Synthesis and characterization of light-emitting and hole-transporting polymers

Abstract

Novel conjugated-nonconjugated multiblock copolymers (CNMBCs), poly[(9-carbazol-3-yl)vinylene-2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene-(9-butylcarbazol-3-yl)] (PCAR-MEH) and poly[(9-carbazol-3-yl)vinylene-2,5-trimethylsilyl-1,4-phenylenevinylene-(9-butylcarbazol-3-yl)] (PCAR-Si) have been synthesized by Wittig reaction. The purpose of the synthesis of these polymers that contain carbazole units with good hole transporting abilities is for the use as hole-transporting materials in light-emitting diodes (LEDs). Because the turn-on voltage of the device depends on the hole transporting ability of the material that is in direct contact with the anode from which the holes are injected, these new materials lower the turn-on voltage when applied to LED fabrication. The results show that PCAR-MEH is better suited as hole-transporting layer than PCAR-Si. PCAR-MEH starts to emit yellowish green light at an applied voltage of ca. 6 V (ITO/PCAR- MEH/Al). This turn-on voltage is much lower compared to that of the device made of Alq3 layer only, which is ca. 16 V. As possible future applications of polymer lasers, relaxation of photoexcitations and spectral narrowing of photoluminescence for poly(para-phenylenevinylene) have been investigated. Photoinduced absorption spectra and their temporal profiles on poly(para-phenylenevinylene) thin film exhibit a strong correlation with an appearance of spectrally narrowed emission bands. The dependencies of transient absorption signals on excitation and probe power denote much lower saturation intensity in photoinduced absorption than that in stimulated emission from the identical photoexcitations, singlet excitons. We interpreted the peculiar emission bands in terms of superradiance or superfluorescence with a time constant of around 1 picosecond rather than amplification of spontaneous emission from uncorrelated emitters. The efforts to increase the quantum efficiency of the LED have been made by many research groups. ...