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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.
Relatively little progress has been made in the field of pressure or bending recognition compared to the areas of gas and chemical sensing, mainly because mechanical sensing requires attributes of conformability and flexibility as well as three-dimensional large area shaping that in many cases are difficult to achieve even for organic devices.

On 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.
Organic transistors should be inherently mechanically flexible, but many of the devices reported in the literature are fabricated on rigid mechanical supports such as silicon or glass. Often, even in devices assembled on plastic films, the presence of a mechanical support results in a reduction of flexibility. In addition, if the whole device is flexible, the mechanical stimulus can be applied to the semiconductor through the support itself (thus avoiding the risk of damaging the semiconductor layer).


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.

Fig.1: (a) IDVD curves for different pressure states. (b) ID vs.Time curves for different applied pressures. (c) Reversibility of ID vs. Time


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.
The main drawbacks of these devices seem to be the high operating voltage and a limited stability that could be overcome by using a proper flexible encapsulation layer in order to protect the semiconductor layer from exposure to ambient (humidity, light, etc.) as these factors are known to cause drift and in general to negatively affect the device performance.



I. Manunza, and A. Bonfiglio, Pressure sensing using a completely flexible, organic transistors, Biosensors and Bioelectronics 22, 2775-2779, (2007)
I. Manunza, A. Sulis and A. Bonfiglio, Pressure sensing by flexible, organic, field effect transistors, Applied Physics Letters, 89, 143502 (2006)
I. Manunza, and A. Bonfiglio, Organic Field-Effect based devices for pressure detection, Materials Research Society. Symp. Proc., 965, 0965-S10-03 (2007)





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