Transparent conductive coatings for optoelectronic and electronic devices

Abstract

The invention provides processes for the manufacture of conductive transparent films and electronic or optoelectronic devices comprising same.

Claims

1. A process for the manufacture of a conductive transparent film on a substrate, the process comprising: coating a substrate by a first material to form a wet film of said first material on at least a region of a surface of said substrate; treating the film with at least one second material capable of displacing the first material in the film at the point of contact; thereby leading to displacement of the film material from the point of contact and the exposure of the substrate, to provide an array of spaced apart ring-voids in said film; and optionally treating the film to render the first material conductive.

2. The process according to claim 1, wherein the substrate is of a material selected from the group consisting of glass, paper, a semiconductor inorganic or organic material, a polymeric material and a ceramic material.

3. The process according to claim 2, wherein the substrate is an inorganic semiconductor material selected from the group consisting of silicon, tin, compounds of boron, tellurium, geranium, gallium, gallium arsenide (GaAs), gallium phosphide (GaP), cadmium telluride (CdTe), gallium aluminum arsenide (GaAlAs), indium phosphide (InP), gallium arsenide phosphide (GaAsP), cadmium sulfide (CdS), copper indium gallium diselenide (CIGS), mercury cadmium telluride (HgCdTe), copper indium sulfide and copper indium selenide.

4. The process according to claim 2, wherein the substrate is of a polymeric material selected from the group consisting of a polyimide, polyester, a polyacrylate, a polyolefin, a polyimide, a polycarbonate and polymethyl methacrylate.

5. The process according to claim 1, wherein the first material is selected from the group consisting of a metal, a transition metal, a semiconductor, an alloy, an intermetallic material, a conducting material, and a carbon-based material.

6. The process according to claim 5, wherein the conductive material is a conductive polymer.

7. The process according to claim 6, wherein the conductive polymer is selected from the group consisting of poly(3,4-dioctyloxythiophene) (PDOT), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PDOT:PSS), polyaniline and polypyrrole.

8. The process according to claim 5, wherein the conductive material is a combination of two or more different conductive materials, being deposited on the substrate to form a plurality of ring structures, each being of a different conductive material, or are deposited step-wise to form a conductive multilayer, each layer being composed of a plurality of ring structures and being of a different material or material form.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 presents an array of material-free voids (holes) in a film of conductive metallic nanoparticles, the voids are separated by a film of metallic nanoparticles. The voids were obtained by printing an aqueous solution onto a previously formed film of a solvent-based metallic ink.

(3) FIG. 2 presents a SEM image of the ring periphery composed of Ag and Cu nanoparticles on a Si substrate.

(4) FIG. 3A is an SEM image of a single ring structure according to an embodiment.

(5) FIG. 3B is a zoomed in image of the rim of the ring in FIG. 3A.

(6) FIG. 3C is another zoomed in image of the rim of the ring in FIG. 3A.

(7) FIG. 3D is a height profile measurement of the ring in FIG. 3A.

(8) FIG. 4 presents an SEM image of an array of non-intersecting ring structures.

(9) FIG. 5A is a topographic image of the rim of a single Ag ring based on conductive AFM measurement.

(10) FIG. 5B is a corresponding current image to the image in FIG. 5A, acquired at tip bias of 0.5 V.

(11) FIG. 5C presents a corresponding current image to the image in FIG. 5B in the cross section of the silver line measured with tip bias of 1 V where the current image range is 0-40 nA (the saturation current in the measurement was 40 nA).

(12) FIG. 6A is a light microscope image of a chain of rings according to an embodiment.

(13) FIG. 6B is an SEM image of the image in FIG. 6A showing a closer look on the junction between two rings where it is clearly seen that close-packing of the particles is not damaged by the new junction.

(14) FIG. 6C is another SEM image of the image in FIG. 6A showing a closer look on the junction between two rings where it is clearly seen that close-packing of the particles is not damaged by the new junction.

(15) FIG. 7A presents a 40×40 μm.sup.2 topographic image according to an embodiment.

(16) FIG. 7B presents a corresponding current image to the image in FIG. 7A measured with tip bias of 1 V where the current image range is 0-40 nA (the saturation current in the measurement was 40 nA).

(17) FIG. 8A presents an array of interconnected rings at a first magnification.

(18) FIG. 8B presents an array of interconnected rings at a second magnification.

(19) FIG. 9A presents the electro-luminescent glow from the rings in FIGS. 8A and 8B in a 2 mm×1 cm device at a first magnification.

(20) FIG. 9B presents the electro-luminescent glow from the rings in FIGS. 8A and 8B in a 2 mm×1 cm device at a second magnification.

DETAILED DESCRIPTION OF EMBODIMENTS

(21) The transparent conductive patterns provided in accordance with the processes of the present invention are better alternatives to the widely used transparent conductive oxides, such as ITO, and can be utilized in optoelectronic devices such as solar cells. The new transparent conductive coatings are achieved by forming a 2-D array of interconnected rings or array of holes, while the rim of the ring and the spaces in between the holes is composed of a conducting material such as metallic nanoparticles. The rim of the individual rings has a width of below 50 microns and a height below 300 nm, which surrounds a “hole” with a controllable diameter for example diameter of about 150 micron, and therefore the whole array of the interconnected rings is invisible to the naked eye. In case of metallic materials, the rims of the rings are composed of self-assembled, closely packed nanoparticles, which make the individual rings and the resulting array electrically conductive.

(22) The formation of arrays of holes is based on controlled wetting which induces, in case of metallic nanoparticles, their self-assembly into predetermined narrow patterns around two-dimensional empty cells. In case of dissolved conducting polymer, the conducting material concentrates at the rims (in case of ring formation) or in between empty spaces (in case of holes formation).

(23) The performance of the new transparent conductive coatings which are obtained by printing metallic (or semiconductor) particles was demonstrated while using it as the transparent electrode on a plastic electroluminescent device, demonstrating the applicability of this concept in plastics electronics. Such transparent conductive coatings can be used in a wide range of applications such as displays (LCD, Plasma, touch screens, e-paper), lighting devices (electroluminescence, OLED) and solar cells.

Example 1

Printing Holes

(24) A glass slide was coated by a solvent based silver ink (solvent with dispersed silver nanoparticles) by draw down. Before the total evaporation of the solvent, an aqueous solution was printed on top of the coat by ink-jet printing (MicroFab, 60 um wide nozzle). The solution contained 0.05% of a wetting agent (BYK 348) The printed droplets led to the de-wetting of the silver ink at the printing zone, i.e., to the formation of holes (an area which does not contain the conducting material). The area with holes can be further treated if required to obtain conductivity. For example, if the conducting material is silver nanoparticles, the substrate can be sintered at elevated temperature, or by other chemical means.

Example 2

Single Rings

(25) Single rings with a diameter of ˜150 micrometer were obtained by ink-jetting a silver ink (prepared as described elsewhere [14]) on polyethylene terphthalate (PET) substrate with the single nozzle print-head.

(26) Silver ink: The aqueous ink contained 0.5 wt % dispersed silver nanoparticles, with a diameter of 5 to 20 inn, stabilized by polyacrylic acid.

(27) The surface tension of the ink was adjusted to 30 mN/m by the use of Byk®348 (BykCheime). The pH was set to 10 using amino methyl propanol.

(28) Printing: The printing of the dispersion was performed by Microfab®JetDrive™ III printer with a 60 micrometer wide single nozzle. The applied waveform for all the printing experiments was: voltage 110 V, rise time 3 microsec, echotime 15 microsec, dwell time 30 microsec fall time 5 microsec. As expected, the voltage increased (from the minimal voltage of 20 V), the droplet size increased as well, thus enlarging the ring diameter. Further increase above 110V, was not tested due to device limitation. The dwell time was also increased in steps of 5 microsec up to a value of 40 microsec (in each change the echo-time was double the value of the dwell time), which also caused the droplet and ring diameter to increase. Changing the dwell time and the voltage did not affect the edge profile, as it remained in a parabolic shape. The movement of the substrate was performed by a DMC-21×3XY table (Galil Motion Control, Inc.).

(29) The substrate temperature was set to 30° C. with a Peltier heater/cooler, and humidity within the printing chamber was 30-40% RH.

(30) Preliminary printing tests performed by varying the waveform, surface tension, and metal load of the dispersion indicated that during the evaporation of individual droplets circular rings were formed. As presented in FIG. 3A, the ring diameter is about 150 micrometers, while most of the metallic particles are concentrated at the rim of the ring. SEM evaluation (FIGS. 3B and 3C) shows that the rim is composed of closely packed silver nanoparticles. Profilometer measurement (FIG. 3D) reveals that the height of this layer of nanoparticles is about 250 nm. As shown in FIG. 4, the ring formation process can be, repeated for a large number of droplets, while the formed rings are very similar in size and shape.

(31) In order to obtain a conductive array composed of rings, each ring should be conductive. Therefore, at the first stage, resistivity measurements of individual rings were performed by connecting the ring to microelectrodes obtained by vapor deposition of Au/Cr bi-layer through a suitable mask.

(32) In order to achieve low resistivity without heating the plastic substrate, a method that causes close packing of the particles due to surface charge neutralization of the nanoparticles [15] upon exposure to HCl vapors was employed. The resistivity of such individual rings (calculated from the measured resistance and the ring cross-section area and length), was 4.3 (±0.7).Math.10.sup.−7 Ohm*m, which is only 7 times greater than that for bulk silver. This value remained constant for at least three months.

(33) Further insight into the structural conductance properties of the silver ring is provided by the C-AFM data presented in FIGS. 5A-C. The topographic 3-D (FIG. 5A) image shows a continuous line having maximal height of 400 nm and width of about 7 μm. The corresponding 2-D current map (FIG. 5C) and image (FIG. 5B) show that the line is indeed conductive. It should be emphasized that even though a small area of the ring is scanned, the fact that this area of the ring is conductive proves electrical continuity over a much larger range, at least as far as the distance to the Au/Cr counter electrode.

Example 3

Rings Composed of Two Different Metals

(34) Two different population of silver and copper NPs were mixed, Ag (˜10 nm) and Cu (˜100 nm) dispersed in water. A 1 μL droplet of this homogenous dispersion was dispensed on a Si substrate. After the evaporation of the water a single ring with a diameter of ˜2 mm was formed. HR-SEM characterization of the ring periphery revealed that the rim is composed of two separate rims (FIG. 2), each rim is consisted mainly of one type of metallic NP; the larger particles (Cu) were at the outer rim while the inner rim consisted mainly of the smaller particles (Ag). Therefore, this approach enables formation of rings composed on different materials.

Example 4

Chains of Connected Rings

(35) Chains composed of overlapping printed rings printed as described in Example 2, were obtained by printing a first forward line of rings with pre-determined spaces between the rings, followed by printing a backward, second line of rings, with a proper distance adjustment between the two lines. Optimization of the chain formation process was achieved by adjusting the jetting frequency (35 Hz) and the substrate movement (10000 micrometer/s). Part of such a chain is shown in FIGS. 6A-C.

(36) It should be noted that the deposition of one ring on top of another was previously reported to lead to the destruction of the first ring due to its re-dispersion [5]. However, by adjusting various ink and printing parameters such as concentration of silver nanoparticles in the ink, delay between line printing, and substrate temperature, close packing of the silver particles was achieved enabling, while dried, to overcome the possible re-dispersion of the pre-deposited rings. By controlling the positioning of the rings, continuous chains with fine contact between the rings were formed (FIG. 6B and FIG. 6C). Indeed, comparison between the topographic AFM image (FIG. 7A) and the current map image (FIG. 7B) of the same area (by C-AFM) reveal that the junction between the rings is not only geometrically continuous, but also has high electrical connectivity.

(37) Resistance measurements performed for various chains composed of 4 to 20 connected rings revealed that the (average) resistivity is 5.1 (+0.5) 10−7 Ohm*m, which is close to the resistivity of an individual ring (described in Example 2), further demonstrating the high quality of the junctions between the rings. This resistivity was constant for at least three months at room temperature. It should be noted that this resistivity is much lower than that obtained by ITO, which is typically in the range of 10.sup.−6 Ohm*m.

Example 5

2-D Arrays

(38) Two-dimensional (2-D) arrays of such rings were formed by repeating the chain formation procedure for a large number of lines, while keeping a constant distance between the lines. As shown in FIGS. 8A and 8B, 2-D arrays composed of connected chains could be obtained. The array is actually composed mainly of holes (the inner part of each ring), ˜150 micrometer in diameter, which are connected by narrow lines, about 5 micrometer in width, located around each hole. The sheet resistance of such a 2-D array (sample area of 0.5 cm.sup.2) was very low, 4±0.5 Ohm/square. It should be noted that qualitative bending experiments show that these values remained constant even after bending the substrate at angles below about 20°, showing that the arrays may be suitable for applications in which flexibility is required. For comparison, the typical sheet resistance of ITO thin films that have more than 80% transparency is much greater, in the range of 20-100 Ohm/square.

(39) As may be realized, the 5 micrometer lines are almost invisible to the naked eye, thus the 2-D pattern is almost transparent. Quantitatively, the transmittance measured by a spectrophotometer at 400-800 nm was as great as 95(±3) % T.

Example 6

Electroluminescent Device

(40) In order to further test these ring patterns as a transparent conductive oxide replacement, these conductive arrays were evaluated as the transparent electrode in an electroluminescent device. The device was fabricated on top of the transparent ring array by depositing layers of ZnS, and BaTiO.sub.3 by conventional screen printing, followed by deposition of a second silver electrode. As demonstrated in FIG. 9A, for a 2 mm×1 cm device, the printed ring array is indeed conductive and transparent. As shown in FIG. 9B, in the areas in which the rings are connected there is a uniform light emission by the device (the decay length for emission was estimated to be about 20 μm).