Cathode including a mixture of a metal and an insulator for...

Details Cathode including a mixture of a metal and an insulator for... Cathode including a mixture of a metal and an insulator for...

2000-04-07

2003-02-25

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

With luminescent solid or liquid material

Solid-state type

C313S506000

Reexamination Certificate

active

06525466

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cathode that may be incorporated into organic light-emitting devices, display panels, organic transistors, and organic solid state lasers.
2. Description of the Related Art
Organic light-emitting devices (OLEDs) have recently become a prime focus of numerous researchers because of their relative simplicity of fabrication, large viewing angle, ultra-thin structure, mechanical flexibility, light weight, and faster response time. In particular, OLEDs are being investigated as candidates for commercial display applications, such as ultra-thin flat panel displays (FPDs), roll-up displays, and head-mounted displays, such as virtual reality and cockpit displays. The utility of OLEDs is expected to be especially important in FPDs incorporated in high definition televisions, personal computers, and portable computers.
A cross-section of a conventional bilayer (two organic layers) OLED
10
structure is shown in FIG.
1
. The operation of the OLED
10
can be described as follows: upon the application of a voltage, holes (represented by open ovals) are injected from the anode
20
into the highest occupied molecular orbital (HOMO) of the molecules of a first organic layer
30
, called the hole transport layer (HTL)
30
, and electrons (represented by closed ovals) are injected from the cathode
40
into the lowest unoccupied molecular orbital (LUMO) of the molecules of a second organic layer
50
, called the electron transport layer (ETL)
50
. The charges drift under the influence of the external field and recombine in the emitting layer
60
, which can be the HTL
30
or the ETL
50
, thereby generating excited molecules. Some of the excited molecules decay radiatively, thus releasing light (represented by arrows). The materials forming HTL
30
and the ETL
50
are thus electroluminescent organic (ELO) materials. An example of a material for HTL
30
is TPD, the structure of which is shown in FIG.
2
(A), and an example of a material for ETL
50
is 8-tris-hydroxyquinoline (Alq
3
), the structure of which is shown in FIG.
2
(B).
One of the electrodes, for example the anode
20
, can be transparent to allow the transmission of light to the outside environment so that a viewer can see it. Due to its relatively high transparency to visible light and its relatively good electrical conductivity, indium tin oxide (ITO) can be used as the material for the anode
20
. A substrate
70
, for example made of glass, can support the anode
20
. The material of the cathode
40
is often a metal such as Al or Mg, although other variances have been used as discussed below.
In general, organic materials have a higher hole mobility than electron mobility. This relatively high hole mobility, as well as the presence of a high barrier (&PHgr;
e
) for electron injection at the cathode-organic layer interface, lead to an imbalance between the hole charge density and the electron charge density near the interface of the two organic layers. This behavior has a negative effect on the device external quantum efficiency, which is defined as the ratio of the number of photons collected (for example measured with a calibrated silicon photodetector) in the forward direction to the number of charges injected in the device.
One way to enhance the external quantum efficiency is to increase the number of injected electrons. This can be achieved by decreasing the barrier height between the work function of the metal cathode and the LUMO of the ETL. To that end, OLED cathodes based on metals with a relatively low work function such as lithium, calcium, or magnesium, are used and show higher external quantum efficiency than similar devices with cathodes such as aluminum (Al), copper, or silver. However, the major drawback of using low work function metals is their readily reactive nature, especially in air atmosphere, which results in unreliable OLEDs.
More environmentally stable cathodes such as Al are sometimes used. Aluminum is cheaper, more abundant, relatively resistant to full oxidation and corrosion, when exposed to atmospheric conditions, than either calcium or magnesium. Moreover, the compatibility of Al with silicon microelectronic circuits has made it a material of choice for micro-pixel OLEDs displays driven by thin film transistor or complimentary metal-oxide-semiconductor circuits. However, due to the high work function of Al, OLEDs with Al cathodes are inefficient, and their light output, at a given voltage, is an order of magnitude less than OLEDs with reactive metal cathodes.
A thin insulating layer, such as lithium fluoride (LiF), cesium fluoride (CsF), sodium fluoride (NaF), silicon dioxide, deposited between the Al cathode
30
and the organic layer
40
as a buffer layer has also been used and has lead to some improvements in performance. However, the deposition of a buffer layer requires very careful thickness control. Any thickness variation over the active area of the device leads to uneven electric field distribution, which results in nonuniform brightness, an unacceptable feature in display applications.
Cathodes of Al—Li alloy (a two-metal alloy) have also been tried since they do not require an insulating buffer layer. However, these cathodes are not very reproducible, mainly due to the strict Li content that must be maintained at 0.1% for optimum performance.
Although the devices have shown some progress in the reliability of OLEDs, in terms of operational lifetime, higher operational lifetime are desirable. Furthermore, there is a need for an OLED having enhanced external quantum efficiency and injected electrons densities, and at the same time being reliable, stable, not requiring a buffer layer and having an increased efficiency, reproducibility, and lifetime.
Examples of light-emitting devices are disclosed in U.S. Pat. No. 5,399,502; in Tang et al, “Organic Electroluminescent diodes” Appl. Phys. Lett. 51 (12) 1987; and in Baigent et al, “Conjugated Polymer Light-emitting Diodes on Silicon Substrates” Appl. Phys. Lett. 65 (21) 1994; the entire content of these three references being hereby incorporated by reference.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a cathode having an increased injected electrons densities; and to provide a method for making the same.
Another object of the present invention is to provide an OLED having enhanced external quantum efficiency and injected electrons densities; and to provide a method for making the same.
Yet another object of the present invention is to provide an OLED that is reliable, stable, that does not require a buffer layer; and to provide a method for making the same.
A further object of the present invention is to provide an OLED with an increased efficiency, reproducibility, and lifetime; and to provide a method for making the same.
Another object of the present invention is to provide a display device with OLEDs having increased performance; the display being incorporated, for example, in FPDs for high definition televisions, personal computers, and portable computers; in roll-up displays; or in head-mounted displays, such as virtual reality and cockpit displays.
A further object of the present invention is to provide an organic transistor with a cathode having an increased injected electrons densities.
It is a further object of the present invention to provide an organic solid state laser with a cathode having an increased injected electrons densities.
In a first embodiment, the present invention provides a device including a layer of organic material and a cathode in contact with the organic material layer and including a mixture of a metal and an insulator. The mixture can be either an alloy, a composite, or a combination of both. The metal can be for example Al, Mg, silver (Ag), gold (Au), copper (Cu), nickle (Ni), iron (Fe), chrominum (Cr), indium (In), calcium (Ca), or a combination thereof. The insulator can be an inorganic insulator such as but not limited to alkali, and alkaline compounds, e.g.,: l

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