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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 .)

Material	ρ (10^-8Ω-m)		Material	ρ (10^-8Ω-m)
Silver	1.59		Iron	10
Copper	1.68		Platinum	10.6
Gold	2.44		Tin	10.9
Aluminum	2.82		1010 Carbon steel	14.3
Calcium	3.36		Lead	22
Tungsten	5.60		Titanium	42
Zinc	5.90		Stainless Steel †	69
Nickel	6.99		Mercury	98
Lithium	9.28		Nichrome	110

† 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 nanowire

Image 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 nanowires

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

(Duke University image by Zuofeng Chen.)

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