Spectral conversion materials

Abstract

Composite comprises polymer and lanthanide complex Au+[Ln(L1)w(L2)x(L3)y(L4)z)]-u. A is a metal cation, preferably potassium. Ln is a lanthanide, preferably Europium. L1, L2, L3, L4 are dibenzoylm

Description

Field of the invention The invention relates to complexes that increase the wavelength of electromagnetic radiation and their use in agriculture and photovoltaic devices. The invention also relates to composites and compositions comprising the complexes, methods of forming the composites and compositions, and products and devices comprising the composites and / or compositions. Background Many biological and synthetic processes utilise solar radiation as their energy source. For example, plants and photovoltaic cells convert light energy into chemical and electrical energy, respectively. The majority of the electromagnetic radiation which is received from the Sun has a wavelength in the range of 20 nm to 2500 nm (i.e., from ultraviolet to infrared radiation). However, many processes are only able to utilise a narrow bandwidth of this radiation, meaning a large proportion of the light energy from the Sun is wasted. In photosynthesis, plants primarily utilise longer wavelengths of light, in the region of around 400 nm to 700 nm. This is known as photosynthetically active radiation (PAR). Wavelengths outside this region can contribute to heat build-up in plant tissues, subsequent damage to the plant, and even interference with photosynthesis. One method to reduce the incidence of undesired radiation and to increase the energy available for photosynthesising plants is to use artificial light sources which emit a desired spectrum of light, such as LEDs or lamps. Although these have been effective at improving plant growth, particularly in areas with limited sunlight and indoor environments, they can be expensive to purchase and run, require an energy source to use, and may not always be suitable for use on a large scale. Photovoltaic (PV) cells also utilise a narrow bandwidth of electromagnetic radiation. Photons from the Sun which have wavelengths outside the required bandwidth for a particular photovoltaic cell (as determined by its semiconductor bandgap), will either not be absorbed, or be converted into heat energy rather than useable electrical energy. PV cells could be made more productive if the incident light were of optimum wavelength. While the productivity of processes which utilise light for their energy can be increased through supplementation with artificial light, it would be desirable to be able to harness a greater proportion of solar radiation. One method of achieving this would be to convert any non-utilised wavelengths to a desirable wavelength for the specific application, thereby improving the efficiency of the process. Summary of Invention In a first aspect, there is provided a composite comprising a polymer and a lanthanide complex, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, wherein A is a metal cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane (dbm) and derivates thereof, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3. In a second aspect, there is provided a film-forming coating composition comprising a liquid and a lanthanide complex dissolved or dispersed in the liquid, the coating composition comprising a lanthanide complex, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, wherein A is a cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3. In a third aspect, there is provided a lanthanide complex, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]-u, wherein A is a cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof, wherein at least one of L1 to L4 is avobenzone, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3. In a fourth aspect, there is provided a use of a lanthanide complex as a wavelength shifter in an agricultural composite material, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, wherein A is a cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3. In a fifth aspect, there is provided a use of a lanthanide complex as a wavelength shifter in a photovoltaic device coating, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, wherein A is a metal cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3. In a sixth aspect, there is provided a method of forming the composite according to the first aspect, comprising: (i) compounding the lanthanide complex and the polymer to form a mixture; and (ii) extruding the mixture to form the composite. In a seventh aspect, there is provided an agricultural sheet comprising the composite according to the first aspect, a film formed from the coating composition according to the second aspect, or the lanthanide complex according to the third aspect. In an eighth aspect, there is provided a windowpane comprising a first major surface and a second major surface; and the composite according to the first aspect, a film formed from the coating composition according to the second aspect, or the lanthanide complex according to the third aspect, disposed on at least one major surface. In a ninth aspect there is provided a photovoltaic (PV) device comprising a coating disposed thereon, wherein the coating comprises the composite according to the first aspect, a film formed from the coating composition according to the second aspect, or the lanthanide complex according to the third aspect. The effectiveness of photosynthesis is limited by the fact that plants can only utilise a narrow bandwidth of incident solar radiation. Other processes, including electrical energy generation with photovoltaic devices, suffer from the same problem. The inventors wished to efficiently convert this wasted spectrum of solar radiation into photons with wavelengths which could be utilised in photosynthesis and other applications, to enhance plant growth and crop yields. They aimed to achieve this through the use of a spectral conversion material. It was desirable for the spectral conversion material to be able to convert wavelengths of light across a broad range of the solar radiation spectrum, in particular wavelengths of higher energy than those useful for, for example, photosynthesis and photovoltaic devices. Advantageously, the spectral conversion materials could be incorporated into other materials to make products which are useful for their specific application whilst maintaining desirable spectral conversion properties as well as not compromising the structural integrity of the materials. A further advantage is that the spectral conversion materials and products made therefrom could be manufactured in a time- and cost-effective manner, ideally using cheap and abundant materials. The inventors found that the lanthanide complexes of the invention are able to efficiently convert solar radiation into wavelengths appropriate for photosynthesis. In particular, lanthanide complexes comprising a lanthanide coordinated to four bidentate ligands independently selected from dibenzoylmethane (dbm) and derivates thereof and an associated counterion were found to be effective. These lanthanide complexes are able to absorb high energy wavelengths of solar radiation which are not utilised in photosynthesis and subsequently re-emit them at longer wavelengths within the photosynthetically active radiation (PAR) region. This downconversion effectively increases the intensity of radiation incident on plants which they can then utilise for photosynthesis. Importantly, due to the nature of the ligands, a broad spectrum of solar radiation is absorbed by the complexes, resulting in a greater proportion of the solar spectrum being utilised. Lanthanide complexes which comprise at least one avobenzone ligand were found to be particularly effective at absorbing a broad spectrum of solar radiation. Furthermore, the lanthanide complexes of the present disclosure exhibit a narrow emission spectrum, meaning that a significant proportion, if not all, of the emitted light is of a desirable wavelength for photosynthesis. If alternative wavelengths are desired for specific applications, the wavelengths of emission can be controlled through the choice of lanthanide. The lanthanide complexes therefore can be used as wavelength shifters in agricultural composite materials and in photovoltaic device coatings. The inventors also found that composites comprising the lanthanide complex and a polymer can be formed. Importantly, these composites were found to retain the downconverting properties of the lanthanide complexes whilst not significantly affecting the structural properties of the polymer. The inventors also found that composites with excellent downconverting properties could be readily formed by compounding extrusion. Without wishing to be bound to a particular theory, it is believed that the use of a potassium counterion for the lanthanide complexes of the invention, especially when the four ligands are all dibenzoylmethane, may provide the complexes with sufficient thermal stability to survive the elevated temperatures typically used during compounding extrusion. Other downconverting complexes may degrade during compounding extrusion, limiting their usefulness as downconverters and / or significantly limiting the materials in which the complexes can be incorporated into. The composites of the invention can be formed in a cost and time efficient process and are suitable for use in or on the surface of, for example, polytunnel sheeting, windowpanes (such as greenhouse windowpanes), and photovoltaic devices. Film-forming coating compositions, such as paints, which comprise the lanthanide complexes of the invention can also be produced. These coating compositions were found to be stable and can form films which have excellent downconverting properties. Accordingly, the film-forming coating compositions are suitable for retrofitting materials to be used in downconverting applications. For example, the film-forming coating composition can be applied to greenhouse windowpanes or polytunnel sheeting for use in agriculture to improve plant growth, or to photovoltaic devices for improving device effectiveness. Brief Description of Figures Figure 1 shows a schematic of use of the lanthanide complexes in agriculture. Figure 2 shows a schematic of an alternative use of the lanthanide complexes in agriculture. Figure 3 shows images of composite pellets of K+[Eu(dbm)4]' and polyethylene formed by compounding extrusion at 170 °C (left) and 250 °C (right). Figure 4 shows images of composite pellets as visualised under UV light. Figure 5 shows images of films produced from composite pellets of K+[Eu(dbm)4]' and polyethylene which were formed by compounding extrusion at 170 °C (left) and 250 °C (right). Figure 6 shows images of films produced from composite pellets as visualised under UV light. Figure 7 shows TGA data for K+[Eu(dbm)4]'. Detailed Description Unless otherwise stated, any optional or preferred feature may be combined with any other optional or preferred feature, and with any of the aspects of the invention mentioned herein. When discussing the products, methods, or uses of the present disclosure, each of these discussions can be considered applicable to other examples whether or not they are explicitly discussed in the context of that example. Thus, for example, in discussing a component related to the composite, such disclosure is also relevant to and directly supported in context of the method, and vice versa. Additionally, for example, in discussing a component related to the composite, such disclosure is also relevant to and directly supported in context of the film-forming coating composition and vice versa. Definitions As used herein, the terms “about” or “approximately” mean within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. For example, “about” can mean within three or more than three standard deviations, per the practice in the art. Alternatively, “about” can mean a range of up to 20%, preferably up to 10%, more preferably up to 5%, and more preferably still up to 1% of a given value. Also, particularly with respect to systems or processes, the term can mean within an order of magnitude, preferably within five-fold, and more preferably within two-fold, of a value. Wt% herein, unless otherwise specified, is w / w%. “Spectral conversion” describes shifting electromagnetic radiation from one wavelength to another. “Spectral conversion materials” are defined as materials capable of spectral conversion. “Downconversion” as used herein describes spectral conversion from a first wavelength to a second wavelength, wherein the second wavelength is longer than the first wavelength. “Downconverter” as used herein describes a material that facilitates spectral conversion to a longer wavelength. “Wavelength shifter” as used herein describes a substance or material that facilitates the conversion of radiation from a first wavelength to a second wavelength. “Lambda max”, also termed Amax, refers to the wavelength of which a material exhibits its highest level of absorption or emission of electromagnetic radiation. In other words, the wavelength at which a substance or material has its strongest photon absorption or emission. Amax of absorption refers to the wavelength of which a material exhibits its highest level of absorption. Amax of emission refers to the wavelength of which a material exhibits its highest level of emission. “Binder”, also termed “resin”, as used herein refers to a polymer which hardens under certain conditions. Upon hardening, cross-link bonds may form between the polymer chains constituting the resin. Hardening of the resins may be initiated by, for example, light, heat, moisture, setting agents, catalysts, solvent evaporation, or combinations thereof. Resins may form via condensation of monomers. Binders may be present in coating compositions such as paint compositions. They serve a number of roles, including, but not limited to, assisting the coating composition to adhere to a surface and holding the components of the coating composition together. Resins may be natural resins, such as rosins, or synthetic resins, such as polyester resins. The resins may be biodegradable resins. The resins may be bioderived resins. “Film-forming”, as used in the context of the film-forming composition of the invention, refers to a liquid formulation which forms a film when applied to a surface in a layer. The mechanism of film formation is not restricted and could be evaporation, solidification, polymerisation, cross-linking, or other chemical reaction or physical transformation. For example, a film-forming composition according to the invention may be sprayed or painted onto a windowpane in a thin layer and allowed to dry, thereby forming a film. Composite and Lanthanide Complex In a first aspect, there is provided a composite comprising a polymer and a lanthanide complex, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, wherein A is a metal cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane (dbm) and derivates thereof, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3. The inventors found that the lanthanide complexes of the invention, which comprise a saturated coordination sphere and a cationic counterion, exhibit a higher PLQY than neutral complexes such as Eu(dbm)3 with an unsaturated coordination sphere. An unsaturated coordination sphere can lead to the PLQY being quenched, for example by water when in solution. PLQY is defined as the number of photons emitted as a fraction of the number of photons absorbed. Complexes with higher PLQY values are desirable as they will be more efficient at downconverting light. The inventors also found that complexes comprising a potassium cation had improved thermal stability. The metal cation may be a monovalent cation, a divalent cation, or a trivalent cation (i.e., as defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, u may be 1, 2, or 3). The metal cation may be selected from an alkali metal cation or an alkaline-earth metal cation. The metal cation is preferably an alkali metal cation. The metal cation may be selected from Li+, Na+, K+, Rb+, Cs+, Be+, Mg+, Ca+, Sr+, or Ba+. The metal cation may be selected from Li+, Na+, K+, Mg+, or Ca+. The metal cation is preferably a potassium cation. The thermal stability of lanthanide complexes comprising a metal cation, particularly a potassium cation and particularly when the four ligands are all dibenzoylmethane, were found to be significantly higher than other complexes, in particular those comprising organic cations such as ammonium cations. The charge on the lanthanide-ligand complex is -1, so the charge on the counterion matches the number of complexes with which it is associated. The Lanthanide may be any lanthanide element (also known as lanthanoid elements). The lanthanide may be a lanthanide that has an emission spectrum suited for the application in which the lanthanide complex is used. For example, if used to increase the effectiveness of photosynthesis with solar radiation, the lanthanide may have an emission spectrum which is primarily within the PAR region of between about 400 nm to about 700 nm. The lanthanide may be selected from the group comprising Samarium, Terbium, Erbium, Ytterbium, and Europium. The lanthanide is preferably Europium. The emission spectrum of Eu is well-matched to the wavelength requirements for photosynthesis. Accordingly, lanthanide complexes comprising Eu are preferred for converting higher energy photons of solar radiation to longer wavelengths of light within the PAR region. According to the invention, L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane (dbm) and derivates thereof. The ligands act to absorb radiation and transfer the energy into the f electronic state of the lanthanide atom to which it is coordinated. In some embodiments, all the ligands are the same. It may be preferable for the complex to have 4 of the same ligand such that it can be synthesised in a more cost and time efficient process. In some embodiments, at least one ligand is dbm, optionally at least two ligands are dbm, optionally at least 3 ligands are dbm, optionally all ligands are dbm. In some embodiments, all the ligands are dbm. The dbm derivatives may be dbm ligands with substituents in any position around the phenyl rings. Different substituents and combinations of substituents will affect the absorption spectrum of the ligands and lanthanide complex, as well as the efficiency of the energy transfer from the ligands to the lanthanide. Dbm ligands can be modified with different substituents to readily control these properties. The substituents may be independently selected from linear alkyl or alkoxy and branched alkyl or alkoxy, halides, cyclic alkyl, amines, / V-alkylated amines, aryl, nitro, and hydroxyl; preferably the substituents are independently selected from linear alkyl or alkoxy and branched alkyl or alkoxy, and hydroxyl. The substituents may be in the 2, 2’, 3, 3’, 4, 4’, 5, 5’, 6, and / or 6’ position. The substituents may be in the 2, 2’, 4, and / or 4’ position. The dbm derivatives may be derivates wherein at least one phenyl ring is replaced with a naphthalene ring. The naphthalene ring may be a substituted naphthalene ring. The dbm derivates may be 2,2’-dinapthyl ketone. The complex may comprise at least one avobenzone ligand, optionally at least two avobenzone ligands, optionally at least three avobenzone ligands, optionally all ligands are avobenzone. In some embodiments, all the ligands are avobenzone. Avobenzone is able to absorb radiation over a wide range of wavelengths and has a Amax of absorption at about 357 nm. It is therefore particularly effective for use as a light-absorbing ligand for use in downconverting lanthanide complexes as the greatest intensity of solar radiation is between about 200 nm to about 400 nm. The ligands preferably have a broad absorption spectrum. The ligands preferably absorb wavelengths of solar radiation which are too short to be utilised in photosynthesis (i.e., wavelengths of light less than about 400 nm). The ligands may have a Amax of absorption of from about 100 nm to about 500 nm, optionally from about 150 nm to about 450 nm, optionally from about 200 nm to about 400 nm. It is beneficial for the ligands to have a Amax of absorption within this region in order to maximise the energy absorbed and subsequently re-emitted at a longer wavelength. The lanthanide complex may have a Amax of emission of from about 400 nm to about 700 nm, optionally about 450 nm to about 675 nm, optionally about 500 nm to about 650 nm. The optional and preferred features relating to the light absorbing properties of the ligands are equally applicable to the light absorbing properties of the lanthanide complex. For example, the lanthanide complex may have a Amax of absorption of between about 200 nm and about 400 nm. The lanthanide complex may have a thermal decomposition temperature of greater than about 150 °C, optionally greater than about 160 °C, optionally greater than about 170 °C, optionally greater than about 180 °C, optionally greater than about 200 °C, optionally greater than about 220 °C, optionally greater than about 240 °C, optionally greater than about 260 °C, optionally greater than about 280 °C, optionally greater than about 300 °C. The thermal decomposition temperature may be defined as the temperature at which decomposition of the complex occurs. Decomposition may be defined as the breakdown or change in chemical structure. A complex may be termed thermally stable at a temperature if, at that temperature, thermal degradation does not occur. Thermal stability and thermal decomposition temperature may be determined by thermogravimetric analysis (TGA), for example according to ISO 11358-1:2022. If a complex is thermally stable within a temperature range, no significant mass change will be observed over the temperature range by TGA. The lanthanide complex may be thermally stable up to at least 150 °C, optionally at least 160 °C, optionally at least 170 °C, optionally at least 180 °C, optionally at least 200 °C, optionally at least 220 °C, optionally at least 240 °C, optionally at least 260 °C, optionally at least 280 °C, optionally at least 300 °C, as determined by TGA. The lanthanide complex may be thermally stable at a temperature for at least about 15 seconds, optionally at least about 20 seconds, optionally at least about 30 seconds, optionally at least about 40 seconds, optionally at least about 60 seconds, optionally at least about 80 seconds, optionally at least about 2 minutes. The lanthanide complex may be thermally stable at a temperature of at least 180 °C for at least about 10 seconds, optionally at least about 20 seconds, optionally at least about 30 seconds. The inventors found that the lanthanide complexes of the invention have sufficient thermal stability such that they could be incorporated into polymers using processes which employ elevated temperatures, such as compounding extrusion. Importantly, after incorporation into polymers at elevated temperatures, the lanthanide complexes still had desirable luminescence and light converting properties. The PLQY of the lanthanide complexes outside a polymer matrix is usually greater than that achieved within a polymer matrix. However, the lanthanide complexes of the invention can be incorporated into polymers and still have good PLQY. The composite comprises a polymer and a lanthanide complex. The lanthanide complex may be dispersed throughout the polymer matrix, preferably homogenously dispersed throughout the polymer matrix. Alternative arrangements of the polymer and the lanthanide complex include: a thin layer of solid comprising or consisting of the lanthanide complex and being disposed between polymer layers; a solution or gel comprising the lanthanide complex and being disposed between polymer layers; agglomerates of or comprising the lanthanide complex distributed in the polymer; nanoparticles of or comprising the lanthanide complex distributed in the polymer. However, it is preferred that the lanthanide complex is dispersed throughout the polymer matrix. The lanthanide complexes may be incorporated into any polymer that can be made into a substantially transparent film, coating, or sheet. The polymer may comprise a polymer selected from the list comprising polyethylene (PE), polypropylene (PP), poly(ethyl acrylate) (PEA), polymethylmethacrylate (PMMA), polycarbonate (PC), polyether ether ketone (PEEK), ethylene-vinyl acetate (EVA), acrylate, polyvinylacetate (PVA), polyvinyl chloride (PVC), polyolefin elastomer (POE), silicones (e.g. PDMS), and combinations thereof. For example, the polymer may comprise a polymer selected from the list comprising polyethylene (PE), polypropylene (PP), poly(ethyl acrylate) (PEA), polymethylmethacrylate (PMMA), polyether ether ketone (PEEK), ethylene-vinyl acetate (EVA), acrylate, polyvinylacetate (PVA), polyvinyl chloride (PVC), polyolefin elastomer (POE), silicones (e.g. PDMS) and combinations thereof. The polymer preferably comprises PE. The PE may comprise or be low-density PE (LDPE), high-density PE (HDPE), linear low-density PE (LLDPE), or ultra-high molecular weight PE (LIHMWPE). The composites comprising the polymer and the lanthanide complex are preferably substantially transparent in the visible spectrum. In some embodiments, the polymer is not polycarbonate. The polymer may be or comprise a biodegradable polymer. A “biodegradable” polymer is capable of mineralising completely on burial within a given period of time (often by being decomposed by bacteria and other living organisms), leaving no traces of polymer in addition to a total absence of hazardous or toxic residue, unlike a degradable polymer. The biodegradable polymer may be selected from the list comprising polylactic acid (PLA), polyhydroxyalkanoate (PHA), polybutylene succinate (PBS), poly(vinyl alcohol) (PVA), cellulose acetate (CA), polyglycolic acid (PGA), polybutylene adipate terephthalate (PBAT), and polypropylene carbonate (PPG). The polymer may be or comprise a bioderived polymer. As used herein, the term “bioderived” means derived from or synthetised by a biological feedstock, such as, for example, an agricultural, forestry, plant, bacterial, or animal feedstock. For example. The polymer may be a polymer derived from algae or plants. The bioderived polymer may be selected from the list comprising bio-polyethylene (bio-PE), bio-polypropylene (bio-PP), bio-polyethylene terephthalate (bio-PET), bio-polyamide (bio-PA), bio-polyfluoro alcohol (bio-PFA), polylactic acid (PLA), polyhydroxyalkanoate (PHA), bio-polybutylene succinate (bio-PBS), cellulose acetate (CA), polyglycolic acid (PGA), and polyethylene furan-2,5-dicarboxylate (PEF). The polymer may be a bioderived and biodegradable polymer. The PLQY of the composite is dependent on the materials in which the lanthanide complex is dispersed. The lanthanide complex may be present in the composite in an amount of from about 0.1 wt% to about 15 wt%, optionally from about 0.5 wt% to about 12 wt%, optionally from about 1 wt% to about 10 wt%, optionally from about 2 wt% to about 8 wt%, optionally from about 3 wt% to about 7 wt%, optionally from about 4 wt% to about 6 wt%, optionally about 5 wt%, based on the solids content of the composite (i.e., the total weight of solids, excluding any solvent or water). The lanthanide complex may be present in the composite in an amount of at least about 0.1 wt%, optionally at least about 0.25 wt%, optionally at least about 0.4 wt%, optionally at least about 0.75 wt%, optionally at least about 1 wt%, optionally at least about 1.5 wt%, optionally at least about 2 wt%, optionally at least about 3 wt%, optionally at least about 4 wt%, optionally at least about 5 wt%, based on the solids content of the composite (i.e., the total weight of solids, excluding any solvent or water). The lanthanide complex may be present in the composite in an amount of no more than about 15 wt%, optionally no more than about 13 wt%, optionally no more than about 11 wt%, optionally no more than about 10 wt%, optionally no more than about 9 wt%, optionally no more than about 8 wt%, optionally no more than about 7.5 wt%, optionally no more than about 6 wt%, based on the solids content of the composite (i.e., the total weight of solids, excluding any solvent or water). Whereas the final composite product may comprise no more than 15 wt% of the lanthanide complex, a masterbatch may comprise a higher concentration of the lanthanide complex. A masterbatch is a feedstock component which may be diluted with additional polymer feedstock before extrusion. This is a known technique in the polymer processing industry, for example when incorporating a pigment into a polymer component, and the same benefits may be realised when incorporating the lanthanide complexes of the invention into a polymer. Accordingly, when a masterbatch is produced, as a feedstock material for the composite of the invention or for other purposes, the lanthanide complex may be present in the masterbatch in an amount of from about 15 wt% to 80 wt%, for example from about 20 wt% to about 60 wt%. The masterbatch may comprise the lanthanide complex and a matrix polymer. The matrix polymer may be the same as or different than the polymer of the composite. The composite may be substantially free of solvent, for example water. Accordingly, the of lanthanide complex may be present in the composite as defined above but based on the total weight of the composite. The composite may have a weight ratio of lanthanide complex to polymer of from about 1:100 to about 1:5, optionally from about 1:50 to about 1:8, optionally from about 1:40 to about 1:10, optionally from about 1:30 to about 1:15, optionally about 1:20. Typically, composites comprising a higher lanthanide complex content will be able to absorb (and therefore re-emit) a greater intensity of radiation. This is desirable as the composites will have improved effectiveness as spectral conversion materials. However, a higher lanthanide complex content may be detrimental to the structural and physical properties of the composites. Furthermore, higher levels of lanthanide complex increase the cost of manufacture. In some embodiments, the composites comprising the lanthanide complexes were found to have good structural properties and spectral conversion properties. Indeed, in some embodiments, there was no significant detriment to the strength and tensile properties of the polymers upon incorporation of the lanthanide complexes. The composite may comprise additives to impart desirable properties to the composite. The additives may be antimicrobials, biostabilisers, antioxidants, antistatic agents, blowing agents, fillers, flame retardants, heat stabilisers, reinforcement agents, and / or impact modifiers. The composite may comprise additives in an amount of no greater than about 20 wt%, optionally about 15 wt%, optionally about 10 wt%, optionally about 8 wt%, optionally about 5 wt% based on the solids content of the composite. The composite may be a masterbatch. A masterbatch may be a granule form of a plastic to be used in plastic manufacturing. The masterbatch may be added during the compounding step in plastic manufacture to impart properties on the final plastic product. For example, the masterbatch may be a granule comprising the lanthanide complex and the polymer which can be added to additional materials in plastic manufacture to impart light-converting properties to the plastic product. The composite may be in the form of a polytunnel sheeting. The composite may be in the form of a film or sheet for adhesion to, for example, greenhouse panels, greenhouse windows, or polytunnel sheeting. A polytunnel sheeting may be a multilayer film, for example a 7-layerfilm. The composite may be a greenhouse panel. The composite may be in the form of a film or sheet for adhesion to the surface of a photovoltaic device. The composite may be sandwiched between layers of another material, deposited on at least one surface of another material, or dispersed throughout another material. The composite may be overlayed over a second material. The thickness of the composite may be selected based on the end use application. For example, polytunnel sheeting may have a thickness of from about 50 pm to about 500 pm, optionally about 50 pm to about 500 pm, optionally about 75 pm to about 400 pm, optionally about 100 pm to about 300 pm, optionally about 125 pm to about 250 pm, optionally about 150 pm to about 200 pm. In a preferred embodiment of the first aspect, there is provided a composite comprising a polymer and a lanthanide complex, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, wherein A is a metal cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane (dbm) and derivates thereof, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3, wherein the polymer is polyethylene, and wherein the lanthanide complex is present in the composite in an amount of from about 1 wt% to about 10 wt%, based on the solids content of the composite. In a preferred embodiment of the first aspect, there is provided a composite comprising a polymer and a lanthanide complex, wherein the lanthanide complex is defined by K+[Ln(L1)4]', wherein Ln is any lanthanide, and wherein L1 is dibenzoylmethane (dbm) and derivates thereof, wherein the polymer is polyethylene, and wherein the lanthanide complex is present in the composite in an amount of from about 1 wt% to about 10 wt%, based on the solids content of the composite. In a preferred embodiment of the first aspect, there is provided a composite comprising a polymer and a lanthanide complex, wherein the lanthanide complex is defined by K+[Eu(L1)4]', wherein L1 is dibenzoylmethane (dbm) and derivates thereof, wherein the polymer is polyethylene, and wherein the lanthanide complex is present in the composite in an amount of from about 1 wt% to about 10 wt%, based on the solids content of the composite. In a preferred embodiment of the first aspect, there is provided a composite comprising a polymer and a lanthanide complex, wherein the lanthanide complex is defined by K+[Eu(L1)4]', wherein L1 is dibenzoylmethane (dbm) and derivates thereof, and wherein the lanthanide complex is present in the composite in an amount of from about 1 wt% to about 10 wt%, based on the solids content of the composite. In another preferred embodiment of the first aspect, the lanthanide complex is K+tEutUM-, wherein L1 is avobenzone. Method of Making the Composite According to the present disclosure, there is also provided a method of forming the composite according to the first aspect, comprising: (i) compounding the lanthanide complex and the polymer to form a mixture; and (ii) extruding the mixture to form the composite. The composite of the present disclosure, in accordance with any of the preferred and optional features previously discussed, and combinations thereof, may be produced according to the method of the sixth aspect. Compounding and extrusion may be performed by any standard method known to those skilled in the art. Compounding and extrusion may occur in any suitable device known to those skilled in the art, for example, an extruder. Compounding extrusion may occur in an extruder which comprises a feeder, at least one barrel, at least one screw that rotates within the barrel, and a die. The extruder may be a twin screw extruder. The components of the composite may be mixed, melted, and / or homogenised during compounding extrusion, for example within the barrel of a twin screw extruder by action of the screw rotating. The temperature of the mixture may be controlled. The temperature of the barrel may be controlled to melt the components of the mixture. If present, the die plate may control the shape of the extruded material (also termed the extrudate). The design of the die plate may determine the dimensions of the extrudate. The mixture may be extruded to form the composite in the form of a film or a sheet. In some embodiments, the extruded mixture may be cut into pellets using a pelletiser. Any suitable pelletiser known to those skilled in the art may be used. The extrudate may be allowed to cool prior to cutting into pellets. The pellets may be a masterbatch. The masterbatch may be further processed through compounding with additives and subsequently extruded and shaped to produce the composite in the form of a film or a sheet. According to the present disclosure, compounding in step (i) refers to mixing the lanthanide complex and the polymer to form a mixture. The mixing during compounding may be blending. The lanthanide complex in step (i) may be added as a free flowing powder. The lanthanide complex may be homogenised prior to compounding. The lanthanide complex may be added as a homogenised free flowing powder. In some embodiments, the temperature during compounding and / or extrusion is from about 100 °C to about 450 °C, optionally from about 100 °C to about 400 °C, optionally form about 100 °C to about 350 °C, optionally from about 100 °C to about 250 °C, optionally from about 120 °C to about 240 °C, optionally from about 130 °C to about 230 °C, optionally from about 140 °C to about 220 °C, optionally from about 150 °C to about 200 °C, optionally about 170 °C. The temperature during compounding and / or extrusion may be at least 100 °C, optionally at least 120 °C, optionally at least 140 °C, optionally at least 150 °C, optionally at least 170 °C. In some embodiments, the temperature refers to the temperature of the barrel in the compounding extruder. In some embodiments, the temperature refers to the temperature of the mixture. In some embodiments, the composite formed after extrusion from step (ii) may be a masterbatch. The masterbatch may be in the form pellets of the composite comprising the lanthanide complex and the polymer. The masterbatch may be further compounded with or without additives and then extruded to form the composite. As described above, a masterbatch would comprise a higher concentration of lanthanide complex than a finished composite product. This compounding and extrusion steps may be carried out as described previously. The extrudate may be made into a film or a sheet. The extrudate may be made into a film using a blown film extrusion machine. The composite may be formed into a sheet or a film by pressing the molten extrudate between to plates. Due to the thermal stability of the lanthanide complexes of the present disclosure, they can be incorporated into polymers by the method of the present disclosure. In particular as they can survive the elevated temperatures required to melt the polymers during compounding extrusion. Film-forming coating Composition In a second aspect, there is provided a film-forming coating composition comprising a liquid and a lanthanide complex dissolved or dispersed in the liquid, the film-forming coating composition comprising a lanthanide complex, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, wherein A is a cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3. The film-forming coating composition is also referred to herein as the “coating composition”, for brevity. The preferred and optional features and combinations thereof for the lanthanide complex of the composite apply equally to the lanthanide complex of the film-forming coating composition. The liquid may comprise or be an organic liquid, an aqueous liquid, or a combination thereof. The liquid may be termed a solvent. The solvent may be selected from the list comprising water, methyl ethyl ketone, turpentine, methylated spirits, methanol, ethanol, isopropanol, diethyl ether, xylene, toluene, acetone, petroleum ether, methyl acetate, ethyl acetate, propyl acetate, hexane, cyclohexane, and combinations thereof. According to the present disclosure, the lanthanide complex is dissolved or dispersed in the liquid. The film-forming coating composition may be an emulsion, a solution, or a suspension. The film-forming coating composition may be applied to a surface to form a film after evaporation of the solvent. The film may also be termed a coating. The film-forming coating composition may therefore be used to coat a material with a film comprising the lanthanide complexes of the present disclosure. Importantly, the film-forming coating composition may be used to impart materials with spectral downconverting properties. In some embodiments, the film-forming coating composition may be applied to the surface of a greenhouse, the surface of a greenhouse window, or the surface of a polytunnel sheeting. The film-forming coating composition may allow retrofitting of existing greenhouses or polytunnels in order to gain the benefits of the lanthanide complexes of the present disclosure without needing to replace the panels, greenhouses, or polytunnels. For example, a greenhouse or polytunnel can be coated with the filmforming coating composition to improve plant growth within the greenhouse or polytunnel. This may be more practical and sustainable than replacing existing panels or polytunnels and provides a quick and convenient method by which to increase agricultural efficiency. It is also envisaged that the film-forming coating compositions of the present disclosure may be applied to the surfaces of other materials and devices, for example, to the surface of photovoltaic devices. The film-forming coating composition may comprise a binder. The binder may also be termed a resin. One role of the binder is to allow the film-forming coating composition to adhere to the surface to which the composition is applied. The binder may be selected from a synthetic resin or a natural resin. In some embodiments, the binder is or comprises a resin selected from an acrylic resin, alkyd resin, vinyl-acrylic resin, vinyl acetate / ethylene resin, polyurethane resin, polyester resin, melamine resin, epoxy resin, silane resin, siloxane resin, or combinations thereof. The binder is preferably an acrylic resin. The resin may be a biodegradable resin. The resin may be a bioderived resin. The film formed from the film-forming coating composition is preferably a substantially transparent or translucent film. Nevertheless, the film-forming coating composition may comprise a pigment. The pigment may also be termed a dye. The film-forming coating composition may comprise additives. The additives are additional components of the film-forming coating composition, other than the binder, solvent, and pigment, which are added to provide the film-forming coating composition and / or the film formed from the film-forming coating composition with desirable properties. The use of additives depends upon the application of the film-forming coating composition. The additives may serve to provide or improve properties such as, but not limited to, stain resistance, scuff protection, sag prevention, shelf-life, and drying speed. The film-forming coating composition may comprise additives selected from wetting agents, dispersants, emulsifiers, defoamers, anti-setting agents, anti-skinning agents, anti-freezing agents, preservatives, curing agents, levelling agents, matting agents, antisag agents, film-forming aids, heat stabilisers, scratch-resistant agents, hydrophobic agents, thixotropic agents, and combinations thereof. In some embodiments the film-forming coating composition is a paint. The film-forming coating composition may be a sprayable film-forming coating composition, for example a spray paint. The film-forming coating composition may be applied to a surface by spray coating, painting, spreading, casting, flood coating, roll coating, spin coating, immersion coating, or drop coating. For example, in some embodiments the film-forming coating composition is applied to the surface of a greenhouse panel or polytunnel by spray coating. In some embodiments the paint may be a biodegradable paint. In some embodiments, the paint may be a bioderived paint. A bioderived paint comprises either wholly or substantially bioderived materials. The lanthanide complex may be present in the film-forming coating composition in an amount of from about 0.1 wt% to about 15 wt%, optionally from about 0.5 wt% to about 12 wt%, optionally from about 1 wt% to about 10 wt%, optionally from about 2 wt% to about 8 wt%, optionally from about 3 wt% to about 7 wt%, optionally from about 4 wt% to about 6 wt%, optionally about 5 wt%, based on the solids content of the film-forming coating composition (i.e., the total weight of solids, excluding any solvent or water). The lanthanide complex may be present in the film formed from application of the filmforming coating composition in an amount of from about 0.1 wt% to about 15 wt%, optionally from about 0.5 wt% to about 12 wt%, optionally from about 1 wt% to about 10 wt%, optionally from about 2 wt% to about 8 wt%, optionally from about 3 wt% to about 7 wt%, optionally from about 4 wt% to about 6 wt%, optionally about 5 wt%, based on the solids content of the film (i.e., the total weight of solids, excluding any solvent or water). The PLQY of the film-forming coating composition is typically lower than that of the film formed from the film-forming coating composition. The PLQY of the lanthanide complex is typically lower in solution than when solid. The PLQY of the films formed from the filmforming coating composition were found to be sufficient for use as a downconverting material. The PLQY of the film-forming coating composition and film formed from the film-forming coating composition is affected by the materials in which the lanthanide complex is dispersed. In a preferred embodiment of the second aspect, the film-forming coating composition is an acrylic paint. In a preferred embodiment of the second aspect, there is provided a film-forming coating composition comprising a liquid and a lanthanide complex dissolved or dispersed in the liquid, the film-forming coating composition comprising a lanthanide complex, wherein the lanthanide complex is defined by K+[Ln(L1)4]', wherein Ln is any lanthanide, and wherein L1, is dibenzoylmethane and derivates thereof, and wherein the film-forming coating composition comprises a binder, wherein the binder is an acrylic resin. In a preferred embodiment of the second aspect, there is provided a film-forming coating composition comprising a liquid and a lanthanide complex dissolved or dispersed in the liquid, the film-forming coating composition comprising a lanthanide complex, wherein the lanthanide complex is defined by K+[Eu(L1)4]', wherein L1, is dibenzoylmethane and derivates thereof, and wherein the film-forming coating composition comprises a binder, wherein the binder is an acrylic resin. In a preferred embodiment of the second aspect, there is provided a film-forming coating composition comprising a liquid and a lanthanide complex dissolved or dispersed in the liquid, the film-forming coating composition comprising a lanthanide complex, wherein the lanthanide complex is defined by K+[Eu(L1)4]', wherein L1, is avobenzone, and wherein the film-forming coating composition comprises a binder, wherein the binder is an acrylic resin. Lanthanide Complex In a third aspect, there is provided a lanthanide complex, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]-u, wherein A is a cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof, wherein at least one of L1 to L4 is avobenzone, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3. The preferred and optional feature, and combinations thereof for the lanthanide complex of the composite and the lanthanide complex of the film-forming coating composition apply equally to the lanthanide complex of the third aspect. The inventors found that lanthanide complexes comprising at least one avobenzone ligand have particularly good spectral converting properties. Avobenzone has the ability to absorb radiation over a wide range of wavelengths, particularly high energy solar radiation. Avobenzone is able to absorb significant amounts of radiation with wavelengths in the range of 200 nm to 400 nm and in particular at about 260 nm and about 357 nm. Due to its wide use as a UV absorber in sunscreen, avobenzone is an abundant and affordable ligand. Complexes formed with avobenzone may therefore be produced in a cost-effective manner. In a preferred embodiment of the third aspect, the lanthanide complex is defined by K+[Ln(L1)4]‘, wherein Ln is any lanthanide, wherein L1 is avobenzone. In another preferred embodiment of the third aspect, the lanthanide complex is K+tEutUM-, wherein L1 is avobenzone. Products, Devices, and Uses Agricultural sheet There is also provided an agricultural sheet comprising the composite according to the first aspect, a film formed from the film-forming coating composition according to the second aspect, or the lanthanide complex according to the third aspect. The agricultural sheet may be a polytunnel sheeting, a windowpane, a greenhouse panel, or a film for application to a windowpane, polytunnel sheeting, or greenhouse panel. The agricultural sheet may be or comprise plastic or glass. Plants that are grown in polytunnels or greenhouses may benefit from the increased amounts of light in the wavelength range that is useful for photosynthesis when the lanthanide complexes are implemented in the greenhouse panels or in polytunnels films. Accordingly, the growth of plants may be improved when grown inside of polytunnels or greenhouses according to the invention. The agricultural sheet may be or may comprise the composite. The agricultural sheet may comprise the composite disposed on another layer of material, wherein the other material may comprise a glass or a plastic. The composite may be sandwiched between two layers of other material, wherein the other materials may comprise a glass and / or plastic. The agricultural sheet may comprise a film formed from the film-forming coating composition. The agricultural sheet may comprise a first major surface and a second opposing major surface, wherein the film is present on at least one major surface of the agricultural sheet. Windowpane There is also provided a windowpane comprising a first major surface and a second major surface; and the composite according to the first aspect, a film formed from the film-forming coating composition according to the second aspect, or the lanthanide complex of according to the third aspect, disposed on at least one major surface. Photovoltaic device There is also provided a photovoltaic (PV) device comprising a coating disposed thereon, wherein the coating comprises the composite according to the first aspect, a film formed from the film-forming coating composition according to the second aspect, or the lanthanide complex according to the third aspect. Use of the Lanthanide Complexes in Agriculture There is also provided a use of a lanthanide complex as a wavelength shifter in an agricultural composite material, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, wherein A is a cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3. The preferred and optional features of the lanthanide complex of the first aspect, the second aspect, and the third aspect, and combinations thereof, apply equally to the use of the lanthanide complex as a wavelength shifter in an agricultural composite material. The agricultural composite material may be or may comprise the composite material of the first aspect, the film formed from the film-forming coating composition of the second aspect, the lanthanide complex of the third aspect, the agricultural sheet of the seventh aspect, or the windowpane of the eight aspect. A schematic of how the lanthanide complex may be implemented as a wavelength shifter in an agricultural composite material is shown in Figure 1. An agricultural composite material 100 is placed between the sun 102 and a plant 112. The agricultural composite material 100 comprises lanthanide complexes 108 dispersed in a film, panel, or sheeting 106. For example, the agricultural composite material 100 may be a greenhouse panel, greenhouse window, or polytunnel sheeting comprising the composite comprising a polymer 106 with lanthanide complexes 108 dispersed therein. Incident solar radiation has a wavelength spectrum 104. Some of the incident light passes through agricultural composite material 100 unchanged. Some of the light is absorbed by the lanthanide complexes 108 and emitted at longer wavelength 110. The proportion of useful wavelength radiation for photosynthesis is increased for the plant 112 by the lanthanide complexes 108 in the agricultural composite 100. The energy available for photosynthesis is therefore increased compared to a panel, window, or sheeting without lanthanide complexes 108. Figure 2 shows a schematic of another embodiment of the how the lanthanide complex may be implemented as a wavelength shifter in an agricultural composite material. An agricultural composite material 200 is placed between the sun 202 and a plant 212. The agricultural composite material 200 comprises a film 206 with lanthanide complexes 208 dispersed therein, wherein the film 206 is disposed on the surface of a panel, window, or sheeting 214. For example, the film 206 may be a film formed from the film-forming coating composition and the panel, window or sheeting 214 may be a greenhouse panel, a greenhouse window, or a polytunnel sheeting. Incident solar radiation has a wavelength spectrum 204. Some of the incident light passes through agricultural composite material 200 unchanged. Some of the light is absorbed by the lanthanide complexes 208 and emitted at longer wavelength 210. The proportion of useful wavelength radiation for photosynthesis is increased for the plant 212 by the lanthanide complexes 208 in the agricultural composite 200. The energy available for photosynthesis is therefore increased compared to a panel, window, or sheeting without lanthanide complexes 208. Use of the Lanthanide Complexes in Photovoltaic Devices There is also provided a use of a lanthanide complex as a wavelength shifter in a photovoltaic device coating, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, wherein A is a metal cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3. The preferred and optional features of the lanthanide complex of the first aspect, the second aspect, and the third aspect, and combinations thereof, apply equally to the use of the lanthanide complex as a wavelength shifter in a photovoltaic device coating. The photovoltaic device coating may comprise the composite material of the first aspect, the film formed from the film-forming coating composition of the second aspect, or the lanthanide complex of the third aspect. Preferably the photovoltaic device coating is placed on at least one surface of the photovoltaic device in order improve the effectiveness of the device. Examples Example 1: Production of composite comprising a lanthanide complex and a polymer by compounding extrusion: Composites were produced where the wt% of lanthanide complex and compounding extrusion conditions were varied, the details of which are outlined in table 1. Polyethylene (PE) was Revolve® N-250 polyethylene (rotational moulding polyethylene general purpose grade; Matrix Polymers, United Kingdom). K+[Eu(dbm)4]'was supplied as a relatively free-flowing powder with few visible hard agglomerates. To maximise dry mixing, this was further homogenised using an IKA A11 analytical homogenising mill before being mixed in various amounts with polyethylene and dry blended. 100 g of each of the resultant mixtures were then compound extruded on a Collins 16 mm twin screw extruder operating at a 60% of its maximum feeder speed. Each extruded strand was cut into pellets for further processing. Table 1. Processing conditions for the production of composite pellets with PE. Composite pellets sample Wt% K+[Eu(dbm)4]- Barrel temperatures (°C) Screw speed (rpm) Residence / dwell time in barrel (s) 1 1 170 500 30 2 3 170 500 30 3 5 170 500 30 4 3 250 500 30 The pellets produced with barrel temperatures of 170 °C (samples 1-3) were pastel yellow in colour; the pellets produced with barrel temperature of 250 °C (sample 4) were a darker yellow. This can be seen in Figure 3 which shows the pellets which were extruded at 170 °C (left) are lighter than the pellets which were extruded at 250 °C (right). Pellet samples 1, 3, and 4 were viewed under UV light to determine their luminescence. Samples 1 and 3 showed significant luminescence. The luminescence of sample 4 was less than that observed for samples 1 and 3. This can be seen in Figure 4 which shows photographs of the pellets of sample 1 (left), sample 2 (middle) and sample 4 (right), as visualised under UV light. The photographs of samples 1 and 2 in Figure 4 show bright red fluorescence. The photograph of sample 4 in Figure 4 shows a weaker fluorescence. Example 2: Formation of films from composite pellets: The pellets from Example 1 were used to form films of the composite. The samples were moulded on a Collins Platen Press under the conditions outlined in table 3 to achieve films with a thickness of approximately 250 pm. Table 2. Formation of films from the pellets in Example 1. Film sample Composite pellets sample Platen temperatures (°C) Screw speed (rpm) Residence / dwell time in barrel (s) 1 1 180 500 30 2 2 180 500 30 3 3 180 500 30 4 4 180 500 30 All film samples comprising PE and K+[Eu(dbm)4]' had good strength. Film samples 1-3 were pastel yellow in colour whilst film sample 4 (produced from composite pellets which were extruded at an elevated temperature 250 °C) was a darker yellow colour. This can be seen in Figure 5 which shows that a film produced from pellets which were extruded at 170 °C (left) is lighter than a film produced from pellets which were extruded at 250 °C (right). The film samples were viewed under UV light to determine their luminescence. Samples 1-3 showed significant luminescence, with samples 2 and 3 being visually brighter than sample 1. The luminescence of sample 4 was lower than that observed for samples 1-3. This can be seen in Figure 6 which shows images of the film samples (from left to right, samples 1 to 4) as visualised under UV light. The photograph of film sample 1 shows a bright red fluorescence from the film. The photographs of film samples 2 and 3 shows a very bright red fluorescence from the films. The photograph of film sample 4 in Figure 4 shows a weaker fluorescence. Example 3: Thermogravimetric analysis of KTEufdbm)^-: A sample of K+[Eu(dbm)4]' was subjected to thermogravimetric analysis (TGA) under an inert atmosphere to determine the thermal stability of the complex. The TGA data in Figure 7 shows the complex was stable at temperatures exceeding 350 °C. This is advantageous as polymers are often processed at elevated temperatures, for example temperatures of greater than 200 °C. Indeed, during compounding extrusion, elevated temperatures of greater than 200 °C may be required to melt the polymers. Thermal stability at a temperature of greater than 350 °C means that the complexes can be readily incorporated into a variety of polymers which require elevated temperatures during processing, whilst still retaining their spectral converting properties.

Claims

1. A composite comprising a polymer and a lanthanide complex,wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u,wherein A is a metal cation,wherein Ln is any lanthanide,wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane (dbm) and derivates thereof,wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4,wherein W+X+Y+Z = 4, andwherein u = 1, 2 or 3.

2. A film-forming coating composition comprising a liquid and a lanthanide complex dissolved or dispersed in the liquid,the film-forming coating composition comprising a lanthanide complex, whereinthe lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u,wherein A is a cation,wherein Ln is any lanthanide,wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof,wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4,wherein W+X+Y+Z = 4, andwherein u = 1, 2 or 3.

3. A lanthanide complex, wherein the lanthanide complex is defined byAu+[Ln(L1)w(L2)x(L3)Y(L4)z]-u,wherein A is a cation,wherein Ln is any lanthanide,wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof,wherein at least one of L1 to L4 is avobenzone,wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4,wherein W+X+Y+Z = 4, andwherein u = 1, 2 or 3.

4. Use of a lanthanide complex as a wavelength shifter in an agricultural composite material, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, wherein A is a cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3.

5. Use of a lanthanide complex as a wavelength shifter in a photovoltaic device coating, wherein the lanthanide complex is defined by Au+[Ln(L1)w(L2)x(L3)Y(L4)z]'u, wherein A is a metal cation, wherein Ln is any lanthanide, wherein L1, L2, L3, and L4 are ligands that are independently selected from dibenzoylmethane and derivates thereof, wherein W, X, Y, and Z are independently selected from 0, 1,2, 3, or 4, wherein W+X+Y+Z = 4, and wherein u = 1, 2 or 3.

6. The composite of claim 1, the film-forming coating composition of claim 2, the lanthanide complex of claim 3, or the use of any of claims 4 and 5, wherein the Ln is selected from the group consisting of Europium, Samarium, Terbium, Erbium, and Ytterbium, preferably wherein the Ln is Europium.

7. The film-forming coating composition of claim 2 or claim 6, the lanthanide complex of claim 3 or claim 6, or the use of any of claims 4-6, wherein A is a metal cation.

8. The composite of claim 1 or claim 6, the film-forming coating composition of claim 2 or claim 6, the lanthanide complex of claim 3 or claim 6, or the use of any of claims 4-6, wherein A is an alkali metal, preferably potassium, and wherein u = 1.

9. The composite of any of claims 1,6, and 8, the film-forming coating composition of any of claims 2 and 6-8, the lanthanide complex of any of claims 3 and 6-8, or the use of any of claims 4-8, wherein all the ligands are the same.

10. The composite of any of claims 1,6, and 8, the film-forming coating composition of any of claims 2 and 6-8, or the use of any of claims 4-8, wherein all the ligands L1, L2, L3, and L4 are dibenzoylmethane .

11. The composite of any of claims 1,6, and 8, the film-forming coating composition of any of claims 2 and 6-8, the lanthanide complex of any of claims 3 and 6-8, or the use of any of claims 4-8, wherein all the ligands L1, L2, L3, and L4 are avobenzone.

12. The composite of any of claims 1, 6, and 8-11, wherein the polymer is selected from the list comprising polyethylene, polypropylene, poly(ethyl acrylate), polymethylmethacrylate, polycarbonate, polyether ether ketone, ethylene-vinyl acetate, acrylate, polyvinylacetate, polyvinyl chloride and combinations thereof.

13. The composite of any of claims 1, 6, and 8-12, wherein the lanthanide complex is dispersed in the polymer matrix.

14. The composite of any of claims 1,6, and 8-13, wherein the composite is sandwiched between layers of another material, deposited on at least one surface of another material, or dispersed throughout another material.

15. The composite of any of claims 1, 6, and 8-14, the film-forming coating composition of any of claims 2 and 6-11, the agricultural composite material of any of claims 4, and 6-11, or the photovoltaic device coating of any of claims 5-11, wherein the lanthanide complex is present in an amount of from about 0.1 wt% to about 15 wt%, optionally from about 0.5 wt% to about 12 wt%, optionally from about 1 wt% to about 10 wt%, optionally from about 2 wt% to about 8 wt%, optionally from about 3 wt% to about 7 wt%, optionally from about 4 wt% to about 6 wt%, optionally about 5 wt%, based on the solids content of the composite, film-forming coating composition, agricultural composite material, or photovoltaic device coating.

16. The film-forming coating composition of any of claims 2, 6-11, and 15, wherein the film-forming coating composition comprises a binder, wherein the binder is or comprises a resin selected from an acrylic resin, alkyd resin, vinyl-acrylic resin, vinyl acetate / ethylene resin, polyurethane resin, polyester resin, melamine resin, epoxy resin, silane resin, siloxane resin, or combinations thereof.

17. The film-forming coating composition of any of claims 2, 6-11, 15, and 16, optionally wherein the film-forming coating composition is a paint, optionally wherein the liquid is selected from an aqueous liquid or an organic liquid.

18. A method of forming the composite of any of claims 1, 6, and 8-15, comprising:(i) compounding the lanthanide complex and the polymer to form a mixture; and(ii) extruding the mixture to form the composite.

19. The method of claim 18, wherein the lanthanide complex is provided in step (i) as a homogenised free-flowing powder.

20. The method according to any of claims 18 and 19, wherein the temperature during compounding and / or extrusion is of from about 100 °C to about 250 °C, optionally from about 120 °C to about 240 °C, optionally from about 130 °C to about 230 °C, optionally from about 140 °C to about 220 °C, optionally from about 150 °C to about 200 °C, optionally about 170 °C.

21. An agricultural sheet comprising the composite of any of claims 1, 6, and 8-15, a film formed from the film-forming coating composition of any of claims 2, 6-11, 15, and 16, or the lanthanide complex of any of claims 3, 6-9, and 11.

22. The agricultural sheet of claim 22, wherein the agricultural sheet is a polytunnel sheeting, a windowpane, a greenhouse panel, or a film for application to a windowpane, polytunnel sheeting, or greenhouse panel.

23. A windowpane comprising a first major surface and a second major surface; and the composite of any of claims 1,6, and 8-15, a film formed from the film-formingcoating composition of any of claims 2, 6-11, 15, and 16, or the lanthanide complex of any of claims 3, 6-9, and 11, disposed on at least one major surface.

24. A photovoltaic (PV) device comprising a coating disposed thereon, wherein the coating comprises the composite of any of claims 1, 6, and 8-15, a film formed from the film-forming coating composition of any of claims 2, 6-11, 15, and 16, or the lanthanide complex of any of claims 3, 6-9, and 11.

25. The use of the lanthanide complex of any of claims 1-4, 6-9, and 11 as a wavelength shifter in an agricultural composite material, wherein the agricultural composite material is or comprises the composite of any of claims 1, 6, and 8-15, or the film formed from the film-forming coating composition of any of claims 2, 6-11, 15, and 16, or the agricultural sheet according to any of claims 21 and 22, or the windowpane of claim 23.

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