Page 154 - Organic Electronics in Sensors and Biotechnology

Integrated Pyr oelectric Sensors 131

where A is the surface area of the capacitor and Q is the charge
generated.
The spontaneous polarization creates an electric displacement
D in the material. This generates a net positive or negative charge at
the surface of the sample. In air these charges are more or less
shielded by free charge carriers from the surrounding atmosphere.
When electrodes are situated on the surface, the potential is com-
pensated by free charges in the conductive material. For a stable
temperature, no current can be observed since the electric displace-
ment in the sensor material remains constant. As soon as the material
undergoes a temperature variation, the spontaneous polarization
changes due to changes in the dipole orientation and by a change in
the density of the dipoles per volume. The change in the spontane-
ous polarization induces a change of the charge density at the sur-
face of the pyroelectric layer. This is compensated by charge carriers
in the electric circuit, thus producing a measurable current via the
connecting electrodes.

4.2.3 Pyroelectric Polymer Materials

Structure of Poly(Vinylidene Fluoride)
The various properties of poly(vinylidene fluoride) or PVDF make it
suitable for a wide range of applications. Depending on the way of
fabrication the PVDF can crystallize in several different phases which
are called α, β, γ, and δ phases. Vinylidene fluoride has a large molecular
−30
dipole moment of μ = 7 × 10 C ⋅ m that occurs between the positively
v
charged hydrogen and the negatively charged fluorine atoms in the
(⎯CF CH ⎯) section of the polymer. In the ferroelectric β-phase, the
2 2
polymer chain is in the all-trans (TTTT) conformation, having all
the dipoles oriented in the same direction, leading to a polar axis over
the whole polymer chain. In the crystalline structure of the β-phase, the
orientation of the polar axis is the same for all the polymer chains, lead-
2 24
ing to a macroscopic spontaneous polarization of 130 μC/m .
Since the properties such as ferro-, piezo-, and pyroelectricity
depend on the existence of a macroscopic spontaneous polarization,
the generation of samples with a high amount of the ferroelectric β-
phase is important. The crystallization as paraelectric α- or ferroelectric
β-phase depends on the way in which the thin films are fabricated. To
get the ferroelectric phase directly from the melt, some amount of the
copolymer poly(trifluoro ethylene) (PTrFE) has to be added. This is so
because a substitution of one hydrogen atom by the larger fluorine
atom in TrFE (tri-fluoroethylene) favors the formation of the all-trans
conformation. From a theoretical point of view, the copolymer reduces
the possible maximum of the macroscopic polarization because it is a
less polar molecule than the PVDF, but it increases the overall crystal-
linity of the semicrystalline polymer material.