Page 181 - Organic Electronics in Sensors and Biotechnology

158 Cha pte r F o u r

1.E–05
V D = –3 V
1.E–06
–1.2E–06
–4 V
–1.0E–06 1.E–07
–8.0E–07 1.E–08
I Drain (A) –6.0E–07 –3 V I ds (A) 1.E–09

–4.0E–07
1.E–10
–2.0E–07
–2 V
–1 V
0.0E+00 1.E–11
0.0 –0.5 –1.0 –1.5 –2.0 –2.5 –3.0 –3.5 –4.0 –4.5 –5.0 –4 –3 –2 –1 0 1
V Drain (V) V (V)
gs
FIGURE 4.27 (Left) Output and (right) subthreshold characteristics of a pentacene-
based OTFT with 30 nm Al O + 10 nm PαMS as gate dielectric and Au-source and drain
3
2
electrodes.
ZrO -based layers, and consequently the input impedance is some-
2
what smaller (~150 MΩ @ V = −3 V).
D
Therefore we also produced a thicker Al O layer and, similar to
2 3
the other device, a PαMS layer of about 10 nm, that resulted in an
2
overall area-related gate capacitance of 80 nF/cm . The output and
subthreshold characteristics of this device are depicted in Fig. 4.27.
The drain current level of this device is smaller than that of the one
with thinner Al O which is partly due to the smaller gate capacitance
2 3
and partly due to the slightly worse pentacene morphology that results
in a smaller charge carrier mobility of about μ = 0.2 cm /(V ⋅ s). The
2
other relevant parameters are V = −1 V and S = 200 mV/dec and are
on
extracted from the subthreshold characteristic displayed in Fig. 4.27.
Clearly, the smaller gate leakage of ~1 to 2 × 10 −10 A results in an input
impedance of the order of about 5 to 6 GΩ.
Integrated Sensor
The fully flexible pyroelectric sensor element shown schematically in
Fig. 4.28a illustrates the potential of such OTFTs in new high-end
35
applications of organic electronics. The sensing principle is based on
the pyroelectric effect in a ferroelectric P(VDF-TrFE) copolymer. The
sensor element is composed of a 2 μm thick pyroelectric P(VDF-TrFE)
copolymer film that is fabricated by spin coating. The pyroelectric
element is sandwiched between 70 nm thick Al electrodes directly
integrated on glass or on a flexible PET substrate. The sensing capac-
itor is fabricated prior to the deposition of the OTFT, and both devices
are integrated via the bottom electrode of the sensor serving as the