Light-emitting devices

#This invention relates to light-emitting devices, especially such devices that include phosphorescent material that can be stimulated by organic light-emitting material of the device.

#One class of light-emitting devices is those that use an organic material for light emission. Light-emitting 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-emitting 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/WO90/13-148 the organic light-emitting material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emitting 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 the typical cross-sectional structure of an organic light-emitting device (“OLED”). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent anode electrode 2 of a material such as indium-tin-oxide (“ITO”) that is suitable for injecting positive charge carriers. Such coated substrates are commercially available. This ITO-coated substrate is covered with at least a layer of a thin film of an electroluminescent organic material 3 and a final layer forming a cathode electrode 4 of a material that is suitable for injecting negative charge carriers. The cathode electrode is typically of a metal or alloy. Other layers can be included in the device, for example to improve charge transport between the electrodes and the electroluminescent material.

#The present invention will now be described by way of example with reference to the accompanying drawings, in which:

#FIG. 2 shows a cross-section of a display device;

#FIG. 3 shows a plan view of the device of FIG. 2; and

#In the display device of FIGS. 2 and 3 a substrate 1 carries phosphorescent material 2, 3, 4. The regions of phosphorescent material are separate. Regions 2 comprise red phosphorescent material. Regions 3 comprise green phosphorescent material. Regions 4 comprise blue phosphorescent material. The device of FIG. 2 is intended to be viewed in the viewing direction indicated at 5. Behind the phosphorescent regions (with respect to the viewing direction) are individually controllable regions 6 of organic light-emissive material. Anode 7 and cathode 8 electrodes are arranged on either side of each controllable region of light-emitting material to allow a voltage to be applied across it. When a suitable voltage is applied across it each region of light-emitting material emits light of a wavelength that is capable of stimulating phosphorescence from the corresponding phosphorescent region. Therefore a selected one of the phosphorescent regions can be caused to phosphoresce, and thereby emit light of the colour with which it phosphoresces, by activation of the corresponding region of light-emitting material.

#Phosphorescence is the emission of radiation by a material due to bombardment by particles or radiation from another source which excites carriers to a triplet state, as opposed to the singlet state associated with fluorescence. The lifetime of triplet states is generally longer than singlet states, meaning that the phosphorescent emission generally continues after the bombardment has ceased. Conventionally, the lifetime of the excitation of the atoms etc. in phosphorescence is taken to be greater than 10−8 s.

#The device of FIGS. 2 and 3 has great advantages over prior colour displays. In the device of FIGS. 2 and 3, unlike many prior colour displays employing organic light-emitting materials, there is no need to precisely tailor the emission colours of the organic light-emitting regions to obtain the precise red, green and blue colours that are needed for a high quality full colour RGB display. In the device of FIGS. 2 and 3 the emission colours are dependant on the phosphors that are used; and the phosphor technology required to produce those emission colours is very well established after many years of research in the CRT field. Therefore, it is straightforward to design the device to produce desired red, green and blue emission colours for accurate full colour display. Unlike conventional phosphorescent cathode ray tube displays the device of FIGS. 2 and 3 can be made very thin, allowing it to be used for flat panel displays in, for example, portable computers.