Delivered by Ingenta to: McMaster University IP: 188.68.3.199 On: Wed, 15 Jun 2016 18:50:59 Copyright: American Scientific Publishers RESEARCH ARTICLE Copyright © 2008 American Scientiﬁc Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 8, 2990–2995, 2008 Solution-Processed High-Efﬁciency Organic Phosphorescent Devices Utilizing a Blue Ir(III) Complex Eunsil Han, Yi-Yeol Lyu, Tae-Woo Lee ∗ , Younghun Byun, Ohyun Kwon, Alexander Tikhonovsky, Youngnam Kwon, Gyeong-Su Park, and Rupasree Ragini Das ∗ Samsung Advanced Institute of Technology, Mt. 14-1, Nongseo-dong, Giheung-Gu, Youngin-Si, Gyeonggi-Do, 446-712, Korea We report high-efﬁciency blue phosphorescence organic light-emitting devices by solution process utilizing a blue Ir(III) complex [(F 2 ppy) 2 Ir(ph-imz)CN] (F 2 ppy = 2 ′ ,6 ′ -diﬂuoro-2-phenyl pyridine and ph-imz = N-phenyl imidazole) blended with the host mCP (NN ′ -dicarbazolyl-3,5-benzene), and the inert polymers polystyrene (PS) and polymethylmethacrylate (PMMA). The effects of the dopant conﬁnement in the PS and PMMA matrix on the device performance are studied by ﬁeld emis- sion transmission electron microscopy (FE-TEM) and atomic force microscopy (AFM). The complex shows photoluminescence peaked at 458 nm with CIE color coordinates (0.14, 0.21) in solution and (0.14, 0.18) in doped PMMA ﬁlm. The PS based device showed better device performance than the PMMA based device with a maximum luminous efﬁciency of ( L ) 5.11 cd/A with CIE color coordinates (0.17, 0.29) (at 10 mA/cm 2 ) and a maximum luminance of 9765 cd/m 2 . Keywords: Phosphorescent, Blue Emitting Iridium(III) Complex, Dipolar Cloud. 1. INTRODUCTION The cyclometalated complexes of heavy transition met- als with large spin-orbit coupling constant have offered a wide range of phosphors emitting colors from red to blue from the triplet metal-to-ligand-charge-transfer ( 3 MLCT) states 1–3 that are suitable for use in organic light emit- ting devices (OLED) as triplet emitters. 4–6 Although green and red emitting OLEDs using these complexes furnish very high efﬁciency and incredible lifetime, 7 there have been a less number of reports on blue emitting phos- phors with high efﬁciency and longer lifetime. 8–10 Blue phosphorescence can be achieved either by the structural tuning of the cyclometalating ligand, or by a proper selec- tion of the ancillary ligands, 11 or both. The availability of a wide gap host for efﬁcient energy transfer to the triplet MLCT state of the phosphorescent metal complex is one of the most critical issues for the blue phosphores- cent OLEDs. Sato, 9 Forrest, and Thompson 4 8 groups have obtained high efﬁciencies by using hosts with high triplet energies. Their devices are fabricated by vapor evapora- tion method using small organic molecules as the hosts. So far there have been only a few reports 12–14 on blue emitting phosphorescent polymer OLEDs. The emissive ∗ Authors to whom correspondence should be addressed. layers can be spin-coated utilizing such complexes blended with emitting or non-emitting polymers. 15–17 Unfortunately polymeric hosts with high triplet energies are not yet known to fabricate devices utilizing deep blue phosphors. The use of low triplet energy polymeric hosts will not give desired efﬁciency, rather this will lead to phospho- rescence quenching due to the back transfer of energy from the 3 MLCT state of the phosphor to the triplet energy level of the host. A blend of inert polymers like polymethyl methacrylate (PMMA) and polystyrene (PS) and a high triplet energy organic molecule like mCP (N ,N ′ -dicarbazolyl-3,5-benzene) can be used as the emitting layer in the solution-processed device. Effec- tive exciton conﬁnement in a nanometer-order conﬁned geometry by mixing the emitting dyes with an inert host matrix (e.g., PS or Clay) can lead to enhanced photolu- minescence and electroluminescence quantum yield. 18 19 Therefore, the inert host matrices (PMMA and PS) play two important roles in increasing exciton conﬁnement in the matrix as well as in making solution process pos- sible. Here, we report a new blue emitting phospho- rescent Ir(III) complex, [(F 2 ppy) 2 Ir(ph-imz)CN], where F 2 ppy = 2 ′ ,6 ′ -diﬂuoro-2-phenyl pyridine and ph-imz = N -phenyl imidazole, and the solution-processed electro- luminescent device with high efﬁciency. The complex is doped in a blend of mCP and PS or PMMA at different 2990 J. Nanosci. Nanotechnol. 2008, Vol. 8, No. 6 1533-4880/2008/8/2990/006 doi:10.1166/jnn.2008.153