Silver Nanowire Transparent Electrodes for Soft Optoelectronic and Electronic Devices

Abstract

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