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Beyond Doping and Charge Balancing: How Polymer Acid
TemplatesImpact the Properties of Conducting Polymer ComplexesMelda
Sezen-Edmonds*,† and Yueh-Lin Loo*,†,‡
†Department of Chemical and Biological Engineering, Princeton
University, Princeton, New Jersey 08544, United States‡Andlinger
Center for Energy and the Environment, Princeton University,
Princeton, New Jersey 08544, United States
ABSTRACT: Polymer acids are increasingly used as
dopants/counterions to access andstabilize the electrically
conducting states of conducting polymers. Beyond doping
and/orcharge balancing, these polymer acids also serve as active
components that impact themacroscopic properties of the conducting
polymer complexes. Judicious selection ofthe polymer acid at the
onset of synthesis or manipulation of the interactions between
thepolymer acid and the conducting polymer through processing
significantly impacts theelectrical conductivity, piezoresistivity,
electrochromism, mechanical properties, andthermoelectric
efficiency of conducting polymers. As polyelectrolytes, these
polymer acidsenable conducting polymer complexes to transport ions
in addition to electrons/holes.Understanding the role of the
polymer acid and its interactions with the conductingpolymer
generates processing−structure−function relationships for
conducting polymer/polymer acid complexes, which can help overcome
challenges that were associated withthese materials, such as low
electrical conductivity and sensitivity to humidity, and enablethe
design of conducting polymer complexes with desired
functionalities.
Electrically conducting polymers belong to a class ofmaterials
that possess electrical conductivities approachingthose of metals
while having plastic-like mechanical properties.Their mechanical
compliance with flexible substrates coupledwith their unique
optoelectronic properties, such as mixed ionicand electronic
conduction, and electrochromism, have enabledtheir use in a broad
range of applications, including confor-mable neural recording
devices, transparent electrodes, bio-sensors, strain gauges, and
smart windows.1−4 Although thediscovery of their synthesis goes
back to the 19th century,breakthroughs in conducting polymer
research came in the1970s, when Heeger, MacDiarmid, and Shirakawa
showed thatpolyacetylene’s conductivity can be increased by more
than6 orders of magnitude upon iodine doping.5,6 This
transforma-tion of polyacetylene from its electrically insulating
form toan electrically conducting form stems from delocalization
ofcharges along polyacetylene’s conjugated backbone upon
iodineincorporation. One substantive drawback despite high
conductiv-ities with these materials is that iodine-doped
polyacetylene iseasily oxidized in air and is thus environmentally
not stable.7,8
Other conjugated polymers, such as polyaniline
(PANI),poly(3,4-ethylenedioxythiophene) (PEDOT), and
polypyrrole,
have shown promise with their improved environmental
stability.9,10
Accessing the electrically conducting states of these
polymersalso requires delocalization of charges along their
conjugatedbackbones. In some conducting polymers, such as
PEDOT,delocalization of charges is achieved through electron
trans-fer by oxidizing or reducing the conducting polymer. In
otherconducting polymers, such as PANI, proton doping is neededto
access their electrically conducting states. In both
systems,incorporation of counterions is needed in order to
preservecharge neutrality of the conducting polymer complex.11−13
Inthe early development of these conducting polymers,
small-molecule acids were employed as proton dopants and/or
thecounterions. The volatility of small-molecule acids,
however,limits the ambient stability of these conducting
polymers.14
Moreover, these conducting polymers are insoluble in
commonsolvents due to the extensive conjugation along the
polymerbackbone. This intractability necessitates simultaneous
thin-filmdeposition during synthesis like electropolymerization on
con-ducting substrates. Though an exciting discovery, the
necessityof an underlying conducting substrate has limited the use
ofthese early generation conducting polymers in practical
applica-tions.15,16
Replacing small-molecule acids with polymer acid
dopants/counterions, such as poly(acrylic acid),
poly(styrenesulfonate)(PSS), and
poly(2-acrylamido-2-methyl-1-propanesulfonic
acid)(2-acrylamido-2-methyl-1-propanesulfonic acid)
(PAAMPSA),enhances the ambient stability of conducting polymers due
to
Received: July 11, 2017Accepted: August 30, 2017Published:
August 30, 2017
Beyond doping and/or chargebalancing, these polymer acidsalso
serve as active componentsthat impact the macroscopicproperties of
the conducting
polymer complexes.
Perspective
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the nonvolatile nature of the polymer acids. Excess acid
groupsalso introduce water dispersibility to the resulting
conductingpolymer complex.14−17 In addition to ambient
electricalstability, the use of polymer acids also enhances the
electro-chemical and physiological pH stability of conducting
polymersand opens the possibility of their use in electrochemical
bio-sensors, artificial muscles, and smart windows.2,18,19
Thisenhanced stability and water dispersibility, however, has
tradi-tionally come at the expense of electrical conductivity.
Theincorporation of polymer acids generally introduces
structuraldisorder that is correlated with reduced electrical
conductivity(
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forms particles with smaller hydrodynamic diameter and
PANIbecomes more crystalline.21,26 These structural differences
inturn result in conductivity improvements in PANI−PAAMPSAthin
films from 0.4 to 1.1 S/cm as the molecular weight of thePAAMPSA
template is reduced from 724 to 45 kg/mol, asshown in Figure 1b
(unfilled circles).27 Yoo et al. showed thatby narrowing the size
distribution of the PAAMPSA template,PANI crystallinity and the
connectivity between conductingdomains can be further improved, and
the conductivity ofas-cast PANI−PAAMPSA films can be increased to
2.4 S/cm(filled circles in Figure 1b).25
Given mass transport limitations, the molecular weight ofPAAMPSA
necessarily impacts the extent of doping, whichalters the
conductivity of PANI−PAAMPSA, albeit to a smallerextent than the
structural influence discussed above. That dop-ing is incomplete is
evidenced by control experiments inwhich different PANI−PAAMPSA
grades are exposed to HClvapor. When PANI−PAAMPSA synthesized with
a 255 or724 kg/mol PAAMPSA template is exposed to HCl vapor,
therespective conductivities increase from 0.55 and 0.4 S/cm to0.65
and 0.55 S/cm, respectively. This modest increase in con-ductivity
indicates that these PANI−PAAMPSA grades werenot fully doped
as-synthesized. Conversely, the conductivitiesof PANI−PAAMPSA that
is synthesized with 106 or 45 kg/molPAAMPSA templates remain
unchanged upon HCl exposure,an observation that indicates these
PANI−PAAMPSA gradesto be fully doped as-synthesized. Results from
solid-state NMRexperiments suggest that mass transport limitations
during PANItemplate synthesis in the presence of high molecular
weightPAAMPSA results in the incorporation of a small fraction
ofaniline to the polymer chain in its deprotonated quinoid
form.27
As opposed to protonic doping, the conductive state ofPEDOT is
accessed through redox doping. The polymer acid,in this case, does
not play the role of a dopant. Instead, itbalances the charge of
the electrically conductive oxidized stateof PEDOT. The choice of
counterion similarly affects the mor-phology, the conformation of
the PEDOT chain, and the stabil-ity of the oxidized PEDOT and
results in differences in theelectrical conductivity of the
resulting PEDOT:counterioncomplexes.28−30 The use of hyaluronic
acid, for example, resultsin PEDOT:hyaluronic acid complexes with
negligible con-ductivities, whereas PEDOT:PSS has conductivities
that rangefrom 10−3 to 103 S/cm, depending on the synthesis and
pro-cessing conditions.20,30 Collectively, these examples
providestrong evidence that implicate the role of the polymer
acidbeyond doping and charge balancing.Beyond electrical
conductivity, the choice of the polymer
acid template also impacts the piezoresistive properties ofthe
conducting polymer complexes. Piezoresistivity definesthe change in
electrical resistance of conducting materials withmechanical
deformation and is quantified by a unitless parametercalled the
gauge factor that relates changes in electrical resis-tance to
applied strain. The gauge factor of conducting mate-rials
critically impacts their applicability as the active com-ponents in
flexible sensors. High gauge factors are needed forstrain sensors,
whereas near-zero gauge factors are needed forflexible
thermoresistive or chemoresistive sensor applications toprevent
mechanical deformation-related drifts in the
measuredresistance.4,31,32 The ability to tune the piezoresistivity
of con-ducting polymers can therefore augment their deployment
inthese different applications. Figure 1c (black data points)
showsthe impact that the molecular weight of PAAMPSA has onthe
gauge factor of the resulting PANI−PAAMPSA thin films.
The gauge factor decreases linearly and becomes negative
withdecreasing PAAMPSA molecular weight.21 The dependence ofthe
gauge factor of PANI−PAAMPSA on the molecular weightof the PAAMPSA
template is reminiscent of the anticorrelationbetween the
electrical conductivity of PANI−PAAMPSA andthe PAAMPSA molecular
weight.27 Similarly, this change ingauge factor can be correlated
with structural differences betweenthe different PANI−PAAMPSA
grades. A positive gauge factoris observed in samples comprising
high molecular weightPAAMPSA (255 and 724 kg/mol); these samples
have thelargest hydrodynamic radii as-synthesized, and the
resultingfilms are largely amorphous (Figure 1a). A negative
gaugefactor, on the other hand, is observed in the thin films
ofPANI−PAAMPSA that are synthesized with lower molecularweight (45
and 106 kg/mol) PAAMPSA templates; thesePANI−PAAMPSA grades are
more crystalline than those synthe-sized with higher molecular
weight templates. We believe thenegative gauge factor to be a
result of strain-induced alignment ofPANI crystallites that can
bridge neighboring PANI−PAAMPSAparticles.21 A lack of PANI
crystallites that connect individualPANI−PAAMPSA particles causes
the mostly amorphousPANI−PAAMPSA samples to have positive gauge
factors; stretch-ing of these films results in an increase in their
resistance due toincreased separation between the conducting
domains undertensile strain. The ability to tune the piezoresistive
response ofPANI−PAAMPSA has enabled the use of this material in
abroad range of flexible sensor applications, such as strainsensors
that require high gauge factors, and thermoresistive
orchemoresistive sensors that require near-zero gauge factorsfor
accurate sensing under mechanical deformation.4,21
Although the examples we have given thus far focus onthe
electrical properties of PANI that is template synthesizedon
PAAMPSA, the choice of polymer acid dopant/counterionimpacts the
macroscopic properties of electropolymerizedconducting polymers as
well. The polymer acid that isused to dope polypyrrole during its
electropolymerization, forexample, determines its hydrophobicity,
roughness, Young’smodulus, and the brittleness of the resulting
electricallyconducting polypyrrole films. Polypyrrole when doped
withhyaluronic acid forms rough films with high moduli
(Young’smodulus of 706 MPa) that are not suited for muscle
celldifferentiation. In contrast, doping polypyrrole with
poly-(2-methoxyaniline-5-sulfonic acid) results in conducting
polymerfilms with smoother surfaces and lower moduli
(Young’smodulus of 30 MPa) that are capable of electrically
stimulatingcell differentiation in tissue engineering
applications.33,34
As an alternative to altering the synthetic parameters to
affectthe structure of conducting polymers, the incorporation
ofcosolvents or postdeposition solvent annealing has been shownto
impact the macroscopic properties of these conducting poly-mer
complexes.3,35−37 Commercially available PEDOT:PSS, forexample, is
synthesized through oxidative polymerization
of3,4-ethylenedioxythiophene (EDOT) along PSS, where the
PSStemplate acts as a counterion to charge balance the oxida-tively
doped PEDOT. This template polymerization results inPEDOT:PSS
particles with conducting PEDOT-rich cores andinsulating PSS-rich
shells that are dispersible in water. CastingPEDOT:PSS dispersions
forms structurally heterogeneous filmswith PEDOT-rich domains
separated by PSS-rich shells in amatrix of free-PSS.38,39 The
electrical conductivity of PEDOT:PSSis highly dependent on how
PEDOT:PSS particles and thefree-PSS matrix are distributed in thin
films, with contiguityof PEDOT-rich domains providing conducting
pathways for
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charge transport. One route to improve the conductivity
ofPEDOT:PSS is the addition of cosolvents, such as
dimethylsulfoxide (DMSO) and ethylene glycol (EG), which induce
struc-tural changes in PEDOT:PSS at both molecular and
interparticlelevels. The addition of cosolvents increases the PEDOT
crystallitesize and decreases the π−π stacking distance. At the
inter-particle level, this cosolvent treatment increases the size
and thepurity of PEDOT-rich domains, redistributes PEDOT:PSS
par-ticles and free PSS, and results in formation of thin films
withmore contiguous conducting pathways.20,35−37 These struc-tural
changes have been correlated with an increase in theelectrical
conductivity of PEDOT:PSS (Clevios PH1000,Heraeus) from about 10 to
103 S/cm with the addition of5 wt % EG prior to deposition.20,35−37
Similarly, postdepositionsolvent annealing of PEDOT:PSS thin films
has also shownto effectively increase the conductivity of
PEDOT:PSS. Theexposure of PEDOT:PSS thin films to cosolvents, such
asdichloroacetic acid (DCA), for example, removes free PSS
andeffectively enhances the connectivity between PEDOT-richdomains.
Accordingly, the electrical conductivity increases by2 orders of
magnitude.3,35 Structural modification of conduct-ing polymer
complexes induced by the addition of cosolvent orby postdeposition
solvent annealing go beyond improving theelectrical conductivity;
they can similarly enhance their thermo-electric properties. The
figure of merit (ZT) of PEDOT:PSSafter EG exposure, for example,
has been reported to be com-parable to those of nanostructured and
epitaxially growninorganic thermoelectric materials at room
temperature.40−42
With ZT of 0.42, solvent-annealed PEDOT:PSS is promising asan
active material in waste-heat recovery devices.41,43 Coupledwith
its solution processability and mechanical compliancewith flexible
substrates, this improved ZT makes the fabricationof lightweight,
flexible, and large-area thermoelectric devicespossible.In the case
of PANI−PAAMPSA, conductivity improvements
with postdeposition solvent annealing are more sensitive
tosolvent properties. Although both DCA and DMSO exposuresenhance
the conductivity of PEDOT:PSS, only treatment withDCA among these
two solvents increases the electrical conduc-tivity of PANI−PAAMPSA
by more than two orders of mag-nitude (to 40 S/cm).3 In
PANI−PAAMPSA thin films, becausethe polymer acid also proton dopes
PANI, there are stronginteractions between the aniline repeat units
and −SO3− groupsof PAAMPSA. DCA is a good solvent for PAAMPSA and
hasan ionization constant, pKa, lower than that of PAAMPSA;exposing
PANI−PAAMPSA thin films to DCA thereforeplasticizes PAAMPSA and
induces structural rearrangementby disrupting the electrostatic
interactions between anilinerepeat units and −SO3− groups of
PAAMPSA. This structural
rearrangement results in the elimination of the
particulatenature of PANI−PAAMPSA, increases PANI crystallinity,
andenhances the connectivity of the conducting domains.3,4
DMSO, with a pKa higher than that of PAAMPSA, on theother hand,
cannot induce such structural rearrangementsdespite being a good
solvent for PAAMPSA, and no conductivityimprovement is observed in
PANI−PAAMPSA thin films uponDMSO exposure. As yet another control
experiment, Yoo et al.also tested HCl, a bad solvent, but it has a
lower pKa thanPAAMPSA and no structural rearrangement is observed
whenHCl is used.3 These examples show that the choice of solventfor
manipulating structure in conducting polymer complexesnot only
depends on the solubility of the polymer acid in thesolvent of
choice but is also affected by the type and relativestrength of
intermolecular interactions between the polymeracid, conducting
polymer, and solvent.3,44
Seeing the effects of postdeposition DCA annealing on
theelectrical conductivity of PANI−PAAMPSA thin films andknowing
the sensitivity of the piezoresistive response of PANI−PAAMPSA to
its microstructure, in our more recent work wealso tested the
piezoresistivity of DCA-treated PANI−PAAMPSAfilms.4,21 Because DCA
treatment increases the crystallinity ofPANI and enhances the
connectivity of the conducting domains,a negative gauge factor is
expected from these samples, aresponse that is similar to the
piezoresistive response of theas-cast PANI−PAAMPSA films that are
synthesized with lowmolecular weight PAAMPSA templates. Figure 1c
(blue datapoints) shows that DCA treatment of PANI−PAAMPSA
thinfilms results in a negative gauge factor of comparable
mag-nitude for all grades independent of the molecular weight
ofPAAMPSA used in the synthesis.4,21 By redefining the
inter-molecular interactions between the conducting polymer and
thepolymer acid dopant through postdeposition solvent anneal-ing,
we are able to significantly impact the morphology,
andconsequently, the electrical conductivity and the
piezoresistivityof PANI−PAAMPSA.Separately, Tarver et al. showed
that morphological control
of PANI−PAAMPSA through postdeposition DCA annealingeliminates
hysteresis during electrochromic switching of PANI−PAAMPSA.
Moreover, this treatment results in PANI−PAAMPSAadopting its
emeraldine salt form that shows significant near-IRabsorption,
opening the possibility of using this postdepositionprocessed
material in smart windows that can regulate heattransmission in
addition to visible light transmission to increasethe energy
efficiency of residential and commercial build-ings.2,22 Examples
given in this section show that tunability ofthe structure of the
conducting polymer/polymer acid com-plexes not only enables
optimization of their optoelectronicperformance but also provides
access to new functionalitiesinaccessible with conducting polymers
doped with small-molecule acids for different applications.Mixed
Ionic and Electronic Conductivities in Conducting
Polymer/Polymer Acid Complexes. Materials that can transportions
in addition to electrons/holes form an intermediate classof
materials between liquid and polymer electrolytes that showonly
ionic conductivity and inorganic semiconductors andmetals that only
have electronic conductivity (in the form ofelectron or hole
transport). This mixed conductivity can beemployed in many
different applications in which the ionicallyconducting media
interface with electronically conductingmaterials, such organic
electrochemical transistors (OECTs),1
actuators,45 supercapacitors,46 controlled drug-delivery
devi-ces,47 and neural monitoring devices.48 Conducting
polymers
Tunability of the structure of theconducting polymer/polymer
acidcomplexes not only enables opti-mization of their
optoelectronicperformance but also providesaccess to new
functionalities
inaccessible with conducting pol-ymers doped with
small-moleculeacids for different applications.
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that have polymer acid dopants/counterions are promising
can-didates as mixed ionic and electronic conductors.46 Havinga
polymer acid dopant/counterion that in itself is a poly-electrolyte
enables ion transport in these conducting polymercomplexes while
still rendering them electronically conduc-ting, environmentally
stable, and solution-processable.46,49−51
Such mixed ionic and electronic conduction differentiates
theseconducting polymers from inorganic semiconductors and
small-molecule acid-doped conducting polymers that typically
onlytransport electrons and/or holes.Figure 2a illustrates mixed
conduction in PEDOT:PSS; this
mechanism is contrasted to Figure 2b, which illustrates
hole-only conduction in boron-doped silicon when they are incontact
with a liquid electrolyte. At the interface betweenPEDOT:PSS and
the electrolyte solution, ions can move intoand through PEDOT:PSS
with relative ease because PSSsupports ion transport.52 Moreover,
the free volume created bythe disordered PSS matrix further
facilitates ion transport. Thetight packing of the covalently bound
silicon network, on theother hand, limits ion transport.
Additionally, the presence of anative oxide layer atop boron-doped
silicon prevents theelectrolytes from coming into direct contact
with silicon andcreates an additional barrier to ion transport.23
These differ-ences make conducting polymer complexes uniquely
capableof mixed conduction. In this case, PSS is responsible
fortransporting ions and PEDOT is responsible for
transportingholes.This feature of mixed ionic and electronic
conductivity
enabled by the presence of the polymer acid template
facilitatescommunication between ion-containing biological
environ-ments and electronically conducting inorganic
semiconductor-or metal-based electronic devices.53 Coupled with
their bio-compatibility and mechanical compliance, conducting
polymersare ideal candidates to directly and conformably interface
withliving tissues. Figure 3a shows a micrograph of a flexible
brainmonitoring device based on PEDOT:PSS-coated microelec-trodes.
Leveraging the mechanical compliance of PEDOT:PSS,this
microelectrode array can conform to the topography of
the brain surface. Figure 3b shows that the
PEDOT:PSSmicroelectrode array has at least an order of magnitude
lowerimpedance than either the gold or silicon electrodes that
arestandard electrodes in use today. A low impedance in
neuralrecording devices is correlated with a higher
signal-to-noiseratio. The PEDOT:PSS microelectrode array is able to
providesingle-neuron-resolution monitoring of human brain
activityfrom the brain surface without the need to penetrate
braintissues.54 Similarly, conducting polymer-based OECTs haveshown
promise as noninvasive neural recording devices.Highly conformable
arrays of PEDOT:PSS-based OECTs thatare placed on the somatosensory
cortex, for example, are shownto record electrophysical signals
with little to no invasion, withmore sensitivity than other surface
electrodes and comparablesensitivity as electrodes that penetrate
brain tissues.1 Becauseconducting polymers can conduct ions through
the bulk, theircapacitance is defined by their volume. In contrast,
the capac-itance of inorganic semiconductors is limited to their
surfacearea. This difference in capacitance can be significant; it
hasbeen shown that the volumetric capacitance can be almost2 orders
of magnitude higher than its double-layer capacitancefor the same
material.55,56 Bulk ion transport thus dramaticallyincreases the
sensitivity of the brain monitoring devices com-pared to
conventional inorganic brain monitoring devices.The enhanced
sensitivity of these PEDOT:PSS-based neuralrecording devices is
attributed to the PEDOT:PSS active layer’sability to capture and
transport ions generated in the neuralsystem in addition to its
ability to detect local field potentialswith higher signal-to-noise
ratio than the conventional electro-des due to the low impedance of
PEDOT:PSS electrodes.54,57
Having a combination of ion-conducting and
electron/hole-conducting regions in these polymer acid templated
conductingpolymers can also enable easy fabrication of
electronicallycontrolled drug delivery devices. Figure 3c shows the
devicearchitecture of an ion pump based on PEDOT:PSS interfac-ing
with an electrolyte solution. The ion pump is composed oftwo
electronically (hole transport) and ionically conductingPEDOT:PSS
electrodes (dark blue) that are connected by an
Figure 2. Scheme of (a) the conducting polymer (PEDOT:PSS) and
(b) inorganic semiconductor (boron-doped silicon) interfacing a
biologicalenvironment (electrolyte). The hydrated ion sizes are the
same in both cases. Reprinted with permission from ref 23.
Copyright 2014 AmericanChemical Society.
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overoxidized PEDOT:PSS region (pink) that only conductsions. The
overoxidized PEDOT loses its electrical conductivitydue to
disruption of its conjugation pathway, but the PSSportion of this
polymer complex remains ionically conductive.PSS thus facilitates
ion transfer and forms a solid-state saltbridge between the two
electrodes in this ion pump. When apotential bias is applied
between the two electrodes, thePEDOT on the left electrode is
oxidized, and its PSS releases a
cation (denoted as M+ in Figure 3c) in order to charge
balancethe oxidized PEDOT. The PEDOT on the right electrode
issimultaneously reduced. Upon PEDOT oxidation/reduction onthe
left/right electrodes, respectively, free M+ cations movefrom the
left electrode to the right electrode through theoveroxidized
PEDOT:PSS bridge. Ion transfer continues withan applied potential
bias until PEDOT is completely oxidizedand reduced on the left and
right electrodes, respectively.58
Critical to operation of this ion pump is the ability for PSS
totransport ions and for PEDOT to exhibit an electronic
(hole)conductivity that is redox-controlled. Figure 3d shows
thetransport of ions across a concentration gradient between
twoelectrolytes in this ion pump. The electrodes are
initiallybrought in contact with the identical electrolyte
solutions con-taining potassium ions, K+ (solid black bars). After
applying abias for 25 min, a decrease in the K+ concentration is
observedin the left electrolyte with a concomitant increase in the
K+
concentration in the right electrolyte (cross hatched
bars).58
The ion pump has been successfully demonstrated in vivo
todeliver neurotransmitters to the auditory system of a guinea
pig,and has also been used in combination with
PEDOT:PSS-basedneural monitoring devices to monitor epileptiform
dischargesinduced in mouse hippocampus and then to deliver
aninhibitory neurotransmitter to stop epileptiform
activity.47,59
These examples show that beyond enabling electron con-duction in
conducting polymers, polymer acid dopants/counterions, with their
ion conduction, add critical functionalitythat would otherwise be
missing in small-molecule acid-dopedconducting polymers.
Given the applicability of mixed ionic and electronic
con-ductivity in various electronic applications, especially as
theypertain to human health, it is important to understand
whatdetermines the extent of ion and electron/hole transport
inconducting polymers. Ion transport is facilitated by the
freevolume created by the structurally disordered polymer
aciddopants/counterions; yet the same structural disorder
limitselectron/hole transport. That this structural disorder aids
iontransport but impedes electron/hole transport suggests that
therelative contributions of ionic and electronic conductivity
canbe controlled by tuning the morphology of these
conductingpolymer systems.49,50 In order to understand how
structuralchanges affect ion transport in PEDOT:PSS thin films,
Rivnayet al. performed one-dimensional moving front experimentsto
monitor the movement of K+ ions in PEDOT:PSS thinfilms.37,51 Figure
4a shows the dependence of the electronic(hole) conductivity (blue)
and K+ ion mobility in PEDOT:PSSthin films on the amount of EG
added to PEDOT:PSS prior todeposition. While the addition of
cosolvents, such as EG orDMSO, increases the electronic
conductivity of PEDOT:PSS,this cosolvent addition significantly
retards ion mobility.
Figure 3. (a) Optical micrograph of a conformable
PEDOT:PSS-basedneural interface array. The scale bar is 200 μm. (b)
Electrochemicalimpedance of the PEDOT:PSS microelectrode array
shown in(a) (filled circles) and conventional Au-based electrodes
(emptycircles). The inset shows the impedance of the
PEDOT:PSSmicroelectrode array (blue) and of conventional
implantable siliconprobes (red) composed of different arrays of
electrodes. Reprinted bypermission from Macmillan Publisher Ltd.:
Nature Neuroscience(ref 54), copyright (2014). (c) Scheme of an ion
pump based onPEDOT:PSS electrodes (dark blue) connected with an
overoxidizedPEDOT:PSS bridge (pink). Light blue regions represent
the elec-trolytes that are brought into contact with the PEDOT:PSS
electrodes.When a potential bias is applied between the two
electrodes, ionsfrom one electrode can be carried to the other
electrode through theionically conducting but electronically
insulating overoxidizedPEDOT:PSS bridge. (d) K+ ion concentration
in the electrolytesplaced on the left (AB) and right (CD)
electrodes of the ion pumpshown in (c). The black bar represents
the K+ concentration in theinitial electrolyte solution, and the
horizontal hatched bar representsthe K+ concentration of the
electrolyte that is in contact with the left(AB) and right (CD)
electrodes, respectively, when the ion pump isoff. The cross
hatched bar represents the K+ concentration 25 minafter the pump is
turned on. The K+ concentration decreases in the left(AB)
electrolyte and increases by the same amount in the right
(CD)electrolyte. Reprinted by permission from Macmillan Publisher
Ltd.:Nature Materials (ref 58), copyright (2007).
Having a combination ofion-conducting and
electron/hole-conducting regions inpolymer acid templated
conducting polymers can alsoenable easy fabrication of
electronically controlled drugdelivery devices.
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The decrease in ionic conductivity upon EG addition is
attributedto a loss in free volume with increased connectivity of
PEDOT-rich domains.37 This postdeposition processing thus impartsan
ability to tailor the relative extent of ionic and electronic
conductivities per application needs. When building OECTsbased
on conducting polymers in which high ionic conduc-tivity is needed,
for example, the anticorrelation betweenthe electronic and ionic
conductivities in PEDOT:PSS with EGaddition must be kept in mind in
optimizing the device per-formance.
Ef fect of Relative Humidity on the Electrical Properties
ofConducting Polymer/Polymer Acid Complexes. Polymer acids
arehygroscopic; one disadvantage of using polymer acids to
dopeand/or charge balance conducting polymers is how
waterabsorption impacts macroscopic properties. Humidity
affectsionic conductivity and the extent of solvation of the
polymeracid and can also change the morphology of the
conductingpolymer due to plasticization of the polymer acid
template.Figure 4b shows changes in the electronic and ionic
con-ductivities of PEDOT:PSS thin films with humidity.24 As
rela-tive humidity increases, ionic conductivity increases
becausewater facilitates ion transport.60 Interestingly, the
electronicconductivity decreases with humidity up to 40%
relativehumidity and then recovers and even exceeds the
electronic
Figure 4. (a) Change in the electronic conductivity (blue) and
K+ ionmobility (red) in PEDOT:PSS thin films with the addition of
EG tothe PEDOT:PSS dispersion prior to deposition. Reprinted
withpermission from ref 37. (b) Conductivity of PEDOT:PSS thin
filmsmeasured by impedance spectroscopy (blue) as a function of
%relative humidity. Electronic (black) and ionic (red)
conductivities arecalculated from the impedance spectra by
equivalent circuit modelfittings. Reprinted with permission from
ref 24.
The change in the piezoresistivepolarity of PEDOT:PSS thin
filmsupon drying indicates that waterabsorption induced
morphologi-cal changes not only impact thestatic electrical
properties of
PEDOT:PSS but also affect howthe conducting PEDOT domains
rearrange under dynamicalmechanical deformation.
Figure 5. Change in the relative resistance (black) of as-cast
PEDOT:PSS films under cyclic tensile strain (blue) (a) in air and
(b) afterencapsulation in a N2-filled glovebox. Change in the
relative resistance (black) of as-cast PANI−PAAMPSA-724 films under
cyclic tensile strain(blue) (c) in air and (d) after encapsulation
in a N2-filled glovebox.
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conductivity of the dry film as the relative humidity
continuesto increase. This nonmonotonic change in the electronic
con-ductivity with relative humidity suggests that the presence
ofwater has competing effects on the structure of PEDOT:PSSthin
films. Water uptake by PSS is manifested as an increasein the
PEDOT:PSS film thickness by more than 10% withina few minutes of
exposing dry PEDOT:PSS films to 70%relative humidity.61 The initial
decrease in electronic con-ductivity up to 40% relative humidity is
attributed to swell-ing of the electrically insulating PSS-rich
shell and PSS matrix,which in turn increases the separation between
PEDOT-rich conducting domains. As the relative humidity
furtherincreases from 40%, increased solvation of the
counterionsreduces the electrostatic interactions between PEDOT
andPSS, and results in an increase in the hole
mobility.24,44,62
This increase in hole mobility counterbalances the decreasein
the conductivity due to increased separation betweenconducting
domains, and the electronic conductivity recoversand exceeds what
is reported for the dried PEDOT:PSSfilms.24
Inspired by this work showing how morphological changesinduced
by water uptake of the polymer acid affect the elec-tronic
conductivity and knowing that the piezoresistiveresponse of
conducting polymers is also highly sensitive totheir thin-film
morphology, we conducted experiments tostudy the effect of humidity
on the piezoresistive response ofPEDOT:PSS (Clevios PH 1000,
Heraeus) thin films. Figure 5ashows the change in the electrical
resistance of as-castPEDOT:PSS (black) under cyclic applied strain
(blue) in air.The resistance increases with increasing tensile
strain andrecovers when the strain is removed (positive gauge
factor).When the film is kept under vacuum for a few hours andthen
encapsulated in a N2-filled glovebox to eliminate fur-ther
absorption of water, we instead observe a negativegauge factor. The
resistance of the encapsulated-PEDOT:PSSfilm decreases with
increasing tensile strain (Figure 5b).This change in the
piezoresistive polarity of PEDOT:PSS thinfilms upon drying
indicates that water absorption inducedmorphological changes not
only impact the static electricalproperties of PEDOT:PSS but also
affect how the conduct-ing PEDOT domains rearrange under dynamical
mechanicaldeformation.The positive gauge factor observed in
PEDOT:PSS in air is
correlated with the separation between conducting domainswith
stretching. When PEDOT:PSS films are kept in air,water absorption
causes swelling of PSS-rich domains andthis phenomenon in turn
disrupts the connectivity betweenPEDOT-rich conducting domains. Due
to a lack of connectivitybetween individual conducting domains,
PEDOT:PSS particlesbehave like separate entities.63 Upon
stretching, they can slideby each other, and the conducting domains
become furtherseparated, which results in an increase in resistance
with tensiledeformation. When water is removed from the
PEDOT:PSSfilms, on the other hand, PEDOT:PSS particles come
closer,and PEDOT chains from one particle can interact withthe
neighboring PEDOT-rich domains.63,64 The interactionsbetween
neighboring PEDOT:PSS particles at low relativehumidity have been
reported to be very strong, so thateven upon fracture, the crack
propagates through particlesas opposed to along interparticle
boundaries.63 These stronginteractions connect conducting domains
so they no longerbehave as individual particles. Although the exact
mechanism ofthe reversible decrease in the resistance of the
encapsulated
PEDOT:PSS films with tensile strain is unknown, stretchingmust
cause rearrangement or alignment of this already-con-nected
conducting PEDOT domains in a way to promotecharge transport.
As a control experiment, we also tested the effect of humidityon
the piezoresistive response of PANI−PAAMPSA. UnlikePEDOT:PSS,
conducting PANI is preferentially located on theexterior of
PANI−PAAMPSA particles.3 We therefore hypo-thesized that water
uptake from air is not the reason for thepositive gauge factor
observed in high molecular weight PANI−PAAMPSA samples (shown in
Figure 1b).26 Swelling of thepolymer acid should instead bring the
conducting domainscloser, as opposed to separating them. Electrical
conductivitymeasurements support our hypothesis. Unlike
PEDOT:PSSthin films, PANI−PAAMPSA films show increased
electricalconductivity in air in comparison to films tested under
vacuumdue to increased ionic conductivity and increased solvation
ofthe polymer acid with increased humidity.3 In PEDOT:PSSthin
films, on the other hand, water uptake from air disrupts
theconduction pathway and results in a decrease in the elec-trical
conductivity.24 In order to understand the effect ofhumidity on the
piezoresistive response of PANI−PAAMPSA,we performed cyclic strain
tests on PANI synthesized on724 kg/mol PAAMPSA, PANI−PAAMPSA-724,
because itforms the largest particles and the least crystalline
films(Figure 1a).27 In order to understand if this positive
gaugefactor is a result of water uptake, we compared the
piezo-resistive response of PANI−PAAMPSA-724 films tested inair and
after keeping the same film under vacuum andencapsulating in a
N2-filled glovebox. Figure 5c,d shows therelative change in the
resistance of PANI−PAAMPSA-724films with applied cyclic strain in
air and after encapsula-tion, respectively. In both films, the
resistance increases withapplied tensile strain and recovers when
the strain is removed.We do not observe a change in the polarity of
the piezo-resistive response of PANI−PAAMPSA-724 upon drying
thefilms. These experiments show that PANI−PAAMPSA is morerobust in
its electrical properties in response to humidity com-pared to
PEDOT:PSS. We believe this robustness is cor-related with the
morphology of PANI−PAAMPSA with theconducting PANI on the exterior
of the hygroscopic polymeracid. These findings can also implicate
the possibility of usingprocessing routes, as opposed to using
different conductingpolymer complexes, that determine the
distribution of the con-ducting polymer and the polymer acid
dopant/counterion in
The choice of the polymer acidand/or manipulation of the
inter-molecular interactions betweenpolymer acid, conducting
poly-mer, and solvent through pro-cessing can significantly
impactoptoelectronic properties, relativecontributions of ionic and
elec-tronic conductivity of conducting
polymer complexes, and thesensitivity of the electrical
prop-
erties to varying humidity.
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thin films to control how humidity affects the morphology
andconsequently the electrical properties of conducting
polymer/polymer acid complexes.Beyond a template, a dopant, and/or
a charge balancing
agent, polymer acids are active components that bring
addi-tional functionalities to conducting polymer complexes.
Themacroscopic properties of these conducting polymer complexesare
not only determined by the conducting polymer itselfbut also depend
on the molecular properties of the polymeracid. The choice of the
polymer acid and/or manipulation ofthe intermolecular interactions
between polymer acid, con-ducting polymer, and solvent through
processing can sig-nificantly impact optoelectronic properties,
relative contribu-tions of ionic and electronic conductivity of
conducting poly-mer complexes, and the sensitivity of the
electrical prop-erties to varying humidity. With this understanding
of therole of polymer acids, we can begin to think about
tailoringmacroscopic properties by properly designing
conductingpolymer/polymer acid complexes.
■ AUTHOR INFORMATIONCorresponding Authors*E-mail:
[email protected] (M.S.-E.).*E-mail: [email protected]
(Y.-L.L.).ORCIDMelda Sezen-Edmonds: 0000-0003-0476-6815Yueh-Lin
Loo: 0000-0002-4284-0847NotesThe authors declare no competing
financial interest.
■ ACKNOWLEDGMENTSThe authors are grateful for funding from the
Princeton Centerfor Complex Materials that is supported by
NSF-MRSEC underNSF Award DMR-1420541.
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