Page 175 - Organic Electronics in Sensors and Biotechnology

152 Cha pte r F o u r

the activated sensor element. The adjacent sensor elements are not
activated, but due to the heat flux from the excited sensor element a
thermal variation is also present in these elements. The middle line
corresponds to the first neighbor and the lower line to the second
neighbor. The different subfigures represent the temperature lift for
various material compositions.
A direct quantitative comparison between the one- and the two-
dimensional models is complicated. In the one-dimensional model, the
starting point is the intensity of the laser, and the absorption at the top
of the sensor element is already a part of the modeling process. The
starting point in the two-dimensional model is the temperature lift at
the front side of the sensor element. Since the temperature lifts for the
different layers can be computed with the one-dimensional model, this
value can be taken as a starting point for the two-dimensional model.
In Fig. 4.22a, a calculation of a P(VDF-TrFE) layer on a silicon substrate
is made, whereas in Fig. 4.22b PET is used as the substrate. The upper-
most lines correspond to the time development of the average tem-
perature in the sensor element excited. The average temperature was
calculated by summing over all cells in the pyroelectric layer of the sen-
sor element and dividing them by the number of cells. In the case of the
silicon substrate the maximum average temperature is simulated to be
0.03 K (assuming the excitation temperature at the top side of 0.1 K and
the frequency of 1 Hz). In the case of the PET foil substrate the maxi-
mum temperature lift is 0.06 K. This corresponds to the result of the
one-dimensional model. The high thermal conductive substrate is act-
ing as a heat sink, lowering the average temperature in the sample. It
takes too much time to calculate this temperature over the whole fre-
quency range as it was done in the one-dimensional model, but the
result at the frequency of 1 Hz corresponds to the values obtained in
the one-dimensional model.

Considerations on the Lateral Resolution Since the heat flux limits the
spatial resolution of a sensor array, the heat flux to adjacent sensor
elements has been calculated. The response of the first and second
neighbor element is also displayed in Fig. 4.22. The ratio of the tem-
perature difference T /T is given in Table 4.3 for different
neighbor excited
substrates calculated. The determination of the allowed crosstalk
between adjacent sensor elements depends on the readout electron-
ics. However, this value can be seen as a lower limit for the tempera-
ture difference detectable by adjacent sensor elements. Even if smaller
differences could be detected by the readout electronics, the heat con-
ductivity becomes the resolution limiting factor for the device. It
becomes obvious that a low thermally conductive substrate is advan-
tageous for PVDF as the pyroelectric layer. However, the best results
are obtained when using a high thermally conductive pyroelectric
layer (see Fig. 4.22). The temperature lift in the pyroelectric layer is the
highest, and the thermal crosstalk is low. This result is in contradiction