www.advmat.deDual-Mode Organic Electrochemical Transistors Based on
Self-Doped Conjugated Polyelectrolytes for Reconfigurable
Electronics
Tung Nguyen-Dang, Sangmin Chae, Jirat Chatsirisupachai, Hiba Wakidi,
Vinich Promarak, Yon Visell, and Thuc-Quyen Nguyen*
result in further improvements in energy
efficiency, or to provide significant economic benefits. Several emerging technologies employing new materials andnew device structures have been identified as possible candidates for post-CMOS
electronics.[1–7] Among these, reconfigurable logic circuits are of particular interest
because they make it possible to simplify
circuit design while increasing energy
efficiency.[8–13] The design of reconfigurable logic circuits is based on the use ofindividual transistors that exhibit multiple
functionalities. The most common way
to achieve such capabilities is to employ
ambipolar field-effect transistors withdynamically gate-tunable doping types.
For instance, using double-gate 2D homojunction devices, Pan et al. showed thatthe polarities of gate voltages determined
the major carrier types in ambipolar 2D
semiconductors, thus altering the transistors into n-type or p-type.[8] Using different
combinations of applied electrical input
signals, these 2D transistors were used to fabricate configurable
ON/OFF switching circuits. Similar strategies for achieving
reconfigurable logic functionality with ambipolar transistors
have been demonstrated using metal–graphene–silicon–graphene barristors or using ambipolar organic transistors.[9,10]
These excellent works leverage advances in microfabrication inorder to create complex device structures, such as vertical heterojunctions or multi-gate transistors, that enable their behavior.
The complexities involved could, however, limit their integration in large-scale devices, and result in high fabrication costs.
We believe that to simplify the fabrication process of reconfigurable electronics, it is desirable to have devices that could exhibit
dynamically configurable functionalities using simpler structures, such as conventional top-gate bottom-contact designs.
Organic electrochemical transistors (OECTs) comprise arecently developed class of organic transistors that has emerged
as an electronic platform that is well-suited for reconfigurable
electronics. Recently, OECTs have been investigated for manyelectronic applications, including bio-sensing, wearable devices,and neuromorphic computing systems, where they can serve
as artificial synapses or nonvolatile memories.[5,14–18] A typical
OECT consists of three electrodes (source, drain, and gate), a

Reconfigurable organic logic devices are promising candidates for next
generations of efficient computing systems and adaptive electronics. Ideally,such devices would be of simple structure and design, be power efficient,and compatible with high-throughput microfabrication techniques. This
work reports an organic reconfigurable logic gate based on novel dual-modeorganic electrochemical transistors (OECTs), which employ a self-doped conjugated polyelectrolyte as the active material, which then allows the transistors to operate in both depletion mode and enhancement mode. Furthermore,mode switching is accomplished by simply altering the polarity of the applied
gate and drain voltages, which can be done on the fly. In contrast, achieving
similar mode-switching functionality with other organic transistors typicallyrequires complex molecular design or multi-device engineering. It in shownthat dual-mode functionality is enabled by the concurrent existence of anion
doping and cation dedoping of the films. A device physics model that accurately describes the behavior of these transistors is developed. Finally, theutility of these dual-mode transistors for implementing reconfigurable logic
by fabricating a logic gate that may be switched between logic gates AND to
NOR, and OR to NAND on the fly is demonstrated.

1. Introduction
Efforts to miniaturize conventional complementary metal–
oxide–semiconductor (CMOS) transistors are not expected toT. Nguyen-Dang, S. Chae, J. Chatsirisupachai, H. Wakidi, T.-Q. Nguyen
Center for Polymers and Organic Solids
University of California, Santa Barbara
Santa Barbara, CA 93106, USA
E-mail: quyen@chem.ucsb.edu
J. Chatsirisupachai, V. Promarak
Department of Materials Science and Engineering
School of Molecular Science and Engineering
Vidyasirimedhi Institute of Science and Technology
Wangchan, Rayong 21210, Thailand
Y. Visell
RE Touch Lab
California NanoSystems InstituteDepartment of Electrical and Computer Engineering
University of California, Santa Barbara
Santa Barbara, CA 93106, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.202200274.

DOI: 10.1002/adma.202200274

Adv. Mater. 2022, 34, 2200274

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Research Article

www.advmat.dechannel comprising a mixed organic ionic–electronic semiconductor connecting the source and the drain, and an electrolyte
connecting the semiconductor and the gate electrode.[19] Insuch a transistor, the application of a gate voltage results in ion
migration into the semiconductor channel. This induces electrochemical doping/dedoping reactions of the channel material,
thereby modulating the source–drain current.[20] Advantages of
OECTs include low operating voltages (<1V), very fast switchingtime compared to other organic devices, and very low power
consumption projected of 35 aJ per switching event for achannel of 0.3 μm × 0.3 μm large (compared to ≈10 fJ for biological synapses and ≈1.23 fJ for nanowire-based organic fieldeffect transistors).[5,21–24] As organic-based devices, OECTs canbe simply and easily printed into large-area electronic circuits
using room temperature organic materials.[25,26] Due to these
attributes, OECTs are attractive for reconfigurable electronics.
However, the development of long-term stable ambipolar
OECTs is currently at an early stage as most n-type active materials in OECTs can easily be degraded.[27,28] Despite recent progress in creating complementary from ambipolar OECTs,[28–31]
reconfigurable OECT transistors with dynamically gate-tunable
doping types have not been demonstrated. Another viableapproach would be to dynamically tune the operation mode of
OECTs. OECTs operate either as enhancement-mode transistors or as depletion-mode transistors. Depletion-mode transistors are in an ON state at zero gate voltage, while enhancementmode transistors are in an OFF state at this condition. Enhancement mode OECTs have the active channel of an organic mixed
ionic-electronic semiconductor that can be doped following theion uptake, for example, poly(2-(3,3′-bis(2-(2-(2-methoxyethoxy)
ethoxy)ethoxy)-[2,2′-bithiophen]-5-yl)thieno[3,2-b]thiophene)(p(g2T-TT)),[32] while depletion-mode OECTs are based on semiconductors that are in a doped state, but become dedoped upon
ion uptake, as in poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonate) (PEDOT:PSS).[20,33] The operation mode of an OECT
is usually an intrinsic and static property that can be designed
but cannot be dynamically changed. For instance, to switch
between operation modes using PTEB-S, Zeglio et al. blended
an enhancement-mode material, sodium poly(2-(3-thienyl)
ethoxy-4-butylsul- fonate) (PTEB-S), with a depletion-mode
material, poly(4-(2,3-dihydrothieno-(3,4-b)-(1,4)dioxin-2-yl-methoxy)-1-butanesulfonic acid, sodium salt) (PEDOT-S), and controlled the operation-mode of conjugated polyelectrolyte-based
OECTs by precisely balancing stoichiometry of the two active
materials.[34] More recently, Cea et al. showed that by adding
PEI in PEDOT:PSS, a common material for depletion-mode
OECTs, the new blend becomes an excellent active material forenhancement-mode OECTs.[35] Another route to switching theoperation mode of OECTs is via device engineering. Doris et al.
created a separate compartment for the gate in their OECTs, inwhich several redox couples are used to control the gate electrochemical potential. This allowed them to vary the thresholdvoltage of the OECTs. Depending on the specific redox couple,
and therefore the threshold voltage, the operation mode of their
OECTs can be either depletion or enhancement with the sameactive material, PEDOT:PSS.[36] Relying on the same principle,
Tan et al. designed two-chamber OECTs, in which the electrochemical potential of the gate can be controlled in an independent separate electrochemical cell containing redox-active

Adv. Mater. 2022, 34, 2200274

electrodes and analytes.[37] Their OECTs can in principle used
as mode switching transistors, but the slow response time ofredox reactions (on the order of 100 s) makes this approachunattractive for electronics. Finally, the only work using OECTs
for reconfigurable electronics is found in microwave resonators, where Bonachini and Omenetto employed PEDOT:PSSbased OECTs in conjunction with metamaterial structures
to modulate the bulk conductivity of the channel material.[38]
While not directly related to reconfigurable logic circuits, theirwork highlights the simplicity of OECTs, the robustness of thetransistors’ performance, and the possibility of effectively using
the transistors in electronics.
In this work, we take advantage of the rich flexibility in material design and the simplicity in device engineering that OECTs
have to offer, and report the fabrication of dynamically operation-mode switching OECTs with a self-doped conjugated polyelectrolyte poly[2,6-(4,4-bis-potassium butanylsulfonate-4H-cyclopenta-[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)](PCPDTBT-SO3K, aka, CPE-K) as the active material. We showthat a CPE-K OECT can operate in both enhancement modeand depletion mode, depending on the polarity of the applied
voltages. This dual-mode behavior is attributed to the tunability
in electrical conductivity of the active material CPE-K: it canboth be doped by injection of anions and dedoped by injectionof cations. We develop a device physics model that explains thedevices’ working mechanism and correctly predicts its behavior
in both operation modes. To highlight the potential of thesedual-mode organic transistors for reconfigurable logic circuits,
we demonstrate a reconfigurable logic gate that can switchfrom a NOR gate to an AND gate on the fly, by simply reversing
the voltage polarity.

2. Results and Discussion
2.1. Device Structure and Dual-Mode Behavior
The structure of a typical OECT in this study (Figure 1A) consists of an interdigitated contact, a spin-coated CPE-K that connects the source and drain, an electrolyte (100 × 10−3 m KCl),and an Ag/AgCl gate electrode (see Experimental Section andSupporting Information Section S1 for details on the fabrication). A drain voltage Vd is applied on the source and drain tocreate a source–drain current Id. To control the ion injections
from the electrolyte to the channel, a gate voltage Vd is applied
from the gate to the source electrode. We observed that CPE-K
OECTs operate in both enhancement mode and depletion
mode. To operate CPE-K OECTs in the enhancement mode, thedrain voltage and gate voltage were set to negative values (Vd < 0and Vg < 0). From the transfer characteristics of an OECT at different drain voltages, shown in Figure 1B, it is observed thatthe drain current increased with the increase in the amplitudeof the gate voltage. The higher the drain current, the steeper
the slope of the transfer curves, with the change of slope from
Vd = −0.2 V to Vd = −0.4 V being more pronounced than thatfrom Vd = −0.4 to Vd = −0.6 V, indicating that the device reaches
saturation at more negative drain voltage (semi-log plot in
Figure S2A, Supporting Information). In an ON/OFF switchingexperiment (Figure 1C), in which the gate voltage is switched

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www.advmat.deFigure 1. Dual operational mode organic transistors with CPE-K OECTs. A) Top: The chemical structure of PCPDTBTSO3K (aka CPE-K); Center:
Schematics of an OECT based on CPE-K; Bottom: the CPE-K OECT can work in two operation modes depending on voltage polarity. B–E) Transfer
characteristics and ON/OFF switching of a CPE-K OECT operating in enhancement mode ((B) and (C), respectively), and in depletion mode ((D) and(E), respectively).

between 0 V and −0.3 V, the transistor switches between the
OFF state (Vg = 0 V) to the ON state (Vg = −0.3 V). The amplitude of the switching experiment was chosen so that the device
is not in the saturation regime to avoid the risk of sample degradation or instability.[39] The ON/OFF switching of the output
current was in phase with the switching of the gate voltage,
which is a characteristic of an enhancement-mode transistor.
With the same device, we observed that under a reversed
polarity, Vd > 0 V and Vg > 0 V, the operation mode changed
to depletion. The transfer characteristics of the transistors inthese conditions are presented in Figure 1D, showing the draincurrent being suppressed as the gate current increases, withthe rate of change (the slope of the transfer curves) increasing
with the drain voltage (semi-log plot in Figure S2B, Supporting
Information). The depletion-mode characteristics are evidenced
further with an ON/OFF switching experiment (Figure 1E): withthe gate voltage alternating between 0 and 0.3 V, the output current switched between the ON state (Vg = 0 V) to the OFF state(Vg = 0.3 V). As a consequence, CPE-K OECTs can operate inboth operation modes, depending on the polarity of the applied
voltages. It is noted that OECTs based on PEDOT:PSS operate
in depletion mode and do not switch to enhancement mode byswitching voltage polarity as CPE-K OECT (Section S3, Supporting Information).
2.2. Device Physics Model of the Dual-Mode Single-Device
OECTs
CPE-K is a self-doped conjugated polyelectrolyte that has
recently been used in OECTs.[40] The dependence of theAdv. Mater. 2022, 34, 2200274

operation mode on the polarity suggests that both cation injection and anion injection into the CPE-K film would result inelectrochemical reactions that change the doping state of theactive channel. To investigate this behavior, we conducted aspectroelectrochemical study of a CPE-K film. Our spectroelectrochemical cell (Figure 2A) consists of a cuvette containing anelectrolyte KCl 100 × 10−3 m (the same electrolyte used in the
OECTs). A CPE-K film deposited on an ITO substrate was used
as a working electrode. As a self-doped p-type semiconductor,
the positive polarons in the CPE-K backbone are stabilized by
the SO3− groups.[24] In the cell, an Ag/AgCl pellet was used as
the counter electrode. A voltage is applied between the working
and the counter electrode: a positive voltage would draw anions(Cl−) into the CPE-K film and therefore a higher doping levelis expected while a negative voltage would result in cation (K+)
migration into the film leading to a lower doping level due tothe compensation of SO3− by the K+ ions. These ion migrations
would in turn result in electrochemical reactions, changing the
CPE-K between a doped state to a dedoped state indicated in
Figure 2A (bottom), which could be tracked with the UV–vis
absorption of the film. The absorbance of the CPE-K at different biases is shown in Figure 2B. We observed three typicalpeaks: the one at ≈400 nm corresponds to the π–π transition,
the one ≈700 nm corresponds to the intra-charge transfer, while
the one at ≈1180 nm is a polaron absorption.[41] The intensityof the polaron peak reflects the level of charge carrier density
inside the CPE-K film.[41,42] With a voltage sweeping from −0.8
to 0.8 V, the three peaks follow a reverse trend: the π–π andcharge transfer peaks decrease and the polaron peak increases.
The increase of the polaron peak compared at the expense of
the other two peaks indicates the increase of polarons in the2200274 (3 of 9)© 2022 Wiley-VCH GmbH

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www.advmat.deFigure 2. Spectroelectrochemical behavior of CPE-K. A) Left: Schematics of the spectroelectrochemical experiment; Top right: Schematics of ion
uptake in CPE-K; Bottom right: Doping and dedoping electrochemical reactions of CPE-K. B) Absorption of CPE-K depending on an applied voltage(vs Ag/AgCl). C) The relative absorption of CPE-K at different V with respect to that of V = 0 V. D) The relative change in absorption of the polaron peak
(λ = 1180 nm) as a function of applied voltage V.

backbone of CPE-K, i.e., higher doping level. This can be
explained with the following mechanism: under the applicationof a voltage, ions in the electrolytes would migrate into the film,
which then interact with the anion SO3− group. If cations K+ are
injected, the Coulombic interaction between K+ and the SO3−
groups would compensate for the ionic sidechains and would
destabilize the polarons. As a result, the number of polaronsdecreases and the film is dedoped (less conductive). In case ofanion Cl− migration, Cl− anions would stabilize the positive
polarons as well as interact with the cationic counter-ions in
the CPE-K and free up SO3− groups to stabilize the polarons on
the backbone, hence increasing the hole carrier density of the
CPE-K film. These electrochemical processes can be describedas the following:
Dedoping : PCPDTBT +SO3− + K + + e − → PCPDTBT 0SO3− K + (1)Doping : PCPDTBT 0SO3− K + + Cl − → PCPDTBT +SO3− + KCl + e − (2)This reversed trend between the charge transfer peak andthe polaron peak upon a sweeping voltage is evidenced further in Figure 2C, plotting the adjusted absorption value withrespect to the absorption at V = 0 V. From this curve, the difference in intensity of the polaron peak to that at V = 0 V isplotted with regard to the applied voltage (Figure 2D). In thisplot, around the negative value of the applied voltage, corresponding to cation injection into the film, we observed thatthe polaron peak plateaued at high amplitude V (more negative). This could be attributed to an exhaustion of available
polaron inside the CPE-K film. Indeed, the concentration of
the polaron in dehydrated CPE-K is in the order of 1018 cm−3,[42]whereas the concentration of K+ in 100 × 10−3 m KCl is in theorder of 6 × 1019 cm−3. The excess of cation in the film could
compensate for all SO3− sidechains, impeding the self-doping
Adv. Mater. 2022, 34, 2200274

of CPE-K. As the voltage goes to the positive range, we observea linear increase of the polaron peak intensity. This behavior
could be attributed to the volumetric capacitance characteristics of the organic semiconductor,[43] with the number of net
ion density written as: −[K+] + [Cl−] = q = (c*/e)V, c* being thevolumetric capacitance of the CPE-K films, and e the elemental
unit charge (1.6 × 10−19 C). In this experiment, the voltage range
stayed not higher than 0.8 V to avoid the risk of electrolysisof the electrolyte. However, one could expect another saturation, in which all backbones in CPE-K possess a mobile charge
and the conductivity cannot increase further upon further ion
injection. We note that the proportional change in charge carrier density, indicated by the polaron peak, at both polarities ofapplied voltages, has not been shown in conjugated polymers(e.g., p(g2T-TT)),[44] conjugated polyelectrolyte copolymers (e.g.,poly[6-(thiophen-3-yl)hexane-1-sulfonate-co-3-(hexylthiophene)]
copolymers PTHS–TMA−-P3HT/PTHS–TMA+-co-P3HT),[45] or
conjugated polymer blends (e.g., PEDOT:PSS).[46] Only selfdoped polyelectrolytes, such as CPE-K (this work) and PEDOTS and their derivatives are reported with this feature.[34,47]
Using this understanding from the spectroelectrochemicalmeasurements of the CPE-K, we developed a model to describe
the dual-mode behavior of CPE-K OECTs behavior based on
Bernards’ model for depletion mode OECTs.[20] A transistor,
whose structure shown in the inset of Figure 3A, is considered,with KCl as the electrolyte and Ag/AgCl gate. As shown above,
the hole density in CPE-K can be described asc*p ( x ) = − K +  + Cl −  − p0 =(Vx (x ) − Vg ) − n0 (3)ewith n0 being a constant related to the initial charge carrier
density of the CPE-K film, c* the volumetric capacitance of thematerial, e the unit charge (1.6 × 10−19 C), and with Vx(x) being

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www.advmat.deFigure 3. Output characteristics of a CPE-K OECT. A) Output curves, full range, with device structure and dimension of the channel in the inset atthe top-right corner. B) Output curves of the quadrant Vd < 0 V and Vg < 0 V, showing enhancement-mode behavior. C) Output curves of the quadrant
Vd > 0 and Vg > 0, showing depletion-mode behavior. The solid lines in (B) and (C) are calculated from the device physics model (Equation 8) with thesame fitting parameters of Vp = 32.9 mV and G = 1.66 mS.

the voltage along the x-direction (inset of Figure 3A). Note thatthe charge carrier density in Equation 3 is a positive number,when Vd < Vg + p0e/c*, the charge carrier density becomes 0.
Using the microscopic form of Ohm’s law, the current density along the x-direction jx is given as:
jx = σ ( x )∂Vx ( x )  C * ∂V ( x )=(Vx (x ) − Vg ) − p0  µ e x (4)
 e
∂x
∂x

Here, we assume that the hole mobility μ is constantthroughout the film, then the conductivity of CPE-K is givenas σ(x) = p(x)μe. By definition, jx = −Id/(Wd), with W and d thewidth and the thickness of the CPE-K film, we obtain:

Adv. Mater. 2022, 34, 2200274

C *

Id
dx = (Vx (x ) − Vg ) − p0  µ edVx (x ) (5) eWd
By integrating Equation 5 from 0 to L along the x-direction, we
get:
L

Vd

Id
C *

∫0 Wd dx = ∫0  e (Vx ( x ) − Vg ) − p0  µedVx ( x ) (6)or
C *  1


Id
L =
 Vd − Vg  − p0  µ eVd (7) e 2

Wd

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www.advmat.de

Wd
µ ep0
L
and a threshold voltage Vp = (p0e)/c*; thus, Equation 7 becomesTo simplify Equation 7, we define a conductance G = Vg 
1G 2
I d =− G  1 + Vd +Vd (8)2 Vp
 Vp 
For validation of the model, we obtained the output characteristics of the OECT at different gate voltages (Figure 3A), andused the Equation 8 to fit experimental data. The experimental
output curves of the transistors show two quadrants of saturation: when Vg > 0 V and Vd < 0 V, the film showed a very low
conductivity, attributed to exhaustion of polarons due to excess
K+ cation injections. When Vg < 0 V and Vd > 0 V, the film
showed very high conductivity, reaching saturation of polarons
due to the excess of Cl− anions in the film. Only in the othertwo conditions, namely (Vg > 0 V, Vd > 0 V) in Figure 3B and(Vg < 0 V, Vd < 0 V) in Figure 3C that the device showed gating
behaviors. Figure 3B shows a typical output characterizationof an enhancement-mode OECT: increasing the amplitude of
the gate voltage leads to an increase in the drain current. On
the other hand, the depletion-mode behavior is observed in
Figure 3C: higher amplitude gate voltage lowers the output
curve. The governing Equation 8 can be used to fit both behaviors using the same set of fitting parameters Vp = 32.9 mV and
G = 1.66 mS (solid lines in Figure 3B,C).
Equation 8 provides another way to validate the modelwith experimental data. The transconductance of the transistor ∂Id/∂Vg can be both calculated from this governing
equation and can be measured from the slope of the transfer
curves (Figure S4, Supporting Information). An expression of
the transconductance at non-saturation can be derived from
Equation 8
gm =

∂I d
G
= − Vd (9)∂Vg
Vp

This linear dependence is observed with both enhancementmode (Vd < 0 V, Vg < 0 V) and depletion mode (Vd > 0 V, Vg > 0 V),
showing the slope gm/Vd = (32.85 ± 0.25) × 10−3 F cm−1 V−1 s−1 forenhancement mode and gm/Vd = (34.90 ± 0.20) × 10−3 F V−1 s−1
for depletion mode. These similar fitted values gm/Vd obtained
from the transfer characteristics of two modes are in the sameorder of magnitude with the one obtained from the output
characteristics gm/Vd = G/Vp = 45 × 10−3 F V−1 s−1. This good
fit confirms the validity of our model, which then allows forunderstanding the principle of the dual-mode OECTs. On the
one hand, for the device to operate in enhancement mode,
the initial condition is set in the way that the conductivity of
the film is very low. It is achieved by applying a negative Vd,
so that cations are present in the film even without the gate
voltage. A negative small-amplitude Vd (≈1 V) might be enoughfor the film to reach exhaustion of polarons (evidenced by thespectroelectrochemical measurement). From this initial condition, applying a negative gate voltage (Vg < 0 V) would draw K+
cations out of the film and to drive Cl− anions into the film,
allowing for more SO3− groups to be available for compensating
polarons in the backbone of the CPE-K, thereby increasing further the conductivity of the film and the drain current. On theAdv. Mater. 2022, 34, 2200274

other hand, to operate the transistor in depletion-mode, a necessary condition is to have a positive drain voltage (Vd > 0 V). In
this condition, at zero gate voltage there is an excess numberof Cl− anions in the CPE-K film, assuring a high conductivity.
Increasing the amplitude of a positive Vg would result in more
K+ cations in the films which interact with the sulfonate SO3−in the sidechains, weakening the self-doping efficiency of the
CPE-K. This results in a lower conductivity of the channel, andtherefore, a smaller drain current.
2.3. Demonstration of a Dynamically Reconfigurable Logic Gate
With the OECTs operating in both enhancement mode anddepletion mode, and switching between modes being doneon-the-fly via switching the polarity of applied voltage, we constructed reconfigurable logic gates which are the basic building
blocks of digital electronics. A logic gate produces a binary
output from one or more binary inputs, and any complex logic
operation can be built from the most common logic gates, suchas AND, OR, NAND, and NOR. Reconfigurable logic gatesallow to modify the binary output from the same set of digital
input without interrupting its operation and without modifying
its physical built.[2] We designed a logic gate from CPE-K OECTs
as shown in Figure 4A. The inputs of the logic gate Vin(1) andVin(2) were applied to the gate electrode of the two transistors,
respectively, and the output signal is the drain current Iout. With
a VDD = −0.5 V and the input voltage Vin(1) = Vin(2) = −0.3 V, thetransistors operated in the enhancement mode. In this condition, the logic gate works as an AND gate. At zero gate voltages in one of the two transistors, the current was suppressed(output = 0). Only when both gate voltages were ON the transistors are both in an ON state, allowing for the current to be nonzero (output = 1). Figure 4B shows the input signals, the analog
output signal and the digital output signal over time. In theanalog signal, the ON/OFF switching may be observed fromthe spike of the output current. This spike can be eliminatedby device engineering (e.g., using thinner active materials).[26]Here, the spike helps to illustrate the operation of two individual transistors. The switching time of the transistors in thislogic gate in the AND gate is 3.8 ms (Figure S5A, Supporting
Information).
When the polarity of the applied voltage was reversed,
without any hardware alteration, the logic gate became a NOR
gate. Indeed, for VDD = 0.5 V and Vin(1) = Vin(2) = 0.3 V, thetransistors operated in depletion mode. As a consequence, atzero gate voltage, the output current was in an ON state. Whenone of the transistors was turned into an OFF state by a positive
applied input voltage, the current was zero. Figure 4C presents
the input voltage sequences and the response of the output
signal in analog and in digital forms, respectively. Similar to
the AND gate configuration, in the NOR gate configuration, theON/OFF switching of each transistor could be seen by a spikein the output current, and the digital output was obtained usinga binary filter. The switching time of the NOR gate was 8.4 ms(Figure S5B, Supporting Information).
Similarly, we realized an OR gate with two enhancementmode OECTs in parallel. This device was reconfigured to a
NAND gate when the transistors were switched to depletion

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www.advmat.deFigure 4. Dynamically reconfigurable logic gates with CPE-K OECTs. A) The circuit diagram of the logic gate (top) and its output based on the sign ofgate and drain voltages (bottom). B,C) For the same input pattern, the logic gate is an AND gate for VDD = −0.5 V and Vin(1) = Vin(2) = −0.3 V (B), anda NOR gate for VDD = 0.5 V and Vin(1) = Vin(2) = 0.3 V (C).

mode (Figure S6, Supporting Information). To our knowledge,
this is the first demonstration of logic based on OECTs. Compared to previous reconfigurable organic logic gates based onorganic field-effect transistors (OFETs),[9] this OECT-based gate
requires far smaller voltages (the amplitude of Vg and VDD is
0.3 V and 0.5 V here, respectively, compared to 60 V and 60 V
for the OFETs), yet achieves similar output (ON) current to
OFET devices (Ids ≈0.2 mA). This feature is the result of thehigher transconductance of transistor components of the twosystems. The average transconductance of OECTs in our circuit
at Vd = 0.3 V is 10.74 ± 0.45 mS, while the average transconductance of the p-type OFETs in the previous work was 1.6 μS(a four order of magnitude difference).[9] The low-operatingvoltage and high-transconductance of OECTs stem from theirinherent ion-to-electronic transductions as well as from thebulk conductivity of the channel materials,[48,49] highlighting
the advantages of using this type of transistor as platforms forreconfigurable electronics.
After successfully demonstrating the reconfigurability of thelogic gate with CPE-K OECTs, we believe that there are someareas for further improvement of these devices. For instance,
for the circuit to work, the voltage signs of the logical inputs
would need to be changed. This is non-standard in digital
logic, and would require additional components in order toachieve. This can be addressed by configuring the functionalityof the circuit with appropriate connections. Another point of
improvement is to replace liquid electrolytes with solid-state
OECTs. Indeed, because the operation of OECTs is based onion migration, liquid electrolytes are used for their high ionic
mobility. Highly ion-conducting solid electrolytes, such as gelsor ionic liquid will allow for more stable and easier to miniaturize devices, with a possibility of integration into the largescale high-throughput printing process.[23,25,26,35] For example,
one particular interest lies in the fabrication of reconfigurable instrumentation amplifier circuits, which are relevant forphysiological sensing such as electroencephalography measurement[50] and noninvasive hormone monitoring.[51] Another

Adv. Mater. 2022, 34, 2200274

promising development lies in combining these reconfigurable
CPE-K OECT circuits with organic solar cells for self-powered
flexible on-skill integrated electronics.[52]3. Conclusions
We have reported in this communication the dual operation
mode behavior of organic transistors with single-device conventional top-gate bottom contact architecture. The transistors are
CPE-K- OECTs that operate in both depletion mode and enhancement mode. Dual-mode single-device transistors have not been
demonstrated with organic counterparts because operation
modes in organic transistors are static and switching between
two modes requires complex molecular design or device engineering. As for our transistor, this is done via simply changing
the polarity of the applied gate and drain voltages. We conducteda study of the underlying working mechanism using spectroelectrochemistry and developed an analytical model to describe thisbehavior that matched well with experimental data. The model
provided insights into the transistor operation, and revealed thatthe dual operation mode feature stems from the unique concurrent doping following anion injection and dedoping following
cation injection into the CPE-K film. The dual-mode transistors
were used to construct a configurable logic gate, for example,
one that switches from an AND to a NOR gate and vice versa.
This finding paves the way toward a simple and low-cost fabrication of low-power consumption reconfigurable electronics, andopens new opportunities for organic transistor designs that are
relevant to currently emerging applications of OECTs in neuromorphic computing and biosensing.

4. Experimental Section
Materials: CPE-K was synthesized following a protocol detailed
elsewhere.[41] KCl and Ag/AgCl pellets were bought from Sigma–Aldrichand used as received.

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www.advmat.de

OECT Fabrication: Interdigitated contacts were fabricated on Silicon
substrates with a lift-off process, using two photoresists (SF11 and
AZP4110, MicroChemical). Photoresist patterning was done with a maskaligner SUSS MA6, and photoresist development was done with AZ400k(for AZP4110) and MicroChem 101A Developer (for SF11). Au evaporation
was done via thermal evaporation and lift-off was done by immersing theevaporated wafers in an N-methyl-2-pyrrolidone (NMP) bath in 60 °C.
Detailed parameters of the photolithography and lift-off process could
be found elsewhere.[53] Active material CPE-K (10 mg mL−1 with 0.1% v.v.
(3-glycidyloxypropyl)trimethoxysilane (GOPS)) was spin-coated on theinterdigitated contact, with a thickness of 75 nm, the channel area wasdefined via solution removal with a Q-tip, then was annealed at 110 °C
for 30 min. Devices were kept in a desiccator if not used.
Spectroelectrochemical Characterization: UV–vis–NIR spectra wereobtained using a Perkin-Elmer Lambda 750 UV–vis–NIR spectrometerusing a tungsten lamp for the visible to near-IR region and a deuterium
lamp for the UV region. A 100% transmission blank was obtained using
a clean indium tin oxide/glass substrate in KCl 100 × 10−3 m for all
spectra.
OECT Electrical Characterization: Characteristics of OECTs were
measured with a Keithley semiconductor parametric analyzer (model
4200-SCC). Inputs for the logic gates were done with two function
generators and were recorded using a Teledyne Lecroy Waveace 2034
oscilloscope and a function generator. The voltage sweeps of thetransfer curves in both modes started at 0 V, the voltage sweep of theoutput curves started at Vd = −0.3 V. The hold time for each data point
was 100 ms, two orders of magnitude higher than the switching timeof the devices. Forward and backward sweeps of the transfer curves inboth modes showed negligible hysteresis (Figure S2C,D, Supporting
Information).

Supporting Information
Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements
This work was supported by the National Science Foundation (DMR1922042). T.N.-D. acknowledge the UCSB’s “Otis Williams Fellowship”
for financial support. J.C. thanks Vidyasirimedhi Institute of Science andTechnology (VISTEC) for the Ph.D. scholarship. The authors thank Prof.
Guillermo C. Bazan for fruitful discussion.

Conflict of Interest
The authors declare no conflict of interest.

Data Availability Statement
The data that support the findings of this study are available from thecorresponding author upon reasonable request.

Keywords
dynamically gate-tunable transistors, organic electrochemical transistors,organic electronics, reconfigurable logic gate
Received: January 11, 2022
Revised: March 10, 2022
Published online: April 28, 2022

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