One class of opto-electrical devices is that using an organic material for light emission or as the active component of a photocell or photodetector (a “photovoltaic” device). The basic structure of these devices is a semiconducting organic layer sandwiched between a cathode for injecting or accepting negative charge carriers (electrons) and an anode for injecting or accepting positive charge carriers (holes) into the organic layer.
In an organic electroluminescent device, electrons and holes are injected into the semiconducting organic layer where they combine in to generate excitons that undergo radiative decay. In WO 90/13148 the organic light-emissive material is a polymer, namely poly (p-phenylenevinylene) (“PPV”). Other light emitting polymers known in the art include polyfluorenes and polyphenylenes. 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) aluminum (“Alq3”). In a practical device one of the electrodes is transparent, to allow photons to escape the device.
A organic photovoltaic device has the same construction as an organic electroluminescent device, however charge is separated rather than combined as described in, for example, WO 96/16449.
FIG. 1 illustrates the cross-sectional structure of a typical organic light-emissive device (“OLED”). The OLED is typically fabricated on a glass or plastic substrate 1 coated with a transparent first electrode 2 such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material 3 covers the first electrode. Finally, a cathode 4 covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminum, or a plurality of layers such as calcium and aluminum. Other layers can be added to the device, for example to improve charge injection from the electrodes to the electroluminescent material. For example, a hole injection layer such as poly(ethylene dioxythiophene)/polystyrene sulfonate (PEDOT-PSS) or polyaniline may be provided between the anode 2 and the electroluminescent material 3. When a voltage is applied between the electrodes from a power supply one of the electrodes acts as a cathode and the other as an anode.
The nature of the electrodes has a strong influence on the efficiency, and also lifetime, of the device. For the cathode electrode a number of materials have been proposed, with materials having a low work-function being generally preferred. The inclusion of a high dipole dielectric layer between the cathode and the electroluminescent layer has been shown to improve device efficiency by assisting electron injection. For example, EP 0822603 discloses a thin fluoride layer between the EL layer and a thick conductive layer. The fluoride can be selected from the group of alkali fluorides and alkali earth fluorides. The conductive layer can be selected from the group of elemental metals, metal alloys and conductive materials. For the fluoride layer thicknesses in the range 0.3 nm to 5.0 nm are taught. Similarly, Applied Physics Letters 79(5), 2001, 563-565 discloses metal fluoride/Al cathodes. In addition, WO 00/48257 describes an arrangement comprising a metal fluoride layer, a layer of calcium and a layer of aluminum.
A focus in the field of OLEDs has been the development of full color displays utilizing organic red, green and blue (RGB) electroluminescent materials. To this end, a large body of work has been reported in the development of both small molecule and polymeric red, green and blue emitters. These emitters comprise aromatic moieties which may carry substituents. Appropriate selection of the aromatic moiety, and/or the substituents therefor, enables tuning of the color of emission. Electroluminescent materials comprising sulfur, such as polymers comprising thiophene or benzothiadiazole repeat units, have been reported. For example, red and green emitters comprising these units are disclosed in WO 00/46321.
Full color OLEDs have been disclosed in, for example, Synthetic Metals 111-112 (2000), 125-128. A difficulty with these devices is poor overall device performance (i.e. efficiency, lifetime, etc.) resulting from incompatibility of the cathode with at least one of the red green and blue emitters. For example, the cathode disclosed in Synthetic Metals 111-112 (2000), 125-128 is LiF/Ca/Al which is particularly efficacious with respect to the blue emissive material but which shows poor performance with respect to green and, especially, red emitters. A particular problem of degradation in green and red efficiency has been observed when pixels of these colors are not driven.
The present inventors have identified deleterious interactions between the cathode and sulfur containing materials in the aforementioned devices. In addition to the deleterious effect of this interaction on an OLED, the same deleterious interaction will affect the semiconducting properties of the organic material in an organic photovoltaic device. It is therefore a purpose of the invention to provide a cathode that has improved compatibility with organic semiconducting materials comprising sulfur. It is a further purpose of the invention to provide a cathode which has improved compatibility with all of red, green and blue electroluminescent organic semiconducting materials.