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

Abstract

We find that conjugated polymers can undergo reversible structural phase transitions during electrochemical oxidation and ion injection.We study poly[2,5-bis(thiophenyl)-1,4-bis(2-(2-(2methoxyethoxy)ethoxy)ethoxy)benzene] (PB2T-TEG), a conjugated polymer with glycolated side chains. Using grazing incidence wide angle X-ray scattering (GIWAXS), we show that, in contrast to previously known polymers, this polymer switches between two structurally distinct crystalline phases associated with electrochemical oxidation/reduction in an aqueous electrolyte. Importantly, we show that this unique phase change behavior has important physical consequences for ion transport. Notably, using moving front experiments visualized by both optical microscopy and super-resolution photoinduced force microscopy (PiFM), we show that a propagating ion front in PB2T-TEG exhibits non-Fickian transport, retaining a sharp step-edge profile, in stark contrast to the Fickian diffusion more commonly observed. This structural phase transition is reminiscent of those accompanying ion uptake in inorganic materials like LiFePO 4 . We propose that engineering similar properties in future conjugated polymers may enable the realization of new materials with superior performance in electrochemical energy storage or neuromorphic memory applications.

Time‐Resolved Structural Kinetics of an Organic Mixed Ionic–Electronic Conductor

“…Across applications, many OMIEC‐based devices under operating conditions undergo changes in structure and system composition due to the uptake and expulsion of ions and solvent, and the modulation of electronic charge on the conjugated OMIEC backbone. [ 6–8 ] This greatly complicates the establishment of predictive structure–property relationships with which to guide the design of next‐generation OMIECs, as static (often dry) structure is no longer sufficient for drawing these relationships. This has motivated the investigation of OMIECs and related materials with ex situ, in situ, and operando methods.…”

“…Whereas, for polyalkylthiophenes, further doping is likely to drive dopant anions into the sidechain lamellae, thus further expanding the lamellae spacing with increasing degree of doping, [ 8,11,12 ] here the dopant anion is tethered to the PSS chains already present between PEDOT layers, thus increasing the degree of doping occurs by depleting the PSS chains of some of their charge balancing cations. Specifically, the further doping of PEDOT:PSS requires the loss of material (cations and their solvating water molecules) from between the PEDOT layers, resulting in a decrease in the lamellar d ‐spacing.…”

A Low‐Swelling Polymeric Mixed Conductor Operating in Aqueous Electrolytes

“…[ 13 ] In turn, the torsional backbone order and, hence, optoelectronic features significantly vary with temperature. [ 13 ] Structural [ 14 ] and mechanical properties also will be affected, rendering systematic establishment of structure/property relations intricate. In this communication, we demonstrate that in poly[3‐(6‐hydroxy)hexylthiophene] (P3HHT, see inset in Figure a for chemical structure), hydroxylated alkyl side chains introduce mixed conduction when operated with aqueous electrolytes, as work on random copolymers of poly(3‐hexyl thiophene) (P3HT) and P3HHT already indicated, [ 15 ] but without excessive swelling and/or unwanted plasticizing effects observed in materials with oligoethylene‐glycol side chains.…”

“…Important for its usage in devices such as OECTs, the ion uptake during application of a positive bias results in chemical doping of P3HHT. We used electrochemical absorption spectroscopy, [ 25,26 ] which is a commonly used method in the organic mixed conductor field [ 9,14–17 ] for the purpose of tracking ion uptake/doping (details on this methodology are given in the Supporting Information). Data obtained using a 0.1 mol L −1 KCl solution as electrolyte and a similar measurement geometry as employed for the e‐QMC‐D experiments, reveals bleaching of the ground‐state absorption centered around 550 nm upon application of a 0.75 V bias while, simultaneously, an increase of the polaron band (centered around 800 nm), characteristic for doped thiophene‐based polymers, is observed (see Figure a).…”

Charge-Transfer Intermediates in the Electrochemical Doping Mechanism of Conjugated Polymers

“…In some cases, heterogeneity occurs over extremely small length scales comprising only a few chromophore segments, which greatly limits the utility of conventional characterization techniques that rely on longer-range order and/or periodicity (e.g. X-ray and scanning probe methods [26][27][28][29][30] ) for understanding the doping process. It is therefore necessary to develop new approaches for capturing the dynamic picture underlying electrochemical doping to provide a molecular description of charged states in weakly-coupled, structurally irregular, and heterogeneous polymer-electrolyte systems.…”