Highly twisted biphenyl-linked carbazole–benzimidazole hybrid bipolar host materials for efficient PhOLEDs

Abstract

Three new highly twisted bipolar host materials (PhczNBI, PhczCBI, and PhczDCBI) comprising of a biphenyl core linking a hole-transport 9-phenylcarbazole (donor) and an electron-transport N-phenylbenzimidazole (acceptor) with different topologies were synthesized, characterized, and applied as host materials for full-color phosphorescent OLEDs. The highly twisted conformations imparted through a 2,2′-biphenyl linkage effectively disrupt the π-conjugation and electronic coupling between the donor and the acceptor, thus giving the bipolar hosts promising properties such as bipolar charge transport features (μ_h ≈ μ_e ≈ 10⁻⁶ to 10⁻⁴ cm² V⁻¹ s⁻¹) and high triplet energies (E_T = 2.55–2.71 eV). The PhOLED device gave a maximum external quantum efficiency (η_ext) of 19.6% for the PhczNBI host with a yellow phosphor (Bt)₂Ir(acac). A two-color, single-emitting-layer and all-phosphor WOLED hosted by PhczDCBI achieved high efficiencies (13.9%, 34.9 cd A⁻¹, and 24.8 lm W⁻¹) and a CRI up to 65.5.