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

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 (MW≈50 000 g mol ) 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 (MW≈10 000 g mol ) and almost 30 % higher than the electrical conductivity of P3HT-M (MW≈100 000 g mol ). Consequently, the maximum TE power factor of P3HT-M at room temperature was as high as (22.6±0.6) μW mK , which is much higher than that of either P3HT-M or P3HT-M . 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.

The Molecular Weight Dependence of Thermoelectric Properties of Poly (3-Hexylthiophene)

“…However, the surface topography at higher MWs seems to become slightly smoother, which could be attributed to the increase of polymer chain lengths. It is also consistent with previously published reports describing the effect of MW on the electrical properties of P3HT films [42][43][44]. Variations in the topography among samples could be attributed to the difference in the evaporation rate of the solvent and the solidification of the polymer in different solutions.…”

“…As mentioned before, the electrical properties of P3HT films with various MWs have been investigated in the literature. However, to the best of our knowledge, there is only one report which compares the thermoelectric properties of P3HT thick films with three different MWs [42]. Regardless of that report, the correlation between chain lengths, morphology, and thermoelectric parameters has not yet been completely studied.…”

Polar Side Chain Effects on the Thermoelectric Properties of Benzo[1,2‐b:4,5‐b′]Dithiophene‐Based Conjugated Polymers

“…Compared to inorganic thermoelectric materials, these organic thermoelectric materials exhibit some unique advantages, such as a low thermal conductivity, highly adjustable molecular structures, the potential for low cost, and facile processing into versatile forms . Until recently, several conducting polymers, including polythiophene, polypyrrole, polyaniline, polyfluorene, and their derivatives, have been widely studied as thermoelectric materials, and some conjugated polymers with new structures have also been explored . For instance, Yang et al designed and synthesized two donor–acceptor (D–A)‐type conjugated polymers and effectively enhanced the thermoelectric properties of D–A conjugated polymers through donor engineering; Liu et al tailored the density of states of D–A conjugated polymers, and as a result, improved the power factor dramatically .…”

Enhanced Thermoelectric Performance of Indacenodithiophene-Benzothiadiazole Copolymer Containing Polar Side Chains and Single Wall Carbon Nanotubes Composites

“…[8][9][10][11][12] due to their low toxicity, low thermal conductivity and mechanical flexibility. Recent results have shown that CPs are among the most promising materials for application in flexible thermoelectric generators [13][14][15][16][17]. For example, poly(3,4-ethylenedioxy-thiophene) (PEDOT) with a high thermoelectric figure of merit (ZT) of 0.45 has been reported [18], which stimulated the investigation of CPs to be used for thermoelectric generators.…”

“…Organic materials generally possess large Seebeck coefficients (S) and low thermal conductivity [13][14][15][16][17][18][19], therefore, the main challenge of achieving high-performance OTEs is how to improve the electrical conductivity of CPs without sacrificing their large Seebeck coefficients and low thermal conductivity [20,21]. Many strategies have been attempted to improve the conductivities of CPs (such as structural manipulations [22], strengthening the ordering of polymer packing [23], efficient doping [24], and forming composites with highly conductive inorganic materials [25], etc.…”