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

Article abstract of DOI:10.1021/jacs.9b12769

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-(2-methoxyethoxy)ethoxy)ethoxy)benzene] (PB2T-TEG), a conjugated polyme

Full text of DOI:10.1021/jacs.9b12769

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Article
A Reversible Structural Phase Transition by Electrochemically-
Driven Ion Injection into a Conjugated Polymer
Connor G Bischak, Lucas Q. Flagg, Kangrong Yan, Tahir Rehman, Daniel W. Davies, Ramsess J.
Quezada, Jonathan W Onorato, Christine K. Luscombe, Ying Diao, Chang-Zhi Li, and David S Ginger
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b12769 • Publication Date (Web): 30 Mar 2020
Downloaded from pubs.acs.org on April 4, 2020
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A Reversible Structural Phase Transition by
Electrochemically-Driven Ion Injection into a
Conjugated Polymer
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Connor G. Bischak,1 Lucas Q. Flagg,1 Kangrong Yan,2 Tahir Rehman,2 Daniel W. Davies,3 Ramsess J. Quezada,1
Jonathan W. Onorato,4 Christine K. Luscombe,4,5 Ying Diao,3 Chang-Zhi Li,2 David S. Ginger1*
1Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States
2MOE Key Laboratory of Macromolecular Synthesis and Functionalization, State Key Laboratory of Silicon
Materials, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, P.R. China
3Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana−Champaign, 600 South
Mathews Avenue, Urbana, Illinois 61801, United States
4Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, United
States
5Department of Molecular Engineering and Sciences, University of Washington, Seattle, Washington 98195, United
States
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-(2-
methoxyethoxy)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-polaron pair transport. Notably, using moving
front experiments visualized by both optical microscopy and super-resolution photoinduced force
microscopy (PiFM), we show that a laterally-propagating ion-polaron pair front in PB2T-TEG exhibits non-
Fickian transport, retaining a sharp step-edge profile, in stark contrast to the Fickian diffusion more
commonly observed in polymers like P3MEEMT. This structural phase transition is reminiscent of those
accompanying ion uptake in inorganic materials like LiFePO4. 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.
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INTRODUCTION
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The movement of ions within materials, and the transport of ions across material interfaces are
fundamental processes that underlie many technologies, including bioelectronics,1,2 mechanical actuators,3,4
light emission,5,6 neuromorphic computing,7,8 and including energy storage.9,10 Electrochemically-driven
ion insertion is often accompanied by a reorganization of the host material, with the host expanding to
accommodate the volume of the additional ions. In inorganic materials, such as lithium ion battery
electrodes, ion insertion often leads to a structural phase transition of the host material, in which the
presence of ions induces a new thermodynamically-favorable structural phase.11,12 A consequence of the
structural phase transition is phase separation between ion-rich and ion-poor regions of the host material at
particular ion concentrations. In lithium ion battery electrode materials, this phase separation results in a
flat voltage profile upon battery discharging.11 On the other hand, soft materials, such as conjugated
polymers or polymer electrolytes, which are often semi- or paracrystalline, generally deform without
undergoing a structural phase transition upon ion insertion, and phase separation between the ion-rich and
ion-poor regions is not observed.13–17
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Conjugated polymers are an important class of organic electronic materials that are capable of
transporting both electronic and ionic charge carriers.18 Compared to inorganic materials, such as silicon,
conjugated polymer have lower electronic mobilities, yet are capable of fast, reversible, volumetric charge
injection.18,19 Recently, the ability of these mixed conductors to uptake and transport both ionic and
electronic charges has led to their study in applications including bioelectronics,20–22 thermoelectrics,23
neuromorphic computing,7 and energy storage.24–26 Many of these technologies rely on ion injection through
the application of an electrochemical bias, which oxidizes (or reduces) the polymer, driving anions (or
cations) into the conjugated polymer where they provide charge compensation for the injected electronic
carriers, allowing the realization of capacitances as high as ~1000 F/g.25 Although this process provides the
foundation for many technologies, the mechanism of ion injection and transport in mixed conductors is still
an active area of research.1,2
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The incorporation of molecular dopants and ions in conjugated polymers alters the electronic
conductivity of the conjugated polymer host and impacts its crystal structure.27,28 The structural changes
that conjugated polymers undergo upon electrochemically-driven ion insertion have been investigated in a
number of cases.16,17,29 For instance, in poly(3-hexylthiophene-2,5-diyl) (P3HT), the lamellar spacing of the
polymer increases continuously and the π-π stacking distance decreases as the concentration of injected
ions increases.15–17 Although structural changes in P3HT have been widely studied, P3HT is a relatively
poor conjugated polymer for transporting most ions due to its hydrophobic alkyl sidechains.30 Recently,
new conjugated polymers and small molecule semiconductors designed for enhanced ion transport with
ethylene glycol or charged side chains have been synthesized.29,31–37
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Here, we investigate structural changes in poly[2,5-bis(thiophenyl)-1,4-bis(2-(2-(2-
methoxyethoxy)ethoxy)ethoxy)benzene] (PB2T-TEG), a conjugated polymer with glycolated sidechains,
upon electrochemical oxidation using both ex situ and in operando X-ray scattering. We find that, unlike
other polymers studied to date, PB2T-TEG undergoes a reversible structural phase transition upon oxidation
in an aqueous electrolyte. Using moving front experiments monitored by both optical transmission and
nanoscale infrared imaging with photoinduced force microscopy (PiFM),29,38 we observe a sharp ion-
polaron pair front, inconsistent with Fickian diffusion. We propose that structural phase transitions in
conjugated polymers may be beneficial for many applications ranging from energy storage and to post-von
Neumann computing.
RESULTS AND DISCUSSION
Figure 1A shows the chemical structure of PB2T-TEG, a conjugated polymer consisting of a
repeating backbone unit of two thiophenes and one benzene with two glycolated sidechains to facilitate ion
uptake and transport. The synthesis and characterization of PB2T-TEG is described in the Supplementary
Information, which also includes cyclic voltammetry, absorption spectra, and thermogravimetric analysis
of PB2T-TEG (Figure S1 and Figure S2). Derivatives of PB2T-TEG have been used previously as hole
extraction materials in lead halide perovskite solar cells.39 Figure 1B shows cyclic voltammetry (CV)
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sweeps of PB2T-TEG in 100 mM KCl, indicating that the polymer is indeed electrochemically active in
polymer solution, with an oxidation onset of +0.2 V relative to Ag/AgCl (Figure 1B). The
spectroelectrochemistry measurements in Figure 1C show a nearly complete bleach by +0.4 V relative to
Ag/AgCl, indicating that PB2T-TEG can be rapidly oxidized throughout the entire volume of the polymer
with a time constant of 530 ± 20 ms (Figure S3). Upon increasing the bias in 0.1 V increments from 0 V
to +0.7 V, anions compensate for injected holes on the polymer backbone, the π-π* absorption peak
decreases by approximately 80% and a redshifted polaron appears (Figure 1D). Volumetric oxidation is
also supported by mass changes of the film, as measured by electrochemical quartz crystal microbalance
(EQCM) (Figure S4). Based on the electrochemical characterization of PB2T-TEG, we find that it behaves
similarly to other conjugated polymers with glycolated sidechains in its ability to effectively take up ions
from an aqueous solution at a relatively fast rate.29,31,32 To determine the electronic mobility (µ) and
volumetric capacitance (C*) of PB2T-TEG, we fabricated organic electrochemical transistors (OECTs) and
performed electrochemical impedance spectroscopy (EIS) measurements. Figure S5 shows output and
transfer curves of a PB2T-TEG OECT, as well as a µC* plot. We find a C* of 195 ± 40 F cm-3 (Figure S6)
and a µC* product of 25 ± 4 F cm-1 V-1 s-1, resulting in a µ of 0.13 ± 0.05 cm2 V-1 s-1 at a gate voltage (VG)
of 0.7 V. The OECT electronic mobility agrees reasonably with the dry electronic mobility measured with
a four-point probe (0.06 ± 0.02 cm2 V-1 s-1).
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Figure 1: Electrochemical and spectroelectrochemistry characterization of PB2T-TEG. (A) Chemical structure of PB2T-TEG.
(B) Cyclic voltammetry (CV) of PB2T-TEG in 100 mM KCl at a rate of 0.1 V/s. (C) Absorption spectra as a function of applied
voltage (vs. Ag/AgCl), showing a decrease in the π-π* absorption peak and an increase of the polaron absorption peak. (D)
Normalized absorption versus applied voltage (vs. Ag/AgCl) for the π-π* absorption peak (512 nm) and the polaron absorption
(720 nm).
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We next investigate structural changes in PB2T-TEG as a function of applied bias and compare the
observed structural changes to those found in two other conjugated polymers, P3HT and poly(3-{[2-(2-
methoxyethoxy)ethoxy]methyl}thiophene-2,5-diyl) (P3MEEMT), that exemplify more typically observed
behavior. Figure 2 compares voltage-dependent X-ray diffraction (XRD) data on a series of polymer films
taken at different points in the process of oxidation and ion insertion. We include P3HT and P3MEEMT
as examples of typical conjugated polymer behavior: P3HT has been widely studied as a material for
organic transistors,40,41 while P3MEEMT is a closely related derivative with the same backbone but
possessing ethylene-oxide side chains that exhibits facile redox chemistry and ion uptake in aqueous
environments.29,42 For these XRD measurements, the polymers are electrochemically oxidized (0 V to +0.7
V vs. Ag/AgCl) in 100 mM KCl (or 100 mM KPF6 for P3HT), rinsed with deionized water, and dried with
N2 prior to measurement. Figure 2A-C shows the (100) diffraction peak as a function of applied voltage
for P3HT, P3MEEMT, and PB2T-TEG with the corresponding peak positions shown in Figure 2D-E. The
expansion in the lamellar spacing of P3HT is continuous as a function of applied voltage, expanding by
11% from 16.6 to 18.5 Å from 0 V to +0.7 V, which is consistent with previous studies using ionic liquids
or polymeric ionic liquids as the electrolyte.16,17 In contrast, the lamellar spacing of P3MEEMT changes
minimally by 2% from 18.8 to 19.2 Å from 0 V to +0.7 V, consistent with previous studies and grazing
incidence wide angle X-ray scattering (GIWAXS) measurements (Figure S7).29,42 In contrast, the lamellar
stacking of PB2T-TEG expands discontinuously upon applying a higher potential, expanding by 27% from
14.3 to 18.1 Å from 0 to +0.7 V. Strikingly, almost all of this expansion occurs over a small window
between +0.2 and +0.3 V and the dopant concentration in the film increases by approximately an order of
magnitude from 0.56 ± 0.14 x 1019 cm-3 to 4.7 ± 1.2 x 1019 cm-3 (Figure S8). . This discontinuity in the
structural data suggests that the polymer undergoes a sharp structural phase transition associated with ion
injection. This sharp structural transition is also supported by EQCM data (Figure S4), which shows a
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much sharper increase in mass uptake around +0.2 V in comparison to the more gradual onset of ion uptake
with bias we have observed with other organic mixed conductors.29,43 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 S9).
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Figure 2: Structural changes upon electrochemically-driven ion insertion for P3HT, P3MEEMT, and PB2T-TEG measured
ex situ. (A) XRD spectra of the (100) lamellar spacing diffraction peak for (A) P3HT, (B) P3MEEMT, and (C) PB2T-TEG as a
function of applied voltage (vs. Ag/AgCl). Peak positions as a function of applied voltage (vs. Ag/AgCl) for (D) P3HT, (E)
P3MEEMT, and (F) PB2T-TEG. The pink dotted line in F shows the discontinuity in lattice spacing between the neutral and
oxidized states of PB2T-TEG.
We further investigate the neutral and oxidized structures of PB2T-TEG using ex situ GIWAXS,
focusing on the discontinuity in lattice spacing occurring between +0.2 and +0.3 V (vs. Ag/AgCl). Figure
3A shows the GIWAXS diffraction patterns of neutral PB2T-TEG, PB2T-TEG oxidized at +0.23 V, and
PB2T-TEG oxidized at +0.4 V relative to Ag/AgCl. For the neutral PB2T-TEG, we find that the lamella
stacking (100 and 200 peaks) corresponds to a distance equal to one fully extended TEG chain distance (d
= ~14.1 Å), suggesting side-chain interdigitation.44 The 010 rod corresponds well to the expected distance
between the TEG chains along the backbone, or equivalently the length of the repeat unit (d = ~11.3 Å).
We propose that the rod at 1.41 Å-1 originates from interchain side chain stacking, as this feature
corresponds well to typical alkyl stacking distances.45 While the side-chain stacking peaks are present in
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the XRD data, a π-π stacking peak is absent. Further, we speculate that the two peaks at Qxy = 0.38 Å-1
(unlabeled in Figure 3A) are the result of twisting in the polymer.46–48
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Once PB2T-TEG is oxidized at +0.4 V, the out-of-plane (100) spacing significantly increases. At
the same time, the glycolated chains lose their crystallinity seen by the disappearance of the h10, h20 and
h01 Bragg rods and become less interdigitated as the ions disrupt the sidechain interactions. At the same
time, the backbones now show evidence of π-π stacking, with a 010 peak at Qxy = 1.78 Å-1, suggesting that
the backbone untwists (either due to planarization of the quinoidal structure, or due to loss of order from
the side-chains). GIWAXS spectra acquired at higher bias (+0.6 and +0.8 V) show a similar diffraction
pattern as that acquired at +0.4 V (Figure S10). For PB2T-TEG oxidized at +0.23 V, we observe a linear
superposition of the GIWAXS patterns acquired at 0 V and +0.4 V. This superposition is also clear from
line-cuts taken from the in-plane and out-of-plane directions of the GIWAXS patterns in Figure 3B and
Figure 3C. This linear superposition of the oxidized (+0.4 V) and neutral states (0 V) suggests that the
polymer is composed of a phase-separated blend of ion-rich and ion-poor structural phases.
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Figure 3: GIWAXS of neutral, phase-separated, and oxidized PB2T-TEG. (A) GIWAXS diffraction patterns of dry PB2T-
TEG neutral, phase-separated, and oxidized. Line cuts along the (B) out-of-plane (Qz) and (C) in-plane (Qxy) directions. (D)
Schematic showing the polymer unzipping/zipping mechanism for ion insertion in PB2T-TEG. The green spheres are hydrated
Cl- (d = ~6 Å).49
Overall, in the neutral form, the polymer structure is driven by side-chain crystallization (perhaps
because the backbone does not support strong π-π stacking), while in the oxidized/ion compensated form,
the polymer crystal structure is driven by π-π stacking, with the side-chains being more disordered. Next,
we discuss a possible mechanism to account for the observed structural changes. We suggest that PB2T-
TEG undergoes an unzipping/zipping mechanism occurring upon oxidation and reduction, as shown in
Figure 3D. Initially in its neutral structural phase, PB2T-TEG has interdigitated, sidechains and no
significant π-π stacking. Following oxidation of the backbone under an applied bias, the glycolated side
chains lose their crystallinity, and the backbone (which becomes more quinoidal upon oxidation) forms π-
π stacking interactions through what we hypothesize is an unzipping and potentially untwisting motion of
the polymer. Such an unzipping mechanism could also underpin the origin of the phase-separated state. We
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propose that it should be energetically-favorable to electrochemically oxidize regions of the polymer that
are already unzipped, presumably leading to nucleation and growth-like kinetics of ion insertion.
Temperature-dependent XRD measurements on the phase-separated film show that the two phases mix with
increasing temperature and then demix as the temperature is lowered (Figure S11). This observation is
consistent with the proposed unzipping mechanism, as the sidechains most likely melt at higher temperature,
leading to reversible mixing of previously phase-separated regions of the polymer.
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Finally, we show that the structural phase transition persists under working electrochemical
conditions using in operando WAXS. Figure 4A shows the in operando WAXS electrochemical cell for
interrogating changes in polymer structure upon electrochemical oxidation in an aqueous electrolyte
environment (with additional images of the liquid cell shown in Figure S12). The cell consists of an Au-
coated polyimide window with the conjugated polymer layer located between the Au layer and the aqueous
electrolyte (100 mM KCl). A Ag/AgCl pellet electrode is used as the counter electrode, which is placed in
a well filled with aqueous electrolyte and surrounded by a PDMS spacer. The X-ray beam penetrates the
Au/polyimide window at a 2.0˚ incidence angle to probe changes in the polymer structure upon applying
an electrochemical bias. Figure 4B shows line cuts of the WAXS diffraction pattern in the lamellar
direction (Qz). The line cuts show the disappearance of the peak corresponding to the neutral state of PB2T-
TEG and appearance of the peak correspond to its oxidized state (the full WAXS patterns are shown in
Figure S13). Upon returning to 0 V, the peak corresponding to neutral PB2T-TEG reappears, indicating
that this process is also reversible in operando (Figure S14). In Figure 4C, we compare the lattice spacing
in the lamellar direction (Qz) for ex situ and in operando (GI)WAXS characterization. We observe the same
discontinuity between +0.2 and +0.3 V with the in operando measurements, yet the presence of the
electrolyte does cause some swelling of the crystalline regions of the polymer (Figure S15). Compared to
that of the dry polymer, the lattice spacing for the in operando measurements with electrolyte present is
larger for both oxidized and neutral samples by approximately 12% and 8%, respectively. This observation
is consistent with previous reports showing that polymer lattice spacing can increase in a hydrated
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environment. Importantly, the in operando WAXS measurements demonstrate that the structural phase
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transition occurs in a working device-relevant environment.
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Figure 4: In operando GIWAXS measurements of electrochemical oxidation in PB2T-TEG. (A) Schematic of the custom
liquid cell used for in operando WAXS measurements. (B) Line cuts of the WAXS diffraction pattern along the Qz axis for +0.1,
+0.2, and +0.3 V. (C) Comparison of the lamellar spacing for the in operando and ex situ (GI)WAXS measurements, showing a
larger lattice spacing for the in operando measurements in both the oxidized and neutral phases.
The structural phase transition has significant implications on how ion-polaron pairs move through
the crystalline polymer matrix. To interrogate how the electrochemically-induced phase transition impacts
ion-polaron pair motion, we compare ion transport in PB2T-TEG and P3MEEMT. We choose to compare
PB2T-TEG to P3MEEMT because P3MEEMT shows minimal changes in lattice spacing upon ion injection.
To directly visualize ion-polaron pair transport, we use moving front diffusion experiments and observe the
lateral movement of the interface between oxidized and neutral regions of the polymer thin film. Unlike
previous moving front experiments,50–53 we do not apply an electrochemical bias in situ. Instead, we
electrochemically oxidize half the sample and watch the interface relax to equilibrium through ion diffusion
under dry conditions without an applied bias. To create an interface between oxidized and neutral regions
of the polymers, we electrochemically oxidize half of a polymer sample (+0.7 V vs. Ag/AgCl) by
submerging half of the sample in an aqueous electrolyte (100 mM KCl), applying an electrochemical bias
(+0.7 V vs. Ag/AgCl), and then removing the sample after 30 s, resulting in a sample that is half oxidized
and half neutral (Figure 5A). We then watch the interface relax to equilibrium using optical transmission
imaging of the polaron absorption peak (at 750 ± 25 nm) as ion-polaron pairs diffuse. Figure 2B and Movie
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S1 shows the interface motion for PB2T-TEG and P3MEEMT (with line profiles shown in Figure S16).
The P3MEEMT interface between oxidized and neutral regions broadens and becomes less sharp out with
time, with a 50% transmission threshold at 720 µm after 38 hrs, as expected for Fickian diffusion of the
ions. In contrast, the interface between the oxidized and neutral regions in PB2T-TEG remains sharp with
time (< 1 µm), moving 140 µm at 38 hrs. This interface remains sharp for as long as six months after
electrochemical oxidation (Figure S17).
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Figure 5: Ion front movement in PB2T-TEG and P3MEEMT. (A) Example image of the samples used for ion front imaging.
The gray area is oxidized in 100 mM KCl at +0.7 V (vs. Ag/AgCl) and the red area is neutral. (B) Optical transmission images of
the interface between oxidized and neutral regions of the film for PB2T-TEG and P3MEEMT. The transmission images are acquired
in the same spectral region as the polaron absorption (750 ± 25 nm). (C) Time series of spatially correlated PiFM and topography
images as the interface migrates in PB2T-TEG. The white line designates the 50% intensity threshold from the PiFM channel.
To confirm that the ion front remains sharp, even below the diffraction limit, we also tracked ion
motion as a function of time in PB2T-TEG at the nanoscale using super-resolution photoinduced force
microscopy (PiFM). In doing so, we find that the ion front is also sharp at the nanoscale as the interface
moves (Figure 5D). For imaging ion motion, we electrochemically oxidize the half of the film from a 100
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mM KPF6 solution, so that we can use the P-F IR-active vibrational mode of PF6 to detect the presence of
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ions within the PB2T-TEG film (Figure S18). We note that oxidizing the polymer with PF6 shifts the
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threshold voltage for oxidation in agreement with previous studies (Figure S19).29,30,54,55 We capture
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simultaneous images of the infrared signal of PF6 and the sample topography as the interface between the
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oxidized and neutral regions of the polymer transits the field-of-view (Figure 5D, Figure S20, Movie S2,
Movie S3). We find that the ion front remains sharp (~700 nm 90/10 width of a line-cut) as the phase front
propagates. Along with changes in the PiFM signal, we also observe noticeable changes in the film
topography as ions move through the materials (Figure S17, Movie S4).
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While this non-Fickian transport appears startling at first, we hypothesize that the polymer phase
change, rather than the intrinsic diffusivity of the ions, is what limits ion motion in PB2T-TEG and hence
maintains a sharp interface between neutral and oxidized regions of the film. We emphasize that we are
observing ion-polaron pairs, which require sufficiently high ion and electron mobilities to diffuse in organic
mixed conductors. Based on the XRD results, we observe ions in the crystalline regions of the polymer film
and, therefore, hypothesize that ion-polaron pairs diffuse through the crystalline regions of the polymer,
maintaining a sharp interface because of a difference in the ion-polaron pair diffusion constant in each
structural phase. This hypothesis is consistent with numerical calculations of concentration-dependent
diffusion, which show a sharp moving front can be preserved when the diffusion constant of the ions in the
oxidized and neutral region differs by at least three orders of magnitude (Figure S22 with additional details
in the Supplementary Information). The observed sharp interface is reminiscent the sharp propagating front
observed upon solvent infiltration into some glassy polymers, which is often called Case II diffusion and
results from the mechanical response of the polymer limiting diffusion.56 Similar non-Fickian diffusion has
also been observed in 2D materials57 and inorganic materials58 that undergo structural phase transitions
upon ion insertion. Although similar sharp interfaces have previously been observed in inorganic materials
for energy storage, such as in interfaces between lithium-rich and lithium-poor iron phosphate (LiFePO4 /
FePO4),12 they have not been observed in organic materials.
CONCLUSIONS
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We have demonstrated that conjugated polymers can undergo reversible structural phase transitions
upon electrochemical insertion of ions, in analogy with inorganic materials like LiFePO4. We suggest that
the structural phase transition occurs due to melting of the glycolated side-chains upon ion insertion,
accompanied by the formation π-π stacking interactions of the polymer backbone in the oxidized state. The
kinetics of the phase transition can be described by an unzipping motion of the polymer backbone. At
intermediate biases, the films exhibit phase-separation between ion-rich and ion-poor regions of the
polymer film. Using a custom liquid cell for in operando WAXS measurements, we also show that the
phase transition occurs in aqueous electrolytes. In addition to its fundamental interest as a structural motif,
this phase transition has important implications for the mixed-conduction behavior of conjugated polymers:
whereas the polymers P3MEEMT that does not exhibit a phase transition exhibits normal Fickian diffusion,
ions in PB2T-TEG propagate as a sharp front, that does not spread out in time over periods of days to
months. We speculate that this behavior could be used to engineer desirable electrochemical properties,
perhaps aiding retention of electrochemical states in computing applications,7,59,8 or exploited to improve
energy storage in conjugated polymers, where structural phase transitions might enable flatter voltage
profiles during discharge due to phase coexistence between the oxidized and reduced phases. As a result,
engineering structural phase transitions in conjugated polymers could become an important new design
principle.
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ASSOCIATED CONTENT
Supporting Information
Details of PB2T-TEG and P3MEEMT synthesis and characterization, electrochemical quartz crystal
microbalance data, OECT device data, EIS data, additional (GI)WAXS data, simulation methods, and
additional PiFM data.
AUTHOR INFORMATION
Corresponding Author
* ginger@chem.washington.edu
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ORCID
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Connor G. Bischak: 0000-0002-3071-4069
Lucas Q. Flagg: 0000-0002-2798-5650
Kangrong Yan: 0000-0001-9255-9917
Tahir Rehman: 0000-0001-6862-7051
Daniel W. Davies: 0000-0001-5170-4559
Ramsess Quezada: 0000-0002-2212-8422
Jonathan W. Onorato: 0000-0003-1349-8277
Christine. K. Luscombe: 0000-0001-7456-1343
Ying Diao: 0000-0002-8984-0051
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Chang-Zhi Li: 0000-0003-1968-2032
David S. Ginger: 0000-0002-9759-5447
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
L. Q. F and R. J. Q.’s contributions began as exploratory work under the expired National Science
Foundation award, NSF DMR-1607242, and were subsequently supported partly from NWIMPACT SEED
award funding. C. G. B. is a Washington Research Foundation Postdoctoral Fellow. Y. K., T. R and C.-Z.
L. thank the support from National Natural Science Foundation of China (Nos. 21722404 and 21674093)
for supporting synthesis of the PB2T-TEG polymer. Y.D. and D.W.D. acknowledge support by the Sloan
Foundation through the Sloan Research Fellowship in Chemistry and 3M Nontenured Faculty Award
contributing to analysis of the XRD data and interpretation of the phase-change mechanism. J. O. and C.
K. L.’s contributions are based in part on work supported by the National Science Foundation, NSF CBET-
1922259. The photoinduced force microscopy (PiFM) and X-ray diffraction (XRD) were conducted at the
Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure site at the University
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of Washington that is supported in part by the National Science Foundation (grant ECC-1542101), the
University of Washington, the Molecular Engineering & Sciences Institute, the Clean Energy Institute and
the National Institutes of Health. This research used beamline 7.3.3 of the Advanced Light Source (ALS),
which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. The authors
thank C. Zhu and E. Schaible at the ALS for assistance with (GI)WAXS data acquisition and analysis.
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