1 Introduction
Printed organic electronic technology is a new interdisciplinary technology, which involves different disciplines such as synthetic chemistry, organic semiconductor materials, device physics, printing technology and electronic circuits
[1]. Printed organic electronic technology currently has many Synonyms, including plastic electronics, large area electronics, thin film electronics, flexible electronics, and combinations of Synonyms such as organic large area electronic devices and flexible organic large area electronic devices
[2][3,4][5][6]. Common to all of these statements is that these electronic components are fabricated and patterned by printing methods on flexible substrates, as opposed to lithographic fabrication (rather than printing) techniques on rigid substrates (single crystal silicon wafers). In this context, printed electronics refers in particular to the fabrication of electronic devices based on organic semiconductor materials.
Printed electronics technology has many potential applications, such as energy devices, biosensors, and display devices. Printed electronics is an ideal low-cost, green, print-anywhere, on-demand, scalable novel manufacturing technology for printing on mechanically flexible substrates
[7,8]. Printed electronics products are not competitors to complementary metal-oxide-semiconductor (CMOS) process devices, but are complementary to them. Low cost is one of the most important attributes of printed electronics technology. Printed electronics is an integrated technology that integrates flexible and rigid materials and components onto flat or curved surfaces, enabling unique, on-demand, customized products at low cost. Based on this, manufacturers can use more new materials and new designs. Printed electronic products have the characteristics of large area and flexibility, and have shown great development potential and broad market prospects in wearable electronics, flexible display and lighting, health care and other fields
[9]. Over the past few decades, printed flexible electronics have demonstrated the ability to be mass-produced, such as organic light-emitting diode (OLED) displays, radio frequency identification (RFID) tags, flexible sensors, amplifiers, and electronic skin. In addition, electronic components such as sensors and data processing circuits can be integrated on the surface of objects through printing technology to monitor environmental parameters during logistics.
Integrated circuit (IC) chips and printed circuit boards (PCBs) fabricated by traditional CMOS technology are assembled on PCBs with silicon-based CMOS circuits and other rigid passive components encapsulated in rigid epoxy resin through a series of complex processing steps such as lithography, development and etching. Printed electronic products usually have mechanical flexibility, and the use of printing methods can effectively solve the problems of poor extensibility of traditional silicon-based devices based on the limitation of their own silicon materials. At present, the process of printing and manufacturing is much simpler, and only two steps of printing and sintering are needed. In addition, the printing method also has the advantages of low cost, large area preparation, low environmental pollution, low temperature and wide substrate applicability, so it is widely used in the preparation of electronic devices. Figure 1 contrasts the significant differences between silicon and printed electronics
[10,11]. Printed electronics are often referred to as smart, lightweight electronics based on cheap, abundant materials and ubiquitous printing processes, printed with material mechanical flexibility form factors, with desirable attributes including:
(1) Low cost-only a fraction of silicon-based electronic devices;
(2) Green or "sustainable"-the use of environmentally friendly or earth-rich chemicals, the efficient use of materials, the reduction of chemical waste generation, and recycling where possible;
(3) Printing on demand-fast printing with production time ranging from months/weeks to minutes, printing at any time on simple printing equipment, processing in atmospheric environment and low temperature;
(4) Mechanical Flexibility — Thin substrates are mechanically flexible and can be bent to adhere to a curved surface, conformable, stretchable, bendable, crimpable, etc.;
(5) Extensible-large format printing, such as wallpaper and electronic skin.
Because of the unique advantages and broad application prospects of printed electronics technology, in order to further understand this interdisciplinary field, this paper summarizes the recent development of organic digital circuits fabricated by printing methods and their applications.The recent important work progress is summarized from the point of view of printing technology, organic digital circuits and applications, and the existing problems and future development in this field are prospected.
2 Printed materials and technology
Printed Electronics is a multi-disciplinary and multi-technical cross-application technology based on printing. Any electronic or optoelectronic manufacturing can be called Printed Electronics (PE) after using printing processing methods. In the early days, printing was not widely used in the preparation of electronic devices. Until 2000, Sirringhaus et al. Published an all-polymer organic field-effect transistor and an inverter with a gain of 2 prepared by inkjet printing in Science, which attracted the attention of scholars
[12]. After 2005, the number of articles in the field of printed electronics increased gradually, and scientists began to explore the use of printing methods to prepare organic circuits in order to produce electronic devices with large area, high integration and fast speed
[13].
Printed electronic materials are mainly divided into semiconductor materials, dielectric materials and electrode materials according to their functions. Semiconductor materials are mainly organic small molecule materials and polymer materials. Small molecule materials usually have better crystallinity, which is conducive to the transport of carriers to achieve better device performance
[14]. The development of high performance soluble organic small molecule materials and the modification of the structure of traditional organic small molecule materials to obtain better solubility are the main directions of the development of printed organic semiconductor materials
[15]. The printable dielectric layer materials mainly include organic insulating polymer materials and metal oxide dielectric layer materials. The former has good printing performance, but its relative dielectric constant is low, which makes the driving voltage of the transistor too high
[16]. Sol-gel printing of metal oxide precursor materials is an effective method to improve the relative dielectric constant, but the solidification of sol-gel usually requires high temperature, so the research on the solidification of metal oxide sol-gel at room temperature is an urgent problem to be solved in this field. The printed electrode materials include metal nanoparticles, conductive ink and conductive polymer materials. The metal electrode usually determines the channel length of the transistor device, that is, the feature size. The higher resolution printing method can effectively improve the integration of organic circuits and the frequency response of a single device
[17]. From a material perspective, the resolution is determined in part by the size of the metal nanoparticles in the ink. Therefore, the development of metal electrode materials suitable for high resolution and good conductivity has become a hot topic in this field.
Printing electronics technologies can be classified according to different categories, including (1) digital printing processes (such as inkjet printing and 3D printing) and non-digital printing processes (transfer printing and screen printing), depending on whether templates are used; (2) contact printing (e.g., screen printing, gravure printing, and transfer printing) and non-contact printing (e.g., 3D printing, inkjet printing), depending on whether the print head is in direct contact with the substrate; (3) Print on a single sheet or roll-to-roll basis, depending on throughput. This paper mainly introduces the non-contact printing represented by inkjet printing and the contact printing represented by screen printing and transfer printing.
2.1 Printing method
2.1.1 Ink-jet printing
Inkjet printing is a representative method of non-contact printing, which is widely used to prepare printed electronic devices. Inkjet printing involves five processes: ejection, flight, impact, diffusion, and drying, as shown in Figure 2
[18]. Inkjet printing requires inks with low viscosity, so metal nanoparticle inks and semiconductor inks are commonly used. The low viscosity of the ink causes the droplets to spread after they are deposited on the substrate, which limits the resolution of the ink-jet printed pattern. The diffusion of ink is affected by the surface energy of the substrate, the temperature of the substrate, and the properties of the ink. Although a low substrate surface energy may prevent the spreading process, a very low surface energy makes it difficult to obtain a stable pattern and causes the print line to break up into individual droplets
[19]. Therefore, the ink properties (i.e., solvent, ink concentration) and substrate surface properties must be optimized to achieve a clear pattern
[20]. When used in a continuous ink jet printing process, especially the solvent and ink concentration must be specifically selected to avoid nozzle clogging. Substrate surface properties and ink properties can significantly affect the drying behavior of the droplets, which determines the final morphology and structure of the deposited ink dots. The upper right inset in fig. 2 shows a typical coffee ring effect formed in an inkjet printed polymer film. This coffee ring effect is caused by convection within the droplets, so the entire process must be optimized to achieve the desired pattern through inkjet printing. Inkjet printing is widely used to fabricate source/drain electrodes and semiconductor layers in organic field effect transistors. The functional differences between the semiconductor and the source/drain electrodes impose different requirements on the inkjet printing parameters of the material. It is important to control the morphological/structural characteristics of ink-jet printed organic semiconductors during the drying process, as these characteristics can significantly affect the electrical performance of organic thin film transistors (OTFTs). Inkjet printing is a very efficient method of manufacturing source/drain electrodes, with a minimum pattern size of about 20 μm produced by an inkjet printer of standard pL level drop volume. For ink-jet printed source/drain electrodes, the electrode pattern size and the electrode conductivity affect the electrical performance of the OTFT. Therefore, properly processing the printed electrode to reduce the energy barrier between the organic semiconductor layer and the source/drain electrode is beneficial to obtain a high-performance OTFT
[21].
2.1.2 Screen Printing
Screen printing is a technology between direct writing printing and transfer printing, which can transfer ink to the substrate according to the design pattern through the screen. Fig. 3 is a schematic diagram of screen printing in which ink is pressed onto a screen with a squeegee and then printed onto a substrate through the open areas of the mesh. The mesh size affects the precision of printing: the larger the mesh size, the higher the resolution. In addition, the viscosity, polarity and wettability of printing ink will affect the precision of printing. As the viscosity of the ink increases, the difficulty of getting a high-resolution, well-edged pattern increases. In addition, the simplicity of the process makes it possible to print a variety of inks on various types of substrates, such as rigid substrate silicon, glass, flexible substrate polyvinyl alcohol and other substrates, which provides a new preparation idea for the preparation of flexible electronics, wearable electronics and so on. For different inks and substrates, in order to get a better pattern effect, on the one hand, different materials and different mesh numbers should be selected; On the other hand, the adhesion of the ink to the substrate can be improved by adjusting the viscosity, wettability, and the like of the ink.
2.1.3 Transfer Printing
Transfer printing uses a transfer medium, and the printed material is transferred by direct contact between the material and the target substrate. This high-speed, low-cost and simple printing process is quite suitable for mass production of devices such as OTFTs, and provides a new method for the production of organic transistors. Two types of transfer methods can be used depending on the phase of the material to be transferred. One method is to transfer the material between two different substrates, and the other is to use an embossing/engraving plate with transfer ink. The former includes soft lithography, such as microcontact, transfer printing, elastic stamps (such as polydimethylsiloxane (PDMS)), and rigid stamps (such as silicon wafers or glass), among others. Elastomeric stamps such as PDMS are mainly used as stamps with a surface resolution of less than 1.m
[23]. The inked stamp is brought into contact with a receiver (target substrate) and the ink is transferred to the receiver (fig. 4A), forming a pattern directly on the surface of the receiver. The resolution of this printing technique depends on the size of the pattern on the stamp. The latter type of transfer printing using relief/engraving plates includes intaglio printing (fig. 4B)
[24],
Flexographic printing and offset printing, etc. At present, OTFTs and other electronic devices can be mass-produced by a roll-to-roll process. Regardless of the transfer method, damage to the substrate during successive transfers should be avoided and the adhesion energy between the stamp and the receiver should be optimized
[25,26]. This requires precise control of basic material properties such as the surface energy of the substrate, the surface tension of the ink, and the cohesion between particles in the ink. In addition, the moving speed of the stamp is an important parameter for transfer printing because it strongly affects the adhesion strength of the pattern, and the stamp must move slowly during the transfer process to take advantage of the weak adhesion between the stamp and the film
[27].
A wide range of novel prin technologies, such as inkjet prin, screen printing, and transfer printing, are now being use to fabricate organic electronic devices. Each printing technique requires different ink parameters and provides different resolutions and thicknesses of the patterns formed. Table 1 gives a comparison of the print parameters for the various print methods. The ink viscosity required for inkjet printing is low, not exceeding 1000 mN·m-1, while the ink viscosity range for screen printing and transfer printing is as high as 100 000 mN·m-1, which greatly increases the range of materials that can be applied. The pattern resolution of ordinary inkjet printing can be as small as 30 μm, while the electrofluidic inkjet printing method (E-jet) can reduce the resolution to the size of 1 μm. The minimum resolution is 30 μm for screen printing and 10 μm for intaglio printing. Therefore, in order to obtain the desired film, it is essential to select the most appropriate ink and printing method.
2.2 Printed electronic device
With the rise of printed electronics in the printing world, printed electronic devices are the most extensive research direction in PE. For printed electronic devices, instead of printing traditional organic or inorganic pigments, different kinds of electronic functional materials such as conductors, semiconductors and insulators are printed. In addition, the structural characteristics of electronic devices determine higher printing accuracy requirements, which is an important feature of printed electronic devices different from the traditional printing industry. At present, the development of most of the printed electronic devices is still at a relatively early stage, which is reflected in the fact that the reports and studies on the electrical properties of devices only involve the most basic performance parameters, and there are still many deficiencies in the printing realization of high-performance devices. For example, for an inverter circuit device, in addition to obtaining a good inverter gain to enable the device to have a basic inversion function, another important indicator, the inverter noise margin performance, is far from ideal. Second, most printed digital circuits are based on p-type organic transistors, and this unipolar design results in low device gain, complex structure design, low noise tolerance, and so on. For example, the output signal swing of a p-type inverter with a diode-connected load is reduced, that is, the output cannot be pulled to the supply voltage (VDD), thereby reducing the noise margin. Thirdly, the operating frequency of printed electronic devices is very slow, which is related to the low resolution of printed manufacturing methods. For example, the highest frequency of the reported printed organic transistor ring oscillator is 80 kHz, while the highest frequency of the existing silicon-based device ring oscillator is 10 GHz. Fourth, the ION/IOFF of the current switching ratio parameter in printed electronic devices, especially in printed organic transistors, is generally low (typically 103~104), several orders of magnitude lower than in silicon-based devices (typically about 108). Because the definition of "on" and "off" States is not strict enough, and the noise tolerance is affected, the performance of printed electronic devices is still unreliable, and their application in higher integration and more complex circuits is limited. Fifthly, a large scale integrated circuit containing many logic gates is an indispensable part of realizing complex digital systems. In the reported work, the number of printed electronic digital logic gates is small, and the types of logic gates are limited. Sixth, the reliability and lifetime of printed organic electronic devices remain suboptimal compared to silicon-based devices.
In addition, the choice of materials is a crucial step in the preparation of printed electronic devices. The performance of the OTFT largely determines the performance of the subsequently fabricated circuit. Organic semiconductor material is an important part of OTFT, and it is a key task to select the appropriate material for device preparation. For organic semiconductor materials, the main challenge is the charge carrier mobility, which is expected to be high in order to achieve higher operating frequencies for devices and circuits. Organic semiconductor materials are classified as p-type, n-type, and bipolar materials. At present, the p-type materials commonly used in the preparation of printed electronic products include 29-DPP-TVT, diF-TES-ADT(0.20 cm
2/Vs), BTBT and its derivatives (C
8-BTBT,7.5 cm
2/Vs), DNBDT and its derivatives (C
10-DNBDT,3.0 cm
2/Vs) and TIPS-pentacene
[29][30][31][32][33]. The n-type materials include :P(NI2OD-T2)(0.12 cm
2/Vs), TU-3(0.069 cm
2/Vs), BASF(0.22 cm
2/Vs) and the like
[29][30,34][35]; Bipolar materials have :PTVPHI-Eh(μ
h=0.29 cm
2/Vs,μ
e=0.001 cm
2/Vs)
[36]. From the above, it can be seen that the mobility level of semiconductor materials used in printed electronic devices is much lower than the 10 cm
2/Vs, in most cases, below the 1 cm
2/Vs. The development of air-stable n-type organic semiconductors with excellent mobility lags far behind that of p-type organic semiconductors. The mobility of n-type semiconductors is usually several orders of magnitude lower than that of p-type semiconductors, and the electron mobility of bipolar materials is also several orders of magnitude lower than that of hole mobility.
3 Organic digital circuit
3.1 Inverter
The basic building block of an integrated circuit is a logic gate circuit. The simplest NOT gate in a logic gate circuit is also called an Inverter. It can reverse the phase of an input signal by 180 degrees. This circuit is often used in audio amplification, clock oscillators, etc. Inverters are often used in the design of electronic circuits
[37]. Inverters are mainly divided into two types: CMOS and Pseudo-CMOS
[38⇓~40][41,42]. CMOS inverters are complementary inverters based on PMOS and NMOS, while Pseudo-CMOS is a design style based on unipolar thin film transistors (p-type or n-type), which has been widely used in digital, analog and power circuits. Here, two widely used inverter types are introduced. In fig. 5, enhancement mode (Pseudo-E) and depletion mode (Pseudo-D) inverters based on p-type thin film transistors are shown
[31]. The main difference between these two design types is the connection of the transistors, with the Pseudo-E and Pseudo-D inverters using diodes and a zero gate-to-source voltage (V
gs) connection, respectively. The zero Voltage Transfer Characteristics connection of the Pseudo-D inverter results in a high gain in the Voltage Transfer Characteristics (VTC) curve. The VTC curve of the Pseudo-E inverter has a very smooth transition with a constant slope over a wide input range and a low gain.
In 2016, Shiwaku and Takeda of Yamagata University in Japan successfully printed short-channel OTFT devices with high mobility, low-voltage operation and uniform electrical characteristics on glass substrates by inkjet printing using a mixed solution of C
6-DTBDT and polystyrene. Based on this p-type OTFT, 15 Pseudo-D inverters with excellent performance, uniformity and low-voltage operation were prepared
[35]. All inverters can operate with a V
DD of 1 V, indicating that the inverter circuit can operate properly in the low power state. Figure 6B shows the voltage inversion point (V
Trip) and inverter gain (dV
OUT/dV
IN) versus V
DD. The standard deviation of the V
Trip for each V
DD was 0.09, indicating that the OTFT characteristics were very uniform. Since the average gain of the inverter is greater than 100 at a V
DD of 3 to 5 V, this inverter can be used to make an amplifier for a sensor. This work successfully fabricated high performance inverters with high throughput, uniform V
Trip, and low voltage operation, demonstrating that printed OTFT devices can be used in the fabrication of complex integrated circuits that require reduced manufacturing costs over large areas. These results will help establish the reliability of printed electronics.
3.2 NAND gate
A NAND gate is a digital logic gate equivalent to an and gate followed by a not gate, with two or more inputs and one output. A low level is output if both that first input and the second input of the NAND are high. NAND is one of the general gate circuits, and any logic circuit can be implemented with NAND. Multiple NANDs can be designed to form circuit modules such as flip-flops, data selectors, and demultiplexers, which are used to form more sophisticated components such as memories and microprocessors, and are important basic components in integrated circuits
[43][44,45].
In 2019, Kwon and Takeda's research on the preparation of three-dimensional (3D) monolithic integrated circuits using flexible printed organic transistors was published in Nature communications
[36]. Interconnected stacked 2-transistor (2-T) and 3-transistor (3-T) devices were prepared by inkjet printing of silver nanoparticles as electrodes to fabricate inverter and NAND circuits. In the 3D NAND gate, the symmetrical bottom and top gate operation of the p-type double-gate transistor makes the two input and output characteristics almost the same (fig. 7B). To optimize the static noise margin in 3D NAND gates, the drain current (I
D) of a double-gate n-type transistor in double-gate mode is matched to the I
D of a double-gate p-type transistor in bottom-gate or top-gate mode by adjusting the thickness of the dielectric layer as well as the channel length. A scale fabrication process then produce a large 3D array of NAND gates (fig. 7c),
When the 3D NAND design is composed of four transistors (Figure 8), the recording transistor density is about 60 transistors per square centimeter. Based on this technology, up to 2700 programmable transistors can be fabricated in the size of a standard credit card (85.60 × 53.98 mm), which is compatible with the transistor count of the first commercial 4-bit microprocessors. 3D monolithic integration is a promising strategy to scale the technology while improving the performance of flexible printed transistor devices. This approach will open up possibilities for innovative flexible printed ICs for the Internet of Things, wearable medical monitoring, and smart packaging, where sophisticated device performance and the ability to integrate hundreds of transistors onto a limited plastic or flexible substrate are essential.
3.3 Ring Oscillator
The ring oscillator is a closed loop consisting of an odd number (N) of inverters connected in series in a positive feedback fashion, and the output oscillates between two voltage levels (1 or 0). If the oscillator has 3 inverters, it is called a three-stage ring oscillator, and if the number of inverters is n, it is an n-stage ring oscillator. The number of inverter stages in the oscillator depends primarily on the frequency to be generated from the oscillator. Ring oscillators and related circuits can be used as basic building blocks such as clock oscillators in computers and carrier frequency generators in wireless communications. It is also the basic circuit for evaluating the intrinsic speed of CMOS logic processing
[46,47]. The ring oscillator uses an odd number of inverters to produce the effect of a single inverting amplifier with a gain greater than one. Instead of a single delay element, each inverter results in a delay of the signal around the inverter ring, hence the name ring oscillator. Adding a pair of inverters to the loop increases the total delay, which reduces the oscillator frequency. Changing the supply voltage changes the delay of each inverter, with higher voltages typically reducing the delay and increasing the oscillator frequency
[48,49]. If "t" denotes the delay of a single inverter and "n" denotes the number of inverters in the inverter chain, the oscillation frequency is given by:
The oscillation frequency is inversely proportional to the number of stages and the propagation delay time.
In 2010, Jung et al. Printed single-walled carbon nanotube thin film transistors (SWCNT-TFTs) and other devices by inkjet printing. In order to add clock signals at the maximum frequency, they used SWCNT-TFTs with mobility of 5.24 cm
2/Vs, threshold voltage of -4 V and transconductance (gm) of 0.307 μS to prepare a five-stage ring oscillator
[50]. Oscillation frequencies of 1, 12, and 60 Hz were observed at 10 V direct current (dc), and an oscillation frequency of 67.4 Hz was observed at 12 V dc (Figure 9). On the basis of this ring oscillator, they continue to prepare 1-bit RFID tag precursors, which fully demonstrates that more complex functional printed electronic systems can be realized through reasonable design on the basis of excellent ring oscillator components.
3.4 D flip-flop
Flip-flop usually refers to the circuit that can be level triggered or edge triggered, and it is the basic component of sequential logic circuit. It is a circuit that has two stable States and can store one bit of state information. The circuit can be caused to change state by applying a signal to one or more control inputs and has one or two outputs
[51]. Triggers are the basic building blocks of digital electronic systems used in computers, communications, and many other types of systems. D-flip-flop (DFF) is the most widely used type of flip-flops, because DFF is the basis of digital counters such as frequency divider counters, ring counters (shift registers), and counters are an important part of many electronic devices
[52,53].
In 2015, Takeya, Uno, Kanaoka, et al. Of the University of Tokyo ingeniously fabricated single crystal films of C
10-DNBDT and GSID104031-1 (BASF) in parallel on a substrate by a continuous crystal growth method of edge-cast to prepare p-type and n-type transistors with short channels in order to prepare complementary logic circuits, as shown in fig. 10a
[32]. The average mobility of the p-type transistor is the average mobility of the -type transistor is (0.16±0.04)cm
2/Vs. These devices have a high on/off ratio, greater than the 10
6, which is critical for proper operation of CMOS logic circuits. Subsequently, a CMOS circuit consisting of p-type and n-type transistors was prepared. To measure its dynamic response, a 5-stage ring oscillator with a response speed of 110 kHz per stage was designed and fabricated. The DFF circuit was then designed based on NOR logic, which can be made with a smaller number of transistors (26). Fig. 10C is a signal of a DFF circuit, and the expected response of the circuit is observed when a rectangular wave is input. The output signal represents the response to the input signal. These findings will accelerate the development of organic logic circuits, demonstrate a way to construct organic monocrystalline complementary logic circuits, and contribute to the future realization of practical applications of organic electronics.
4 Application
After the realization of printed organic electronic digital circuits with a limited number of devices, the development of functional printed electronic systems by further improving the integration level is the focus of development
[54,55]. Because of the unique properties of printed organic electronics, especially its thin size and mechanical flexibility, it enables a wide variety of applications and functions, including radio frequency identification tags (RFID), electronic skin, OLED display driver backplanes, and flexible microphone arrays for separate audio sources, among other applications.
4.1 RFID tags
Of all the printed electronics applications, RFID is the most popular. This may be because its thin and flexible form factor properties provide a range of applications beyond rigid RFID, for example, RFID placed on a curved surface. At present, a large number of RFID tags based on printed electronics have been reported, but they are all near-field RFID, and far-field RFID has not yet been reported. This is because the operating frequency of the near-field RFID is significantly lower than the operating frequency of the far-field RFID, typically 128 kHz and 13.56 MHz, while the latter is greater than 100 MHz, such as 2.45 GHz. These frequencies are related to printed diodes (rectifiers) based on diodes or diode-connected TFTs
[56⇓~58].
In 2010, Suncheon University of Korea cooperated with MICE to print the antenna, electrode and dielectric layer by roll-to-roll gravure printing, and to print the SWCNT-TFT device by inkjet printing. Finally, a 1-bit RF tag precursor with a working frequency of 13.56 MHz was prepared. The schematic circuit diagram of the tag is shown in Figure 11A
[50]. The fabrication of the RF tag opens a practical way to develop low-cost RFID tags, RF sensors, and RF-powered displays. In 2014, Myny et al. Of IMEC, Belgium, prepared an RFID tag based on pentacene and In
2O
3 metal oxide TFT rectifiers by combining evaporation, atomic deposition and spin coating
[59]. The thin film RFID tag technology combines metal oxide n-type transistors with organic p-type transistors to form a hybrid complementary circuit. The design of the RFID tag includes two metal oxide transistor rectifiers and a comparator (Figure 11B). Complementary, mixed-solution processed metal oxide/OTFT technology demonstrates a feasible approach to fabricating low-cost RFID tags at 13.56 MHz. In 2015, Fiore et al. Of the University of Catania fabricated complementary TFTs on a polyethylene-naphtalate (PEN) substrate by screen printing, and implemented an integrated 13.56 MHz RFID tag according to the design in Figure 10c
[60]. The RFID has a bit rate of up to 50 bit/s over a reading range of 2 to 5 cm at an internally generated 24 V supply voltage, proving its RFID capability. This work, with more than 250 transistors on the same sheet (Figure 11 d), is the most complex circuit ever reported for printed organic complementary TFT technology, demonstrating the feasibility of implementing very complex printed electronic systems.
4.2 Electronic Skin
Electronic skin is a flexible, stretchable and self-healing electronic product that can imitate the function of human or animal skin, with the advantages of high sensitivity, mechanical flexibility, stretchability, multi-function and large area. In addition, electronic skin is an ultra-thin, flexible and wearable electronic product that combines information and communication technology with innovative materials, microelectronics and sensors, with many potential applications in robotics, artificial intelligence, prosthetics and health monitoring technology (Figure 12)
[61⇓~63]. Researchers are developing new materials and manufacturing methods to create skin-like electronics with mechanical compatibility and versatility, and to transmit skin signals to brain/machine interfaces in the body
[64]. In addition, the preparation of the electronic skin is closely related to the printing of electronics, because the printing method has low requirements on the substrate and a wide range of applicable materials, a sensor for monitoring the surface signal of human skin can be printed on a flexible stretchable or self-healing material, and then the electronic skin can be prepared after steps such as packaging.
In 2013, Takao Someya et al. Of the University of Tokyo chose DNTT, a stable organic semiconductor material, to fabricate a hybrid electronic system including integrated circuits, sensors and power supplies on a 1 μm thick PEN substrate based on common organic electronic manufacturing methods
[65]. With a total thickness of only 2 μm, these electronic devices are very lightweight, like plastic wrap, and can be easily applied to curved and active surfaces, so they can be used to prepare electronic skins for healthcare and biomedical systems (Fig. 13A, B). In 2015, Bao Zhenan and Tee of Stanford University prepared DiTact sensors consisting of pressure-sensitive tactile elements and organic ring oscillators on PEN and polyurethane by spin-coating and evaporation
[66]. The maximum frequency of the DiTact sensor is 200 Hz, which is comparable to the upper limit of action potential generation during human tactile stimulation, and the output of the sensor can electrically and optically stimulate neurons in vitro using engineered optogenetic neurons that respond to the frequency range of human action potentials. As shown in Fig. 13C, a schematic of a model hand depicts how the sensor is incorporated into the prosthesis, with the electrical components connected to an external circuit with stretchable silver nanowires. In addition, DiTact sensors are equipped with wearable gloves to display the frequency changes of voltage signals after pressure is applied in wearable systems, which can be integrated into electronic skin on the surface of conventional neuroprostheses or optogenetics-based prostheses. In 2019, Liu Jing of Tsinghua University and others used gallium-based functional liquid metal (LM) to prepare an electronic skin for multi-site tumor treatment, which has a good magnetocaloric effect.The success of wireless power transmission in alternating magnetic fields for multi-site cancer therapy is expected to expand the development of cancer therapeutics and develop non-invasive and high-performance cancer treatment methods in the future
[67].
4.3 OLED display
As a new generation of flat panel display devices, OLED has a simple structure and does not need background light and filters, so it can produce ultra-thin, lightweight and portable products. However, in recent years, the flat panel display market has not continued to expand due to the gradual saturation of the display market, shrinking TV demand and fierce competition among display manufacturers. To break this stagnation, new displays are needed to build new markets. Flexible displays will become one of the new display forms in the future, because flexibility can meet the requirements of various forms of future displays that can be mounted on any shape object, such as circular dome ceilings, columns, large windows and even walls. In addition, the future need to access information anytime and anywhere will require many displays, so the price of display panels should be cheap. In short, flexibility and low price will be the key features of future displays. These key features can be achieved by flexible displays manufactured by printing technology. From the perspective of devices, the combination of OTFT and OLED is the most promising combination, which is expected to achieve ideal flexible display as a driving transistor and a display mode, respectively. Therefore, OLED display driven by OTFT has become an important direction of the development of printed electronics technology.
In 2013, Ryu et al. Of Dong-a University in Korea applied spin coating, screen printing, inkjet printing and other printing technologies on glass substrates to manufacture OTFT backplanes for AMOLED
[68]. The gate electrode, the scan bus and the contact pad were fabricated by screen printing and photolithography, and the Ag metal lines with a width of 20 μm, a thickness of 60 nm and a resistivity of 3.0×10
-5Ω·cm were obtained. The OTFT backplane using Ag gate electrodes and ink-jet printed TIPS-pentacene achieved more than 17,500 pixels on a 7-inch panel with uniform performance, successfully demonstrating its ability to display patterns. In 2016, Kim and Song used inkjet printed TIPS-pentacene OTFTs to drive OLEDs on flexible polyethylene (PET) substrates
[69]. The mobility of the OTFT is a 0.3 cm
2/Vs,OLED and the luminance is a 64 459 cd/m
2, and the luminance of each pixel can be changed according to the applied voltage (fig. 14B). In 2020, Zhou Hang of Peking University and others published an article on the preparation of OLED displays by inkjet printing of quantum dots (QD). In this work, a novel full-color display based on blue organic light emitting diodes (BOLEDs) and patterned red and green QD color conversion layers (CCLs) is proposed. Based on this QD-BOLED architecture, a 6.6 inch full-color display has been successfully fabricated.
5 Conclusion and prospect
As an emerging technology, printed organic electronics technology has attracted extensive research interest, mainly because of its unique technical advantages, in addition to low-cost, large area manufacturing, printing technology can also be applied to a variety of different types of substrates, including three-dimensional curved surfaces. These characteristics will change the production mode of the whole electronic device and broaden the application of electronic devices. Inkjet printing, in particular, can provide flexible and inexpensive production for customized low-volume products, making "tabletop manufacturing" possible. Printed electronics technology has demonstrated the printing ability of organic semiconductor materials, insulating layer materials and electrode materials, constructed basic components such as thin film transistors, diodes, capacitors and basic logic gate circuits such as inverters, and demonstrated its application potential in RFID, electronic skin, display driving backplane and other fields. However, this technology is still in the early stage of research and development, and there are still many problems. First, the ink used in the printing method is relatively unstable, and the performance of different batches of products varies greatly, which makes the prepared printed electronic devices relatively unreliable. Secondly, compared with silicon-based circuit components, the performance of printed OTFT, diode, rectifier and other basic components is low, which makes the performance of printed electronic products difficult to compare with that of similar silicon products. However, because of the unique flexibility of printed electronics, printed electronic devices can be used as a supplement to silicon-based electronic devices. Third, printed electronic systems have limited integration, meaning that the overall device count is still low and far below that of their silicon-based counterparts. If printed electronic products really want to occupy a place in the market, they still need to develop and study key scientific and technological issues such as new printing materials (semiconductor materials, dielectric materials and electrode materials), printing process and printing device integration. Optimistically, people have high expectations for printed electronics. IDTechEx Consulting predicts that by 2025, the global sales of all kinds of printed electronics manufacturing products will reach 300 billion US dollars. At that time, the maturity of printed electronics technology will affect and change people's lives in many ways.
The work was supported the National Key R&D Programs(2017YFA0204503)
The work was supported the National Key R&D Programs(2016YFB0401100)
the National Natural Science Foundation of China(91833306)
the National Natural Science Foundation of China(51633006)
the National Natural Science Foundation of China(51703159)
the National Natural Science Foundation of China(51703160)
the National Natural Science Foundation of China(51733004)
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