Influence of doping on thermoelectric behavior of conjugated polymers and their application

Abstract

Recently, emerging interest in flexible and wearable electronics has inspired a great focus on developing flexible thermoelectric (TE) systems for energy harvesting. Despite recent advances in flexible TE devices using organic TE materials such as conductive polymers, carbon nanotubes (CNTs), and their hybrids, many challenges still remain to be overcome. In this study, the TE, chemical and mechanical properties of the conjugated polymer (CP) and carbon nanotube (CNT) are systemically controlled using doping and hybridization of CNT and elastomer. The first, the influence of the doping of a conjugated polymer on the thermoelectric properties and stability is analyzed in depth in terms of comprehensive chemical, crystallographic, and nanostructural methods. Although, various doping techniques of CPs using small molecules and lewis-acid based dopants have been widely investigated, the electrical conductivity with a level of a few S·cm$^{-1}$ and weak doping stability in atmosphere of CPs should be further improved for practical application of organic thermoelectric generator (OTEG). Hence, electrochemical and chemical behaviors of dopants in polymer chain and their effects on TE and nanostructural properties of CPs should be in-depth understood. From such point of view, we highly improved TE performance and doping stability of poly(3-hexylthiophene) (P3HT) using two dopants based on lewis acid. Furthermore, the fabrication of OTEG using stably doped P3HT and its characteristic power generation are successfully demostrated. Even FeCl$_3$ and AuCl$_3$ are used for doping into P3HT polymer by the identical redox reaction, the AuCl$_3$ significantly exhibits the higher doping efficiency and stability in the atmosphere than FeCl$_3$. As a results, AuCl$_3$-doped P3HT exhibits the twice higher electrical conductivity and four times better air stability than FeCl$_3$-doped P3HT. The Second, TE nanocomposite system is systemically investigated by incorporating an elastomer with CNTs to improve TE performance and mechanical durability at the same times. Although conventional organic TE materials possess mechanical flexibility, almost organic TE generators have been typically designed in planar layout because organic TE material is difficult to pile up to a thickness of over several hundred micrometers for vertical layout, which adversely yields a low current output and inefficient use of in-plane thermal gradient. Hence, the simultaneous improvement of TE performance and the mechanical flexibility of the TE material itself are crucial for a practical application of TE generator to wearable devices. In the present study, we prepare elastic and durable (CNT)/polydimethylsiloxane (PDMS) foam materials with high TE performance using a rapid solvent evaporation method. Furthermore, a highly flexible and durable CNT/PDMS foam power generator with a vertical structure is successfully fabricated and its power generation is demonstrated. The CNT/PDMS foam generator exhibits good mechanical stability and durability, and it provides stable relative resistance values after cyclic bending more than 6000 times and under harsh vibrational stress. The incorporation of PDMS into CNT foam effectively decreases the thermal conductivity of the CNT/PDMS foam without the electrical loss because of the creation of conducting percolation structures in the CNT/PDMS foam. As a result, a CNT/PDMS foam exhibits a zT value twice as high as pristine CNT foam, and it has the extremely low thermal conductivity of 0.13 W m$^{−1}$ K$^{−1}$. In addition, the PDMS plays a significant role by providing a self-supporting structure that does not require an additional scaffold matrix, and this greatly improves mechanical properties such as the stiffness and elasticity of the CNT/PDMS foam. These approaches by doping and hybridization of CNT and elastomer suggest a new strategy for improving TE, chemical and mechanical properties of organic TE materials.