Rubbing and drawing: generic ways to improve the thermoelectric power factor of organic semiconductors?
Dorothea Scheunemann a, Martijn Kemerink a
a Linköping University, Sweden, SE-581 83, Linköping, Sweden
nanoGe Fall Meeting
Proceedings of nanoGe Fall Meeting19 (NGFM19)
#CharDy19. Charge Carrier Dynamics
Berlin, Germany, 2019 November 3rd - 8th
Organizers: Marcus Scheele and Maksym Yarema
Poster, Dorothea Scheunemann, 395
Publication date: 18th July 2019

Semiconducting organic materials have attracted increasing interest as thermoelectric (TE) converters in the recent years due to their potentially low fabrication costs and non-toxicity. As in other organic electronic devices, mastering morphology and crystallinity of polymer semiconductors is an essential tool to control the performance of organic TEs. One specific case of morphology, which has a strong impact on the thermoelectric properties of organic TEs, is structural anisotropy. Experimentally anisotropy can be induced by orienting of the polymer backbones using high temperature rubbing or tensile drawing. Such highly oriented conducting polymer films can show considerable increases in the electrical conductivity σ along the direction of orientation, while the influence on the Seebeck coefficient S is less clear.

Here, we use kinetic Monte Carlo (MC) simulations to rationalize the impact of anisotropy on the thermoelectric properties of disordered organic semiconductors. We find that variable-range hopping (VRH) can consistently describe charge and energy transport in both the parallel and the perpendicular direction, which contrasts with the interpretation drawn from experiments on ordered PBTTT [1]. Furthermore, we clarify why some materials show a simultaneous enhancement in conductivity and thermopower in the direction of orientation, while for others the Seebeck coefficient remains largely unaffected. By analyzing the effect of the parameters affecting the length scales and the structural order of the system we demonstrate under which conditions structural anisotropy is a suitable strategy to enhance the power factor of bulk materials beyond values predicted by the empirical power law S∝σ-1/4.

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