Light-induced Degradation Behaviors And Performances Of Silver Nanowire Electrode For Flexible Electronic Devices

Research on flexible and printed electronic technologies for next-generation advanced electronic devices have emerged and developed rapidly in recent years. Transparent electrode, which is an important device in electronic industries, usually play an indispensable role in various applications including solar cells, touch screens, light-emitting diodes and sensors. However, indium tin oxide(ITO), which is the most widely used material for transparent electrode, is n ot suitable for flexible and printed electronics due to its brittleness, high-cost fabrication process, and also indium scarcity. Therefore, it is necessary to develop high-performance ITO alternatives. Transparent electrode based on silver nanowire(Ag NW) networks is featured by high conductivity and transparency, and also performs well in flexibility and processing cost, thus is believed to be the ideal ITO alternative. Nevertheless, the outstanding performance of Ag NW electrode usually degrades due to it s poor long-term reliability, which also becomes the key problem for wide application. Since light-induced degradation behaviors and performances of Ag NW transparent electrode are newly emerged cutting-edge research fields, there are still few studies until now.In this study, high-performance Ag NW electrode has been prepared firstly, and light-induced degradation behaviors of Ag NW networks and relevant mechanism have been studied. New protecting solutions for improving degradation-resisted reliability have been designed and processed, and various protecting mechanism have been proposed in addition to physical isolation. Finally, highly sensitive flexible pressure sensor application based on Ag NW electrode has been successfully realized.Sheet resistance of Ag NW electrode with high transmittance increased rapidly. Sheet resistance is susceptible to the contact resistance at junctions between nanowires. The number of junctions was effectively reduced using long Ag NWs with average length of 89.5μm, which were synthesized by polyol method at 110°C without stirring. Formation mechanism of long Ag NWs involving effects of reaction temperature and stirring rate was provided. Furthermore, the organic coating layer on nanowire surface was effectively tailored by innovative pre-treatment, and contact resistance decreased. Accordingly, high-performance Ag NW ink was developed. No post-treatment is needed and the ink fit for various substrates. Sheet resistance was 15.57Ω/sq(T=90%), while sheet resistance was 92.97Ω/sq(T=95%).After long-term light illumination, both large nanoparticles(Large NP, diameter > 70nm) and small nanoparticles(Small NP, diameter < 30nm) emerged in Ag NW networks. Large NPs emerged at many contact junctions between Ag NWs, while Small NPs always distributed on the edges of pentagonal nanowire. Increase of Small NPs induced weight loss of Ag NWs and undermined sheet resistance continuously. At initial stage of Large NP formation, contact resistanc e decreased because emerged Large NPs increased the contact area. But when Large NPs continued growing, Ag NWs became thinner or even broken due to silver migration, increasing resistance instead. Since the susceptibility to contact resistance change is related to transmittance of Ag NW electrode, change trend of sheet resistance after long-term light illumination is also related to transmittance of Ag NW electrode.Four highly reliable protecting solutions, i.e. hydrophobic coating, Pt plating, in situ patterned barrier fabricated by reversed photolithography, and self-protection by soy protein isolate(SPI) substrate, have been designed. Stearic acid and octadecyltrichlorosilane(OTS) coatings effectively improved the hydrophobic property of electrode surface, and normalized sheet resistance deceased from 3.8 to 1.02 after accelerating test for 600 h. Normalized sheet resistance also deceased to 1.15 with plated Pt coating. However, transmittance of Ag NW electrode deceased obviously with these coatings. The in situ patterned barrier was fabricated by reversed photolithography, and transmittance loss was only 1.9%. Normalized sheet resistance decreased from 5.52 to 1.72 after 720 h accelerating test. The hydrophobic property and photo-absorption of photoresist were also believed to be involved in protecting mechanism. Ag NW electrode based on SPI substrate was fabricated by cold transfer method. Sheet resistance was improved by high intensity pulsed light(HIPL) treatment, which also embedded Ag NWs into SPI substrate and thus realized self-protection by SPI substrate.Capacitive flexible pressure sensor based on Ag NW electrode was designed and fabricated, and the best sensitivity could reach 5.54×10-3Pa-1. Since not only the thickness of elastomeric dielectric interlayer decreased under pressure, but relative permittivity increased also due to pressing, the sensitivity was accordingly improved. Great reproducibility, flexibility and performance in detecting non-contact pressure were also verified. Finally, wearable device based on pressure sensor was realized to monitor movement of human body.