Fabrication and Characterization of Transparent Conductive Electrodes for Flexible Electronic Devices

Abstract

Transparent conductive electrodes (TCEs) are important for many electronic devices such as solar cells, liquid crystal displays (LCDs), organic light-emitting diodes (OLEDs), and touch screen panels (TSPs). Indium-tin-oxide (ITO) has been widely used as a TCE material because of its excellent properties, such as its high transmittance and low sheet resistance. On the other hand, the ITO has difficulty being applied to the next-generation flexible and portable electronic devices due to the various problems such as brittle nature, high processing temperature, low adhesion with a flexible substrate, and fluctuation of the indium prices. Therefore, researches on the materials for replacing ITO, such as carbon nanotubes (CNTs), graphene, metal meshes, metal nanowires, and conductive polymers, are actively being progress. In this thesis, TCEs applicable for the flexible electronic devices are fabricated using the aforementioned various materials via solution-based processes, which have a low process cost, a short process time, and the advantages of a simple process and easy mass production, because the processes can be conducted at room temperature and under atmospheric pressure. First, the overall solution-based processes for fabricating TCEs based on CNTs were described. The CNT-TCEs were fabricated by depositing the CNTs on flexible substrates via spray-coating and the their electrical and optical characteristics were analyzed. Corona treatment was performed to improve the adhesion between the CNTs and the PET flexible substrates by varying the corona energies, the PET feeding directions, and the numbers of corona treatment. Also, PEDOT:PSS thin layers were coated using the various methods such as spin-coating and electrophoretic deposition (EPD). The PEDOT:PSS-coated CNT-TCEs revealed the improved characteristics in both electrical properties and chromatic properties. Next, the flexible TCEs based on metal grid-meshes were fabricated with the various dimensions of line-to-line spacings and line widths. The effects of post-washing on the characteristics of the Cu mesh TCEs were analyzed. The fabricated metal grid-mesh TCEs were applied to the flexible film heaters and touch screen panels several devices, and their device characteristics were presented. In order to solve the visibility problem of the conventional grid-type metal mesh TCEs, various hybrid-type TCEs were fabricated by coating Cu meshes with the nano-structured carbon materials such as CNT and graphene. For minimizing the oxidation of metal mesh TCEs, other hybrid-types of TCEs were fabricated by coating Cu meshes with CNT/nickel (Ni) or with graphene/aluminum-oxide (AlOx). The hybrid-type metal mesh TCEs proposed in this study revealed the electrical, optical, and chromatic properties, which were superior to those of the conventional metal mesh TCEs, and at the same time they showed much improved flexibility and oxidation-stability. Furthermore, the random metal mesh TCEs were fabricated via solution processes including spray-coating of crack gel, crack solidification, pressing, and baking. It was confirmed that the Cu random mesh TCEs fabricated in this study could resolve the poor visibility problem due to diffraction originated from the regular pattern that was often observed in the widely-used grid-type metal mesh TCEs. Finally, a new figure-of-merit (FoM) model for evaluating metal mesh TCEs was designed and proposed to overcome limitations of the conventional FoM models. In the proposed FoM model, two more characteristic parameters, such as the reflectance and the color difference, were considered in addition to the electrical sheet resistance and the transmittance. The validity of the proposed FoM model was verified by evaluating the characteristics of the various metal mesh TCEs fabricated in this study.