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

Abstract

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. Our work provides a platform to predict and study the influence of polymer chemistry on the ionic conductivity of conjugated polymers.

Balancing Ionic and Electronic Conduction for High‐Performance Organic Electrochemical Transistors

“…Recently, conjugated polymers comprised of conjugated backbones functionalized with hydrophilic side chains have emerged as promising OECT materials . Unlike PEDOT:PSS, these materials exhibit synthetic flexibility, which furnishes them with specific properties that promise broader applications of OECTs, e.g., functionalization of the material for selective sensing of biomarkers or promoting interactions with target cells .…”

Mini-Review: Mixed Ionic–Electronic Charge Carrier Localization and Transport in Hybrid Organic–Inorganic Nanomaterials

“…The voluminous dopant anions are generally more fixed to the polymer chain, allowing the electronic exchange process (doping) to take place but contributing in a lesser extent to the ionic conductivity except for a few particular cases (Cheng et al, 2005 ). Pursuing an increase in the ionic conduction of MIECs, blending and co-polymerization (including functionalization of side chains) of electronic conducting polymers with good ionic conducting polymers [e.g., poly(ethylene oxide) (PEO)], has shown enhancement of ionic conductivities up to σ i ~ 10 −5 -10 −4 S cm −1 (Li and Khan, 1991 ; Barthet et al, 1997 ; Ghosh and Inganäs, 2000 ; Zhang et al, 2002 ; Patel et al, 2012 ; Ju et al, 2014 ; Kang et al, 2014 ; Dong et al, 2019 ; Sengwa and Dhatarwal, 2020 ). Another strategy includes the simultaneous doping and blending of electronic conducting polymers with polymeric dopants, particularly observed for protons and lithium-ion charge carriers (Murthy and Manthiram, 2011 ; Fu and Manthiram, 2012 ; Liu et al, 2012 ).…”

A Reversible Structural Phase Transition by Electrochemically-Driven Ion Injection into a Conjugated Polymer

“…14,15 In contrast, the lamellar spacing of P3MEEMT changes minimally by 2% from 18.8 to 19.2Å from 0 V to +0.7 V, consistent with previous studies and GIWAXS measurements ( Figure S5). 27,39 In contrast, the lamellar stacking of PB2T-TEG expands discontinuously upon applying a higher potential, expanding by 27% from 14.3 to 18.1Å from 0 to +0.7 V. Strikingly, almost all of this expansion occurs over a small window between +0.2 and +0.3 V. This discontinuity in the structural data suggests a fundamentally different mechanism for lattice expansion upon ion insertion for PB2T-TEG, one in which the polymer switches between two structurally-distinct states. Importantly, we note that this discontinuous lamellar expansion is reversible, as the lamellar spacing switches between these two states upon repeated oxidation and reduction ( Figure S6).…”