Advanced Materials for Printed Flexible Electronics

Three-dimensional (3D) printing, known as additive manufacturing, includes a family of technologies consisting of novel ink materials, flexible substrates, and unique processing methods that can be integrated to create flexible, stretchable, and wearable electronics. These technologies can be used to fabricate components and full systems mainly in a layer-by-layer manner and offer various options regarding cost, feature details, and organic and inorganic materials. The most popular materials are printable organic, inorganic, and hybrid semiconductors with various functional structures (i.e., 1D, 2D and 3D, even 4D), including polymers, metals, composites, ceramics, and nanomaterials. 3D printing enables the creation of complex geometric shapes and merging of selected functional components into any configuration thus supplying an innovative approach for the fabrication of multifunctional end-use devices that can potentially combine mechanical, optical, chemical, electronic, electromagnetic, fluidic, thermal, and acoustic features. On the other hand, rapid advances in modern electronics place ever-accelerating demands on innovation towards more robust and versatile functional components. In the flexible electronics domain, novel material solutions often involve creative uses of common materials to reduce cost, while maintaining uncompromised performance. Moreover, mechanically durable and highly stretchable materials are fundamentally important to the development of flexible and stretchable devices. Therefore, there has been enormous progress in the materials, designs, and associated assembly techniques as well as manufacturing processes for flexible/stretchable electronic systems and subcomponents, such as transistors, amplifiers, sensors, actuators, light-emitting diodes, photodetector arrays, photovoltaics, energy generation and storage devices, and bare die integrated circuits. This chapter will highlight the fundamentals and design guides for 3D-printed flexible electronics, including historical perspectives, printing requirements for printable materials, design strategies, and advanced fabrication technologies for printed flexible electronics.

Integration of additive manufacturing (AM) and 3D printing technologies with electronics fabrication processes have achieved better advancement in the electronics industry. In additive manufacturing or 3D printing of flexible electronics, for instance, the fabrication process and design of the structures and functional components strongly affect the material properties and device performance. This is especially critical for novel and innovative structures that approach the limit of what is possible nowadays, such as stretchable and wearable electronics or adaptive structures in 4D printing. It is therefore crucial to include the material characterization in the design loop: material properties can differ due to build orientation (anisotropy), size of the object (scaling), and many other factors. The typical materials used in AM or 3D printing are polymers, metals, ceramics, and composites (as well as biomaterials). The raw materials used can be broadly classified based on their forms in either liquid, solid, or powder. The aims of this chapter are to address the most efficient characterization methods for 3D printing processes, characterization of AM and 3D printable materials through testing, and material models that can be used in process design and optimization methods to develop durable flexible electronics.

Flexible circuits can be produced on polymer film, metal foil, paper, or textile using printing processes and permit futuristic designs with curved diodes or input elements. This requires printable electronic materials that can be printed on the curved substrate surfaces and retain a high level of conductivity during usage even after specified folding and/or stretching. Different materials and their composites have been developed for 3D prinitng and additive manufacturing to fabricate conductive features. These conductive materials could be mainly categorized into metal-based, carbon-based, and organic-based materials as well other conductive materials. The applications have covered photovoltaics, touch screen edge electrodes, automotive and in-mold electronics, PCB, electronic textile and wearable electronics, 3D antennas and conformal printing, EMI shielding, printed sensors, RFID (HF, UHF), TFT and memory, OLED and large-area LED lighting, and more. This chapter will provide an overview of the current status and future trends of typical conductive materials for printed flexible electronics.

Inorganic, organic, and hybrid composite semiconducting materials are critical for developing active flexible electronics. Inorganic materials have superior properties in terms of performance and stability while solution processable organic semiconductors are attractive due to low-cost processing at ambient environment and flexibility. Examples of inorganic semiconductors commonly used for flexible electronics are Si, oxides of transition metals, and chalcogenides. From the printability point of view, the solubility and proper dispersion of organic semiconductors are important parameters. Commonly used solution-processed organic semiconductors having acceptable charge transport and mobility include regioregular poly(3-hexylthiophene) (P3HT), poly(triarylamine), poly(3,3-didodecyl quaterthiophene) (PQT), poly(2,5-bis(3-tetradecyllthiophen-2-yl) and thieno[3,2-b]thiophene) (PBTTT). Fullerenes and solution processable derivatives such as phenyl-C61-butyric acid methyl ester (PCBM) blended with P3HT are some of the commonly used electron donors and acceptors in the bulk heterojunction devices. Additionally, carbon nanotubes and graphene are also under investigation due to their high mobility. Besides, three-dimensionally confined semiconductor quantum dots and nanoconfinement of semiconductors have emerged to be a versatile material system with unique physical properties for a wide range of device applications including flexible electronics. This chapter will provide a brief review on the perspectives and prospects of semiconducting materials for printed flexible electronics, including inorganic, organic semiconductors and their composite systems.

Substrate, dielectric, and encapsulation materials are critically important as their properties can dominate those of the integrated flexible electronic system. A flexible substrate should be highly deformable and mechanically robust and must exhibit high tolerance levels of bending repeatability. They are also required to possess properties such as dimensional stability, thermal stability, low coefficient of thermal expansion (CTE), excellent solvent resistance, and good barrier properties for moisture and gases. The substrate materials mainly include plastic films, metal foils, and fibrous materials (including paper and textiles). Moreover, a uniform layer of dielectric is needed to promote the activation of the medium caused by electric fields or other transduction phenomena. Inorganic materials such as silica, alumina, and other high permittivity oxides often used in electronics on flexible substrates are usually not printable. Low-cost organic dielectric materials that are available in large quantities and can be dissolved in various solvents and solutions can be printed easily as compared to inorganic counterparts. Some of the commonly used organic dielectric materials in printed electronics are poly (4-vinylphenol) (PVP), poly(methyl methacrylate), Polyethylene Terephthalate, Polyimide, Polyvinyl alcohol, and Polystyrene. Besides dielectric layers in electronic devices, solution processed organic dielectric materials are also used for final encapsulation of printed devices. There is a wide range of permeation requirements for different encapsulation materials. To ensure protection of flexible devices, conventional encapsulation methods are not suitable due to their inherent rigidity, and organic/inorganic hybrid thin-film encapsulation (TFE) has been considered as the most promising technology. This chapter will provide a brief review on substrate, dielectric, and encapsulation materials for flexible electronics applications.

The ability to realize flexible thin-film transistors (TFTs) which are key driving/switching components of wearable/stretchable electronics, offers much freedom on the target substrates. Therefore, a variety of functional materials focusing on semiconductors have been extensively explored for realizing competitive flexible TFTs, including traditional silicon, organics, and inorganics (such as oxides, carbon nanotubes (CNTs), graphene, and other emerging 2D materials). In particular, additive printing has great advantages for realizing stack-structured TFTs consisting of conductive, insulation, and semiconductor layers on flexible substrates with a low thermal budget, even below 200 °C when organic or nanoparticle-type functional inks are used. For obtaining high-performance printed TFTs, there is lots of research focused on printable semiconductor/dielectric/electrode materials, surface and interface properties, as well as printing techniques. With the in-depth research on materials, device structure, and manufacturing processes, TFTs gradually realize the fabrication on flexible substrates with printing techniques. This chapter will give a brief review on printed flexible thin-film transistors, including types of transistors, structure and operation of thin-film transistors, printing techniques and printed components of thin-film transistors, printed organic thin-film transistors, and printed inorganic thin-film transistors.

Printed flexible organic light-emitting diodes (OLEDs) hold great promise because of their self-emitting property, high luminous efficiency, wide viewing angle, full-color capability, high contrast, lower power consumption, light weight, and flexibility. They are revolutionizing next-generation flat-panel display technology. While OLED displays can be fabricated on large area and flexible substrates, a stream of new OLED products has reached the marketplace. Although this field is growing well, some grand challenges still remain. More multidisciplinary studies would be needed to address the critical issues, from fluorescent, phosphorescent and Thermally Activated Delayed Fluorescence (TADF) emitters for OLEDs, blue and white OLEDs as well as QOLEDs, to metal-containing nanomaterials in the optimization of OLED performance. Many aspects of the field use a number of materials design and device tactics. This chapter will address working principles and device structure, materials, and components used for general OLEDs, white lighting OLEDs, and flexible quantum dot OLEDs.

Printable photovoltaic modules, along with other printed electronic devices, such as light-emitting diodes, thin-film transistors, capacitors, coils, and resistors, are a low-cost alternative to the conventionally deposited devices. Due to its fabrication simplicity and the feasibility of using large-area flexible substrates, the printable solar cell (PSC) is a prospective candidate in many application fields. Furthermore, the light-absorbing layer of PSC is usually several orders of magnitude thinner than widely used conventional Si solar cells; thus, production of PSC requires much less material, and in the case of printing deposition there is very little waste of material in comparison to other deposition methods. The possibility of using flexible large-scale substrates opens the door to multiple advanced application opportunities such as smart textiles and photovoltaic window shades. This chapter addresses a topic regarding key aspects of development, fabrication, and application with respect to PSC, mostly including solution-processed organic polymeric cells, inorganic thin-film solar cells, and organic–inorganic hybrid perovskite solar cells. Moreover, working principles, applicability of PSC, their merits and demerits, and future application possibilities are considered. Additionally, the chapter contains a description of a variety of different solution processable materials and their deposition and printing technologies.

Printed flexible electronic devices can be portable, lightweight, bendable, and even stretchable, wearable, or implantable and therefore have great potential for applications such as roll-up displays, smart mobile devices, wearable electronics, implantable biosensors, and so on. To realize fully printed flexible devices with matchable or integrable power sources, printed flexible electrochemical energy storage units with high energy storage and power density have been investigated. Many works are dedicated to exploring suitable and effective electrode/electrolyte materials as well as more preferable cell configuration and structural designs. As a result, exciting progress has been achieved in developing high-performance printed flexible electrochemical energy storage devices, mainly including lithium-ion and zinc-based batteries, and supercapacitors. In addition, printing nanomaterials have made significant advances for energy electrochemical storage applications. With these advancements, future flexible power sources that combine both outstanding electrochemical and mechanical performance will boost the development and commercialization of next-generation flexible electronics. This chapter will briefly review the advances of printed flexible electrochemical energy storage devices, including evolution of electrochemical energy storage, working principles of battery and supercapacitor, as well as various printed flexible batteries and supercapacitors, covering printable organic, inorganic materials and nanomaterials, printed components, integration processes, and suitable applications.

Printed flexible sensors have the potential to be seamlessly applied to soft and irregularly shaped surfaces such as human skin or textile fabrics. Owing to its varied range of applications in the field of flexible and wearable electronics, soft robotics, human–machine interaction, and biomedical devices, it is required of these sensors to be flexible and stretchable conforming to the arbitrary surfaces of their soft or stiff counterparts. The challenges in maintaining the fundamental features of these sensors, such as flexibility, sensitivity, repeatability, linearity, and durability, are tackled by the progress in the fabrication techniques and customization of the material properties. As a result, materials and structures for innovative flexible sensors, as well as their integration into systems, continue to be in the spotlight of research. This chapter outlines the current state of flexible sensor technologies and the impact of material developments on this field. Special attention is given to stress, strain, temperature, chemical, electropotential, and magnetic sensors, as well as their respective applications.

Flexible hybrid electronics (FHE) are heterogeneous electronics embodying both conventional silicon electronics and printed electronics, which is advantageous compared to conventional silicon electronics and the emerging printed electronics—FHE features better mechanical flexibility/conformability and lower cost compared to conventional silicon electronics, and higher performance compared to printed electronics. While silicon ICs thrive at low-power high-performance computing, creating flexible and large-area electronics using silicon remains a challenge. On the other hand, flexible and printed electronics use intrinsically flexible materials and printing techniques to manufacture compliant and large-area electronics. Nonetheless, flexible electronics are not as efficient as silicon ICs for computation and signal communication. Flexible hybrid electronics (FHE) leverage the strengths of these two dissimilar technologies. They use flexible and printed electronics where flexibility and scalability are required, i.e., for sensing and actuating, and silicon ICs for computation and communication purposes. Combining flexible electronics and silicon ICs yields a very powerful and versatile technology with a vast range of applications. This chapter will provide a brief review about the fundamental building blocks of an FHE system, printable materials and circuits, thinned silicon ICs, emerging applications, current challenges, and future trends related to FHE.

Printed electronics will play a more and more important role in the electronics industry due to advantages in high-throughput production and customizability in terms of material support and system process. The printing of traces and interconnects, passive and active components such as resistors, capacitors, inductors, and application-specific electronic devices have been a growing focus of research in the area of additive manufacturing electronics. Adaptation of novel 3D-printing technologies and manufacturing methods are potentially transformative in flexible/stretchable/wearable electronics, wireless communications, efficient batteries, solid-state display technologies, and so on. Other than printing new and reactive functional electronic materials, the functionalization of the printing substrates with unusual geometries apart from the conventional planar circuit boards will be a challenge. Building the substrate, printing the conductive tracks, pick-and-placing or embedding the electronic components, and interconnecting them are fundamental fabrication protocols of 2D- and 3D-printing systems, which should be adopted for a more integrated fabrication system. Moreover, adaptive 4D-printed systems have been developed with highly versatile multidisciplinary applications, including medicine, in the form of assisted soft robots, smart textiles as wearable electronics, and other industries such as agriculture and microfluidics. The adaptive 4D-printed systems incorporated synergic integration of three-dimensional (3D)-printed sensors into 4D-printing and control units, which could be assembled and programmed to transform their shapes based on the assigned tasks and environmental stimuli. This chapter gives a brief review on perspectives of various 2D-, 3D-, and 4D-printing methods, and describes the state-of-the-art in printed electronics and their future growth.