New naphthyridine-based bipolar host materials for thermally activated delayed fluorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2019.04.003

A series of bipolar host materials, namely, o-, m-, p-NPCz and o-, m-, p-NPDa, composing of electron-transporting naphthyridine (NP) and phenylene bridge with ortho-, meta-, and para-substituted hole-transporting carbazole (Cz)/diphenylamine (Da) were synthesized and characterized. By adjusting the linkage topology, the physical properties are subtly tuned. The characteristics of devices employing these new bipolar hosts with green thermally activated delayed fluorescence (TADF) emitter 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) employed as emitter were investigated under the same device structure. Among these hosts, carbazole-based NPCz can perform efficient energy transfer from the host to the dopant and better exciton confinement in the emitting layer due to higher triplet energy and deeper HOMO/LUMO level than those of diphenylamine-based NPDa. The device with o-NPCz as bipolar host exhibits the best device performance with external quantum efficiency of 18.4% and low efficiency roll-off. However, o-, m-, p-NPDa with lower E_T and shallower HOMO levels than 4CzIPN exhibit inferior host to dopant energy transfer. Instead, the exciplex formation between host and 4CzIPN, which was verified by TRPL, led the resulting EL spectra of the NPDa-based devices broad with yellow to orange emissions.