Synthesis and electrochemical and electroluminescent properties of N-phenylcarbazole-substituted poly(p-phenylenevinylene)

An N-phenylcarbazole-containing poly(p-phenylenevinylene) (PPV), poly [(2-(4'-carbazol-9-yl-phenyl)-5-octyloxy-l,4-phenylenevinylene) -alt-(2-(2′-ethylhexyloxy)-5methoxy-l,4-phenylenevinylene)] (Cz-PPV), was synthesized, and its optical, electrochemical, and electroluminescent properties were studied. The molecular structures of the key intermediates, the carbazole-containing boronic ester and the dialdehyde monomer, were crystallographically characterized. The polymer was soluble in common organic solvents and exhibited good thermal stability with a 5% weight loss at temperatures above 420 °C in nitrogen. A cyclic voltammogram showed the oxidation peak potentials of both the pendant carbazole group and the PPV main chain, indicating that the hole-injection ability of the polymer would be improved by the introduction of the carbazole-functional group. A single-layer light-emitting diode (LED) with a simple configuration of indium tin oxide (ITO)/Cz-PPV (80 nm)/Ca/Al exhibited a bright yellow emission with a brightness of 1560 cd/m² at a bias of 11 V and a current density of 565 mA/cm². A double-layer LED device with the configuration of ITO/poly(3,4-ethylenedioxy-2,5-thiophene):poly (styrenesulfonic acid) (60 nm)/Cz-PPV (80 nm)/Ca/Al gave a low turn-on voltage at 3 V and a maximum brightness of 6600 cd/m² at a bias of 8 V. The maximum electroluminescent efficiency corresponding to the double-layer device was 1.15 cd/A, 0.42 Im/W, and 0.5%. The desired electroluminescence results demonstrated that the incorporation of hole-transporting functional groups into the PPVs was effective for enhancing the electroluminescent performance.