Electroluminescent device

Details Electroluminescent device Electroluminescent device

1997-09-02

2001-02-13

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

With luminescent solid or liquid material

Solid-state type

C313S506000, C313S503000, C257S040000

Reexamination Certificate

active

06188175

ABSTRACT:

FIELD OF THE INVENTION RECEIVED
This invention relates to electroluminescent devices.
BACKGROUND OF THE INVENTION
The most popular flat panel display technology currently in use is based on liquid crystal devices, which are effectively light shutters used in combination with illumination sources. In graphic displays there are many different pixels that must be independently driven. Typically this is achieved through matrix addressing, where each pixel is addressed by application of a suitable switching voltage applied between row and column conducting tracks on either side of the liquid crystal. Each row is selected by applying a voltage to the row track, and individual pixels within the row are selected by application of column data voltages to the column tracks. The rows are addressed sequentially, each for a line address time such that the whole frame is addressed within the frame time. However, because the speed of switching of the liquid crystals is slow relative to the line addressing time, when video frame rates are required (<20 ms), special circuitry has to be added to each pixel. This arrangement is called active matrix addressing and often involves the use of thin-film transistors at each pixel. Because of the increased complexity of the active matrix displays they are much more expensive to make than passive matrix devices.
Electroluminescent devices are made from a layer of a suitable material between two conductive electrodes. The material emits light when a suitable voltage is applied across the electrodes. One class of such materials is semiconductive conjugated polymers which have been described in our earlier Patent U.S. Pat. No. 5,247,190, the contents of which are herein incorporated by reference. The electrodes can be patterned to form a matrix of rows and columns so that matrix addressing can take place. There are several potential advantages over liquid crystal graphic displays. Because the polymers are directly emissive, no backlight is required. Also polymers of different colours can be fabricated so that a suitably patterned matrix of polymers can be used for a colour display without the use of colour filters as required by a liquid crystal display. Perhaps most significantly, the light emitting polymers are very fast, easily achieving switching times of 1 microsecond, and therefore they are able to react directly when a particular row is selected. Unfortunately, when the row voltage is removed they immediately switch off. To achieve a given average brightness for the display as a whole, each individual line needs to be driven at a peak brightness that is higher by a factor L, where L is the number of lines. The peak brightness that a given emitting area can achieve is limited by the amount of current that can be injected into the semiconductor due to space charge effects.
So-called thin film inorganic electroluminescent devices are also known, as described for example by M. J. Russ and D. I. Kennedy in the Journal of the Electrochemical Society, vol. 114 (1967) page 1066, whose contents are herein incorporated by reference. These too can suffer from the same problem. Phosphor materials are sandwiched between dielectric layers and conducting electrodes, and high ac fields are applied across the structure. When used in displays with a matrix addressing scheme, the average luminance of the display decreases with the number of lines due to a limitation in current densities. One way that this problem has been tackled is by the use of a photoconductor layer integrated with the device (e.g. an amorphous silicon layer deposited between one of the electrodes and the normally adjoining dielectric layer) as described by P. Thioulouse and I. Solomon in IEEE Transactions on Electron Devices, vol. ED-33, (1986), page 1149. The photoconductor layer provides a “memory effect” which allows a device to be turned on and driven with a given light output; subsequently the voltage can be reduced without a reduction in light output, but with the new voltage still below the original turn-on threshold voltage.
SUMMARY OF THE INVENTION
To manufacture an electroluminescent device using the thin film technology discussed above in relation to the prior art is relatively costly because of the high cost of depositing the phosphor layers and the amorphous silicon photoconductor layers. An electroluminescent device using a semiconductive conjugated polymer is much easier to manufacture.
According to one aspect of the present invention there is provided an electroluminescent device comprising first and second electrodes and, arranged between said first and second electrodes, a first layer of a semiconductive conjugated polymer acting as an electroluminescent layer and a second layer of a semiconductive conjugated polymer acting as a light dependent voltage regulating layer the conductivity of which varies with light incident thereon from the electroluminescent layer, wherein the bandgaps of the semiconductive conjugated polymers constituting the first and second layers are selected to be close to one another but with offset energy levels.
Moreover, by selecting the bandgaps of the semiconductive conjugated polymers to be as close as possible, the sensitivity of the device is maximised. Furthermore, because the energy levels of the first and second layers are offset, charge carriers of a given type will accumulate at the interface between the polymers. In this way, recombination of charge carriers in the electroluminescent layer is maximised, with the second layer acting as a charge transport layer from the associated one of the first and second electrodes to the electroluminescent layer.
The light dependent voltage regulating layer acts to regulate the voltage across the electroluminescent layer in accordance with the amount of light falling on it. For a given potential difference between the first and second electrodes, initially most of the potential difference will fall across the light dependent voltage regulating layer as a result of its low conductivity. However, as light emitted from the electroluminescent layer falls on the light dependent voltage regulating layer, the conductivity of the light dependent voltage regulating layer increases thus reducing the voltage across it and also introducing more charge carriers into the electroluminescent layer. Therefore, light emitted from the electroluminescent layer rapidly increases.
Preferably the semiconductive conjugated polymers are selected from the family of polyphenylenevinylene (PPV) and its derivatives. In one example, the first polymer is PPV and the second polymer is blue-shifted PPV (or dimethoxy PPV).
More than two semiconductive conjugated polymer layers could be used. In such as case, it would be possible to arrange for light emission from the semiconductive conjugated polymer having the second largest bandgap, while the semiconductive conjugated polymer with the lowest bandgap would constitute the photoconductive layer. The extra layer acts as a charge transport layer.
In the described embodiment, the first electrode comprises a plurality of electrode strips extending column-wise of the device and the second electrode comprises a plurality of electrode strips extending row-wise of the device, pixels being defined in the device where the row-wise extending strips and the column-wise extending strips respectively overlap.
For use of the electroluminescent device in an addressing scheme, it can comprise addressing means for applying row select voltages to the row-wise extending electrode strips and column data voltages to the column-wise extending electrode strips thereby to selectively address pixels of the display.
Advantageously, these addressing means are operable to apply dc voltages. The prior art discussed above using phosphors requires an ac voltage. Moreover, the thickness of the layers in the prior art is relatively great and therefore to achieve sufficient fields, high voltages (for example of the order of 100 V) are required. An electroluminescent device constructed in accordance with the present inventio

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