Polycarbazoles: Relationship between intra- and intermolecular charge carrier transports

We investigated the optical and electronic properties of three kinds of polycarbazole with linkages located at 2,7-, 3,6-, and alternating positions, corresponding to poly(N-dodecyl-2,7-carbazolediyl), poly(N-dodecyl-3,6-carbazolediyl), and poly(N-dodecyl-2,7-carbazolediyl-alt-N-dodecyl-3,6-carbazolediyl), respectively. Fluorescence from the latter two polymers in chloroform showed time-dependent changes, which were examined in terms of solvent, concentration, UV light irradiation, and the presence of oxygen. Flash photolysis time-resolved microwave conductivity (FP-TRMC) and space charge limited current (SCLC) measurements revealed that poly(3,6-carbazole) had the highest intra- and intermolecular charge carrier mobilities, at 0.3 and 1.8 × 10−5 cm2 V−1 s−1, respectively, which were higher than those of the copolymer (0.2 and 2.0 × 10−7 cm2 V−1 s−1) and poly(2,7-carbazole) (0.1 and 3.4 × 10−9 cm2 V−1 s−1). Although poly(2,7-carbazole) was expected to show superior electronic properties based on X-ray diffraction (XRD), it showed the lowest value among the polycarbazoles investigated. The surface roughness, based on atomic force microscope (AFM) images, indicated that the deterioration of charge carrier transport properties arose from an unusually high degree of crystallinity. This study underlines the importance of the balance between intermolecular interaction (crystallinity), solubility, and delocalization of radical cations along the conjugated backbone, which could be of help in designing the molecular structures of highly efficient organic electronic devices.