A New Bis -Cyclometalated Iridium (III) Complex as a Triplet Emitter in Organic Light Emitting Devices
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A New Bis-Cyclometalated Iridium (III) Complex as a Triplet Emitter in Organic Light Emitting Devices. Rupasree R Das*, Ohyun Kwon, Younghun Byun, Yi-Yeol Lyu, Myeong Suk Kim, Heekyung Kim, Eun Sil Han, Mu Gyum Kim, Jong-Jin Park, Lyong Sun Pu Samsung Advanced Institute of Technology, P. O. Box 111, Suwon, 440-600, Korea ABSTRACT We report a new Iridium(III) complex and the study of its optical, electrochemical and electroluminescence properties. The crystal structure shows an octahedral environment around Ir(III) center. Density functional theory (DFT) calculations indicate the contribution of the d-orbitals of Ir and the π-orbitals of the cyclometalating and ancillary ligands toward HOMO, whereas LUMO is concentrated on only the cyclometalating ligand. These complexes emit in the sky blue color region from an admixture of triplet metal-to-ligandcharge-transfer (3MLCT) and ligand π-π* states. A maximum external (ηex) quantum efficiency and luminance efficiency of 2.4% and 5.5 cd/A at 0.12 mA/cm2 was obtained from the device consisting of a 5% doped polymeric and low molecular host. A maximum brightness of 10,200 cd/m2 at 14.8 V was obtained from the device. INTRODUCTION Recently, there is much importance on the phosphorescent organic light emitting devices (POLEDs) consisting of heavy metal complexes as the triplet emitters in the emitting layer [1]-[4]. Due to their strong spin-orbit coupling, these complexes can burrow intensity from the singlet metal-to-ligand-charge-transfer (1MLCT) state and emit effectively from their 3 MLCT states. This helps in utilizing both of the singlet and triplet excitons on the fluorescent host doped with these complexes. Several small organic molecule based on blue emitting POLEDs have been reported with high efficiency. But less number of polymers based on POLEDs are reported [5]. To this end, we tried to dope the complex in a blend of small organic molecule mCP (m-bis(N-carbazolyl benzene)) and a polymer P. EXPERIMENTAL The starting material, [Ir (F2ppy)2Cl]2 was synthesized from IrCl3·3H2O (Acros Chemicals), and the cyclometalating ligand 4´,6´-difluoro-2-phenylpyridine by the literature procedure [6]. The target complex, FZN ([Ir (F2ppy)2 (3,5-dimethylpyrazole-1carboxamido)]) was then obtained by refluxing the 1 mmol of the dimer [Ir(F2ppy)2Cl]2 with 2.5 mmol of 3,5-dimethylpyrazole-1-carboxamide (ZN) in tetrahydrofuran in the presence of sodium methoxide. The synthesis scheme is given in Figure 1. The resulting complex was purified by a silica column and dichloromethane, methyl alcohol and nhexane as the eluent. 1H NMR and mass spectra confirmed the synthesis of the complex. Single crystals were grown from a solution of THF and methanol. A 5% FZN doped OLED
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was fabricated on a glass substrate coated with ITO. A 50 nm thick hole injecting layer was spin coated on the precleaned ITO surface and baked at 120 oC for 30 minutes in vacuum followed by the deposition of the emissive layer consisting of the polymer P (25%), mCP(70%) and FZN (5%). Then a 40 nm thick B
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