Transparent conductive electrodes are important components of thin-film solar cells, light emitting diodes, touch screens and many display technologies. For this purpose, doped metal oxides such as indium tin oxide (ITO) are commonly used, but their optical transparency is limited for films with a low sheet resistance. Furthermore, they are prone to cracking when deposited on flexible substrates, are costly, and require a high-temperature fabrication step for the best performance. Recent discoveries regarding the optical properties of nanostructured metals have opened up an important opportunity to develop a new class of transparent electrodes suitable for optoelectronic devices operating in the visible and near-infrared regime. These new type of electrodes involve networks of silver nanostructures that have been shown to match the optical transparency and electrical sheet resistance of ITO, but are much more flexible and less expensive.
It is well known that composite materials made of conductive fillers randomly dispersed in an insulating matrix exhibit an abrupt insulator-to-conductor transition caused by the formation of long-range networks. This transition depends on the shape and the concentration of the conductive dispersed filler. For the targeted application, we must obtain the highest electronic conductivity compatible with the lowest conductive filler concentration to ensure transparency of the electrode. Percolation concepts are used to describe the abrupt transition from insulating to conductive behavior. The percolation threshold depends on the shape of the percolating objects and on their spatial dispersion. For example, in the case of metallic nanowires with finite L/D ratios (L being the length and D the diameter of the metallic nanowire), it is well known from percolation theory that the electrical percolation threshold decreases with increasing L/D.
Relying on the above considerations, we plan to investigate various types of conductive fillers and propose to improve the design of the electrodes in two ways. Firstly, we will study the optimal nanostructure morphology, particularly the length and diameter of silver nanowires, to obtain conductive films with maximum transparency. 2D branched architectures will also be considered, as they may decrease resistance through decreasing the number of highly resistive junctions where nanowires overlap. Secondly, in contrast to the random arrangement of nanostructures used by other groups, we will control the spatial orientation of the nanostructures by using an evaporator-based microfluidic device. Overall, this PhD project aims to render silver nanostructured electrodes a viable and attractive alternative to conventional transparent conductive materials.
Université Bordeaux 1: Institute for Condensed Matter Chemistry (ICMCB-CNRS) and Institut des Sciences Moleculaires (ISM): optical characterization; plasmonics
University of Waterloo: Waterloo Institute of Technology (WIN): electronic transport, nanofabrication
Rhodia-Solvay; -Laboratory of Future (Bordeaux): Microfluidics.