A Flexible Temperature Sensor Array with Polyaniline/Graphene–Polyvinyl Butyral Thin Film

sensors

Article A Flexible Temperature Sensor Array with Polyaniline/Graphene–Polyvinyl Butyral Thin Film

Jin Pan 1, Shiyu Liu 2 , Hongzhou Zhang 1 and Jiangang Lu 1,*

1 National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; [email protected] (J.P.); [email protected] (H.Z.) 2 Shenzhen Goodix Technology Co. Ltd., Shenzhen 518000, China; [email protected] * Correspondence: [email protected]; Tel.: +86-21-3420-7914

Received: 21 August 2019; Accepted: 18 September 2019; Published: 23 September 2019

Abstract: Thermal-resistance temperature sensors generally employ temperature-sensitive materials as active layers, which are always deposited on a flexible substrate to improve flexibility. Such a temperature sensor is usually integrated in wearable devices with other sensors, such as pressure sensors and stretchable sensors. In prior works, the temperature and pressure sensors are usually located in different layers in a multifunction sensor, which results in a complicated fabrication process, as well as a large thickness of devices. Meanwhile, many temperature sensors are based on large areas of non-transparent materials, leading to difficulties in integrating display applications. In this paper, we demonstrate a flexible temperature sensor based on polyaniline/graphene (GPANI)–polyvinyl butyral (PVB) thin film and indium tin oxides (ITO)- polyethylene terephthalate (PET) substrates. The GPANI particles embedded in PVB film not only contribute to temperature detection, but also response to external pressures, due to weak deformations. In addition, the thin composite film (2.7 µm) highly improved the transparency. By optimizing the device structure, the sensor integrates temperature and pressure detection into one single layer, which shows a wide temperature range of 25–80 ◦C, a pressure range of 0–30 kPa, and a high transparency (>80%). The temperature sensor offers great potential for applications in emerging wearable devices and electronic skins.

Keywords: temperature sensor array; temperature coeﬃcient of resistivity (TCR); polyaniline/graphene (GPANI); polyvinyl butyral (PVB) thin ﬁlm

1. Introduction Various studies on wearable devices and electronic skins based on pressure sensors and stretchable sensors have been carried out. Temperature detection is actually another important factor to be integrated in wearable devices, in order to monitor the body temperature and response to the ambient temperature, similar to real skins. There are many kinds of temperature sensors, such as thermal-couple sensors [1], infrared temperature sensors [2], optic fiber temperature sensors [3], thermal-resistance temperature sensors [4,5], and some sensors based on thermal response field-effect transistors [6,7]. The infrared and optic fiber sensors are usually for non-contact temperature detection. The thermal-couple type, thermal-resistance type, and transistor-based type are actually the appropriate candidates for electronic skins, because they can detect contact temperature. A thermal-resistance temperature sensor is generally based on temperature-sensitive materials, such as metals [8], semiconductors [9], and polymers [10–13]. The metal and semiconductor are usually deposited on a flexible substrate to improve flexibility. Compared with metal and semiconductors, a polymer with conductive temperature-sensitive materials shows greater flexibility for a low Young’s Modulus. Such a temperature sensor based on polymers can usually detect temperature in two mechanisms. One

Sensors 2019, 19, 4105; doi:10.3390/s19194105 www.mdpi.com/journal/sensors Sensors 2019, 19, x FOR PEER REVIEW 2 of 8 mechanisms. One is the deformation of the active layer [10,11], and another is the conductivity change of temperature sensitive materials [12,13]. Both mechanisms always work at the same time. In prior creations, the pressure array and temperature array are fabricated separately in different layers and then packaged into a system, which results in a complicated fabrication process as well as a larger thickness of devices. Meanwhile, conventional temperature sensors are usually based on large areas of non-transparent conductive materials, such as Pt, graphene, and poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), leading to poor transparency. To get a facile fabrication and high transparency, in this paper, we proposed a temperature sensor based on Sensors 2019, 19, 4105 2 of 8 polyaniline/graphene (GPANI)–polyvinyl butyral (PVB) composite film, and optimized the sensor’s structure. The GPANI shows high conductivity and is sensitive to ambient temperatures, making it ais suitable the deformation material offor the the active temperature layer [10 ,sensor.11], and The another sensor is theintegrates conductivity the temperature change of temperature layer and pressuresensitive layer materials into one [12 single,13]. Both layer, mechanisms and the thin always composite work film at the with same PVB time. and GPANI particles shows a highIn transparency prior creations, (>80%). the The pressure sensor array shows and a wide temperature temperature array range are fabricatedof 25–80 °C, separately which can in detectdifferent body layers temperature. and then Meanwhile, packaged the into sensor a system, can detect which both resultspressure in and a complicatedtemperature under fabrication low pressure,process asand well only as temperature a larger thickness under ofhigh devices. pressure, Meanwhile, which exhibits conventional the potential temperature to be used sensors in wearableare usually devices based in on the large future. areas of non-transparent conductive materials, such as Pt, graphene, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), leading to poor transparency. 2.To Materials get a facile and fabrication Methods and high transparency, in this paper, we proposed a temperature sensor basedBased on polyaniline on our previous/graphene work (GPANI)–polyvinyl [14], we optimized butyral the sensor’s (PVB) composite structure. film,In previous and optimized design, the a GPANI–PVBsensor’s structure. film is sandwiched The GPANI between shows high two conductivity ITO-PET films. and Because is sensitive the bezel to ambient tapes contacting temperatures, the topmaking and bottom it a suitable substrates material have for a the larger temperature thickness sensor. than the The GPANI– sensor integratesPVB composite the temperature film, there layeris a thinand air pressure gap between layer into the oneGPANI–PVB single layer, film and and the the thin topcomposite PET substrate film (Figure with PVB 1a). and Such GPANI a thin particlesair gap usuallyshows ashows high transparencyexpansion and (> shrinkage80%). The when sensor pressure shows achanges, wide temperature which results range in a ofchange 25–80 of◦C, contact which areascan detect between body the temperature. active layer Meanwhile, and ITO theelectrodes sensor canand detect improves both pressurethe sensitivity and temperature of the device. under However,low pressure, the air and gap only shows temperature adverse under effects high when pressure, the sensor which is exhibitsused for the detecting potential temperatures. to be used in Becausewearable the devices air shows in the low future. thermal conductivity, it is not good for heat conduction but is suitable for heat preservation. 2. Materials and Methods To eliminate the air gap between the top ITO-PET substrate and the GPANI–PET composite film, we designedBased on two our structures. previous In work the first [14], structur we optimizede, two GPANI–PET the sensor’s films structure. are fabricated In previous between design, top a andGPANI–PVB bottom electrodes film is sandwiched (Figure 1b). between Although two the ITO-PET thickness films. of the Because two films the is bezel the same tapes as contacting the bonding the tapes,top and such bottom a structure substrates shows have a large a larger resistance thickness (>20 thanMΩ), the due GPANI–PVB to the difficulties composite in self-alignment film, there isof a GPANIthin air particles gap between on the the top GPANI–PVB and bottom film films. and theIn the top second PET substrate structure, (Figure the 1bezela). Such tapes a thin are airat gapthe outsideusually periphery shows expansion of the andPET shrinkage substrates when (Figure pressure 1c), changes,which showed which resultsa great in aconductivity change of contact and apparentlyareas between reduced the active the thickness layer and of ITO the electrodes air gap. Ther andefore, improves the thestructure sensitivity in Figure of the 1c, device. with However,a single activethe air layer gap and shows outside adverse bonding, effects is when the candidate the sensor for is usedour temperature for detecting sensor. temperatures. Because the air shows low thermal conductivity, it is not good for heat conduction but is suitable for heat preservation.

(a) (b) (c)

FigureFigure 1. 1. DifferentDifferent structures structures of of sensors. sensors. (a ()a )Top Top and and bottom bottom substrates substrates directly directly bonded bonded with with bezel bezel tapestapes or or spaces spaces in in between, between, with with only only one one poly polyanilineaniline/graphene/graphene (GPANI)–polyvinyl (GPANI)–polyvinyl butyral butyral (PVB) (PVB) filmfilm between between the the top top and and bottom bottom electrodes; electrodes; (b (b) )two two GPANI–PVB GPANI–PVB films films between between the the top top and and bottom bottom electrodes;electrodes; ( c) toptop andand bottom bottom substrates substrates fixed fixed by bezelby beze tapesl tapes at the outsideat the outside periphery periphery of PET substrates, of PET substrates,with one GPANI–PVB with one GPANI–PVB film. film.

PolyanilineTo eliminate (PANI) the air is gap a temperature between the sensitive top ITO-PET material substrate and it andis easy the to GPANI–PET aggregate [15]. composite However, film, GPANIwe designed can tackle two the structures. aggregation In the of first polyaniline structure, and two retain GPANI–PET high conductivity films are fabricatedand dispersity. between To get top and bottom electrodes (Figure1b). Although the thickness of the two films is the same as the bonding

tapes, such a structure shows a large resistance (>20 MΩ), due to the difficulties in self-alignment of GPANI particles on the top and bottom films. In the second structure, the bezel tapes are at the outside periphery of the PET substrates (Figure1c), which showed a great conductivity and apparently reduced the thickness of the air gap. Therefore, the structure in Figure1c, with a single active layer and outside bonding, is the candidate for our temperature sensor. Polyaniline (PANI) is a temperature sensitive material and it is easy to aggregate [15]. However, GPANI can tackle the aggregation of polyaniline and retain high conductivity and dispersity. To get a Sensors 2019, 19, x FOR PEER REVIEW 3 of 8 a homogenous solution with polyaniline uniformly dispersed in the solution, we used GPANI as the temperatureSensors 2019, 19 ,sensitive 4105 material. Generally, GPANI can be synthesized with graphene oxide, reduced3 of 8 graphene oxide, or graphene. Among the three kinds of graphene, the GPANI based on graphene oxidehomogenous shows the solution highest with dispersity polyaniline and lowest uniformly conductivity. dispersed The in the GPANI solution, based we on used graphene GPANI shows as the thetemperature lowest dispersity sensitive and material. highest Generally, conductivity. GPANI The candispersity be synthesized and conductivity with graphene of the oxide,GPANI reduced based ongraphene reduced oxide, graphene or graphene. oxide are Among between the those three of kinds other of graphene,two. Since thewe GPANIneed a basedfilm with on graphene GPANI particlesoxide shows uniformly the highest dispersed dispersity within and it, lowestthe GPANI conductivity. based on Thegraphene GPANI oxide based was on used. graphene The oxidation shows the statelowest of dispersityPANI is emeraldine, and highest due conductivity. to its high stability. The dispersity and conductivity of the GPANI based on reducedThe graphenefabrication oxide process are betweenis shown those in Figure of other 2. two. First Since of all, we the need GPANI a film with(HQNANO-GR-030, GPANI particles Tanfenguniformly Tech. dispersed Inc., Suzhou, within China), it, the GPANI PVB (30153960, based on Sinopharm graphene oxideChemical was Reagent used. The Co., oxidation Ltd, Shanghai, state of China),PANI is and emeraldine, absolute due ethyl to itsalcohol high stability.(80059490, Sinopharm Chemical Reagent Co., Ltd, Shanghai, China)The were fabrication stirred for process 4 h and is shownultrasonicated in Figure for2. 30 First min of to all, make the GPANI a homogenous (HQNANO-GR-030, solution. The Tanfeng weight ratioTech. of Inc., the Suzhou, GPANI, China), PVB, and PVB absolute (30153960, ethyl Sinopharm alcohol Chemicalwas 0.1:5:100. Reagent The Co., concentration Ltd., Shanghai, of GPANI China), showedand absolute no influence ethyl alcohol on the(80059490, sensitivity Sinopharmof the temp Chemicalerature sensor, Reagent since Co., the Ltd., temperature Shanghai, only China) affected were thestirred mobility for 4 hof and the ultrasonicatedGPANI particles. for 30The min resistance to make of a homogenousthe GPANI particles solution. can The be weight expressed ratio as of Equationthe GPANI, (1), PVB, where and RGPANI absolute represents ethyl alcoholthe resistance was 0.1:5:100. of the GPANI The concentration particle, ρ is of the GPANI resistivity showed of the no GPANI, σ is the conductivity of the GPANI, μ is the mobility of the GPANI, n is the number of influence on the sensitivity of the temperature sensor, since the temperature only affected the mobility carriers, L is the length of the GPANI particle, and S is the cross-section area of the particle. The of the GPANI particles. The resistance of the GPANI particles can be expressed as Equation (1), where resistance of the sensor can be defined as Equation (2), where D represents the concentration of the RGPANI represents the resistance of the GPANI particle, ρ is the resistivity of the GPANI, σ is the GPANI particles. The sensitivity of the sensor can be expressed as Equation (3), where Ssensor is the conductivity of the GPANI, µ is the mobility of the GPANI, n is the number of carriers, L is the length sensitivity of the sensor, Rs is the resistance of the sensor, Rs0 is the initial resistance of the sensor, and of the GPANI particle, and S is the cross-section area of the particle. The resistance of the sensor can be μ0 represents the initial mobility of the GPANI. According to Equation (3), the sensitivity Ssensor only defined as Equation (2), where D represents the concentration of the GPANI particles. The sensitivity depends on the mobility (μ) and the number of carriers (n). Both μ and n are affected by temperatures. of the sensor can be expressed as Equation (3), where Ssensor is the sensitivity of the sensor, Rs is the Therefore, the sensor can be used for temperature detection, and the sensitivity of the sensor is not resistance of the sensor, Rs0 is the initial resistance of the sensor, and µ0 represents the initial mobility related to the concentration of GPANI on the device. of the GPANI. According to Equation (3), the sensitivity Ssensor only depends on the mobility (µ) and the number of carriers (n). Both µ and n areLL affected by L temperatures. Therefore, the sensor can be RGPANI ===ρ , (1) used for temperature detection, and the sensitivityS σS ofμ theneS sensor is not related to the concentration of GPANI on the device. L L L RGPANI = ρ =DL = , (1) RDRs=× GPANIS = σS , µneS (2) μneS DL Rs = D RGPANI = , (2) DL× DL µneS − − − RRss0μneS DLμ00neSDLμ00n μn Ssensor ==Rs Rs0 µneS − µ0 =n0eS µ0n0. µn (3) S = s0 = DL = − sensor R − DL μn . (3) Rs0 µn μ00neSµ0n0eS

Figure 2. The fabrication process of the sensor. Figure 2. The fabrication process of the sensor.

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When the ratio of GPANI increased, the transparency decreased. When the ratio of GPANI decreased, the resistance of the sensor increased. To get a high transparency and low resistance, we used 0.1 wt % GPANI. Meanwhile, as a large thickness of PVB will decrease the transparency, the PVB of 5 wt % was used. The ethyl alcohol was served as a third solvent, because GPANI particles can be well dispersed in alcohol. Figure 3 is the scanning electron microscopy (SEM) (WT2ZSEM01, Zeiss Co., Ltd, Jena, Germany) image, and the GPANI particles are uniformly distributed in the PVB film. Sensors 2019, 19, 4105 4 of 8 The mixture was then uniformly coated on an ITO-PET substrate with a Mayer rod (Bar No. 30, R.D.SPECIALTIES. Inc., New York, NY, United States) to make a wet GPANI–PVB film. The PET film had a Whenthickness the of ratio 0.13 of mm, GPANI an area increased, of 75 mm the × 75 transparency mm, and 10 decreased.strip ITO electrodes When the with ratio a width of GPANI of 4 mm.decreased, The electrodes the resistance were parallelly of the sensor aligned increased. on the PET To film. get with a high an transparencyinterval of 1 mm. and The low wet resistance, film of thewe composite used 0.1wt was % then GPANI. annealed Meanwhile, at 80 °C asfor a 15 large min thickness to eliminate of PVBthe alcohol, will decrease and the the thickness transparency, of the drythe composite PVB of 5 wt film % waswas used.2.7 μm. The A ethylsecond alcohol ITO-PET was servedwas then as coated a third directly solvent, on because the film GPANI without particles any spacers,can be welland dispersedthe ITO electrodes in alcohol. on Figurethe top3 PET is the film scanning were aligned electron perpendicularly microscopy (SEM) to the (WT2ZSEM01, electrodes on theZeiss bottom Co., PET Ltd., film. Jena, The Germany) top and image,bottomand substrates the GPANI were particlesfinally fixed are by uniformly bezel tapes distributed at the outside in the peripheryPVB ﬁlm. of the PETs. Then, a temperature sensor with GPANI–PVB composite film was fabricated.

(a) (b) (c)

FigureFigure 3. 3. TheThe scanning scanning electron electron microscopy (SEM) imageimage ofof thethe distribution distribution of of GPANI GPANI in in PVB PVB ﬁlms films on onthe the scale scale of of (a )(a 100) 100µm; μm; (b )(b 20) 20µm; μm; (c )(c 4) µ4m. μm.

3. ResultsThe mixtureand Discussion was then uniformly coated on an ITO-PET substrate with a Mayer rod (Bar No. 30, R.D.SPECIALTIES. Inc., New York, NY, USA) to make a wet GPANI–PVB film. The PET film had a Compared with polyaniline with poor conductivity, the GPANI shows a high conductivity and thickness of 0.13 mm, an area of 75 mm 75 mm, and 10 strip ITO electrodes with a width of 4 mm. good dispersity. The conductivity of GPANI× is sensitive to ambient temperature, which is caused by The electrodes were parallelly aligned on the PET film. with an interval of 1 mm. The wet film of the two competing effects. On the one hand, there are more and more additional carriers when the composite was then annealed at 80 C for 15 min to eliminate the alcohol, and the thickness of the temperature increases. One the other ◦hand, the scattering of the lattice gets more and more intense as dry composite film was 2.7 µm. A second ITO-PET was then coated directly on the film without any the temperature increases, which decreases the mobility of carriers. Generally, only one of these two spacers, and the ITO electrodes on the top PET film were aligned perpendicularly to the electrodes on effects dominates the conductivity of GPANI particles. Under low temperature, the conductivity is the bottom PET film. The top and bottom substrates were finally fixed by bezel tapes at the outside dominated by the first effect. The mobility, as well as the numbers of carriers, increases dramatically, periphery of the PETs. Then, a temperature sensor with GPANI–PVB composite film was fabricated. due to more and more absorbed energy. As the temperature increases to a threshold temperature, the mobility3. Results decreases and Discussion dramatically due to lattice scattering, and the second effect dominates. The GPANI particles were embedded in and penetrated through the transparent PVB films, leading to a top-downCompared conduction with polyaniline of sensors. with Since poor GPANI conductivity, particles the have GPANI slight shows deformations a high conductivity under low and pressure,good dispersity. the resistance The conductivity is subject ofto GPANIboth the is sensitivepressure toand ambient temperature. temperature, Therefore, which the is caused sensor by with two onecompeting single active effects. layer On thecan onebe used hand, to there detect are pressure more and as morewell as additional temperature carriers under when 30 thekPa. temperature When the pressureincreases. is Onelarger the than other 30 hand, kPa, thethere scattering is no furt ofher the latticedeformation gets more in GPANI and more particles, intense asand the the temperature pressure hasincreases, almost whichno effect decreases in the the resistance mobility of of sensors. carriers. The Generally, resistance only of one the of sensor these two is only effects affected dominates by temperatures,the conductivity which of GPANI leads to particles. a single temperature Under low temperature,measurement. the conductivity is dominated by the firstThe effect. measurement The mobility, was as performed well as the numbersby applying ofcarriers, certain pr increasesessure on dramatically, the sensor duewith to a morepressure and metermore absorbed(ZQ-20B-1, energy. Zhiqu As Co., the Dongguan, temperature China). increases We to also a threshold used a temperature,temperature thecontroller mobility (HCS302, decreases Instecdramatically Co., Boulder, due to CO, lattice USA) scattering, to apply and different the second temperatures. effect dominates. The resistance The GPANI was measured particles by were a multimeterembedded (Vx890C+, in and penetrated Victory throughCo., Shenzhen, the transparent China) at PVB a humidity films, leading of 75%. to a top-down conduction of sensors.As Figure Since GPANI4a shows, particles the resistance have slight decreased deformations in the under temperature low pressure, range the of resistance 25–80 °C is at subject 75% humidity,to both the since pressure the absorbed and temperature. energy accelerated Therefore, thethe sensor carriers with and one increased single active the number layer can of be carriers. used to detect pressure as well as temperature under 30 kPa. When the pressure is larger than 30 kPa, there is no further deformation in GPANI particles, and the pressure has almost no effect in the resistance of sensors. The resistance of the sensor is only affected by temperatures, which leads to a single temperature measurement. The measurement was performed by applying certain pressure on the sensor with a pressure meter (ZQ-20B-1, Zhiqu Co., Dongguan, China). We also used a temperature controller (HCS302, Sensors 2019, 19, 4105 5 of 8

Instec Co., Boulder, CO, USA) to apply diﬀerent temperatures. The resistance was measured by a multimeterSensors 2019, 19 (Vx890C, x FOR PEER+, Victory REVIEW Co., Shenzhen, China) at a humidity of 75%. 5 of 8 Sensors 2019, 19, x FOR PEER REVIEW 5 of 8 As Figure4a shows, the resistance decreased in the temperature range of 25–80 ◦C at 75% humidity, sinceWe did the not absorbed measure energy the acceleratedperformance the of carriers the temp anderature increased sensor the above number 80 of °C, carriers. because We the did PET not We did not measure the performance of the temperature sensor above 80 °C, because the PET measuresubstrate theshows performance irreversible of deformation the temperature above sensor that temperature. above 80 ◦C, As because the GPANI the PET particles substrate show shows weak substrate shows irreversible deformation above that temperature. As the GPANI particles show weak irreversibledeformations deformation under 30 kPa, above the resistance that temperature. varies from As the0–30 GPANI kPa at particlesthe same showtemperature, weak deformations which leads deformations under 30 kPa, the resistance varies from 0–30 kPa at the same temperature, which leads underto a detection 30 kPa, theof both resistance pressure varies and from temperature. 0–30 kPa atWhen the samethe pressure temperature, is higher which than leads 30 kPa, to a detectionthere are to a detection of both pressure and temperature. When the pressure is higher than 30 kPa, there are ofno bothmore pressure deformations and temperature.in the GAPNI Whenparticles. the The pressure resistance is higher stays thannearly 30 stable, kPa, there and only are nochanges more no more deformations in the GAPNI particles. The resistance stays nearly stable, and only changes deformationswith temperature, in the which GAPNI allows particles. for a single The resistance detection staysof temperature. nearly stable, The and normalized only changes resistance with with temperature, which allows for a single detection of temperature. The normalized resistance temperature,change (R−R0)/ whichR0 is shown allows in for Figure a single 4b. detection of temperature. The normalized resistance change change (R−R0)/R0 is shown in Figure 4b. (R R )/R is shown in Figure4b. − 0 0

(a) (b) (a) (b) Figure 4. (a) The resistance and (b) the normalized resistance change, (R−R0)/R0, under different Figure 4.4. ((aa)) TheThe resistanceresistance andand ( b(b)) the the normalized normalized resistance resistance change, change, (R (R−RR0)/)/R0,, under didifferentﬀerent pressures, at 25–80 °C on temperature sensors and with humidity at 75% in− (5,5).0 0 pressures, atat 25–8025–80 ◦°CC onon temperaturetemperature sensorssensors andand withwithhumidity humidityat at75% 75%in in(5,5). (5,5).

The sensor allows multi-points measurements, since the normalized resistance change (R−R0)/R0 The sensor allows multi-pointsmulti-points measurements,measurements, since since the the normalized normalized resistance resistance change change (R (R−RR00))//RR00 is consistent on different points in the sensor array, as shown in Figure 5a. Figure 5b shows− the is consistentconsistent on on di differentfferent points points in thein the sensor sensor array, ar asray, shown as shown in Figure in Figure5a. Figure 5a.5 Figureb shows 5b the shows average the average resistance change in (5,5), (3,9), (8,9), and (9,4) under different pressures and different resistanceaverage resistance change in change (5,5), (3,9), in (5,5), (8,9), (3,9), and (9,4)(8,9), under and di(9,4)fferent under pressures different and pressures different temperatures,and different temperatures, and the small error bar represents the variance of the average resistance changes, andtemperatures, the small errorand barthe representssmall error the bar variance represents of the th averagee variance resistance of the changes,average whichresistance shows changes, a high which shows a high consistency on different points of the sensor array. consistencywhich shows on a dihighfferent consistency points of on the different sensor array. points of the sensor array.

(a) (b) (a) (b) Figure 5. ((a)) The The resistance resistance change in (5,5 (5,5),), (3,9), (8,9), and (9,4) at 25–80 °C,C, 70 70 kPa, kPa, and and 75% 75% humidity; humidity; Figure 5. (a) The resistance change in (5,5), (3,9), (8,9), and (9,4) at 25–80 °C,◦ 70 kPa, and 75% humidity; ((b)) thethe averageaverage resistanceresistance changeschanges underunder 3,3, 10,10, 30,30, andand 7070 kPakPa atat 25–8025–80 °C,C, withwith aa humidityhumidity ofof 75%.75%. (b) the average resistance changes under 3, 10, 30, and 70 kPa at 25–80 ◦°C, with a humidity of 75%. The response of thethe sensorsensor underunder bendingbending isis shownshown inin FigureFigure6 6a.a. TheThe sensorsensor showedshowed aa similarsimilar The response of the sensor under bending is shown in Figure 6a. The sensor showed a similar response when the bendingbending radiusradius waswas 2.52.5 andand 3.253.25 cm.cm. The humidityhumidity shows little inﬂuenceinfluence on thethe response when the bending radius was 2.5 and 3.25 cm. The humidity shows little influence on the sensor, forfor thethe sensorsensor has has an an encapsulation, encapsulation, with with bezel bezel tapes tapes bonding bonding the the top top and and bottom bottom PET PET ﬁlms films at sensor, for the sensor has an encapsulation, with bezel tapes bonding the top and bottom PET films theat the outside outside periphery; periphery; in addition,in addition, the the PET PET is waterproof,is waterproof, as shownas shown in Figurein Figure6b. 6b. at the outside periphery; in addition, the PET is waterproof, as shown in Figure 6b.

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(a) (b)

Figure 6. The response of the sensor at 25–80 °C◦C in (5,5) ( a)) with with a a different diﬀerent bending radius at 70 kPa, and (b)) withwith didifferentﬀerent humidityhumidity atat 33 kPa.kPa.

As thethe resistance resistance is aisff ectedaffected by bothby both temperature temperature and pressure and pressure change change under 30 under kPa, we30 proposedkPa, we aproposed method toa getmethod the target to get temperature the target and temperat pressure.ure First, and a modelpressure. showing First, thea relationshipmodel showing between the temperature,relationship between pressure, temperature, and resistance pressure, can be obtained and resistance by fitting can the be measured obtained data by fitting with multiple the measured linear regressiondata with functionmultiple inlinear simulation regression software function Matlab. in simulation The model software shows a Matlab. quadratic The equation model ofshows binary a variablesquadratic temperature equation of binary (T) and variables pressure temperature (P), as Equation (T) and (4) shows. pressure Then (P), the as Equation measured (4) resistances shows. Then can bethe substituted measured resistances into Equation can (1) be tosubstituted get the corresponding into EquationT (1)and toP get. For the example, corresponding in a heating T and process P. For withexample, constant in a pressure,heating process since the with conduction constant of pre temperaturessure, since takes the aconduction certain amount of temperature of time, we cantakes get a Ncertainresistances amount [R of0 ... time,, Ri we... ,canRN ],get where N resistancesRi represents [R0…, the Ri…, resistance RN], where at an Ri intermediate represents the temperature. resistance Then,at an intermediate we can substitute temperature. these N resistances Then, we intocan Equationsubstitute (4) these to get N theresistances target pressure into Equation and temperature. (4) to get the target pressure and temperature. (R R)/R = 2.9983 10 4T2 + 1.1206 10 4P2 2.8391 10 6TP + 0.0429T 0.0115P 0.6011. (4) 0 0 −−−42− 42− 6 − (RRR00−=−×+×−×+−− ) /− 2.9983− 10 T× 1.1206 10 P× 2.8391− 10 TP× 0.0429 T 0.0115− P 0.6011.− (4) The temperature coefficient of resistivity (TCR) α is defined in Equation (5). Our temperature The temperature coefficient of resistivity (TCR) α is defined in Equation (5). Our temperature sensor shows a negative TCR of 1.2% C 1 at 25–80 C, which is higher than the temperature sensors sensor shows a negative TCR of− −1.2% ◦°C−1 at 25–80 °C,◦ which is higher than the temperature sensors with Pt and PEDOT:PSS–carbon nanotube (CNT). The TCR of the sensor is also larger than that of with Pt and PEDOT:PSS–carbon nanotube (CNT). The TCR of the sensor is also larger than that of the sensor based on polyaniline nanofibers [4]. Moreover, our temperature sensor has a much high the sensor based on polyaniline nanofibers [4]. Moreover, our temperature sensor has a much high transparency, as shown in Table1. transparency, as shown in Table 1.

1 1 RT()R−( RTT () 0 )R(T0) α α= = × − . . (5) R0(T0) ×TT− T0 T0 R(T ) −

Table 1.1. Comparation of the sensor in this studystudy andand sensorssensors inin priorprior works.works.

Materials of Active Layer in Sensors |TCR|(%·°C1 −1) Transparency Materials of Active Layer in Sensors |TCR|(% ◦C− ) Transparency GPANI–PVB ·1.2 >80% GPANI–PVB 1.2 >80% PtPt [ 16[16]] 0.385 0.385 No No PEDOT:PSS-CNTPEDOT:PSS-CNT [12 [12,13],13] 0.2–0.6 0.2–0.6 No No PolyanilinePolyaniline [4 [4]] 1.0 1.0 No No MultiwalledMultiwalled carboncarbon nanotube nanotube [17 [17]] 0.07 0.07 No No

The sensor shows aa littlelittle hysteresishysteresis in in a a cycle cycle of of heating heating and and cooling cooling from from 25 25 to to 80 80◦C, °C, as as shown shown in Figurein Figure7a. Compared7a. Compared with manywith previousmany previous works, the works, temperature the temperature sensor shows sensor a high shows transparency a high (transparency>80%). The high (>80%). transparency The high contributes transparency to both cont theributes small to diameters both the of small GPANI diameters particles (10–20of GPANIµm) andparticles the transparent (10–20 μm) thin and PVB the ﬁlms transparent (2.7 µm). thin As a result,PVB films the sensor (2.7 μ showsm). As potential a result, for the being sensor integrated shows withpotential other for interactive being integrated applications, with other such asinteractive displays. applications, such as displays.

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(a) (b)

FigureFigure 7.7. ((aa)) TheThe resistanceresistance inin aa cyclecycle ofof heatingheating andand coolingcooling inin (5,5)(5,5) atat 25–8025–80◦ °C.C. ((bb)) TheThe resistanceresistance ofof thethe temperaturetemperature sensor sensor with with ITO–glass ITO–glass substrates substrates in in (5,5) (5,5) at at 30–170 30–170◦ °C.C.

ToTo confirmconfirm thethe mechanismmechanism ofof thethe temperaturetemperature sensorsensor basedbased onon GPANI-PETGPANI-PET films,films, wewe replacedreplaced ITO–PETITO–PET substratessubstrates withwith ITO–glassITO–glass substrates,substrates, whichwhich cancan sustainsustain temperaturestemperatures higherhigher thanthan 100100 ◦°C.C. Thethe performance of the sensor is showed in Figure7 7b.b. TheThe resistanceresistance decreaseddecreased atat lowlow temperatures,temperatures, andand beganbegan to to increase increase until until the temperaturethe temperature became beca higherme higher than 120 than◦C, 120 which °C, implies which thatimplies the absorbed that the energyabsorbed increased energy the increased conductivity the conductivity of carriers below of carrier 120 s◦ C,below and then120 °C, lattice and scattering then lattice began scattering to dominate began andto dominate decreased and the decreased conductivity theof conductivity GPANI particles of GP aboveANI particles 120 ◦C. Therefore,above 120 the°C. GPANI–PVBTherefore, the composite GPANI– filmPVB can composite detect a film larger can temperature detect a larger range temperature when using range electrodes when with using higher electrodes heat resistance. with higher heat resistance. 4. Conclusions 4. ConclusionsIn summary, we proposed a temperature sensor based on GPANI–PVB composite film. The sensor shows a negative temperature coefficient of resistivity ( 1.2% C 1) in the temperature range of In summary, we proposed a temperature sensor based− on·◦ GPANI–PVB− composite film. The 25–80sensor◦ C.shows The a resistance negative oftemperature the GPANI–PVB coefficient film of decreased resistivity below (−1.2%·°C 120 ◦−1C) in and the increased temperature when range the temperatureof 25–80 °C. The was resistance higher than of the GPANI–PVB threshold temperature, film decreased which below enables 120 °C the and detection increased of a when broader the temperaturetemperature rangewas higher by using than substrates the threshold with highertemper heatature, resistance. which enables The temperature the detection sensor of a allowedbroader detectiontemperature of both range pressure by using and substrates temperature with under higher low heat pressure resistance. (<30 kPa),The temperature and integrated sensor the pressure allowed anddetection temperature of both sensors pressure into and one temperature single layer. under Meanwhile, low pressure the sensor (<30 can kPa), only and detect integrated temperature the pressure under high pressure ( 30 kPa), because the GPANI particle shows no more deformation under such pressure. and temperature≥ sensors into one single layer. Meanwhile, the sensor can only detect temperature Moreover,under high high pressure transparency (≥30 kPa), (> 80%)because made the the GPANI temperature particle sensor shows a no potential more deformation candidate for under wearable such devices,pressure. which Moreover, can be high well transparency integrated with (>80%) other made visual, theinteractive temperature applications, sensor a potential such as candidate displays. for wearable devices, which can be well integrated with other visual, interactive applications, such as Author Contributions: Formal analysis, J.L.; investigation, J.P., S.L., and J.L.; software, J.P. and H.Z.; validation, J.P.;displays. writing–original draft, J.P. Funding:Author Contributions:This research wasFormal funded analysis, by National J.L.; investigation, Natural Science J.P., S.L., Foundation and J.L.; of software, China under J.P. and Grant H.Z.; No. validation, 61727808 andJ.P.; 61775135.writing – original draft, J.P. Conflicts of Interest: The authors declare no conflict of interest. Funding: This research was funded by National Natural Science Foundation of China under Grant No. 61727808 and 61775135. References Conflicts of Interest: The authors declare no conflict of interest. 1. Park, J.-J.; Taya, M. Design of Micro-Temperature Sensor Array with Thin Film Thermocouples. J. Electron. ReferencesPackag. 2005 , 127, 286. [CrossRef] 2. Ramakrishna, M.V.S.; Karunasiri, G.; Neuzil, P.; Sridhar, U.; Zeng, W.J. Highly sensitive infrared temperature 1. Park, J.-J.; Taya, M. Design of Micro-Temperature Sensor Array with Thin Film Thermocouples. J. Electron. sensor using self-heating compensated microbolometers. Sens. Actuators A Phys. 2000, 79, 122–127. [CrossRef] 3. Shao,Packag. L.-Y.; 2005 Dong,, 127, X.;286, Zhang, doi:10.1115/1.1997157. A.P.; Tam, H.-Y.;He, S. High-resolution strain and temperature sensor based on

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