Electrochemistry Communications 13 (2011) 597 – 599
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j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
fi
⁎
Institute for Physical Chemistry & Center for Nanotechnology, University of Muenster, Heisenbergstr. 11, D-48149 Muenster, Germany a r t i c l e i n f o
Article history:
Received 31 January 2011
Received in revised form 14 March 2011
Accepted 15 March 2011
Available online 23 March 2011
Keywords:
Electrochemical transistor
Enhancement-mode
Depletion-mode a b s t r a c t
Until now, electrochemical fi eld-effect transistors using PEDOT:PSS have operated as normally-on depletionmode devices. However, for many electronic applications, it is highly desirable to use also enhancement-mode transistors that remain in the OFF state until a gate voltage is applied. We report the fi rst realization of this second transistor type using PEDOT:PSS. This represents a step towards electrochemical transistors serving complementary electronic functions as existing CMOS technology.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Many electronic devices employed in CMOS technology have functional counterparts made from organic materials. Active components like transistors, diodes, sensors, transducers, and memory elements can be combined with passive elements such as conductive leads, antennas, gate dielectrics, interlayer dielectrics, and storage capacitors made from conductive and dielectric polymers [1 – 3] . One of the most studied conducting polymers used in organic electronics is
PEDOT:PSS, which consists of poly(3,4-ethylenedioxythiophene) that is p-doped with poly(4-styrenesulfonate). Polymer FETs using PEDOT:
PSS as the active material have already been fabricated [4,5] . In particular, electrochemical FETs [6] can be constructed using this polymer. In an electrochemical cell, PEDOT can be de-doped to its insulating neutral state:
PEDOT
þ
PSS
−
þ M
þ
þ e
−
→ PEDOT o þ M
þ
PSS
− where M + is a metal ion supplied by the electrolyte and e
− the electron transported inside the PEDOT:PSS fi lm. This switching can be the basis of electrochemical organic fi eld effect transistors [7,8] or electrochromic display cells operating at low voltages [9] . By switching the oxidation state of PEDOT, the electrical conductivity can be controlled over up to fi ve orders of magnitude, with a concomitant change in optical absorption characteristics. These color and electronic conductance switching properties are attractive features, especially in light of the fact that only one electroactive material is required.
⁎ Corresponding author.
E-mail address: knoll@uni-muenster.de
(M. Knoll).
1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi: 10.1016/j.elecom.2011.03.019
In recent years, concerted efforts have been focused on the goal of
“ electronic paper ” technology, where organic electronic elements are printed on plastic or thin metal substrates. Anticipated applications for this technology range from fl exible displays for use in consumer electronics to very large video billboards [9] .
However, organic electrochemical transistors made from PEDOT:
PSS have, to this point, been limited to depletion-mode con fi gurations only. When the gate voltage, V
GS
, is zero, the transistor is in its conducting regime (normally on). For the electronic applications mentioned above, it is very advantageous to have the option of an electrochemical FET working in enhancement-mode. This would allow the transistor to remain insulating at zero gate voltage
(normally off), and would be brought into its conducting regime by an applied V
GS b 0.
Since electrochemical cells that use PEDOT:PSS as the active channel and aluminum as the gate electrode possess a rather high potential difference (PEDOT:PSS: E = 0.197 V and aluminum: E =
− 1.676 V [10] ), a novel enhancement-mode fi eld effect transistor can be constructed from the same material as the depletion-mode FET. A shift of 1.87 V can be anticipated for the transfer characteristic of an enhancement-mode FET.
In the past electrochemical FET used a 4-terminal con fi guration with a gate voltage applied between the gate electrode and the back gate terminal [7] . Two operation modes are possible. If the back gate is not connected to the source contact the gate voltage is fl oating against the source potential. In this case only the gate leakage current is fl owing through the back gate terminal. The back gate can also be connected to the source electrode and the device operates similar to a MOS-FET made from silicon. We did not change this traditional T-shaped structure and connected the back gate to the source contact. In this con fi guration the drain current is fl owing through the source contact and the back gate terminal.

598 a)
M. Knoll, M. Thämer / Electrochemistry Communications 13 (2011) 597 – 599
b)
GS
DS
c) d)
Fig. 1.
Electrochemical organic fi eld effect transistor. a) Channel area with source, drain, and back gate (top view). b) Channel area covered with the gate electrode (top view) and applied gate voltage V
GS and drain voltage V
DS
. c) Cross-sectional view of the depletion transistor. d) Cross-sectional view of the enhancement transistor.
2. Experimental
A 200 nm fi lm of PEDOT:PSS on 125 μ m polyester foil was used as received from AGFA-Gevaert (Orgacon EL-350). A scalpel was used to pattern the fi
lm into a T-shaped structure ( Fig. 1 a) as described in
[6] .
We employed a 4-terminal architecture with front- and back-gate contacts. Instead of a lateral gate electrode, we used a vertical con fi
guration with the gate electrode on top of the gate area ( Fig. 1
b and c).
Calcium chloride (0.2 g) and polyvinyl alcohol (PVA) (0.6 g) were dissolved in deionized water (10 ml) at 80 °C to form the electrolyte gel. This was applied to the channel area between source and drain and covered by the gate electrode (
Fig. 1 c).
For the depletion-mode transistor, a piece of PEDOT:PSS fi lm on polyester foil was placed face-down to serve as the gate electrode. The enhancement mode transistor was realized by using aluminum foil
(10 μ m) as the gate. Between the channel and the gate electrode, a frame of polyester foil with thickness 500 μ m was inserted as a spacer to avoid direct contact between the gate and channel. The thickness of the electrolyte layer was 500 μ m. The transistor was then encapsulated with a silicon coating and outer contacts using silver paste were applied to achieve good ohmic contact. The robustness of the devices is given by the silicon coating.
During the measurement phase, a gate voltage V
GS between the gate and the source contact with V
GS was applied
N 0 for the depletion-mode FET and V
(
Fig. 1
GS b 0 for the enhancement-mode FET
b). Negative drain voltages (V
DS b 0) were applied between the drain and source contacts and the resultant drain current I
D was recorded every 5 s. A constant current was achieved 2 s after changing the voltage.
V
GS
The transfer characteristics (I
D vs. V
GS
) are presented in
Figs. 2
a and 3 a. For V
shown ( Fig. 2
GS
N 0, the curve for the depletion-mode transistor is
a). With an applied drain voltage V
DS
= − 1.8 V and
= 0, a nonzero drain current was observed. If a positive gate voltage V
GS was applied, the channel area under the gate electrode was electrochemically reduced, resulting in an increase in electrical resistance, causing a decrease in I
D
.
The enhancement-mode transistor exhibited a markedly different behavior, and the curve for its transfer characteristic is shown for
V
GS b 0 ( Fig. 3 a). With an applied drain voltage V
DS
= − 1.8 V and
V
GS
= 0, the transistor is in an insulating state because the channel area is electrically reduced due to the built-in voltage of the electrochemical cell between channel material (PEDOT:PSS) and gate material (aluminum). A negative gate voltage V
GS b 0 is required to oxidize and dope the channel to observe a drain current. Comparing both transfer characteristics, we see an almost identical trace except that the gate voltage is shifted by Δ V
GS
= 1.5 V. This is less than the theoretical shift of 1.87 V calculated from the potential difference
(PEDOT:PSS: E = 0.197 V and aluminum: E = − 1.676 V [10] ). This can be explained by the fact that in the depletion-mode FET the gate
Fig. 2.
Depletion-mode FET. a) Transfer characteristics I
D vs. V
GS
, V
DS
= − 1.8 V. b) Output characteristics I
D vs. V
DS
.

180321 An enhancement-mode electrochemical organic field-effect transistor
3