New Phenanthro[9,10‐d]oxazole‐Based Fluorophores with Hybridized Local and Charge‐Transfer Characteristics for Highly Efficient Blue Non‐doped OLEDs with Negligible Efficiency Roll‐Off

It is vital to develop highly efficient non‐doped blue organic light‐emitting diodes (OLEDs) with high color purity and low‐efficiency roll‐off for applications in display and lighting. Herein, two blue D‐A fluorophores TPA‐PO and TPA‐DPO are designed and synthesized, in which phenanthro[9,10‐d]oxazole (PO) acts as the acceptor and triphenylamine as the donor. TPA‐PO and TPA‐DPO display good thermal stability and efficient luminescence efficiency in neat film. Results based on photophysical property and theoretical calculation demonstrate that TPA‐PO and TPA‐DPO possess the hybridized local and charge‐transfer (HLCT) feature, which can utilize the triplet exciton to achieve highly efficient electroluminance (EL). The non‐doped OLEDs with TPA‐PO/TPA‐DPO as pure emissive layer show the uniform EL emission peak at 468 nm, corresponding to CIE coordinates of (0.168, 0.187) and (0.167, 0.167), respectively. The TPA‐DPO‐based non‐doped OLEDs provide the maximum external quantum efficiency (EQE) of 7.99% and high exciton utility efficiency of 48.4%~72.6%. Moreover, the TPA‐DPO‐based device exhibits low‐efficiency roll‐off, still maintaining the EQE of 6.03% at the high luminance of 5000 cd m‐2. Those findings state clearly that PO is a promising building block of blue fluorophore with a potential HLCT feature to be applied in non‐doped OLEDs.