Publication date: 15th December 2025
In this work, we investigate the polymorphism of a new PBTTT-based polymer bearing oligo(ethylene-glycol) side chains separated from the conjugated backbone by a methylene spacer. We identified in rub-aligned films two distinct structures of the polymer, that coexist at room temperature: a mesophase and a crystalline polymorph. The mesophase has a well-planarized backbone and disordered side chains. In the crystalline phase, the side chains conformations are mostly antiperiplanar but at the expense of backbone planarity. We describe how to obtain aligned films of the pure LC and Cryst. phases and identify the structural signatures of the two polymorphs using Transmission Electron Microscopy (TEM) and Fourier Transform Infrared Spectroscopy (FTIR). These two phases were sequentially doped with the molecular acceptor F4TCNQ. The changes induced by doping, both structural and in the resulting transport/thermoelectric (TE) properties, are presented for the two polymorphs. Notably, despite being chemically identical, these two forms display markedly different thermoelectric properties and behaviours upon doping, emphasizing the importance of a strict control of the structure of organic PSCs to obtain reproducible TE properties.
We thank Nicolas Boyron for performing the SEC measurements of the polymer. M. B. thanks the French national research agency (ANR) for funding through contract Thermopolys (ANR-22-CE50-0020) and TRIPODE (ANR-23-CE06-032). Support and technical assistance from the TEM platform at ICS (M. Schmutz), for AFM measurements (C. Contal) and for high temperature rubbing (L. Herrmann) is gratefully acknowledged.
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