Optimizing the Thermoelectric Performance of Poly(3-hexylthiophene) through Molecular-Weight Engineering

Sanyin Qu; Qin Yao; Bingxue Yu; Kaiyang Zeng; Wei Shi; Yanling Chen; Lidong Chen

doi:10.1002/asia.201801080

Optimizing the Thermoelectric Performance of Poly(3-hexylthiophene) through Molecular-Weight Engineering

Chem Asian J. 2018 Nov 2;13(21):3246-3253. doi: 10.1002/asia.201801080. Epub 2018 Oct 2.

Authors

Sanyin Qu¹, Qin Yao¹, Bingxue Yu², Kaiyang Zeng², Wei Shi^{1

3}, Yanling Chen^{1

3}, Lidong Chen^{1

4}

Affiliations

¹ State Key Laboratory of High Performance Ceramics and Superfine Microstructure and CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China.
² Department of Mechanical Engineering, National University of Singapore, Singapore, 117576, Singapore.
³ University of Chinese Academy of Sciences, Beijing, 100049, China.
⁴ Shanghai Institute of Materials Genome, Shanghai, 200050, China.

PMID: 30105791
DOI: 10.1002/asia.201801080

Abstract

Poly(3-hexylthiophene) (P3HT) films with various molecular weights (MWs) were successfully prepared, and both their molecular structures and thermoelectric (TE) properties were investigated. It was found that the molecular weight of P3HT had an important effect on the carrier-transport properties by affecting the molecular structure and, as a result, also had an effect on the TE performance. The electrical conductivity of the P3HT films first increased upon increasing the molecular weight and then decreased at high molecular weights, whereas the Seebeck coefficient remained at the same level. As a result, the P3HT-M_50k (MW≈50 000 g mol^-1 ) film reached an electrical conductivity of (103.8±1.2) S cm, which is more than 20 times higher than the electrical conductivity of P3HT-M_10k (MW≈10 000 g mol^-1 ) and almost 30 % higher than the electrical conductivity of P3HT-M_100k (MW≈100 000 g mol^-1 ). Consequently, the maximum TE power factor of P3HT-M_50k at room temperature was as high as (22.6±0.6) μW mK^-2 , which is much higher than that of either P3HT-M_10k or P3HT-M_100k . Microstructure analysis combined with C-AFM suggested that carrier transport between most of the ordered and amorphous regions was unconnected in films with low molecular weights, and this resulted in a high migration barrier and poor carrier mobility. Upon increasing the molecular weight, the long molecular chain provided enough connectivity for the charge to move through the ordered regions, which decreased the carrier barrier and increased carrier mobility. Therefore, both the conductivity and Seebeck coefficient were significantly improved. However, a too-high molecular weight could cause more folding of the polymer chain, which would deteriorate the electrical-transport properties. The experimental results not only reveal the intrinsic effect of molecular weight on the electric transporting properties of conducting polymers but also suggest that molecular-weight engineering is an effective way to design and screen high-performance polymer TE materials.

Keywords: electrical transport; molecular weight; organic thermoelectrics; polymers; thin films.

Abstract

Grants and funding