U.S. patent application number 15/724055 was filed with the patent office on 2018-04-05 for enhanced oled outcoupling by suppressing surface plasmon modes.
This patent application is currently assigned to Regents of the University of Michigan. The applicant listed for this patent is Regents of the University of Michigan. Invention is credited to Stephen R. Forrest, Yue Qu.
Application Number | 20180097202 15/724055 |
Document ID | / |
Family ID | 61758596 |
Filed Date | 2018-04-05 |
United States Patent
Application |
20180097202 |
Kind Code |
A1 |
Forrest; Stephen R. ; et
al. |
April 5, 2018 |
ENHANCED OLED OUTCOUPLING BY SUPPRESSING SURFACE PLASMON MODES
Abstract
A number of new solutions for enhancing the extraction of
waveguided mode and suppressing surface plasmon polariton mode in
OLEDs are disclosed.
Inventors: |
Forrest; Stephen R.; (Ann
Arbor, MI) ; Qu; Yue; (Ann Arbor, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Regents of the University of Michigan |
Ann Arbor |
MI |
US |
|
|
Assignee: |
Regents of the University of
Michigan
Ann Arbor
MI
|
Family ID: |
61758596 |
Appl. No.: |
15/724055 |
Filed: |
October 3, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62403454 |
Oct 3, 2016 |
|
|
|
62403490 |
Oct 3, 2016 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 51/504 20130101;
H01L 51/5271 20130101; H01L 51/5275 20130101; H01L 2251/558
20130101; H01L 51/5268 20130101; H01L 51/5265 20130101; H01L
2251/5369 20130101; H01L 51/5212 20130101; H01L 51/5072
20130101 |
International
Class: |
H01L 51/52 20060101
H01L051/52; H01L 51/50 20060101 H01L051/50 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] This invention was made with government support under Award
No. DMR-1411064 awarded by NSF and Award No. DE-EE0007626 awarded
by Office of Energy Efficiency and Renewable Energy (EERE) and
United States Department of Energy. The government has certain
rights in the invention.
Claims
1. An organic light emitting device (OLED), comprising: a
transparent substrate having a first side and a second side; a
transparent organic light scattering layer disposed over the first
side of the substrate, wherein the organic light scattering layer
is a continuous layer having a randomly corrugated surface texture
with surface texture height between 5 nm-10 .mu.m with a lateral
feature size of 100-1000 nm; and an emissive region disposed on the
organic scattering layer, the emissive region comprising: a
transparent anode; a cathode; and at least one organic emissive
layer between the transparent anode and the cathode, wherein the
transparent anode, the organic emissive layer, and the cathode each
have a randomly corrugated structure produced by the randomly
corrugated surface texture of the underlying transparent organic
light scattering layer and the randomly corrugated structure in the
emissive region extracts waveguided mode.
2. The OLED of claim 1, wherein the surface texture height of the
organic light scattering layer is between 5-500 nm.
3. The OLED of claim 1, wherein the surface texture height of the
organic light scattering layer is between 5-300 nm.
4. The OLED of claim 1, wherein the average surface texture height
of the organic light scattering layer is between 130-170 nm.
5. The OLED of claim 1, wherein the lateral feature size of the
randomly corrugated surface texture is 100 nm-10 .mu.m.
6. The OLED of claim 1, further comprising an optical diffuser
layer provided on the second side of the transparent substrate.
7. The OLED of claim 6, wherein the optical diffuser layer
comprises a microlens array or a nanoparticle diffuser.
8. The OLED of claim 1, wherein the emissive region further
comprises an electron transport layer having a thickness of at
least 30 nm but no more than 400 nm disposed between the cathode
and the at least one organic emissive layer.
9. A organic light emitting device (OLED), comprising: a
transparent substrate having a first side and a second side; an
emissive region disposed over the first side of the transparent
substrate, the emissive region comprising: a transparent first
electrode disposed over the transparent substrate; at least one
organic emissive layer disposed over the transparent first
electrode; and a transparent second electrode disposed over the at
least one organic emissive layer; an optical grating layer having a
grating structure having a sub-wavelength periodicity disposed on
the transparent second electrode; and a reflective layer disposed
over the optical grating layer.
10. The OLED of claim 9, wherein the grating structure layer has a
feature size of 300 nm and a thickness of no more than 10 nm.
11. The OLED of claim 9, further comprising an optical diffuser
layer provided on the second side of the transparent substrate.
12. The OLED of claim 11, wherein the optical diffuser layer
comprises a microlens array or a nanoparticle diffuser.
13. The OLED of claim 9, wherein the emissive region further
comprises an electron transport layer having a thickness of at
least 50 nm disposed between the cathode and the at least one
organic emissive layer.
14. A organic light emitting device (OLED), comprising: a substrate
having a first side and a second side; a reflective layer disposed
over the first side of the substrate; a grid layer consisting of
two optically transparent materials with different refractive
indices disposed on the reflective layer; a transparent first
electrode provided over the grid layer; an organic emissive layer
provided over the transparent bottom electrode; and a transparent
second electrode provided over the organic emissive layer, wherein
the grid layer scatters trapped waveguided modes from the organic
emissive layer.
15. The OLED of claim 14, wherein the two optically transparent
materials forming the grid layer are SiO.sub.2 and TiO.sub.2.
16. The OLED of claim 14, wherein the grid layer is electrically
conductive, and provides an extension of the first electrode.
17. The OLED of claim 14, wherein the reflective layer is a metal
layer.
18. The OLED of claim 14, wherein the reflective layer is
positioned at least 100 nm from the organic emissive layer and
inhibits excitation of surface plasmon polaritons.
19. The OLED of claim 14, further comprising a spacer layer
provided between the grid layer and the transparent first
electrode, wherein cavity resonant frequency in the OLED can be
tuned by varying the spacer layer's thickness.
20. The OLED of claim 14, further comprising an optical diffuser
layer provided on the second electrode layer.
21. The OLED of claim 20, wherein the optical diffuser layer
comprises a microlens array or a nanoparticle diffuser.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a non-provisional application claiming
priority to U.S. Provisional Application Ser. No. 62/403,454, filed
on Oct. 3, 2016, and U.S. Provisional Application Ser. No.
62/403,490, filed Oct. 3, 2016, the entire contents of which are
incorporated herein by reference.
PARTIES TO A JOINT RESEARCH AGREEMENT
[0003] The claimed invention was made by, on behalf of, and/or in
connection with one or more of the following parties to a joint
university corporation research agreement: Regents of the
University of Michigan, Princeton University, University of
Southern California, and Universal Display Corporation. The
agreement was in effect on and before the effective filing date of
the presently claimed invention, and the claimed invention was made
as a result of activities undertaken within the scope of the
agreement.
FIELD
[0004] The present invention relates to methods of enhancing
outcoupling in organic light emitting devices.
BACKGROUND
[0005] Opto-electronic devices that make use of organic materials
are becoming increasingly desirable for a number of reasons. Many
of the materials used to make such devices are relatively
inexpensive, so organic opto-electronic devices have the potential
for cost advantages over inorganic devices. In addition, the
inherent properties of organic materials, such as their
flexibility, may make them well suited for particular applications
such as fabrication on a flexible substrate. Examples of organic
opto-electronic devices include organic light emitting
diodes/devices (OLEDs), organic phototransistors, organic
photovoltaic cells, and organic photodetectors. For OLEDs, the
organic materials may have performance advantages over conventional
materials. For example, the wavelength at which an organic emissive
layer emits light may generally be readily tuned with appropriate
dopants.
[0006] Generally, an OLED comprises at least one organic layer
disposed between and electrically connected to an anode and a
cathode. When a current is applied, the anode injects holes and the
cathode injects electrons into the organic layer(s). The injected
holes and electrons each migrate toward the oppositely charged
electrode. When an electron and hole localize on the same molecule,
an "exciton," which is a localized electron-hole pair having an
excited energy state, is formed. Light is emitted when the exciton
relaxes via a photoemissive mechanism. In some cases, the exciton
may be localized on an excimer or an exciplex. Non-radiative
mechanisms, such as thermal relaxation, may also occur, but are
generally considered undesirable.
[0007] Several OLED materials and configurations are described in
U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are
incorporated herein by reference in their entirety.
[0008] FIG. 1 is a representation of an organic light emitting
device (OLED) 100 in which each stated component is not drawn to
scale. Device 100 may include a substrate 110, an anode 115, a hole
injection layer 120, a hole transport layer 125, an electron
blocking layer 130, an emissive layer 135, a hole blocking layer
140, an electron transport layer 145, an electron injection layer
150, a protective layer 155, a cathode 160, and a barrier layer
170. Cathode 160 is a compound cathode having a first conductive
layer 162 and a second conductive layer 164. Device 100 may be
fabricated by depositing the layers described, in order. The
properties and functions of these various layers, as well as
example materials, are described in more detail in U.S. Pat. No.
7,279,704 at cols. 6-10, which are incorporated by reference.
[0009] FIG. 2 shows an inverted OLED 200. The device includes a
substrate 210, a cathode 215, an emissive layer 220, a hole
transport layer 225, and an anode 230. Device 200 may be fabricated
by depositing the layers described, in order. Because the most
common OLED configuration has a cathode disposed over the anode,
and device 200 has cathode 215 disposed under anode 230, device 200
may be referred to as an "inverted" OLED. Materials similar to
those described with respect to device 100 may be used in the
corresponding layers of device 200. FIG. 2 provides one example of
how some layers may be omitted from the structure of device
100.
[0010] One application for phosphorescent emissive molecules is a
full color display. Industry standards for such a display call for
pixels adapted to emit particular colors, referred to as
"saturated" colors. In particular, these standards call for
saturated red, green, and blue pixels. Alternatively the OLED can
be designed to emit white light. In conventional liquid crystal
displays emission from a white backlight is filtered using
absorption filters to produce red, green and blue emission. The
same technique can also be used with OLEDs. The white OLED can be
either a single EML device or a stack structure. Color may be
measured using CIE coordinates, which are well known to the
art.
[0011] As used herein, the term "organic" includes polymeric
materials as well as small molecule organic materials that may be
used to fabricate organic opto-electronic devices. "Small molecule"
refers to any organic material that is not a polymer, and "small
molecules" may actually be quite large. Small molecules may include
repeat units in some circumstances. For example, using a long chain
alkyl group as a substituent does not remove a molecule from the
"small molecule" class. Small molecules may also be incorporated
into polymers, for example as a pendent group on a polymer backbone
or as a part of the backbone. Small molecules may also serve as the
core moiety of a dendrimer, which consists of a series of chemical
shells built on the core moiety. The core moiety of a dendrimer may
be a fluorescent or phosphorescent small molecule emitter. A
dendrimer may be a "small molecule," and it is believed that all
dendrimers currently used in the field of OLEDs are small
molecules.
[0012] As used herein, "top" means furthest away from the
substrate, while "bottom" means closest to the substrate. Where a
first layer is described as "disposed over," "provided over," or
"deposited over" a second layer, the first layer is disposed
further away from substrate. There may be other layers between the
first and second layer, unless it is specified that the first layer
is "in contact with," "disposed on," "provided on," or "deposited
on" the second layer. For example, a cathode may be described as
"disposed over" an anode, even though there are various organic
layers in between.
[0013] As used herein "bottom-emitting OLED" refers to an OLED in
which the light emitting from the emissive region (a.k.a. the
active region) of the OLED stack exits the OLED stack through the
glass substrate. In comparison, "top-emitting OLED" refers to an
OLED in which the light emitting from the emissive region of the
OLED stack exits the OLED stack in the direction away from the
glass substrate, generally through the cathode layer.
[0014] The total external quantum efficiency (EQE), which is the
product of the internal quantum efficiency (IQE) and the
outcoupling efficiency (.eta..sub.out), is regarded as one of the
critical device parameters because it directly describes the amount
of emitted photons per consumed electrical energy. The inherent
layered structure of OLEDs causes a low outcoupling efficiency
since generated photons become trapped in waveguided modes and are
wasted in the excitation of surface plasmon polaritons (SPPs). The
major loss channels for trapped light (beyond the modes trapped in
the substrate (i.e., glass mode)) are waveguide and SPPs. Waveguide
modes propagate tens of micrometers and can be efficiently
scattered out of the device with appropriate outcoupling
structures. In contrast, SPP modes are excited primarily in the
metal cathode, propagate only a few micrometers, and dissipate
before scattering. Thus, improving the extraction of waveguided
modes and preventing SPPs are desired to enhance the external
quantum efficiency of OLEDs.
SUMMARY
[0015] An OLED is disclosed which comprises a transparent substrate
having a first side and a second side, a transparent organic light
scattering layer disposed over the first side of the substrate,
wherein the organic light scattering layer is a continuous layer
having a randomly corrugated surface texture with surface texture
height between 5 nm-10 .mu.m with a lateral feature size of
100-1000 nm. The OLED includes an emissive region disposed on the
organic light scattering layer, the emissive region comprising a
transparent anode, a cathode, and at least one organic emissive
layer between the transparent anode and the cathode. The
transparent anode, the organic emissive layer, and the cathode each
have a randomly corrugated structure produced by the randomly
corrugated surface texture of the underlying transparent organic
light scattering layer and the randomly corrugated structure in the
emissive region extracts waveguided mode.
[0016] An OLED is disclosed which comprises a transparent substrate
having a first side and a second side, an emissive region disposed
over the first side of the transparent substrate. The emissive
region comprises a transparent first electrode disposed over the
transparent substrate, at least one organic emissive layer disposed
over the transparent first electrode; and a transparent second
electrode disposed over the at least one organic emissive layer, an
optical grating layer having a grating structure having a
sub-wavelength periodicity disposed on the transparent second
electrode, and a reflective layer disposed over the optical grating
layer.
[0017] A TEOLED is disclosed which comprises a substrate having a
first side and a second side, a reflective layer disposed over the
first side of the substrate, a grid layer consisting of two
optically transparent materials with different refractive indices
disposed on the reflective layer, a transparent first electrode
provided over the grid layer, an organic emissive layer provided
over the transparent bottom electrode, and a transparent second
electrode provided over the organic emissive layer, wherein the
grid layer scatters trapped waveguided modes from the organic
emissive layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 shows an organic light emitting device.
[0019] FIG. 2 shows an inverted organic light emitting device that
does not have a separate electron transport layer.
[0020] FIG. 3A shows the structure of a sub-anode grid.
[0021] FIG. 3B shows the fraction of power (at 550 nm) vs. ETL
thickness, along with measured performance of the sub-anode grid;
the upper (grey) portion corresponds to fraction of light coupled
into SPPs.
[0022] FIG. 4 shows a modal analysis of a WOLED. The solid dashed
and dotted lines correspond to the power distribution of spectral
peaks at wavelengths of 460 nm, 510 nm and 610 nm. The shaded areas
show the optical total WOLED power distribution according to the
ration of B:G:R=0.25:0.3:0.45. The brown circles show the white
spectrum due to comparable light extraction efficiencies of the
three colors.
[0023] FIG. 5A shows a schematic of WOLED with organic scattering
layer.
[0024] FIG. 5B shows a laser interference microscopic image of
rubene deposited using OPVD at 0.5 nm/s at 0.1 Torr.
[0025] FIG. 5C shows a laser interference microscopic image of
rubene deposited using OPVD at 0.5 nm/s at 10 Torr.
[0026] FIG. 6 shows a complete WOLED showing sub-anode 2D TiO2 grid
for outcoupling waveguide and SPP modes, glass substrate (faintly
shaded), and diffusing microlens array.
[0027] FIG. 7 is a schematic illustration of a bottom-emitting OLED
stack 700 that is provided with a shallow, non-perturbative
periodic nanoscale optical grating structure according to the
present disclosure.
[0028] FIG. 8 shows simulation data comparing the EQE of the
bottom-emitting OLED structure 700 of FIG. 7 against the EQE of a
control device that has the same OLED stack layers without the
optical grating structure.
[0029] FIG. 9 shows the sequence of fabricating PDMS flexible mold
for stamping the optical grating structure onto an organic layer in
the OLED stack.
[0030] FIGS. 10A and 10B are illustrations of an TEOLED configured
for extracting light trapped in the organic and ITO layers using a
patterned grid placed between a substrate and an electrode.
[0031] FIG. 11A shows the outcoupling efficiency vs. bottom ITO
thickness.
[0032] FIG. 11B shows the structure simulated using COMSOL
simulation software.
[0033] FIG. 12A shows the dependence of outcoupling efficiency on
grid layer thickness.
[0034] FIG. 12B shows the device structure.
[0035] FIGS. 13A, 13B, and 13C show the experimental data of a
device architecture shown in FIG. 13D which incorporates the grid
structure and a control device. PEDOT:PSS thickness is 100 nm.
Organic layer from substrate side: 40 nm TAPC/30 nm CBP:Ir(ppy)3/60
nm Bphen/10 nm BPhen doped with lithium. The control device has the
same structure as FIG. 13D but without the grid layer. FIG. 13A
shows the electrical characteristics of the two devices. FIG. 13B
is a plot of the EQE for the two devices. FIG. 13C shows the
spectra of the devices. The blue line is the control device and the
red line is the grid.
[0036] FIGS. 14A, 14B, and 14C illustrate other possible geometry
for the sub-anode grid.
[0037] FIG. 15 shows a schematic cut away view of TEOLED with
indium zinc oxide (IZO)/MoO3 electrodes and a metal-coated
subelectrode grid. The low refractive index antireflection layer
reduces microcavity effects. The Au layer bonds the subelectrode
grid to the substrate.
[0038] FIG. 16A is a plot of the modal power distribution vs.
spacer layer thickness. The refractive index of the spacer is
1.5.
[0039] FIG. 16B shows SPP magnetic field intensity across the
control device structure with different refractive indexes, n. Also
shown is the SPP mode propagation length, .delta.. The structure is
Ag/656 nm dielectric with variable n/60 nm IZO and
MnO.sub.3/organic layers (grid line area of the metal-coated grid).
Emission layer position is denoted by the red dotted line.
[0040] FIG. 16C shows field intensity over deeper grid region, with
dielectric layer thickness of 245 nm and n=1.5 (depression area of
the metal-coated grid).
[0041] FIGS. 17A, 17B, and 17C are color maps showing modal power
distributions within the cavities. Here, u is the in-plane
component of the wavevector, k.sub..parallel., for light
propagating within the organic layers with a refractive index of
n.sub.org, normalized to the wavevector k itself. The waveguide
modes, and SPP modes at u>n.sub.air/n.sub.org, of the cavities
over the grid lines is shown in FIG. 17A. The waveguide modes, and
SPP modes at u>n.sub.air/n.sub.org, of the cavities over the
depression is shown in FIG. 17B. The power distribution of a
conventional TEOLED optimized over the same spectral range, with
the structure Ag/90 nm organic layers/20-nm-thick top Ag layer,
with a 20 nm thick EML centered in the organic active region is
shown in FIG. 17C. The plots above each color map are the power
distributions at a wavelength of .lamda.=540 nm.
[0042] FIG. 18A is a color map of the simulated angle and
wavelength dependence of the control device without a 70-nm-thick
low-refractive (n=1.37) antireflection (AR) coating.
[0043] FIG. 18B is a color map of the simulated angle and
wavelength dependence of the control device with a 70-nm-thick
low-refractive (n=1.37) antireflection (AR) coating.
[0044] FIG. 19A is a plot of current density-voltage
characteristics and (inset) the angular intensity profiles of the
control (black) and metal-coated grid (red) devices.
[0045] FIG. 19B is a plot of EQE of the control and metal-coated
grid devices.
[0046] FIG. 19C shows the emission spectra of the control device
with a MgF.sub.2 AR coating at 0.degree., 30.degree., and
60.degree. with 2.degree. error.
[0047] FIG. 19D shows the emission spectra of the metal-coated grid
device with a MgF.sub.2 AR coating at 0.degree., 30.degree., and
60.degree. with 2.degree. error.
[0048] FIG. 20A-20C show the fabrication sequence of the
metal-coated grid. FIG. 20D shows an atomic force microscope image
of the grid surface. There is deformation over the grid line area
(indicated by dashed lines), and the root-mean-square surface
roughness is 1.2 nm. The height difference due to the deformation
is <5 nm.
DETAILED DESCRIPTION
[0049] The OLED structures of interest can deviate from the common
layered structure illustrated in FIGS. 1 and 2. For example, the
substrate may include an angled reflective surface to improve
out-coupling, such as a mesa structure as described in U.S. Pat.
No. 6,091,195 to Forrest et al., and/or a pit structure as
described in U.S. Pat. No. 5,834,893 to Bulovic et al., which are
incorporated by reference in their entireties.
[0050] Unless otherwise specified, any of the layers of the various
embodiments may be deposited by any suitable method. For the
organic layers, preferred methods include thermal evaporation,
ink-jet, such as described in U.S. Pat. Nos. 6,013,982 and
6,087,196, which are incorporated by reference in their entireties,
organic vapor phase deposition (OVPD), such as described in U.S.
Pat. No. 6,337,102 to Forrest et al., which is incorporated by
reference in its entirety, and deposition by organic vapor jet
printing (OVJP), such as described in U.S. Pat. No. 7,431,968,
which is incorporated by reference in its entirety. Other suitable
deposition methods include spin coating and other solution based
processes. Solution based processes are preferably carried out in
nitrogen or an inert atmosphere. For the other layers, preferred
methods include thermal evaporation. Preferred patterning methods
include deposition through a mask, cold welding such as described
in U.S. Pat. Nos. 6,294,398 and 6,468,819, which are incorporated
by reference in their entireties, and patterning associated with
some of the deposition methods such as ink-jet and OVJD. Other
methods may also be used.
[0051] Devices fabricated in accordance with embodiments of the
invention can be incorporated into a wide variety of electronic
component modules (or units) that can be incorporated into a
variety of electronic products or intermediate components. Examples
of such electronic products or intermediate components include
display screens, lighting devices such as discrete light source
devices or lighting panels, etc. that can be utilized by the
end-user product manufacturers. Such electronic component modules
can optionally include the driving electronics and/or power
source(s). Devices fabricated in accordance with embodiments of the
invention can be incorporated into a wide variety of consumer
products that have one or more of the electronic component modules
(or units) incorporated therein. A consumer product comprising an
OLED that includes the compound of the present disclosure in the
organic layer in the OLED is disclosed. Such consumer products
would include any kind of products that include one or more light
source(s) and/or one or more of some type of visual displays. Some
examples of such consumer products include flat panel displays,
computer monitors, medical monitors, televisions, billboards,
lights for interior or exterior illumination and/or signaling,
heads-up displays, fully or partially transparent displays,
flexible displays, laser printers, telephones, mobile phones,
tablets, phablets, personal digital assistants (PDAs), wearable
devices, laptop computers, digital cameras, camcorders,
viewfinders, micro-displays (displays that are less than 2 inches
diagonal), 3-D displays, virtual reality or augmented reality
displays, vehicles, video walls comprising multiple displays tiled
together, theater or stadium screen, and a sign. Various control
mechanisms may be used to control devices fabricated in accordance
with the present invention, including passive matrix and active
matrix. Many of the devices are intended for use in a temperature
range comfortable to humans, such as 18 degrees C. to 30 degrees
C., and more preferably at room temperature (20-25 degrees C.), but
could be used outside this temperature range, for example, from -40
degree C. to +80 degree C.
[0052] Applicant describes a number of solutions for enhancing the
extraction of waveguided mode and suppressing SPP mode in OLEDs are
disclosed. In one embodiment, a sub-anode grid with thick electron
transport layer (ETL) is utilized. In another embodiment, a
corrugated textured (i.e. rough) transparent organic layer is
introduced between the anode and the glass substrate. The
corrugation is random and has a low profile. In a third embodiment,
a planarized optical grating is provided between the anode and the
glass substrate coupled with an optical diffuser at the
substrate-air interface. Moreover, any one or more of the above
respective embodiments can be used in combination. Also, any one,
or combination, of the described embodiments can be coupled with a
substrate light extraction strategy, such as microlens array, to
achieve external quantum efficiency greater than 70%.
[0053] In theory, electrophosphorescent organic light emitting
devices (PHOLEDs) can yield 100% internal quantum efficiency (IQE).
However, even in such an instance, one at best can hope to achieve
an external quantum efficiency of EQE-20% on conventional glass
substrates in the absence of an outcoupling strategy. Much of the
generated light remains trapped within the substrate due to total
internal reflection at the glass-air interface, trapped within the
organic material layers and the transparent anode due to their high
refractive indices compared to glass, and/or dissipates at the
organic/cathode interface by exciting SPPs. The optical power
trapped inside the active region excites two different modes: the
waveguide mode (power guided within the organic layer and
transparent anode), and SPPs consisting of power confined at the
metal/organic interface. The waveguide mode propagates tens of
microns before dissipation, whereas SPPs can survive only
microns.
[0054] To account for some of the internal reflection at the glass
substrate-air interface, microlens arrays can be used. The arrays
outcouple the majority of substrate mode photons, but have no
effect on optical power confined within the high-index organic and
anode regions (waveguide modes), or at the metal/organic interface
(SPPs). The waveguided light can be extracted by inserting a planar
grid layer consisting of two transparent materials with different
refractive indices between the indium tin oxide (ITO) anode and
glass substrate (called a sub-anode grid, see FIG. 3A). Outcoupling
by this grid (whose spacing is significantly greater than the
wavelength of interest) has minimal impact on wavelength and
viewing angle. Also, by positioning the grid external from or
outside of the OLED's active region, the approach allows for
complete freedom in varying its dimensions and materials without
impacting the optical and electrical characteristics of the OLED.
Hence, both the grid and the OLED can be independently optimized,
separate from an optimized organic device structure to deliver
optimal external quantum efficiencies.
[0055] However, as indicated in FIG. 3B, a large fraction of the
light (.about.40%) still remains lost to surface plasmons.
Disclosed herein are Applicant's solutions for extracting light
from OLEDs by means that are non-intrusive to the active OLED
structure. The solutions involve making of a substrate for front
plane organic film deposition, and the solutions are wavelength
and/or viewing angle independent. By combining sub-anode SPP
extraction solution disclosed herein with microlens arrays to
diffuse and outcouple all of the glass modes, up to 80% of the
total light generated in the active OLED emission region can be
extracted providing the needed increase in OLED efficiency.
[0056] FIG. 4 shows the calculated optical power distribution
(assuming IQE=100%) inside a white organic light emitting diode
(WOLED) as a function of ETL thickness. In this example WOLED, the
color rendering index (CRI) is equal to 91 and correlated color
temperature (CCT) is equal to 3100K based on the emission of known
Blue, Green, and Red iridium or platinum phosphorescent emitters
used in OLED displays and lighting. The lines correspond to the
power distribution of each spectral peak at wavelengths of 460 nm
(solid), 510 nm (dashed), 610 nm (dotted). The shaded areas show
the optical total WOLED power distribution according to the ratio
of B:G:R=0.25:0.3:0.45. The different area shows the different
modes of the device. The circles show the white spectrum due to
comparable light extraction efficiencies of the three colors. The
inset shows the structure of the WOLED. The modal power
distributions vary for the different wavelengths, especially for
air and substrate modes. The sub-anode grid is capable of
extracting all optical power but SPPs (dotted line in FIG. 4)
without distortion of the original spectrum of the emitters. In
fact, one can achieve a light extraction efficiency of .about.70%
or more. As the dotted line in FIG. 4 demonstrates, a good white
color balance and high light extraction efficiency is attainable at
electron transport layer (ETL) thicknesses of 130 nm or 260 nm,
where the efficiency of each phosphor is comparable. As disclosed
herein, the SPPs can be decoupled by using thick,
conductivity-doped ETLs to achieve high luminance efficiency even
at ETL thicknesses approaching 260 nm. The preferred ETL thickness
is at least 30 nm. In other embodiments, the ETL can have a
thickness of at least 50 nm, 75 nm, 100 nm, 125 nm, 150 nm, 175 nm,
or at least 200 nm and no more than 400 nm. Organic dopants such as
Li-quinoline (Liq) and Li doped into Alq3 are both known for
reducing layer resistance. The thick ETL is shown to be helpful in
reducing shorts, and hence increasing device yield for large area
PHOLEDs.
[0057] [Random Corrugated Texture]
[0058] According to another aspect of the present disclosure, a
bottom-emitting OLED is disclosed which comprises a transparent
substrate having a first side and a second side, a transparent
organic light scattering layer disposed over the first side of the
substrate, wherein the organic light scattering layer is a
continuous layer having a randomly corrugated surface texture with
surface texture height between 5 nm-10 .mu.m with a lateral feature
size of 100-1000 nm. The OLED includes an emissive region disposed
on the organic light scattering layer, the emissive region
comprising a transparent anode, a cathode, and at least one organic
emissive layer between the transparent anode and the cathode,
wherein the transparent anode, the organic emissive layer, and the
cathode each have a randomly corrugated structure stemming from the
randomly corrugated surface texture of the underlying transparent
organic light scattering layer. The randomly corrugated structure
in the emissive region extracts waveguided mode.
[0059] An example of such bottom-emitting OLEDs incorporating a
random but very low profile corrugated textured organic layer is
introduced in a sub-anode position is illustrated in FIG. 5A. FIG.
5A shows a schematic illustration of such a bottom-emitting OLED
structure 500. The OLED structure 500 comprises a glass substrate
510 and an OLED active region comprising a transparent anode 520,
organic emissive layer 530, and a metal cathode 540. An organic
light scattering layer 515 having a randomly corrugated surface
texture is provided on the glass substrate 510 between the
transparent anode 520 and the substrate 510. The random corrugation
on the organic light scattering layer 515 is formed by randomly
arranged surface features produced by the deposition process for
the organic light scattering layer 515.
[0060] The corrugated texture of this sub-anode organic light
scattering layer 515 is replicated in the subsequently deposited
active region layers 520, 530, 540 thus introducing roughness to
the layer interfaces. This texture does not change the photon state
density in the active region and, thus, the emission spectrum is
not distorted, and the power distribution is comparable to a
conventional device (cf. FIG. 4). The resulting active region,
together with the randomly corrugated organic light scattering
layer 515 form a waveguide structure. The generated photons trapped
in the waveguided mode will be extracted out of the active region
by the bends and the structural interruptions at the layer
interfaces produced by the random corrugation texture in the
waveguide structure. Corrugations at the metal cathode surface
scatter the power in the waveguide mode as well as prevent coupling
to SPPs. Furthermore, light trapped in the glass substrate (glass
modes) also will be extracted by the corrugated surface and
reflected back by the metal cathode surface acting as a mirror, to
further improve light extraction as well. By incorporating the
random corrugation in the active region, inventors believe that
light extraction efficiency can be improved to greater than
70%.
[0061] The dimensional size of the random corrugation features can
be important. If the corrugation feature size is too large, it will
introduce diode leakage current. If the corrugation feature size is
too small, it will not efficiently extract light. The height of the
surface texture of the randomly corrugated organic light scattering
layer 515 is between 5 nm-10 .mu.m with a lateral feature size of
100-1000 nm. Preferably, the surface texture height is between
5-500 nm. More preferably, the surface texture height is between
5-300 nm. Most preferably the surface texture height is 10 nm.
Preferably, the lateral feature size of the randomly corrugated
surface texture is approximately 400 nm.
[0062] Interestingly, Applicant learned that unlike periodic
structures, the randomly corrugated structure of the present
disclosure do not have wavelength dependency nor angle dependency.
Therefore, the lateral feature size of the corrugation can be small
to outcouple the trapped light by scattering before the light
unretrievably dissipates.
[0063] Two fabrication methods are described for depositing the
randomly corrugated organic light scattering layer 515 on the glass
substrate prior to ITO transparent anode 520 sputter deposition
using organic vapor phase deposition (OVPD). FIGS. 5B and 5C show
the example growth of rubrene nano-pillars in the sub-ITO region
where the features are controlled by deposition conditions. Rubrene
was used here to illustrate that OVPD process can be used to
deposit small molecule organic film having randomly corrugated
surface texture. Some organic material candidates that are UV
absorbing and do not absorb visible light spectrum that can be used
for the random corrugated organic light scattering layer 515 are
examples such as NPD and many hole conductors that can easily also
tolerate subsequent deposition of ITO. Another example method for
depositing the randomly corrugated organic light scattering layer
515 is spin-coating a thin film of the organic material on the
glass substrate and then stamped with a mold, cured together with a
"strain-buckled" mold and then peeled off the mold. The mold
fabrication is as follows: a thin Ag film (.about.10 nm) is first
deposited and then baked. Due to differences in surface energies,
the Ag film spontaneously forms randomly arranged islands. Then
another thin glass film is deposited over the Ag islands thus
creating a mold with the glass film. This glass film mold is used
to stamp the spin-coated organic film replicating the randomly
distributed Ag islands. Since the texture height is on the order of
10 nm with lateral feature size of several 100 nm, the aspect ratio
of the texture is small .about.0.1, which is quite easy to
form.
[0064] [Optical Grating+Mirror Provided on Topside of an OLED in a
BEOLED]
[0065] A bottom-emitting OLED is disclosed which comprises a
transparent substrate having a first side and a second side, an
emissive region disposed over the first side of the transparent
substrate. The emissive region comprises a transparent first
electrode disposed over the transparent substrate, at least one
organic emissive layer disposed over the transparent first
electrode; and a transparent second electrode disposed over the at
least one organic emissive layer, an optical grating layer having a
grating structure having a sub-wavelength periodicity disposed on
the transparent second electrode, and a reflective layer disposed
over the optical grating layer.
[0066] Optical band gap structures or optical grating structure
embedded near or inside the bottom-emitting OLED active region
inhibit SPPs and enhance extraction of photons from waveguided
mode. The optical grating structure is non-perturbative (i.e., does
not introduce rough texture or corrugation to the OLED stack) and
has a periodic structure with nanoscale periods. Although the
provision of the optical grating structure within the OLED stack
near or inside the OLED's active region introduces pronounced
sensitivity to wavelength and angular emission, the sensitivity can
be completely eliminated by using an external diffuser structures
at the glass-air interface. The optical gratings near the active
regions then scatter and extract light from waveguided mode into
the glass substrate, ultimately resulting in the desired Lambertian
angular emission profile. The enhancement achieved by the optical
grating is attributed to a fundamentally different effect from the
described sub-anode grid structure and the corrugated OLED stack
structure because the optical grating prevents the generated
photons from exciting SPPs to begin with. The optical grating
proximal to the OLED active region eliminates the waveguided mode
and SPP modes, yet enhancing optical power directed into the
substrate, thereby eliminating all but intrinsic absorption losses.
A feature size of approximately 300 nm is needed to direct the
waveguided modes into the light cone of the air mode to achieve an
efficiency greater than 70%. The height of the grating structure is
approximately 10 nm, resulting in a minimal perturbation of the
OLED structure.
[0067] An example of such OLED incorporating an optical grating and
a reflective layer (a mirror) provided on the topside of a
bottom-emitting OLED is illustrated in FIG. 6. In the embodiment
shown in FIG. 6, the sub-anode optical grating 615 (unlike the grid
in FIG. 3) is provided buried into or fabricated directly into the
glass substrate at the glass substrate-anode interface. The optical
grating 615 can be fabricated directly into the glass substrate 610
by interference lithography. A layer of photoresist is deposited
over the glass substrate 610 then patterned with recesses
representing the grating pattern 615 via interference lithography.
The glass substrate 610 is then etched through the patterned
recesses in the photoresist. Then a transparent semiconductor
material such as TiO.sub.2 is blanket deposited over the substrate
filling the etched pattern of recesses in the glass substrate 610.
The photoresist and the excess TiO.sub.2 is removed in a liftoff
step then the glass substrate 610 is planarized and polished
leaving behind a surface in which the optical grating is patterned
with TiO.sub.2. Next, the transparent ITO anode 620 and organic
layers of the active region 630 are deposited onto the glass
substrate with the embedded optical grating 615. The cathode 640 is
then deposited onto the active region 630. A diffuser such as
microlens array 605 can be incorporated at the glass substrate-air
interface to achieve wavelength independence and to extract light
from glass mode.
[0068] Referring to FIG. 7, in another embodiment, the shallow,
non-perturbative periodic nanoscale optical grating structure 740
is provided in proximity of the active region but on the side
opposite from the glass substrate in the bottom-emitting OLED 700.
The nanoscale optical grating structure 740 is provided in
combination with a metal mirror to reflect and redirect the stray
light extracted from the waveguided mode toward the bottom side
(i.e. the glass substrate side) of the OLED stack 700. The OLED
stack 700 comprises a glass substrate 710. An emissive region
provided above the glass substrate where the emissive region
comprises a first electrode (ITO) layer 715 and a second electrode
(IZO) layer 725, and an organic emissive layer 720 disposed between
the two electrode layers. Provided over the second electrode
(indium zinc oxide (IZO)) layer 725 is an organic layer 730. Formed
on top of the organic layer 730 is the shallow, non-perturbative
periodic nanoscale optical grating structure 740 and the metal
mirror layer 750. In addition to the optical grating 740 extracting
the waveguided mode, the metal mirror layer 750 also prevents
SPPs.
[0069] The optical grating 740 and the mirror layer 750 can be
formed from the typical cathode material such as Al and Ag metal.
Other reflective metal can be used for the mirror layer. The
optical grating 740 has subwavelength grating periodicity. The
periodicity of the grating needs to be subwavelength to cause
optical interference.
[0070] FIG. 8 shows simulation data (using COMSOL Multiphysics
software) comparing the EQE of the bottom-emitting OLED structure
700 against the EQE of a control device which had the same OLED
stack structure as the structure 700 but without the optical
grating 740. The devices were cavity tuned for green emission and
the simulation was run for emission wavelengths of 460 nm, 510 nm,
and 600 nm. The solid lines (1), (2), and (3) represent the EQE of
the control device at the emission wavelengths 460 nm, 510 nm, and
600 nm, respectively. The EQE of the OLED structure 700 at the same
emission wavelengths are shown by the data points represented by
.box-solid. for 460 nm, .tangle-solidup. for 510 nm, and
.circle-solid. for 600 nm. The EQE data for the OLED structure 700
was generated for optical grating period of 300 nm, 400 nm, and 500
nm. The data shows substantial increase in EQE for the OLED
structure 700. The data also shows that the grating period can be
used to optimize different emission wavelengths. The emission
wavelength of 460 nm exhibited the highest EQE at the grating
period of 300 nm. The emission wavelength of 510 nm also exhibited
the highest EQE at the grating period of 300 nm. The emission
wavelength of 600 nm exhibited the highest EQE at the grating
period of 500 nm.
[0071] In the simulation, the OLED stack had the following layers:
the substrate/100 nm of ITO/190 nm of the emissive organic layer/50
nm of ITO/200 nm of organic layer/Al for the optical grating and
mirror. The depth (or height) of the optical grating structure 740
into the 200 nm thick organic layer 730 was 120 nm.
[0072] According to an aspect of the present disclosure, FIG. 9
shows an example of the method for fabricating the optical grating
structure 740 that utilizes flexible polydimethylsiloxane (PDMS)
mold to stamp the optical grating structure 740 directly onto the
organic layer 730. First, a layer of photoresist 915 is deposited
on a temporary substrate 910 and patterned to create recesses 920
having the desired optical grating pattern. (See FIG. 9, step (a)).
Next, a layer of PDMS 925 is deposited over the photoresist 915
filling the optical grating pattern recesses 920. (See FIG. 9, step
(b)). Thus, the PDMS forms a mold 925 for the optical grating
pattern. Next, the PDMS mold 925 is removed from the temporary
substrate 910 via a photoresist liftoff process and the PDMS mold
925 is used to stamp the optical grating structure pattern directly
onto the organic layer 730. (See FIG. 9, step (c)). Next, a metal
such as Al used for cathodes is deposited over the patterned
organic layer 730 to form the optical grating structure 740 and the
metal mirror layer 750 resulting in the OLED stack 700. (See FIG.
9, step (d)). The
[0073] Using the methods just described, the optical gratings with
a feature size in the order of 300 nm can be readily
fabricated.
[0074] [Patterned Grid+Mirror in a TEOLED]
[0075] A TEOLED is disclosed which comprises a substrate having a
first side and a second side, a reflective layer disposed over the
first side of the substrate, a grid layer consisting of two
optically transparent materials with different refractive indices
disposed on the reflective layer, a transparent first electrode
provided over the grid layer, an organic emissive layer provided
over the transparent bottom electrode, and a transparent second
electrode provided over the organic emissive layer, wherein the
grid layer scatters trapped waveguided modes from the organic
emissive layer.
[0076] An example of such TEOLEDs configured for extracting light
trapped in the organic and ITO layers using a patterned grid placed
between a substrate and an electrode is illustrated in FIGS. 10A
and 10B. In a conventional TEOLED architecture, light is produced
within the organic layers (the emissive region) and is emitted
through the transparent electrode (mostly thin metal films) into
the air away from the carrier substrate. Because of the high
refractive index of the organic layers, a large portion of the
produced light is trapped in so-called waveguide modes which reside
mainly in these layers. The waveguide mode light travels parallel
to the layer and is eventually lost to material absorption. By
coupling this light out of the device one can significantly enhance
efficiency and operational lifetime of the OLED.
[0077] Spectral narrowing and a pronounced angular dependence of
the emission characteristics result in reduced efficiency and color
quality for white TEOLEDs, compared to their bottom-emitting
counterparts. Typical methods of enhancing efficiency of TEOLEDs
include using microcavity effect, through the use of silver
electrode or capping layer, for example, and introducing a rough
layer right next to the emissive region. However, using microcavity
effect can only enhance efficiency by limited amount,
leaving>30% optical power generated in the organic layers. Until
now, no effective ways to extract these fraction of light were
known.
[0078] A grid consisting of two optically transparent materials
having different refraction index can be used to extract the
waveguided optical power in TEOLEDs. Instead of using silver or
aluminum as bottom-side electrode, the typical cathode material
silver or aluminum is used only as a mirror and ITO or other
transparent conductor is used as the bottom-side electrode. Such
TEOLED stack 1000 is shown in FIG. 10A. The TEOLED stack is formed
on a glass carrier substrate 1010. The active emissive region is
comprised of a transparent first electrode (e.g. ITO, IZO, etc.)
1030 and a transparent second electrode (e.g. ITO, IZO, etc.) 1050
with an emissive organic layer 1040 provided between the two
electrode layers. A metal mirror (e.g. Al or Ag film) 1015 is
provided on the glass substrate 1010. A grid 1020 consisting of two
optically transparent materials with different refractive indices
is positioned between the metal mirror 1015 and the transparent
bottom electrode 1030. In a typical TEOLED stack, the top electrode
1050 is a cathode and the bottom electrode 1030 is an anode. Thus,
the grid 1020 can be referred to as a sub-electrode grid. The grid
1020 is constructed of two optically transparent materials having
different refractive index where the materials for the grid can be
conductive. In this way, the grid can be a part of or an extension
of the bottom electrode 1030. In one embodiment, the first material
is SiO.sub.2 and the second material is TiO.sub.2. The grid layer
1020 scatters the trapped waveguided modes from the organic layers.
The metal mirror 1015 is positioned at a distance at least 100 nm
from the organic emissive layer 1040 to prevent or inhibit
excitation of SPPs. In some embodiments the metal mirror 1015 is
positioned at a distance greater than 100 nm from the organic
emissive layer 1040. Inventors believe that the provision of the
combination of the sub-electrode grid and the metal mirror in a
TEOLED stack according to the present disclosure is able to extract
substantial amount of waveguide mode efficiently. The TEOLED stack
1000 can include a spacer layer 1025 whose thickness can be varied
for tuning the cavity effects. The spacer layer thickness is one
parameter that can be used to tune the cavity resonant frequency
based on the emitter color. This adds one more degree of freedom to
enhance the efficiency.
[0079] As shown in FIG. 10B, the grid layer 1020 is formed of two
transparent materials having different refractive indices where one
material forms a grid structure within (or embedded in) the second
material as the host. The refractive index of the host material and
the embedded grid can be varied independently. Moreover, all of the
geometric parameters of the grid (width, periodicity, height, and
shape) can be varied. The thickness and refractive index of the
grid layer can be used to tune the optical field (i.e. shape and
distribution of the waveguide modes) within the OLED, while the
parameters of the grid are optimized for maximum scattering of
light from the waveguide.
[0080] Referring to FIGS. 11A and 11B, inventors calculated the
outcoupling efficiency vs. bottom ITO thickness for a control
TEOLED device (i.e, with a Ag minor layer but without the grid
layer) shown in FIG. 11B by varying the bottom ITO thickness and
using a COMSOL simulation software. The plot in FIG. 11A shows that
without introducing the grid layer, the OLED achieved the maximum
outcoupling efficiency of 25% with ITO of 180 nm thick.
[0081] Based on that information, inventors formulated a TEOLD
structure shown in FIG. 12B including the grid layer according to
the present disclosure. The device was configured with the optimal
bottom ITO thickness of 180 nm. The grid layer consisted of two
transparent dielectric materials M1 and M2 which were SiO.sub.2 and
TiO.sub.2, respectively. The plot of the outcoupling efficiency as
a function of the grid layer thickness in FIG. 12A shows the
dependence of outcoupling efficiency on the grid layer thickness.
As shown, with the exception of the three data points at grid
thickness of 100 nm, 160 nm, and 180 nm, the inventive TEOLED
exhibited outcoupling efficiency exceeding 25% which was the
maximum outcoupling efficiency exhibited by the control device of
FIG. 11B. This confirmed the beneficial effects of providing the
combination of a mirror layer and a grid layer below the bottom
transparent electrode in a TEOLED.
[0082] FIGS. 13A-13C show the experimental data of a TEOLED
architecture shown in FIG. 13D which incorporates the sub-electrode
grid and Al minor layer and a control device. The spacer layer
between the grid layer and the bottom transparent electrode (ITO)
was 50 nm of SiO.sub.2. The 100 nm PEDOT layer provided in this
example is a hole injection layer. Organic emissive layer was
structured as follows (listing from the substrate side): 40 nm
TAPC/30 nm CBP:Ir(ppy)3/60 nm Bphen/10 nm BPhen doped with lithium.
The control device was configured to have the same structure as
FIG. 13D but without the grid layer. FIG. 13A shows the electrical
characteristics of the two devices. FIG. 13B is a plot of the EQE
for the two devices. FIG. 13C shows the spectra of the devices. The
blue line is the control device and the red line is the grid.
[0083] In some embodiments, the pattern of the grid in the grid
layer comprises a plurality of space-filling polyhedra
substantially aligned in rows and columns. In one embodiment, sides
of the space-filling polyhedra have a step height of less than 10
nm. In one embodiment, sides of the space-filling polyhedral have a
step height of less than 8 nm. In some embodiments, the
space-filling polyhedra are substantially one of square,
triangular, or hexagonal. In other embodiments, as shown in FIGS.
14A-14C, the grid structure in the grid layer 1020 can be in the
form of a variety of geometry. For example, as shown in FIG. 14A,
the grid lines can be in the form of a polygon, such as a
hexagon.
[0084] The fabrication of the grid layer 1020 can be performed in a
variety of conventional ways including lithography followed by
etching and planarization of the substrate or host material;
nanoimprint patterning of the grid followed by planarization; or
pattering of a void grid followed by lamination of a thin planar
overlayer. Subsequent transparent electrode (ITO) deposition and
OLED fabrication can be done using known conventional means.
[0085] Another embodiment of TEOLED with a configuration to enhance
light extraction is described below. The TEOLED comprises a
transparent conductive oxide on the surface of a nondiffractive,
reflecting metal-coated scattering grid located beneath the organic
active region. The grid scatters light trapped in waveguide modes
without changing the device electrical properties or causing
significant plasmonic losses. This results in an increase in EQE
for green PHOLED devices from 20.+-.1% to 30.+-.2%, for structures
without and with the reflecting grid. Adding a low refractive index
capping layer reduces the spectral angular dependence
characteristic of TEOLEDs. The improvement in light extraction by
substrate modification allows for optimization of the optical
design without necessitating changes in the design or structure of
the OLEDs themselves.
[0086] Unlike bottom-emitting OLEDs, TEOLEDs emit through a
semitransparent electrode into air and do not suffer from optical
power trapping within the substrate. However, the higher
reflectivity of the semitransparent top electrode creates a strong
optical cavity that introduces additional lossy waveguide modes
along with undesirable angle and wavelength dependences of the
emission spectrum. Furthermore, since both electrodes in
top-emitting devices are often composed of metal, it is not
possible to entirely suppress SPP modes using thick organic layers
without also lowering efficiency.
[0087] In one instance, an outcoupling scheme for TEOLEDs can
include replacing both anode and cathode with indium zinc oxide
(IZO)/molybdenum trioxide (MoO3) transparent contacts and placing a
reflective and scattering corrugated metal-coated dielectric mirror
beneath the electrically active organic emissive region (EML). This
is combined with a low refractive index antireflection (AR) layer
to reduce microcavity effects.
[0088] A schematic diagram of the device is shown in FIG. 15. The
design spaces the active organic emissive region away from the
metal reflector (Ag) to minimize coupling to SPP modes while
scattering out the waveguided optical power without disturbing the
planarity of the device itself. Employing a scattering structure
within the substrate while retaining a planar substrate surface
allows complete freedom for optimizing the scattering layer
dimensions without affecting the electrical properties of the OLED.
The Ag reflector is a patterned grid of raised rectangles whose
periodicity is on the order of several wavelengths to avoid angle-
and wavelength-dependent effects. A dielectric spacer (SiO.sub.2)
fills in the depressions between the Ag patterned grid and extends
above the rectangular grid, providing a planar surface for the
subsequent deposition of the electrodes and organic layers. The
thick and thin spacer regions couple differently to the microcavity
modes by locally creating both thick and thin cavity regions
beneath the electrode.
[0089] To optimize the light extraction from the two cavity
regions, the effects of the spacer (SiO.sub.2) thickness are
determined using Green's function analysis, as shown in FIG. 16A.
FIG. 16A shows the modal power distribution vs. spacer layer
thickness. The refractive index of the spacer is 1.5. The EML
region used in simulating this structure comprises a 130-nm-thick
organic layer (norg, (the refractive index of the organic
layer)=1.8) sandwiched between 80-nm-thick transparent electrodes
(50 nm IZO and 30 nm MoO.sub.3, the refractive indices nIZO and
nMoO.sub.3 are=2), with the emission layer (EML) placed 60 nm above
the bottom electrode. A randomly oriented green-emitting molecular
dipole is placed at the position of the EML. The spacer refractive
index is n=1.5. The optical outcoupling efficiency is >20% when
spacer layer thicknesses are 65 and 245 nm. Most of the optical
power is lost into waveguide modes due to the elimination of SPPs,
as shown in FIG. 16A.
[0090] SPP coupling in the thick and thin cavity regions are
determined by calculating the local electric field of the plasmon
mode at the EML, E(z). The emission rate into the SPP mode is found
using Fermi's golden rule:
.GAMMA. ( .omega. ) = 2 .pi. i d E ( z ) f 2 .rho. ( .omega. ) ( 1
) ##EQU00001##
where d denotes the exciton dipole moment, i and f the initial and
final exciton state wave functions, and .rho.( .omega.) the
plasmonic mode density. The distance of the exciton from the metal
interface is z, and E(z) is the electrical field of the SPP mode
normalized to a half-quantum for zero-point fluctuations. The
magnetic field intensity profiles of SPP modes in the OLED cavities
are shown in FIGS. 16B and 16C, found using the dispersion
relation
k SPP 2 = m d m + d ( 2 .pi. .lamda. ) 2 ( 2 ) ##EQU00002##
where wavelength .lamda.=510 nm. Since the skin depths of the modes
are comparable or even smaller than the dielectric thickness, we
assume a semi-infinite metal layer having a dielectric constant
.epsilon..sub.m in contact with a semi-infinite dielectric layer
with dielectric constant .epsilon..sub.d.
[0091] FIG. 16B shows the calculated mode profiles in the regions
where the cavity is thin (corresponding to the areas where the grid
lines are raised) for different values of the index of refraction
of the spacer layer, n. The calculated propagation lengths of SPP
modes (.delta..sub.SPP=1/[2Im(kspp)]) are also shown. Both the
field intensity in the EML whose position is denoted by the black
dotted line and the propagation length decrease as the refractive
index of the spacer layer increases. For comparison, the red dotted
line indicates the distance from the EML to the metal in a
conventional TEOLED. In that case, the local field of the SPP mode
in the EML is larger than in our design (black dotted line),
leading to a faster exciton coupling rate to SPPs. In FIG. 16C, we
observed that the SPP decays before reaching the organic layers in
a thick cavity with n=1.5. The coupling to SPP modes decays
exponentially as the distance increases between the EML and the
metal surface. Thus, SPP coupling can be avoided with nonmetallic
electrodes and the appropriate choice of spacer material and
thickness.
[0092] The simulated modal power distributions of the cavities
using Green's function analysis are shown in FIGS. 17A and 17B. We
define u as the ratio of the in-plane component of the wavevector,
k.sub..parallel., for light propagating in the organic layer with a
refractive index of n.sub.org, to the total wavevector k, i.e.,
u=k.sub..parallel.,/k.
[0093] Thus, the region at u>1 corresponds to evanescent waves
in the near field. Usually, modes with u<n.sub.air/n.sub.org are
radiative, those with n.sub.air/n.sub.org<u<1 are waveguided,
and modes in the region u>1 are SPPs. For the thin cavity, there
are four waveguide modes in the emission spectrum and a weak SPP
mode. The thick cavity adds three more waveguide modes but has no
SPP modes. FIG. 18C shows the power distribution of a conventional
TEOLED optimized over the same spectral range, with the structure
Ag/90 nm organic layers/20-nm-thick top Ag layer, with a
20-nm-thick EML centered in the organic active region. This
structure does not support waveguide modes, but has two SPP modes.
The first SPP mode at u.apprxeq.0.6 and .lamda..apprxeq.540 nm is
supported by the top thin Ag film. According to equation (2) and
given n.sub.air<n.sub.org, this SPP mode lies in the region of
u<1 rather than u>1 for organic/metal SPPs. The second SPP
mode supported by the Ag/organic interface lies at u>1.5. FIGS.
17A-17C show that, compared to conventional devices, both the
control and metal-coated grid devices successfully suppress SPP
modes while coupling more power into the waveguide modes.
[0094] The scattering by the grid is a consequence of the mismatch
between the waveguide modes supported by the two cavity regions
(thick and thin) above the grid lines and depressions, which can be
estimated by the overlap of the wavevectors of these modes. The TE1
and TM1 modes in the thin SiO2 cavity (FIG. 17A) have some overlap
with the TE1 and TM1 modes in the thick cavity (FIG. 17B), and thus
these modes are inefficiently scattered. The modes that are not
aligned between cavities are scattered by the grid. The spacer
thicknesses determine the mismatch of the modes, but freedom to
optimize the spacer thickness is limited when matching the cavity
resonance to the OLED emission spectrum.
[0095] FIG. 18A is a color map of the simulated angle and
wavelength dependence of the control device without a 70-nm-thick
low-refractive (n=1.37) antireflection (AR) coating. FIG. 18B is a
color map of the simulated angle and wavelength dependence of the
control device with a 70-nm-thick low-refractive (n=1.37)
antireflection (AR) coating. The device with the AR coating shows a
broader spectrum and smaller blue shift at larger angles. The
simulated output of a dipole in the thin SiO2 cavity regions using
Green's function analysis, shown in FIG. 18A is strongly angle
dependent. The spectral peak wavelength ranges from .lamda.=545 nm
normal to the plane (0.degree.) to .lamda.=460 nm at 90.degree.. A
70-nm-thick low-refractive AR layer between air and the top IZO
electrode reduces the cavity effects, as is apparent by the spread
in wavelength emission and reduced blue shift with angle in FIG.
18B. An example AR coating material is MgF.sub.2, whose refractive
index is nMgF.sub.2=1.38 in the range of the emission spectrum,
close to the optimized value of n=1.4 for IZO/air interfaces.
Experiment
[0096] An OLED was fabricated using IZO/MoO3 electrodes as the
control and another OLED was fabricated with a disclosed
metal-coated grid, each device having the same active layer
according to the present disclosure. The active layers in the
devices were as follows starting from the substrate: 50 nm IZO/30
nm MoO.sub.3/30 nm 4,7-diphenyl-1,10-phenanthroline
(BPhen):Li(molar 1:1)/30 nm BPhen/30 nm Ir(ppy).sub.3 doped at 8
vol. % in 4,4'-bis(carbazol-9-yl)biphenyl (CBP)/40 nm
4,4'-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine]
(TAPC)/30 nm MoO.sub.3/50 nm IZO. The bottom MoO.sub.3 layer
reduces electrical shorts that penetrate the spacer layer created
by protrusions left behind from the grid lithography process. The
top MoO.sub.3 layer prevents damage to the organic active region
during the IZO sputtering process (discussed below).
[0097] The control device was made with a 150 nm thick, planar Ag
layer covered with a 65 nm SiO.sub.2 spacer layer beneath the
active region. The grid reflector is an array of 1.times.3 .mu.m
raised rectangles surrounding 3.times.3 .mu.m and 1.times.1 .mu.m
square depressions, with the spacer thicknesses given above. Both
devices are capped with a 70 nm thick MgF.sub.2 AR coating.
[0098] The current density--voltage curves of both devices are
identical above turn on (.about.3 V), as shown in FIG. 19A.
Furthermore, .eta..sub.EQE is increased from 20.+-.1% to 30.+-.2%
using the metallic scattering grid, as shown in FIG. 19B.
[0099] The angular intensity profiles of the devices with the AR
layer are broadened from a simple Lambertian emission profile. The
peak intensity of the control device is normal to the surface,
whereas the grid OLED intensity is at a maximum at 20.degree. from
normal. The spectra of the control (FIG. 19C) and the metal-coated
grid (FIG. 19D) OLEDs are shown at 0.degree., 30.degree., and
60.degree.. Both devices have spectral peaks at .lamda..apprxeq.550
nm. Compared with the control, the grid device shows a slightly
increased blue shift at large angles.
[0100] The emission intensity is a function of the overlap between
the cavity resonance and the emission spectrum. The microcavity
resonance peak (at .lamda..apprxeq.540 nm, see FIGS. 18A and 18B)
is red-shifted from the emission spectral peak (.lamda..apprxeq.510
nm). Thus, the blue shift with angle results in broader than the
Lambertian angular intensity profiles for the control and grid
devices shown in FIG. 18A, inset. The broadening of the grid OLED
spectrum is more severe because the capping layer thickness, which
reduces the cavity quality, was optimized for the thin cavity
region of the grid devices, which is the same as the spacer layer
thickness used in the control device.
[0101] Although a significant enhancement in efficiency is obtained
using the reflecting grid, 60% of the optical power is still lost
in the device. In addition to the limited grid scattering
efficiency due to the spacer thickness used, the scattered light
incurs losses at each reflection from the metal surface. A diffuser
film or microlens array comprised of high refractive index
materials added to the top surface of the control devices should
also generate higher efficiencies by reducing the cavity quality
factor. The weaker cavity produced by these strategies is also
beneficial for outcoupling white light. Using the fact that a
microlens array foil extracts more than half of the optical power
going into the foil, the Green's function analysis shows that a
microlens array on the device emitting surface could further
improve the efficiency by at least 30%.
[0102] In summary, a TEOLED with IZO/MoO.sub.3 electrodes has
achieved .eta..sub.EQE=20.+-.1%, with almost no excitation of SPP
modes. The efficiency is increased to 30.+-.2% by using a
metal-coated scattering grid layer beneath the anode without
impacting the OLED electrical characteristics. The efficiency can
be further improved using a microlens array or diffuser on the
device emitting surface. The grid scatters the waveguided power and
reduces plasmonic losses. The metallic scattering grid is
fabricated within the substrate and hence, is totally separate from
the organic active layers, allowing for considerable freedom in
both the OLED and grid optical designs. Note that if the insulating
spacer layer is replaced with a low-resistance transparent metal
oxide, it can be used as conductive layer used for addressing OLED
pixels in an active matrix display. This is just one of several
possibilities allowed by this subelectrode light-scattering
approach.
[0103] The fabrication sequence for the metallic scattering layer
is shown in FIGS. 20A-20C. A 245 nm SiO.sub.2 film 2120 was
deposited by electron-beam evaporation on a glass substrate 2110
precoated with sacrificial lift-off resist 2115 (MicroChem LOR 10B)
(4000 rpm, 180.degree. C.). Photoresist (Microposit S1813)(not
shown) was subsequently coated at 4000 rpm and cured at 115.degree.
C. for 90 s. The pattern was photolithographically defined using an
AutoStep exposure system (GCA AS200) with an exposure time of 0.33
s. The 180 nm deep etch of the SiO.sub.2 film 2120 was done using a
1:1 CF.sub.4/CHF.sub.3 plasma at 100 W. The photoresist was removed
by exposure to oxygen plasma for 3 min at 800 W. Referring to FIG.
20B, then a 150 nm thick Ag film 2125 was deposited by thermal
evaporation after a 2-nm-thick Ti wetting layer (not shown). Next a
5-nm-thick Ge wetting layer (not shown) followed by a 200-nm-thick
Au film 2130A, 2130B was deposited by electron-beam evaporation at
10 .ANG./s onto the surface of a clean glass 2140 and
Ag/SiO.sub.2/glass substrate, respectively. The two glass
substrates were then sealed together via cold-weld bonding by
applying heat (200.degree. C.) and pressure (4 MPa) for 5 min under
vacuum (10-3 Torr) using an EVG 510 wafer bonder. The bonding is
sufficiently robust to survive sonication, although the Au surface
on the grid is irregular due to the SiO.sub.2 trenches, leaving
vacancies at the bonding interface. The bonded glass slabs are
diced into 1.times.1 in. squares, which were soaked in Remover PG
(MicroChem at 80.degree. C.) to dissolve the sacrificial LOR layer
to leave the metallic-coated grid. After the grid preparation, a 50
nm thick IZO layer was deposited at 60 W in a chamber with an Ar
pressure of 2 mTorr at a rate of 0.6 .ANG./s using a radio
frequency magnetron sputterer.
[0104] The control substrate was prepared as follows: A glass
substrate was cleaned using sonication in tergitol, deionized
water, acetone, and isopropyl alcohol (IPA). A 2-nm-thick Ti
wetting layer and 150-nm-thick Ag layer were sequentially deposited
by thermal evaporation, followed by a 65-nm-thick SiO.sub.2 film by
electron-beam evaporation and 50-nm-thick IZO by sputtering (23
ohm/sq). The area was defined by a shadow mask without breaking
vacuum between depositions.
[0105] The IZO-coated substrates were cleaned for 3 min by
sonication in IPA and exposed to ultraviolet-ozone before PHOLED
layer deposition by vacuum thermal evaporation in a system with a
base pressure of 10-7 Torr. The first MoO.sub.3 layer was deposited
at 0.5 A/s and the top MoO.sub.3 layer at 0.05 .ANG./s for the
first 5 nm and at 0.2 .ANG./s for the remaining thickness in the
same chamber as the organic layers. The top IZO electrode was
sputter-deposited in a chamber with an Ar pressure of 5 mTorr at
0.05 .ANG./s for the first 10 nm and 2 mTorr at 0.2 .ANG./s for the
remaining thickness. Finally, the MgF.sub.2 capping layer was
thermally deposited. The refractive indices and thicknesses of
materials were measured using a variable-angle spectroscopic
ellipsometer (J. A. Woollam WVASE32). Current-voltage-luminance
characteristics were collected using a semiconductor parameter
analyzer (HP-4156A) and a calibrated Si photodiode. The
electroluminescence spectra were measured using an Ocean Optics
miniature spectrometer. The .eta..sub.EQE was calculated using
standard methods.
[0106] In the various embodiments of enhancing outcoupling and
inhibiting SPPs disclosed herein, preferably an optical diffuser
structure is provided at the glass substrate-air interface to
achieve wavelength independence and to extract light from the glass
mode. For example, microlens array can be provided at the glass
substrate-air interface. Since the microlens surface angles vary
for different incident positions, the refracted output angles are
also different, making these arrays effective diffusers. Another
example of a diffuser is a nanoscale scattering layer consisting of
a transparent polymer film with a suspension of .about.100 nm
diameter, high index TiO.sub.2 nanoparticles. Because of the
difference in the refractive index, the nanoparticles scatter the
incident photons into random angles.
[0107] In some embodiments, the OLED incorporating the novel
structures disclosed herein has one or more characteristics
selected from the group consisting of being flexible, being
rollable, being foldable, being stretchable, and being curved. In
some embodiments, the OLED is transparent or semi-transparent. In
some embodiments, the OLED further comprises a layer comprising
carbon nanotubes.
[0108] In some embodiments, the OLED further comprises a layer
comprising a delayed fluorescent emitter. In some embodiments, the
OLED comprises a RGB pixel arrangement or white plus color filter
pixel arrangement. In some embodiments, the OLED is a mobile
device, a hand held device, or a wearable device. In some
embodiments, the OLED is a display panel having less than 10 inch
diagonal or 50 square inch area. In some embodiments, the OLED is a
display panel having at least 10 inch diagonal or 50 square inch
area. In some embodiments, the OLED is a lighting panel.
[0109] The OLED disclosed herein can be incorporated into one or
more of a consumer product, an electronic component module, and a
lighting panel. The organic layer can be an emissive layer and the
compound can be an emissive dopant in some embodiments, while the
compound can be a non-emissive dopant in other embodiments.
[0110] It is understood that the various embodiments described
herein are by way of example only, and are not intended to limit
the scope of the invention. For example, many of the materials and
structures described herein may be substituted with other materials
and structures without deviating from the spirit of the invention.
The present invention as claimed may therefore include variations
from the particular examples and preferred embodiments described
herein, as will be apparent to one of skill in the art. It is
understood that various theories as to why the invention works are
not intended to be limiting.
* * * * *