Shamrock-Shaped D2–A–Cz Materials Featuring Trifluoromethyl Carbazole and Pyrimidine: a Tailored Host for Green Phosphorescent OLEDs

Abstract

A new class of shamrock-shaped D₂–A–Cz and D₂–A–D′ bipolar host materials is specifically designed to optimize performance in green phosphorescent organic light-emitting diode (OLED) devices. These materials utilize 3,6-bis(trifluoromethyl)-9H-carbazole (CF₃Cz) as the donor (D), and pyrimidine (Pym) or pyridine (Py) as the acceptor, with features of structural modifications aimed at enhancing stability and luminescence efficiency. In particular, the introduction of strong electron-withdrawing CF₃ groups helps stabilize the highest occupied molecular orbital (HOMO) energy level, while electron-donating groups like tert-butyl group, carbazole, and N,N-dimethylamine destabilize the lowest unoccupied molecular orbital (LUMO), resulting in high triplet energy levels crucial for green OLED performance. CF₃Cz–2CzPym exhibits outstanding electroluminescent properties when doped with Ir(ppy)₃, achieving a maximum external quantum efficiency (EQE) of 18.5% and a luminance efficiency of 64.6 cd A⁻¹ with a low turn-on voltage of 2.6 V. Additionally, when combined with the electron-transporting material bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PyMPM) to form a co-host system, these materials further improve charge balance and green OLED efficiency, attaining a maximum EQE of 19.8% and a peak luminescence efficiency of 68.4 cd A⁻¹. The strong electron-withdrawing CF₃ substituents at the 3 and 6 positions of carbazole firmly stabilizes the frontier molecular orbitals with high triplet energy.