METHOD FOR THE PREPARATION OF A TRANSPARENT AND CONDUCTIVE AUTO-SUPPORTED SILVER NANOWIRE FILM AND APPLICATIONS THEREOF

Abstract

The invention generally relates to the field of transparent electrodes. In particular, the invention relates to a method for producing a transparent and conducting auto-supported silver nanowire film, to the transparent and conducting silver nanowire film obtained by said method and to the use of said film as a transparent and flexible electrode in an electric device, in particular in a photovoltaic cell (solar cell).

Claims

1. A method for producing a transparent and conductive auto-supported silver nanowire film, said method comprising at least the following steps: A) a first step of preparing a dispersion of silver nanowires in a liquid medium comprising at least one solvent and triphenylphosphine (TPP) or a derivative thereof, B) a second step of depositing the dispersion obtained in step A) onto a substrate to obtain an assembly comprising a silver nanowire film supported on said substrate, C) a third step of drying the assembly obtained in step B) to obtained a dried assembly, D) a fourth step of separating the silver nanowire film from the substrate of the dried assembly obtained in step C) to obtain a transparent and conductive auto-supported silver nanowire film.

2. The method according to claim 1, wherein silver nanowires used according to step A) are chosen among those having a mean diameter ranging from 5 to 200 nm and a length ranging from 1 to 100 m.

3. The method according to claim 1, wherein the liquid medium used to disperse the silver nanowires during step A) is chosen among water, isopropyl alcohol, ethylene glycol and epoxy resin.

4. The method according to claim 1, wherein the amount of silver nanowires in the dispersion of step A) varies from 0.1 to 4 mg.Math.mL.sup.1.

5. The method according to claim 1, wherein the amount of triphenylphosphine in the dispersion of step A) varies from 0.05 to 2 mg.Math.mL.sup.1.

6. The method according to claim 1, wherein the weight ratio of silver nanowires to TPP varies from about 2:1 to 1:2.

7. The method according to claim 1, wherein the substrate is a porous filter and step B) is carried out by filtering of the dispersion obtained in step 1) onto said porous filter.

8. The method according to claim 7, wherein the porous filter is an anodized aluminum oxide (AAO) membrane.

9. The method according to claim 1, wherein the drying of step C) is performed by heat treatment of the assembly obtained in step B) in air, at a temperature ranging from 60 to 90 C., and for a period of time varying from 5 to 30 minutes.

10. The method according to claim 8, wherein the fourth step of separating the silver nanowires film from the substrate is performed by dissolving the substrate of the dried assembly obtained in step C) in a dissolving medium.

11. A transparent and conductive auto-supported silver nanowires film directly obtained by the method as claimed in claim 1, wherein said film comprises a web of percolated silver nanowires, and wherein the surface of said nanowires is coated by an oxidation resistant layer comprising triphenylphosphine or a derivative thereof.

12. The film of claim 11, wherein said film has a thickness ranging from 10 to 300 nm.

13. The auto-supported silver nanowires film as defined in claim 11, said film configured to be employed as a transparent and flexible electrode in an electric device.

14. The auto-supported silver nanowires film of claim 13, wherein said auto-supported silver nanowires film is configured to be employed as a transparent and flexible electrode in a photovoltaic cell, in plasma screens, in liquid crystals displays, in touch screens, or in organic light-emitting devices.

15. An electric device comprising at least one auto-supported silver nanowires film as defined in claim 11, as a transparent and flexible electrode.

Description

EXAMPLES

[0038] The raw materials used in the examples are listed below: [0039] Silver nanowires (average diameter 26 nm, average length 24 m) sold under the trade name Silver Nanowire AgNW-25 from Seashell Technologies at a concentration of 5.4 mg.Math.mL.sup.1 in isopropyl alcohol, [0040] Triphenylphosphine (Aldrich, 99%)

[0041] All other products used in these examples were of analytical grade. These raw materials have been used as received from the manufacturers, without any step of purification, unless otherwise stipulated.

Example 1: Preparation and Properties of Transparent and Conductive Silver Nanowire Films According to the Method of the Invention

[0042] In this example, different silver nanowire transparent and conductive films have been prepared according to the method of the invention, i.e. using triphenylphosphine (TPP) as an agent for preventing oxidation of the silver nanowires (AgNW-TPP films). A comparative silver nanowire transparent and conductive film has also been prepared according to a method not forming part of the present invention, i.e. without using triphenylphosphine as an agent for preventing oxidation of the silver nanowires (AgNW film). They have then been compared with regards to their respective optical properties, sheet resistivity and resistance to accelerated oxidation.

[0043] The dispersion of silver nanowires as purchased was then diluted with isopropyl alcohol to obtain a concentration of silver nanowires of 0.054 mg.Math.mL.sup.1. This diluted dispersion of silver nanowires was then used in the overall process.

[0044] A triphenylphosphine (Aldrich, 99%) solution at 3.4 mg.Math.mL.sup.1 in isopropyl alcohol was prepared.

[0045] Silver nanowires and silver nanowires+triphenylphosphine dispersions were then prepared as follows: [0046] Dispersion 1: 0.5 mg of Silver nanowires without TPP [0047] Dispersion 2: 0.5 mg of silver nanowires+0.55 mg of TPP, [0048] Dispersion 3: 0.1 mg of silver nanowires without TPP, [0049] Dispersion 4: 0.1 mg of silver nanowires+0.11 mg of TPP.

[0050] Each of the dispersions was vacuum filtered on an Anodized Aluminium Oxide (AAO) filter (47 mm, 0.02 m, Whatman).

[0051] The resulting film+membrane assembly was then dried during 5 minutes at 60 C. in air. The film transfer was carried out by dissolving the alumina membrane with a NaOH solution and deposition of the film onto a PET substrate.

[0052] To do so, 20 mL of a 1.5M NaOH solution was prepared and put in a Petri dish. The alumina membrane was firstly put on deionized water to fill the pores with water and drive the dissolution softer. The wet membrane was then put on the NaOH solution and dissolved within 15 minutes. The solution was partially taken out of the Petri dish, as well as the alumina fragments. The solution has been changed several times with deionized water in order to reach a neutral pH.

[0053] A 55 cm.sup.2 PET substrate was put at the bottom of the Petri dish. The remaining water was taken out with a pipette, allowing the film to deposit onto the PET membrane. The PET supporting the films was then dried at 60 C. in air.

[0054] 4 films were prepared respectively with Dispersions 1 to 4: [0055] Film called 0.5AgNW-0TPP not forming part of the present invention, i.e. prepared with Dispersion 1 comprising only silver nanowires and no TPP; [0056] Film called 0.5AgNW-0.55TPP according to the invention, i.e. prepared with Dispersion 2 comprising both silver nanowires and TPP; [0057] Film called 0.1AgNW-0TPP not forming part of the present invention, i.e. prepared with Dispersion 3 comprising only silver nanowires and no TPP; and [0058] Film called 0.1AgNW-0.11TPP according to the invention, i.e. prepared with Dispersion 4 comprising both silver nanowires and TPP.

[0059] In these denominations, the numbers correspond to the mass (in mg) of each component used in the dispersion. These films were stored in normal conditions of pressure and temperature

[0060] The transmittance and sheet resistivity measurements were tested on films 0.5AgNW-0TPP and 0.5AgNW-0.55TPP.

[0061] The transmittance of the films was measured at a wavelength of 550 nm with a Cary 100 Bio UV-vis spectrophotometer.

[0062] The sheet resistivity of the films was measured with a four-point probe method as a function of time. Four equidistant gold electrodes are evaporated through a mask all along the substrates prior to carrying out the measurements. An electrical assembly composed of four flexible conductive fingers has been used. These fingers are put in touch with the gold electrodes. Current is applied to the external electrodes while voltage is measured at the central electrodes (range of current from a few nA to some mA is applied). The use of Ohm's law gives resistance which is converted in sheet resistance by normalizing it with the ratio of the length to the spacing of the electrodes.

[0063] For both, measurements have been carried out for 108 days.

[0064] The corresponding results are given by FIG. 1 annexed on which the sheet resistivity (in Ohms/sq) and transmittance (in %) are expressed as a function of time (in days). On this figure, the two higher curves correspond to transmittance while the two lower curves correspond to sheet resistivity. For each, the triangles correspond to the measurements made on the 0.5AgNW-0TPP film not forming part of the invention while the dark dots correspond to the 0.5AgNW-0.55TPP film according to the invention.

[0065] This figure clearly shows an increase of the sheet resistivity when the nanowires are not coated with TPP (0.5AgNW-OTTP film not forming part of the present invention). In addition, it can be concluded that the presence of TPP doesn't affect the transmittance which is comparable for materials with or without TPP, films made of TPP protected nanowires (0.5AgNW-0.55TPP) exhibit a constant resistivity as a function of time.

[0066] The resistance to accelerated oxidation was tested on films 0.1AgNW-0TPP and 0.1AgNW-0.11TPP.

[0067] The method to test the resistance of the films to accelerated oxidation consists in depositing films on an anodic aluminium oxide (AAO) membranes and then to expose them to nitric acid vapours. Mother solutions of fuming nitric acid in water from 0.001 to 0.1M are prepared. A small amount of the mother solution is placed in a petri Dish at the bottom of a dessicator before putting the films in the latter. Films are taken out of the dessicator to carry out measurements before putting them back in the dessicator. The small amount of nitric acid solution is replaced every time the samples are taken out.

[0068] The results are reported in FIGS. 2 and 3 annexed on which the resistance to accelerated oxidation (in kOhms: k) is given as a function of time (in min). FIG. 2 represents the evolution in time of the resistance of the 0.1AgNW-0TPP (dark squares) and 0.1AgNW-0.11TPP (dark dots) films exposed to 0.005 M nitric acid vapours and FIG. 3 represents the evolution in time of the resistance of the 0.1AgNW-0TPP (dark squares) and 0.1AgNW-0.11TPP films (dark dots) exposed to 0.01 M nitric acid vapours.

[0069] FIGS. 2 and 3 clearly show that the 0.1AgNW-0TPP film not forming part of the invention gets oxidized very quickly. Indeed its electrical resistance increases by several orders of magnitude after a few hours (from 10 kQ to 10.sup.5 k). By contrast, the resistance of the 0.1AgNW-0.11TPP film according to the present invention remains constant in the same conditions, even after several hours in this highly oxidative medium.

[0070] The above detailed results clearly demonstrate the efficacy of the method of the invention at protecting AgNW against oxidation, even in highly oxidative conditions.