Highly Efficient Multifunctional Phosphorescent Dendrimers Consisting of an Iridium-Complex Core and Charge-Transporting
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Highly Efficient Multifunctional Phosphorescent Dendrimers Consisting of an Iridium-Complex Core and Charge-Transporting Dendrons for Organic Light-Emitting Devices Toshimitsu Tsuzuki1, Nobuhiko Shirasawa2, Toshiyasu Suzuki2, and Shizuo Tokito1 1
Science and Technical Research Laboratories, Japan Broadcasting Corporation (NHK), 1-10-11 Kinuta, Setagaya-ku, Tokyo 157-8510, Japan 2 Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8787, Japan ABSTRACT We report a novel class of emitting materials for use in the organic light-emitting devices (OLEDs): multifunctional phosphorescent dendrimers that have a phosphorescent core and have charge transporting dendrons. We have synthesized first-generation and second-generation dendrimers consisting of a fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] core and hole transporting phenylcarbazole-based dendrons. Smooth amorphous films of these dendrimers were formed by spin-coating them from solutions. The OLEDs using the dendrimer exhibited bright green or yellowish-green emission from the Ir(ppy)3 core. The external quantum efficiency of the OLED using the mixture film of the first-generation dendrimer and an electron-transporting material was as high as 7.6%. INTRODUCTION Organic light-emitting devices (OLEDs) have attracted considerable attention because they can be used in full-color flat-panel displays. An advantage of OLEDs based on polymers is that they can be fabricated by solution processes such as spin-coating and inkjet printing, which are suitable for making large fine-pixel displays. There have recently been some attempts to use dendrimers [1] consisting of a luminous core, dendrons and surface groups as the solutionprocessable emitting material for OLEDs [2-10]. An advantage of the dendrimer structure is that significant properties such as solubility and electronic and luminescence properties can be adjusted by appropriately designing the molecules of each component (core, dendrons, and surface groups). Another advantage is that the large dendrons surrounding the luminescent core would improve the luminous efficiency of the OLED by isolating that core from the cores of the adjacent dendrimers. A simple single-layer OLED using dendrimers consisting of a fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] [11] phosphorescent core, phenylene-based dendrons, and 2-ethylhexyloxy surface groups demonstrated a high external quantum efficiency of 8.1% [6]. The phenylene-based dendrons in these phosphorescent dendrimers prevent concentration quenching by isolating the Ir(ppy)3 core but do not transport charge well. A dendrimer with dendrons that transport charge well would be a promising high-efficiency emitting material that could be used in OLEDs made using a solution process. In this paper we report a novel class of emitting materials for use in the OLEDs: multifunctional phosphorescent dendrimers that have a phosphorescent core and have dendrons based on charge-transporting building blocks. In OLEDs using these dendrimers, charge carriers are transported on t
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