Tetraphenylmethane-based nonconjugated bipolar hosts with tunable donor-acceptor ratios for high-performance phosphorescent and TADF OLEDs

The development of versatile host materials that combine high triplet energy, efficient bipolar charge transport, and excellent morphological stability remains essential for realizing high-performance organic light-emitting diodes (OLEDs). Herein, three nonconjugated bipolar host materials, Cz-TBI, DCz-DBI, and TCz-BI, were designed and synthesized by integrating carbazole donor units and benzo[d]imidazole acceptor units through a tetrahedral tetraphenylmethane (TPM) core. Varying the donor-to-acceptor ratio enabled systematic regulation of the charge-transport properties of the host materials. All three compounds exhibited excellent thermal stability, with decomposition temperatures above 500 °C, high glass transition temperatures of 170-187 °C, smooth amorphous films, and a high triplet energy of 3.02 eV. Hole-only and electron-only device measurements confirmed their bipolar charge-transport characteristics and demonstrated that the carrier-transport properties could be tuned by adjusting the donor-to-acceptor ratio. The compounds were evaluated as universal hosts in blue TADF OLEDs and blue, green, and red phosphorescent OLEDs (PhOLEDs). Among them, DCz-DBI delivered the best device performance, achieving maximum external quantum efficiencies (EQEs) of 22.7% for the blue TADF OLED and 21.5%, 24.3%, and 21.3% for the blue, green, and red PhOLEDs, respectively. A single-host white OLED based on DCz-DBI achieved a maximum EQE of 24.2%, an EQE of 22.4% at 1000 cd m-2, and a color-rendering index of 81, with negligible voltage-dependent color variation. These results establish TPM as an effective core for constructing versatile host materials for high-performance multicolor OLEDs.