Effects of the Ionic Currents in Electrolyte-gated Organic Field-Effect Transistors

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DOI: 10.1002/adfm.200701251
PAPER

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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E. Said et al. / Ionic Currents in Organic Field-Effect Transistors

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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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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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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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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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


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


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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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Effects of the Ionic Currents in Electrolyte-gated Organic Field-Effect Transistors

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