Periodic two-dimensional networks of silver nanowires can act as transparent conducting electrodes (TCE) with low sheet resistance and high transparancy, outperforming the standard TCE material ITO (Indium Tin Oxide). After publishing their research results on the influence of the wire diameter and the pitch between the wires in 2012, van de Groep et al. proceeded their work and showed that, when geometrically optimized, these silver nanowire networks can additionally act as photonic light-trapping structures.

 In many optoelectronic devices, as for example displays, solar cells and LEDs, optical transparent and electrical conductive layers are needed. Commonly ITOs (Indium Tin Oxides) are sputtered on a substrate to realize this transparent conductive electrodes (TCE). But ITO has several disadvantages, such as high material costs, damage of organic substrates, brittleness and absorption in the UV/blue spectrum. This motivated van de Groep et al. to search for alternative materials. They published their research results in NanoLetter's May edition of 2012 under the title Transparent Conducting Silver Nanowire Networks.

Several Ag NW networks were fabricated and characterized by van de Groep et al. with wire widths between 45 nm and 110 nm and pitches of 500, 700 and 1000 nm. Averaged over the AM1.5 spectrum, a normalized transmittance up to 91% showed up for their best transmitting Ag NW network. Their best conducting Ag NW network showed a sheet resistance of 6.5 Ω/sq (ca. 0.699 Ω/m²). Their best performing Ag NW network could outperform a 80 nm thick ITO layer sputtered on glass in both disciplines, conductance and transmittance.

 

Van de Groep et al. proceeded their work and published their new results on 19th June 2015 in Nature under the title Large-area soft-imprinted nanowire networks as light trapping transparent conductors. In their previous work they already mentioned the potential of NW networks to function as a light trapping structure in addition to their function as TCE. Now they tried to use this additional feature in a real application device, an organic solar cell.

With substrate conformal imprint lithography (SCIL), using a PDMS stamp, the substrates were patterned. Briefly, a PMMA sacrificial layer and a silica sol-gel layer were applied on a glass substrate by spincoating. With an SCIL stamp the nanowire pattern was applied to the silica sol-gel, which then hardened during drying. Subsequent, the PMMA was removed by reactive ion etching and Ag was applied via thermal evaporation. After a last liftoff process, the Ag NW networks were completed. The wires were all 30 nm high and differed in width from 55 nm too 130 nm, while the pitches differed from 300 nm too 1000 nm in steps of 100 nm. 40 Combinations of pitches and widths were carried by one SCIL stamp and applied on wafer-scale device assemblies.

They successfully implemented these Ag NW networks in P3HT-PCBM organic solar cells as a model system to make use of them as TCEs as well as light trapping structures which were assumed to increase the light absorption of the solar cells. Similar ITO based reference devices were fabricated for comparison. Again they could show that, after geometrical optimization, Ag NW neworks clearly outperform ITO as a material for TCEs  -  now in an actual application, not only in theory  -  and that they additionally scatter the light into guided modes, what increases the light absorption of the solar cell.

 

Silver Nano Wire Networks provide threefold functionality picture 1
Figure 1. Nanoimprinted silver nanowire networks as transparent electrodes.
(a) SEM image of large area 500 nm pitch Ag NW network on glass.

(b) High resolution SEM image of 800 nm pitched network with NW width ~85 nm.
Picture: Van de Groep et al.; DOI: 10.1038/srep11414

 

Silver Nano Wire Networks provide threefold functionality picture 2
Figure 2. Silver NW network based polymer solar cells.
(a) Schematic cross sections of the layer stack of the NW network device (top) and ITO reference device (bottom). (b) FIB cross section of a device with p = 500 nm, clearly showing the Ag NW network embedded in the cell.
Picture: Van de Groep et al.; DOI: 10.1038/srep11414

 

Original Publications:

Transparent Conducting Silver Nanowire Networks, May 3, 2012, NanoLetters
Jorik van de Groep, Pierpaolo Spinelli, and Albert Polman
http://dx.doi.org/10.1021/nl301045a

Large-area soft-imprinted nanowire networks as light trapping transparent conductors, June 19, 2015, Nature
Jorik van de Groep, Dhritiman Gupta, Marc A. Verschuuren, Martijn M. Wienk, Rene A. J. Janssen & Albert Polman
http://dx.doi.org/10.1038/srep11414

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