Sensor and Circuit Solutions for Organic Flexible

A.K.M. Mahfuzul Islam1,2, Hiroshi Fuketa1,3, Koichi Ishida1,4, Tomoyuki Yokota1,2, Tsuyoshi Sekitani1,2,5, Makoto Takamiya1,2, Takao Someya1,2, and Takayasu Sakurai1,2

1 University of Tokyo, Tokyo, Japan 2 JST/ERATO, Tokyo, Japan

3 Now with National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan 4 Now with Dresden University of Technology, Dresden, Germany

5 Now with Osaka University, Osaka, Japan

Abstract

This paper describes several sensor and circuit solutions for organic flexible electronic devices. Organic field effect transistors (OFET) enable low-cost, high-flexibility and large-area which can be utilized to implement smart sensors. These sensors can be in contact with the physical objects of any shape. Furthermore, as the devices are very light and flexible, they can even be placed in contact with the human skins. However, design of organic flexible circuits poses great challenges because of several intrinsic properties of organic materials. Thus, newer circuit techniques need to be adopted for robust operation. We give an overview of various sensor and circuit techniques developed in our research group along with other literature reports.

1. Introduction With the emergence of IoT (Internet of Things) era, more and more physical objects are being connected to the internet providing information of our surrounding to enable a more secure and comfortable life. The gap between physical world and cyber world is being narrowed continuously. Newer devices are being investigated that can provide tighter integration between the two worlds. In order to gather information from the physical world, various forms of devices are required including devices capable of sensing large area. Especially for health-care and bio-medical devices, flexible and large area devices that can be attached to human skin for example will provide greater options. Organic transistors can be an attractive option to fulfil the above needs.

Organic transistors can be fabricated on a plastic or a glass sheet, and because of the material properties such transistors are not as rigid as the silicon transistors. Thus, flexible, large- area, low-cost and transparent electronic devices become possible. However, because the materials used for organic transistors are inherently different from silicon, newer circuit solutions need to be established to realize smart sensors with various functions integrated [1, 2].

In Sec. 2, we present several examples of sensor solutions developed in our group. Then, we discuss some digital and analog circuit solutions to realize the sensors in Secs. 3 and 4. Finally, Sec. 5 concludes the paper.

2. Sensor Solutions As the key feature of organic electronics is its flexibility, they are most suitable for sensor devices that require flexibility. Fig. 1 shows four such sensor devices to show the capability and versatility of organic electronics.

Fig. 1(a) shows an example of energy harvesting device realized on organic electronics [3]. This is an insole pedometer sheet that is self-powered with energy harvesting capability. The device uses a polyvinylidene difluoride (PVDF) sheet as piezoelectric energy harvester. Then all-pMOS organic circuits are used to utilize the harvested energy and count the stepping. Pseudo-CMOS circuit techniques are used to reshaping the pulses generated from the energy harvester and to implement the counters.

Fig. 1(b) shows an electromyogram measurement sheet which can be used for prosthetic hand control [4]. The key feature is to amplify the small signals from the electrodes. Details of the amplifier techniques will be discussed in Sec. 4.

Fig. 1(c) shows a flexible fever alarm sheet with energy harvesting capability from a flexible solar cell sheet for medical application [5]. The device features an active voltage limiter circuit to protect the device from excess voltage levels. The device also features a piezoelectric speaker sheet for alarming. As the whole system consists of flexible sheets, the device can be used as an armband so that the need for constant monitoring of fever manually is eliminated.

Piezoelectric energy harvester (PVDF sheet)

2V organic PMOS circuits

8 x 8 EMG electrode array

40mm

45

mm

EMG electrodes

Electrode pitch = 5mm

8 x 2 amplifier arrayStacked 2 sheets

1m-thickultra-flexible PEN film

Surface electromyogram measurement sheet (SEMS)

(0.7mm x 0.7mm)

Prosthetic hand

Organic transistors

Patient

Temperature detector(15mm x 5mm)

Organic circuits

(under speaker)

Solar cells (11cm x 13cm)

Piezoelectricspeaker

(10cm x 5cm)

30 cm

Fever alarm armband

All flexible components 18

cm

Underarm temperature is measured.

Diaper

78 m

m

40 mm

53 mm

Organic circuits

12.5m-thickflexible PCB(polyimide)

Sensor electrodes

(a) An insole pedometer sheet withpiezoelectric energy harvester [3].

(b) An electromyogram measurement sheet with integrated amplifiers [4].

(c) Fever alarm sheet with energy harvesting from solar cell sheet [5].

(d) Wireless wet sensor sheet with wireless power and data transmission capability [6].

Figure 1. Various sensor applications developed in our group.

Invited Paper 47-1 / A.K.M. M. Islam

SID 2016 DIGEST • 629ISSN 0097-966X/16/4702-0629-$1.00 © 2016 SID

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47-1 / A.K.M. M. Islam Invited Paper

630 • SID 2016 DIGEST

workaround is to use the transistors in the triode region as resistors [4]. Fig. 6 shows IDS–VDS characteristics of pMOS transistor under large and small VDS regions. Fig. 6(b) shows that pMOS transistor can be used as resistors under small VDS region. Although this workaround has some limitations such as the reduced operating range, high resistance values can be realized because of the intrinsically lower mobility.

Inductors are essential components especially for wireless communication. Because of low material properties, on-sheet inductors are not available. As an alternative, flexible printed circuit board (PCB) can be used where the PCB sheet and the organic sheet are stacked together to realize flexible devices. A successful operation of wireless power and data transmission using stacked inductor sheet is demonstrated in [4]. On-sheet capacitors can be realized by MIM capacitors. MIM capacitor with capacitance of 700 nF/cm2 is reported in [10].

4.2 Floating Voltage Regulation Energy harvesting devices such as a solar cell may generate excessive power which may damage the large organic devices. Thus, voltage regulation is required for devices with energy harvesting capability. Conventionally, a diode clamp is used to limit the voltage level. However, because of the material properties, voltage references and Zener diodes are not available. As an alternate solution, a transistor-based active voltage limiter circuit is proposed for voltage regulation [5]. Fig. 7 shows the schematic of the proposed voltage limiter. Here, the first four

pMOS transistors from the left realizes a constant voltage generator. Followed by two CMOS inverters and a power pMOS transistor, the circuit acts as a diode but with higher current sensitivity. The right hand figure shows the I–V characteristics of the active voltage limiter along with the conventional diode connected pMOS transistor. The active voltage limiter shows higher sensitivity than the conventional diode clamp.

4.3 Amplifier with AC Coupled Load Amplifier is a basic circuit block especially for smart sensor devices as the sensed signals are often small analog signals. Conventional diode connected load does not give the required gain for OFET based amplifiers. In order to improve amplifier’s gain and performance, an AC-coupled load technique is proposed [4]. Fig. 8(a) shows an amplifier schematic, and Fig. 8(b) shows the gain of the amplifier with two types of load. One is the diode connected pMOS load and the other is the proposed AC-coupled load. As explained in [4], amplifier with AC-coupled load shows larger gain which is demonstrated in the figure. However, AC-coupled load has a disadvantage that the gain decreases at low frequency region. The lower cut-off frequency is determined by R2C2, thus their values need to be set to meet application requirements.

4.4 pMOS-only Rectifier For wireless power and data transmission transponders, rectifying circuits are required. Fig. 9 shows a pMOS-only rectifier circuit [11]. Cross-coupled topology is used here instead of conventional diode-connected topology to improve the driving current. Thus,

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respectively. (a) Large VDS, (b) Small VDS.

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Invited Paper 47-1 / A.K.M. M. Islam

SID 2016 DIGEST • 631

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