Flexible transparent electrodes with silver nanowires

Abstract

Transparent conductive electrodes (TCEs), which transmit almost every range of visible light and transport electric current, are vital components for various optoelectronic devices, such as liquid-crystal displays (LCDs), organic light-emitting diodes (OLEDs), Photovoltaics (PVs), and so forth. Particularly, indium tin oxide (ITO) has been widely employed as representative TCEs owing to its high electrical conductivity and high transmittance. Recently, with rapid evolution of optoelectronics, there is an increasing demand for the function as flexibility to impart the devices to be bendable or stretchable, where traditional technologies cannot be adapted. This requirement leads to an unprecedented shift of technological paradigm. Particularly, flexible transparent electrodes (FTEs) are regarded as one of the indispensable components of the flexible optoelectronic devices. Among various FTEs, silver nanowires (AgNWs) with a one-dimensional structure are the most promising candidate owing to the suitable features, namely high electrical conductivity, high transparency, reasonable mechanical flexibility, and low-cost solution-based fabrication. However, achieving superior optical and electrical properties beyond those of ITO based TCEs is still necessary. Inspired by this research gap, one-step multifunctional chemical treatment, which is our earlier work to fulfill all the requirements for FTEs, has been developed. Nevertheless, there is a critical problem with respect to the stability of the chemical process, that is, the short reaction time, which is the time capable of sustaining the constant welding effect. As my first research project in this thesis, by clarifying the mechanism of the instability derived from the solvent, in turn, replacing with the proper one, the reaction time is dramatically prolonged for over 5 times than that of the conventional process. The FTEs treated by the developed chemical treatment show the rigid mechanical stability under 10,000 bending cycles (Bending radius is 1 mm.) with continuous electrical bias. Moreover, based on the FTEs developed in this work, flexible LEDs are demonstrated. Secondly, in order to further pursue characteristic enhancement of FTEs, merging techniques related to reducing percolation threshold, namely, high-aspect ratio nanowires, composite with conducting polymer, and alignment of AgNWs, is proposed on the basis of percolation theory. As a preliminary study to confirm the potential of this idea, based on AgNWs with high-aspect ratio, ~2 % higher optical transmittance is confirmed by combining cross-aligned AgNWs coated by a Mayer rod with the conducting polymer (PEDOT:PSS) than that of randomly formed AgNWs on PEDOT:PSS under the fixed sheet resistance. The further enhancement of the optical transmittance is expected by the introduction of “highly cross-aligned AgNWs”. Finally, mechanically stable stretchable TCEs based on buckled structures with superior electrical and optical properties are demonstrated by taking the full advantages of two techniques developed in my study. The proposed stretchable TCEs deliver better characteristics in terms of both conductivity and optical transmittance than those of the previously reported work. Simultaneously, the developed stretchable TCEs withstand 10,000 alternative stretch-release cycles between 0 % and 50 % deformation. The work done here will pave the way for the further development of flexible electronics.