Flexible tactile sensor based on PVDF-TrFE integrated on organic TFT for robotic applications

Nowadays organic electronics can be successfully adopted as an enabling technology for the development of a new generation of smart flexible sensors. In fact the ceaseless progress concerning organic materials and low cost fabrication technologies is pushing organic electronics towards applications difficult to conceive only a few years ago. In particular, in robotics, where the development of tactile sensor array is a critical issue, flexible sensing techniques based on new materials result mostly appealing for the fabrication of electronic skin. Tactile sensing can enhance the cognitive input that a robot can collect for acquiring fundamental information of the environment in which the robot is operating [1-3]. In this work we investigated flexible tactile sensors fabricated by integrating an ultrathin piezoelectric capacitor based on PVDF-TrFE film with an OTFT made on a PEN substrate by using a multi-foil approach [4-5] (see fig.1). The sensing element is a non-coplanar circular structure with an area of 2 mm2 and the thickness of PVDF-TrFE is 2 um. The sensor has been fabricated on a flexible polyimide substrate reaching a final thickness of 7 um. P-channel OTFTs, with staggered top-gate configuration were fabricated on a PEN foil, 125 um thick, adopting the following materials: 1) evaporated gold for source-drain contacts; 2) solution processed pentacene derivative, namely SmartKem? p-FLEX(TM) supplied by SmartKem Ltd, about 30nm thick, as organic semiconductor; 3) a fluoropolymer (CYTOP) film, 500nm thick as gate dielectric; 4)aluminium as gate electrode. The active material and the dielectric were deposited by spin-coating technique and lithographically patterned. The OTFTs dimensions were designed ranging from 2 to100 um as channel length and 50, 100 and 200 um as channel width. The devices show very high performance with field effect mobility up to 3cm2/Vs, low threshold voltages (0.2 V) and sub-threshold slope (2 V/dec) and excellent stability, making them good candidates to drive a sensing system. The sensor was then mounted on a flexible PCB and inserted into a mini-shaker to measure its response at different mechanical stimuli in terms of applied force (up to 3 N) and working frequency (up to 1.1 kHz). The device was analysed in thickness mode, thus exploiting the higher piezoelectric coefficient d33 that for PVDF-TrFE is of about 30 pC/N. The sensor was then connected in a common-source amplifier configuration by using a load resistance of 8 MOhm. Due to the non negligible value of the impedance used in the circuit the output signal was read through a trans-resistance amplifier (see Fig.2) placed on the source terminal of the DUT. Virtual ground has been set at the input to make a more reliable measure. The linear response of the sensor measured for a sinusoidal stimulus at 200 Hz is shown in fig.3. The offset of about 10mV has been verified to be due to an internal offset of the trans-resistance amplifier only. In fig.4 the behaviour of the sensor at increasing working frequencies is analysed for an incoming stimulus of 1 N. The output increases with the frequency due to the circuital high-pass filter composed by CPVDF-TrFE and RGG at the gate