Fluorinated poly( N -vinylcarbazole) host for triplet energy confinement on phosphorescent emitter in organic light-emit
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1197-D02-08
Fluorinated poly(N-vinylcarbazole) host for triplet energy confinement on phosphorescent emitter in organic light-emitting diodes Yukitami Mizuno1, Isao Takasu1, Shuichi Uchikoga2, Shintaro Enomoto1, Tomoaki Sawabe1, Akio Amano1, Atsushi Wada1, Jiro Yoshida1 and Tomio Ono1 1 Corporate Research & Development Center, Toshiba Corp., 1, Komukai-Toshiba-cho, Saiwaiku, Kawasaki, 212-8582, Japan 2 Toshiba Research Europe, Ltd, Toshiba corp., 208 Cambridge Science Park, Milton Road, Cambridge, CB40GZ, U.K. ABSTRACT Fluorinated carbazoles as host materials have been investigated for highly efficient organic light emitting diodes (OLEDs). By molecular orbital calculations, we found that fluorinations at position 2, 4, 5 and 7 of carbazole ring were effective for widening HOMOLUMO energy gap. The energy gaps of our synthesized 2,7-difluorocarbazole (F2-Cz) and 2,4,5,7-tetrafluorocarbazole (F4-Cz), were estimated to be 3.71 eV and 3.87 eV by the absorption spectra, respectively. These energy gaps were higher than that of the non-substituted carbazole (Cz, 3.59 eV). We synthesized poly(N-vinyl-2,7-difluorocarbazole) (F2-PVK) and poly(N-vinyl-2,4,5,7-tetrafluorocarbazole) (F4-PVK) as solution processable polymer host materials. However, the F4-PVK was found to be an unsolved polymer. The F2-PVK could be compared with non substituted poly(N-vinylcarbazole) (PVK) in OLEDs. The emission layer (EML) contained iridium(III) bis [(4,6-di-fluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) as a blue phosphorescent dopant, and iridium(III) bis [2-(9,9-dihexylfluorenyl)-1-pyridine] acetylacetonate as a yellow dopant. The white OLED with the F2-PVK showed 1.4 times higher luminous current efficiency (24 cd/A) than the PVK (17 cd/A). These data show that the excitation energy is confined on dopants by using fluorinated polymer host material with higher T1 corresponding to wider HOMO-LUMO energy gap. INTRODUCTION Solution processable OLEDs have attracted attention because of their low cost and easy fabrication for display or lighting applications. For their applications, high luminous efficient OLEDs are required. OLEDs with phosphorescent dopants can theoretically achieve 100% internal quantum efficiency [1-3]. The reason why the efficiency decreases is dissipation of the excitation energy without emission. To confine the energy on the dopants, triplet energy level (T1) of host materials is needed to be higher than that of the dopants [4,5]. However, blue dopants have relatively high T1 (~2.6 eV) corresponding to the short wavelength emission. In comparison to host materials for green and red dopants, host materials for blue dopants should have higher T1. Therefore, candidate host materials are limited to fabricate blue or white OLEDs with high luminous efficiency by solution process such as spin-coating, ink-jet printing and so on. One of the host materials with high T1 is carbazole derivatives [2-6]. We consider that T1 depends on HOMO-LUMO (highest occupied molecular orbital–lowest unoccupied molecular orbital) energy gap. HO
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