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Copper Nanowires for Solar Cells

December 6, 2013

You need electrical conductors to make electrical devices. You can use copper wire for larger devices, like table lamps and motors, but integrated circuits need to have small conductive traces of metal applied directly to the semiconductor chip. Although aluminum, and now copper, are used as conductors on integrated circuits, they aren't the only high conductivity (low resistivity) metals, as the following table shows.

Electrical Conductors (Data at 20°C from Wikipedia)
(ρ is the resistivity, the reciprocal of conductivity.)
Materialρ (10-8Ω-m) Materialρ (10-8Ω-m)
Silver1.59 Iron10
Copper1.68 Platinum10.6
Gold2.44 Tin10.9
Aluminum2.82 1010 Carbon steel14.3
Calcium3.36 Lead22
Tungsten5.60 Titanium42
Zinc5.90 Stainless Steel †69
Nickel6.99 Mercury98
Lithium9.28 Nichrome110
† 18% Cr, 8% Ni Austenitic

As you can see from the table, alloys of metals have a higher resistivity than pure metals, a fact that's easily explained by a classical model of electrical conductance called the Drude model.

So, we have quite a panoply of materials to use in our electrical devices; but, a problem arises when you need an electrical conductor that's also transparent for use in displays and solar cells. Although metals are good electrical conductors because of their mobile electrons, the same electrons responsible for the conductivity also scatter photons, so metals are reflective, not transparent. Some "simple metals," copper, silver, and gold, are both transparent and conductive, but only when they are thin films, and not that conductive.

I reviewed a number of transparent conductor alternatives in three previous articles: Transparent and Conductive (June 10, 2011), Silver Nanowire Transparent Electrodes (December 19, 2011), and Nickel-Copper Nanowires (June 5, 2012).

If you want a reasonably conductive material that's transparent at visible wavelengths, then indium-tin oxide (ITO) is the most developed candidate. ITO's resistivity, about 10−4 ohm-cm, is adequate for both display and solar cell applications, but the primary problem with ITO is the price of indium, as high as $650/kilogram in 2012.[1] Not surprisingly, many scientists are looking for alternatives to ITO.

One approach is to use a low concentration dispersion of carbon nanotubes in a transparent material. While it's desirable to have a low nanotube concentration for greatest transparency, the concentration must be higher than the threshold for electrical percolation. Even then, the conductivity is not that large.

A network of silver nanowires has been used instead of a carbon nanotube dispersion. The nanowires, overcoated with the conductive polymer, PEDOT:PSS (poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)), to increase wire-wire connection, form a high quality transparent electrode.[2-3]

As I wrote in a previous article (Nickel-Copper Nanowires, June 5, 2012), Benjamin Wiley, an assistant professor of chemistry at Duke University, and his students have been doing research on copper nanowire transparent electrodes. Copper is as conductive as silver, but much more abundant and a lot less expensive (less than $4.00/lb. vs more than $400/lb.).

The problem with copper is that it will oxidize when exposed to air. Last year, the Duke research team solved this problem by a process in which the copper nanowires are coated with a thin layer of nickel (see figure).[4-5] The nickel layer was found to be an effective means of maintaining the copper conductivity for hundreds of years.[4]

Nickel-coated copper nanowireImage of a nickel-coated copper nanowire.

(Image supplied by Benjamin Wiley, used with permission).

Research on these nickel-coated copper nanowires has continued with application as catalytic electrodes for water photolysis, and the results have been published in a recent issue of Angewandte Chemie.[6-7] Nickel and cobalt-coated copper nanowire network electrodes were fabricated with seven times better transparency than the 80% transparency of ITO. The electrodes, which function at the oxidation half-reaction of the water-splitting reaction, are also flexible.[7]

Dark-field optical microscopy of a network of copper nanowires.Dark-field optical microscopy of a network of copper nanowires.

(Duke University image by Zuofeng Chen.)

The Duke research was funded by the National Science Foundation.[7] Wiley has started NanoForge Corp., a Durham, North Carolina, startup company, to develop such materials.[4]

Scanning electron micrograph of nickel-coated copper nanowiresScanning electron micrograph of nickel-coated copper nanowires intended for water photolysis.

(Duke University image by Zuofeng Chen.)

References:

  1. US Geological Survey, Indium - USGS Mineral Resources Program, Mineral Commodity Summaries, 2013 (201 page PDF file).
  2. Jennifer Marcus, "UCLA team develops highly efficient method for creating flexible, transparent electrodes," UCLA Press Release, November 21, 2011.
  3. Rui Zhu, Choong-Heui Chung, Kitty C. Cha, Wenbing Yang, Yue Bing Zheng, Huanping Zhou, Tze-Bin Song, Chun-Chao Chen, Paul S. Weiss, Gang Li and Yang Yang, "Fused Silver Nanowires with Metal Oxide Nanoparticles and Organic Polymers for Highly Transparent Conductors," ACS Nano, vol. 5. no. 12 (October 28, 2011), pp. 9877-9882.
  4. Ashley Yeager, "Copper-Nickel Nanowires Could Be Perfect Fit For Printable Electronics," Duke University Press Release, May 29, 2012.
  5. Aaron R Rathmell, Minh Nguyen, Miaofang Chi and Benjamin John Wiley, "Synthesis of Oxidation-Resistant Cupronickel Nanowires for Transparent Conducting Nanowire Networks," Nano Letters, vol. 12, no. 6 (May 29, 2012), pp. 3193-3199.
  6. Zuofeng Chen, Aaron R. Rathmell, Shengrong Ye, Adria R. Wilson and Benjamin J. Wiley, "Optically Transparent Water Oxidation Catalysts Based on Copper Nanowires," Angew. Chem. Int. Ed., Online before Print (October 18, 2013), DOI: 10.1002/anie.201306585.
  7. Copper Promises Cheaper, Sturdier Fuel Cells, Duke University Press Release, November 22, 2013.

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Linked Keywords: Electrical conductor; electronics; electrical device; copper; wire; light fixture; table lamp; electric motor; integrated circuit; metal; semiconductor; aluminum; copper interconnect; copper; conductivity; resistivity; silver; iron; copper; platinum; gold; tin; aluminium; aluminum; 1010 carbon steel; calcium; lead; tungsten; titanium; zinc; stainless steel; nickel; mercury; lithium; nichrome; austenitic stainless steel; alloy; chemical element; pure metal; kKinetic theory; classical model; Drude model; material; transparency; transparent; display; solar cell; metal; electron; photon; reflection; thin film; visible spectrum; visible wavelength; indium-tin oxide; ITO; ohm; centimeter; cm; indium; kilogram; scientist; concentration; dispersion; carbon nanotube; threshold for electrical percolation; nanowire; conductive polymer; PEDOT:PSS; Benjamin Wiley; assistant professor; chemistry; Duke University; pound mass; lb.; oxide; oxidize; atmosphere of Earth; air; catalysis; catalytic; electrode; water; photodissociation; photolysis; Angewandte Chemie; oxidation half-reaction; chemical reaction; stiffness; flexible; Dark-field optical microscopy; Zuofeng Chen; National Science Foundation; NanoForge Corp.; Durham, North Carolina; scanning electron microscope; electron micrograph; US Geological Survey, Indium - USGS Mineral Resources Program, Mineral Commodity Summaries, 2013.

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