Rubbing and Drawing: Generic Ways to Improve the Thermoelectric Power Factor of Organic Semiconductors?

Highly oriented polymer films can show considerable anisotropy in the thermoelectric properties leading to power factors beyond those predicted by the widely obeyed power law linking the thermopowerSand the electrical conductivity sigma asS proportional to sigma(-1/4). This has led to encouraging practical results with respect to the electrical conductivity, notwithstanding that the conditions necessary to enhance sigma andSsimultaneously are less clear. Here, kinetic Monte Carlo simulations are used to study the impact of structural anisotropy on the thermoelectric properties of disordered organic semiconductors. It is found that stretching is a suitable strategy to improve the conductivity along the direction of strain, whereas the effect on the power factor depends on the morphology the polymer crystallizes. In general, crystalline polymers show a simultaneous increase in sigma andSwhich is not the case for amorphous polymers. Moreover, it is shown that the trends resulting from simulations based on variable-range hopping are in good agreement with experiments and can describe the different functional dependencies in theSversus sigma behavior of different directions.
Funding Agencies|European Unions Horizon 2020 research and innovation program under the Marie SklodowskaCurie grantEuropean Union (EU) [799477]; ANR AnisothermFrench National Research Agency (ANR) [ANR-17-CE05-0012]; CNRS grant PEPS Thermobody