Application of bipolar host materials with serendipitously CN-modulated HOMO levels in phosphorescent OLEDs

Abstract

In this study, two novel bipolar host materials, PNTrzCz and CN-PNTrzCz, were rationally designed and successfully synthesized to systematically investigate the impact of incorporating a cyano (–CN) moiety into the donor unit on molecular electronic structure engineering, which distinctly adjusts the HOMO/LUMO distribution, thereby significantly influencing the hole transport characteristics. Driven by the –CN modification, the HOMO electron density shifts significantly: localized on phenanthrene in PNTrzCz but on carbazole in CN-PNTrzCz. Both compounds exhibit exceptional thermal stability, with glass transition temperatures (T_g) of 135 °C and 157 °C, and decomposition temperatures (T_d) of 387 °C and 407 °C, respectively. When employed in red-light-emitting devices, PNTrzCz achieves a high external quantum efficiency (EQE) of 23.22% at 100 cd m⁻² with a low turn-on voltage (V_on) of 2.6 V and a narrowed electroluminescence (EL) spectral full width at half maximum (FWHM) of 46.10 nm. In contrast, CN-PNTrzCz exhibits suppressed efficiency roll-off. In green-light co-host systems, PNTrzCz delivers an EQE of 18.77% at 140 cd m⁻² (V_on = 2.6 V), whereas CN-PNTrzCz significantly reduces the efficiency roll-off to 3.07%, marking a 19.2% improvement over the unmodified device (3.80%). These findings underscore the significance of strategic molecular design in precisely controlling hole transporting properties through subtle modifications of the donor architecture.