Efficient Solution-Processed Deep-Blue Organic Light-Emitting Diodes Based on Multibranched Oligofluorenes with a Phosphine Oxide Center

A series of multibranched oligofluorenes with a phosphine oxide center were designed and synthesized through Suzuki cross-coupling reaction. Their thermal, photophysical, and electrochemical properties were investigated. The phosphine oxide linkage can disrupt the conjugation, and then allow the molecule system to extend to improve the solution processability and photoluminescent quantum yields without depreciating the deep blue emission. The noncoplanar molecular structures resulting from the phosphine oxide triangular pyramidal configuration can suppress the intermolecular interaction. All compounds display strong deep-blue emission both in solution and the solid state. Solution-processed devices based on these oligofluorenes exhibit highly efficient deep-blue electroluminescence, and the device performances are significantly enhanced with the extension of the oligofluorene branches. The double-layered device featuring PPO-TF3 as emitter shows a maximum current efficiency of 1.88 cd A –1 and a maximum external quantum efficiency of 3.39% with Commission Internationale de l′Eclairage (CIE) coordinates of (0.16, 0.09) that are very close to the National Television Standards Committee’s blue standard.