New Conjugated Polymers Derived from Carbazole as Thermoelectric Materials
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New Conjugated Polymers Derived from Carbazole as Thermoelectric Materials Isabelle Lévesque 1,2, Xing Gao 1, Christopher I. Ratcliffe 1, Dennis D. Klug 1, John S. Tse 1, Nicolas Blouin 2, Jean-François Morin 2, Mario Leclerc 2 1 Steacie Institute for Molecular Sciences, National Research Council of Canada 100 Sussex Drive, Ottawa (Ontario), K1A 0R6 Canada 2 Canada Research Chair in Electroactive and Photoactive Polymers, Département de chimie, Université Laval, Quebec (Quebec), G1K 7P4 Canada ABSTRACT Novel poly(3,6-hexyl-2,7-N-octylcarbazole) derivatives and poly(diindolocarbazole)s were synthesized. Optical, electrochemical, electrical and thermoelectric properties were investigated. Band structure calculations were used to predict which polymers were promising as thermoelectric materials. The best combination of Seebeck coefficient and conductivity (power factor) was 9,4 x10-8 Wm-1K-2 with a copolymer of carbazole and thiophene. This corresponds to a ZT at room temperature of 0.0003. Optimization of the polymer structure and doping level should lead to an increased ZT. INTRODUCTION Conjugated polymers such as polyacetylene, polythiophenes, polyaniline, polypyrrole, polyphenylenevinylenes, polyfluorenes, poly(3,6-carbazole)s and more recently poly(2,7carbazole)s show interesting optical and electrical properties [1,2]. The incorporation of flexible side chains in the structure is now a common way of improving processing by increasing solubility or fusibility. Conjugated polymers have been extensively studied in applications such as sensors, light-emitting diodes, transistors and photovoltaic cells [1]. Our objective is to study conjugated polymers for their use as thermoelectric materials for power generation. To obtain an efficient thermoelectric, the materials need to have a high Figure of Merit, ZT=S2σT/κ, where S stands for the Seebeck coefficient, σ and κ are the electrical and thermal conductivities, and T is the temperature [3]. Polymers inherently possess a low thermal conductivity, which gives them one significant advantage over conventional thermoelectric materials. Thermoelectric data has been reported previously in the literature [3] for polyacetylene, polyaniline, polypyrrole and polythiophene. Polyacetylene shows a high ZT (0.6-6) but poor stability in the doped state even in an inert atmosphere. The other polymers show better stability but a low Seebeck coefficient. There is a need to find other stable polymers in the doped state having a high Seebeck coefficient as well as high conductivity. We have recently reported a high Seebeck coefficient for a poly(carbazolenevinylene) derivative [4]. However, the conductivity of this polymer was still too low to obtain a reasonable ZT. We are using an approach combining band structure calculations and experimental measurements to predict and design new promising materials. Good thermoelectric materials should have a flat band near the Fermi level for high Seebeck coefficient and a dispersive band for high electrical conductivity. In this paper, we
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