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