Abstract
A novel bipolar conjugated copolymer poly[(2, 5-diethynylthienylene)-alt-co-(9-hexyl-3,6-carbazole)] (PCT) bearing electron-affinitive moiety of 2,5-diethynylthiophene and hole-transport moiety of carbazole was synthesized via a Pd-catalyzed coupling reaction. The polymer possesses a weight average molecular weight of 23 000 with a polydispersity index of 2.5. PCT exhibits good thermal stability with a glass-transition temperature of 103°C and an onset decomposition temperature of 338°C. The HOMO and LUMO energy levels of PCT were determined to be –5.36 and –2.46 eV, respectively, using cyclic voltammetry. Single-layer and multilayer light-emitting diodes emit yellow light (peak at ≈ 580 nm). Double-layer electroluminescent device with the structure of ITO/copper phthalocyanine/PCT/Ca/Ag has an external quantum efficiency of 0.01%, higher than that of three-layer device with the structure of ITO/copper phthalocyanine/PCT/Alq3/Al, indicating that the electron injection is the limiting factor for the performance of PCT in polymer light-emitting diodes.
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