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Materials and Process Engineering for Printed and Flexible Optoelectronic Devices

ANTONIO FACCHETTI
Polyera Corporation and Northwestern University

Printed optoelectronics is a revolutionary technology to fabricate mechanically flexible, low-cost, lightweight, and large area electronic devices by using electronic inks and processes borrowed from the graphic arts industry (Kim et al. 2012; Loo and McCulloch 2008).1 Devices such as light emitters, light harvesters, circuits, and sensors will be based on a new materials set for the semiconductor, dielectric, electrical contact, emitter components (Facchetti 2007). Applications such as flexible displays, plastic radio frequency identification tags, disposable diagnostic devices, rollable solar cells, and simple consumer products and games represent a future multibillion-dollar market. Smart objects (e.g., packages that integrate multiple printed devices) are further examples of printed optoelectronics.

Companies, startups, research institutions, and government agencies are investing in research and development in this field. Progress will also depend on close collaboration among chemists, materials scientists, and engineers—whether they are material providers, equipment makers, producers, or system integrators— to ensure the success of printed electronics in the marketplace.

This paper provides an overview of the materials and process requirements and the applications of this technology.

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1Printed electronics and optoelectronics are used interchangeably in this paper, although printed electronics generally refers to electronic circuits, whereas optoelectronics embraces photonic devices such as light-emitting diodes, light-emitting transistors, and solar cells. Also, because all printable materials were originally organic, organic electronics was used instead of printed electronics; several inorganic materials are now printable.



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Materials and Process Engineering for Printed and Flexible Optoelectronic Devices Antonio Facchetti Polyera Corporation and Northwestern University Printed optoelectronics is a revolutionary technology to fabricate mechani- cally flexible, low-cost, lightweight, and large area electronic devices by using electronic inks and processes borrowed from the graphic arts industry (Kim et al. 2012; Loo and McCulloch 2008).1 Devices such as light emitters, light harvesters, circuits, and sensors will be based on a new materials set for the semiconductor, dielectric, electrical contact, emitter components (Facchetti 2007). Applications such as flexible displays, plastic radio frequency identification tags, disposable diagnostic devices, rollable solar cells, and simple consumer products and games represent a future multibillion-dollar market. Smart objects (e.g., packages that integrate multiple printed devices) are further examples of printed optoelectronics. Companies, startups, research institutions, and government agencies are investing in research and development in this field. Progress will also depend on close collaboration among chemists, materials scientists, and engineers—whether they are material providers, equipment makers, producers, or system integrators— to ensure the success of printed electronics in the marketplace. This paper provides an overview of the materials and process requirements and the applications of this technology. 1  Printed electronics and optoelectronics are used interchangeably in this paper, although printed electronics generally refers to electronic circuits, whereas optoelectronics embraces photonic devices such as light-emitting diodes, light-emitting transistors, and solar cells. Also, because all printable materials were originally organic, organic electronics was used instead of printed electronics; several inorganic materials are now printable. 113

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114 FRONTIERS OF ENGINEERING Background The goal of printed electronics is not to replace the conventional inorganic- based electronic industry. Rather, it offers opportunities for new products and/or reduces the cost of certain devices (Table 1). Traditional thin film materials deposition is accomplished with chemical vapor deposition, physical vapor deposition, and sputtering. Although these processes are performed in a vacuum, they are not intrinsically “low speed.” For instance, polymer webs over 2 m in width are metallized at 18 m/s for food packaging at the cost of pennies per square meter. Conventional printing presses used in the graphic art industry commonly run at speeds of m/s, with webs several meters wide, and are used to deposit several different types of color inks. Simi- larly, at the highest degree of sophistication liquid crystal display production is based on processing large glass plates (>4 m2) with a takt time of a few minutes. Film patterning on glass (for display) and on silicon wafers uses conven- tional photolithography. In this subtractive process active film is deposited over the entire substrate area and then selected regions of it are removed by coating the film with photoresist, exposing the photoresist film to contact or projection optics (or electrons for e-beam lithography), developing the photoresist, etching the underlying layer, and stripping the resist. This process produces the resolution and reliability required for the high-tech integrated circuit industry. But photo­ lithography is costly, using extremely expensive equipment (a new FAB line, the infrastructure to fabricate electronic devices, costs $2–3 billion), and requiring several batch-to-batch steps. TABLE 1  Conventional versus organic electronics. Conventional Electronics Organic/Printed Electronics Advantage or disadvantage High performance Low performance Small area/feature size Large area/feature size High cost/unit area Low cost/unit area High capital investment Low capital investment Long production run Short production run Durable Disposable Rigid Flexible Selected markets Everywhere Photolithography Printing Materials Semiconductor, conductor, dielectric, passive, substrate Devices/applications Transistors, circuits, memory, diodes, sensors, displays, batteries, photovoltaics, conductive traces, antennas, resistors, capacitors, inductors

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MATERIALS AND PROCESS ENGINEERING 115 The advantage of using printable materials, most of which are organic (see below), is to replace conventional processes for device fabrication. The formula- tion of organic materials into inks (active material + solvent + additives) would enable roll-to-roll printing or printing-like processes. Furthermore, the additive process of printing minimizes material waste. If similar processes could be used for functional materials, high-volume inexpensive devices could be fabricated. This is a goal of printed (or organic) electronics. The main obstacle to the realization of this technology is on the materials side, particularly the semiconductor (charge-transporting material), because highly processable semiconductors exhibit poor charge transport performance. Furthermore, it is unlikely that the same printing presses used for newspapers and magazines can be used for processing functional materials, so modification and optimization on the processing side are also necessary. Finally, device design architectures and circuit engineering are necessary to address the poorer perfor- mance of organic materials and/or take advantage of their unique properties. Devices and Applications Transistors and Circuits The field-effect transistor (FET) is essential in almost all electronic devices (Facchetti 2007), and it is the building block of the circuits (a collection of con- nected FETs) necessary for logic operations, memory functions, displays, and sensors. FETs based on organic (OFET) or other printable semiconductors have the structure of a thin film transistor (Figure 1), a three-terminal device composed of source, drain, and gate electrodes, a dielectric layer, and a semiconducting layer. The transistor is essentially an electronic valve or switch, and the flow of current between the source and drain electrodes (for a given source-drain bias) is controlled by the magnitude of the source-gate voltage (or electric field dropped through the dielectric layer). The charge flow in the transistor channel can be dominated by positive charges or negative charges (electrons), which define whether the semiconductor is p- or n-type, respectively. The two most important transistor performance parameters are the charge carrier mobility (m, how fast electrons move) and the current on-off ratio (Ion:Ioff, how efficiently the current can be modulated by the source-gate bias). To maxi- mize the transistor speed, the carrier mobility should be as high as possible and the distance between the source and drain electrodes (channel length) as small as possible. The carrier mobility of printable semiconductors is about two orders of magnitude lower than that of crystalline inorganic materials, and typical resolu- tion for the OFET channel length in printed devices is larger by the same order of magnitude. Thus OFET circuit speeds cannot compete with those based on silicon or gallium arsenide and fabricated using photolithographic processes. However,

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116 Organic Field-Effect Transistor (OFET) ApplicaƟons Id , Vd Source Drain Semiconductor Vg p- or n-type Dielectric Printed Circuits, Rollable Gate Rollable Display RFID Newspapers Sensors Substrate Organic Photovoltaic Cell (OPV) Metal cathode g pp I Interlayer Semiconductor Donor-Acceptor Blend Interlayer ITO anode Flexible Modules Power Textiles Power Window Substrate FIGURE 1  Structure of an organic field-effect transistor and photovoltaic cell and corresponding applications. ITO = tin-doped indium oxide; . RFID = radio frequency identification; Id = Drain current; Vd = drain voltage; Vg = gate voltage; I = Photocurrent.

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MATERIALS AND PROCESS ENGINEERING 117 when the performance requirements are relaxed and/or it is necessary to add device functions (e.g., flexibility, easy integration) and/or reduce costs, OFETs may be very competitive. Displays One great area of opportunity for printed electronics is in the fabrication of backplane circuits for flexible displays. Electrophoretic displays, in which the image is formed by black and white charged particles, are well suited for printed transistors because of the slow switching time and minimal current flow needed to drive them. Furthermore, they are bi-stable, meaning that the image is retained without power (power is required only during refresh). In these displays the contrast is independent of viewing angle and significantly better than newsprint. Polymer-dispersed liquid crystal and electrochromic-based displays can also be driven by printed transistors. In addition to transistors and backplanes, organic electronic materials, some of which can be solution processed, are used to fabricate emissive devices such as organic and polymer light-emitting diodes (OLEDs and PLEDs, respectively). Radio Frequency Identification Tags Radio frequency identification (RFID) tags use radio frequency transmissions to identify, track, sort, and detect persons and items based on communication between a reader (interrogator) and a transponder (a chip connected to an antenna, often called a tag). RFID tags can be either active (powered by battery) or ­passive (powered by the reader field) and come in various forms such as smart cards, tags, labels, watches, and even embedded in mobile phones. The communication frequencies used range from 125 KHz to 2.45 GHz, depending to a large extent on the application. Regulations imposed by most countries control emissions and prevent interference with other industrial, scientific, and medical equipment. RFID is not expected to replace bar codes in the supply chain because tags are still too expensive, even though their prices have fallen to around 20 cents in volume (versus 0.2 cents for a bar code label). Adoption is therefore likely first for expensive items, then as technology advances and costs decline tags will probably appear on more and more products. It is commonly thought that the only way to reduce the price sufficiently, and produce billions or trillions of tags per year, is by printing both the circuitry (using solution-processable materials) and the antenna in an integrated process. The major obstacle to printed RFID applications is to achieve high circuit frequency operation and enable efficient rectification (convert the ac-voltage detected and generated by the antenna at the targeted base carrier frequency to a dc voltage).

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118 FRONTIERS OF ENGINEERING Sensors Another important opportunity for printed electronics lies in the fabrication of sensors, which can be used to detect a variety of stimuli such as temperature, pressure, radiation, and chemical identity. An integrated temperature and pressure sensor array has been utilized as an artificial skin, and organic actuators have been fabricated. Sensors are also used in tamper-detecting packaging, data-logging pill dispensers, chemical sensors, electronic noses and tongues, photodiodes, and light scanners. Photovoltaics Another exciting application of printed electronics is in photovoltaics, a field currently dominated by silicon. The vision is to fabricate inexpensive, lightweight, flexible, conformal, and efficient production of energy from the sun. Photovoltaic devices are composed of a charge-transporting donor-acceptor semiconductor blend sandwiched between two electrical contacts (Figure 1) (Facchetti 2011). Light exposure produces free carriers that are collected at the electrodes as electrical energy. The most important metric is that of power con- version efficiency (showing how efficiently the solar energy is converted into electrical energy). To be more widely adopted, the photovoltaic semiconductor needs to provide high power conversion efficiencies (solar to electrical energy efficiency) while remaining stable and inexpensive. Printing Technologies Enabling the use of roll-to-roll and high-throughput device fabrication tech- nologies is the most relevant aspect of printed electronics. Throughputs greater than 1 m2/s are considered “high volume”; most printing methods fall in this category. There are several considerations to determine what process can be used based mainly on the viscoelastic properties of materials and the desired feature sizes (lateral resolution, film thickness, surface morphology, surface energy) required for device assembly. Table 2 shows some of the most important printing techniques and relevant specifications for use in electronics; probably the most used processes are inkjet, screen, and gravure printing. Inkjet Printing This process uses a stepper motor to control the position of the print head along a stabilizer bar; as the print head slides back and forth along the bar, ink drops are ejected from the nozzle to create a pattern on the substrate. There are two primary mechanisms for ejecting the ink: in a thermal inkjet, evaporation of

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TABLE 2  Typical ink requirements, printing features, and throughputs of conventional printing methods. Ink Viscosity Film Thickness Resolution Registration Throughput Features/ Electronic Technique (mPas) (µm) (µm) (µm) (m2/s) Issues Materials Printed Lithography (10–4) × 102 1.5–0.5 50–10 >10 0.1 High quality; Conductor Need for additives Screen (5–0.5) × 102 100–30 100–30 >25 2–3 Wide range of inks; Conductor Medium quality Flexography 5000–50 2.5–0.8 70 <200 10 Wide range of substrates; Conductor Medium quality Semiconductor Dielectric Gravure 200–50 5–0.5 5 60 Large run length; Dielectric High quality Semiconductor Pad >50 2–1 20 >10 0.1 Nonplanar objects Conductor Laser/thermal N/A <1 5 ~10 0.002 N/A Conductor Semiconductor Inkjet 30–1 <0.5 <10 20–5 0.5–0.01 Digital data; Conductor Local registration Semiconductor N/A = not applicable 119

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120 FRONTIERS OF ENGINEERING a small portion of the ink solvent forces ink out of the nozzle; in a piezoelectric inkjet, voltage applied to a piezoelectric material causes it to expand, forcing ink out of the nozzle. Inkjet printing is now one of the most widespread graphic arts printing m ­ ethods, and in recent years has received attention as a technique to deposit func- tional materials with specific electrical, optical, chemical, biological, or structural functionalities at well-defined locations on a substrate. One of the most useful features of inkjet printing is its capacity to vary the printing pattern without the need to make a new printing plate. Using a camera and image analysis software, the printed image can be adjusted “on the fly” to compensate for many of the registration errors that plague other types of printing process. Organic semiconductors are commonly inkjet patterned for transistor and OLED fabrication. Recently a variation on inkjet printing, self-aligned printing, was used to pattern features as small as 60 nm. Inkjet printing has some limitations, such as susceptibility of the printing head to corrosion from aggressive solvents, a liability to high mechanical shears in piezoelectric print heads, and high temperatures in thermal inkjet heads. In addition, fluctuations in droplet volume or trajectory can adversely affect film uniformity (creating a “coffee-stain” effect) and materials performance. Screen Printing Screen printing consists of three key elements—the screen, which is the image carrier; the squeegee; and the ink—and uses a porous polyester or stainless steel mesh stretched tightly over a wood or metal frame. A stencil, produced on the screen either manually or photochemically, defines the image to be printed (in other printing technologies this is called an image plate). The technique usually produces relatively thick films (thinner films fabricated using less viscous inks often result in poorly defined printed patterns); the resolution is poor and limited by the screen size, although recent presses perform much better. In organic electronics this technique has been used to fabricate top-level interconnects and contacts where the thickness of the printed film is not a critical factor. It can also be used to print thick dielectric layers and passive materials for device encapsulation. Indeed, the first report of a “printed organic thin film transistor” described screen-printed carbon paste electrodes for source, drain, and gate contacts (Horowitz et al., 1996). Gravure Printing In gravure printing the image areas consist of honeycomb-shaped cells that typically are etched or engraved on a copper cylinder. The cylinder rotates into an ink pan and, as it turns, excess ink is removed by a flexible steel doctor blade so that the ink remaining in the recessed cells forms the image by direct transfer to

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MATERIALS AND PROCESS ENGINEERING 121 the substrate as it passes between the plate cylinder and the impression cylinder. The process uses fluid inks with relatively low viscosity. Because it is one of the highest-volume printing processes, gravure printing is often used commercially to produce high-quality graphic materials such as magazines. Depending on the nature (depth, surface energy, shape, etc.) of the engraved cells, different amounts of material used to fabricate the devices (semi- conductor, conductor, dielectric) can be deposited in different substrate regions, enabling tuned film thicknesses. The drawback is that the printed pattern edges may be wavy. Gravure has recently been used to fabricate dielectric and semiconducting layers for organic transistors. Electronic Materials Printed electronic devices need a set of core materials for charge accumula- tion, injection, and transport as well as specific materials to enable particular device functions (Facchetti 2013; Facchetti et al. 2005; Usta et al. 2011). Every type of electronic device needs memory capacity and a control for cur- rent flow, both of which are based on FETs, which in turn need three fundamental materials: a conductor, dielectric, and semiconductor (Table 3). Depending on the specific device functions, additional active materials may be needed. OLEDs, for example, require an emissive material for efficient conversion of electricity to light, whereas organic photovoltaics need a photosensitizer and/or efficient light absorber for photon absorption and dissociation (in addition to the materials needed for efficient charge transport). Displays may be based on different technologies for pixel fabrication, such as organic emitters, electrophoretic inks (proper dyes are necessary), liquid crystals (LC molecules are used), and electrochromic compounds. Many other types of chemicals/materials may also be necessary for device fabrication, such as small molecules for interfacial layers to ensure efficient charge injection or surface energy match, additives for use as dopants or stabilizers, and polymers for encapsulation. The properties of conductors, semiconductors, and dielectrics are summa- rized below. Semiconductors The semiconductor is the most important material in optoelectronic devices, although its key function varies depending on the device application, and different electronic devices use different types of semiconductors. When used in organic transistors, it is the material where, at the interface with the dielectric, charges are accumulated and transported. The semiconductor must satisfy a number of requirements for charge injection and transport related to the device structure.

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TABLE 3  Properties of current-generation solution-processed materials for printable electronics. 122 Materials Conductor Semiconductor Dielectric Performance/ Metal/metal Small needs oxides Polymers molecules Polymers Inorganics Polymers Inorganics Current s > 104 S/cm s > 1 S/cm m = 1–30 cm2/Vs; m = 0.1–10 cm2/Vs; m = 1–100 cm2/Vs; J < 10-8 A/cm2; J < 10-7 A/cm2; performance (~104 S/cm) Ion:Ioff > 106 Ion:Ioff > 106 Ion:Ioff > 106 BF > 6 MV/cm BF > 5 MV/cm Current Ag, Au, Cu PEDOT:PSS; Fused thiophenes; Polythiophenes; In2O3, ZnO, IZO, PMMA; Sol-gel oxides; materials nanoparticles; PANI; Polymer + Heteroarenes; Naphthalene IGZO P-UV; Oxides-epoxy ITO CNT; Graphene Perylenes diimides; Cross-linked DPPs PVP Advantages Good Good Easy purification; Easy ink High mobility Easily Tunable processability; processability; Facile scale-up formulation printable permittivity High Sufficient conductivity conductivity Limitations Costly Low-speed Difficult ink Difficult High processing Low Leaky; application formulation for purification; temp; Few permittivity; Rough surface; printing; Few n-channels p-channels High Thick films Ambient stability available; available; Ambient permeability; Limited ambient stability Thick films stability Next- Reduce cost Enhance Increase m Increase m Reduce T; enhance Reduce film Enhance generation conductivity uniformity thickness printability targets NOTE: s = conductivity; m = charge carrier mobility; Ag = silver; Au = gold; BF = breakdown field; CNT = carbon nanotube; Cu = copper; DPP = diketopyrrolo- pyrrole; IZO = indium-zinc-oxide; IGZO = indium-gallium-tin-oxide; ITO = tin-doped indium oxide; J = leakage current density; PANI = polyaniline; PMMA = poly(methylmetacrylate); PEDOT:PSS = polyethylenedioxothiophene:polystyryl sulphonate; P-UV: A UV-vis crosslinkable polymer; PVP = poly(vinylphenole); T = temperature.

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MATERIALS AND PROCESS ENGINEERING 123 The design of highly efficient and easily printable organic semiconductors for OFETs is the key challenge in roll-to-roll electronics. Optimization of charge transport (and thus carrier mobility) requires that the semiconductor molecules be planar so that the molecular orbitals can overlap efficiently. However, this molecular design usually leads to poorly soluble materials, which are very difficult to print. Obviously, for roll-to-roll printing fabrication, the organic semiconductor must be solution processable so that it can be formulated into an ink. But good molecular design and solution processability are only two factors. The charge transport in organic semiconductor films is highly dependent on the film deposition conditions—printing process, solvent used in formulating the ink, active/ additive component concentrations, deposition temperature, substrate morphology, and surface energy. Environmental conditions during film deposition can also affect materials performance, although some organic semiconductors are air stable and do not require a controlled environment during film processing. Several organic semiconductors for OFETs have been synthesized, including those based on small molecules and polymers. P-type organic semiconductors have been shown to have carrier mobilities of ~10 cm2/Vs as thin film and up to 35 cm2/Vs for single crystals. These mobility numbers are greater than that of amorphous silicon (~ 0.1-1 cm2/Vs), which is commonly used for large area display backplanes, but very few semiconductors exhibit good charge transport and solution processability. In this respect, probably the most promising semi­ conductor families are those based on thiophene-containing polymers. A common drawback in organic electronics is the limited availability of electron-transporting (n-type) semiconductors, which are needed for comple- ­ mentary circuit applications; very few air-stable n-type organic semiconductors are known. Sol-gel and nanoparticulate inorganic semiconductors or hybrid organic-inorganic semiconductor materials have been investigated; these materials promise both the superior carrier mobility of inorganic semiconductors and the processability of organic materials. Conductors All electronic devices have electrical contacts, which should satisfy a number of requirements—high conductivity, appropriate work function, chemical stability, and appropriate surface energy characteristics and morphology. Materials used as conductors are metals/metal oxides and conducting p-­ onjugated polymers. Metallic features can be fabricated by thermal evapora- c tion, sputtering, and printing (the latter is essential for organic electronics). Print- ing metal is usually achieved by using inks that contain metal particles, which may differ substantially in morphology, size, and type/amount of stabilizers; gold, silver, and other noble metals are typically used. Alternatively, nanoparticle-based inks can be printed and subsequently ­sintered at temperatures (<150°C) compatible with inexpensive plastic foils, or metal pre-

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124 FRONTIERS OF ENGINEERING cursors can be used, sometimes in combination with other materials, and similarly thermally cured. Metal oxides are another class of conductive materials often used for electrodes (tin-doped indium oxide is by far the most commonly used). Even though certain p-conjugated polymers are highly conductive, they typi- cally exhibit far lower conductivity than metals. The most common conducting polymers used in printing conducting lines are polyaniline, polythiophenes, and polypyrroles; of these, PEDOT:PSS (a polythiophene-based polymer) is the most widely used because it is commercially available and exhibits good conductivity (<400 S/cm). Dielectrics The dielectric film is extremely important for OFETs because it enables the creation of induced charges in the semiconductor upon application of the gate voltage. A good dielectric material should exhibit high dielectric strength, low leakage current, and high capacitance. The latter allows higher charge density to be induced at lower gate voltages and thus reduces power consumption. The dielectric layer capacitance can be enhanced by using thinner films and/ or by increasing the material dielectric constant. Unfortunately, when the layer becomes too thin, film defect density and leakage current increase. Furthermore, the dielectric film surface in contact with the semiconductor should be very smooth. Because charge transport in organic semiconductors is confined in the semiconductor within a few nanometers of the semiconductor-dielectric interface, rough interfaces generate charge scattering and reduce carrier mobility. Again, for printed electronic applications, the dielectric material must be solu- tion processable. Various dielectric materials have been used to fabricate OFETs; absurdly, they have mostly been inorganic films such as silica, alumina, and titanate, which are not generally printable. A variety of organic polymers—­ olypropylene, p polyvinyl alcohol, poly(vinyl phenol), poly(methyl ­ ethacrylate)—have been used m as dielectrics; most of them are widely used for other purposes and available in bulk quantities quite inexpensive. More recently more complex formulations have been printed to fabricate OFETs. These dielectric films are usually cured either thermally or photo­ chemically to enhance mechanical strength and improve dielectric properties. Summary and Outlook Printed electronics has the potential to become a significant industry within the next decade and, contrary to common opinion, many of the forecasted applica- tions will be new and not created by eroding today’s electronics markets. The challenges for this technology are mainly related to materials perfor- mance for circuits and proper engineering of printing processing methods for electronic device assembly. The semiconductor is the weakest of the current

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MATERIALS AND PROCESS ENGINEERING 125 generation of materials. The most important limitations are the lack of high carrier mobility, environmentally stable semiconductors (particularly n-type), high-performance solution-deposited semiconducting films over a large area, and temporal performance stability. Despite these difficulties, initial important successes in device fabrication using organic materials and roll-to-roll processes are encouraging and several companies strongly believe that organic electronics is already a reality. REFERENCES Facchetti A. 2007. Semiconductors for organic transistors. Materials Today 10(3):28–37. Facchetti A. 2011. p-Conjugated polymers for organic electronics and photovoltaic cell applications. Chemistry of Materials 23:733–758. Facchetti A. 2013. Polymer donor-polymer acceptor (all polymer) solar cells. Materials Today 16:123–132. Facchetti A, Yoon M-H, Marks TJ. 2005. Gate dielectrics for organic field-effect transistors: New opportunities for organic electronics. Advanced Materials 17:1705–1725. Horowitz G, Kouki F, Spearman P, Fichou D, Nogues C, Pan X, Garnier F. 1996. Evidence for n-type conduction in a perylene tetracarboxylic diimide derivative. Advanced Materials 8:242–245. Kim J, Ng TN, Kim WS. 2012. Highly sensitive tactile sensors integrated with organic transistors. Applied Physics Letters 101:103308-1–103308-5. Loo Y-L, McCulloch I. 2008. Progress and challenges in commercialization of organic electronics. MRS Bulletin 33:653–662. Usta H, Facchetti A, Marks TJ. 2011. n-Channel semiconductor materials design for organic comple- mentary circuits. Accounts of Chemical Research 44:501–510.

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