Low Turn-on Voltage, High-Power-Efficiency, Solution-Processed Deep-Blue Organic Light-Emitting Diodes Based on Starburst Oligofluorenes with Diphenylamine End-Capper to Enhance the HOMO Level

Two new star-shaped oligofluorenes, HFB-diF-Dpa and HFB-terF-Dpa, with a hexakis(fluoren-2-yl)benzene core and six diphenylamine end-cappers were designed and synthesized. The peripheral diphenylamine groups enhance the HOMO energy levels of the materials, and the bulky star-shaped structures efficiently suppress the intermolecular interaction. Their thermal, photophysical, and electrochemical properties were investigated. The two compounds display strong deep-blue emission both in solution and solid state. Solution-processed devices based on these starbursts exhibit highly efficient and stable deep-blue electroluminescence. Their high-lying HOMO energy levels match very well with that of the hole-injecting material. The double-layered device featuring HFB-diF-Dpa as emitter shows a low turn-on voltage of 3.6 V, a maximum current efficiency of 6.99 cd A –1, and a maximum external quantum efficiency of 5.45% with the CIE coordinate of (0.154, 0.136). In particular, the combination of low driving voltage and high EQE provides an outstanding maximum power efficiency of 6.10 lm W –1, which is the highest for nondoped deep-blue OLEDs based on solution-processable materials. Moreover, these devices present small values of efficiency roll-off at high brightness up to 1000 cd m –2 .