Flexible and printed organic transistors: From materials to integrated circuits

Journal Pre-proof Flexible and Printed Organic Transistors: from Materials to Integrated Circuits

Hiroyuki Matsui, Yasunori Takeda, Shizuo Tokito PII:

S1566-1199(19)30451-3

DOI:

https://doi.org/10.1016/j.orgel.2019.105432

Article Number:

105432

Reference:

ORGELE 105432

To appear in:

Organic Electronics

Received Date:

11 June 2019

Accepted Date:

24 August 2019

Please cite this article as: Hiroyuki Matsui, Yasunori Takeda, Shizuo Tokito, Flexible and Printed Organic Transistors: from Materials to Integrated Circuits, Organic Electronics (2019), https://doi.org /10.1016/j.orgel.2019.105432

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

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Organic Electronics Review

Flexible and Printed Organic Transistors: from Materials to Integrated Circuits Hiroyuki Matsuia, Yasunori Takedaa and Shizuo Tokitoa,* a

Research Center for Organic Electronics (ROEL), Yamagata University, Yonezawa, 992-

8510, Japan

ABSTRACT This article reports on the recent progress in the research and development of flexible and printed organic thin-film transistor (OTFT) devices, including organic materials, fabrication processes, electronic devices, and integrated circuits, and highlights their application to healthcare sensors. The fabrication process flow for printed OTFT devices is described with various printing methods such as inkjet printing, dispensing, reverse-offset printing, and spincoating. Silver nanoparticle inks are commonly used to form interconnect and electrode layers with inkjet printing and reverse-offset printing. Highly crystalline small-molecule organic semiconductors are patterned on the printed source and drain electrodes. Various integrated circuits such as inverters, D flip-flops (D-FFs), and operational amplifiers (OPAs) are fabricated based on the printed OTFT devices, and their electrical performance is discussed in detail. The potential applications of printed integrated circuits to biosensors are successfully demonstrated on plastic film substrates, enabling various flexible and printed sensor systems for potential use in the Internet of things (IoT) society.

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1. Introduction Organic semiconductors are intrinsically printable at low temperatures, making them compatible with low-cost plastic film substrates. Therefore, printed organic electronics enables the environmentally-friendly manufacturing of inexpensive and flexible electronic devices. Recently, flexible displays [1, 2], radio-frequency identification (RFID) tags [3, 4], smart labels [5, 6] and a variety of sensors [7-9] based on thin-film transistor (TFT) devices have been fabricated on thin plastic film substrates with various printing processes that made excellent progress in research and development. In particular, TFT devices based on organic semiconductors (OSCs) can be fabricated at low temperatures and are more compatible with printing methods than inorganic semiconductors. With printing methods, flexible and printed organic TFT (OTFT) devices and integrated circuits can be realized for various emerging applications. We anticipate the potential use of these flexible and printed organic electronics to the internet of things (IoT) society, which is expected to connect a vast number of electronic devices to cloud computing networks. Because flexible and printed electronics make it possible to mount sensors and wireless communication functions on virtually anything, such as in wearable sensors for healthcare, as well as in factories, automobiles, for item tracking, environmental and agriculture uses. It will produce new value that is difficult to provide using conventional electronics technology. Several research groups have previously reported very high carrier mobilities of up to 10 cm2/(Vs) for solution-processed or printed OTFT devices [10-12]. However, only the OSC or gate dielectric layer was printed or coated on rigid substrates, such as silicon wafer or glass plate, and other essential steps such as the formation of the interconnect, gate, source/drain electrodes were vacuum-deposited and patterned with photolithography. Realizing printed OTFT circuits on thin plastic film substrates, such as those shown in Fig. 1a and 1b, requires a variety of printable materials including not only semiconductors but also metals, insulators, and polymers for use as planarization or passivation layers.

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Fig. 1. a), b) Photographs of flexible and printed organic integrated circuits fabricated on a plastic film substrate. c) Layer thickness and resolution capabilities for various printing methods.

Further progress in the development of printing technologies is also significant to finely print uniform interconnect, electrode, insulator and semiconductor layers. In printed organic electronics, the printing methods used are significant because fine patterns with various thickness can be formed from the solutions or inks containing functional materials. There are a number of printing methods that apply to electronic device fabrication, such as inkjet printing, screen printing, gravure offset printing, flexographic printing and reverse-offset printing [13], as well as spin coating and slot die coating, as shown in Fig. 1c. We have primarily employed inkjet printing technology for the patterning of interconnects and electrodes, but are recently more focused on reverse-offset printing for finer patterning. Required ink viscosities for those printing methods are very different, 5-10 mPas for inkjet printing, 100 mPas for gravure offset printing and 1000-10000 mPas for screen printing. Therefore, the ink formulation is an essential part of printing technology.

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There are few reports on fully-printed OTFT devices or those with printed electrodes. Also, the electrical performance of printed OTFT devices is generally inferior to those formed using vacuum-processed or photolithographically patterned devices. In this research field understanding how to realize practical levels of electrical performance in flexible printed OTFT devices and integrated circuits is essential. These issues have been gradually resolved by using newly printable materials and optimizing fabrication conditions. One of the most important uses of OTFT devices is in integrated circuits for potential use in various electronic applications. In printed integrated circuits, reproducibility and stability of electrical characteristics in the OTFT devices have been significant challenges. Also, circuit design and simulation are recently becoming important, especially since OTFT device reproducibility has been greatly improved. In this review paper, we will report briefly on printable electronic materials, printed OTFT devices and integrated circuits and their application to biosensors. Among the printable materials described are silver nanoparticles and organic semiconductors. The fabrication of printed OTFT devices with the inkjet printing method are also described and their typicallyachieved electrical characteristics are reported. A variety of integrated circuits based on printed OTFT devices are demonstrated in digital and analog circuit applications by using both inkjet and reverse-offset printing methods. Potential integrated circuit implementation will also be described in potentiometric and amperometric biosensor applications to realize flexible printed biosensor systems.

2. Printable Functional Materials 2.1. Metal Nanoparticle Layers The most common printable conductive materials are silver (Ag) pastes, which are widely used in solar cells and touch panels [14]. More recently, Ag nanoparticle inks have become key materials in the fabrication of electrode and interconnect layers for printed electronics because they can be sintered at lower temperatures and reach lower resistivities than conventional silver pastes. Various types of Ag nanoparticle inks for inkjet printing are now commercially

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available, e.g. NPS-JL (Harima Chemicals Group, Inc.) and F-Nano IJ100/200 (Future Ink Corporation). In transistor applications, not only is low resistivity necessary but also low contact resistance between the printed Ag electrodes and the organic semiconductor layer is significant to realize the high-performance OTFT devices. We developed an Ag nanoparticle ink that was optimized for OTFT device applications [15]. The Ag nanoparticles ink is synthesized from a silver complex, and the surface of Ag nanoparticle is chemically modified with small organic molecules to stabilize the dispersion in a solution. The diameter of Ag nanoparticles is around 15 nm and these are quite uniform. Fig. 2 shows the scanning electron microscope (SEM) images of Ag thin film prepared form Ag nanoparticle ink before and after thermal sintering, and the resistivities of ink-jet printed Ag lines sintered by thermal annealing for 30 min. The sintering phenomena is observed in the nanoparticle layer after thermal annealing and a continuous Ag layer is formed. Low resistivities of about 9 µΩcm can be obtained at 120 ˚C, which is much lower than that of conventional Ag pastes. Photonic sintering using a Xenon flash lamp is effective for rapid sintering with less damage to plastic film substrates. With this sintering method, low resistivities of 5 µΩcm can be obtained by irradiation with a light intensity of 5 J/cm2 in ambient conditions. (a) (a)

100nm

(b) (b)

Resistivity (cm)

Printed Ag Electrodes Sintering

(c)

100nm

Sintering temperature (C)

Fig. 2. SEM images of printed Ag nanoparticle films a) before and b) after thermal sintering at 100 C for 30 min. c) Sintering temperature dependence of resistivity of the Ag films.

Copper (Cu) nanoparticle inks are also practically promising because of their lower costs. However, there are few reports on printed OTFT devices that use printed Cu layers for the

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source and drain electrodes. Previously, we have reported on printed OTFT devices with printed Cu electrodes [16]. By optimizing the photonic sintering process, inkjet-printed Cu electrodes with low resistivities below 10 µΩcm could be achieved, even on plastic film substrates in an air ambient. SEM images and resistivity data are shown in Fig. 3. The sintered Cu layer can be seen in the SEM image. 103

(a)

200 nm (b)

200 nm

Resistivity (μΩ cm)

(c) 102

101

100 2

4

6

8

10

Pulse energy (J/cm2)

Fig. 3. SEM images of copper nanoparticle films: a) before and b) after photonic sintering with a light intensity of 6 J/cm2. c) Pulse energy dependence of resistivity in the copper films [16].

2.2. Dielectric Material Layers Gate dielectric layers can be also fabricated at low temperatures from dielectric polymers solutions and with printing methods [17-20]. By the crosslinking of the dielectric polymers, the gate dielectric layer becomes insoluble in organic solvents used in the OSC solutions, so that the OSC layer can also be produced using printing methods. Crosslinked poly(p-vinyl phenol) (PVP), shown in Fig. 4, has been widely used as the gate dielectric layer for the printed OTFT device fabrication. The PVP become insoluble after the crosslinking with a cross-linker, typically poly(melamine-co-formaldehyde) methylated, by thermal heating. Electrical requirements to the dielectric materials are high breakdown strength and low leakage current in the thin layer prepared from the solution. Optimal thickness is around 200 nm to 500 nm to reduce the operating voltage and avoid the electrical shorting. Recently, a variety of new dielectric materials have been reported as shown in Fig. 4. The cross-linking reaction is initiated by a photonic reaction, or thermal reaction after preparation of the thin films. One of

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the best known photoreactive dielectric materials is a commercially available photoresist, SU8, and a low voltage operation was reported in the printed OTFT device [21]. For the thermal reaction, the crosslinked polymers based on thiol-ene chemistry or a cross-linking reaction of azide-containing and alkyne-containing styrenic polymers were reported previously. These can be used to fabricate the gate dielectric layer by thermal heating at relatively low temperatures below 100˚C.

Fig. 4. Printable dielectric materials for gate dielectric layers. Three materials 1-3 can be prepared by thermal reaction at a relatively low temperature and SU8 (4) can be prepared by photo-chemical reaction with UV light irradiation.

2.3. Organic Semiconductor Layers Solution-processable, e.g., printable organic semiconductor materials are the most critical functional material for realizing high-performance printed OTFT devices. A variety of p-type and n-type organic semiconductors have been reported in both small-molecule and polymer varieties, as shown in Fig. 5. These molecules possess π-conjugated chemical structures which have a delocalized electron system. The famous organic semiconductors are a polymeric type, poly(3-hexylthiophene) (P3HT) and its derivatives, which are solution-processable due to flexible long alkyl chains in thiophene rings [22]. Although the mobility and current on/off

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ratio are typically below 0.1 cm2/(Vs) and below 106, respectively, these previous reports expected a possibility of solution-processed or printed OTFT devices and printed integrated circuits which can be fabricated without vacuum deposition and photolithography process. After that, the mobility in the polymer-based OTFT devices was improved up to 1 cm2/(Vs) by modifying the chemical structure of the polymer backbone [23]. On the other hand, many types of excellent small-molecule semiconductors have been reported, such as TIPS-pentacene [24] and C8-BTBT [25, 26] and related materials in the past decade. Very high carrier mobilities as high as 10 cm2/(Vs) were published on the printed OTFT devices using those small-molecule semiconductors by several research groups. These results suggest that organic semiconductors have intrinsically high charge mobility in the crystalline thin film prepared by solution process. However, most of these previously reported solution-processed or printed OTFT devices were fabricated on silicon wafers or glass substrates and with source and drain electrodes that were patterned using vacuum-deposition with a shadow mask or using photolithography.

S

O

C16H33

C6H13

S

S

n

S

S

1, P3HT (p)

C8H17

S

C8H17 N

N C8H17

O

S

C8H17

n

O

O 7, PTCDI-C8 (n)

3, C8-BTBT (p)

C16H33

O

2, PBTTT-C16 (p) C10H21

S

Si S

C10H21

S F

C10H21

S

N

S Si

C6H13

S

NC S

N O

S

N

S

C6H13

S 4, diF-TES-ADT (p)

N O

8, DCy-NTCDI (n)

5, C10-DNTT (p)

F

O

6, DTBDT-C6 (p)

S N

S

NC

N C10H21

9, TU-3 (n)

Fig. 5. Printable polymer and small-molecule organic semiconductors. Materials (1-6) are ptype semiconductors and others (7-9) are n-type semiconductors.

We initially employed a polythiophene derivative, PBTTT, in the printed OTFT device fabrication. The highest mobilities of 1 cm2/(Vs) with an excellent on/off current ratios were achieved in the devices where the OSC layer was prepared with the spin-coating method [23].

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In our latest project, we are primarily employing a newly developed p-type organic semiconductor based on 2,7-dihexyldithieno[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene (DTBDT-C6, Fig. 5). Two-dimensional crystal growth and large crystal domains were clearly observed in the thin DTBDT-C6 layers drop-casted onto a substrate. The highest occupied molecular orbital (HOMO) level is around 5.3 eV indicating high stability in air. Recently, the fabrication of OSC layer based on small molecules has been commonly prepared from the blended solution of a small molecule with a polymer such as polystyrene (PS) or polymethylmethacrylate (PMMA) [10, 12, 27-29]. Due to phase separation during the drying process a more high-quality OSC layer can be formed on the polymer layer as shown in Fig. 6. The electrical performance and uniformity in the printed OTFT devices are significantly improved. The blending of a small molecule and a polymer is commonly used in the OSC ink formulation for the printed OTFT device technology. We observed improvements in the crystalline quality of a thin DTBDT-C6 layer. Fig. 6 shows optical microscope photographs for observations made of a drop-casted thin DTBDT-C6 film from the blended solution with polystyrene. The thin DTBDT-C6 film prepared from the blended solution shows more close packing of crystalline domains compared with pure DTBDT-C6 film. (a)

(a)

OSC molecule

PS

(c) (c)

Pure

PS 0.25 wt% 150 µm

Parylene

Boundary

Under layer

(b) (b)

150 µm

Boundary

40 nm 200 nm

Highly crystalline OSC layer

0 nm

300 nm

300 nm 40 nm

PS 0 nm

1 µm

200 nm

0 nm

0 nm

1 µm

Fig. 6. Organic semiconductor layers fabricated from a blended solution of a small-molecular OSC material with a polymer a) before and b) after drying. c) Cross-polarized optical microscope and atomic force microscope images of drop-casted DTBDT-C6 layer on a Parylene layer with source-drain electrodes [29].

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On the contrary, printable n-type organic semiconductors are limited, and their performance levels in the OTFT devices were low and not generally reproducible because they were not stable in air. Representative molecular structures are naphthalene tetracarboxylic diimide (NTCDI) [30] and perylene tetracarboxylic diimide (PTCDI) derivatives [31] (Fig. 5). In the circuit applications, n-type semiconductors are very important because complementary inverters are necessary for low power consumption and high-speed operation. Previously, we developed a new n-type OSC material, TU-X, which is a benzobisthiadiazole-based molecule with strong electron acceptors on both sides (Fig. 5) [32, 33]. This n-type OSC is solutionprocessable and highly crystalline, as well as stable in air. The lowest unoccupied molecular orbital (LUMO) level of TU-3 is as deep as 4.3 eV, which is effective for electron injection from a printed electrode into the TU-3 layer and the OTFT devices provide for excellent stability in air.

3. Inkjet-Printed OTFT Devices and Integrated Circuits 3.1 Printed OTFT Devices The printable functional materials shown above enable the fabrication electronic devices with printing methods such as inkjet printing. Here, we show an example of inkjet-printed OTFT devices [29, 34-36]. Inkjet printing has two primary advantages among the various printing methods. Firstly, inkjet printing is a plateless digital printing method, which is suitable for on-demand fabrication of electronic circuits, making rapid and cost-efficient prototyping of a variety of circuits possible. Secondly, it can utilize inks with low viscosities in the range of 1-30 mPas, which means that thickening agents that usually degrade the electronic properties are rarely required in inkjet printing. Figure 7a shows the construction of an inkjet-printed OTFT device. The source, drain and gate electrodes were each fabricated using a silver nanoparticle ink (NPS-JL, Harima Chemicals Group, Inc.) and employing a commercial inkjet printer (Dimatix Material Printer DMP-2831, Fujifilm). The resulting silver nanoparticle layers can be sintered at 120 C for 1 hour and exhibit resistivities below 10 cm [37]. The work

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function of the printed silver electrodes is 4.8 eV, and can be modified using a variety of selfassembly monolayer (SAM) treatments [38]. For example, surface treatment with pentafluorobenzenethiol (PFBT) can increase work function to 5.3 eV and improve hole injection at the source and drain electrodes in p-type OTFT devices, whereas surface treatment with 4-methylbenzenethiol (4-MBT) can reduce the work function to 4.0 eV and improve the electron injection in n-type OTFT devices. The semiconductor layer was formed by printing a blended

solution

of

2,7-dihexyldithieno[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene

(DTBDT-C6) and polystyrene (PS) with a dispenser (Image Master 350 PC, MUSASHI Engineering). The blending of insulating polymers such as PS, poly--methylstyrene (PMS), and polymethyl methacrylate (PMMA) in channel layers is known to improve the device performance and layer uniformity [27-29, 39]. Semiconductor layers can also be printed using inkjet printing [40]. Figure 7b shows optical microscope images of inkjet-printed OTFT devices. The linewidth and thickness of an inkjet-printed silver electrode layer were 150-200 m and 30-120 nm, respectively, when an ink cartridge with 10-pL drop volumes was used. A dispenser, on the other hand, ejects larger volumes of ink over larger areas more than 0.1 mm2, and helps to obtain sufficiently thick layers from dilute organic semiconductor solutions. The crosspolarized optical microscope image in Fig. 7c highlights the crystal domains in the semiconductor layer and indicates that the size of each crystal domain ranges from 10 m to a few-hundred m, which is larger than domain sizes of < 10 m typically obtained using vacuum deposition. Since the domain sizes are larger than the channel lengths of 8 or 26 m in these devices, the domain boundaries should not affect the electric properties of the devices. The crystal axes are randomly oriented in the substrate plane, and the mobility of organic semiconductors is usually anisotropic [41]. Thus, randomness in the crystal orientation is one of the sources of device-to-device variations in electric properties.

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(a)

(b)

Encapsulation Semiconductor (Teflon) (DTBDT-C6:PS) Bank (Teflon) (c) Dielectric (parylene)

Gate Source, Drain (Ag ink) (Ag ink) Planarization layer (Cross-linked poly(vinyl phenol))

Substrate (glass)

(d)

(e)

Fig. 7. a) Device structure, b) optical, and c) cross-polarized microscope images of inkjetprinted DTBDT-C6 OTFT devices. d) Transfer and e) output characteristics of the OTFT devices. Reproduced with permission [34].

By optimizing the materials used and processing conditions, textbook characteristics of OTFT devices can be obtained via printing methods. Fig. 7d shows the transfer (IDS-VGS) characteristics of the inkjet-printed OTFT devices. Here, IDS is the drain current and VGS is the gate-source voltage. These characteristics exhibit four ideal properties: (1) The |IDS|1/2-VGS curves are highly linear as expected in trap-free OTFT devices. (2) The difference between turnon voltage VON and threshold voltage Vth is as small as 0.4 V. (3) Hysteresis, the difference between the curves in forward and backward sweeps of the gate voltage, is less than 5 mV. (4) The subthreshold swing SS, the gate voltage needed to change the drain current by one decade,

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is 100 mV, which is close to the ideal value of 60 mV at 300 K. Each of these properties indicate that the density of trap states, whether deep or shallow in energy, is quite low in the inkjetprinted OTFT devices [42-44]. Actually, a rough estimation of the density of deep trap states is Nt = (Ci/e2)[eSS/(kBT ln 10) – 1] = 1  1011 cm-2 eV-1 [45, 46]. This value is comparable to the density of deep trap states, 1  1011 cm-2 eV-1, at the high-quality interface of a rubrene single crystal and an amorphous perfluoropolymer, Cytop [47]. The output (IDS-VDS) characteristics in Fig. 7e shows high linearity at low VDS, where VDS is the drain-source voltage. Owing to the low trap density and high linearity in output characteristics, the printed OTFT devices could be operated at a small voltage of -1 V. They can be used even in inverter circuits with ultra-low operating voltages down to 0.3 V [34]. As seen in all of these results, the notion that the printed OTFT devices have too many trap states for either practical applications or scientific study should be rethought or abandoned. Variability in electronic device performance characteristics is an important measure that determines circuit yields, active-matrix display homogeneity, and the analog sensing accuracy. Fig. 8a-c shows the variability in the transfer characteristics of one hundred inkjet-printed OTFT devices [36]. The mobility was 1.1  0.17 cm2/(Vs) with a relative standard deviation of 15 %. The threshold voltage was -0.01  0.09 V, a variability that is considered small enough in digital circuits and active-matrix backplane applications, while the further reduction in these variability levels is preferred in analog circuits. Minimal variability is also important in designing integrated circuits based on the circuit simulations containing a large number of OTFT devices.

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(b) (a)

(c)

(d)

(e)

Fig. 8. a) Variability of OTFT transfer characteristics, b) mobility, and c) threshold voltage of the inkjet-printed DTBDT-C6 OTFT devices. d) Transfer characteristics, e) maximum drain current and threshold voltage shift under the bias stress of gate voltage VGS = -2 V. Reproduced with permission [35, 36].

Since it had been believed that printed organic circuits are unstable, OTFT device reliability in terms of short-term and long-term stability and resistance to air exposure and heat must also be evaluated. For example, short-term stability can be seen as a difference between the forward and reverse curves in the transfer characteristics, also known as hysteresis. Long-term stability, on the other hand, can be seen as bias stress: the variation in electric characteristics due to the application of gate and/or drain voltages for an extended period of time. Fig. 8d and 8e show an example of bias stress at VGS = -2 V in inkjet-printed OTFT devices [35]. There are

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typically two kinds of bias stress; the first kind decreases drain current, and the other increases drain current. The former is usually considered to originate from deep traps with time constants longer than a time scale of minutes since the trapped charges in the channel layer compensate a part of gate electric field and reduce the number of free carriers in the channel [43]. The latter is thought to originate from the polarization or drift of ions in gate insulators [48]. In other words, the former is the nature of semiconductor layers or semiconductor/insulator interfaces, while the latter is the nature of the bulk gate insulators. Resistance to air exposure strongly depends on the semiconductor materials employed. Although conventional organic semiconductors such as pentacene and C60 were unstable in air and had to be handled in glove boxes filled with inert gases, there are many air-stable organic semiconductors today [32, 33]. All the OTFT devices shown in this paper were fabricated and measured in air, indicating the air-stable nature of the materials. The heat resistance of semiconductor materials is also important because some organic semiconductor materials have rather low melting points (< 100 C). Their molecular designs can improve the heat resistance of the semiconductor molecules [49]. Contact resistance, the resistance at the semiconductor/metal interface, is known as one of the limiting factors of OTFT device performance [50]. In general, the effect of contact resistance is more serious in short-channel OTFT devices. The channel resistance Rch and contact resistance Rc can be measured individually using the transfer line method (TLM) [51] or the four-terminal method [52, 53]. Because of its ease of use, TLM is widely used in the field of OTFT devices. TLM is based on two strong assumptions that the contact has ohmic resistance represented as RC/2 and that the channel resistance is proportional to the channel length: 𝑅ch = (𝐿 𝑊)𝜌ch, where ch is the sheet resistance of channel, L is the channel length, and W is the channel width. If these two assumptions are satisfied, the total source-to-drain resistance is given by 𝑅total = (𝐿 𝑊)𝜌ch + 𝑅C, or 𝑅total𝑊 = 𝜌ch𝐿 + 𝑅C𝑊 in a channel-widthnormalized form. Here, Rtotal, RC, and ch can be dependent on the gate voltage. Thus, the sheet channel resistance ch and contact resistance RCW can be estimated as the slope and intercept

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in a plot of the total source-to-drain resistances RtotalW as a function of channel length L. Figure 9a shows the total resistance plotted as a function of channel length for many OTFT devices with various channel lengths. The estimated channel and contact resistance values in Fig. 9b indicate that the contact resistance is comparable to the channel resistance when the channel length is about 5 m. In other words, the contact resistance becomes dominant if the channel

(a)

(b)

RtotalW (kΩcm)

RchW, RCW (kΩcm)

length is less than 5 m in these kinds of devices.

Channel length (µm)

channel resistance RchW at L = 9 µm contact resistance RC W

VGS (V)

Fig. 9. a) Transfer line method (TLM) for estimating channel and contact resistance values. b) Gate voltage dependence of channel and contact resistance values for inkjet-printed DTBDTC6 OTFT devices. Reproduced with permission [29].

Table 1 summarizes the contact resistance of various OTFT devices in literature, where the fabrication processes of contact electrodes and channel layers are categorized into wet (printed or coated) or dry (vacuum-deposited or chemical-vapor deposited) processes [16, 29, 54-64]. The typical contact resistance values for OTFT devices with wet-processed electrodes and wetprocessed channels are in the range of several to several tens of kcm. These contact resistances are comparable to those of OTFT devices with dry-processed electrodes and dryprocessed channels. The wet-processed electrodes with dry-processed semiconductor have exhibited relatively high contact resistance higher than 100 kcm so far. Quite low contact resistances below 1 kcm have been observed only in the combination of dry-processed

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electrodes and wet-processed semiconductors. All of these low-contact-resistance devices actually use unique techniques such as single-crystalline bilayer [57], liquid crystalline semiconductor [58], and high-k polymer binder [59]. There are no clear dependencies of contact resistance on the staggered OTFT constructions (BGTC and TGBC) and the planar OTFT constructions (BGBC and TGTC).

Table 1 Contact resistance RC in a variety of organic thin-film transistor (OTFT) devices. electrode process wet

channel process wet

wet

dry

dry

wet

electrode material* Ag/lisicon M001 Ag/SAM Ag/F-SAM Cu/F-SAM Ag/F-SAM Au/F4TCNQ Au/F-SAM Au/F-SAM Au/MoO3

dry

dry

Au/NO2-SAM Au Au/FeCl3 graphene

channel material lisicon S1200

structure**

Ref.

BGBC (p)

RC (kcm) 3

lisicon SP400 DTBDTC6:polystyrene pentacene pentacene C8-DNBDT-NW Ph-BTBT-10 TMTES-pentacene: high-k polymer TIPSpentacene:PTAA TIPS-pentacene pentacene C8-BTBT pentacene

TGBC (s) BGBC (p)

5 20

[50] [22]

BGBC (p) BGBC (p) BGTC (s) BGBC (p) TGBC (s)

310 4100 0.0469 0.156 0.3

[12] [51] [52] [53] [54]

TGBC (s)

11

[55]

TGBC (s) BGTC (s) BGTC (s) BGBC (p)

291 1.4 8.8 560

[56] [57] [58] [59]

[49]

* F-SAM: fluorinated self-assembled monolayer. ** p: planar type (BGBC or TGTC), s: staggered type (BGTC or TGBC).

3.2 Printed Digital Circuits Inverters are one of the most common organic circuits employing OTFT devices since it is simple and contains a lot of important principles common to many other circuits. In particular, complementary inverters, which utilize p-type and n-type OTFT devices, have significant advantages in low power consumption and fast response to both low-to-high and high-to-low transitions, wide output voltage swing, and high noise resistance. Patterning of p-type and n-

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type OTFT devices is one of the difficulties in fabricating complementary circuits using printing. The two OTFT device types normally use different semiconductors and different electrodes. However, the SAM treatments shown in section 3.1, for example, require the immersion of the substrates into the solution of PFBT or 4-MBT, where the patterning of the SAM treatments is difficult [65]. One way to solve this problem is to fabricate p-type and n-type OTFT devices in different layers as shown in Fig. 10a [38, 66]. To balance the drain currents in p-type and ntype OTFT devices, 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TESADT) and TU-3 (Fig. 5) were chosen as p-type and n-type semiconductors, respectively. In this structure, the source (drain) electrodes for p-type and n-type OTFT devices can be treated with PFBT and 4-MBT, respectively, by immersion method. Figures 10b and 10c show the circuit diagram of a complementary inverter and output characteristics. When the input voltage is low, the n-type OTFT is switched off and the p-type switched on. As a result, the output node is electrically connected to the voltage supply (VDD), whose voltage is positive in DC, and disconnected from the ground (GND), and consequently, the output voltage becomes high. The high input voltage results in the opposite, low output voltage. Thus, the inverter can invert a voltage between high and low states. In digital circuits, both input and output voltages are supposed to be close enough to the VDD voltage (high) or the GND voltage (low). The operation of other digital logic elements such as NAND and NOR gates can be understood in the same manner. Another way to address p-type and n-type OTFT devices patterning challenges is to choose the semiconductor material that is not sensitive to work function of source and drain electrodes [67]. A kind of donor-acceptor polymer semiconductor (MOP-01, Mitsubishi Chemical Corporation) was reported to be virtually insensitive to of electrode work function. The exact mechanism for the insensitivity to work function is not yet clear and should be investigated in future research work.

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(a)

(c) (b) VDD p-TFT OUT

IN

n-TFT GND

Fig. 10. a) Device structure, b) circuit diagram, and c) output characteristics for inkjet-printed organic complementary inverters. Reproduced with permission [38].

D-flipflops (D-FFs) are an important element in digital circuits; for example, a quarter of TFTs in near-field communication (NFC) tags are used to compose D-FFs [68]. This is because the D-FFs are the most primitive 1-bit memory whose timing of data storage can be controlled by the high-to-low (or low-to-high) transition of the clock signal. Fig. 11a and 11b show the circuit diagram and an optical microscope image of the inkjet-printed organic D-FFs [69]. The D-FF is composed of nine p-type and nine n-type OTFT devices. Figure 11c illustrates how negative-edge-triggered D-FFs operate. When a negative edge comes in the clock terminal (C), the voltage in the data input terminal (D) is copied to the output terminal (Q). The voltage at Q is kept unless another negative edge trigger comes in, which is the memory effect of the DFFs. NQ is the inverted output whose voltage is always opposite to that of the Q terminal.

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Integrating multiple D-FFs can construct further advanced circuits such as counters and shift registers.

(a)

(b)

Data

D

Q

Clock

C

NQ

VDD

D

Output

C

Inverted output GND

(c)

2 mm

NQ Q

D

Voltage

C Q 5V NQ 0

0.2

0.4

0.6

0.8

1

Time [s] Fig. 11. a) Input and output terminals of a D-flipflop (D-FF). b) Photograph and c) transient characteristics of the inkjet-printed organic D-FF. Reproduced with permission [69].

3.3 Printed Analog Circuits In contrast to digital circuits which utilize OTFT devices as on-off switches, analog circuits employ OTFT devices as amplifiers and are one of key technologies for sensor applications. Assuming that OTFT devices are in saturation regime, that is, high enough voltage is supplied between source and drain electrodes, the drain current depends on VGS as 𝐼D = (𝜇𝐶i𝑊 2𝐿)

(𝑉GS ― 𝑉TH)2. Regarding gate voltage as input and drain current as output, an OTFT converts the gate voltage into the drain current, where the conversion ratio is given by a transconductance: 𝑔m ≡ ∂𝐼D ∂𝑉GS = (𝜇𝐶i𝑊 𝐿)(𝑉GS ― 𝑉TH). Typical values of gm for silicon MOSFETs are 1 – 30 mS, and those for OTFT devices are 1 – 100 S. In this way, an OTFT acts as a voltage-to-current amplifier if gm is high enough. To obtain a voltage-to-voltage amplifier,

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the simplest way is to connect an OTFT with a resistor as shown in Fig. 12a. Since the output voltage of this circuit with respect to VSS is given by 𝑉OUT = 𝑅load𝐼D from the Ohm’s law at the resistor, the resistor acts as a current-to-voltage converter with a conversion ratio of 𝑅load. As a whole, the circuit converts the input voltage into the output voltage with the ratio, 𝐴 = ∂𝑉OUT ∂𝑉 = 𝑅 IN load 𝑔𝑚 < 0. This simple circuit configuration is called a common-source amplifier. Approximating the output characteristics near the bias point by a linear function, it can be written by 𝑉OUT ≈ 𝐴(𝑉IN ― 𝑉OS), where VOS is an offset voltage. Although the commonsource amplifiers are easy to fabricate, they have drawbacks that a large offset VOS exists. Since the gain A and the offset voltage Vos depend on many OTFT devices parameters, obtaining the gain and offset voltage in a reproducible and controllable fashion is quite difficult. The uncertain or uncontrollable VOS gives crucial errors in sensor applications. (a)

(b)

VDD

VDD

IN

OUT

M1

CUR

IN-

M2

M4

M3

IN+

M5

OUT

Rload M8

VSS M6

M7

VSS

Fig. 12. a) Single-ended input amplifiers and b) differential input amplifiers. VDD and VSS are positive and negative supply voltages; the voltages at VDD and VSS nodes are constant, e.g., VDD = +2.5 V and VSS = -2.5 V.

Differential amplifiers are commonly used to solve the offset voltage problem [70]. Fig. 12b shows an example. The differential amplifiers amplify the voltage difference between two input terminals: 𝑉OUT ≈ 𝐴(𝑉IN + ― 𝑉IN ― ― 𝑉OS). Although in principle differential amplifiers also have an offset VOS, it can be much smaller than that of the common-source amplifier. This is because the transistor pair, M4, and M5, compensate the offsets of respective inputs. In

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particular, differential amplifiers with high gain A are called operational amplifiers (OPAs). Figure 13a shows optical microscope images of inkjet-printed organic OPAs [71]. The organic OPA consists of five p-type OTFT devices and three n-type OTFT devices as shown in Fig. 12b. A compensation capacitor indicated with dashed lines may be added to improve the stability in feedback circuits. Since OPAs are usually used in combination with passive elements such as resistors and capacitors, packaging the organic OPAs, e.g., in dual in-line package (DIP), facilitates testing (Fig. 13b). The frequency dependence of gain |A| and phase of the organic OPA is shown in Fig. 13c. In the inkjet-printed OPAs, the DC open-loop gain is about 60, and the gain-bandwidth (GB) product is about 50 Hz. The GB product can be improved to 1 kHz by reverse-offset printing, which will be reported elsewhere. As the name suggests, OPAs are not just amplifiers but can be used for a variety of operations including addition, subtraction, integration, and differentiation. Furthermore, they can be used for a variety of signal processing such as active filters (low-pass, high-pass, etc.), impedance converters, current-to-voltage converters, oscillators and comparators. To design and analyze integrated circuits such as D-FFs and OPAs, circuit simulation based on individual device characteristics is essential. Simulation of circuits which include OTFT devices can be carried out by using the open-source analog circuit simulator, SPICE (e.g., LTspice®), and one of the compact models for OTFT devices proposed thus far. For example, a compact model for OTFT devices can be represented by the equivalent circuit in Fig. 14a, where the current through the current source is given by

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(a)

1 mm CUR

VDD

OUT

IN+

VSS

IN-

VDD (+2.5 V) IN+ IN-

4 mm (b)

OUT VSS (-2.5 V)

(c)

Gain

100

OPA only

10 non-inverting amplifier

1 0.1

1 10 Frequency (Hz)

100

Fig. 13. a) Optical microscope images of the inkjet-printed organic operational amplifiers (OPAs). b) Photograph of the organic OPAs in a dual in-line package (DIP). c) Frequency dependence of gain for an organic OPA and non-inverting amplifier (R1 = 100 M, R2 = 500 M). Solid circles and squares denote experimental data, and dashed lines denote circuit simulation based on the compact model in Fig. 14. Reproduced with permission [71].

𝐼D =

𝑊 𝜇 𝐶 (1 + 𝜆|𝑉D1 ― 𝑉S1|) 𝐿 o I 𝑉G ― 𝑉T ― 𝑉S1 ln 1 + exp 𝑉SS

{[

×

(

)] }

𝛾+2

{[

― ln 1 + exp

(

)] }

𝑉G ― 𝑉T ― 𝑉D1 𝑉SS

𝛾+2

𝛾+2

according to Marinov’s model [72]. Here, ID is the drain current, o is the mobility when |VG – VT| = 1 V, CI is the capacitance per unit area, W is the channel width, L is the channel length,  is the channel length modulation coefficient, VG is the gate voltage, VT is the threshold voltage, VSS is the subthreshold swing related to deep traps, and  is the exponent of the gate dependence of mobility related to shallow traps. RS and RD are contact resistances at source and drain electrodes, and can be replaced with other non-linear and gate-dependent elements

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if necessary. The parasitic capacitances of CSG and CDG are essential for AC or transient simulations. Figure 14b-e show how this compact model reproduces the experimental output and transfer characteristics of the inkjet-printed OTFT devices. Good agreement indicates that the compact model can be used for designing and analyzing printed integrated circuits. As an example, we simulated the AC gain of the inkjet-printed organic OPAs and non-inverting amplifiers in Fig. 13c (dashed lines), which have a good agreement with experimental results. In this way, once the electric characteristics of individual printed OTFT devices are known, we can predict the behavior of the integrated circuits based on the OTFT devices.

Fig. 14. a) Compact equivalent circuit model of OTFT devices for circuit simulation. b) Experimental (open circle) and simulated (solid line) output and c) transfer characteristics of a diF-TES-ADT p-type OTFT. d) Experimental (open circle) and simulated (solid line) output and e) transfer characteristics of a TU-3 n-type OTFT.

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4. Fine Patterning and Device Fabrication with Reverse-Offset Printing Screen printing, gravure printing, inkjet printing, and the like are known as printing methods used for printed electronics (PE). In each printing method, the line width/line spacing (resolution), film thickness, the viscosity of ink and the printable area of the printing apparatus are different (Fig. 1 (c)). Therefore, it is necessary to select an appropriate printing method according to the desired resolution and shape. Here, reverse-offset printing which is one of the high-resolution printing methods will be described in detail. Improvement in print pattern resolution is expected to improve both circuit integration and organic transistor performance. The width of the electrode formed by the conventional inkjet printing method is 100 to 200 μm, and the shape of the electrodes at the ends and corners is rounded. For this reason, many reports of OTFT devices using electrodes printed with inkjet printing method have a channel width of 1000 μm or more [71, 73-75]. Thus, the miniaturization and shape control of electrodes are indispensable for size reduction of OTFT devices.

The cutoff frequency, one of the most important figures of merits of OTFT devices,

is related to the channel length and the overlap length, and explained by the following equation [76]: 𝑓c =

𝜇eff(𝑉GS ― 𝑉th) 2𝜋𝐿(𝐿 + 2𝐿c)

Here, 𝜇eff is the effective mobility, 𝑉th is the threshold voltage, 𝑉GS is the gate-source voltage, L is the channel length, and 𝐿c is the gate-source or gate-drain overlap length. This equation explains that reducing the L and the 𝐿c by high-resolution patterning contributes to highspeed operation of the OTFT devices. Therefore, not only the realization of large area electronics but also high-resolution printing methods have attracted significant attention in recent years. Currently, printing methods with a line width of 10 μm or less studied for organic transistors use a patterned printing plate (a cliché), photoactivated surfaces [77], or subfemtoliter inkjet technology [78]. Among the printing methods using a cliché, offset printing is a printing method that can easily realize a high-resolution printing pattern. In offset printing, the pattern to be printed is

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once formed on the blanket (intermediate) made of polydimethylsiloxane (PDMS) and then transferred to the substrate. Since the ink is partially dried and loses its flowability on the blanket before transferring, fine electrodes can be transferred onto the substrates without spreading or deformation. In addition, since the face of ink in contact with the blanket appears on the surface after transferring, it is easy to obtain a flat electrode surface. Reverse-offset printing is a form of offset printing. The process of reverse-offset printing is shown in Fig. 15a [7]. The printing process has three steps. The first step is coating of ink on the blanket, after which the ink layer is dried partially for optimum patterning. Next, the thin semi-dried ink layer on the blanket is patterned using a printing plate. The last step is to transfer the patterned thin film from the blanket to the substrate.

(a)

Ink

Roll (c)

Blanket

Engraved glass Substrate (d)

(b)

(e)

Fig. 15. Schematic illustrations of the reverse-offset printing: a) coating (left), patterning (middle), and transferring steps (right). b) A magnified illustration of ink patterning step. Patterning step for the cases of c) ideal patterning, d) incomplete removal, and e) excess removal. Reproduced with permission [13].

In order to obtain thin layers, the viscosity of the ink used for reverse-offset printing is relatively low, in the order of 1 to 5 mPas. In addition, the ink should have a low surface tension so that it can be coated on a PDMS blanket without dewetting or peeling from the blanket after being partially dried. Representative inks for reverse-offset printing reported in the past

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include silver nanoparticle inks, insulating polymer inks, and metal-oxide semiconductor precursor inks. Furthermore, it is necessary to select a solvent for the ink which does not damage the blanket mainly composed of PDMS. Polar solvents are chosen in many cases. Printability varies with the force applied to the film coated on the blanket in the pattern formation process (Fig. 15b) [13].　The forces to be considered here are the adhesive force of the ink/blanket interface (fb), the adhesive force of the ink/cliché interface (fg), and the internal cohesive force of the film (fc). When both fb and fg exceed fc, the ink is removed properly from the blanket (Fig. 15c). However, when the force fc exceeds the force fg, removal of the film becomes insufficient (Fig. 15d). When the force fc exceeds fb, excessive film removal occurs (Fig. 15e). In order to form electrodes having a fine gap, a cliché having a fine pattern is necessary. This means that the contact area between the cliché and the film decreases. At the same time, this leads to a decrease in fg and deteriorates printability. Also, a blanket employing PDMS as the primary material is soft and deforms when it contacts the cliché. When patterning wide electrodes, it is necessary to consider that the blanket does not contact with the engraved area of cliché. Therefore, the depth of the cliché needs to be comparable to or larger than the electrode width. Since reverse-offset printing has the advantage of high resolution, its applications to various device manufacturing have been reported. The first study on the electrodes formed by reversed offset printing for TFTs was reported by Moon et al. in 2009, where the gate electrodes of aSi:H transistors were printed [79]. The application to source and drain (SD) electrodes can take advantage of the narrow gap of reverse-offset printing, and has been reported since 2011 as summarized in Table 4. M. Kim et al. realized the SD electrodes with a channel length of 3 μm for OTFT devices by reverse-offset printing [80]. The OTFT had a carrier mobility of 0.003 cm2/(Vs) using top-gate bottom-contact geometry and P3HT as the semiconductor layer. The sintering temperature of the silver nanoparticle ink in the study was 450 oC, which was not compatible with most organic materials. OTFT devices with the further miniaturization of channel length and lower sintering temperature was reported by K. Fukuda et al. in 2015 (Fig.

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16) [13]. The OTFT devices was a channel length of 0.6 μm at minimum and the sintering temperature was 180 oC. A mobility of 0.29 cm2/(Vs) with a channel length of 2.4 μm and a mobility of 0.009 cm2/(Vs) with a channel length of 0.6 μm were achieved with a driving voltage of -20 V using diF-TES-ADT for the organic semiconductor. SD electrodes formed by reverse-offset printing were applied to organic integrated circuits by our group in 2017 (Fig. 17) [81]. Complementary integrated circuits (inverter) have been demonstrated using diF-TESADT and TU-3 as p-type and n-type organic semiconductors, respectively. Channel lengths were 1.6 to 70 μm, and the complementary inverter circuits exhibited a gain higher than 20. In addition, owing to the low sintering temperature of 120 oC, it could be applied to a flexible substrate. Furthermore, the driving voltage of the integrated circuits was reduced down to 2.5 V. In 2018, Y. Kusaka et al. fabricated n-type InGaZnO (IGZO) TFTs using reverse-offsetprinted electrodes and precursor inks for metal oxide semiconductor (Fig. 18) [82]. The mobility was 0.17 cm2/(Vs) at a channel length of 60 μm and a drive voltage of 40 V. Since many of the printed OTFT devices having high carrier mobilities are p-type, printed n-type transistors with high mobilities contribute greatly

to higher performance complementary

organic integrated circuits.

(a)

(b)

Fig. 16. a) Top-view photograph of reverse-offset printed electrodes formed on 120×120 mm glass substrates. b) AFM image of reverse-offset-printed silver electrodes. Reproduced with permission [13].

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Fig. 17. a) Circuit schematic for the complementary inverter circuit. b) A microscope image of the inverter with reverse-offset printed electrodes. c) Transfer and signal gain curves for the inverter at various operating voltages. Reproduced with permission [81].

(b) (a)

Fig. 18. a) Schematic illustration of the fabricated TFT structure with spin-coated IGZO layer, printed MoOx S/D and ZrOx barrier layer. b) Example of a 6-in. scale ROP pattern of metal(acac) inks. c) Transfer and d) output curves of a MoOx/ZrOx/IGZO TFT with a channel length of 60 μm and a width of 240 μm. Reproduced with permission [82].

The application of reverse-offset printing to other devices has been reported increasingly since 2014. Reports on basic mechanisms, such as line and space and mesh patterns, are useful for acquiring knowledge and performance evaluation of reverse-offset printing. Y.-M. Choi et al. reported a very high-resolution silver electrode pattern with L/S = 1 μm [83], and also reported the formation of a metal mesh pattern touch sensor in 2015 (Fig. 19) [84]. An

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alternative to transparent electrodes is also reported, which utilizes the fineness of the metal pattern formed by this reverse-offset printing. In 2018, Z. Jiang et al. applied the reverseoffset-printed transparent electrodes to the organic solar cells, which achieved high flexibility, durability and equivalent performance [85]. In the report, the line width was 5 μm, the line spacing was 45 μm, and the sintering temperature was 210 C. Furthermore, near-field communication (NFC) antennas [86], solder bumps [87], and interlayer contacts [88] have been reported, and the application fields of fine printing electrodes are expanding.

Fig. 19. (Left) SEM images of the printed patterns obtained using optimized printing techniques. (Right) Printed single-layer touch screen sensor based on a metal mesh. Reproduced with permission [84].

5. Potential Application to Sensing Systems 5.1 Multi-Sensing Healthcare Systems One of the more promising applications for printed and flexible OTFT devices is wearable smart sensors for nursing the elderly and for infants who cannot readily perform self-care. Owing to the flexibility, thinness, and light weight of the printed OTFT circuits, such smart sensors can be attached to human skin or clothing with minimal physical discomfort. The sensing parameters for healthcare purposes can be body temperature [89], heart rate [90, 91], blood pressure, blood glucose level [92], ions and lactate in sweat [35, 36, 93], brain waves [94], myoelectricity [95], acceleration, and mental stress [96]. Figure 20 shows a schematic depiction and an example circuit diagram for a wearable multi-sensing device. It has two types

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of electrochemical sensors, for ions and lactate, a heart rate sensor, a temperature sensor, and a wireless communication module. Printed OTFT circuits were inserted between each sensor and a wireless communication module for multiple purposes such as impedance conversion, current-to-voltage conversion, signal amplification, noise filtering, and the active control of counter/reference electrodes for electrochemical sensing. Here we adopted inverter-based circuits with placing a priority on manufacturability and ease of fabrication. Operational amplifiers (OPAs) are more commonly used instead of inverters in conventional silicon electronics technology.

reference counter

sensor 3

sensor 1 sensor 2

printed antenna

sensor 4 printed IC

flexible film

Reference electrode Counter electrode Ion sensor Sweat

Heart rate

Body temperature

Enzymatic sensor

Analog-Digital Converter & Wireless Communication (13.56 MHz, 900 MHz, 2.45 GHz)

Piezoelectric sensor Temperature sensor

Fig. 20. Schematic diagrams for a multi-sensing healthcare system employing printed organic circuits.

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5.2 Biosensors Here we introduce two distinctive biosensor systems using printed OTFT circuits. They are the prototypes of the biosensors to detect biomarkers in various body fluids such as sweat, tear, saliva, urine, and blood. Among a variety of electrochemical measurements, potentiometric and amperometric measurements are two standard approaches. Potentiometric measurement is suitable to, for example, ion sensors, where the open-circuit voltage between the working and reference electrodes should be measured. Amperometric measurement is suitable to enzymatic sensors, where the current through the working electrode should be measured by keeping the voltage between the working and reference electrodes constant, specifically, in a three-electrode cell consisting of working, reference, and counter electrodes.

5.2.1 Potentiometric Electrochemical Sensors The most basic biosensing system using field-effect transistors (FETs) is an extended-gate field-effect transistor (EG-FET), in which a sensor electrode is externally connected to the gate electrode of an FET device (Fig. 21) [93]. The key concept of EG-FETs is to receive the weak voltage signal with high-impedance gate node of FETs and read out the signal as the variation in drain current. The high input impedance of FETs minimizes the current through the sensor electrode, and make the condition nearly open-circuit. The merits of the EG-FETs are (1) the ease of fabrication (only one FET is required), and (2) the high input impedance, which is necessary to measure open-circuit voltage. However, the EG-FETs also have two drawbacks: (1) an additional circuit to detect the small changes in drain current is needed, and (2) an additional circuit to control the potential of the reference electrode is required. Accordingly, the EG-FETs are not a complete system but a part of the overall sensing system.

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(a)

body fluid

A

pyruvate lactate

source

reference

drain enzyme

organic semiconductor gate dielectric gate electrode

(b)

extended gate

(c)

Fig. 21. a) Schematics of an extended-gate organic field-effect transistor (EG-OFET). b) Threshold voltage shift in the transfer characteristics due to the change of electrochemical potential at the extended gate with respect to the reference electrode. c) Response of drain current to the changes in lactate concentration. Reproduced with permission [93].

The drawbacks of the EG-FETs can be overcome by the inverter-based circuit in Fig. 22a [35]. The inkjet-printed organic inverters are again used here. In this circuit, the amplification unit is a voltage-to-voltage amplifier, and then the output is voltage rather than current. The two resistors are attached to determine the voltage-to-voltage gain as R2/R1, and may be omitted if the gain of the inverter itself is finely controllable. The reference unit is used to adjust the potential of the reference electrode so that the input of the amplification unit fits within the appropriate operational window. The working electrode is coated with a potassium-ionsensitive membrane, and the reference electrode is Ag/AgCl. The voltage difference between the working and reference electrode follows the Nernst equation: Δ𝑉 = 𝑉0 ― (𝑅𝑇 𝐹)ln [K + ] ≈ 𝑉0 ―58 mV ⋅ log [K + ] at 293 K. The output voltage at various concentrations of K+ is shown in Fig. 22b. The output voltage exhibited a stepwise response to the changes in [K+], and the

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sensitivity was found to be 160 mV/dec after amplification.

(a)

(b)

Fig. 22. a) Circuit diagram and b) output voltage of the potentiometric electrochemical sensing system with inkjet-printed organic inverters. Reproduced with permission [35].

5.2.2 Amperometric Electrochemical Sensors Some biosensors such as enzymatic sensors generate current in electrochemical cells, and cannot be used in the potentiometric configuration described above. For such amperometric sensors, the current through the working electrode should be measured by keeping the voltage between the working and reference electrodes constant. However, voltages cannot be applied directly between the working and reference electrodes, since the reference electrode must be used in the condition that no current flows through it. This is why a three-electrode cell is used widely in electrochemical measurements. In the cell, the potential of the third electrode called counter electrode is controlled by an active feedback so that the following two conditions are satisfied: (1) the voltage difference between the working and reference electrodes is kept at a

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target value, and (2) the current in the three-electrode cell flows only in the path from the working electrode to the counter electrode. The circuit system for the active feedback control is called a potentiostat. The inkjet-printed organic circuits in Figure 23a behave like a potentiostat for making amperometric measurements [36]. The feedback control unit adjusts the potential of the counter electrode actively so that the two conditions are satisfied. The detection unit, on the other hand, converts the current signal through the working electrode into a voltage signal. Here, lactate oxidase was immobilized on the working electrode to detect lactate in sweat selectively. Figure 23b shows the output voltage at various concentrations of lactate. The output voltage exhibited a stepwise response to changes in lactate concentration, and the sensitivity was found to be 1 V/mM after the current-to-voltage conversion. It was also confirmed that the voltage difference between the working and reference electrodes was kept constant independent of the lactate concentration.

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(a)

(b)

Fig. 23. a) Circuit diagram of the amperometric electrochemical sensing system with inkjetprinted organic inverters. b) Voltages at the output terminal, working electrode, and reference electrodes of the system at a variety of lactate concentrations. Reproduced with permission [36].

5.3 Design of Fully-Printable Contactless Sensor Tags One of the goals of this development field is to develop fully-printed smart sensors which can sense a variety of information for human healthcare, item tracking, environment monitoring, agriculture, etc. and send it by wireless communication. However, realizing the fully-printed organic wireless communication circuits working in a manner exactly the same as the inorganic ones is quite difficult at the present time mainly for two reasons: operating speed and high-density integration. For example, the ISO14443-A specification describes the

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clock division at 13.56 MHz, which requires high performance OFET devices, such as those with effective mobility of 10 cm2/(Vs) with a channel length of 2 m [68]. This specification also requires the integration of at least a thousand OFET devices in each wireless tag. Another strategy for developing smart sensors is to utilize an analog data transmission such as pulse-width-modulation (PWM). PWM can send analog information via the width of periodic rectangular pulses. Fig. 24a shows the concept of fully-printed smart sensors based on PWM. It generates a DC power source by rectifying the radio wave from the antenna. A sensor, and an amplification circuit if necessary outputs a DC voltage which depends on the sensing target such as temperature or the concentrations of biomarkers. Then, the PWM circuit generates a PWM signal whose width is determined by the sensor output. Finally, the PWM signal is transmitted by the load modulation of the antenna. An advantage of this system is that the whole circuit of the smart sensor is very compact. The circuit diagram and the experimental results of the inkjet-printed PWM circuits are shown in Fig. 24b and 24c [71]. The PWM circuit is composed of two organic OPAs (or, more precisely, comparators), four resistors and one capacitor. The first OPA is used to generate a triangular wave. The second OPA generates the PWM signal whose pulse width can be tuned by the DC voltage at VIN terminal. Rectifiers and load modulators based on OTFT devices have also been reported, while they were fabricated by vacuum deposition and photolithography [97]. We believe that this compact system can be one possibility for realizing fully-printed organic smart sensors.

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(a)

Antenna 𝑉���

𝑉��

Rectifier

PWM 𝑉��

Sensor

Load Modulator PWM signal 𝑡� 𝑡�

(b)

VDD

100 MΩ

100 MΩ

VIN VOUT2

100 MΩ VSS

VOUT2 (V)

(c)

VOUT1 (V)

1 nF

1

VOUT1

500 MΩ

VIN = 0 V

-0.2 V

0 -1 2 0 -2 0

1

2

3

Time (s)

Fig. 24. a) Concept of a fully-printable contactless sensor tags based on PWM. b) Circuit diagram and c) output signal of an inkjet-printed organic PWM oscillator. Reproduced with permission [71].

6. Conclusion In this paper, we have reviewed the recent progress of the flexible and printed organic transistors, including printable functional materials, printing processes, OTFT devices, integrated circuits, and sensor applications. There are a lot of printable functional materials these days, and some of them are commercially available. Particularly, the development of lowtemperature sinterable metal nanoparticle ink and air-stable and soluble p-type and n-type organic semiconductors has enabled the fabrication of electronic devices on flexible substrates by printing in air without vacuum chambers and photolithography. Now that the properties of respective materials have been improved significantly, more attention must be paid to the

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engineering of interfaces such as semiconductor/metal and semiconductor/insulator. How to realize efficient carrier injection from metal electrodes to semiconductors, and how to reduce trap states at the semiconductor/insulator interfaces still remain open question. There are also many chioces of printing methods such as inkjet printing, screen printing, gravure printing and reverse-offset printing. Since they have advantages and disadvantages in spatial resolution, the range of film thickness, the range of ink viscosity, and printing speed, it would be necessary to choose different methods for respective layers of electronic devices. The performance of printed OTFT devices, not only mobility but also stability and reproducibility, has been improved significantly in the past decade. After the optimization of materials and process conditions, the printed devices are not necessarily inferior to the devices fabricated by vacuum deposition. Solution processes of organic semiconductors also have advantages in large crystalline domains and a clean semiconductor/insulator interface as a result of phase separation from mixed solution. There had been many reports on the digital circuits of OTFT devices, since they are more resistant to the variation of device parameters than analog circuits. Recent improvement of stability and reproducibility of OTFT devices is paving the way for printed analog circuits, which may be utilized in internet of things (IoT) society. One of the most important building blocks for sensor applications is operational amplifiers (OPAs), which possess high versatility in signal processing. Wearable smart sensors are one of the most promissing applications of printed and flexible OTFT devices. The sensor applications based on printed OTFT devices has been demonstrated successfully for ion and lactate sensing. We believe that further research and development on functional materials, processes, devices, and circuit systems will open a new field of flexible and printed electronics industry.

Acknowledgements We thank Mr. Clay Shepherd for English editing.

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Printable functional materials such as metal nanoparticles, dielectric polymers and organic semiconductors are reviewed

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Device fabrication with on-demand inkjet printing and high-resolution reverse-offset printing are described in detail

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Biosensor application with integrated circuits of organic thin-film transistor devices is demonstrated