Highly Efficient Deep-Blue Electroluminescence from a Charge Transfer Emitter with Stable Donor Skeleton

Organic materials containing arylamines have to been widely used as hole-transporting materials as well as emitters in organic light-emitting devices (OLEDs). However, it has been pointed out that the C-N bonds in these arylamines can easily suffer from degradation in excited states, especially in deep-blue OLEDs. In this work, phenanthro [9,10-d]imidazole (PI) is proposed as a potential donor with higher stability than those of arylamines. Using PI as the donor, a donor acceptor type deep-blue fluorophore 1-phenyl-2-(4-(1-phenyl-1H-benzo[d]imidazol-2-yl-[1,1':4',1-terphenyl]-4-yl)1H-phenanthro[9,10-d]imidazole (BITPI) is designed and synthesized. Results from UV-aging test on neat films of BITPI and other three arylamine compounds demonstrate that PI is indeed a more stable donor comparing to common arylamines. An OLED using BITPI as an emitter exhibits good device performances (EQE over 7%) with stable deep-blue emission (color index: (0.15, 0.13)) and longer operation lifetime than the similarly structured device using arylamine-based emitter. Single-organic layer device based on BITPI also shows superior performances, which are comparable to the best results from the arylamine-based donor acceptor emitters, suggesting that PI is a stable donor with good hole transport/injection capability.