Molecular Engineering of Host Materials for High‐Performance Phosphorescent OLEDs: Zig‐Zag Conformation with 3D Gridding Packing Mode Facilitating Charge Balance and Quench Suppression

A group of bipyridine/carbazole hybrid compounds, namely m‐BPyDCz, p‐BPyDCz, m‐BPySCz, and p‐BPySCz, are designed and developed as host materials for phosphorescent organic light‐emitting diodes (PhOLEDs). By tuning the p/n molar ratio and para‐/meta‐ substitution style, scorpion‐, Y‐, Z‐, and L‐shape molecular conformations are generated. In virtue of intermolecular hydrogen bonds and π–π interaction, these compounds form different molecular packing patterns in their single crystals. Particularly the Z‐shaped m‐BPySCz achieves 3D gridding packing with regular and ordered carbazole and pyridine columns as carrier hoping channels and larger intermolecular distance, which not only guarantees charge balance but also suppresses exciton quenching. Consequently the m‐BPySCz hosted sky‐blue and green PhOLEDs exhibit high external quantum efficiencies of 27.3% and 28.0% and low efficiency roll‐offs of 8.1% (at brightness of 1000 cd m^?2 for blue) and 14.3% (at 10000 cd m^?2 for green), all superior to other analogs and many reported host materials. The excellent performance of m‐BPySCz versus its lowest molecular weight and lowest amorphous stability manifests that the molecular packing style of host material dominates to determine the overall performance of PhOLEDs and the 3D gridding packing mode of zig‐zag conformation may be one ideal strategy to eliminate efficiency roll‐off in PhOLEDs.