This invention relates to a process for producing a transparent electrical conductor comprising of copper-based nanowires with improved

conductivity which involves treatment of a copper-based nanowire network with one or more acids and optionally with a silver containing solution.

Currently several research activities are going on worldwide for

development of transparent conductors that are printable from a solution based ink. The transparent conductor layer is an essential component in several optical-electronic devices such as solar cells, LEDs, displays, touch screens etc. Indium tin oxide (ITO) and Aluminum doped zinc oxide (AZO) have traditionally been used for some of these applications. However ITO, the most commonly used transparent conductor, is relatively expensive due to the high price of indium. Another disadvantage is that a continuous ITO film is brittle creating a significant challenge, for use on flexible substrates.

A new class of alternative transparent conductors prepared from metal nanowires, carbon nanotubes and graphenes have the potential for cost reduction and also can be printed from a solution on flexible substrates at low temperatures. The low cost of production may be achieved by developing high-throughput printable inks based on these materials. Out of the above mentioned materials, transparent conductors made from metal nanowires are promising for ITO replacement owing to the higher DC conductivity of metal. A small amount of randomly organized wires are needed to make a percolated structure and also have enough transparency (owing to low light scattering due to small nanowire dimensions and the large void area in the nanowire network). Carbon nanotube networks have relatively higher sheet resistances due to their 1/3 metallic and 2/3 semiconducting nature which leads to higher junction resistances (Hu et al. ACS Nano, 4(5), 2955-2963). Currently sheet resistance of only up to 200- 1000 Ω/sq and transmittance of 80-90 % have been achieved. Graphene based transparent conductors are also emerging as a promising candidate, however more research efforts are needed for further improvement as currently sheet resistance of only 300 Ω/sq and 80% transmittance have been achieved. (Hu et al. ACS Nano, 4(5), 2955-2963).

Silver nanowire networks have been widely studied and shown to have 20 Ω/sq and ~ 80% specular transmittance, and 8 Ω/sq and ~ 80 % diffusive transmittance in the visible range (Hu et al. ACS Nano, 4(5), 2955- 2963). However, a significant disadvantage from the commercial point of view is the high cost of silver in the inks.

Alternatively, copper is much cheaper and abundantly available in nature. Recently nanowires of copper were synthesized and used for making transparent conductors (Rathmell et al. Adv. Mater, 2010, 22, 3558;

Rathmell et al. Adv. Mater., 2011 , 23, 4798-4803; WO2011071885(A2) ). The sheet resistance and transmittance obtained were 30 Ω/sq and 85 % respectively. However the solution deposited nanowire network or Cu NW electrode was initially non-conducting. The Cu NW electrode was treated in a plasma cleaner for 15 minutes in an atmosphere of 95 % nitrogen and 5 % hydrogen to burn off the polymer shell or ligands (nitrocellulose) on the nanowires. Next the Cu NW electrodes were subjected to annealing at 175 °C in a tube furnace for 30 minutes under a flow of hydrogen to clean the wire surface, reduce any oxides and weld the wires together to reduce the sheet resistance below 200 Ω/sq (Rathmell et al. Adv. Mater., 201 1 , 23, 4798-4803). A similar treatment of Cu-Ni alloy nanowire electrode has been described to produce a transparent conductor in literature (Rathmell et al. Nano Letters, 2012, 12, 3193-3199). As a drawback, hydrogen is a highly flammable gas with handling issues from a safety point of view on a large scale production. Secondly the use of heat may not be suitable for some device applications if the whole device needs to be heated for transparent conductor fabrication. The present invention relates to a process for the preparation of a transparent conductor comprising copper nanowires, said process comprising:

Depositing a layer of copper nanowires, copper alloy nanowires or copper- based core-shell type nanowires (altogether referred to as Cu-based NWs) onto the surface of a substrate, and treating the layer of nanowires with acid or mixtures comprising acid, preferably mixtures with one or more solvents.

In the following 'copper-based nanowire' ('Cu-based NW) denotes copper nanowires, particularly plain copper nanowires, copper alloy nanowires or core-shell type nanowires, wherein the shell is made of copper or of an copper alloy. 'Copper alloy' herein denotes an alloy comprising copper, wherein a content of 10% by weight or greater is preferred, more preferably of 50 % or greater.

For the sake of brevity, copper-based nanowires and copper nanowires are used synonymously herein. The term copper nanowires is inclusive of any copper-based nanowires unless specified differently explicitly or implicitly by the context or technical implications.

In one preferred embodiment the invention relates to a process as described above, wherein the process additionally comprises depositing silver onto the layer of copper nanowires by treating the layer of copper nanowires with a silver containing solution.

The invention further relates to a transparent conductor comprising a layer of electrically interconnected copper nanowires covered with a layer of silver.

One achievement of the current invention is to make a Cu-based NW electrode deposited on a substrate such as glass or plastic or other substrates and to reach a high level of conductivity so that the Cu-based NW electrode can be used as transparent conductor. The Cu-based NW electrode is non-conducting when deposited from solution on to a substrate due to a) oxide formation on nanowires b) surfactants or small molecular or polymeric ligands and/or c) other causes of inter-wire contact resistance. Surprisingly a remarkably high level of conductivity can be achieved by chemical treatment of a conventional film of Cu-based NWs. It eliminates the use of hydrogen gas and heat by using a room temperature based wet chemical treatment. The process, especially the deposition step of the Cu- based NWs, can be performed in air, i.e. ambient atmosphere containing moisture and/or oxygen.

The Cu-based NW electrode is simply treated in an acid solution to etch away any surface oxide and any surfactants/ligands on the metal nanowires. This process is sufficient to make the electrode conducting without affecting the transparency of the electrode. The acid etching works on Cu and copper alloy such as Cu-Ni as demonstrated herein. The acid etching can also work in principle for copper-based core-shell type nanowires (nanowires having a Cu shell or Cu-cores combined with a shell from any of the metals Ag, Zn or Sn), copper alloy nanowires such as Cu- Zn, Cu-Sn and other possible Cu alloy combinations, other metal nanowires such as Ag, Ni, Zn, Sn etc., their alloys.

The process optionally involves an additional step, which improves the conductivity of the Cu NW network further by depositing a thin silver layer on the layer of Cu NWs. The silver shell can be deposited by treating the substrate in a silver salt solution. The removal of the surface coating by acid treatment makes the surface of the silver nanowires suitable to silver deposition by a simple redox displacement reaction:

Cu° + Ag ⁺ -» Cu ⁺ + Ag° and/or Cu° + 2Ag ⁺ - Cu ²⁺ + 2Ag°.

The order of the chemical processing is important. If silver is deposited on the Cu NW electrode without the acid treatment the conductivity is much lower as compared to a sample with acid treatment under similar reaction conditions.

Acid in the context of the current invention means a pure acid or mixtures comprising one or more acids. Acids are preferably protic (Bronsted) acids, and chosen from the following weak acids and strong acids. Weak acids used here are carboxylic acids, R-COOH where R is an organic group and acids with two or more carboxyi group (-COOH) such as dicarboxylic, tricarboxylic, and higher numbers of carboxylic acid groups. Common examples of acids with one carboxyi group are formic acid (HCOOH), acetic acid (CH ₃ COOH), propionic acid (C ₂ H ₅ COOH), butyric acid (C ₃ H7COOH), lactic acid (CH ₃ CH(OH)COOH), pyruvic acid (CH ₃ COCOOH), etc.; acids with two carboxylic acid groups such as oxalic acid (COOH) ₂ , malonic acid CH ₂ (COOH) ₂ , adipic acid HO ₂ C(CH ₂ ) ₄ COOH, etc.; acids with three carboxyi groups such as citric acid (COOH)CH ₂ C(OH)(COOH)CH ₂ (COOH), etc.

Strong acids such as inorganic acids, including preferably hydrochloric acid (HCI), nitric acid (HNO ₃ ), sulfuric acid (H ₂ SO ₄ ), etc. can also be used.

Strong acids are preferably diluted with a solvent or a weak acid, most preferably to a content of strong acid of 1% by weight or less. A mixture comprising acid preferably comprises a solvent or a solvent mixture.

Solvent or solvent mixture in the context of the current invention means water, polar-protic organic solvents like methanol, ethanol etc., polar aprotic organic solvents such as acetone, DMSO, DMF, acetonitrile etc. and mixtures of two or more of the aforementioned solvents.

One advantage of the above chemical treatment of nanowires is that the process can be done at room temperature by simply dipping the substrate comprising the copper nanowire electrode in acid solution or in acid solution followed by silver salt solution.

The process described here is relatively benign, because weak acid, e.g. acetic acid and/or diluted acid, is used, and can be used on a wide variety of substrates including plastics. The process described here eliminates the requirement for processing the Cu-based NW electrode in the N ₂ /H ₂ plasma and use of heat treatment in H ₂ gas environment, a step that may be unsuitable for optoelectronic devices. Thus the new chemical process according to the invention offers a significant advantage over the state of the art Cu-based NW processing technique and yet is very simple and effective.

The Cu-based NW electrode can be deposited from a solution by spray coating, ink-jet printing, dip coating, doctor blading or Meyer rod coating, gravure coating, slit coating and drop casting etc on any substrate. The substrate may be glass, plastic, metal foil or even a semi-finished device such as a solar cell, LED or display with Cu-based NW network as either top electrode or the bottom electrode or any other layer acting as

conducting transparent electrode.

The treatment with acid can be done by dipping the substrate comprising the nanowires into a bath containing the liquid agent. Alternatively the liquid agent is sprayed or otherwise provided as a flow onto the substrate. Acid vapor can be used alternatively to treat the nanowire electrode.

Preferably the acid treatment of the Cu-based NW layer is done in an organic carboxylic acid, most preferably in acetic acid, to make the layer conducting without affecting the transparency. Acetic acid and analogous acids, including mixtures, have a low surface tension and helps to prevent any liftoff of Cu-based NWs from the substrate when dipped in acid bath. Preferably the acid treatment of a Cu alloy NW layers is done in a solution of organic carboxylic acid with addition of up to 1 % of strong acid. For example, preferably the acid treatment of a Cu-Ni alloy NW layer is done in a solution of 0.1 % hydrochloric acid (HCI) in acetic acid by volume to make the layer conducting without affecting the transparency. The term nanowires refers to elongated particles having a diameter of less than 100 nm and an aspect ratio of 5 or more. Preferably nanowires for building a nanowire network on a surface have much higher aspect ratios. Preferable nanowires will have a diameter of the wire shaped part ranging from 1 to 150 nm, more preferably 10 to 90 nm, and an average length of 50 nm to 500 pm, preferably 500 nm to 100 pm. Generally, longer NWs with smaller diameter will improve the transmittance while maintaining low value of sheet resistance.

In a preferred embodiment of the current invention the transparent conductor has a conductivity of less than 1000 Ω/square, more preferably less than 200 Ω/square, and most preferably less than 50 Ω/square. The transparent conductor moreover preferably has a transparency of more than 60 %, more preferably a transparency of more than 75 % and most preferably a transparency of more than 85 %.

The examples below shall illustrate the invention without limiting it. The properties and composition of the methods and materials used in a single example can be applied to variations of the invention, which are not explicitly mentioned, but which are covered by the claims. The skilled person will be able to recognize practical details of the invention not explicitly mentioned in the description, to generalize those details by general knowledge of the art and to apply them as a solution to any special problem or task in connection with the technical matter of this invention.

The steps involving treatment with liquids (acid or silver bath) usually require short reaction times. Typically the layer of Cu NWs is submerged in the acid bath for about 1 to 60 min, and optionally in the silver bath from about 1 to 60 min.

The transmittance value may be improved further by improving the quality of the Cu-based NW network by avoiding aggregation that effectively reduces the transmittance. Improving the stability of the NW suspension ink by optimization of ligands/surfactants and solvent may significantly reduce the number of aggregates on the Cu-based NW electrodes thereby improving the transmittance combined with a low value of sheet resistance.

The invention is now further described by the attached figures, which are briefly described in the following.

Description of the Figures:

Figure 1 shows the specular transmittance of several Cu NW electrodes by using UV-Vis spectroscopy. The numbers (4 to 16) shown in the figure represent the number of coating passes needed to achieve the

corresponding transmittance value for a particular concentration of nanowires in solution. A detailed description of the process of providing the Cu NW electrode is provided in Example 1

Figure 2 Specular transmittance at 530 nm vs sheet resistance for a Cu NW electrode after 30 min glacial acetic acid treatment. The as-deposited Cu NW electrode was non-conducting before acetic acid treatment. The data is shown for several Cu NW electrodes prepared by varying the number of coating passes. A detailed description of the process of providing the Cu NW electrode is provided in Example 1.

Abbreviations:

The following abbreviations are used throughout the specification and examples:

Cu NWs Copper nanowires

Cu-based NWs Copper nanowires, Copper alloys, Copper- based core-shell type nanowire structures

Cu-Ni NWs Copper-Nickel alloy nanowires

Dl water deionized water Examples

The copper nanowires can be synthesized as described in any of the references Rathmell et al. Adv. Mater, 2010, 22, 3558, Rathmell et al. Adv. Mater., 201 1 , 23, 4798-4803 or WO 201 1/071885 A2. However, for this work the copper nanowires and copper nickel alloy nanowires were purchased from Nanoforge Inc. The copper nanowires wires are originally dispersed in water with polyvinylpyrrolidone (PVP) as protective ligand and diethylhydroxylamine (DEHA) as anti-oxidation agent. The copper nickel alloy nanowires were originally dispersed in isopropanol.

For measuring the sheet resistance two parallel bus lines of conductive silver are painted using a commercial silver paint so that the area defined by the bus lines is a square (1 inch by 1 inch) and resistance values were recorded using a multimeter. The sheet resistance measurement is done on 2 to 3 square areas along the glass slide.

Example 1. Acid treatment of Cu NW electrode

5 ml of the commercial nanowire suspension was centrifuged at 3000 rpm for 5 min to precipitate the copper nanowires. The supernatant was discarded and 5 ml of ethanol was added to the Cu NWs to re-suspend the nanowires. The nanowires were re-suspended by shaking or vortexing. The centrifugation and re-suspension cycles were repeated 3 times to remove water, DEHA and excess PVP ligands and to transfer Cu NWs to the organic solvent (here ethanol) so that it could be easily deposited on any substrate. Next, the nanowire suspension was sonicated briefly for 30 seconds to improve the quality of the suspension and reduce any visible aggregation. A few drops of Cu NW suspension was put near the edge of a pre-cleaned glass substrate and coated with a swift rolling motion of a Meyer rod or a glass rod. The solvent is allowed to evaporate before depositing the next coating. The coating process was repeated from 1 up to 16 times to prepare Cu NW electrodes of different nanowire area coverage.

Next the substrates coated with Cu NWs were dipped in glacial acetic acid for 30 min. The substrates are removed from the acetic acid bath and dried in air. All the above processing steps are done at room temperature.

The transmittance of several Cu NW electrodes by using UV-Vis

spectroscopy is shown in Figure 1. The transmittance data was taken by using a blank glass slide as baseline, thus subtracting the contribution of the absorbance of the glass slide. The transmittance data clearly shows that the electrodes are evenly transparent over the UV, visible and IR regions. Increasing the number of coating passes results in a denser nanowire network thereby reducing the transmittance throughout the spectrum. Thus a desired value of transmittance could be achieved by just varying the number of coating passes for a given nanowire concentration in the ink or the nanowire concentration itself. For a higher nanowire concentration ink fewer coating passes are needed to achieve a desired value of transmittance vs. a dilute nanowire ink.

Specular transmittance at 530 nm vs sheet resistance for several measurements is shown in Figure 2. A general trend observed is that as the transmittance increases the sheet resistance increases. For the acetic acid treated Cu NW electrode we were able to achieve an optical transparency of 86.6 % with sheet resistance of 31 Ω/sq. In another instance we were able to achieve an optical transparency of 82 % with sheet resistance of 21 Ω/sq. This is remarkable since the as-deposited Cu NW electrode was nonconducting before the acid treatment. Example 2. Acid treatment combined with silver coating of Cu NW electrode

In this example Cu NW electrodes are prepared by Meyer rod coating similar to that in example 1.Next the substrate coated with Cu NWs is dipped in acetic acid for 30 min.

The substrate is removed from the acetic acid bath and dried in air. The transmittance and sheet resistance measured in this instance is 72 % and 150 Ω/sq. Next the substrate is further dipped in 0.5 mM silver nitrate solution for 5-10 min. Next the substrate is removed from the silver nitrate solution, dipped in Dl water briefly and then dried in air. All the above processing steps are done at room temperature. The optical transparency is measured by UV- Visible spectroscopy to be ~ 64 % with sheet resistance of 21 Ω/sq. Thus the sheet resistance can be reduced

significantly by silver salt treatment.

Example 3. Acid treatment of Cu-Ni alloy NW electrodes

5 mL of a commercial Cu-Ni alloy NW suspension in isopropanol (IPA) was centrifuged at 7000 rpm for 5 minutes to precipitate Cu-Ni NWs. The supernatant was poured off and the Cu-Ni NWs were resuspended in 5 mL of fresh IPA and sonicated for 3 minutes. Next the wires were coated onto glass slides with a Meyer rod using a process described in Example 1. After the films were dried, they were immersed in a solution of 0.1 % HCI by volume in acetic acid for 2 minutes. A film with eight coating passes and no acid treatment showed a sheet resistance of 10 ΜΩ/sq and a transmittance of 64.2% at 530 nm. An equivalent film with eight coating passes and 2 minute acid treatment showed a sheet resistance of 279 Ω/sq and a transmittance of 65.2% at 530 nm. These two films have similar

transmittance, but the effect of the acid treatment is a drop in the sheet resistance by several orders of magnitude. Further combinations of the embodiments of the invention and variants of the invention are disclosed by the following claims.