Page 152 - Organic Electronics in Sensors and Biotechnology

Integrated Pyr oelectric Sensors 129

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review see Bauer and Lang ), and a large variety of amorphous,
semicrystalline, single crystalline and liquid crystalline polymers are
known to show significant pyroelectric response.
A complete pyroelectric polymer sensing pixel typically is thought
of as a capacitive pyroelectric sensor with an input unit comprised of
infrared absorption and focusing elements such as absorption layers
and micro-optics and an output unit that accounts for signal readout,
impedance transforming, and signal amplification. In the angle-
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selective motion sensor developed by Siemens (marketed under the
brand name PID-21), PVDF is used as a freestanding unit in an array
configuration that is glued to an appropriate frame and connected to
the electronics, thus forming a hybrid PCB construction. Another
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possibility is to directly integrate polymer pyroelectric sensor arrays
on silicon substrates, which provide the readout electronic circuits
(impedance transformers, amplifiers). This, however, faces serious
problems concerning thermomanagement. 14–18
Contrary to that, it would be very advantageous to directly inte-
grate polymer sensors and transistors, which means that in the case
of silicon-based electronics, the sensor has to be produced directly on
the silicon wafer, acting as the common substrate for transistor and
capacitive sensing unit. Therefore classical integrated infrared detec-
tors are confined to rigid substrates and planar surfaces and do not
provide continuous panoramic (360°) views. 16–18
With this in mind, organic thin-film transistors (OTFTs) and capac-
itive pyroelectric polymer sensors are easily pulled together to form
integrated flexible pyroelectric sensors if one accounts for the possibil-
ity of both device classes being fabricated on flexible substrates and
large areas in cost-effective production processes, thus opening com-
pletely new application areas. Such applications are found, e.g., in the
context of pedestrian protection in the automotive industry, novel con-
cepts for human/machine interfaces used in mobile electronics, large-
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area security features, low-cost home electronics, and artificial skin.
To date, there is no example of a large-area integrated organic pyroelec-
tric sensor on the market, due to the high demands made on the per-
formance of the OTFTs.

4.2.2 Theoretical Background—Pyroelectricity

Pyroelectrics and Ferroelectrics
Ferroelectrics have raised a lot of interest in the last decades, because
of their wide field of applications especially in flash memories, due to
their high dielectric constants (in the range from 10 to 1000), allowing
high storage densities.
The important property for pyroelectrics and ferroelectrics is the
existence of a spontaneous polarization. This polarization arises from a
polarity in the unit cell of an electrical anisotropic crystal or portions of