Study on enhanced thermoelectric efficiency of organic-based composite materials

Abstract

Thermoelectric (TE) materials, which exhibit a capability to convert heat to electricity vice versa, can be used to develop power generation and cooling refrigeration without any moving parts. It is well known that the TE efficiency can be defined as the dimensionless figure of merit ZT = σ S2T/k or power factor P = σS2, where S is the Seebeck coefficient, σ is the electrical conductivity, k is the thermal conductivity, and T is absolute temperature. However, there has been a critical issue for TE efficiency arising from conflicting relationship of TE factors such as σ, S, and k. Many attempts have been suggested to optimize or overcome conflicting relationship of TE factors through structural modification, nano-structuring, organic based materials, composites, and etc. Among these suggestions, the energy filtering effect, which can be introduced at an interface between two different materials in composite, is a key component for solving conflicting relationship of TE factors due to enhancing the S without significant reduction of σ. The potential barrier formed at an interface between two materials scatters low energy carriers and allows to pass high energy carriers only. It leads to increase average heat transported per carrier by the high energy carriers and result in high S. These phenomena at an interface lead us to the high P value based on TE composites. For further improvement of TE efficiency by energy filtering effect, we have succeeded to precisely tune the barrier energy in poly (3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS)/Bi2Te3 nanowires (NWs) based organic/inorganic nanocomposite films through polar solvent vapor annealing (PSVA). Controlling a PSS/PEDOT ratio as a function of PSVA duration, work function of PEDOT:PSS was tuned, which eventually varied the barrier energy of nanocomposite thin films. Through optimization of PEDOT:PSS/Bi2Te3 barrier energy, the S was maximized up to 47 μV/K. The electrical conductivity was also maximized simultaneously, because of the PSVA-induced π-π stacking among PEDOT chains and templating effect. Density functional theory calculated an optimal barrier energy (0.12 eV) which showed an excellent agreement with our experimentally determined optimal barrier energy (0.11 eV), at which we also maximized a power factor—an efficiency indicator of TE performance. Our feasible strategy on the manipulation of barrier energy in PEDOT:PSS/Bi2Te3 NWs through the PSVA can be extended to other organic/inorganic based TE composites, toward the realization of highly efficient TE devices.