A study on PEDOT:PSS based low dimensional conducting polymer structure for enhanced thermoelectric performance

Abstract

Owing to the worldwide energy crisis due to the depletion of fossil fuels, there exists a huge demand for alternative renewable energy sources. Among them, the thermoelectric (TF) generator, which has the non-moving parts, noiseless, and eco-friendly energy source, has grabbed the attention of researchers. Among the various TE materials, conducting polymers are an appropriate choice for flexible TE devices, due to their intrinsic properties such as non-toxicity, transparency, and flexibility. However, conducting polymers, like conventional inorganic TE materials, also have interdependencies by electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ), which are factors that determine TE efficiency. Most researches of conducting polymer for TE materials are the development of new synthetic methods or the control of the charge concentration through the chemical doping process, and thus have limitations in improving TE efficiency. In order to overcome the efficiency limitation of the material itself, low-dimensional structures such as superlattice, nanowire, or quantum dot, have been applied to inorganic TE materials, and consequently there have been huge effective in improving TE performance. Low-dimensional structures have the advantages as less charge carrier scattering in dimensionally limited conducting path, and increasing phonon scattering at the interface, there have not been many studies dealing with the low-dimensional structure of conducting polymers due to lack of measurement system and complex synthesis process. In this dissertation, a study to improve TE performance through the low-dimensional structure of poly (3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS), a representative conducting polymer, will be described. Through a low-dimensional structure such as multilayers or nanotubes of PEDOT: PSS, we suggest organic TE materials with enhanced efficiency through improved σ by changing the internal bonding structure of the polymer at the generated interface and reduced κ due to generation of scattering points. Firstly, we investigated the TE properties of 2-dimensional(2D) polymers structure by cross-stacking PEDOT: PSS and another conducting polymer, PANI-CSA. 2D deposition for polymer is a structural approach for growing highly integrated functional thin films and for manipulating functionalities of the multilayer films that could be different from the constituent layers. Our multilayer films exhibit enhanced σ attributed to the stretching of both the PEDOT:PSS and PANI–CSA layers along with a hole diffusion from PANI–CSA to PEDOT:PSS. The multilayer films were deposited via multiple solution processes, which exhibit enhanced σ without any significant degradation of the S, in contrast to a coupling behavior between the σ and the S in bulk materials. Secondly, we introduced the TE properties of 1-dimensional polymer structure by fabricating a single PEDOT:PSS nanotube(NT), which has improved σ resulting from stretched chains in linear growth of nanotube and low in-plane κ due to phonon scattering at interfaces. We reported the characterization of the in-plane thermal energy transport in PEDOT:PSS NTs via direct measurement of the in-plane κ. Further, we could alter the in-plane κ by approximately an order of magnitude, from 0.19 to 1.92 W/m·K, via changing the post-treatment solvents. We found that the electrical and thermal transport in PEDOT:PSS NTs is highly correlated, and the slope of the κ–σ plot is approximately 1.2 times that of L0T, which exceeds the limit predicted by the Wiedemann–Franz law. Our work will broaden the perception of the correlation between electrical and thermal transport in conducting polymers and in nanomaterials with highly disordered structures. The H2SO4-NT can be used for electronic applications in modern miniaturized devices, while P-NT and EG-NT can be used in TE applications.