Achieving host-free near-ultraviolet electroluminescence via electronic state engineering with phosphine oxide

Developing near-ultraviolet (NUV) emitters with board band gap and high photoluminescence quantum efficiency (PLQY) is a formidable challenge for organic light-emitting diode (OLED) applications. Herein, two NUV emitters PhImPOCz and PhImPOtBuCz were designed on the basis of phosphine oxide (PO) modification. It is showed that based on crescent phenanthrene (Phen) fused with acceptor imidazole (Im) as NUV chromophore, carbazole (Cz) or 3,6-di-tertbutyl carbazole (tBuCz) donors make the molecule bipolar character to balance carrier recombination in emissive layer (EML). More significantly, besides steric hindrance for aggregation suppression, the introduction of triphenylphosphine enhances localized excited feature of the electronic states, and prevents the red shift induced by intra- and inter- molecular charge transfer, which establishes the basis for fabricating host-free NUV OLEDs. The non-doped devices based on the PhImPOCz and PhImPOtBuCz emitters achieved the low turn-on voltages of 4.2 V and 4.4 V, and the state-of-the-art external quantum efficiency (EQE) up to 2.81% and 3.42%, respectively. This work demonstrates the significance of electronic state engineering for high-energy OLED applications.