Color variable bipolar/AC light-emitting devices

Details Color variable bipolar/AC light-emitting devices Color variable bipolar/AC light-emitting devices

1998-03-11

2001-05-22

Yamnitzky, Marie R. (Department: 1774)

Stock material or miscellaneous articles

Composite

Of inorganic material

C428S917000, C313S504000, C313S506000, C257S040000, C257S089000, C257S103000

Reexamination Certificate

active

06235414

ABSTRACT:

TECHNICAL FIELD
This invention relates to light-emitting devices driven by an electric field and which are commonly referred to as electroluminescent devices.
BACKGROUND
In the past decade, there has been great interest in organic electroluminescent devices, particularly conjugated polymer based light-emitting devices (“LED”s). Electroluminescence (“EL”) combined with other unique properties of polymers, such as solution processibility, band gap tunability, and mechanical flexibility, make conjugated polymers excellent candidates for low cost large area display applications.
Among the most important limitations associated with “conventional” polymer light-emitting diodes is poor stability and so-called “shelf lifetime.” The devices degrade even during storage. This is usually caused by the chemical reactivity of the low work function metal electrodes required for efficient electron injection and/or by the poor oxygen stability of most conjugated polymers. Recently there have been reports of new device configurations such as symmetrically configured AC light-emitting (“SCALE”) devices and light-emitting electrochemical cells (“LEC”s). These devices modify the charge injection and/or transport characteristics such that the device operation is not sensitive to the electrode materials used. As a consequence, more stable metals such as aluminum or even gold can be used as electrodes, potentially improving the device operating stability and storage lifetimes.
To date, a variety of conjugated polymers and/or copolymers have been found to exhibit electroluminescent properties such that all the necessary colors needed for display applications are obtainable.
However, for most devices the color of the emitted light is fixed once the device is fabricated. Recently there has been great interest in developing color variable light-emitting devices, i.e., individual devices that can generate two or more colors of light. In color variable devices based on blends of polythiophene derivatives, different components in the blend emit different colors of light simultaneously, with the strength of each component varying with the applied voltage. Such devices can emit multiple colors of light; however, such devices have very limited control over brightness at a desired color. Color variable light-emitting electrochemical cells which emit two independent colors of light also have been developed. The two-color LECs offer an improved control over the color and brightness: the color is controlled by the polarity and the brightness is controlled by the magnitude of the driving voltage. However, due to the involvement of ionic species in the device operation, the response of the devices is intrinsically slow, making them clearly unsuitable for applications that require rapid switching of colors. More recently, multi-layer light emitting devices which generate two independent colors were achieved at liquid nitrogen temperature by inserting a blocking layer in between two different emitting polymer layers. The two colors can also be controlled by the polarity of the driving voltage. Such an approach improves the device response time, but it raises the device operating voltage due to the introduction of the charge blocking layer and retains the stability concerns of “conventional” polymer LEDs.
At present, most polymer-based LEDs can only be operated under forward DC bias, and require a low workfunction metal in the electron injecting contact. However, since low workfunction metals, such as calcium, are unstable against oxidation, such devices show very poor stability under ambient environment. Also, the conventional polymer LEDs generally only can emit one color of light, and it is not possible to tune the color of light once such LEDs have been fabricated.
The present invention thus is a further improvement upon the bipolar electroluminescent devices described in U.S. Pat. No. 5,663,573, which is incorporated herein by reference.
It is thus an object of the present invention to provide a color variable bipolar light emitting device that can be applied to a variety of display applications requiring a robust and reliable electroluminescent device.
SUMMARY OF THE INVENTION
The present invention includes color-variable light-emitting devices which are capable of generating two independent colors, even at room temperature. The devices comprise a layer of at least one active electroluminescent polymer sandwiched between two different redox-mediating polymer layers. The redox polymer layers modify the charge injection and transport properties such that the device may be operated under both forward and reverse bias. Also, at least one of the redox polymers is capable of modifying the emission properties of the emitting polymer layer at the interface such that the interface emits different colors of light than the bulk of the emitting polymer layer. Thus, the colors of the light may be controlled by selecting the desired emission locations which in turn may be controlled by the polarity of the driving voltage and the charge injection and transport properties of the emitting polymer layer. As movement of ionic species is not required for the device to operate, a relatively fast time response may be achieved , allowing colors to be changed rapidly.
The present invention includes polymer based color-variable bipolar(“CVBP”)/AC light-emitting devices, and their fabrication. The devices of the present invention may have either a single layer or a multi-layer structure. In the single layer structure, the device may be fabricated from a blend of conjugated polymers and/or copolymers as the emitting layer. In the multi-layer configuration, the device may be fabricated as a layer of emissive polymer or a blend of emissive polymers sandwiched between two non-emissive polymers, such as two different redox polymer layers. Indium-tin-oxide (“ITO”) and metals may be used as charge injecting contacts in both configurations.
FIG. 1
shows schematically the structure of the devices of the present invention. The devices of the present invention may be fabricated by spin casting polymer layers on one electrode and then vacuum depositing the other electrode. The spin casting technique is conventional and well-known in the art. However, a wide variety of other well known methods may also be used to obtain the layered structure shown in
FIG. 1
including doctor blading, dip coating, chemical vapor deposition, physical vapor evaporation, sputtering, and Langmuir-Blodgett techniques. Spin casting is preferred due to the ease of fabrication and uniformity of the resulting thin films.
The CVBP devices of the present invention may operate under either polarity of driving voltage with different colors of light being emitted under forward and reverse bias. The relative fast time response allows the rapid switch of colors and AC operation. The fundamental concept of the present invention is quite general, it is applicable to a variety of emitting materials in conjunction with suitable redox materials, as well as suitable electrode materials.
Electron-Injecting Electrodes
With respect to such alternative materials and referring to
FIG. 1
, the classical electron-injecting electrode
1
of either configuration (a), (b) or (c) may be of any appropriate material. The electrode materials may be metals, degenerate semiconductors, and conducting polymers. Electrodes can be fashioned from any suitable conductive material including electrode materials that may be metals, degenerate semiconductors, and conducting polymers. Examples of such materials include a wide variety of conducting materials including, but not limited to, (1) indium-tin-oxide (“ITO”), (2) metals such as gold, aluminum, calcium, silver, copper, indium and magnesium, (3) alloys such as magnesium-silver, (4) conducting fibers such as carbon fibers, and (5) highly-conducting organic polymers such as highly-conducting doped polyaniline, highly-conducting doped polypyrrole, or polyaniline salt (such as PAN-CSA) or other pyridyl nitrogen-containing polymer, such as polypyridylvinylene. O

Affiliated with

Also associated with

Rating

Color variable bipolar/AC light-emitting devices does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Color variable bipolar/AC light-emitting devices, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Color variable bipolar/AC light-emitting devices will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2504218