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.