WO2000057446A1

WO2000057446A1 - High efficiency electrodes for organic light emitting diode devices

Info

Publication number: WO2000057446A1
Application number: PCT/US2000/006929
Authority: WO
Inventors: Gary W. Jones; Munisamy Anandan
Original assignee: Fed Corporation
Priority date: 1999-03-19
Filing date: 2000-03-17
Publication date: 2000-09-28

Abstract

The present invention is directed to an organic light emitting diode device (10) having a substrate (100), a first conductor (200) formed on the substrate (100), an organic stack (300) formed on the first conductor (200), and a second conductor (400) formed on the organic stack (300). In accordance with the present invention, at least one of the first conductor (200) and the second conductor (400) includes a thin layer of dielectric material (410), and a conducting layer (420). The conducting layer (420) may be transparent. The thin layer of dielectric material may include a doped dielectric material. The doped dielectric material may include between 5 % and 50 % of a conducting material. The conducting material may include one of Mg, Ca, Ce, Ba, Al, Sn, Ga, and In. In accordance with the present invention, the thin layer of dielectric material (410) may include one of lithium fluoride, cesium fluoride, and silicon monoxide.

Description

HIGH EFFICIENCY ELECTRODES FOR ORGANIC LIGHT EMITTING DIODE DEVICES

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention relates to and claims priority on U.S. Provisional Patent Application Serial No. 60/125,239. filed on March 19, 1999.

FIELD OF THE INVENTION The present invention relates to organic light emitting diode ("OLED") devices. In particular, the present invention relates to the use of doped conductors in organic light emitting devices.

BACKGROUND OF THE INVENTION OLED devices have been known for approximately two decades. All OLEDs work on the same general principles. An OLED is typically a thin filmed structure formed on a substrate. A light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between a cathode and an anode. The semiconductor layers may be hole-injecting or electron-injecting layers. The light-emitting layer may be selected from any of a multitude of fluorescent organic solids. The light-emitting layer may consist of multiple sublayers.

When a potential difference is applied across the device, negatively charged electrons move from the cathode to the electron-injecting layer and finally into the layer(s) of organic material. At the same time positive charges, typically referred to as holes, move from the anode to the hole- injecting layer and finally into the same organic light-emitting layer. When the positive and negative charges meet in the center layers (i.e., the semiconducting organic material), they combine, and produce photons. The wave-length - and consequently the color - of the photons depends on the electronic properties of the organic material in which the photons are generated.

The color of light emitted from the OLED device can be controlled by the selection of the organic material. White light is produced by generating and mixing blue, red and green lights simultaneously. The precise color of the light emitted by a particular structure can be controlled both by the selection of the organic material, as well as by the selection of dopants. In a typical OLED, either the cathode or the anode is transparent. The cathode is typically constructed of a low work function material. The holes are typically injected from a high work function anode material into the organic material via a hole transport layer. Typically, the devices operate with a DC bias of 2 to 30 volts. The films may be formed by evaporation, spin casting or other appropriate polymer film-forming techniques, or chemical self-assembly. Thicknesses typically range from a few mono layers to about 1 to 2.000 angstroms. OLEDs typically work best when operated in a current mode. The light output is much more stable for constant current drive than for a constant voltage drive. This is in contrast to many other display technologies, which are typically operated in a voltage mode.

An active matrix display using OLED technology, therefore, requires a specific pixel architecture to provide for a current mode of operation. In a typical matrix-addressed OLED device, numerous OLEDs are formed on a single substrate and arranged in groups in a regular grid pattern. Several OLED groups forming a column of the grid may share a common cathode, or cathode line. Several OLED groups forming a row of the grid may share a common anode, or anode line. The individual OLEDs in a given group emit light when their cathode line and anode line are activated at the same time.

OLEDs have a number of beneficial characteristics. These include a low activation voltage (about 5 volts), fast response when formed with a thin light-emitting layer, and high brightness in proportion to the injected electric current. OLEDs also provide high visibility due to self-emission, as well as superior impact resistance, and ease of handling of the solid state devices in which they are used. OLEDs have practical application in television, graphic display systems, and digital printing.

Optimization of charge injection and transport are of critical importance in providing bright and efficient OLEDs. The use of low work function metals, such as lithium, calcium, and magnesium, as cathodes generally results in efficient but unreliable OLEDs. The unreliability is primarily due to the reactive nature of these materials, especially when exposed to uncontrolled atmospheric conditions. More stable materials, such as aluminum, silver, etc., are generally preferred as cathodes. However, OLEDs that include cathodes made from these materials are inefficient, and their light output is very low compared to OLEDs with a reactive metal cathode.

Alternatives that employ an Al:Li alloy (0.1 % Li) have been used to fabricate efficient and stable OLEDs. These devices, however, are difficult to reproduce because the Li content in Al is difficult to control during the co-evaporation process. Other alternatives employ a thin insulating film such as lithium fluoride (LiF), deposited between the organic layer and the Al cathode. These OLED devices are easier to reproduce, but do not produce the desired efficiency. In addition to LiF, other materials such as silicon dioxide, magnesium fluoride, calcium fluoride, sodium chloride, hexatriacontane. cesium carbonate, etc., have been used as insulating layers in OLEDs fabrication. These materials, however, provide no better enhancement in device performance and stability than devices that include a LiF buffer layer. More recent developments in OLEDS include employing a co-deposited composite cathode of AhLiF as disclosed in Jabbour et. al, "Aluminum based cathode structure for enhanced electron injection in electroluminescent organic devices," 73 Applied Physics Letters 1 185 (1998). The OLEDs employ only a small percentage of dielectric material (alkali fluorides e.g. LiF) in the cathode, typically 1 to 3 percent. These OLEDs are down emitting and employ non transparent cathodes.

Although substantial progress has been made in the development of OLEDs to date, substantial additional challenges remain. There is a need for a method of improving the conduction and efficiency of the conductors within OLEDs. A high transparency cathode or anode with good carrier injection efficiency is needed for OLED devices that emit light upward, away from the substrate.

The present invention provides a solution to the current problems by increasing the luminous efficiency for OLED devices that include substrates formed from silicon integrated circuits or metal foil. OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide an organic light emitting device with improved luminous efficiency.

It is another object of the present invention to provide a high efficiency electrode for an OLED device. It is another object of the present invention to provide a high efficiency electrode for an upwardly emitting OLED device.

It is another object of the present invention to provide a display comprising a plurality of organic light emitting devices with improved luminous efficiency.

It is another object of the present invention to improve light transmissions in up-emitting organic light emitting devices.

It is another object of the present invention to provide a conductor with improved carrier injection efficiency in organic light emitting devices.

It is another object of the present invention to provide a transparent cathode with improved carrier injection efficiency in up-emitting organic light emitting devices. It is another object of the present invention to provide a transparent anode with improved carrier injection efficiency in up-emitting organic light devices. It is another object of the present invention to increase the effective light emitting active are on active matrix substrates.

It is another object of the present invention to provide an electrode formed from semitransparent conductor doped dielectric films and a transparent top conductor. Additional objects and advantages of the invention are set forth, in part, in the description which follows, and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.

SUMMARY OF THE INVENTION

In response to the foregoing challenges, applicants have developed an innovative OLED electrode and process for fabricating an OLED. The present invention is directed to an organic light emitting diode device having a substrate, a first conductor formed on the substrate, an organic stack formed on the first conductor, and a second conductor formed on the organic stack. In accordance with the present invention, at least one of the first conductor and the second conductor includes a thin layer of dielectric material, and a conducting layer. In accordance with one embodiment of the present invention, the first conductor may include the thin layer of dielectric material, and the conducting layer. In accordance with another embodiment of the present invention, the second conductor may include the thin layer of dielectric material, and the conducting layer. In accordance with another embodiment of the present invention, each of the first conductor and the second conductor includes the thin layer of dielectric material, and the conducting layer. The second conducting layer may be either a cathode or an anode.

The conducting layer may be transparent. The thin layer of dielectric material may include a doped dielectric material. The doped dielectric material may include between 5% and 50% of a conducting material. The conducting material may include one of Mg, Ca. Ce, Ba, Al, Sn, Ga and

In. In accordance with the present invention, the thin layer of dielectric material may include one of lithium fluoride, cesium fluoride and silicon monoxide.

The present invention is also directed to a conductor layer for an organic light emitting diode display. The conductor layer includes a thin layer of dielectric material, and a conducting layer. The conducting layer may be transparent. The thin layer of dielectric material may include a doped dielectric material. The doped dielectric material may include between 5% and 50% of a conducting material. The conducting material may include one of Mg. Ca. Ce, Ba. Al, Sn, Ga and In. In accordance with the present invention, the thin layer of dielectric material may include one of lithium fluoride, cesium fluoride and silicon monoxide. The present invention is also directed to a method of fabricating an organic light emitting diode device. The method includes the steps of providing a substrate, forming a first conductor layer on the substrate, forming an organic stack on the first conductor layer, and forming a second conductor layer on the organic stack. In accordance with the present invention, one of the step of forming the first conductor layer and the step of forming the second conductor layer includes the steps of forming a doped dielectric material layer and forming a layer of conducting material such that the doped dielectric material layer is located adjacent the organic stack. The step of forming a doped dielectric material layer may include the step of co-evaporating a transparent dielectric material with a conducting material. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.

The accompanying drawings, which are incorporated herein by reference and which constitute apart of this specification, illustrate certain embodiments of the invention, and together with the detailed description serve to explain the principles of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein:

Fig. 1 is a cross-sectional side view of a conventional OLED device; Fig. 2 is a cross-sectional side view of an OLED device according to the present invention; Fig. 3 is a cross-sectional side view of an OLED device according to another embodiment of the present invention; and

Fig. 4 is a cross-sectional side view of an OLED device according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Fig. 1 discloses the basic structure of an organic light emitting device ("OLED"). The OLED

15, shown in Fig. 1, includes a substrate 100. Overlying the substrate 100 is a first conductor 200. Overlying the first conductor 200 is a second conductor 400. Sandwiched between the conductor layers is a layer or a stack of layers of light emitting organic material 300.

The substrate 100 is substantially planar and underlies and provides support for the entire OLED structure. The first and second conductors 200 and 400 function as either electron injecting or hole injecting layers. When the positive and negative charges from the conductors meet in the layer of organic material 300, light is emitted. OLED 15 may emit light through either the second conductor 400 or the substrate 100.

An OLED device 10 according to the present invention is shown in Fig.2. The substrate 100 may be formed from silicon or a metal foil. Furthermore, the substrate 100 may include integrated circuitry for operating the OLED device 10. The OLED device 10 further includes a first conductor 200 and an organic stack 300. The second conductor 400 of the up-emitting OLED device 10 includes a conducting layer 420 and a layer of dielectric material 410. The layer of dielectric material 410 is located between the conducting layer 420 and the organic stack 300. The organic material 300 may include multiple layers of materials (e.g. A1Q/A1Q + dopant/NPB/CuPc). It is contemplated that polymers, small molecule organics or combinations thereof may be employed in the layers of organic material 300.

The dielectric layer 410 is preferably formed of a thin film (<50nm) of transparent dielectric. The transparent dielectric may be formed of lithium fluoride, cesium fluoride, or silicon monoxide. It is contemplated that other suitable materials having similar electrical and physical properties may be used to form the transparent dielectric. The dielectric 410 is preferably doped to include between 5 and 50 percent of conducting material. The conducting material may include, for example, Mg, Ca, Ce, Ba, Al, Sn, Ga, or In. The conducting material serves to insure vertical conduction and to enhance carrier injection. The conductor doping of the dielectric layer 410 also minimizes the need for precise thickness control of the dielectric layer 410 and provides a barrier for damage during subsequent sputter depositions of the transparent top conductor cathode 420 (e.g., sputtered ITO, IAO, or IZO). Thin metal layers (e.g., <20nm) can be used as the transparent electrode 420 instead of ITO or IZO. Thin metal layers, however, are not preferred for most applications since they are more opaque than the intrinsically transparent conductors. It is contemplated that the second conductor 400 may serve as either the anode or cathode in accordance with embodiments of the present invention. It is contemplated that other suitable dielectric host and conductor dopant materials having suitable electrical and physical properties are considered to be well within the scope of the present invention.

The selection of both the dielectric material and the doping material determines the improvement of carrier injection into the OLED device. For example, selecting CsF doped with Ca or Mg works well when second conductor 400 serves a cathode, and selecting either LiF or SiO doped with Al or Sn works well when second conductor 400 functions as an anode. The preferred material selections for anode contacts also work reasonablv well as cathode contacts and the reverse is generally true with the preferred materials for cathode contacts. Below the organic material 300, is the bottom conductor 200. The conductor 200 is preferably formed from 20nm Mo over 40nm Cr.

The present invention may also can be used to increase the effective light emitting active area on active matrix substrates. The present invention improves the up-emitting characteristics of the

OLED device 10, thereby permitting the OLED device to be constructed over the substrate transistors. In prior designs, OLED devices were required to emit between the transistors. The improvement increases increasing peak luminance or life.

The process of forming an OLED device 10 utilizing a conductor in accordance with the present invention will now be described. The basic OLED device stacked structure is constructed by first forming the first conductor 200 on the substrate 100 using conventional techniques. The organic stack 300 is then deposited in layers on the first conductor 200. This includes depositing a final layer of organic light emitting material (e.g., preferably an electron transport layer including Alq₃). The process of forming the second conductor 400 as a cathode will now be described. A layer of doped dielectric material 410 is then co-evaporated on the final layer of the organic stack 300. Preferably a thin layer of 5nm is formed from the co-evaporation in a vacuum of CsF with 30wt% Mg through a shadow mask opening onto the planned cathode area of the device. The co- evaporation of the dielectric layer 410 may begin with a higher dopant concentration such as up to 98% conductor for the first 0.5-3nm before increasing the conductor deposition rate to reach a less than 50 percent conductor concentration overall. The conductor-dielectric concentration change may be abrupt or gradual. The transparent conductor 420 is then deposited over doped dielectric layer 410. The conductor 420 is preferably formed from indium-tin oxide (ITO) that is sputter deposited in a very low pressure argon and ~1% oxygen atmosphere. The process of forming the second conductor 400 as an anode will now be described. When constructing the second conductor 400 as the anode, the doped dielectric material 410 is preferably formed by co-evaporating a thin layer (approximately 5nm) of LiF or SiO with 30 wt% In or Sn through a shadow mask opening onto to the planned anode area of the device. The transparent conductor 420 is then deposited over doped dielectric layer 410. It is further contemplated that in accordance with the present invention, the dielectric material may be placed under the organic stack 300, as shown in Fig. 3. Fig. 3 discloses an OLED device 20 in accordance with another embodiment of the present invention. In the OLED device 20, the first conductor 200 includes a conductor layer 210 and a layer of doped dielectric material 220. This composite layer can be under or on top of the stack of light emitting organic material 300 depending on the desired current direction through the device. When the dielectric material 220 is placed under the organic material 300 and on top of a conducting electrode 210, then the bottom conductor or electrode 210 does not have to be formed from a transparent material (i.e., ITO) because an up- emitting OLED does not require a transparent first conductor 200. The process used to form layers 410 and 420, described above, is performed to form layers 210 and 220.

A semi-transparent OLED device can be formed if transparent conductors (e.g., ITO) are used on both sides of the organic material 300. It is further contemplated that in accordance with the present invention, the dielectric material may be placed on both sides of the organic stack 300, as shown in Fig. 4. Fig. 4 discloses an OLED device 30 in accordance with another embodiment of the present invention. In the OLED device 30, doped dielectric layers 220, 410 are formed on both sides of the light emitting organic material 300.

It will be apparent to those skilled in the art that various modifications and variations may be made in the preparation and configuration of the present invention without departing from the scope and spirit of the present invention. For example, an OLED may include a top cover overlying the conductors and the organic material. Thus, it is intended that the present invention covers the modifications and variations of the invention, provided they come within the scope of the appended claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. In an organic light emitting diode device having a substrate, a first conductor formed on said substrate, an organic stack formed on said first conductor, and a second conductor formed on said organic stack, the improvement comprising: at least one of said first conductor and said second conductor comprising: a thin layer of dielectric material; and a conducting layer.

2. The organic light emitting diode device according to Claim 1 , wherein said first conductor includes said thin layer of dielectric material, and said conducting layer.

3. The organic light emitting diode device according to Claim 1, wherein said second conductor includes said thin layer of dielectric material, and said conducting layer.

4. The organic light emitting diode device according to Claim 1 , wherein each of said first conductor and said second conductor includes said thin layer of dielectric material, and said conducting layer.

5. The organic light emitting diode device according to Claim 1. wherein said conducting layer is transparent.

6. The organic light emitting diode device according to Claim 1. wherein said thin layer of dielectric material includes a doped dielectric material.

7. The organic light emitting diode device according to Claim 6, wherein said doped dielectric material includes between 5% and 50% of a conducting material.

8. The organic light emitting diode device according to Claim 7. wherein said conducting material includes one of Mg, Ca, Ce, Ba, Al, Sn. Ga and In.

9. The organic light emitting diode device according to Claim 1, wherein said thin layer of dielectric material includes one of lithium fluoride, cesium fluoride and silicon monoxide.

10. The organic light emitting diode device according to Claim 9, wherein said thin layer of dielectric material includes a doped dielectric material.

11. The organic light emitting diode device according to Claim 10. wherein said doped dielectric material includes between 5% and 50% of a conducting material.

12. The organic light emitting diode device according to Claim 1 1. wherein said conducting material includes one of Mg, Ca, Ce, Ba. Al, Sn. Ga and In.

13. The organic light emitting diode device according to Claim 1. wherein said second conductor is a cathode.

14. The organic light emitting diode device according to Claim 1, wherein said second conductor is an anode.

15. The organic light emitting diode device according to Claim 1, wherein said thin layer has a thickness of less than 50nm.

16. A conductor layer for an organic light emitting diode display, said conductor layer comprising: a thin layer of dielectric material; and a conducting layer.

17. The conductor layer according to Claim 16, wherein said conducting layer is transparent.

18. The conductor layer according to Claim 16, wherein said thin layer of dielectric material includes a doped dielectric material.

19. The conductor layer according to Claim 18, wherein said doped dielectric material includes between 5% and 50% of a conducting material.

20. The conductor layer according to Claim 19, wherein said conducting material includes one of Mg, Ca, Ce, Ba, Al, Sn, Ga and In.

21. The organic light emitting diode device according to Claim 16, wherein said thin layer of dielectric material includes one of lithium fluoride, cesium fluoride and silicon monoxide.

22. The organic light emitting diode device according to Claim 21, wherein said thin layer of dielectric material includes a doped dielectric material.

23. The organic light emitting diode device according to Claim 22, wherein said doped dielectric material includes between 5% and 50% of a conducting material.

24. The organic light emitting diode device according to Claim 23. wherein said conducting material includes one of Mg, Ca, Ce, Ba, Al, Sn, Ga and In.

25. The organic light emitting diode device according to Claim 16, wherein said thin layer has a thickness of less than 50nm.

26. A method of fabricating an organic light emitting diode device, said method comprising the steps of: providing a substrate; forming a first conductor layer on said substrate; forming an organic stack on said first conductor layer; and forming a second conductor layer on said organic stack, wherein one of said step of forming said first conductor layer and said step of forming said second conductor layer includes the steps of forming a doped dielectric material layer and forming a layer of conducting material such that said doped dielectric material layer is located adjacent said organic stack.

27. The method according to Claim 26, wherein said step of forming a doped dielectric material layer includes the step of co-evaporating a transparent dielectric material with a conducting material.