U.S. patent application number 15/901913 was filed with the patent office on 2019-01-31 for excited state management.
The applicant listed for this patent is The Regents of the University of Michigan, The University of Southern California. Invention is credited to Stephen R. FORREST, Michael SLOOTSKY, Mark E. THOMPSON.
Application Number | 20190036060 15/901913 |
Document ID | / |
Family ID | 54702816 |
Filed Date | 2019-01-31 |
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United States Patent
Application |
20190036060 |
Kind Code |
A2 |
FORREST; Stephen R. ; et
al. |
January 31, 2019 |
EXCITED STATE MANAGEMENT
Abstract
Arrangements and techniques for providing organic emissive
layers are provided, in which the emissive layer includes a first
dopant having a dissociative energy level. A second dopant in the
emissive layer provides a solid state sink energy level, to which
doubly excited excitons and/or polarons may transition instead of
to the dissociative energy level, thereby decreasing the
undesirable effects of transitions to the dissociative energy
level.
Inventors: |
FORREST; Stephen R.; (Ann
Arbor, MI) ; SLOOTSKY; Michael; (Ann Arbor, MI)
; THOMPSON; Mark E.; (Anaheim Hills, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Regents of the University of Michigan
The University of Southern California |
Ann Arbor
Los Angeles |
MI
CA |
US
US |
|
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20180190927 A1 |
July 5, 2018 |
|
|
Family ID: |
54702816 |
Appl. No.: |
15/901913 |
Filed: |
February 22, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14724238 |
Mar 27, 2018 |
9929365 |
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15901913 |
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62003591 |
May 28, 2014 |
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62095904 |
Dec 23, 2014 |
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62103483 |
Jan 14, 2015 |
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62108100 |
Jan 27, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C09K 2211/1088 20130101;
C09K 2211/1011 20130101; C09K 2211/1044 20130101; C09K 11/06
20130101; C09K 2211/1007 20130101; H01L 51/5016 20130101; H01L
51/5028 20130101 |
International
Class: |
H01L 51/50 20060101
H01L051/50; C09K 11/06 20060101 C09K011/06 |
Goverment Interests
STATEMENT REGARDING GOVERNMENT RIGHTS
[0002] This invention was made with government support under
Contract No. DOE DESC00001013 awarded by the Department of Energy,
and under FA9550-14-1-0245 awarded by the U.S. Air Force Office of
Scientific Research. The government has certain rights in the
invention.
Claims
1.-18. (canceled)
19. An emissive layer of an opto-electronic device, the emissive
layer comprising: a host material; a first dopant; and a second
dopant that has a solid state sink energy level, wherein the first
dopant is a blue-emitting, phosphorescent dopant that has a
multiply-excited energy level from which doubly excited excitons
and/or polarons can undergo a dissociative reaction, and a triplet
energy level T1 that is lower than the solid state sink energy
level of the second dopant, wherein the solid state sink energy
level is at least 0.2 eV below the multiply-excited energy level of
the first dopant.
20. The emissive layer of claim 19, wherein the multiply-excited
energy level of the first dopant is a doubly-excited triplet energy
level, or a doubly-excited polaron energy level.
21. The emissive layer of claim 19, wherein the concentration of
the first dopant is graded within the host material.
22. The emissive layer of claim 19, wherein the second dopant
absorbs light in the ultraviolet region of the electromagnetic
spectrum.
23. The emissive layer of claim 22, wherein the second dopant
comprises 1,4,5,8-naphthalenetetracarboxylic dianhydride
(NTCDA).
24. The emissive layer of claim 21, wherein the emissive layer is
disposed between an electron-transport layer (ETL) and a
hole-transport layer (HTL), and the concentration of the first
dopant is graded from about 20% at the HTL/emitting layer interface
to about 8% at the ETL/emitting layer interface.
25. The emissive layer of claim 19, wherein the opto-electronic
device is a white-emitting OLED (WOLED).
26. The emissive layer of claim 19, wherein the opto-electronic
device is selected from the group consisting of: a flat panel
display, a computer monitor, a medical monitor, a television, a
billboard, a light for interior or exterior illumination and/or
signaling, a heads-up display, a fully or partially transparent
display, a flexible display, a laser printer, a telephone, a cell
phone, a tablet, a phablet, a personal digital assistant (PDA), a
laptop computer, a digital camera, a camcorder, a viewfinder, a
micro-display, a 3-D display, a smart watch, a vehicle, a large
area wall, theater or stadium screen, and a sign.
27. An opto-electronic device that includes an emissive layer
comprising: comprising: a host material; a first dopant; and a
second dopant that absorbs light in the ultraviolet region of the
electromagnetic spectrum, and has a solid state sink energy level,
wherein the first dopant is a blue-emitting, phosphorescent dopant
that has a multiply-excited energy level from which doubly excited
excitons and/or polarons can undergo a dissociative reaction, and a
triplet energy level T1 that is lower than the solid state sink
energy level of the second dopant, wherein the solid state sink
energy level is at least 0.2 eV below the multiply-excited energy
level of the first dopant, wherein the opto-electronic device is
selected from the group consisting of: a flat panel display, a
WOLED, a computer monitor, a medical monitor, a television, a
billboard, a light for interior or exterior illumination and/or
signaling, a heads-up display, a fully or partially transparent
display, a flexible display, a laser printer, a telephone, a cell
phone, a tablet, a phablet, a personal digital assistant (PDA), a
laptop computer, a digital camera, a camcorder, a viewfinder, a
micro-display, a 3-D display, a smart watch, a vehicle, a large
area wall, theater or stadium screen, and a sign.
28. The device of claim 21, wherein the emissive layer is disposed
between an electron-transport layer (ETL) and a hole-transport
layer (HTL), and the concentration of the first dopant is graded
from about 20% at the HTL/emitting layer interface to about 8% at
the ETL/emitting layer interface.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application Nos. 62/003,591, filed May 28, 2014; 62/095,904, filed
Dec. 23, 2014; 62/103,483, filed Jan. 14, 2015; and 62/108,100,
filed Jan. 27, 2015, the entire contents of each of which is
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 the Universal Display Corporation. The
agreement was in effect on and before the date the claimed
invention was made, and the claimed invention was made as a result
of activities undertaken within the scope of the agreement.
FIELD OF THE INVENTION
[0004] The present invention relates to organic light emitting
devices (OLEDs) and, more specifically, to arrangements and
techniques that provide relatively high performance and lifetime to
various types and components of OLEDs, and devices such as organic
light emitting diodes and other devices, including the same.
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 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] OLEDs make use of thin organic films that emit light when
voltage is applied across the device. OLEDs are becoming an
increasingly interesting technology for use in applications such as
flat panel displays, illumination, and backlighting. 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.
[0007] 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. Color may be measured using
CIE coordinates, which are well known to the art.
[0008] One example of a green emissive molecule is
tris(2-phenylpyridine) iridium, denoted Ir(ppy).sub.3, which has
the following structure:
##STR00001##
[0009] In this, and later figures herein, we depict the dative bond
from nitrogen to metal (here, Ir) as a straight line.
[0010] 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.
[0011] 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" 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" the second
layer. For example, a cathode may be described as "disposed over"
an anode, even though there are various organic layers in
between.
[0012] As used herein, "solution processible" means capable of
being dissolved, dispersed, or transported in and/or deposited from
a liquid medium, either in solution or suspension form.
[0013] A ligand may be referred to as "photoactive" when it is
believed that the ligand directly contributes to the photoactive
properties of an emissive material. A ligand may be referred to as
"ancillary" when it is believed that the ligand does not contribute
to the photoactive properties of an emissive material, although an
ancillary ligand may alter the properties of a photoactive
ligand.
[0014] As used herein, and as would be generally understood by one
skilled in the art, a first "Highest Occupied Molecular Orbital"
(HOMO) or "Lowest Unoccupied Molecular Orbital" (LUMO) energy level
is "greater than" or "higher than" a second HOMO or LUMO energy
level if the first energy level is closer to the vacuum energy
level. Since ionization potentials (IP) are measured as a negative
energy relative to a vacuum level, a higher HOMO energy level
corresponds to an IP having a smaller absolute value (an IP that is
less negative). Similarly, a higher LUMO energy level corresponds
to an electron affinity (EA) having a smaller absolute value (an EA
that is less negative). On a conventional energy level diagram,
with the vacuum level at the top, the LUMO energy level of a
material is higher than the HOMO energy level of the same material.
A "higher" HOMO or LUMO energy level appears closer to the top of
such a diagram than a "lower" HOMO or LUMO energy level.
[0015] As used herein, and as would be generally understood by one
skilled in the art, a first work function is "greater than" or
"higher than" a second work function if the first work function has
a higher absolute value. Because work functions are generally
measured as negative numbers relative to vacuum level, this means
that a "higher" work function is more negative. On a conventional
energy level diagram, with the vacuum level at the top, a "higher"
work function is illustrated as further away from the vacuum level
in the downward direction. Thus, the definitions of HOMO and LUMO
energy levels follow a different convention than work
functions.
[0016] More details on OLEDs, and the definitions described above,
can be found in U.S. Pat. No. 7,279,704, which is incorporated
herein by reference in its entirety.
SUMMARY OF THE INVENTION
[0017] According to an embodiment, an organic emissive layer is
provided that includes a host material, a first dopant having a
dissociative first energy level at which doubly excited excitons
and/or polarons undergo a dissociative reaction, and a second
dopant having a solid state sink energy level that between a
singly-excited energy level of the first dopant and a
multiply-excited energy level of the first dopant. Alternatively or
in addition, the solid state sink energy level may be lower than
the dissociative energy level. The multiply-excited state may be,
for example, a doubly-excited triplet energy level or a
doubly-excited polaron energy level of the first dopant. The solid
state sink energy level may be at least 0.2 eV lower than the
multiply-excited state of the first dopant. The rate of transition
of the excitons and/or polarons from a doubly-excited triplet
energy level of the first dopant to the solid state sink energy
level of the second dopant may be higher than the rate of
transition from the double-excited triplet energy level of the
first dopant to the dissociative energy level. The first dopant may
have an emissive first triplet energy level that is lower than the
solid state sink energy level of the second dopant. The
concentration of the first dopant may be graded within the host
material. The first dopant may be a thermally assisted delayed
fluorescence dopant. The second dopant may comprise a material that
absorbs light in the ultraviolet region of the electromagnetic
spectrum, such as NTCDA. The first dopant may include a
phosphorescent dopant or a fluorescent dopant. The dopant may
provide thermally assisted delayed fluorescence in the emissive
layer.
[0018] In an embodiment, a device is provided that includes an
emissive layer as previously described. The device may include an
OLED, such as a white-emitting OLED (WOLED). The emissive layer may
emit any color of light, such as light in the blue region of the
visible spectrum.
[0019] According to an embodiment, a first device comprising a
first organic light emitting device is also provided. The first
organic light emitting device can include an anode, a cathode, and
an organic layer, disposed between the anode and the cathode. The
organic layer can include a first dopant having a dissociative
first energy level at which doubly excited excitons and/or polarons
undergo a dissociative reaction, and a second dopant having a solid
state sink energy level that is lower than the dissociative energy
level. The first device can be a consumer product, an organic
light-emitting device, and/or a lighting panel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 shows an organic light emitting device.
[0021] FIG. 2 shows an inverted organic light emitting device that
does not have a separate electron transport layer.
[0022] FIGS. 3A-3B show two mechanisms resulting in PHOLED failure
via the dissociation of molecular species into non-emissive and
permanent trap states; FIG. 3A shows exciton-polaron, dissociation;
FIG. 3B shows exciton-exciton annihilation.
[0023] FIG. 4 shows an example energy level arrangement including a
solid state sink energy level as disclosed herein.
[0024] FIG. 5A shows a structure suitable to measure
aza-substitution in an H3P material.
[0025] FIG. 5B shows features of the compounds resulting from the
structure in FIG. 5A.
DETAILED DESCRIPTION
[0026] 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.
[0027] The initial OLEDs used emissive molecules that emitted light
from their singlet states ("fluorescence") as disclosed, for
example, in U.S. Pat. No. 4,769,292, which is incorporated by
reference in its entirety. Fluorescent emission generally occurs in
a time frame of less than 10 nanoseconds.
[0028] More recently, OLEDs having emissive materials that emit
light from triplet states ("phosphorescence") have been
demonstrated. Baldo et al., "Highly Efficient Phosphorescent
Emission from Organic Electroluminescent Devices," Nature, vol.
395, 151-154, 1998; ("Baldo-I") and Baldo et al., "Very
high-efficiency green organic light-emitting devices based on
electrophosphorescence," Appl. Phys. Lett., vol. 75, No. 3, 4-6
(1999) ("Baldo-II"), which are incorporated by reference in their
entireties. Phosphorescence is described in more detail in U.S.
Pat. No. 7,279,704 at cols. 5-6, which are incorporated by
reference.
[0029] FIG. 1 shows an organic light emitting device 100. The
figures are not necessarily 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.
[0030] More examples for each of these layers are available. For
example, a flexible and transparent substrate-anode combination is
disclosed in U.S. Pat. No. 5,844,363, which is incorporated by
reference in its entirety. An example of a p-doped hole transport
layer is m-MTDATA doped with F.sub.4-TCNQ at a molar ratio of 50:1,
as disclosed in U.S. Patent Application Publication No.
2003/0230980, which is incorporated by reference in its entirety.
Examples of emissive and host materials are disclosed in U.S. Pat.
No. 6,303,238 to Thompson et al., which is incorporated by
reference in its entirety. An example of an n-doped electron
transport layer is BPhen doped with Li at a molar ratio of 1:1, as
disclosed in U.S. Patent Application Publication No. 2003/0230980,
which is incorporated by reference in its entirety. U.S. Pat. Nos.
5,703,436 and 5,707,745, which are incorporated by reference in
their entireties, disclose examples of cathodes including compound
cathodes having a thin layer of metal such as Mg:Ag with an
overlying transparent, electrically-conductive, sputter-deposited
ITO layer. The theory and use of blocking layers is described in
more detail in U.S. Pat. No. 6,097,147 and U.S. Patent Application
Publication No. 2003/0230980, which are incorporated by reference
in their entireties. Examples of injection layers are provided in
U.S. Patent Application Publication No. 2004/0174116, which is
incorporated by reference in its entirety. A description of
protective layers may be found in U.S. Patent Application
Publication No. 2004/0174116, which is incorporated by reference in
its entirety.
[0031] 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.
[0032] The simple layered structure illustrated in FIGS. 1 and 2 is
provided by way of non-limiting example, and it is understood that
embodiments of the invention may be used in connection with a wide
variety of other structures. The specific materials and structures
described are exemplary in nature, and other materials and
structures may be used. Functional OLEDs may be achieved by
combining the various layers described in different ways, or layers
may be omitted entirely, based on design, performance, and cost
factors. Other layers not specifically described may also be
included. Materials other than those specifically described may be
used. Although many of the examples provided herein describe
various layers as comprising a single material, it is understood
that combinations of materials, such as a mixture of host and
dopant, or more generally a mixture, may be used. Also, the layers
may have various sublayers. The names given to the various layers
herein are not intended to be strictly limiting. For example, in
device 200, hole transport layer 225 transports holes and injects
holes into emissive layer 220, and may be described as a hole
transport layer or a hole injection layer. In one embodiment, an
OLED may be described as having an "organic layer" disposed between
a cathode and an anode. This organic layer may comprise a single
layer, or may further comprise multiple layers of different organic
materials as described, for example, with respect to FIGS. 1 and
2.
[0033] Structures and materials not specifically described may also
be used, such as OLEDs comprised of polymeric materials (PLEDs)
such as disclosed in U.S. Pat. No. 5,247,190 to Friend et al.,
which is incorporated by reference in its entirety. By way of
further example, OLEDs having a single organic layer may be used.
OLEDs may be stacked, for example as described in U.S. Pat. No.
5,707,745 to Forrest et al, which is incorporated by reference in
its entirety. The OLED structure may deviate from the simple
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.
[0034] 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. The materials to be deposited may be
modified to make them compatible with a particular deposition
method. For example, substituents such as alkyl and aryl groups,
branched or unbranched, and preferably containing at least 3
carbons, may be used in small molecules to enhance their ability to
undergo solution processing. Substituents having 20 carbons or more
may be used, and 3-20 carbons is a preferred range. Materials with
asymmetric structures may have better solution processibility than
those having symmetric structures, because asymmetric materials may
have a lower tendency to recrystallize. Dendrimer substituents may
be used to enhance the ability of small molecules to undergo
solution processing.
[0035] Devices fabricated in accordance with embodiments of the
present invention may further optionally comprise a barrier layer.
One purpose of the barrier layer is to protect the electrodes and
organic layers from damaging exposure to harmful species in the
environment including moisture, vapor and/or gases, etc. The
barrier layer may be deposited over, under or next to a substrate,
an electrode, or over any other parts of a device including an
edge. The barrier layer may comprise a single layer, or multiple
layers. The barrier layer may be formed by various known chemical
vapor deposition techniques and may include compositions having a
single phase as well as compositions having multiple phases. Any
suitable material or combination of materials may be used for the
barrier layer. The barrier layer may incorporate an inorganic or an
organic compound or both. The preferred barrier layer comprises a
mixture of a polymeric material and a non-polymeric material as
described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos.
PCT/US2007/023098 and PCT/US2009/042829, which are herein
incorporated by reference in their entireties. To be considered a
"mixture", the aforesaid polymeric and non-polymeric materials
comprising the barrier layer should be deposited under the same
reaction conditions and/or at the same time. The weight ratio of
polymeric to non-polymeric material may be in the range of 95:5 to
5:95. The polymeric material and the non-polymeric material may be
created from the same precursor material. In one example, the
mixture of a polymeric material and a non-polymeric material
consists essentially of polymeric silicon and inorganic
silicon.
[0036] 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. 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, cell phones,
tablets, phablets, personal digital assistants (PDAs), laptop
computers, digital cameras, camcorders, viewfinders,
micro-displays, 3-D displays, smart watches, vehicles, a large area
wall, theater or stadium screen, or 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 C to 30 C, and more
preferably at room temperature (20-25 C), but could be used outside
this temperature range, for example, from -40 C to +80 C.
[0037] Generally, different types, arrangements, and colors of
OLEDs may have dramatically different operational lifetimes. For
example, blue phosphorescent OLEDs (PHOLEDs) often have a
relatively short operational lifetime. Blue emission comprises
about 15-25% of the emission in white OLEDs (WOLEDs) depending on
the WOLED color coordinates and the color temperature of the
lighting. Since the blue PHOLED lifetime typically is far shorter
than that of red and green, it therefore also presents the primary
limitation to the WOLED lifetime.
[0038] The most fundamental source of degradation in PHOLEDs is the
presence of a high density of energetic molecular excited states,
or excitons, which may dissipate their energy by breaking molecular
bonds, thereby destroying the active electronic molecular species
and at the same time forming a non-radiative defect. Particularly,
excitons on the phosphor can encounter free electrons (i.e.
"electron-polarons") or excitons on the host molecules, resulting
in an instantaneous doubling of the polaron (or exciton) energy as
shown, for example, in FIGS. 3A-3B. FIGS. 3A-3B show two mechanisms
resulting in PHOLED failure via the dissociation of molecular
species into non-emissive and permanent trap states, R (dashed
line)-exciton polaron, i.e. charge, as shown in FIG. 3A, and
exciton-exciton annihilation, as shown in FIG. 3B. Solid lines
correspond to the molecular ground (S0) and excited states (singlet
S0, triplet T1). Horizontal dashed lines represent the molecular
vibronic energies. Here, .DELTA.r represents the average of atomic
distances within the molecule, and E the energy. The lowest
E(.DELTA.r) corresponds to the equilibrium state.
[0039] If concentrated onto a single molecular bond on the host,
the excess energy can lead to molecular decomposition, or
fragmentation, creating the non-radiative trap. This is a
fundamental energy-driven process. It is expected the highest
energy (blue) excitons lead to the highest energy polarons with the
greatest probability for breaking bonds. This is precisely what is
observed: blue PHOLEDs typically have orders of magnitude shorter
lifetimes compared with green and red. This ultimately determines
the lifetime of white PHOLEDs that may need to emit approximately
25% of their light in the blue to generate certain types of white
color.
[0040] According to embodiments disclosed herein, the effects of
doubly excited triplets may be mitigated by "sinking" them with a
manager material. The manager material provides a "solid state
sink" energy level, to which the doubly excited triplets or
polarons may transition instead of transitioning to the
dissociative energy level. As further disclosed herein, this
concept may be implemented by doping a blue PHOLED guest/host
system with a second dopant, which removes doubly excited triplets
and/or polarons from the system by drawing them to ground state.
FIG. 4 shows an example energy level arrangement that may be used
to "sink" the highly excited states before they result dissociative
reactions with the host or phosphor dopant, using such a solid
state sink energy level. In FIG. 4, the desired outcome for light
emission is a transition from the T1 to the S0 state in the blue
phosphor. However, this transition competes with triplet-triplet
(TTQ) and triplet-polaron (TPQ) annihilation that can promote the
exciton to a higher state T2/P2. This higher excitation can result
in a destructive chemical reaction (TTQ or TPQ). According to
embodiments disclosed herein, when the EML is codoped with an
excited sink dopant, this reaction then competes with the
non-dissociative transition, which returns the molecule from Tx/Px
to T1, or to S0 in the manager molecule.
[0041] In such an arrangement, the rate of transition to the
manager excited state or LUMO Tx/Px should be higher than the rate
to the dissociative state D at which damaging TPQ and TTQ may
occur. Thus, to effectively manage the multiply excited triplets,
both the energetic and spatial properties of the system should
favor the transition from T2/P2 to Tx/Px over the transition to D.
This suggests that the energy of the Tx/Px state should be lower
than the D state. Alternatively or in addition, it may suggest that
the manager molecules should be positioned relatively close to the
phosphor through a high doping concentration to allow for rapid
energy and/or charge transfer. As previously described, the rate of
transition of doubly excited excitons and/or polarons from to the
solid state sink energy level may be higher than the rate of
transition to the dissociative energy level. The relative efficacy
of "sinking" need not be perfect, as removing only a fraction of
the double excited states can have a significant effect on the
PHOLED lifetime. For example, it has been shown that about only 1
in 1000 excitations leads to molecular dissociation that then forms
a non-radiative recombination center. Appropriate molecules for the
"manager" material include UV absorbers such as
1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTCDA).
[0042] More specifically, according to an embodiment disclosed
herein, an organic emissive layer such as a blue emissive layer in
a PHOLED or other OLED may include a host material that is doped
with at least two dopants. The first dopant, typically the emissive
dopant, has a dissociative energy level at which doubly excited
excitons and/or polarons undergo a dissociative reaction as
previously described, which may cause damage to the host molecule.
In general, the most damaging reaction is when an exciton on the
dopant and a polaron on the host undergo triplet-polaron quenching,
which typically will cause damage to the host. The first dopant
also may have an emissive first triplet energy level Ti that is
lower than the solid state sink energy level of the second dopant,
as shown in FIG. 4. As described in further detail below, the first
dopant also may be one that is selected to provide thermally
assisted delayed fluorescence. More generally, the first dopant may
be a phosphorescent or a fluorescent dopant. The dopant may be one
that emits light of any color, though it is expected that the
benefits described herein will be particularly suited to
blue-emitting materials.
[0043] In some embodiments, the manager material, i.e., a second
dopant, may have a solid state sink energy level that is between
the singly- -excited triplet or polaron energy level of the
emissive material and a multiply-excited triplet or polaron energy
level of the emissive material, typically a doubly-excited triplet
or polaron energy level. For example, referring to FIG. 4, the
Tx/Px energy level of the manager is between the T1 and T2/P2
energy levels of the emissive material. Such an arrangement may
provide the benefits disclosed herein even where the sink energy
level Tx/Px is not substantially lower than, equal to, or slightly
higher than the dissociative energy level D, because competing
rates of transition to the two levels may reduce degradation in the
host and/or emissive materials, as disclosed herein. In general, it
may be preferred for the solid state sink energy level to be at
least 0.2 eV below the multiply-excited triplet or polaron energy
level of the emissive material, to prevent transfers back to the
multiply-excited state. For example, referring to FIG. 4, it may be
preferred for the solid state sink energy level Tx/Px to be at
least 0.2 eV lower than the doubly-excited triplet or polaron
energy level T2/P2.
[0044] In the annihilation process in FIG. 3, an exciton and
polaron combine to generate "hot" states via:
T*+H.sup.-.fwdarw.T.sub.0+H.sup.-/*
or T*+H.sup.-.fwdarw.T*.sup./-+H.sub.0
where "*" indicates a multiply excited state, "-" a polaron on the
corresponding molecule, and host (H.sup.-/*) or
phosphor.sup.-(T.sup.-/*) represent a hot polaron or hot triplet on
the host or phosphor, respectively. It may not be possible to
quantify this reaction directly, since the amount of degraded
material in the thin film typically is too low to be analytically
measured. The hot polaron states may be quantized similarly to the
techniques described in I. Ghosh et al., Science 346, 725-728
(2014), in which where a reduced organic compound in solution was
photolyzed to generate the equivalent hot state. This state can
reduce chloro-aromatic compounds, ultimately producing halide ions
and aryl radicals. A similar approach may be used in which
solutions of T.sup.- and H.sup.- ions are irradiated while
monitoring their photostability to determine whether a hot polaron
on the phosphor or host is the "weak link" in a given phosphor-host
pair. The excited state lifetimes of H.sup.-/* and T*.sup./- may be
characterized using transient absorption (TA) spectroscopy, and the
hot polaron lifetime correlated with its photostability. In
addition, high performance liquid chromatography, mass spectroscopy
and Fourier transform infrared spectroscopy may be used to identify
the decomposition products. With this information, "hardened"
materials may be designed that are less susceptible to
fragmentation in the H.sup.-/* or T*.sup./- state, further
decreasing the likelihood of destructive decay from the hot
polaron.
[0045] A similar approach may be used to estimate the energy of
H.sup.-/* or T.sup.-/* by examining the photostability of H.sup.-
and T.sup.- in the presence of a range of trapping agents. By using
trapping agents with graded reduction potentials the reducing power
of the hot polaron may be estimated. This estimate then allows for
a selection of appropriate candidates from libraries of H3 and H2P
materials, for example as shown in FIG. 5, to quench the hot
polaron, while not trapping charge or triplets. FIG. 5A shows a
structure suitable to measure aza-substitution in an H3P material;
the variation provides 37 different compounds, as shown in FIG. 5B.
Other materials also may be used as triplet-triplet and
triplet-polaron quenchers, such as pyridyl, pyrazine and triazine
based materials which are reducible, stable, and have high triplet
energies.
[0046] The described approach is focused on the anionic host and
dopant. This is because in most cases the phosphor dopant traps and
transports holes, while electrons are transported by the host
matrix. Hence, the electron density in the EML tends to
significantly exceed that of higher mobility holes. Nevertheless,
the methodology can be used equally well to study the stability of
cathodic H.sup.+/* and T.sup.+/* states if the primary carrier in
the host is the hole.
[0047] As previously described, the model illustrated in FIG. 3
suggests that PHOLED operational lifetimes can be greatly extended
by reducing triplet-polaron quenching (TPQ) and triplet-triplet
quenching (TTQ) annihilation events that lead to molecular
destruction. Even removing only a small fraction of the highly
excited states may have a remarkable effect on PHOLED lifetime
since there is a probably of only 1 in 2.times.10.sup.-9 that an
excitation will lead to molecular dissociation. Thus, alternatively
or in addition to using a manager material having a solid state
sink energy level as previously described, in embodiments disclosed
herein, the rate of TPQ and TTQ may be reduced through extending
the device emission zone via phosphor dopant grading across the
PHOLED emissive layer. That is, the concentration of a dopant
within a host-dopant system as previously described with respect to
FIG. 4 may be graded within the host material. Such a configuration
may provide up to a tenfold improvement in blue PHOLED operational
lifetime relative to conventional designs. Strategic grading
reduced the exciton and polaron densities (and hence their tendency
to create traps through energetic encounters) while also increasing
efficiency at high brightness by reducing triplet annihilation.
This approach has been found to triple the lifetime of a relatively
stable blue emitting phosphor from LT.sub.80=50 hrs at an initial
luminance of L.sub.0=1000 cd/m.sup.2. When used in a stacked
structure, a lifetime of LT.sub.80>600 hr may be achieved. In
this example, the phosphor Ir(dmp).sub.3, a hole conductor, is
paired with a stable, electron conducting host, such as mCBP.
Ordinarily, the doping of the PHOLED EML is uniform, in which case
almost all excitons form at the EML/electron transport layer (ETL)
interface, resulting in the local pile-up of a high density of
electrons and excitons where TPQ and TTQ is frequent. To lower
local exciton and polaron densities, the phosphor concentration may
be graded from, for example, about 20% at the hole transport layer
(HTL)/EML interface, to about 8% at the opposite EML/ETL interface.
For example, an emissive layer as described with respect to FIGS.
1-2 may be graded between two adjacent layers, such that the
concentration of a phosphorescent or other emissive dopant in the
emissive layer is graded vertically across the emissive layer. Such
a structure has been found to increase the lifetime of a blue
PHOLED by a factor of up to 10 when used in a stacked blue
PHOLED.
[0048] Different grading profiles from those used to optimize
simple guest-host doped EMLs may be used, since the presence of the
sink molecule may change the exciton and charge distributions. It
is anticipated that the combination of graded doping and the use of
excited state sinks can increase the lifetime of blue PHOLEDs by a
factor of 1000 or more, compared to current conventional devices.
This approach is general, and hence can also be applied to red and
green phosphorescent devices as well as blue devices as disclosed
herein. Hence, excited state sinking may result in phosphor
lifetimes exceeding 1 million hours in some cases.
[0049] Excited state management as disclosed herein also may be
used in devices and arrangements that make use of phosphor
sensitized fluorescence and/or thermally activated delayed
fluorescence (TADF). Both techniques typically require sensitizers
having energies higher than the emissive energy, especially
"hyperfluorescence" in which a TADF sensitizer transfers energy to
a blue fluorophore. In both cases, EML grading and/or excited state
management as disclosed herein may provide improved lifetimes of
the devices. For example, the first or primary dopant in a
guest/host emissive layer as disclosed herein may be a dopant
suitable for use with TADF, in which excitons are maintained in the
triplet manifold of the dopant. A UV sensitizer is then used to
access the fluorescence dopant energies. However, the longer
triplets are maintained at a relatively high energy, the higher the
chance that they collide with a triplet or polaron from another
molecule and undergo a destructive transition as previously
described. The excited state management techniques disclosed herein
may prevent such destructive transitions, thereby improving the
lifetime of the TADF or similar system.
[0050] Techniques disclosed herein may be suitable for use in any
arrangement or device that includes organic emissive layers, such
as OLEDs, white-emitting OLEDs (WOLEDs), and the like. Similarly,
any device that can incorporate an OLED as disclosed herein, also
may incorporate OLEDs having emissive layers as disclosed
herein.
[0051] Layers, arrangements, and techniques as disclosed herein may
be used with any device that uses organic emissive layers, such as
the OLEDs described with respect to FIGS. 1-2. For example, one or
more emissive layers in an OLED, such as emissive layers 135, 220,
may be doped with first and second dopants as disclosed herein to
provide a solid state sink energy level within the emissive layer
as previously described. Such a device may be a pixel or sub-pixel
in a device such as a full-color display, or any other device that
incorporates structures such as shown in FIGS. 1-2 and disclosed
herein. 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.
* * * * *