Abstract
Organic materials have been found to be promising candidates for low-temperature thermoelectric applications. In particular, poly (3-hexylthiophene) (P3HT) has been attracting great interest due to its desirable intrinsic properties, such as excellent solution processability, chemical and thermal stability, and high field-effect mobility. However, its poor electrical conductivity has limited its application as a thermoelectric material. It is therefore important to improve the electrical conductivity of P3HT layers. In this work, we studied how molecular weight (MW) influences the thermoelectric properties of P3HT films. The films were doped with lithium bis(trifluoromethane sulfonyl) imide salt (LiTFSI) and 4-tert butylpyridine (TBP). Various P3HT layers with different MWs ranging from 21 to 94 kDa were investigated. UV–Vis spectroscopy and atomic force microscopy (AFM) analysis were performed to investigate the morphology and structure features of thin films with different MWs. The electrical conductivity initially increased when the MW increased and then decreased at the highest MW, whereas the Seebeck coefficient had a trend of reducing as the MW grew. The maximum thermoelectric power factor (1.87 μW/mK2) was obtained for MW of 77 kDa at 333 K. At this temperature, the electrical conductivity and Seebeck coefficient of this MW were 65.5 S/m and 169 μV/K, respectively.
Highlights
Conducting polymers have become promising materials for a wide range of applications, including thermoelectric generators, actuators, and supercapacitors
We found out that by increasing the molecular weight (MW), the electrical conductivity initially increases, there is an inflection point, after which it decreases
Our results highlight the importance of MW tuning in high-performance organic thermoelectric materials
Summary
Conducting polymers have become promising materials for a wide range of applications, including thermoelectric generators, actuators, and supercapacitors. Crispin et al [14] reported that polymers can be semimetallic, without having an energy bandgap, and a very low density of states at the Fermi level They observed an enhancement of the Seebeck coefficient and electrical conductivity through molecular organization. When considering the intrinsic thermoelectric properties of P3HT, it is commonly found in the literature that this material has a high Seebeck coefficient compared to other conducting polymers, such as PEDOT:PSS [37,38]. It has low electrical conductivity for thermoelectric applications, so it is important to increase electrical conductivity in P3HT layers. Our results highlight the importance of MW tuning in high-performance organic thermoelectric materials
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