Highly efficient organic LEDs made by solution processing are reported. It is shown that the dendritic architecture (see Figure) can be used to solubilize luminescent chromophores and form uniform films of blends. The simple device structures containing a light‐emitting chromophore are amongst the most efficient solution‐processed devices reported. Thanks to this technique, the inkjet printing of phosphorescent materials becomes feasible.

We demonstrate very high-efficiency green phosphorescence from a single-layer dendrimer organic light-emitting diode formed by spin-coating. A first generation fac-tris(2-phenylpyridine) iridium cored dendrimer doped into a wide-gap 4,4′-bis(N-carbazole) biphenyl host displays a peak external quantum efficiency of 8.1% (28 Cd/A) at a brightness of 3450 Cd/m2 and a current density of 13.1 mA/cm2. A peak power efficiency of 6.9 lm/W was measured at 1475 Cd/m2 and 5 mA/cm2. We attribute this exceptionally high quantum efficiency for a single-layer device to the excellent film forming properties and high photoluminescence quantum yield of the dendrimer blend and efficient injection of charge into the emissive layer. These results suggest that dendrimers are an effective method for producing efficient phosphorescent devices by spin-coating.

High-efficiency single-layer-solution-processed green light-emitting diodes based on a phosphorescent dendrimer are demonstrated. A peak external quantum efficiency of 10.4% ͑35 cd/A͒ was measured for a first generation f ac-tris(2-phenylpyridine) iridium cored dendrimer when blended with 4,4Ј-bis(N-carbazolyl)biphenyl and electron transporting 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene at 8.1 V. A maximum power efficiency of 12.8 lm/W was measured also at 8.1 V and 550 cd/m 2 . These results indicate that, by simple blending of bipolar and electron-transporting molecules, highly efficient light-emitting diodes can be made employing a very simple device structure. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1586999͔Over the past ten years, tremendous advances in the area of organic light-emitting diodes ͑OLEDs͒ have been achieved mainly through the synthesis of efficient lumophores and the development of improved device structures. 1-4 Thermally evaporated devices have been demonstrated to be the most efficient with quantum efficiencies approaching 20% and power efficiencies in the region of 60-70 lm/W. 4,5 These devices implement efficient phosphorescent dopants as the light-emitting medium 4 -7 capable of harvesting light from both singlet and triplet excitons. The best performing phosphorescent dopants have been shown to be those based on iridium ͑Ir͒ complexes, which can emit from the metal-ligand charge transfer state. 4,5,7 These organometallic complexes are highly suitable due to their relatively short excited state lifetime, 7 high photoluminescence efficiencies, and excellent color tunability. 8,9 However, in order to achieve the very good performance, complex device structures are required with several charge transport and exciton confinement layers being used.Solution processible materials such as conjugated polymers 1,2,10 or dendrimers 11,12,13 have also demonstrated high efficiency but in much simpler device structures that can be realized by highly cost effective fabrication techniques such as spin coating 11 and ink-jet printing. 14 We have recently shown that efficient electrophosphorescence can be obtained from single-layer OLEDs employing a solution processible dendrimer compound doped into a suitable host material. 11 We have found that the charge balance can be improved further at some expense of complexity by introducing an electron transporting/hole blocking layer to give a highly efficient bilayer device. 15 Even simpler device structures are desirable and, in this letter, we demonstrate device efficiency enhancement in a single-layer OLED structure. This is achieved by means of blending the components used in two-layer devices, thereby overcoming the limitations of poor charge injection and balance that are usually encountered in simple single-layer devices, and so giving major improvements in device performance, especially in terms of power efficiency.The molecular structure of the first-generation f ac-tris(2-phenylpyridine) iridium cored dendrimer ͑G1-Ir͒ and the host mater...

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