Sterically hindered fluorenyl-substituted poly(p-phenylenevinylenes) for light-emitting diodes

A new series of processable dihexylfluorenyl-substituted poly(p-phenylenevinylene) derivatives (DHF-PPVs) were synthesized by dehydrohalogenation-condensation polymerization (GILCH polymerization). The polymers were characterized by NMR, FT-IR, and elemental analysis and were completely soluble in common organic solvents, All of the polymers demonstrated bright blue-green emission with high photoluminescence (PL) efficiencies of 68-71% in chloroform, good thermal stability with decomposition temperatures above 322degreesC, and high glass transition temperatures in the range of 113-148degreesC. Cyclic voltammetry studies revealed that these polymers undergo both irreversible oxidation and reduction onsets around 0.9 and -1.5 V, which are attributed to the conjugated backbone. Polymer light-emitting diodes (PLEDs), fabricated with DHF-PPVs, as the emitting layer, poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonic acid) (PSS) (PEDOT:PSS), as the hole injection/ transporting layer, Ca cathodes, and indium-tin oxide (ITO) anodes, (ITO/PEDOT:PSS/DHF-PPVs/Ca), emitted blue-green emission with a maximum peak around 500 nm and a shoulder peak around 532 nm. For the poly[2-(9,9-dihexylfluorenyl)-1,4-phenylenevinylene] (DHF-PPV) device, a low turn-on voltage of 2.8 V (0.13 MV/cm), a maximum luminance of 12 000 cd/m(2) at 0.4 MV/cm, and a maximum external quantum efficiency of 0.53% were obtained, The latter was 33-37 times brighter and 1.6-2.8 times more efficient than those devices with poly[2-(7-methoxy-9,9-dihexylfluorenyl)-1,4-phenylenevinylene] (MDHF-PPV) and poly[2-(7-cyano-9,9-dihexylfluorenyl)-1,4-phenylenevinylene] (CNDHF-PPV).