Page 122 - Organic Electronics in Sensors and Biotechnology

Strain and Pressure Sensors Based on OFET 99

PVA
Pentacene

Source Drain
PVP gate dielectric
Gate
Glass substrate

FIGURE 3.5 Schematic of the device structure reported in Ref. 11; PVA is
poly(vinyl alcohol). (Reprinted with permission from Ref. 11. Copyright 2005,
American Institute of Physics.)

not flexible, therefore not suitable for applications such as robot skin,
e-textiles, etc.
11
Darlinski and coworkers studied the possibility of realizing
pressure sensors based solely on organic transistors, without the need
of any additional sensing element. In this way the organic device itself
acts as a sensing element. To study the pressure dependence of the
electrical performance of these devices, the authors applied mechani-
cal force directly on the transistors using a tungsten microneedle
moved by a step motor, as shown in Fig. 3.5. During measurements,
the device substrate is placed on a balance to measure the applied
pressure.
The authors explained the force-induced change in the drain cur-
rent in terms of the variation in the distribution and activity of trap
states at or near the semiconductor/dielectric interface. A major
drawback of the reported device is that, being realized on a stiff glass
substrate, it is not flexible.
Strain sensors using an organic semiconductor as the sensitive
(resistive) element of a strain gauge have been also reported by Jung
and Jackson. In conventional strain gauges, the large stiffness mis-
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match generated between the inorganic semiconductor element and
the flexible (polymeric) substrate may lead to irreversible plastic sub-
strate deformations, and this can be problematic. The stress in the
sensitive element (i.e., the inorganic semiconductor or the metal, with
a high Young modulus) is not representative of the stress present in the
substrate (i.e., the polymeric material with the low Young modulus).
In this way, as a result, the sensor’s performance is reduced in terms
of reliability and reproducibility. In contrast, it is expected that the
use of organic semiconductors with low Young modulus (on the order
of 5 GPa) as the sensing element would minimize the induced stress
concentration. The sensor cross section is shown in Fig. 3.6.
For these sensors, 2 nm thick Ti and 20 nm thick Au were depos-
ited on 50 μm thick polyimide substrates by thermal evaporation.