Unique polymorph of P3HT by Brønsted acid doping for highly conductive and stable organic thermoelectric via solution mixing
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Molecular doping is crucial for improving thermoelectric (TE) properties of conjugated polymers. Although solution mixing (i.e. mixing of the polymer and the dopant in a single solvent) provides a facile route to the uniform doping of farily thick polymer films suitable for TE applications, low solubility of dopants restricts high quality film fabrication. Here, we report on the effects of Brønsted acid doping of poly(3-hexylthiophene) (P3HT) with a largely soluble molecular dopant. Interestingly, P3HT formed a unique type II polymorph upon doping in ambient condition, while a conventional crystal structure is observed in inert conditions. Moreover, the doped P3HT films showed a conformational change to quinoid structure, increasing the backbone planarity. As a result, the Brønsted acid doped P3HT films exhibited outstanding electrical and TE properties, which are at least an order of magnitudes higher than those of P3HT films doped with F4TCNQ, with excellent air stability.