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Pressure and strain sensors
Keywords: Organic Field-Effect Sensors, Mechanical Sensors, Strain and pressure sensing.
Organic thin-film transistors have excellent potential for applications in low-cost, large area and flexible electronics. Organic semiconductors offer several advantages due to easy processing, good compatibility with a wide variety of substrates including flexible plastics, and great opportunities in terms of structural modifications. Furthermore, thin films of organic semiconductors are mechanically robust and flexible, and this characteristic offers new possibilities for non-planar flexible electronics. Sensors seem to be the optimal candidate for fully profiting from the unique properties of these materials.
the other hand, in terms of applications for these devices, artificial sense
of touch is considered an essential feature for future generations of robots
and wearable electronics have become one of the hottest themes in electronics
aimed at the design and production of a new generation of garments with distributed
sensors and electronic functions. For both of these fields, conformable strain/pressure
sensors are highly desirable.
We have recently proposed a support-free and totally flexible structure for mechanical
sensors. The effect of strain on the mechanical and electronic properties of
organic semiconductors is an emerging research topic in fundamental physics
and applications. The marked sensitivity of the drain current to an elastic
deformation induced by a mechanical stimulus on the device channel has been
exploited for detecting a pressure applied by means of air flow on the gate
side of the free-standing device.
Figure 1(a) shows the drain current ID versus the drain voltage VD at different values of VG and with different pressures applied. The characteristic shows a decrease in the current when pressure is applied. Figures 1(b) and 1(c) show the time variation of ID (in the linear region) while pressure is applied to the device. In Fig. 1(b) the current variation in response to different value of the applied pressure is shown, while Fig. 1(c) shows what happens when the same value of pressure is applied and removed according to a time sequence.
In order to investigate the sensitivity of the device to different mechanical stimuli, we also performed experiment in order to detect the bending of a substrate. These experiments were performed by fixing the device on a substrate and by bending it as shown in Fig. 2(a). The sensor was bended 50 times and the signal recorded .is shown in Fig. 2(b)
Fig.2: (a) Experimental set-up for bending detection. (b) Reversibility of ID vs.Time for 50 bending
A careful analysis of the pressure dependence of the current shows that this
dependence can be explained in terms of variation in the mobility, the threshold
voltage and in the contact resistance of the transistor. The variation of the
mobility may be attributed to a direct dependence of the semiconductor conductivity
on the pressure applied to the device while the distribution and activity of
trap states are typically responsible for variations in contact resistances
and threshold voltages.
I. Manunza, and A. Bonfiglio, Pressure sensing using a completely flexible,
organic transistors, Biosensors and Bioelectronics 22, 2775-2779, (2007)