Candidate: Alexandra Madeira
Title: Enhancing the performance of transparent electrodes through the design of new silver nanostructures.
Date: July 10, 2018
Time: 9:00 AM
Place: MC 2009
Supervisor(s): Goldthorpe, Irene
Abstract:
Transparent electrodes are a necessary component in a number of devices such as solar cells, flat panel displays, touch screens and light emitting diodes. The most commonly used transparent conductor, indium tin oxide (ITO), is expensive and not adaptable to up-and-coming flexible devices. Films consisting of random networks of solution-synthesized silver nanowires have emerged as a promising alternative material. Silver nanowire films have transparency and conductivity values comparable to ITO while being less expensive, much more mechanically flexible, and compatible with roll-to-roll deposition techniques. This PhD work makes contributions to this burgeoning topic in the important areas of electrode conductivity and electrode stability. As with all transparent electrode materials, conductivity should be maximized for a given transparency. The strategy pursued here to achieve this was to synthesize two silver nanostructures different than what is commercially available: (i) ultra-long nanowires and (ii) branched nanowires. These structures were chosen to reduce the number of high resistance junctions in the nanowire network and therefore lower the sheet resistance of the electrode. Key parameters in the nanowire synthesis process, such as the concentration of silver salt precursor and the nature of the polyol solvent, are modified to enable the elaboration of ultra-long silver nanowires. An average length of 130 µm with an average diameter of 124 nm was achieved. These are the highest aspect-ratio silver nanowires ever achieved in the literature by the polyol process and lead to more conductive transparent electrodes.
‘Y’-shaped and boomerang-shaped nanowires with branch lengths of 6 µm were synthesized by modifying the classical polyol process with ultra-sound vibrations. However, the short length and the shape of these nanostructures are not ideal for a percolated system and future work is required. Regarding electrode stability, the corrosion of silver nanowires and the resulting electrode resistance increase over time is a severe problem hindering their use in commercial devices. 11-mercaptoundecanoic acid (MuA) is identified here as a promising passivation agent of silver nanowires. Lifetime testing showed that the electrode resistance increased more slowly than the resistance of unpassivated electrodes. After 4 months of storage in daylight, the resistance of MuA passivated electrodes increased by a factor of 6 instead of 110 when not passivated. Furthermore, unlike many other passivation methods, the MuA molecule itself does not negatively affect the conductivity or transparency of the electrode and is very inexpensive, all contributing to the commercial viability of the passivation method.