Hole‐rich host materials for blue‐phosphorescent OLEDs

Abstract— Stable and efficient organic light‐emitting devices (OLEDs) are an integral part of the future of lighting and displays. The hole accumulation at the hole‐transport/emissive‐layer interface in such devices is considered to be a major pathway for degradation and efficiency loss. Here, the design and synthesis of two charge‐transporting host materials, based on the phosphine oxide (PO) moiety, engineered to improve hole transport of the emissive layer, will be reported. The compounds are an extension of a molecular design strategy which incorporates a hole‐transporting moiety and an electron‐transporting moiety. These materials were designed with two hole‐transport moieties (HTms) to further improve hole transport, compared to the first‐generation host materials that were designed with one hole‐transport functional group. The triplet exciton energy was maintained at a level greater than that of FIrpic (2.7 eV) to prevent exciton quenching. The EHOMO and ELUMO of the two classes of molecules (i.e., 1 HTm vs. 2 HTms) were similar; however, their device performance varied greatly. Emission zone experiments were conducted to further characterize the difference in charge transport between the molecules.