Organic Bendable and Stretchable Field Effect Devices ... - IEEE Xplore

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IEEE SENSORS JOURNAL, VOL. 13, NO. 12, DECEMBER 2013

Organic Bendable and Stretchable Field Effect Devices for Sensing Applications Alberto Loi, Laura Basiricò, Piero Cosseddu, Stefano Lai, Massimo Barbaro, Annalisa Bonfiglio, Perla Maiolino, Emanuele Baglini, Simone Denei, Fulvio Mastrogiovanni, and Giorgio Cannata

Abstract— In this paper we propose a detailed investigation on the electrical response to mechanical deformations of organic field effect transistors assembled on flexible plastic substrates. We demonstrate that through applying a surface deformation by an external mechanical stimulus we induce morphological and structural changes in the organic semiconductor giving rise to a marked, reproducible and reversible variation of the device output current. We show how the intrinsic properties of the employed active layers play a crucial role in determining the final sensitivity to the mechanical deformation. Finally we also demonstrate that the fabricated flexible system can be successfully employed for different applications that go from the detection of bio-mechanical parameters (e.g., joint motion, breath rate, etc.) in the wearable electronics field to tactile transduction for the realization of artificial robot skin Index Terms— OFETs, strain sensors, inkjet printing, robot skin, bio-parameters monitoring.

I. I NTRODUCTION

O

RGANIC semiconductor-based electronics has achieved a wide consideration in the past decades, as this new class of materials has definitely opened the way for the fabrication of electronic devices over large areas with cost-efficient technologies and remarkable electrical properties [1]–[5]. One of the main advantages of employing conjugated polymers is the fact that, thanks to their mechanical properties, it is possible to fabricate highly flexible electronic systems using these materials. However, as recently highlighted Manuscript received December 13, 2012; revised June 14, 2013; accepted July 2, 2013. Date of publication July 11, 2013; date of current version October 9, 2013. This work was supported by the European Commission’s Seventh Framework Programme project ROBOSKIN under Grant 231500. The work of A. Loi and S. Lai was supported in part by Regione Autonoma della Sardegna under the POR Sardegna FSE. This is an expanded paper from the IEEE SENSORS 2012 Conference. The associate editor coordinating the review of this paper and approving it for publication was Dr. Alexander Fish. A. Loi, P. Cosseddu, S. Lai, M. Barbaro, and A. Bonfiglio are with the Dipartimento di Ingegneria Elettrica ed Elettronica, University of Cagliari, Cagliari I-09123, Italy (e-mail: [email protected]; [email protected]; [email protected]; barbaro@diee. unica.it; [email protected]). L. Basiricò was with the Dipartimento di Ingegneria Elettrica ed Elettronica, University of Cagliari, Cagliari I-09123, Italy. She is now with the Istituto per lo Studio dei Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche, Bologna I-40129, Italy (e-mail: [email protected]). P. Maiolino, E. Baglini, S. Denei, F. Mastrogiovanni, and G. Cannata are with the Dipartimento di Informatica, Bioingegneria, Robotica ed Ingegneria dei Sistemi, University of Genova, Genova I-16145, Italy (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2013.2273173

by several research groups [6]–[13], electronic transport in organic semiconductors is affected by mechanical deformations. The surface strain, induced on the semiconducting layer by mechanical stress, typically leads to variations of its structural/morphological properties, which strongly influence the charge carrier transport in Organic Field Effect Transistors (OFETs) [6]–[10]. For applications where this sensitivity is undesired (typically for electronic circuitry), several approaches have been introduced for reducing the effect of mechanical deformation on the electrical behavior of the fabricated devices. In some cases it is reported that by properly modifying the device structure, i.e. by placing the organic semiconductor film on a zero strain layer, the effect of mechanical stress can be eliminated [11]. In another example it is also demonstrated that the employment of alkylated molecules could be also a valuable solution, as the alkyl chain terminations can help in accommodating most of the surface strain induced on the semiconductor film [12]. Very recently, Sokolov et al. [13] reported that not only the organic semiconductor, but also the employed dielectric film plays a crucial role in the device response to the applied strain. In particular, by employing different polymeric gate dielectrics, they observed a clear correlation between the sensitivity to strain and the surface energy of the insulating film, and have been able to fabricate devices with a very small dependence on mechanical deformations. On the other hand, if the electrical response of the devices is reproducible, possibly linear, and reversible, OFETs can be employed as mechanical deformation sensors. In fact, there are several advantages for using OFETs in comparison with piezoresistive sensors: i) transistors are multiparametric devices, in which different electronic parameters, not only one as for piezoresistive sensors, can be extracted from their electrical characterization, offering the possibility of using a combination of variables in order to characterize their response to the parameter to be sensed; ii) in OFETs the electrical response can be intrinsically amplified by the transistor itself; iii) OFETs join the sensing properties with the switching features of a transistor, allowing the fabrication of sensors matrices in which every single element can be addressed without additional devices. Moreover, thanks to the intrinsic mechanical properties of organic polymers, highly flexible and compliant systems, which can be easily transferred on different surfaces, can be fabricated and employed for a wide range of applications. For these reasons, there are several examples reported in the literature on the employment of OFETs for mechanical sensing, and in particular for reproducing the sense of

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LOI et al.: ORGANIC BENDABLE AND STRETCHABLE FIELD EFFECT DEVICES FOR SENSING APPLICATIONS

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another dielectric layer was deposited in order to protect the structure from the damage caused by external agents, like oxygen and humidity. B. Plastic Substrates Fig. 1. Schematic representation of the bottom-gate/bottom-contact OFET structure. (a) Mylar acts at the same time as flexible substrate and as gate dielectric. (b) All the layers are deposited on the PET substrate.

touch ([14]–[16]). For instance Someya et al. [16] fabricated matrices of pressure sensors in which a piezoresistive rubber was connected in series with the source electrode of each OFET. In this way, changes in the series resistance induced by pressure lead to a variation of the output current of the OFET. More recently, Mansfeld et al. and Schwartz et al. ([14], [15]) reported on the fabrication of an OFET-based system for pressure sensing fabricated by a micro-structuring of the gate dielectric, in which the capacitance changes induced by pressure on the dielectric layer lead to a variation of the OFET output current. The approach we have employed for fabricating mechanical sensors is slightly different. We took advantage of the sensitivity of the organic semiconductor films to surface deformation, and exploited this property for the fabrication of mechanical sensors, without adding any other fabrication step. In particular, in this paper we have performed a detailed electromechanical characterization on devices realized with two very different organic semiconductors, namely a thermally evaporated small molecule (Pentacene) and a solution processable polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT). Interestingly enough, it was found that the intrinsic morphological properties of the two employed semiconducting films play a crucial role in the electrical response of the devices to mechanical deformation. We also demonstrate that, since such a response is reproducible and reversible within a certain range of mechanical deformation, these devices can be employed for the fabrication of mechanical sensors on a wide range of applications, going from tactile transduction for the realization of artificial “robot skin” to the detection of bio-mechanical parameters (e.g., joint motion, breath rate, etc.) in the wearable electronics field. II. E XPERIMENTAL A. Fabrication Steps All OFETs have been fabricated in a bottom-gate/bottomcontact configuration, as shown in Fig. 1. The base of the final structure is a highly flexible substrate, where the gate electrode was deposited by thermal evaporation or inkjet printing. The gate dielectric was deposited from vapour phase, except for the structure reported in Fig. 1(a), in which a very thin, freestanding, plastic film acts at the same time as flexible substrate and as gate dielectric. Then, the source and drain electrodes were patterned by inkjet printing or thermal evaporation. After that, the organic semiconductor was deposited from solid phase, by thermal evaporation, or liquid phase, by drop casting or spin coating. Finally, in the fabrication of the robot skin,

Several substrates have been used, which differ in thickness and thermal resistance. For the electromechanical characterization of single OFETs, a very thin, 1.5 μm thick, free-standing poly(ethylenetherephthalate) (PET) foil (Mylar, DuPont) was employed. In this particular case, being the film free-standing, the gate electrode was patterned on one side of the film, whereas source and drain and the organic semiconductor were deposited and patterned on the opposite side (Fig. 1(a)). For the realization of the electronic skin (structure shown in Fig. 1(b)), three substrates were employed: 175 μm thick PET (boPET) films (Goodfellow), 125 μm thick poly(ethylenenaphthalate) (PEN) films (Goodfellow), and 50 μm and 13 μm thick polyimmide films (Kapton, Goodfellow). All these substrates are flexible, transparent and have good resistance to chemical agents, but Kapton is more thermoresistant than PET and PEN. Therefore Kapton substrates are more suitable when high-temperature annealing is required, since no deformation occurs until temperatures as high as 500 °C. In all cases, the substrates (175 μm and 125 μm) were cleaned by subsequent 15 min ultrasonic baths in acetone and isopropyl alcohol, then washed with deionized water and finally dried under nitrogen flow. C. Electrodes Gate, source and drain electrodes were fabricated using two different approaches. In one case, gold electrodes have been deposited by thermal evaporation in high-vacuum condition and patterned using a shadow mask (for the gate) and standard photolithography (for source and drain). For the fabrication of matrices of OFETs, electrodes were deposited by means of inkjet printing of a silver ink (Cabot Conductive Ink, CCI-300), which contains surface modified ultra-fine (average size 20 nm) silver nanoparticles dispersed in a liquid vehicle composed of ethanol and ethylene glycol. Before filling the printer cartridge, CCI was subjected to 15 min ultrasonic bath and filtered with 0.2 μm nylon filter in order to avoid agglomeration of nanoparticles. D. Gate Dielectric The gate dielectric layer, namely polypara-xylylene (Parylene C, purchased by Specialty Coating Systems), was deposited from the vapor phase, using a Labcoater 2 SCS PDS 2010, according to the standard procedure described in [7]. In order to promote the dielectric polymer adhesion on the metallic gate electrode, γ -methylacryloxypropyltrimethoxysilane (Sylane A-147, Specialty Coating Systems) was deposited by thermal evaporation before the deposition of Parylene. On one hand, in order to prevent high leakage currents caused by fractures and/or pinholes in the Parylene layer, the thickness of the gate dielectric was set to 1.5 μm and the resulting capacitance was 1.86 nF/cm2 . On the other hand, the layer deposited to encapsulate the final structure was 2.5 μm thick.

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Fig. 2. Picture of the interdigitated (left) and continuous (right) patterns printed for the realization of the OFETs. The channel length is 50 μm.

E. Organic Semiconductors Three different organic semiconductors have been employed in this work. For the electromechanical characterization of single OFETs, two different organic semiconductors have been employed, namely P3HT (OS2100, purchased from Plextronics) and Pentacene (Sigma Aldrich). The first one was dissolved in chlorobenzene (0.5 wt.%) and deposited by spin coating at 3000 rpm for 60 s. After deposition the film was annealed for 1 hour at 40 °C in order to remove residual solvent. Pentacene was deposited by thermal vapor deposition in high vacuum condition. For the fabrication of the robot skin, a solution-processable version of Pentacene, namely 6,13-bis(triisopropylsilylethynyl)-Pentacene (TIPS-Pentacene, Sigma Aldrich), was deposited by drop casting. The solution was prepared at 0.5 wt.% in toluene and stirred for 1 hour at 90 °C. 1 μL drops were deposited on the transistors’ channel while keeping the substrate on a hotplate at 90 °C, in order to promote fast solvent evaporation and subsequently to obtain good crystallization of the semiconductor. F. Inkjet Printing The main deposition technique adopted for the realization of the robot skin was inkjet printing, performed by the piezoelectric drop-on-demand Dimatix Materials Printer 2800 (DMP2800). We used DMC-11610 cartridges, which contain 16 nozzles with a diameter of 21.5 μm corresponding to 10 pL drops of ink. Since drop spacing (distance between two contiguous drops), firing voltage (needed for inducing deformation of the piezoelectric crystals) and jetting frequency (related to the speed of the printing cartridge) strongly affect the quality of the printed pattern, a fine preliminary tuning of these printing parameters was mandatory. In order to determine the resolution of the printed pattern, single drops of CCI-300 ink were printed, showing an uniform value of 30 μm ± 2 μm for the drop diameter on all the substrates. When printing continuous layers, the upper value that can be used for the drop spacing is 20 μm, because for higher values some discontinuities occur due to the capillarity of the ink. During printing, the substrates were kept at 60 °C in order to promote faster solvent evaporation. After printing, the devices were annealed at high temperature to promote the material sintering: 1 hour at 100 °C for OFETs on Kapton, 24 hours at 60 °C for OFETs on PET. Two main types of patterns, i.e. interdigitated and continuous patterns (Fig. 2), have been printed employing different


printing setup. The interdigitated source and drain electrodes are the core of the devices. This pattern consists of a series of 18 lines, 50 μm thick at a distance of 50 μm, as shown in Fig. 2 on the left hand side. We always used this configuration in order to increase the form factor W/L, W being the channel width and L being the channel length. We obtained W = 50 mm and L = 50 μm. On the one hand, for this type of pattern we always printed with one nozzle using a drop spacing of 20 μm, a firing voltage of 25 V or lower to avoid short circuits, a jetting frequency of 2 kHz or lower to avoid pattern discontinuities. On the other hand, printing continuous patterns, as for example the gate electrodes, is quite easier and does not require high precision (Fig. 2, on the right hand side). We printed these patterns with 2–3 nozzles, using a drop spacing of 15 μm to create continuous patterns, firing voltage of 30 V or higher and jetting frequency of 5–10 kHz to avoid discontinuities. G. PDMS Substrates Two types of polydimethylsiloxane (PDMS) were employed for the realization of the electronic skin, both provided as twopart liquid component kit comprised of an elastomer (A) and a curing agent (B). For the first very soft (shore 00) PDMS (Ecoflex 00–30, purchased from Smooth-on), components A and B were mixed together by hand in the ratio 1:1 by volume and, subsequently, in order to avoid a detriment of mechanical properties, air bubbles were extracted from the obtained compound by means of a vacuum pump. The compound was deposited over the Kapton substrate with the printed organic array in an ad hoc mold and was polymerized at room temperature for 4 hours. For the second, more rigid (shore 50) PDMS (Sylgard 184, purchased from Dow Corning), components A and B were mixed by ratio 10:1 by weight, and the compound was degassed in a vacuum pump. In this case, the PDMS was polymerized at 100 °C for 1 hour. H. Electrical Characterization Electrical characterization of the OFETs was carried out by means of a Keithley Sourcemeter 2600 in air, at room temperature. The hole mobility μp and the threshold voltage VT was derived from the analytical expression of the drain current ID in saturation regime: W · μ p · Cins · (VG S − VT ) . ID = (1) 2L III. E LECTROMECHANICAL C HARACTERIZATION OF OFET S In a first experiment, we have investigated the correlation between intrinsic morphological properties of the employed organic semiconductor films and the electrical response to mechanical deformation. In this case, devices have been fabricated on a 1.5 μm thick, free-standing poly(ethylenetherephtalate) (PET) foil (Mylar, DuPont), which acts at the same time as gate dielectric and as flexible mechanical support for the whole device, as shown in Fig. 3(a). Such a film is glued on a circular frame so that it can be very


Fig. 3. Schematic representation of the experimental setup employed for the electromechanical characterization (a) of single OFETs (b). AFM micrographs of Pentacene and P3HT films (c), and electrical response to mechanical deformation of the two different sets of samples (d).

easily patterned on both sides. To investigate the electrical response to mechanical deformation, the apparatus shown in Fig. 3(a) was employed. It consists of a pressurized chamber provided with an air inlet and a circular aperture of radius R = 1.1 cm, on the top side of which the free-standing, patterned film can be fixed (the film is glued only on the borders of the circular frame). When the air flows into (or out from) the chamber, it deforms the free-standing device, inducing an isotropic surface strain, as shown in Fig. 3(b). The applied pressure can be monitored by a manometer, and at the same time, taking into consideration the deformation of the substrate, the induced surface strain can be calculated using the methodology described by Hsu et al. [17]. Two different organic semiconductors have been employed in this case, Pentacene and P3HT, as they are the most common representatives of two different classes of organic semiconductors, i.e. small molecules and polymers respectively. As we have already recently demonstrated [9], the electrical response of organic semiconductors was found to be linear, reproducible and fully reversible for a range of deformation that goes from 0% to 2% of surface strain. Moreover, as recently reported in [8], the electrical response of small molecule-based systems (Pentacene) is much more pronounced than the one observed in polymer-based devices (P3HT). In this paper we demonstrate that this difference is related to the different morphological properties of the employed organic semiconductors. In fact, in polycrystalline systems like those used in this experiment, charge transport is dominated by hopping [18]. Therefore, a tensile deformation may induce a strain of the active layer, thus leading to a possible increase of spacing between molecules within the active layer as stated by Yang et al. [19], and also, as suggested by

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Fig. 4. Example of a OFET sensor transferred on a ribbon (a); the sensor is then sued on an elastic bend for elbow motion monitoring (b); electrical response of the sensor to different bending angles (c) and to a cycled measurement (d).

Sekitani et al. [6] and demonstrated by Scenev et al. [20], to an increase in distance between adjacent crystal domains. As a result, applying a tensile strain should lead to an increase in the hopping barrier, which in turns causes a decrease of the OFET mobility. As a consequence, the very different morphological features observed in Pentacene and P3HT systems (Fig. 3(c)), which strongly influence the transport properties of the transistors, are the main reasons for the reported differences in the observed sensitivity to strain. Interestingly, recently it was also found that by properly modulating the morphological features of the deposited active layer (average grain dimensions), it is possible to tune the sensitivity to mechanical deformation going from insensitive devices, that could be employed for flexible electronics applications, to highly sensitive devices that can be used for fabricating mechanical sensors [9]. IV. A PPLICATIONS Starting from the previous considerations, we have developed a highly flexible system for sensing mechanical deformation. In this case, devices have been fabricated on 175 μm thick substrates, using Parylene C as gate dielectric, following the procedures reported before in Section II. Thanks to the noticeable flexibility of the employed structure, these devices can be easily transferred onto garments or elastic bends for monitoring bio-mechanical parameters such as breath rhythm or joint motion. A possible application of the introduced system is reported in Fig. 4. In this case, as shown in Fig. 4(a), the OFET-based sensor has been transferred on a ribbon, and the final system has been sewed on an elastic bend. After that, the sensor was placed in the elbow and the electrical response of the device during the elbow motion was monitored. The sensor clearly detects the joint motion. The output current decreases when the elbow is bent, accordingly to the fact that bending the elbow induces a surface strain on the

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Fig. 5.


High-flexible Kapton substrates where devices are fabricated.

Fig. 7. Electrical stress induced degradation of the semiconductor performances over 380 minutes. OFETs are biased with square-wave gate voltage (VGS from +5V to –15V) and constant VDS (–5V).

Fig. 6.

Output (a) and transfer (b) characteristics of a printed OFET.

OFET active layer. The elbow has been bent at different bending angles, and as shown in Fig. 4(c), these different states can be clearly detected by employing our sensing system. Moreover, the response of the device is stable and reproducible and did not show any significant degradation even after several hundreds of applied bending cycles (Fig. 4(d)). Moreover, we have also developed and tested a system based on an array of OFET-based sensors for the realization of a robot skin, i.e. the reproduction of the sense of touch in robotic applications. The realization of this system has been carried out by means of the inkjet printing technique with a silver-based ink, as described in the Section II. Starting from a single OFET, with W/L = 1000 and Cins = 1.86 nF/cm2 , we fabricated an array of 8 OFETs, with a common gate electrode, a common source electrode and 8 independent drain electrodes, so that each device can be addressed independently, and with a lateral pitch of 5 mm. The most critical step of the fabrication process is the inkjet printing of the interdigitated source and drain electrodes, which represent the core of each device: the fine tuning of the printing parameters, calibrated in order to avoid short circuit and discontinuity problems, allowed us to obtain a yield, i.e. number of working devices, higher than 90%, and a very good reproducibility of the electrical performances between different devices. Organic semiconductor, TIPS-Pentacene, was deposited by means of drop-casting. We used different types of substrates with different thickness (as small as 13 μm), leading to the possibility of high flexible and compliant systems (Fig. 5). The output and the transfer characteristics of an inkjet printed OFET are shown in Fig. 6, where a large field effect and a very small hysteresis can be noticed. We obtained a good mobility (0.07 ± 0.03 cm2 /(V· s), up to 0.1 cm2 /(V· s)), slightly negative threshold voltages (−1.5 ± 1.2 V) and high ION /IOFF (≈105). The employment of the OFETs as strain sensors required preliminary dynamic tests in order to investigate the real-time response and the electrical performance over long time.

It is well known [21] that applying a DC voltage for long time causes the degradation of the electrical performances of the semiconductor, i.e. a reduction of the output current. For this reason, we applied square-wave gate voltage alternatively positive and negative, in order to reduce the bias stress effects. We found that the best tradeoff between signal saturation and low bias stress is obtained at 100 Hz frequency and 50% duty cycle. Fig. 7 shows the relative variation of the output drain current with respect to the initial on-current, measured during a dynamic test with a square-wave gate voltage (VGS from +5 V to –15 V) and constant VDS (–5 V). The effects of the bias stress are not very relevant: after an initial decrease, the level of the maximum drain current does not change significantly over more than 6 hours of biasing. The read-out circuitry, developed ad hoc in our laboratory, consists of 8 input channels, and analog block (I/V converter and amplifier) and a digital block (microcontroller and analog to digital converter, with 500 Hz sampling frequency). The real-time Graphic User Interface evaluates the average value of 5 acquisitions for each device and shows the relative current variation ID of the “pressed” state with respect to the initial state. The result is represented in Fig. 8, where the response of the flexible robot skin is shown. When a certain pressure is applied on the device structure, a variation of the output current can be detected. In this case, an arbitrary pressure has been exerted with a finger on a single element of the array. It is noteworthy that, when the indenter is moved across the array surface each device responds to the applied mechanical deformation independently, and a very small crosstalking (both electrical and mechanical) can be detected. In order to gain more detailed information about the sensitivity and the reliability of the robot skin, calibration and stress tests have been performed by means of a mechanical indenter, shown on Fig. 9. This home-made indenter consists of a metal scaffold and a mechanical finger, controlled in vertical position, which can move up and down with high spatial resolution. A linear DC-servomotor (LM-1247040-02, purchased from Faulhaber) is employed to calibrate the applied force. On the bottom of the mechanical finger,


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Fig. 9. Mechanical indenter employed for the sensitivity and reliability tests.

Fig. 8. Electrical response of the flexible system fabricated for reproducing the sense of touch.

a load cell (Microswitch Force Sensor FS Series, purchased from Farnell) measures the force exerted on the device. The cell can detect forces up to 4.2 N. The finger ends with a hemispherical indenter of 8 mm diameter, that is a plausible value of the diameter of a human finger. The hemispherical indenter was realized by rapid prototyping a general-purpose resin (FullCure 720, purchased from Artcorp) using a Objet Eden 260. In summary, the input parameters of the system are the vertical position of the finger with respect to the “zero” point (z) and the force applied (F), measured by the load cell. The output is the relative drain current variation of the transistors (I/I), that is proportional to the strain induced to the substrate. Moreover, in these experiments the plastic substrates have been covered with Ecoflex PDMS, and in one case also with Sylgard PDMS, in order to recreate the consistency of the human skin and to protect the sensors from fatal damages. Four different configurations for the PDMS-active array complex have been tested, in order to investigate which could be the best layout that maximizes the response of the devices. For each configuration, shown in Fig. 10 and later labelled from (a) to (d), stimuli of increasing pressures were applied to the PDMS-Kapton system, covering the range 0–3 N or 0–4 N in (c). Each cycle of increasing pressure was therefore replied twice (four times in the last configuration) in order to analyze the reproducibility of the sensor.

In the first configuration (a), Kapton was simply placed on a 5 mm thick Ecoflex layer and the deformation was directly applied to the back side of the Kapton film. We applied 2 cycles and 10 steps per cycle. This led to a high mortality of the devices for high pressures. Sensitivity was very good, but repeatability was poor for forces above 1.25 N. Results are shown in Fig. 10(a). In the second configuration (b), the Kapton substrate was placed above a Sylgard PDMS layer realized ad hoc as follows. Sylgard was patterned on top of a rigid substrate where a square relief was realized: in this way, after peeling off the Sylgard stamp and realizing the conformal contact with the Kapton layer, each device channel corresponds to the empty space between two PDMS micro-pillars and is therefore free to deform during pressure application. After this, the whole structure was covered with another 5 mm thick Ecoflex layer. We applied 2 cycles and 10 steps per cycle. In this case, a lower sensitivity can be noticed with respect to the previous configuration, but the protective upper Ecoflex substrate covered the devices and avoided mortality. The reproducibility was very good over the two cycles. Results are shown in Fig. 10(b). In the third configuration (c), Kapton was totally embedded on Ecoflex: the bottom layer was 2 mm thick and the top layer was 0.5 mm thick. We applied 2 cycles of increasing force and 15 steps per cycle. In Fig. 10(c) the results are shown. In this case we obtained a very high sensitivity, up to 15 %, but reproducibility was not good for pressures above 1 N. This was probably due to the small thickness of the upper layer of Ecoflex. In the last configuration (d), Kapton was totally embedded in Ecoflex, as in configuration (c), but with a thicker upper layer (2 mm). This turned out to be the best configuration. We applied 4 cycles, 10 steps per cycle. Repeatability was very good over the four cycles, as shown in Fig. 10(d). Since configuration (d) was the best one, leading to very high sensitivity and good resolution, i.e. a minimum detectable

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Fig. 11. Sensitivity of the OFET sensors. a): 10 cycles of increasing pressures applied on one sensor. b): average of 10 cycles of pressure each one applied on 10 different devices.

Fig. 12. Stress tests. In graph (a) the percentage variation of the output current versus the progressive number of the stimuli applied by the mechanical indenter is shown; in graph (b) the relative variation of the output current versus time is shown.

Fig. 10. Configurations of Kapton, Ecoflex PDMS and Sylgard PDMS and calibration curves. In (a) Kapton is simply placed on a 5 mm thick Ecoflex layer, and posed in direct contact with the indenter. In (b) Kapton is placed above a rigid windowed Sylgard layer (one window under each device) and covered with a 5 mm thick Ecoflex layer. In (c) Kapton is totally embedded on Ecoflex: the bottom layer is 2 mm thick and the top layer is 0.5 mm thick. In (d) Kapton is totally embedded in Ecoflex with both top and bottom layer 2 mm thick.

force of 0.1 N, it was used for the calibration of the robot skin. Fig. 11(a) shows the statistics on 10 consecutive cycles of pressure applied on one single OFET. The narrow error bands highlight a very high reproducibility of the device response. In Fig. 11(b) the overall statistics, referred to 10 cycles of pressure each one applied on 10 different devices, thus a total of 100 pressure events for each force, is shown. Although each device has its peculiar sensitivity and response, a general trend can be noticed, with a good linearity and an average maximum sensitivity up to 6% for 3 N force applied. Finally, mechanical stress degradation tests have been performed, by means of the mechanical indenter and the human finger, in order to investigate the durability and reliability of the printed OFETs. In Fig. 12(a) the results of the tests performed by means of the mechanical indenter are shown. In this experiment we exerted consecutive mechanical stimuli over time, each one having a square-wave profile: pressure released for 2.5 s and then applied for 2.5 s at a constant force of 3.7 N, as measured by the load cell of the

indenter. As output signal, the percentage variation of the device response, both in the pressed (red) and released (black) states, is evaluated with respect to the initial released state. Up to 2000 consecutive stimuli have been exerted: no degradation is noticed before 1000 stimuli, and although a slight current increase, as important result the difference between the pressed and the released state remained constant during the whole test, showing therefore a good response before irreversible degradation. In Fig. 12(b) the same experiment has been carried out by applying the force with the finger in dark conditions, thus avoiding the current increase. The variation of the output current with respect to the initial unstressed state (highlighted by the red line) is represented. Over 1000 pressure events for 600 s were exerted and the response remained constant over the whole experiment, showing a good reliability of this system as electronic skin over the time and under mechanical stress conditions. V. C ONCLUSION In this article we have demonstrated that the electrical response of OFETs to mechanical deformation is dramatically influenced by the morphological properties of the organic semiconductor film. In particular the device response can be enhanced or almost neglected just by choosing semiconductor films with the proper morphological properties. Moreover, we have also demonstrated that OFETs can be successfully employed for the fabrication of mechanical sensors, giving rise to a pronounced, reproducible and linear (within a certain range) response to the applied mechanical stimulus. The fabricated structure can be employed for the fabrication of smart wearable systems for detecting joint motion, or as tactile transducers. These results represent a step forward for the fabrication, at low costs and over large areas, of flexible and compliant systems for wearable electronics and robot skin applications.


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Alberto Loi received the M.Sc. degree in electronic engineering from the University of Bologna, Bologna, Italy, in 2010, discussing the thesis "Analysis of a 30 V multi-finger LDMOS power device." In 2011, he joined the Department of Electrical and Electronic Engineering, University of Cagliari, Cagliari, Italy, where he is currently pursuing the Ph.D. degree. His current research interests include inkjet printing of organic materials, fabrication and characterization of organic field effect transistors, and fabrication and characterization of mechanical sensors and sensing systems based on organic devices.

Laura Basiricò received the M.S. degree in physics from the University of Bologna, Bologna, Italy, in 2008. In 2009, she joined the Department of Electrical and Electronic Engineering, University of Cagliari, Cagliari, Italy, as the Ph.D. Student. She received the Ph.D. degree in 2012. Her research activity focused on the development of inkjet printing technique for the realization of organic field effect transistors and organic electrochemical transistors for the sensing application. She is currently a Researcher with ISMN-CNR, Bologna. Her current research interests include the field of organic electronic devices on the fabrication and physical characterization of organic field effect transistors for light emission and sensing applications.

Piero Cosseddu received the M.Sc. degree in electronic engineering and the Ph.D. degree in electronic and computer science engineering from the University of Cagliari, Cagliari, Italy, in 2003 and 2007, respectively. Since May 2007, he has been a PostDoctoral Fellow with the Department of Electrical and Electronic Engineering, University of Cagliari. His current research interests include the design, realization, and characterization of organic semiconductor based devices for innovative applications, as artificial electronic skin, biomedical sensing, and wearable electronics.

Stefano Lai was born in Cagliari, Italy, in 1984. He received the master’s degree (magna cum laude) in electronic engineering from the University of Cagliari, Cagliari, in 2010. He joined the Department of Electrical and Electronic Engineering, University of Cagliari in 2011 as a Ph.D. Student. His current research interests include chemical and biological sensors in CMOS and organic technology and highperformances organic devices for the realization of sensors and actuators.

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Massimo Barbaro received the M.Sc. and Ph.D. degrees in electronic engineering from the University of Cagliari, Cagliari, Italy, in 1997 and 2001, respectively. He is an Assistant Professor of analog microelectronics with the University of Cagliari. His current research interests include the design and realization of CMOS imagers with computational capabilities, CMOS and organic biosensors, and implantable neural interfaces. He has published more than 30 papers and holds two international patents.

Annalisa Bonfiglio received the Laurea degree in physics from the University of Genoa, Genoa, Italy, in 1991, and the Ph.D. degree in bioengineering from Politecnico di Milano, Milano, Italy, in 1995. She is currently an Associate Professor of electronics and electronic bioengineering with the University of Cagliari, Cagliari, Italy. She is a member of the Institute of Nanoscience-National Research Council. She is the author of more than 130 papers on international journals, conference proceedings, book chapters, and five patents. Her current research interests include innovative materials and electronic devices for wearable electronics and bioengineering and several international and national research projects.

Perla Maiolino is a Post-Doctoral Fellow with the Mechatronic and Control Laboratory, Department of Communication, Computer and System Sciences, University of Genoa, Genoa, Italy. She received the M.S. degree in robotics and automation and the Ph.D. degree in robotics from the University of Genoa in 2006 and 2010. Her current research interests include materials and in the design of technological solutions related to the development of distributed tactile sensors for robots.

Emanuele Baglini received the M.Sc. degree in computer engineering from the University of Genova, Genova, Italy, in 2010. He is a Researcher with the Department of Informatics, Bioengineering, Robotics and Systems Engineering, University of Genova, where he works on tactile sensors for humanoid robots and tactile systems. His current research interests include tactile sensors, realtime networks and software architectures, embedded hardware, and distributed systems.

Simone Denei received the M.Sc. degree in computer engineering and the Ph.D. degree in robotics from the University of Genova, Genova, Italy, in 2009 and 2013. He is a Post-Doctoral Researcher with the Department of Informatics, Bioengineering, Robotics and Systems Engineering, University of Genova, where he works on tactile sensors for humanoid robots and tactile systems representations. His current research interests include humanoid robots, tactile sensor, automatic control systems, real-time software architectures, robotics middleware, robotics, robot control theory, and embedded device development.

Fulvio Mastrogiovanni received the Computer Science Engineering degree (Hons.) and the Ph.D. degree in robotics from University of Genoa, Genoa, Italy, in 2003 and 2008, respectively. He has been a Visiting Professor with the Asian Institute of Technology, Thailand, in 2010, Jiao Tong University, China, in 2012, and Karlsruhe Institute of Technology, Germany, in 2013. Currently, he is an Assistant Professor with the University of Genoa. His current research interests include perception and cognitive representation processes, reasoning, sensory-motor strategies, human behaviour understanding, and human-robot interaction. He served as an Automation Information Co-Chair for the IEEE CASE in 2012, an EU Program Chair for IEEE RO-MAN in 2013 and a Program Chair for URAI in 2013. He received the Best Paper Award at DARS in 2008 and IEEE RO-MAN in 2010, and the IFSA Award in 2013. He is the co-author of more than 70 peer-reviewed publications in international journals or conferences and he is co-editor of one book.

Giorgio Cannata received the Laurea degree in electronic engineering from the University of Genova, Genova, Italy, in 1988. From 1989 to 1995, he has been a Research Scientist with the Naval Automation Institute, Italian National Research Council, working in the area of underwater robotics. From 1995 to 1998, he has been an Assistant Professor with the Department of Communication, Computer and System Sciences, University of Genova. He is currently an Associate Professor of automatic and digital control with the Faculty of Engineering, University of Genova. His current research interests include humanoid robots, automatic control systems and control architectures for robotic and mechatronic systems, robotics and robot control theory, control of mechanical systems, and dynamic simulation.