Patent Application
20170098494
The present invention relates to methods and apparatus for manufacturing a conductive thin film that has high performance in terms of its optical and electrical properties, and low cost. The invention relates in particular to films that are also transparent.
Indium tin oxide (ITO) is one of the most used materials in applications where there is a need for transparent and conductive thin films due to its low sheet resistance, RS, (RS<100 Ω/sq), and high optical transmittance (T>90%). However, ITO suffers from several important drawbacks, such as high cost, brittleness, and a requirement for high processing temperatures.
Silver nanowires (AgNWs) are a potential candidate to replace ITO. High electrical and thermal conductivities, as well as excellent optical transmittance of AgNW networks make them one of the most promising materials to be used as transparent electrodes in optoelectronic applications. Many groups have reported using AgNWs to produce flexible, transparent, and conducting thin films. Polymer substrates coated with thin films of AgNW are available commercially from various suppliers e.g. Toray, Carestream, Okura and Hitachi Chemical.
There are many techniques available to fabricate thin films of AgNWs on substrates, such as vacuum filtration, Langmuir-Blodgett (LB), drop casting, Meyer-rod-coating, spray deposition and slot die coating.
De and co-workers have produced AgNW thin films by vacuum filtering aqueous dispersions of AgNWs onto a cellulose membrane, achieving a transmittance, T, of 85% and a RS of 13 Ω/sq. (van de Groep, J., P. Spinelli, and A. Polman, Transparent Conducting Silver Nanowire Networks. Nano Letters, 2012. 12(6): p. 3138-3144). Hu et al. have reported the use of the Meyer rod coating technique to produce AgNW thin films with T=80% and RS=20 Ω/sq (Hu, L., et al., Scalable Coating and Properties of Transparent, Flexible, Silver Nanowire Electrodes. ACS Nano, 2010. 4: p. 2955-2963). Scardaci et al. have reported large-scale deposition of AgNWs with spray coating. They showed T=90% and RS=50 Ω/sq for their deposited films (Scardaci, V., et al., Spray Deposition of Highly Transparent, Low-Resistance Networks of Silver Nanowires over Large Areas. Small, 2011. 7(18): p. 2621-2628, Scardaci, V., R. Coull, and J. N. Coleman. Spray deposition of Silver Nanowire transparent conductive networks. in Nanotechnology (IEEE-NANO), 2012 12th IEEE Conference on. 2012).
Although AgNW networks demonstrate comparable electrical conductivity and optical transmittance to ITO thin films, some challenges remain. For example, it would be desirable to increase the in-plane electrical conductivity for a given quantity of AgNWs. AgNWs are currently very expensive (˜$5/mg), so reducing the quantity of AgNWs will tend to reduce manufacturing costs. Furthermore, reducing the quantity of AgNWs will help to increase optical transmittance. It would also be desirable to reduce or eliminate anisotropy in the in-plane electrical conductivity.
It is an object of the invention to provide methods and apparatus that allow for improved manufacture of conductive thin films, at least partially addressing one or more of the problems with the prior art discussed above.
According to an aspect of the invention, there is provided a method of manufacturing a conductive thin film, comprising: providing first and second compositions of nanowires, each composition being initially separate from the other; applying the first and second compositions to a substrate in order to form a layer of nanowires on the substrate, the first and second compositions being mixed together before or during the application, wherein: a mean aspect ratio, defined as the mean ratio of the length to the cross-sectional diameter of a nanowire, of the nanowires in the first composition is larger than the mean aspect ratio of the nanowires in the second composition.
According to an alternative aspect of the invention, there is provided a method of manufacturing a conductive thin film, comprising: providing a composition of nanowires in which each nanowire has an aspect ratio defined as the ratio of the length to the cross-sectional diameter of the nanowire; processing the composition of nanowires so as to increase the variance in the distribution of aspect ratios in the composition by a factor of at least 1.5; and applying the composition of nanowires having the increased variance to a substrate in order to form a layer of nanowires on the substrate.
The inventors have found that using the mixture of nanowire compositions, or the nanowire composition that has been treated to increase the aspect ratio variance, provides an improved balance of electrical conductivity to transparency (generally increasing conductivity for a given quantity of nanowires). Additionally, analysis by the inventors shows that the isotropy of the in-plane electrical conductivity should be improved. This makes the approach particularly advantageous where manufacturing methods tend intrinsically to introduce spatial anisotropy into the nanowire layer, which can result in electrical anisotropy. For example, it is known that the slot-die coating process imparts significant shear forces on the nanowires. It is known that this can result in significantly anisotropic in-plane electrical properties. The present invention will tend to reduce or eliminate such anisotropy. The improvement in isotropy is thought to occur because of the tendency for shorter nanowires to be rotated by shear stresses to a lesser extent than longer nanowires. Where a mixture of shorter and longer nanowires are provided (as in the present invention), the longer nanowires will tend to be effective generally for maintaining high electrical conductivity and the shorter nanowires will tend to maintain cross-linking (and therefore percolation) between the longer nanowires even in the presence of shear forces.
According to an alternative aspect of the invention, there is provided an apparatus for manufacturing a conductive thin film, comprising: a first storage device containing a first composition of nanowires; a second storage device containing a second composition of nanowires; an applicator configured to apply the first and second compositions to a substrate in order to form a layer of nanowires on the substrate, the first and second compositions being mixed together before or during the application, wherein: a mean aspect ratio, defined as the mean ratio of the length to the cross-sectional diameter of a nanowire, of the nanowires in the first composition is larger than the mean aspect ratio of the nanowires in the second composition.
According to an alternative aspect of the invention, there is provided an apparatus for manufacturing a conductive thin film, comprising: a storage device containing a composition of nanowires in which each nanowire has an aspect ratio defined as the ratio of the length to the cross-sectional diameter of the nanowire; a nanowire processor configured to process the composition of nanowires so as to increase the variance in the distribution of aspect ratios in the composition by a factor of at least 1.5; an applicator configured to apply the processed composition of nanowires to a substrate in order to form a layer of nanowires on the substrate.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols indicate corresponding parts, and in which:
The present inventors have recognised that it is possible to alter the percolating properties of nanowire films by adjusting the aspect ratio distribution (for a given cross-sectional diameter this may be referred to equivalently as the length distribution) of the nanowires.
Transmission-Sheet resistance experiments were carried out and confirmed the idea that variation of the aspect ratio distribution produces a significant effect on the percolative behaviour of nanowire thin films. A bi-modal mixing of longer and shorter nanowires was performed by mixing of unsonicated and sonicated silver nanowire dispersions in different volume ratios. The resulting compositions were sprayed onto a polymer substrate to produce an isotropic film. Example results are depicted in
The following fitting function can be used to describe the data more formally and has been used to generate the three broken lines in
Here T is the optical transmission of the film at 550 nm; Π is the percolative figure of merit; Z0 is the impedance of free space (377Ω); RS is the sheet resistance of the film; n is the percolative exponent for the electrical properties of the film. Superior films are characterised by larger values of Π; this constitutes the coupling of high optical transmission and low sheet resistance. As shown in
Theory suggests that Π should be related to the length distribution (and therefore, equivalently, the aspect ratio distribution) of the particles forming the film as follows:
where A is a constant for the materials used; <l> is the mean nanowire length; σ2 is the variance in the nanowire length; dNW is the nominal diameter of the nanowires. This expression shows that nanowires with smaller diameter and larger mean length will produce superior films by increasing Π. The present inventors have also recognised that increasing the variance in the length (or aspect ratio) distribution can also be used to improves the film properties.
The inventors have performed simulations of nanowire networks using a Monte-Carlo process. A list of nanowires is generated which have an isotropic arrangement on a 2-dimensional domain, and an isotropic orientational arrangement. The distribution of aspect ratios can be tailored to an arbitrary distribution, by using the appropriate cumulative probability density (CPD) function. The distribution of nanowires is illustrated schematically in
This process allows the resistance of layers formed by arbitrary distributions of nanowires to be predicted by simulation. Based on such simulations the inventors have confirmed that increasing the variance in aspect ratio significantly reduces the sheet resistance for a given density of nanowires. The model can also be used to simulate how the electrical properties of the network behave as the size or shape of the domain containing the nanowires (i.e. the size and/or shape of the layer of nanowires being simulated) changes. For example, the model can be used to predict how narrow tracks of nanowire film will behave. Such narrow tracks often need to be formed in commercial applications, for example in capacitive touch sensors or LCD pixel electrodes. The inventors have shown that tailoring the aspect ratio distribution can reduce the rate of track failure at small track widths, which is a strong advantage.
It is understood that some methods of depositing nanowire films introduce large anisotropy into the resulting electrical properties. For example, slot die coating used in current commercial film production can produce perpendicular resistances of near 1.5 times that in the parallel direction. This is a result of the shear forces in the nanowire solution as it is dragged through the slot die; torque is applied to the nanowires causing them to align. Since the torque applied scales with nanowire length (and therefore aspect ratio) it is possible to reduce this electrical anisotropy by tailoring the nanowire length distribution; shorter nanowires will align less, and therefore bridge between longer nanowires in the perpendicular direction. This in turn will reduce the anisotropic effect in the sheet conductivity.
In an embodiment, a method of manufacturing a conductive thin film is provided.
The method comprises providing first and second compositions of nanowires (steps S1 and S2). Each composition is initially separate from the other (e.g. such that there is no mixing between the two compositions).
In a subsequent step, step S3 and/or step S4, the first and second compositions are applied to a substrate in order to form a layer of nanowires on the substrate. The first and second compositions may be mixed together before the application or during the application. The first and second compositions may be applied to the substrate simultaneously or one after the other (sequentially). In the case where the first and second compositions are applied to a given portion of the substrate at different times, the mixing together of the first and second compositions may occur as the later applied composition is applied. The mixing may be partial and/or inhomogeneous mixing. For example, in the case where the first composition is applied first and the second composition applied afterwards, on top of the first composition, a greater degree of mixing may occur at the upper surface of the first composition where the first and second compositions are first brought into contact than at the lower extremity of the applied first composition, for example.
An example apparatus 2 for manufacturing a conductive thin film is shown schematically in
In the embodiment of
In the arrangements of
The applicator 8 or sub-applicators 8A, 8B may be moved/scanned relative to the substrate 6 (either by moving the applicators/sub-applicators or by moving the substrate).
Each nanowire in the compositions has an aspect ratio defined as the ratio of the length of the nanowire to the cross-sectional diameter of the nanowire (or mean or maximal cross-sectional diameter where the cross-sectional diameter varies significantly along the length of the nanowire). It is understood that the cross-sectional diameter is a cross-sectional diameter perpendicular to a longitudinal axis of the nanowire. Typically, the cross-section takes a circular or approximately circular form and the diameter is the diameter of that circle or average diameter when the diameter varies as a function of angle within the cross-section. The distribution of nanowire shapes and sizes in the compositions may be characterized by reference to the mean aspect ratio of the nanowires contained in the compositions. In an embodiment, the first and second compositions are formulated such that the mean aspect ratio is larger in the first composition than in the second composition.
In an embodiment the first composition is a typical, commercially available composition having nanowires of a relatively uniform size and shape (and therefore aspect ratio). The distribution of aspect ratios in the first composition may therefore be described by a curve of concentration against aspect ratio that is sharply peaked around the mean aspect ratio. An example distribution for the first composition is illustrated schematically in
In an embodiment the second composition is a typical, commercially available composition having nanowires of relatively uniform size and shape (and therefore aspect ratio). Alternatively, the second composition may be obtained by processing a third composition of nanowires (which may itself be a typical, commercially available composition having nanowires of relatively uniform size and shape, for example). For example, as shown in
In
In
In the examples of
In the embodiments discussed above a composition of nanowires having a range of nanowire lengths is created by a method comprising mixing together two distinct compositions, either before or during application of the nanowires to the surface of a substrate. However, it is not essential that the composition of nanowires be created in this way. The nanowire processor 18 for example may be configured to take as input a distribution of nanowires having a small variance (e.g. a distribution having a narrow peak as shown in
Regardless of how the distribution of nanowires in the nanowire layer is created, performance that is significantly superior to typical, commercially available nanowire distributions can be achieved if the variance in the aspect ratio of the nanowires is increased by a factor of 1.5 or more, preferably 2 or more, preferably 5 or more, preferably 10 or more, preferably 50 or more. The variance in the aspect ratio can be achieved by mixing together different compositions of nanowires, as described above with reference to
The above methods and apparatus are particularly effective where the layer of nanowires is deposited at a density that causes the optical and/or electrical properties of the film to exhibit percolative behaviour to a greater extent than bulk behaviour. The nanowire layer 4 thus formed may or may not be a monolayer. A characteristic feature of the percolative regime is that the following power law relationship is satisfied between the conductivity of the layer and the concentration of the nanowires within the layer:
σ=σ0(φ−φc)t
for
φ>φc
where σ is the conductivity of the nanowire layer (S/cm), σ0 is a proportionality constant, and φ is the concentration of nanowires in the layer, φc is the critical concentration marking the lower bound of the percolative behaviour, and t is a power law exponent in the range of 1-1.33.
The behaviour of percolative networks can be described using percolation theory (Stauffer, D., Introduction to percolation theory. Taylor & Francis: London; Philadelphia, 1985, Kulshreshtha, A. K.; Vasile, C., Handbook of polymer blends and composites. Rapra Technology Lt.: Shawbury, Shewsbury, Shropshire, [England], 2002).
In a specific example in which the metal nanowires are AgNWs, the nanowires are provided at a density which causes the sheet resistance RS of the metal nanowires to be equal to or greater than 100 Ω/sq, preferably equal to or greater than 103 Ω/sq, preferably equal to or greater than 104 Ω/sq, preferably equal to or greater than 105 Ω/sq, preferably equal to or greater than 106 Ω/sq, preferably equal to or greater than 107 Ω/sq, preferably equal to or greater than 108 Ω/sq, preferably equal to or greater than 10 9 Ω/sq, preferably equal to or greater than 1010 Ω/sq, preferably equal to or greater than 1011 Ω/sq, preferably equal to or greater than 1012 Ω/sq. Embodiments of the invention may also be applied where the sheet resistance takes lower values, for example lower than 100 Ω/sq.
The substrate may be transparent, for example where the thin film is to be used to implement a touch screen display. Alternatively, the substrate may be opaque, which may be acceptable where the thin film is to be used in batteries or fuel cells, for example.
The metal nanowires may be formed from one or more of a range of different metals, including one or more of the following: Ag, Au, Pt, Cu, Pd, Ti, Al, Li.
In an embodiment, the applying of the nanowires S4 or S103 on the substrate 6 by the applicator 8 is performed by spray deposition. Alternatively or additionally, the applying of the nanowires S4 or S103 on the substrate 6 by the applicator 8 is performed by slot die coating. Both of these methods have a tendency to produce an anisotropic spatial distribution of the nanowires on the substrate (generally to a greater extent in slot die coating than in spray deposition). The anisotropic spatial distribution may lead to anisotropic electrical properties (e.g. greater resistance parallel to a first direction in the plane of the layer than in a second direction perpendicular to the first direction), which are undesirable. In spray deposition, the anisotropy may arise due to non-uniform evaporation of the solvent (leaving so-called “tide marks” for example in the spatial distribution of the nanowires). In slot-die coating it is well known that significant shear forces are applied to the nanowires and anisotropic electrical properties arising due to this have been observed in practice. Embodiments of the present invention are particularly advantageous in the context of methods of application such as these because the distribution of nanowires is more resistant to shear forces and/or other effects that tend to cause electrical anisotropy because the wide variance of aspect ratios ensures that longer (higher aspect ratio) nanowires are present to enhance conductivity generally while shorter (lower aspect ratio) nanowires, which tend to be aligned with each other less by effects such as shear forces and can therefore cross-link between different ones of the longer nanowires and maintain percolation, are also present.
Methods and apparatus according to embodiments may be applied to manufacturing a wide range of devices comprising conductive thin films, for example touch screen panels, photovoltaic panels, and batteries or fuel cells comprising conductive thin films.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1408949.4 | May 2014 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2015/051378 | 5/11/2015 | WO | 00 |
