Highly Sensitive Flexible Pressure Sensors Based on Printed Organic

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Highly Sensitive Flexible Pressure Sensors Based on Printed Organic Transistors with Centro-Apically SelfOrganized Organic Semiconductor Microstructures So Young Yeo, Sangsik Park, Yeonjin Yi, Do Hwan Kim, and Jung Ah Lim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15960 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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ACS Applied Materials & Interfaces

Highly Sensitive Flexible Pressure Sensors Based on Printed Organic Transistors with CentroApically Self-Organized Organic Semiconductor Microstructures So Young Yeo, Sangsik Park, Yeon Jin Yi, Do Hwan Kim,*, Jung Ah Lim* ––––––––– S. Y. Yeo, Dr. J. A. Lim Center for Opto-Electronic Materials and Devices, Post-Silicon Semiconductor Institute, Korea Institute of Science and Technology, Seoul 02792, Korea S. Y. Yeo, Prof. Y. J. Yi Department of Physics, Yonsei University, Seoul 03722, Korea Prof. D. H. Kim Department of Chemical Engineering, Hanyang University, Seoul 04763, Korea E-mail: [email protected] S. Park Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul 06978, Korea Dr. J. A. Lim Division of Nano and Information Technology, KIST School, Korea University of Science and Technology (KUST), Daejeon, 34113, Korea E-mail: [email protected]

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––––––––– Abstract A highly sensitive pressure sensor based on a printed organic transistors with threedimensionally self-organized organic semiconductor microstructures were demonstrated. A unique organic transistor with semiconductor channels positioned at the highest summit of printed cylindrical microstructures was achieved simply by printing an organic semiconductor and polymer blend on the plastic substrate without the use of additional etching or replication processes. A combination of the printed organic semiconductor microstructure and an elastomeric top-gate dielectric resulted in a highly sensitive OFET pressure sensor with a high pressure sensitivity of 1.07 kPa–1 and a rapid response time of < 20 ms with a high reliability over > 1000 cycles. The flexibility and high performance of the 3D OSC FET pressure sensor were exploited in the successful application of our sensors to real-time monitoring of the radial artery pulse, which is useful for healthcare monitoring, and to touch sensing in the e-skin of a realistic prosthetic hand.

Keywords: flexible pressure sensor, organic field-effect transistors, printing, selforganization of organic semiconductor, electronic skin


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1. Introduction Highly sensitive flexible pressure sensors are at the forefront of the development of future mobile applications, such as rollable touch displays1, health monitoring2, and electronic skin3. The last few years have seen a dramatic increase in the number of academic research efforts geared toward designing and fabricating flexible pressure sensors based on piezoresistive, piezocapacitive, and pieozoelectric operating principles. Comprehensive reviews of the state of the art in flexible pressure sensors are available2, 4. Recently, monolithic organic fieldeffect transistors (OFETs) combined directly with pressure-sensitive components have been demonstrated as actively workable pressure sensors

5-11

. This is because OFETs-based

flexible pressure sensor platform shows tremendous advantages such as cost effectiveness, good flexibility, and large-area solution processing, which effectively enables conformal large-area contact with a surface3. Moreover, monolithic OFET pressure sensors are amenable to a low impedance-output active matrix design, which results in reducing power consumption because the current flows only when the OFET is switched on 8. A novel approach to incorporating OFETs with a pressure-sensing element has involved utilization of a microstructured elastomer as a pressure-sensitive gate dielectric. In a first attempt, Mannsfeld et al. reported a monolithic pressure sensitive organic transistor prepared using microstructured polydimethylsiloxane (PDMS) as a dielectric layer8. In their work, microstructuring and thinning of the PDMS dielectric layer improved the sensitivity (0.55 kPa–1) and shortened the response time on the milliseconds range by minimizing viscoelastic creep of the PDMS. Motivated by this work, Schwartz et al. successfully demonstrated an extremely pressure-sensitive OFET with a maximum sensitivity of 8.4 kPa–1 using a microstructured PDMS dielectric and a high-mobility organic semiconductor12. In an analogous approach, Kim et al. recently reported the bimodal sensing of pressure and temperature using an OFET, achieved by adapting a microstructured ferroelectric gate 3 ACS Paragon Plus Environment


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dielectric9. Also, Zhang et al. demonstrated ultrasensitive OFET pressure sensors with a sensitivity of 192 kPa–1 and a low limit-of-detection pressure of < 0.5 Pa using a flexible suspended gate dielectrics in a top-gated OFET 7. Despite these pioneering efforts, the low cost and easy fabrication of OFET devices cannot be fully exploited without the development of OFET-based pressure sensor printing processes. Fabrication methods that utilize printing processes are optimal for bringing pressure sensor matrix fabrication into the production scale. Few printed OFET-based pressure sensors, however, have been reported to date. Kim et al. demonstrated the fabrication of inkjet-printed OFETs with a nanoscale needle-structured gate dielectric as a pressure sensor with a high sensitivity at low pressures ( 1000 cycles. We demonstrated that our sensors are applicable to the real-time monitoring of radial artery pulse waves and as touch sensors for use in realistic prosthetic hands. It should be noted that proposed 3D OSC OFET pressure sensors, compared to the previously reported OFET-based pressure sensors with a microstructured dielectric top layer, have substantial advantages of (i) facile fabrication without introducing etching or replication processes and (ii) no need of precise alignment of the micro-pillar structure of dielectric in the OFET device. Regarding the latter point, when the OFET pressure sensor is prepared by assembling the microstructured dielectric and organic semiconductor channel, at least one or more micro-pillar (with several micrometer size) in the dielectric film should be positioned exactly on the semiconductor channel region (typical channel length is from several to tens of micrometer) of the FET in order to obtain high pressure-sensibility. This can be ultimately problematic when the sensor is applied to the high-resolution integrated array because downscaling of the channel length in the FET device is limited by the size of dielectric micropillar. Furthermore, bending or mechanical deformation of the sensor can cause misalignment



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of the micro-pillar structures from the semiconductor channel, restricting potential application of the sensors to the substance with curved surface.

2. Experimental Section 2.1. Materials 2,8-Difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene

(diF-TESADT)

was

purchased from Lumtec in Taiwan. Poly(methyl methacrylate) (PMMA, Mw 996 kgmol–1), 1,2,4-trichlorobenzene (1,2,4-TCB), and indium tin oxide-coated PET (ITO-PET, 60 Ωsq–1) were purchased from Sigma-Aldrich. The PDMS elastomer and crosslinking agent (Sylgard 184) was purchased from Dow Corning. 2.2. Printing of the 3D OSC Prior to printing the polymer solution, the PET substrate was rinsed with ethanol and subjected to 20 min UV-ozone treatment to improve the hydrophilicity. PMMA dissolved in 1,2,4-TCB in a 10 wt% concentration was printed onto the PET substrate using a picoliter fluidic dispenser (Sonoplot Inc.) with a 50 µm orifice glass tip at a 1000 µm/sec printing speed. The height of the printed cylindrical microstructure was controlled by repeatedly printing the PMMA structures 3 or 5 times. A distinct apical diF-TESADT semiconductor microstructure, referred to as a 3D OSC, was formed by printing a 10 wt% diFTESADT:PMMA blend (1:4 w/w) solution in 1,2,4-TCB solvent onto the pre-printed PMMA baselines at a 1000 µm/sec printing speed. 2.3. Preparation of the OFET pressure sensors based on the 3D OSC


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An OFET pressure sensor based on the 3D OSC with a top gate configuration was fabricated by assembling two separate components via lamination: a top gate component with an elastomeric dielectric layer was laminated to the bottom component of the 3D OSC, bearing source and drain electrodes (Figure 1c). The bottom component was prepared by depositing the source and drain electrodes onto the printed 3D OSC via thermal evaporation of Au using a shadow mask. Thickness of Au source and drain was 100 nm. The elastomeric dielectric in the top gate component was formed by mixing the PDMS prepolymer and crosslinking agent in a 5:2 w/w ratio, and the mixture was diluted in hexane in a 1:1 weight ratio. The PDMS solution was spin-cast at 5000 rpm and spun for 120 s on the ITO-deposited PET substrate, and the film was cured in the vacuum oven at 120°C for 4 hours. The ITO electrode acted as a transparent gate electrode. The PDMS dielectric was 2.5 µm thick. In order to make sure the interconnect of ITO gate electrode, 50 nm thin Al film was thermally evaporated on both the edge and top surfaces of the ITO PET substrate. Then, this Al film was connected with a copper cable using a silver conductive adhesive.

2.4. Characterization The height profiles of the printed patterns were measured using a surface profiler (AlphaStep AS-IQ). diF-TESADT crystal formation on the PMMA base structure was observed via optical microscopy (Nikon ECLOPSE LV100). The pressure dependent electrical signal change was observed by a custom-built sensor probe station with a motorized xy- and z-axis stage to apply an exact pressure, and a force gauge (Mark-10) measured the load. The changes in the drain current and the capacitance were measured using Keithley 4200-SCS and Agilent E4980A units, respectively. The artery pulse was measured using a HewlettPackard 4155B unit operated in the dynamic current mode. Field-effect mobility of the transistor (µ) in the saturation regime was calculated from the transfer curve using following 7 ACS Paragon Plus Environment


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equation; ID =

WC

G

2L

µ(V g −V th )2 , where CG is the capacitance of the PDMS gate dielectric,

and W and L are the channel width and length, respectively. The value of CG was directly measured under the various pressure. W was estimated by the summation of the widths of diF-TESADT crystalline stripes in the channel region and L was 100 µm.

3. Result and discussion Figure 1a schematically illustrates the preparation of the 3D OSC via centro-apical selforganization of the organic semiconductor in a line-printed organic semiconductor:polymer blend. 2,8-Difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TESADT), a small molecule semiconductor with a high carrier mobility blended with an insulating polymer, poly(methyl methacrylate) (PMMA), were used to obtain the 3D OSC structure. A 10 wt% diF-TESADT:PMMA blend (1:4 w/w) dissolved in 1,2,4-trichlorobenzene solvent was printed on a flexible PET substrate using a picoliter fluidic dispenser. Unlike typical inkjet printing methods, which cannot dispense highly viscous solutions, the picoliter fluidic dispenser enables deposition of micropatterns with high aspect ratios from the one-step printing of a highly concentrated solution (viscosity ~ 1000 cP). In a previous study, we found that the combination of energetically favorable vertical phase separation and the presence of hydrodynamic fluids inside the drying droplet produced a centro-apical selforganized organic semiconductor layer with a line-printed microstructure (Figure 1b)13. Unlike previous approaches, the height of the 3D OSC could be controlled by printing the diF-TESADT:PMMA blend solution onto pre-printed PMMA baselines, the heights of which could be varied by reprinting the PMMA solution 1, 3, and 5 times. The PMMA bottom baseline (BBL) structures with different numbers of overlayers were referred to as B1, B3,


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and B5. Figure 2a shows the height profiles of the printed 3D OSC patterns. Depending on the number of PMMA overlayers, the 3D OSC height could be varied from 0.8 to 2.8 µm without changing the printed line width. Optical microscopy images shown in Figure 2c clearly reveal 25 µm wide diF-TESADT crystalline stripes formed at the center region of 100 µm wide B5 PMMA printed baselines. The width of the diF-TESADT crystalline stripe remained constant, regardless of the thickness of the BBL structure. As shown in Figure 2b, the printed 3D OSC formed a bell-shaped height profile, the center region of which consisted of diF-TESADT crystals that rose upward. After selective dissolution of diF-TESADT by dipping in cyclohexane, the hemispherical height profile of the printed microstructure was revealed. This observation confirmed that the thickness of the 3D positioned diF-TESADT was approximately 140 nm. These results clearly revealed that the centro-apically selforganized diF-TESADT layers were successfully formed on the BBL structure without additional dissolution steps. The centro-apical positioning of diF-TESADT on the PMMA underline structure required the use of the diF-TESADT:PMMA blend solution. Figure 2d exhibits optical microscopy images of a layer prepared by printing a diF-TESADT solution alone on the PMMA baseline structure. Aggregation of diF-TESADT and the formation of crystalline stripes that were misaligned with the central position of the pre-printed PMMA baseline were observed. These features suggested that the centro-apical self-organization of diF-TESADT was driven by the combinatorial effect of the vertical phase separation between diF-TESTADT and PMMA and the inward flow within the cylindrically printed line pattern during the solvent drying process 13. An OFET pressure sensor based on a 3D OSC with a top gate configuration was fabricated by assembling two separate components via lamination. The top gate component featured an elastomeric dielectric and the bottom component of the 3D OSC featured source and drain electrodes, as shown in Figure 1c. A 2.0 µm thick polymethyldisiloxane (PDMS) layer was 9 ACS Paragon Plus Environment


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deposited to form a pressure-deformable gate dielectric on a transparent indium tin oxide (ITO) gate electrode. The PDMS dielectric layer had a capacitance of 700 pF/cm2 at 1 kHz and was highly durable between –100 and 100 V without electric field breakdown. The complete 3D OSC FET pressure sensor was flexible and transparent (Figure 3a). The design rationale for our 3D OSC FET pressure sensor is illustrated in Figure 1d. Laminating the top gate component induced direct contact between the 3D OSC and the PDMS gate dielectric to form an intimate interface for charge carrier injection and transport. The elastic PDMS dielectric directly deformed as the pressure was applied, leading to a decrease in the dielectric layer thickness and increasing the areal capacitance of the gate dielectric (C G ) based on the following relationship: C G =

ε rε 0 , where ε r is the relative dielectric constant of the material, d

ε 0 is the permittivity of free space, and d is the distance between the two electrodes. The drain ( IDS ) current of the FETs depended linearly on the dielectric capacitance in the saturation regime according to the following equation, IDS =

1 W µsatC G (V −VTH )2 , where 2 L G

µsat is the field-effect mobility, W and L are the channel width and length, respectively, and VTH is the threshold voltage. This relationship describes how the capacitance change in the gate dielectric, driven by a pressure load, was reflected in the drain current of the device. The sensing capabilities of the 3D OSC FET pressure sensor were investigated, as shown in Figure 3. The pressure was applied to the entire device area (1ⅹ1 mm2), inclusive of the 3D OSC channel and source and drain electrode regions. Figure 3b plots the transfer characteristics of the 3D OSC FET with a B5 BBL structure at a constant source–drain bias in terms of applied pressures. Without a pressure load, small on–off switching modulation was observed. As the pressure load increased, on- and off-current modulation was conspicuously


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improved. These results were attributed to the formation of a more intimate contact interface between the 3D OSC and the gate dielectric, and the enhanced influence of the electric field controlled by the gate bias under the applied pressure. In fact, the on/off current ratio approached 105 at a pressure load of 5.5 kPa, indicating the successful demonstration of a 3D OSC FET with a current modulation that was controlled by the gate bias. For the 3D OSC FET under pressure of 8.0 kPa, the field-effect mobility in the saturation regime calculated from the transfer curve was approximately 0.32 cm2/Vs, which closely corresponds with the previously reported values of the device based on diF-TESADT:PMMA blends13. A linear scale plot of the transfer curve clearly reveals that the on-current of the 3D OSC FET varied directly with the applied pressure load. Figure 3c shows the pressure response of the drain current at a constant source–drain (–80 V) and source–gate (–80 V) voltage for the 3D OSC FET pressure sensors with different BBL structures. The pressure sensitivity S, defined as S= (∆ID/I0)/∆P, where P denotes the applied pressure, and ID and I0 denote the drain current with and without an applied pressure, respectively, was estimated based on the slope of the trace shown in Figure 3c. At pressures below 1 kPa, the pressure sensitivity of the 3D OSC FET varied according to the height of the BBL structure, indicating that the sensitivity of the device was controllable. The 3D OSC FET pressure sensors prepared with the B5 BBL structure showed a sensitivity of 1.07 kPa–1, a 10-fold improvement in the pressure sensitivity compared to the unstructured planar device fabricated by spin-coating the diF-TESADT:PMMA blend (0.09 kPa–1). The sensitivity of the 3D OSC FET was comparable to the recently reported pressure-responsive OFET devices prepared with microstructured gate dielectrics

8-9

. To the best of our knowledge, this report

indicates the highest sensitivity yet achieved among OFET pressure sensors fabricated using direct additive-printing technologies without introducing etching or replication processes.



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The pressure response curve shown in Figure 3c displayed two distinguishable sensitivity regimes, with a high sensitivity at low pressures < 1 kPa and a saturation in sensitivity at high pressures above 5 kPa. The distinct dependences of the sensitivities on the pressure range were related to the anisotropic hemispherical structure of the 3D OSC pattern, which has been observed in sensors consisting of pressure-responsive pyramid microstructures

8-9, 12, 14-

15

. At low pressure loads, a decrease in the PDMS dielectric thickness and an increase in the

contact area between the PDMS and 3D OSC dramatically enhance the on-current of the device. At high pressure load, additional pressure loads can only induce small decreases in the PDMS dielectric layer thickness, and its deformation tends to saturate. These results suggest that the pressure responsivity of the 3D OSC FET relies primarily on the printed 3D OSC microstructure. The origin of the pressure responsivity of the 3D OSC FET was explored by measuring the areal capacitance change (∆CG) in the PDMS dielectric in a 3D OSC FET under various applied pressures. Based on the relation between the drain current and the gate dielectric capacitance, if the drain current variations (∆ID) with the applied pressure originate from the areal capacitance change, the two ∆ID and ∆CG curves as a function of the applied pressure will agree 8. Figure 3d shows that ∆ID and ∆CG did, in fact, agree well, indicating that the capacitance change in the gate dielectric originated from the pressure applied to the 3D OSC. For the 3D OSC FET pressure sensor design to be effective in practical applications, it should be highly sensitive and also provide a rapid responsivity, good durability, and high reliability. Our pressure sensors showed rapid response and relaxation times (18 ms) (Figure 4a). Figure 4b shows the current change in the 3D OSC FET sensor under a series of pressures: i) 0.4, ii) 0.8, iii) 1.6, iv) 2.4 kPa applied repeatedly to the sensor and then released. The current change nearly retained its original levels without hysteresis under the repeated pressure loading and relaxation cycles, indicating good reliability of the 3D OSC FET. The 12 ACS Paragon Plus Environment

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reliability of the sensor was further illustrated by repeatedly loading and unloading a pressure of 1.0 kPa >1000 times at a forcing frequency of 0.5 Hz (Figure 4c). The drain current difference derived from the applied pressure remained constant (15 nA) during the cycling test (about 20 min), indicating a good long-term stability of the 3D OSC FET sensor. Regarding the durability of the 3D OSC crystalline structures, we confirmed that diFTESADT crystalline stripes were not damaged even after such high pressure of 80 kPa was applied to the 3D OSC pressure sensors (see SI Figure S1(a)). In addition, the bending stability of 3D OSC FET was tested. When the device was bended with a bending radius of 0.5 cm in a vertical direction to the printed 3D OSC line pattern, the transfer characteristic maintained almost identical to that of the flat device (see SI Figure S1(b)).. This means that the 3D OSC structures have certain tolerance towards the bending and normal pressure. The sensitivity and response time of the 3D OSC FET pressure sensor are useful for the precise detection of the wrist artery pulse. The flexibility of the 3D OSC FET pressure sensor enabled facile sensor attachment to a rubber band (a PDMS band in this work) to enable wearing around the wrist, as shown in Figure 5a. To achieve artery pulse recording, source, drain, and gate electrodes were connected with a copper cable using a silver conductive adhesive. The current responses to a real-time pulse wave and an enlarged pulse wave are shown in Figures 5b and 5c. A typical pulse pressure shape was obtained, with three distinguishable peaks16. These peaks are known to be caused by the superposition of a pressure wave ejected from the left ventricle and a set of waves reflected from the hand and the lower body. The first two peaks provide clinically useful parameters, such as the radial artery augmentation index (AIr), defined as the ratio of two peaks (ΔI2/ΔI1), and the time delay (ΔTDVP) between the two peaks. AIr and ΔTDVP , measured from the test person, were



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estimated to be 0.56 and 140 ms, respectively, which corresponded to the values characteristic of a healthy male in his 30’s. The 3D OSC FET pressure sensor was demonstrated in electronic skin by deploying and testing the touch sensing capabilities of the sensors in a prosthetic hand. Figure 5d shows that the 3D OSC FET pressure sensor was attached to the index finger of a prosthetic hand. The prosthetic hand was fabricated simply using a 3D printer. The motions of the prosthetic hand were controlled using electronic motors. The changes in the drain current were observed as the prosthetic hand held a tumbler, as shown in Figure 5e. A sudden current decrease during the grip motion might indicate slippage and additional bending motions of the index finger immediately after contact with the subject surface (Supporting Information. Movie S1). After releasing the finger touch, the drain current recovered its initial value, and these touch sensing characteristics were observed of repeated touch motion cycles. We also tested the capacity to sense the touch of a tiny (microliter volume) droplet corresponding to the approximate volume of a raindrop (which are typically 0.5–33 µL, 0.5 – 4 mm in diameter)17. Due to the top gate configuration of the 3D OSC FET sensor, in which the top laminated component acted as a passivation layer, we expected the sensor to detect the pressure of the water droplet, in analogy to raindrop detection by the human skin. As shown in Figure 5f, the water droplet was loaded onto a sensor attached to the palm of the prosthetic hand. Dropping an approximately 20 microliter (21.30 mg) droplet onto the sensor produced a distinct increase in the drain current (Figure 5e). A live video of droplet sensing using the 3D OSC FET is provided in the Supporting Information (Movie S2). The applied pressure was estimated to be 16.61 Pa based on the weight and contact area of the droplet on the sensor. Because the water droplet contacted the top gate component on the PET substrate surface, the drain current signal was attributed to the pressure load of the water droplet, not the change in the effective electric field or the introduction of organic semiconductor doping by the water 14 ACS Paragon Plus Environment

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molecules. These results indicated that the 3D OSC FET pressure sensor was highly sensitive to the detection of low pressures below 20 Pa. These results suggested that the 3D OSC FET pressure sensor may be useful in wearable devices for healthcare monitoring and electronic skin applications.

4. Conclusions In summary, we demonstrated a pressure-sensitive OFET fabricated using the facile printing

of

cento-apically

positioned

organic

semiconductors

onto

half-cylinder

microstructures. Printing an organic semiconductor and polymer blend, diF-TESADT:PMMA, onto pre-printed PMMA baseline structures produced centro-apically crystallized diFTESADT stripes on the highest summit of the PMMA baseline structure, producing a 3D OSC. Lamination of the 3D OSC and the top gate component using an elastomeric gate dielectric film successfully yielded a unique pressure-sensitive OFET. The 3D OSC FET device showed typical current modulation behavior via the gate bias and good on/off switching on the order of 105 under a pressure load. The pressure sensitivity was controlled by varying the height of the bottom PMMA baseline. The 3D OSC FET pressure sensors exhibited the high sensitivity of 1.07 kPa–1, a rapid response, short relaxation times of 18 ms, and good reliability over > 1000 cycles, characteristics that are comparable to those of previously reported OFET-based pressure sensors prepared with a microstructured gate dielectric layer. The flexibility and high performance of the 3D OSC FET pressure sensor were exploited in the successful application of our sensors to real-time monitoring of the radial artery pulse, which is useful for healthcare monitoring, and to touch sensing in the eskin of a realistic prosthetic hand. To the best of our knowledge, this report provides the highest sensitivity yet achieved among OFET pressure sensors fabricated using direct additive printing technologies without the use of additional etching or replication processes. 15 ACS Paragon Plus Environment


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The novel device designs and fabrication processes proposed in this work will enable the low-cost, large-area mass production of FET-based pressure sensors.

ASSOCIATED CONTENT Supporting Information Optical microscopy images of 3D OSC microstructures based on the centro-apically crystallized diF-TESADT after 80 kPa was applied to the device. Comparison of transfer curves of the 3D OSC FET before and after bending of the device. Photograph showing the bending of the device with a bending radius of 0.5 cm. Movie S1. A live video of touch sensing using the 3D OSC FET attached to the index finger of a prosthetic hand. Movie S1. A live video of droplet sensing using the 3D OSC FET. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest

Acknowledgements This work is supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT & Future Planning (MSIP) (2017R1A2B2002721) and Korea Institute of Science and Technology (KIST) institutional program (2V05530). Also, this work was supported by the Center for Advanced Soft-Electronics under the Global Frontier Project 16 ACS Paragon Plus Environment

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(NRF-2014M3A6A5060932)

and

the

Basic

Science

Research

Program

(NRF-

2017R1A2B4012819) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning.

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Figure 1. Schematic illustration of (a) the formation of a 3D OSC microstructure, printing of the diF-TESADT:PMMA blend solution onto the pre-printed PMMA baseline (BBL), and (b) centro-apical self-organization of the diF-TESADT molecules in a line-printed diFTESADT:PMMA blend during solvent evaporation. (c) Schematic diagram of the 3D OSC FET pressure sensor consisting of a bottom component printed 3D OSC structure and a top gate component with an elastomeric gate dielectric film. (d) Schematic illustration of the pressure-sensing process displayed by the 3D OSC FET.


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Figure 2. (a) Height profile of the 3D OSC microstructures prepared with various PMMA BBL overlayers. (b) Comparison of the height profiles of a 3D OSC and B5 BBL structure before and after the selective removal of diF-TESADT by cyclohexane. (c) Optical microscopy images showing (c) the crystalline diF-TESADT stripe formed at the centroapical position of the printed B5 BBL via printing of the diF-TESADT:PMMA blend solution, and (d) misaligned diF-TESADT crystalline layer formed by printing only diF-TESADT onto the BBL structure.



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Figure 3. (a) Photograph of the 3D OSC FET pressure sensor microstructures, and magnified image of the electrode and 3D OSC patterns. (b) Transfer curves obtained from the 3D OSC FETs under different pressure loads. (c) Pressure response curves at a constant source–drain (–80 V) and source–gate (–80 V) voltage of the 3D OSC FET pressure sensors prepared with different BBL structures and of an unstructured planar device fabricated by a spin-coating process. (d) Plot of the drain current, , (left y-axis, red circle) and areal capacitance of the PDMS gate dielectric, , (right y-axis, blue line) as a function of the applied pressure.


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Figure 4. (a) Time-resolved response of a 3D OSC FET to a pressure of 80 Pa at a constant source–drain (–80 V) and source–gate (–80 V) voltage. (b) Source–drain current change in the 3D OSC FET under a series of pressures: i) 0.4, ii) 0.8, iii) 1.6, iv) 2.4 kPa applied and released repeatedly to the sensor. The inset shows the repeatability of the pressure-responsive current change in the sensor. (c) The durability of the 3D OSC FET sensor under the repeated loading and unloading of a pressure (1.0 kPa) over 1000 cycles at a forcing frequency of 0.5 Hz.



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Figure 5. (a) Photograph (left) and schematic diagram (right) of a 3D OSC FET pressure sensor affixed above the radial artery in the wrist using a PDMS band. The current response to (b) the real-time pulse wave obtained at a constant source–drain (–60 V) and source–gate (–40 V) voltage, and (c) an enlarged pulse wave, showing three distinguishable peaks. (d) Photograph of a 3D OSC FET sensor affixed to the index finger of a prosthetic hand. (e) The source-drain current response as a function of the motion of the prosthetic hand during gripping and release of a tumbler. (f) Photograph showing a water droplet loaded onto a 3D OSC FET sensor affixed to the palm of a prosthetic hand. (g) The source–drain current response of the device during loading of a 20 µL water droplet (21.30 mg).


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FIGURE FOR ToC ABSTRACT


Highly Sensitive Flexible Pressure Sensors Based on Printed Organic

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