KUNALA DURGA MALLIKHARJUNA RAO (IN)
NINGAPPA PUJAR RAJASHEKHAR (IN)
| US20160009928A1 | 2016-01-14 | |||
| US20130342221A1 | 2013-12-26 |
| WE CLAIM 1. A synergistic mixture of Water and Isopropyl alcohol in ratio 15:85 to induce crack precursor to produce interconnected crack template to obtain wire network of width ranging from about 200nm to about 800nm and wire spacing ranging from about 4μπι to about 16μπι on a substrate. 2. The synergistic mixture as claimed in claim 1 , wherein the crack precursor is acrylic resin nano particle dispersion. 3. The synergistic mixture as claimed in claim 1, wherein the wire is selected from a group comprising of metal and alloy. 4. A substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μπι to about 16μπι. 5. The substrate as claimed in claim 4, wherein the substrate is selected from a group comprising of transparent and translucent substrates. 6. The substrate as claimed in claim 4, wherein the substrate transmits light ranging from about 92% to about 96%. 7. A method of fabricating a substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μπι to about 16μπι, said method comprising acts of a) obtaining a solution of crack precursor and synergistic mixture of claim 1 ; b) coating the solution on a substrate to obtain a crack template of crack precursor; c) implanting metal into the crack template on the substrate; and d) removing the coat of crack precursor to obtain a substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μηι to about 16μηι. 8. A transparent conducting electrode comprising interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 8μηι to about 16μηι. 9. The transparent conducting electrode as claimed in claim 8, wherein the transparency is ranging from about 92% to about 96%. 10. A Hybrid transparent electrode comprising substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 8μηι to about 16μηι along with transparent conducting materials selected from a group comprising indium tin oxide, fluorine tin oxide, and zinc oxide and current collection grid. 11. The Hybrid transparent electrode as claimed in claim 10, wherein the conductivity is ranging from about 4.0Q/sq to about 7 Ω/sq, preferably 4.3Q/sq; fill factor ranging from about 18% to about 22%, preferably 21.1%. |
TECHNICAL FIELD
The present invention is in relation to the field of electronics. The invention provides a transparent conducting electrode of transparency more than 92%. The invention also provides a method of fabricating the transparent conducting electrode by embedding wire network of nano dimensions on transparent substrates.
BACKGROUND AND PRIOR ART
The pandemic usage of electronic devices across different fields has influenced the research and development enormously in the area of optoelectronics. Focus has been to develop new products and methods thereof which are economical, flexible and importantly reconcilable for large scale and large areas. Transparent conducting electrode is one of the principle components in optoelectronic devices like touch screens, sensors, solar cells and the like. Metal oxides of Indium are the coveted ingredients in the preparation of the transparent conducting electrodes. The Indium tin oxide (ITO), the cynosure of the optoelectronic devices is being seriously considered to be replaced with alternatives because ITO films are expensive due to finite supply of Indium, flexibility of ITO films are restricted and they cannot be used over large areas.
The literature provides documents wherein graphene coating (Tung V et al; Nano lett, 2009, 9, 1949-1955, Wang et al Nano Lett, 2008, 8, 323-327); conducting nanotubes have been examined (M Zang et al Science 2005,309 1215; R.C. Tenent; Adv. Mater.2009, 21, 3210) to a great extent. However these materials have failed to gain consistent success as their usage is expensive owing to the problems associated with its purity, stability, bulk preparation and cumbersome methods of application. In the recent past the fine wire network of nano and micro dimensions has been the cynosure of scientific community as it finds numerous applications in the optoelectronics industries. Lithographical methods are the most coveted techniques for obtaining nano and micro patterned templates. Commonly adopted lithographical techniques like, photo electron beam lithography (H. He, et al, Nanoscale, 2012,4,2101-2108; C. H.Duan and A. Majumdar, Nat Nanotechnol,2010, 5,848), imprint lithography( L. J, Guo, J Phys D Applphys, 2004, 37, R123- R141; L. J. Guo, Adv Mater, 2007, 19, 495; C. Peroz et al; nanotechnology, 2012, 23, 15305), ion beam lithography (Z. M. Han et al, Nanotechnology, 2014, 25, 115302) are expensive, elaborate, highly skilled processes. The adoptions of the techniques to large areas are difficult to achieve.
Cracks in various substances are being extensively studies for their use as templates for fabrication of metal wire networks of nano and micro dimensions. The alluring results have prompted extensive studies to understand the dependencies of the crack pattern and fabricate intrinsically fine metal wire networks. In the recent past crack lithography has substantially gained prominence among the various lithographical techniques. The method has been found to be economical in terms of cost as well as time more importantly it is applicable for large areas (K. H. Nam et al, Nature, 2012, 485, 221). The crack lithography has been found to be elaborately applicable on and with various substrates which makes it viable for developing fine metal network over large areas;S. Kiruihika, et al, Fabrication of Oxidation-Resistant Metal Wire Network-Based Transparent Electrodes by a Spray-Roll Coating Process , ACS Appl. Mater, inter. (2015); S. Kiruthika, et al, Metal wire network based transparent conducting electrodes fabricated using interconnected cracked layer as template, Mater. Res. Express 1, 026301 (2014) and S. Kiruthika, et al Large area solution processed transparent conducting electrode based on highly interconnected Cu wire network, J. Mater. Cheni. C 2, 2089■■ 2094 (2014): provides methods for developing nano and micro sized crack networks of developing nano and micro sized wires 1 micron to 100 micron dimensions are developed and used as templates for obtaining metal wires. Owing to the fineness of the dimensions of the metal wires they have gained importance as transparent conductors (E. C. Garnett et al; Nature materials Vol 11, 2012).
In the current scenario, the versatility of the optoelectronic devices demands materials and methods which are highly transparent, conducting, economical and easy to fabricate over large areas. It is desirable to have fine, well connected wire network over large areas to fabricate transparent conducting electrodes which are better than the known devices. The present invention is aimed to develop a transparent conducting electrode fabricated with nano wires imparting transparency of more than 92% to the material on which it is fabricated, by adopting a simple and economical crack lithography method.
STATEMENT OF INVENTION: Accordingly the present invention provides a synergistic mixture of Water and Isopropyl alcohol in ratio 15:85 to induce crack precursor to produce interconnected crack template to obtain wire network of width ranging from about 200nm to about 800nm and wire spacing ranging from about 4μηι to about 16μηι on a substrate; a substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μηι to about 16μηι; a method of fabricating a substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μηι to about 16μηι, said method comprising acts of (a) obtaining a solution of crack precursor and mixture of present invention, (b) coating the solution on a substrate to obtain a crack template of crack precursor, (c) implanting metal into the crack template on the substrate; and (d) removing the coat of crack precursor to obtain a substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μπι to about 16μπι, a transparent conducting electrode comprising interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μπι to about 16μπι, and Hybrid transparent electrode comprising substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μπι to about 16μπι along with transparent conducting materials selected from a group comprising indium tin oxide, fluorine tin oxide, and zinc oxide and current collection grid.
BRIEF DESCRIPTION OF FIGURES:
Figure- 1 : Figure- 1(a) to (d) shows optical microscopic images of dispersion of colloidal solution in water as diluter and cracks formed with 0.7 to 1.0 gm/ml concentration respectively. Figure - 1(e) shows plot of crack width against crack precursor concentration;
Figure -2: Figure- 2(a) to (f) shows optical microscopic images of dispersion of colloidal solution in different diluters like Chloroform, Petrol, Dimethylformamide, Isopropyl alcohol (IP A), Ethyl acetate, Acetone respectively to form crack pattern;
Figure-3: Figures 3(a) and 3(b) shows plot of cracks formed with concentration 0.3g/ml and concentration 0.35g/ml using different Water-Isopropyl alcohol ratio respectively. Light blue colour strip indicates cracks are not formed; Figure -4: shows optical microscopic images of dispersion of colloidal solution and formation of cracks with Water-IPA combination ranging from about 12:88-23:77 as diluter with crack precursor concentration of 0.3 g/ml;
Figure-5: shows optical microscopic images of dispersion of colloidal solution and formation of cracks with Water-IPA combination ranging from about 11 :89-30:70 as diluter with crack precursor concentration of 0.35 g/ml;
Figure-6: Figures- 6 (a-d) shows optical microscopy and optical profiler images of cracks formed with different speeds of spin coater ranging from about 1000 to about 4000 rpm at crack precursor concentration of 0.4 gm/ml andwith Water-IPA mixture of ratio (15:85) as diluter. Figures- (e) and (f) shows height profile measured at 2000 and 3000 rpm respectively. Figure - (g) shows plot of crack width against speed of spin coater as in (a-d). Figure-(h) shows plot of template height against speed of spin coater;
Figure -7: Figures-(a-f) shows optical microscopy and optical profiler images of cracks formed with different speeds of spin coater ranging from about 1000 to about 6000 rpm respectively at crack precursor concentration of 0.45 g/ml with Water-IPA mixtureof ratio (15:85) as diluter. Figure- (g) shows plot of crack width against speed of spin coater as in (a-f). Figure- (h) shows plot of template height measured from optical profiler against speed of spin coater;
Figure-8: Figures-(a-d) shows magnified optical microscopic images of cracks formed when thinner is used as diluter to disperse colloidal solution. The concentration of crackle precursor being 0.45 g/ml. Figures- (e-h) shows cracks formed when Water-IPA of (15:85) ratio is used as diluter to disperse colloidal solution; Figure -9: Figures- (a-d) shows magnified optical microscopy of metal deposited on Glass template using thinner as diluter. Figures-(e-h) shows the metal deposited on Glass template using Water- IP A of ratio 15:85 as diluter;
Figure-10: FESEM images for wire width measurements at various positions on template with Silver metal on Glass substrate;
Figure- 11 : Histogram (a) of counts observed in different wire width measurements using Water - IPA (15:85) as diluter. Histogram (b) of counts observed in different wire width measurements using thinner as diluter. Histogram (c) and (d) observed in different wire width measurements using thinner as diluter. Histogram (e) and (f) showing counts of average distance between wires and cell area respectively for thinner as diluter;
Figure-12 (a): shows magnified FESEM images, Figures-12(a-j) shows cracks from cross- sectional view, and Figures 12 (k-o) shows cracks from top view;
Figure-12 (b): shows magnified SEM images of cross sectional views of (a) V shaped and (b) U shaped groves; Figure-13: shows magnified FESEM images of Copper metal wire deposited on Glass substrate; Figure-14: shows magnified FESEM images of Copper metal wire deposited on PET substrate; Figure-15: shows magnified FESEM images of Silver metal wire deposited on PET substrate; Figure-16: shows magnified FESEM images of Silver metal wire deposited on Glass substrate; Figure-17: Figures 17 (a) and (b) are plots of transmittance against wavelength of Copper metal on Glass and PET respectively. 17(c) and (d) are plots of transmittance against wavelength of silver metal on Glass and PET respectively;
Figure-18: shows AFM data, Figures- (a) & (d) shows height AFM (wire width=800nm wire height=120nm), (b) & (e) showing conducting AFM at-4V, (c) &(f) showing conducting AFM at 0V;
Figure-19: shows SEM images. Figures- (a-c) are Type-2 network on PET, and Figures- (d-f) are Type-1 network on PET and Hybrid (Type-2 on Type-1) network on PET;
Figure -20: shows transmittance plot of Hybrid network; Figure-21 : shows Bitmap of metal wire network derived from FESEM images for (a) Hybrid network and (b) Type-2 (fine wire) network and (c) Type-1 (broadwire) network; and
Figure -22: shows electrical measurements, Figure-22(a) shows change in sheet resistance for each cycle and Figure-22 (b) shows the strain applied each time when bending.
DETAILED DESCRIPTION OF INVENTION:
The foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the drawings, description and claims.
The present invention is in relation to a specific composition of solvents for obtaining crack network of 200nm-800nm dimensions. The crack network is used for obtaining metal wire network on transparent material to obtain transparent conducting electrodes. The various embodiments of the present invention however has been studied and described for exemplary purpose.
The various embodiments of the present invention along with the method of fabrication of transparent conducting electrode is described below with reference to the Figures. It may further be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art.
The present invention is in relation to a synergistic mixture of Water and Isopropyl alcohol in ratio 15:85 to induce crack precursor to produce interconnected crack template to obtain wire network of width ranging from about 200nm to about 800nm and wire spacing ranging from about 4μπι to about 16μπι on a substrate.
In another embodiment of the present invention, the crack precursor is acrylic resin nano particle dispersion. In still another embodiment of the present invention, the wire is selected from a group comprising of metal and alloy.
The present invention is also in relation to a substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μπι to about 16μπι. In still another embodiment of the present invention, the substrate is selected from a group comprising of transparent and translucent substrates.
In still another embodiment of the present invention, the substrate transmits light ranging from about 92% to about 96%.
The present invention is also in relation to a method of fabricating a substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μπι to about 16μπι, said method comprising acts of a) obtaining a solution of crack precursor and synergistic mixture of present invention; b) coating the solution on a substrate to obtain a crack template of crack precursor;
c) implanting metal into the crack template on the substrate; and
d) removing the coat of crack precursor to obtain a substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μπι to about 16μπι.
The present invention is also in relation to a transparent conducting electrode comprising interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μπι to about 16μπι.
In yet another embodiment of the present invention, the transparency is ranging from about 92% to about 96%.
The present invention is also in relation to a Hybrid transparent electrode comprising substrate implanted with interconnected wire network of width ranging from about 200nm to about 800nm and with wire spacing ranging from about 4μηι to about 16μηι along with transparent conducting materials selected from a group comprising indim tin oxide, fluorine tin oxide, and zinc oxide and current collection grid.
In another embodiment of the present invention, the conductivity is ranging from about 4.0Q/sq to about 7 Ω/sq, preferably 4.3Q/sq; fill factor ranging from about 18% to about 22%, preferably 21.1%.
Definitions
Crack precursor material- is an acrylic resin nano particle dispersion.
Crack- Crack produced down to substrate with V shape ranging crack width of 200nm-800nm. The experimentation for obtaining the transparent conducting electrodes is optimized in two different stages. In the stage one, a method to obtain the crack template which can yield self- welded, interconnected metal wire network of nano dimensions ranging from about 200nm to about 800nm on transparent materials over large areas of 5x5cm ''" is optimized and in the stage two the crack template is used for the fabrication of the metal wire network over large areas of transparent materials to obtain transparent conductors of transparency ranging from about 92% to about 96% .
The cracking of crack precursors is dependent on various parameters like temperature, nature of crack precursor, diluter, and method of application of the crack precursor. However for the current study, the dependency on the diluter for cracking is analyzed. The experimentation is carried out using commercially available acrylic resin of nano particles as the crack precursor. The crack precursor is diluted with different diluters and the cracking pattern is analyzed. The crack formations are observed using 60μ1 of solution of crack precursor in different diluters, spin coated at 1000 rpm over l l inch 2 Glass and PET substrate. The crack precursor cracked differently with different types of diluters.
The results of usage of water as the diluter is depicted in Figure- 1. Initially a solution of 0.6 g/ml and lesser concentrations of colloidal dispersions are spin coated at lOOOrpm on Glass substrate and drying did not result in crack formation. However, increase in the concentration of the crack precursor exhibited crack formation with inter-connectivity specifically at 0.9 g/ml concentration forming crack width of 200 -800nm range and further increase in concentration resulted in broad cracks of width ranging from 1-1.5μηι.
The crack formation with different diluters like Acetone, Isopropyl alcohol, Chloroform, Petrol is also analyzed using acrylic resin of nano particles as the crack precursor. However, the efforts are futile as none of them could get dispersed well in any composition and as an individual solvent when applied over various substrates. The images in Figure-2 show that different diluters could only form big particles or lumps did not allow crack formation acrylic resin of nano particles.
However, the stable dispersion of crack precursor in water and isopropyl alcohol directed the research to focus and analyse the usage of mixture of water and isopropyl alcohol for crack formations. It is also observed that with water per se cracks formed are not uniform all over the film and with isopropyl alcohol per se cracks are not interconnected. Preliminary experiments with mixture of Water and isopropylalcohol resulted in better crack formation as compared to crack formation in Isopropylalcohol or Water er se. L Optimization of Water-Isopropylaicohol ratio:
The crack formation is analyzed keeping concentration fixed at 0.3g/ml (Figure- 3-5). The results are depicted in the graph of Figure- 3(a) and optical images of Figure -4. It is observed that cracks do not form when the volume of water in the Water-Isopropyl alcohol solutions is upto 13 %. However when the concentration of water is increased to 14% intermittently connected cracks are formed. The ratio of 15:85 Water-Isopropyl alcohol gives well connected cracks and further increase in water concentration in the ratio of Water-Isopropylaicohol composition do not yield any crack. Figure -4.
Similarly experimentation is carried out with higher concentration of crack precursor i.e., 0.35g/ml. From the graph of Figure-3(h) and optical images in Figure -5 it is observed that cracks do not form when the concentration of the water in the composition ranges from 1-12%. However for the ratios 13:87 and 14:86, crack formation are noticed and Water-Isopropylalcobol in the ratio 15:85 yielded well connected cracks. Further, increase in the volume of water in the Water-Isopropylaicohol composition do not yield any crack as observed from Figure-(5). The crack dimensions are observed to be of 200nm to about SOOnra (Figures- 4 and 5). Lower concentrations i.e., 0.2 and 0.25g/ml though yield cracks, are not well connected and are not formed uniformly. The results thus helped to optimize that, a synergistic combination of Water-Isopropylaicohol in the ratio 15:85 as a diluter help in the formation of fine cracks of crack width ranging from about 200nm to about 800nm with a concentration of 0.3-0.4g/ml. Table-1 below provides the tabulated results under various conditions. Table- 1 showing the formation of crack width under different conditions.
II. Effect of speed of spin coating on the crack formation: Experiments are also performed to analyse the effects of change in the speed of spin coater. The Figures- 6 and 7 depicts the results obtained keeping ratio of diluter i.e. Water-Isopropylalcohol as 15:85. The experimentation is done keeping the concentration fixed at 0.4 g/ml. As observed in Figure-6, the cracks are wide and are in range of 1-1.5 μπι. However increase in rpm from 1000 to 2000 results in crack formation in the range of 200 to 800nm and further increase in rpm do not result in any crack formation (Figure-6).
Similarly for concentration of 0.45g/mi (Figure-7), crack formation in the range of 1 -1.5μηι range is observed when the rpm is 1000-2000 rpm. Further increase from 2000rpm to SQQQrpni result in well interconnected narrow cracks of width ranging from 200-800nm and however at 600Qrpm crack formation is not observed. Higher concentrations produced fine crack network when the rpm is more than 1000.
Experiments are also performed to compare two different diluters, a thinner and Water- Isopropylalcohol composition in the ratio 15:85 with same concentration of crack precursor to form cracks. The crack formed when concentration of 0.45g/ml is used with Water- Isopropyl lcohol as diiuterat lOOOrpm (Figure-8),axe dense, well i terconnected as compared to tliinner. Comparison of Figures-8 and 9 of metal network with Water-Isopropylalcohol and thinner as diluter respectively clearly indicates that the fine wire network formed by the Water - isopropyl alcohol is dense and uniformly well distributed all over the substrate. The histogram plot of 11 (a) also indicates good interconnectivity of wires for water-IPA as diluter. The SEM images from Figure- 10 indicates that the average wire width measured is in the range of 200- 400nm. The histogram figures indicate that average cell area is ranging from 10-600μηι as in histogram plot 11 (b) and average distance between the wires is ranging from 3 -30μηι as in histogram plot 11(c) for Water -Isopropyl alcohol as diluter. Also, the Histogram Figures l l-(d- f) indicates that the wire width, average cell area and average distance between the wires is 2-14 μπι, 50-4000 μηι , 12-150 μπι respectively. The values clearly indicate that with Water - Isopropyl alcohol as the diluter forms nano meter width crack leading to less cell area and reduced distance between wires.
III. Shape of tSie cracks:
SEM images in Figure-12 (a) and 12 (b) shows cracks formed by6(^l crack precursor spin coated at lOOOrpm on PET and Glass substrate of lxl inch size. The SEM images indicates that the cracks formed using Water-Isopropylalcohol in the ratio 15:85 as diluter are in V shape helping in formation of fine crack width of 200-800nm and U shaped cracks yield cracks of width 1-20μηι.
IV. Characterization of Transparent Conducting Electrode:
A. Instrumentation:
Transmittance is measured using a Perkin Elmer Lambda 800 UV/visible/near-IR spectrophotometer. Sheet resistance is measured using a 4-Point Probe Station (Jandel Model RM3, London and Techno Science Instruments, India). Optical images are acquired with the optical microscope (Labenjndia). SEM is carried out using a Nova Nano SEM 600 instrument (FEI Co., The Netherlands). AFM measurements are performed using di Innova (Bruker, USA) in contact mode. Standard Si cantilevers are used for normal topography imaging. Wyko NT9100 Optical Profiling System (Bruker, USA) is used for height and depth measurements. ImageJ software is used to perform analysis of the crack patterns. Flexibility test is done using a self-developed setup.
B, Measurement of transmittance:
The studies on the transmittance are conducted using wire network of different metals lodged on either Glass or polyethylene terephthalate (PET) under different conditions. Table-! shows results of copper nano wire network lodged on Glass, Table-2 corresponds to the results obtained by copper wire network lodging on PET, Table 3 provides information of transmittance of silver nano wire lodged on Glass and Table-4 is related to the transmittance studies of silver nano wire network on PET. Table- 1 : Characteristics of transmitting conductor electrodes formed by lodging of copper nano wire network on Glass,
TabIe-2: Characteristics of transmitting conductor electrodes formed by lodging of copper nano wire network on PET
Wire
CP condition Speed Sheet Transmittance %
SI. No. network g/ml (rpm) resistance(i sq)
width (nm)
1 0.4 1000 192.7 93.92 600-800
2 0.5 4000 138.6 92.73 650-800
0.5 3000 10.14k 94.83 400-600
4 0.55
3200 178.72 94.47 650-800
0.55
5 3200 775.16 94.82 600-800
0.55
6 3000 290.05 94.32 600-850
7 0.55
3200 3.29k 96.37 250-450
Blank
8 - - 99.92 - Table 3: Characteristics of transmitting conductor electrodes formed by lodging of silvernano wire network on Glass using 0.45 g/ml of crack precursor.
Wire network
Si No Speed (rpm) Sheet resistance (Ω/sq) Transmittance %
width(nm)
1 6000 182.8 93.98 500-700
2 6000 93.13 93.92 450-600
3 6000 159.0 94.1 400-600
4 6000 446.0 94.53 400-600
5 6000 698.3 93.69 600-800
6 6000 206.2 94.69 450-650
7 6000 226.4 94.61 600-800
8 4000 203.7 94.03 350-550
9 6000 269.3 93.31 500-700
10 5500 144.1 93.95 500-700
11 6000 196.3 94,05 600-700
12 6000 3.3k 94.86 600-800
13 Blank - 99.91 - Table 4: Characteristics of transmitting conductor electrodes formed by lodging of silver nano wire network on PET.
It is seen that transmittance ranging from about 92.36% to about 96.50% is observed for the wire network of width measuring from 200nm-800nm indicating high transmittance of the conducting materials. While Figures-13-16 shows the SEM images of nano wire network of metals silver and copper deposited on crackle template to obtain transparent conducting electrode (TCE),the Figures-17 (a) and (b) are plots of transmittance against wavelength of Copper metal on Glass and PET respectively and 17(c) and (d) shows plot of transmittance against wavelength of Silver metal on Glass and PET respectively. C. Conducting mode Atomic Force Microscopic measurement (C-AFM):
Figure- 18 indicates that the conductivity follows the topography of the wire network on the substrate, however it is noticed that the wire width in C-AFM measurement is marginally increased as compared to the topographical wire network which indicate that the conducting channels are wider than the actual wire network, which indicate that the conducting channels are wider than the actual wire network. Such conducting channels are more desirable for optoelectronic devices. It is also observed that the current in C-AFM is uniform all around the network including the junction and edges indicating seamless wire network.
D. General preparation of embedded wire network on Glass or PET: Colloidal systems are used as sacrificial layer to produce crack pattern, typically an acrylic resin water based dispersion available commercially as crack nail polish (Ming Ni Cosmetics Co., Guangzhou, China) is used. A dispersion of about 0.3-0.55g/ml is diluted in water-isopropyl alcohol composition of ratio of 15:85. The diluted dispersion is ultrasonicated at 37Hz for lOmin prior to spin coating on a substrate i.e., commonly available Glass or PET sheets (2mm and 125 μιτι thickness, respectively). The substrates are washed with water and isopropyl alcohol and purged with nitrogen or hot air gun before they are coated with theColloidal dispersion. Crack network pattern is formed spontaneously on the coated layer. The crack network is deposited with metals like copper and silver by physical vapour deposition system. In the final step of liftoff, the cracked sacrificial layer is dissolved by dipping in chloroform. The transparent conductor electrodes thus formed are measured for their transparency and conductivity. Example 1
0.4 g/ml acrylic resin water based dispersion available commercially as crack nail polish (Ming Ni Cosmetics Co., Guangzhou, China) is diluted in water-isopropyl alcohol composition of ratio of 15:85. The diluted dispersion is ultrasonicated at 37Hz for lOmin and spin coated at rpm 1000 on a clean PET sheet of thickness 125μιτι thickness. Crack network pattern is formed spontaneously on the PET layer. The crack network is deposited with copper by physical vapour deposition system. In the final step of lift-off, the cracked sacrificial layer is dissolved by dipping in chloroform to obtain the transparent conducting electrode.
The transparent conductor electrode thus formed is found to be 93.92% transparent and of resistance 192.7 Ω-sq.
Example 2:
0.5 g/ml acrylic resin water based dispersion available commercially as crack nail polish (Ming Ni Cosmetics Co., Guangzhou, China) is diluted in water-isopropyl alcohol composition of ratio of 15:85. The diluted dispersion is ultrasonicated at 37Hz for lOmin and spin coated at 4000 rpm on a clean Glass of thickness2mm thickness. Crack network pattern is formed spontaneously on the Glass layer. The crack network is deposited with copper by physical vapour deposition system. In the final step of lift-off, the cracked sacrificial layer is dissolved by dipping in chloroform to obtain the transparent conducting electrode.
The transparent conductor electrode thus formed is found to be 92.73% transparent and of resistance 138.6 Ω-sq. Example 3:
0.45 g/ml acrylic resin water based dispersion available commercially as crack nail polish (Ming Ni Cosmetics Co., Guangzhou, China) is diluted in water-isopropyl alcohol composition of ratio of 15:85. The diluted dispersion is ultrasonicated at 37Hz for lOmin and spin coated at 6000rpm on a clean Glass of thickness 2mm thickness. Crack network pattern is formed spontaneously on the Glass layer. The crack network is deposited with copper by physical vapour deposition system. In the final step of lift-off, the cracked sacrificial layer is dissolved by dipping in chloroform to obtain the transparent conducting electrode.
The transparent conductor electrode thus formed is found to be 93.92% transparent and of resistance 93.12.
Example 4:
0.5 g/ml acrylic resin water based dispersion available commercially as crack nail polish (Ming Ni Cosmetics Co., Guangzhou, China) is diluted in water-isopropyl alcohol composition of ratio of 15:85. The diluted dispersion is ultrasonicated at 37Hz for lOmin and spin coated at 1200rpm on a clean Glass of thickness2mm thickness. Crack network pattern is formed spontaneously on the Glass layer. The crack network is deposited with copper by physical vapour deposition system. In the final step of lift-off, the cracked sacrificial layer is dissolved by dipping in chloroform to obtain the transparent conducting electrode.
The transparent conductor electrode thus formed is found to be 91.97% transparent and of resistance 51.6 i sq. Hybrid network preparation:
Hybrid networks corresponding to two wire networks lodged one above the other is prepared to enhance the charge collection in optoelectronic devices. In the present invention a wire network of width 200-800 nm formed by using Water -Isopropyl alcohol of ratio 15:85 as diluter is lodged above a wire network of width 3-20μη formed using thinner as diluter. SEM images and transmission plot of the Hybrid network are in the Figures- 19 and 20. Images in Figure-19(a-c) corresponds to fine wire network of T pe-2 and Figures- 19(d-f) corresponds to broad wire network of Type-1 and Figures 19-(g-i) corresponds to Hybrid network where both types of networks are present. Table-5: Characteristics of Hybrid transparent electrode.
Using ImageJ software Bitmap image (Figure-21(a) (b) and (c) is derived from FESEM imagesof wire network, and the fill factor values obtained correlates with the transmission loss occur in Glass or PET due to metal wire network lodged. The table 6 below provides the comparison.
Table 6: Comparison of Type-1, Type -2 and Hybrid electrodes
From the values in the table 6 it is concluded that among Type-1, Type-2 and Hybrid networks, the Hybrid networks is advantageous as sheet resistance is less as compared to Type-1 and Type- 2, however the percentage of transmittance is less compared to both types of network. The transmission observed is about 78% and the resistance is less than 10Ω. Hybrid network of the present invention also provides the advantage of better conductivity, higher fill factor and efficient charge collection if used as electrode in an optoelectronic device.
Example 5: Typically 60 μΐ solution of 0.85g/ml of acrylic colloidal solution available commercially as crack nail polish (Ming Ni Cosmetics Co., Guangzhou, China) diluted using thinner is spin coated at lOOOrpm on PET substrate to obtain crack network; copper is deposited by physical vapor deposition and lift off is done to obtain Type-1 metal network on PET. Later 60 μΐ solution of 0.4 g/ml composition using Water-Isopropyl alcohol in 15:85 ratio as diluter is spin coated on copper-Type- 1 network on PET at 2000rpm to form Type-2 crack network. Copper metal is deposited by physical vapour deposition and liftoff is done to obtain the Hybrid network. The results are tabulated in Table -5. Figure -20 shows the transmission plot where it is seen that the percentage transmission is around 78% in case of Hybrid network.
Flexibility tests of Hybrid network- Experiment is performed using a Hybrid network to understand the effects of applying stain to the substrate. The results indicated that even after applying strain the resistance do not alter substantially recommending the usage of the wire network in flexible opto-electronics devices. Figure -22 shows the optical images and electrical characteristics of Hybrid network. Figure-22 (a)) shows that the TCE is flexible in nature and Figure-22(b) shows the optical image of tension position of TCE and strain applied is 1.25% for each cycle. The strain percentage applied to the sample is calculated using the formula given below.
E=(t X 100)/r
where 'r' is the bending radius of the substrate 't' is the thickness of substrate. Length of nanowire Formation of long wires of length of about 7.33cm are observed in the wire mesh network of dimensions 5x5 cm on both PET/Glass samples.
Thus the present invention provides a simple and commercially viable method of producing transparent conducting electrodes of transparency greater than 92% and resistance of 50 Ω/sq. This wire network can be combined with continuous thin film such as ITO, ZnO, FTO or current collection grid with big wire to enhance the charge collection efficiency. The transparent conducting electrodes thus can be easily adopted in various optoelectronic devices in a economical way.
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