Pyridine linked fluorene hybrid bipolar host for blue, green, and orange phosphorescent organic light-emitting diodes toward solution processing

Abstract

Herein, a new series of ambipolar host materials is synthesized, which features a pyridine core and triphenylamine or carbazole arm combined in a non-conjugated configuration. Theoretical calculation results show that their HOMO/LUMO distribution is adjusted from an overlapped state to partially and thoroughly separated states, and their hole/electron transportation ability and energy levels are also tuned, in addition to their triplet energy level varying from 2.38 to 2.87 eV. Thermogravimetric analysis and DSC measurement results display a high thermal decomposition temperature of 420–490 °C and a wide glass transition temperature range of 160–173 °C, along with excellent reversible redox behavior in electrochemical processes. Utilizing the pure host of 4′,4′′′-(pyridine-2,6-diylbis(9H-fluorene-9,9-diyl))bis(N,N-diphenyl-[1,1′-biphenyl]-4-amine), blue organic light-emitting diodes (OLEDs) show a high efficiency of 18.5 cd A⁻¹ and 13.6 lm W⁻¹ compared with 16.3 cd A⁻¹ and 11.6 lm W⁻¹ for the cohost diodes, respectively. Additionally, orange phosphorescent device from the N,N′-((pyridine-2,6-diylbis(9H-fluorene-9,9-diyl))bis(4,1-phenylene))bis(N-phenylnaphthalen-1-amine) cohost shows higher efficiency than the single host devices with a maximum luminous efficiency of 37.3 cd A⁻¹ in a high brightness region of around 12 680 cd m⁻², which makes it one of the most efficient orange OLEDs containing a solution processed single emissive layer structure. Significantly, their efficiency roll-off is greatly suppressed at high luminance, particularly for the orange device with a 3.21% decrease in efficiency from the maximum to a high brightness of 10 000 cd m⁻². This work provides a useful guide for molecular design toward solution processed OLEDs.