Abstract
The fundamental understanding of the influence of molecular structure on the carrier transport properties in the field of organic thermoelectrics (OTEs) is a big challenge since the carrier transport behavior in conducting polymers reveals average properties contributed from all carrier transport channels, including those through intra-chain, inter-chain, inter-grain, and hopping between disordered localized sites. Here, combining molecular dynamics simulations and experiments, we investigated the carrier transport properties of doped highly oriented poly(3-hexylthiophene) (P3HT) films with different side-chain regioregularity. It is demonstrated that the substitution of side chains can not only take effect on the carrier transport edge, but also on the dimensionality of the transport paths and as a result, on the carrier mobility. Conductive atomic force microscopy (C-AFM) study as well as temperature-dependent measurements of the electrical conductivity clearly showed ordered local current paths in the regular side chain P3HT films, while random paths prevailed in the irregular sample. Regular side chain substitution can be activated more easily and favors one-dimensional transport along the backbone chain direction, while the irregular sample presents the three-dimensional electron hopping behavior. As a consequence, the regular side chain P3HT samples demonstrated high carrier mobility of 2.9 ± 0.3 cm2/V·s, which is more than one order of magnitude higher than that in irregular side chain P3HT films, resulting in a maximum thermoelectric (TE) power factor of 39.1 ± 2.5 μW/mK2 at room temperature. These findings would formulate design rules for organic semiconductors based on these complex systems, and especially assist in the design of high performance OTE polymers.
Highlights
Organic materials have attracted increased attention in thermoelectric (TE) research due to their properties of light weight, low cost, and flexibility
Both the regioregular P3HT (rg-P3HT) and random poly(3-hexylthiophene) poly(3-hexylthiophene) (ra-P3HT) materials were purchased from Sigma-Aldrich (Shanghai, China) with molecular weight (Mw) of 87 kg/mol and used as received
For rg-P3HT, only one sharp band corresponding to the thiophene proton at δ = 6.98 is observed in the 1 H-NMR spectrum, which denotes the HT-HT structure
Summary
Organic materials have attracted increased attention in thermoelectric (TE) research due to their properties of light weight, low cost, and flexibility. We obtained highly oriented P3HT films through the combination of organic small-molecule epitaxy method using 1,3,5-trichlorobenzene (TCB) as the template and a temperature gradient assisting the crystallization process [20] This allowed us to study the macroscopic carrier transport properties of quasi-one-dimensional P3HT films along or across the backbone chain orientation by suppressing or diminishing the effects of structural disorder in the polymer chains. A combination of molecular dynamics simulations and experimental investigations was performed to understand the side chain effect on the carrier transport properties in the doped highly oriented P3HT films Two such films were prepared through the small molecule epitaxy method using two typical P3HT polymers with different side chain regularity These findings may assist in the design of high performance TE polymers
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