Copolyfluorenes containing pendant bipolar groups: Synthesis, optoelectronic properties and applications

This article reports the synthesis of copolyfluorenes (P1-P3) containing pendant bipolar groups (2.1-8.2 mol%), directly linked to hole-transporting triphenylamine and electron-transporting aromatic 1,2,4-triazole, by the Suzuki coupling reaction and their application in improving PLED performance of conventional MEH-PPV. The bipolar groups not only suppress undesirable green emission of polyfluorene under thermal annealing, but also promote electron- and hole-affinity of the resulting copolyfluorenes. Blending bipolar model compound M0 with MEH-PPV results in enhancement of device performance [ITO/PEDOT:PSS/M0 + MEH-PPV/Ca(50 nm)/Al(100 nm)]. Maximum luminance and luminance efficiency were increased from 3160 cd/m² and 0.24 cd/A of MEH-PPV-only device to 6830 cd/m² and 0.50 cd/A. Moreover, blending the bipolar copolyfluorenes with MEH-PPV further improves the device efficiency, with the maximum luminance and luminance efficiency being significantly enhanced up to 11090 cd/m² and 0.56 cd/A (ca. 0.4 wt% of bipolar residue), respectively. Our results demonstrate the efficacy of the bipolar copolyfluorenes and model compound M0 in enhancing emission efficiency of MEH-PPV.