Fabrication and Applications of Flexible Transparent Electrodes Based on Silver Nanowires

There has been an explosion of interests in using flexible transparent electrodes for next-generation flexible electronics, such as touch panels, flexible lighting, flexible solar cells, and wearable sensors. Silver nanowires (AgNWs) are a promising material for flexible transparent electrodes due to high electrical conductivity, optical transparency and mechanical flexibility. Despite many efforts in this field, the optoelectronic performance of AgNW networks is still not sufficient to replace the present material, indium tin oxide (ITO), due to the high junction resistance. Also, the environmental stability and the mechanical properties need enhancement for future commercialization. Many studies have attempted to overcome such problems by tuning the AgNW synthesis and optimizing the film-forming process. In this chapter, we survey recent progresses of AgNWs in flexible electronics by describing both fabrication and applications of flexible transparent AgNW electrodes. The synthesis of AgNWs and the fabrication of AgNW electrodes will be demonstrated, and the performance enhanced by various methods to suit different applications will be also discussed. Finally, technical challenges and future trends are presented for the application of transparent electrodes in flexible electronics

Silver Nanowire Transparent Electrodes for Soft Optoelectronic and Electronic Devices

School of Energy and Chemical Engineering (Energy Engineering)Recently, with an increasing importance of human-machine interface along with the rapid growth of Internet of Things (IoT), various flexible and stretchable electronic and optoelectronic devices have been developed for the wide range of multifunctional and wearable applications such as touch screen panels, organic solar cells, organic light-emitting diodes, thin-film loudspeakers, microphones, interactive displays, and electronic skins. High mechanical flexibility/stretchability, optical transparency, and electrical conductivity are the critical properties that transparent conductive electrodes (TCEs) should possess for the realization of high-performance flexible/stretchable electronics and optoelectronics. While indium tin oxide (ITO) has been widely used in commercial TCEs, the further development and application of ITO have been limited by the high cost and inherent brittleness of the material. One promising alternative to ITO as a TCE material is silver nanowire (AgNW) networks having good flexibility and stretchability, which can provide lower sheet resistance (Rs) and higher optical transmittance (T) than other TCE candidates such as carbon nanotubes, graphene, and conducting polymers. Moreover, AgNW networks can be readily prepared by low-cost solution-based process, enabling the mass production of next-generation optoelectronic and electronic applications. The integration of AgNW networks with the flexible/stretchable substrates can provide powerful platforms to realize highly stable and high-performance soft optoelectronic and electronic devices with the superior transparency and stable supply of electrical conductivity during mechanical deformations. This thesis covers our recent studies about flexible/stretchable AgNW TCEs and their applications in various soft optoelectronic and functional electronic devices. First, chapter 1 introduces research trends in flexible/stretchable transparent electrodes and several issues of AgNW networks that should be carefully considered for their future soft optoelectronic and electronic device applications. In chapter 2, we demonstrated a simple and efficient assembly strategy for the large-area, highly cross-aligned AgNW arrays for TCE applications through a modified bar-coating assembly. As opposed to conventional solvent-evaporation-induced assemblies, which are slow and produce nonuniform conductive networks, our modified bar-coating strategy enables fast, efficient, and uniform alignment of AgNWs in a large-area by simply dragging the Meyer rod over the AgNW solution on the target substrates. For the potential applications, we demonstrated large-scale, flexible, and transparent resistive-type touch screens and force-sensitive mechanochromic touch screens using cross-aligned AgNW transparent electrodes which exhibited highly uniform and precise touch sensing performance across the entire region. In chapter 3, we introduced ultrathin, transparent, and conductive hybrid nanomembranes (NMs) with nanoscale thickness, consisting of the orthogonal AgNW arrays embedded in a polymer matrix. Here, we present a skin-attachable NM loudspeaker and wearable transparent NM microphone, which can emit thermoacoustic sound and can provide excellent acoustic sensing capabilities. In chapter 4, solution-processable, high-performance flexible alternating-current electroluminescent (ACEL) devices are developed based on high-k nanodielectrics and cross-aligned AgNW transparent electrodes. The solution-processed La-doped barium titanate (BTO:La) nanocuboids are fabricated as high dielectric constant nanodielectrics, which can enhance the dielectric constant of an ACEL devices, enabling the fabrication of high-performance flexible ACEL devices with a lower operating voltage as well as higher brightness. In chapter 5, we fabricated transparent, flexible, and self-healable thermoacoustic loudspeakers based on AgNW/poly(urethane-hindered urea) (PUHU) conductive electrodes. Our self-healable AgNW/PUHU electrodes exhibit a great self-healing property for the surface damages by means of the dynamic reconstruction of reversible bulky urea bonds in PUHU. In chapter 6, synesthetic bimodal generation of sound and color is demonstrated by stretchable sound-in-display devices consisting of strain-insensitive stretchable AgNW electrodes and field-induced inorganic EL phosphor emissive layers. The stretchable sound-in-display devices show highly robust and reliable EL and sound generating performances that can be repeatedly stretched and released without severe performance degradation because of the use of strain-insensitive AgNW electrodes. Finally, in chapter 7, we summarize this thesis along with the future perspective of flexible/stretchable transparent electrodes that should be considered for next-generation soft electronic and optoelectronic device applications. In this thesis, studies on flexible/stretchable AgNW transparent electrodes and their device applications could be further expanded for diverse soft and wearable optoelectronic and electronic applications such as wearable sensors, healthcare monitoring devices, and human-machine interfaces with better convenience, appearance, and reusability.ope

Fabrication of an autonomously self-healing flexible thin-film capacitor by slot-die coating

Flexible pressure sensors with self-healing abilities for wearable electronics are being developed, but generally either lack autonomous self-healing properties or require sophisticated material processing methods. To address this challenge, we developed flexible, low-cost and autonomously self-healing capacitive sensors using a crosslinked poly(dimethylsiloxane) through metal-ligand interactions processed into thin films via slot-die coating. These films have excellent self-healing properties, approximately 1.34 × 105 μm3 per hour at room temperature and 2.87 × 105 μm3 per hour at body temperature (37 °C). Similarly, no significant change in capacitance under bending strain was observed on these flexible thin-films when assembled on poly(ethyleneterephthalate) (PET) substrates; capacitors showed good sensitivity at low pressure regimes. More importantly, the devices fully recovered their sensitivity after being damaged and healed, which is directly attributed to the rapid and autonomous self-healing of the dielectric materials

Review of materials and fabrication methods for flexible nano and micro-scale physical and chemical property sensors

The use of flexible sensors has tripled over the last decade due to the increased demand in various fields including health monitoring, food packaging, electronic skins and soft robotics. Flexible sensors have the ability to be bent and stretched during use and can still maintain their electrical and mechanical properties. This gives them an advantage over rigid sensors that lose their sensitivity when subject to bending. Advancements in 3D printing have enabled the development of tailored flexible sensors. Various additive manufacturing methods are being used to develop these sensors including inkjet printing, aerosol jet printing, fused deposition modelling, direct ink writing, selective laser melting and others. Hydrogels have gained much attention in the literature due to their self-healing and shape transforming. Self-healing enables the sensor to recover from damages such as cracks and cuts incurred during use, and this enables the sensor to have a longer operating life and stability. Various polymers are used as substrates on which the sensing material is placed. Polymers including polydimethylsiloxane, Poly(N-isopropylacrylamide) and polyvinyl acetate are extensively used in flexible sensors. The most widely used nanomaterials in flexible sensors are carbon and silver due to their excellent electrical properties. This review gives an overview of various types of flexible sensors (including temperature, pressure and chemical sensors), paying particular attention to the application areas and the corresponding characteristics/properties of interest required for such. Current advances/trends in the field including 3D printing, novel nanomaterials and responsive polymers, and self-healable sensors and wearables will also be discussed in more detail

Stretchable Self-Healable Semiconducting Polymer Film for Active-Matrix Strain-Sensing Array

Skin-like sensory devidces shoud be stretchable and self-healable to meet the demands for future electronic skin applications. Despite recent notable advances in skin-inspired electronic materials, it remains challenging to confer these desired functionalities to an active semiconductor. Here, we report a strain-sensitive, stretchable, and autonomously self-healable semiconducting film achieved through blending of a polymer semiconductor and a self-healable elastomer, both of which are dynamically cross-linked by metal coordination. We observed that by controlling the percolation threshold of the polymer semiconductor, the blend film became strain sensitive, with a gauge factor of 5.75 x 105 at 100% strain in a stretchable transistor. The blend film is also highly stretchable (fracture strain, \u3e1300%) and autonomously self-healable at room temperature. We proceed to demonstrate a fully integrated 5 x 5 stretchable active-matrix transistor sensor array capable of detecting strain distribution through surface deformation

Highly Sensitive Soft Foam Sensors for Wearable Applications

Due to people’s increasing desire for body health monitoring, the needs of knowing humans’ body parameters and transferring them to analyzable and understandable signals become increasingly attractive and significant. The present body-sign measurement devices are still bulky medical devices used in settings such as clinics or hospitals, which are accurate, but expensive and cannot achieve the personalization of usage targets and the monitoring of real-time body parameters. Many commercial wearable devices can provide some of the body indexes, such as the smartwatch providing the pulse/heartbeat information, but cannot give accurate and reliable data, and the data could be influenced by the user’s movement and the loose wearing habit, either. In this way, developing next-generation wearable devices combining good wearable experience and accuracy is gathering increasing attention. The aim of this study is to develop a high-performance pressure/strain sensor with the requirements of comfortable to wear, and having great electromechanical behaviour to convert the physiological signal to an analyzable signal

Materials, Mechanics, and Patterning Techniques for Elastomer-Based Stretchable Conductors

Stretchable electronics represent a new generation of electronics that utilize soft, deformable elastomers as the substrate or matrix instead of the traditional rigid printed circuit boards. As the most essential component of stretchable electronics, the conductors should meet the requirements for both high conductivity and the capability to maintain conductive under large deformations such as bending, twisting, stretching, and compressing. This review summarizes recent progresses in various aspects of this fascinating and challenging area, including materials for supporting elastomers and electrical conductors, unique designs and stretching mechanics, and the subtractive and additive patterning techniques. The applications are discussed along with functional devices based on these conductors. Finally, the review is concluded with the current limitations, challenges, and future directions of stretchable conductors

Nanocellulose-based sensors in medical/clinical applications: The state-of-the-art review

In recent years, the considerable importance of healthcare and the indispensable appeal of curative issues, particularly the diagnosis of diseases, have propelled the invention of sensing platforms. With the development of nanotechnology, the integration of nanomaterials in such platforms has been much focused on, boosting their functionality in many fields. In this direction, there has been rapid growth in the utilisation of nanocellulose in sensors with medical applications. Indeed, this natural nanomaterial benefits from striking features, such as biocompatibility, cytocompatibility and low toxicity, as well as unprecedented physical and chemical properties. In this review, different classifications of nanocellulose-based sensors (biosensors, chemical and physical sensors), alongside some subcategories manufactured for health monitoring, stand out. Moreover, the types of nanocellulose and their roles in such sensors are discussed.This work was supported by the University of the Basque Country (Training of Researcher Staff, PIF 20/197)

SENSING MECHANISM AND APPLICATION OF MECHANICAL STRAIN SENSOR: A MINI-REVIEW

This study reviews the potential of flexible strain sensors based on nanomaterials such as carbon nanotubes (CNTs), graphene, and metal nanowires (NWs). These nanomaterials have excellent flexibility, conductivity, and mechanical properties, which enable them to be integrated into clothing or attached to the skin for the real-time monitoring of various activities. However, the main challenge is balancing high stretchability and sensitivity. This paper explains the basic concept of strain sensors that can convert mechanical deformation into electrical signals. Moreover, this paper focuses on simple, flexible, and stretchable resistive and capacitive sensors. It also discusses the important factors in choosing materials and fabrication methods, emphasizing the crucial role of suitable polymers in high-performance strain sensing. This study reviews the fabrication processes, mechanisms, performance, and applications of stretchable strain sensors in detail. It analyzes key aspects, such as sensitivity, stretchability, linearity, response time, and durability. This review provides useful insights into the current status and prospects of stretchable strain sensors in wearable technology and human–machine interfaces