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

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-M50k (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-M10k (MW≈10 000 g mol-1 ) and almost 30 % higher than the electrical conductivity of P3HT-M100k (MW≈100 000 g mol-1 ). Consequently, the maximum TE power factor of P3HT-M50k at room temperature was as high as (22.6±0.6) μW mK-2 , which is much higher than that of either P3HT-M10k or P3HT-M100k . 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.