The efficiency of electroluminescent organic light-emitting devices 1,2 can be improved by the introduction 3 of a fluorescent dye. Energy transfer from the host to the dye occurs via excitons, but only the singlet spin states induce fluorescent emission; these represent a small fraction (about 25%) of the total excited-state population (the remainder are triplet states). Phosphorescent dyes, however, offer a means of achieving improved light-emission efficiencies, as emission may result from both singlet and triplet states. Here we report high-efficiency ( Ռ 90%) energy transfer from both singlet and triplet states, in a host material doped with the phosphorescent dye 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP). Our doped electroluminescent devices generate saturated red emission with peak external and internal quantum efficiencies of 4% and 23%, respectively. The luminescent efficiencies attainable with phosphorescent dyes may lead to new applications for organic materials. Moreover, our work establishes the utility of PtOEP as a probe of triplet behaviour and energy transfer in organic solid-state systems.When the absorption spectrum of the acceptor (dye) overlaps the emission spectrum of the donor (host), efficient energy transfer from the host to the dye can occur via a singlet-allowed, induceddipole coupling between the molecular species. Hence, for a fluorescent emitter, the maximum external quantum efficiency (photons extracted in the forward direction per electron injected) is 4,5 :The fraction of charge carrier recombinations in the host resulting in singlet excitons is x, which from spin statistics is presumed to be ϳ1/4, Φ fl is the photoluminescent efficiency of the dye, h e is the fraction of emitted photons that are coupled out of the device, and h r is the fraction of injected charge carriers that form excitons. As both the recombination and fluorescent efficiencies can approach unity for an optimized device, the efficiency is primarily limited by coupling losses and a restriction to singlet excitons imposed by spin conservation in the induced-dipole energy-transfer process.Although the output coupling of photons can be increased by using shaped substrates 6 , further efficiency improvements require that both singlet and triplet excited states contribute to luminescence. It has been proposed that intersystem crossing in lanthanide complexes may achieve this with an intramolecular energy transfer from a triplet state of the organic ligand to the 4f energy state of the ion 7 . However, a more general and efficient solution to the problem is to use phosphorescent emissive materials.Phosphorescence is the forbidden relaxation of an excited state with spin symmetry different from the ground state; in organic molecules it typically results from a triplet to a singlet ‡ Permanent address:

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Organic light-emitting diodes (OLEDs) are currently receiving a great deal of attention, both academically and commercially. These devices have promise in applications ranging from low information content alphanumeric displays, to high resolution, large area flat panel displays. The most common OLED structure consists of a substrate coated with a transparent conductor (indium tin oxide, ITO) coated with a thin film of organic material(s), followed by a vapor deposited metal cathode. When a potential is applied to the device, holes are injected into the organic material(s) from the ITO electrode and electrons from the metal cathode. The holes and electrons recombine in the organic material, giving excitons, which radiatively relax to give off light. The efficiency of the device is best if multiple organic layers are used, i.e. separate hole and electron transporting layers.The choice of anode material for OLEDs is based on several criteria. The anode in a conventional OLED must have good optical transparency, good electrical conductivity and chemical stability, and a work function that lies near the HOMO levels of the organic materials to which it will inject holes. ITO fits these criteria and is the most widely used anode material in OLEDs. ITO films combine high transparency (» 90 %) with low resistivity (1´10 ±3 ±71 0 ±5 W cm). [1,2] Thin films of ITO can be prepared by a variety of methods, including sputtering, chemical vapor deposition (CVD) and sol-gel techniques.[2] The work function of ITO (4.4±4.7 eV based on ultraviolet photoemission spectroscopy measurements) [3±4] lies near the HOMO levels of typical OLED hole transporting or injecting material, which leads to a small barrier for hole injection into the organic material. It has been found that the ITO electrode can be treated to reduce this energy barrier. For example, devices with lower drive voltage and higher brightness have been prepared by treating the ITO surface with an oxygen plasma [5] or aqua regia. [6] The authors stressed the relationship between the surface morphology of the ITO and the device behavior. The device stability and efficiency strongly depend on the nature of the anode/organic interface. [7] One of the causes of long term OLED degradation involves the diffusion of metal ions or oxygen from the ITO into the organic film.[8±10]By introducing an ultra-thin interlayer of different organic materials at the interface, the efficiency of the devices could be improved.[11] Application of ITO covered with polyaniline film in the same type of devices also gives better results in terms of carrier injection and electroluminescence efficiency. [12,13] Polyaniline film serves as a barrier that prevents oxygen from diffusing out of the ITO and thus stabilizes the electrode±organic interface. In addition to polymers, copper phthalocyanine (CuPc) [3] and aluminum oxide [14] have been used as hole-injecting buffer layers between the ITO electrode and hole-transporting materials in OLEDs. Although the ITO electrode can be used efficiently with m...