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Proceedings Paper

Improvement in triplet exciton confinement of electrophosphorescent device using fluorinated polymer host
Author(s): Isao Takasu; Yukitami Mizuno; Shuichi Uchikoga; Shintaro Enomoto; Tomoaki Sawabe; Akio Amano; Atsushi Wada; Jiro Yoshida; Tomio Ono
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Paper Abstract

Host materials for phosphorescence organic light-emitting diodes (OLEDs) are required to have wide energy gap to prevent back energy transfer and confine triplet exciton on guest molecules. Carbazole (Cz) has been widely used as a building block for host materials because of its relatively high energy gap. We found that the energy gap of Cz can be widened by fluorination at specific positions. A characteristic of the energy gap widening by fluorination is its controllability by the number and position of fluorine substituents. We synthesized 2,7-difluorocarbazole (F-Cz) and estimated the energy gap of Cz and F-Cz from absorption spectra to be 3.59 eV and 3.71 eV, respectively. To confirm the wide-gap effect of F-Cz on OLED device, we synthesized a solution-processable polymer host, poly(N-vinyl-2,7-difluorocarbazole) (F-PVK), which has F-Cz as pendant groups, and compared it with poly(N-vinylcarbazole) (PVK). The OLED devices investigated consisted of an ITO/PEDOT:PSS/EML/CsF/Al multilayered structure. The poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) layer was spin-coated onto an indium tin oxide (ITO) coated glass substrate. Subsequently, the emission layer (EML) composed of PVK or F-PVK, 1,3-bis[(4-tertbutylphenyl)-1,3,4-oxidiazolyl]phenylene (OXD-7), and a blue phosphorescent dopant, iridium(III)bis [(4,6-difluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) was spin-coated, and the CsF and Al layers were vapor-deposited. The OLED device with F-PVK showed 1.8 times higher maximum current efficiency (27 cd/A) than that with PVK (15 cd/A). The improved efficiency of F-PVK device can be rationalized by the enhanced triplet confinement effect of polymer host composed of fluorinated carbazole.

Paper Details

Date Published: 27 August 2009
PDF: 8 pages
Proc. SPIE 7415, Organic Light Emitting Materials and Devices XIII, 74150B (27 August 2009); doi: 10.1117/12.826688
Show Author Affiliations
Isao Takasu, Toshiba Corp. (Japan)
Yukitami Mizuno, Toshiba Corp. (Japan)
Shuichi Uchikoga, Toshiba Corp. (Japan)
Shintaro Enomoto, Toshiba Corp. (Japan)
Tomoaki Sawabe, Toshiba Corp. (Japan)
Akio Amano, Toshiba Corp. (Japan)
Atsushi Wada, Toshiba Corp. (Japan)
Jiro Yoshida, Toshiba Corp. (Japan)
Tomio Ono, Toshiba Corp. (Japan)

Published in SPIE Proceedings Vol. 7415:
Organic Light Emitting Materials and Devices XIII
Franky So; Chihaya Adachi, Editor(s)

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