Highly efficient single-layer dendrimer light-emitting diodes with balanced charge transport

Abstract

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...

Highly simplified small molecular phosphorescent organic light emitting devices with a solution-processed single layer

“…The solution-processed means for fabrication of OLEDs using soluble materials would be simple and opens up a pathway toward large area, low cost and high efficiency OLEDs in the future. Previous attempts to the solutionprocessing phosphorescent devices have been fabricated using phosphorescent polymers [8][9][10][11] and dendrimers [12][13][14] with high efficiency. However, satisfactory fabricating cost and device performance has not yet been achieved.…”

Energy and charge transfer in blends of dendronized perylenes with polyfluorene

“…Various studies have shown that the addition of hole-and electrontransporting molecules to dendrimer-based active layers improves the device performance. [59][60][61] In this sense, dendrimers carrying charge-transporting moieties in the scaffold shall be very applicable for the realization of efficient molecular devices. 62 For all blends, we observe a significant blueshift of the EL spectrum with bias.…”

Optimum Structure Adjustment for Flexible Fluorescent and Phosphorescent Organic Light Emitting Diodes

“…Improving device efficiency, the triplet state excitons must be confined in the emitting layer to increase the chance for energy transfer from host to guest. The material that achieves this effect is called the hole blocking layer (HBL), CF-X [14], CF-Y [14], BCP [12], TPBi [15], and BAlq [16]. These materials have higher ionization energy and band gap that can block the diffusion of excitons.…”