Page 151 - Organic Electronics in Sensors and Biotechnology

128 Cha pte r F o u r

4.2 Integrated Pyroelectric Sensors

4.2.1 Introduction
Infrared sensors are used to detect thermal radiation in the mid- to far-
infrared wavelengths. The wavelength region around 10 μm is of par-
ticular interest for it is there that the thermal radiation of living species
reaches maximum intensity (at room temperature). Thermal radiation
can be converted to electric signals by two groups of infrared detectors.
The first group is formed by the photon detectors, which are wavelength-
selective. They can be based on the photovoltaic, photoconductive, or
photoelectric effect. They are made of semiconductor materials with a
narrow energy gap, such as indium antimonide, and are extremely fast
and sensitive. However, these quantum detectors require a minimal
energy per photon for their operation and therefore often are cooled to
cryogenic temperatures to obtain sufficient performance. Thermal detec-
tors form the second group. They indicate the temperature rise of the
sensor material by a change in resistance or thermoelectric power and
are characterized by a slower response (and hence a low-frequency
bandwidth) than that of the quantum detectors. They are sensitive to the
entire absorbed radiation, regardless of its spectral composition, and are
therefore particularly well suited for the detection of IR radiation. Their
performance is limited solely by the spectral transmittance of the
entrance window and of the optical imaging elements. However, ther-
mal detectors are inferior to quantum detectors especially in sensitivity
by several orders of magnitude. Bolometers, thermocouples and ther-
mopiles, and pyroelectric detectors belong to this group.
The pyroelectric detector is the fastest of the thermal detectors
since temperature changes at the molecular level are directly respon-
sible for the detection process. Pyroelectricity is the electrical response
of a material to a change in temperature. It is found in any dielectric
material containing spontaneous or frozen polarizations resulting
from oriented dipoles and occurs in 10 crystal classes, certain ceram-
ics, and polymers that have been submitted to a special treatment. As
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discussed by S. B. Lang, the pyroelectric effect has been known for
24 centuries when the Greek philosopher Theophrastus probably
gave the earliest known description of the pyroelectric effect in his
treatise “On Stones.” Although pyroelectricity of polymers was
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already discovered in the 1940s, it was not before 1971, when strong
pyroelectricity was discovered in polyvinylidene fluoride (PVDF) by
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Bergmann et al., that the polymers received any serious attention
due to the initially weak effects. Early applications then emerged
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very soon—Glass et al. and Yamaka reported on polymeric pyroe-
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lectric infrared sensors, while Bergmann and Crane demonstrated a
pyroelectricity-based xerography process. Nowadays the nature of
pyroelectricity in polymers is reasonably well understood (for a