(Invited) Synthesis of Thermally and Chemically Stable Silver Nanowires for Transparent Electrode

Abstract

Transparent conductive electrodes (TCEs) are important components of various optoelectronic devices and commonly used in solar cells, touch screens and other display technologies. The most widely used material for industrial applications of TCEs is indium tin oxide (ITO), which has a low sheet resistance of less than 100 Ω sq−1, and high optical transparency of ~90%. However, ITO-based TCEs are relatively expensive owing to the scarcity of the indium supply and the high sputtering processing costs. In addition, ITO films are brittle, which limits their application as flexible and stretchable components in future electronic devices. In order to address these concerns, various emerging materials such as graphene, carbon nanotubes (CNTs), conductive polymers, and metal NWs are being considered as alternatives to ITO. Among these candidate replacements, Group 11 metal NWs, especially silver NWs (Ag NWs) have garnered special interest owing to their excellent electrical and optical properties; and, solution-processability. Ag NWs are usually prepared and suspended in a solution and the resultant colloidal ink is used to coat plastic film or other substrates by spin-coating with Mayer bar or roll-to-roll processing to create transparent conducting sheets. Highly transparent and conductive Ag NW based thin films have been fabricated with a sheet resistance of <20 Ω sq−1 and a transmittance of ∼90%, which is similar to the performance of commercial ITO substrates. However, Ag NWs-based electrodes suffer from inherent limitations that restrict their applicability. One major issue is that Ag NWs are prone to oxidation, which limits their functional processability and lifetime. Coating the Ag NWs with a protective metal oxide (e.g. TiO2, SnO2, ZnO) has the potential to substantially improve the chemical, thermal and mechanical stabilities of Ag NWs. However, the preparation of a metal oxide protection coating usually requires depositing the metal oxide in a vacuum on a prepared film, which is not suitable for roll-to-roll continuous production of films with large areas. Fabrication of highly conductive and air-stable Ag NWs-based electrodes via an all-solution process for flexible and transparent applications remains a challenge. Here, we developed a nucleation mediated method using nanowire seeds for the selective synthesis of uniform Ag NWs via polyol syntheses. The seed-mediated polyol approach gives Ag NWs with high reproducibility (>90% in numerous repetitions of the synthesis), high processing efficiency (grams or more are possible for each synthesis run), and high aspect ratios (>2000), with average dimensions of L> 100 µm and D < 50nm.The Ag@TiO2 NWs subsequently created by surface modification of the Ag NWS have a substantially higher thermal stability than Ag NWs which have the same diameter, and can be annealed to 300 ˚C. Transparent conductive films can be prepared by simple vacuum filtration or by spin coating onto an appropriate filter paper and then by direct transfer to the desired glass or plastic substrate.Therefore, Ag@TiO2 nanowires are ideal candidates for transparent electrodes with good conductivity, high transmittance and low haze, and are demonstrated to be thermally and chemically stable for long-term applications.