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Review
Flexible Field-Effect Transistor-TypeSensors Based on Conjugated MoleculesYoon Ho Lee,1,2 Moonjeong Jang,1,2 Moo Yeol Lee,1 O. Young Kweon,1 and Joon Hak Oh1,*
The Bigger Picture
With an attempt to prepare for the
imminent fourth industrial
revolution, which will bring new
ways of embedding technology
within societies and even within
the human body, the
development of flexible and
stretchable high-performance
sensors for wearable devices is
attracting tremendous interest.
Organic field-effect transistor
(OFET)-based sensors can
contribute to these goals because
of their high sensitivity,
mechanical flexibility,
biocompatibility, property
tunability, and low-cost
fabrication. This review provides
an overview of the sensing
mechanisms and recent progress
of OFET-based sensors, including
chemical, biological, photo,
pressure, and temperature
sensors. Further development of
flexible and stretchable OFET-
based sensors holds great
potential for the
commercialization of next-
generation sensor devices in a
variety of fields such as smart
health care and environmental
monitoring applications for
biomedical, manufacturing, and
military industries.
With the advent of the Internet of Things (IoT) era, flexible sensors are regarded
as one of the most important technologies for the development of human-
friendly wearable devices. Organic field-effect transistors (OFETs) based on
conjugated polymers or small molecules are promising sensor platforms
because they have various advantages, including high sensitivity, mechanical
flexibility, and low-cost fabrication processes. OFET-based sensors enable
continuous monitoring of external stimuli or target analytes with superior
detection capabilities. This review describes the working principles and sensing
mechanisms of various OFET-based sensors, including chemical, biological,
photo, pressure, and temperature sensors, and introduces the recent progress
in this field. In addition, the technical challenges and future outlook of OFET-
based sensors for next-generation flexible electronics are briefly discussed.
INTRODUCTION
Intensive development of devices and services in the ‘‘Internet of Things (IoT)’’ era is
expected to expedite the fourth industrial revolution. In this respect, device plat-
forms that are lightweight, portable, and easily adaptable to human life are of great
importance for promoting and guiding the development of next-generation ubiqui-
tous electronic and energy devices. Their applications include roll-up flexible dis-
plays, wearable devices, smart clothing and shoes, electronic skins, implantable
medical devices, and so on.1–6 In particular, as the application area of the IoT rapidly
develops, sensors capable of detecting various external stimuli are in increasing
demand. In recent years, considerable research efforts have been aimed at the
development of mechanically durable, flexible, and stretchable sensors, and much
progress has been achieved. Molecule-based semiconductors, covering conjugated
polymers and small molecules, have attracted great interest recently because of
their high potential for use in flexible, low-cost, large-scale, and lightweight elec-
tronic applications.7–11 The functional properties of molecular semiconductors and
conductive materials, such as electrical, chemical, and optical properties, can be
tuned for specific charge-transport characteristics via rational molecular design or
surface functionalization.12–15 In particular, amperometric sensors based on molec-
ular semiconductors are viewed as key electronic elements in advanced flexible and
wearable sensors because they can enable continuous monitoring of external stimuli
for long periods of time and have superior flexibility and mechanical robustness. In
addition to their typical use as switching elements for logic circuits, organic field-ef-
fect transistors (OFETs) are currently playing an important role in realizing large-area
flexible and stretchable sensors. Typical OFETs consist of three main components:
an active semiconductor layer, a dielectric (or insulator), and three terminals (i.e.,
source, drain, and gate electrodes). Such field-effect transistor (FET)-type sensors
enable easy amplification and fine-tuning of detected electrical signals by control-
ling the applied voltage on the third terminal, the gate electrode, in comparison
with conventional sensing devices composed of only two terminals.15–17 In most
724 Chem 3, 724–763, November 9, 2017 ª 2017 Elsevier Inc.
120
Surveillance
Environmental Pollution
Urban Warming
Blood Test
Cardiac Impulse
Health Care Environmental Monitoring
OFET-Based Sensors
UV rays
CCCCCCCCCCCCCard
OOFET-Based Sensorsss
Figure 1. Potential Applications of OFET-Based Sensors in Health Care and Environmental Monitoring
OFET-based sensors, the active semiconductor layers are exposed to external envi-
ronments in which the channel conductivity and interfacial properties can be
changed by trapping or doping via the interactions with target analytes. In addition
to changes in semiconductor properties, the properties of the gate dielectric layer,
source and drain electrodes, and gate electrodes of OFETs can be altered by various
methods.18–24 Molecule-based sensors, in particular, are expected to be applicable
to a variety of sensors, such as chemical, biological, photo, pressure, temperature,
humidity, and magnetic sensors, depending on the type of substance to be de-
tected. Notably, the number of research papers related to molecule-based flexible
sensors has recently increased very rapidly.
In this review, we focus on the development status of OFET-based sensors with
viable approaches to achieving sensitive detection of external stimuli, including an-
alytes (chemical and biological species), light, pressure, and temperature (Figure 1).
In addition, sensor applications involving mechanical flexibility and stretchability are
discussed, because they are highly promising for smart health care and environ-
mental monitoring devices. Recent progress regarding unconventional sensor plat-
forms, such as dual-gated configurations, is also described.
1Department of Chemical Engineering, PohangUniversity of Science and Technology, Pohang,Gyeongbuk 37673, Republic of Korea
2These authors contributed equally
*Correspondence: [email protected]
https://doi.org/10.1016/j.chempr.2017.10.005
OPERATION MECHANISM OF OFET-BASED SENSORS
Various types of OFET-based electronic devices have been demonstrated through
a variety of fabrication methods, such as vacuum evaporation,25–27 single-crystal
growth,16,28 and solution-based processes.12,29,30 In general, OFETs control the
current between source and drain electrodes by modulating the voltage of a
gate-source terminal (VGS), where charge carriers transport in the channel region
near the interface between the semiconductor and dielectric layers (Figure 2A).
Holes (h+) and electrons (e�) in the semiconductor act as charge carriers mostly
depending on energy-level relationships between the semiconductor and the
source/drain electrodes, exhibiting different operating behaviors, including
p-channel, n-channel, and ambipolar operations. Current-voltage (I-V) characteris-
tics of typical OFETs are calculated in saturation regime (Equation 1) or linear
regime (Equation 2), depending on the relative applied voltages of the electrodes
(Figure 2B):
IsatD =1
2
W
LmsatCiðVGS � VTÞ2; VDS >VGS � VT; (Equation 1)
Chem 3, 724–763, November 9, 2017 725
A B
C
Figure 2. Operation of OFETs and OFET-Based Sensors
(A) Schematic illustration of a device structure for a bottom-gate top-contact p-type OFET. Hole
(h+) as charge carriers which transport from the source to the drain electrode at the interface
between the semiconducting layer and gate dielectric via the channel region formed by negative
bias applied to the gate electrode.
(B) Typical transfer (left) and output (right) characteristics of OFETs operated in a saturation regime
(VDS > VGS – VT).
(C) Factors that cause the change in the ID of OFET-based sensors (i.e., signal of sensors) and
representative OFET-based sensors.
IlinD =W
LmlinCiðVGS � VTÞVDS; VDS <VGS � VT; (Equation 2)
where ID is the drain current, W and L are the width and length of a channel, m is the
field-effect mobility,Ci is the capacitance of the gate dielectric per unit area, VT is the
threshold voltage of the transistor, and VDS is the applied voltage between drain and
source. The key factors for evaluating the performance of a transistor include m, the
ON/OFF current ratio, and VT.
Among various amperometric sensors, FET-type sensors typically exhibit higher
sensitivity than two-terminal-based sensors, owing to the signal amplification capa-
bility by controlling the voltage applied to the gate electrode.15,17 OFET-based sen-
sors have been especially highlighted as core components of future electronics, not
only because of the superior mechanical flexibility of organic materials but also
because of the ability to tune the properties by rational molecular design, structural
engineering, and functionalization of the semiconducting layers.31 The structure and
components of OFET-based sensors can be customized depending on the proper-
ties of external stimuli through facile structural engineering and functionalization.
Multiple factors can affect signal response in OFET-based sensors, such as changes
in the charge density of the semiconducting layers, the capacitance of the gate
dielectric, the conductivity of the electrodes, and the applied gate voltages (Fig-
ure 2C). External stimuli could alter these factors, leading to changes in the drain cur-
rent, which is often used as the signal of the sensor. Therefore, various OFET-based
sensors can be fabricated according to the type of external stimulus (e.g., chemical
and biological species, light, mechanical deformation, temperature, magnetic and
electromagnetic forces, etc.) and operating mechanisms. This diversity of stimuli
andmechanisms has expanded the applications of OFET-based sensors; the change
in charge carrier density by interactions between analytes and semiconducting or
726 Chem 3, 724–763, November 9, 2017
dielectric layers and the photogeneration of charge carriers are used for chemical/
biosensors and photosensors, respectively. The conductivity variations induced by
mechanical deformation and temperature change are used for pressure sensors
and temperature sensors, respectively. The change in capacitance of a dielectric
layer by mechanical deformation can be used for pressure sensors. The voltages
generated from piezoelectric materials or thermoelectric materials can also be
used for pressure sensors and temperature sensors, respectively.
OFET-BASED CHEMICAL SENSORS AND BIOSENSORS
Sensor applications of OFETs for detecting chemical and biological species have
received great attention, because organic semiconducting layers can be easily
tailored by molecular design and surface functionalization according to the chemical
and/or physical properties of the target analytes, and OFET-based sensors enable
signal amplification by modulating the voltage at the third terminal, i.e., gate elec-
trode.14,15,27,29,32–34 In chemical/biological OFET sensors, electrical signals, such as
the output current, threshold voltage, and field-effect mobility, are generally
changed as a result of the doping or trapping effects of the analytes. The typical
sensing mechanism of OFET-based chemical sensors and biosensors is as follows:
first, the target analyte is injected onto the sensing layer of the OFET-based sensors.
Then, the injected analytes affect the organic semiconducting or dielectric layers
with various molecular interactions, changing the distribution of the charge carrier
density in the organic semiconducting layer. Finally, a change in charge carrier den-
sity results in a change in the drain current of the sensors.
Chemical Sensors
Chemical sensors can be used as environmental monitoring tools in daily life and
industrial fields. The conductivity of the active channel of an OFET is sensitive to
chemical reactions or interactions with analytes. Various demonstrations of
OFET-based chemical sensors, including the detection of volatile organic com-
pounds (VOCs),26,27,35 aqueous-phase analytes,12,13,33 and liquid-phase organic
solvents,29 have been reported, mainly because of the advantages of OFET-based
chemical sensors, such as simple customization and functionalization of active
components.
Pristine OFET-Based Chemical Sensors
In general, the sensing behaviors of chemical sensors originate from the results of
complex interactions between analytes and sensors, including hydrogen bonding,
hydrophobic/hydrophilic interactions, charge transfer (CT), and dipole-dipole inter-
actions. In the case of chemical sensors based on small-molecule organic semicon-
ductors, infused analytes penetrate into the adjoining grains of the semi-crystalline
semiconducting layer via the grain boundaries rather than through the grain because
of the dense molecular packing; this gives rise to a change in the charge-transport
characteristics by affecting the charge carrier density or changing the molecular
packing properties. The permeated analytes can act as a charge-trapping site,
decreasing the conductivity of the semiconductors. Li et al.36 demonstrated
OFET-based chemical sensors for detecting gas-phase organic solvents by utilizing
an ultra-thin organic semiconducting layer. The ultra-thin microstripes of the organic
semiconducting layer of dialkyl tetrathiapentacene (DTBDT-C6) were fabricated via
dip-coatingmethods and showed a decrease in the drain current toward various gas-
phase organic solvents because of the increased density of the charge traps (Fig-
ure 3A). The sensors showed high sensitivity (the ratio of the drain currents before
and after the analyte injection: Igas-off/Igas-on) over 100 toward ammonia vapor
(NH3) at a concentration of 50 parts per million (ppm) (Figures 3B and 3C). The
Chem 3, 724–763, November 9, 2017 727
Figure 3. Pristine OFET-Based Chemical Sensors
(A‒C) Molecular structure of DTBDT-C6 and atomic force microscopy (AFM) image (50 mm 3 50 mm) and section profile along the line (A). Sensing
response of the OFET-based sensors with an ultra-thin DTBDT-C6 layer toward 50 ppm of NH3 (B) and enlarged graph of the switching signal with
on/off switching of analyte stream (C). Reproduced with permission from Li et al.36 Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
(D‒F) Device structure of an OFET based on a pentacene/TSB3 heterointerface and molecular structure of TSB3 (D). AFM images of pentacene film
(35 nm) on a TSB3 layer (15 nm) and pentacene film (45 nm) on a silicon wafer (scale bars, 200 nm) (E). Sensing response of pentacene/TSB3-based
chemical sensors toward methanol vapor (F). Reproduced with permission from Kang et al.26 Copyright 2014 Nature Publishing Group.
(G) Sensing responses of the P3HT-based copolymer chemical sensors toward vapors of acetone and toluene. Reproduced with permission from
Li et al.37 Copyright 2006 American Chemical Society.
(H and I) Different responses of OFET-based sensors depending on various gate voltages (H). Color map representing sensing responses toward the
ten analytes injected with various applied gate voltages (I). Reproduced with permission from Li and Lambeth38 Copyright 2008 American Chemical
Society.
ultra-thin semiconducting layers facilitated interactions between infused analytes
and organic semiconductors in the channel, indicating that sensors with ultra-thin
semiconducting layers respond with greater sensitivity than those with typical
thin-film layers. Kang et al.26 reported high-performance chemical sensors based
on a highly crystalline nanoporous pentacene film grown on a small-molecule
728 Chem 3, 724–763, November 9, 2017
m-bis(triphenylsilyl)benzene (TSB3) dielectric with a low glass transition temperature
(Figure 3D). By sequential thermal evaporation of TSB3 and pentacene without
breaking the vacuum, a nanoporous pentacene layer with indistinct grain boundaries
was formed on the dewetted nanoporous TSB3 dielectric layer. The highly crystalline
nanoporous pentacene thin film facilitated charge carrier transport and improved
sensing ability because of the significantly reduced grain boundary and the facile
diffusion of gaseous analytes into the channel region via the nanopores (Figure 3E).
The sensor devices with a highly crystalline nanoporous active layer showed dramat-
ically enhanced sensitivity and fast response and recovery times toward vaporized
methanol, in comparison with the sensors based on conventional pentacene thin
films grown on a silicon wafer (Figure 3F).
In OFET-based chemical sensors using polymer semiconductors, on the other hand,
injected analytes can permeate to the grain because typical polymer semiconduc-
tors have a lamellar packing structure in which molecular packing is relatively looser
than in small-molecule semiconductors. Therefore, permeated analyte molecules
can either affect the molecular packing structures within the grain or act as trapping
sites and doping or de-doping agents at the grain boundary.17,39 Li et al.37 investi-
gated the sensing responses of polythiophene-based copolymers toward VOCs by
using a two-terminal-based sensor system at first to investigate intrinsic sensing re-
sponses of the sensors toward VOCs, depending on various morphologies of semi-
conducting layers and interactions between analytes and semiconductors arising
from the different compositions of polythiophene-based copolymers. Because the
polymer block used in the synthesis of these copolymers interacted with the injected
analytes in different ways, different sensing responses were observed for each
copolymer. The copolymer-based chemical sensors showed different sensing re-
sponses from each other upon exposure to ten different VOCs, including polar
and non-polar organic solvents. It was suggested that the sensing responses could
have resulted from multiple mechanisms. Among the various factors affecting
sensing responses, the main factor for each sensing demonstration could differ de-
pending on the second polymer block in the copolymer as well as on the properties
of the analytes (Figure 3G). They suggested that themain mechanism for a homopol-
ymer (i.e., poly(3-hexylthiophene) [P3HT]) could be the reduced molecular packing
distance of polar analytes for positive responses and the enlarged spacing of the
polymer molecules according to the swelling effect of non-polar analytes for nega-
tive responses, respectively. Along with the main factor for the sensing responses of
P3HT FETs, other factors, such as interactions between second polymer blocks in co-
polymers and analytes or the morphologies of the copolymer films, could also influ-
ence the signals, resulting in various sensing responses, depending on the types of
copolymers and analytes. Then, the sensing responses of P3HT FETs toward various
VOCs as a function of operation voltage (i.e., VGS) were further investigated (Fig-
ure 3H).38 In contrast with the previous report that described the independence of
sensing responses toward VGS,39 it was observed that the changes in gate voltages
in OFET-based chemical sensors induced a competition among the multiple mech-
anisms of the sensors. As VGS increased, most of the charge transport of a transistor
took place at the interface between the semiconductor and the gate dielectric layer,
not in the bulk of the film. The difference in charge carrier transport behaviors
depending on VGS resulted in a change in the sensing response. When VGS was
small, charge transport occurred in the bulk of the films, where mainly intragrain
effects are observed. The grain boundary effects, on the other hand, were distinctly
observed at large VGS. These intragrain and grain boundary effects compete with
each other, resulting in different sensory responses toward various analytes on the
basis of the magnitude of VGS (Figure 3I). Various interactions between injected
Chem 3, 724–763, November 9, 2017 729
A B C
D E F
Figure 4. Surface-Functionalized OFET-Based Chemical Sensors
(A‒C) Molecular structure of PII2T-Si and the conformational change of DNA-functionalized gold nanoparticles upon binding with Hg2+ (A). Sensing
responses of chemical sensors with DNA toward heavy-metal ions in seawater (i.e., Hg2+, Zn2+, and Pb2+) and response of sensors without DNA toward
Hg2+ (B). Photograph of a flexible PII2T-Si OFET-based chemical sensor (C). Reproduced with permission from Knopfmacher et al.12 Copyright 2014
Nature Publishing Group.
(D‒F) Molecular structure of cross-linked P3HT-azide as a solvent-resistant semiconductor and calix[8]arene as a surface functionalization material (D).
Sensing responses of chemical sensors toward liquid-phase analytes (i.e., methanol, ethanol, DI water, toluene, and n-hexane) (E). Photograph of
flexible P3HT-azide OFET-based chemical sensors (F). Reproduced with permission from Lee et al.29 Copyright 2015 Wiley-VCH Verlag GmbH & Co.
KGaA, Weinheim.
analytes and the dielectric layer can also change the drain current of OFET-based
chemical sensors in the same manner as those between analytes and the semicon-
ducting layer.40,41 For enhancement of the sensitivity or selectivity of the OFET-
based chemical sensors based on the analyte-dielectric layer interactions, several
approaches have been demonstrated; these include UV irradiation to modify the
chemical properties of the surface of the dielectric layer,41 introduction of gas
dielectric,42 and additional functionalization at the semiconductor-dielectric
interface.43,44
Surface-Functionalized OFET-Based Chemical Sensors
Surface functionalization of the semiconducting layer is a widely used approach
for enhancing the sensitivity and selectivity of OFET-based chemical sensors.17
Based on the surface functionalization of the semiconducting layers, various
demonstrations of high-performance OFET-based chemical sensors have been
reported.12,27,29 Knopfmacher et al.12 demonstrated OFET-based chemical sensors
with functionalized polymer semiconducting layers for detecting heavy-metal ions in
seawater. A polyisoindigo-based polymer functionalized with siloxane-containing
solubilizing side chains (PII2T-Si) was utilized as the semiconducting layer for
water-stable OFET-based chemical sensors (Figure 4A). The fabricated chemical
sensors showed higher operational stability than alkyl-terminated polyisoindigo-
based polymers (PII2T-Ref) not only in ambient conditions but also in aqueous
solutions (i.e., deionized [DI] water and seawater). For precise detection of heavy-
metal mercury ions (Hg2+), DNA-functionalized gold nanoparticles (NPs) that could
selectively bind with Hg2+ were introduced on the surface of the PII2T-Si thin-film
layer. As a result, the functionalized OFET-based sensors exhibited sensitive Hg2+
detection in seawater, with a detection limit of 10 mM. Moreover, selective sensing
responses toward Hg2+ were demonstrated in the presence of other seawater
730 Chem 3, 724–763, November 9, 2017
Table 1. OFET-Based Chemical Sensors
Semiconductor Mobility (cm2
V�1 s�1)Target Stimuli Sensitivity Detection Limit Additional Strategy Transistor Part
for AdditionalStrategy
Flexibility(SubstrateMaterials)
DTBDT-C636 0.3 NH3 vapor 1,000 (100 ppmNH3), >30(10 ppm NH3)
10 ppm NA NA NA
Pentacene26 6.3 methanol vapor NA NA NA NA NA
P3HT38 0.026 ten vapor organiccompounds
NA NA NA NA NA
PII2T-Si12 �0.04 mercury ion (Hg2+) NA NA functionalization(DNA-functionalizedAu NPs)
semiconductor NA (PI)
P3HT-azide29 0.032 liquid-phase solvents(methanol, ethanol,DI water, toluene,and n-hexane)
NA 1 vol % of methanol,ethanol, andn-hexane in toluenebase medium
functionalization(container molecule)
semiconductor NA (PEN)
NA, not available.
contaminants (i.e., Zn2+ and Pb2+) (Figure 4B). In the case of functionalized OFET-
based sensors with gold NPs without DNA aptamer, no response was observed
upon exposure to Hg2+, confirming that the sensingmechanism was related to direct
binding between DNA and Hg2+. Flexible devices based on a flexible polyimide (PI)
substrate and a polymer dielectric (Figure 4C) were also demonstrated.
Lee et al.29 reported solvent-resistant OFET-based chemical sensors that can directly
detect liquid-phase analytes, including harmful organic solvents. A containermolecule,
calix[8]arene (C[8]A), which has functional groups and a cavity structure for interacting
with analytes, was introduced onto the semiconducting layer, a cross-linked P3HT-
azide copolymer OFET,45 in order to enhance the sensing ability of the chemical
sensors (Figure 4D). When comparing the sensing performance of sensors with and
without C[8]A, the methanol-sensing response of sensors with C[8]A was enhanced
by about an order of magnitude over the non-functionalized sensors. The sensing
responses to various liquid-phase analytes, including polar and non-polar solvents
(i.e., methanol, ethanol, DI water, toluene, and n-hexane), were investigated, showing
distinct responses toward each solvent (Figure 4E). The surface-functionalized chemical
sensors had the ability to detect 1 vol % of methanol, ethanol, and n-hexane in toluene
medium. The trend in sensing the responses of flexible chemical sensors based on a
cross-linked poly-4-vinylphenol (PVP) gate dielectric and polyethylene naphthalate
(PEN) substrate was consistent with that of chemical sensors prepared on a siliconwafer
(Figure 4F). Some representative OFET-based chemical sensors are summarized in
Table 1.
Furthermore, the sensitivity, response time, stability, and selectivity of sensors can
be improved by adopting or engineering more suitable fabrication methods and
active materials. Among them, reusability is one of the crucial criteria determining
the performance of OFET-based sensors. The reusability of OFET-based chemical
sensors and biosensors, including signal recovery and water or chemical resistance,
is important because the typical operation of OFET-based sensors is carried out as
real-time signal monitoring with continuously applied voltage bias. The reusability of
sensors can be enhanced by the molecular engineering of semiconductors as well as
by effective immobilization methods. In the case of OFET-based gas sensors, the re-
covery time of the sensors usually increased because of the trapped analytes perme-
ated in the semiconducting layer or semiconductor-dielectric interface. Several
experimental approaches, such as the introduction of ultra-thin36,46 or porous26,47
Chem 3, 724–763, November 9, 2017 731
Figure 5. Pristine OFET-Based Biosensors
(A and B) Schematic diagram of an OFET device under a water droplet (the two dashed-line areas
indicate hydrophobic materials, such as fluorinated polymers) (A). Channel current of a CuPc OFET
as a function of time (the concentration of the lactic acid changes from 10 mM to 2 mM) (B).
Reproduced with permission from Someya et al.49 Copyright 2002 American Chemical Society.
(C and D) Schematic diagram of a pentacene-based OFET in a bottom-contact bottom-gate
configuration (C). Transfer characteristics of the pentacene OFETs at various DNA concentrations
(D). Reprinted from Stoliar et al.50
semiconducting layers, have been demonstrated to expedite the removal of the
trapped analytes in sensing layers. The operational stability of OFET-based chemical
sensors upon exposure to aqueous analytes or organic solvents is also a big obstacle
for the expansion of the types of analytes for OFET-based chemical sensors and
biosensors from gas-phase to aqueous- or liquid-phase analytes. Several methods
for the fabrication of highly stable OFET-based chemical sensors, such as the
introduction of long alkyl chains at the side chain of semiconductors,13 the use of
passivation layers,48 and physical or chemical cross-linking agents,12,29,35 have
been reported.
Biosensors
A variety of biosensors that monitor the binding of specific biomolecules to solid-
state substrates have been developed for biological and medical diagnostic appli-
cations. In the case of OFET-based biosensors, biomolecules adsorbed onto the
active layer can change the charge carrier density and the doping level in the organic
semiconductor film. The following section on transistor-based biosensors is divided
into two parts: non-enzymatic mechanism-based biosensors and specific receptor-
based biosensors.
Non-enzymatic OFET-Based Biosensors
Someya et al.49 developed non-specific adsorption-based biosensors by using
OFETs based on different semiconductors, which were stable in stationary and flow-
ing water (Figure 5A). The source and drain electrodes were covered with a
732 Chem 3, 724–763, November 9, 2017
hydrophobic fluorinated insulator. Active organic semiconductors, including penta-
cene, a-sexithiophene (a6T), dihexyl a6T (DHa6T), and copper phthalocyanine
(CuPc), were used to detect a variety of analytes, such as lactic acid, pyruvic acid,
and glucose in aqueous solutions (Figure 5B). The sensing response was due to
the traps induced by the interaction between the analyte and the semiconductor
grain. Subsequently, many other types of non-enzymatic FET-based biosensors
have been reported in the literature.50–52 Stoliar et al.50 demonstrated a label-free
DNA sensor based on a pentacene FET only two monolayers thick (Figure 5C).
The DNA molecules were adsorbed on the ultra-thin pentacene film surface without
any binding agents or immobilization. They found that FET devices exhibited a shift
of pinch-off voltage during the detection of DNA at different concentrations and the
sensitivity reached 1 mg/mL (Figure 5D). The sensing response was attributed to the
negative charge of DNA adsorbed on the pentacene via electrostatic interactions,
leading to an increase of positive charge carriers in the pentacene film.
Receptor-Functionalized OFET-Based Biosensors
This section focuses on biosensors utilizing chemical modification of specific recep-
tors on the active layer surface. First, OFET platforms functionalized with biological
recognition materials, such as enzymes, have been highlighted in the field of bio-
sensing applications because of the biocompatibility of the organic materials. Liu
et al.53 developed sensors based on OFETs to detect glucose enzymatically by
using glucose oxidase (GOx). They immobilized GOx on the surface of the semicon-
ductor, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-con-
ducting polymer film and encapsulated the sensor within a cellulose acetate
membrane. Therefore, the PEDOT:PSS matrix and GOx could not be dissolved in
the glucose solution. The increase in current observed with increasing glucose con-
centration was due to the redox property of PEDOT:PSS. Hydrogen peroxide
(H2O2) produced by the reaction of glucose with GOx can react electrochemically
with PEDOT:PSS, changing the drain current. The device detected target glucose
down to millimolar concentrations. Khan et al.32 showed selective detection of
DNA by using OFET-based biosensors functionalized with peptide nucleic acid
(PNA) sequences. The surface of the OFETs was modified with a thin polymaleic an-
hydride (pMA) layer to confer the carboxylic acid groups on the device surfaces.
Then amine-terminated PNA probes were covalently attached to the pMA layer
for real-time in situ detection of target DNA molecules (Figure 6A). A change in
channel current was observed after the hybridization of complementary target
DNA with PNA probes. The device detected target DNA with a detection limit of
1 nM concentration (Figure 6B). In these biosensor platforms, biological analytes
and biologically derived recognition elements are typically active in aqueous
media.54 Among the different types of organic transistors, ion-sensitive FETs
(ISFETs) operate in an aqueous electrolyte-gated medium, which is constructed
by an electrolyte solution and a reference electrode used as the gate contact
of the transistor. The change in the electrical potential at the interface between
the electrolyte and the semiconducting layer affects the charge transport in the
transistor. Recently, another interesting ISFET-like OFET sensing platform that
does not require a reference electrode, namely organic charge-modulated FET
(OCMFET), has been developed.55 In this extended gate OFET, the gate sensing
area and channel region are physically separated, creating an ultra-low operating
voltage sensor for detecting DNA hybridization in liquids (Figure 6C). OCMFETs
have two gate electrodes, including a floating gate, and the floating gate can
change the charge density in the channel upon exposure to analyte solution, thus
affecting charge transport in the channel of the OFET. The label-free detection of
DNA in the sub-nanomolar range has been reported.55
Chem 3, 724–763, November 9, 2017 733
Figure 6. Surface-Functionalized OFET-Based Biosensors
(A and B) Schematic representation of an in situ PNA attachment and target DNA hybridization (A). Channel current responses to DNA sequences of T2-
MM0, T1-MM1, and T3-MM2 and a control experiment using OFET sensors for the hybridization of complement DNA at the minimum concentration
(�1 nM) (B). Reproduced with permission from Khan et al.32 Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
(C) Schematic diagram for a DNA sensor device with OCMFETs. The sensing area is electrically connected to the floating gate. Reproduced with
permission from Lai et al.55 Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
(D) Device structure and transfer characteristics for an SA functional biointerlayer OFET at VDS = �80 V measured in pure water and at different biotin
concentrations. Reproduced from Angione et al.56
(E) Molecular structure of (allyloxy)12CB[6] (AOCB[6]) as a synthetic receptor. Photograph of a flexible OFET-based sensor. Reprinted with permission
from Jang et al.34 Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
(F) Current responses of FETs with different bitterness perception of a human bitter taste receptor (PAV and AVI type)-functionalized CPNT and pristine
CPNT toward phenylthiocarbamide (PTC). Reprinted with permission from Song et al.57 Copyright 2012 American Chemical Society.
(G) Optical image of silk-based graphene FET biosensors attached to the human wrist (inset: SEM image of the graphene FET channel; scale bar,
100 mm). Current responses to the sequential glucose concentration increase at VGS = 0 V. Reprinted from You and Pak.58
OFETs with desired functionalities can be developed by tailoring functional inter-
layers for use as label-free, ultra-selective, and sensitive biological sensors. In this
respect, the new OFET structure is based on a functional biological interlayer placed
directly at the interface between the dielectric and the organic semiconductor. An-
gione et al.56 integrated streptavidin (SA) into an OFET device by spin-coating SA
layers between the SiO2 dielectric and the P3HT. SA is well known for having an
extraordinarily high-affinity and selective binding to biotin, and the strong binding
734 Chem 3, 724–763, November 9, 2017
between SA and biotin confers an effective change in the electronic properties of
P3HT. The SA-embedded P3HT OFET showed label-free biotin detection with su-
per-high selectivity and sensitivity at a concentration of 10 parts per trillion (ppt)
level, which was about nine times higher than its response to pure water (Figure 6D).
In a follow-up paper, the same research group reported the detection of biotin at
3 ppt or 15 pM with OFET-based biosensors embedding an SA-capturing layer
through a controllable layer-by-layer (LbL) assembly method.59 The SA layer was
also successfully replaced with the biotin antibody and enzyme. The functional
bio-interlayer-OFET represents a promising platform for a variety of biorecognition
elements.
Thus far, most OFET-based biosensors have immobilized biosubstances such as en-
zymes, protein receptors, or DNA to enhance the selectivity of metabolic processes.
Unfortunately, these approaches normally have several shortcomings, such as a slow
response, low metabolic stability, and high-cost and complicated fabrication pro-
cesses. One way to solve such problems is to develop detection methods that use
biological recognition elements instead of bioenzymes.60 Recently, Jang et al.34
demonstrated highly sensitive organic transistor-based sensors that can detect the
target neurotransmitter acetylcholine (ACh+) without enzyme immobilization. A syn-
thetic receptor, a cucurbit[6]uril (CB[6]) derivative film, was deposited on the surface
of the OFET device with 5,50-bis-(7-dodecyl-9H-fluoren-2-yl)-2,20-bithiophene(DDFTTF) as the semiconductor layer. This type of OFET-based biosensor, prepared
on a rigid Si wafer as well as on a flexible plastic substrate, can successfully detect
low concentrations of ACh+ at a detection limit of 1 pM (Figure 6E). Moreover, these
sensors with CB[6] film showed high discrimination of ACh+ over choline as an inter-
fering species, selectively detecting ACh+. This work provides a good example of
well-combined sensor devices with highly sensitive OFET device configuration
and highly selective synthetic receptors. More recently, an FET-based biosensor
using thiamine as a probe molecule for the specific detection of human prion
proteins (PrP) was reported by Wustoni et al.61 The FET gate surface was modified
with glutaraldehyde for the attachment of the amino group of thiamine molecules.
They then designed a dual-ligand binding approach and further amplified the
FET detection signal by adding Cu2+ ions, which induced an additional positive
charge on the gate surface of the device when bound to the adsorbed PrP-thiamine
complex. They successfully detected PrP in the range of 40 pM in a serum
sample, which can be used for practical diagnosis at relevant concentrations less
than 2 nM.
Biosensor platforms based on one-dimensional (1D) and two-dimensional (2D)
nanomaterials have also been widely investigated by some researchers. 1D nano-
structures, such as nanowires (NWs) and nanotubes, have emerged as the most
promising building blocks for biosensors as a result of having direct paths for charge
transport and high surface areas for ultra-sensitive detection. Song et al.57 demon-
strated FET devices used as transducers combined with 1D conducting polymers in
taste receptor-based biosensors for detecting bitter compounds. They functional-
ized taste receptors on the surface of carboxylated polypyrrole nanotubes (CPNTs)
by using covalent anchoring to detect the target taste compounds. Specific binding
between taste receptors and target taste compounds altered the surface charge
density of the sensor, which theymeasured in real time bymonitoring changes in cur-
rent output. The fabricated device detected target bitter tastants at concentrations
as low as about 1 fM and displayed high selectivity in mixtures and real food samples
(Figure 6F). Khatayevich et al.62 developed a protein sensor based on a highly
sensitive 2D-FET by using graphene as the active material. A self-assembled
Chem 3, 724–763, November 9, 2017 735
Table 2. OFET-Based Biosensors
Semiconductor Mobility(cm2 V�1 s�1)
Target Stimuli Sensitivity DetectionLimit
Additional Strategy Transistor Part forAdditional Strategy
Flexibility(SubstrateMaterials)
Pentacene, a6T,DHa6T, CuPc49
NA lactic acid,pyruvic acid,glucose
NA NA NA NA NA
Pentacene50 0.014 DNA 74 ng cm�2 650 ng mL�1 NA NA NA
PEDOT:PSS53 NA glucose 1.65 mA mM�1 1.1 mM functionalization (GOxenzyme)
semiconductor NA
DDFTTF32 �0.5 DNA NA 1 nM functionalization (PNA) semiconductor NA
TIPS-pentacene55 NA DNA NA 0.1 nM functionalization (PNA) semiconductor NA
P3HT56 0.001 biotin NA 10 ppt functionalization (SAinterlayer)
semiconductor NA
P3HT59 NA biotin 0.07 decade�1 3 ppt functionalization (SAinterlayer by LbLassembly)
semiconductor NA
DDFTTF34 0.042 ACh+ NA 1 pM functionalization(CB[6] derivatives)
semiconductor NA (PEN)
Thiamine61 NA PrP NA 40 pM functionalization(thiamine)
gate electrode(dual-ligand bindingapproach)
NA
CPNT57 NA PTC NA 1 fM functionalization(taster PAV-hTAS2R38)
semiconductor NA
Graphene62 NA SA NA 50 mg mL�1 functionalization(probe peptide)
semiconductor NA
Graphene58 NA glucose 2.5 mA mM�1 0.1 mM functionalization(GOx enzyme)
semiconductor NA (silksubstrate)
NA, not available.
biotinylated peptide was engineered to bind to graphene for biosensor functional-
ization. The device detected SA against a BSA solution of less than 50 ng/mL. The
designed nanosensor allowed for restoration of the graphene surface and utilization
of each sensor in multiple sensor measurements. Furthermore, graphene FETs can
be used easily as flexible biosensors, because graphene is a flexible 2D carbon crys-
tal. You et al.58 demonstrated a chemical vapor deposition-grown graphene FET-
based glucose biosensor that utilizes silk-fibroin protein as both an enzyme immobi-
lization material and a device substrate. A silk-fibroin gate dielectric film was
embedded with GOx enzymes to act as a glucose-sensitive layer, inducing a change
in the conductance of graphene under the glucose catalytic reaction. The flexible
graphene FET biosensor fabricated on flexible and biocompatible silk substrates
showed a linear response to glucose with a detection limit of 0.1 3 10�3 M, which
is promising for applications of implantable continuous glucose-monitoring biosen-
sors (Figure 6G). Table 2 summarizes the characteristics of these OFET-based
biosensors.
OFET-BASED PHOTOSENSORS
Organic phototransistors (OPTs), which is a term referring to OFET-based photosen-
sors, have great advantages for light detection over organic photosensors based on
photodiode systems as a result of their current amplification characteristics. A
comprehensive review of organic semiconductor materials for OPTs has already
been published.63,64 In this review, we focus on recent developments in OPTs based
on various approaches to improving photosensing performance and mechanical
flexibility.
736 Chem 3, 724–763, November 9, 2017
Charge Generation Mechanism
Photogeneration of charge carriers in pristine semiconductors is briefly described in
Figures 7A and 7B.65 The excitonic state is separated from the charge pair (CP) state
because of a non-negligible potential barrier by the relatively large exciton binding
energy. Application of an external electric field could reduce this barrier by modifi-
cation of the CP state and enhance photogeneration of charge carriers; the yield
dependence on the electric field shows a threshold-like behavior. Two models
have been proposed in the literature. First, Geacintov and Pope66 presented a
model for excess photon energy: during the relaxation process of exciton from a
high-energy singlet state (Sn) to the lowest singlet state (S1), CP branching can occur.
This autoionization process occurs on a very short timescale (<100 fs). Second, Arkhi-
pov et al.67 presented a model for vibrationally hot singlet S1 states: vibrationally hot
singlet S1 states generated by exciton annihilation processes or by decaying pro-
cesses from Sn to S1 can utilize their excess vibrational energy to split into charges.
This process is strongly time dependent as a result of competition with the S1 cooling
process. Figure 7A shows the CP generation model of pristine materials. CPF is the
CP modified by the applied electric field, lowering the barrier for exciton breaking.
Process 1 optically generates S1, process 2 generates Sn, process 3 represents
charge generation starting from a thermalized S1 state, process 4 is relaxation
from Sn to S1 or CP (autoionization), process 5 is charge generation starting from a
vibrationally hot S1 state.67
To enhance photogeneration yield, donor (D)-acceptor (A) systems have been uti-
lized widely to exploit the phenomenon of photo-induced charge separation (Fig-
ure 7B). The term ‘‘donors’’ refers to molecules that are defined by a low ionization
potential, and ‘‘acceptors’’ are molecules characterized by a high electron affinity.
Upon photo-excitation of D (or A), an exciton reaching a D/A interface decays into
a CT state. As a result, the e� transfers on the lowest unoccupied molecular orbital
of A and the h+ transfers on the highest occupied molecular orbital of D.63 The
advantage of CT states for photogeneration is that e�/h+ pairs show reduced
coulombic binding and hence are more prone to dissociation. Process 1 refers to
exciton generation upon light illumination in D; process 2 refers to charge separation
at the D/A interface; process 3 refers to recombination between the e� in A and
the h+ in D.
In OPTs, there are two different effects in the active layer under light illumination
because of their photovoltaic effect (VGS < VT for a p-channel device) and photocon-
ductive effects (VGS > VT).68 In photovoltaic mode, photogenerated charge carriers
cause a shift of VT toward more positive (negative) values for p-channel (n-channel)
devices. If we consider the case of a p-channel OPT under illumination conditions,
the photogenerated h+ flows easily to the drain electrode, whereas e� are accumu-
lated under the source electrode, effectively lowering the h+ injection barrier be-
tween the source and the semiconductor channel. The lowered injection barrier
makes an effective decrease in contact resistance (Rc) and a positive shift in VT and
leads to a significant increase in the drain current. The photocurrent (Iph) caused
by the photovoltaic effect can be expressed as Equation 3:69
Iph =gmDVT =AkT
qln
�1+
hqlPopt
Ipdhc
�; (Equation 3)
where gm is the transconductance, DVT is the threshold voltage shift, A is a propor-
tionality parameter, k is the Boltzmann constant, T is the temperature, q is the
e� charge, h is the photogeneration quantum efficiency, Popt is the incident optical
power, Ipd is the dark current for minority charges, and hc/l is the photon energy. In
Chem 3, 724–763, November 9, 2017 737
C
BPTT Pentacene
CuPc F8T2
4(HPBT)-benzene 4(HP3T)-benzene
BODIPY-BF2 P(DPP4T-co-BDT)
IP
vacuum
LUMO
LUMO
HOMODonor
Acceptor
1
3
2 EA
Charge separation
ygrenE
Sn
2
135
4
A B
HOMO
S1
CPF
CP
Figure 7. Charge-Carrier Generation Mechanism of OPTs upon Light Illumination and Molecular
Structures of Representative Semiconductors Used in OPTs
(A) CP generation model in pristine materials. Process 1 (2) optically generates S1 (Sn), process
3 represents charge generation starting from a thermalized S1 state, process 4 is relaxation from
Sn to S1 or CP, and process 5 is charge generation starting from a vibrationally hot S1 state.
(B) Charge generation model in D/A interfaces. Process 1 refers to exciton generation upon light
illumination in D, process 2 refers to charge separation at the D/A interface, and process 3 refers to
738 Chem 3, 724–763, November 9, 2017
Figure 7. Continued
recombination between e� in A and h+ in D (IP, ionization potential of the D; EA, electron
affinity of the A; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular
orbital).
(C) Representative small-molecule and polymer semiconductors used in OPTs.
photoconductive mode, when the device is in the OFF state (VGS > VT for the p-chan-
nel), photogenerated ID shows a linear increase with optical power because of a
photoconductive effect, but with the additional gate terminal responsible for a trans-
verse electric field enhancing photogeneration. The current can bemodeled accord-
ing to Equation 4:70
Iph =�qmppE
�WD =BPopt; (Equation 4)
where mp is themobility of majority charge carriers, p is the charge concentration, E is
the electric field in the channel,W is the gate width, D is the depth of the absorption
region, and B is a proportionality factor.
In addition to the fundamental OFET characteristics, such as m, VT, and subthreshold
swing, the figures of merit specific to OPTs are external quantum efficiency (EQE),
photoresponsivity (R), and the photocurrent/dark current ratio (P). The EQE and
R can be defined by Equations 5 and 6, respectively:
EQE=
�ID;ph � ID;dark
�hc
ePintAl; (Equation 5)
lq I I � I
R =EQEhc=
ph
Popt=
D;ph D;dark
Popt; (Equation 6)
where Pint is the incident light intensity (i.e., the incident optical power density), A is
the active area, and ID,ph and ID,dark are the drain current under illumination and in the
dark, respectively.
Semiconducting Materials for Organic Phototransistors
Theperformanceof anOPT ismost influencedby thephotogeneration characteristicsof
its photoconductive semiconductor material. This section outlines the use of small mol-
ecules and polymers as high-performance organic semiconducting materials in OPTs.
Small-Molecule-Based Phototransistors
Noh et al.71 developed an OPT based on 2,5-bis-biphenyl-4-yl-thieno[3,2-b] thio-
phene (BPTT) by the vapor deposition method (for chemical structure, see Fig-
ure 7C). The BPTT OPTs showed a m of 0.082 cm2 V�1 s�1, a maximum R of
82 A W�1, and Iph/Idark of 2.0 3 105. Under UV light illumination, the channel current
was increased to 20 mA at VGS = �20 V by the photovoltaic effect. The Iph/Idark value
was higher than that reported in amorphous Si-based phototransistors (R z
300 A W�1, Ion/Ioff = 103). They also developed high-performance OPTs with small
molecular organic semiconductors (pentacene or CuPc).72 The pentacene and
CuPc OPTs exhibited high R values of 10–50 AW�1 and 1.5–2.4 AW�1, respectively.
In contrast to the vapor deposition method, Cho et al.73 demonstrated highly photo-
responsive OPT devices by a solution process using soluble star-shaped oligothio-
phenes with four-armed p-conjugation paths: 1,2,4,5-tetra(50-hexyl-[2,20]-bithio-
phenyl-5-vinyl)-benzene (4(HPBT)-benzene), and 1,2,4,5-tetra(50-hexyl-[2,20]
terthiophenyl-5-vinyl)-benzene (4(HP3T)-benzene) (for chemical structures, see Fig-
ure 7C). The R was typically more than 2,500 A W�1 (maximum R = 4,300 A W�1) un-
der low incident light power (<30 mW cm�2) with high Iph/Idark of 4 3 104 and a short
Chem 3, 724–763, November 9, 2017 739
response time (<1 s). The higher OPT performance in comparison with that of amor-
phous Si-based phototransistors was due to the unique star-shaped molecular
structure. The planar core parts with four-armed p-conjugation in these molecules
led to efficient light absorption and photogeneration. Li et al.74 reported organic
small-molecule 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY-BF2)-based
OPTs for near-infrared (NIR) detection (for chemical structures, see Figure 7C).
The OPTs showed R up to 11,400 A W�1 and Iph/Idark of 1.04 3 104 under NIR light
illumination (500 mW cm�2).
Polymer-Based Phototransistors
Polymer-based OPT devices have the advantage of better compatibility with plastic
substrates than small-molecule-based devices. These promising features could lead
to lightweight and high-flexibility optoelectronic integrated circuits based on poly-
mer semiconductors. Poly(9,9-dioctylfluorene-co-bithiophene) (F8T2)-based OPT
devices were reported to exhibited very high light sensitivity, with the maximum
R value as high as 18.5 A W�1 at 5 mW cm�2 light intensity (for chemical structure,
see Figure 7C).75 In these devices, the OFF current of OPT increased with light illu-
mination (the photo-induced charge carrier density was changed from 5 3 1011
to 1 3 1013 cm�2). Ma et al.76 reported OPTs based on D-A copolymer 3,6-bis
(5ʹ-bromo-[2,2ʹ-bithiophen]-5-yl)-2,5-bis(2-octyldodecyl)pyrrolo-[3,4-c]-pyrrole-1,4
(2H,5H)-dione and 2,6-bis(trimethyltin)-4,8-dimethoxybenzo[1,2-b:3,4-bʹ]dithio-
phene (P(DPP4T-co-BDT)) (for chemical structures, see Figure 7C). The OPTs based
on (P(DPP4T-co-BDT)) exhibited a m of 0.047 cm2 V�1 s�1, a maximum R of
4,000 A W�1, and Iph/Idark of 6.8 3 105 under white light (9.7 mW cm�2).
Approaches to High-Performance Phototransistors
Many approaches have been reported for enhancing the performances of OPTs by
using various semiconductor materials. This section introduces the representative
methodologies for achieving high-performanceOPTs, which are related to engineer-
ing the photoactive layer, D/A interfaces, light-trapping effects, and CT systems.
Single Crystals
Organic single crystals do not have grain boundaries that act as energetic barriers to
charge transport and have frequently been used as photoactive materials for OPTs
with a view to systematically investigating their intrinsic properties and enhancing
the optoelectronic performance. Organic single crystals are typically synthesized
by solution growth methods and physical vapor transport (PVT) methods. Tang
et al.77 developed copper hexadecafluorophthalocyanine (F16CuPc) sub-micro-
and nanometer ribbons by using PVT methods. The OPTs based on F16CuPc
ribbons exhibited high-performance (maximum Iph/Idark = 4.53 104). Guo et al.78 re-
ported solution-phase self-assembled nano- andmicrometer ribbons of 6-methylan-
thra[2,3-b]benzo[d]thiophene (Me-ABT). The Me-ABT single-crystal-based OPTs ex-
hibited extremely high performance, such as m of 1.66 cm2 V�1 s�1, R of 1.2 3
104 A W�1, and Iph/Idark of 6 3 103 under low incident light power (30 mW cm�2).
Kim et al.79 reported that J-aggregated single-crystal compounds consisting of an
anthracene core and containing two 2-ethynyl-5-hexyldithieno[3,2-b:20,30-d] thio-phene (DTT) (A-EHDTT) groups in 9,10 positions of the anthracene core. The
A-EHDTT-based OPTs showed a high R, exceeding 1 3 104 A W�1 under very low
incident light power (1.4 mW cm�2). Yu et al.16 developed OPTs based on single-
crystalline N,N0-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-
PTCDI) NWs. The OPTs with a high m (1.13 cm2 V�1 s�1) showed a high R of 1.4 3
103 A W�1 and a maximum P of 4.96 3 103 under red and green light irradiation,
respectively (Figures 8A‒8C). Moreover, the normalized EQEs of the NW-OPTs
740 Chem 3, 724–763, November 9, 2017
Figure 8. Approaches to High-Performance OPTs (Single Crystals and D/A Interface Systems)
(A‒C) Optical microscopy image of BPE-PTCDI NWs (scale bars, 100 mm) (A). Transmission electron
microscopy (TEM) image of BPE-PTCDI NW (scale bar, 500 nm) (B). EQE of BEP-PTCDI single NW-
OPT under red, green, and polychromatic light (C). Reproduced with permission from Yu et al.16
Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Chem 3, 724–763, November 9, 2017 741
Figure 8. Continued
(D and E) Chemical structures of chiral PTCDI semiconductors (D). Transfer characteristics in the
dark or under CPL illumination (l = 460 nm, power = 50 mW cm�2) for OPTs based on (S)-CPDI NWs
(left) and (R)-CPDI-Ph NWs (right) at VDS = 70 V (E). Error bars represent the SD values obtained for
at least four devices from more than two different batches. Reproduced with permission from
Shang et al.80 Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
(F‒H) Device structure of OPT with BPE-PTCDI-rGO core/shell p-n heterojunction NW (scale bar, 1
mm) (F). Typical transfer characteristics of OPTs with pure BPE-PTCDI NW and BPE-PTCDI-rGO
core/shell at VDS = 10 V (G). EQEs as a function of VDS of pure BPE-PTCDI NW-OPT and BPE-PTCDI/
rGO NW-OPT under polychromatic light with irradiation power of 17 mW cm�2 at VGS = 0 V and
VDS = 100 V (H). Reproduced with permission from Yu et al.81 Copyright 2015 Wiley-VCH Verlag
GmbH & Co. KGaA, Weinheim.
(I‒L) Device structure of OPT with a P3HT:PEHTPPD-BT layer (I). Transfer characteristics of OPTs
with the P3HT:PEHTPPD-BT layers according to the incident light density under illumination with
visible (550 nm) and NIR (900 and 1,000 nm) light at VDS = �80 V (J). Photographs of flexible OPTs
based on PEN substrates (bent at an angle of �30� for performance measurement) (K).
Photoswitching characteristics of bent OPTs (L). Reproduced with permission from Han et al.82
Copyright 2015 Nature Publishing Group.
showed ultra-high values in comparison with thin-film OPTs (EQE z 7,900-fold
larger) under polychromatic light. More recently, Shang et al.80 reported that self-
assembled NWs of chiral PTCDI derivatives show amplified chirality because of su-
pramolecular chirality and can be used for chiroptical sensing toward the selective
detection of circularly polarized light. The R values of these OPTs were 21 times
higher (334 A W�1) than those of thin-film OPTs. Furthermore, OPTs based on the
(S)-homochiral NWs showed higher sensitivity toward left-handed circularly polar-
ized light than toward right-handed circularly polarized light (Figures 8D and 8E),
making this platform highly promising for practical applications such as image sen-
sors, optical scanners, and security-enhanced optical communication.
D/A Interfaces
TheD/A system is highly advantageous for increasing the performance ofOPTs, leading
to high R values. From a device point of view, various D/A interface systems have been
adopted including bi-, multi-,81 and bulk heterojunction systems.82 In the bulk hetero-
junction system, D/A-blended nanomorphology has a critical impact on device perfor-
mance. Yu et al.81 developed core/shell p-n heterojunction NWs by using BPE-PTCDI
and reduced graphene oxide (rGO). As a result of the synergistic optical and electrical
properties of the core and shell materials, the charge compensation rate of the hetero-
junctionNW-OPT device was three to five times higher upon light on-and-off switching,
and R was approximately 1.5 times larger than that of the pure BPE-PTCDI NW-OPT
(5.63 A W�1 for BPE-PTCDI/rGO and 3.97 A W�1 for BPE-PTCDI; Figures 8F‒8H).
Han et al.82 fabricated all-polymer bulk heterojunction phototransistors by using
P3HT and poly[{2,5-bis-(2-ethylhexyl)-3,6-bis-(thien-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-
diyl}-co-{2,20-(2,1,3-benzothiadiazole)]-5,50-diyl}] (PEHTPPD-BT). The phototransistors
displayed R values of �450 mA W�1 under visible light and �250 mA W�1 under NIR
light, according to the high NIR region absorption characteristics of semiconductor
layers based on bulk heterojunction systems (Figures 8I and 8J). Furthermore, they
used PEN substrates for flexible OPTs and demonstrated stable sensing performance
at a bending angle of 30� (Figures 8K and 8L).
Light-Trapping Effects
Utilization of light-trapping effects is an effective approach to enhance light absorp-
tion for diode-structured optoelectronic devices, such as photovoltaics, photosen-
sors, and light-emitting diodes. However, the application of light-trapping effects
in OPTs has only recently been actively reported. In particular, Au NPs and nanorods
(NRs) with localized surface plasmons are known to increase the excitation efficiency
742 Chem 3, 724–763, November 9, 2017
and m of OPTs.28,83 For example, Zakaria et al.83 first reported enhanced perfor-
mance of OPTs utilizing Au NPs. The m and R were enhanced by more than 40%
(0.0167 to 0.0237 cm2 V�1 s�1) and 100% (9 to 21 A W�1 at VGS = +10 V), respec-
tively, by integrating Au NPs into P3HT OPTs. Jung et al.28 reported a more than
10-fold increase in R from BPE-PTCDI NW-based OPTs after functionalization with
Au NRs at 350-nm wavelength of light. The Au NR-attached OPTs also showed R
values of 10.7 AW�1 under 980-nmNIR light, whereas pristine OPTs were insensitive
to the NIR spectral region (Figures 9A‒9C). In addition, Lee et al.84 first reported the
utilization of multiple-patterned Au gate electrodes composed of various nanoscale
posts and grating patterns. R was enhanced by more than 950% (0.85 to 8.11 AW�1)
at 460-nm light (Figures 9D‒9F) in comparison with OPTs based on flat gate elec-
trodes, whereas those of the grating-patterned OPTs and nanopost-patterned
OPTs were 62% (1.38 A W�1) and 211% (2.64 A W�1), respectively.
Ligand Systems
Metal ligands are also utilized for enhancing the performance of OPTs by intro-
ducing metal-ligand CT from a metal ligand to the active component of the device.
Liu et al.85 reported a high-performance of OPTs by applying ruthenium (Ru) com-
plexes (Figures 9G and 9H). They fabricated devices by simple drop-casting the
Ru-complex on BPE-PTCDI films. The BPE-PTCDI OPTs functionalized with Ru-com-
plex exhibited �5,000 times higher R values than pristine BPE-PTCDI OPTs. In addi-
tion, a 10 3 10 array of the OPTs was fabricated, and it was highly flexible and twist-
able. The devices exhibited no significant changes in m after 6,000 bending cycles at
a 10-mm bending radius and with twisting of up to 40�.
Approaches to Flexible Phototransistors
Nanofiber Systems
In general, 1D organic semiconducting nanomaterials are suitable for applications in
flexible and stretchable sensors because of their higher flexibility and stretchability
and surface-to-volume ratios than film-type materials, which is beneficial for
enhancing sensitivity. Lee et al.86 fabricated highly flexible organic nanofiber photo-
transistors based on a highly flexible poly(ethylene terephthalate) (PET) textile/pol-
y(dimethylsiloxane) (PDMS) composite substrate (Figures 9I‒9K). Organic nanofibers
were obtained by electrospinning with a mixture of poly(3,3%-didodecylquarterthio-
phene) (PQT-12) and poly(ethylene oxide) (PEO) as the semiconducting polymer and
processing aid, respectively. The nanofiber phototransistors fabricated on the PET/
PDMS textile composite substrate showed highly stable device performance (ON-
current retention up to 82.3% G 6.7%) under extreme bending conditions, with a
bending radius down to 0.75 mm and repeated tests over 1,000 cycles, whereas
those prepared on film-type PET- and PDMS-only substrates exhibited much poorer
performances. Themaximum R, P, and EQE values under blue light illumination were
930 mA W�1, 2.76, and 246%, respectively. Furthermore, highly flexible 10 3 10
photosensor arrays were fabricated to detect incident photonic signals with high
resolution. Representative examples of OFET-based photosensors are summarized
in Table 3.
OFET-Based Pressure Sensors
In this section, we introduce several OFET-based pressure sensors used to convert
force into an electric quantity involving variation of an electrical property (resis-
tance, capacitance) and generating charge displacement (piezoelectric property),
as shown in Figure 10.87,88 OFETs have been widely developed for detecting
external force and strain. OFET-based tactile-perception systems are highly prom-
ising for flexible artificial intelligence products. Flexible pressure sensors are a vital
Chem 3, 724–763, November 9, 2017 743
Figure 9. Approaches to High-Performance OPTs (Light-Trapping Effects and Ligand Systems)
and Flexible OPTs (Nanofiber System)
(A‒C) TEM image of BPE-PTCDI/Au NR hybrid NWs (scale bar, 1 mm) (A). Real-time photoresponse
behaviors of hybrid NW-based OPTs as a function of time under the illumination of monochromatic
light with a wavelength of 350 nm (UV), 460 nm (blue), 532 nm (green), 670 nm (red), and 980 nm
(NIR) (B). EQEs of the hybrid NW-based OPT (C). Reproduced with permission from Jung et al.28
Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
(D‒F) Device structure of OPTs utilizing multiple-patterned gate electrodes (D). SEM image of
multiple-patterned gate electrodes (scale bars, 500 nm) (E). EQEs of OPTs as a function of gate
voltage under 460, 532, and 670 nm wavelength of light with flat, grating, nanopost, and multiple
patterns at VDS = 100 V (500 mW cm�2). Reproduced with permission from Lee et al.84 Copyright
2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
(G and H) Device structure of OPTs utilizing a Ru-complex ligand (inset: photograph of a 10 3 10
OPT array; scale bar, 15 mm) (G). R values of Ru-complex 1/BPE-PTCDI/SiO2/Si and BPE-PTCDI/
SiO2/Si devices as a function of gate voltage (l = 450 nm, 1.5 mW cm�2) (H). Reproduced with
permission from Liu et al.85 Copyright 2016 American Chemical Society.
(I‒K) Cross-section of PDMS-buffered PET textile-based OFETs (scale bar, 50 mm) (I). Transfer
characteristics of PDMS-buffered textile-based OPTs under light illumination at various
wavelengths (J). Photograph of textile-based OPTs upon bending (bending radius z 0.75 mm) (K).
Reproduced with permission from Lee et al.86 Copyright 2016 Wiley-VCH Verlag GmbH & Co.
KGaA, Weinheim.
744 Chem 3, 724–763, November 9, 2017
Table 3. OFET-Based Photosensors
Semiconductor Mobility(cm2 V�1 s�1)
Wavelengthof Light (nm)
Photoresponsivity (Light Source,Intensity)
AdditionalStrategy
Transistor Partfor AdditionalStrategy
Flexibility(SubstrateMaterials)
BPTT71 0.082 300‒500 82 AW�1 (380 nm, 1.55 mW cm�2) NA NA NA
CuPc72 0.02 365‒650 2 A W�1 (365 nm, 1.55 mW cm�2) NA NA NA
4(HPBT)-benzene73 0.0013 436 4,300 A W�1 (6.8 mW cm�2) NA NA NA
4(HP3T)-benzene73 NA 436 390 A W�1 (6.8 mW cm�2) NA NA NA
BODIPY-BF274 0.113 760‒960 11,400 A W�1 (850 nm,500 mW cm�2)
NA NA NA
F8T275 0.003 white light 1 A W�1 (2.9 mW cm�2) NA NA NA
P(DPP4T-co-BDT)76 0.047 white light 4,000 A W�1 (9.7 mW cm�2) NA NA NA
F16CuPc77 0.1 white light NA nanoribbon semiconductor NA
Me-ABT78 1.66 white light 12,000 A W�1 (30 mW cm�2) nanoribbon semiconductor NA
A-EHDTT79 1.6 400 14,000 A W�1 (1.4 mW cm�2) single crystal semiconductor NA
BPE-PTCDI16 1.13 450‒680 14,000 A W�1 (635–680 nm,14 mW cm�2)
nanowire semiconductor NA
BPE-PTCDI/rGO81 0.23 white light 5.63 A W�1 (36 mW cm�2) nanowire, D/Ainterface
semiconductor NA
P3HT/PEHTPPD-BT82 0.0003 550‒1,000 450 mA W�1 (665 nm,1.4 mW cm�2)
D/A interface semiconductor bending angle:30� (PEN)
P3HT83 0.0237 solar radiation 27 A W�1 (NA) light-trapping(Au NPs)
semiconductor NA
BPE-PTCDI28 0.175 350‒980 770,000 A W�1 (670 nm,2.5 mW cm�2)
light-trapping(Au NPs)
semiconductor NA
BPE-PTCDI84 0.044 460‒670 8.11 A W�1 (460 nm,500 mW cm�2)
light-trapping(Au nanopatterns)
gate electrode NA
BPE-PTCDI85 0.0136 450 7230 A W�1 (450 nm,1.5 mW cm�2)
ligand System(Ru-complex)
semiconductor twisting angle:40�, 10 mmbending radius (PI)
PQT-12:PEO86 0.0225 470‒670 930 mA W�1 (450 nm,1.5 mW cm�2)
nanofiber system semiconductor bending radius:0.75 mm (PETtextile/PDMS)
NA, not available.
component of electronic skins that would allow future biomedical prostheses and
robots to interact with human beings and the environment. Mobile health moni-
toring and remote medical diagnostics are attractive potential applications for
these sensors.
OFET-Based Resistive-Type Pressure Sensors
One of the most promising OFET-based pressure sensors is a resistive-type sensor.
Its transduction mechanism is based on transducing the resistance change of the
device into an electrical signal. The difference in resistance between two materials
created by an external force (F) is the main source of the electrical signal change.
Because of the simple structure, easy readout mechanism, and high pixel density,
resistive-type pressure sensors have been intensively investigated.89 They also
have the advantages of facile fabrication with flexible materials and relevance for
use over a wide range of pressures.90 Lee et al.91 reported the fabrication of
ultra-flexible, optically transparent OFET-based resistive-type pressure sensors by
using composite nanofibers. Dinaphtho[2,3-b:20,30-f]thieno[3,2-b]thiophene(DNTT) was used as the active layer on the parylene gate insulator, as shown in
Chem 3, 724–763, November 9, 2017 745
Figure 10. Schematic Images of Transduction Methods of OFET-Based Pressure Sensors
Various transduction mechanisms, including resistivity, capacitance, and piezoelectricity, for
converting applied pressure into electrical signals. The resistive-type, capacitance-type, and
piezoelectric-type sensors detect the pressure by utilizing the changes in the conductivities of the
electrodes, the capacitance changes of the dielectric layers, and the additionally applied voltages
to the electrodes, respectively.
Figures 11A‒11C. Then, a pressure-sensitive sheet consisting of a fluorinated
copolymer nanofibers with a mixture of carbon nanotube (CNT) and graphene
was fabricated and connected to a ground. The resistance of the pressure-sensitive
sheets decreased drastically by a factor of a million, revealing their applicability at a
very small pressure (�800 Pa), under a wide range of bending conditions (bending
radius from 15 mm to 80 mm). In contrast, the resistance of the thin-film sensors
decreased by a factor of 10 with the application of 10 kPa of pressure, as shown
in Figures 11D and 11E. The extremely large change of resistance enabled the sen-
sors with a pressure-sensitive sheet to easily detect very small pressure signals,
such as blood pressure and other biological signals. The excellent uniformity of
this sensing performance was confirmed for a 9 3 9 cm2 sensor array. Furthermore,
a number of research groups have developed microstructured composites of
numerous materials, such as elastic polymers and conductive carbon materials,
to create highly sensitive wide-impedance piezoresistors. Tee et al.92 developed
OFET-based resistive-type sensors composed of a CNT composite. To create a
wide-impedance piezoresistor, CNT was dispersed in a polyurethane (PU) elas-
tomer molded into pyramidal microstructures. Incorporation of microstructured py-
ramidal elastomers reduced the effective modulus and concentrated the electric
field, resulting in improved piezoresistive properties in relation to those of an un-
structured film. Moreover, the size and spacing of the pyramidal microstructures
were controlled to optimize the sensitivity and working range of the sensors. The
integration of OFETs with pressure-sensitive materials, which can be produced
by low-cost processing technologies such as large-area manufacturing technology,
will provide an ideal solution for realizing practical OFET-based pressure sensors.
Someya et al.93 fabricated 32 3 32 arrays of flexible pressure sensors by using
746 Chem 3, 724–763, November 9, 2017
Figure 11. OFET-Based Resistive-Type Pressure Sensors
(A‒E) Schematic illustration of vertical structure of a single pressure sensor (A). Optical image of a
single OFET (W/L = 100; scale bar, 100 mm) (B). Photograph of an integrated sensor array attached
to the surface of a soft balloon, to which a pressure was applied by a pinching motion (scale bar, 1
cm) (C). Pressure versus resistance curve of the nanofiber sensor (red) and thin-film sensor (black)
Chem 3, 724–763, November 9, 2017 747
Figure 11. Continued
sandwiched between Au electrodes (inset: pressure from 0 to 1 kPa) (D). Typical output
characteristics of a transistor (E). Reprinted with permission from Lee et al.91 Copyright 2016 Nature
Publishing Group.
(F‒J) Manufacturing process flow of a pentacene-based pressure sensitive sensor (F). Output
characteristics of the transistors with pentacene as a channel layer, showing p-type conduction (G).
Photograph of electronic artificial skin (H). A magnified image of a sensor cell (scale bar, 0.5 mm) (I).
Transfer characteristics under various pressures from 0 to 30 kPa (inset: the circuit diagram of each
sensor, where VBL, bit line; VWL, word line; VDD, supply voltage) (J). Reprinted with permission from
Someya et al.93 Copyright 2004 National Academy of Sciences.
(K‒M) Demonstration of a pressure sensor active matrix on an arbitrary curved surface (K). Pressure
sensor matrix spread over an egg. The pressure spatial distribution of a sensor matrix over an egg
(L). The spatial distribution of temperature that is converted from the temperature-dependent
current in the thermal sensor network; a copper block (15 3 37 mm2) with the temperature
maintained at 50�C is positioned diagonally (indicated by the dotted line) (M). Reprinted with
permission from Someya et al.94 Copyright 2005 National Academy of Sciences.
(N‒P) Circuit diagram of an ultra-flexible active-matrix pressure-sensor array in the shape of a
tightly wound helix (N). Photographs of an OFET array fabricated on a shape-memory polymer film
and permanently transformed into a helix (O). Transfer characteristics of an individual sensor cell
measured at two different pressures (P). Reprinted with permission from Sekitani et al.25 Copyright
2010 Nature Publishing Group.
photolithography. The complete devices, including rubber pressure sensors, con-
tained a pentacene active channel layer and the pressure-sensitive flexible layer
consisted of electrically conductive graphite particles (Figures 11F‒11J). As a pres-
sure ranging from 0 to 30 kPa was applied to the device, the resistance of the rub-
ber sheet varied from 10 MU to 1 kU, and the transconductance and current
increased. The electrical performance did not change when the bending radius
was changed from 50 to 10 mm. Well-resolved pressure-mapping images were
obtained with the OFET-based active-matrix sensor array, and the overall configu-
ration was similar to a memory cell and pixel of a charge-coupled device. Someya
et al.94 also demonstrated pentacene-based OFETs with a net-shaped structure for
both pressure (using conductive rubber) and temperature (utilizing organic diodes)
sensing, which allowed the E-skin films to be extended by 25% (Figure 11K). The
magnitude of the drain current with and without applying pressure was not
changed when the tension increased up to 25% stretching, whereas the device
was out of function over 30% stretching as a result of electronic disconnections
at the bridge wiring. Furthermore, as shown in Figures 11L and 11M, the device
showed stable pressure-mapping properties on curved egg surfaces and worked
well as a temperature sensor over a wide temperature range (30�C–80�C). Sekitaniet al.25 demonstrated pentacene-based OFETs and complementary circuits that
continued to operate without degradation at a bending radius of 100 mm. On
the basis of these ultra-flexible OFETs, they manufactured a thin catheter (a tightly
wound pressure-sensor helix) that measured the spatial distribution of mechanical
pressure (Figures 11N and 11O). The sensor was fabricated by laminating three
sheets, a foldable 4 3 36 array of OFETs, a pressure-sensitive rubber sheet, and
a 12.5-mm-thick PI sheet with an Au counter electrode. When mechanical pressure
was applied on the catheter, the electrical resistance between the top and bottom
surfaces of the rubber sheet decreased. A potential of �3 V applied to the counter
electrode was supplied to the OFETs at the positions where pressure was applied,
and thus the spatial distribution of pressure was obtained by interrogating the
OFETs in the active-matrix array (Figure 11P).
OFET-Based Capacitive-Type Pressure Sensors
A capacitor is a passive electronic component that stores energy in the form of an
electrical charge. Capacitors consist of two conducting plates separated by a
748 Chem 3, 724–763, November 9, 2017
dielectric material. OFET-based capacitive-type sensors can be built with flexible
materials, making them a suitable choice for pressure measurements. In a parallel
plate capacitor, capacitance is commonly changed as a function of applied force.
Capacitance is given by the equation C = 3r 30Ȃ/d, where C is the capacitance, 3r is
the relative static permittivity of the materials, 30 is the electric constant, Ȃ is the
area of overlapped region of the devices, and d is the thickness of the dielectric
layer. Recently, several groups have developedOFET-based pressure sensors by us-
ing microstructure-engineered dielectric materials. The capacitance upon compres-
sion of the dielectric is significantly changed because the constants Ȃ and d are
changed easily with deformation of the dielectric layer. Furthermore, the pressure
sensitivity is tunable by controlling the film thickness and using different microstruc-
tures. The combination of microstructured polymers and high-mobility semicon-
ducting materials in an OFET design is a highly promising approach to realize
high-performance flexible pressure sensors that can be used for mobile health moni-
toring and remote mobile diagnostics.95
Microstructured films have been integrated into OFETs as the dielectric layer, form-
ing a new type of active sensor device with highly sensitive and fast response times.
Mannsfeld et al.18 fabricated single-crystal rubrene and microstructured PDMS films
for OFET-based pressure sensors. A single-crystal semiconductor was used because
of the high mobility, low surface density of traps, and suitability for low-voltage op-
erations. The highly pressure-sensitive microstructured PDMS films were prepared
and used as dielectric layers. The sensing mechanism is related to the direct depen-
dence of the electrical current in the transistor on the gate dielectric capacitance. In
addition, pressure-sensitive OFET arrays with low-impedance output, active-matrix
design, and low power consumption were successfully demonstrated. The device
configuration and performance characteristics of the OFET-based pressure sensors
are shown in Figures 12A‒12C. A rubrene single crystal, grown with PVT, was lami-
nated on top of bottom-contact Au electrodes on a Si wafer; these crystals typically
exhibited a field-effect hole mobility of 1 cm2 V�1 s�1, and the maximum slope of the
relative capacitance change of the pyramidal film in the 0.2 kPa range was
0.55 kPa�1. In addition, Kim et al.96 reported a highly sensitive capacitive pressure
sensor composed of a polymer dielectric film with a nanoneedle structure. Highly
sensitive pressure sensors were fabricated by inkjet printing and patterning of the
dielectric filler. Dielectric microstructures were analyzed for selection of the optimal
geometry to record sensitivity for pressure sensors, especially in the range of less
than 1 kPa. The pressure-sensitive nanoneedle PU film was sandwiched between
two Al electrodes, and the change in capacitance of the sensors was measured.
When pressure was applied to the top electrode of the integrated OFET sensor,
the thickness of the nanoneedle film placed above the gate electrode decreased,
and then the total capacitance between the semiconductor layer and the electrode
increased. This capacitance change led to increased charge accumulation in the
OFET channel, which in turn increased the drain current.
Schwartz et al.19 constructed flexible pressure-sensitive OFETs by assembling sepa-
rate layers via lamination. Polyisoindigobithiophene-siloxane (PII2T-Si)-based semi-
conductors were placed with bottom-contact electrodes on PI substrates. They
reported the fabrication of flexible pressure-sensitive OFETs with a maximum sensi-
tivity of 8.4 kPa�1, a fast response time of <10 ms, high stability over >15,000 cycles,
and a low power consumption of <1 mW. The combination of the high-mobility
semiconducting PII2T-Si and a microstructured PDMS dielectric in a monolithic tran-
sistor design enabled the devices to be operated in a subthreshold regime, where
the capacitance change upon compression of the dielectric layer was strongly
Chem 3, 724–763, November 9, 2017 749
Figure 12. OFET-Based Capacitive-Type Pressure Sensors and Response of an OFET with a Microstructured PDMS Dielectric Layer
(A‒C) Layout of pressure-sensing organic single-crystal transistors, consisting of thin rubrene single crystals and structured PDMS dielectric films (A).
Output characteristics of a transistor-based sensor with different external pressures applied; the legend lists the applied loads in the order of the
original loading cycle (B). Relative current change versus pressure of sensing results and fitted data; the change in ID, DID (diamond symbols), is
proportional to the measured relative change in capacitance as expected in the OFET saturation regime (inset: time-resolved measurements with
excellent response and relaxation times) (C). Reprinted with permission from Mannsfeld et al.18 Copyright 2010 Nature Publishing Group.
(D‒F) SEM pictures (scale bars, 10 mm) of four different designs of microstructured PDMS: pyramids with height of 3 mm and spacing of 1.33, 3.79, 8.85,
and 13.61 mm and taken at 45� tilt angle (D). Relative capacitance change of capacitors with the PDMS microstructures as the dielectric (E). Relative
current change of a transistor with integrated microstructured PDMS dielectric with 8.85 mm spacing at different transistor source-drain voltages and
source-gate voltages (F). Reprinted with permission from Schwartz et al.19 Copyright 2013 Nature Publishing Group.
(G‒K) Schematic diagram of the pressure sensor prepared with graphene and ion gel dielectric (G). Integration of the top PET cover and the matrix
backplane (H). Relative current change of the pressure sensor with graphene and ion gel dielectric; error bars represent the SD obtained for the 16
devices (I). Spatial pressure mapping of the pressure sensor matrix (J). Pressure-sensing characteristics of the sensor mounted on a PDMS rubber
substrate (inset: attached sensor matrix with a human hand or a table tennis ball) (K). Reprinted with permission from Sun et al.97 Copyright 2014 Wiley-
VCH Verlag GmbH & Co. KGaA, Weinheim.
750 Chem 3, 724–763, November 9, 2017
amplified (Figures 12D‒12F). Sun et al.97 fabricated a transparent flexible coplanar
gate OFET pressure-sensor matrix (4 3 4 pixels), which consisted of graphene and
an ion gel gate dielectric on a plastic or rubber substrate for E-skin applications (Fig-
ures 12G and 12H). When pressure was applied, the sensor matrix induced contact
between the square-type graphene on the top cover and the bottom interdigitated-
type graphene on the OFET backplane, causing a decrease in the resistance be-
tween the source and drain electrodes and a higher transconductance. The devices
exhibited excellent pressure-sensing characteristics, including a high-pressure
sensitivity of 0.12 kPa�1, excellent mechanical durability over 2,500 cycles, and
spatial sensing ability (Figures 12I‒12K).
OFET-Based Piezoelectric-Type Pressure Sensors
Piezoelectricity is also a commonly used transduction method for pressure sensors.
Piezoelectric-type sensors are based on the piezoelectric effect of certain types of solid
materials, which generate an electrical charge when amechanical stress is applied. The
piezoelectric effect caused by the occurrence of electric dipole moments in solids and
the parasitic effects tend to recombine the charge, making the material briefly neutral.
Because of their high sensitivity and fast response, piezoelectric-type sensors are
widely used for the detection of dynamic pressures, such as vibrational signals. How-
ever, these transducers can barely measure static or slow-varying forces; this can be
solved by combining information from other kinds of pressure sensors or OFETs. In
addition, piezoelectric-type sensors are good candidates for developing self-powered
and low power consumption sensing devices. Apart from utilizing a single piezoelectric
material as a transducer, combination with a piezoelectric material and an amplifier
element, such as a dual-gate (DG) OFET, can also be used to build flexible pressure
sensors with improved sensitivity.98 Kim et al.99 presented the precise detection of
unknown and small amount of stimuli for both dynamic and static pressures by
measuring output signals from an OFET platform with highly sensitive microstructured
functional gate dielectrics (Figures 13A and 13B). Because of the larger flexoelectricity-
enhanced piezoelectric effects in pyramidal microstructures, organic piezoelectric
material poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) greatly improved
the responsivity of the OFET sensor to detect pressures as low as 20 Pa.
In addition, as described above, piezoelectric transducers can be used to measure
static or slow-varying forces with high sensitivity by combining other kinds of pres-
sure sensors or OFETs. Tsuji et al.23 reported an OFET-based piezoelectric-type
sensor with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) and
P(VDF-TrFE) layers (Figures 13C and 13D). As the piezoelectric layer of the sensing
capacitor, the P(VDF-TrFE) layer was blade coated onto a Si wafer. The electrical
properties of the OFETs were characterized by changing the orientation of the
PVDF-TrFE chain. The pressure response of the sensor device was evaluated by
measuring the change in ID of the OFET under pressure loading on the piezoelectric
layer. Consequently, highly sensitive pressure sensors with relative current change of
155 versus a pressure load of 300 kPa and a low operation voltage of �5 V were
demonstrated. Furthermore, Thuau et al.98 reported an organic micro electro-me-
chanical system (MEMS) sensor with a cutting-edge electro-mechanical transducer
consisting of P(VDF-TrFE) piezoelectric polymer as the active gate dielectric (Figures
13E and 13F). Such an advanced scheme and poling process enabled highly efficient
integrated electro-mechanical transduction for physical and humidity sensing appli-
cations. As shown in Figure 13G, the large enhancement of the strain sensitivity by a
factor of 18 after poling clearly demonstrates the benefit offered by the piezoelectric
effect. In the low strain regime (<0.3%), this device showed higher sensitivity than
poly(methyl methacrylate) (PMMA) or unpoled P(VDF-TrFe) dielectric devices
Chem 3, 724–763, November 9, 2017 751
Figure 13. Piezoelectric-Type Pressure Sensors and Their Responses under Different Applied Pressure
(A and B) The structure of an OFET and a transistor array composed of microstructured P(VDF-TrFE) (A). Relative current change in a piezoelectric-type
pressure sensor with error bars after repeated conduction up to five times (B). Reprinted with permission from Kim et al.99 Copyright 2014 Wiley-VCH
Verlag GmbH & Co. KGaA, Weinheim.
(C and D) Schematic structure of the dual-gate organic pressure sensor (C). Relative current change in the OFET at VGS = �1.8 V; the pressure load was
incrementally applied onto the device from 0 to 300 kPa with a step pressure load of 100 kPa (inset: log ID as a function of the pressure load at VGS =�1.8
V) (D). Reprinted with permission from Tsuji et al.23 Copyright 2017 Japan Society of Applied Physics.
(E‒I) Illustrative schematic images of an OFET-embedded micro-cantilever sensor and 1.5 cm 3 1.5 cm chip containing six OFETs (E). SEM image of a
fabricated OFET-embedded cantilever sensor (scale bar, 1 mm) (F). Real-time measurements of relative current changes in the device submitted to
different applied tensile strains of 0%, 0.045%, 0.12%, 0.16%, 0.18%, and 0.28% (G). Relative current change as a function of applied strain ( 3) for different
coupled gate dielectric layers in a polarized OFET-embedded cantilever (H). Relative humidity control as a function of time (middle) and relative
changes of ID plotted (bottom); optical images of the OFET-embedded sensor with a humidity-sensitive layer on top at 20% RH and 80% RH are
displayed on top (I). Reprinted with permission from Thuau et al.98 Copyright 2016 Nature Publishing Group.
752 Chem 3, 724–763, November 9, 2017
Table 4. OFET-Based Pressure Sensors
Semiconductor Mobility(cm2 V�1 s�1)
Pressure Sensitivity DetectionLimit
Additional Strategy TransistorPart forAdditionalStrategy
Flexibility (SubstrateMaterials)
DNTT91 0.9 0‒10 kPa NA 0.4 g, 80 mmHg resistive-type (nanofibersystem)
gate 80 mm bendingradius (PET)
P3HT92 0.1 0‒100 kPa NA 10 kPa resistive-type (pyramidalCNT composite)
gate NA (PU)
Pentacene93 1.4 0‒30 kPa NA NA resistive-type(conductive rubber)
gate 2 mm bendingradius (PEN)
Pentacene94 1 0‒30 kPa NA NA resistive-type(conductive rubber)
gate 2 mm bendingradius (PET)
Pentacene,F16CuPc25
0.5 0‒1 kPa NA NA resistive-type(conductive rubber)
drain source 0.1 mm bendingradius (PI)
Rubrene18 1 0‒20 kPa 0.55 kPa�1 0.2 Pa capacitive-type(pyramidal PDMS)
dielectric NA (PET)
NA96 NA 0‒7 kPa 1.76 kPa�1 0.2 Pa capacitive-type(nanoneedle PU)
dielectric NA (PEN)
Pil2TSi19 2.45 0‒20 kPa 8.4 kPa�1 <1 kPa capacitive-type(pyramidal PDMS)
dielectric NA (PET)
Graphene97 NA 0‒60 kPa 0.12 kPa�1 NA capacitive-type (ion gel) dielectric NA (PET)
Pentacene99 NA 0‒80 Pa 1.02 kPa�1 <0.013 Pa piezoelectric-type(microstructured P(VDF-TrFE))
gate NA (PET)
TIPS-pentacene23 0.27 0‒300 kPa 0.51 kPa�1 NA piezoelectric-type(polarized P(VDF-TrFE))
gate NA
DNTT/pentacene98
0.1 0%–0.28%(tensile strain)
NA 0.05% strain piezoelectric-type(P(VDF-TrFE))
gate strain: 0.05% (PEN)
NA, not available.
(Figure 13H). Also, the fabricated MEMS devices were tested as humidity sensors.
The sensitivity of the humidity sensor was estimated to be 7,500 ppm/% relative hu-
midity (RH) with an extracted limit of detection of 0.2% RH and good repeatability
and reversibility (Figure 13I). Representative examples of OFET-based pressure sen-
sors are listed in Table 4.
OFET-BASED TEMPERATURE SENSORS
OFET-based temperature sensors have also been developed by utilizing various ap-
proaches. First, changes in the resistivity between source and drain electrodes upon
changing temperature can be utilized for temperature sensors. Yokota et al.20 re-
ported flexible OFET-based temperature sensor arrays for multi-point measure-
ments by using positive temperature coefficient (PTC) polymers as temperature
sensors with PI substrates. The PTC effects in the polymer with conductive fillers
are generally due to an increase in specific volume as the temperature increases dur-
ing its progression through the melting point of the crystalline region. Thus, resistiv-
ity changes drastically with temperature changes of only a few degrees. Figure 14A
shows a schematic diagram of individual sensors comprising an OFET and a temper-
ature sensor connected to the source electrode of the OFET. The resistivity of the
temperature sensor gradually increased as the temperature increased, leading to
a gradual decrease in the drain current. Furthermore, the device showed
outstanding mechanical stability when bent to a radius of 700 mm and over 200 cy-
cles of bending at a 1-mm radius (Figures 14B and 14C). Moreover, these ultra-flex-
ible sensors detected small physiological temperature changes in rat lung
Chem 3, 724–763, November 9, 2017 753
(Figure 14D). Ren et al.21 constructed flexible organic transistor temperature sensor
arrays by serially connecting thermistors to the drain-source electrode of the switch-
ing OFET in each sensing element based on ultra-thin 12-mm-thick PEN substrate
(Figure 14E). They embedded a thin layer of discontinuous Ag NPs in pentacene
thin film to form a two-terminal temperature-sensing thermistor device. Integrating
AgNPs into pentacene film allowed sensitive hopping-dependent electrical conduc-
tivity with regard to temperature. The integrated temperature sensor showed a large
change in output current (>20 times) upon changing the operating temperature from
20�C to 100�C (Figures 14F and 14G). The temperature sensor also showedmechan-
ical stability under 104 bending cycles at a 3.2-mm bending radius. Moreover, the
16 3 16 OFET array showed a 100% yield with clear temperature mapping property
of contacted irregular-shaped objects (Figure 14H).
A second approach utilizes thermoelectric voltage generators to apply gate bias. Zhao
et al.22 reported an external heat-gated OFET consisting of an electrolyte-gated tran-
sistor and an ionic thermoelectric supercapacitor (ITESC) for sensing temperature.
They fabricated electrolyte-gated transistors by using P3HT as the active semiconduct-
ing layer and poly(vinylphosphonic acid-co-acrylic acid) (PVPA-AA) as the polyanionic
electrolyte insulator for driving voltages of only a few hundreds of millivolts. Then, ionic
thermoelectric voltage generators utilizing PEO-NaOH electrolytes with a remarkable
Seebeck coefficient of 7 mV K�1 were fabricated and combined with the electrolyte-
gated transistors for sensing temperature. Figure 14I shows a schematic diagram of
the ionic thermoelectric gated transistors. One electrode (working electrode) of the
thermoelectric device is connected to the gate of the transistor, whereas the other elec-
trode (grounded electrode) is connected to the source and grounded together.When a
temperature difference is applied, the voltage of the gate is the same as the working
electrode of the thermoelectric device. Because the operating voltage of electrolyte-
gated transistors is of the same order of magnitude as the variations in the thermal
voltage generated by the ITESC, DT changes the transistor output current by more
than two orders of magnitude. Moreover, they demonstrated for the first time that a
heat signal can act as input for logic circuits.
A third approach is based on the intrinsic change of charge-transport characteristics
of organic semiconductors in OFETs with respect to temperature changes. Jung
et al.100 developed an OFET-based temperature sensor by using pentacene as the
semiconductor. They utilized change in the charge-transport characteristics of
OFETs for temperature sensing, which includes temperature and VGS dependence
behaviors in the saturation and subthreshold regimes of the OFET (Figure 14K).
The OFETs were prepared in a bottom-contact configuration, as shown in the inset
of Figure 14K. The drain current increased with increasing temperature, from 273 to
453 K, as a result of the positive shift in threshold voltage with increasing tempera-
ture. Figure 14L shows the temperature-dependent transfer curve and drain current
as a function of temperature in the saturation and subthreshold regions of the pen-
tacene-based OFET. The representative characteristics of OFET-based temperature
sensors are listed in Table 5.
DUAL AND SUSPENDED-GATE-TYPE OFET-BASED SENSORS
The first attempt to develop OFET-based sensors has mostly relied on the direct
functionalization of OFETs as electronic readout elements for sensory components.
However, sensing performance, in the case of chemical/biosensors, was limited
because of the continuous damage occurring during the direct contact of electrodes
or semiconductors with analytes. The application of OFETs as sensors in aqueous
754 Chem 3, 724–763, November 9, 2017
Chem 3, 724–763, November 9, 2017 755
Figure 14. OFET-Based Temperature Sensors
(A‒D) A flexible temperature sensor sheet and cross-sectional illustration of a flexible large-area
active-matrix sensor with 12 3 12 temperature pixels (scale bar, 1 cm) (A). Flexibility of the
temperature sensor (B). Flexible cycle test of the temperature sensor (C). Temperature
measurement of respirated lung as heat is exchanged between the lung tissue and air, and time
dependency of the temperature of the living lung (D). Reproduced from Yokota et al.20
(E‒H) Schematic diagram of a flexible temperature sensor device and enlarged schematic diagram
of a single temperature sensor with electrical sign (E). Linear fitting of output currents of 256
devices on an Arrhenius plot of temperature (F). Transfer characteristics of the OFET at different
temperatures (solid line, ID; dashed line, gate leakage currents) (G). Schematic diagram of the
temperature measurement setup and temperature distribution measured from the sensor array
(the dashed lines indicate where the Peltier heater is located) (H). Reproduced with permission from
Ren et al.21 Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
(I and J) The structure of the ionic thermoelectric gated transistor (I). Output characteristics at a
different fixed gating DT (J). Reproduced with permission from Zhao et al.22 Copyright 2017 Nature
Publishing Group.
(K and L) Temperature-dependent transfer characteristic of a pentacene OFET (VDS = �60 V) (K). IDas a function of temperature in the saturation and subthreshold regions (L). Reproduced with
permission from Jung et al.100 Copyright 2007 Applied Physics Letters.
solution inevitably leads to Faradaic leakage currents because of electrolysis. To
avoid this direct contact, the semiconductor must be covered with a barrier layer.
DG FETs, a type of FET with a second gate electrode, are an excellent sensor plat-
form for detecting analytes in aqueous systems or biological applications, because
the semiconducting layer is separated from the loading part of the analyte solution
and direct exposure to the analyte solution is prevented. The first DG OFET was
developed in 2005 by Cui and Ling.101 Since then, a number of studies on DGOFETs
have been reported. Typical semiconductors used for DG OFETs included penta-
cene, poly(9,9-dioctylfluorene-co-)phenylene-(N-4-sec-butylphenyl)-iminopheny-
lene (TFB), and F8T2. The dielectrics utilized commodity-insulating polymers such
as divinyltetramethyldisiloxane bis(benzocyclobutene) (BCB), parylene, PI, and
PVP. Both rigid substrates, such as Si/SiO2 and glass, and flexible substrates, such
as polyethylene naphthalate and PI, were used.
The schematic layout of a DG FET is shown in Figure 15A.102 The DG FET consists of
top- and bottom-gate electrodes and gate insulators, semiconductor, and source/
drain electrodes. Both gate electrodes can deplete or accumulate charges in the
semiconductor, and the accumulated charges are concentrated between the dielec-
tric layer and the semiconductor. The current in the DG FET at a given source-drain
bias is determined by the interplay between the biases on the two gate electrodes.
Thus, DG FETs are highly suitable for use in chemical sensors and biosensors.103
Sensitivity can be enhanced by DG systems, and the indirect detection of analyte
charges helps sensors maintain their performance for a longer time. Spijkman
et al.104 reported the operating mechanism of a DG transistor based on elastic poly-
mers as sensors for detecting analytes in solution. The transducer is displayed sche-
matically in Figure 15B. For the top dielectric, a dual layer of 700-nm-thick polyiso-
butylmethacrylate and amorphous Teflon was used to prevent water penetrating to
the semiconductor layer. Amplification of small changes in surface potential caused
by capacitive coupling between the two gate dielectric layers makes DG FETs attrac-
tive for sensing applications, for which ISFETs are currently used.
Another approach to developing and modifying the electrical signals of OFET sen-
sors is to use suspended gate (SG) OFETs. SGOFETs are DG systems constructed by
combining two OFETs. Zang et al.105 fabricated a flexible SG OFET that can be con-
structed with a versatile platform for ultra-sensitive pressure detection. Diketopyrro-
lopyrrole (DPP)-based polymer PDPP3T-based SG OFET pressure sensors, with an
756 Chem 3, 724–763, November 9, 2017
Table 5. OFET-Based Temperature Sensors
Semiconductor Mobility(cm2 V�1 s�1)
Temperature (�C) Sensitivity Resolution Additional Strategy Transistor Part forAdditional Strategy
Flexibility(SubstrateMaterials)
DNTT20 2.2 25‒40 NA D 0.1�C resistive-type (copolymerwith graphite filler)
drain source 700 mm bendingradius (PI)
Pentacene21 0.5 20‒100 20.4 DR/R/D 0.2�C resistive-type (Ag NPs) semiconductor 3.2 mm bendingradius (PEN)
P3HT22 NA 15‒45 NA NA thermoelectric voltagegenerators
gate NA
Pentacene100 0.15 0‒180 NA D 20.0�C NA NA NA
NA, not available.
elastic rubber dielectric layer on indium tin oxide (ITO)-coated PET film and 10-mm-
thick aluminum foil fixed onto the support layer, serving as the SG, are shown in Fig-
ure 15C. Moreover, the combination of an air dielectric layer and SG electrodes
enabled the intrinsic limitations of the polymeric dielectric (pressure-sensitive) layer
in OFET-based pressure sensors to be overcome. By fine-tuning the properties of
the SG, the SG OFET was optimized to achieve an unprecedented sensitivity of
192 kPa�1 and a low limit-of-detection pressure of <0.5 Pa; OFET-based pressure
sensors based on a polymeric dielectric layer have not yet reached ultra-high sensi-
tivity (>100 kPa�1). In real-time sensing, these highly sensitive SG OFETs detected
very low pressures, such as acoustic waves and wrist pulses, thus allowing their appli-
cation in artificial intelligence and health care devices. In addition, Zang et al.106
developed a dual organic-transistor-based tactile-perception element (DOT-TPE)
with biomimetic functionality by using organic synaptic transistors with integrated
sensing transistors. The unique geometry of the textile sensor permitted instanta-
neous sensing of pressure stimuli and synapse-like processing of electric signals in
a single element. By taking advantage of the pressure-electricity transduction of
SG transistors and proton-electron coupling at the dielectric-organic layer interfaces
of organic synaptic transistors, the proposed DOT-TPE consisted of a pressure-
sensing OFET and a signal-processing OFET to combine signal transduction and
processing functions within a single element (Figures 15D‒15F).
Incorporating the novel emergent technology of circuit design into previously devel-
oped OFET-based sensors could contribute to the realization of sensing devices
with new features. In the near future, the development of various types of sensors
will lead to multifunctional sensors that can detect different signals with one device.
These possible applications make OFET-based sensors an exciting field of research.
MATERIAL DEVELOPMENT FOR OFET-BASED SENSORS: CHEMISTRYASPECT
Because OFET-based sensors are based on organic materials, advances in chemis-
try (chemical approaches) typically play a key role in enhancing the performance
and stability of OFET-type sensors. In the case of OFET-based chemical sensors
and biosensors, poor solvent resistance of typical organic materials has limited
their applications; for instance, it has been challenging to develop OFET-based
sensors to detect liquid-phase analytes, such as analytes dissolved in water or
organic solvents. The environmental and operational stabilities of OFET-based
chemical sensors and biosensors have usually been achieved by advancements in
chemistry, which has significantly expanded the range of their practical applica-
tions. Among several approaches to enhance the chemical robustness of OFET-
based chemical sensors, physical or chemical cross-linking in the semiconducting
Chem 3, 724–763, November 9, 2017 757
Figure 15. Device Structure of DG ISFET-, DG OFET-, and SG OFET-Based Tactile-Perception Systems
(A) A cross-sectional schematic image of the DG ISFET. Reprinted with permission from Jang and Cho.102 Copyright 2014 Nature Publishing Group.
(B) Device structure for the Braille sheet display with the DG FET of the SRAM cell on the bottom. Reproduced with permission from Spijkman et al.104
Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
(C) SG OFET-based array with magnified device geometry. Reproduced from Zang et al.105
(D‒F) Schematic illustration of the suspended gate OFET (D). Relative current changes of the sensing device (top) and the Ipost responses of the synaptic
transistor (bottom) under different pressures (Ipost, postsynaptic current) (E). Photograph (left) and the transfer curves (right) of the synaptic OFET under
the flat and bent states (F). Reprinted with permission from Zang et al.106 Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
layer for enhancement of chemical robustness is a representative chemistry-based
approach. Azide functional groups have been used as effective photo-initiated
cross-linking agents for the formation of chemically robust semiconducting layers,
and the cross-linked semiconducting layers have been applied to solvent-resistant
OFETs45 and OFET-based chemical sensors for directly detecting liquid-phase an-
alytes.29 Furthermore, small molecular organic semiconductors, such as DDFTTF
with hydrophobic long dodecyl alkyl chains attached to the conjugated backbone,
have been developed to fabricate water-stable organic electronic devices.32–34,107
This molecular design strategy can provide a waterproof surface on an organic
758 Chem 3, 724–763, November 9, 2017
semiconducting layer with dense molecular packing, which is highly effective for
the fabrication of water-stable organic chemical sensors and biosensors. In the
case of OPTs, rational molecular design in the chemistry aspect is highly desirable
for achieving high-performance OPTs. For example, low band-gap molecules
obtained via rational molecular design make it possible to absorb a broader range
of wavelength in light and enhance the R of OPTs.108 The rigid and planar molec-
ular structures of conjugated molecules typically result in a large internal conver-
sion quantum yield and a high R because of the more efficient absorption of
incident light than with three-dimensional molecules.109 More recently, the supra-
molecular chirality concept, developed via supramolecular chemistry, has shown
great promise for developing chiral organic optoelectronics; for instance, supramo-
lecular nanowires composed of chiral perylene imides exhibited amplified chirality
that can selectively absorb circularly polarized light depending on the enantiomeric
handedness.80 Unlike OFET-based chemical, biological, and photoactive sensors,
most OFET-based pressure and temperature sensors do not detect the stimuli
directly from the semiconducting layers; they detect them by changes in other
components, such as resistance of electrodes,20,25,94 capacitance of dielectric
layers,18,96 or additional voltages generated by piezoelectric23 or thermoelectric22
materials. Therefore, the development of organic semiconducting materials that
can directly detect pressure or heat by molecular design is expected to have
high potential because it has the advantage of simplifying the structure of the
device. Advances in chemistry will continue to provide a variety of tools and plat-
forms for OFET-based sensors, which will expand the range of their applications
and expedite their commercialization.
CONCLUSIONS AND OUTLOOK
Organic sensors are emerging as a next-generation sensor suitable for the IoT era
because of their various advantages such as low cost, large-area processability,
superior mechanical flexibility, and tailorable optoelectronic properties. Among
them, OFET-based sensors, in which stimuli detection and signal amplification
are combined in a single device, are attracting great interest because of their
high sensitivity and selectivity. In this article, we presented a review of the working
mechanisms, current research progress, and applications of OFET-based sensors.
OFET-based sensors have been investigated widely for chemical, biological,
photo-, pressure-, and temperature-sensing applications. The performances of
OFET-based sensors have greatly improved with the development of more suitable
high-performance materials, device architecture, and fabrication techniques. More-
over, OFET-based sensors have shown high mechanical flexibility and operational
stability, revealing their high potential for use in next-generation flexible
electronics.
On the other hand, although the progress in OFET-based sensors is impressive,
their lack of stretchable characteristics must be remedied for their future applica-
tion in wearable electronic devices that can be easily mounted on clothing or
directly attached to the body. To overcome this hurdle, development of stretch-
able, yet electrically stable materials for OFET-based sensors is necessary.
Recently, approaches to developing stretchable organic semiconductor-based
OFETs, such as introducing nanoconfinement effects,110,111 sophisticated molecu-
lar designs for polymer semiconductors,112 and a polymer blend system,113 have
been suggested. However, application of stretchable organic semiconductors to
highly stable organic sensors has scarcely been reported. In addition, develop-
ment of stretchable dielectric114 and electrode115 materials is also necessary for
the realization of fully stretchable OFET-based sensors. Improvement in low
Chem 3, 724–763, November 9, 2017 759
ambient stability of organic semiconductor materials116 is an additional problem to
be solved.
Despite the issues remaining with OFET-based sensors, they have certainly made
great contributions to the development of the sensor field. Their great advantages
as next-generation sensor devices, such as cost-effectiveness, high performance,
and mechanical flexibility, are particularly appealing. Once the remaining issues
have been resolved, OFET-based sensors could eventually contribute to making hu-
man life convenient and healthy and herald a bright future for the fourth industrial
revolution.
AUTHOR CONTRIBUTIONS
J.H.O. proposed the topic of the review and supervised the writing. Y.H.L. and M.J.
equally contributed to writing the followingmain sections: Summary, Bigger Picture,
Introduction, Operation Mechanism of OFET-Based Sensors, OFET-Based Chemical
Sensors and Biosensors, OFET-Based Photosensors, OFET-Based Pressure Sensors,
OFET-Based Temperature Sensors, Dual and Suspended-Gate-Type OFET-Based
Sensors, Material Development for OFET-Based Sensors: Chemistry Aspect, and
Conclusions and Outlook. M.Y.L. and O.Y.K. studied the literature and prepared
the following main sections of the draft manuscript: Operation Mechanism of
OFET-Based Sensors and OFET-Based Chemical Sensors (M.Y.L.) and OFET-Based
Pressure Sensors and Dual and Suspended-Gate-Type OFET-Based Sensors
(O.Y.K.). Y.H.L., M.J., and J.H.O. discussed and revised the manuscript.
ACKNOWLEDGMENTS
This work was supported by the Center for Advanced Soft Electronics under the
Global Frontier Research Program (2013M3A6A5073175) and by the Nano Material
Technology Development Program (2017M3A7B8063825) through the National
Research Foundation, funded by the Korean Ministry of Science and ICT.
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