Publication date: 15th December 2025
Polymorphism is a common feature of polymer semiconductors such as polythiophenes, and it plays a crucial role in determining their optical and electronic properties. Controlling polymorphism is thus an elegant means to probe structure-property correlations in doped polymer semiconductors. In this contribution, we focus on PBTTT bearing single-ether side chains, namely PBTTT-8O.[1-3]This polymer forms two distinct polymorphs under controlled growth conditions: form 1 and form 2 are prepared by thermal annealing and solvent treatment in n-hexane of rub-aligned thin films, respectively. Polarized UV-vis-NIR spectroscopy demonstrates that backbone planarization is reduced in form 2 whereas regular π-stacking and extended planarization is present in form 1. Upon doping with the strong acceptor F6TCNNQ, the aligned polymorphs show substantially different charge transport and thermoelectric properties. The best thermoelectric properties are observed for the doped form 1 with a very high power factor of 3.6±0.6 ×10³ µW.m-1.K-2 versus 2.4±0.5 ×10² µW.m-1.K-2 for the doped films of form 2. Notably, blends of the two polymorphs show even better TE performances than form 1.
This work was financially supported by the European Commission through the Marie Sklodowska-Curie project HORATES (GA-955837). S.O. thanks the French national research agency (ANR) for funding through contract Thermopolys (ANR-22-CE50-0020) as well as University of Strasbourg.
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