U.S. patent application number 09/234457 was filed with the patent office on 2002-01-31 for organic electroluminescent device with improved hole injecting structure.
Invention is credited to DENG, ZHEN-BO, GAMBLING, WILLIAM ALEXANDER, LEE, SHUIT-TONG.
Application Number | 20020011782 09/234457 |
Document ID | / |
Family ID | 22881476 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020011782 |
Kind Code |
A1 |
LEE, SHUIT-TONG ; et
al. |
January 31, 2002 |
ORGANIC ELECTROLUMINESCENT DEVICE WITH IMPROVED HOLE INJECTING
STRUCTURE
Abstract
An organic electroluminescent device (OELD) with improved
luminescent efficiency has been fabricated upon inclusion of an
inorganic buffer layer in the hole injecting or electron injecting
regions. The structure of the device can be as follows (from bottom
to top): ITO.backslash.buffer
layer.backslash.NPB.backslash.Alq.backslash.Mg:Ag. In comparison
with devices without the buffer layer, the present OELD may be more
efficient in a proper bias condition.
Inventors: |
LEE, SHUIT-TONG; (KOWLOON,
HK) ; DENG, ZHEN-BO; (KOWLOON, HK) ; GAMBLING,
WILLIAM ALEXANDER; (KOWLOON, HK) |
Correspondence
Address: |
JACOBSON PRICE HOLMAN & STERN
400 SEVENTH STREET NW
WASHINGTON
DC
20004
|
Family ID: |
22881476 |
Appl. No.: |
09/234457 |
Filed: |
January 21, 1999 |
Current U.S.
Class: |
313/504 |
Current CPC
Class: |
Y10S 428/917 20130101;
H01L 51/5088 20130101; H01L 51/5092 20130101 |
Class at
Publication: |
313/504 |
International
Class: |
H05B 033/14 |
Claims
1. An organic electroluminescent device including an anode, a
cathode and an organic electroluminescent element disposed between
the anode and cathode and wherein at least one buffer layer is
provided between said anode and cathode.
2. A device as claimed in claim 1 wherein said organic
electroluminescent element includes one hole transporting layer and
one fluorescent emitting layer.
3. A device as claimed in claim 1 wherein said organic
electroluminescent element includes one fluorescent emitting layer
and one electron transporting layer.
4. A device as claimed in claim 1 wherein said organic
electroluminescent element includes one hole transporting layer,
one fluorescent emitting layer and one electron transporting
layer.
5. A device as claimed in claim 1 wherein said buffer layer is
disposed between the anode and the hole transporting layer.
6. A device as claimed in claim 1 wherein the buffer layer is
disposed between the cathode and the electron transporting
layer.
7. A device as claimed in claim 1 wherein said buffer layer
comprising a material selected from the group consisting of
insulating metal oxides, semiconductor oxides, diamond-like carbon
and nitrogen-doped carbon.
8. A device as claimed in any one of claims 1 to 7 wherein said
metal oxides and semiconductor oxides including Ta.sub.2O.sub.5,
Y.sub.2O.sub.3 and SiO.sub.x, with x from 1 to 2.
9. A device as claimed in any one of claims 1 to 7 wherein said
buffer layer has a thickness ranging from 0 to 20 nm.
10. A device as claimed in any one of claims 1 to 7 wherein said
buffer layer has been further treated by radiation.
11. A device as claimed in any one of claims 1 to 7 wherein said
organic electroluminescent element comprises materials including
molecules selected from Alq, TPD, NPB and TPBI and polymers.
12. A device as claimed in claim 11 wherein said polymers include
poly (paraphenylene vinylene) (PPV), PPV copolymers and
derivatives.
13. A device as claimed in any one of claims 1 to 7 wherein said
anode is selected from transparent conducting oxides including
indium-tin-oxide, aluminum- or indium-doped zinc oxide, tin oxide,
magnesium-indium-oxide, and cadmium-tin-oxide.
14. A method of manufacturing an organic electroluminescent device
comprising the steps of providing an anode, a cathode and an
organic electroluminescent element between said anode and cathode
and wherein a buffer layer is positioned between said anode and
cathode.
15. A method as claimed in claim 14 wherein said step of
positioning said buffer layer comprises depositing a buffer layer
onto said anode and/or cathode.
16. A method as claimed in claim 15 wherein said buffer layer is
deposited onto said anode.
17. A method as claimed in claim 15 wherein said deposition is
performed by evaporation.
18. A method as claimed in claim 14 wherein said buffer layer is
provided having a thickness in a range of 0 to 200 Angstorms.
Description
FIELD OF THE INVENTION
[0001] The present invention pertains to organic thin-film
electroluminescent devices and a method of manufacture of such
devices.
BACKGROUND TO THE INVENTION
[0002] Fabrication of organic electroluminescent devices (OELDs)
can be traced back to 1960's. Representatives of early OELDs are
Gurnee et al U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee
U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; and Dresner U.S. Pat.
No. 3,710,167, issued Jan. 9, 1973. These devices comprised a
single organic emitting layer having thicknesses above 1 mm and two
electrodes (cathode and anode). Alkali metals were used as the
cathode materials as they had the lowest work function values.
[0003] Tang et al U.S. Pat. No. 4,356,429 disclosed an OELD with
two organic layers, in which the organic luminescent medium
consisted of two extremely thin layers separating the anode and the
cathode. These comprised one layer injecting and transporting holes
and the other layer injecting and transporting electrons and also
acting as the organic luminescent zone of the device.
[0004] Improvements were then achieved by using different cathode
materials. Tang et al U.S. Pat. No. 4,885,211 disclosed an OELD
with its cathode comprising a layer of a plurality of metals other
than single alkali metals, at least one of the metals having a work
function of less than 4 eV. Hung et al U.S. Pat. No. 5,608,287
disclosed OELDs having a conductive electron injector. Shi et al
U.S. Pat. No. 5,668,438 disclosed an OELD in which the materials
making up the electron transporting layer (ETL) and the hole
transporting layer (HTL) were selected so that the energy barrier
for hole injection from the HTL to the ETL was substantially higher
than the energy barrier for electron injection from the ETL to the
HTL. In that case, light was emitted from the HTL rather than the
ETL.
[0005] More recently, Hung et al U.S. Pat. No. 5,776,623 disclosed
an OELD containing a transparent electron-injecting electrode. The
electrode included a thin non-conductive layer contacting the
electroluminescent layer and a conductive transparent overcoat
layer. The thickness of the nonconductive layer was selected so
that the bilayer acted as an electron injecting contact and
provided stability against atmospheric corrosion.
[0006] However, only very few patents have dealt with the hole
injecting structure. Vanslyke et al U.S. Pat. No. 5,061,569
disclosed an internal junction OELD in which the hole injecting and
transporting zone included a tertiary amine containing at least two
tertiary amine moieties and including, attached to a tertiary amine
nitrogen atom, an aromatic moiety containing at least two fused
aromatic rings. Tokito et al U.S. Pat. No. 5,783,292 disclosed an
OELD in which organic-inorganic composite thin film was used.
OBJECT OF THE INVENTION
[0007] It is an object of the present invention to provide an OELD
with an increased electroluminescent efficiency and/or improved
stability or which will at least provide the public with a useful
choice.
SUMMARY OF THE INVENTION
[0008] This invention comprises an OELD and method of manufacture
in which an inorganic buffer layer is included in the hole
injecting region.
[0009] Preferably, the device comprises in order: an ITO-covered
glass substrate which behaves as the hole injector and is
transparent and transmissive to optical radiation, a layer of
inorganic material as a buffer to the hole injection, an organic
single layer or multilayer structure for electroluminescence and
carrier confinement, and a layer of low-work-function material as
an electron injector which is stable relative to atmospheric
corrosion. The buffer layer may also be included in the electron
injecting region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Preferred embodiments of this invention can be better
appreciated by reference to the following description considered in
conjunction with the drawings in which:
[0011] FIGS. 1 and 2 which are schematic diagrams of an embodiment
of the OELD of the invention;
[0012] FIG. 3 is a graphical representation of the results of
different thickness of buffer layer in terms of brightness; and
[0013] FIG. 4 is a graphical representation of the results of FIG.
3 in terms of electroluminescent efficiency.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] An electroluminescent (EL) device 100 of the invention is
schematically illustrated in FIG. 1. Anode 102 is separated from
cathode 104 by an inorganic buffer layer 106 and an organic
luminescent medium 108, which, as shown in this embodiment,
consists of two superimposed layers. Layer 110, which is located
above the buffer layer 106, forms a hole injecting zone of the
organic luminescent medium. Above layer 110 is layer 112, which
forms an electron transporting zone of the organic luminescent
medium. The anode and the cathode are connected to an external AC
or DC power source 114 by conductors 116 and 118, respectively. The
power source can be pulsed or continuous wave.
[0015] When the EL device 100 is forward biased, i.e., the
potential of the anode is higher than that of the cathode, there
occurs injection of holes (positive charge carriers) into the lower
organic layer, as schematically shown at 120, while electrons are
injected into the upper organic layer, as schematically shown at
122, in the luminescent medium. The injected holes and electrons
each migrate toward the oppositely charged electrode, as shown by
the arrows 124 and 126, respectively. This results in hole-electron
recombination. When a migrating electron drops from its conduction
potential to a valence band in filing a hole, energy is released as
light. Hence the organic luminescent medium forms between the
electrodes a luminescent zone receiving mobile charge carriers from
each electrode. Depending upon the choice of alternative
constructions, the released light can be emitted from the organic
luminescent material through one or more edges 128 of the organic
luminescent medium separating the electrodes, through the anode,
through the cathode, or through any combination of the
foregoing.
[0016] Organic EL device 200 shown in FIG. 2 is illustrative of one
preferred embodiment of the invention. Same as in previous
OELDs'fabrication, a transparent anode electrode is used in the
present invention. Layer 202 is a transparent and insulating
support onto which is deposited a conductive and light transmissive
204. Typically, this layer is made from metal or metal oxide such
as ITO. This layer, as an anode electrode, possesses a relatively
high work function. Adjacent to 204 is the anode-combined buffer
layer 206, which, in this embodiment, is selected from the group
consisting of SiO.sub.x, Y.sub.2O.sub.3, and Ta.sub.2O.sub.5, where
x is from 1 to 2. Above 206 is the organic luminescent medium 208,
consisting of hole injecting zone 210 and electron transporting
zone 212 in which luminescence occurs. The top electrode 214 acts
as the electron injector with a relatively low work function. It is
usually either a single metal or a multi-metal mixture formed by
codeposition in vacuum.
[0017] The preferred embodiments above describe an OELD which
positions the buffer layer between the anode and the luminescent
medium. The buffer layer may also increase efficiency when
positioned between the cathode and the luminescent medium, for
example, between the cathode and electron transporting layer.
Furthermore, multiple buffer layers such as a buffer layer adjacent
the anode and a further buffer layer adjacent the cathode are
possible. There is no requirement for identical buffer layers in
such a multi-layer structure with the buffer layers on opposed
sides of the luminescent medium able to be of different materials
or mixtures as required.
[0018] The buffer layers described have been selected from a
particularly group of oxides. In preferred embodiments, the buffer
layer is chosen from insulating metal oxides, semiconductor oxides,
diamond-like carbon and nitrogen doped carbon. Of course, a number
of different materials may be used in the buffer layer to perform
the same function. The list provided is not considered
exhaustive.
[0019] The electroluminescent element contained within the device
can comprise variety of materials found suitable for such purposes.
These include the inclusion of molecules selected from Alq, TPD,
NPB and TPBI as well as a variety of polymers. The polymers may
include such polymers as poly (paraphenylene vinylene) (PPV), PPV
copolymers and derivatives. Again, this is not an exhaustive list
of possible materials for this element.
[0020] Similarly, the anode can be made from many suitable
materials for the purpose. Typically such anodes are made from
transparent conducting oxides. These include indium-tin-oxide,
aluminum- or indium-doped zinc oxide, tin oxide,
magnesium-indium-oxide and cadmium-tin-oxide.
EXAMPLES
[0021] The invention and its advantages are further illustrated by
the specific examples which follow:
Example 1
[0022] Fabrication Procedure
[0023] An OELD satisfying the requirements of the invention was
constructed in the following manner. The device structure has an
inorganic buffer layer and a two organic-layer stack, namely hole
transporting layer and fluorescent emitting and electron
transporting layer.
[0024] An indium-tin-oxide coated glass substrate was sequentially
ultrasonicated in a commercial detergent, rinsed in deionized
water, degreased in organic solvent, such as ethanol and acetone,
and exposed to ultraviolet light and ozone for a few minutes.
[0025] An inorganic buffer layer (0-200 Angstroms) was then
deposited on top of the ITO coated substrate by evaporation.
[0026] Onto the buffer layer was deposited a hole transporting
layer of TPD or NPB (500-1,500 Angstroms), evaporated from a
tantalum boat.
[0027] A fluorescent emitting and electron transporting layer of
Alq (600 Angstroms) was then deposited onto the hole transporting
layer.
[0028] On top of the Alq layer was deposited a 1,500-Angstrom thick
cathode formed of a 10:1 atomic ratio of Mg and Ag.
[0029] The above sequence completed the deposition of the OELD. The
device was then packaged in a dry glove box for protection against
ambient environment.
Example 2
[0030] OELDs with SiO.sub.2 Buffer Layer
[0031] The OELDs were fabricated according to the procedure of
Example 1. The buffer layers with different thicknesses of
SiO.sub.2 were used. Table 1 lists the EL luminance quantum
efficiency measured in unit of candela per ampere, and luminance
output under a constant current bias of 20 mA/cm.sup.2.
1TABLE 1 SiO.sub.2 Thickness (nm) 0 0.5 1.0 1.5 CIEx 0.334 0.338
0.339 0.339 CIEy 0.562 0.563 0.562 0.562 Luminance 1,140 960 1,820
1,680 Luminance Eff. (cd/A) 5.7 4.8 9.1 8.4
[0032] Referring to the luminance efficiency, it can be observed
that there may be a reduction in efficiency with a particularly
thin layer. However, as shown in this example using SiO.sub.2, the
efficiency increases towards the thickness of 1 nm. There may then
be a tailing off of efficiency as the thickness increases. This
particular buffer layer is an insulating layer and will inhibit
current as the layer thickness increases. The thickness as provided
in this table shows the performance of SiO.sub.2. Different
thicknesses may provide different results with alternative
materials used as the buffer layer.
Example 3
[0033] Brightness-Current-Voltage (B-I-V) Characteristics
[0034] The OELDs were fabricated according to the procedure of
Example 1. FIG. 3 shows the B-I-V curves of four devices with
different thicknesses of buffer layer (SiO.sub.2)
Example 4
[0035] Quantum Efficiency of the Devices with Different Thicknesses
of SiO.sub.2
[0036] The OELDs were fabricated according to the procedure of
Example 1. FIG. 4 shows the electroluminescent efficiency of the
four devices with different thicknesses of buffer layer
(SiO.sub.2).
Example 5
[0037] OELDs with Different Buffer Layers
[0038] The OELDs were fabricated according to the procedure of
Example 1. Buffer layers made of different inorganic materials,
such as SiO.sub.2, Y.sub.2O.sub.3 and Ta.sub.2O.sub.5, were used.
Table 2 lists the EL luminance quantum efficiency measured in unit
of candela per ampere and luminance output under a constant current
bias of 20 mA/cm.sup.2 (the thickness of the inorganic materials is
about 1.0 nm).
2 TABLE 2 Materials SiO.sub.2 Y.sub.2O.sub.3 Ta.sub.2O.sub.5 CIEx
0.339 0.334 0.335 CIEy 0.562 0.562 0.562 Luminance 1,820 1,280
1,360 Luminance Eff. (cd/A) 9.1 6.4 6.8
Example 6
[0039] Further Treatment of the Buffer Layer
[0040] The OELDs were fabricated according to the procedure of
Example 1. The substrate with buffer layer was further treated by
radiation, which effectively protects the surface of the buffer
layer against harmful contamination. Even higher EL efficiency can
be obtained thereby.
[0041] The fabrication procedure provided in the examples again
discloses a method of manufacturing an OELD which incorporates a
buffer layer between the cathode and anode. The examples given
deposit the buffer layer onto the intended anode of the device.
Again, it should be noted that the layer may be deposited prior to
deposition of the cathode or onto the cathode be the device is
deposited in reverse. The order of the layers in the examples are
for these preferred embodiments only.
[0042] Further aspects of this invention may become apparent to
those skill in the art to which the invention relates. It should be
noted that integers referred to throughout the specification are
deemed to incorporate known equivalents and the disclosure of the
preferred embodiments did not be considered limiting to the scope
of the invention as defined by the appended claims.
* * * * *