Carbazole–benzimidazole hybrid bipolar host materials for highly efficient green and blue phosphorescent OLEDs

Abstract

In this study, we synthesized a series of bipolar hosts (CbzCBI, mCPCBI, CbzNBI, and mCPNBI) containing hole-transporting carbazole and electron-transporting benzimidazole moieties and then examined the morphological, thermal, and photophysical properties and carrier mobilities of these bipolar host materials. Altering the linking topology (C- or N-connectivity of the benzimidazole) changed the effective conjugation length and led to different excited-state solvent relaxation behavior. The N-connected compounds (CbzNBI, mCPNBI) possessed higher triplet energies (E_T) than those of their C-connected analogues (CbzCBI, mCPCBI) by 0.23 eV. The higher values of E_T of CbzNBI and mCPNBI endowed them with the ability to confine triplet excitons on the blue-emitting guest. A blue PhOLED device incorporating mCPNBI achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 16.3%, 35.7 cd A⁻¹, and 23.3 lm W⁻¹, respectively; confirming the suitability of using N-connected bipolar hosts for the blue phosphor. The donor/acceptor interactions of the C-connected analogue resulted in a lower triplet energy, making it a suitable bipolar host for green phosphors. A green-phosphorescent device incorporating CbzCBI as the host doped with (PBi)₂Ir(acac) achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 20.1%, 70.4 cd A⁻¹, and 63.2 lm W⁻¹, respectively.