Efficient deep-blue OLEDs based on phenanthro[9,10-d]imidazole-containing emitters with AIE and bipolar transporting properties
Highly efficient deep-blue emitters, especially those matching the NTSC (National Television Standards Committee) blue standard CIE (Commission Internationale de L’Eclairage) coordinate of (0.14, 0.08), are crucial for full-colour displays and white lighting. However, deep-blue emitters with excellent electroluminescence performance are rare. According to the strategy of developing highly luminescent solids by constructing aggregation-induced emission (AIE) systems, two AIE-type blue emitters, mTPE-PPI and mTPE-DPI, have been synthesized by integrating a triphenylethylene (TPE) group with one or two phenanthro[9,10-d]imidazole (PI) groups. The adoption of the PI unit with a large optical bandgap, the meta-linkage of the PI and the TPE units on a benzene ring, and the twisted molecular conformations of present AIE systems endow these compounds with bright blue emission in the solid state. In addition, these compounds are capable of transporting both electrons and holes and possess appropriate HOMO and LUMO levels for carrier injections. These features enable them to serve as excellent blue-emitting materials in OLEDs. A non-doped OLED based on mTPE-PPI displayed deep-blue light with the CIE coordinate of (0.15, 0.09) and a maximum external quantum efficiency (EQEmax) of 2.30%. An analogous device with mTPE-DPI as the emitting layer emitted blue light (CIE: 0.15, 0.14) with a high EQEmax of 3.69% and low efficiency roll-off. Further improvement of device performance has been achieved by the doping technique. The doped device of mTPE-DPI was able to realize deep-blue emission (CIE: 0.15, 0.10) matching well the blue standard, meanwhile reaching a EQEmax of 5.52% that represents the highest value reported for deep-blue OLEDs based on AIE compounds.