HIGH REFRACTIVE INDEX HIGH SCATTERING NANO COMPOSITE FOR
ENHANCING LIGHT EXTRACTION
FIELD OF INVENTION
[0001] The invention disclosed herein relates, in general, to lighting devices. More
specifically, the present invention relates to lighting devices providing improved light extraction.
BACKGROUND
[0002] Lighting devices and new display and lighting technology, provide high resolution
and high definition display and lighting applications. For example, many organic light emitting
based devices (OLEDs) include a thin film of electroluminescent organic material sandwiched
between a cathode and an anode. When a voltage is applied across the device, electrons and
holes are injected from their respective electrodes and recombine in the electroluminescent
organic material through the intermediate formation of emissive light.
[0003] However, in OLEDs, a large percentage of light is lost within the device structure.
Due to the significant refractive-index mismatches at air/substrate and substrate/transparent
oxide interfaces, OLED internal emission usually suffers total internal reflection, and hence most
of internal radiation is trapped and guided inside the device. For example, trapping of light at the
interfaces between the higher refractive index organic and electrode layers and the lower
refractive index substrate layers is a major cause of this poor efficiency. The majority of the light
undergoes internal reflections, resulting in trapping of light within the device.
[0004] Current solutions to the above mentioned problems include providing a textured
intermediate layer between the higher index organic and electrode layers and the lower index
substrate layers. This textured layer increase scattering of the light being emitted and causes a
reduction in internal reflections. However, there is still room for improvements in terms of there
being a substantial difference in refractive indices of different layers.
1
[0005] Recent studies reveal that the light extraction of OLEDs can be enhanced by use of
the external light extraction structure (EES) and/or internal light extraction structure (IES). All
these methods, however, increase complexity in device construction and cost. At the same time
these methods produce light outputs that have high wavelength dependence which is not suitable
for many practical applications
[0006] In light of the above discussion, there is a need for a lighting device that can enable
better and more efficient light extraction and overcome one or more drawbacks associated with
the prior art.
BRIEF DESCRIPTION OF FIGURES
[0007] The features of the present invention, which are believed to be novel, are set forth
with particularity in the appended claims. The invention may best be understood by reference to
the following description, taken in conjunction with the accompanying drawings. These
drawings and the associated description are provided to illustrate some embodiments of the
invention, and not to limit the scope of the invention.
[0008] FIG. 1 is a diagrammatic illustration of various components of an exemplary lighting
device, in accordance with an embodiment of the present invention;
[0009] FIG. 2a is a diagrammatic illustration of an exemplary lighting device having a first
light extraction layer including a first set of particles and a second set of particles, in accordance
with an embodiment of the present invention;
[0010] FIG. 2b is a diagrammatic illustration of an exemplary lighting device having a
second light extraction layer including a first set of particles and a second set of particles, in
accordance with an embodiment of the present invention;
[0011] FIG. 3 represents different formulation examples prepared for analysis, in
accordance with an embodiment of the present invention;
2
[0012] FIGs. 4a, 4b, 5a, and 5b are surface topographical study of two functional coatings
through AFM study, wherein FIGs. 4a, and 5a show roughness analysis and FIGs. 4b and 5b
are AFM 3D height images of the samples, in accordance with an embodiment of the present
invention;
[0013] FIG. 6 depicts SEM image of a curable nanocomposite layer of a sample, in
accordance with an embodiment of the present invention;
[0014] FIG. 7 represents a refractive index plot for different formulation samples, in
accordance with an embodiment of the present invention; and
[0015] FIGs. 8a and 8b represents total transmittance and scattering plot respectively for
different formulation samples and Lacquer without any nanoparticles, in accordance with an
embodiment of the present invention.
[0016] FIG. 9 depicts a table of optical performance data for coated and reference white
OLEDs upon application of different formulations, in accordance with an embodiment of the
present invention;
[0017] FIG. 10 represents a table of optical performance data for coated and uncoated pixels
for samples, in accordance with embodiments of the invention;
[0018] FIGs. 11a and l ib show OLED pixels pictures with and without scattering layer
application, respectively for samples in accordance with an embodiment of the present invention;
[0019] Those with ordinary skill in the art will appreciate that the elements in the figures are
illustrated for simplicity and clarity and are not necessarily drawn to scale. For example, the
dimensions of some of the elements in the figures may be exaggerated, relative to other
elements, in order to improve the understanding of the present invention.
3
[0020] There may be additional structures described in the foregoing application that are not
depicted on one of the described drawings. In the event such a structure is described, but not
depicted in a drawing, the absence of such a drawing should not be considered as an omission of
such design from the specification.
SUMMARY
[0021] The instant exemplary embodiments provide a lighting device having improved light
extraction property.
[0022] The instant exemplary embodiments provide a hybrid nano-composite material for
application on a light emitting surface of a lighting device to provide improved light extraction.
[0023] In some embodiments of the present invention, a UV curable nano-composite layer
is provided. The UV curable nano-composite layer have dual property, high refractive index (to
reduce mismatch between ITO/substrate layer) and light scattering effect. Further, the UV
curable nano-composite layer is cost effective and more environment friendly than UV curable
organic matrix (solvent free). Furthermore, rapid drying and easy fabrication process makes it an
attractive option for mass production at the industry level.
[0024] Some embodiments of the present invention provide a hybrid nano-composite
material for use on an emissive surface of a lighting device. The hybrid nano-composite material
includes a UV curable organic material, which includes a first set of particles and a second set of
particles. The first set of particles are uniformly distributed in the UV curable organic material
and have a loading fraction of 8-30%. Further, each of the first set of particles has dimensions of
up to avg. 10 nm and are configured to increase overall refractive index of the curable organic
material. Similarly, the second set of particles are uniformly distributed in the UV curable
organic material and have a loading fraction of 1-10%. Further, each of the second set of
particles have dimensions ranging from 200 nm to 600 nm, and are configured to increase
scattering property of the curable organic material.
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[0025] Some embodiments of the present invention provide a hybrid nano-composite
material where surface treatment for both set of nanoparticles done to achieve better particle
dispersion and long storage stability.
[0026] Some embodiments of the present invention provide a hybrid nano-composite
material for application on an emissive surface of a lighting device to improve light extraction in
the lighting device. The hybrid nano-composite material includes a UV curable organic material.
The UV curable organic material includes a first set of smaller particles that have higher
refractive index than the UV curable organic material and are configured to increase the overall
refractive index of the hybrid nano-composite material. The UV curable organic material also
includes a second set of larger particles that are configured to increase scattering property of the
UV curable organic material.
DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0027] Before describing the present invention in detail, it should be observed that the
present invention utilizes apparatus components related to a lighting device such as an organic
light emitting device. Accordingly the apparatus components have been represented where
appropriate by conventional symbols in the drawings, showing only specific details that are
pertinent for an understanding of the present invention so as not to obscure the disclosure with
details that will be readily apparent to those with ordinary skill in the art having the benefit of the
description herein.
[0028] While the specification concludes with the claims defining the features of the
invention that are regarded as novel, it is believed that the invention will be better understood
from a consideration of the following description in conjunction with the drawings, in which like
reference numerals are carried forward. !
[0029] As required, detailed embodiments of the present invention are disclosed herein;
however, it is to be understood that the disclosed embodiments are merely exemplary of the
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I invention, which can be embodied in various forms. Therefore, specific structural and functional
details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims
and as a representative basis for teaching one skilled in the art to variously employ the present
invention in virtually any appropriately detailed structure. Further, the terms and phrases used
herein are not intended to be limiting but rather to provide an understandable description of the
invention.
[0030] The terms "a" or "an", as used herein, are defined as one or more than one. The term
"another", as used herein, is defined as at least a second or more. The terms "including" and/or
"having" as used herein, are defined as comprising (i.e. open transition). The term "coupled" or
"operatively coupled" as used herein, is defined as connected, although not necessarily directly,
and not necessarily mechanically.
[0031] Referring now to the drawings, there is shown in FIG. 1, an exemplary lighting
device 100, in accordance with an embodiment of the present invention. Examples of the lighting
device 100 include an Organic Light Emitting Devices (OLEDs), lighting displays, and organic
displays.
[0032] Some real life examples of an OLED can include, but are not limited to, White
Organic Light Emitting Diode (W-OLED), Active-matrix Organic Light Emitting Diodes
(AMOLED), Passive-matrix Organic Light Emitting Diodes (PMOLED), Flexible Organic Light
Emitting Diodes (FOLED), Stacked Organic Light Emitting Diodes (SOLED), Tandem Organic
Light Emitting Diode, Transparent Organic Light Emitting Diodes (TOLED), Top Emitting
Organic Light Emitting Diode, Bottom Emitting Organic Light Emitting Diode, Fluorescence
doped Organic Light Emitting Diode (F-OLED) and Phosphorescent Organic Light Emitting
Diode (PHOLED).
[0033] For the purpose of the description, the lighting device 100 illustrated here has been
shown to include only those layers that are pertinent to the description of the invention.
However, it should be understood that the invention is not limited to the layers listed in the
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description here. In some cases, the lighting device 100 may include additional layers to enhance
efficiency or to improve reliability, without deviating from the scope of the invention.
[0034] The optoelectronic device 100 is shown to include a substrate 102, a first light
extraction layer 104, a second light extraction layer 106 and a functional layer stack 108. The
functional layer stack 108 is configured to have one or more functional layers therein. The one or
more functional layers include a first electrical contact 110, one or more organic layers 112 and
114, a second electrical contact 116 and a cover substrate 118.
[0035] The substrate 102 functions to provide strength to the optoelectronic device 100.
Examples of material useful as the substrate include, but are not limited to, glass, flexible glass,
polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC) and other
transparent or translucent material. The substrate 102 is defined by a first surface and a
substantially parallel second surface. Further, the substrate 102 is configured to receive the first
; light extraction layer 104 on the first surface and the second light extraction layer 106 on the
second surface. In an embodiment, the substrate may only include one of the first light extraction
layer 104 and the second light extraction layer 106.
[0036] The first light extraction layer 104 and the second light extraction layer 106 are made
up of a hybrid nano-composite material that is provided over the substrate 102. It should be
appreciated that, the first light extraction layer 104 may be interchangeably referred to as an
internal light extraction layer and the second light extraction layer 106 may be interchangeably
referred to as an external light extraction layer, for the purpose of the description. The internal
light extraction layer 104 and the external light extraction layer 106 can be provided on the
substrate 102 by using a brush or roller, dispensing, screen printing, slot dye coating, spincoating,
spray coating, diverse replication techniques, or even printing.
[0037] The hybrid nano-composite materials are composites including two or more
constituents at the nanometer or molecular level. Usually, the constituents are a mixture of
inorganic and organic materials. Mixing of these constituents at the microscopic scale leads to a
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more homogeneous material that shows characteristics in between the original constituents or
even new properties.
[0038] According to the embodiments of the present invention, the hybrid nano-composite
material is such that is has a high refractive index and good light scattering properties. The
hybrid nano-composite primarily includes an ultraviolet (UV) curable organic material. The UV
curable organic material has a property to retain any light extraction texture embossed on it when
it is cured by using mediums such as heat or light. The curable material can include, but is not
limited to, a ultra-violet curable material, a photo-polymer material, an acrylate, or a mixture of
curable oligomer and monomer mixture. However, it should be understood that the curable
organic material can be formed of any material having similar characteristics without deviating
from the scope of the invention. In an embodiment, a refractive index of a material forming the
curable organic material ranges between 1.60 to 1.85. The UV curable material also includes a
first set of particles and a second set of particles uniformly distributed in the curable organic
material.
[0039] A UV curable organic material can be an oligomer, a monomer or a combination. An
oligomer material can be epoxy acrylates, polyester acrylates, aliphatic/aromatic urathene
acrylates etc. A monomer material can be ethoxylated and propoxylated etc. For example, the
UV curable organic material can be a combination of an oligomer component such as epoxy
acrylates and monomeric component such as ethoxylated.
[0040] The light scattering property of particles is a general physical phenomenon in which
light radiation is forced to deviate from a straight line of path by one or more localized nonuniformities
in a medium. With respect to this invention, the localized non-uniformities can be
caused by presence of the second set of particles in the UV curable organic material, reflective
index and density of the UV curable organic material. Also, the light scattering depends on the
size of the second set of particles dispersed in the UV curable organic material.
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[0041] For example, in real life applications, surface treatment of nanoparticles provides
stable dispersions in organic materials. Preferably surface treatment stabilizes the nanoparticles
in cross-linkable binder medium and helps to achieve a homogeneous mixture. Surface of the
nanoparticles can be partially / fully modified using a surface treatment agent having copolymerizable
groups so that stabilized particles can co-polymerize with cross linkable material
during curing. Usually the surface treatment agent has some functionality at one end that could
attach to the particle surface through ionic or covalent bond or through physical adsorption
process. At the same time, the other end helps it to be compatible with the organic material or
react during curing step. Examples of the surface treatment agents include, but are not limited to
carboxylic acids, various silanes, amines, sulphonic or phosphoric acids, silanes carboxylic acid
functional compounds, methacrylic acid, acrylic acid, beta-carboxyethylacrylate, caproic acid,
oleic acid, methoxyphenylacetic acid, propyldimethoxysilane, vinyltriphenoxysilane,
mercaptopropyltrimethoxysilane, vinylmethylethoxysilane, propyldimethylethoxysilane,
vinyldimethylethoxysilane, 3-(trimethoxysilyl) propylmethacrylate,3-
(acryloxypropyl)trimethoxysilane, 3-(methacryloyloxy)propyltriethoxysilane, 3-
(methacryloyloxy)propyldimethylethoxysilanephenyltrimethoxysilane,
vinylmethyldiethoxysilane, and mixture thereof.
[0042] In some embodiments, a texture may also be applied to the internal light extraction
layer 104 and the external light extraction layer 104. The texture facilitates light extraction in the
lighting device 100. For example, when the lighting device 100 is an OLED, a light emitted by
organic layers of the OLED, needs to pass through the substrate 102. However, when light is
incident from a high refractive index material onto an interface with a lower refractive index
material or medium, the light undergoes total internal reflection (TIR) for all incidence angles
greater than the critical angle 9C, defined by 6C = sin"1 (n2/ni), where ni and n2 are the refractive
indices of the high refractive index material and low refractive index material, respectively. Due
to the same reason, when the light emitted by the one or more organic layers 112 and 114
reaches an interface between the substrate 102 and an ambient medium, a substantial amount of
light is reflected back into the OLED.
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[0043] In accordance with the present invention, the internal light extraction layer 104 and
the external light extraction layer 106 are deposited in a manner such that the texture can be
provided on a surface of the internal light extraction layer 104 and the external light extraction
layer 106. The examples of the texture include, but are not limited to, ID or 2D periodic Ushaped
features, a ID or 2D periodic sinusoidal grating, ID or 2D periodic upright or inverted
pyramids, random upright or inverted pyramids, ID or 2D periodic inverted cones, and other
micro and nano-sized structures.
[0044] In real life applications, the internal light extraction layer 104 and the external light
extraction layer 106 may be deposited by screen printing or spin coating process. The internal
light extraction layer 104 and the external light extraction layer 106 may be deposited using a
screen printing process which uses a paste of the curable material having a viscosity of 3000-
8000cps @ 25° C. In this, a polyester screen of 300-450 mesh size may be used. A squeeze
pressure of 2.0 to 3.0 bar may be applied and a squeeze angle of 60 to 75° may be maintained.
Further once applied the curable nano composite layer may be dried through UV Curing
technique using a 3.2KW medium pressure Hg Vapour or a metal halide lamp within 0.5 to 3.0
seconds.
[0045] Once the first light extraction layer 104 has been deposited, the first electrical contact
110 is provided over the nano composite layer 104. In an embodiment, the first electrical contact
110 can be implemented using a transparent conducting oxide (TCO). TCOs are doped metal
oxides, examples of TCOs include, but are not limited to, Zinc Oxide, Tin Oxide, Aluminumdoped
Zinc Oxide (AZO), Boron doped Zinc Oxide (BZO), Gallium doped Zinc Oxide (GZO),
Fluorine doped Tin Oxide (FTO), Indium Zinc Oxide and Indium doped Tin Oxide (ITO). TCOs
have more than 80% transmittance of incident light and have conductivities higher than 103 S/cm
for efficient carrier transport. The transmittance of TCOs, just as in any transparent material, is
limited by light scattering at defects and grain boundaries. In another embodiment, the first
electrical contact 110 can also be implemented with a PEDOT-PSS, or any other transparent
polymers, or thin metal layers.
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t
[0046] Even though the light extraction has been explained in association with OLEDs, it
should be appreciated that the light extraction layers can be provided on other lighting devices
like lighting displays, organic displays, bulbs, general LED lightings etc. The extraction layer
described above can be provided on the light emitting surfaces of these lighting devices to
enhance the light extraction.
[0047] The first electrical contact 110 may be deposited by processes such as Physical
Vapor Deposition (PVD), Low Pressure Chemical Vapor Deposition (LPCVD), Atmospheric
Pressure Chemical Vapor Deposition (APCVD), or even Plasma Enhanced Chemical Vapor
Deposition (PECVD) and the like.
[0048] Moving on to the next set of layers, the one or more organic layers 112 and 114.
Generally, the one or more organic layers 112 and 114 are deposited using methods such as dip
coating, spin coating, doctored blade, spray coating, screen printing, sputtering, glass mastering,
photoresist mastering, all kinds of CVD, electroforming, and evaporation. In an embodiment, the
one or more organic layers 112 and 114 can be implemented with any organic electroluminescent
material such as a light-emitting polymer, evaporated small molecule materials, light-emitting
dendrimers or molecularly doped polymers. For the purpose of this description, the organic
layers are shown to include only two layers, however, it will be readily apparent to those skilled
in the art that the optoelectronic device 100 can include or exclude one or more organic layers
without deviating from the scope of the invention.
[0049] Following the one or more organic layers 112 and 114, the second electrical contact
116 is deposited. In one embodiment, the second electrical contact 116 can includes a second
layer of TCO, a layer of silver, and a layer of aluminum. In other embodiments, the second
electrical contact 116 can be implemented with metals with appropriate work function to make
injection of charge carriers, for example, calcium, aluminum, gold, and silver. In an embodiment,
the first electrical contact 110 and the second electrical contact 116 act as electrode layers. For
example the first electrical contact 110 acts as an anode and the second electrical contact 116
acts as a cathode. Thereafter, all the above mentioned layers are encapsulated using a cover
substrate 118 between the substrate 102 and the cover substrate 118.
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i
[0050] Moving on there is illustrated in FIGs. 2a and 2b, two exemplary views of the
lighting device 100, in accordance with two embodiments of the present invention.
[0051] In FIG. 2a, for example, the lighting device 100 is shown to include the first light
extraction layer 104 having a first set of particles 202 and a second set of particles 204
distributed across the first light extraction layer 104. The lighting device 100, in this
embodiment, is shown not to include the second light extraction layer 106.
[0052] Similarly, in FIG. 2b, the optoelectronic device 100 is shown to include the first
light extraction layer 104 and the second light extraction layer 106. Further, the second light
extraction layer 106 is shown to include the first set of particles 202 and the second set of
particles 204 but not the first light extraction layer 104. In other embodiments, both the first light
extraction layer 104 and the second light extraction layer 106 can include the first set of particles
202 and a second set of particles 204.
[0053] The first set of particles have dimensions up to avg 10 nm and have a refractive index
that is substantially greater than RI of said curable organic material. The higher refractive index
of the first set of particles increases the overall refractive index of the hybrid nano-composite
material and tries to reduce the mismatch in refractive indices of glass and the one or more
organic layers, thereby reducing chances of Total Internal Reflection (TIR) and enabling higher
quantity of light to pass through. Further, the first set of particles has dimensions up to avg 10
nm which configures them to not interfere with visible light spectrum. Similarly, the second set
of particles has dimensions ranging from 200 nm to 600 nm and is configured to scatter the
visible light. The first set of particles are configured to increase the refractive index of the
internal light extraction layer 104 and the external light extraction layer 106 and the second set of
particles are capable of increasing the scattering property of the internal light extraction layer
104 and the external light extraction layer 106. Further, details of the structure and composition
of the particles has been provided with reference to FIGs. 2a and 2b.
[0054] The internal light extraction layer 104 and the external extraction layer 106 are
formed from a hybrid nano-composite material that includes a paste of curable organic material.
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In an embodiment the curable organic material can be a mixture of oligomeric and monomeric
components. For example, in real life applications the oligomeric component can include a UV
curable di/tri/tera functional epoxy acrylates of MW 400-1500(g/mol), a UV curable di/tri/tera
functional polyester acrylates of MW 500 - 5000(g/mol), a UV curable di/tri/tera functional
aliphatic/aromatic urathene acrylates acrylates of MW 1000-3 500(g/mol), and a UV curable
di/tri/tera functional full acrylates MW 1000-5000(g/mol). In another embodiment, the polyester
acrylates in the oligomeric component may sometime be amine modified or chlorinated polyester
acrylate types. In an embodiment, the loading fraction of the oligomeric component may be from
10 to 50% and the monomeric component may be from 30 to 60%. In real life applications, for
example, if the loading fraction of the oligomeric component is 20% and the monomeric
component is 50%, then it would mean that if the weight of the hybrid nano-composite material
| is lOOg, the oligomeric component constitutes 20g and the monomeric component constitutes
50g of the lOOg of the hybrid nano-composite material.
[0055] Similarly, real life applications of the monomeric component can include
photocurable / UV Curable mono/bi/tri/tetra/hexa functional aliphatic/aromatic acrylates of MW
150- 600(g/mol). Further, in another embodiment the monomeric component may include
photocurable / UV Curable mono/bi/tri/tetra/hexa functional aliphatic/aromatic acrylates of MW
150- 600(g/mol) that are ethoxylated, propoxylated -and/or modified with ether functionality.
[0056] Further, the first set of particles 202 and the second set of particles 204 are inorganic
particles that include but are not limited to metal oxides MxOy with high refractive index
property, wherein M can be any one of Ti, Zr, Zn, Al, Si and mixtures thereof. In an
embodiment, the inorganic particles may be reacted with coplymers having acidic functionalities
or some silane compounds to stabilize them. Also, the shape of the first set of particles 202 and
the second set of particles 204 are spherical, rod, square or elongated spherical shaped. However,
it should be appreciated that the shape can be in any other suitable form not limited to the ones
mentioned. In an embodiment, a loading fraction of the first set of particles 202 is from 8 to 30%
and that of the second set of particles 204 is from 1 to 10%. In continuation to the example
provided above, if the loading fraction of the oligomeric component is 50%, the loading fraction
of the monomeric component is 20%, the loading fraction of the first set of particles is 10% and
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the loading fraction of the second set of particles is 10%, then it would mean that if the weight of
the hybrid nano-composite material is lOOg, the oligomeric component constitutes 30g, the
monomelic component constitutes 50g, the first set of particles constitutes lOg, and the second
set of particles constitutes lOg of the lOOg of the hybrid nano-composite material.
[0057] Further, the first set of particles 202 and the second set of particles 204 are such that
they are of two different size ranges. For example, the first set of particles 202 have a size less
than the wavelength of visible light and each of the first set of particles 202 are in a first
dimension range of avg lOnm The first set of particles 202 are configured to increases overall
refractive index of the light extraction layer. Similarly, the second set of particles 204 have a size
same as the wavelength of visible light and each of the second set of particles 204 are in a first
dimension range of 200 to 600 nm. Further, the second set of particles are configured to
increases the scattering property of the light extraction layers. Also, the first light extraction layer
104 and the second light extraction layer 106 can have a thermal stability of up to 250° C.
[0058] The first set of particles 202 and the second set of particles 204 may be incorporated
into the curable organic material by dispersing the stable inorganic particles into curable organic
layer. This may be carried out by using an ultrasonic homogenizer. In an embodiment, an
ultrasonic frequency of 20 to 50 KHz and a power of 100-500 watt may be maintained during the
dispersion step. Further, in another embodiment, the ultrasonic frequency of 20 to 50 KHz may
be applied to the mixture over several cycles, for about 10 to 20 minutes on each cycle. In an
embodiment the number of cycles can vary from 3-10 for optimum particle dispersion.
[0059] In an embodiment the first light extraction layer 104 and the second light extraction
layer 106 can also include a catalyst for photochemically induced curing process of the nanocomposite
material. Photoinitiators such as Benzophenone, 2-Hydroxy-2-methyl-lphenylpropanone,
1-Hydroxy-cyclohexyl-phenyl-ketone, Diphenyl (2,4,6 trimethylbenzoyl) -
phosphine oxide and their mixture could be used for the coating formulation that can be cured
under UV radiation. Some cationinc photoinitiator like Idonium salts and metallocenes,
sulphonium salts may be used for the curing process of cationic system epoxy resins formulatins.
14
Depending upon final film characteristics, oligomer/monomer ratio photoinitiator loading may
be controlled, usually it is in the range of 1 to 7% of overall formulation.
[0060] In an embodiment, the first light extraction layer 104 and the second light extraction
layer 106 can also include some surface control agents that helps to achieve proper film
morphology after the curing stage. There are usually some defoaming, slip and leveling agents
that can be used to produce defect free film. Examples of such agents include, but are not limited
to, cross linkable or noncrosslinkable organo silicones, some epoxy or polyether modified
silicones; UV curable normal polyacrylate or fluromodified polyacrylate solutions, organomodified
polysiloxane, or some fatty acid solution in monomeric mixture can be used for coating
compositions. Such agents added based on formulation are usually within 0.5-5% of overall the
composition.
[0061] Further, as illustrated in the FIGs. 2a and 2b, the first set of particles 202 and the
second set of particles 204 may be uniformly distributed across the light extraction layers 104
and 106. In some embodiments, the first set of particles 202 and the second set of particles 204
may not be uniformly distributed across the light extraction layers 104 and 106 and instead be
randomly distributed or have a biased distribution based upon specific requirements, without
deviating from the scope of the inventions.
[0062] In an embodiment, the hybrid nano-composite material thus formed has a refractive
index of 1.82 that is high refractive index comparable to the refractive indices of the glass/first
electrical contact layer. The hybrid nano-composite material is formed by mixing 85% of
oligomer and monomer. The titania particles of around lOnm and around 500nm are mixed in the
ratio of 4:6 %. The particles are mixed using an ultrasonic homogenizer. Ultrasonic frequency of
50 KHz is applied to the mixture for over 8 cycles, for about 18 minutes in each cycle. Then a
photo initiator mixture solution 6% is added and mixed again for 10 minutes. Finally, 3% of
surface control agents are added and mixed for another 10 minutes to prepare the hybrid nanocomposite
material. As can be seen, the titania particles of around lOnm significantly enhance
refractive index of the hybrid nano-composite material and reduces refractive indices mismatch
at air/substrate and substrate/transparent oxide interfaces and minimize overall total internal
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reflection (TIR) loss. Also, titania particles of around 500nm interfere with the visible light
wavelength and significantly enhance the scattering effect of the hybrid nano-composite material
made layer.
[0063] Moving on, provided below are details of a few different experiments carried out to
test out the hybrid nano-composite material provided in accordance with the embodiments of the
present invention.
Examples
[0064] Surface Treatment of Titania nanoparticles:
[0065] In an experimental set up, a first nano-Ti02 powder mixed with Ethyl Acetate
solvent was stirred vigorously for 30 minutes. As per the surface area of particle, appropriate
amount of surface treatment agent was added into this solution. To get effective surface coating
on nano-particles, the mixture solution was refluxed near its boiling point for 8-12 hrs. Then, the
resulting solution was centrifuged to remove excess solvents. The coated nano-particles were
washed with fresh ethyl acetate/or some other alcohol 3-4 times to remove excess surface
treatment agent and contaminants. Finally the mixture was re-centrifuged to collect precipitated
Ti02. The resultant surface modified Ti02 nano- particles were dried at 125°C under inert
atmosphere for 3-6 hrs. The dry surface treated particles were used for dispersions process.
[0066] Functional coating formulation, for Examples A-E
[0067] Example Blank: A mixture of Ethoxylated tripropylene glycol diacrylate, aliphatic
urethane acrylate in 1, 6 hexanedioldiacrylate and polyhydroxy propyl methacrylate (30%) and a
mixture of Pentaerythritol tri/tetra acrylate, Triacrylate of ethoxylated trimethylol propane and
Hexamethylene diacrylate (60.5%) were mixed in a suitable container by a high speed mixer and
then dispersed with an ultrasonic homogenizer. Ultrasonic frequency of 20 to 50 KHz was
applied to the mixture over several cycles, for about 10 to 20 minutes on each cycle. Then
16
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i
mixture of photo initiators (Darocur TPO and Darocur 1173) solution 4.5% was added and
mixed again for 10 minutes. Finally surface control agents (dispersing agents, de-foamers and
leveling agents) 5.0% were added and mixed for another 10 minutes to prepare a blank organic
coating material without any inorganic nano-particles.
[0068] Example A: A mixture oligomers (27.5 %) containing Ethoxylated tripropylene
glycol diacrylate and aliphatic urethane acrylate in 1,6 hexanedioldiacrylate and mixture of
monomers (56.0%) triacrylate of ethoxylated trimethylol propane and hexamethylene diacrylate
were mixed in a suitable container using a high speed mixer. Then surface treated Set 1 of
titania particles 3% and Set 2 of titania particles 4% was mixed through that high speed mixer
and then dispersed with an ultrasonic homogenizer. Ultrasonic frequency of 20 to 50 KHz was
applied to the mixture over several cycles, for about 10 to 20 minutes on each cycle. The number
of cycles can vary for optimum particle dispersion. Then mixture of photo initiators solution
4.5% (Darocur TPO and Darocur 1173) was added and mixed again for 10 minutes. Finally
surface control agents (dispersing agents, defoamers and leveling agents) 5.0% were added and
mixed for another 10 minutes to prepare Example A.
[0069] Example B: A mixture of oligomers (26%) containing polyhydroxy propyl
methacrylate and aliphatic urethane acrylate in 1,6 hexanedioldiacrylate and a mixture of
monomers (54.0%) containing triacrylate of ethoxylated trimethylol propane and
hexamethylene diacrylate were mixed in a suitable container by a high speed mixer. Then
surface treated Set 1 of titania particles 7.5 % and Set 2 of titania particles 4% was mixed
through that high speed mixer and then dispersed with an ultrasonic homogenizer. Ultrasonic
frequency of 20 to 50 KHz was applied to the mixture over several cycles, for about 10 to 20
minutes on each cycle. The number of cycles can vary for optimum particle dispersion. Then a
mixture of photo initiators (Darocur TPO and Darocur 1173) solution (4.5%) was added and
mixed again for 10 minutes. Finally surface control agents (dispersing agents, defoamers and
leveling agents) 5.0% were added and mixed for another 10 minutes to prepare Example B.
[0070] Example C: A mixture of oligomers (27.5 %) containing ethoxylated tripropylene
glycol diacrylate and aliphatic urethane acrylate in 1,6 hexanedioldiacrylate and mixture of
17 I
triacrylate of ethoxylated trimethylol propane and pentaerythritol tri/tetra acrylate of monomers
(49.0%) were mixed in a suitable container by a high speed mixer. Then surface treated Set 1 of
titania particles 12.5% and Set 2 of titania particles 4.5% was mixed through that high speed
mixer and then dispersed with an ultrasonic homogenizer. Ultrasonic frequency of 20 to 50 KHz
was applied to the mixture over several cycles, for about 10 to 20 minutes on each cycle. The
number of cycles can vary for optimum particle dispersion. Then mixture (Darocur TPO and
Darocur 1173) of photo initiators solution 4.5% was added and mixed again for 10 minutes.
Finally surface control agents (dispersing agents, defoamers and leveling agents) 5.0% were
added and mixed for another 10 minutes to prepare Example C.
[0071] Example D: A mixture of oligomers (24.5 %) containing amine modified polyester
acrylate in 1,6 hexanediol diacrylate and aliphatic urethane acrylate in 1,6 hexanedioldiacrylate
and mixture of monomers (47.0%) containing triacrylate of ethoxylated trimethylol propane and
pentaerythritol tri/tetra acrylate) ware mixed in a suitable container by a high speed mixer. Then
surface treated Set 1 of titania particles 16.5 % and Set 2 of titania particles 2.5% ware mixed
using high speed mixer and then dispersed with an ultrasonic homogenizer. Ultrasonic
frequency of 20 to 50 KHz was applied to the mixture over several cycles, for about 10 to 20
minutes on each cycle. The number of cycles can vary for optimum particle dispersion. Then
mixture of photo initiators solution (4.5%) containing Darocur TPO and Darocur 1173 was
added and mixed again for 10 minutes. Finally surface control agents (dispersing agents,
defoamers and leveling agents) 5.0% were added and mixed for another 10 minutes to prepare
Example D.
[0072] Example E: A mixture of oligomers (22.1 %) containing amine modified polyester
acrylate in 1,6 hexanediol diacrylate and aliphatic urethane acrylate in 1,6 hexanedioldiacrylate)
and a mixture of monomers (44.0%) containing triacrylate of ethoxylated trimethylol propane
and dipropylene glycol diacrylate were mixed in a suitable container by a high speed mixer.
Then surface treated Set 1 of titania particles 22.0% and Set 2 of titania particles 2.4% were
mixed through that high speed mixer and then dispersed with an ultrasonic homogenizer.
Ultrasonic frequency of 20 to 50 KHz applied to the mixture over several cycles, for about 10 to
20 minutes on each cycle The number of cycles can vary for optimum particle dispersion. Then
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a mixture of photo initiators solution (4.5%) contacting Darocur TPO and Darocur 1173 was
added and mixed again for 10 minutes. Finally surface control agents (dispersing agents,
defoamers and leveling agents) 5.0% were added and mixed for another 10 minutes to prepare
Example E.
[0073] Functional coating with multiple formulation sample example A-E prepared as
mentioned are reported in FIG. 3.
[0074] The nanocomposite mixture thus produced (for any of the examples A-E) is applied
on glass or some other appropriate substrate 102 using for example screen printing or spin
coating or other known in the art methods. The applied coating is then cured under UV light for
0.5 to 3.0 seconds depending upon formulation and process requirement. According to an
embodiment of the present invention, coating thickness after curing can be in the range of 200nm
to 6000nm.
[0075] The first light extraction layer 104 and the second light extraction layer 106 thus
formed may have thickness of 3 to 6 microns and a very low surface roughness of less than 6nm.
Surface morphology of the cured composite film is characterized by Atomic force Microscopy
(AFM) ,images captured under tapping mode on Digital instrument Dimension 3100 AFM. For
example, FIGs. 4a and 5a show surface roughness analysis of two formulations Examples A and
E respectively. As can be noted, mean roughness has increased from 0.572nm to 2.739nm with
increase in loading% of first set of particles but that is well in range for further OLED
processing. So overall inorganic particle loading is critical for controlling surface roughness of
the cured film.3D images. Moving on FIGs. 4b and 5b show surface topography of above
mentioned examples respectively and supports uniformity of the coating surface, very much
essential for OLED making. The present invention enables preparation of a controlled coating
surface by optimizing particle loading, balancing surface control agent in formulation and tuning
coating process parameters/conditions.
[0076] Moving on, FIG. 6 illustrates the SEM image of the first light extraction layer 104
and the second light extraction layer 106. It confirms the formation of inorganic domains in
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1
nanometer range. As can be seen, both set of particles are evenly distributed in the organic
matrix .That homogenious dispersion gives better storage stability of the wet composite paste.
[0077] Also, the first light extraction layer 104 and the second light extraction layer 106
thus formed has a refractive index of 1.60-1.85 that is comparable to the refractive indices of the
glass/first electrical contact layer 110. This is further illustrated in FIG.7 that shows the
refractive index values of different composite formulation for example, samples A to E and
corresponding lacquer medium against wavelength measured using ETA/RT technique. If we
consider a particular wavelength range for example, at 550nm wavelength range refractive index
increased from 1.5914 (lacquer medium) to 1.684 (Sample E). As can be seen even a small
particle loading significantly helped enhance refractive index of the functional coating and
reduce the refractive indices mismatch between above two layers, and minimize overall TIR loss.
[0078] Further, optical transmittance of nanocomposite layers herein is characterized using
UV Visible spectroscopy the first light extraction layer 104 and the second light extraction layer
106 can have a transmittance of greater than 70% and a scattering transmittance 40-50% for a
cured film with layer thickness of around 4.0 micron. FIGs. 8a and 8b indicate the total
scattering transmittance of the formulation examples from A-E and A-D respectively. It depicts
that light absorption property tends to increase slowly upto a certain percentage loading for Set 1
and Set 2 particles and then changes rapidly. So to maintain a proper balance between light
scattering and light transmission property in the composite film optimum particle loading is very
important.
[0079] OLED performance of the composite film was also tested for the formulation
examples A-E. In one experiment, white OLED was prepared by applying individual coatings on
emitting glass surface using screen printing process. Another reference OLED device was made
without any composite layer coating keeping all other process parameters fixed. Optical
performance (luminance current efficacy) was compared under 3.5 volt applied voltage for all
formulations against the reference device. All the formulation showed luminance current efficacy
gain against the reference device, gain of up to 60% improvement was observed in light output.
Referring to FIG. 9 there is shown a table indicating the optical gain achieved with different
20
formulation examples A-E for white OLED. It is apparent from the results reported in the table
that luminance current efficacy gain under 3.5V for Examples A,B,C,D,E are 17%, 22%, 43%,
57%, and 48% respectively.
[0080] In another experiment two of the composite coating formulation Examples B and E
are selectively applied through screen printing on two different emitting glass substrates. Then
all the pixels, both with and without scattering layer coating were processed for making further
white OLEDs. Luminance output was tested under same 3.5 volt applied voltage, pixel 1, 2, 5, 6
and 3,4 in FIG. 11a refer to the coated and without coated area of an embodiment of the present
invention, specifically for Example E. At the same time, pixels 1, 2, 5, 6 and 3, 4 in FIG. l ib
refer to coated and uncoated area of specifically example B. Luminance current efficacy and
luminance current efficacy gain for couple of pixels in above experiment has been reported in the
FIG. 10.
[0081] Various embodiments, as described above, provide a lighting device that has several
advantages. The lighting device according to the invention provides improved light extraction to
a lighting device. Further, the particles provided in the light extraction layer ensure an increase in
the refractive index of the layer thus reducing the total internal reflection and also increase the
scattering property thereby again improving light extraction.
[0082] While the invention has been disclosed in connection with the preferred
embodiments shown and described in detail, various modifications and improvements thereon
will become readily apparent to those ordinarily skilled in the art. Accordingly, the spirit and
scope of the present invention is not to be limited by the foregoing examples, but is to be
understood in the broadest sense allowable by law.
[0083] All documents referenced herein are hereby incorporated by reference.
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CLAIMS
What is claimed is:
l
1. A hybrid nano-composite material for enhancing light extraction, said hybrid nanocomposite
material having high refractive index and light scattering properties, said
hybrid nano-composite material comprising an ultraviolet (UV) curable organic material,
said hybrid nano-composite material further comprising:
a first set of particles uniformly distributed in said UV curable organic material,
each of said first set of particles having dimensions up to avg 10 nm, wherein the
refractive index (RI) of said first set of particles is substantially greater than RI of
said UV curable organic material, further wherein a loading fraction of said first
set of particles is between 8-30% by weight, said first set of particles configured
to not interfere with visible light spectrum; and
a second set of particles uniformly distributed in said UV curable organic
material, each of said second set of particles having dimensions ranging from 200
nm to 600 nm, wherein a loading fraction of said second set of particles is 1-10%
by weight, said second set of particles being configured to scatter the visible light.
2. The hybrid nano-composite material of claim 1, wherein said UV curable organic
material comprise an oligomer component and a monomelic component, said oligomer
component having a loading fraction of 10-50% by weight and said monomeric
component having a loading fraction of 5-30% by weight.
3. The hybrid nano-composite material of claim 1, wherein said first set of particles and said
second set of particles are inorganic metal oxide.
4. The hybrid nano-composite material of claim 1, wherein said first set of particles and said
second set of particles and said second set of particles can be selected from one of Ti02,
Zr02, ZnO, AhCh, and Si02.
(kJn
22
V
5. The hybrid nano-composite material of claim 1, wherein said first set of particles and
said second set of particles having a refractive index of 2.4 to 2.6.
6. The hybrid nano-composite material of claim 1, wherein said hybrid nano-composite
material has atlcast one of a refractive index is 1.6 to 1.85, total transmittance of at least
more than 75% and a roughness of less than 6 nm.
7. The hybrid nano-composite material of claim 1, wherein said first set of particles and said
second set of particles are spherical, rod, square or elongated spherical shaped.
8. The hybrid nano-composite material of claim 1, wherein said oligomeric component
comprises :
a. a UV curable di/tri/tera functional epoxy acrylates of MW 400-1500(g/mol);
b. a UV curable di/tri/tera functional polyester acrylates of MW 500 - 5000(g/mol);
c. a UV curable di/tri/tera functional aliphatic/aromatic urethane acrylates of MW
1000-3500(g/mol); and
d. a UV curable di/tri/tera functional full acrylates MW 1000-5000(g/mol).
9. The hybrid nano-composite material of claim 1, wherein said monomeric component
comprises curable mono/bi/tri/tetra/hexa functional aliphatic/aromatic acrylates of MW
150-600(g/mol).
10. The hybrid nano-composite material of claim 1, wherein said hybrid nano-composite
material enables enhancement of light extraction in a lighting device, wherein said
lighting device is selected from the group comprising a light emitting device, an organic
light emitting device, a liquid crystalline display, and other organic displays.
•