Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Patent
1990-12-28
1993-09-21
Crane, Sara W.
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
257103, 313504, H01L 2929
Patent
active
052471900
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to electroluminescent (EL) devices, and in particular EL devices in which the light-emitting layer is a semiconductor.
BACKGROUND OF THE INVENTION
Electroluminescent (EL) devices are structures which emit light when subject to an applied electric field. The usual model for the physical process in a semiconductor used in this way is through the radiative combination of electron-hole pairs which are injected into the semiconductor from opposite electrodes. Common examples are light-emitting diodes based on Gap and similar III-V semiconductors. Though these devices are efficient and widely used, they are limited in size, and are not easily or economically used in large area displays. Alternative materials which can be prepared over large areas are known, and among the inorganic semiconductors most effort has been directed to ZnS. This system has considerable practical drawbacks, primarily poor reliability. The mechanism in ZnS is believed to be one where acceleration of one type of carrier through the semiconductor under a strong electric field causes local excitation of the semiconductor which relaxes through radiative emission.
Among organic materials, simple aromatic molecules such as anthracene, perylene and coronene are known to show electroluminescence. The practical difficulty with these materials is, as with ZnS, their poor reliability, together with difficulties in deposition of the organic layers and the current-injecting electrode layers. Techniques such as sublimation of the organic material suffer from the disadvantage that the resultant layer is soft, prone to recrystallisation, and unable to support high temperature deposition of top-contact layers. Techniques such as Langmuir-Blodgett film deposition of suitably-modified aromatics suffer from poor film quality, dilution of the active material, and high cost of fabrication.
An electroluminescent device utilising anthracene is disclosed in U.S. Pat. No. 3,621,321. This device suffers from high power consumption and low luminescence. In an attempt to provide an improved device, U.S. Pat. No. 4,672,265 describes an EL device having a double layer structure as its luminescent layer. However, the suggested materials for the double layer structure are organic materials which suffer from the disadvantages mentioned above.
SUMMARY OF THE INVENTION
The present invention seeks to provide an electroluminescent device in which the above mentioned drawbacks are obviated or at least mitigated.
The present invention provides in one aspect an electroluminescent device comprising a semiconductor layer in the form of a thin dense polymer film comprising at least one conjugated polymer, a first contact layer in contact with a first surface of the semiconductor layer, and a second contact layer in contact with a second surface of the semiconductor layer, wherein the polymer film of the semiconductor layer has a sufficiently low concentration of extrinsic charge carriers that on applying an electric field between the first and second contact layers across the semiconductor layer so as to render the second contact layer positive relative to the first contact layer charge carriers are injected into the semiconductor layer and radiation is emitted from the semiconductor layer.
The invention is based on the discovery by the present inventors that semiconductive conjugated polymers can be caused to exhibit electroluminescence by the injection of charge carriers from suitable contact layers.
Semiconductive conjugated polymers per se are known. For example, their use in an optical modulator is discussed in EP-A-0294061. In that case, polyacetylene is used as the active layer in a modulating structure between first and second electrodes. It is necessary to place an insulating layer between one of the electrodes and the active layer so as to create a space charge region in the active layer which gives rise to the optical modulation effect. Such a structure could not exhibit electroluminescence since the presence of
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Bradley Donal D.
Burroughes Jeremy H.
Friend Richard H.
Cambridge Capital Management Limited
Cambridge Research and Innovation Limited
Crane Sara W.
Lynxvale Limited
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