U.S. patent application number 10/932761 was filed with the patent office on 2005-02-10 for cascaded organic electroluminescent devices with color filters.
This patent application is currently assigned to Eastman Kodak Compamy. Invention is credited to Arnold, Andrew D., Cok, Ronald S., Liao, Liang-Sheng, Tang, Ching W..
Application Number | 20050029933 10/932761 |
Document ID | / |
Family ID | 27660276 |
Filed Date | 2005-02-10 |
United States Patent
Application |
20050029933 |
Kind Code |
A1 |
Liao, Liang-Sheng ; et
al. |
February 10, 2005 |
Cascaded organic electroluminescent devices with color filters
Abstract
An OLED device is described comprising a substrate having
thereon a cascaded organic electroluminescent device comprising: a)
an anode; b) a cathode; c) a plurality of cascaded organic
electroluminescent units disposed between the anode and the
cathode, wherein each organic electroluminescent unit includes at
least one light-emitting layer and wherein the plurality of
cascaded units includes at least two units that emit light of
different colors; and d) a connecting unit disposed between each
adjacent cascaded organic electroluminescent unit; and e) a colored
filter that filters the emitted light.
Inventors: |
Liao, Liang-Sheng;
(Rochester, NY) ; Tang, Ching W.; (Rochester,
NY) ; Cok, Ronald S.; (Rochester, NY) ;
Arnold, Andrew D.; (Hilton, NY) |
Correspondence
Address: |
Paul A. Leipold
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Compamy
|
Family ID: |
27660276 |
Appl. No.: |
10/932761 |
Filed: |
September 2, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10932761 |
Sep 2, 2004 |
|
|
|
10077270 |
Feb 15, 2002 |
|
|
|
Current U.S.
Class: |
313/504 ;
313/500; 313/503 |
Current CPC
Class: |
H01L 27/3244 20130101;
H01L 51/5036 20130101; H01L 51/0062 20130101; H01L 51/0077
20130101; H01L 27/3281 20130101; H01L 51/0052 20130101; H05B 45/60
20200101; H01L 51/5278 20130101; Y10T 428/26 20150115; H01L 51/0059
20130101; H01L 27/3211 20130101; H01L 51/0084 20130101; H01L
51/5016 20130101; H01L 51/0085 20130101; H01L 2251/5361 20130101;
Y02B 20/30 20130101; H01L 27/322 20130101; H01L 51/0089 20130101;
Y10S 428/917 20130101 |
Class at
Publication: |
313/504 ;
313/503; 313/500 |
International
Class: |
H05B 033/00 |
Claims
1. An OLED device comprising a substrate having thereon a cascaded
organic electroluminescent device comprising: a) an anode; b) a
cathode; c) a plurality of cascaded organic electroluminescent
units disposed between the anode and the cathode, wherein each
organic electroluminescent unit includes at least one
light-emitting layer and wherein the plurality of cascaded units
includes at least two units that emit light of different colors;
and d) a connecting unit disposed between each adjacent cascaded
organic electroluminescent unit; and e) a colored filter that
filters the emitted light.
2. The OLED device of claim 1, comprising a plurality of anodes
defining independently controlled light-emitting areas of the OLED
device, wherein the plurality of cascaded organic
electroluminescent units are disposed between the plurality of
anodes and a common cathode.
3. The OLED device of claim 2, wherein at least two independently
controlled light-emitting areas have differently colored filters
that filter the emitted light.
4. The OLED device of claim 3, wherein the colors of the light
transmitted by the differently colored filters are matched to the
colors of the light emitted by one of the plurality of organic
electroluminescent units.
5. The OLED device of claim 4 wherein the cascaded organic
electroluminescent units include units that individually emit red,
green, and blue light.
6. The OLED device of claim 3 wherein the cascaded organic
electroluminescent units include units that in combination emit
white light.
7. The OLED device of claim 6 wherein the color filters transmit
red, green, or blue light.
8. The OLED device of claim 3 wherein the efficiency of light
emission from or lifetime of one of the cascaded organic
electroluminescent units is different from the efficiency of light
emission from or lifetime of another of the organic
electroluminescent units.
9. The OLED device of claim 8 wherein the relative size of an
independently controlled light-emitting area and associated color
filter is inversely related to the relative efficiency of light
emission from or lifetime of the cascaded organic
electroluminescent units.
10. The OLED device of claim 8 wherein the cascaded organic
electroluminescent units are ordered in a stack to correspond to
the relative efficiency of light emission from the organic
electroluminescent units in the stack.
11. The OLED device of claim 8 wherein the cascaded organic
electroluminescent units are ordered in a stack to minimize the
relative absorption of light emission from the organic
electroluminescent units in the stack as the light passes through
the stack.
12. The OLED device of claim 3 further comprising an independently
controlled light-emitting area and cascaded organic
electroluminescent unit without a corresponding color filter.
13. The OLED device of claim 12 wherein the independently
controlled light-emitting area without a corresponding color filter
emits white light.
14. The OLED device of claim 1 wherein two of the cascaded organic
electroluminescent units emit light of the same color.
15. The OLED device of claim 1 wherein the light is emitted through
the substrate.
16. The OLED device of claim 1 further comprising a cover formed
over the cascaded organic electroluminescent units wherein the
light is emitted through the cover.
17. The OLED device of claim 1 comprising two cascaded organic
electroluminescent units that emit green and white light
respectively.
18. The OLED device of claim 1 comprising two cascaded organic
electroluminescent units that emit blue and yellow light
respectively.
19. The OLED device of claim 1 comprising two cascaded organic
electroluminescent units that emit red and cyan light
respectively.
20. The OLED device of claim 1 comprising two cascaded organic
electroluminescent units that emit green and magenta light
respectively.
21. The OLED device of claim 1, wherein the connecting unit
comprises a doped organic layer.
22. The OLED device of claim 21, wherein the connecting unit
comprises, in sequence, an n-type doped organic layer, and a p-type
doped organic layer.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of copending,
commonly assigned U.S. patent application Ser. No. 10/077,270 filed
Feb. 15, 2002 by Liang-Sheng L. Liao et al., entitled "Providing an
Organic Electroluminescent Device Having Stacked Electroluminescent
Units", the disclosure of which is herein incorporated by
reference. Reference is also made to copending, commonly assigned
U.S. Ser. Nos. 10/437,195 and 10/845,038, the disclosures of which
are also incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The present invention relates to providing a OLED device
having a plurality of organic electroluminescent (EL) units in the
form of a cascaded organic electroluminescent device with color
filters.
BACKGROUND OF THE INVENTION
[0003] Organic electroluminescent (EL), or organic light-emitting
diode (OLED), devices are electronic devices that emit light in
response to an applied potential. The structure of an OLED
comprises, in sequence, an anode, an organic EL medium, and a
cathode. The organic EL medium disposed between the anode and the
cathode is commonly comprised of an organic hole-transporting layer
(HTL) and an organic electron-transporting layer (ETL). Holes and
electrons recombine and emit light in the ETL near the interface of
HTL/ETL. Tang et al., "Organic electroluminescent diodes", Applied
Physics Letters, 51, 913 (1987), and commonly assigned U.S. Pat.
No. 4,769,292, demonstrated highly efficient OLEDs using such a
layer structure. Since then, numerous OLEDs with alternative layer
structures have been disclosed. For example, there are three-layer
OLEDs that contain an organic light-emitting layer (LEL) between
the HTL and the ETL, such as that disclosed by Adachi et al.,
"Electroluminescence in Organic Films with Three-Layer Structure",
Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang
et al., "Electroluminescence of doped organic thin films", Journal
of Applied Physics, 65, 3610 (1989). The LEL commonly includes of a
host material doped with a guest material wherein the layer
structures are denoted as HTL/LEL/ETL. Further, there are other
multilayer OLEDs that contain a hole-injecting layer (HIL), and/or
an electron-injecting layer (EIL), and/or a hole-blocking layer,
and/or an electron-blocking layer in the devices. These structures
have further resulted in improved device performance.
[0004] Color, digital image display devices are well known and are
based upon a variety of technologies such as cathode ray tubes,
liquid crystal and solid-state light emitters such as Organic Light
Emitting Diodes (OLEDs). In a common OLED color display device a
pixel includes red, green, and blue colored OLEDs. By combining the
illumination from each of these three OLEDs in an additive color
system, a full-color display having a wide variety of colors can be
achieved.
[0005] OLEDs may be used to generate color directly using organic
materials that are doped to emit energy in desired portions of the
electromagnetic spectrum. However, to create a color OLED device
using different organic materials requires the patterning of these
materials over the surface of the OLED device substrate. This
patterning is a difficult and problematic task, especially for
large substrates, for example substrates having a diagonal greater
than about 50 cm. An alternative method utilizes a white-light
emitting material in combination with color filter arrays to
provide color emission, much as conventional LCD displays do.
White-light emitting OLED devices that are known in the art are
typically formed by doping multiple, individual emitting layers
such that each doped layer produces light within a specific
spectral frequency band. White-light emitting devices may be formed
from either two or three individual emitting materials. However,
this approach suffers from efficiency problems because the white
light emitters tend to be relatively broadband so that most of the
light emitted by the white-light emitter is absorbed by the color
filters, reducing the efficiency of the OLED device. Additionally,
some of the white light emitters may age more rapidly than other
due to the relative efficiencies for which they emit different
colors.
[0006] In order to further improve the performance of the OLEDs, a
new kind of OLED structure called stacked OLED, which is fabricated
by stacking several individual OLED vertically, has also been
proposed. Forrest et al. in U.S. Pat. No. 5,703,436 and Burrows et
al. in U.S. Pat. No. 6,274,980 disclosed their stacked OLEDs. In
their inventions, the stacked OLEDs are fabricated by vertically
stacking several OLEDs, each independently emitting light of a
different color or of the same color. Using their stacked OLED
structure can make full color emission devices with higher
integrated density in the display, but each OLED needs a separate
power source. In an alternative design, Jones et al. in U.S. Pat.
No. 6,337,492 proposed a stacked OLED structure by vertically
stacking several OLED without individually addressing each OLED in
the stack. Jones et al. believe that their stacked structure could
increase the luminance output and operational lifetime. These OLEDs
use individual OLEDs (anode/organic medium/cathode) as building
blocks to fabricate the stacked OLEDs. The complex architecture in
these designs presents serious fabrication problems for patterned
multi-colored display devices. Moreover, these designs do not
address inefficiencies of white light emitters in combination with
color filters, or the consequent differential aging associated with
differential efficiencies. This reduces the overall device
efficiency.
[0007] There is a need therefore, for an improved OLED device
structure providing simplicity of manufacture with efficiency of
operation.
SUMMARY OF THE INVENTION
[0008] In accordance with one embodiment, the present invention is
directed towards an OLED device comprising a substrate having
thereon a cascaded organic electroluminescent device comprising: a)
an anode; b) a cathode; c) a plurality of cascaded organic
electroluminescent units disposed between the anode and the
cathode, wherein each organic electroluminescent unit includes at
least one light-emitting layer and wherein the plurality of
cascaded units includes at least two units that emit light of
different colors; and d) a connecting unit disposed between each
adjacent cascaded organic electroluminescent unit; and e) a colored
filter that filters the emitted light.
ADVANTAGEOUS EFFECT OF THE INVENTION
[0009] Various embodiments of the present invention provide color
OLED devices with simplified manufacturing, improved efficiency,
and reduced aging.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 depicts a schematic cross sectional view of a
cascaded OLED device according to the present invention;
[0011] FIG. 2 depicts the emission spectrum of a white-light
emitting OLED; and
[0012] FIG. 3 depicts a schematic cross sectional view of an
alternative cascaded OLED device according to the present
invention.
[0013] It will be understood that FIGS. 1 and 3 are not to scale
since the individual layers are too thin and the thickness
differences of various layers too great to permit depiction to
scale.
DETAILED DESCRIPTION OF THE INVENTION
[0014] The layer structure of a cascaded OLED (or stacked OLED)
comprises an anode, a cathode, a plurality of organic EL units and
a plurality of organic connectors (or connecting units thereafter),
wherein each of the connecting units is disposed between two
organic EL units. The organic EL unit includes at least one
light-emitting layer, and typically comprises, in sequence, a
hole-transport layer, a light-emitting layer, and an
electron-transport layer, denoted in brief as HTL/LEL/ETL.
[0015] To function efficiently, the connecting unit for the
cascaded OLED should provide electron injection into the
electron-transporting layer and hole injection into the
hole-transporting layer of the two adjacent organic EL units. A
variety of materials may be used to form the connecting units. In
preferred embodiments, connecting unit materials are selected to
provide high optical transparency and excellent charge injection,
thereby providing the cascaded OLED high electroluminescence
efficiency and operation at an overall low driving voltage.
[0016] The connecting unit may comprise doped organic connectors
provided between adjacent organic EL units. Each doped organic
connector may include at least one n-type doped organic layer, or
at least one p-type doped organic layer, or a combination of
layers, thereof. Preferably, the doped organic connector includes
both an n-type doped organic layer and a p-type doped organic layer
disposed adjacent to one another to form a p-n heterojunction. It
is also preferred that the n-type doped organic layer is disposed
towards the anode side, and the p-type doped organic layer is
disposed towards the cathode side. The choice of using n-type doped
organic layer, or a p-type doped organic layer, or both (the p-n
junction) is in part dependent on the organic materials that
include the organic EL units. Each connector can be optimized to
yield the best performance with a particular set of organic EL
units. This includes choice of materials, layer thickness, modes of
deposition, and so forth.
[0017] An n-type doped organic layer means that the organic layer
has semiconducting properties after doping, and the electrical
current through this layer is substantially carried by the
electrons. A p-type doped organic layer means that the organic
layer has semiconducting properties after doping, and the
electrical current through this layer is substantially carried by
the holes. A p-n heterojunction means an interfacial region (or
junction) formed when a p-type layer and an n-type layer contact
each other.
[0018] N-type doped organic layers may include a host organic
material and at least one n-type dopant. The host material in the
n-typed doped organic layer can include a small molecule material
or a polymeric material, or combinations thereof, and it is
preferred that it can support electron transport. The p-type doped
organic layers may include a host organic material and at least one
p-type dopant. The host material can include a small molecule
material or a polymeric material, or combinations thereof, and it
is preferred that it can support hole transport. In some instances,
the same host material can be used for both n-typed and p-type
doped organic layers, provided that it exhibits both hole and
electron transport properties set forth above. The n-type doped
concentration or the p-type doped concentration is preferably in
the range of 0.01-10 vol. %. The total thickness of each doped
organic connector is typically less than 100 nm, and preferably in
the range of about 1 to 100 nm.
[0019] The organic electron-transporting materials used in
conventional OLED devices represent a useful class of host
materials that may be employed for the n-type doped organic layer.
Preferred materials are metal chelated oxinoid compounds, including
chelates of oxine itself (also commonly referred to as 8-quinolinol
or 8-hydroxyquinoline), such as tris(8-hydroxyquinoline) aluminum.
Other materials include various butadiene derivatives as disclosed
by Tang (U.S. Pat. No. 4,356,429), various heterocyclic optical
brighteners as disclosed by Van Slyke and Tang and others (U.S.
Pat. No. 4,539,507), triazines, hydroxyquinoline derivatives, and
benzazole derivatives. Silole derivatives, such as
2,5-bis(2',2"-bipridin-6-yl)-1,1-dimethyl-3,4-diphenyl
silacyclopentadiene as reported by Murata and others [Applied
Physics Letters, 80, 189 (2002)], are also useful host
materials.
[0020] Materials useful as n-type dopants in the n-type doped
organic layer of a doped organic connector include metals or metal
compounds having a work function less than 4.0 eV. Particularly
useful dopants include alkali metals, alkali metal compounds,
alkaline earth metals, and alkaline metal compounds. The term
"metal compounds" includes organometallic complexes, metal-organic
salts, and inorganic salts, oxides and halides. Among the class of
metal-containing n-type dopants, Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba,
La, Ce, Sm, Eu, Th, Dy, or Yb, and their compounds, are
particularly useful. Materials useful as n-type dopants in the
n-type doped organic layer of a doped organic connector also
include organic reducing agents with strong electron-donating
properties. By "strong electron-donating properties" we mean that
the organic dopant should be able to donate at least some
electronic charge to the host to form a charge-transfer complex
with the host. Non-limiting examples of organic molecules include
bis(ethylenedithio)-tetrathiafulval- ene (BEDT-TTF),
tetrathiafulvalene (TTF), and their derivatives. In the case of
polymeric hosts, the dopant can be any of the above or also a
material molecularly dispersed or copolymerized with the host as a
minor component.
[0021] The hole-transporting materials used in conventional OLED
devices represent a useful class of host materials for p-type doped
organic layers. Preferred materials include aromatic tertiary
amines having at least one trivalent nitrogen atom that is bonded
only to carbon atoms, at least one of which is a member of an
aromatic ring. In one form the aromatic tertiary amine can be an
arylamine, such as a monoarylamine, diarylamine, triarylamine, or a
polymeric arylamine. Other suitable triarylamines substituted with
one or more vinyl radicals and/or comprising at least one active
hydrogen-containing group are disclosed by Brantley and others
(U.S. Pat. No. 3,567,450 and U.S. Pat. No. 3,658,520). A more
preferred class of aromatic tertiary amines are those which include
at least two aromatic tertiary amine moieties as described by Van
Slyke and Tang and others (U.S. Pat. No. 4,720,432 and U.S. Pat.
No. 5,061,569). Non-limiting examples include as
N,N'-di(naphthalene-1-yl- )-N,N'-diphenyl-benzidine (NPB) and
N,N'-diphenyl-N,N'-bis(3-methylphenyl)- -1,1-biphenyl-4,4'-diamine
(TPD), and N,N,N',N'-tetranaphthyl-benzidine (TNB).
[0022] Materials useful as p-type dopants in p-type doped organic
layers of doped organic connectors include oxidizing agents with
strong electron-withdrawing properties. By "strong
electron-withdrawing properties" we mean that the organic dopant
should be able to accept some electronic charge from the host to
form a charge-transfer complex with the host. Some non-limiting
examples include organic compounds such as
2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F.sub.4-TCNQ)
and other derivatives of TCNQ, and inorganic oxidizing agents such
as iodine, FeCl.sub.3, SbCl.sub.5, and some other metal chlorides.
In the case of polymeric hosts, the dopant can be any of the above
or also a material molecularly dispersed or copolymerized with the
host as a minor component.
[0023] Examples of materials that can be used as host for either
n-type or p-type doped organic layers include, but are not limited
to: various anthracene derivatives including those described in
U.S. Pat. No. 5,972,247; certain carbazole derivatives, such as
4,4-bis(9-dicarbazolyl)- -biphenyl (CBP); and distyrylarylene
derivatives such as 4,4'-bis(2,2'-diphenyl vinyl)-1,1'-biphenyl and
as described in U.S. Pat. No. 5,121,029.
[0024] The materials used for fabricating doped organic connectors
are preferably substantially transparent to emitted light.
[0025] In a preferred embodiment, the connecting unit comprises, in
sequence, an n-type doped organic layer and a p-type doped organic
layer. Thus, in this structure, the ETL of the EL unit is adjacent
to the n-type doped layer of the connecting unit and the HTL of the
EL unit is adjacent to the p-type doped connecting unit. In this
cascaded device structure only a single external power source is
needed to connect to the anode and the cathode with the positive
potential applied to the anode and the negative potential to the
cathode. No other electrical connections are needed to connect the
individual organic EL units to external electrical power
sources.
[0026] In a further specific cascaded OLED device embodiment, the
physical spacing between adjacent electroluminescent zones may be
more than 90 nm and the connecting unit disposed between each
adjacent organic electroluminescent unit may comprise an n-type
doped organic layer and a p-type doped organic layer forming a
transparent p-n junction structure wherein the resistivity of each
of the doped layers is higher than 10 i-cm, as described in
commonly assigned U.S. patent application Ser. No. 10/437,195 filed
May 13, 2003 entitled "Cascaded Organic Electroluminescent Device
Having Connecting Units with n-Type and p-Type Organic Layers", the
disclosure of which is herein incorporated by reference.
[0027] For a cascaded OLED to function efficiently, it is necessary
that the optical transparency of the layers constituting the
organic EL units and the connecting units be as high as possible to
allow for radiation generated in the organic EL units to exit the
device. Furthermore, for the radiation to exit through the anode,
the anode should be transparent and the cathode can be opaque,
reflecting, or transparent. For the radiation to exit through the
cathode, the cathode should be transparent and the anode can be
opaque, reflecting or transparent. The layers constituting the
organic EL units are generally optically transparent to the
radiation generated by the EL units, and therefore their
transparency is generally not a concern for the construction for
the cascaded OLEDs.
[0028] The operational stability of cascaded OLED is dependent to a
large extent on the stability of the connecting units. In
particular, the driving voltage will be highly dependent on whether
or not the connecting unit can provide the necessary electron and
hole injection. It is generally known that the close proximity of
two dissimilar materials may result in diffusion of matters from
one into another, or in interdiffusion of matters across the
boundary between the two. In the case of cascaded OLEDs employing
an n-type doped organic layer and a p-type doped organic layer, if
such diffusion were to occur in the connecting unit between the
n-type doped organic layer and the p-type doped organic layer, the
injection properties of this organic connecting unit may degrade
correspondingly due to the fact that the individual n-type doped
layer or p-type doped layer may no longer be sufficiently
electrically conductive. Diffusion or interdiffusion is dependent
on temperature as well as other factors such as electrical field
induced migration. The latter is plausible in cascaded OLED devices
since the operation of OLED generally requires an electric field as
high as 10.sup.6 volt per centimeter. To prevent such an
operationally induced diffusion in the connecting units of a
cascaded OLED, an interfacial layer which provides a barrier for
interfusion may be introduced in between the n-type doped layer and
the p-type doped layer, as described in U.S. Pat. No. 6,717,358,
the disclosure of which is incorporated herein by reference.
[0029] Interfacial layers useful in the connecting unit may
comprise at least one inorganic semiconducting material or
combinations of more than one of the semiconducting materials.
Suitable semiconducting materials should have an electron energy
band gap less than 4.0 eV. The electron energy band gap is defined
as the energy difference between the highest occupied molecular
orbital and the lowest unoccupied molecular orbital of the
molecule. A useful class of materials can be chosen from the
compounds of elements listed in groups IVA, VA, VIA, VIIA, VIIIA,
IB, IIB, IIIB, IVB, and VB in the Periodic Table of the Elements
(e.g. the Periodic Table of the Elements published by VWR
Scientific Products). These compounds include the carbides,
silicides, nitrides, phosphides, arsenides, oxides, sulfides,
selenides, and tellurides, and mixture thereof. These
semiconducting compounds can be in either stoichoimetic or
non-stoichiometic states, that is they may contain excess or
deficit metal component. Particularly useful materials for the
interfacial layer are the semiconducting oxides of titanium,
zirconium, hafnium, vanadium, niobium, tantalum, chromium,
molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium,
cobalt, rhodium, iridium, nickel, palladium, platinum, copper,
zinc, cadmium, gallium, thallium, silicon, germanium, lead, and
antimony, or combinations thereof. Particularly useful materials
for the interfacial layer also including zinc selenide, gallium
nitride, silicon carbide, or combinations thereof.
[0030] The interfacial layer useful in a connecting unit also can
comprise at least one or more metallic materials, where at least
one of these metallic materials has a work-function higher than 4.0
eV as listed by Sze, in Physics of Semiconducting Devices, 2.sup.nd
Edition, Wiley, N.Y., 1981, p. 251. The thickness of an interfacial
layer suitable for the construction of a connecting unit is
preferably in the range of 0.05 nm to 10 .mu.m, more preferably
between 0.1 nm to 5 nm for inorganic semiconducting materials and
between 0.05 nm to 1 nm for metallic materials.
[0031] In a further embodiment, the connecting unit disposed
between each adjacent organic electroluminescent unit in the
cascaded device may include at least a high work function metal
layer having a work function of no less than 4.0 eV and a metal
compound layer, wherein the intermediate connector has a sheet
resistance of higher than 100 k.OMEGA. per square, such as
described in copending, commonly assigned U.S. Ser. No. 10/857,516,
filed May 28, 2004, the disclosure of which is incorporated herein
y reference. The use of such high work function metal layer in a
connecting unit of a cascaded OLED device improves the operational
stability of the OLED.
[0032] As discussed above, for a cascaded OLED to function
efficiently, it is necessary that the intermediate connector should
provide good carrier injection into the adjacent organic EL units.
Due to their lower resistivity than that of organic materials,
metals, metal compounds, or other inorganic compounds can be good
for carrier injection. However, low resistivity can cause low sheet
resistance resulting in pixel crosstalk. If the lateral current
passing through the adjacent pixels to cause pixel crosstalk is
limited to less than 10% of the current used to drive a pixel, the
lateral resistance of the intermediate connector (R.sub.ic) should
be at least 8 times the resistance of the cascaded OLED. Usually,
the static resistance between two electrodes of a conventional OLED
is about several k.OMEGA.s, and a cascaded OLED should have a
resistance of about 10 k.OMEGA. or several 10 k.OMEGA.s between the
two electrodes. Therefore R.sub.ic should be greater than 100
k.OMEGA.. Considering the space between each pixel is smaller than
one square, the sheet resistance of the intermediate connector
should be then greater than 100 k.OMEGA. per square (lateral
resistance equals to sheet resistance times the number of square).
Because the sheet resistance is determined by both the resistivity
and the thickness of the films (sheet resistance equals to film
resistivity divided by film thickness), when the layers
constituting an intermediate connector are selected from metals,
metal compounds, or other inorganic compounds having low
resistivity, a sheet resistance of the intermediate connector
greater than 100 k.OMEGA. per square can still be achievable if the
layers are thin enough.
[0033] Another requirement for the tandem OLED to function
efficiently is that the optical transparency of the layers
constituting the organic EL units and the intermediate connectors
be as high as possible to permit for radiation produced in the
organic EL units to exit the device. According to a simple
calculation, if the optical transmission of each intermediate
connector is 70% of the emitting light, a tandem OLED will not have
much benefit because no matter how many EL units there are in the
device, the electroluminance efficiency can never be doubled when
comparing to a conventional device. The layers constituting the
organic EL units are generally optically transparent to the
radiation produced by the EL units, and therefore their
transparency is generally not a concern for the construction of the
tandem OLEDs. As is known, metals, metal compounds, or other
inorganic compounds can have low transparency. However, when the
layers constituting an intermediate connector are selected from the
metals, metal compounds, or other inorganic compounds, an optical
transmission higher than 70% can still be achievable if the layers
are thin enough. Preferably, the intermediate connector has at
least 75% optical transmission in the visible region of the
spectrum.
[0034] In accordance with one specific embodiment, the intermediate
connectors may comprise, in sequence, a low work function metal
layer, a high work function metal layer, and a metal compound
layer. Herein, a low work function metal is defined as a metal
having a work function less than 4.0 eV. Likewise, a high work
function metal is defined as a metal having a work function no less
than 4.0 eV. The low work function metal layer is preferably
disposed adjacent to the ETL of an organic EL unit towards the
anode side, and the metal compound layer is preferably disposed
adjacent to the HTL of another organic EL unit towards the cathode
side. The low work function metal layer may be selected to provide
efficient electron injection into the adjacent
electron-transporting layer. The metal compound layer may be
selected to provide efficient hole injection into the adjacent
hole-transporting layer. Preferably, the metal compound layer
comprises, but is not limited to, a p-type semiconductor. The high
work function metal layer is selected to improve the operational
stability of the OLED by preventing a possible interaction or
interdiffusion between the low work function layer and the metal
compound layer.
[0035] In accordance with another specific embodiment, the
intermediate connectors may comprise, in sequence, an n-type
semiconductor layer, a high work function metal layer, and a metal
compound layer. The n-type semiconductor layer is preferably
disposed adjacent to the ETL of an organic EL unit towards the
anode side, and the metal compound layer is preferably disposed
adjacent to the HTL of another organic EL unit towards the cathode
side. Herein, an n-type semiconductor layer means that the layer is
electrically conductive having electrons as the major charge
carriers. Likewise, a p-type semiconductor layer means that the
layer is electrically conductive having holes as the major charge
carriers. Similar to a low work function metal layer, the n-type
semiconductor layer may be selected to provide efficient electron
injection into the adjacent electron-transporting layer. The metal
compound layer again may be selected to provide efficient hole
injection into the adjacent hole-transporting layer, and the high
work function metal layer is selected to improve the operational
stability of the OLED by preventing a possible interaction or
interdiffusion between the n-type semiconductor layer and the metal
compound layer.
[0036] In the case such that the ETL in the EL unit is an n-type
doped organic layer, the layer structure of the intermediate
connector can be simplified by comprising, in sequence, a high work
function metal layer disposed adjacent to the n-type doped ETL of
an organic EL unit towards the anode side, and a metal compound
layer disposed adjacent to the HTL of another organic EL unit
towards the cathode side. The metal compound layer may be selected
to provide efficient hole injection into the adjacent
hole-transporting layer, and the high work function metal layer is
selected to improve the operational stability of the OLED by
preventing a possible interaction or interdiffusion between the
n-type doped ETL and the metal compound layer. Herein, an n-type
doped organic layer means that the layer is electrically
conductive, and the charge carriers are primarily electrons. The
conductivity is provided by the formation of a charge-transfer
complex as a result of electron transfer from the dopant to the
host material. Depending on the concentration and the effectiveness
of the dopant in donating an electron to the host material, the
layer electrical conductivity can change by several orders of
magnitude. With an n-type doped organic layer as an ETL in the EL
unit, electrons can be efficiently injected from the adjacent
intermediate connector into the ETL.
[0037] In order for the intermediate connectors to have good
optical transmission (at least 75% optical transmission in the
visible region of the spectrum), good carrier injection capability,
and good operational stability, the thickness of the layers in the
intermediate connectors has to be carefully considered. The
thickness of the low work function metal layer, when employed, in
the intermediate connectors is preferably in the range of from 0.1
nm to 5.0 nm, more preferably in the range of from 0.2 nm to 2.0
nm. The thickness of the high work function metal layer, when
employed, in the intermediate connectors is preferably in the range
of from 0.1 nm to 5.0 nm, more preferably in the range of from 0.2
nm to 2.0 nm. The thickness of the metal compound layer, when
employed, in the intermediate connectors is preferably in the range
of from 0.5 nm to 20 nm, more preferably in the range of from 1.0
nm to 5.0 nm. The thickness of the n-type semiconductor layer, when
employed, in the intermediate connectors is preferably in the range
of from 0.5 nm to 20 nm, more preferably in the range of from 1.0
nm to 5.0 nm.
[0038] The materials used for the fabrication of intermediate
connectors are basically selected from nontoxic materials. Low work
function metal layers may include, e.g., Li, Na, K, Rb, Cs, Mg, Ca,
Sr, Ba, La, Ce, Nd, Sm, Eu, Th, Dy, or Yb. Preferably, the low work
function metal layer includes Li, Na, Cs, Ca, Ba, or Yb. High work
function metal layers may include, e.g., Ti, Zr, Ti, Nb, Ta, Cr,
Mo, W, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt. Cu, Ag, Au, Zn, Al,
In, or Sn. Preferably, the high work function metal layer includes
Ag, Al, Cu, Au, Zn, In, or Sn. More preferably, the high work
function metal layer includes Ag or Al.
[0039] The metal compound layer, when employed, can be selected
from the stoichiometric oxides or nonstoichiometric oxides of
titanium, zirconium, hafnium, niobium, tantalum, molybdenum,
tungsten, manganese, iron, ruthenium, rhodium, iridium, nickel,
palladium, platinum, copper, zinc, silicon, or germanium, or
combinations thereof. The metal compound layer can be selected from
the stoichiometric sulfides or nonstoichiometric sulfides of
titanium, zirconium, hafnium, niobium, tantalum, molybdenum,
tungsten, manganese, iron, ruthenium, rhodium, iridium, nickel,
palladium, platinum, copper, silicon, or germanium, or combinations
thereof. The metal compound layer can be selected from the
stoichiometric selenides or nonstoichiometric selenides of
titanium, zirconium, hafiium, niobium, tantalum, molybdenum,
tungsten, manganese, iron, ruthenium, rhodium, iridium, nickel,
palladium, platinum, copper, silicon, or germanium, or combinations
thereof. The metal compound layer can be selected from the
stoichiometric tellurides or nonstoichiometric tellurides of
titanium, zirconium, hafnium, niobium, tantalum, molybdenum,
tungsten, manganese, iron, ruthenium, rhodium, iridium, nickel,
palladium, platinum, copper, silicon, or germanium, or combinations
thereof. The metal compound layer can be selected from the
stoichiometric nitrides or nonstoichiometric nitrides of titanium,
zirconium, hafnium, niobium, tantalum, molybdenum, tungsten,
manganese, iron, ruthenium, rhodium, iridium, nickel, palladium,
platinum, copper, zinc, gallium, silicon, or germanium, or
combinations thereof. The metal compound layer can also be selected
from the stoichiometric carbides or nonstoichiometric carbides of
titanium, zirconium, hafnium, niobium, tantalum, molybdenum,
tungsten, manganese, iron, ruthenium, rhodium, iridium, nickel,
palladium, platinum, copper, zinc, aluminum, silicon, or germanium,
or combinations thereof. The metal compound layer can be selected
from MoO.sub.3, NiMoO.sub.4, CuMoO.sub.4, WO.sub.3, ZnTe,
Al.sub.4C.sub.3, AIF.sub.3, B.sub.2S.sub.3, CuS, GaP, InP, or SnTe.
Preferably, the metal compound layer is selected from MoO.sub.3,
NiMoO.sub.4, CuMoO.sub.4, or WO.sub.3.
[0040] The n-type semiconductor layer, when employed, may include,
e.g., ZnSe, ZnS, ZnSSe, SnSe, SnS, SnSSe, LaCuO.sub.3, or
La.sub.4Ru.sub.6O.sub.19. Preferably, the n-type semiconductor
layer includes ZnSe or ZnS.
[0041] Other intermediate connector materials may also be employed
in the OLED cascaded devices of the present invention. For example,
Tanaka et al., U.S. Pat. No. 6,107,734, demonstrated a 3-EL-unit
OLED using In--Zn--O (IZO) films or Mg:Ag/IZO films as intermediate
connectors and achieved a luminous efficiency of 10.1 cd/A from
pure tris(8-hydroxyquinoline)aluminum emitting layers. Kido et al.
U.S. Patent Publication 2003/0189401 A1 discloses the use of
light-emissive units partitioned from each other by at least one
charge generation layer, the charge generation layer constituting
an electrically insulating layer having a resistivity of not less
than 1.0.times.10 2 .OMEGA.cm. Kido et al., "High Efficiency
Organic EL Devices having Charge Generation Layers", SID 03 Digest,
964 (2003), fabricated 3-EL-unit OLEDs using In--Sn--O (ITO) films
or V.sub.2O.sub.5 films as intermediate connectors and achieved a
luminous efficiency of up to 48 cd/A from fluorescent dye doped
emitting layers. The disclosures of the above references with
respect to intermediate connector materials are herein incorporated
by reference.
[0042] The intermediate connectors layers, including interfacial
layers, can be produced, e.g., by thermal evaporation, electron
beam evaporation, or ion sputtering technique. Preferably, the
intermediate connectors are fabricated from materials which allow
for a thermal evaporation method for the deposition of all the
materials in the fabrication of the cascaded OLED, including the
intermediate connectors.
[0043] FIG. 1 shows a cascaded bottom emitting OLED device 100 in
accordance with one embodiment of the present invention. This
cascaded OLED has a plurality of independently controlled anodes
110 located over a substrate 105 and a common cathode 140, at least
one of which is transparent. Disposed between the anode and the
cathode are a stack 120 of three organic EL units 121, 122, and
123. These organic EL units are cascaded serially to each other and
to the anode and the cathode. Unit 121 is the first EL unit
(adjacent to the anode) and 123 is the third unit (adjacent to the
cathode). EL unit 122 is an intermediate organic EL unit disposed
between unit 121 and 123. Disposed between any two adjacent organic
EL units is a connecting unit 130. Each anode 110 in the cascaded
OLED 100 is externally connected to a voltage/current source 150
through electrical conductors 160 and can be individually powered
to provide current for the associated light emitters, typically
through either a passive-matrix or active-matrix control
scheme.
[0044] The number of the organic EL units in the cascaded OLED is
in principle equal to or more than 2. Preferably, the number of the
organic EL units in the stacked OLED is such that the luminance
efficiency in units of cd/A is improved or maximized. In further
preferred embodiments, three or more cascaded organic EL units
providing independent optical emission peaks may be employed to
provide a white emission having a combination of relatively narrow
band emitters rather than a single broad-band white light
emission.
[0045] According to the present invention, EL units 121, 122, and
123 emit light of different colors having different efficiencies,
for example red light 128, green light 129, and blue light 127. The
plurality of cascaded organic electroluminescent units are disposed
between a plurality of anodes 110 and a common cathode 140. Each of
the plurality of anodes defines an independently controlled
light-emitting area of the OLED device. Each independently
controlled light-emitting area is associated with a complementary
color filter 124, 125, 126 that transmits the light emitted by only
one of the three EL units. By providing at least two independently
controlled light-emitting areas having differently colored filters
that filter the emitted light, and more preferably by providing a
color filter complementary and matched to each of the different
colors and associated with a different light emitting area, each
light emitting area can be made to emit light of a different color,
thus providing a full-color OLED device.
[0046] Cascaded OLED 100 is operated by applying an electric
potential generated by a voltage/current source 150 between a pair
of contact electrodes, anodes 110 and cathode 140, such that anode
110 is at a more positive potential with respect to the cathode
140. This externally applied electrical potential is distributed
among the three organic EL units in proportion to the electrical
resistance of each of these units. The electric potential across
the cascaded OLED causes holes (positively charged carriers) to be
injected from anodes 110 into the first organic EL unit 121, and
electrons (negatively charged carriers) to be injected from cathode
140 into the third organic EL unit 123. Simultaneously, electrons
and holes are generated in, and separated from, each of the
connecting units 130. Electrons thus generated in a connecting unit
130 are injected towards the anode and into the adjacent organic EL
unit. Likewise, holes generated in a connecting unit 130 are
injected towards the cathode and into the adjacent organic EL unit.
Subsequently, these electrons and holes recombine in their
corresponding organic EL units to produce light, which is observed
via the transparent electrode or electrodes of the OLED through the
corresponding color filter 124, 125, 126. In other words, the
electrons injected from cathode are energetically cascading from
the third organic EL unit 123 to the first organic EL unit 121, and
emit light in each of the organic EL units. Therefore, we prefer to
use the term "cascaded OLED" instead of "stacked OLED" in the
present invention.
[0047] Each organic EL unit in the cascaded OLED 100 is capable of
supporting hole and electron transport, and electron-hole
recombination to produce light. Each organic EL unit can include a
single layer or a plurality of layers. Organic EL multiplayer
structures include HTL/ETL, HTL/LEL/ETL, HIL/HTL/LEL/ETL,
HIL/HTL/LEL/ETL/EIL, HIL/HTL/electron-blocking layer or
hole-blocking layer/LEL/ETL/EIL, HIL/HTL/LEL/hole-blocking
layer/ETL/EIL. Organic EL unit can be formed from small molecule
OLED materials or polymeric LED materials, both known in the art,
or combinations thereof. There are many organic EL multilayer
structures and materials known in the art that can be used as the
organic EL unit of this invention. Each organic EL unit in the
cascaded OLED device can be the same or different from other units.
Some organic EL units can be polymeric LED and other units can be
small molecule OLEDs. Each organic EL unit can be selected in order
to optimize performance or achieve a desired attribute, for example
light transmission through the OLED stack, driving voltage,
luminance efficiency, light emission color, manufacturability,
device stability, and so forth.
[0048] The layer structure of the organic EL unit adjacent to the
anode preferably is of HIL/HTL/LEL/ETL, and the layer structure of
the organic EL unit adjacent to the cathode preferably is of
HTL/LEL/ETL/EIL, and the layer structure of the intermediate
organic EL units preferably are of HTL/LEL/ETL. Connectors
facilitate hole injection into the HTL of one organic EL unit and
electron injection into ETL of the adjacent organic EL unit.
[0049] Within each organic EL unit, the transport of the hole and
electron carriers is supported by the HTL and ETL, respectively.
The LEL may itself be an ETL. Recombination of the hole and
electron carriers in the vicinity at or near the HTL/ETL interface
within each organic EL unit causes light to be produced
(electroluminescence). The HTL in each organic EL unit can be the
same or different in terms of materials used, layer thickness,
method of deposition, and so forth. The properties of the HTL in
the device can be individually optimized to achieve the desired
performance or feature, for example light transmission through the
OLED stack, driving voltage, luminance efficiency, light emission
color, manufacturability, device stability, and so forth. The same
is true for the ETL and LEL. Although not necessary, it is
preferable that a hole-injecting layer (HIL) be provided between
the anode and the first HTL. It is also preferable, but not
necessary, that an electron-injecting layer (EIL) be provided
between the cathode and the last ETL. Both the HIL and EIL improve
charge injection from the electrodes. Organic EL units can
optionally include a HIL between a HTL and a doped organic
connector. Similarly, organic EL units can optionally include an
EIL between an ETL and a doped organic connector.
[0050] In order to minimize driving voltage for the cascaded OLED,
it is desirable to make each organic EL unit as thin as possible
without compromising the electroluminescence efficiency. It is
preferable that each organic EL unit is less than 500 nm thick, and
more preferable that it be 2-200 nm thick. It is also preferable
that each layer within the organic EL unit be 200 nm thick or less,
and more preferable that it be 0.1-100 nm.
[0051] The cascaded OLED 100 of the present invention is typically
provided over a supporting substrate 105 where either the cathode
140 or anode 110 can be in contact with the substrate 105. The
electrode in contact with the substrate is conveniently referred to
as the bottom electrode. Conventionally, the bottom electrode is
the anode, but the present invention is not limited to that
configuration. The substrate can either be light transmissive or
opaque, depending on the intended direction of light emission. The
light transmissive property is desirable for viewing the EL
emission through the substrate (a bottom emitter configuration).
Transparent glass or plastic is commonly employed in such cases.
For applications where the EL emission is viewed through the top
electrode (a top emitter), the transmissive characteristic of the
bottom support is immaterial, and therefore can be light
transmissive, light absorbing or light reflective. Substrates for
use in this case include, but are not limited to, glass, plastic,
semiconductor materials, silicon, ceramics, and circuit board
materials. Of course, it is necessary to provide in these device
configurations a light-transparent top electrode. In such a
configuration, the color filters 124-126 may be provided over the
cathode 140, any protective layers located over the cathode 140, or
on an encapsulating cover (not shown) provided over the OLED
materials and affixed to the substrate 105.
[0052] When EL emission is viewed through anode 110, the anode
should be transparent or substantially transparent to the emission
of interest. Common transparent anode materials used in the present
invention are indium-tin oxide (ITO), indium-zinc oxide (IZO) and
tin oxide, but other metal oxides can work including, but not
limited to, aluminum- or indium-doped zinc oxide, magnesium-indium
oxide, and nickel-tungsten oxide. In addition to these oxides,
metal nitrides, such as gallium nitride, and metal selenides, such
as zinc selenide, and metal sulfides, such as zinc sulfide, can be
used as the anode. For applications where EL emission is viewed
only through the cathode electrode, the transmissive
characteristics of anode are immaterial and any conductive material
can be used, transparent, opaque or reflective. Example conductors
for this application include, but are not limited to, gold,
iridium, molybdenum, palladium, and platinum. Typical anode
materials, transmissive or otherwise, have a work function higher
than 4.0 eV. Desired anode materials are commonly deposited by any
suitable means such as evaporation, sputtering, chemical vapor
deposition, or electrochemical means. Anodes can be patterned using
well-known photolithographic processes. Optionally, anodes may be
polished prior to application of other layers to reduce surface
roughness so as to minimize electrical shorts or enhance
reflectivity.
[0053] While not always necessary, it is often useful to provide a
HIL in the first organic EL unit to contact the anode 110. The HIL
can serve to improve the film formation property of subsequent
organic layers and to facilitate injection of holes into the HTL
reducing the driving voltage of the cascaded OLED. Suitable
materials for use in the HIL include, but are not limited to,
porphyrinic compounds as described in U.S. Pat. No. 4,720,432,
plasma-deposited fluorocarbon polymers as described in U.S. Pat.
No. 6,208,075, and some aromatic amines, for example, m-MTDATA
(4,4',4"-tris[(3-ethylphenyl)phenylamino]triphenylamine). A p-type
doped organic layer for use in the aforementioned connecting unit
is also useful for the HIL as described in U.S. Pat. No. 6,423,429
B2. Alternative hole-injecting materials reportedly useful in
organic EL devices are described in EP 0 891 121 A1 and EP 1 029
909 A1.
[0054] Referring to FIG. 2, the emission spectrum of a white-light
emitting OLED made by applicant is shown. While this is a
relatively efficient white-light emitting material, it can be seen
that most of the energy in the spectrum is cyan (peak 200) and
yellow (peak 202). When combined with red, green, and blue color
filters, the amount of light emitted through the filters of the
desired color will be relatively low. In particular, the green
emission (peak 204) and red emission (peak 206) are relatively
low.
[0055] The present invention can provide a higher efficiency
full-color OLED device by providing a cascaded RGB architecture
having a common control for all light-emitting units in the stack,
where white light is generated by each pixel and filtered using RGB
filters. This simplifies manufacturing because it is generally
easier to apply a RGB filter after OLED device fabrication than to
pattern RGB emitting pixels. However, since some of the light
emitting units may be less efficient, they may need to be driven
harder to provide comparable levels of light to produce, for
example, a white or gray color. This may cause the less efficient
materials to age more rapidly, thereby causing color differential
aging and a color-OLED device whose white point will change over
time and whose luminance will decrease. In a preferred embodiment
of the invention, this may be addressed by differential sizing of
the independent controlled light emitting areas and corresponding
differentially colored filters.
[0056] To illustrate the advantages of a preferred embodiment of
the present invention in a practical example, a set of four
materials developed by applicant may have efficiencies as listed in
the table below:
1 Color Efficiency (cd/A) Relative Size White 13 Green 28 1 Red 9
3.1 Blue 6 4.7
[0057] As can be seen from the table above, the relative
light-emitting efficiency of the various color light emitters
varies. If all of the light emitting pixels were of the same size
and the color filters were of equal efficiency over the output
bandwidth of the light emitters, the green light would be much
brighter because it is more efficient and the blue light would be
dimmer because it is less efficient. The relative efficiency of the
light-emitting materials and the associated color filter can be
accommodated by creating anodes and color filters of relatively
different sizes corresponding to the relative efficiencies. The
more efficient units will have smaller associated color filters and
anodes, the less efficient units will have larger color filter and
anodes. For example, as shown in FIG. 1, the green filter is
smallest and the blue filter is largest. Using the example of the
materials cited in the table above, the relative sizes of the
anodes and associated color filters are 1 for green, 3.1 for red,
and 4.7 for blue. The relative sizes of the anodes and associated
color filters should reflect the relative efficiency of the light
emitting units in combination with the color filters.
Alternatively, the relative sizes of the anodes and associated
color filters may reflect the relative aging and lifetime of the
light emitting units. Typically, the relative size of an
independently controlled light-emitting area and associated color
filter will be inversely related to the relative efficiency of
light emission from or lifetime of the cascaded organic
electroluminescent units. E.g., an efficient long-lived emitter
will be relatively smaller than an inefficient short-lived emitter.
As shown in the embodiment of FIG. 1, filters 124-126 may be a
conventional color filter array such as is used in the liquid
crystal display industry composed of light-absorbent material that
only permits the desired color of light to pass through. The
frequency of light passed through the filter from the stack 120 of
light emitting units should match the emission spectrum of the
light from the corresponding desired unit. For example, the light
transmitted by a green filter should match the emission of the
green light emitting unit.
[0058] For maximum benefit, both the emission of the light emitting
units and the color filters will be as narrow as possible. This
will optimize the power emitted through the color filters while
providing an improved color gamut. Moreover, a narrow emission
spectrum will provide improved contrast to the OLED device by
absorbing more of the ambient light.
[0059] As illustrated in FIG. 1, the OLED device has three
light-emitting layers, e.g., one each for red, green, and blue.
However, it is also possible to combine a color emitter with a
broadband emitter and utilize only two light emitting units. As
noted above, the white-light emitting material that produces the
spectrum illustrated in FIG. 2 is very deficient in green light
emission. An OLED device according to another embodiment of the
present invention can have a white-light emitting unit in
combination with a green-light emitting unit, as illustrated in
FIG. 3. This arrangement reduces the number of light-emitting units
while providing improved light output over a single, white-light
emitting unit. In this arrangement, the green color filter 126
permits green light 129 from both the green unit 123 and the white
unit 170 to pass while absorbing the blue and red light from the
white unit 170. The red light 128 emitted from the white unit 170
will pass through the red color filter 125 while blocking the green
light emitted from the green light-emitting unit 123 and blue light
from the white-light emitting unit 170. The blue light 127 emitted
from the white unit 170 will pass through the blue color filter 124
while blocking the green light emitted from the green
light-emitting unit 123 and red light from the white-light emitting
unit 170.
[0060] In general, a plurality of combinations of light-emitting
units having different colors may be employed. There are many other
combinations of organic EL units in addition to red, green and blue
that can be used to yield light that appears white. For example,
two-layer structures that emit blue and yellow light, or that emit
red and cyan light, or that emit green and magenta light, can be
used to generate white light. In all cases these units can be
combined multiple times. Further, any combination of colored-light
and white-light emitting units that provide improved efficiency of
output may be included in the present invention. Applicant has
found that a variety of solutions for various applications may be
possible that include various color filters combined with OLED
materials to provide a good color gamut and efficiency.
[0061] The present invention may also be employed in an RGBW
configuration, that is, one that has four light-emitting pixels,
one each for red, green, blue, and white. In this arrangement, the
white pixel need not have any filter at all, or may only have a
filter necessary to achieve the desired white point of the OLED
device. It is also possible to provide two layers that emit the
same color of light; this technique can be used to optimize the
color balance of the white emitter or the relative amount of light
emitted by the various colors.
[0062] The order of layer deposition may be controlled to optimize
the structure of the present invention to improve performance.
Applicant has demonstrated that the organic materials within an
OLED are themselves light absorbing. Hence, some of the light
emitted from the bottom of a stack (the side farthest from the side
from which light is emitted from an OLED) will be absorbed by
layers above it. Therefore, it is useful to put the most efficient
emitter or the one that emits light that is least absorbed by the
other layers at the bottom. In the example cited above, green is
the most efficient and thus may be advantageously placed at the
bottom of the stack. Likewise, models of the light absorption in
the various organic layers indicate that blue light is absorbed
most readily, so that the blue light-emitting unit may be located
at the top of the stack, as illustrated in FIG. 1.
[0063] The present invention provides improved manufacturability of
a color OLED device by enabling unpatterned deposition of organic
materials onto the substrate. No masking is needed because all of
the layers are deposited across the entire light emitting area of
the substrate. Moreover, improved efficiency over the use of a
conventional white-light emitting OLEDs may be obtained by
providing an output spectrum for each of the colors that is closely
matched to its associated color filter.
[0064] The entire contents of the patents and other publications
referred to in this specification are incorporated herein by
reference.
[0065] The above examples demonstrate that significant increase in
luminance efficiency can be achieved by using a cascaded OLED
structure of the present invention comparing the conventional OLED.
If operated with the same luminance, significant increase in
operational lifetime can also be achieved by using the cascaded
OLED structure of the present invention comparing the conventional
OLED. Moreover, during operation, the driving voltage can be
stabilized due to the insertion of the interfacial layer in the
connecting unit. The invention has been described in detail with
particular reference to certain preferred embodiments thereof, but
it will be understood that variations and modifications can be
effected within the spirit and scope of the invention.
2 PARTS LIST 100 cascaded OLED 105 substrate 110 anode 120 EL stack
121 blue EL unit 122 red EL unit 123 green EL unit 124 blue color
filter 125 red color filter 126 green color filter 127 blue light
128 red light 129 green light 130 connecting unit 140 cathode 150
voltage/current source 160 electrical conductors 170 white EL unit
200 cyan peak 202 yellow peak 204 green peak 206 red peak
* * * * *