Host-guest energy level matching criteria for cyano-based host materials in phosphorescent and TADF OLEDs

The development of universal host materials with balanced bipolar transport properties and appropriate energy level alignment for diverse dopant emitters remains highly challenging for high-performance OLEDs. This study presents two bipolar host materials, 3CN-mCBP and 4CN-mCBP, by introducing cyano (CN) groups at the 3- or 4-positions of carbazole rings in the well-known hole-transport host mCBP (3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl). Compared to mCBP, the modified hosts exhibit comparable or slightly reduced triplet energy levels, along with deeper HOMO and LUMO energy levels. Charge transport characterization demonstrates enhanced electron injection/transport in the order of 4CN-mCBP > 3CN-mCBP ≫ mCBP, with hole transport capabilities following the reversed sequence: mCBP > 4CN-mCBP > 3CN-mCBP. In green phosphorescent OLEDs using (ppy)₂Ir(acac), the CN-modified hosts achieve superior efficiencies (21.6 % and 19.9 % EQE) versus mCBP (11.8 %), while blue devices with FIrpic show opposite performance trends. For green TADF emitter 4tCzDOXD, all hosts yield comparable efficiencies (15.3–17.9 %). These results highlight the challenges in developing universal host materials for diverse emitter systems.