A carbazole–phenylbenzimidazole hybrid bipolar universal host for high efficiency RGB and white PhOLEDs with high chromatic stability

Abstract

A novel bipolar molecule CNBzIm comprised of electron-donating carbazole and electron-accepting phenylbenzimidazole is designed, synthesized and characterized for RGB phosphorescent organic light emitting diodes (PhOLEDs). CNBzIm exhibits promising properties such as high triplet energy (E_T = 2.71 eV) due to disrupted π-conjugation, bipolar and balanced charge transport ability (μ_h ≈ μ_e ≈ 10⁻⁵ cm² V⁻¹s⁻¹), and high morphological stability (T_g = 185 °C). PhOLEDs adopted CNBzIm as a host respectively doped with Os(bpftz)₂(PPh₂Me)₂, (PPy)₂Ir(acac) and FIrpic under the same device structure show excellent performance with external quantum efficiencies (η_ext) as high as 19.1% for red, 17.8% for green, and 12.7% for blue. A two-color, all-phosphor and single-emitting-layer WOLED hosted by CNBzIm was achieved with a maximum η_ext of 15.7%, current efficiency (η_c) of 35 cd A⁻¹, and power efficiency (η_p) of 36.6 lm W⁻¹. Utilizing a bipolar host with balanced mobilities, WOLED can effectively broaden its recombination zone and show high color stability. The Commission Internationale de L'Eclairage (CIE) coordinates of the WOLED remain almost constant when the brightness goes from 1700 cd m⁻² (0.32, 0.41) to 49 900 cd m⁻² (0.31, 0.41).