Page 160 - Organic Electronics in Sensors and Biotechnology

Integrated Pyr oelectric Sensors 137

about 80 MV/m, the applied coercive field should be taken well
above this value (around 150 MV/m). A stepwise enhancement of the
voltage and intermediate times with zero voltage applied to the sam-
ple was used as reported in the literature. The effectiveness of the
16
poling with respect to dipole alignment is strongly influenced not
only by the poling voltage (the coercive field) but also by the tem-
perature during poling as is depicted in Fig. 4.10a for a sample with
2.9 μm layer thickness and VDF content of 70%. According to this, the
highest response (current and voltage) corresponding to the highest
polarization can be achieved by increasing the poling field stepwise
to about 140 MV/m at 130°C.

Phenomenology of the Pyroelectric Response
The pyroelectric response as obtained by the measurement setup
shown in Fig. 4.10a can be detected in the voltage and current mode.
The frequency dependence of the voltage as well as the current response
for the copolymer sample with d = 2.9 μm is shown in Fig. 4.10b. The
maximum voltage response values for the setup with the lock-in
amplifier are achieved at 5 Hz, whereas the maximum current
response is achieved between 2 and 6 kHz. According to the equiva-
lent circuit and the relation

R ⎛ 1 1 ⎞ −1
|V |=| I |⋅ C = C + C R = ⎜ ⎜ + ⎟ (4.25)
pyro pyro p i
1 + ω 2 RC 2 ⎝ R p R i⎠
2

the voltage response varies as IR below and I/(ωC) above the cutoff.
The cutoff frequency ω is determined by the RC time constant of the
c
whole equivalent circuit as ω = 1/RC. Here R and C correspond to
c p p
the resistance and capacitance of the pyroelectric element and R and
i
C to the input resistance and capacitance of the measurement circuit,
i
respectively.
From an inspection of Fig. 4.10 it becomes clear that, apart from
influences of the dipole alignment (poling voltage and temperature),
the absorption of the incident light (existence of absorber structure)
and the waveform of the excitation have an influence on the magni-
tude of the response. With a graphite absorber layer, the voltage
response is more than doubled, and the use of a square waveform of
the laser excitation additionally yielded 30% signal (compare values
for 500 V poling voltage in Fig. 4.10).
For the calculation of the voltage and current sensitivities of
the pyroelectric sensor element, the magnitude of the pyroelectric
response has to be divided by the power of the incident radiation. This
is done in Fig. 4.11 for a thin pyroelectric sensor element fabricated on
Melinex substrate. Here the inverse frequency dependence of the
voltage response with respect to that of the current response [which
is basically expected from Eq. (4.25) for ω > ω ] is nicely observed.
c