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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External quantum efficiencies of up to (5.6±0.1)% at low brightness and (2.2±0.1)% at 100 cd/m2 are obtained from a red electrophosphorescent device containing the luminescent dye 2,3,7,8, 12,13,17,18-octaethyl-21H23H-phorpine platinum(II) (PtOEP) doped in a 4,4′-N,N′-dicarbazolebiphenyl (CBP) host. Due to weak overlap between excitonic states in PtOEP and CBP, efficiency losses due to nonradiative recombination are low. However, energy transfer between the species is also poor. In compensation, a thin layer of 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline is used as a barrier to exciton diffusion in CBP, improving the energy transfer to PtOEP. This technique may be applied to improve the efficiency of other electrophosphorescent devices.

The origin of electrophosphorescence from a doped polymer light emitting diode ͑LED͒ has been investigated. A luminescent polymer host, poly͑9,9-dioctylfluorene͒ ͑PFO͒, was doped with a red phosphorescent dye, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum͑II͒ ͑PtOEP͒. The maximum external quantum efficiency of 3.5% was obtained at a concentration of 4% PtOEP by weight. Energy transfer mechanisms between PFO and PtOEP were studied by absorption, photoluminescence, and photoinduced absorption spectroscopy. Even though electroluminescence spectra were dominated by PtOEP at a concentration of only 0.2 wt % PtOEP, Förster transfer of singlet excitons was weak and there was no evidence for Dexter transfer of triplet excitons. We conclude that the dominant emission mechanism in doped LED's is charge trapping followed by recombination on PtOEP molecules.

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