FULL
DOI: 10.1002/adfm.200701251PAPER
Effects of the Ionic Currents in Electrolyte-gated Organic Field-Effect Transistors**
By Elias Said,* Oscar Larsson, Magnus Berggren, and Xavier Crispin*
Polyelectrolytes are promising materials as gate dielectrics in organic field-effect transistors (OFETs). Upon gate bias, their
polarization induces an ionic charging current, which generates a large double layer capacitor (10–500mF cm�2) at the
semiconductor/electrolyte interface. The resulting transistor operates at low voltages (<1V) and its conducting channel is
formed in �50ms. The effect of ionic currents on the performance of the OFETs is investigated by varying the relative humidity
of the device ambience. Within defined humidity levels and potential values, the water electrolysis is negligible and the OFETs
performances are optimum.
1. Introduction
Organic field-effect transistors[1–3] (OFETs) are promising
for printed electronics in applications such as chemical
sensors,[4] logics,[5] RFID tags,[6] and displays.[7,8] The OFETs
need to fulfill several key features to be implemented in a
product. Firstly, the materials (semiconductor, insulator, and
metal) need to be solution processible to be transferred to
printing technologies. Secondly, portable applications such as
sensors for smart packaging and displays in cell phones require
OFETs with low operational voltages to insure compatibility
with printable batteries. Thirdly, the current throughput of the
transistor needs to be large to control display elements; and
the OFETs must respond fast to be used in circuits. Finally, the
electrical characteristic of the transistor must be stable.
Although electrolytes have been demonstrated as gate
insulators in silicon-based transistors[9] or as ion-transporting
layer in polymer-based electrochemical transistors[10,11] for
more than two decades ago; this is only recently that solid
electrolytes have been introduced as gate insulators in
OFETs.[12–18] Electrolytes are electrons and holes insulators
but ionic conductors. Upon contact with a charged electrode,
the oppositely charged ions migrate toward the electrode
[*] Prof. X. Crispin, E. Said, O. Larsson, Prof. M. BerggrenDepartment of Science and Technology (ITN),Linkoping University, Organic ElectronicsSE-601 74 Norrkoping (Sweden)E-mail: [email protected]; [email protected]
[**] The authors gratefully acknowledge the Swedish Foundation forStrategic Research (SSF), VINNOVA, the Royal Swedish Academyof Sciences (KVA), the Swedish Research Council, Knut och AliceWallenbergs Stiftelse, COE@COIN, and Linkoping University forfinancial support of this project. In addition, the authors wish tothank Frank Louwet at AGFA for providing the PSSH material. Thiswork was supported by the EU Integrated Project NAIMO (No NMP4-CT-2004-500355). Supporting Information is available online fromWiley Interscience or from the author.
Adv. Funct. Mater. 2008, 18, 3529–3536 � 2008 WILEY-VCH Verlag
surface thus creating two charged sheets distant by few
angstroms: the Helmholtz double layer. The resulting electric
double layer capacitor (EDLC) formed at the electrolyte/metal
interface has a capacitance value up to three orders magnitude
larger than conventional oxide-based dielectrics for similar
thicknesses (up to 500mF cm�2[19]). Similar EDLC can be
formed at the electrolyte/semiconductor interface in an
electrolyte-gated OFET. The high capacitance of EDLCs
confers a low operating voltage (<2V) to the OFETs [12–18]
and an extremely high field-induced charge carrier densities
(1014–1015 jej cm�2).[20] Panzer and Frisbie[21] have demon-
strated p- and n-channel operations in electrolyte-gated OFETs;
thus suggesting the possibility to built complementary circuits.
Various types of solid electrolytes can be envisaged as gate
insulator for an OFET (polymer electrolyte, polyelectrolyte,
ionic liquids, etc.) However, the intimate nature of the solid
electrolyte affects the mechanism of current modulation, as
well as the performances of the transistor.[12–18] In a p-channel
OFETs, if small mobile anions are present in the electrolyte
layer, they are susceptible to penetrate into the organic
semiconductor layer and lead to its bulk electro-oxidation
upon hole injection from the source. A transistor displaying
current modulation upon bulk electrochemistry is an electro-
chemical transistor, which has typically slower response.[22] To
prevent electrochemistry of the semiconductor, large and
immobile anions from polyelectrolytes have been successfully
used in p-type OFETs.[12,14] Up to now, these are the poly-
electrolytes and the ionic liquids that confer the fastest response
(�1 ms) in the family of electrolyte-gated OFETs.[23–25]
This novel class of transistors is far to be fully characterized
and the underlying physics of channel formation is barely
understood. The ionic current is a key observable to enlighten
the mechanism of channel formation since the electrolyte
polarization involves themigrationof ions from the semiconductor/
electrolyte interface to the electrolyte/gate interface. Importantly,
the presence of residual water in hydroscopic electrolyte favors
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Figure 1. The chemical structures of a) PSSH, b) the polymer semicon-ductor, regio-regular P3HT, and c) PMMA. d) A schematic cross-section ofan OFET with channel length L¼ 3.5mm and width W¼ 200mm. Thesketch displays the ions in the polyelectrolyte and the charge carriers in thesemiconductor channel upon device operation.
3530
electrochemical side-reactions at electrodes that are expected
to contribute to the transistor electrical characteristics and to
the degradation of its performance.
In this work, we have investigated the effect of the ionic
currents on the electrical characteristics of a polyelectrolyte-
gated OFET. The OFET is fabricated with poly(styrene
sulfonic acid) (PSSH) (Fig. 1a) as gate insulator and regio-regular
poly(3- hexylthiophene) (P3HT) (Fig. 1b) as the thin film
semiconductor. The ionic current level is controlled by varying
the humidity level in a climate chamber. The ionic conductivity
in polymer electrolytes and polyelectrolytes is known to
depend drastically on the relative humidity (RH).[26] This is the
basic principle of operation of some humidity sensors.[27,28]
2. Results and Discussions
2.1. Performances of Polyelectrolyte-gated OFETs
The OFET, illustrated in Figure 1d, is composed of gold
source and drain electrodes, which here define a channel length
L of 3.5mm and width W of 200mm; and, a top titanium gate
electrode. A 20 nm-thick P3HT semiconductor layer is coated
with an 85 nm-thick layer of PSSH. The polyelectrolyte PSSH
is a strong acid (pH� 1.5) with labile protons as mobile cations,
while the anions are the sulfonate pendant groups attached to
the polymer chains (Fig. 1a). The mechanism involved in the
channel formation in the OFET is briefly summarized and
described in Figure 1d. By applying a negative electrical
potential to the gate, protons migrate toward the gate and form
an electric double layer at the polyelectrolyte/gate electrode
interface. Subsequently, an excess of immobile polyanionic
www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH
chains, PSS�, located at the polyelectrolyte/semiconductor
interfaces, induces the formation of a positively charged
conducting channel between source and drain (p-chan-
nel).[12,14]
The transfer characteristics and the square root evolution of
ID versus the VG are given in Figure 2a for various humidity
levels of the environment. At a first glance, the humidity has a
strong influence on the transistor characteristics in the low
voltage region. A V-shaped curve is visible at high humidity
levels with an increase in the drain current for positive gate
bias. As proven below, the V-shape is not due to an ambipolar
electronic current behavior of this OFET.[29] At higher
negative voltages, the current level is less affected by the
humidity. From the I�1/2D versus VG curves, the threshold
voltage VT is estimated to �0.3V and it does not change
significantly with humidity. Figure 2b shows that the off-
current IOFF (ID atVG¼ 0V) increases exponentially with RH;
while at VG¼�1V, the drain current ION is almost constant
versus RH. As a result, the on/off current ratio decreases from
2500 at RH¼ 20% to 15 at RH¼ 80%.
The drain current is composed of a current passing from
source to drain (ISD); and a current crossing the polyelectrolyte
layer between gate and drain (IGD). At low gate voltages, the
electronic contribution to the drain current is vanishingly small
and the drain current is therefore due to the ionic current
coming from motions of ions within the PSS layer. In contrast,
at higher gate potentials, the semiconductor channel is open
and the electronic contribution to ID overwhelms the ionic
contribution at low humidity levels. Above 50% RH, the ionic
contribution IGD to the drain current becomes close to the
same order of magnitude than electronic contribution
(compare Fig. 2c and a). Hence, the mobility of charge carrier
cannot be estimated from the OFET characteristics in a safe
fashion for humidity levels above 50% RH. Figure 2d displays
the mobility at saturation versus gate potential for various
humidity levels. The hole mobility decreases with humidity in
agreement with previous studies using SiO2 as gate dielec-
tric.[30,31] This is attributed to the presence of water molecules
diffusing within the P3HT film; which create dipolar charge
traps and/or disrupt the intermolecular p–p electronic coupling
between the thiophene units. The mobility increases versus
gate voltage because of an increase in charge carrier
concentration in the channel.[32,33]
In addition to current–voltage characteristics, the turn-on
and turn-off responses of the OFET have been measured at
different RH levels (Fig. 3). The transient measurements are
performed by applying a 10Hz square-shaped pulse [curve
(i) in Fig. 3a], with an associated duty cycle of 50 ms and
amplitude of�1V, to the gate electrode. The drain electrode is
held at a constant potential of �0.8V. The potential drop over
the resistance (10 kV) that is connected to the source is
recordedwith an oscilloscope. At each step of the square-pulse,
the recorded source current includes two major contributions:
Firstly, a charging current originating from the capacitor due to
gate–source overlap. This is the stray capacitive current.
Secondly, the electronic current passing through the channel.
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E. Said et al. / Ionic Currents in Organic Field-Effect Transistors
Figure 2. a) Transfer characteristics of the drain current (ID) versus gate voltage (VG) at different RHs. b) On/offcurrent ratio (left axis) and on- and off-current (right axis) at VG¼�1 V (on-state) and 0 V (off-state) versus RH.c) Absolute value of the gate current (IG) versus VG at different RHs. The filled circles are negative current whilethe empty circles are positive current. The applied drain voltage (VD) was�0.8 Vand the scan speed 0.1 V s�1. d)Field-effect mobility at saturation (VD¼�0.8 V) versus gate voltage for three humidity levels (20% RH, 30% RH,and 40% RH).
Since, the capacitive current peak exceeds the actual electronic
current at the early moments of the rise and fall of the source
current, it hides information related to the channel formation.
The capacitive current contribution is recorded by biasing the
gate but leaving the drain unaddressed [VD¼ 0V, see curve (ii)
in Fig. 3a] to eliminate the electronic current contribution. The
capacitive current is subtracted from the current transient
recorded with VD¼�0.8V [see curve (iii)] to extract the
current contribution due to the formation of the channel [curve
(iv)]. Figures 3b and c display the rise characteristics of the
source current, after subtracting the capacitive peak.
A zoom on the evolution of the source current versus time is
reported in Figure 3b for humidity levels below 50%. Despite
some uncertainty on the current recorded before 15ms, the
qualitative trend is clear: at higher humidity levels, the current
rises faster. Hence, the conducting channel is quickly formed in
humid electrolyte due to a larger proton concentration and
mobility. The positive channel forms because a charging
(displacement) current of protons crosses the PSS layer from
the P3HT/PSS interface to the PSS/Ti interface. The response
time of the device has reached the fundamental limit of
polarization of the polyelectrolyte. Indeed, previous impe-
dance spectroscopy data indicate that the formation of the
electrical double layer (impedance phase angle equals �45 8)takes about 50ms.[14] In principle, with vanishingly small stray
Adv. Funct. Mater. 2008, 18, 3529–3536 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
capacitances, the OFETs response
would be limited by the polariza-
tion of the electrolyte and could
operate at maximum 20kHz. Note
that the continuous decrease in the
drain current (at 500ms) as theRH
varies from 10 to 50% is mostly
attributed to the presence of water
molecules in theP3HT layer and at
the P3HT/PSS interface resulting
in a decreased hole mobility.
At humidity levels above
50%, the source current shows
a large current peak that slowly
decays in 10ms and reaches
a constant value (Fig. 3c). The
maximum of that peak reaches
higher current and longer time
for high humidity levels. Similar
signals are observed when an
electrochemical reaction is inves-
tigated by chronoamperometry.[34]
This current peak is attributed to
the electrolysis of water in acidic
environment. The solvated pro-
tons, attracted by the negatively
charged gate, are reduced in
dihydrogen gas because, simulta-
neously, water molecules likely
cross the 20 nm-thin hydropho-
bic P3HT layer and are oxidized
in dioxygen at the source electrode. When increasing the
humidity levels, the concentration of water is larger; which
leads to an enhanced current. At steady state (>10 ms), the
electrochemical current is limited by the diffusion of reactants
toward the electrodes. The current levels reached at steady
state (6� 10�7 A for 90% RH, 4� 10�7 A for 80% RH, and
2� 10�17 A for 65% RH) reflect the flow of reactants for
various humidity levels. This water electrolysis occurs when the
potential difference between the electrodes is outside the
potential window of water in acidic environment.[34] At low
gate potential and in dry atmosphere, this electrochemical
current is vanishingly small; but increases with voltage and
humidity. In practice, the voltage and humidity domains for
which this side electrochemical reaction is minimized define
the optimum operating conditions for the OFET.
2.2. Decoupling the Electronic and Ionic Currents
In order to separate the electronic and the ionic contribu-
tions in the drain current of the OFET, a three terminal device
with identical electrode configurations as the reported OFET,
is fabricated but the P3HT layer is replaced with an insulating
poly(methyl methacrylate) (PMMA) (Fig. 1c) layer of the
same thickness. In this device, the contributions to the drain
current are the ionic charging current IchargingD and the
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E. Said et al. / Ionic Currents in Organic Field-Effect Transistors
Figure 3. a) Transient measurements are performed by applying the 10Hz square-shaped gate potential pulse[curve (i)] for VD¼ 0 V [curve (ii)] and VD¼�0.8 V [curve (iii)]. Curve (iv) is the difference between curve (iii) andcurve (ii). It represents the current response passing through the channel, without the stray capacitive currentcontribution. The inset shows the measurement setup. The potential was measured over a resistance of 10 kV.b) The transient response when the OFET is turned on for humidity levels from 10 to 50%. c) Above 50% RH,the current increases sharply due to the electrolysis of water. The increase in source current for higher humiditylevels is not due to the charge transport in the channel of the transistors.
3532
electrochemical current IelectrochemD (water electrolysis). The
electronic current between source and drain is only a small
leakage current due to the insulating character of the PMMA.
The equivalent of the transfer characteristic is recorded at
different humidity levels (Fig. 4a), as well as the gate current
versus VG (Fig. 4b). Those two currents (ID and IG) show
similar evolution with the gate voltage since the electronic
current between source–drain is only a constant leakage
current. Note that ID changes sign at a specific voltageVS which
varies between �0.3 and �0.5V depending on RH. VS is seen
as the dip in ID curves given in Figure 4a. Because of their
similar behavior with humidity, ID and IG have contributions
coming from the side electrochemical reaction. It is interesting
www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
to note that at 20% RH, the
electrochemistry is almost sup-
pressed and the resulting gate and
drain current is of the order of
10�11 A over the whole gate
potential range used in the
OFETs. This indicates that the
polyelectrolyte is a good electron
insulator, but the large increase in
ID and IG with humidity points
directly to potential problems of
using the OFET in humid envir-
onments.
In the transfer characteristics
of the OFETs, see Figure 2a,
there are several other contribu-
tions to the drain current besides
the electronic current (Ielect) pas-
sing in the P3HT channel: (i) The
ionic charging (displacement)
current of the EDLCs created
between drain and gate electro-
des (Icharging), (ii) the electronic
leakage current through the
dielectric I leakageGD ; (iii) and, the
additional ionic current due to an
electrochemical side-reaction at
the electrodes (Ielectrochem). All
contribute to ID to various
extends depending on the gate
potential region and the humidity
level. A measurement of the sum
of those contributions is recorded
in the PMMA device. It is
opposite to the electronic con-
tribution IelectD of the OFETs at
potentials larger than VS. How-
ever, the electronic contribution
overwhelms the others in that
region of potentials (compare
Figs. 2a and 4a). They add to
IelectD between VS and the thresh-
old voltage (VT¼��0.2V).
Since, those additional current contributions have the same
order of magnitude as IelectD , the transfer characteristic is
significantly modified. This results in a V-shape in the transfer
characteristic. For VG between �0.2 and þ0.2V, ID continues
increasing due to the large ionic contributions at high humidity
levels.
In a simple form, the drain current in the OFET can be
written: ID ¼ IelectD þ Icharging þ IleakageGD þ Ielectrochem. For the
PMMA device, the electronic contributions between source
and drain are negligible, such that ID ¼ Icharging þ I leakageGD þI leakageSD þ Ielectrochem. Since, at 20% RH, the ID in the PMMA
device varies between 10�11 and 10�10 A, the drain current
measured for the P3HT transistor is almost purely electronic at
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E. Said et al. / Ionic Currents in Organic Field-Effect Transistors
Figure 4. a) Absolute value of the ID versus gate voltage at different RHsfor the PMMA device. The device architecture is similar to Figure 1d withPMMA replacing the P3HT layer. The scan speed is 0.1 V s�1 andVD¼�0.8 V. b) Absolute value of the IG versus gate voltage at differentRHs for the PMMAdevice. c) Simulation of the transfer characteristic of theOFETs by adding the ionic current measured in graph (a) to the extractedelectronic drain current, IelectD for the OFET.
20% RH over the complete range of gate potential scanned.
The electronic contribution IelectD of the drain current in the
OFET can be estimated as the difference between the drain
current of the OFETs at 20% RH and the drain current of the
PMMA device at 20% RH. The extracted IelectD is plotted in
Adv. Funct. Mater. 2008, 18, 3529–3536 � 2008 WILEY-VCH Verl
Figure 4c (empty circles). To demonstrate that the electronic
current and other ionic contributions (charging and electrolysis
currents) can be indeed decoupled, the current of the PMMA
device is added to the extracted IelectD transfer curve of the
OFET. The sum of those currents should represent the
measured OFETs characteristics for various humidity levels.
The comparison between Figures 2a and 4c shows clear
similarities both qualitatively and quantitatively. The only
difference is a clear variation in the drain current in the actual
OFET at VG>�0.2V versus humidity, which is not due to the
ionic current (below 50%RH) but to the change in themobility
of charge carriers in P3HT.
2.3. Electrochemical Side Reaction
Ideally, OFETs with thick non-electrolytic gate dielectrics
possess IG versus VG behaviors that reflect the charging of the
semiconductor channel and stray capacitance (formed by
the overlap between source–drain and the gate electrodes).
The charging current is equal to C (dVG/dt), where C is the
capacitance and dVG/dt is the scan rate (SR). Hence, upon
tuning VG from �1 to 0V, IG is constant over the entire
potential sweep; but it changes sign as soon as the electric
potential is scanned in reverse direction (from 0 to �1V). The
capacitive current follows a square box when the potential is
scanned back and forth. For the polyelectrolyte-gated OFETs
presented in Figure 1d, the gate current strongly deviates from
the ideal charging current behavior, thus indicating the
importance of other contributions to the gate current.
To simplify the interpretation of the gate current in the
OFET, only two electrodes of the transistor are used: the gate
and the drain while the source remains unconnected. The gate
potential is swept at different SRs in ambient atmosphere
(40% RH; Fig. 5a) and in vacuum (P¼ 4.0� 10�6 Torr, Fig.
5b). When the device operates in vacuum, the current is one
order of magnitude smaller than in air. Additionally, the IGversus VG curves are quasi-linear: a feature of an electronic
leakage current. Those observations support the hypothesis of
an electrolysis, which is significantly reduced in vacuum
because of the low water concentration (and slow proton
diffusion). In contrast, I–V curves are nonlinear in 40% RH
due to the electrolysis. Moreover, the current, although higher
than in vacuum, increases for faster potential scan (at same
potential). This is attributed to the progressive consumption of
the reactants close to the electrodes. Note that in the actual
OFETs, the electrolysis takes place at various electrodes
(source–gate, drain–gate, or source–drain) depending on the
applied bias scheme. For instance, at VG¼�0.3V and
VD¼�1V, water can be oxidized in O2 at the gate electrode,
while protons penetrate through the P3HT layer and are
reduced to H2 at the drain.
The impact of the potential on the water electrolysis rate,
i.e., on the electrochemical current, is best illustrated in
Figure 6. A constant potential is applied and the current is
recorded versus time in 40% RH and in vacuum (see Fig. 6a
and b, respectively). In 40%RH, the current drops significantly
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E. Said et al. / Ionic Currents in Organic Field-Effect Transistors
Figure 6. IGD versus log(time) (source is unconnected) for differentapplied gate potentials: a) in 40% RH, b) in vacuum with pressure4.0� 10�6 Torr.
Figure 7. A summary of the gate–drain current contributions in thetransistor structure. Curve a represents the theoretical charging currentIcharging estimated to �0.32 nA. Curve b is the electronic leakage measuredbetween the gate and drain in vacuum. The current recorded in 40% RH isdisplayed in curve d. The latter includes an electrochemical current due tothe electrolysis of water. The difference of curve d and curve b gives theelectrochemical current contribution (curve c) at 40% RH. The SR is100mV s�1.
Figure 5. Gate-drain current IGD versus VG. The source electrode isunconnected. The gate potential is swept at different SRs, a) in 40%RH, b) in vacuum with pressure 4.0� 10�6 Torr. The zoom in the insetsshow currents between 0 and �0.2 V.
3534
within 10 s. In that time scale, most watermolecules close to the
electrode are consumed, the electrochemical current is
reduced to the few un-reacted water molecules diffusing
toward the electrode. In vacuum, the current is small even at
high potentials and quasi constant with time. The current level
in vacuum represents the electronic leakage current. Note that
at about �0.2V, the transient of the current in 40% RH and in
vacuum are similar. The water electrolysis is barely taking
place at a low voltage even in a 40% humid atmosphere.
The current required to charge the electric double layers at
the Ti/PSS and PSS/P3HT-Au interfaces is estimated from the
capacitance of a PSS-based EDLC. The capacitance of the
EDLC is 3.2 nF as calculated by the product of the surface
capacitance of the EDLC (20mF cm�2 at 1Hz measured by
impedance spectroscopy[14]) and the area overlap between the
two electrodes (1.6� 10�4 cm2). This is in good agreement with
the capacitance of j2.4j nF extracted from the slope of the
current versus dVG/dt at �0.2V (see Supporting Information).
The charging current Icharging equals �0.32 nA for SR of
100mV s�1. Hence, the ideal features of a charging current
versus VG can only be observed if the electronic leakage
current and the electrochemical current are reduced to few
nanoamperes. This condition is encountered at low voltages. In
Figures 5a and b, the zooms in the insets show currents between
www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH
0 and �0.2V. Two features of the charging current are visible:
the current changes sign by reversing the direction of the
sweep; and, it increases with the SR.
Figure 7 summarizes the contributions to the gate–drain
current in the OFET structure. The theoretical charging
& Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18, 3529–3536
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E. Said et al. / Ionic Currents in Organic Field-Effect Transistors
current Icharging is constant with potential (curve a). The
vacuum measurement gives Icharging þ IleakageGD with a resistance
of 90MV of the 85 nm-thin polyelectrolyte layer (curve b). The
electrochemical current contribution for 40% RH (curve c) is
the difference of the currents measured in 40% RH and in
vacuum.
3. Conclusions
We have investigated the effect of the ionic currents on the
electrical performances of electrolyte-gated OFETs by varying
theRHof the environment. The high capacitance (20mF cm�2)
of the electric double layers created at the electrolyte/
semiconductor and electrolyte/gate interfaces allows to run
the transistor below 1V. The response time of the transistor
decreases for higher RH levels and reaches below 20ms at 50%
RH. Polyelectrolyte-gated OFETs works surprisingly well in
low humidity level environments. The large current throughput
and reasonable switch time makes this transistor attractive to
drive electronic ink displays or electrochemical displays. The
response of the OFETs is limited by the polarization of the
polyelectrolyte, i.e., by the formation of the electrical double
layers. Hence, fast OFETs can be fabricated with solid
electrolyte possessing a high ionic mobility and/or ion
concentration.
Between 10% and 40% RH, water molecules in the
hydroscopic acidic electrolyte are present at the electrolyte/
semiconductor interface and deteriorate the hole mobility of
the semiconductor. In more humid environments, the water
molecules or protons (depending on the applied voltage
scheme) diffuse through the thin hydrophobic semiconductor
film and lead to a non-desired water electrolysis current that
modifies the performances of the OFETs. At low humidity
level and low voltages, this detrimental side reaction is
negligible and the OFETs operate with a maximum on/off
current ratio, while the channel formation remains fast
(�80ms). This side electrochemical reaction is expected to
be significantly reduced simply by replacing the protons by
alkali metal cations more difficult to reduce. Also, dry solid
electrolytes with high ionic mobility, such as ionic liquids, are
promising candidates as gate insulating layers.
From a processing point of view, the creation of an interface
between the hydrophobic conjugated polymer layer and the
hydrophilic polyelectrolyte layer is challenging. There are
many routes to improve the stability of such an interface, e.g.,
by using more hydrophilic organic semiconductors or more
hydrophobic electrolytes. Novel hybrid electrolytes, such as
copolymers made of conjugated polymer segments and
polyelectrolyte segments will likely play an important role
for the development of this class of OFET insulators.
4. Experimental
To construct theOFET’s sketched in Figure 1d, regio-regular P3HT(electronic grade fromSigma–Aldrich) is dissolved in chloroform (3mg
Adv. Funct. Mater. 2008, 18, 3529–3536 � 2008 WILEY-VCH Verl
mL�1) and spin-coated on a thermally oxidized Si substrate patternedwith gold source and drain electrodes. The channel length L is 3.5mmand its widthW is 200mm. The 20 nm-thick P3HT film was annealed at120 8C for 10min under nitrogen. The polyelectrolyte used is a protonconductor called PSSH. A fluorosurfactant (Zonyl FS-300 from Fluka)was added to the aqueous PSSH solution to spin-coat PSSH (�85 nm-thick) on top of the hydrophobic P3HT layer. A titanium gate electrodewas finally vacuum-deposited on top of the PSSH layer to finish thetransistor. The fabrication of the PMMAdevices is similar to theOFETarchitecture. PMMA is dissolved in acetone (3mg mL�1) and spin-coated on the top of the gold electrodes. The 10 nm-thick PMMA layerwas annealed at 85 8C for 90 s. PSSHwas then spin-coated on the top ofthe PMMA film, followed by deposition of the titanium electrodes.Before starting the measurements, the devices were conditioned in theclimate chamber during 90min at 20%RH and 22 8C.When increasingthe humidity level, the drain current ID was followed. When ID wasconstant versus time, the device was in equilibrium with the climatechamber and measurements could start. Ramping up the RH requiredtypically about 25min of reconditioning before each measurement.The characteristic of the OFET and the PMMA device were measuredusing a Hewlett Packard 4155B parameter analyzer with scan speed0.1V s�1. Transient Measurements were recorded by measuring thepotential drop over the 10 kV resistance via an oscilloscope (InfiniiumStation). A keithley 2400 source meter was used to apply a voltage tothe drain electrode, while an hp waveform generator applied a 10Hzpulse to the gate. All measurements were made in a controlled airhumidity exposure, which were performed in a Challenge 160environmental chamber from Angelantoni Industries.
Received: October 30, 2007Revised: June 6, 2008
Published online: October 17, 2008
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