PEDOT:PSS: From conductive polymers to sensors

Abstract

HIGHLIGHTS
•
The morphology of PEDOT:PSS, including in the forms of aqueous dispersions, solid films, and hydrogels, is outlined, and the application potential of PEDOT:PSS hydrogels is described.
•
Fabrication techniques for PEDOT:PSS-based devices are introduced, including coating, printing, conventional lithography, and soft lithography.
•
The latest developments in four main categories of PEDOT:PSS-based physical sensors, for humidity, temperature, pressure, and strain, respectively, are introduced.
•
The development prospects for PEDOT:PSS, from materials to fabrication techniques to physical sensors, are outlined.

Full text

Nanotechnol. Precis. Eng. 4, 045004 (2021); https://doi.org/10.1063/10.0006866 4, 045004
© 2021 Author(s).
PEDOT:PSS: From conductive polymers to
sensors
Cite as: Nanotechnol. Precis. Eng. 4, 045004 (2021); https://doi.org/10.1063/10.0006866
Submitted: 15 May 2021 • Accepted: 28 June 2021 • Published Online: 22 November 2021
Xiaoshuang Zhang, Wentuo Yang, Hainan Zhang, et al.
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Nanotechnology and
Precision Engineering REVIEW scitation.org/journal/npe
PEDOT:PSS: From conductive polymers to sensors
Cite as: Nano. Prec. Eng. 4, 045004 (2021); doi: 10.1063/10.0006866
Submitted: 15 May 2021 •Accepted: 28 June 2021 •
Published Online: 22 November 2021
Xiaoshuang Zhang, Wentuo Yang, Hainan Zhang, Mengying Xie,a) and Xuexin Duana)
AFFILIATIONS
State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072, China
a)Authors to whom correspondence should be addressed: mengying_xie@tju.edu.cn and xduan@tju.edu.cn
ABSTRACT
PEDOT:PSS conductive polymers have received tremendous attention over the last two decades owing to their high conductivity, ease of
processing, and biocompatibility. As a flexible versatile material, PEDOT:PSS can be developed into various forms and has had a signifi-
cant impact on emerging sensing applications. This review covers the development of PEDOT:PSS from material to physical sensors. We
focus on the morphology of PEDOT:PSS in the forms of aqueous dispersions, solid films, and hydrogels. Manufacturing processes are
summarized, including coating, printing, and lithography, and there is particular emphasis on nanoimprinting lithography that enables
the production of PEDOT:PSS nanowires with superior sensing performance. Applications to various physical sensors, for humidity, tem-
perature, pressure, and strain, are demonstrated. Finally, we discuss the challenges and propose new directions for the development of
PEDOT:PSS.
©2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/10.0006866
KEYWORDS
PEDOT:PSS, Conductive polymer, Sensor, Physical sensor, Manufacturing process, Morphology
I. INTRODUCTION
Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)
(PEDOT:PSS) is one of the most promising conductive poly-
mers.1–4 During the last two decades, research on PEDOT:PSS
has increased dramatically.5–7 PEDOT:PSS polymers with various
compositions, doping, and chemical modifications have been
developed to meet different application requirements. PEDOT:PSS
has attracted much attention commercially, and its market con-
tinues to grow, with an estimated annual value of 50 billion
dollars.8The key advantages of PEDOT:PSS include adjustable
conductivity,9–13 good transparency to visible light,14–17 excel-
lent thermal stability,18 and a high level of biocompatibility,19
which makes it successfully applied in electrostatic coatings,
flexible electronics,20–22 bioelectronics,23,24 energy storage,14 and
tissue engineering.25–27 Since commercial PEDOT:PSS is usually
in the form of an aqueous dispersion,28 it is compatible with
many solution-based manufacturing processes, such as coating
techniques (e.g., dip-coating, drop-coating, spin-coating, and
spray-coating), printing techniques (e.g., inkjet printing and screen
printing), and lithography (e.g., soft lithography and nanoimprint
lithography).29–32
In recent years, there has been a dramatic increase in the
number of studies of PEDOT:PSS-based devices for sensing appli-
cations, including gas,33,34 biological,35,36 electrochemical,37–39 and
physical40–42 sensing. PEDOT:PSS biological and chemical sens-
ing have been reviewed in Refs. 1and 43. Liao et al.44 reviewed
the development of biological and chemical sensors for OECTs
using PEDOT:PSS as the channel. Gao et al.45 reviewed the lat-
est developments in the use of PEDOT:PSS and composites based
upon it in chemical sensors. However, there has been no review
summarizing the use of PEDOT:PSS in physical sensing applica-
tions, such as in humidity,46–48 temperature,29,49,50 pressure,51–53
and strain sensors.54–56 Therefore, this review focuses on physi-
cal sensors using PEDOT:PSS and on methods to improve the
sensing performance, especially with regard to the enhancement
of the sensing performance of PEDOT:PSS at the nanometer
scale.29,57,58
In this review, we discuss the latest developments concern-
ing PEDOT:PSS, from materials to sensors, covering basic mate-
rial properties, sensor fabrication processes, and sensing princi-
ples, focusing especially on the influence of micro/nanostructures
on sensor performance. First, the polymer structure of PEDOT:PSS
is discussed, including aqueous dispersions, solid films, and the
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emerging PEDOT:PSS hydrogels. Second, we introduce the man-
ufacturing process of PEDOT:PSS-based devices, such as coating,
printing, conventional lithography, and alternative lithography. The
advantages and disadvantages of these fabrication techniques in
terms of resolution, cost, and efficiency are compared. Finally, we
summarize the latest developments in PEDOT:PSS-based physical
sensors, including those for humidity, temperature, pressure, and
strain.
II. MORPHOLOGY OF PEDOT:PSS
A. PEDOT:PSS aqueous dispersions
The chemical structures of PEDOT and PSS are shown in
Fig. 1(a). PEDOT:PSS aqueous dispersions with different ratios of
PEDOT and PSS can be obtained through oxidative polymerization
of 3,4-ethylenedioxythiophene (EDOT) monomers in the presence
of a PSS matrix. Completely oxidized PEDOT chains under ideal
conditions possess one charge carrier per three monomers.59 Neg-
atively charged PSS serves as a counterion to balance the positive
charges of PEDOT, and, together with the PEDOT matrix, forms a
homogenous and stable aqueous dispersion.
PEDOT:PSS aqueous dispersion presents a typical tertiary
structure.60 As shown in Fig. 1(b), EDOT and styrene sulfonate
units represent the primary structure of the polymer. Relatively
short positively charged PEDOT chains attach electrostatically to
the longer negatively charged PSS chains, forming the secondary
structure. The polyionic complex is eventually dispersed in water
in the form of colloidal particles, with a hydrophilic PSS-rich shell
and a hydrophobic PEDOT-rich core (tertiary structure). As a ref-
erence, the particle size of PEDOT:PSS in aqueous solution ranges
from 20 to 200 nm for the commercially available Clevios P.61 The
hydrodynamic radius of colloidal particles in PEDOT:PSS aqueous
solution for Orgacon ICP 1050 is about 250 nm.62 The colloidal gel
particles repel each other owing to the electronegative PSS shell,
forming a uniform and stable water dispersion. The interactions
between the colloidal gel particles are dominated by electrostatic
forces.62
B. PEDOT:PSS solid films
Pristine PEDOT:PSS solid films in general have a multilayered
structure. Randomly distributed colloidal gel particles precipitate
out during the process of water evaporation, forming a pancake-like
structure [Fig. 2(a)].61,63 Each “pancake” possesses a hydrophobic,
conductive PEDOT-rich core and a hydrophilic, insulating PSS-rich
shell. As a reference, the average diameter of PEDOT:PSS “pancakes”
is 30–50 nm, with a 5–10 nm-thick PSS-rich shell (a 25 nm-thick
solid film made from unfiltered Clevios P). The cohesive strength
between “pancakes” is based on hydrogen bonding between sulfonic
acid groups61 and π-orbital stacking between PEDOT chains. This
pancake model is in accordance with the granular structure observed
under bright-field transmission electron microscopy (TEM), as
shown in Fig. 2(b). Secondary doping64,65 and post-treatment66,67
alter the microstructures of PEDOT:PSS solid films, enhancing their
conductivity. For example, counterion exchange between an ionic
liquid and PEDOT:PSS leads to rearrangement of PEDOT and PSS
chains. Interconnected PEDOT crystalline nanofibrils are observed
in ionic-liquid-doped PEDOT:PSS solid films under TEM, as shown
in Fig. 2(c). In addition, it has been shown that PEDOT:PSS-
produced film has excellent thermal stability, chemical stability, and
durability.68–70 For instance, it retains a stable high electrical con-
ductivity after being tested for 60 days in an environment of high
humidity at 65 ○C.71–73
C. PEDOT:PSS hydrogels
Conductive hydrogels can be used to compensate for
the mechanical mismatch between biomaterials and inorganic
materials. Their superior mechanical properties, together with
FIG. 1. (a) Chemical structures of PEDOT and PSS. (b) Hierarchical structure of PEDOT:PSS.
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FIG. 2. (a) Schematic structure of PEDOT:PSS solid films. (b) Bright-field TEM
image of pristine PEDOT:PSS solid film. (c) Bright-field TEM image of EMIM TCB
(1-ethyl-3-methylimidazolium tetracyanoborate)-doped PEDOT:PSS film.74
their stability in water, make conductive hydrogels one of the
best candidates for applications in neuronal interfaces and tissue
engineering.25–27 Compared with other conductive hydrogels,
PEDOT:PSS-based hydrogels are endowed with high conductivity
even at a low solid content (46 S/m at a solid content of 0.78%).75
PEDOT:PSS-based hydrogels with high water content normally
exhibit a porous 3D structure, as shown in Fig. 3. This porosity
of PEDOT:PSS hydrogels makes them compatible with a variety
of post-processing methods to tune their mechanical performance
or to introduce new capabilities. For example, polymer monomers
can be injected into the porous structure to form a double-network
hydrogel. In addition, PEDOT:PSS is biocompatible and nontoxic,
allowing it to act as a scaffold for cell proliferation and differentia-
tion.25,26 Besides, PEDOT:PSS has superb environmental stability in
acidic, alkaline, high-temperature, and other harsh environments.
PEDOT:PSS-based hydrogels are expected to have a wide range of
applications in the fields of tissue engineering, drug delivery, seal-
ing, wound dressing, interface engineering, environmental pollu-
tion control, sensors, actuators, bioelectronics, energy storage, and
catalysis.76–78
Currently, available PEDOT:PSS-based hydrogels described
in the literature include PEDOT:PSS hydrogels themselves,75,79,80
PEDOT:PSS polymer blend hydrogels made from combinations of
PEDOT:PSS and one or more other polymer components,81,82 and
PEDOT:PSS composite hydrogels83–85 consisting of PEDOT:PSS
and inorganic materials. PEDOT:PSS polymer blend hydrogels have
been applied successfully in tissue engineering and other domains.86
However, PEDOT:PSS hydrogels are still in their early stage, with
new fabrication techniques being developed and new applications
emerging.87,88
FIG. 3. Hydrogels based on PEDOT:PSS. (a) Cu2+ions promoting the gelation of PEDOT:PSS microgel particles. (b) Scanning electron micrograph showing the porous
structure of a PEDOT:PSS hydrogel.78
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III. FABRICATION OF PEDOT:PSS-BASED DEVICES
A. Coating
As PEODT: PSS can be dispersed in aqueous solution, it is
compatible with many solution processes. Coating is a simple pro-
cess with low cost and high material utilization rate. It has been
widely used in research laboratories and industrial production.89–91
At present four coating processes are available, namely, dip coat-
ing,29,92 drop coating,93,94 spray coating,95,96 and spin coating,97,98 as
shown in Fig. 4. Dip coating can be used for large-area film forma-
tion on a variety of substrates, but is limited by the viscosity of the
material solution.99,100 Ding et al.101 used dip coating to make con-
ductive sponges by immersing melamine sponge in an aqueous dis-
persion of PEDOT:PSS so that the latter adhered to the micrometer-
scale framework of the sponge. As a much simpler process, drop
coating needs only a one-step operation. For instance, a transpar-
ent PEDOT:PSS electrode was fabricated by dropping PEDOT:PSS
solution directly on the surface of a polyethylene terephthalate sub-
strate without special equipment.102 However, owing to the influ-
ence of solvent evaporation, the thickness and size of the films
formed by drop coating are difficult to control.103,104 Spray coat-
ing has also been used.105,106 Kumar et al.107 sprayed a suspen-
sion of PEDOT:PSS/reduced graphene oxide (rGO) onto a carbon
cloth using a single-step spray deposition technique and thereby
fabricated large-area flexible electrodes. However, spraying has
higher instrumentation requirements and relatively high costs.108–110
Compared with the above coating methods, spin coating can pro-
duce uniform PEDOT:PSS films with good repeatability.111,112 A
simple strategy of cryo-controlled quasi-congealing spin coating was
developed by Chen et al.113 to prepare flat and smooth PEDOT:PSS
films with high conductivity and moisture resistance. However, this
approach requires a flat substrate and leads to some wastage of
materials.114,115
B. Printing
Printing is a straightforward way to deposit materials on a
wide variety of substrates. It has the advantages of high efficiency,
low cost, and less material waste.27,30,116 Current printing pro-
cesses include screen printing,117,118 inkjet printing,119,120 and 3D
printing,121,122 as shown in Fig. 5. Screen printing is a mature
process and suitable for various substrates.123,124 A mixed solu-
tion of PEDOT:PSS and poly(dimethylsiloxane-b-ethylene oxide)
was screen-printed on knitted cotton to prepare conductive tex-
tiles.125 However, the thickness of the formed film is large, and
this method is not suitable for solutions with low viscosity and
high volatility.126–128 Lo et al.129 used inkjet printing to prepare
a PEDOT:PSS gas-sensitive film with FeCl3additive on a poly-
imide film. It has been shown that inkjet printing is a simple
and low-cost method to prepare thin films with controllable thick-
ness.130,131 However, the charged ink droplets will affect the reso-
lution and performance of the device.132–134 Recently, 3D printing
FIG. 4. Four types of coating processes. (a) Dip coating.101 (b) Drop coating.102 (c) Spray coating.107 (d) Spin coating.113
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FIG. 5. Three types of printing processes. (a) Screen printing.125 (b) Inkjet printing.129 (c) 3D printing.138
has emerged as a technique to rapidly print arbitrary PEDOT struc-
tures, benefitting from its freedom with regard to design, its abil-
ity to print complex structures, and its compatibility with many
materials.135–137 One typical application is that by Lei et al.,138
who used 3D printing technology with hybrid electrohydrody-
namics to construct multiscale conductive scaffolds for cardiac
tissue engineering. The scaffolds were composed of micrometer-
scale fibers of polycaprolactone and sub-micrometer-scale conduc-
tive fibers of PEDOT:PSS/polyethylene oxide. However, with 3D
printing, it is difficult to print high-precision complex curved sur-
faces,139 and the anisotropy of the product will affect its mechanical
properties.140,141
C. Conventional lithography
Conventional top-down lithography is a common technique for
making micro- or nanoscale structures on flat substrates.142,143 For
example, photolithography is a key technology in the manufacture
of integrated circuits and microchips, and it has made a revolution-
ary contribution to the semiconductor industry.31,144 Figure 6 shows
three commonly used conventional lithography techniques, includ-
ing ordinary lithography,145,146 ion beam lithography,147,148 and
electron beam lithography.149,150 Zhu et al.46 proposed a one-step
photopatterning method based on photolithography, using ultravi-
olet rays to transfer microstructures from a polydimethylsiloxane
(PDMS) photomask to a PEDOT:PSS/polyethylene glycol hybrid
film, thereby producing a microstructured thin film with a resolution
of 2 μm. In ion beam lithography, the collision of high-momentum
ions [e.g., gallium ions or hydrogen ions (protons)] with the resist
layer leads to the removal of the resist particles.151,152 A PEDOT:PSS-
coated silica single-mode fiber was etched into a patterned phase
photonic sieve by focused ion beam lithography to improve the
optical coupling between the fiber and a silicon photonic wave-
guide.153 Their high momentum means that the ions propagate in a
straight line, resulting in high-resolution etching.154,155 However, it
also damages the substrate and reduces the device yield.156,157 Elec-
tron beam lithography has extremely high resolution (below 10 nm)
owing to the relatively small wavelength of the electron beam.158
Santoro et al.159 used electron beam lithography to fabricate quartz
nanostructures, and then spin-coated a composite film containing
PEDOT:PSS to make a cell–material interface with nanometer reso-
lution. However, electron beam lithography is time-consuming160,161
and the process is complicated, which increases its cost, and thus it
is only suitable for a small-scale production involving high-precision
processing.162,163
D. Alternative lithography
Alternative lithographic techniques have been developed that
have advantages of low cost, convenience, and high throughput, and
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FIG. 6. Three types of conventional lithography. (a) Ordinary lithography.46 (b) Ion beam lithography.164 (c) Electron beam lithography.165
that can replicate micro-to nanostructures on a large scale.166 After
decades of development, alternative lithography has become a viable
commercial process and is expected to overcome the limitations of
conventional lithography.167,168
Soft lithography is a typical alternative lithographic technique
that uses elastomer impressions for printing, molding, transferring,
and embossing, relying on conformal contact between the elas-
tomer and the target substrate [Fig. 7(a)].169 Soft lithography has
FIG. 7. Alternative lithographic techniques. (a) Soft lithography.184 (b) NIL.185 (c) Hybrid nanoscale printing approach combining soft lithography and nanoimprinting.29
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been widely applied to the fabrication of microfluidics, microelec-
tromechanical systems, and flexible electronics/photonics. However,
owing to the mechanical instability of the elastomers, such tech-
niques are usually limited to patterning micrometer-scale struc-
tures.170,171
Nanoimprint lithography (NIL) is another commonly used
alternative lithographic technique and is able to achieve high-
resolution nanoscale patterning.172,173 Typically, it can be classi-
fied into thermal and ultraviolet (UV) imprinting, as shown in
Fig. 7(b).174,175 Hot embossing was the earliest NIL technique, and
requires just a simple one-step heating and pressing process to trans-
fer a pattern from a hard mold to a thermoplastic.32,176 During the
process, the structure on the mold directly contacts the substrate
in a molten state, and the “opposite” structure is replicated on the
substrate.177,178
Although NIL can pattern very small structures (down to
5 nm), the high-temperature or UV processes cannot fulfill the
requirements of PEDOT patterning. To solve this problem, we
developed a hybrid nanoscale printing approach by combining the
advantages of soft lithography and nanoimprinting. Thermal NIL
was used to pattern nanochannels on PDMS and then transfer
the nanostructures to the substrate through capillary action.179–181
As shown in Fig. 7(c), the soft flat PDMS was coated with
Mr-I T85 imprint resist, and was then heated and pressurized to
replicate the nanostructures from a rigid silicon mold. Next, the
PDMS was combined with a flat substrate to form nanochannels.
The material solution was sucked into the nanochannel under the
action of capillary force. Finally, the imprint resist was removed
after the solvent had evaporated.182,183 This approach has been
successfully applied to patterning PEDOT:PSS nanowires on both
hard and flexible substrates.29,57 This hybrid approach can be
performed at room temperature without a cleanroom require-
ment, greatly improves process efficiency and safety, and can pro-
duce nanopatterns on a larger scale with high speed and high
yield.
IV. SENSORS BASED ON PEDOT:PSS
PEDOT:PSS in the form of an aqueous dispersion is compati-
ble with a variety of manufacturing processes, and this has promoted
its application in electrical devices. Figure 8 shows some electri-
cal devices based on PEDOT:PSS, namely, a polymer light-emitting
device (PLED), a supercapacitor, a perovskite solar cell (PSC), a ther-
moelectric device, and an electrochromic window. As a common
hole injection layer in PLEDs, PEDOT:PSS with a high work func-
tion is spin-coated between the emitter layer and the anode to reduce
the energy barrier and improve the surface roughness of the anode
[e.g., indium tin oxide (ITO)], thereby producing a more efficient
PLED.186,187 PEDOT:PSS is also used as an electrode in supercapac-
itors, since it can increase the specific capacitance compared with
carbon materials.188 Khasim et al.189 doped rGO into PEDOT:PSS to
increase its conductivity fourfold, thereby showing that secondary
doping can help improve the performance of PEDOT:PSS-based
supercapacitors. PEDOT:PSS is also used as the electrode in PSCs
to solve the problems of high cost and poor stability associated with
traditional electrodes (Au and Ag) and thus promote the commercial
FIG. 8. PEDOT:PSS-based electrical devices. (a) PLED.186 (b) Supercapacitor.189 (c) PSC.190 (d) Thermoelectric device.191 (e) Electrochromic window.192
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development of PSCs.190 In addition, PEDOT:PSS has proved to be
a promising organic thermoelectric material because of its enhanced
high electrical conductivity and low thermal conductivity. Through
doping and dedoping treatment, the thermoelectric quality factor
and response speed of PEDOT:PSS-based thermoelectric devices
can be enhanced.191 In electrochromic windows, PEDOT:PSS can
be used as a solid proton source for the electrochromic material
WO3. When the protons released by PEDOT:PSS under the action
of an electric field are inserted into the WO3lattice, transition
between the small polarization states associated with W ions of dif-
ferent valences results in optical absorption that causes coloration of
the WO3.192
As one of its most important applications, PEDOT:PSS has had
a profound impact as a sensor material, owing to its excellent and
adjustable electrical conductivity, hygroscopicity, and flexibility.1
There are many reviews of the use of PEDOT:PSS-based devices to
detect biological and chemical substances, but few have summarized
research on physical sensing.44,45 Therefore, this section focuses on
PEDOT:PSS-based physical sensors, including those for humidity,
temperature, pressure and strain.
A. Humidity sensors
PEDOT:PSS has been shown to be a hygroscopic material,193
making it promising for sensing humidity.194,195 The sensing
principle is dependent on water adsorption and desorption by
the PSS.196,197 In high-humidity condition, the insulating and
hydrophilic PSS shell absorbs water and swells, resulting in an
increase in the distance between adjacent conductive and hydropho-
bic PEDOT cores, which leads to an increase in the resistivity of
PEDOT:PSS for the resistive type of sensing device, as shown in
Fig. 9(a). By contrast, low humidity causes water to desorb out of
the PSS, reducing the volume of the PSS and the distance between
adjacent PEDOTs and thereby reducing the resistivity.57 So far,
PEDOT:PSS has been used to develop various types of humidity
sensors, including resistance,47,57,198–200 capacitance,46,201 and res-
onator48,195,196 types.
A resistive-type humidity sensor converts changes in humid-
ity into changes in the resistivity of PEDOT:PSS.41,57 Zhou et al.57
developed a PEDOT:PSS humidity sensor with high sensitivity
(5.46%) and ultrafast response (0.63 s) on a flexible substrate
using PEDOT:PSS nanowires as shown in Fig. 9(b). Owing to
their ultrahigh surface-to-volume ratio, the PEDOT:PSS nanowires
were readily exposed to the environment and further accelerated
the water absorption/release rate. Such a device has been success-
fully applied for human breath testing. In addition, PEDOT:PSS
has been used in combination with other moisture-sensitive mate-
rials to overcome the shortcomings of a relatively small dynamic
response range from a single material. Siddiqui et al.202 pro-
posed a humidity sensor with dual sensing elements composed of
PEDOT:PSS and MoS2, as shown in Fig. 9(c). MoS2and PEDOT:PSS
respond to humidity changes in the ranges 0%RH–40%RH and
40%RH–80%RH, respectively. By combining these two sensing
materials in series in the detection circuit, a very high sensitivity
(50 kΩ/%RH) and a wide response range (0%RH–80%RH) were
obtained.202 This strategy has been applied to make a humidity
sensor that can detect the full humidity range (0%RH–100%RH)
through a combination of three humidity-sensitive materials:
graphene oxide (GO), PEDOT:PSS, and C15H15N3O2(methyl
red).200
Capacitive humidity sensors have also been reported.
Figure 9(d) shows a typical humidity sensor combining PEDOT:PSS
film and interdigital capacitors.201 The sensor used PEDOT:PSS
film as the dielectric layer, which demonstrated good sensitivity
(3.4 pF) to humidity at 52.0%RH–93.4%RH. The sensing prin-
ciple is that humidity causes a change in the effective dielectric
constant of the PEDOT:PSS-based dielectric layer.201 Zhu et al.46
proposed a one-step photopatterning method to manufacture a
PEDOT:PSS/polyethylene glycol capacitive humidity sensor, with
PEDOT:PSS being used for the humidity electrodes to enhance
sensitivity.
A piezoelectric resonator coated with a PEDOT:PSS thin film
will respond to humidity changes owing to adsorption and desorp-
tion of water molecules by the film through a typical mass loading
effect.196 As shown in Fig. 9(e), a microwave resonator coupled with
PEDOT:PSS film was developed for humidity sensing.48 The trans-
mission coefficient and resonance frequency of the sensor changed
with the humidity and showed high repeatability in the humidity
range of 40%RH–60%RH.48 Julian et al.195 developed a quartz crys-
tal microbalance (QCM)-based humidity sensor in which the sens-
ing chips were coated with PEDOT:PSS mixed with polyvinyl alco-
hol (PVA) nanofibers. The sensor demonstrated excellent sensitivity
(up to 33.56 Hz/%RH), fast response/recovery times (5.6 s/3.5 s),
long-term stability, and low hysteresis (<1.3%).
B. Temperature sensors
PEDOT:PSS has been shown to be one of the most promis-
ing candidates for wearable temperature sensors,203,204 owing its
good flexibility and high thermal response.63,205 PEDOT:PSS itself
is very sensitive to temperature changes,206 since these result in
microstructural changes.207 Owing to the hygroscopicity of PSS,
temperature changes affect the water content and volume of the
insulating and hydrophilic PSS-rich shell.29,208 At room tempera-
ture, the hydrophilic PSS absorbs moisture from the air by form-
ing hydrogen bonds between the sulfonic acid groups of the PSS
chains and water molecules. The insulating PSS chains, together
with nonconductive water, form boundaries to prevent electrons
hopping between PEDOT:PSS nanoparticles. At high temperature,
water loss from PSS not only reduces the total number of particle
boundaries but also decreases the effective “size” of these bound-
aries. When the temperature decreases, the electrons may not have
enough energy to overcome these barriers caused by the parti-
cle boundaries, and thus the resistance increases,208 as shown in
Fig. 10(a).
In recent years, attempts have been made to improve the
thermal sensitivity of PEDOT:PSS-based devices.29,49,50 The most
commonly used techniques are secondary doping198,209 and ther-
mal expansion.49,50 It has been found that secondary doping of
PEDOT:PSS using materials with higher thermal conductivity
[e.g., GO,29 rGO,209 multiwalled carbon nanotubes (MWCNTs),198
and silver nanoparticles (AgNPs)210] is a relatively easy strategy.
Zhou et al.29 optimized the temperature-sensing performance of
a PEDOT:PSS nanodevice by doping the PEDOT:PSS with GO.
Nanoscale GO flakes filled the gaps between adjacent PEDOT:PSS
nanoparticles, serving as temperature-dependent conductive paths
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FIG. 9. Humidity sensors based on PEDOT:PSS. (a) Analytical diagram of the action of PEDOT:PSS in humidity sensing.57 (b) Resistive humidity-sensitive nanowires based
on PEDOT:PSS.57 (c) Resistive humidity sensor connected in series with PEDOT:PSS and MoS2.202 (d) Capacitive humidity sensor combining PEDOT:PSS film and an
interdigital capacitor.201 (e) Humidity sensor based on microwave resonator and PEDOT:PSS film.48
for electron hopping. The temperature coefficient of resistance
(TCR) of the PEDOT:PSS/GO nanowire reached a maximum of
−1.2%/○C when the mixing ratio of PEDOT:PSS and GO was
13:1. There is a threshold at which the GO flakes completely fill
the gaps between adjacent PEDOT:PSS nanoparticles. Excess GO
will then lead to a deterioration in the cohesiveness of adjacent
PEDOT:PSS nanoparticles, while insufficient GO will reduce the
path of electronic transition, thereby reducing electron mobility
and the sensitivity of the sensor,29 as shown in Fig. 10(b). A second
method is thermal expansion of the substrate, which can create
microcracks in the sensing layer to improve the temperature sensi-
tivity of the sensor. Yu et al.49 fabricated a PEDOT:PSS sensing layer
with microcracks on a PDMS substrate through pre-stretching and
sulfuric acid treatment, as shown in Fig. 10(c). Increasing tempera-
ture caused the PDMS substrate to swell and expanded the cracks in
the PEDOT:PSS sensing layer, resulting in an increase in resistance.
The positive TCR caused by PDMS expansion is much larger than
the inherent TCR of PEDOT:PSS (about −0.45%/○C), resulting in
a high temperature sensitivity of 4.2%/○C of the entire sensor. By
contrast, the crack-free PEDOT:PSS-PDMS sensor has a rather
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FIG. 10. Temperature sensors based on PEDOT:PSS. (a) Analytical diagram of the action of PEDOT:PSS in temperature sensing.208 (b) The sensor has the greatest
temperature sensitivity when the GO flakes completely fill the gaps between adjacent PEDOT:PSS nanoparticles. An excess of GO flakes will affect the connection of
adjacent PEDOT:PSS nanoparticles, and fewer GO flakes will be unable to completely fill the gap between the adjacent PEDOT:PSS nanoparticles.29 (c) Micrographs of
PSS:PEDOT-PDMS sensors with microcracks and a heatmap of the sensor’s TCR and crack morphology.49 (d) Structure of a multilayer temperature sensor based on
pNIPAM/PEDOT:PSS/CNT and PDMS, and a performance comparison with other sensing materials.50
low sensitivity of only 0.1%/○C.49 In addition, a method that
combines secondary doping and material swelling has been
reported to improve the sensitivity of temperature sensors.50
Oh et al.50 fabricated an Au-poly(N-isopropylacrylamide)
(pNIPAM)/PEDOT:PSS/carbon nanotube (CNT) multilayer
structured temperature sensor on a PDMS substrate. The
enhanced electron hopping between PEDOT:PSS and CNTs
in the PEDOT:PSS/CNT composites due to increased temperature
improved the thermal sensitivity. Besides, at high temperature, the
microstructure of pNIPAM was coiled and hydrophobic, leading to
expulsion of absorbed water. The reduced volume of the pNIPAM
made the closely attached PEDOT:PSS/CNT network more densely
packed, which further reduced the resistance of the sensor. By
contrast, at low temperature, the microstructure of the pNIPAM
became hydrophilic, absorbed water, and swelled. The closely
attached PEDOT:PSS/CNT network was loosened, increasing the
resistance of the sensor. This approach helped to improve the
sensitivity of the sensor to −2.6%/○C. The structure of the sensor is
shown in Fig. 10(d).
C. Pressure sensors
The compressibility of pure PEDOT:PSS is limited by its
rigid conjugated backbone,211 limiting the potential of pristine
PEDOT:PSS as a sensing element in pressure sensors.212,213 How-
ever, Wang et al.51 did develop a pure PEDOT:PSS pressure sen-
sor. In this sensor, 0.87–1.88 μm thick pure PEDOT:PSS film was
directly compressed to test its pressure response. Figure 11(a) shows
the rapid decrease in sensor sensitivity with increasing pressure
caused by the fairly small compressibility in the thickness direc-
tion. Therefore, a prerequisite for PEDOT:PSS-based pressure sen-
sor design is to endow the PEDOT:PSS with greater compressibil-
ity.214 When a compressible elastic structure with PEDOT:PSS as
the main conductive path is deformed under pressure, its electri-
cal conductivity is changed, which makes it able to sense pres-
sure. We classify PEDOT:PSS pressure sensors into three cate-
gories according to the method used to realize “compressible”
PEDOT:PSS, namely, doping with soft polymers or plasticizers,52
making compressible skeletons or dielectrics,215–217 and designing
microstructures.218,219
The first approach involves changing the compressibility of
PEDOT:PSS directly through doping soft polymers or plasticiz-
ers into it.42 Tseng et al.52 introduced poly (acrylic acid) (PAA)
into the PEDOT:PSS and obtained an increased stretchability (from
20% to 40%) and a much-decreased Young’s modulus, as shown in
Fig. 11(b). A pyramidal-microstructure-based pressure sensor was
then developed using this compressible and conductive material.
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FIG. 11. (a) Response of PEDOT:PSS film pressure sensor with interdigital electrodes and ITO conducting film.51 (b) Improved stretchability of PEDOT:PSS compared
with pristine PEDOT:PSS and a schematic illustration of a PEDOT:PSS/PAA pressure sensor.52 (c) PEDOT:PSS/PU foam pressure sensor.53 (d) Pyramidal-structure-based
pressure sensor.220 (e) Schematic of a pleat-based pressure sensor.218
Here, a noteworthy problem is that soft polymers or plasticizers,
nonconductive in most cases, will lead to a decrease in the over-
all conductivity. There is always a trade-off between compressibil-
ity and conductivity. Methanol treatment was adopted to solve this
problem.52 The sensitivity of the pressure sensor doubled after such
treatment.
Increased compressibility of PEDOT:PSS can also be
achieved indirectly by attaching it to a compressible skeleton
or dielectrics.215,216 The simplest and easiest method is to soak
a sponge in PEDOT:PSS solution, as shown in Fig. 11(c).53,101
PEDOT:PSS is then anchored to the sponge as the water evaporates.
An advantage of a sponge-based sensor is that its sensitivity can
be adjusted by changing the size and density of the pores in the
sponge over a large range.53 In another study, Lee et al.220 coated
a thin layer of PEDOT:PSS onto a pyramidal PDMS substrate to
achieve high-sensitivity pressure detection, since the point contact
condition at the beginning of compression enabled a large response
under a small pressure, as shown in Fig. 11(d). The ultrasensitivity
of this pressure sensor was demonstrated by tracking the blood
pulse at the wrist.220
Structural design is another strategy to improve the compress-
ibility of PEDOT:PSS.219 Out-of-plane buckling is frequently used
in PEDOT:PSS stretchability enhancement. In-plane tensile strain
is relieved when the buckling PEDOT:PSS film becomes flat.218 In
fact, pleats on PEDOT:PSS films not only improved the in-plane
stretchability but also increased the compressibility along the thick-
ness direction.218 A pressure sensor based on pleated PEDOT:PSS
film was developed by Tan et al.218 and exhibited a high sensitiv-
ity of 642.5 kPa−1and a rapid response time (200 μs), as shown in
Fig. 11(e).
D. Strain sensors
PEDOT:PSS-based strain sensors have attracted widespread
attention in various applications, such as artificial electronic
skin and health monitoring/diagnosis,54,56 owing to the excellent
electrical conductivity and mechanical properties of PEDOT:PSS.20
Intrinsic PEDOT:PSS is not suitable for stretchable strain sensors
because of its brittleness and susceptibility to breakage.55 However,
the stretchability of PEDOT:PSS can be improved by mixing
with elastomers55,221,222 or by forming textile fibers56,223,224 or
aerogel.54,209,211 PEDOT:PSS can be incorporated into a stretchable
elastic structure to make the latter conductive. Strain changes
the shape of the PEDOT:PSS-based stretchable structure, thereby
changing its electrical conductivity. According to this principle,
the PEDOT:PSS-based structure can be used for strain sensing.
For example, latex55 has been mixed with PEDOT:PSS to form
an elastomer matrix at an appropriate mixing ratio to produce
a conductive film with excellent stretchability.221 Panwar and
Anoop222 mixed PEDOT:PSS with a biocompatible polymer poly
(vinylidene fluoride–trifluoroethylene–chlorotrifluoroethylene)
P(VDF-TrFE-CTFE) and heat-cured the mixture to create a
conductive elastomer, as shown in Fig. 12(a). Owing to the
molecular bonding between the biocompatible polymer and the
PEDOT:PSS, the strain sensor can achieve a maximum strain
of 230%.
A dip-coating method has been used to facilitate attach-
ment of PEDOT:PSS from solution to realize textile-fiber-based
strain sensors.225,226 Excellent flexibility and skin adhesion makes
these conductive textiles good candidates for constructing wear-
able flexible sensors.227 For example, PEDOT:PSS was dipped into
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FIG. 12. Strain sensors based on PEDOT:PSS. (a) An elastomer strain sensorbased on a biocompatible polymer and PEDOT:PSS.222 (b) A PU fiber strain sensor based
on PEDOT:PSS/CNTs. PEDOT:PSS improves the sensitivity of the sensor.56 (c) An aerogel strain sensor based on rGO/PEDOT:PSS. The bridging effect of PEDOT:PSS
nanoparticles helps to improve the sensitivity of the sensor.209
a polyurethane (PU) fiber core coated with CNTs. Here, the PU
fiber was pre-stretched to produce cracks on the PEDOT:PSS,
which reduced the initial resistance of the strain sensor and
improved its strain sensitivity (by about six times), as shown in
Fig. 12(b). The CNTs bridging the PEDOT:PSS prevent complete
rupture of the conductive path when the sensor is subjected to
a large strain, thereby increasing the sensing range (0.1%–150%
strain).56
Zhang et al. prepared rGO/PEDOT:PSS aerogel by doping GO
into PEDOT:PSS and then reducing GO, as shown in Fig. 12(c).
A small amount of PEDOT:PSS nanoparticles greatly improved the
strain sensitivity of aerogels (from 580% to 5760%) due to the spe-
cific cell size of the aerogel and the bridging effect of PEDOT:PSS
nanoparticles.209 Zhou and Hsieh54 developed another strong and
highly conductive aerogel based on cellulose nanofibrils (CNFs)
and PEDOT:PSS in which surface carboxylates of the CNFs were
protonated into carboxyls and hydrogen-bonded with the PSS. The
strength of the aerogel was increased by an order of magnitude,
which is attributed to the change in the PEDOT benzenoid struc-
ture. Annealing in ethylene glycol vapor increased the conductiv-
ity of the aerogel by two orders of magnitude, and a strain sensor
formed by injecting PDMS into the aerogel had a good stretchable
range (a maximum strain of 90%) and sensitivity (a gauge factor
of 14.8).
Ultrasmall sensors (<1μm) have emerged that reduce the min-
imum detection limit of strain sensors.228 Tetsu et al.229 developed
an ultrathin (thickness ∼1μm) strain sensor that conforms to the
epidermal structure. It can detect small deformations of the human
skin (∼2%) and reduce the influence of the natural deformation of
the skin during measurement. Tang et al.58 prepared highly aligned
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PEDOT:PSS nanowires that can cover a large area through a mech-
anism involving capillary action. A nanomorphology-based strain
sensor using these nanowires had greatly improved sensitivity (a
gauge factor of 35.8). It could detect slight bending changes up to
200 μm with a rapid response (230 ms) and had a negative or posi-
tive response to inward or outward bending, respectively. This is due
to the fact that the inward bending of the sensor reduces the distance
between adjacent PEDOT:PSS nanocrystals, thereby reducing the
charge transfer distance and the sensor resistance. The opposite is
true for outward bending. In addition, the nanowire sensor exhibits
anisotropy, i.e., it is only sensitive to the bending strain parallel to
the nanowire, but not to the bending strain perpendicular to the
nanowire. However, such PEDOT:PSS-based strain sensors exposed
to the air will be affected by the environment owing to the sensi-
tivity of PEDOT:PSS to humidity. A processing method in which
PEDOT:PSS is injected into microchannel can prevent interference
by air and humidity, as well as solving the problem of connec-
tion damage caused by frequent bending.230 Bhattacharjee et al.231
injected a conductive solution of PEDOT:PSS into a microchannel
with a diameter of 275 μm formed by PDMS to make a strain sen-
sor. When the sensor was stretched, the axial deformation of the
microchannel reduced the effective volume fraction of the inter-
nal PEDOT:PSS. The creation of an electrical discontinuity led to a
decrease in the conductive path inside the channel, thereby increas-
ing the resistance of the sensor. This structure enabled the sensor to
achieve an ultrahigh gauge factor of about 12 000, which is 400 times
higher than that of most strain sensors.
V. CONCLUSIONS AND OUTLOOK
Over the years, PEDOT:PSS, as the most commercially suc-
cessful conductive polymer, has been extensively developed and
studied. PEDOT:PSS in the form of aqueous dispersions is com-
patible with a variety of fabrication techniques. It can be fabricated
into high-performance sensors through simple and low-cost meth-
ods. In this review, the morphology of PEDOT:PSS in the forms
of aqueous dispersions and solid films has been described, and the
potential of emerging hydrogels has also been discussed. Tradi-
tional processes (coating, printing, and lithography) and emerging
processes (soft lithography and nanoimprint lithography) used to
prepare PEDOT:PSS-based equipment have been presented, and
the latest developments of PEDOT:PSS in physical sensors (for
humidity, temperature, pressure and strain sensors) have been
described. We have specifically pointed out that PEDOT:PSS-
based nanowires made by nanoscale patterning improves the
sensitivity and response speeds of humidity and temperature
sensors.
Although much progress has been made in these areas, there
are still issues that need to be resolved to allow further development.
First, the uniformity of the films is important for stability and
consistency of performance in each area. However, to date, there
have been no studies of the control of uniformity during the prepa-
ration and conductivity enhancement of PEDOT:PSS solid films.
Particular attention needs to be paid to the heterogeneity caused by
phase separation during the drying process, which is also a problem
that needs to be solved when PEDOT:PSS films are applied to
precision instruments and metal-free devices in the future. Second,
the high-precision micro/nanoprocessing currently used for
PEDOT:PSS-based devices has the disadvantages of high cost and
low yield, which limits their application. In the future, it will be
necessary to develop low-cost and high-precision processes that
can be used in industrial mass production. Third, sensors based
on PEDOT:PSS are susceptible to environmental interference,
since PEDOT:PSS is sensitive to temperature, humidity, and strain.
Reasonable packaging and appropriate compensation are the major
directions for the development of PEDOT:PSS-based sensors to
take in the future. Finally, the microstructure of PEDOT:PSS-based
hydrogels is not yet completely clear. More efforts are required to
study the polymer chain system of PEDOT:PSS-based hydrogels on
the microscopic scale. In general, therefore, PEDOT:PSS research,
from material to manufacturing process to sensors, still has a long
way to go.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (Grant Nos. 62001325, 91743110, 52075384,
and 21861132001), the National Key R& D Program of China
(Grant No. 2018YFE0118700), Tianjin Applied Basic Research and
Advanced Technology (Grant No. 17JCJQJC43600), the Foundation
for Talent Scientists of Nanchang Institute for Microtechnology of
Tianjin University, and the “111” Project (Grant No. B07014).
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
X.Z. and W.Y. contributed equally to this work.
DATA AVAILABILITY
Data sharing is not applicable to this article as no new data were
created or analyzed in this study.
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