RESEARCH ARTICLE
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High-Hole-Mobility Fiber Organic Electrochemical
Transistors for Next-Generation Adaptive Neuromorphic
Bio-Hybrid Technologies
Paula Alarcon-Espejo, Ruben Sarabia-Riquelme, Giovanni Maria Matrone,
Maryam Shahi, Siamak Mahmoudi, Gehan S. Rupasinghe, Vianna N. Le,
Antonio M. Mantica, Dali Qian, T. John Balk, Jonathan Rivnay, Matthew Weisenberger,
and Alexandra F. Paterson*
The latest developments in fiber design and materials science are paving the
way for fibers to evolve from parts in passive components to functional parts
in active fabrics. Designing conformable, organic electrochemical transistor
(OECT) structures using poly(3,4-ethylenedioxythiophene):polystyrene
sulfonate (PEDOT:PSS) fibers has excellent potential for low-cost wearable
bioelectronics, bio-hybrid devices, and adaptive neuromorphic technologies.
However, to achieve high-performance, stable devices from PEDOT:PSS
fibers, approaches are required to form electrodes on fibers with small
diameters and poor wettability, that leads to irregular coatings. Additionally,
PEDOT:PSS-fiber fabrication needs to move away from small batch processing
to roll-to-roll or continuous processing. Here, it is shown that synergistic
effects from a superior electrode/organic interface, and exceptional fiber
alignment from continuous processing, enable PEDOT:PSS fiber-OECTs with
stable contacts, high μC* product (1570.5 F cm−1 V−1 s−1 ), and high hole
mobility over 45 cm2 V−1 s−1 . Fiber-electrochemical neuromorphic organic
devices (fiber-ENODes) are developed to demonstrate that the high mobility
fibers are promising building blocks for future bio-hybrid technologies. The
fiber-ENODes demonstrate synaptic weight update in response to dopamine,
as well as a form factor closely matching the neuronal axon terminal.
P. Alarcon-Espejo, R. Sarabia-Riquelme, M. Shahi, S. Mahmoudi,
G. S. Rupasinghe, V. N. Le, D. Qian, M. Weisenberger, A. F. Paterson
Department of Chemical and Materials Engineering
Centre for Applied Energy Research
University of Kentucky
Lexington, KY 40506, USA
E-mail: alexandra.paterson@uky.edu
G. M. Matrone, J. Rivnay
Department of Biomedical Engineering
Northwestern University
Evanston, IL 60208, USA
A. M. Mantica, T. J. Balk
Department of Chemical and Materials Engineering
University of Kentucky
Lexington, KY 40506, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202305371
DOI: 10.1002/adma.202305371
Adv. Mater. 2023, 2305371
1. Introduction
Fibers are intricately woven throughout
the fabric of human history. The latest
developments in fiber design and materials science are paving the way for fibers
to evolve from parts in passive goods to
highly complex, functional components
in active fabrics. Conductive materials,
in particular, are being explored for a
wide range of fiber-based applications,
including mechanical sensors to detect
audible sounds or transpose mechanical vibrations into electrical signals,[1,2]
power systems,[3–6] electronic devices,[7,8]
machine-learning interfaces combining
multidimensional sensing,[9–11] storage,
communication, logic, and processing.[12]
Fibers are uniquely equipped to preserve
their mechanical properties under different
types of deformation and stimuli, enabling
technologies that function even while being
stretched, bent, and twisted in 3D.[13] Additionally, they can be sewn together to form
fabrics and create customized electronic
textiles that are light weight, flexible,
breathable, and comfortable.[14–16] One area that would benefit
greatly from these unique properties is wearable biotechnologies:
the human body contains a wealth of information, which computers integrated within clothing could access, enabling machine
learning and fabric artificial intelligence to process, analyze, and
return digital health insights.[17]
However, Loke et al. identified in their “Moore’s law in
fibers”,[17] that realizing such sophisticated, wearable bioelectronic platforms requires conductive fibers that can also operate as neurons and/or transistors.[17] Although various materials, such as carbon nanotubes, graphene, metallic nanoparticles, and nanowires,[18] are being explored as fibers, organic electronic materials are competetive candidtates for
mass-market, affordable wearable bioelectronics. One such
material poly(3,4ethylenedioxythiophene):polystyrene sulfonate
(PEDOT:PSS), is a commercially available conjugated polymer
with good ambient stability, high electrical conductivity, and the
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Figure 1. PEDOT:PSS fiber-OECTs. a) Scanning electron microscopy image of a single, continuously spun PEDOT:PSS fiber with a diameter of 7.12 μm.
b) Schematic of the PEDOT:PSS fiber-based OECTs. c) OECT transfer curves where VD = −1 V and d) transconductance for the various contact materials
explored with the fibers. e) Mobilities measured for the PEDOT:PSS fiber-based OECTs with different contact materials. Statistics for graphite and silver
were taken from size OECTs each, whereas μ for only one OECT is shown for Sn63:Pb37, because the contact interface was so poor that it did not produce
any additional working devices.
flexibility that is compatible with textiles.[19,20] In addition to
promising conductive fibers,[8,20–22] PEDOT:PSS has been used
in developing artificial synapses and synaptic circuits for organic neuromorphic electronics,[23–25] and is also the most widely
studied material for organic bioelectronics[26–28] and organic
electrochemical transistors (OECTs).[19] The latter OECTs have
been identified as fundamental building blocks for advanced
electronics, body–machine interfaces, drug delivery, smart textiles, bioelectronics, and adaptive healthcare.[19,29–33] Indeed, PEDOT:PSS fiber-OECTs have already demonstrated their potential
for biosensors/biomonitoring devices,[34] and Kim et al. reported
PEDOT:PSS fiber-OECTs with high μC* product values (≈1500 F
cm−1 V−1 s−1 ) and high hole mobilities (12.9 cm2 V−1 s−1 ).[8] Employing PEDOT:PSS fibers to design conformable OECT structures may therefore offer a highly promising option for future,
wearable bio-hybrid devices, at affordable costs.[7]
Before scalable, bio-hybrid devices made from PEDOT:PSS
fiber-OECTs can be realized, there are key challenges to overcome. First, in the context of wearable technologies, ≈4 km of
thread/fiber is required to make a single T-shirt (Supporting Information). It is therefore important to move away from small
batch processing (5–10 cm length for each fiber[8] ), to roll-toroll or continuous processing. Second, forming good contacts
is crucial to achieve high-performance, stable OECTs,[35–37] yet
creating good electrical contacts on fibers is challenging because their small radius of curvature leads to wettability issues
and irregular coatings.[38–40] Here, we address these core challenges by combining continuous PEDOT:PSS fiber-processing—
that can align hundreds of meters of PEDOT:PSS fiber per run—
Adv. Mater. 2023, 2305371
with contact engineering. We demonstrate that the synergistic
effects from exceptional fiber alignment and the superior contact/organic interface enable PEDOT:PSS fiber-OECTs with high
𝜇C* product (1570.5 F cm−1 V−1 s−1 ) and high hole mobility over
45 cm2 V−1 s−1 —the highest charge carrier mobility measured in
an organic transistor to-date.[41–43] Finally, we demonstrate that
the high mobility fibers are promising building blocks for future
bio-hybrid integration, by designing electrochemical transistors
with neurotransmitter-mediated plasticity, and a form factor that
closely matches the neuronal axon terminal.
2. Results
We span the PEDOT:PSS fibers as the active channel for an
OECT (Figure 1a,b) using a previously reported continuous wetspinning process, that can continuously collect hundreds of meters of fibers per run (Figure S1, Supporting Information).[22] In
short, a 2.5 wt% dispersion of PEDOT:PSS in water was extruded
through a single-hole capillary spinneret into a coagulation bath
containing 10 vol% dimethyl sulfoxide (DMSO) in isopropanol
(IPA). The coagulated filament was then driven by rollers into a
pure DMSO drawing bath followed by an in-line drying step with
an air temperature of 170 °C. The fibers were stretched throughout the entire duration of the fabrication process, with a total
draw ratio of 1.82, i.e., 1.82 m of fiber collected per each 1 m
of filament formed in the coagulation bath.[22] Next, we identified materials to use as the contacts/electrodes for the OECTs.
Contact resistance (RC ) in transistors is well-documented; the
Schottky barrier formed from the difference between the
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electrode work function (WF) and channel transport inhibits the
injection and extraction of charge carriers, which reduces operating voltage, operating frequency, measured mobility, and prevents miniaturization.[44] However, the role of RC in OECTs is
convoluted and under debate: on the one hand, it is reportedly so
dominant that it causes the monotonic transconductance (gm )[45]
and Bernards and Malliaras capacitive model to break down.[46]
On the other hand, gm remains monotonic when four-point probe
measurements are used to avoid RC ,[47,48] and electrochemical
doping is reported to reduce Schottky barrier widths to produce
the lowest ever RC values measured in organic transistors.[35,49]
We therefore chose contact materials with a broad range of WF
values to further explore RC in OECTs, namely: graphite paint,
silver paint, a 63% tin and 37% lead alloy (Sn63:Pb37), and a
52% indium and 48% tin alloy (In52:Sn48). The measured WF
values (relative to the PEDOT:PSS highest occupied molecular
orbital (HOMO)) are shown in Figure S2a in the Supporting
Information. The silver and graphite WF values are found to
be similar, measured at −5.01 and −5.03 eV, respectively, and
higher than the In52:Sn48 WF, which measures at −4.91 eV,
and the Sn63:Pb37 WF which is −3.72 eV. We also note that the
value for PEDOT:PSS is around 5.0 eV.[50] Based on these values, it would therefore be expected that the silver and graphite
paints yield comparable device performance because they minimize RC equally, and that the silver and graphite paints surpass the performance of the alloys. We then used copper and
graphite foils/tapes to complete the fabrication process by mediating between the measurement probe and aforementioned
contact materials, as previously reported (Figure S2b, Supporting Information).[8,22] Further information on both fiber continuous processing and fiber-OECT fabrication are in the Experimental Section and Figures S3 and S4 in the Supporting
Information.
The transfer curves for representative fiber-OECTs fabricated
with the different contact materials are shown in Figure 1c and
Figure S5 in the Supporting Information. The peak transconductance (gm_peak ) for representative fiber-OECTs is 1162.0,
1651.7, and 2093.5 S cm−1 , for Sn63:Pb37, silver, and graphite
contacts, respectively (Figure 1d). The In52:Sn48 contacts did
not produce functioning fiber-OECTs. We note that gm was
normalized to account for channel geometry and enable comparison with other values with the literature,[51] following the
formula gmnormalized = gm AL ,[51] where L is the channel length and
A denotes the fiber cross-section. The average gm (gm_avg ) over six
fiber-OECTs per system is 1408.8 and 1843.9 S cm−1 for silver
and graphite contacts, respectively. Statistics for the Sn63:Pb37
contacts are not presented because only one device out of six
worked. In addition to showing high transconductance values,
the measured charge carrier mobility (μ) values are remarkable:
the time-of-flight or transient method outlined by Bernards
and Malliaras[52] was used to measure the hole 𝜇, giving 21.2,
31.1, and 45.1 cm2 V−1 s−1 , for Sn63:Pb37, silver, and graphite,
respectively (Figure S6, Supporting Information), with average
values of 23.0 and 32.6 cm2 V−1 s−1 for silver and graphite (Figure
S7, Supporting Information), respectively. Although 𝜇 extracted
using this transient technique is not influenced by the channel
cross-sectional area, it is essential to precisely determine L to
extract the hole transient time between the source and the drain
electrodes; see Figure S3 for further details.[52] Next, we explored
Adv. Mater. 2023, 2305371
the impact of the high μ on the independently derived OMIEC
figure-of-merit, the μC* product.[53] C* was found to be ≈34.8 F
cm−3 , measured using electrochemical impedance spectroscopy
(see the Experimental Section for more detail; Figure S8 in the
Supporting Information shows cyclic voltammetry for a representative PEDOT:PSS fiber-OECT), which is similar to the values
reported for PEDOT:PSS thin films (≈40 F cm−3 ).[54,55] The maximum μC* products for best-performing fiber-OECTs are 634.9,
1233.9, and 1570.5 F cm−1 V−1 s−1 , for the Sn63:Pb37, silver, and
graphite contacts, respectively. The corresponding, average values over six fiber-OECTs per system are 914.3 and 1134.6 F cm−1
V−1 s−1 , for the silver and graphite contacts, respectively. Figure
S16 compares the fibre-OECTs with 𝜇 values from a standard
thin-film OECT. Overall, at time of writing this is the state-of-theart μC* product, the highest PEDOT:PSS mobility, and the highest hole mobility measured in an organic transistor to-date for
the graphite contacts in the PEDOT:PSS fiber-OECTs.[41–43,56,57]
3. Discussion
Interestingly, as indicated later in this Section, the measured
mobility strongly depends on the contact material. Therefore,
we explored the role of the contacts to begin elucidating the
reasons for the extraordinarily high μ. The measured-μ has
a strong relationship with the contact/organic interface because efficient carrier injection/extraction is enabled between a
well-matched contact WF and organic transport level. The WF
values (Figure S2a, Supporting Information) and μ (Table 1)
indicate that Sn63:Pb37 has the largest energetic mismatch at
the interface, and the Sn63:Pb37 fiber-OECT performance and
metrics are lowest out of the systems. However, surprisingly,
the silver and graphite contacts have comparable energetic
interfaces (Figure S2a, Supporting Information), yet different
measured-μ (31.1 and 45.1 cm2 V−1 s−1 for silver and graphite,
respectively) and μC* products (1233.9 and 1570.5 F cm−1 V−1
s−1 for silver and graphite, respectively). Correspondingly, extracting resistance per length for the silver and graphite contacts
using two-probe measurements gives 108.8 and 103.8 Ω mm−1 ,
respectively. We note that at the time of writing we were unable
to extract reliable data using the transmission line method,
due to the variability (device uniformity) of the fiber-OECTs
across the various channel lengths.[59] A possible reason for
the lack of relationship between the WF-HOMO, measured-μ,
and two-probe resistance is that the quality of the fiber/contact
interface depends on the contact material fluid type, drying
dynamics, thermal expansion, and commonly used nonNewtonian suspensions can lead to irregular coatings on fibers
due to surface tension;[39,40] poor wettability, high curvature,
and surface contamination.[38,39] In particular, the silver paint
is in methyl isobutyl ketone and the graphite paint is in an
isopropanol-based solution, suggesting that there may be differences in wetting between the two contact paints and the
fiber channel surface. The best way to approach the wettability
of the unique fiber surface is to use a dynamic contact angle
measurement, such as the Wilhelmy plate method.[60,61] This
technique would take into account the fiber curvature that
will substantially influence the droplet spreading dynamics,
and therefore also impacts the accuracy of the measured contact angles.[38,39,62] However, in this case, our systems present
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Table 1. Table comparing published PEDOT:PSS mobility values with best-performing graphite contact mobility values.
OECT
Young’s modulus
[GPa]
Contact material
μmax [cm2 V−1 s−1 ]
Fiber
14.6
Graphite
Fiber
8.7
Silver
Thin film
N/A
Gold
Analysis technique
μC*
Reference
45.1
Transient
1570.5
This work
12.9
Impedance
matching
1500
[8]
11.7 (EgVDP)
Linear regime
195.6
[48]
gm
157.5
[58]
6.7 (OECT)
Thin film
N/A
Gold
4.5
practical challenges from rapid paint drying times (30 s) which
impact the advancing and receding contact angles, and the
different particles suspended in the inks and their interaction
with the fiber surface also impact the wettability phenomena
and accuracy of the measured contact angles.
We therefore explored the relationship between measuredμ and the quality of the mechanical contact/organic interface,
using scanning electron microscopy (SEM). Figure 2a,b and
Figure S9 in the Supporting Information show that there is a
strong correlation between interface quality and measured-μ; we
find that the In52:Sn48 does not coat the fibers, explaining the
nonfunctioning fiber-OECTs. Additionally, it is clear that the sil-
ver does not completely coat the fiber surface, resulting in a poor
mechanical interface (Figure 2a). In stark contrast, the graphite
completely covers the fiber (Figure 2b). We performed a pixel
count analysis on a representative fiber OECT made with silver
contacts, to better quantify the contact area, in relation to a representative graphite contact which covers 100% of the fiber (i.e.,
Figure 2b). Figure S10 in the Supporting Information shows the
pixel count, revealing that ≈31.1% of the fiber is exposed to the
silver. This can also be compared to Figure S9 in the Supporting Information, which shows that as little as 0% of the fiber
is coated by the Sn63:Pb37 and In52:Sn48 alloys. Figure S11 in
the Supporting Information shows the impact of the contact area
Figure 2. The role of contacts in fiber-OECTs. Scanning electron microscopy images showing the morphological aspects of the a) silver and b) graphite
contacts. The primary images show the electrode material coating the fiber. One inset shows how the surface of the PEDOT:PSS fiber is covered with the
silver or graphite at the middle point of the fiber–electrode contact, and the other inset shows the exit point of the fiber–electrode contact. c) Real-time
change in resistance, normalized to the minimum resistance, after applying the silver and the graphite paints/inks to the PEDOT:PSS fiber to fabricate
the contacts. Two probe resistance measurements showing the degradation of the d) silver and e) graphite contacts over time. f) Two probe resistance
measurements showing the lack of degradation in the PEDOT:PSS fiber over time, and demonstrating high electrical stability of the fibers on the spool
used in this study. For each measurement, the fiber was unwound from the same spool and measured using contacts deposited on the indicated day of
testing.
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on μ for best-performing graphite contacts (at 1.5, 3, and 7 mm
length) Overall, these findings indicate the importance of the contact/organic interface quality for achieving high measured-μ in
transistors.[41,59,63]
Considering that the organic/contact interface was recently
shown to play a dominant role in OECT stability,[37] we investigated the stability of the coating quality over short and long
timescales. We monitored the resistance (R) normalized to the
minimum measured resistance in a two-probe configuration
for silver and graphite contacts in the PEDOT:PSS fiber-OECTs.
Figure 2c shows the results: in both cases, there is a sharp decrease in R when the contact material is applied to the fiber, i.e.,
at t = 0. After the sharp reduction in R, that corresponds to the
application and drying of the paints/inks containing the contact
material, it is clear that the silver contacts start to degrade In
contrast, the graphite contacts are highly stable. We extended the
two-probe conductivity to longer, shelf-life stability studies on the
contacts (see Supporting Information). Again, the silver contacts
demonstrate poor stability leading to complete loss of contact
beyond the 30 days (Figure 2d), while none of the graphite
contacts failed. We note that the graphite contacts showed on
average a 40% increase in resistance after 20 days, with a further
3% increase after 2 months, indicating that an aging-related
equilibrium may have been reached (Figure 2e). We also measured PEDOT:PSS fiber conductivity at 3 month intervals for
6 months; Figure 2f shows the fiber electrical conductivity was
consistent and stable, giving an average value of 2170 ± 60 S
cm−1 , indicating that the observed degradation (Figure 2d) is
occurring at the contact itself. As there was no coverage change
over time for the silver contacts, one possible mechanism for the
degradation is the acidity of the PEDOT:PSS, which may have
corroded the fiber/contact interface via corrosion of common
contact metals like silver[64] or copper.[65] In other cases, this acid
corrosion has been reported to degrade and limit performance of
PEDOT:PSS devices,[66,67] which may suggest a key difference between the silver and graphite, and origin of the difference in the
resistance changes over time (Figure 2). Namely, the PEDOT:PSS
behaves as an acid and will readily accept electrons from contact
materials with a high tendency to donate electrons, such as
silver. If the silver donates electrons to the doped PEDOT:PSS, it
equivalently removes holes and therefore effectively de-dopes the
PEDOT:PSS at the contact/fiber interface region as a function of
time. This in turn oxidizes the silver and increases the temporal
resistance of the contact interface, where the application of current may act as a catalyst for the oxidation. On the other hand, for
graphite, which has little tendency to donate electrons, we would
observe a more stable contact/fiber interface. To investigate
further whether oxidation increasing over time is the origin of
the degradation at the silver/fiber contact interface, we used
X-ray photoelectron spectroscopy (XPS) to look for shifts in the
Ag5/2 peak to lower binding energy. However, the study indicated
that XPS is not sensitive enough to probe for silver oxidation
specifically at the silver/fiber interface. The measurement and
data analysis were further complicated by the presence of oxygen
in all samples (silver/fiber interface, silver paint only, the fiber
only), the fact XPS only probes 5–10 nm at the sample surface,
and the size of the probe relative to the fiber diameter meaning
that—although the probe is incident on the contacts—it is
not only investigating the interface between the silver and PE-
Adv. Mater. 2023, 2305371
DOT:PSS fibers. Although beyond the scope of the current paper,
going forward more sensitive techniques such as high-resolution
transmission electron microscopy and selected area electron
diffraction could be used to probe the interface between PEDOT
and both the silver and the graphite contacts. Finally, Figure S12
in the Supporting Information further demonstrates superior
mechanical resilience and electrical contact for the graphite
contacts compared to silver. The contacts were subjected to a
brief mechanical distress that clearly unveiled the mechanical
deficiencies at the silver/fiber interface, which are then recovered
by applying pressure. On the other hand, the graphite contacts
were unaffected, maintaining noise free, initial resistance values.
Overall, the data indicate that the contact/organic interface is the
source of instability rather than the fiber/channel itself.
Next, we explored the role of the fiber channel on the high
OECT mobility. Table 2 shows the hole μ measured in the
PEDOT:PSS fiber-OECTs is significantly higher than μ reported
for thin-film OECTs (6.7 cm2 V−1 s−1 ),[48] and higher than μ
reported for PEDOT:PSS thin films measured using a fourprobe technique (11.7 cm2 V−1 s−1 ).[48] Furthermore, μ is 2.5 ×
greater than previously reported for a PEDOT:PSS fiber OECT
(silver contacts: 12.9 cm2 V−1 s−1[8] compared to 31.1 cm2 V−1
s−1 measured here). A key difference between the previously
reported values and the fibers used in this work is the drawing
in the continuous processing. On the one hand, drawing in
continuous processing applies a permanent extensional deformation along the axis of the fiber,[68] which, in turn, induces
preferential orientation of the polymer chains by aligning them
with the fiber-axis direction. On the other hand, the previously
reported high mobility fibers were fabricated discontinuously,
i.e., without applied draw. In the discontinuous case, the polymer
chains were orientated by drying the sulfuric acid-treated fibers
and using hanging weights, limiting the amount of force applied
and therefore the level of polymer chain orientation achieved.[8]
Indeed, improved chain orientation in continuously processed
fibers is reported to give exceptional mechanical properties and
enhanced thermal conductivity, where thermal conductivity is
also considered a good indicator of alignment. For example, oriented polyethylene nanofibers have reported values up to 104 W
m−1 K−1 ,[69] and PEDOT:PSS-fibers with values 3–4 W m−1 K−1 ,
over one order of magnitude higher than typical PEDOT:PSS
thin films.[22] Additionally, increased electrical conductivity with
increasing draw has been reported for conducting polymer fibers
made from poly(3-alkylthiophene),[70,71] polyaniline,[72,73] and
PEDOT:PSS,[22] as a result of polymer chains aligned along the
fiber axis, and charge carrier transport happens fastest along the
backbone of the polymer chains in conjugated polymers.[74]
We therefore hypothesize that the drawing in continuous
processing is the reason for the higher μ compared to discontinuously processed fibers. To investigate this further, we first
measured the Young’s modulus of the continuously processed
PEDOT:PSS fibers, where the Young’s modulus is a key indicator
that can be used as a proxy to compare polymer chain alignment
and orientation in fibers made from the same polymer. Figure
S13 in the Supporting Information shows the Young’s modulus
of the continuously processed fibers is 14.2 GPa—almost twice
the value of the optimized (i.e., dried under the maximum
tensile stress) discontinuously processed fibers (8.7 GPa[8] ).[21,22]
We measured the electrical conductivity and break stress to be
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Table 2. Comparative data for maximum charge carrier mobilities for different contact materials in PEDOT:PSS-fiber OECTs. Silver and graphite contact
statistics are taken over N = 6 devices. Statistics for the Sn63:Pb37 contacts are not presented because only one device worked. Work function values
and interface quality are compared to demonstrate their collective impact on μ.
Material
μmax
μavg
Work Function [eV]
Interface quality
−4.9
Extremely poor
In52:Sn48
-
Sn63:Pb37
21.2
-
−3.7
Poor
Silver
31.1
23.0
−5.0
Poor to moderate
Graphite
45.1
32.6
−5.0
Excellent
2130 S cm−1 and 357 MPa, respectively. We note that changing
the solvent that the PEDOT:PSS is drawn from, i.e., changing
the DMSO to concentrated sulfuric acid, has been reported
to further enhance the Young’s modulus (22 GPa[21,22] ) and
therefore may offer an approach to further enhance the mobility
when combined with the optimized graphite contacts. However,
we also note that sulfuric acid may degrade the channel material
over time.[8]
Wide-angle X-ray scattering (WAXS) was then used to study
the alignment in the continuously drawn fibers (Figure 3a–c).
The 2D-WAXS spectrum of the fibers is shown in Figure 3a,
where the intense arcs observed along the horizontal line of the
spectrum are an indication of the anisotropy of the fibers. Since
the fibers were placed vertically and normal to the transmitted
incident X-ray beam, diffracted horizontal arcs are an indication
of preferential orientation of crystal planes in the fiber axis direction. Figure 3b shows the 2D-WAXS 2𝜃 integrated intensities for
the fibers. The first peak at 3.6° corresponds to the (100) lamella
stacking of PEDOT and PSS chains with a d-spacing of 24.2
Å.[75,76] The peak appearing at 26.3° is attributed to the (020) re-
flection of the 𝜋–𝜋 stacking of PEDOT chains. The 𝜋–𝜋 stacking
distance was 3.4 Å, in agreement with previous reports.[22,75,76]
Lastly, the peak observed at 17.8° corresponds to amorphous
PSS.[75,76] The azimuthal intensity profiles for the three reflections are shown in Figure 3c. For PSS, the azimuthal profile reflection is flat, indicating random orientation. However, the peaks
observed at 0° and 180° for the (100) and (020) reflections indicate
a preferential orientation of those planes parallel to the axial direction of the fiber, meaning that the PEDOT polymer backbones
are also oriented in that direction. Finally, we studied the surface
morphology of the fibers with SEM and atomic force microscopy
(AFM). Figure 3d,e shows longitudinal ridges and striations running parallel to the fiber axis along the length of the fibers, and
Figure 3f shows that these aligned fibrils are also visible in AFM
surface topography. We note that, based on previous reports, the
ridges and striations in the SEM images are characteristic of wetspun fibers that undergo a solvent diffusion-induced coagulation
process,[77,78] and are associated with polymer chain alignment
and orientation of polymer fibrils on the fiber surface in the fiber
axis direction. Additionally, AFM was used to analyze the surface
Figure 3. Continuous processing enables highly aligned PEDOT:PSS fibers. a) 2D WAXS pattern of the fibers. b) 2 theta scan integrated intensity with
the main peaks for PEDOT:PSS noted. c) Azimuthal intensity profiles of (100) lamella stacking, PSS, and (020) 𝜋–𝜋 stacking reflections. d,e) SEM image
showing the ridges and striations running along the length of the fibers indicative of fibril orientation. f) AFM topography image showing fibril orientation
in the fiber axis direction.
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Figure 4. An artificial bio-hybrid synapse with dopamine-dependent plasticity. a) Schematic showing the double PEDOT:PSS fiber-ENODe configuration.
b) Measurements of conductance modulation in the channel terminal while periodically pulsing VG . In the presence of dopamine, the conductance
modulation is permanent and depends directly on the concentration of the neurotransmitter in the electrolyte solution.
roughness of the PEDOT:PSS fiber, where a root mean square
(RMS) value was found to be 17.5 nm with a standard deviation
of 3.8 nm. This is considerably smaller than the particle size distribution of the graphite and the silver paints, as reported by the
manufacturer, which ranges from 0.4 to 1 μm. Overall, the collective Young’s modulus, WAXS, SEM, and AFM evidence that continuously processed PEDOT:PSS fibers are highly aligned and
orientated, which contributes to the high μ measured in the fiberOECTs, and may play a role in increasing the intrinsic mobility
of the PEDOT:PSS fiber.
Finally, active elements based on building blocks, such as these
high-performance fibers and their associated OECTs, could offer
unprecedented fiber-based sensors, circuits, and neuromorphic
components. Artificial bio-hybrid synapses are one emergent
bioelectronic platform that show synaptic conditioning from
neuronal physiological biochemical signaling activity. For example, electrochemical neuromorphic organic devices (ENODes)
have been reported to work as artificial synapses by emulating
neurotransmitter synaptic weight modulation and recycling
machinery at the synaptic cleft,[79] where the gate represents the
Adv. Mater. 2023, 2305371
pre-synapse and the channel acts as the post-synapse. Although
ENODes made from PEDOT:PSS exploit the reversible dedopingdoping (depletion mode) for conductance modulation,[80,81] the
oxidaton electro-active neurotransmitters can produce protons
and electrons that cause problematic, nonreversible decreases
in conductance. The conductance modulation is important in
bio-hybrid interfacing platforms because it represents the weight
of the artificial synapse, that is directly updated in response to
the biological element’s neurochemical signal activity (i.e., the
neuronal pre-synapse). Additionally, unavoidable molecular
crosstalk can occur in some cell-interfacing scenarios, e.g.,
Keene et al. translated chemical activity into post-synaptic weight
modulation[79] by seeding neuronal model cells (PC12) on a gate
electrode. However, the neuron-to-neuron signal transmission
was lost because the neuon ensembles expressed multiple
neurotransmitters. Overall, it is of interest to design neuromorphic platforms with the sensitivity and physical dimension
and to match biological pre-to-post synaptic terminals, so that
pre-to-post synaptic chemical activity can be combined with
neuromorphic processors for in-sensor computing.
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Inspired by the fact that the fibers approach the aspect ratio of neuron axons, we developed neuromorphic fiber-ENODes,
finding that the fiber-ENODes display synaptic weight update
(conductance modulation) in response to a key biological cue,
dopamine (DA) (Figure 4a).[82] The results are shown in Figure 4b
and Figures S14 and S15 in the Supporting Information. First, in
presence of NaCl electrolyte only, periodically pulsing 0.3 V for
0.85 s, followed by an 8 s delay, causes cations to penetrate and
decrease the conductance (conditioning). When the gate bias is
removed, the cations return to the electrolyte (extinction), and the
initial conductance level of the transistor channel is restored. On
the other hand, when DA is present, a permanent conductance
modulation is recorded, which we find increases with the number of pulses. Additionally, we demonstrate that this permanent
conductance modulation depends on the concentration of the
neurotransmitter in the electrolyte. For 0.025 × 10−3 and 0.05 ×
10−3 m DA solutions, conductance values of 2.5 × 10−2 and 4.9 ×
10−2 mS are recorded, respectively. Overall, the remarkably stable
conductance modulation emulates synaptic conditioning mechanisms from neural signaling activity on the pre-synaptic terminal, and the biological electrical-to-chemical signal transduction.
4. Conclusion
We have fabricated organic electrochemical transistors using PEDOT:PSS fibers as the active channel material, and measured a
hole mobility of 45.1 cm2 V−1 s−1 —the highest reported mobility measured in an organic transistor to date. Additionally, the
independently derived μC* product, 1570.5 F cm−1 V−1 s−1 . We
find that the underlying fiber-OECT performance arises from a
synergistic combination of the drawing step in continuously processing the fibers and optimizing the mechanical interface between the source/drain electrodes and the channel. On the one
hand, Young’s modulus measurements, X-ray scattering, and microscopy imaging show the former leads to highly aligned, polymer chains that are preferentially orientated along the fiber axis,
i.e., in the charge transport direction, increasing the intrinsic mobility. On the other hand, SEM, mechanical distress, and stability interface testing indicate that the latter, superior mechanical
interface improves the measured mobility, and we find that it
is equally or more important than the good electronic/energetic
match (Kelvin probe and two-probe resistance measurements).
Finally, we show that the fiber-OECTs could enable neuromorphic devices, such as artificial synapses, with reduced molecular
crosstalk effects. Overall, the collective evidence indicates synergistic effects from contact engineering and processing optimization produce organic electrochemical transistors that are compatible with both large area processing and neuromorphic computing, with high μC* product and high charge carrier mobility.
5. Experimental Section
PEDOT:PSS Fiber Fabrication: PEDOT:PSS fibers were fabricated
following a procedure previously described by Sarabia-Riquelme, et al.[22]
Aqueous PEDOT:PSS dispersion was purchased from Heraeus (PH1000;
solid content 1.3 wt%). The dispersion was concentrated to 2.5 wt% by
placing it on a hot plate at 90 °C while magnetically stirring. Then, 5 wt%
Adv. Mater. 2023, 2305371
of DMSO was added and the dope was stirred for 2 more hours at room
temperature. Lastly, the dope was sonicated for 30 min and degassed
in a vacuum oven for 5 min at room temperature. Once the dope was
finished, it was transferred to a 5 mL syringe and placed in a syringe pump
(KD Scientific). The dope was extruded at a constant rate of 0.25 mL h−1
through a nylon syringe filter with average pore size of 5 μm and a capillary
spinneret with 100 μm in diameter into a coagulation bath of 10 vol%
DMSO in IPA. After coagulation, fibers were drawn in a DMSO bath and
then dried passing through a keyhole-cylinder-shaped oven kept at 170
°C before being taken up on a spool. Both baths were kept at room temperature. The total draw ratio applied during processing was 1.82, which
corresponds to the ratio of velocities at which the fiber was being collected
on a spool and the velocity at which the dispersion was being extruded,
i.e., per each 1 m of nascent filament being extruded, 1.82 m of fiber was
being continuously collected. A spool of a single-filament PEDOT:PSS
fibers was characterized before implementing the fibers in devices.
PEDOT:PSS Fiber Mechanical Characterization: The mechanical properties of the fiber were extracted using an automatic single-fiber test system, FAVIMAT+ from Textechno. Pretension was 0.50 cN tex−1 , test speed
5.0 mm min−1 over a gauge length of 25.4 mm. Each value obtained was
the average for five fibers. Young’s modulus was then extracted from the
slope of the linear section of the stress–stress curve.
PEDOT:PSS Fiber Electrical and Contact Stability Characterization: The
fibers were characterized electrically by placing a single fiber between
two graphite foil electrodes and contacted using graphite paint. The resistance was then measured using a two-probe method with a Keithley
2100 microvoltmeter. By conducting resistance measurements on specimens of different lengths, the resistance of the fibers and the contact resistance could be extracted from the slope and intercept, respectively, by plotting the total resistance measured versus the length,
as shown in Figure 2d–f, where Rmeasured = Rspecimen + Rcontact . This
method was commonly used to determine the conductivity of the specL
. The cross-sectional area was thereimen, where Rspecimen = 𝜎 1
A
specimen
fore not required for the two-probe measurements to obtain values of total resistance per length fibers from the same spool and fabricated during the same run. However, to obtain the electrical conductivity of the
fibers, the cross-sectional area became necessary. To determine the crosssectional area, a bundle of fibers was cut from the spool and placed with
their cross-sections perpendicular to the electron beam in the SEM. The
diameter of ten fibers was measured to calculate an average, and to obtain
the electrical conductivity from the length, resistance, and cross-section of
the fiber.
Organic Electrochemical Transistor Fabrication and Characterization: Six
OECT devices per each pair of electrode–contact material were fabricated
using the continuously processed highly aligned PEDOT:PSS fibers as the
channel. Figure 1b shows a scheme of the PEDOT:PSS fiber-based OECTs
and Figure S4 in the Supporting Information shows a photograph of actual
devices using graphite foil–graphite paint as contacts. To fabricate the devices, the chosen electrode (copper tape or graphite foil) was first placed
on a 3D-printed support structure (see Figure 1b). Next, a PEDOT:PSS
fiber was placed on top of the electrodes and, then, the contact material (silver paint (Silver Conductive adhesive 503, Electron Microscopy
Sciences) or graphite paint (Graphite Conductive Adhesive 154, Electron
Microscopy Sciences)) was applied to make electrical contact between
the fiber and the electrodes. The fiber was left suspended above a well that
was then filled with the electrolyte solution in which the Ag/AgCl gate was
immersed. The curvature of the electrolyte solution at the top edges of
the well, formed due to surface tension, prevented the electrical contacts
from being wetted while ensuring an almost complete coverage of the
fiber (see the Experimental Section and Supporting Information for more
details). A Keysight B2912A Precision Source/Measure Unit was used to
obtain current–voltage characteristics. The devices were then placed on
a microprobe station. With two microprobes, contacts were made to the
electrodes, called source and drain. OECTs were measured with 0.1 m
NaCl electrolyte, where de-doping was caused by the application of a
positive gate bias at the gate, so that cations from the NaCl electrolyte
were injected into the channel terminal and compensated the anions on
PSS chains decreasing the number of holes and so the drain current.[55]
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OECTs were measured in depletion mode with a drain–source voltage
from 0 to −1.2 V with a gate voltage from −0.57 to 0.7 V in 0.212 V steps
for output characteristics, and a gate voltage from −0.57 to 0.7 V at −0.1 V
drain voltage for the linear region and at −1 V for the saturation region for
transfer characteristics. To obtain carrier mobilities, a gate pulse current
with a specific drain voltage (0.15 V) was applied, then the source–drain
current was measured. Extracting the slopes of each plot, called the
transient slopes of source–drain current, and plotting them versus the
gate current applied in each case, the electronic transient time could be
obtained from the slope of this plot which could be correlated directly
to mobility ( 𝜏 e = L2 /μVD ).[52] Finally, to measure μ using the transient
method in a control sample, thin-film PEDOT:PSS OECTs were prepared
using a PEDOT:PSS dispersion, by mixing Clevios PH 1000, 5 vol%
ethylene glycol (EG), 1 vol% 3-glycidoxypropyltrimethoxysilane, and 0.002
vol% 4-dodecylbenzenesulfonic acid.[55,80,83,84] OECTs were fabricated in
a cleanroom environment and patterned with photolithography.[85] The
PEDOT:PSS dispersion was then spin-coated onto OECT substrates at
500 RPM for 15 s, followed by 2500 rpm for 45 s. The coated substrates
were then baked at 140 °C for 45 min. The device dimensions for the
control OECT were 500 × 500 μm.
Electrochemical Impedance Spectroscopy and Cyclic Voltammetry: Electrochemical impedance spectroscopy was used to determine the PEDOT:PSS fiber capacitance. The impedance spectra were extracted from
each set of electrodes with the contact material having the fiber submerged
in 0.1 m NaCl aqueous electrolyte solution to function as the working electrode. An Autolab potentiostat was used to take the measurements in ambient conditions using a Pt wire as the counter electrode and an Ag/AgCl
pellet as the reference electrode, applying a sine wave of 10 mV at frequencies from 105 to 0.1 Hz, and a 0.32 V DC offset potential. The data
analysis was performed using Metrohm Autolab NOVA software. Capacitance was then normalized by the fiber volume to give volumetric capacitance (C*). An average C* was calculated for every contact system. Cyclic
voltammetry data were obtained at a scan rate of 0.05 V s−1 for three
cycles.
Scanning Electron Microscopy: Images were obtained using a Hitachi
S-4800 field emission scanning electron microscope at 10 keV accelerating voltage and 10 μA beam current. PEDOT:PSS fibers and contacts were
imaged. Gold sputtering was not required for any of the samples due to
their conductive nature. SEM was also used to obtain an average diameter
of the fiber. 10 diameters were measured at different points of the sample
and then they were averaged to obtain an average cross-section. Since the
fibers had a circular cross-section, the area was calculated by 𝜋4 diameter2 .
To image the cross-section, fibers were cut by a razor blade in liquid nitrogen to avoid deformation, then the bundle was positioned perpendicular
to the electron beam.
Energy-Dispersive Spectroscopy: Energy-dispersive spectroscopy (EDS)
spectra were acquired using a Thermo Scientific Quanta 250 microscope,
operating at an accelerating voltage of 20 kV for EDS analysis. Data from
the EDS measurements were collected and analyzed using Oxford Instruments’ Aztec software.
Wide-Angle X-ray Scattering: Experiments were performed in transmission mode using the Xenocs Xeuss 2.0 SAXS/WAXS system located at the
Electron Microscopy Center at the University of Kentucky. The system uses
a source of GeniX3D Cu ULD of 8 keV and a wavelength of 1.54189 Å. Several fibers were grouped together and placed on an aperture card. The sample was located in a vacuum chamber to obtain the 2D diffraction pattern.
The sample-to-detector distance was 108.070 mm and the exposure time
was 600 s. Data analysis was processed using the software Foxtrot from
Xenocs.
Atomic Force Microscopy: Topographical information was obtained using a Cypher S atomic force microscope operating in tapping mode. Igor
Pro was used for image processing. PEDOT:PSS fibers were fixed using
conductive carbon adhesive tab on a glass substrate.
Kelvin Probe Microscopy: The WF of the materials was quantified by
measuring contact potential difference (CPD) with an ambient Kelvin
probe system, manufactured by KP Technology, LTD. The WF of the Kelvin
probe tip was calibrated by making a CPD measurement on a sputtered
thin film of gold with largely Au (111) orientation, and has a WF of 5.16 eV
Adv. Mater. 2023, 2305371
according to the Materials Project.[86] The confidence of the WF measurements was within less than 0.01 eV.[87]
X-ray Photoelectron Spectroscop: Measurements were performed on
Thermo Scientific K-Alpha, which uses an aluminum mono-chromatic Xray source (1486.69 eV) and an electron flood gun for charge neutralization. XPS data were collected on three points of each sample, with X-ray
spot size of 30 μm. At each point, wide survey scans were performed at
a pass energy of 160 eV, and high-resolution scans were performed at a
pass energy of 20 eV. Thermo Scientific software “Avantage” was used to
transfer raw XPS data into excel format for further peak fittings.
ENODes and Artificial Synapse Measurements: For this series of experiments, a 3D-printed stage containing two fibers was used. With the use of
a Keithley, one of the two fibers was used as a gate terminal, instead of the
Ag/AgCl pellet, while the other was used as a channel in a transistor configuration. Periodic voltage pulses were applied (pulses of 0.85 s, delay 8 s,
0.3 V amplitude) and the drain current was recorded, then converted into
conductance. NaCl solutions with different concentrations of DA were prepared (0.025 × 10−3 and 0.05 × 10−3 m). For each experiment, the stage
well was filled with the target solution in order to completely cover the
fibers while avoiding contact wetting. Before each DA experiments, the
fibers were rinsed in a pristine NaCl solution at least three times.
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
The authors gratefully acknowledge the support by the National Science
Foundation under Cooperative Agreement no. 1849213. Parts of the work
were supported by Excet contract ID-08180074, Advanced Fiber Technologies, Inc. P.O. 22205UKY, and US Army Combat Capabilities Development
Command, Chemical Biological Center. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the
authors and do not necessarily reflect the views of the National Science
Foundation. G.M. and J.R. acknowledge the support of the Alfred P. Sloan
Foundation under award FG-2019-12046.
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Keywords
bio-hybrid technologies, contact engineering, hole mobility, organic electrochemical transistors, organic electronics
Received: June 5, 2023
Revised: September 29, 2023
Published online:
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