Available online at www.sciencedirect.com
Procedia Engineering 00 (2009) 000–000
Proc. Eurosensors XXIV, September 5-8, 2010, Linz, Austria
Fully printed, flexible, large Area Organic Optothermal Sensors for
Zirkl M.a,*, Scheipl G.a, Stadlober B.a, Haase A.a, Kuna L.a, Magnien J.a, Jakopic G.a,
Krenn J.R.a, Sawatdee A.b, Bodö P.b, Andersson P.b*
aInstitute of Nanostructured Materials and Photonics, Joanneum Research, Franz Pichler Straße 30, 8160 Weiz, Austria
bAcreo AB Bredgatan 34,,Box 787, 601 17 Norrköping, Sweden
Pyroelectric sensors presented in this work are based on polymers from the PVDF family which are comprised of a piezo- and/or
pyroelectric polymer thin film capacitor integrated with either high performance organic thin film transistors (OTFTs) or
electrochemical transistors (ECTs) acting as impedance converters, signal amplifiers and conditioners.
For flexible integration with diverse electronic devices, large area processes such as screen printing applicable for industrial
partners have been used for the fabrication of the sensors and ECT´s. With respect to the intended purpose for detection of human
body radiation the absorbance of the impinging IR-light is dramatically increased by the application of printed carbon/Pedot top-
electrodes, hence meeting the requirements for low-cost large area processibility.
Here we present good working integrated sensor devices based on two components, being an organic thin film transitor with a
high-k-nanocomposite gate dielectric or a fully printed electrochemical transistor and a PVDF-copolymer based sensor.
© 2009 Published by Elsevier Ltd.
c ? 2010 Published by Elsevier Ltd.
Keywords: PVDF-TrFE; IR-sensors; OTFTs; ECTs; pyroelectricity; ferroelectricity; piezoelectricity
The interest in the application of polymer ferroelectric thin films has continuously grown during the last decade,
especially in terms of fabrication of non-volatile memory cells , high-performance organic thin film transistors
, detectors for infrared radiation and temperature , and sensors for pressure and motion, pointing towards full
organic actuators and artificial skin . In the field of thermal imaging and pressure sensing, that usually is based on
high-impedance capacitive sensor elements, a suitable read-out electronics for data representation and processing is
highly recommended associated especially whit electronic skin and actuator applications.
Skin-like sensitivity, including the recognition and processing of touch and temperature is one of the essential
features of future generations of robots. Requirements for electronic skin are high flexibility on large area, high
* Corresponding author. Tel.: +43-316-876-2709; fax: +43-316-876-2710.
E-mail address: firstname.lastname@example.org.
Procedia Engineering 5 (2010) 725–729
1877-7058 c ? 2010 Published by Elsevier Ltd.
2 Zirkl M./ Procedia Engineering 00 (2010) 000–000
pressure and temperature sensitivity and their realization on a three-dimensional surface. It has been shown recently,
that conformable, flexible, large-area networks of thermal and pressure sensors based on organic semiconductors
and a net-shaped structure of organic transistor-based electronic circuits can simultaneously detect various
distributions of pressure and temperature .
In order to significantly facilitate the fabrication, to increase stability and improve reproducibility a direct
integration of sensor elements and organic thin film transistor-based electronics on one flexible substrate would be
highly recommendable. Further on, if the sensing material itself were temperature and pressure sensitive, which is
the case for pyro- and piezoelectric PVDF-based polymers , the number of different elements could be
dramatically reduced thus enabling a simultaneous detection of changes and modulations in temperature and
pressure which is interesting for a large number of applications in robotics and human detection. In order to enable
better discrimination between pyro- and piezoelectric response it could also be beneficial to use a sensor material
with intrinsically decreased cross-sensitivity .
In this context we have developed an integrated, printed pyro- and piezoelectric sensor element on the basis of a
PVDF-TrFE copolymer capacitor that is either combined with a high-performance low-voltage organic thin film
transistor based on pentacene or a printed electrochemical transistor. This is the first application of OTFTs or ECTs
in a pyroelectric polymer sensor operating as an opto-thermal light detector. We demonstrate that bottom gate
OTFTs based on the organic semiconductor pentacene and high-k nanocomposite gate dielectrics directly integrated
on the polymer sensor layer exhibit transistor performances with very low gate leakage currents, subthreshold
swings close to the theoretical limit and low-voltage operation. Furthermore the direct integration of printed sensors
and printed electrochemical transistors on flexible substrates has been realized.
In the following the transistor, the temperature sensitive fluorinated polymer, their combination to an integrated
circuit (Fig 1) and the application of this circuit as a thermal infrared sensor and as a switch that can be operated e.g.
by a laser pointer is described. The circuit is composed of a capacitive sensor element with one electrode serving as
the gate electrode of the transistor. The voltage output of the sensor controls the transistor.
2.1. P(VDF-TrFE) Sensors
P(VDF-TrFE) copolymers have become very attractive as functional materials for high-tech applications due to a
number of excellent inherent physical properties. Apart from the usage as high-k gate dielectrics in logic gates based
on miniaturized organic thin film transistors, a remnant polarization charge of more than 100 mC/m2qualifies these
copolymers as charge storage dielectrics in non-volatile memory elements , and high piezo- and pyroelectric
coefficients (up to 40 μC/Km2)  make them attractive for sensor- and transducer based organic devices. However,
no efforts have been undertaken until now to integrate P(VDF-TrFE) based pyroelectric sensors and OTFTs/ECTs,
thus comprising a flexible, integrated organic sensor device.
Fig. 1 Schematic view of the fully flexible sensor circuit based on an organic TFT and a ferroelectric polymer sensor (left) and of a printed
electrochemical transistor (right).
M. Zirkl et al./Procedia Engineering 5 (2010) 725–729
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For realizing such a sensor device, varying sensor stripes have been fabricated in a first step. These sensors were
based on different solutions, electrodes and substrates and been extensively evaluated with respect to their electric
properties. The PVDF-TrFE copolymer pellets with a composition of 70mol% PVDF and 30mol% TrFE purchased
by Solvay Solexis turned out to be the most suitable precursor for our requirements. The standard Sol-Gel process
used for the fabrication of solution based PVDF-TrFE layers for IR-sensors is published elsewhere . The obtained
solutions are suitable for spin-coating, bar-coating and screen printing. The fabrication of the sensor element starts
with screen printing of PEDOT SV3 (HC Starck) bottom electrodes. In the second step the PVDF-TrFE solution is
screen printed resulting in homogeneous films with a standard thickness of 5 μm. Finally top electrodes are screen
printed using either PEDOT SV3 or Carbon paste 7102 (DuPont).
As mentioned before, two different types of transistors have been used. OTFTs have been fabricated by lab-scale
methods and ECTs have been fabricated by large area printing techniques. Both fabrication routines are described
The gate electrode is formed by thermal evaporation of 50 nm Al through shadow masks on a flexible substrate
or directly onto the P(VDF-TrFE) layer of the sensor. A 50-65 nm thick high-k metal oxide layer is fabricated by
reactive oxygen sputtering of Zr under high vacuum conditions thus forming club-shaped ZrO2 grains ,.
Afterwards a thin layer of PVCi (poly-vinyl-cinnamate) is spin-coated on the metal oxide layer to form a dense
metal oxide polymer nano-composite. On the nanocomposite gate dielectric a 50 nm thick pentacene layer is
thermally evaporated via a shadow mask at a rate of 0.1 nm min-1. The devices are finalized by e-beam evaporation
of gold source and drain electrodes on top of the semiconductor.
The ECTs shown in Fig 1 are having a lateral design as described elsewhere . The carbon electrodes are
screen printed (black) in the first step followed by screen printing of the PEDOT:PSS (poly(3,4-
ethylenedioxythiophene):poly(styrene sulfonic acid)) channel (blue) and the isolating laquer (brown). In the last step
the electrolyte (yellow) is inkjet printed on top of the channel.
The superior performance of pentacene transistors with high-k nanocomposite gate dielectrics is clearly indicated
in the output and transfer characteristics shown in Fig 2(a). The output characteristics are virtually hysteresis free
Fig. 2. (a) Output and Transfer characteristics of an OTFT; (b) Output and Transfer characteristics of a screen printed, lateral ECT.
M. Zirkl et al./Procedia Engineering 5 (2010) 725–729
4 Zirkl M./ Procedia Engineering 00 (2010) 000–000
Fig. 3. (a) Sensor output (left top) and drain current modulation (left bottom) of an integrated OTFT sensor during a long time measurement. The
blue arrow indicates the gate voltage supplied by the sensor when being exited with a power of 70 mW at a wavelength of 808 nm; (b) Sensor
output (left top) and drain current modulation (left bottom) of an integrated ECT. The sensor was excited at a wavelength of 808 nm and an
intensity of 70 mW.
and Fig 2(a) demonstrates that the OTFT can be operated at gate voltages below 2 V. Moreover, the charge carrier
mobility is reasonably high (0.2 cm2/Vs) and the swing is substantially smaller than 1 V/dec (0.75 V/dec). The gate
leakage currents are around 10 nA/cm2. The high performance of these devices is remarkable since the organic thin
film transistors having an ultra-thin (70-85 nm) gate dielectric are directly integrated on the rather rough PVDF-
TrFE layer (rms = 10 nm).
If these low-voltage OTFTs are directly integrated on the pyroelectric sensors for an amplified, low-impedance
read-out one has to account for the equivalent circuit of the whole configuration. The modulated incident laser light
generates charge waves at the sensor electrodes and thus the sensor acts as a modulated current source. Via its
intrinsic (very high) impedance the sensor can be operated in the voltage mode at least for frequencies higher than
the cut-off frequency fco. With realistic parameters of the input stage of the transistor (gate resistance RG = 1 G? and
gate capacitance CG = 5 nF) and of the output stage of the sensor (sensor impedance RSens > 10 G? and sensor
capacitance CSens~ 0.5 – 5 nF) the cut-off is calculated to be about fco ~ 0.01 Hz. For f > 0.01 Hz which is the
interesting regime, the sensor is working in the voltage mode.
The performance of printed ECTs can be derived from its output and transfer characteristics shown in Fig 2(b).
The working principle of these chemical transistors is described in . The switching current of ECTs shown in the
transfer characteristics in Fig 2(b) must be treated analogous to the leakage current of OTFTs with respect to the
equivalent circuit of the integrated sensor device described in the next paragraph.
Due to the equivalent circuit the effective sensor output voltage is determined by the overall impedance that is
summed over all resistances connected in parallel. If only the sensor is characterized by a parametric analyser
(Rmeas =10 G?) the overall impedance Rtot is given by 1/Rtot = 1/Rmeas + 1/Rsens with Rmess and Rsens being in the same
order of magnitude whereas the overall impedance is 1/Rtot = 1/RG + 1/Rsens ~ 1/RG is mainly determined by the gate
resistance if the transistor is connected in parallel to the high resistive sensor.
In case of integration with a transistor the sensor´s output voltage is equivalent to a gate voltage biasing the
transistor. In Fig 3 it can be seen how the modulated sensor voltage (Fig 3 top) translates into a modulated output
current of the transistor (Fig 3 bottom). For the OTFT a complete switching could be realized because of the good
impedance matching as described before. The integrated sensor device shows a very stable behavior in long time
experiments as shown in Fig 3(a). The integration with ECTs shows a reasonable drain current modulation but the
sensor signal is reduced because of the current consumption of the electrochemical switching process thus not
enabling a complete switching of the transistor Fig 3(b). The switching currents can be reduced by decreased
channel sizes which will be realized by ink-jet printing of the next generation of ECTs. Preliminary experiments
revealed that the impedance matching between sensor and ECT can be optimized by this way and a complete
switching of the ECT can be expected.
M. Zirkl et al./Procedia Engineering 5 (2010) 725–729
Zirkl M./ Procedia Engineering 00 (2010) 000–000 Download full-text
In conclusion we have shown that ferroelectric capacitive sensors combined with organic field effect transistors
on flexible substrates are suitable for versatile sensor applications. By direct integration of an OFET/ECT with a
fluoropolymer sensor element, a pyroelectric sensor response is obtained, thereby demonstrating that organic
pyroelectric sensor elements with organic signal processing electronics can be achieved.
A wet – chemical production process for obtaining ferroelectric thin films based on polyvinylidene fluoride and
its copolymer trifluoroethylene has been developed by means of a sol – gel method. The solutions show good
properties regarding different kinds of coating/printing processes. The obtained layers show good remnant
polarizations and sufficient properties regarding the thermally induced voltage and current responses.
Moreover we managed the reduction of the threshold voltage and the subthreshold swing of organic thin film
transistors for low voltage operation. Due to the low leakage current of these ZrO2 – PVCi nanocomposite gate
dielectric OTFTs a suitable impedance matching between the read out transistor and the biasing polymer-sensor
could be achieved. Direct integration of these devices on a flexible polymer PET substrate and onto the pyroelectric
sensor layer was performed successfully appearing as a complete on-off switching of the transistor due to light
stimulation of the sensor.
Furthermore a fully printed sensor device could be combined with a fully printed electrochemical transistor on
flexible substrates. The light stimulation of the sensor results in a reasonable drain current modulation of the ECT.
Due to the required switching currents of the ECT the impedance matching between the capacitive sensor and the
transistor is not yet optimized and the transistor could not be totally switched off. The next generation of printed
ECTs will be fabricated by ink-jet printing resulting in thinner channels and reduced current consumption during the
switching process thus enabling a complete switching of the transistor by the sensor signal.
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