A DEVICE FOR EMITTING LIGHT

Abstract: A device for emitting light comprises a body (3) consisting of a light-emissive polymer or an electroluminescent organic material, and a plurality of electrodes (2, 4) electrically coupled to the body. A layer of non-crystalline tetrahedrally bonded carbon (6) is located between the body and at least one electrode (2). The layer of tetrahedrally bonded carbon has an electron affinity sufficiently low such as to promote the emission of electrons into the body in use. The carbon layer also promotes adhesion between the electrodes and the body. The carbon layer is a poor electrical conductor, and need not be removed between adjacent electrodes.

Description: 1 A DEVICE FOR EMITTING LIGHT This invention relates to a device for emitting light, comprising a body consisting of a light-emissive polymeric or organic material, and a plurality of electrodes electrically coupled to the body. Known devices of this type, used for backlighting liquid crystal displays, are described in JP-08031573-A and US 5,710,484. In these devices a layer of sputtered carbon is provided between the body and the anode (hole injection electrode) to improve adhesion between the body and the anode. According to a first aspect of the present invention, there is provided a device for emitting light as specified in claims 1 - 7. According to a second aspect of the invention there is provided a display unit as specified in claims 8 and 9. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which:- Figure 1 shows a cross-section of a prior art device, and Figure 2 shows a cross-section of a first embodiment of the present invention. Light emitting materials which are polymeric or organic are currently being developed for use in displays. Two types of display are presently under development, one type is based on light emitting polymers, and the other is based on organic electroluminescent materials which need not be polymers. The structures used for these two types of known display device are very similar and are shown in Figure 1. The light emissive body (3) comprises a layer of either a polymer which emits light in response to an electrical impulse, or an organic electroluminescent material. This layer is sandwiched between a pair of electrodes, namely an anode (4) and a cathode (2), which are carried by a substrate (1). Hole carriers are injected from the anode into the body, whilst electrons are injected from the cathode into the body. These injected carriers ultimately undergo radiative recombination, resulting in the emission of light from the body in use. The anode in such prior art devices is generally made from a layer of a high work function material such as a metal or indium tin oxide (which is light transmissive). The anode may also be supplied with other layers to aid hole transport such as CN X , or a conductive polymer such as polyaniline. The cathode is usually made from a material having a low electron affinity such as a low work function metal or alloy such as Mg:Ag, Li:Al, CaOx, or LiF:Al. 2 The use of lithium or its alloys as a cathode can provide a number of disadvantages. For example, the cathode surface is susceptible to chemical reactions with water and/or oxygen which impairs its electron injection properties. Lithium also reacts with some organic layers. These problems are mitigated in the present invention by the provision of a tetrahedrally bonded carbon layer between the cathode electrode and the polymeric or organic body. This feature can provide the advantage that reactive metals such as lithium or its alloys (which are commonly used for this electrode because of the low electron affinity required) may be substituted by less reactive conductors, thereby improving reliability of operation. Figure 2 shows a device according to the present invention for emitting light, comprising a body (3) consisting of a light-emissive material which is polymeric and/or organic, a plurality of electrodes (2, 4) which are electrically coupled to the body, and a layer of tetrahedrally bonded carbon (6), which layer is located between the body and at least one electrode (2) and which is in electrical contact with both the body and the at least one electrode. Such tetrahedrally bonded material has been disclosed in EP-A-0 175 980. The layer of tetrahedrally bonded carbon is adapted to have an electron affinity sufficiently low such as to promote the emission of electrons into the body from the layer adjacent the said at least one electrode in use. To achieve this effect the work function of the tetrahedrally bonded carbon layer is preferably less than that of magnesium, i.e. less than about 3.5 eV. Such layers can be produced by laser ablation (as described by N Kumar et al., in Society for Information Display SID 94 Digest p43 et seq. 1994 (ISSN 1083-1312/97/1701) or using a filtered cathodic vacuum arc (FCVA) as described by W Milne in J. Non-Cryst. Solids, vol 198 - 200, p605 (1996). The work function of the material may be reduced by the addition of n-type dopants such as nitrogen or phosphorus. The device shown in Figure 2 is made in the following way. A Corning 7059 glass substrate (1) has a patterned layer of ITO (4) deposited thereon by vapour deposition and subsequent photolithography and wet etching. The organic or polymer layer (3) is then spin coated to a thickness of 100 nm. A layer of undoped tetrahedrally bonded non-crystalline carbon 100 nm to 2 microns thick is then deposited thereon using a filtered cathodic vacuum arc (FCVA). An aluminium cathode is then evaporated onto the carbon layer. This aluminium layer is patterned by photolithography and wet etching. The whole device is 3 then encapsulated using an encapsulant (7) in the usual way to passivate the device. The light emissive material in the embodiment shown in Figure 2 may comprise, for example, an organic electroluminescent material such as tris (8- quinolinolato) aluminium (known as Alq), or a light emitting polymer such as for example polyphenylenevin lene (known as PPV). The peak emission wavelengths of both materials can be controlled by adding suitable dopants or modifier compounds, or through modification of their structure. The organic or polymeric layer may be produced by spin coating, or spreading on the surface of a substrate using a doctor blade. Devices having a high quantum efficiency in practice usually have a body comprising a plurality of organic and /or polymeric layers, selected layers being doped to promote carrier transport or luminescence (as described for example in the paper by C W Tang, Society for Information Display - Seminar Lecture Notes 1997, volume 2, pF4/3 et seq.(ISSN 0887-915X). The body (3) may therefore comprise a plurality of such layers. The carbon layer (6) also promotes adhesion between the electrodes (2, 4) and the body (3). The carbon layer is a poor electrical conductor, and need not be removed between adjacent electrodes.

Claims: 4 CLAIMS 1. A device for emitting light, comprising a body (3) consisting of a light- emissive material which is polymeric and /or organic, a plurality of electrodes (2, 4) which are electrically coupled to the body, and a layer of tetrahedrally bonded carbon (6), which layer is located between the body and at least one electrode (2) and which is in electrical contact with both said body and said at least one electrode, characterised in that the layer of tetrahedrally bonded carbon has an electron affinity sufficiently low such as to promote the emission of electrons into the body from the layer adjacent the said at least one electrode in use. 2. A device as claimed in claim 1, in which the at least one electrode in electrical contact with the tetrahedrally bonded carbon layer is a cathode. 3. A device as claimed in claim 1, in which the tetrahedrally bonded carbon is non-crystalline. 4. A device as claimed in any preceding claim, in which the tetrahedrally bonded carbon layer includes nitrogen and/or phosphorus. 5. A device as claimed in any preceding claim, in which the tetrahedrally bonded carbon layer is greater than 100 nm in thickness. 6. A device as claimed in claim 5, in which the tetrahedrally bonded carbon layer is greater than 200 nm in thickness. 7. A device as claimed in any preceding claim, in which the layer of tetrahedrally bonded carbon extends along a surface of the body between adjacent electrodes, or adjacent extensions of the same electrode. 8. A display unit comprising a plurality of devices as claimed in any preceding claim, the devices being arranged to form an array of picture elements, and drive means for supplying electrical impulses to the devices. 9. A display unit as claimed in claim 9, in which the light emitted by a given picture element has one of a plurality of predetermined colours.

Inventors: MOSELEY ALAN, MOSELEY, ALAN
Applicants: CENTRAL RESEARCH LAB LTD, MOSELEY ALAN, CENTRAL RESEARCH LABORATORIES LIMITED, MOSELEY, ALAN
Application year: 1999
Main classification: H05B33
IPC codes: H01L51, H05B33