Constructing Low-Triplet-Energy Hosts for Highly Efficient Blue PHOLEDs: Controlling Charge and Exciton Capture in Doping Systems

With the aim to rationally figure out the significance of charge and exciton capture in the electrophosphorescent processes in low-triplet-energy hosts involved doping systems, the frontier molecular orbital (FMO) energy levels of a series low-triplet-energy hosts with diphenylphosphine oxide (DPPO) and triphenylamine (TPA), collectively named DPE(x)POTPA(n), were successfully and gradually tuned on the basis of their constant triplet energy levels (T-1) of 2.63 eV to get rid of interference from host-dopant energy transfer. It was showed that device efficiencies were directly proportional to the depths of carrier traps formed on the dopants and inversely proportional to the exciton capture ability of the hosts, which were evidenced by the highest external quantum efficiency of similar to 15% from FIrpic-based PHOLED of DPESPOTPA with the deepest hole and electron traps and the weakest exciton capture ability among DPE(x)POTPA(n). This work not only demonstrated the great advantages and potential of this kind of host materials for low-driving-voltage application but also clarified the determinants of highly efficient low-triplet-energy hosts for blue PHOLEDs, which are consequentially referable for purposeful molecular design.

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