Influence of Side-Chain Chemistry on Structure and Ionic Conduction Characteristics of Polythiophene Derivatives: A Computational and Experimental Study

Although extensive efforts have been devoted to understanding electronic transport in conjugated polymers, little is known about their ionic conduction characteristics in relation to polymer chemistry, processing, and morphology. This work presents a combined computational and experimental study on morphology and ion transport in thin-film blends of polythiophene derivatives bearing oligoethylene glycol side-chains and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Using molecular dynamics (MD) simulation, we show that in the amorphous phase, the polythiophene derivative P3MEET bearing oligoethylene glycol side-chains with oxygen directly attached to the thiophene rings possesses lower Li⁺ ionic conductivity compared to its analog P3MEEMT that has a methyl spacer between the oxygen and the thiophene rings. Structural characterization of P3MEET and P3MEEMT thin film upon blending with LiTFSI indicates that adding LiTFSI expands the side-chain domains of the polymer crystallites and reduces the total degree of crystallinity at the same time. Moreover, LiTFSI is found to infiltrate both the amorphous and crystalline regimes at low concentrations but preferably resides in the amorphous domain at high LiTFSI concentrations. Ionic transport measured by electrochemical impedance spectroscopy in both P3MEET- and P3MEEMT-LiTFSI thin films is found to occur predominately in the amorphous domain, and ionic conductivity in P3MEEMT-LiTFSI is always higher than in P3MEET-LiTFSI samples, consistent with predictions from MD simulations. As a result, our work provides a platform to predict and study the influence of polymer chemistry on the ionic conductivity of conjugated polymers.