Sterically Hindered Fluorenyl-Substituted Poly(<i>p</i>-phenylenevinylenes) for Light-Emitting Diodes

Abstract

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 322 °C, and high glass transition temperatures in the range of 113−148 °C. 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/m2 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).