Synthesis of new polyfluorene copolymers with a comonomer containing triphenylamine units and their applications in white-light-emitting diodes

Abstract

Novel conjugated polyfluorene copolymers, poly[9,9-dihexylfluorene-2,7-diylco-(2,5-bis(4'-diphenylaminostyryl)-phenylene-1,4-diyl)]s (PIGS), have been synthesized by nickel(0)-mediated polymerization from 2,7-dibromo-9,9-dihexylfluorene and 1,4'-dibromo-2,5-bis(4-diphenylaminostyryl)benzene with various molar ratios of the monomers. Because of the incorporation of triphenylamine (TPA) moieties, PGs exhibit much higher HOMO levels than the corresponding polyfluorene homopolymers and are able to facilitate hole injection into the polymer layer from the anode electrode in light-emitting diodes. Conventional polymeric light-emitting devices with the configuration ITO/PEDOT-.PSS/polymer/Ca/Al have been fabricated. A light-emitting device produced with one of the PG copolymers (PG10) as the emitting layer exhibited a voltage-independent and stable bluish-green emission with color coordinates of (0.22, 0.42) at 5 V The maximum brightness and current efficiency of the PG10 device were 3370 Cd/m(2) (at 9.6 V) and 0.6 cd/A, respectively. To realize a white polymeric light-emitting diode, PG10 as the host material was blended with 1.0 wt % of a red-light-emitting polymer, poly[9,9-dioctylfluorene-2,7-diyl-alt-2,5-bis(2-thienyl-2-cyanovinyl)-1-(2'-ethylhexyloxy)-4-methox- ybenzene-5',5'-diyl] (PFR4-S), and poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV). The device based on PG10:PFR4-S showed an almost perfect pure white electroluminescence emission, with Commission Internationale de I'Eclairage (CIE) coordinates of (0.33, 0.36) at 8 V; for the PG10:MEH-PPV device, the CIE coordinates at this voltage were (0.30, 0.40) with a maximum brightness of 1930 cd/m(2). Moreover, the white-light emission from the PG10:PFR4-S device was stable even at different driving voltages and had CIE coordinates of (0.34, 0.36) at 6 V and (0.31, 0.35) at 10 V. (c) 2007 Wiley Periodicals, Inc.