New host materials with high triplet energy level for blue-emitting electrophosphorescent device

Abstract

A series of new 9-phenylcarbazole (Cz-Ph)-based host materials with 1,2,4-trizole (TAZ) were synthesized for blue-emitting electrophosphorescent device. The substitution position of Cz-Ph on TAZ ring did not influence photoluminescence maximum (band gap) but the triplet energy level (T1). The electron mobility of 3,5-bis-(3-(9-carbazoyl)-phenyl)-4-(4-butyl-phenyl)-4H-[1,2,4]triazole (6) is 10 times higher than the hole mobility. This is due to the electron transporting/hole blocking characteristics of TAZ moiety. The triplet energy level of the new host materials ranges from 2.8 to 3.0 eV which are suitable for blue-emitting electrophosphorescent devices. The time-resolved photoluminescence decay curve of 4% of FIrpic (iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate) doped in the film of compound 6 showed a single exponential decay curve with a lifetime of 1.2 μs. The absolute PL quantum efficiency (ηPL) of 6 doped with 4% of FIrpic was (82 ± 2%), which is significantly higher than the case of commonly used CBP (4,4′-bis-(9-carbazoyl)-biphenyl) (44 ± 2%). These results also strongly support that triplet excitons formed in FIrpic was not transferred to 6. For a device based on 6 (ITO/PEDOT:PSS (40 nm)/NPB (15 nm)/6:6% FIrpic (30 nm)/BAlq (35 nm)/LiF (1 nm)/Al (100 nm), the maximum photometric efficiency was 14.2 cd/A at a current density of 1.1 mA/cm2, which is higher than that observed with a device based on CBP (ITO/PEDOT:PSS (40 nm)/NPB (30 nm)/CBP:6% FIrpic (40 nm)/BAlq (30 nm)/LiF (1 nm)/Al (100 nm)).