Morphology controls the thermoelectric power factor of a doped semiconducting polymer.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 28630931.
- Also identified by DOI 10.1126/sciadv.1700434 and PMC identifier 5473677.
- Licence recorded as CC BY-NC.
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Abstract
The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with F <sub><i>n</i></sub> TCNQ (<i>n</i> = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (σ) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (μ). The Seebeck coefficient (α) is largely independent of OCL. This reveals that, unlike σ, leveraging strategies to improve μ have a smaller impact on α. Our best-performing sample with the largest OCL, vapor-doped PBTTT:F<sub>4</sub>TCNQ thin film, has a σ of 670 S/cm and an α of 42 μV/K, which translates to a large PF of 120 μW m<sup>-1</sup> K<sup>-2</sup>. In addition, despite the unfavorable offset for charge transfer, doping by F<sub>2</sub>TCNQ also leads to a large PF of 70 μW m<sup>-1</sup> K<sup>-2</sup>, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics.