Highly Transparent Conductive Sheet by Self-assembled Silver Network Electrodes with Antireflective Coating via Roll-to-Roll Process

A highly transparent conductive film was fabricated using self-assembled silver electrodes with anti-reflection (AR) film. The micro pore diameter of sliver nanoparticles (ca 50~60nm in diameter) network electrodes expanded to size of 200-300 μm by the increase of air voids with evaporated water. The transmittance of the silver network electrode increased was drastically increased by the optical interference from the AR film on the surface and it showed a maximal value 84% (sheet resistance: 8.0 Ω/□). The surface resistance of silver network electrodes films did not change even after coating with the AR films because thickness of AR films (ca 100~200 nm) are extremely thin compared with the diameter of the silver wire (ca 3 μm). High performance transparent conductive film for electric and optical devic was successfully fabricated by roll to roll layer-by-layer (LBL) process.


Introduction
Self-assembly processes have been well known as one of the effective method for preparing nanoscale multilayer films from molecular scale to microscale by a bottom-up approach.Recently, self-assembly coating technologies have been used to prepare electronic and optical devices under convenient conditions (room temperature and standard pressure) without requiring vacuum processing [1][2][3][4][5][6][7][8][9][10][11].Moreover, Layer-by-layer (LBL) self-assembled multilayer films are constructed from various nano scale order materials on three dimensional substrates such as optical lenses [12], particles [13] and mesh substrate [14].And it can fabricate functional nano structural film via roll-to-roll process [15][16][17].Up to nowadays, most of the transparent conductive films are indium tin oxide (ITO) film which used rare metal, and produced using vacuum processing.Therefore, it is expected for the practical realization of transparent conductive film by simple wet process without using vacuum process.In this research, we report the fabrication of transparent conductive films by selfassemble of silver nano particles to form the microscale network electrode with the deposition of anti-reflection (AR) film to increase the transparency of visible light.

Experimental procedure
The dispersed silver nanoparticles solution was obtained from Cima NanoTech [18][19][20].The concentrations of silver emulsions were adjusted 1 and 0.8 wt% in toluene/H 2 O, it was ultrasonically shaked for 10 min.These silver emulsions was casting on PET film and formed thin films by bar coating method.Fig 1 shows schemeatic illustrations to form highly transparent conductive film using self-assembled silver nanoparticles network electrodes with AR film.
Single-block AR film was deposited by PDDA and SiO 2 solutions and intermediate H 2 O rinsing 3 times for 8 cycles on silver network electrode.Double-block AR was deposited by PDDA and TALH solutions and intermediate H 2 O rinsing for 3 times for 20 cycles on silver network electrode.The fabricated (PDDA/TALH) 20 films were used as the higher-refractive-index layer.After the deposition of the higher-refractive index layer, (PDDA/Na 2 SiO 3 ) 40, layers were deposited by dipping the substrate into PDDA and Na 2 SiO 3 solutions and H 2 O rinsing bath 3 times and in total repeated for 40 cycles.
The film thickness and refractive indexes were estimated by an ellipsometer.The sheet resistance was measured by four terminal probe units.The optical properties were estimated using an ultraviolet-visible (UV-vis) spectroscopy and Haze mater.The surface morphologies were measured by Laser microscopy.

Self-assembled micro structural silver nanoparticles network electrodes
The micro structureal network electrode was fabricated by self assembly of the silver nanoparticles emulsified solutions.Table 1 shows the values of sheet resistance and transmittance of the fabricated silver nanoparticles network electrodes with the emulsified solution conditions.As shown in this table, it was clearly shown that sheet resistance was lower when the silver concentration was the higher ((a)<(c), (b)<(d)).For example, by comparing (b) and (d), the transmittance of (d) increased 3% by increasing the silver concentration.On the oher hand, transmittance of the film was higher, when the ratio of water to dispersion was higher in the solution ((b)>(a), (d)>(c)).

Table 1. Sheet resistance and transmittance of silver network electrode at the silver solution conditions
Surface morphologies of silver network electrodes at the various silver solution conditions were shown in Fig. 2. As show in Fig. 2, the density of silver network electrodes of (b) and (d) are lower than those of (a) and (c).Since toluene quantity of the dispersion of (b) and (d) are smaller than those of (a) and (c), the size of the voids are larger when the quantity of water is larger As show in the photos, the diameter of micro pores were distrubuted from the size of 200 to 300 µm by the the increase of air voids formed by the water evaporation.We consider this phenomenon was caused by the difference of evaporation speed of toluene and water.When the solution evaporation speed is higher, the smaller voids are formed, and when it is lower, the smaller voids are formed.
This was caused also by the affinity difference of silver nanoparticles to the water and toluene.Since the affinity of silver nanoparticles is larger for toluene and smaller for water, phase separated water droplets do not contain silver nano particles.And these phase separated water droplets formes the circle shape voids among the silver wire of the network that were formed by the assembly of silver nanoparticles.The narrow view of silver wire was shown in Fig. 3 network electrodes was increased according with the expanding size of microsize air voids appeared after water evaporation.[18][19][20].

2 . Self-assembled nano heterostructural antireflection film
The refractive index and film thickness of a doubleblock AR film are estimated using eqs.(1)-( 5) [2],where n 0 is the refractive index of the air medium, n 1 is the refractive index of the lower-refractive-index layer, n 2 is the refractive index of the higher-refractive-index layer, d is the film thickness of AR film, and λ is wavelength of incident light.
In this experimet, refractive index and film thickness of higher and lower refractive index layers were adjusted by the deposition cycles of LBL process.Table 2 shows refractive indexes and film thicknesses of (PDDA/SiO 2 ) 8, (PDDA/TALH) 20 and (PDDA/Na 2 SiO 3 ) 40 coated on silicon wafer measured by 632.5 nm of incident light wavelength were estimated by ellipsometer [21,22].On the other hand, the calculated value of the refractive index and film thickness obtaied from the equations (1) to ( 5) was shown in Table 3.
Table 2.The refractive indexes and film thickness of fabricated films measured by ellipsometry.

Table 3. Caluculated film thickness based on the equations (1) ～(5).
By comparing these two tables, the refractive indexes and film thicknesses of the (PDDA/SiO 2 ) 8 film was consistent with the results obtained using eqs.(1)-( 2) for a single-block AR film, and the refractive indexes and film thicknesses of the (PDDA/TALH) 20 and (PDDA/Na 2 SiO 3 ) 40 films were consistent with the values obitained using eqs.( 3)-( 5) for a double-block AR film.
Fig 4 shows the transmittance spectra of single-block AR films and silver network electrode with AR films.These spectra were measured by a UV-vis spectrometer using 300 to 1000 nm of incident light wavelength.The mean value of the transmittance of single-block AR film was 96 % in the visible range (400 ～ 700nm).The transmittance of silver nanoparticles network electrode was increased when it is covered with single-block AR film, and it showed maximal value of 79 % in the visible range.
Fig. 5 shows the transmittance spectra of double-block AR films and silver network electrode with AR films.The transmittance of double-block AR films showed 99 % maximum trasnmission in the visible range.The transmittance of silver network electrode was increased when it is covered with double-block AR film, and it showed maximal value 84 % in the visible range.This increasing transmittance of silver network electrode was affected by optical interference from AR film.The transmitance of silver network electrode covered by double-block AR was higher than that covered by singleblock AR film.

Conclusions
In this paper, we reported highly transparent conductive film, which has self-assembled silver network electrode and anti-reflection film.The enhancement transmittance of silver electrodes was confirmed by the adjustment of microstructure of silver network and anti-reflective interference.
Moreover, self-assemblled highly transparent conductive film was successfully fabricated by a roll-to-roll LBL-SA process.The surface resistance of silver network electrodes films did not change despite coated with AR films.It is consider that surface resistance slightly shift because thickness of AR films are too thin compared with the film thickness and width of silver nanoparticles network electrodes.The transmittance and sheet resistance of highly transparent conductive film was 84 % and 8.0 Ω/□, respectively.These experimental results obtained in this research suggest that high performance transparent conductive film was successfully obtained by the proposed method that can be used for electric and optical devices.

Fig. 1 .
Fig. 1.Schemeatic image of A highly transparent conductive film was fabricated using self-assembled silver network electrodes with anti-reflection (AR) film.

Fig. 4 Fig. 5
Fig. 4 Transmittance of silver network electrode with single block AR film

Fig. 6 .
Fig. 6.Highly transparent conductive sheet by self-assembled silver electrode with AR film (under side) and without AR film (upper side).