Improvement in triplet exciton confinement of electrophosphorescent device using fluorinated polymer host

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.