The effects of percolation in nanostructured transparent conductors
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Introduction In the modern world, transparent conductors (TCs) are extremely common. They are generally found as thin films that have low electrical resistance but are optically transparent and are critically important in many modern electronic devices. In the home or office, they are found in flat panel displays such as in TVs, laptops or digital cameras, as smart windows, heat shields in some oven windows, and as invisible security circuitry on glass windows. They defrost windows in planes, trains, and automobiles and are used in self-dimming rearview mirrors in cars. They are also used in portable electronic devices (phones, tablet computers) that affect our daily lives. Many people hope that as electrodes in solar cells, they will help generate clean, renewable energy. Modern TC technology is dominated by doped metal oxides, of which the most important is tin-doped indium oxide (ITO).1–3 This material has been studied for 80 years3 and has been largely optimized for a range of properties. For example, while metal oxide TCs can display transmittances as high as 95% and sheet resistances as low as 3 Ω/sq, they also have a range of other
useful properties, such as work functions that vary between 4.2 and 5.3 eV, depending on the material, thermal stability, and chemical and mechanical durability.2 In fact, metal oxides are model materials for transparent electrodes. However, even model systems may fall victim to issues related to limited supply and ever growing demand. Partly due to the rapid growth in portable electronics, the demand for ITO is currently growing at 20% per annum. Due to a combination of this demand and the already limited supply, the cost of indium and hence ITO has risen dramatically in recent years (see Reference 3 and references therein). Furthermore, metal oxides have another weakness: brittleness.4–7 It is thought that a significant fraction of future displays will be plastic-based and inherently flexible. As metal oxides tend to fracture at strains of ∼2%, they are completely unsuitable for use in such flexible electronics. Other problems with metal oxides include their high refractive index and the high cost of producing large area metal oxide coated glass. Thus, it is clear that new transparent conducting materials are required. These must have a number of specific properties:
Sukanta De, Center for Research in Adaptive Nanostructures and Nanodevices, and School of Physics, Trinity College Dublin, Ireland; [email protected] Jonathan N. Coleman, Center for Research in Adaptive Nanostructures and Nanodevices and School of Physics, Trinity College Dublin, Ireland; [email protected] DOI: 10.1557/mrs.2011.236
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MRS BULLETIN • VOLUME 36 • OCTOBER 2011 • www.mrs.org/bulletin
© 2011 Materials Research Society
THE EFFECTS OF PERCOLATION IN NANOSTRUCTURED TRANSPARENT CONDUCTORS
they must retain their electrical conductivity after repeated flexing; it must be possible to deposit them over large areas using low-temperature processing; they must have deposition and material costs comparable to ITO; and
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