Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
1999-09-30
2003-04-01
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S504000, C257S043000, C257S101000
Reexamination Certificate
active
06541908
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the applications of co-coped n-type zinc oxide thin film as a chemically stable and low work function cathode to Organic Light Emissive Diode (OLED) display, Field Emission Display (FED) and vacuum microelectronic display devices.
2. Description of Related Art
Display devices are a fast growing segment of the electronics market. Historically, commercially viable displays have relied on cathode ray tube (CRT) and liquid crystal display (LCD) technologies due to reliability and affordability. CRT technology is mature and able to achieve high resolution, high luminance (brightness), low cost and long life. Unfortunately, CRT displays require high operating voltages and are too heavy for portable applications. CRTs also have a large bulky form factor. More recently, flat panel LCDs have gained acceptance in many applications since they operate at power levels compatible with battery operation, are lightweight and have a thin form factor. LCD panels either reflect or transmit light so an external light source is required. LCD panels also have a limited viewing angle so the user may not be able to see the displayed information from an oblique viewing angle. Although the viewing angle for LCD displays has improved over the years, they are still inferior compared to CRTs and other emissive display technologies. Another weakness of LCD displays is that the liquid crystal material response to a stimulus is intrinsically slow at low temperatures. Thus, LCD displays are a poor choice for portable, automotive or military applications where operation at extreme low temperature may be required. Accordingly, there is great need for an inexpensive low-power display technology that exhibits real-time imaging capability in the flat panel form factor over an extended operating temperature range.
There are a number of alternative technologies that offer the luminous efficiency and resolution of a CRT and the flat panel form factor of an LCD that are compatible with portable applications. Light emissive devices have a form factor of an LCD display but are not dependent on external light sources. Emissive devices also have the wide viewing angle of a CRT and will operate over an extended temperature range. Two examples of emissive devices are the organic light emitting diode (OLED) display devices and the field effect display devices. Emissive display devices are lightweight and capable of projecting video rate images with high contrast ratio over an extended temperature range. Emissive displays hold great promise as an alternative to LCDs because they have superior viewing angle characteristics and high video rates. Moreover, unlike LCDs, the response rate of emissive displays is not affected by a low ambient operating temperature.
FIG. 1
illustrates a portion of a prior art OLED device
10
. Device
10
has an opaque cathode electrode
12
in a spaced-apart arrangement with a transparent anode electrode
14
deposited on a transparent substrate
18
. An organic emissive medium
20
is sandwiched between cathode electrode
12
and an anode electrode
14
with pixels defined wherever the cathode electrode overlaps the anode electrode. Layer
23
may be applied over cathode electrode
12
to encapsulate device
10
and protect the device. When electrons injected into medium
20
from the cathode combine with holes injected at the anode, light, denoted as h&ngr; and illustrated as downwardly projected arrows, is generated and transmitted through transparent anode
14
and substrate
18
.
The cathode electrode
12
is usually an opaque reflective low work function metal such as an alkaline earth metal or reactive metal alloy. Examples of prior art cathode electrodes include calcium, magnesium/silver, or aluminum/lithium. Typically, the anode electrode
14
is a high work function thin film of transparent indium tin oxide (ITO). The phrase “work function” refers to the energy difference, in electron volts (eV), between a free electron and an electron at the Fermi level of the material. The phrase “Fermi level” indicates the energy level at which the probability that a state of energy is occupied is equal to one half. To minimize the energy barriers, the work function of the cathode needs to be low so that the Fermi level closely matches the energy level of the lowest unoccupied molecular orbital (LUMO) of the organic medium. Similarly, the work function of the anode needs to closely match the energy level of the highest occupied molecular orbital (HOMO) of the organic medium. Since ITO is the material of choice for the transparent anode, prior art research has focused on use of alkaline earth metal cathodes having a low work function to achieve device efficiency. However, alkaline earth metals are extremely reactive and are not transparent.
A significant problem with prior art displays is that the interface between the electrodes and the emissive medium creates energy barriers that must be overcome before charge can be injected into the medium. Where the energy barrier at one electrode is much greater than at the other electrode, the supply voltage must be sufficient to overcome the larger barrier thereby increasing the power that must be supplied to the device.
FIG. 2
illustrates the potential energy diagram of the prior art OLED device illustrated in FIG.
1
. As indicated, the Fermi level of the ITO (work function is about 4.7 eV as indicated at
22
) is above the HOMO energy level of the organic medium. For example, since the HOMO is about 4.9 eV for the MEH-PPV organic polymer, energy is required to inject holes, represented by h
+
, over the potential energy barrier into the organic medium. Further, the Fermi level of the metal cathode (about 3 eV to 4 eV for typical alkaline earth metals as indicated at
24
) is below the LUMO of the medium (about 2.8 eV), so energy is also required to inject electrons, represented by e
−
, into medium
20
. Thus, the operating voltage must be sufficient to overcome the potential barriers to inject both electrons and holes into the medium before light will be generated. In many prior art OLED devices, the imbalance in the ratio of hole injection and electron injection generates heat dissipation. Such heating contributes to degradation of the medium and low efficiency of the OLED device.
The use of ITO as the transparent electrode (anode) and a reactive low work function metal as the cathode also constrains OLED device design to the traditional architecture illustrated in FIG.
1
. One example is a miniature OLED display integrated monolithically with the driving circuit on a silicon substrate. This device requires that the opaque reactive metal cathode (Ca, Mg) be deposited on the silicon backplane and the ITO anode to be deposited on the organic emissive medium. However, the reactive cathode readily oxidizes and may degrade the organic medium interface. Also, reactive metals are not compatible with the semiconductor processing technology and can degrade driving circuits on silicon substrates. Accordingly, a complicated semiconductor manufacturing process involving barrier layers is required to combine the prior art OLED display device with electronic elements on a common substrate. Clearly, what is needed is an OLED device with a design and a set component materials that may be processed and integrated on a common silicon substrate together with control circuits. It is desirable to have a transparent and stable cathode material with a low work function and a low processing temperature, so it can be used as a substitute for the reactive metal cathode in the prior art.
Field emission display (FED) devices represent another type of emissive display that is similar to traditional CRT display technology in that an independently addressable stream of electrons excites each pixel. Where the CRT uses a single electron source to sweep a single beam of electrons across the back of a phosphor screen, FED devices incorporate an array of emitters (the cathodes), each of whic
Cheung Jeffrey T.
Warren, Jr. Leslie F.
Williams George M.
Zhuang Zhiming
Patel Ashok
Rockwell Science Center LLC
Shinners Craig E.
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