average nanowire size or length
divided by the average nanowire thickness or diameter. In one
exemplary embodiment, the metal nanowire is a silver nanowire,
having an aspect ratio of 500:1. The metal nanowires contemplated
herein have high aspect ratios, such as 500:1 or higher. A 1000:1
aspect ratio may be calculated, for example, by utilizing nanowires
that are 60 microns by 60 nm. Aspect ratios higher than 1000:1 can
be calculated from nanowires having lengths longer than 50 microns
and diameters 50 nm or less.
Synthesis of Super Metal Nanowires
[0091] The method of synthesizing the metal nano wires having high
aspect ratios, disclosed herein, comprises the steps of: [0092]
heating a solvent at 100-150 degree C.; [0093] adding a controlled
amount of nucleating precursor at a controlled speed to the
solvent; [0094] adding the metal salts into the heated mixture;
[0095] reducing metal ions dissociated from the metal salts into
metal by redox reaction in the solvent; [0096] heating the reaction
mixture to allow the reaction to proceed for 4-8 hours; [0097]
adding additional metal ions at a controlled rate to maintain the
desired ionic concentration; and [0098] quenching the redox
reaction with cold water.
[0099] In one aspect of the present invention, the metal nanowire
includes silver nanowire, gold nanowire and any other metal
nanowires made of noble elements.
[0100] In another aspect of the present invention, the solvent
described in the method is a liquid that is capable of generating a
small amount of a redox agent when heated, is not particularly
limited and may be suitably selected according to the intended
purpose. Examples thereof include alcohols such as propanol,
isopropanol; diols such as ethylene glycol, propylene glycol,
diethylene glycol, triethylene glycol, butanediol, and tri-ols such
as glycerol, polyethylene glycol. These solvents may be used alone
or in combination. Preferably, ethylene glycol is used as a
solvent, at >99% purity. Examples of glycols include the ACS
reagent grades that are available from commercial sources.
[0101] In still another aspect of the present invention, the
nucleating precursor described in the above method includes
chemicals that can effectively facilitate the formation of the
metal nanowire, particularly silver nanowire, when silver is
reduced from silver ion to silver metal. Using nucleating
precursors can assist the formation of nucleating centers and
direct the growth of silver nanowire in a preferred orientation,
thereby providing unusually long and thin silver nanowires, which
in turn will improve the conductivity and reduce the haze of the
conductive film and transparent conductive electrode formed from
them. Many different chemical compositions are known for this
purpose. In accordance with the aspects of the present invention,
examples of the nucleating precursors include, but are not limited
to, polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA),
polyethylene oxide (PEO) etc. Other examples of the nucleating
precursors can be found in US patent publication 2009/0196788
A1.
[0102] Besides choosing an appropriate nucleating agent for the
above method, in order to optimize silver nanowire growth to make
unusually long and thin silver nanowires, adding a controlled
amount of nucleating precursor at a controlled speed is critical
too. The ability of the nucleating agent in a solvent, the amount
of it at a given time, and the concentration of the silver metal
ions are important factors that determine the lengths and diameters
of the resultant silver nanowire.
[0103] In yet another aspect of the present invention, the method
of synthesizing the metal nanowire described in the present
invention includes controlling multi-stage of the reaction to allow
the independent control of wire diameter and wire length. The first
stage is primarily directed to control the nanowire diameter
whereas the second stage is primarily directed to grow the wire
length without significant changing the wire diameter; typically
the growth stage continues for 2-4 hours and can be extended
further if longer nanowires are needed. In one example we
synthesize wires with diameter of 30 nm and length of 15 um during
first stage. It then proceed into second stage and the wires formed
in the first stage serve as nucleating seeds. Newly reduced silver
atoms continue to deposit on both ends of nanowire extending its
length without making the wire thicker. At the end of 2 hours,
nanowire dimension become .about.25 um long and 30 nm thick.
[0104] It is further noted that in the second stage the addition of
metal ions must be controlled at a much slower rate to maintain the
desired ionic concentration. If the salt is added all at the
beginning, then the final reaction mixture contains a mixture of
particles with other shapes (nanocubes and low aspect ratio
(<100) nanowires/nanorods), in addition to the desired high
aspect ratio nanowires.
[0105] The metal salt can be used to make metal nanowires is not
particularly limited and may be suitably selected according to the
intended purpose. Examples thereof include nitrate salts, halide
salts, phosphoric salts, sulfate salts, tetrafluoroborates, amine
complexes, chloro complexes, and organic acid salts. Among these,
nitrate salts, tetrafluoroborates, amine complexes, chloro
complexes and organic acid salts are particularly preferred, since
these show high solubility in polar solvents. Silver ion
concentration may also be further controlled by precipitating
silver ions with halides/hydroxide/sulfate ions to form insoluble
silver salts that readily dissociates in the polar solvent at an
equilibrium solubility product.
[0106] The reduction reaction by the addition of a solution of a
metal salt proceeds even at room temperature, but is preferably
performed while heating a solution containing silver nanowires and
a metal salt or a solution of a metal salt. Heating of the solution
promotes the reduction of the metal salt to silver as well as
controls the phase morphology of reduced silver. Optionally,
addition of a reducing agent, or chemical reduction method may
further be used in combination with the heating selected according
to the intended purpose.
[0107] Heating a solution can be performed by means of, for
example, an oil bath, aluminum block heater, hot plate, oven,
infrared heater, heat roller, steam (hot air), ultrasonic wave, or
microwave. The heating temperature is preferably 80 C. to 200.
degree. C., and more preferably 100. degree. C. to 150. degree.
C.
[0108] It is further noted that in the step of the addition of
metal ions, the metal ions must be controlled at a much slower rate
(typically one tenth of the first stage addition rate, in the range
of 0.1 ml/min-1 ml/min or at a rate of <1% of total reaction
volume/min) to maintain the desired ionic concentration, which in
turn determines the length of the nanowire. If the salt is added
all at the beginning, then the final reaction mixture contains a
mix of particles with other shapes (nanocubes and low aspect ratio
(<100) nanowires/nanorods), in addition to the desired high
aspect ratio nanowires. The addition rate must be such that the
aspect ratio of the resultant product is >500:1.
Nanowire Purification
[0109] Further, the present invention is directed to a method of
purifying the transparent conductive electrode. The purification
method comprises a step of filtering the nanowire solution using a
filtration apparatus having a moving contact surface. The
filtration apparatus can be a funnel, or a rotation device.
Optionally, the filtration step is performed under pressure,
with/without heating.
[0110] The present invention is also directed to a method of making
the transparent conductive electrode comprising: [0111]
synthesizing the nanowires; [0112] purifying the nanowires using
pressure or under atmosphere; [0113] formulating nanowires
dispersion using a solvent; [0114] cleaning the pre-treated the
substrate; and [0115] coating a layer of index-matching coating or
a protective coating onto the substrate.
[0116] After the nanowires having an aspect ratio higher than 500:1
are synthesized and purified, a nanowire dispersion is formulated
in order to fabricate a conductive electrode. In one exemplary
embodiment of the invention, the silver nanowires comprise about
0.01% to about 4% by weight of the total dispersion. In a preferred
embodiment of the invention, the silver nanowires comprise about
0.1 to about 0.6% by weight of the dispersion.
[0117] Solvents suitable for preparation in the dispersion comprise
any suitable pure fluid or mixture of fluids that is capable of
forming a dispersion with the metal nanowires and that may be
volatilized at a desired temperature. In addition, the solvent is
desired to provide good quality nanowire film and can be obtained
high purity (>99%) at a relatively low cost. In some examples,
solvents used for the dispersion have a boiling point of less than
about 250.degree. C. In other examples, solvents used for the
dispersion have a boiling point in the range of from about
50.degree. C. to about 250.degree. C. Solvents for the dispersion
also include any mixture of organic solvents.
[0118] Optionally, additives or additional solvents can be added to
nanowire dispersion to improve the overall performance of the
electrode. Such additives include additives to improve film
morphology, reduce the drying time, and improve adhesion to other
layers in the electrode.
EXPERIMENTAL
Example 1
Preparation of an Index Matched Low Haze Transparent Conductive
Film
[0119] Silver nanowires (25 um, 60 nm) were first prepared by
polyol process and followed by purification. Then 0.15 g of
purified nanowires was dispersed in a 50 ml of binder free solvent
such as ethanol or methanol or IPA to prepare a 0.3% w/v of silver
nanowire dispersion. On a PET substrate, a thin layer of SNW
dispersion is spun coated at 1500 rpm spin speed for 30 s. The
coated substrate was baked at 100 C for 1 minute. A layer of 1.8%
wt silica sol-gel solution was applied on top of baked sample by
spin coating at 2500 rpm for 10 s. The resulted sample was further
baked at 100 C for 1 minute.
[0120] The final sample comprises a PET substrate with a connected
silver nanowire network buried or embedded in closely packed silica
particles.
Example 2
Comparison of Optical Properties
[0121] Table 1 lists a comparison of transmittance and haze values
between transparent conductive films with or without index matching
layer. Data on transmittance, haze and transmittance are summarized
for a first sample having only a substrate, a second sample having
a substrate with nanowires and a third sample having nanowire
bearing substrate with an index matching layer. Comparing the
results from the second and third sample, data in Table 1 clearly
established that the extra index matching layer improves the
transmittance, reduces haze and offers a film having less surface
roughness.
TABLE-US-00003 TABLE 1 Summary of optical properties and surface
roughness data (S: bare substrate; S + N: bare substrate having
metal nanowire network; S + N + I: bare substrate having metal
nanowire network, with additional index matching layer) Sample name
Transmittance Haze Roughness S .sup. 90% 0.6% <1Ra S + N 89.8%
1.7% 2Ra S + N + I 91.6% 1.5% <1Ra
Example 3
Nanowire Synthesis and Post Synthesis Characterization
[0122] Nanowires were synthesized by the known literature
techniques then followed by suitable purification procedures using
pressure and filtration. Then the purified nanowires were
subsequently characterized by optical microscopy for their length
and by SEM for their diameters. Optionally, the nanowires
synthesized were further casted into a nanowire film to a piece of
substrate to test for haze values before being assembled into an
electrode.
[0123] In a first example, 5.1 g of PVP is first dissolved in 50 mL
of glycerol solvent and heated to 150 C. In the meantime, 5.1 g of
silver nitrate is separately dissolved in 30 ml of glycerol to form
a silver nitrate solution. Then, the silver nitrate solution is
added to the PVP solution at a constant rate of 2 ml/min for 9
minutes; this is then followed by adding the remaining portion of
silver nitrate solution at 0.4 ml/min for another 30 minutes. The
reaction is allowed to proceed for another 3 hours before it is
quenched by mixing equal volume of cold water (20 C). The final
mixture was then solvent exchanged by repeated centrifuge and
re-dispersion in desired solvents. Choices of solvent for the
centrifuge/re-dispersion processes are determined by the desired
dispersing medium for nanowire in ink formulation, e.g. water or
ethanol. Typical amount of solvent used in re-dispersion is about
50.times. or more of the remaining solid collected by centrifuged.
The whole process is repeated at least 3 times. The process
conditions can be scaled to large volume as illustrated in the
example below.
[0124] In the second example, 51 g of PVP is first dissolved in 500
mL of glycerol solvent and heated to 150 C. In the meantime, 51 g
of silver nitrate is dissolved in 300 ml of glycerol separately to
form silver nitrate solution. Then, the silver nitrate solution is
added to the PVP solution at a constant rate of 20 ml/min for 9
minutes; this is then followed by adding the remaining portion of
silver nitrate solution at 4 ml/min for another 30 minutes. The
reaction is allowed to proceed for another 3 hours before it is
quenched by mixing equal volume of cold water (20 C). The reaction
mixture was then solvent exchanged by repeated centrifuge and
re-dispersion in desired solvents. Choices of solvent for the
centrifuge/re-dispersion processes is determined by the desired
dispersing medium for nanowire in ink formulation, e.g. water or
ethanol. Typical amount of solvent used in re-dispersion is about
50.times. or more of the remaining solid collected by centrifuged.
The whole process is repeated at least 3 times.
[0125] Although few embodiments of the present invention have been
illustrated and described, it would be appreciated by those skilled
in the art that changes may be made in these embodiments without
departing from the principles and spirit of the invention. The
foregoing embodiments are therefore to be considered in all
respects illustrative rather than limiting on the invention
described herein. Scope of the invention is thus indicated by the
appended claims rather than by the foregoing description, and all
changes which come within the meaning and range of equivalency of
the claims are intended to be embraced therein. As used in this
disclosure, the term "preferably" is non-exclusive and means
"preferably, but not limited to." Terms in the claims should be
given their broadest interpretation consistent with the general
inventive concept as set forth in this description.
* * * * *