Balancing carrier injection/transport through CN-modification on classical hole-transport host for enhanced performance in phosphorescent OLEDs

Two bipolar host materials, TCTA-3CN and TCTA-4CN, are developed by incorporating strong electron-withdrawing cyano groups at the 3- or 4-position of carbazole in classical hole-transport tris(4-carbazoyl-9-ylphenyl)amine (TCTA). Their HOMO/LUMO energy levels were significantly reduced to −5.56/−2.32 eV and −5.60/−2.48 eV, respectively, compared to −5.19/−1.92 eV for TCTA. This CN-modification results in a markedly improved carrier balance, as evidenced by a reduction in the hole/electron current ratio from >16,000 for TCTA to ∼45 for both TCTA-3CN and TCTA-4CN in the corresponding single-carrier devices at 4 V. When utilized as host materials for (ppy)₂Ir(acac) based devices, a synergistic enhancement in luminescence and efficiency was observed.