With the ability to modulate electronic properties through molecular doping coupled with ease in processability, semiconducting polymers are promising materials for organic thermoelectrics. In contrast to uniform thermoelectric material properties, an alternative route focuses on functionally graded materials (FGMs) where one spatially controls and optimizes transport properties across the length of a thermoelectric material.&lt;span style="font-size:10.8333px"&gt; &lt;/span&gt;While primarily studied in the context of inorganic materials, the concept of FGMs for organic thermoelectrics has not been explored. In this talk, we introduce how molecular doping of semiconducting polymers enables spatial compositional control for thin film FGMs. Specifically, we utilize sequential vapor doping of poly[2,5-bis(3-tetradecylthiophen-2-yl) thieno [3,2-b]thiophene] (PBTTT) with the small molecule acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) to fabricate various forms of FGMs including double segmented films and continuously graded films. Through a combination of grazing incidence wide angle X-ray scattering (GIWAXS) and Raman spectroscopy, we are able to spatially resolve dopant compositional profiles in FGM thin films. In addition, the lateral distribution of the electronic conductivity and the Seebeck coefficient of the FGM thin films are measured in order to determine spatially averaged transport parameters. Lastly, the experimentally resolved electronic conductivity and Seebeck coefficient profiles are correlated to heat and charge transport models, which reveal theoretical limits of improved thermoelectric cooling performance. Overall, our work showcases how sequential doping is an enabling technique for applying the principles of functional grading and how it further advances the understanding of structure-transport properties and the development of more efficient organic thermoelectric materials.

Semiconducting polymers to enable functionally graded materials for organic thermoelectrics

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ACS Fall 2022

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