Flexible Structures for Mechanical Sensors

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 [32]. 

A careful analysis of the pressure dependence of the current [34] 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. 

In order to clarify the influence of structural effects (in particular at the metal/semiconductor interfaces) on the pressure sensitivity, we have also developed, on the same insulating layer, couples of bottom-contact and 

On the other hand, the mobility had a very similar behavior in top-contact and in bottom-contact devices, indicating also a direct contribution of the semiconductor mobility to the observed sensitivity. Despite the fact that the underlying mechanism of the observed pressure sensitivity is not completely clarified yet, pressure sensitivity seems to result from a combination of mobility variations in the channel and interface effects in the source/drain surrounding areas, likely due to morphological modifications of the pentacene layer under stress. 

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