Silver Nanowire Networks as Flexible, Transparent, Conducting Films: Extremely High DC to Optical Conductivity Ratios Sukanta De, †,‡ Thomas M. Higgins, § Philip E. Lyons, †,‡ Evelyn M. Doherty, †,‡ Peter N. Nirmalraj, ‡, Werner J. Blau, †,‡ John J. Boland, ‡, and Jonathan N. Coleman †,‡, * † School of Physics, Trinity College Dublin, Dublin 2, Ireland, ‡ Centre for Research on Adaptive Nanostructures and Nanodevices, Trinity College Dublin, Dublin 2, Ireland, § School of Chemistry, University of Wollongong, Wollongong NSW 2522 Australia, and  School of Chemistry, Trinity College Dublin, Dublin 2, Ireland T hin, transparent, conducting films are critical for many optoelectronic devices and components. They are most heavily used for electrode applications in devices such as liquid crystal, flat panel or plasma displays, touch panels, organic light-emitting diodes (OLEDs), and solar cells but are also commonly used as anti- static coatings and EMI shielding material. Such films are usually made from doped metal oxides, most commonly indium tin oxide (ITO). However, ITO has a number of drawbacks and is unlikely to be the material of choice in future optoelectronic devices. The difficulties with ITO revolve around the rising cost of indium, the brittleness of ITO, and the high temperature processing used in its production. Attempts to resolve the first problem have involved use of alterna- tive metal oxides, 1,2 thin metal films, 3,4 or metal grids 5,6 to prepare transparent con- ductors. However, the latter two problems are probably more significant; future dis- plays will be larger and will probably reside on a plastic rather than a glass substrate and so must be flexible. Thus, prospective displays will require flexible transparent electrodes that can be produced at low temperature and over large areas at low cost. We note that such requirements are in addition to traditional technical require- ments associated with low sheet resistance and high transparency. It has been known for the past few years that flexibility and low temperature pro- cessing can be achieved by deposition of nanostructured thin films from the liquid phase. These are known to be stable under flexing 7 and can be spray cast, 8 opening the way to large area deposition. While poly- mer 9 and graphene 10-15 films have been studied, the most common material used to date has been carbon nanotubes. 7,8,16-27 While the cost associated with nanotubes is still a factor, it is expected to come down as demand increases to industrial levels. However, the main problem has been achieving transmittance and sheet resis- tance values routinely achievable with ITO. It is straightforward to achieve sheet resis- tance of 10 /▫ for transmittance of 90% with ITO. Such values have not been achieved with nanostructured electrodes. A commonly used figure of merit for transpar- ent conductors is the ratio of DC to optical conductivity,  DC / Op . To achieve R s = 10 /▫ and T = 90% requires  DC / Op = 350 *Address correspondence to colemaj@tcd.ie. Received for review April 6, 2009 and accepted June 11, 2009. Published online June 24, 2009. 10.1021/nn900348c CCC: $40.75 © 2009 American Chemical Society ABSTRACT We have used aqueous dispersions of silver nanowires to prepare thin, flexible, transparent, conducting films. The nanowires are of length and diameter close to 6.5 m and 85 nm, respectively. At low thickness, the films consist of networks but appear to become bulk-like for mean film thicknesses above 160 nm. These films can be very transparent with optical transmittance reaching as high as 92% for low thickness. The transmittance (550 nm) decreases with increasing thickness, consistent with an optical conductivity of 6472 S/m. The films are also very uniform; the transmittance varies spatially by typically <2%. The sheet resistance decreases with increasing thickness, falling below 1 /▫ for thicknesses above 300 nm. The DC conductivity increases from 2  10 5 S/m for very thin films before saturating at 5  10 6 S/m for thicker films. Similarly, the ratio of DC to optical conductivity increases with increasing thickness from 25 for the thinnest films, saturating at 500 for thicknesses above 160 nm. We believe this is the highest conductivity ratio ever observed for nanostructured films and is matched only by doped metal oxide films. These nanowire films are electromechanically very robust, with all but the thinnest films showing no change in sheet resistance when flexed over >1000 cycles. Such results make these films ideal as replacements for indium tin oxide as transparent electrodes. We have prepared films with optical transmittance and sheet resistance of 85% and 13 /▫, respectively. This is very close to that displayed by commercially available indium tin oxide. KEYWORDS: nanowires · electrode · flexible · transparent · conducting ARTICLE www.acsnano.org VOL. 3 ▪ NO. 7 ▪ 1767–1774 ▪ 2009 1767