A study into the electronic and photophysical properties of polycarbazoles.

Abstract

This thesis details an investigation into the electronic and photophysical properties of a series of 2,7-linked polycarbazoles. This class of conjugated polymer has recently attracted interest as a potential blue-emitting material in electroluminescent devices. Polymer light-emitting diodes were fabricated using a variety of device structures. The most promising devices incorporated a triarylamine-substituted carbazoleoxadiazole alternating copolymer (P7). Time-of-flight photocurrent measurements suggested that this material may favour electron transport. By including a thin layer of TFB (that functioned as a complimentary hole transport layer) an EL turn on voltage of 2.8 V, maximum luminance of 2,240 cd/m2 and efficiency of 1.12 cd/A was achieved. The thermal stability of the polymers was investigated by monitoring changes in their photoluminescence spectra after a variety heat treatments. All of the polymers that were studied exhibited reduced emission after being annealed in air at 150 °C. However, polymer P6 (a triarylamine-substituted carbazole homopolymer) exhibited good colour stability over a two hour treatment. The absence of degradation under an inert N2 atmosphere suggested that these polymers may oxidise to form an emission quenching defect. Solution photoluminescence spectra that were obtained from polymer P6 revealed a strong solvatochromic effect. This was attributed to the presence of a charge transfer (CT) state between the carbazole backbone and triarylamine pendant group. By changing the polarity of the solvent, the relative intensities of the intrinsic ‘locally-excited’ (LE) state and low-energy CT state could be controlled. Time-resolved photoluminescence decay measurements also revealed a significant increase in the PL lifetime with solvent polarity