GB2502564A - Method of producing a Plasmon-active electrode - Google Patents

Abstract

A method of producing a film 1 that is suitable for use as a plasmon-active electrode in an organic photovoltaic device 10 is described. The method comprises forming on a substrate 3 at least one binding layer 2, depositing a metallic layer 4 that binds to the binding layer 3, and heating the metallic layer 4 so as to produce therein a plurality of apertures or voids 5, each having a diameter of less than 300 nm, wherein the metallic layer after the heating has a sheet resistance of no more than twice the metallic layer before the heating. The apertures can be arranged randomly and have a large variation in diameter.

Description

Method of producing a film

Description

The invention relates to a method of producing a film that is suitable for use, among other things, as a plasmon-active dectrode in a photovoltaic device, and to such a film.

Organic photovoltaics (OPVs) have strong commercial potential, not least because they can be produced quickly, cheaply, on flexible substrates and at relatively low temperatures.

Conducting oxides such as indium tin oxide (ITO) and fluorine doped tin oxide are commonly used as a transparent electrode material. However, among other things, ITO has the disadvantage that it is generally too brittle at the thicknesses needed for sufficient electrical conductivity for use on flexible substrates. Optically-thin (e.g. sub- 15 nm) metallic films are an attractive alternative to ITO. They are simple, flexible, lightweight, highly electrically conductive and can be easily produced, e.g. using roll-to-roll vacuum evaporation. However, the far-field transparency of optically-thin metallic films is typically to to 20 % lower than ITO-coated glass across the visible spectrum.

Thus, generally, fewer photons will arrive at the photoactive layer and so the efficiency with which incident Bght is converted to electrical energy is lower.

One way that has been used to increase the efficiency of such OPVs is to increase the far-field transparency of the metallic (e.g. gold or silver) film electrode by incorporating a random array of micron-sized circular apertures using microsphere lithography.

However, this makes production more complex and, furthermore, the increase in transparency is at the expense of an increase in sheet resistance.

Another approach is to make use of surface-plasmon-active metallic nanostrtictures in OPVs. These can have the effect of enhancing the optical field intensity in the photoactive layer via localised surface plasmon resonance enhanced absorption and/or increasing the effective path length through the photoactive layer by trapping the incident Ught in surface plasmon pelaritons at the electrode-semiconductor interface.

Plasmon resonances are collective oscillations of the conduction band elections that are particularly prononced in metals owing to the high density of free electrons. Plasmon-active metallic structures in the form of nanoparticles can be provided at electrode interfaces or dispersed within the photoactive layer. Mternatively, electrodes comprising networks of nanowires or films patterned with arrays of sub-wavelength-sized apertures or periodic gratings can be used. Due to the relatively low thickness of the semiconductor layer used in OPVs (c 200 nm) incorporating metal nanoparticles or nanowires increases the risk of short circuiting, and so from a practical perspective nanostructuring that is confined in the pthne of the dectrode is preferable.

Reilly eta!. (Applied Physics Letters, 92, 243304 (2008)) describe films of silver with a random array of nanoholes of the same shape and size created through colloidal lithography techniques and metal vapour deposition by thermal evaporation. The OPV devices with these films as electrodes have a power conversion efficiency of around 1.2 %, which is higher than the same device with a continuous silver electrode (around i%) but only about a third of that of the same device with an ITO electrode. This can be attributed to the relatively high thickness of the silver layer needed for sufficient electrical conductivity (30 nm) and hence its low transparency. Similar methods and i results, albeit in relation to a hexagonal array of nanoholes, were described by Luhman et al. (Applied Physics Letters, 99, 103306 (2011)).

In the context of any photovoltaic device, plasmon-active electrodes can increase light absorption in the photoactive layer. In devices in which the photoactive layer is limited to a thickness that is tess than that required to efficienfly harvest the incident light (i.e. due to the properties of the photoactive material system such as, in the case of organic photovoltaics, the low electrical conductivity or short exciton diffusion length, or in the case of some inorganic semiconductors the thw minority carrier diffusion length), pasmon-active electrodes can increase device efficiency by increasing Ught harvesting.

Reducing the cell thickness not only reduces the materials cost and fabrication speed but also improves the electrical characteristics of the cell.

According to a first aspect of the invention, there is provided a method of producing a film that is suitable for use as a plasmon-active electrode in an organic photovoltaic so device, the method comprising forming on a substrate at least one binding layer, depositing a metallic layer that binds to the binding layer, and heating the metallic layer so as to produce therein a plurality of apertures each having a diameter of less than 300 nm, wherein the metallic layer after the heating has a sheet resistance of no more than twice the metallic thyer before the heating.

Thus, the method may provide a way of producing a film which is plasmon-active and thus improves the performance of a photovoltaic device in which it is used as an electrode. Moreover, the method is such that it is may be carried out relatively quickly and cheaply. In particular, the method need not involve a lithographic step.

The apertures need not be circular and so the word "diameter" is used herein in a more general sense to mean a maximum or an average dimension of an aperture in a direction substantially in the plane of the metallic layer. Such a dimension can be easily measured by suitable microscopy, for example atomic force microscopy or scanning io electron microscopy.

The metallic layer may have a sheet resistance of no more than 20 ohms per square.

The metallic layer may have a sheet resistance of no more than 50 ohms per square or no more than 30 ohms per square or no more than 15 ohms per square or no more than 10 ohms per square or no more than 8 ohms per square or no more than 6 ohms per square or no more than 5 ohms per square.

Thus, the film may have an electrical conductivity which makes it particularly suitable for use as an electrode in an organic photovoltaic device, particularly a large-area device. Moreover, the sheet resistance may be similar to or lower than ITO-coated glass (typically 15 12 sq) or ITO-coated PEN (typically 40 12 sq-i).

The apertures may each have a diameter of no more than 290 nm or no more than 280 nm or no more than 270 nm or no more than 260 nm or no more than 250 nm and/or no tess than 10 nm. The apertures may each have a diameter of no more than 290 nm or no more than 280 nm or no more than 270 nm or no more than 260 nm or no more than 250 nm and/or no tess than 20 nm. The apertures may each have a diameter of no more than 290 nm or no more than 280 nm or no more than 270 nm or no more than 260 nm or no more than 250 nm and/or no less than 30 nm. The apertures may each have a diameter of no more than 290 nm or no more than 280 nm or no more than 270 nm or no more than 260 nm or no more than 250 nm and/or no less than 40 nm. The apertures may each have a diameter of no more than 290 nm or no more than 280 nm or no more than 270 nm or no more than 260 nm or no more than 250 nm and/or no less than 50 nm.

Thus, the film maybe particularly effective at enhancing the absorption of light in the wavelength range from about 300 to 900 nm by a photoactive layer in the device. This corresponds to the most useful part of the solar spectrum for photovoltaic applications.

The area number density of the apertures may be more than about io per pm2. The area number density of the apertures may be more than about 5 per jim2 or more than about 15 per pm2 or more than about 30 per jim2 or more than about 100 per jim2.

The apertures may have a variation in diameter of more than about 25% and/or may io have varying shapes and/or may be randomly arranged. The apertures may have a variation in diameter of more than about 50%.

Thus, the film may be particularly effective at enhancing the absorption of light over a broad range of wavelengths.

Forming the binding layer may comprise performing an oxidative treatment and, in particular, an ultraviolet/ozone treatment or oxygen plasma treatment.

Forming the binding layer may comprise depositing one or more silanes on the surface of the substrate. The one or more silanes may comprise 3-mercaptopropyltrimethoxysilane and/or 3-am]nopropyltnmethoxysilane.

Or the method may comprise forming a binding ayer that comprises an inorganic compound or metal that binds to the metallic layer.

The method may comprise forming a binding layer that comprises a first substance that binds relative'y strong'y to the metallic ayer and/or to the substrate and a second substance that binds relatively weakly to the metallic layer and/or to the substrate, wherein the proportion of the first substance to the second substance is determined so o that the plurality of apertures are formed.

The method may comprise forming a binding layer that is not fully dense and/or that does not fully cover the surface of the substrate.

The metaflic layer may be deposited from the vapour phase.

The metallic layer may have a thickness of between about 5 and 15 nm. The metallic layer may have a thickness of between about 8 and 11 nm or tip to about 20 or 40 nm.

Thus, the film may have a flexibility and a transparency which make it particularly suitable for use as the electrode. Coupling between surface plasmon polaritons on the front and back surfaces of the metallic layer is generally also more efficient in thinner films.

The metallic layer may comprise silver, gold, copper, aluminium, chromium, nickel, platinum, palladium or a combination thereof When the metallic layer comprises gold, a binding layer formed by depositing one or more silanes may be particularly suitable. When the metallic layer comprises silver, a binding layer formed by an oxidative treatment may be particularly suitable.

The film may be mechanically robust and, in particular the sheet resistance of the metallic layer may increase by less than io%, if at all, after ultrasonic agitation in toluene or 2-propanol or after subjecting the film to the Scotch-tape test.

The heating may be performed to a temperature in the range from 150 °C to 450 °C.

The temperature may be 150 °C or 200°C or 250°C or 300°C or 350°C or 400°C or 450°C.

The diameter, area number density and/or shape of the apertures after heating for a time of to minutes may not change substantially after heating for longer times. Here, substantially means a change of 10 % in the diameter or the area number density of the apertures or a qualitative change in shape of the apertures. Instead of to minutes, the time may be 30 minutes.

o There may be provided a method of producing a photovoltaic device, the method comprising producing a first electrode by a method according to any preceding claim and combining the first electrode with an organic photoactive layer and a second electrode.

The method may further comprise providing a hole extracting layer with a thickness of no more than about nm between the first electrode and the organic photoactive layer.

Thus, the h&e extracting layer is thin enough that the organic photoactive layer experiences the enhanced optica' fieM due to plasmon effects.

The steps of the method may be performed in a vacuum or an inert atmosphere which is maintained between the steps.

Thus, the method maybe performed in an efficient manner and the components of the device may be less likely to become oxidised or contaminated.

In some embodiments, the photovoltaic device may have a power conversion efficiency of more than about 5 %.

According to a second aspect of the invention, there is provided a film for use as a plasmon-active e'ectrode in an organic photovoltaic device, the film comprising a substrate, at east one binding layer on the substrate, and a metallic layer bound to the binding ayer, wherein the metallic ayer comprises a plurality of apertures each having a diameter of less than 300 nm, wherein the metallic layer has a sheet resistance of no more than 30 ohms per square.

The metaflic ayer may have a sheet resistance of no more than 20 ohms per square.

The metaflic ayer may have a sheet resistance of no more than 15 ohms per square or no more than 10 ohms per square or no more than 8 ohms per square or no more than 6 ohms per square or no more than 5 ohms per square.

The apertures may each have a diameter of no more than 290 nm or no more than 280 nm or no more than 270 nm or no more than 260 nm or no more than 250 nm and/or no tess than 10 nm. The apertures may each have a diameter of no more than 290 nm or no more than 280 nm or no more than 270 nm or no more than 260 nm or no more than 250 nm and/or no less than 20 nm. The apeitures may each have a diameter of no o more than 290 nm or no more than 280 nm or no more than 270 nm or no more than 260 nm or no more than 250 nm and/or no tess than 30 nm. The apertures may each have a diameter of no more than 290 nm or no more than 280 nm or no more than 270 nm or no more than 260 nm or no more than 250 nm and/or no less than 40 nm. The apertures may each have a diameter of no more than 290 nm or no more than 280 nm or no more than 270 nm or no more than 260 nm or no more than 250 nm and/or no tess than 50 nm.

The area number density of the apertures may be more than about 10 per pm2. In some instances, the area number density of the apertures may be more than about 5 per pm3 or more than about 15 per pm2 or more than about 30 per pm2 or more than about 100 per pm2, The apertures may have a variation in diameter of more than about 25 % and/or may have varying shapes and/or may be randomly arranged. The apertures may have a variation in diameter of more than about 50% The binding layer may comprise oxygen moieties such as carboxylic acid groups or other reactive oxygen groups.

The binding layer may comprise one or more silanes. The one or more silanes may comprise 3-mercaptopropyltrimethoxysilane and/or 3-aminopropyltrimethoxysilane.

Or the binding layer may comprise an inorganic compound or metal that binds to the metallic layer.

The binding layer may comprise a first substance that binds relatively strongly to the metaflic layer and/or to the substrate and a second substance that binds rethtiveiy weakly to the metaflic thyer and/or to the substrate.

The binding ayer may not be fully dense and/or may not hilly cover the surface of the substrate.

The metallic layer may have a thickness of between about 5 and i nm. The metallic layer may have a thickness of between about 8 and 11 nm or up to about 20 or 40 nm.

o The metallic layer may comprise silver, gold, copper aluminium, chromium, nickel, platinum, palladium or a combination thereof.

When the metallic layer comprises gold, a binding layer comprising one or more sflanes may be particularly suitable. When the metallic thyer comprises silver, a binding layer comprising oxygen moieties maybe particifiarly suitable.

The film maybe mechanically robust and, in particular the sheet resistance of the metallic layer may increase by less than 10 %, if at all, after ultrasonic agitation in toluene or 2-propanol or after subjecting the film to the Scotch-tape test.

There may be provided a photovoltaic device comprising a first electrode comprising the film, an organic photoactive layer and a second electrode.

The device may comprise a hole extracting layer with a thickness of no more than about nm provided between the first electrode and the organic photoactive layer.

The device may have a power conversion efficiency of more than about 5 %.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates a method of producing a film according to some example embodiments of the invention.

Figures a to 2d show atomic force microscopy images and cross-sections of two different example films before (a and c) and after (b and d) a heating step such as that illustrated in Figure 1. The first example film (a and b) comprises an il-nm silver layer on a polyethylene terephthalate (PEN) substrate with a binding layer formed by a UV/O treatment. The first example film was heated to 200 DC for 10 minutes. The second example film (c and d) comprises an 8.4-nm gold layer on a glass substrate with a binding layer comprising a mixture of the silanes AFfl1S and MPTMS. The second example film was heated to 350 °C for 10 minutes.

Figure 3 shows the far-field transparency spectra of example films corresponding to those shown in Figures a to 2d.

Figure 4 shows the sheet resistance with increasing temperature of example films comprising an 8.4-nm gold layer supported on glass with a binding layer of APTMS only, MVMS only or a mixed layer of APTMS and MPTMS.

3. Figure 5 shows atomic force microscopy images of examples films produced by a method such as that illustrated in Figure 1. The examples films comprise an 8.4-nm gold layer on a glass substrate with a binding layer comprising a mixture of APTMS and MPTMS. The results are shown for binding layers formed under different conditions.

The substrate was exposed to the APTMS and MPTMS vapour at a background gas pressure of 50 (a), too (b) and 200 (c) mbar for 6o minutes.

Figure 6 shows atomic force microscopy images of examp'es films produced by a method such as that illustrated in Figure 1. The examples films comprise an 8.4-nm gold layer on a glass substrate with a binding layer comprising a mixture of APTMS and MPTMS. The results are shown for binding layers formed under different conditions.

In Figures 6a, 6b and 6c, the substrate was exposed to the APTMS and MPTMS vapour for 6o minutes. The background gas pressure was 50, 100 and 200 mbar respectively.

In Figures 6d, 6e and 6f, the substrate was exposed to the APTMS and MPTMS vapour at a background gas pressure of 100 mbar for 30, 60 and 100 minutes respectively.

i Figure 7 illustrates an example device comprising a film produced by a method such as that illustrated in Figure 1.

Figures Ba to Sd show characteristics of example devices such as that illustrated in Figure 7. The example devices comprise films corresponding to those shown in Figure 2. Current density-voltage characteristics in the dark and under one sun simulated solar iflumination (a for the gold film and c for the silver film) and externa' quantum efficiency spectra (b for the gold film and d for the silver film) are shown. Results are shown for examp'e devices comprising films produced with a heating step, i.e. nanostructured films (N), and without a heating step, i.e. planar films (P). Results are also shown for example devices with different thicknesses of a MoO3 hole-extraction layer.

Example methods

Figure 1 illustrates a method of producing a film 1 according to some example o embodiments of the invention.

Firstly, at step A, a binding layer 2 is formed on a substrate 3.

The substrate 3 may be made of any appropriate material, such as glass or a plastic such as polyethylene terephthalate (PET) or p&yethylene-naphthathte (PEN). -10-

The substrates 3 used when producing some of the example films 1 described herein include a glass microscope slide (Menzel-Gläser), a 1oo-im PET ifim (Hostaphan GN 4600, Mitsubishi Polyester Film GMBH) or a i-pm PEN film (Teonex, DuPont Teijin Films UK Ltd).

The substrate 3 maybe cleaned and/or otherwise prepared before forming the binding layer 2.

When producing some of the example films described herein, the substrate was cleaned by ultrasonic agitation for about 15 minutes firstly in a dilute aqueous solution of a surface active cleaning agent/decontaminant (Decon Neutracon) and then in 2-propanol. The substrate was then exposed to hot acetone vapor for about 10 seconds.

The binding layer 2 may be formed in any suitable way.

In some example embodiments, the binding layer 2 may be formed by depositing one or more silanes on the surface of the substrate. Such a binding layer 2 can be used with glass substrates as described below or, alternatively, with plastic substrates.

The one or more silanes may indude 3-mercaptopropyltrimethoxysilane (MPTMS) and/or 3-aminopropyltrim ethoxysfiane (APTMS). Such a binding layer is paiticdarly effective for binding to g&d. As will be exp'ained in more detail below, a mixed nancilayer of MPTMS and AVI'MS provides particularly robust films, a'though single-component silane layers may a'so be used.

Such a binding layer 2 may consist of a monolayer of molecules.

Such a binding ayer 2 is transparent across the \qsible and near infra-red spectnim.

o Such a binding layer 2 maybe formed by exposing the substrate 3 to a vapour of the one or more silanes. As will be explained in more detail below, the properties of the nanostructured metallic layer 4' depends upon nature of the binding layer 2 and hence, in this instance, upon the silane(s) used (e.g. their binding strength to the substrate 3 and/or to the metaflic layer 4, the background gas pressure and the length of time of 3. the exposure (and hence the coverage). -11-

When producing some of the example films 1 described herein, the substrate 3 was first subject to an ultra-violet/ozone treatment (described in more detail below) and then immediately transferred to a desiccator in which it was exposed to APTMS and/or MPTMS vapour. For some of the example films 1, this was performed at a background gas pressure of 150 mbar for Go mi]lutes.

In some example embodiments, the binding layer 2 may be formed by subjecting the substrate 3 to an oxidative treatment such an ultra-violet/ozone (UV/03) treatment or oxygen plasma treatment.

Such a treatment can have the effect of producing reactive oxygen-containing moieties on the substrate 3 to which the metallic layer 4 can bind.

For example, Table 1, below, shows how the proportion of reactive oxygen-containing i groups to which silver atoms can bind at the surface of PET and PEN substrates 3 increases upon UV/03 treatment. These groups include alcohols, esters and ethers.

7 able 1. High-resolution x-ray photoelectron spectroscopy (XPS) survey scan summaries for PEN and PET substrates 3 before and after UV/03 treatment. Columns 2 and show atomic percentages, while cohimn 4 shows the proportion of the C is peak assigned to C-OH, -C-O-C-and COO-C bonding environments.

Type C is / % 0 is / % C is (alcohol, ester, ether) / Cis PEN 78.9 ± 0.1 21.1 ± 0.1 0.14 PET 72.9 ± 0.1 27.1 ± 0.1 0.21 PEN UV/03 treated 70.0 ± 0.2 29.7 ± 0.2 0.22 PET IJV/O1 treated 68.1 + 0.1 31.9 + 0.1 0.24 It will be appreciated that the oxygen species form a dense binding layer 2, the density of which depends on the UV/03 treatment time and the UV lamp power.

Such a binding ayer 2 is particifiarly effective for binding to silver.

-12 -Example films 1 comprising planar n-nm silver layers 4 on untreated PET and PEN substrates 3 have a sheet resistance of 10 ± 2 12 sq-1. peak transparency of about 73 %, and root mean square (rms) roughness over 5 im x 5 m of about 3 nm. In comparison, example films 1 comprising planar n-nm silver layers 4 on UV/03-treated PET and PEN substrates 3 have a sheet resistance of 5 ± 1 12 sq1, peak far-field transparency of about 78% and an rms roughness of about 1 nm.

The example films 1 comprising the UV/03-treated substrates 3 are also more mechanicafly robust than those comprising the untreated substrates 3. In particular, to the properties of the exampk films 1 comprising the UV/03-treated substrates 3 remain unchanged after ultrasonic agitation in water. Conversely, the example films 1 comprising the untreated substrates 3 show a large increase in sheet resistance after ultrasonication. For example, a 15 minute ultrasonication caused the sheet resistance of 11 nm silver layers 4 on untreated substrates 3 to increase by 50% to about 15 12 sq-1.

To produce the gold films 1 described herein, a 15-minute UV/03 treatment was used for glass substrates 3 prior to deposition of the molecular acthesive layer to which the gold binds. In this case the U\T/03 treatment forms reactive oxygen-containing groups to which the silane adhesive molecules can bind. For plastic substrates 3, e.g. PET or PEN substrates 3, a 5-minute UV/03 treatment was used to form reactive surface oxygen groups that can bind direcfly to silver, removing the requirement for an additional binding layer The IJIT/03 treatment may be performed for onger or shorter times.

In this instance, the length of the UV/03 treatment time and the IJV lamp power determines the nature of the binding layer 2 (e.g. its density) and hence the properties of the nanostructured metallic thyer 4'.

The binding layer 2 may be formed in other ways.

Any material that binds to the substrate 3 and to the metallic layer 4 may be used.

Thus, appropriate binding layers 2 may be selected based upon, for example, the chemical properties of the substrate 3 and the metallic layer 4. When the film is to be used as an electrode in a photovoltaic device, the binding layer 2 may also need to be sufficiently transparent.

-13 -The binding layer 2 may comprise a molecular material having a least one "anchor group" which is a functional moiety capable of binding to the substrate. This binding will typically be via a covalent linkage. Examples of binding groups include chlorosilanes or alkoxysilanes (e.g. methoxysilanes, ethoxysilanes or phenoxysilanes) which form Si-O-C or Si-O-Si Unkages to the substrate.

In addition to the anchor group(s), the molecule must possess at least one "head group" capable of binding to the metallic layer 4. This binding will typically be via a covalent bond. For attachment to gold, the head group may, for example, be a thiol (-SM), isocyanide (-NC), organo-disulphide (-SS-R), primary amine (-NH2) or thioacetate (-SCOCH3). For attachment to silver, the head group may, for example, be a thiol (-SM), organo-disuiphide (-SS-R) or carboxylic acid (-CO2H). For attachment to copper the head group may, for example, be a thiol (-SH), organo-disulphide (-SS-R), carboxylic acid (-CO2M), or primary amine (-NH2). For attachment to aluminiumthe head group may, for example, be methyl ester (-CO2CH3) or carboxylic acid (-CO2H). For attachment to platinum the "head group" may be, for example, a thiol (-SM) or organo-disulphide (-SS-R).

The substrate 3 may carry pendant hydroxy groups to which a molecular binding layer 2 may be attached. Akernativ&y, the substrate 3 may be treated to have reactive moieties, for examp'e by oxygen p'asma treatment or by UV/ 03 treatment as described above.

The binding Thyer 2 may be formed by depositing an ifitra-thin (e.g. ito 10 nm) fflm of a transition metal such as Ge, Ni, Cr or Ta, or metal compound such as, for example, ZnS.

Allernativdy, the binding ayer 2 may comprise a combination of two or more inorganic compounds or metals. One of the compounds or metals (e.g. ZnS or chromium) may bind relatively strongly to the substrate 3 and the metallic layer 4. Another of the o compounds or metals (e.g. molybdenum oxide or tungsten oxide) may bind relatively weakly to the substrate 3 and/or the metallic layer 4. Such a binding layer 2 may be formed by co-depositing the metals or compounds. In this way, by varying the proportion of the two compounds or metals, the effectiveness of the binding layer 2 can be controfled so that apertures 5 form at devated temperature, as will be described bdow.

-14 -The binding layer 2 is particularly important because it increases the thermal stability of the films iand ensures that once the apertures j are formed the nanostructured metallic layer 4' adheres strongly to the substrate 3. Thus, the binding layer 2 enables a high degree of control over the formation of the apertures 5 since it determines the degree of adhesion between the substrate 3 and the metaflic layer 4.

Secondly, at step B, a metallic layer 4 is deposited.

The metallic layer 3 may be formed using a metal that is resistant to corrosion and oxidation, such as gold, silver or another noble metal. However, other metals may also be used, such as aluminium or copper. If relatively highly oxidisable metals such as copper are used, then the metallic layer 3 may need to be deposited and then covered before exposure to air, for example by incorporating into a photovoltaic device in a suitably inert atmosphere.

The metallic layer 3 may also be formed using a mixture or an alloy involving one or two or more such metals.

The metallic layer 4 may be deposited in any suitable way.

In some cases, it may be preferable to deposit the metaflic layer 4 using thermal evaporation or another technique, e.g. dectrochemical deposition or from a solution of metal nanoparticles.

When producing some of the example films 1 described herein, the metallic layer 4 was deposited using an evaporator at a deposition rate of 0.1 nm s1. The thickness of the metaflic layer 4 was measured using a quartz-crystal microbalance mounted adjacent to the substrate.

o Thirdly, at step C, the deposited metallic layer 4 is heated or, in other words, annealed.

This heat treatment is performed in such a way as to form a nanostructured metallic layer 4' which, as will be explained in more detail below, includes a plurality of apertures s each having a diameter ofless than 300 nm.

The heat treatment may be carried out in any suitable way.

-15 -In particular, the heat may be provided by any suitable means, e.g. a furnace. The heat treatment may be carried out with the film 1 in a suitable atmosphere, e.g. under vacuum or in an inert gas.

As will be explained in more detail below, the temperature of the heat treatment can determine the properties of the nanostructured metallic layer 4' and, in particular, the size and area number density of the apertures j.

io When producing some of the example films i described herein, the metallic layers 4 were heat treated at temperatures up to 500 °C for 10 minutes in a nitrogen atmosphere.

Example films

i Referring to Figure 2, atomic force microscopy (AFM) images and cross-sections of first and second example films 1 are shown.

The first example film i comprises an il-nm silver layer 4(') on a PEN substrate 3 with a binding layer 2 formed by a U\T/03 treatment. The first example film i was heat treated at 200 °C for tO minutes. Apertures 5 do not form at heat treatment temperatures below 150 °C, while the maximum temperature of about 200 DC is limited by the plastic substrate 3.

The second examp'e film comprises an 8.4-nm gold layer 4(') on a g'ass substrate 3 with a binding layer 2 comprising a mixture of the silanes APTMS and MPTMS. The second example film 1 was heat treated at 350 DC for 10 minutes.

Tapping-mode AFM measurements were performed in air using an Asylum Research MFP-3D.

Images of the films i before and after the heating step are shown.

As can be seen, the heat treatments result in metallic layers 4' having a plurality of apertures 5.

-16 -In the nanostructured gold layer 4', the apertures 5 have sizes in the range from about to 300 nm. The sheet resistance of the nanostructured gold layer 4' is 12.5 ± 2 fl q-1 In the nanostructured silver layer 4', the apertures 5 have sizes in the range 30 to 100 nm. The sheet resistance of the nanostructured silver thyer 4' is 6 ± 2 2 sq'.

Aperture sizes such as these are known to lead to particular improvements in the harvesting of visible light by OPV devices.

Referring to Figure 3, far-field transparency spectra for the first and second example films before and after the heating step are shown.

The measurements were performed using a Perkin Elmer Lambda 25 UV Spectrometer with the incident beam passing first through the substrate 3 (as is the case in an OPV device such as the device 10 (Figure 7)).

As can be seen, the nanostructuring generally causes the films 1 to have a lower far-field transparency. This is due to the plasmon enhanced absorption and scattering by the nanostructured metaflic layer 4.

The example films 1 and, in particiflar, the nanostructured metaflic layers 4', are stable under ambient conditions and in direct sunlight. The transparency, sheet resistance and morphology remain unchanged after exposure to air and sunBght for one week.

Moreover, the example films 1 are mechanically robust.

For example, as shown in table 2, below, example films 1 are resistant to ulirasonic agitation in various solvents and also to a standard "Scotch tape test". The example o films 1 in this case comprise a non-heat-treated or heat-treated 9-nm gold layer 4 on a glass substrate 3 with an APTMS/MPTMS binding layer 2. The sheet resistances are unchanged or increase by less than io% after ultra-sonic agitation in toluene or 2-propanol or after subject the example films 1 to the Scotch tape test. After ultra-sonic agitation in water, the sheet resistance increases more considerably bitt is still less than 20 12 sqL Table 2. Sheet resistance of example films 1 each comprising a 9-nm gold layer 4 on a glass substrate 3 with an APTMS/MPTMS binding layer 2. Results are shown for non-heat-treated example films 1 and for example films 1 heated to 350 °C (see Fig. 2d) and 500 °C. Results are shown for non-agitated example films 1 and example films 1 subject to ultra-sonic agitation in toluene, water and 2-propan& and also subject to the Scotch-tape test.

Type Sheet resistance (f2 sq') Non-agitated Toluene Water 2-propanol Scotch tape Non-heat-11.0 (± 0.7) 11.2 (±o.8) 12.5 (± i.o) 11.2 (± 0.8) 11 (±i.o) treated Heated to 12.5 ± 2 12.5 16 12.5 350°C Heated to 6.o (± o.) 6.1 (± 0.8) 8.o (± to) 6.2 (± 0.6) 6.2 soo°C As explained above, this mechanical robustness can be attributed to the provision of the binding layer 2.

Film properties The properties of the nanostructured metallic layer 4', and, in particular, the sizes and area number density of the apertures s, depend upon various aspects of the method by which the film 1 is produced. The aspects include, among others, the nature of the is binding layer 2, such as (A) the effectiveness of its component(s) in adhering to the substrate 3 and/or the metallic layer, and (B) its coverage or density. Other aspects indude the temperature and (to a lesser extent) the time of the heat treatment.

The correlation between sheet resistance and the structure of the metallic ifim 3 was confirmed by performing AFI\I measurements on some of the example films 1 in addition to the sheet resistance measurements.

Generally, the apertures 5 do not necessarily form at the expense of increased sheet resistance. Such an effect is inevitable when apertures are formed using lithographic methods. In contrast, the heat treating has the effect of re-distributing the metallic layer 4(') and making it locally thicker and also more highly crystalline. As a result the sheet resistance increases by less than a factor of two. The increase in crystallinity is -18-apparent from increases in grain size and from the straight edges of the apertures j seen in the AFM images.

At the most, the metallic htyer 4' after the heat treatment has a sheet resistance of no more than twice the metalUc thyer 4 before the heat treatment.

Such effects are particularly important for large-area optoelectronic applications, where high sheet resistance (e.g. above 10 or 20 12 sq) is detrimental to device performance, e.g. when scaling to cell areas much greater than 1 cm2.

Referring in particular to Figures 4, the effect of the temperature of the heat treatment, and of the nature of the binding layer 2, on the properties of the nanostructured metallic layer 4' will be described.

Figure 4 shows the sheet resistances of examp'e films 1 comprising 8.4-nm gold layers 4(') on glass substrates 3 with different binding layers 2. The different binding layers 2 are APTMS, MPTMS and the mixture thereof. The sheet resistances are shown for different heat treatment temperatures, the lowest of which corresponds to no heat treating.

In the case of the fflm 1 with the mixed APTMS/MPTMS binding thyer 2, which was deposited using a background gas pressure of 50 mbar for 60 minutes, there are no or very few apertures 5 for heat treatment temperatures lip to 300 °C. At higher temperatures, e.g. at about 400 °C and above, only arger (e.g. 1-tm) apertures 5 are formed (see Fig. 6a). Such apertures are generally not plasmon-active.

As can be seen, the metaflic ayers 4(') deposited on the sing'e-component binding ayers 2 become discontinuous at much tower temperatures than the films 4(') deposited on the mixed binding layer 2. This can be attributed to the single-component o binding layers 2 having weaker substrate-metal bonds (APTMS) or a lower density (MPTMS).

Generally speaking, the heat treatment can be carried out at a suitable temperature (e.g. determined by successively increasing the temperature) so that apertures 5 having a diameter ofless than 300 nm are formed.

-19 -With regard to the heat treatment time, it is evident from AFIVI images (not shown) that significant further changes in the nanostructure of the metallic layers 4' do not occur after heat treatment times of about 10 minutes. For example, comparing a film 1 heat treated for two hours with the ifim 1 heat treated for 10 minutes, there is only a slight decrease in the rms roughness of the film 1 (from about 5 nm to about 4 nm) as the edges of apertures 5 become smoother and no significant change in the area number density or the size of the apertures 5. This is in contrast to known methods in which much thicker films need heat treating for two hours to reach a stationary situation.

io Referring in particular to Figures 5 and 6, further aspects of the effect of the nature of the binding layer 2 on the properties of the nanostructured metallic layer 4' will be described.

Figures 5 and 6 show AFM images of example ifims 1 comprising 8.4-nm gold layers 4' on glass substrates 2 with binding layers 3 prepared under different conditions.

In Figures a to 5c, the substrate 2 was exposed to an APTMS and MPTMS vapour at different background gas pressures, namely 50, 100 and 150 mbar respectively, for 60 minutes. Once the metal was deposited, the films 1 were then heat treated at a temperature of 350 °C.

SimHary, in Figures 6a to 6c, the substrate 2 was exposed to an APTMS and MPTMS vapour at different background gas pressures, nam&y 50, 100 and 200 mbar respectively, for 6o minutes. Once the metifi was deposited, the ifims 1 were then heat treated at a temperature of 400 °C.

As can be seen, the mean size of the apertures 5 decreases and the number density of the apertures 5 increases when the vapour is deposited at higher background gas pressures. Under these circumstances, the vapour pressure of the APTMS and MPTMS o is tower and hence a binding layer 2 that covers a lower percentage of the surface of the substrate 3 is formed. The binding layer 2 formed at a background gas pressure of 50 mbar covers -100 % of the surface of the substrate 3 and is not suitable for the formation of a large number density of sub-300 nm apertures 5. However, by reducing the surface coverage of the binding thyer 2, considerably higher number densities of sub-oo nm apertures s are obtained.

In Figures 6d to 6f, the substrate 2 was exposed to an APTMS and MPTMS vapour at a background gas pressure of too mbar for different times, namely 30, 6o and too minutes respectively. The films 1 were then heat treated at a temperature of 300 °C.

As can be seen, the mean size of the apertures 5 decreases and the area number density of the apertures 5 increases when the vapour is deposited for longer times.

Example devices

Figure 7 illustrates an OPV device 10 according to some example embodiments of the io invention.

The device 10 may include, as its hole-extracting electrode ii for example, a nanostructured film 1 produced as described above.

i The next layer is a hole-extraction layer 12.

In the example devices 10 described herein, the hole-extraction layer 12 comprises a 5-or to-nm layer of MoO3. The MoO3 was thermally deposited at 0.02 nm -1 All chemicals were obtained from commercial sources and used without further purification.

MoQ is an efficient hole-extraction material for OPV devices. In a typical OPV device, the best performance is obtained with a 5-nm ayer of MoO3, although good performance is also obtained with considerably thicker layers.

The next ayer is an organic photoactive ayer 13.

In the example devices 10 described herein, the photoactive layer 13 comprises a 60-nm o layer of a poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1', 3'-benzothiadiazole)]: [6,6]-phenyl-C70-butyric acid methyl ester (PCDTBT:PC70BM) bulk heteroj unction. PCDTBT and PC70BM were combined in a i: ratio at a concentration 8 mg ml-' in chloroform. The blend was stirred for 60 minutes at 65 °C before filtering using an 0.45 jim PTFE filter. The PCDTBT:PC70BM bulk heterojunction films were spin casted at 6000 revolutions per minute for 6o seconds and then annea'ed at 80 °C for 30 minutes.

The PCDTBT:PC70BM material system gives power conversion efficiencies of up to 6 % in optimised device architectures and harvests light across the entire visible spectrum.

The next layer is an exciton blocking layer 14.

In the example devices 10 described herein, the exciton blocking layer 14 comprises an 8-nm layer of bathocuproine (BCP). The BCP layer 14 was deposited at 0.5 to 1 nm s.

io The next layer is a electron-extracting electrode layer 15.

In the example devices 10 described herein, the electron-extracting electrode layer 15 comprises a 100-nm layer of aluminium. The aluminium layer 15 was deposited through a shadow mask.

It will be appreciated that the OPV device 10 described above is only an example and that the nanostructured film i may be included in an OPV device in various different ways.

For example, instead of using the film 1 as a hole-extracting electrode ii, the film 1 may instead be used as an electron-extracting electrode in an inverted OPV architecture. In this case, the hole-extraction layer 12 is replaced with an electron extracting layer such as, for example, Ti02.

Referring to Figures 8a to 8d, the characteristics of six example devices 10 with nanostructured (N) and planar (P) electrodes 11 will be compared.

The six example devices 10 comprise films 1 corresponding to those shown in Figures 2a, zb, c and 2d. There are two different example devices 10 for each of the o nanostructured films 1, one comprising a 5-nm MoO3 hole-extraction layer 12 and another comprising a 10-nm MOO3 hole-extraction layer 12.

Figures Ba and Bc show current density-voltage characteristics in the dark and under one sun simulated solar illumination for the devices 10 comprising gold and silver films 1 respectively.

-22 -As can be seen, the example devices 10 comprising nanostructured films 1 and a 5-nm Mo03 layer 12 have a larger photocurrent density than other example devices 10.

In contrast, the example devices 10 comprising nanostructured films 1 and a 10-nm Mo03 layer 12 have similar photocurrent densities to the example devices 10 comprising planar films 1.

This is consistent with plasmon-induced optical field enhancement effects which extend from the metal electrode into the adjacent semiconductor over relatively short io distances (tens of nanometres), decaying exponentially with increasing distance from the electrode surface. The 5-nm MoO3 layer 12 is thin enough for the photoactive layer 13 to experience the enhanced optical field due to plasmon effects, but thick enough to prevent exciton quenching by the metal electrode ii. However, with thicker MoO3 layers 12, the photoactive layer 13 does not experience a significantly enhanced optical i field. This is also confirmed by the information in Table 3, below.

The nanostructuring may also reduce reflectivity at the interface between the substrate 3 and the metallic layer 4', thus helping to couple more light into the device 10. l'his reflectivity can be particularly high for silver layers 4' on glass substrates 3.

The photocurrent densities of the example devices 10 comprising electrodes without apertures 11 are about 25 % lower than similar devices using ITO/glass and ITO/PEN window electrodes. This is consistent with the lower far-field transparency of the silver or gold film electrodes ii.

The increase in the photocurrent density in the example devices 10 with nanostructured electrodes 11 is larger in the case of silver. This may reflect the fact that silver is a more plasmon active metal than gold. In the example device 10 with the nanostructured silver electrode ii, the photocurrent density is increased (from 8.27 ± 0.40 to 10.62 ± 0.40 mA cm2) such that it is comparable with the photocurrent density in a device with an ITO/PEN electrode (10.50 ± 0.20 mAcr2). This is remarkable when considering that the far-field transparency of the nanostructured silver film 1 is only about 45 %. Moreover, the device with the ITO/PEN electrode has a reduced fill factor and so worse power conversion efficiency than the example device 10 with the nanostructured silver electrode 11. This can be attributed to the higher resistivity of the ITO/PEN electrode which reduces the device fill factor.

-23 -Figures 8b and 8d show external quantum efficiency (EQE) spectra for the examples devices 10 comprising gold and silver films 1 respectively.

The measurements were carried out using AM1.5G solar iflumination, calibrated to a photodiode, and using a Stanford Research SR 830 ock-in amphfier.

The EQE spectra show that the enhancement of the photocurrent in the example device comprising the nanostructured silver electrode 11 occurs at all wavelengths. This is io despite the nanostructured silver film 1 having a much lower far-field transparency than the silver film without apertures 1 (see Fig. 3).

This broad band enhancement can be attributed to the large variation in aperture sizes and shapes.

The enhancement of the photocurrent in the example device 10 with the nanostructured gold electrode ii for wavelengths below about 525 nm could be attributed to the increase in far-field transparency (see Fig. 3). However, for wavelengths greater than about 525 nm, the photocurrent is enhanced despite a lower far-fidd transparency. Indeed, the biggest enhancement is obtained in the wavelength range over which the nanostructured gold film 1 is east transparent.

As can be see from Table 3, below, for the same MoO1 thickness, there are no significant differences in the open-circuit voftage or the fill factor between examp'e devices 10 comprising nanostructured and planar electrodes 11. This implies that the nanostructuring does not adversely impact the efficiency of charge carrier collection.

Table 3. Characteristics of a number of example devices 10 under one sun simulated solar illumination. In (a), the example devices 10 use a film 1 comprising an li-nm o silver layer on a PEN substrate with a binding layer formed by a UV/03 treatment.

These films 1 were nanostructured (N) by heated to 200 °C for 10 minutes, or were not heat treated and so are planar (i.e. have no apertures) (P). In (b), the example devices use a film 1 comprising an 8.4-nm gold layer on a glass substrate with a binding layer comprising a mixture of APTMS and MPTMS. These films 1 were nanostructured (N) by heated to 350 °C for 10 minutes, or were not heat treated and are planar (i.e. have no apertures) (P). The example devices 10 also comprise MOO3 layers 12 with -24 -different thickness. Values of the photocurrent density (Jj, open-circuit voltage (1/), the fill factor (FE) and the power conversion efficiency (ij) are provided.

(a) Silver Type N P N P N P MoO3 (nm) 5 5 10 10 20 20 J 8.56 ± 7.59 ± 7.89 ± 8.14 ± 7.67 ± 7.83 ± (mAcm2) 0.15 0.10 0.30 0.20 0.20 0.20 V0 (V) o.8 ± 0.85 ± o.86 ± 0.85 ± 0.87 ± 0.87 ± 0.01 0.02 0.01 0.02 0.01 0.01 FF 0.59 ± 0.53 ± 0.64 ± 0.61 ± 0.61 ± 0.60 ± 0.03 0.05 0.01 0.01 0.01 0.01 (%) 4.29 ± 3.42 ± 4.34 ± 4.49 ± 4.10 ± 4.05 ± 0.30 0.40 0.30 0.30 0.15 0.15 (14 Gold Type N P N P N P MOO3 (nm) 5 10 10 20 20 J 10.62 ± 8.27 ± 8.36 ± 8.20 ± 7.80 ± 7.44 ± (mAcm-2) 0.25 0.40 0.20 0.30 0.50 o.6o V,jV) 0.86 0.84 ± o.8 ± o.8 ± o.8 ± o.8 ± +0.02 0.02 0.03 0.01 0.02 0.02 FF 0.56 ± 0.55 ± 0.57 ± 0.58 ± 0.57 ± 0.59 ± 0.03 0.03 0.03 0.03 0.02 0.03 I) (%) 5.11 ± 3.82 ± 4.05 ± 4.04 ± 3.78 ± 3.73 ± 0.30 0.35 0.30 0.40 0.50 o.6o It will be appreciated that the above described embodiments are purely ilhistrative and are not limiting on the scope of the invention. Other variations and modifications will be apparent to persons skilled in the art upon reading the application.

For instance, the films 1 need not be used in an organic photovoltaic device and could, for example, be used in another type of optical device.

-25 -For example, the films 1 may be used in many different types of photovoltaic cells (not just organic photovoltaic cells), including dye-sensitized photovoltaic cells and various types of inorganic photovoltaic cells such as those fabricated using inorganic semiconductor nanoparticles. They would be particularly advantageous in photovoltaic devices based on p&ycrystalline semiconductors or inorganic semiconductor nanoparticles which have low minority carrier diffusion lengths.

There may be no need to form a binding layer 2 and/or the binding layer 2 may be an intrinsic part of the substrate 3.

Moreover, the disclosure of the application should be understood to include any novel features or any novel combination of features either explicitly or implicitly disclosed herein or any generalization thereof and during the prosecution of the present application or of any application derived therefrom, new claims may be formulated to i cover any such features and/or combination of such features.

Claims (36)

1. -26 -Claims 1. A method of producing a film that is suitable for use as a plasmon-active electrode in an organic photovoltaic device, the method comprising: forming on a substrate at least one binding thyer; depositing a metallic layer that binds to the binding layer; and heating the metallic layer so as to produce therein a plurality of apertures each having a diameter of less than 300 nrn, wherein the metallic layer after the heating has a sheet resistance of no more than twice the metallic layer before the heating.
2. 2. A method according to claim 1, wherein the metallic layer after the heating has a sheet resistance of no more than 20 ohms per square.
3. 3. A method according to claim 1 or 2, wherein the apertures each have a diameter i of no more than 290 nm and/or no less than 10 nm.
4. 4. A method according to claim 3, wherein the apertures each have a diameter of no more than 250 nm and/or no less than 50 nm.
5. 5. A method according to any preceding claim, wherein the area number density of the apertures is more than 10 per m2.
6. 6. A method according to any preceding claim, wherein the apertures have a variation in diameter of more than 25% and/or have varying shapes and/or are randomly arranged.
7. 7. A method according to any preceding daim, wherein forming the binding thyer comprises performing an oxidative treatment and, in particular, an ultravi&et/ozone treatment or oxygen plasma treatment.
8. 8. A method according to any one of claims ito 7, wherein forming the binding layer comprises depositing one or more silanes on the surface of the substrate.
9. 9. A method according to claim 8, wherein the one or more silanes comprise 3- 3. mercaptopropykrim ethoxysilane and/or 3-aminopropyltrimethoxysilane.-27 -
10. 10. A method according to any one of claims ito 7, comprising forming a binding layer that comprises an inorganic compound or metal that binds to the metallic layer.
11. ii. A method according to any preceding claim, comprising forming a binding layer that comprises a first substance that binds relatively strongly to the metallic layer and/or to the substrate and a second substance that binds relatively weaHy to the metallic layer and/or to the substrate, wherein the proportion of the first substance to the second substance is determined so that the plurality of apertures are formed.
12. 12. A method according to any preceding claim, comprising forming a binding layer that is not frilly dense and/or that does not frilly cover the surface of the substrate.
13. 13. A method according to any preceding claim, wherein the metallic layer is deposited from the vapour phase.
14. 14. A method according to any preceding claim, wherein the metallic layer has a thickness of between 5 and 15 nm.
15. 15. A method according to any preceding claim wherein the metallic layer comprises silver, gold, copper, ahiminium, chromium, nickel, pthtinum, pafladium or a combination thereof.
16. 16. A method according to any preceding claim, wherein the film is mechanicafly robust and, in particular, wherein the sheet resistance of the metallic layer increases by less than about io %, if at all, after ultrasonic agitation in toluene or 2-propanol or after subjecting the film to the Scotch-tape test.
17. 17. A method according to any preceding claim, wherein the heating is performed to a temperature in the range from 150 °C to 450 °C.
18. iS. A method according to any preceding claim, wherein the diameter, area number density and/or shape of the apertures after heating for a time of in minutes does not change substantially after heating for longer times.
19. 19. A method of producing a photovoltaic device, the method comprising: producing a first electrode by a method according to any preceding claim; and combining the first electrode with an organic photoactive layer and a second electrode.
20. 20. A method according to claim 19, comprising providing a hole extracting layer with a thickness of no more than 5 nm between the first electrode and the organic photoactive layer.
21. 21. A method according to claim 19 or 20, wherein the steps of the method are performed in a vacuum or an inert atmosphere which is maintained between the steps.
22. 22. A method according to any one of claims 19 to 21, wherein the photovoltaic device has a power conversion efficiency of more than 5%.
23. 23. A film for use as an electrode in an organic photovoltaic device, the film comprising a substrate, at least one binding layer on the substrate, and a metallic layer bound to the binding layer, wherein the metallic layer comprises a plurality of apertures each having a diameter of less than 300 nm and wherein the metallic layer has a sheet resistance of no more than 30 ohms per square.
24. 24. A film according to claim 23, wherein the metallic layer has a sheet resistance of no more than 20 ohms per square.
25. 25. A film according to claim 23 or 24, wherein the apertures each have a diameter of no more than 290 nm and/or no less than 10 nm.
26. 26. A film according to claim 25, wherein the apertures each have a diameter of no more than 250 nm and/or no less than o nm.
27. 27. A film according to any one of claims 23 to 26, wherein the area number density o of the apertures is more than 10 per pm2.
28. 28. A film according to any one of claims 23 to 27, wherein the apertures have a variation in diameter of more than 25 % and/or have varying shapes and/or are randomly arranged.-29 -
29. 29. A film according to any one of claims 23 to 28, wherein the binding layer comprises oxygen moieties sLich as carboxylic acid groups or other reactive oxygen groups.
30. 30. A film according to any one of claims 23 to 29, wherein the binding layer comprises one or more silanes.
31. 31. A film according to claim 30, wherein the one or more silanes comprise 3-mercaptopropyltrimethoxysilane and/or 3-aminopropyltrimethoxysilane.
32. 32. A film according to any one of claims 23 to 29, wherein the binding layer comprises an inorganic compound or metal that binds to the metallic layer.
33. 33. A film according to claim any one of claims 23 to 32, wherein the binding layer comprises a first substance that binds relatively strongly to the metallic layer and/or to the substrate and a second substance that binds relatively weakly to the metallic layer and/or to the substrate.
34. 34. A film according to any one of claims 23 to 33, wherein the binding layer is not fully dense and/or does not fully cover the surface of the substrate.
35. 35. A ifim according to any one of claims 23 to 34, wherein the metallic layer has a thickness of between sand 15 nm.
36. 36. A film according to any one of claims 23 to 35, wherein the metallic layer comprises silver, gold, copper, aluminium, chromium, nickel, platinum, palladium or a combination thereof.3. A film according to any one of claims 23 to 36, wherein the film is mechanically o robust and, in particular, wherein the sheet resistance of the metallic layer increases by less than 10 %, if at all, after ultrasonic agitation in toluene or 2-propanol or after subjecting the film to the Scotch-tape test.38. A photovoltaic device, comprising a first electrode comprising a film according to any one of claims 23 to 37, an organic photoactive layer and a second electrode.39. A device according to claim 38, further comprising a h&e extracting layer with a thickness of no more than 5 nm provided between the first electrode and the organic photoactive layer.40. A device according to daim 38 or 39, wherein the photovohaic device has a power conversion efficiency of more than 5 %.