The effect of bipolar charge transport of derivatives of 1-phenyl-1H-benzo[d]imidazole with horizontal molecular orientation on the performance of OLEDs based on thermally activated delayed fluorescence or phosphorescence

Abstract

The synthesis and properties of two derivatives of 1-phenyl-1H-benzo[d]imidazole with differing numbers of tert-butylcarbazole electron-donating moieties are reported. The compounds exhibit high thermal stability, with 5% weight loss temperatures exceeding 341 °C and glass transition temperatures of over 149 °C. They display moderate triplet energies of 2.63 and 2.66 eV. The synthesized compounds were employed as host materials in phosphorescence and TADF-based organic light-emitting diodes (OLEDs). An investigation of the angle-dependent emission intensity of light-emitting layers containing the phosphorescent emitter Ir(ppy)₂(acac) doped into the examined compounds revealed a notably high internal outcoupling efficiency in OLEDs, exceeding 30%. This efficiency is attributed to the significant horizontal molecular orientation factor, reaching up to 87%. Based on the characterization of the hosting properties of 1-phenyl-1H-benzo[d]imidazole derivatives, the most significant influence on device performance is attributed to their charge-transporting properties. An OLED with the phosphorescent emitter Ir(ppy)₂(acac) and a host material exhibiting bipolar charge transport demonstrated an external quantum efficiency of 13%. Additionally, the picric acid sensitivity of one of the compounds was examined. Triplet-facilitated emission was completely quenched upon the addition of a nitroaromatic explosive as a guest in a film.