Synthesis and characterization of new light-emitting copolymers in polymeric-light-emitting-diode device fabrications

Abstract

A series of thiophene-containing photoactive copolymers consisting of alternating conjugated and nonconjugated segments were synthesized. The 1H NMR spectra corroborated the well-defined structures, and the copolymers not only were soluble in common organic solvents but also had high glass-transition temperatures (ca. 130 °C) and good thermal stability up to 390 °C. Introducing aliphatic functional groups, such as alkyl or alkoxyl, into chromophores of the copolymers redshifted the photoluminescence spectra and lowered the optical bandgaps. The electrochemical bandgaps calculated from cyclic voltammetry agreed with the optical bandgaps and thus indicated that electroluminescence and photoluminescence originated from the same excited state. The energy levels (highest occupied molecular orbital and lowest unoccupied molecular orbital) of all the copolymers were lower than those of poly[2-methoxy-5-(2′-ethylhexyloxy)-1.4-phenylenevinylene] MEH–PPV, indicating balanced hole and electron injection, which led to improved performance in both single-layer and double-layer polymeric-light-emitting-diode devices fabricated with these copolymers. All the copolymers emitted bluish-green or green light above the threshold bias of 5.0 V under ambient conditions. At the maximum bias of 10 V, the electroluminescence of a device made of poly(2-{4-[2-(3-ethoxy phenyl)ethylene]phenyl}-5-{4-[2-(3-ethoxy,4-1,8-octanedioxy phenyl)ethylene]phenyl}thiophene) was 5836 cd/m2. The external electroluminescence efficiency decreased with the lifetime as the polymer degraded. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3954–3966, 2004