U.S. patent application number 11/028033 was filed with the patent office on 2006-07-06 for tandem oleds having low drive voltage.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Dustin L. Comfort, Tukaram K. Hatwar, Kevin P. Klubek, Liang-Sheng Liao, J. Ramon Vargas.
Application Number | 20060145604 11/028033 |
Document ID | / |
Family ID | 36639603 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060145604 |
Kind Code |
A1 |
Liao; Liang-Sheng ; et
al. |
July 6, 2006 |
TANDEM OLEDS HAVING LOW DRIVE VOLTAGE
Abstract
A tandem OLED includes an anode, a cathode, and at least two
electroluminescent units disposed between the anode and the
cathode, wherein each of the electroluminescent units includes at
least one hole-injecting layer, one hole-transporting layer, one
organic light-emitting layer, one electron-transporting layer, and
one electron-injecting layer. The OLED also includes at least one
intermediate connector, wherein each of the intermediate connectors
includes at least one layer, and wherein each of the intermediate
connectors is disposed between electroluminescent units, wherein
the thickness of each layer in each of the electroluminescent units
and the intermediate connectors is selected to satisfy the test
condition that the voltage drop from the anode to the cathode is
less than 4.0 V.times.N (the number of electroluminescent units) at
20 mA/cm.sup.2.
Inventors: |
Liao; Liang-Sheng;
(Rochester, NY) ; Hatwar; Tukaram K.; (Penfield,
NY) ; Klubek; Kevin P.; (Webster, NY) ;
Vargas; J. Ramon; (Webster, NY) ; Comfort; Dustin
L.; (Webster, NY) |
Correspondence
Address: |
Pamela R. Crocker;Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
36639603 |
Appl. No.: |
11/028033 |
Filed: |
January 3, 2005 |
Current U.S.
Class: |
313/506 |
Current CPC
Class: |
H01L 51/0059 20130101;
C09K 2211/1029 20130101; H01L 51/0081 20130101; H01L 51/5278
20130101; H01L 51/0073 20130101; H01L 51/002 20130101; H01L 51/5076
20130101; H01L 2251/558 20130101; H01L 51/506 20130101; C09K
2211/1037 20130101 |
Class at
Publication: |
313/506 |
International
Class: |
H01J 1/62 20060101
H01J001/62 |
Claims
1. A tandem OLED comprising: a) an anode; b) a cathode; c) at least
two electroluminescent units disposed between the anode and the
cathode, wherein each of the electroluminescent units includes at
least one hole-injecting layer, one hole-transporting layer, one
organic light-emitting layer, one electron-transporting layer, and
one electron-injecting layer; d) at least one intermediate
connector, wherein each of the intermediate connectors includes at
least one layer, and wherein each of the intermediate connectors is
disposed between electroluminescent units; and e) wherein the
thickness of each layer in each of the electroluminescent units and
the intermediate connectors is selected to provide high luminous
efficiency using optical out-coupling while satisfying the test
condition that the voltage drop from the anode to the cathode is
less than 4.0 V.times.N (the number of electroluminescent units) at
20 mA/cm.sup.2.
2. The tandem OLED of claim 1 wherein there are three
electroluminescent units disposed between the anode and the
cathode, wherein each of the electroluminescent units includes at
least one hole-injecting layer, one hole-transporting layer, one
organic light-emitting layer, one electron-transporting layer, and
one electron-injecting layer and two intermediate connectors,
wherein each of the intermediate connectors includes at least one
layer, and wherein each of the intermediate connectors is disposed
between electroluminescent units, and the thickness of each layer
in each of the electroluminescent units and the intermediate
connectors is selected to provide high luminous efficiency using
optical out-coupling while satisfying the test condition that the
voltage drop from the anode to the cathode is less than 12 V at 20
mA/cm.sup.2.
3. The tandem OLED of claim 1 wherein the hole-transporting layer
in contact with the organic light-emitting layer in each of the
electroluminescent units has a thickness of less than or equal to
10 nm, wherein the organic light-emitting layer in each of the
electroluminescent units has a thickness of less than or equal to
20 nm, wherein the electron-transporting layer in contact with the
organic light-emitting layer in each of the electroluminescent
units has a thickness of less than or equal to 10 nm, and wherein
each of the layers in each of the intermediate connectors has a
thickness of less than or equal to 5 nm.
4. The tandem OLED of claim I wherein the hole-transporting layer
in contact with the organic light-emitting layer in each of the
electroluminescent units has a thickness of less than or equal to 5
nm, wherein the organic light-emitting layer in each of the
electroluminescent units has a thickness of less than or equal to
15 nm, wherein the electron-transporting layer in contact with the
organic light-emitting layer in each of the electroluminescent
units has a thickness of less than or equal to 5 nm, and wherein
each of the layers in each of the intermediate connectors has a
thickness of less than or equal to 3 nm.
5. A tandem OLED comprising: a) an anode; b) a cathode; c) at least
two electroluminescent units disposed between the anode and the
cathode, wherein each of the electroluminescent units includes at
least one hole-injecting layer, one hole-transporting layer, one
organic light-emitting layer, one electron-transporting layer, and
one electron-injecting layer; d) at least one intermediate
connector, wherein each of the intermediate connectors includes at
least one layer, and wherein each of the intermediate connectors is
disposed between electroluminescent units; and e) wherein the
thickness of each layer in each of the electroluminescent units and
the intermediate connectors is selected to provide high luminous
efficiency using optical out-coupling while satisfying the test
condition that the voltage drop from the anode to the cathode is
less than 3.5 V.times.N at 1000 cd/m.sup.2.
6. The tandem OLED of claim 5 wherein there are three
electroluminescent units disposed between the anode and the
cathode, wherein each of the electroluminescent units includes at
least one hole-injecting layer, one hole-transporting layer, one
organic light-emitting layer, one electron-transporting layer, and
one electron-injecting layer and two intermediate connectors,
wherein each of the intermediate connectors includes at least one
layer, and wherein each of the intermediate connectors is disposed
between electroluminescent units, and the thickness of each layer
in each of the electroluminescent units and the intermediate
connectors is selected to provide high luminous efficiency using
optical out-coupling while satisfying the test condition that the
voltage drop from the anode to the cathode is less than 10.5 V at
1000 cd/m.sup.2.
7. The tandem OLED of claim 5 wherein the hole-transporting layer
in contact with the organic light-emitting layer in each of the
electroluminescent units has a thickness of less than or equal to
10 nm, wherein the organic light-emitting layer in each of the
electroluminescent units has a thickness of less than or equal to
20 nm, wherein the electron-transporting layer in contact with the
organic light-emitting layer in each of the electroluminescent
units has a thickness of less than or equal to 10 nm, and wherein
each of the layers in each of the intermediate connectors has a
thickness of less than or equal to 5 nm.
8. The tandem OLED of claim 5 wherein the hole-transporting layer
in contact with the organic light-emitting layer in each of the
electroluminescent units has a thickness of less than or equal to 5
nm, wherein the organic light-emitting layer in each of the
electroluminescent units has a thickness of less than or equal to
15 nm, wherein the electron-transporting layer in contact with the
organic light-emitting layer in each of the electroluminescent
units has a thickness of less than or equal to 5 nm, and wherein
each of the layers in each of the intermediate connectors has a
thickness of less than or equal to 3 nm.
9. The tandem OLED of claim 1 wherein the hole-injecting layer in
each of the electroluminescent units is a p-type doped layer
containing at least one hole-transporting material and at least one
p-type dopant.
10. The tandem OLED of claim 9 wherein the hole-transporting
material includes N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine
(NPB),
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1-biphenyl-4,4'-diamine
(TPD), N,N,N',N'-tetranaphthyl-benzidine (TNB),
4,4',4''-tris(N-3-metylphenyl-N-phenyl-amino)-triphenylamine
(m-MTDATA), 4,4',4''-tris(N,N-diphenyl-amino)-triphenylamine
(TDATA), dihydrophenazine compounds, or hexaazatriphenylene
compounds, or combinations thereof.
11. The tandem OLED of claim 9 wherein the p-type dopant includes
oxidizing agents capable of forming a charge transfer complex with
the host material.
12. The tandem OLED of claim 9 wherein the p-type dopant includes
iodine, FeCl.sub.3, FeF.sub.3, SbCl.sub.5, MoO.sub.3, WO.sub.3,
2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane
(F.sub.4-TCNQ), other derivatives of
7,7,8,8-tetra-cyanoquinodimethane (TCNQ), or hexaazatriphenylene
compounds, or combinations thereof.
13. The tandem OLED of claim 1 wherein the organic light-emitting
layer in each of the electroluminescent units individually emits
red, green, blue, or white color, or combinations thereof.
14. The tandem OLED of claim 1 wherein the electron-injecting layer
in each of the electroluminescent units is an n-type doped layer
containing at least one electron-transporting material and at least
one n-type dopant.
15. The tandem OLED of claim 14 wherein the electron-transporting
material includes tris(8-hydroxyquinoline)aluminum (Alq),
4,7-diphenyl-1,10-phenanthroline (Bphen),
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), other
phenanthroline derivatives, or
2,2'-[1,1'-biphenyl]-4,4'-diylbis[4,6-(p-tolyl)-1,3,5-triazine]
(TRAZ), or combinations thereof.
16. The tandem OLED of claim 14 wherein the n-type dopant includes
Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, La, Ce, Nd, Sm, Eu, Th, Dy, or
Yb, or combinations thereof.
17. The tandem OLED of claim 1 wherein each layer in each of the
intermediate connectors includes organic material, metal compound,
or high work function metal, or combinations thereof.
18. The tandem OLED of claim 1 wherein each layer in each of the
intermediate connectors is a doped layer or a non-doped layer.
19. The tandem OLED of claim 1 wherein each of the intermediate
connectors includes a layer of MoO.sub.3, NiMoO.sub.4, CuMoO.sub.4,
WO.sub.3, ZnTe, Al.sub.4C.sub.3, AlF.sub.3, B.sub.2S.sub.3, CuS,
GaP, InP, or SnTe, or combination thereof.
20. The tandem OLED of claim 1 wherein each of the intermediate
connectors includes the layer of 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,
or combinations thereof.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Reference is made to commonly assigned U.S. patent
application Ser. No. 10/437,195 filed May 13, 2003 by Liang-Sheng
Liao, et al., entitled "Cascaded Organic Electroluminescent Device
Having Connecting Units With n-Type and p-Type Organic Layers";
U.S. patent application Ser. No. 10/857,516 filed May 28, 2004 by
Liang-Sheng Liao, et al., entitled "Tandem OLED Having Stable
Intermediate Connectors"; U.S. patent application Ser. No.
10/922,606 filed Aug. 20, 2004 by Liang-Sheng Liao, et al.,
entitled "White OLED Having Multiple White Electroluminescence
Units"; and U.S. patent application Ser. No. 10/970,928 filed Oct.
22, 2004 by Liang-Sheng Liao, et al., entitled "White OLEDs With a
Color Compensated Electroluminescent Unit", the disclosures of
which are herein incorporated by reference.
FIELD OF INVENTION
[0002] The present invention relates to providing a plurality of
organic electroluminescent (EL) units to form a tandem organic
electroluminescent device having low drive voltage.
BACKGROUND OF THE INVENTION
[0003] An organic light-emitting diode device, also called an OLED
or an organic electroluminescent (EL) device, commonly includes an
anode, a cathode, and an organic electroluminescent (EL) unit
sandwiched between the anode and the cathode. The organic EL unit
includes at least a hole-transporting layer (HTL), an organic
light-emitting layer (LEL), and an electron-transporting layer
(ETL). OLEDs are attractive because of their low drive voltage,
high luminance, wide viewing angle and capability for full color
displays and for other applications. Tang, et al. described this
multilayer OLED in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
Since then, numerous OLEDs with alternative layer structures and
using many different types of organic EL materials have been
disclosed.
[0004] An OLED is actually a current driven device. Its luminance
is proportional to current density, but its lifetime is inversely
proportional to current density. In order to achieve high
brightness, an OLED has to be operated at a relatively high current
density, but this will result in a short lifetime. Thus, it is
critical to improve the luminous efficiency of an OLED while
operating at the lowest possible current density.
[0005] In order to dramatically improve luminous efficiency and to
increase the lifetime for OLEDs, a tandem OLED (or stacked OLED, or
cascaded OLED) structure, which is fabricated by stacking several
individual OLEDs vertically and driven by only a single power
source, has been proposed or fabricated. See U.S. Pat. Nos.
6,337,492, 6,107,734, 6,717,358, JP Patent Publication 2003-045676,
U.S. Patent Publications 2003/0189401 A1, and 2003/0170491 A1. In a
tandem OLED having a number of N EL units, the luminous efficiency
can be N times as high as that of a conventional OLED containing
only one EL unit. Therefore, the tandem OLED needs only about 1/N
of the current density used in a conventional OLED to achieve the
same luminance. As a result, the lifetime of the tandem OLED will
be about N times that of a conventional OLED. However, the drive
voltage will be about N times as high as that of a conventional
OLED at the same current density.
[0006] In display applications, there is a voltage limit for the
circuitry. For example, in an active matrix OLED display based on a
thin film transistor (TFT) backplane or drive circuitry, the
maximum permitted voltage drop across the OLED component can be
less than 12 volts (V). A tandem OLED structure may not be suitable
for use in display applications if its drive voltage is higher than
12 V to produce an intended luminance. Therefore, it is important
to make a tandem OLED with a drive voltage suitable for display
applications. However, the aforementioned prior art does not
disclose a tandem OLED containing more than two EL units and with a
drive voltage suitable for the common TFT drive circuitry.
SUMMARY OF THE INVENTION
[0007] In the present invention, tandem OLEDs are constructed using
a plurality of EL units and intermediate connectors, and the
devices are driven only using a single power source.
[0008] It is an object of the present invention to make tandem
OLEDs useful in display applications.
[0009] It is another object of the present invention to make tandem
OLEDs having a low drive voltage compatible with TFT drive
circuitry.
[0010] These objects are achieved by a tandem OLED comprising:
[0011] a) an anode;
[0012] b) a cathode;
[0013] c) at least two electroluminescent units disposed between
the anode and the cathode, wherein each of the electroluminescent
units includes at least one hole-injecting layer, one
hole-transporting layer, one organic light-emitting layer, one
electron-transporting layer, and one electron-injecting layer;
[0014] d) at least one intermediate connector, wherein each of the
intermediate connectors includes at least one layer, and wherein
each of the intermediate connectors is disposed between
electroluminescent units; and
[0015] e) wherein the thickness of each layer in each of the
electroluminescent units and the intermediate connectors is
selected to satisfy the test condition that the voltage drop from
the anode to the cathode is less than 4.0 V.times.N (the number of
electroluminescent units) at 20 mA/cm.sup.2.
[0016] The present invention makes use of a five-layer EL unit
structure with thin LELs, instead of a conventional three-layer EL
unit structure with thick LELs, in conjunction with thin
intermediate connectors in a tandem OLED. By considering both the
carrier transporting capability in most of the organic layers and
the carrier injecting capability at each interface, both EL units
and intermediate connectors in a tandem OLED can have low voltage
drops. Due to low drive voltage and high luminous efficiency, this
tandem OLED can be made useful for display applications.
[0017] It is a feature of the present invention to provide a tandem
OLED with high luminance efficiency and high brightness.
[0018] It is another feature of the present invention to provide a
tandem OLED with improved lifetime.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 depicts a schematic cross sectional view of a tandem
OLED, having N (N.gtoreq.1) EL units connected in series by N-1
intermediate connectors, in accordance with the present
invention;
[0020] FIG. 2 depicts a schematic cross sectional view of a
specific tandem OLED, having three EL units connected in series by
two intermediate connector, in accordance with the present
invention;
[0021] FIG. 3 depicts a schematic cross sectional view of a
specific EL unit, having HIL, HTL, LEL, ETL, and EIL, in accordance
with the present invention;
[0022] FIG. 4 depicts a schematic cross sectional view of an
intermediate connector, having a metal compound layer, in the
tandem OLED in accordance with the present invention;
[0023] FIG. 5 depicts a schematic cross sectional view of another
intermediate connector having a high work function metal layer in
the tandem OLED in accordance with the present invention;
[0024] FIG. 6 depicts a schematic cross sectional view of another
intermediate connector having a metal compound layer and a high
work function metal layer in the tandem OLED in accordance with the
present invention;
[0025] FIG. 7 is a graph of normalized luminance vs. the thickness
of organic light-emitting layer in a conventional green color
OLED;
[0026] FIG. 8 is a schematic of a computerized test setup for
measurement of the current density-voltage (J-V) characteristics
and the luminance-voltage (L-V) characteristics of OLEDs;
[0027] FIG. 9 is a graph showing the current density-voltage (J-V)
characteristics of the Examples 5-7 in accordance with the present
invention;
[0028] FIG. 10 is a graph showing the luminance-voltage (J-V)
characteristics of the Examples 5-7 in accordance with the present
invention; and
[0029] FIG. 11 is a graph showing the luminous efficiency-current
density (.eta.-J) characteristics of the Examples 5-7 in accordance
with the present invention.
[0030] It will be understood that FIGS. 1-6 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, and that
FIG. 8 is also not to scale.
DETAILED DESCRIPTION OF THE INVENTION
[0031] FIG. 1 shows a tandem OLED 100 in accordance with the
present invention. This tandem OLED has an anode 110 and a cathode
170, at least one of which is transparent. Disposed between the
anode and the cathode are N EL units and N-1 intermediate
connectors (each of them indicated as "int. connector" in the
figure), where N is an integer greater than 1. The EL units,
stacked and connected serially, are designated 120.1 to 120.N where
120.1 is the first EL unit (adjacent to the anode), 120.2 is the
second EL unit, 120.N-1 is the (N-1)th EL unit, and 120.N is the
Nth EL unit (nearby the cathode). The intermediate connectors,
disposed between the EL unit, are designated 130.1 to 130.(N-1)
where 130.1 is the first intermediate connector disposed between EL
units 120.1 and 120.2; 130.2 is the second intermediate connector
in contact with EL unit 120.2 and another EL unit (not shown in the
figure); and 130.(N-1) is the last intermediate connector disposed
between EL units 120.(N-1) and 120.N. The tandem OLED 100 is
externally connected to a voltage/current source 180 through
electrical conductors 190.
[0032] Tandem OLED 100 is operated by applying an electric
potential produced by a voltage/current source 180 between a pair
of contact electrodes, anode 110 and cathode 170. Under a forward
bias, this externally applied electrical potential is distributed
among the N EL units and the N-1 intermediate connectors in
proportion to the electrical resistance of each of these units and
connectors. The electric potential across the tandem OLED causes
holes (positively charged carriers) to be injected from anode 110
into the 1.sup.st EL unit 120.1, and electrons (negatively charged
carriers) to be injected from cathode 170 into the N.sup.th EL unit
120.N. Simultaneously, electrons and holes are produced in, and
separated from, each of the intermediate connectors,
(130.1-130.(N-1)). Electrons thus produced, for example, in
intermediate connector 130.1 are injected towards the anode and
into the adjacent EL unit 120.1. Likewise, holes produced in the
intermediate connector 130.1 are injected towards the cathode and
into the adjacent EL unit 120.2. Subsequently, these electrons and
holes recombine in their corresponding EL units to produce
light.
[0033] In order to fabricate a low voltage and a high efficiency
tandem OLED, it is important to carefully design the thickness of
each layer in each of the EL units and the intermediate connectors.
Generally, the thickness of each layer in each of the EL units and
the intermediate connectors is selected to satisfy the test
condition that the voltage drop from the anode to the cathode of
the tandem OLED is less than 4.0 V.times.N (the number of EL units)
at 20 mA/cm.sup.2, or selected to satisfy the test condition that
the voltage drop from the anode to the cathode of the tandem OLED
is less than 3.5 V.times.N at 1000 cd/m.sup.2.
[0034] Shown in FIG. 2 is a tandem OLED 200. This tandem OLED has
an anode 110 and a cathode 170, at least one of which is
transparent. Disposed between the anode and the cathode are three
EL units (EL units 120.1, 120.2, and 120.3) and two intermediate
connectors (st Int. Connector 130.1 and 2.sup.nd Int. Connector
130.2). The tandem OLED 200 is externally connected to a
voltage/current source 180 through electrical conductors 190. The
tandem OLED 200 is a tandem OLED simplified from FIG. 1.
[0035] In order to achieve a low drive voltage within this specific
tandem OLED 200 shown in FIG. 2, the thickness of each layer in
each of the EL units and the intermediate connectors is selected to
satisfy the test condition that the voltage drop from the anode to
the cathode of the tandem OLED is less than 12 V at 20 mA/cm.sup.2,
or selected to satisfy the test condition that the voltage drop
from the anode to the cathode of the tandem OLED is less than 10.5
V at 1000 cd/m.sup.2.
[0036] Each of the EL units in the tandem OLEDs 100 and 200 is
capable of supporting hole injection, hole transport, electron
injection, electron transport, and electron-hole recombination to
produce light. Each of the EL units can comprise a plurality of
layers. There are many organic EL multilayer structures known in
the art that can be used as EL units of the present invention.
These include, HTL/(LEL or LELs)/ETL, hole-injecting layer
(HIL)/HTL/(LEL or LELs)/ETL, HIL/HTL/(LEL or
LELs)/ETL/electron-injecting layer (EIL), HIL/HTL/electron-blocking
layer or exciton-blocking layer/(LEL or LELs)/ETL/EIL, HIL/HTL/(LEL
or LELs)/hole-blocking layer/ETL/EIL. Each of the EL units in the
tandem OLED can have the same or different layer structures from
other EL units provided that it can enable low drive voltage and
high luminous efficiency. Preferably, the layer structure of the EL
units is of HIL/HTL/(LEL or LELs)/ETL/EIL. Considering the number
of the LELs within a specific EL unit in the tandem OLEDs 100 or
200, the number of LELs in each of the EL units can be changed
typically from 1 to 3. Moreover, each of the EL units in the tandem
OLED can emit the same or different color.
[0037] FIG. 3 shows one embodiment of the EL unit structures
according to the present invention. EL unit 320 includes HIL 321,
HTL 322, LEL 323, ETL 324, and EIL 325.
[0038] The HIL 321 in EL unit 320 is a p-type doped layer
containing at least one hole-transporting material as a host
material and at least one p-type dopant (this HIL can also be
called a p-type doped HIL 321). The term "p-type doped layer" means
that this layer has semiconducting properties after doping, and the
electrical current through this layer is substantially carried by
the holes. The host material is capable of supporting hole
injection and hole transport. Hole-transporting materials used in
conventional OLEDs represent a useful class of host materials for
the p-type doped HIL 321. 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, et al. in U.S.
Pat. Nos. 3,567,450 and 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, et al.
in U.S. Pat. Nos. 4,720,432 and 5,061,569. Non-limiting examples
include N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB),
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1-biphenyl-4,4'-diamine
(TPD), and N,N,N',N'-tetranaphthyl-benzidine (TNB). The combination
of the aforementioned materials is also useful to form the p-type
doped layer. More preferably, the host material in the p-type doped
HIL 321 includes NPB, TPD, TNB,
4,4',4''-tris(N-3-metylphenyl-N-phenyl-amino)-triphenylamine
(m-TDATA), or 4,4',4''-tris(N,N-diphenyl-amino)-triphenylamine
(TDATA), or hexaazatriphenylene compounds, or combinations
thereof.
[0039] The p-type dopant in the p-type doped HIL 321 includes
oxidizing agents with strong electron-withdrawing properties. By
"strong electron-withdrawing properties" it is meant that the
dopant should be able to accept some electronic charge from the
host to form a charge-transfer complex with the host material. Some
non-limiting examples include organic compounds such as
2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane
(F.sub.4-TCNQ), other derivatives of
7,7,8,8-tetracyanoquinodimethane (TCNQ), and hexaazatriphenylene
compounds, and inorganic oxidizing agents such as iodine,
FeCl.sub.3, FeF.sub.3, SbCl.sub.5, some other metal chlorides, and
some other metal fluorides. Combining p-type dopants is also useful
when forming the p-type doped HIL 321. The p-type dopant
concentration is preferably in the range of 0.01-30 vol. %. The
thickness of the p-type doped organic layer is typically less than
200 nm, and preferably in the range of less than 150 nm.
[0040] The HTL 322 in EL unit 320 contains at least one
hole-transporting material such as an aromatic tertiary amine,
where the aromatic tertiary amine is understood to be a compound
containing 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. Exemplary monomeric triarylamines are illustrated by
Klupfel, et al. in U.S. Pat. No. 3,180,730. Other suitable
triarylamines substituted with one or more vinyl radicals and/or
comprising at least one active hydrogen-containing group are
disclosed by Brantley, et al. in U.S. Pat. Nos. 3,567,450 and
3,658,520.
[0041] A more preferred class of aromatic tertiary amines are those
which include at least two aromatic tertiary amine moieties as
described by Van Slyke, et al. in U.S. Pat. Nos. 4,720,432 and
5,061,569. The HTL can be formed of a single or a mixture of
aromatic tertiary amine compounds. Illustrative of useful aromatic
tertiary amines are the following:
[0042] 1,1-Bis(4-di-p-tolylaminophenyl)cyclohexane;
[0043] 1,1-Bis(4-di-p-tolylaminophenyl)-4-phenylcyclohexane;
[0044] 4,4'-Bis(diphenylamino)quadriphenyl;
[0045] Bis(4-dimethylamino-2-methylphenyl)-phenylmethane;
[0046] N,N,N-Tri(p-tolyl)amine;
[0047]
4-(di-p-tolylamino)-4'-[4(di-p-tolylamino)-styryl]stilbene;
[0048] N,N,N',N'-Tetra-p-tolyl-4-4'-diaminobiphenyl;
[0049] N,N,N',N'-Tetraphenyl-4,4'-diaminobiphenyl;
[0050] N,N,N ',N '-tetra-1-naphthyl-4,4'-diaminobiphenyl;
[0051] N,N,N',N'-tetra-2-naphthyl-4,4'-diaminobiphenyl;
N-Phenylcarbazole;
[0052] 4,4'-Bis[N-(1-naphthyl)-N-phenylamino]biphenyl;
[0053] 4,4'-Bis[N-(1-naphthyl)-N-(2-naphthyl)amino]biphenyl;
[0054] 4,4''-Bis[N-(1-naphthyl)-N-phenylamino]p-terphenyl;
[0055] 4,4'-Bis[N-(2-naphthyl)-N-phenylamino]biphenyl;
[0056] 4,4'-Bis[N-(3-acenaphthenyl)-N-phenylamino]biphenyl;
[0057] 1,5-Bis[N-(1-naphthyl)-N-phenylamino]naphthalene;
[0058] 4,4'-Bis[N-(9-anthryl)-N-phenylamino]biphenyl;
[0059] 4,4''-Bis[N-(1-anthryl)-N-phenylamino]-p-terphenyl;
[0060] 4,4'-Bis[N-(2-phenanthryl)-N-phenylamino]biphenyl;
[0061] 4,4'-Bis[N-(8-fluoranthenyl)-N-phenylamino]biphenyl;
[0062] 4,4'-Bis[N-(2-pyrenyl)-N-phenylamino]biphenyl;
[0063] 4,4'-Bis[N-(2-naphthacenyl)-N-phenylamino]biphenyl;
[0064] 4,4'-Bis[N-(2-perylenyl)-N-phenylamino]biphenyl;
[0065] 4,4'-Bis[N-(1-coronenyl)-N-phenylamino]biphenyl;
[0066] 2,6-Bis(di-p-tolylamino)naphthalene;
[0067] 2,6-Bis[di-(1-naphthyl)amino]naphthalene;
[0068] 2,6-Bis[N-(1-naphthyl)-N-(2-naphthyl)amino]naphthalene;
[0069] N,N,N',N'-Tetra(2-naphthyl)-4,4''-diamino-p-terphenyl;
[0070] 4,4'-Bis
{N-phenyl-N-[4-(1-naphthyl)-phenyl]amino}biphenyl;
[0071] 4,4'-Bis[N-phenyl-N-(2-pyrenyl)amino]biphenyl;
[0072] 2,6-Bis[N,N-di(2-naphthyl)amine]fluorene;
[0073] 1,5-Bis[N-(1-naphthyl)-N-phenylamino]naphthalene; and
[0074]
4,4',4''-tris[(3-methylphenyl)phenylamino]triphenylamine.
[0075] Another class of useful hole-transporting materials includes
30 polycyclic aromatic compounds as described in EP 1 009 041.
Tertiary aromatic amines with more than two amino groups can be
used including oligomeric materials. In addition, polymeric
hole-transporting materials can be used such as
poly(N-vinylcarbazole) (PVK), polythiophenes, polypyrrole,
polyaniline, and copolymers such as
poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) also
called PEDOT/PSS.
[0076] As is noticed in the EL unit 320 of the tandem OLED in the
present invention, both HIL 321 and HTL 322 can use the same or
different hole-transporting materials.
[0077] The LEL 323 in EL unit 320 includes at least one host
material doped with at least one dopant where EL is produced as a
result of hole-electron recombination in this region, wherein the
light emission comes primarily from the dopant and can be of red,
green, blue, or white, or combinations thereof. The host material
in the LEL can be an electron-transporting material, a
hole-transporting material, or other material, or combinations of
materials that support hole-electron recombination. The dopant is
typically selected from highly fluorescent dyes, but phosphorescent
compounds, e.g., transition metal complexes, are also useful.
Dopants are typically coated as 0.01 to 20% by weight into the host
material. Polymeric materials such as polyfluorenes and
polyvinylarylenes, e.g., poly(p-phenylenevinylene), PPV, can also
be used as the host material. In this case, small molecule dopants
can be molecularly dispersed into the polymeric host, or the dopant
can be added by copolymerizing a minor constituent into the host
polymer.
[0078] An important relationship for choosing a dye as a dopant is
a comparison of the electron energy band gap. For efficient energy
transfer from the host to the dopant molecule, a necessary
condition is that the band gap of the dopant is smaller than that
of the host material. For phosphorescent emitters it is also
important that the triplet energy level of the host be high enough
to enable energy transfer from host to dopant.
[0079] Host and emitting molecules known to be of use include, but
are not limited to, those disclosed in U.S. Pat. Nos. 4,768,292,
5,141,671, 5,150,006, 5,151,629, 5,405,709, 5,484,922, 5,593,788,
5,645,948, 5,683,823, 5,755,999, 5,928,802, 5,935,720, 5,935,721,
and 6,020,078.
[0080] Metal complexes of 8-hydroxyquinoline (oxine) and similar
derivatives constitute one class of useful host compounds capable
of supporting electroluminescence. Illustrative of useful chelated
oxinoid compounds are the following:
[0081] CO-1: Aluminum trisoxine [alias,
tris(8-quinolinolato)aluminum(III)];
[0082] CO-2: Magnesium bisoxine [alias,
bis(8-quinolinolato)magnesium(II)];
[0083] CO-3: Bis[benzo{f}-8-quinolinolato]zinc (II);
[0084] CO-4:
Bis(2-methyl-8-quinolinolato)aluminum(III)-.mu.-oxo-bis(2-methyl-8-quinol-
inolato) aluminum(III);
[0085] CO-5: Indium trisoxine [alias,
tris(8-quinolinolato)indium];
[0086] CO-6: Aluminum tris(5-methyloxine) [alias,
tris(5-methyl-8-quinolinolato) aluminum(III)];
[0087] CO-7: Lithium oxine [alias,
(8-quinolinolato)lithium(I)];
[0088] CO-8: Gallium oxine [alias,
tris(8-quinolinolato)gallium(III)]; and
[0089] CO-9: Zirconium oxine [alias,
tetra(8-quinolinolato)zirconium(IV)].
[0090] Other classes of useful host materials include, but are not
limited to, derivatives of anthracene, such as
2-(1,1-dimethyethyl)-9,10-bis(2-naphthalenyl) anthracene (TBADN),
9,10-di-(2-naphthyl) anthracene (AND), and derivatives thereof as
described in U.S. Pat. No. 5,935,721, distyrylarylene derivatives
as described in U.S. Pat. No. 5,121,029, benzazole derivatives, for
example, 2,2',2''-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole],
and blue emitting metal chelated oxinoid compounds, for example,
Bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (B-Alq).
Carbazole derivatives are particularly useful hosts for
phosphorescent emitters.
[0091] Useful fluorescent dopants include, but are not limited to,
derivatives of anthracene, tetracene, xanthene, perylene, rubrene,
coumarin, rhodamine, and quinacridone, dicyanomethylenepyran
compounds, thiopyran compounds, polymethine compounds, pyrilium and
thiapyrilium compounds, fluorene derivatives, periflanthene
derivatives, indenoperylene derivatives, bis(azinyl)amine boron
compounds, bis(azinyl)methane compounds, and carbostyryl
compounds.
[0092] The ETL 324 in EL unit 320 contains metal chelated oxinoid
compounds, including chelates of oxine itself, also commonly
referred to as 8-quinolinol or 8-hydroxyquinoline. Such compounds
help to inject and transport electrons, exhibit high levels of
performance, and are readily deposited to form thin films.
Exemplary oxinoid compounds are the following:
[0093] CO-1: Aluminum trisoxine [alias,
tris(8-quinolinolato)aluminum(III)];
[0094] CO-2: Magnesium bisoxine [alias,
bis(8-quinolinolato)magnesium(II)];
[0095] CO-3: Bis[benzo {f}-8-quinolinolato]zinc (II);
[0096] CO-4:
Bis(2-methyl-8-quinolinolato)aluminum(III)-.mu.-oxo-bis(2-methyl-8-quinol-
inolato) aluminum(III);
[0097] CO-5: Indium trisoxine [alias,
tris(8-quinolinolato)indium];
[0098] CO-6: Aluminum tris(5-methyloxine) [alias,
tris(5-methyl-8-quinolinolato) aluminum(III)];
[0099] CO-7: Lithium oxine [alias,
(8-quinolinolato)lithium(I)];
[0100] CO-8: Gallium oxine [alias,
tris(8-quinolinolato)gallium(III)]; and
[0101] CO-9: Zirconium oxine [alias,
tetra(8-quinolinolato)zirconium(IV)].
[0102] Other electron-transporting materials include various
butadiene derivatives as disclosed in U.S. Pat. No. 4,356,429 and
various heterocyclic optical brighteners as described in U.S. Pat.
No. 4,539,507. Benzazoles, oxadiazoles, triazoles,
pyridinethiadiazoles, triazines, phenanthroline derivatives, and
some silole derivatives are also useful electron-transporting
materials.
[0103] The EIL 325 in EL unit 320 is an n-type doped layer
containing at least one electron-transporting material as a host
material and at least one n-type dopant (this EIL can also be
called an n-type doped EIL 325). The term "n-type doped layer"
means that this layer has semiconducting properties after doping,
and the electrical current through this layer is substantially
carried by the electrons. The host material is capable of
supporting electron injection and electron transport. The
electron-transporting materials used in conventional OLEDs
represent a useful class of host materials for the n-type doped EIL
325. 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 (Alq). Other materials include
various butadiene derivatives as disclosed by Tang in U.S. Pat. No.
4,356,429, various heterocyclic optical brighteners as disclosed by
Van Slyke, et al. in U.S. Pat. No. 4,539,507, triazines,
hydroxyquinoline derivatives, benzazole derivatives, and
phenanthroline derivatives. Silole derivatives, such as
2,5-bis(2',2''-bipridin-6-yl)-1,1-dimethyl-3,4-diphenyl
silacyclopentadiene are also useful host organic materials. The
combination of the aforementioned materials is also useful to form
the n-typed doped EIL 325. More preferably, the host material in
the n-type doped EIL 325 includes Alq,
4,7-diphenyl-1,10-phenanthroline (Bphen),
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), or
2,2'-[1,1'-biphenyl]-4,4'-diylbis[4,6-(p-tolyl)-1,3,5-triazine]
(TRAZ), or combinations thereof.
[0104] The n-type dopant in the n-type doped EIL 325 includes
alkali metals, alkali metal compounds, alkaline earth metals, or
alkaline earth metal compounds, or combinations thereof. 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. The materials used as the n-type dopants in
the n-type doped EIL 325 also include organic reducing agents with
strong electron-donating properties. By "strong electron-donating
properties" it is meant 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)-tetrathiafulvalene
(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. Preferably, the n-type dopant in the n-type
doped EIL 325 includes Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, La, Ce,
Nd, Sm, Eu, Th, Dy, or Yb, or combinations thereof. The n-type
doped concentration is preferably in the range of 0.01-20 vol. %.
The thickness of the n-type doped EIL 325 is typically less than
200 nm, and preferably in the range of less than 150 nm.
[0105] As is noticed that in the EL unit 320 of the tandem OLED in
the present invention, both ETL 324 and EIL 325 can use the same or
different electron-transporting material.
[0106] The layers in the EL unit 320 can be formed from small
molecule OLED materials, polymeric LED materials, or inorganic
materials, or combinations thereof. The corresponding layer in each
of the EL units in the tandem OLED can be formed using the same or
different materials from those of the other corresponding layers.
Some EL units can be polymeric and other units can be of small
molecules (or non-polymeric), including fluorescent materials and
phosphorescent materials.
[0107] Using the aforementioned low voltage EL unit 320 cannot
guarantee the formation of a low voltage tandem OLED. The
intermediate connectors connecting each of the EL units in the
tandem OLED also play an important role for achieving low voltage
and high luminous efficiency in the device.
[0108] For a tandem OLED to function efficiently, it is necessary
that the intermediate connector should provide effective carrier
injection into the adjacent EL units. It is also necessary that the
optical transparency of the layers constituting the intermediate
connector should be as high as possible to permit for radiation
produced in the EL units to exit the device. In the aforementioned
tandem OLED according to the present invention, an organic
intermediate connector can be formed when an HIL of an EL unit is
in contact with the EIL of the preceding EL unit during the device
fabrication process. This readily formed organic intermediate
connector having an n-type doped organic EIL and a p-type doped
organic HIL, denoted as EIL/HIL intermediate connector, can provide
reasonably effective carrier injection into the adjacent EL units
and have excellent optical transparency in the visible region of
the spectrum. However, in order to fabricate a low voltage tandem
OLED, the carrier injection capability of this EIL/HIL intermediate
connector is not effective enough to further improve the carrier
injection.
[0109] As is known, due to their lower resistivity than that of
organic materials, metals, metal compounds, or other inorganic
compounds can be effective for carrier injection. Therefore, if a
metal layer, a metal compound layer, or other inorganic compound
layer is used as an intermediate connector which has a thickness
thin enough to meet the requirement of possessing high optical
transparency, it is possible to further reduce the drive voltage of
the tandem OLED.
[0110] In the present invention, each layer in each of the
intermediate connectors can include organic material, a metal
compound, or a high work function metal, or combinations thereof.
"High work function metal" is defined as a metal having a work
function no less than 4.0 eV. (Likewise, "low work function metal"
is defined as a metal having a work function less than 4.0 eV).
Preferably, each layer in each of the intermediate connectors
includes a metal compound, or a high work function metal, or
combinations thereof. Moreover, each layer in each of the
intermediate connectors can be a doped layer or a non-doped layer.
Furthermore, each of the intermediate connectors in the tandem OLED
can be the same or different. In other words, each of the
intermediate connectors can have the same or different layer
structures.
[0111] Shown from FIG. 4 to FIG. 6 are typical embodiments of the
intermediate connectors for low drive voltage purpose in the
present invention, wherein the intermediate connector disposed
between each adjacent EL unit includes at least one layer and
without direct connection to an external power source. FIG. 4 shows
an intermediate connector 430 including a metal compound layer 431.
FIG. 5 shows an intermediate connector 530 including a high work
function metal layer 532. Shown in FIG. 6 is an intermediate
connector 630 including both a metal compound layer 431 and a high
work function metal layer 532. The high work function metal layer
532 is in contact with the metal compound layer 431. Either the
metal compound layer 431 or the high work function metal layer 532
can be first deposited in contact with the preceding EL unit. In
other words, the layer order of the intermediate connector 630 can
be altered when this intermediate connector is disposed between two
EL units. For example, Ag/MoO.sub.3, MoO.sub.3/Ag, Ag/WO.sub.3,
WO.sub.3/Ag, and LiF/Al are included in the intermediate connector
630.
[0112] The metal compound layer 431 in the intermediate connectors
430 as shown in FIG. 4 and in the intermediate connector 630 as
shown in FIG. 6 is mainly used to further improve the carrier
injection. Moreover, this metal compound layer can also stop the
possible interdiffusion or interaction between the n-typed doped
EIL in the preceding EL unit and the p-type doped HIL of the
adjacent EL unit to stabilize the drive voltage during operation.
The metal compound layer 431 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 431 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 431 can be selected from the
stoichiometric selenides or nonstoichiometric selenides 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 431 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 431 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 431 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 431
can also be selected from the stoichiometric carbides or
nonstoichiometric fluorides of alkali metals when it was used with
a metal capable of reducing the metal ion contained in the
fluorides, such as Al, Zr, Ti, Y, Sc, or Si. The metal compound
layer 431 can be selected from MoO.sub.3, NiMoO.sub.4, CuMoO.sub.4,
WO.sub.3, ZnTe, Al.sub.4C.sub.3, AlF.sub.3, B.sub.2S.sub.3, CuS,
GaP, InP, or SnTe. Preferably, the metal compound layer 431 is
selected from MoO.sub.3, NiMoO.sub.4, CuMoO.sub.4, or WO.sub.3.
[0113] The high work function metal layer 532 in the intermediate
connectors 530 as shown in FIG. 5 and in the intermediate connector
630 as shown in FIG. 6 is mainly used to further improve the
carrier injection. Moreover, this high work function metal layer
can also stop the possible interdiffusion or interaction between
the n-typed doped EIL in the preceding EL unit and the p-type doped
HIL of the adjacent EL unit to stabilize the drive voltage during
operation. The high work function metal used to form this layer has
a work function no less than 4.0 eV and includes 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, or combinations thereof. Preferably, the high
work function metal layer 532 includes Cu, Ag, Au, Zn, Al, Zn, In,
or Sn, or combinations thereof.
[0114] As aforementioned, in order to fabricate a low voltage and
high efficiency tandem OLED, it is important to carefully design
the thickness of each layer in each of the EL units and the
intermediate connectors. It is desirable to make each of the EL
units and the intermediate connectors as thin as possible. However,
making an EL unit as thin as possible is not practically useful.
From an optical out-coupling point of view, in order to achieve
higher luminous efficiency, the optimal device thickness is mainly
determined by the emission color. For example, giving a tandem OLED
having an S-nm-thick ITO (on glass substrate) as a transparent
anode, a Ag layer as an opaque cathode, an emission wavelength of
.lamda. (nm), and N EL units, the optical distance from the LEL in
the first EL unit adjacent to the anode to the glass /ITO interface
is about .lamda./2, the distance between two LELs in the adjacent
EL units is about .lamda./2, and the distance from the LEL of the
N.sup.th EL unit adjacent to the cathode to the Ag cathode is about
0.672.times..lamda./4 (having considered the wavelength phase shift
as about 122.degree.). Therefore, the total optical distance
(D(optical)) between glass/ITO interface and the Ag cathode surface
is
D(optical)=N.times.(.lamda./2)+0.672.times..lamda./4=(N/2+0.168).times..l-
amda.. Provided that "n" is the average refractive index of all the
organic materials and the ITO, the real optimal thickness can be
obtained between the ITO/HIL interface and the Ag cathode surface
D(real)=D(optical)/n-S=(N/2+0.168).times..lamda./n-S. If N=1 (which
means the tandem OLED has only one EL unit), .lamda.=524 nm,
n=1.75, and S=50 nm, D(real)=150 nm. This estimation tells us that
the optimal thickness of the EL unit is about 150 nm for a green
emissive device. If the thickness of an EL unit deviates too much
from its optimal thickness, there will be interference of the light
beams, which will reduce the out-coupling resulting in low luminous
efficiency.
[0115] With the optimal thickness in mind, it is then understood
that the best way to reduce drive voltage is to increase the
thickness of both the HIL and the EIL, and to decrease the
thickness of the HTL, LELs, and ETL, while keeping the total
thickness basically unchanged. Since it was reported that the
exciton diffusion length is about 5 nm in an OLED, the thickness of
the HTL and the ETL can be as thin as 5 nm to effectively block
exciton diffusion from the LEL into the HIL or EIL and to maintain
the radiative recombination rate in the LEL. In considering the
thickness of the LEL, the width of the intensive emission zone
needs to be known in the LEL before its thickness can properly be
designed. FIG. 7 shows the relationship between the normalized
luminance and the thickness of the LEL in a conventional green
OLED. It indicates that more than 80% of the total luminance is
occurring from the first 10 nm portion of the LEL near the HTL/LEL
interface. It is known from our other experiments that the voltage
drop in a doped EIL can be less than 0.02 V/nm, while the voltage
drop in a non-doped ETL can be higher than 0.07 V/nm. Therefore, if
the LEL thickness is reduced from 30 nm to 20 nm, the luminance
decrease is less than 10%, while the voltage decrease can be more
than 10% resulting in improved power efficiency.
[0116] Therefore, in the tandem OLED of the present invention, the
HTL in contact with the LEL in each of the EL units can have a
thickness of less than or equal to 10 nm, the LEL in each of the EL
units can have a thickness of less than or equal to 20 nm, and the
ETL in contact with the LEL in each of the EL units can have a
thickness of less than or equal to 10 nm. Preferably, in the tandem
OLED of the present invention, the HTL in contact with the LEL in
each of the EL units can have a thickness of less than or equal to
5 nm, the LEL in each of the EL units can have a thickness of less
than or equal to 15 nm, and the ETL in contact with the LEL in each
of the EL units can have a thickness of less than or equal to 5
nm.
[0117] The thickness of each layer in each of the intermediate
connectors is not very critical in terms of bulk resistance, and
thicker intermediate connectors cannot cause much voltage increase
because of their low resistivities. However, the thickness of each
layer in each of the intermediate connectors is still important in
terms of optical transparency. Therefore, in considering both
carrier injection and optical transparency, the thickness of each
layer in each of the intermediate connectors can be less than or
equal to 5 nm. Preferably, the thickness of each layer in each of
the intermediate connectors can be less than or equal to 3 nm.
[0118] The tandem OLED of the present invention is typically
provided over a supporting substrate where either the cathode or
anode can be in contact with the substrate. 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. Transparent glass or plastic is
commonly employed in such cases. For applications where the EL
emission is viewed through the top electrode, 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.
[0119] 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 the anode are immaterial and any
conductive material can be used, regardless if it is 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 no less than 4.0 eV. Desired anode
materials are commonly deposited by any suitable way such as
evaporation, sputtering, chemical vapor deposition, or
electrochemical means. Anodes can be patterned using well known
photolithographic processes. Optionally, anodes can be polished
prior to the deposition of other layers to reduce surface roughness
so as to reduce electrical shorts or enhance reflectivity.
[0120] When light emission is viewed solely through the anode, the
cathode 170 used in the present invention can be comprised of
nearly any conductive material. Desirable materials have effective
film-forming properties to ensure effective contact with the
underlying organic layer, promote electron injection at low
voltage, and have effective stability. Useful cathode materials
often contain a low work-function metal (<4.0 eV) or metal
alloy. One preferred cathode material is comprised of a Mg:Ag alloy
wherein the percentage of silver is in the range of 1 to 20%, as
described in U.S. Pat. No. 4,885,221. Another suitable class of
cathode materials includes bilayers comprising a thin inorganic EIL
in contact with an organic layer (e.g., organic EIL, or organic
ETL), which is capped with a thicker layer of a conductive metal.
Here, the inorganic EIL preferably includes a low work-function
metal or metal salt and, if so, the thicker capping layer does not
need to have a low work function. One such cathode is comprised of
a thin layer of LiF followed by a thicker layer of Al as described
in U.S. Pat. No. 5,677,572. Other useful cathode material sets
include, but are not limited to, those disclosed in U.S. Pat. Nos.
5,059,861, 5,059,862, and 6,140,763.
[0121] When light emission is viewed through the cathode, the
cathode should be transparent or nearly transparent. For such
applications, metals should be thin or one should use transparent
conductive oxides, or include these materials. Optically
transparent cathodes have been described in more detail in U.S.
Pat. Nos. 4,885,211, 5,247,190, 5,703,436, 5,608,287, 5,837,391,
5,677,572, 5,776,622, 5,776,623, 5,714,838, 5,969,474, 5,739,545,
5,981,306, 6,137,223, 6,140,763, 6,172,459, 6,278,236, 6,284,393,
JP 3,234,963, and EP 1 076 368. Cathode materials are typically
deposited by thermal evaporation, electron beam evaporation, ion
sputtering, or chemical vapor deposition. When needed, patterning
can be achieved through many well known methods including, but not
limited to, through-mask deposition, integral shadow masking, for
example as described in U.S. Pat. No. 5,276,380 and EP 0 732 868,
laser ablation, and selective chemical vapor deposition.
[0122] The EL units and the intermediate connectors can be produced
by thermal evaporation, electron beam evaporation, ion sputtering
technique, or spin coating. Preferably, a thermal evaporation
method is used for the deposition of all the materials in the
fabrication of the tandem OLED including the EL units, the
intermediate connectors, and the electrode over the top EL
unit.
[0123] Most OLEDs are sensitive to moisture or oxygen, or both, so
they are commonly sealed in an inert atmosphere such as nitrogen or
argon, along with a desiccant such as alumina, bauxite, calcium
sulfate, clays, silica gel, zeolites, alkaline metal oxides,
alkaline earth metal oxides, sulfates, or metal halides and
perchlorates. Methods for encapsulation and desiccation include,
but are not limited to, those described in U.S. Pat. No. 6,226,890.
In addition, barrier layers such as SiOx, Teflon, and alternating
inorganic/polymeric layers are known in the art for
encapsulation.
[0124] In order to measure the EL characteristics of the devices,
the following test condition was set up for use in the present
invention:
1. Computerized Test Method:
[0125] a) measuring the emission size (Z) of the OLED to be tested.
An optical microscope, luminescence microscope, or an imaging
detector, or other suitable tools can be used to measure the
emission size of the OLED. The emission size will be used to
calculate the applied current density;
[0126] b) measuring the lead resistance (R.sub.lead) from the anode
of the OLED to the positive test contact, and from the cathode of
the OLED to the negative test contact. If the lead resistance is
measurable, a voltage drop on the lead (V.sub.lead) needs to be
taken away from the voltage measurement value;
[0127] c) using a calibrated KEITHLEY 2400 SourceMeter as a
voltage/current current source, using a calibrated PHOTO RESEARCH
SpectraScan PR 650 as a photometer, using suitable software in a
computer to control the communication among the voltage/current
source, the photometer, a printer, and the computer;
[0128] d) arranging a test setup as shown in FIG. 8, wherein the
setup 800 includes computer 801, voltage/current source 802,
photometer 803, sample holder 804, printer 805, communication line
806, and electrical conductor 807;
[0129] e) mounting the OLED onto a sample holder at room
temperature, wherein two electrical conductors from the output of
the voltage/current source are connected to the OLED in such a way
that the positive test contact connecting to the anode of the OLED
is well connected to the positive electrical conductor and the
negative test contact connecting to the cathode of the OLED is well
connected to the negative electrical conductor, so that forward
bias (directed current) can be applied on to the OLED; and making
sure that the photometer is focused on the emission surface of the
OLED perpendicularly and there is no color filter or optical
polarizer between the photometer and the emission surface of the
OLED except the transparent substrate to hold the OLED;
[0130] f) presetting a series of testing current densities (J),
such as, J=0.5, 1.0, 2.0, 6.0, 20, 40, and 100 mA/cm.sup.2,
imputing the test current (I)=J.times.Z into the testing program,
running the computer program to measure the
luminance-current-voltage characteristics at room temperature; and
obtaining a series of voltage (V) and luminance (L) values
corresponding to the given current density, from which the current
density-voltage (J-V) curve and the luminance-voltage (L-V) curve
can be plotted by the computer and printed from the printer;
[0131] g) determining the voltage (V.sub.oled) across the anode and
the cathode of the OLED, V.sub.oled=V-I.times.R.sub.lead, if the
"I.times.R.sub.lead" is not negligible;
[0132] h) plotting J-V.sub.oled curve and L-V.sub.oled curve;
[0133] i) obtaining the V.sub.oled value corresponding to 20
mA/cm.sup.2 and V.sub.oled corresponding to 1000 cd/M.sup.2 from
the J-V.sub.oled curve and L-V.sub.oled curve, respectively;
and
[0134] j) repeating the steps from e) to h) by 3 times, and the
average V.sub.oled value corresponding to 20 mA/cm.sup.2 and the
average V.sub.oled corresponding to 1000 cd/m.sup.2 can finally be
obtained.
2. Manual Test Method:
[0135] a) measuring the emission size (Z) of the OLED to be tested.
An optical microscope, luminescence microscope, or CCD imagine
detector, or other suitable tools can be used to measure the
emission size of the OLED. The emission size will be used to
calculate the applied current density;
[0136] b) measuring the lead resistance (R.sub.lead) from the anode
of the OLED to the positive test contact, and from the cathode of
the OLED to the negative test contact. If the lead resistance is
measurable, a voltage drop on the lead (V.sub.lead) needs to be
taken away from the voltage measurement value;
[0137] c) using any calibrated voltage/current source, but
preferably using a KEITHLEY 2400 SourceMeter as a voltage/current
current source, applying a directed current I.sub.o (I.sub.o=20
mA/cm.sup.2.times.Z) as a forward bias on the OLED in such a way
that the positive test contact connecting to the anode of the OLED
is well connected to the positive electrical conductor of the
source and the negative test contact connecting to the cathode of
the OLED is well connected to the negative electrical conductor of
the source; obtaining and recording the corresponding voltage
reading (V.sub.o); calculating the voltage drop (V.sub.oled) across
the OLED, V.sub.oled=V.sub.o-I.sub.o.times.R.sub.lead;
[0138] d) using any calibrated photometer, but preferably using a
calibrated PHOTO RESEARCH SpectraScan PR 650 as a photometer;
making sure that the photometer is focused on the emission surface
of the OLED perpendicularly and there is no color filter or optical
polarizer between the photometer and the emission surface of the
OLED except the transparent substrate to hold the OLED; applying
directed current as a forward bias on the OLED as aforementioned on
step c) and slowly tuning the current (I) up, at the same time
measuring the luminance (L) from the emission surface; recording
the I and V from the voltage/current source when L=1000 cd/m.sup.2
is obtained from the photometer; and calculating the voltage drop
(V.sub.oled) across the OLED, V.sub.oled=V-I.times.R.sub.lead;
and
[0139] e) repeating the steps from c) to d) by 3 times, and the
average V.sub.oled value corresponding to 20 mA/cm and the average
V.sub.oled corresponding to 1000 cd/m.sup.2 can be finally
obtained.
[0140] For example, there is an OLED for test. Its emission area is
measured as Z=10 mm.sup.2 and its lead resistance is measured as
500 .OMEGA.. In order to obtain a voltage reading at 20
mA/cm.sup.2, a directed current of I=2 mA (I=J.times.Z=20
mA/cm.sup.2.times.10 mm.sup.2=2 mA) from a KEITHLEY 2400
SourceMeter is applied as a forward bias to the device through the
leads. The voltage reading is 4.8 V corresponding to 2 mA.
Therefore, the voltage drop from the anode to the cathode of the
OLED at the test condition of 20 mA/cm.sup.2 is
V.sub.oled=V-I.times.R.sub.lead=4.8 V-2 mA.times.500 .OMEGA.=3.8
V.
EXAMPLES
[0141] The following examples are presented for a further
understanding of the present invention. In the following examples,
the thickness of the organic layers and the doping concentrations
were controlled and measured in situ using calibrated thickness
monitors (INFICON IC/5 Deposition Controller). The EL
characteristics of all the fabricated devices were evaluated using
a constant current source (KEITHLEY 2400 SourceMeter) and a
photometer (PHOTO RESEARCH SpectraScan PR 650) at the room
temperature. The color will be reported using Commission
Internationale de l'Eclairage (CIE) coordinates.
Example 1
(Comparative)
[0142] The preparation of a conventional OLED is as follows: [0143]
A.about.1.1 mm thick glass substrate coated with a transparent
indium-tin-oxide (ITO) conductive layer was cleaned and dried using
a commercial glass scrubber tool. The thickness of ITO is about 42
nm and the sheet resistance of the ITO is about 68 .OMEGA./square.
The ITO surface was subsequently treated with oxidative plasma to
condition the surface as an anode. A layer of CFx, 1 nm thick, was
deposited on the clean ITO surface as the hole-injecting layer by
decomposing CHF.sub.3 gas in an RF plasma treatment chamber. The
substrate was then transferred into a vacuum deposition chamber for
deposition of all other layers on top of the substrate. The
following layers were deposited in the following sequence by
evaporation from a heated boat under a vacuum of approximately
10.sup.-6 Torr: 1. EL Unit:
[0144] a) an HTL, 100 nm thick, including
"4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl" (NPB);
[0145] b) a LEL, 20 nm thick, including
"tris(8-hydroxyquinoline)-aluminum" (Alq) doped with 1.0 vol %
10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,
11H(1)benzopyrano(6,7,8-ij)quinolizin-11-one (C545T); and
[0146] c) an ETL, 40 nm thick, including Alq.
2. Cathode: approximately 210 nm thick, including MgAg (formed by
coevaporation of about 95 vol. % Mg and 5 vol. % Ag).
[0147] After the deposition of these layers, the device was
transferred from the deposition chamber into a dry box (VAC Vacuum
Atmosphere Company) for encapsulation. The OLED has an emission
area of 10 mm.sup.2.
[0148] This conventional OLED requires a drive voltage of about 6.8
V to pass 20 mA/cm.sup.2. Under this test condition, the device has
a luminance of 2164 cd/m.sup.2, and a luminous efficiency of about
10.8 cd/A. Its CIEx and CIEy are 0.284, 0.653, respectively, with
an emission peak at 520 nm. The EL performance data are summarized
in Table 1. The EL unit in this device is a three-layer structure
without a HIL and EIL. As a result, its drive voltage is higher
than 4.0 V under the test condition of 20 mA/cm.sup.2.
Example 2
(Comparative)
[0149] A tandem OLED was constructed with the similar fabrication
methods to those used in Example 1, and the deposited layer
structure is:
1. 1.sup.st EL Unit:
[0150] a) an HTL, 100 nm thick, including NPB;
[0151] b) a LEL, 20 nm thick, including Alq doped with 1.0 vol %
C545T;
[0152] c) an ETL, 10 nm thick, including Alq; and
[0153] d) an EIL, 30 nm thick, including Alq doped with 1.2 vol %
lithium.
2. 1.sup.st Intermediate Connector:
[0154] An EIL/HIL organic intermediate connector will be
automatically formed after completing the 2.sup.nd EL unit.
3. 2.sup.nd EL Unit:
[0155] e) an HIL, 65 nm thick, including NPB doped with 4 vol %
2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane
(F.sub.4-TCNQ);
[0156] f) an HTL, 20 nm thick, including NPB;
[0157] g) a LEL, 20 nm thick, including Alq doped with 1.0 vol %
C545T; and
[0158] h) an ETL, 40 nm thick, including Alq.
4. Cathode: approximately 210 nm thick, including MgAg.
[0159] This tandem OLED requires a drive voltage of about 15.9 V to
pass 20 mA/cm.sup.2. Under this test condition, the device has a
luminance of 4690 cd/m.sup.2, and a luminous efficiency of about
23.5 cd/A. Its CIEx and CIEy are 0.290, 0.659, respectively, with
an emission peak at 524 nm. The EL performance data are summarized
in Table 1. The two EL units in this tandem OLED are the same as
the EL unit in Example 1 except that the layers d) and e) form an
EIL/HIL organic intermediate connector between the two EL units.
Compared to the device in Example 1, both the drive voltage and the
luminous efficiency of this tandem OLED in Example 2 are more than
double of those the device in Example 1. Since the drive voltage of
the device in Example 1 is about 6.8 V under the test condition of
20 mA/cm.sup.2, the drive voltage of the device in Example 2 should
be about 6.8.times.2=13.6 V under the test condition of 20
mA/cm.sup.2 if there were no voltage drop across the EIL/HIL
organic intermediate connector. However, there are an extra 2.3 V
across the device, which can indicate that the EIL/HIL organic
intermediate connector does not have optimal carrier injection
capability. Therefore, it is predicted that even if each of the EL
units has low drive voltage when it is used to form a conventional
OLED, the tandem OLED having these EL units may not be able to
achieve low voltage by only using the EIL/HIL organic intermediate
connector in the device.
Example 3
(Comparative)
[0160] Another conventional OLED was constructed with the similar
fabrication methods to those used in Example 1, and the deposited
layer structure is:
1. EL Unit:
[0161] a) an HIL, 95 nm thick, including
4,4',4''-tris(N-3-metylphenyl-N-phenyl-amino)-triphenylamine
(m-TDATA);
[0162] b) an HTL, 10 nm thick, including NPB;
[0163] c) a LEL, 20 nm thick, including Alq;
[0164] d) an ETL, 10 nm thick, including Alq; and
[0165] e) an EIL, 30 nm thick, including Alq doped with 1.2 vol %
lithium.
2. Cathode: approximately 210 nm thick, including MgAg (formed by
coevaporation of about 95 vol. % Mg and 5 vol. % Ag).
[0166] This conventional OLED requires a drive voltage of about 6.4
V to pass 20 mA/cm.sup.2. Under this test condition, the device has
a luminance of 943 cd/m.sup.2, and a luminous efficiency of about
4.7 cd/A. Its CIEx and CIEy are 0.332, 0.559, respectively, with an
emission peak at 528 nm. The EL performance data are summarized in
Table 1. Although the EL unit in this device is a five-layer
structure with an HIL and an EIL, the HIL and the EIL are not doped
with p-type dopant and n-type dopant, respectively. Therefore, the
conductivity of the HIL and the EIL is not high enough resulting in
high drive voltage in this device.
Example 4
(Comparative)
[0167] Another tandem OLED was constructed with the similar
fabrication methods to those used in Example 1, and the deposited
layer structure is:
1. 1.sup.st EL Unit:
[0168] The 1.sup.st EL unit is the same as the EL unit in Example
3.
2. 1.sup.st Intermediate Connector:
[0169] a) a metal compound layer, 10 nm thick, including
MoO.sub.3.
3. 2.sup.nd EL Unit:
[0170] The 2.sup.nd EL unit is the same as the 1.sup.st EL unit
except that the thickness of the HIL is 70 nm, instead of 95 nm,
for optical out-coupling consideration.
4. Cathode: approximately 210 nm thick, including MgAg.
[0171] This tandem OLED requires a drive voltage of about 16.0 V to
pass 20 mA/cm.sup.2. Under this test condition, the device has a
luminance of 1995 cd/M.sup.2, and a luminous efficiency of about
10.0 cd/A. Its CIEx and CIEy are 0.332, 0.582, respectively, with
an emission peak at 532 nm. The EL performance data are summarized
in Table 1. The two EL units in this tandem OLED are the same as
the EL unit in Example 3, and are connected by an intermediate
connector containing a MoO.sub.3 layer. Compared to the device in
Example 3, both the drive voltage and the luminous efficiency of
this 2-EL unit tandem OLED in Example 4 are more than double of
those of the device in Example 3. However, this tandem OLED is not
considered to be a low voltage tandem OLED because its drive
voltage under the test condition of 20 mA/cm.sup.2 is much higher
than 8.0 V.
Example 5
[0172] A low voltage OLED was constructed with the similar
fabrication methods to those used in Example 1, and the deposited
layer structure is:
1. EL Unit:
[0173] a) an HIL, 70 mn thick, including m-TDATA doped with 4 vol %
F.sub.4-TCNQ;
[0174] b) an HTL, 5 nm thick, including NPB;
[0175] c) a LEL, 12 nm thick, including Alq doped with 1.0 vol %
C545T;
[0176] d) an ETL, 5 nm thick, including
4,7-diphenyl-1,10-phenanthroline (Bphen); and
[0177] e) an EIL, 40 nm thick, including Bphen doped with about 1.5
vol % lithium.
2. Cathode: approximately 210 nm thick, including MgAg (formed by
coevaporation of about 95 vol. % Mg and 5 vol. % Ag).
[0178] This OLED requires a drive voltage of about 3.8 V to pass 20
mA/cm.sup.2. Under this test condition, the device has a luminance
of 2298 cd/m.sup.2, and a luminous efficiency of about 11.5 cd/A.
Its CIEx and CIEy are 0.289, 0.649, respectively, with an emission
peak at 521 nm. Moreover, the drive voltage is about 3.5 V under
the test condition of 1000 cd/m.sup.2. The EL performance data are
summarized in Table 1. Its current density--voltage (J-V) curve,
luminance--voltage (L-V) curve, and luminous efficiency--current
density (.eta.-J) curve are shown in FIGS. 9, 10, and 11,
respectively. The EL unit in this device has a five-layer structure
with a p-type doped HIL and an n-typed doped EIL, wherein the doped
HIL and doped EIL can effectively reduce the drive voltage.
Compared to the conventional device in Example 1 (Device 1),
although the drive voltage of the device in this Example (Device 5)
is 3 V lower than that of Device 1 under the test condition of 20
mA/cm.sup.2, and the LEL thickness is 7 nm thinner than that of
Device 1, the luminous efficiency is still higher than that of
Device 1. Since the drive voltage of Device 5 is lower than 4.0 V
under the test condition of 20 mA/cm.sup.2, and is about 3.5 V
under the test condition of 1000 cd/m.sup.2, this OLED is
considered to be a low voltage OLED. A low voltage tandem OLED
might be formed using this EL unit.
Example 6
[0179] A tandem OLED according to the present invention was
constructed with the similar fabrication methods to those used in
Example 1, and the deposited layer structure is:
1. 1.sup.st EL Unit:
[0180] The 1.sup.st EL unit is the same as the EL unit in Example
5.
2. 1.sup.st Intermediate Connector:
[0181] a) a metal compound layer, 2 nm thick, including
WO.sub.3.
3. 2.sup.nd EL Unit:
[0182] The 2.sup.nd EL unit is the same as the 1.sup.st EL unit
except that the thickness of the HIL is 80 nm, instead of 70 nm,
for optical out-coupling consideration.
4. Cathode: approximately 210 nm thick, including MgAg.
[0183] This tandem OLED requires a drive voltage of about 7.6 V to
pass 20 mA/cm.sup.2. Under this test condition, the device has a
luminance of 5266 cd/m.sup.2, and a luminous efficiency of about
26.3 cd/A. Its CIEx and CIEy are 0.285, 0.657, respectively, with
an emission peak at 522 nm. Moreover, the drive voltage is about
6.2 V under the test condition of 1000 cd/m.sup.2. The EL
performance data are summarized in Table 1. Its J-V curve, L-V
curve, and .eta.-J curve are shown in FIGS. 9, 10, and 11,
respectively. Since the drive voltage is lower than 4.0
V.times.2=8.0 V under the test condition of 20 mA/cm.sup.2, and is
lower than 3.5 V.times.2=7.0 V under the test condition of 1000
cd/m.sup.2, this 2-EL unit tandem OLED is considered a low voltage
tandem OLED.
Example 7
[0184] Another tandem OLED according to the present invention was
constructed with the similar fabrication methods to those used in
Example 1, and the deposited layer structure is:
1. 1.sup.st EL Unit:
[0185] The 1.sup.st EL unit is the same as the EL unit in Example
5.
2. 1.sup.st Intermediate Connector:
[0186] a) a metal compound layer, 2 nm thick, including
WO.sub.3.
3. 2.sup.nd EL Unit:
[0187] The 2.sup.nd EL unit is the same as the 1.sup.st EL unit
except that the thickness of the HIL is 80 nm, instead of 70 nm,
for optical out-coupling consideration.
4. 2.sup.nd Intermediate Connector:
[0188] a) a metal compound layer, 2 nm thick, including
WO.sub.3.
5. 3.sup.rd EL Unit:
[0189] The 3.sup.rd EL unit is the same as the 2.sup.nd EL unit in
this Example.
6. Cathode: approximately 210 nm thick, including MgAg.
[0190] This tandem OLED requires a drive voltage of about 10.9 V to
pass 20 mA/cm.sup.2. Under this test condition, the device has a
luminance of 7147 cd/m.sup.2, and a luminous efficiency of about
35.8 cd/A. Its CIEx and CIEy are 0.272, 0.667, respectively, with
an emission peak at 522 nm. Moreover, the drive voltage is about
8.7 V under the test condition of 1000 cd/m.sup.2. The EL
performance data are summarized in Table 1. Its J-V curve, L-V
curve, and .eta.-J curve are shown in FIGS. 9, 10, and 11,
respectively. Since the drive voltage is lower than 4.0
V.times.3=12.0 V under the test condition of 20 mA/cm.sup.2, and is
lower than 3.5 V.times.3=10.5 V under the test condition of 1000
cd/m.sup.2, this 3-EL unit tandem OLED is considered a low voltage
tandem OLED which is suitable for the common TFT drive
circuitry.
[0191] Shown in Table 1 is the summary of the EL performance of the
examples discussed above. TABLE-US-00001 TABLE 1 Example(Type)
Luminous (EL measured at Voltage Luminance Efficiency CIE x CIE y
Peak 20 mA/cm.sup.2) (V) (cd/m.sup.2) (cd/A) (1931) (1931) (nm) 1
(Comparative) 6.8 2164 10.8 0.284 0.653 520 2 (Comparative) 15.9
4690 23.5 0.290 0.659 524 3 (Comparative) 6.4 943 4.7 0.332 0.559
528 4 (Comparative) 16.0 1995 10.0 0.332 0.582 532 5 3.8 2298 11.5
0.289 0.649 521 6 7.6 5266 26.3 0.285 0.657 522 7 10.9 7174 35.8
0.272 0.667 522
[0192] 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.
PARTS LIST
[0193] 100 tandem OLED
[0194] 110 anode
[0195] 120.1 1.sup.st EL unit
[0196] 120.2 2.sup.nd EL unit
[0197] 120.3 3.sup.rd EL unit
[0198] 120.(N-1).sup.th EL unit
[0199] 120.N N.sup.th EL unit
[0200] 130.1 1.sup.st intermediate connector (or 1.sup.st int.
connector)
[0201] 130.2 2.sup.nd intermediate connector (or 2.sup.nd int.
connector)
[0202] 130.(N-1).sup.th intermediate connector (or (N-1).sup.th
int. connector)
[0203] 170 cathode
[0204] 180 voltage/current source
[0205] 190 electrical conductors
[0206] 200 tandem OLED having three EL units
[0207] 320 EL unit
[0208] 321 hole-injecting layer (or HIL)
[0209] 322 hole-transporting layer (or HTL)
[0210] 323 light-emitting layer (or LEL)
[0211] 324 electron-transporting layer (or ETL)
[0212] 325 electron-injecting layer (or EIL)
[0213] 430 intermediate connector
[0214] 431 metal compound layer
[0215] 530 intermediate connector
[0216] 532 high work function metal layer
[0217] 630 intermediate connector
[0218] 800 computerized test setup
[0219] 801 computer
[0220] 802 voltage/current source
[0221] 803 photometer
[0222] 804 sample holder
[0223] 805 printer
[0224] 806 communication line
[0225] 807 electrical conductor
* * * * *