Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
2002-01-29
2004-07-20
O'Shea, Sandra (Department: 2875)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S504000
Reexamination Certificate
active
06765350
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to opto-electrical devices, for example devices for emitting or detecting light.
DESCRIPTION OF RELATED ART
One specific class of opto-electrical devices is those that use an organic material for light emission or detection, Light-emissive organic materials are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of which are incorporated herein by reference. The basic structure of these devices is a light-emissive organic layer, for instance a film of a poly(p-phenylenevinylene (“PPV”), sandwiched between two electrodes. One of the electrodes (the cathode) injects negative charge carriers (electrons) and the other electrode (the anode) injects positive charge carriers (holes). The electrons and holes combine in the organic layer generating photons. In PCT/WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline)aluminium (“Alq3”). In a practical device one of the electrodes is typically transparent, to allow the photons to escape the device.
FIG. 1
shows a typical cross-sectional structure of such an organic light-emissive device (“OLED”). The OLED is typically fabricated on a glass or plastic substrate
1
coated with a transparent material such as indium-tin-oxide (“ITO”) to form an anode
2
. Such coated substrates are commercially available. The ITO-coated substrate is covered with at least a thin film of an electroluminescent organic material
3
and a final cathode layer
4
, which is typically a metal or alloy.
Some particularly attractive applications of such devices are as displays in battery-powered units such as portable computers and mobile phones. Therefore, to extend the battery life of such units, there is a particularly strong need to increase the efficiency of the light-emissive devices. One route to improving efficiency is by careful choice and design of the light-emissive material itself. Another is by optimising the physical layout of the display. A third is by improving the conditions for charge injection into and charge recombination in the emissive layer.
To improve the conditions for charge injection into and charge recombination in the emissive layer it is known to include a charge transport layer of an organic material such as polystyrene sulphonic acid doped polyethylene dioxythiophene (“PEDOT-PSS”) between one or both of the electrodes and the emissive layer. A suitably chosen charge transport layer can enhance charge injection into the emissive layer and resist reverse flow of charge carriers, which favours charge recombination. It is also known to form the electrodes from materials having work functions that aid the desired flow of charge carriers. For example, a low work function material such as calcium or lithium is preferred as the cathode. PCT/WO97/08919 discloses a cathode formed of a magnesium:lithium alloy.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided an opto-electrical device comprising an anode electrode; a cathode electrode: and an opto-electrically active region located between the electrodes; the cathode electrode including: a first layer comprising a material having a work function below 3.5 eV; a second layer of a different composition from the first layer, comprising another material having a work function below 3.5 eV, the second layer being further from the opto-electrically active region than the first layer; and a third layer comprising a material having a work function above 3.5 eV, the third layer being further from the opto-electrically active region than the first layer.
According to a second aspect of the present invention there is provided a method for forming an opto-electrical device, the method comprising; depositing an anode electrode; depositing over the anode electrode a region of an opto-electrically active material; depositing over the region of opto-electrically active material a material having a work function below 3.5 eV to form a first cathode layer; depositing over the first cathode layer another material having a work function below 3.5 eV to form a second cathode layer of a different composition from the first cathode layer; and depositing over the second cathode layer a material having a work function above 3.5 eV to form a third cathode layer.
The first layer may be adjacent to the opto-electrically active region or there may be one or more other layers (preferably electrically conductive layers) between the first layer and the opto-electrically active region. The opto-electrically active region is suitably in the form of a layer, preferably a layer of an opto-electrically active material. The opto-electrically active region is suitably active to emit light or to generate an electrical field in response to incident light The device is preferably an electroluminescent device.
The thickness of the first layer is suitably less than 50 Å, optionally less than 30 Å, or less than 25 Å or 20 Å. The thickness of the first layer could be less than 15 Å or 10 Å. The thickness of the first layer may be in the range from 5 Å to 20 Å, possibly around 15 Å. More generally, it is preferred that the thickness of the first layer is in the range from 10 Å to 140 Å. The first layer is preferably, but not necessarily thinner than the second layer.
The thickness of the second layer is suitably less than 1000 Å, and preferably less than 500 Å. The thickness of the second layer is suitably more than 40 Å or 100 Å, and optionally more than 150 Å or 200 Å. The thickness of the second layer is preferably in the range from 40 Å to 500 Å.
The said material having a work function below 3.5 eV of which the first layer is comprised (“the first low work function material”) preferably has a higher work function than the said material having a work function below 3.5 eV of which the second layer is comprised (“the second low work function material”), or could alternatively have a tower work function than it. The work functions of the materials as referred to herein are preferably their effective work functions in the device, which may be different from their bulk work functions. Thus the first low work function material preferably has an effective work function in the device of less than 3.5 eV and/or the second low work function material preferably has an effective work function in the device of less than 3.5 eV.
One of the first and second low work function materials is preferably a compound or complex of a group
1
, group
2
or transition metal. That material is preferably a compound—for example a halide (e.g. a fluoride), oxide, carbide or nitride). That material is preferably a compound of a metal such as Mg, Li, Cs or Y.
The second low work function material may be a metal selected from the following list: Li, Ba, Mg, Ca, Ce, Cs, Eu, Rb, K, Sm, Y, Na, Sm, Sr, Tb or Yb; or an alloy of two or more of such metals; or an alloy of one or more of such metals together with another metal such as Al, Zr, Si, Sb, Sn, Zn, Mn, Ti, Cu, Co, W, Pb, In or Ag.
The first and second low work function materials are preferably different materials. In one preferred embodiment the first low work function material is calcium and the second low work function material is lithium fluoride. In another preferred embodiment the second low work function material is calcium and the first low work function material is lithium fluoride.
The first low work function material suitably has a (effective) work function less than 3.4 eV, or less than 3.3 eV or less than 3.2 eV, or less than 3.2 eV or less than 3.1 eV or less than 3.0 eV. The second low work function material suitably has a (effective) work function less than 3.4 eV, or less than 3.3 eV or less than 3.2 eV, or less than 3.2 eV or less than 3.1 eV or less than 3.0 eV.
The first low work function material preferably does not cause s
Burroughes Jeremy H.
Carter Julian C.
Gunner Alec G.
Heeks Stephen K.
Millard Ian S.
Cambridge Display Technology Ltd.
Krishnan Sumati
Marshall & Gerstein & Borun LLP
O'Shea Sandra
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