Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2003-10-03
2004-11-16
Garrett, Dawn (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C313S504000, C313S506000
Reexamination Certificate
active
06818329
ABSTRACT:
FIELD OF INVENTION
The present invention relates to improving the performance of an organic electroluminescent (EL) device, especially relates to improving the luminous efficiency of an EL device.
BACKGROUND OF THE INVENTION
Organic electroluminescent (EL) devices or organic light-emitting devices (OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED comprises, in sequence, an anode, an organic EL medium, and a cathode. The organic EL medium disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al. demonstrated highly efficient OLEDs using such a layer structure in “Organic Electroluminescent Diodes”,
Applied Physics Letters,
51, 913 (1987) and in commonly assigned U.S. Pat. No. 4,769,292. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”,
Japanese Journal of Applied Physics,
27, L269 (1988), and by Tang et al., “Electroluminescence of Doped Organic Thin Films”,
Journal of Applied Physics,
65, 3610 (1989). The LEL commonly consists of a host material doped with a guest material. Further, there are other multilayer OLEDs that contain additional functional layers, such as a hole-injecting layer (HIL), and/or an electron-injecting layer (EIL), and/or an electron-blocking layer (EBL), and/or a hole-blocking layer (HBL) in the devices. At the same time, many different types of EL materials are also synthesized and used in OLEDs. These new structures and new materials have further resulted in improved device performance.
One of the ways to improve luminous efficiency is to modify hole-transporting region (HTR) in OLEDs. A conventional OLED structure is shown in
FIG. 1A
, wherein OLED
100
includes an anode
120
, an HTR
131
, a LEL
134
, an ETL
138
, and a cathode
140
. This device is externally connected to a voltage/current source
150
through electrical conductors
160
. This device only has one HTL in the HTR, and it usually cannot produce high luminous efficiency due to un-balanced carrier injection into the LEL. In order to obtain high luminous efficiency, people are trying to use more than one HTL in OLEDs. Shown in
FIG. 2B
is another type of OLED structure disclosed in prior art, wherein the HTR
131
of OLED
200
contains more than one HTLs, i.e. HTL 1, . . . HTL n, (n>1, an integer). HTL 1 is denoted as HTL
131
.
1
, HTL 2 is denoted as HTL
131
.
2
, and HTL n is denoted as HTL
131
.
n
in the Figures. Shirota et al. reported in “Multilayered Organic Electroluminescent Devices Using a Novel Starburst Molecule, 4,4′,4″-Tris(3-Methylphenylphenylamino)Triphenylamine, as a Hole Transport Material”,
Applied Physics Letters,
65, 807 (1994) that an OLED with dual HTLs could increase efficiency and lifetime. They achieved higher efficiency and longer lifetime when using dual HTLs, 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (m-TDATA)/4,4′-bis(3-methylphenylphenylamino)biphenyl (TPD) than those when using a single TPD layer as an HTL in their device. Egusa et al. in U.S. Pat. No. 5,343,050 shows an OLED structure containing more than two hole-transporting layers. Moreover, doped HTL (or doped HIL) is used in HTR to improve the luminous efficiency of OLED. For example, Zhou et al. reported in “Very-Low-Operating-Voltage Organic Light-Emitting Diodes Using a p-Doped Amorphous Hole Injection Layer”,
Applied Physics Letters,
78, 410 (2001) that high luminance efficiency and low drive voltage can be achieved in an OLED having an HIL comprising tetrafluoro-tetracyano-quinodimethane (F4-TCNQ) doped 4,4′,4″-tris(N,N-diphenyl-amino)triphenylamine (TDATA) in contact with both an anode and an HTL.
Using the aforementioned methods can effectively enhance the luminous efficiency of an OLED. However, multiple HTL structures cannot substantially improve the lifetime of the device. Instead, it usually shortens the lifetime of the device. Shown in
FIGS. 1B and 2B
are the schematic electron energy band diagrams of
FIGS. 1A and 2A
, respectively. As is known, the most commonly used anode, indium tin oxide (ITO), can have a work-function of about 5.0 eV with some proper surface treatments, and a TPD layer has an ionization potential (Ip) of about 5.6 eV. When a single TPD layer is used as an HTL in adjacent to ITO in an OLED, it creates an energy barrier of higher than 0.6 eV for hole-injection at the interface of ITO/TPD, and this high energy barrier can cause a fast interface damage during operation, resulting in short operational lifetime. In Shirota's paper, an m-TDATA layer is used as another HTL between ITO and the TPD layer. Since m-TDATA layer has an Ip of about 5.1 eV, it forms an energy barrier of about 0.1 eV at the interface of ITO/m-TDATA. This low hole-injection barrier will not easily cause interface damage during operation. Therefore, the half-brightness lifetime of the OLED using m-TDATA/TPD layers as dual HTLs increases from 150 hrs to 300 hrs with an initial luminance of 300 cd/m
2
. Moreover, another energy barrier of about 0.5 eV is formed at the interface of m-TDATA/TPD. Although this barrier accumulates holes and slow down the transport of holes into the LEL resulting in high luminous efficiency, this barrier is also limits further improvement of device lifetime. A conventional OLED, having an N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) layer as a single HTL and a tris(8-hydroxyquinoline)aluminum (Alq) layer as an ETL, usually has a half-brightness life time longer than 5,000 hrs with an initial luminance of 300 cd/m
2
. The substantial increase in lifetime is mainly due to the fact that NPB has an Ip of about 5.4 eV, and it forms a lower energy barrier at the interface of ITO/NPB compared to ITO/TPD, as well as due to the higher glass transition temperature of NPB than that of TPD. When NPB HTL is replaced by dual HTLs, the lifetime of the device can actually be reduced. If more than two HTLs are used in OLED and if the hole-injection barriers are not low enough to reduce interface damage at each interface in the HTR, lifetime of the OLED may not be improved. Moreover, F4-TCNQ is not thermally stable in its host layer, and it can diffuse from its host layer into the LEL to quench the luminance and to shorten the device lifetime. Furthermore, the fabrication of multiple HTLs or doped HTL (or doped HIL) needs more material sources in an evaporation chamber and needs longer device fabrication time, especially when more than two HTLs are fabricated.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to improve the luminous efficiency of an OLED.
It is another object of the present invention to simplify the layer structure of an OLED.
It is yet another object of the present invention to reduce the number of the materials sources needed for the fabrication of an OLED.
These objects are achieved by an organic electroluminescent device comprising:
a) an anode;
b) a hole-transporting region disposed over the anode; wherein the hole-transporting region contains at least one hole-transporting material;
c) a metal sub-layer disposed within the hole-transporting region; wherein the metal sub-layer contains at least one metal selected from group 4 through group 16 of the Periodic Table of Elements and the selected metal has a work-function higher than 4.0 eV;
c) a light-emitting layer formed in contact with the hole-transporting region for producing light in response to hole-electron recombination;
d) an electron-transporting layer disposed over the light-emitting layer; and
e) a cathode disposed over the electron-transporting la
Liao Liang-Sheng
Madathil Joseph K.
Eastman Kodak Company
Garrett Dawn
Owens Raymond L.
LandOfFree
Organic electroluminescent devices having a metal sub-layer... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Organic electroluminescent devices having a metal sub-layer..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Organic electroluminescent devices having a metal sub-layer... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3346570