Atomistic investigation of conjugated polymers for thermoelectric applications: from morphology to transport properties

CAPPAI, ANTONIO

2021-02-09

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Abstract

Two centuries after its first discovery, thermoelectricity, i.e. the phenomenon of direct conversion of thermal power into electrical power, has only recently reached the possibility of implementation in a vast number of practical applications. This breakthrough was undoubtedly determined by the advent and diffusion of numerical atomistic simulation techniques, allowing a quick survey of large classes of materials as possible candidates for the realization of active parts for thermoelectric generators. To this aim, two classes of materials have been essentially identified, (I) inorganic thermoelectrics, based on metal alloys, and (II) organic conductive polymers. The latter ones are well suited for implementation in a large-scale economy because of their superior mechanical properties, such as flexibility and low specific weight, as well as simpler synthesis process, as spin coating, and the possibility of tuning the electrical conductivity through chemical doping. Among the most common conducting polymers, polyethylenedi-oxythiophene (PEDOT), the subject of this Thesis work, has clearly emerged as one of the most promising thermoelectric material. Despite its wide use, however, an unanimous and well-established understanding of the link existing between the synthesis process and the corresponding final thermoelectric properties is still missing and it is thus an active field of scientific investigation. The present Thesis represents a part of this research stream, specifically aiming to shed a light on the role and effect of the combinations of most commonly used polymerizing reagents for PEDOT in determining the micromorphology and the resulting thermal and electrical transport properties. In this respect, the description and development of a new computational algorithm, based on a multiscale approach, is presented, combining a purely quantum description based on the Density Functional Theory (DFT) with Classical Molecular Dynamics (MD). The comparison of the results obtained by numerical simulation with the experimental data currently available demonstrates the effective possibility of including the chemical description of the synthesis process in the context of an MD simulation, and allows to demonstrate and quantify the impact of the combination of reagents used on (i) micromorphological properties, such as chain length distribution and crystallinity, (ii) thermal transport properties, in particular thermal conductivity, and (iii) carrier transport properties, mainly hole mobility and conductivity, estimated by Marcus' theory of electron transport.