Page 343 - Organic Electronics in Sensors and Biotechnology

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for 1 s and a constant drain voltage of -100 V. Retention times up to
200 s at zero gate voltage were achieved although the on/off ratio is on
the order of 10. As a summary of this section, it is obvious that the field
of organic memory elements is in its infancy and requires additional
extensive research to reach practical interest in real-world applications.
As another example with potential practical applications, we next
describe briefly organic electronics-based sensors for monitoring
physical parameters and for photodetection.

8.2.2 Single-Element Temperature and Pressure Sensors
There is a large amount of literature on chemical sensing with OFETs,
which we will not review here. Rather we limit our discussion to sen-
sors recording changes in physical environmental parameters such as
changes in temperature and pressure. Such sensors may be useful in
a wide range of applications and may also form the basis for large
area electronic skin.
The field of temperature and pressure sensing with OFETs is also in
its infancy, and only a limited number of publications are available,
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despite the large prospect for applications. Darlinski et al. have out-
lined a pressure sensitivity in polyvinylphenol-based OFETs, which
they ascribe to changes in carrier mobility, threshold voltage, and con-
tact resistances. The origin of the effects measured has not been com-
pletely clear yet, so there is still research required to clarify the inherent
pressure dependence of OFETs. Trapped charges may play an impor-
tant role, but more experiments are needed to elucidate this sugges-
tion. Jung et al. exploited thermal transport in the subthreshold regime
of organic thin-film transistors to demonstrate temperature sensing
with pentacene-based OFETs with silicon dioxide gate dielectrics on
silicon substrates. Maccioni et al. have suggested the use of OFETs as
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sensors for environmental properties in smart textiles. Graz et al. and
Zirkl et al. employed functional polymers such as ferroelectrets or
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ferroelectric copolymers for pressure and temperature sensing. In
their approach, the functional polymer is not directly employed as gate
dielectric, but is connected to the gate. Temperature and pressure sen-
sitivity is obtained by means of the pyro- and piezoelectric effect of
ferroelectrets or ferroelectric polymers. Thereby piezoelectric switches,
pressure sensors, and paper-thin microphones could be demonstrated,
as well as optothermal switches and infrared sensors. There is plenty of
room for further work on sensing principles with OFET-based devices,
which may be used in applications such as mobile appliances, but also
sensitive skin in robotics, etc.

8.2.3 Light Sensors
Light response in OFETs is of interest from both a fundamental science
and an application point of view. 90–109 A scheme of a light-responsive
OFET is shown in Fig. 8.17. From the fundamental point of view, so