TRANSPARENT ELECTRODE MATERIALS AND METHODS FOR FORMING SAME

Abstract

A transparent electrode material including a conductive layer having an active surface and a second surface, and an adjacent base layer, wherein: ∘ the conductive layer includes a conductive network formed by metallic nanowires and carbon nanotubes encapsulated in a conductive material; ∘ the second surface of the conductive layer has encapsulated nanowires and/or nanotubes projecting therefrom; and ∘ the encapsulated nanowires and/or nanotubes projecting from the second surface of the conductive layer are embedded in the adjacent base layer; whereby the active surface of the conductive layer is smooth and electrically active, and the transparent electrode material has a sheet resistance less than 50 Ω/sq and a transparency greater than 70%.

Claims

1. A transparent electrode material including a conductive layer having an active surface and a second surface, and an adjacent base layer, wherein: the conductive layer includes a conductive network formed by metallic nanowires and carbon nanotubes encapsulated in a conductive material; the second surface of the conductive layer has encapsulated nanowires and/or nanotubes projecting therefrom; and the encapsulated nanowires and/or nanotubes projecting from the second surface of the conductive layer are embedded in the adjacent base layer; whereby the active surface of the conductive layer is smooth and electrically active, and the transparent electrode material has a sheet resistance less than 50 Ω/sq and a transparency greater than 70%.

2. The transparent electrode material according to claim 1, wherein at least some of the metallic nanowires and carbon nanotubes of the conductive network are electrically exposed at the active surface.

3. The transparent electrode material according to claim 1, wherein the metallic nanowires are gold, silver or platinum nanowires, or a combination thereof.

4. The transparent electrode material according to claim 1, wherein the nanowires have a length in the range of 1 to 50 μm and a diameter in the range of 15 to 300 nm.

5. The transparent electrode material according to claim 1, wherein the carbon nanotubes are single-wall, double-wall or multi-wall carbon nanotubes, or a combination thereof.

6. The transparent electrode material according to claim 1, wherein the carbon nanotubes have a bundle diameter in the range of 5 to 150 nm, with individual nanotubes being in the range of 1 to 60 nm diameter.

7. The transparent electrode material according to claim 1, wherein the nanotube weight fraction in the conductive network is between 1 and 80 wt %.

8. The transparent electrode material according to claim 1, wherein the nanotube weight fraction in the conductive network is between 5 and 50 wt %, or between 15 and 30 wt %.

9. The transparent electrode material according to claim 1, wherein the area loading concentration (mg/m.sup.2) of nanowires in the conductive layer is between 10 and 250 mg/m.sup.2.

10. The transparent electrode material according to claim 1, wherein the area loading concentration (mg/m.sup.2) of nanowires in the conductive layer is between 50 and 200 mg/m.sup.2, or between 70 and 150 mg/m.sup.2, or between 80 and 130 mg/m.sup.2.

11. The transparent electrode material according to claim 1, wherein the conductive material is selected from a group of materials comprising semi-conducting polymers, metal oxides, and ultrathin metal films.

12. The transparent electrode material according to claim 1, wherein the conductive material is selected from a group of materials comprising poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, poly(3-hexylthiophene-2,5-diyl), poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)], poly(9,9′-dioctyluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine), poly(9,9′-dioctyluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine), ZnO, MoO, aluminium doped ZnO, Ni doped MoO, and ultrathin metal films of Au, Ag and Al.

13. The transparent electrode material according to claim 1, wherein the base layer is not a conductive material.

14. The transparent electrode material according to claim 1, wherein the base layer is selected from a group of materials comprising thermosetting materials, including epoxy resins and polyurethanes, and thermoplastic materials, including ethylene vinyl acetate (EVA).

15. The transparent electrode material according to claim 1, wherein the conductive layer has a thickness of between 5 nm and 300 nm.

16. The transparent electrode material according to claim 1, wherein the base layer has a thickness of between 1 micron and 1000 microns.

17. The transparent electrode material according to claim 1, wherein the active surface has a surface topography with a height profile having a peak-to-trough height of less than about 50 nm.

18. The transparent electrode material according to claim 1, wherein the active surface has a root mean square surface roughness of less than about 10 nm measured over a region of up to 10 micrometres.

19. A transparent electrode material including a first conductive layer having an active surface and a second surface, a second conductive layer having an active surface and a second surface, and a base layer between the first and second conductive layers, wherein: the first and second conductive layers each include a conductive network formed by metallic nanowires and carbon nanotubes encapsulated in a conductive material; the second surface of the first conductive layer and the second surface of the second conductive layer both have encapsulated nanowires and/or nanotubes projecting therefrom; and the encapsulated nanowires and/or nanotubes projecting from the second surface of the first conductive layer and the second surface of the second conductive layer are embedded in the base layer; whereby the active surface of the first conductive layer and the active surface of the second conductive layer are both smooth and electrically active, and the transparent electrode material has a sheet resistance less than 50 Ω/sq and a transparency greater than 70%.

20. The electrode formed from a transparent electrode material according to claim 1 or claim 19.

21. The optoelectronic device having an electrode in accordance with claim 20.

22. A method of forming a transparent electrode material, the method including: forming a network of metallic nanowires and carbon nanotubes; encapsulating the nanowire and nanotube network in a conductive material to form a conductive network in a conductive layer such that the conductive layer has a smooth active surface that is electrically active and a second surface, and such that the second surface has encapsulated nanowires and/or nanotubes projecting therefrom; forming a base layer upon the second surface of the conductive layer to embed the projecting nanowires and nanotubes in the base layer; whereby the transparent electrode material has a sheet resistance less than 50 Ω/sq and a transparency greater than 70%.

23. The method according to claim 22, wherein the method includes an acid reflux of the carbon nanotubes prior to forming the conductive network.

24. The method according to claim 22, wherein the forming of the network of metallic nanowires and carbon nanotubes includes co-depositing nanotubes with nanowires.

25. The method according to claim 22, wherein the forming of the network of metallic nanowires and carbon nanotubes is by way of a stamp transfer step.

26. A method of forming a transparent electrode material, the method including: forming a first network of metallic nanowires and carbon nanotubes; encapsulating the first nanowire and nanotube network in a conductive material to form a first conductive network in a conductive layer such that the first conductive layer has a smooth active surface that is electrically active and a second surface, and such that the second surface has encapsulated nanowires and/or nanotubes projecting therefrom; forming a base layer upon the second surface of the first conductive layer such that the nanowires and nanotubes projecting from the second surface of the first conductive layer are embedded in the base layer; forming a second network of metallic nanowires and carbon nanotubes; encapsulating the second nanowire and nanotube network in a conductive material to form a second conductive network in a second conductive layer upon the base layer such that the second conductive layer has a smooth active surface that is electrically active and a second surface, and such that encapsulated nanowires and/or nanotubes project from the second surface of the second conductive layer to be embedded in the base layer; whereby the transparent electrode material has a sheet resistance less than 50 Ω/sq and a transparency greater than 70%.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0072] In the accompanying drawings:

[0073] FIG. 1 is a schematic representation of an embodiment of a transparent electrode material in accordance with the present invention.

[0074] FIG. 2 is a schematic representation of the instrumental set-up used for the purposes of the experimental work conducted to provide the following examples.

[0075] FIG. 3 is a schematic representation of a preferred laminator stamp and epoxy transfer method used for the fabrication of exemplary planar AgNW/SWCNT electrodes on a substrate.

[0076] FIG. 4 is a graphical representation of the impact of area loading variations in AgNWs showing the percolation threshold and variations in sheet resistance and specular transparency.

[0077] FIG. 5 is a graphical representation of comparative transmission (% T) and the reflectivity (% R) results for a prior art ITO electrode and an exemplary planar AgNW/SWCNT 80/20 w/w % electrode, with the substrate contribution removed. The sheet resistance, shown on the right, are an average of 15 measurements on 3 separate 25 mm.sup.2 samples.

[0078] FIGS. 6(a) and 6(d) are tilted scanning electron microscopy images (SEMs) of (a) non-planarised AgNW and SWCNTs on a glass substrate and (d) AgNWs/SWCNT electrode after the planarisation process embedded into PEDOT:PSS and epoxy. Scale bars are 2 μm.

[0079] FIGS. 6(b) and 6(e) are topographical atomic force microscopy measurements (AFMs) of (b) non-planarised AgNW and SWCNTs on glass and (e) AgNWs and SWCNTs after the planarisation process embedded into PEDOT:PSS and epoxy. Scale bars are 2 μm.

[0080] FIGS. 6(c) and 6(f) are the height profiles along the dotted lines in FIGS. 5(b) and 5(e) respectively.

[0081] FIGS. 7(a) and 7(b) are (a) height and (b) peak force current maps of an exemplary planarised AgNW/SWCNT electrode surface with a bias voltage of 2 V.

[0082] FIG. 8 is a graphical representation of the JV characteristics of OPV devices on exemplary planarised AgNW/SWCNT electrodes with P3HT:PCBM and PCDTBT:PC70BM active layers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0083] The following description outlines experimental work for the preparation of exemplary transparent electrode materials in accordance with the present invention. Specifically, exemplary transparent electrode materials of these embodiments are as illustrated in FIG. 1 and include a single conductive layer 10 and a base layer 12 that is non-conductive. The conductive layer 10 is a conductive network formed by metallic nanowires 14 and carbon nanotubes 16, which in these embodiments are the preferred silver nanowires (AgNW) and single-walled carbon nanotubes (SWCNT) encapsulated in a preferred conductive material 18.

[0084] The conductive layer has a smooth active surface 20 and a second surface 22, noting that the second surface 22 of the conductive layer 10 has encapsulated nanowires 24 and/or nanotubes 26 projecting therefrom. In this respect, the projecting nanowires and/or nanotubes are embedded in the base layer 12, and are shown to be almost completely encapsulated by the conductive material, with the exception of some breaks 28 in the encapsulation coating.

[0085] In particular, the following paragraphs report on the production of a conductive network of AgNW/SWCNT encapsulated in a PEDOT:PSS conductive material, with an epoxy base layer. The following paragraphs also describe a preferred epoxy adhesion lift-off technique that is suitable for the fabrication of OPV devices.

Materials and Methods for the Experimental Work

[0086] AgNWs were purchased from Seashell Technologies (San Diego, USA), which were supplied as a suspension (20.4 mg/mL) in isopropyl alcohol (IPA). An aliquot of the AgNW suspension was diluted to 0.1 mg/mL with IPA and stored until use. Carboxylate functionalized (P3 type) SWCNTs with purity of >90% were purchased from Carbon Solutions (California, USA). 50 mg of the carboxylate functionalized SWCNT were further purified by refluxing the SWCNTS in 3M HNO.sub.3 for 12 hours and collecting via vacuum filtration (0.4 μm polycarbonate, Millipore). In this respect, mild acid treatment of SWCNTs improves aqueous dispersibility and performance of interwoven AgNW/SWCNT films.

[0087] A sample of the acid refluxed SWCNTs was suspended in water via probe sonication (Sonics Vibracell™) at 40% amplitude for 2 minutes before being diluted to a concentration of 0.25 mg/mL with deionized water. The lengths of the as-purchased AgNWs were shown to be in the order of 5 to 50 μm, with a diameter of approximately 100 to 200 nm. After the mild oxidation treatment, the SWCNTs were found to exist in bundles with a bundle diameter in the range of from 5 to 15 nm.

[0088] Sheet resistance measurements were performed using a four point probe (KeithLink® Technology Co., Ltd., New Taipei City, Taiwan). The values reported were an average of 10 measurements on two separate 64 mm.sup.2 samples.

[0089] Transmission and reflectivity were measured on samples (25 mm.sup.2) using a Perkin-Elmer LAMBDA 950 UV/Vis/NIR Spectrophotometer with integrating sphere. The average transmission reported was for a wavelength range between 800-400 nm.

[0090] Scanning electron microscopy (SEM) images were acquired using a CamScan MX2500 (CamScan Optics, Cambridge, UK) working at an accelerating voltage of 10 kV and a distance of 10 mm.

[0091] Topographical atomic force microscopy (AFM) measurements were acquired using a Bruker Multimode AFM with Nanoscope V controller. NSC15 Mikromasch Silicon tapping mode probes with a nominal spring constant of 40 N/m, resonant frequency of 325 kHz and tip diameter equal to 20 nm were used. AFM images were acquired in tapping mode with all parameters including set-point, scan rate and feedback gains adjusted to optimize image quality and minimize imaging force.

[0092] Conductivity of the AgNW/SWCNT electrode eventually formed was mapped using peak force tunnelling AFM (PF-TUNA).sup.22 on a Bruker Multimode AFM with Nanoscope V controller. The software used to acquire all AFM data was control software version 8.15.

[0093] The cantilevers used to obtain the PF-TUNA images were Bruker SCM-PIT conducting probes with a spring constant of 1-5 N/m. The entire cantilever and tip was coated with 20 nm of platinum and iridium resulting in a total tip diameter of approximately 40 nm. Root mean square roughness (R.sub.rms) values were obtained from plane fitted image scans of 10 μm.sup.2.

[0094] The samples surface was electrically connected via copper tape and the instrumental set up shown in FIG. 2.

[0095] PF-TUNA imaging parameters including set-point, scan rate, feedback gains, current sensitivity and applied bias were adjusted to optimize height and current image quality. The scanner was calibrated in x, y and z directions using silicon calibration grids (Bruker model numbers PG: 1 μm pitch, 110 nm depth and VGRP: 10 μm pitch, 180 nm depth).

[0096] For testing the AgNW/SWCNT based electrode materials of these embodiments in devices, two types of devices were fabricated using two different photoactive blends. The devices had the following structures (with schematic representations of these structures inset into FIG. 8) of substrate, conductive layer (including conductive network and conductive material), and base layer: [0097] (1) Glass/AgNW/SWCNT/MoOx/poly(3-hexylthiophene-2,5-diyl) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM)/Al [0098] (2) Glass/AgNW/SWCNT/MoOx/poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)](PCDTBT):[6,6]-phenyl C70-butyric acid methyl ester (PC70BM)/Al

[0099] Once the electrodes were fabricated, all steps of device fabrications and testing were carried out in an inert nitrogen environment (MBraun glove box, O2<0.1 ppm; H2O<0.1 ppm). A thin film of MoOx (Sigma Aldrich) was deposited as a buffer layer on the active surface of the electrodes using a thermal evaporator at a pressure ˜1×10-6 mbar. For devices with the P3HT:PCBM conductive layer (structure (1) above), a blend of P3HT:PCBM (1:1 w/w) was prepared by mixing equal amounts of individual solutions of P3HT (Merck) and PCBM (American Dye Source) in dichlorobenzene (DCB) (anhydrous grade). Both individual solutions had a concentration of 30 mg/mL.

[0100] The P3HT:PCBM blend was then filtered (0.22 μm PTFE filter, Membrane Solutions) and spin coated (500 rpm for 3 s, then 1400 rpm for 17 s) on top of the MoOx layer.

[0101] In a separate experiment for structure (2) above, a blend was prepared by mixing a 6 mg/mL solution of PCDTBT (SPJC, Canada) in DCB in a 24 mg/mL solution of PC70BM (Nano-C) in DCB. The PCDTBT:PC70BM (1:4 w/w) blend was then spin-coated (500 rpm for 3 s, then 800 rpm for 77 s) onto the MoOx layer. In this respect, a person skilled in the art will appreciate that a range of materials can be placed directly onto the conductive layer to alter the electronic properties of a device, such as work function, including, but not limited to, MoO, ZnO and PEDOT:PSS, to create an electrode with application dependent electronic properties while maintaining the conductive attributes of the conductive materials.

[0102] Both the structure (1) blend (P3HT:PCBM (1:1 w/w)) and the structure (2) blend (PCDTBT:PC70BM (1:4 w/w)) films were dried at 60° C. for 20 min on a hot plate. Finally, a thick layer of Al was deposited by thermal evaporation at a pressure of ˜1×10-6 mbar to complete the fabrication and form the opposite electrode for the device, in this configuration being the cathode. The final devices had an active area of 0.2 cm.sup.2, which was defined using a shadow mask.

[0103] Finally, for testing device characteristics, an Abet Triple-A (Abet Technologies) solar simulator was used as the source. The solar mismatch of the Xenon lamp (550 W Oriel) spectrum was minimized using an AM1.5G filter. Light intensity at ˜100 mW/cm2 AM1.5G was calibrated by using a National Renewable Energy Laboratory (NREL) certified standard silicon photodiode (2 cm2), with a KG5 filter. A Keithley® 2400 source measurement unit was used for current density-voltage measurements.

Preparation of Exemplary Planar AgNW/SWCNT Electrodes

[0104] The raw base nanocomposite material for the conducting network (AgNW with 20 wt % SWCNT interwoven therewith) was prepared via vacuum filtration through mixed cellulose ester membranes (MF-Millipore Membrane, USA, mixed cellulose esters, hydrophilic, 0.4 μm, 47 mm). Reference here is made to the steps illustrated in the schematic of FIG. 3.

[0105] AgNW/SWCNT interwoven networks were prepared via vacuum filtration through mixed cellulose ester membranes (MF-Millipore Membrane, USA, mixed cellulose esters, hydrophilic, 0.4 μm, 47 mm). Various volumes of the prepared AgNW (0.1 mg/mL) and SWCNT (0.25 mg/mL) solutions were added to 300 mL of deionised water so that a AgNW are loading of 100 mg/m.sup.2 was achieved in the final nanocomposite electrode.

[0106] In this respect, from the data shown in FIG. 4, it is apparent that in using the experimental method described above, there is a precipitous decrease in sheet resistance above a certain area loading of silver nanowires. It will be appreciated that as the size of the silver wires changes (length and width), and as the method of deposition changes (variations on the method described here being, such as, for example, spraying the solution), the preferred minimum area loading may vary. Similarly, as the density of the nanowires changes, the area loading also appears to change. The optimum area loading of silver nanowires is just above the point of the precipitous decrease in sheet resistance, as it will also typically correspond to the highest transmission for a conductive network

[0107] Returning to a discussion of the arrangement of FIG. 3, electrode patterning was achieved by placing a smaller pore size mixed cellulose ester template (MF-Millipore Membrane, mixed cellulose esters, hydrophilic, 0.025 μm, 47 mm) under the 0.4 μm membrane during filtration (FIG. 3(a)). After filtration, the patterned electrodes were then placed on untreated polyethylene naphthalate (PEN) (FIG. 3(b)). The PEN and patterned electrodes were then passed through a laminator at 130° C. (FIG. 3(c)). The mixed cellulose ester filter paper was subsequently removed with tweezers leaving behind the patterned AgNW/SWCNT nanocomposite on the surface of the PEN substrate.

[0108] Subsequently, 100 μL of 2:1 v/v PEDOT:PSS:IPA, was spin-cast on top of the AgNW/SWCNT nanocomposite at 500 rpm for 5 s then 3000 rpm for 30 s. The nanocomposite was then annealed at 140° C. for 10 min (FIG. 3(d)). 150 μL of Epotek 301 epoxy resin (T=99%) was then placed on top of the PEDOT:PSS coated AgNW/SWCNT electrode. A PEN sheet with surface treatment for adhesion was placed on top of the epoxy to create a PEN-AgNW/SWCNT-PEDOT:PSS-epoxy-PEN, stack (FIGS. 3(e and f)). The stack was heated at 65° C. for 1 h in an oven (Memmert, Germany) to cure the epoxy. The untreated PEN was peeled away to expose the smooth active surface of the electrode.

[0109] In order to achieve a suitable smooth active surface with good electron collection, a thin layer of solution processable conductive material is deposited which acts as both a charge distribution layer for free charges to migrate towards, and be collected by, the interwoven AgNW/SWCNT network, as well as a work function modification layer for subsequent layers in a device. This conductive layer also achieves “planarization” of the active surface, such that the active surface is smooth, assisting with the deposition of subsequent layers to create a required device.

[0110] In one embodiment, PEDOT:PSS was chosen as a conductive layer as it is a conducting polymer that also has the useful property that it can act as an electron blocking layer in OPV devices. This conductive material also encapsulates, and thus substantially coats, the conductive network which, without being bound by theory, is believed ensures that good electrical contact is maintained at the metal-metal interfaces as well as at the metal-nanotube interfaces.

[0111] In order to form the base layer, 150 μL of Epotek 301 epoxy resin (T=99%) was then placed on top of the PEDOT:PSS coated AgNW/SWCNT conductive layer. A glass substrate for transfer was placed on top of the epoxy to create a AgNW/SWCNT-PEDOT:PSS-epoxy-glass, or PEN, stack. The stack was heated at 65° C. for 1 h in an oven (Memmert, Germany) to cure the epoxy. In the case of Si, the stack was then put into liquid nitrogen in order to cleave the silicon-PEDOT:PSS interface, resulting in a smooth active surface attached to the glass substrate. In the case of a PEN planar template, the smooth active surface is exposed by peeling away the electrode from the planar template. The transmission of the resulting electrode was then measured using an integrating sphere and the results are shown in FIG. 4.

[0112] FIG. 5 (and Table 1, which will be described below) shows for the exemplary AgNW/SWCNT electrode over 800-400 nm that the average transmission was 86±1.4% and the average reflectivity was 3.4±0.3%. In contrast, for the prior art ITO electrode the average transmission was 93±6.5% and the average reflectivity was 7.2±4.3%.

[0113] The measured average sheet resistance of the exemplary AgNW/SWCNT electrode (6.56Ω/□) was almost half of that reported by prior art attempts at AgNW only electrodes (at 12Ω/□) and ITO electrodes (at 18.3Ω/□). Importantly, the exemplary AgNW/SWCNT electrode also has an average sheet resistance much lower than the prior art.

[0114] In order to quantify how well transparent electrodes formed using the materials of the present invention perform as conductors, a figure of merit value was calculated (see the results in Table 1 below). Such a figure of merit regularly used is the electrical to optical conductivity ratio (σ_DC/σ_OP) deduced from equations 1 and 2 below. A larger ratio indicates a better transparent conductor.

[00001]$\begin{array}{cc}T={\left(1+\frac{188.5}{R_{\mathrm{sh}}}.Math.\frac{{\sigma }_{\mathrm{OP}}}{{\sigma }_{\mathrm{DC}}}\right)}^{-2}.Math..Math.\mathrm{or}& \left(1\right)\\ \frac{{\sigma }_{\mathrm{DC}}}{{\sigma }_{\mathrm{OP}}}=\frac{188.5}{R_{\mathrm{sh}}.Math.\left(T^{-1/2}-1\right)}& \left(2\right)\end{array}$

[0115] where T=the average transmission at the wavelength of 500 nm

[0116] and R.sub.sh is the sheet resistance.

[0117] Typically, for planar electrodes with a polymer matrix, the σ_DC/σ_OP ratio lies between 186Ω-1 and 240Ω-1. The exemplary electrodes formed in these experiments have a σ_DC/σ_OP ratio of 367Ω-1, which are significantly improved over those of the Takada document mentioned above, as shown in Table 1.

[0118] The properties of an exemplary AgNW/SWCNT electrode and a prior art ITO electrode are summarised below in Table 1.

TABLE-US-00001 TABLE 1 Average Average Sheet Figure of % T (800- % R (800- resistance merit 400 nm) 400 nm) (Ω/□) (σ.sub.DC/σ.sub.OP) AgNW/SWCNT 86 ± 1.4 3.4 ± 0.3 6.56 ± 0.02 367 Large scale 80-85 — 4-8 400-557 5 × 5 cm AgNW/SWCNT ITO 93 ± 6.5 7.2 ± 4.3 18.30 ± 0.51  267 Prior Art - 84 — 10 207 Takada Document

[0119] SEM (FIGS. 6(a) and 6(d)) and AFM (FIGS. 6(b) and 6(e)) images reveal that without the adoption of the structure of the conductive layer of the present invention, and thus without the smooth active surface of the present invention (FIG. 6(a)), the AgNW/SWCNT electrode has a complex active surface topography and exists as a simple interwoven network of AgNWs and SWCNTs with no binding matrix and a multitude of spaces and gaps therebetween. The reason for there being any type of association between the AgNWs and the SWCNTs, in these comparative examples, has been determined to be due to a solution phase interaction between the AgNWs and SWCNTs prior to deposition onto the cellulose ester membranes, which is different to a structure obtained by the sequential deposition of nanowires followed by nanotubes, which can have poorer performance.

[0120] With the adoption of the structure of the conductive layer of the present invention, the SEM reveals a significantly smoother active surface and substantially all of the AgNWs and SWCNTs are encapsulated in the PEDOT:PSS conductive material, with projecting (and encapsulated) AgNWs and SWCNTs embedded into the epoxy base layer (FIG. 6(d)), in the manner described more generally above.

[0121] The change in active surface morphology due to the encapsulation of the conductive network, and the active surface being smooth was also monitored via tapping mode AFM and the images are shown in FIG. 6(b) for a comparative (non-smooth) active surface and FIG. 6(e) for the smooth active surface. FIGS. 6(c) and 6(f) are the height profiles along the dotted lines in the AFM images of FIGS. 6(b) and 6(e). The height profile of a comparative AgNW/SWCNT active surface, not in accordance with the present invention, (FIG. 6(c)) shows that the surface topography of the comparative active surface exceeds a peak-to-trough height of 200 nm. This height profile is far above a preferred operational height for an efficient OPV device. In fact, all OPV devices fabricated from these comparative non-smooth active surfaces (those shown in FIGS. 6(a), 6(b) and 6(c)) displayed electrical characteristics of a short-circuited device.

[0122] On the other hand, FIG. 6(e) shows the surface morphology of an active surface of an exemplary AgNW/SWCNT electrode material, being in accordance with the present invention. It will be apparent that the height profile along the dotted line is significantly smoother than for the comparative active surface of FIG. 6(c), despite the fact that the height profile is positioned over the crossing point of two AgNWs. The roughness (Rq) of the exemplary active surface over a plane fitted image scan of 10 μm.sup.2 was measured to be 3.5 nm. It should be noted that SWCNTs were still observed in the top right hand quadrant of FIG. 6(d) (see the arrow), indicating that the SWCNTs should participate in charge collection, passing collected charges to the more conductive AgNW network.

[0123] In this respect, PF-TUNA also provides evidence of the ability for SWCNTs to contribute to the charge collecting ability of the conductive network of an exemplary electrode as a secondary charge collecting network. The SWCNTs form part of the conductive network at the active surface of the electrode material of the present invention, being at least partly responsible for charge collection in OPV devices, and will presumably result in higher charge extraction efficiency and thus power conversion efficiency of an OPV device.

[0124] FIG. 7(a) shows the height image of the active surface of an exemplary electrode material where a silver nanowire is observed crossing the top right hand quadrant of the image. FIG. 7(b) shows the peak force current map of the active surface of an exemplary electrode material at a 2 V applied bias. It is apparent from FIG. 7(b) that the SWCNTs are electrically connected to the AgNW and are present in a significant density at the top surface (the active surface) of the electrode material, rendering the active surface electrically active. Importantly, at least some of the SWCNTs and the AgNWs remain electrically exposed at the active surface and are not completely covered by an epoxy, such as might be used as an adhesive layer in a traditional process of transferring a AgNW/SWCNT network to a glass substrate. If a non-conductive material, such as an epoxy, were to completely cover the SWCNTs and AgNWs, current would not be extractable from the active surface of the electrode and the sheet resistance would be in the mega-ohm range and completely unacceptable.

[0125] In one example of a device fabricated from the electrode material of the present invention, OPV devices were successfully fabricated on exemplary electrode materials using P3HT:PCBM and PCDTBT:PC71BM photoactive layers. Current density (J) and voltage (V) characteristics of the devices are shown in FIG. 8. P3HT:PCBM devices reached an efficiency of 1.01% while PCDTBT:PC70BM devices reached an efficiency of 2.09%. Device parameters including the open circuit voltage (VOC), short circuit current density (JSC), fill factor (FF) and efficiency are shown in FIG. 7.

[0126] In summary, it has been shown that electrodes based on interwoven AgNW and SWCNT's can be fabricated with a superior figure of merit—being a measure of the combination of transparency and conductivity—than prior art ITO electrodes on glass, and significantly better than prior art ITO electrodes on flexible substrates.

[0127] It has also been shown that the preferred SWCNTs are electrically connected to the preferred AgNWs, and are therefore expected to be able to act as extra charge collectors. The preferred method of fabrication is envisaged to be usable for a wide range of nanocomposite electrode compositions and potentially could be extended to use with other nanomaterials which have previously been overlooked due to surface topography. The preferred AgNW/SWCNT electrode materials of the present invention were used to fabricate efficient low temperature (annealing free) devices using two layer systems, demonstrating the potential of these electrodes to function with a range of semi-conducting polymer bulk heterojunctions.

[0128] It can be hypothesised that the inter-particle resistance, that is the resistance between the nanowires and between nanotubes and nanowires in an intimate mixture, will dominate the overall electrical conductivity. The relatively low viscosity of liquid polymer precursors, such as epoxy resins and the like, combined with the excellent surface wetting properties of these adhesive materials, can be expected to form an interfering layer between nanowires and nanotubes, thereby creating resistive elements in the non-conductive layer into which the nanowires penetrate. By encapsulating all of the nanowires and nanotubes with a conductive material, it is believed that the interparticle resistance can be reduced, and that the total current carrying capacity of the electrode increased, by effectively providing a thicker conductive element.

[0129] A person skilled in the art will understand that there may be variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all steps, features, compositions and compounds referred to, or indicated in this specification, individually or collectively, and any and all combinations of any two or more of the steps or features