Copolyfluorenes containing bipolar groups: Synthesis and application to enhance electroluminescence of MEH-PPV

Enhancing electroluminescence of conventional MEH-PPV {poly[2-methoxy-5- (2′-ethylhexyloxy)-1,4-phenylenevinylene]} is desirable due to its popularity in polymeric emitting materials. The enhancement is much readily attained by simple blending with functional polymers instead of tedious structural modification. In response to this, three copolyfluorenes (P1-P3) containing bipolar groups (3.1-11.2 mol %), directly linked hole-transporting triphenylamine and electron-transporting 1,2,4-triazole, are synthesized by Suzuki coupling reaction. The bipolar groups not only suppress undesirable green emission of polyfluorene under thermal annealing but also increase hole and electron affinity of the resulting copolyfluorenes. Blending the bipolar copolyfluorenes with MEH-PPV effectively improves the emission efficiency of its electroluminescent devices [ITO/PEDOT: PSS/polymer blend/Ca(50 nm)/Al(100 nm)]. The maximum luminance and maximum luminance efficiency are significantly enhanced from 3120 cd/m² and 0.49 cd/A (MEH-PPV device) to 19 560 cd/m² and 1.08 cd/A (blend device with ca. 0.5 wt % of bipolar groups), respectively. Our results demonstrate the efficacy of the bipolar copolyfluorenes in enhancing electroluminescence of MEH-PPV.