Organic Optoelectronic Diodes as Tactile Sensors for Soft-touch

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Organic Optoelectronic Diodes as Tactile Sensors for Soft-touch Applications Marcin Kielar, Tasnuva Hamid, Liao Wu, Francois Windels, Pankaj Sah, and Ajay K. Pandey ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04671 • Publication Date (Web): 27 May 2019 Downloaded from http://pubs.acs.org on May 29, 2019

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ACS Applied Materials & Interfaces

Organic Optoelectronic Diodes as Tactile Sensors for Soft-touch Applications Marcin Kielar1,2*, Tasnuva Hamid1, Liao Wu1, François Windels2, Pankaj Sah2 and Ajay K. Pandey*1 1Robotics

and Autonomous Systems, School of Electrical Engineering and Computer

Science, Science and Engineering Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia 2Queensland

Brain Institute, The University of Queensland, St Lucia, QLD 4072, Australia

* Email: [email protected]; [email protected]

Keywords: optical tactile sensing, soft touch, elastomers, rubrene, organic photodetectors, organic light-emitting diodes

Abstract The distributed sense of touch forms an essential component that defines real-time perception and situational awareness in humans. Electronic skins are an emerging technology in conferring artificial sense of touch for smart human-machine interfaces. However, assigning conformably distributed sense of touch over a large area has been challenging to replicate in modern medical, social and industrial robots. Herein, we present a new class of soft tactile sensors that exploit the mechanisms of triplet-triplet annihilation, exciton harvesting and small Stokes shift in conjugated organic semiconductors such as rubrene. By multiplexing the electroluminescence and photosensing modes, we show that a compact optoelectronic array of multifunctional rubrene/fullerene diodes can accurately measure pressure, position and surface deformation applied to an elastomeric layer. The dynamic sensing range is defined by

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mechanical properties of the elastomer. This paves the way for soft, conformal and large-area compatible electronic skins for medicine and robotics.

Introduction In humans, skin is by far the largest sensory system and the sense of touch plays a vital role in exploring our environment. This sense in particularly relevant in distinguishing between soft and hard surfaces and manipulating fragile objects. Mimicking the multimodal sensory network found in human skin has long been a source of inspiration for the development of so-called electronic skin (e-skin).1–5 Robotic and medical applications are two of the main drivers behind the demand for the conformal and robust tactile sensing technology,6–8 bringing together researchers from computer science, electronics, robotics, biomedicine and neuroscience backgrounds.

9,10

In robotics for example, it is widely accepted that adding a sense of touch

would remove uncertainties in dealing with soft and deformable objects that are hard to model in dynamic and unstructured environments.11 In medicine, restoring sensory feedback to patients with skin damage, amputations or peripheral neuropathy could significantly improve their overall quality of life.12–14 Smart sensing interfaces are yet to be used for training in minimally invasive procedures such as laparoscopy,4,7,8 for tele-presence surgery,15 and for robotic-assisted interventional radiology.15,16 Whether it is for robotics, prosthetics or robotic-assisted surgery, the requirements for tactile sensing are similar. The key element is the capability of quantifying touch (applied pressure) accurately over a few orders of magnitude while maintaining high sensitivity, i.e. the weakest force level that can be detected by a sensor. In most of the applications, knowing the amount of external force is crucial but not sufficient as the information about the position, angle or surface deformation is also required. Most common e-skin sensors reported to date are based


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on triboelectric,17 capacitive,18 piezoresistive,19 piezoelectric,20,21 and magnetic,22 modes of sensing. It is worth emphasizing that some of these sensors are bio-inspired,23,24 flexible and stretchable,17,18 and even self-healable.25,26 The sense of touch is generally measured and restored by transducing external input into quantifiable electrical signals based on different sensing mechanisms. Despite being able to sense different levels of pressure, these electronic devices show a number of limitations. One can note complexity of fabrication, slow response time, poor reproducibility and modest cycling stability, the biggest limitation being the difficulty to fabricate large-area e-skins as these sensing units often require microstructured patterns and layers to be used.18,27 Hence, it is of great interest to engineer cost-effective and intelligent tactile interfaces to fill the current need for soft and conformal sensing. Here, we demonstrate a novel approach to the tactile sensing technology by designing and fabricating a large-area compatible, organic optical tactile sensor that is capable of accurately sensing pressure, position and surface deformation. A number of key features set it apart from all other sensors reported to date: i) tactile sensing is achieved through spatio-temporal changes in light intensity caused by surface deformation; ii) sensor is based on organic optoelectronics and materials are highly compatible with flexible interfaces for prosthetics and e-skins; iii) owing to the physical mechanism of triplet-triplet annihilation, exciton harvesting and small Stokes shift, all diodes of the sensing array are fully reversible, i.e. they can work as organic light emitting diodes (OLEDs) or as organic photodetectors (OPDs), and therefore are capable of not only emitting but also detecting the emitted light; iv) force of touch is measured through changes in open-circuit voltage; and v) dynamic range of the sensor can be further customized to address specific applications. To date, light has already been used as vector for creating artificial muscles for microrobotics including a biomimicked sensitive plant or a light-powered walking bot.28



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Results and discussion Light as vector for tactile sensing technology The organic tactile sensor reported here is composed of an array of multifunctional diodes being covered by an elastomeric surface. The schematic of the array can be seen in Figure 1A. Five organic diodes composed of rubrene and fullerene (C60) are fabricated on a 25 mm square substrate and arranged in a way that the central diode is equidistant and surrounded by the other diodes. For optical force sensing, the central diode is arbitrarily selected to operate as constantcurrent light source while four surrounding diodes operate as OPDs.

Figure 1. Schematic and working principle of the organic tactile sensor. (A) 3D view of the organic sensor where each colour corresponds to a different layer, and a photograph of the array. The total square surface is 6.25 cm2. The

magnified

inset

shows

the

cross-sectional

device

structure

of

one

of

five

sensors:

ITO/PEDOT:PSS/rubrene/C60/Ba/Ag. (B) 2D, front and upside-down view of the sensor at resting state. Opaque, elastomeric dome is placed above the array and equipped with a reflective coating on the inside. Light emitted from


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the central diode B is uniformly distributed across all other diodes resulting in an equal response in open-circuit voltage Voc from diodes A and C. (C) Once an exterior force is applied, the light distribution under the dome changes and the out-of-phase diode A receives less light than the in-phase diode C, resulting in different responses Voc.

An elastomeric dome, 20 mm in diameter, is placed and fixed below the array, as it can be seen in Figure 1B (upside-down, front view). The dome is equipped with a reflective coating on the inside so that the light emitted from the central diode is backscattered to the substrate. Four diodes surrounding the central emitting diode operate in detection mode, each producing an electrical signal proportional to the light intensity received (the stronger light intensity, the stronger electric signal). Care is taken in the design and fabrication processes to ensure that the structure of the array including highly reflective electrodes is symmetrical under the dome so in a resting state (no force applied) all detecting diodes receive the same amount of light, as shown in Figure 1B. In addition, the outside of the dome is opaque to eliminate any external light, thus reducing ambient noise. Application of an external force to the elastomeric surface (touching finger, needle or any other stimulus) modifies its shape as shown in Figure 1C. As a consequence, the distribution of reflected light under the dome will change resulting in changes in electric signals across the detecting diodes. Relative differences in responses from different photodiodes allow us to determine the position of the deformation, while a simple calibration with a haptic robotic arm can give information about a level of force applied. Inorganic LEDs and silicon detectors can be used to fabricate the array and to sense pressure. However, the properties of inorganic technology prevent the low-impact incorporation of sensing units into dynamically-mobile systems. Thus, we took advantage of the differentiating properties of organic electronics to solve this issue. This technology combines carbon-based, conjugated semiconductors with low cost fabrication techniques to create devices that can be remarkably thin and lightweight, flexible, semi-transparent, wearable that can be manufactured



in large sizes.29,30 Organic electronics is widely applied in the development of next generation photovoltaics (organic photovoltaic cells, OPVs),31 OPDs,29 and display screens.32 Recently, wearable and ultra-flexible organic optoelectronic sensors have also been reported for diagnosis and smart prosthesis.33,34 An interesting feature of rubrene,35 a classical organic small molecule used in this work, is the ability to reversibly generate singlet excitons via triplet-triplet annihilation.36,37 These excitons can undergo radiative decay to the ground state and give electroluminescence. In addition, absorption of a photon in rubrene generates a strongly bounded exciton that can be dissociated for photosensing. To achieve this, a second material acting as electron acceptor, fullerene (C60) in this study, is required. Devices based on rubrene/C60 heterojunctions can thus be used for either light emission or detection,37 and these two features are the key element for the tactile sensing described further.

Figure 2. Working principle of rubrene/fullerene organic diodes. (A) Energy level diagram of the fabricated device highlighting the electroluminescence process: (1) positive voltage is applied resulting in charge injection at electrodes, (2) charge transport and charge transfer at rubrene/C60 interface, (3) triplet-triplet annihilation (TTA) generating a singlet exciton, (4) singlet decay to the ground state by electroluminescence. (B) Detection process: (1) photon absorption in rubrene, creation of an exciton, (2) exciton diffusion towards rubrene/C60 interface, (3) charge transfer and charge dissociation of the exciton, (4) free charges collection at electrodes. EA(A) stands for


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electron affinity of the electron acceptor material, IP(D) for ionisation potential of the electron donor. For clarity, no charge loss mechanisms such as trapping or recombination are shown.

The working principles for electroluminescence and absorption in rubrene/C60 diodes are shown in Figure 2, more details about the materials used in this work including their chemical structures can be found in Note S1 and Fig. S1.

Multifunctionality of rubrene/fullerene diodes Current density-voltage-luminance (J-V-L) characteristics of the optimized diode in emission mode are shown in Figures 3A and 3B. The turn-on voltage, i.e. the voltage applied to produce a luminance of 1 cd m-2, is measured at 1.04 V and is in accordance with the previous studies.36,37 Luminance levels of 5, 50 and 500 cd m-2 are measured at 1.21, 1.69 and 3.15 V, respectively. In addition, since the diode enters the mA regime at ultra-low voltages, the overall power consumption is highly reduced and below 80 mW for the maximum brightness (see Fig. S2 for more details).



Figure 3. Multifunctionality of rubrene/C60 organic diodes. (A) Current density-voltage (J-V) characteristics of the diode in emission mode. The corresponding driving currents of 1, 5 and 10 mA are shown to highlight low power consumption of the device. (B) Luminance of the diode as a function of applied voltage. The turn-on voltage is 1.04 V. (C) Semi-log plot showing the open-circuit voltage (Voc) of the fabricated diode as a function of incident light intensity from a calibrated, high-power green LED. The inset highlights the logarithmic dependence between

Voc and irradiance. (D) J-V characteristics of the photodiode under 1-Sun, AM1.5 spectrum (100 mW cm-2). The resulting power is also plotted.

Whereas light emission is a straightforward process (a constant current is applied to the diode), light detection requires more fundamental understanding of the physics of organic


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semiconductors. In organic photodiodes, absorbed photons are converted into photogenerated charges that give rise to an important figure of merit, which is the open-circuit voltage (Voc). It is defined as the voltage across the output terminals at zero current. The open-circuit voltage increases with increasing irradiation,38,39 as well as the photocurrent (Jph) does.40 Therefore, both figures of merit can be used for light detection. However, Voc offers a major advantage over the Jph that can be easily seen through a combination of Marcus theory for electron transfer and the generalized Shockley theory of the dark current density versus voltage (J-V) characteristics.38 Shortly, the photocurrent of the photodiode can be expressed as: (1)

𝐽𝑝ℎ(𝑉) = 𝑃0𝑅0𝐻(𝑉)

where P0 is the incident optical power density, R0 is the responsivity of the device and H the collection function related to the absorption rate of the semiconductor materials used. One can note that Jph is linearly dependent on light intensity. For state-of-the-art organic photodiodes, this linear behaviour is accurate over seven orders of magnitude, as reported in our previous work,41 and many others.30 In parallel to Jph, the open-circuit voltage can be expressed as: 𝑉𝑜𝑐 =

𝑛𝑘𝐵𝑇 𝑞

ln

(

𝐽𝑝ℎ(𝑉𝑜𝑐) 𝐽𝑆

𝑉𝑜𝑐

+ 1 ― 𝐽𝑆𝑅𝑃

)

(2)

where n is the diode ideality factor, JS the reverse saturation current, kB Boltzmann’s constant, T the temperature, and Rp parallel resistance. Since the product 𝐽𝑆𝑅𝑃 ≫ 𝑉𝑂𝐶 and 𝐽𝑝ℎ ∝ 𝑃0,41 it is clear from equation (2) that Voc increases logarithmically with light intensity. This logarithmic dependence gives a major advantage over the Jph expressed in (1) since even at low light intensities the voltage signal is very strong. Figure 3C shows the Voc logarithmic dependence across different monochromatic light intensities spread over five orders of magnitude. One can note that Voc is very sensitive at lower irradiances. For example, a low monochromatic irradiance of 25 W cm-2 produces a Voc signal of 176 mV that can be easily read with a simple lab multimeter. In contrast, the same irradiance generates the photocurrent in the ultra-low nanoampere (nA) regime, as shown in Fig. S3, which is difficult to detect



unless using expensive source-measure or lock-in units. We note that Voc readouts can be performed for light intensities even below 10 W cm-2, as shown in Fig. S4. A similar advantage of Voc over Jph can also be seen in inorganic photodiodes (Fig. S5), however siliconbased diodes do not show the bifunctionality and thus are unable to emit light. Even though only the emission and detection modes of the rubrene/C60 diodes are used for tactile sensing in this work, it is worth emphasizing that the fabricated diodes can also act as photovoltaic solar cells, as shown in Figure 3D, and thus they can generate power. Standard current density-voltage (J-V) characteristics in dark and under illumination are measured with a solar simulator featuring the AM1.5 spectrum as light source. The diode shows large opencircuit voltage (918 mV), high short-circuit current (1.59 mA cm-2) which results in power conversion efficiency of 0.93%. Therefore, the proposed diode structure can be considered as multifunctional. To date, OLED and photovoltaic functions of the rubrene/fullerene diodes have been studied independently.42,43

Reversibility of rubrene/fullerene diodes Absorption and electroluminescent (EL) spectra of the rubrene/C60 thin film and the fabricated diode are shown in Figure 4A. The absorption spectrum shows three well-resolved vibronic peaks at 462, 494 and 528 nm respectively. The rubrene/C60 active layer absorbs visible photons up to 575 nm, with the maximum absorption being measured for green light. The light emission occurs at higher wavelengths, corresponding to yellow and orange colours. The diode emission spectrum peaks at 565 nm demonstrating a large spectral overlap focused at 549 nm. The highest energy peak in the EL spectrum and the lowest energy peak in the absorption feature the highest intensities. These two peaks, which are related to the zero-phonon (0–0) transition, show a small energy difference, so-called Stokes shift.44 A small Stokes shift measured here (37 nm) illustrates an interesting property of the diode: photons emitted by the


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rubrene/fullerene OLED can be partially absorbed by the rubrene/fullerene OPD having the same device structure but operating differently. To demonstrate whether this absorption can significantly contribute to an electric signal, two identical rubrene/fullerene diodes were placed side by side, in dark, at a constant distance. Standard J-V characteristics were performed under different light intensities generated by one of the diodes. As it can be seen in Figure 4B, the detecting diode shows high Voc signals at low luminance levels. For example, a luminance of 197 cd m-2 is sufficient to raise the photodiode Voc signal to 95 mV. In contrast to voltage readouts, photocurrent levels remain in the low nA range, as previously discussed, reflecting a poor responsivity of the diode under the receiving spectrum, and highlighting the advantage of Voc over Jph. The reversibility of the diode alternating OLED and OPD functions is shown in Figure 4C. To measure the diode performance and stability, calibrated luminance meter and monochromatic were used. A constant current of 4.5 mA is applied to the organic diode to reach, at t0, the luminance of 498 cd m-2, while the LED is arbitrary set to deliver 15 W cm-2 of incident power and thus generate the Voc of 118 mV across the organic photodiode. Six periods are shown along with 10 measurements per period to illustrate the reversibility. One can note that null degradation nor hysteresis are observed when switching between emission and detection functions. To further demonstrate the stability of rubrene/fullerene diodes, 5000 transitions OLED/OPD were performed at the frequency of 0.5 Hz, as presented in Figure 4D. Interestingly, the diode performance remains stable excepting a minor increase in luminance over time (4.52 cd m-2) accompanied with a small loss in Voc (3.97 mV). Since it is possible to detect the emitted light via changes in Voc in rubrene/fullerene diodes, no separate light sources and detectors are needed to construct the tactile sensor shown in Figure 1A and discussed further in this work. This greatly simplifies the fabrication process as depositing different device structures with different organic materials side by side for



integrating OLED and OPD functions for tactile sensing, respectively, would increase fabrication times and costs.

Figure 4. Reversibility of rubrene/fullerene organic diodes. (A) Normalized absorption spectrum of the rubrene/C60 thin film and electroluminescence (EL) spectrum of the fabricated device. The overlapping area defines the region where the diode can be multiplexed to operate either in emission or detection mode. The intersection point for highest dual efficiency is identified at 549 nm (green colour). (B) J-V characteristics of the rubrene/C60 OPD illuminated by the rubrene/C60 OLED driven at different currents. The Voc region is magnified for clarity. (C) Reversibility of the rubrene/C60 diode alternating the emission (top) and detection (bottom) modes. (D) Long-term performance study of the bifunctional diode alternating OLED (top) and OPD (bottom) functions.


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To date, this unique bifunctional and reversible behavior and its potential possible applications, have not been fully explored. In addition, as demonstrated by Kim et al., a reversible diode operation can also be achieved by appropriate band engineering of perovskite materials.45 Hence, the reported below tactile sensing approach could be applied across different material platforms.

Tactile sensor based on bifunctional and reversible organic diodes The tactile sensing capability of the organic optoelectronic array was measured with a calibrated haptic, robotic arm capable of delivering any force level from 0 to 3.3 N. To put it into context, a soft touch requires 0.3 to 1 N of force, whereas a hard push or slap corresponds to 10 N.46 Since the robotic arm can move with six degrees of freedom (6DoF) featuring a spatial resolution of 55 m, it is capable of mimicking human-like soft touch with high accuracy. The haptic robot applying a 3 N force can be seen in Figure 5A. A thermoplastic rubber and white paint were used to fabricate the dome that matches the force range of the robot (details in Fig. S6). For example, a silicon-based dome (see Fig. S7) would be too stiff for the robot and thus could be used to sense higher force levels. Once fabricated and coated with reflective paint (reflectivity data can be seen in Fig. S8), the dome was placed and fixed above the bifunctional organic array with the central diode acting as emitter and the surrounding diodes acting as detectors. The OLED driving current is chosen in a way that the corresponding Voc responses from the surrounding photodiodes are located at the lower end of the logarithmic dependence between voltage and light intensity (see the sensing region box in Figure 3C). The OLED current of 5 mA ensures low power consumption (17 mW) while keeping high luminance (537 cd m-2) and large voltage signals (>150 mV) as described below. According to Figures 1B and C, different Voc responses are expected from the orthogonally opposite diodes once the force has been applied. When the tip of the robotic arm delivering



force input to the dome is placed directly above one of the detecting diodes, inward deformation of the dome enhances reflected light reaching the orthogonally opposite diode. The voltage response obtained from this orthogonally opposite diode is defined as in-phase. Thus, the diode experiencing the force applied receives low light and is defined as out-of-phase.

Figure 5. Sensitivity of the organic pressure sensor. (A) Sensing output of the tactile sensor when a force of 3 N was applied by the haptic robotic arm. The dimensions of the robotic tip are 1.35 mm in diameter at the bottom and 8 mm wide at the top with a total height of 6.5 mm. (B) Open-circuit voltage of the in-phase and out-of-phase diodes at different force levels delivered by the haptic robotic arm. (C) Current-voltage characteristics of the in-phase and

out-of-phase diodes highlighting their Voc. (D) Response of the organic sensor as a function of applied force between 0 and 1 N. The sensor output is defined by the equation shown in the inset. (E) Sensitivity of the organic sensor represented in terms of significance (p-value).


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In our study, since both Voc responses feature opposite directions (the in-phase signal is increasing, the out-of-phase signal is decreasing), it is more straightforward to use the normalized gap between them to define force as follow: F = 1N ×

∆𝑉𝑖𝑛 ― ∆𝑉𝑜𝑢𝑡

(3)

∆𝑉𝑚𝑎𝑥

where F is force in newtons, Vin and Vout are relative contributions of the in-phase and outof-phase diodes, and Vmax is the maximum voltage gap between both signals at 1N. Taking relative contributions to the force offers a major advantage as it eliminates the variance in absolute voltage readouts observed across multiple devices. For example, when no force is applied, the Voc varies between 184 and 201 mV depending on photodiode position and on sensor. We attribute this variance to i) films defects and thickness variations during fabrication, ii) minor asymmetry of the sensor due to a non-perfect alignment of shadow masks during deposition processes, iii) asymmetry of the dome featuring a non-uniform reflective coating. Also, by taking relative and not absolute voltage readouts, the influence of minor changes in OLED/OPD performance over time (see Figure 3D) can be minimized. Lastly, using relative contributions of the photodiodes in equation (3) offers another advantage. It significantly reduces the variations in temperature for each individual sensor according to equation (2). These variations are even further reduced by the geometry of the sensor since the photodiodes are protected by the dome with the air acting as poor conductor of heat. Figure 5B illustrates relative voltage contributions of the in-phase and out-of-phase diodes for 1, 2 and 3 N where at a resting state (no force is applied) both signals are considered indistinguishable. One can note that higher the force applied, larger the gap between the in-phase and out-of-phase signals. The differences in Voc of 17.4±3.4, 36.3±3.7 and 51.5±3.8 mV are measured at 1, 2 and 3 N, respectively. Interestingly, both signals show a linear dependence on the force applied (R2=0.995 and 0.913 respectively) and each of them, once calibrated, could in principle be used to sense pressure individually. Definition of force in equation (3) is thus an arbitrary choice



and one could suggest a different methodology. The current-voltage characteristics of the photodiodes highlighting the Voc region at 3 N are presented in Figure 5C (see Fig. S9 for extended J-V data). One can note that the in-phase voltage increase is slight larger (+18.8%) than the voltage loss of the out-of-phase diode (-8.5%). Given the logarithmic dependence between voltage and irradiance (as seen in Figure 3C), the corresponding contributions reflect a gain of 38.2% in light intensity for the in-phase diode and a loss of 17.7% in light intensity for the out-of-phase diode. It is interesting to note that a major contribution to the absolute Voc readouts is not a result of different light distribution under the dome. We attribute this intrinsic voltage signal to a direct absorption of OLED light by the OPD via the shortest path between two diodes. This can be possible since i) the OLED emission is omnidirectional, and ii) emitted photons are waveguided inside different layers of the diode including the substrate.47,48 Further optimization of the geometry of the sensor along with the presence of additional layers for improved light extraction could increase the contrast between the in-phase and out-of-phase signals.49 Following the equation (3), the sensor output was tested with a robotic arm applying back and forward force levels between 0 and 1 N, as shown in Figure 5D (10 periods are averaged). One can note that the sensor response is overestimated (~10%) at low force levels. This undesirable behavior is most likely due to a hysteresis of the dome that does not return to its initial stage at rest when the stress is removed. The presence of the hysteresis along with small variances seen in the sensor response will affect the sensitivity of the sensor, i.e. the lowest amount of force that can be seen by the sensor. Figure 5E shows the results of the t-test (see Note S2 for details) revealing the sensitivity of 110 mN for the optical tactile sensor. The reported value is enough for distinguishing soft touch but cannot compete with the remarkable sensitivity of human skin (0.07 mN).5,50 The sensitivity of the sensor could be further improved by designing more robust elastomers.


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In order to quantify the dynamic touch response, a sinusoidal force output oscillating between 0 and 3 N was delivered by the arm to the dome at the frequency of 159 mHz. The calibrated signal along with the responses from the organic sensor over 10 cycles of applied force are shown in Figure 6A. The in-phase and out-of-phase diodes show time-locked phase differences with the Pearson correlation coefficients of 0.96 and -0.89 respectively. The applied force pattern was updated at the frequency of ~50 Hz and one can note that the spatio-temporal response of the sensor follows the pattern without delay. The transient voltage measurements, shown in Fig. S10, reveal the response time of 6 ms, i.e. the time required for the sensor to see a change in light intensity. Our result is consistent with the transient voltage measurements reported for photosensors.50 Similar response times are observed for piezoelectricity-based (~5 ms), capacitance-based (~10 ms) and piezoresistivity-based (~20 ms) sensors.1 Human skin also shows similar sampling rates.51



Figure 6. Sensing capability and angular resolution of the organic tactile sensor. (A) Cycling repeatability of the organic pressure sensor. A robotic arm is delivering a sinusoidal force from 0 to 3 N. (B) Response of the out-ofphase organic diode when a 3 N of force is applied at different angles with a step of 15. The experiment is repeated 10 times. (C) Calibration curve for all 4 diodes developed from the results in (B). The angle uncertainty is calculated numerically.

In addition, since four detecting diodes surround the OLED, it is not only possible to measure the amount of force applied but also the position of the deformation. The proposed arrangement of diodes allows the spatial calibration similar to the trilateration problem with 4 points. Figure 6B shows the response of the photodiode as a function of the angle at which a force of 3 N has


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been applied. 10 periods are averaged and one can note that the diode is considered as out-ofphase at 0 (Vout = -15.93.2 mV), becomes in-phase at 180 (Vout =35.23.4 mV) and features symmetrical responses when approaching 180 and returning back to 0. Averaged response for each angle generates a fit curve that can be used as calibration for angle measurements (see the fit in Figure 6B). We note here that resolving the angle with only one fit would not be possible since i) the diode response is not unique between 0 and 180 and between 180 and 359 (but rather symmetrical), and ii) there is no significant difference in the diode responses between 135 and 225 given the overall uncertainty of 5.1 mV). That being said, the fit obtained in Figure 6B allows to construct the calibration pattern for the remaining 3 photodiodes. Here, we take advantage of the fact the sensor structure under the dome is symmetrical and the responses of the remaining 3 photodiodes are shifted by 90, 180 and 270 with respect to the response seen in Figure 6B. The curve fitting is detailed in Fig. S11, the resulting calibration pattern for 4 photodiodes is presented in Figure 6C. One can note that a combination of four responses is now unique between 0 and 359 which allows to resolve the angle and thus the position of deformation. For a given set of 4 relative voltage contributions, the angle of deformation is found by matching the signals with the calibration pattern, and thus by minimizing the sum of 4 mismatches between the measured diode response and the expected diode response from the calibration pattern. In parallel to the position of deformation, the force can also be measured. In this case, the equation (3) would be upgraded to take into account the relative contributions of all 4 photodiodes. The angle uncertainty, visible in Figure 6C, has been simulated numerically (details in Note S3 and Fig. S12). Two trends are visible: i) lower uncertainty (1.3) when the responses from 4 photodiodes differ from each other, and ii) higher uncertainty (2.6) in the case two or more signals are similar or overlap each other. The lowest accuracy corresponds to a spatial resolution of 0.45 mm, which is higher than the spatial



resolution of an average human fingertip (1 mm).51 If required, different geometries of the array with higher number of detecting diodes would allow better spatial resolution.

Conclusions We show that organic diodes based on rubrene/fullerene heterojunctions can act as light emitters or detectors on demand. The small Stokes shift between absorption and electroluminescent spectra allows the multifunctionality and reversibility. This unique advantage replaces the need of having separate light sources and detectors. We have exploited this ability in achieving a compact design for pressure sensing that greatly mitigates the bottleneck issues when it comes to design, fabrication and implementation of large-area compatible and distributed touch sensors. The optoelectronic sensor reveals excellent sensitivity, response time and spatial resolution for measuring soft touch. Depending on the stiffness of the dome, different levels of force could be detected without modifying the structure of the array. As a consequence, a range of elastomers can be used to address different applications. For example, ultra-flexible thermoplastic rubbers could be used to sense gentle touch, and hardly-deformable polymers for manipulating heavy objects. Hence, it would be possible to adjust the dynamic range of the sensor. By changing the geometry of the array, it would be possible to further improve the spatial resolution of the sensor. Multiple stimuli could also be detected with a large number of photodiodes and with improved algorithms. In addition, the spatial distribution of the deformation could be used to predict the shape of the stimulus. Therefore, the sensing platform is highly compatible for measuring force and position in form of soft, conformal and large-area compatible electronic skins. Finally, the tactile sensor concept described here could be extended unlimitedly by alternating emitting and detecting diodes to


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create large-area, smart tactile interfaces. In this case, the proposed dome would change to a flat elastomeric surface spread all over the diodes. In summary, this work demonstrates a new concept that organic optoelectronic devices can be implemented in touch sensing systems in form of intelligent electronic skins. All above features are highly desired for medical, social and robotic applications including electronic skins and smart prosthetics.

Experimental section Materials. PEDOT:PSS aqueous dispersion (1.3-1.7%) was purchased from Heraeus and used without any further purification. Rubrene (purity 99.99%, sublimed grade), fullerene (C60, purity 99.5%), barium (purity 99.9%) and silver (purity 99.9%) were purchased from Sigma Aldrich and used as received. Optoelectronic array fabrication. The indium tin oxide (ITO) coated glass substrates (10 Ω per square, squares of 6.25 cm2) were first cleaned in deionized water with a detergent (Alconox from Sigma Aldrich), then sequentially cleaned in an ultrasonic bath with deionized water, acetone and isopropanol (10 min each) and dried under nitrogen flow. For photolithography process, a negative photoresist AZ2020 was spin-coated on the ITO samples following an annealing process at 110 °C for 2 min. Samples covered with a metal mask with desired pattern were then exposed to UV light (12.3 mW cm-2) for 8.1 s and returned on a hot plate at 110 °C for 2 min. A developer AZ726 MiF was used as a 1 min bath solution to remove the unexposed photoresist. Samples were then dried with nitrogen gun. The etching process was carried out with hydrochloric acid (32%) for 10 min following by a conductivity check with a multimeter. All remaining traces of the photoresist were then removed through a 30 min ultrasonic bath with PG remover.



For the deposition process, PEDOT:PSS solution (30 nm) was spin-coated at 5,000 rpm for 60 s on the ITO substrates patterned by photolithography and recleaned. Samples were then dried at 120 °C for 10 min in air and moved into a nitrogen-filled glovebox for the thermal evaporation chamber. Rubrene (32.5 nm), fullerene (25 nm), barium (10 nm) and silver (60 nm) were then evaporated at slow rate (< 0.02 nm s-1) and under high vacuum (10-6 mbar), different shadow masks were used to achieve desired patterns. Reflective elastomer fabrication. Thermoplastic rubber (TPE) was used as elastomer. Waterbased, acrylic white paint (Mont Marte, PMSA0010, Titanium White) was applied with a paintbrush to the elastomer surface to create a single-layer reflective coating. Device characterization. Open-circuit voltage (Voc) measurements were recorded by using a high power, green (528 nm) LED supplied by Intelligent LED Solutions and calibrated with a silicon diode (Osram BPW21). The same silicon diode was used to quantify electroluminescence of the diodes. A Keithley 2604B dual channel source measure unit was used to power the LED and record data from organic and inorganic photodiodes. Optoelectrical characterization of diodes was carried out in a shielded metal box (Faraday cage). Film thickness was measured with DektakXT surface profilometer. UV-Vis absorption spectra were recorded with the Cary-5000 spectrometer. USB4000 spectrometer from Ocean Optics was used to acquire electroluminescence spectra. A Chroma meter CS-200 (Konica Minolta) was used for luminance measurements. A calibrated Phantom Omni portable haptic robotic arm (Sensable Technologies) was used to deliver force. The organic optoelectronic array was used without encapsulation, all tactile sensing measurements were performed in air with the organic array being exposed to nitrogen flow. A data acquisition system based on the PXIe platform (PXIe-1071, PXIe-8840, National Instruments) was used to power the OLED (PXIe-4136 card) and acquire data (PXIe-6365 card) from organic photodiodes. LabVIEW scripts were written to acquire and interpret different input signals from the organic diodes and thus retrieve


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information about position, angle and force applied. C++ scripts were written to control the haptic robotic arm.

Associated content Supporting information Additional experimental and methodical details: Note S1.

About the materials used in this work

Fig. S1.

Chemical structures of the organic materials used in this work

Fig. S2.

Power consumption of the OLED

Fig. S3.

Linearity (Jsc versus light intensity) of the OPD

Fig. S4.

Voc of the OPD as a function of ultra-low light intensities

Fig. S5.

Voc and Jsc as a function of irradiance for an inorganic diode

Fig. S6.

Fabrication details of the dome for the sensing array

Fig. S7.

Photograph of the organic sensor with a silicone-based dome

Fig. S8.

Reflectance of the reflective coating (white paint)

Fig. S9.

J-V characteristics of the OPD at rest and at 3 N

Note S2.

Details of the t-test performed to determine the sensitivity

Fig. S10.

Transient voltage characteristics of the OPD

Fig. S11.

Fitting curve for the angular calibration pattern

Note S3.

Details on Numerical simulation of the angle uncertainty

Figure S12.

Results of numerical simulation for determining the angle uncertainty

Table S1.

Thicknesses of different layers of the organic diode



Author information Corresponding authors *E-mail: [email protected] *E-mail: [email protected] ORCID Marcin Kielar: 0000-0002-9802-1234 Ajay K. Pandey: 0000-0002-6599-745X

Author contributions AKP conceptualized the idea. AKP and MK designed the optical force sensing device layout. MK fabricated the sensing prototypes and performed the experiments and did the initial data analysis. TH performed the absorption/PL measurements and assisted with the thermal evaporation of rubrene and fullerene. AKP, FW and PS supervised the project. LW programmed the haptic robotic arm to work as a force input to tactile sensor assembly. MK and AKP drafted the manuscript and all authors contributed to the interpretation of results and assisted with preparation of the manuscript.

Competing interests The authors declare no competing financial interests.


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Acknowledgements The joint research program between Robotics and Autonomous Systems at Queensland University of Technology (QUT) and Queensland Brain Institute at the University of Queensland funded this research. This work has also been partially funded by Australian Renewable Energy Agency (Project F022-6). This work was performed in part at the Queensland node of the Australian National Fabrication Facility Queensland Node (ANFF-Q) - a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and microfabrication facilities for Australia’s researchers. We also acknowledge Central Analytical Research Facility (CARF) at QUT for experimental facilities.

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Based on Optical Fibers and an Image Sensor. Int. J. Adv. Robot. Syst. 2018, 15 (4), 1– 13.

Table of Contents/Abstract Graphics


Organic Optoelectronic Diodes as Tactile Sensors for Soft-touch

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