NL2006211C2

NL2006211C2 - Luminescent solar concentrator and solar device comprising such luminescent solar concentrator.

Info

Publication number: NL2006211C2
Application number: NL2006211A
Authority: NL
Inventors: Erik Kolk; Diederik Ben Bart Lammerse
Original assignee: Univ Delft Tech
Priority date: 2011-02-16
Filing date: 2011-02-16
Publication date: 2012-08-20

Description

Luminescent solar concentrator and solar device comprising such luminescent solar concentrator

Field of the invention 5 The invention relates to a solar concentrator and to a solar device comprising such solar concentrator.

Background of the invention

The sun is an inexhaustible source of clean power. The major impediment to 10 widely deployed solar-power systems are the costs. Up to now, unsubsidized solar electricity is substantially more expensive than electricity derived from conventional energy sources. Hence, cost reductions are needed. Clean renewable electricity at affordable prices would be an attractive alternative to conventional electricity and the related fossil-fuel dependence, greenhouse-gas emissions and peak-time grid 15 constraints.

Solar cells (or photovoltaic cells “PV’s”) are an example of clean renewable energy. Solar cells transform sunlight into electricity by using a semiconductor device, typically made of silicone. Solar cells are packaged into solar panels, which can be installed on rooftops or large fields. The solar cells are typically some of the most 20 expensive parts of an installed solar panel.

Sometimes solar concentrators are used. Solar concentrators collect light over large areas and focus it onto smaller areas of solar cells. This increases the electrical power obtained from each solar cell. Solar concentrators can reduce the cost of solar power since more electricity is obtained per solar cell, and fewer solar cells are needed. 25 Conventional solar concentrators track the sun to generate high optical intensities, often by using large mobile mirrors that are expensive to deploy and maintain. Solar cells at the focal point of the mirrors must be cooled, and the entire assembly wastes space around the perimeter to avoid shadowing neighbouring concentrators.

A type of solar concentrators are luminescent solar concentrators (LSC). They 30 may for instance consist of a piece of transparent glass or plastic plate with a thin film of dye molecules deposited on the face and inorganic solar cells attached to the edges. Light is absorbed by the dye coating and reemitted into the glass or plastic for collection by the solar cells.

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An example of a photovoltaic cell with solar concentrator is for instance described in WO2010067296. In WO2010067296, a luminescent photovoltaic generator and a waveguide for use in such a photovoltaic generator is described. The photovoltaic generator comprises a photovoltaic cell and a waveguide comprising a 5 transparent matrix having particles of an inorganic luminescent material dispersed therein and/or an inorganic luminescent material disposed at at least one side thereof. The waveguide is associated with the photovoltaic cell, such that, in use, at least some of the light emitted from the luminescent material passes into the photovoltaic cell to generate a voltage in the cell. In preferred embodiments of W02010067296, the 10 inorganic luminescent material is a line emitter and the emission is due to a forbidden electronic transition within the material. The inorganic luminescent material may be selected from an inorganic phosphor, an inorganic fluorescent material and quantum dots, quantum rods and quantum core/shell systems. The photovoltaic generator is an alternative to or an improvement upon known photovoltaic generators, which generally 15 suffer from a lack of power yield per area.

Summary of the invention

The present invention, as further elucidated below, provides (1) a solar device comprising a solar concentrator (herein also indicated as “luminescent solar 20 concentrator”), wherein the solar concentrator comprises a waveguide comprising a transmissive viscous matrix containing an inorganic luminescent material, and (2) a photovoltaic device (PV) optically coupled to an outcoupling surface of the solar concentrator.

The viscous matrix may function as an optical coupler, such as an optical 25 coupling paste. The luminescent material may actually be immersed in a transparent matrix, such as an optical coupling paste, which may increase the optical coupling efficiency between the luminescent material and for instance a transmissive support or closure, such as glass plates. The light that is emitted by the luminescent material would not be able to enter such transmissive support or closure without this paste. This 30 viscous matrix, such as a paste may, may have an index of refraction matching the index of refraction of the luminescent material, and may therefore reduce considerable or even fully remove scattering of emitted light by the luminescent particles. This matrix, such as a paste, may also be convenient for easy processing like spin coating to 3 deposit the luminescent material comprising matrix on a support (waveguide) in the form of a thin layer.

Advantages of such solar device may for instance be that such a solar device may not suffer from loss of efficiency due to photo-bleaching. Also such a solar device may 5 employ inorganic luminescent materials with a large Stokes shift (such as at least 50 nm, or even at least 100 nm), which may result in reduction or even absence of reabsorption losses or self-absorption losses. Further, such a solar device may have a very low or even negligible waveguide parasitic absorption and surface losses.

It is surprising that by using a particulate luminescent material it is still possible 10 to concentrate a useful fraction of sunlight since at first thought one would think that the scattering of luminescent light by the luminescent material particles prevents efficient concentration (light guiding) of sunlight.

It surprisingly appears that (a) scattering can be reduced considerably by matching the index of refraction of the luminescent particles with that of the viscous 15 transmissive matrix (b), scattering can be reduced further by using particles with a cubic crystal structure symmetry, (c) scattering can also be reduced considerably by using luminescent particles that are considerable smaller than the wavelength of emission (like nanoparticles or quantum dots), and (d) that any loss due to remaining scattering can be effectively reduced by adopting suitable wavelength selective mirrors 20 on one or both sides of the solar concentrator.

As indicated above, a problem with prior art systems may be that it is currently not possible to generate electricity from sunlight in an economical way or a way that can be commercially exploited on a large scale without additional funding. The most important reason is that currently solar cells are too expensive, mainly because of the 25 use of ultra pure crystalline silicon. More precisely, the production cost versus the energy conversion efficiency is too high.

The problem does not seem to have been resolved till now, but much research and development is going on to enable large scale world wide application of solar energy by lowering its cost and increasing the efficiency of solar cells. For example, a 30 promising activity is the development of "thin film solar cells" that use typically 10-100 times less silicon per unit area. Also other more exotic solar cell materials like InGaAs or CuInGaSe2 are being developed as thin film solar cells. Large scale production is under way but the $/Watt ratio is still too high.

4

Other developments include organic materials, such as polymer ("plastic") solar cells and dye sensitized solar cells. Both have simple manufacturing process based on the use of cheap and widely available materials but efficiency and stability need to be improved.

5 Further developments include lenses or mirror systems to focus sunlight on a small surface area of the photovoltaic cell (solar concentration). In this way the same amount of sunlight can be converted to electricity by a much smaller surface area photovoltaic cell, thereby lowering cost. However these systems require complex and expensive solar tracking mechanisms to keep lenses or mirror directed to the sun.

10 Also luminescent solar concentrators (LSC) are suggested. LSC are under development in various forms. They have the promise of a cheap high efficiency solar device. The most developed LSC typically consists of a flat polymer plate, containing fluorescent dyes or particles like quantum dots, with one or more solar cells connected along the edges (see for instance also WO2010067296). The dyes or quantum dots 15 absorb the incoming sunlight and emit light at a slightly lower energy. Part of the emitted light is trapped inside the polymer plate and is guided to the solar cells at the side. The main components of such an LSC are the polymer plate, the fluorescent particles like dyes or quantum dots inside or as a layer at the surface of the plate and the attached solar cells.

20 The development of the LSC has recently received renewed interest. The increasing research effort has resulted in new record power conversion efficiencies ranging around 7%. Based on these numbers, people have already started the discussion on market development of the LSC. However, substantial power conversion efficiency is not the only important requirement for market introduction. Lifetime and cost are 25 two other aspects that can make or break the LSC. It was however shown that the LSC can be an interesting approach for future photovoltaics. Outdoor lifetimes of more than one year and indoor lifetimes of five years for LSC based on fluorescent dyes have been reported. Although the lifetimes are increasing, they are far too short for outdoor power generation applications, for which a minimum lifetime of 20 years might be 30 required. The limiting component with respect to the lifetime are the fluorescent dyes or quantum dots is that are not photo-stable or not stable in air, as photo oxidation reactions can take place. Besides that, also reactions with monomer residues and additives of the polymer matrix can occur. A serious efficiency limiting factor of the 5 use of quantum dots or dye molecules in luminescence solar concentrators may be the effect of re-absorption of emitted luminescence light by the quantum dots or dye molecules themselves. Re-absorption causes light scattering out of the waveguide and in addition an increase in luminescence quenching.

5 Hence, disadvantages of prior art solutions for solar devices and solar concentrators are that they are expensive, not economic, not stable, and a not optimised efficiency etc.

Therefore, it is an aspect of the invention to provide an alternative solar device and/or solar concentrator, which preferably further at least partly obviate one or more 10 of above-described drawbacks. As defined above, the invention provides a solar device comprising (1) a solar concentrator, wherein the solar concentrator comprises a waveguide comprising a transmissive viscous matrix (herein also indicated as “matrix”) containing an inorganic luminescent material (herein also indicated as “luminescent material”), and (2) a photovoltaic device (herein further also indicated as “PV”) 15 optically coupled to an outcoupling surface of the solar concentrator.

The invention also provides in a further aspect a solar concentrator per se, i.e. a solar concentrator which comprises a waveguide comprising a transmissive viscous matrix containing an inorganic luminescent material.

In a specific embodiment, the waveguide surrounds a cavity, wherein the cavity 20 contains the matrix with luminescent material. In a further embodiment, the solar concentrator comprises a sandwich structure of a fist waveguide, the transmissive viscous matrix containing the inorganic luminescent material, and the second waveguide.

Unlike other luminescent solar concentrator concepts this invention makes use of 25 a luminescent material that may not suffer from photo-degradation, and which is embedded in a viscous matrix.

The luminescent material may absorb a portion of the solar spectrum (ideally between a substantial part of the 300 and 1350 nm wavelength range) and emit light ideally at for instance around 1450 nm, (or the largest possible wavelength for which 30 the applied solar cells still have a high conversion efficiency). Since the matrix is transmissive it may thus also serve as a very good waveguide that transports the emitted light to the solar cell(s) (i.e. photovoltaic cell). Alternatively the luminescent material may absorb between 300 and 950 nm and emit light at for instance 1000 nm, 6 suitable for a Si PV. As will be clear to the person skilled in the art, other absorption and emission wavelengths than those given by example may be applied.

Optionally a wavelength selective mirror can be deposited on (one or) both sides of the solar concentrator that reflects the long wavelength luminescence light (such as 5 1450 nm or longer) but is transparent for the incoming sunlight (especially 300-1350 nm). Such wavelength selective mirror (also called hot-mirror, dichroic mirror or infrared cut-off mirror) may especially serve the purpose of increasing light trapping efficiency. Alternatively the wavelength selective mirror reflects light at around 1000 nm (such as 950-1050 nm) but may be transparent for incoming sunlight between 300 10 and 950 nm.

The solar cell(s) or PV(‘s) can be of any type and may for instance be coupled by an optical coupling paste to the solar concentrator, such as to part of the edge of a solar concentrator. Due to the waveguiding properties, luminescence light of the luminescent material may travel to an outcoupling surface of the solar concentrator and reach the 15 PV. The PV converts at least part of the luminescence light into electricity. Non-converted solar light might also be outcoupled to the PV and also be converted into electricity. Solar cells particularly adapted to the wavelength of the luminescence light may increase the performance of the solar device.

Such solar device may couple solar light in and transfer to the PV efficiently, 20 without too much loss of (converted) light and against reasonable costs. Further, such solar device, especially the waveguide, more precisely the viscous matrix containing an inorganic luminescent material, may be stable, in contrast to some polymer or quantum particle comprising luminescent solar concentrators. Other advantages may be a large Stokes shift, such as at least 50 nm, especially at least 100 nm, which may result in no 25 re-absorption losses or self-absorption losses, very low or even negligible waveguide parasitic absorption or surface losses. The term Stokes shift (of luminescent materials) is known in the art.

General 30 The solar device comprise two essential elements, the solar concentrator and the PV. The term “solar device” herein especially refers to the combination of solar concentrator and photovoltaic cell, wherein these are configured to concentrate (during use of the solar device) solar light in the solar concentrator and transport the 7 concentrated light and/or luminescence generated in the solar concentrator to the photovoltaic cell. The solar concentrator per se is also part of the invention.

The solar concentrator may have any shape, but will in general have a plate-like shape. Herein, the term “plate” is used to indicate that the waveguide has a length 5 and/or width (or diameter(s)) that are in general substantially larger than the height of the plate. The term plate does thus not necessarily imply “flat” (on a macroscopic scale; see also above).

Assuming a plate, the plate may have a first surface, an (opposite) second surface and an edge surface between the first surface and the second surface. To illustrate the 10 aspect of plate, the first surface and second surface are herein indicated as opposite surfaces. The solar concentrator may also have a tubular shape comprising a fiber, as for instance described in W02006088369 and W02006088370. In the former embodiment, the PV will optically be coupled to the edge; in the latter embodiment the PV will in general optically be coupled to an end face of the fiber (which is also an 15 edge).

The waveguide may comprise a cavity wherein the matrix with luminescent material is arranged. This cavity will in general be closed, in order to prevent air entering the cavity. Air or other gas bubbles in the matrix may reduce the efficiency of the waveguide, since undesired scattering may occur (see further also below). In an 20 embodiment, the waveguide has a sandwich structure (see also below).

The term in “optical contact” or “optically coupled” may in an embodiment imply that part of the surfaces of two items, such as of the solar concentrator (especially the outcoupling surface thereof) and of the PV make physical contact. In another embodiment, a transparent medium (or optical coupler) may be arranged between part 25 of the surfaces of the two items, such as a transparent waveguide or a transparent medium like an (immersion) oil, gel, grease, viscous fluid, gum, resin or silicone between the waveguide and PV. An example of a transparent silicone is the GE silicone viscasil 60M. An example of an oil may be a type A Cargill Immersion Oil. The transparent medium may be used to optically couple two items, such as the outcoupling 30 surface of the solar concentrator and the PV.

The terms “transparent” or “transmissive” herein may especially refer to material that has a light transmission in the range of 50-100 %, for light having a wavelength selected from the visible wavelength range and/or infrared (IR) wavelength range.

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Herein, the term “visible light” especially relates to light having a wavelength selected from the range of 380-780 nm and IR light especially refers to light having a wavelength selected from the range of larger than 780 and, in the context of this invention, especially selected from the range of larger than 780 and up to 2000 nm, 5 especially up to 1350 nm.

The transmission can be determined by providing a parallel beam of light at a specific wavelength with a first intensity to the waveguide under perpendicular radiation and relating the intensity of the light at that wavelength measured after transmission in the same/original direction through the material, to the first intensity of 10 the light provided at that specific wavelength to the material (see also E-208 and E-406 of the CRC Handbook of Chemistry and Physics, 69th edition, 1088-1989). Note that the waveguide plate may be colored or black, due to the presence of luminescent material (see also below).

Transparent is herein further distinguished from translucent, in the sense that 15 transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through a material; translucency (also called translucence or translucidity) only allows light to pass through diffusely. The opposite property of transparency is opacity or translucence. Transparent materials are clear, while translucent ones cannot be seen through clearly. In translucent materials, light might be 20 transmitted, but substantially only in a scattered way.

The matrix per se may be transparent. However, when luminescent material is contained by the matrix, the transparency may, also dependent upon the particle size and content of the luminescent material, decrease and translucency may increase. However, the matrix containing the luminescent material may still be transmissive 25 especially when the index of refraction of the matrix and the luminescent particles are sufficiently identical. The matrix containing the luminescent material may be even more transmissive when particles with a cubic crystal structure are used. For non-cubic systems, nanoscale particles are preferred, for best transmissivity. The fact that the matrix containing the luminescent material may still be transmissive allows the matrix 30 to have waveguiding properties and transport (part of the) luminescent material light to the PV.

Herein, the term “nanoscale” refers to particles having dimensions smaller than 1000 nm. The number averaged particle size may for instance be in the range of 0.5- 9 900 nm, such as 1-500 nm. Herein, the term “microscale” refers to particles having dimensions in smaller than 100 pm, such as equal to or larger than 1 pm and equal to or smaller than 100 pm. The number averaged particle size may for instance be in the range of 5-90 pm.

5 Especially, the transmission of the matrix containing the luminescent material for light travelling 0.1 cm through the matrix may be in the order of 40-100 %, especially 80-99.5 %, for at least part of the range of 350-1350 nm.

Wavelength selective mirror 10 The solar device may further comprise a wavelength selective mirror which is transmissive for at least part of the solar light, arranged upstream of a solar light incoupling surface of the solar concentrator, wherein the wavelength selective mirror is further reflective for at least part of the luminescence generated in the solar concentrator.

15 As indicated above, such mirror may also be known as hot-mirror, dichroic mirror or infra-red cut-off mirror. Examples of such mirrors and configuration are for instance described in W02006088369 and W02006088370. In an embodiment, the wavelength selective mirror may be a modulated photonic-crystal structured broadband reflector, which may have the advantage of very high reflection (nearly 100%) and a 20 reduced number of layers. Examples are described by Krc et al. in Appl. Phys. Lett. 94 153501-1 - 153501-3 (2009). In a specific embodiment, such modulated photonic-crystal structured broadband reflector comprises alternating layers of amorphous silicon and amorphous silicon nitride.

The terms “upstream” and “downstream” relate to an arrangement of items or 25 features relative to the propagation of the light from a light generating means (here the especially the sun), wherein relative to a first position within a beam of light from the light generating means, a second position in the beam of light closer to the light generating means is “upstream”, and a third position within the beam of light further away from the light generating means is “downstream”. For instance, assuming the 30 solar concentrator with a light incoupling surface for the sun and a light outcoupling surface to the PV, the light incoupling surface is upstream of the light outcoupling surface, and the latter is downstream of the former.

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By arranging such wavelength selective mirror upstream of the light incoupling surface of the solar concentrator, solar light might enter the solar concentrator, but luminescence may essentially not escape from the solar concentrator, unless (substantially) through the light outcoupling surface. For instance, the wavelength 5 selective mirror may be a (multi-)layer (see also above) on the light incoupling surface.

The solar device may have a first surface, which may be the light incoupling surface, and an opposite second surface. By way of example, the wavelength selective mirror may be upstream of the light incoupling surface, for instance as multi-layer on such surface. The opposite surface, the second surface, may comprise a mirror, 10 especially configured to reflect the (remaining) solar light and the converted light, thereby preventing outcoupling of the solar light and/or luminescence from the second surface.

Hence, in an embodiment, the solar device comprises in an embodiment a solar concentrator plate enclosing the transmissive viscous matrix containing the inorganic 15 luminescent material, the plate comprising a first surface, a second surface and an edge surface, wherein the wavelength selective mirror is arranged upstream of the first surface, and wherein the photovoltaic device is optically coupled to at least part of the edge surface.

In yet another embodiment (see also below), the solar concentrator comprises a 20 sandwich structure of a fist waveguide (first part), the transmissive viscous matrix containing the inorganic luminescent material and a second waveguide (second part). Here, the first waveguide comprises the first surface and the second waveguide comprises the second surface, and the edge of the sandwich is considered the edge surface. Hence, the invention also provides an embodiment of the solar device 25 comprising said solar concentrator, said solar concentrator comprising said sandwich structure of the first waveguide (first part), the transmissive viscous matrix containing the inorganic luminescent material and the second waveguide (second part), wherein the wavelength selective mirror is arranged upstream of the first surface (of the first waveguide), and wherein the photovoltaic device is optically coupled to at least part of 30 the edge surface (of the sandwich).

Optionally, a mirror may thus be configured downstream from the second surface, to reflect light back into the waveguide.

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Alternatively, it might be desirable to use both the first and second surface as incoupling surfaces. This may allow an even higher influx of the solar light, since two surfaces may be used to couple solar light in. This may also reduce or even prevent the need to use a solar concentrator that moves dependent upon the position of the sun.

5 Hence, the invention also provides an embodiment wherein the solar device comprises such solar concentrator plate (or sandwich structure), wherein the wavelength selective mirror is arranged upstream of the first surface, and further comprising a second wavelength selective mirror, arranged opposite of the first wavelength selective mirror (with the solar concentrator plate in between).

10 In this embodiment, the second wavelength selective mirror is especially transmissive for at least part of the solar light and reflective for at least part of the luminescence generated in the solar concentrator. Such second wavelength selective mirror may be of the same type of mirrors as indicated above, and may for instance be a modulated photonic-crystal structured broadband reflector.

15 The phrase “reflective for at least part of the luminescence generated in the solar concentrator” especially indicates that one or more wavelengths within the visible and/or IR are reflected by the mirror. As indicated above, such mirror may be transmissive for incoming solar light, for instance having one or more wavelengths in the range of for instance 300-1350 nm and may be reflective for luminescence that tries 20 to escape from the solar device.

Such wavelength selective mirror, which may for instance be disposed at at least part of the first surface and/or at at least part of the second surface, may thus be configured to allow transmission of light into the waveguide within the electromagnetic region that is absorbed by the luminescent material and selectively reflect light within 25 the electromagnetic region that is emitted from the luminescent material.

Solar concentrator

In an embodiment, the solar concentrator comprises a solar concentrator plate (see also above) comprising a first surface, a second surface and an edge surface, 30 wherein the cavity is surrounded by the first surface, the second surface and the edge surface. The cavity hosts the transmissive viscous matrix containing the inorganic luminescent material. The first surface, the second surface and the edge surface are external surfaces of the solar concentrator.

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In yet another embodiment, the solar concentrator comprises a sandwich structure of a fist waveguide (first part), the transmissive viscous matrix containing the inorganic luminescent material and a second waveguide (second part). The layer of transmissive viscous matrix containing the inorganic luminescent material may for instance be in the 5 range of 100 pm and 1 mm.

Here, the first waveguide comprises the first surface and the second waveguide comprises the second surface, and the edge of the sandwich is considered the edge surface. The solar concentrator (plate) may thus comprise two plates, configured in a sandwich structure with the viscous matrix material in between.

10 At least part of one of the first surface and the second surface is transmissive, preferably transparent. Further, at least part of the edge surface is transmissive, preferably transparent, and may have the function of outcoupling surface (to the PY).

The concentrator may for instance comprise a first part (as top part), a second part (as bottom part), and optionally a third part (as edge part), which enclose the 15 cavity. The first part and/or second part may function as waveguide; for instance, they may be waveguides.

The parts or waveguiding parts may comprise one or more materials selected from the group consisting of a transmissive organic material support, such as selected from the group consisting of PE (polyethylene), PP (polypropylene), PEN 20 (polyethylene napthalate), PC (polycarbonate), polymethylacrylate (PMA), polymethylmethacrylate (PMMA) (Plexiglas or Perspex), cellulose acetate butyrate (CAB), polycarbonate, polyvinylchloride (PVC), polyethyleneterephthalate (PET), (PETG) (glycol modified polyethyleneterephthalate), PDMS (polydimethylsiloxane), and COC (cyclo olefin copolymer). However, in another embodiment parts or 25 waveguiding parts may comprise an inorganic material. Preferred inorganic materials are selected from the group consisting of glasses, (fused) quartz, transmissive ceramic materials, and silicones. Especially, the parts may be of glass or of another transparent support like PMMC or PC, etc.

30 Luminescent material

High quality luminescent materials are especially but not necessarily chosen from cubic crystal system, such as YAG (Y3AI5O12) and Y2O3, or derivatives thereof (see also below). For non-cubic materials, for example AI2O3 and AIN, birefraction is in 13 general unavoidable when light transmits through a grain boundary. So, their in-line transmission may be very low (-15% for AI2O3).

Examples of suitable materials are for instance YAG; YSAG; YSAG; Y2O3, Sc203, Lu203, Hf02-Y203, Y203-Zr02, A203, Al203-Mg0 etc.

5 Therefore, in an embodiment, the waveguide comprises luminescent material having a cubic crystalline symmetry.

Suitable luminescent materials may for instance be doped with one or more of a transition metal ion, a lanthanide ion and a s2 ion (such as ions of Cu, Pb, Sb, Tl), such as especially one or more of Cr (chromium), Fe (iron), Ti (titanium), Ni (nickel), Mn 10 (manganese), Bi (bismuth), Pb (lead), Tl (tellurium), Sn (tin), Sb (antimony), Ce (cerium), Pr (praseodymium), Nd (neodymium), Sm (samarium), Eu (europium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium) and Yb (ytterbium). Especially, the dopant (ion) may be a lanthanide (ion), including in an embodiment a plurality of lanthanides. In another embodiment, the dopant (ion) may be 15 a transition metal (ion), including in an embodiment a plurality of transition metal. Herein, the term “doping” may in an embodiment also include “co-doping”, i.e. the doping of two or more dopants, like in YAG:Ce,Cr. For further information on luminescent materials and dopants is also referred to Blasse et al., Luminescent Materials, Springer Verlag 1994, especially chapters 1-5. The terms transition metal 20 ion, a lanthanide ion and a s2 ion, may in embodiments also refer to a plurality of transition metal ions, lanthanide ions and a s2 ions, respectively.

Especially, the luminescent material may comprise a garnet doped with one or more of the above indicated dopants, especially one or more of cerium, neodymium, ytterbium, and chromium. Garnets are for instance YAG (Y3AI5O12) or derivatives 25 thereof. Garnets may (thus) also include systems wherein one or more of Y, Al and O are at least partly replaced by other types of ions, such as Sc, Gd, or Tb for Y, Ga for Al and N or S for O, such as YSAG (yttrium scandium aluminium garnet), GGG (gadolinium gallium garnet), etc. Examples of garnets are for instance described in U.S. patent application Ser. No. 10/861,172 (US2005/0269582), U.S. patent application Ser. 30 No. 11/080,801 (US2006/0202105), W02006/097868, W02007/080555, US2007/0126017 and W02006/114726.), and especially in Messing et al., Science vol. 322 (2008), p 383-384, and Ikesue et al., Nature photonics, vol. 21 (2008), 721-727 (see also above), or W00108452 and W00108453, which are all herein incorporated 14 by reference. The dopant ions in general substitute only part of the cations in the crystalline luminescent material, such as for instance YAG:Ce3+, wherein Ce3+ replaces for instance 0.5-10 % of the Y3+ ions. However, the term dopant may in some instance also mean a total replacement. In general, however, the dopant concentration will be in 5 the range of 0.2-20 %.

When substituting ions with other ions that do not have the same charge, compensator ions may be applied. For instance, when substitution at least part of the O in YAG for N, at least part of the A1 may be substituted with Si, as known in the art.

In another embodiment, the luminescent material comprises a sesquioxide doped 10 with one or more of the above indicated dopants, especially one or more of cerium, neodymium, ytterbium, and chromium. Sesquioxide are for instance Y2O3, but also SC2O3, or LU2O3. Sesquioxides may (thus) also include systems wherein one or more of Y and optionally O are at least partly replaced by other types of ions, such as Sc, Gd or Lu for Y or optionally S. Further examples of suitable systems are also indicated in the 15 above cited documents.

Optionally, also materials like MgAhCL, or variants thereon in which Mg is at least partly replaced by Sr or Ba, may be applied. Such materials may for instance be doped with one or more lanthanides and/or one or more transition metal ions (dopants), such as indicated above.

20 Further, also materials like GOS (Gd202S) or variants thereon (for instance with

Gd at least partly replaced by one or more of Sc, Y, La, Gd, and Lu) may be applied. Such materials may for instance be doped with one or more lanthanides (dopants), such as indicated above, or alternatively or additionally with transition metal ions, or alternatively or additionally with or s2-ions.

25 In a further embodiment, the luminescent material comprises a nitride or oxynitride, like a silicium nitride or a silicium oxynitride, especially doped with Eu2+ (and optionally Mn2+) or Ce3+. Hence, in an embodiment, the luminescent material comprises one or more of a nitride material and an oxynitride material, doped with one or more of europium, manganese and cerium For instance, nitride luminescent 30 materials may be chosen from those described in US2003094893, US2002105269, W02009050171, W02005049763, W02009003988 and W02005103199. Especially, M2S15N8 doped with Eu2+ and/or LnShN5 doped with Ce3+, may be applied, where M represents one or more selected from the group consisting of Mg, Ca, Sr, and Ba, and 15

Ln represents one or more selected from the group consisting of La, Gd, Y and Lu. Especially, M comprises at least one or more of Ca, Sr and Ba (such as (Ca,Sr)2Si5N8:Eu2+. Further, especially Ln comprises at least La. Especially, M is > 95 mole % Ca; especially Ln is > 95 mole % La. Especially, Eu2+ doped IVLSisNs might be 5 applied.

Another material that may be applied is La2Hf207, or variants thereon (for instance wherein La is at least partly replaced by one or more of Sc, Y, Gd and Lu). Such materials may for instance be doped with one or more lanthanides (dopants), such as indicated above, or alternatively or additionally with transition metal ions, or 10 alternatively or additionally with or s2-ions.

Preferably, the luminescence light is at a substantial longer wavelength than the wavelength of absorption. This may prevent substantial self-absorption. Hence, in an embodiment, the luminescent material comprises luminescent material having a Stokes-shift that is preferably equal to or larger than FWHM (full width half maximum) of the 15 excitation line or band of the luminescent material, preferably at least 2 times or larger than the FWHM, even more especially equal to or larger than 5 times the FWHM. In an embodiment, the luminescent material has a Stokes-shift that is preferably equal to 50 nm, such as at least 100 nm. In another embodiment, the luminescent material comprises luminescent material having a Stokes-shift that is preferably equal to or 20 larger than FWHM (full width half maximum) of the emission line or band of the luminescent material. The luminescent material may comprise activators, such as Nd, Eu and Ce, etc., that absorb the solar light and emit the luminescence, but the luminescent material may also comprise a sensitizer (or sensitizer dopant), which absorb the solar light and transfer energy to activators. For instance, a suitable 25 sensitizer may be Ce (thus besides potentially being an activator), which may transfer at least part of the absorbed energy to for example Yb, which may luminescence at about 900-1100 nm. Another option may be the combination of Cr and Fe. Hence, in an embodiment, the luminescent material comprises inorganic luminescent material comprising a sensitizer dopant.

30 Most PV’s may absorb in at least part of the range of 300-1200 nm, especially 300-900 nm, but some may also absorb in the range of about 300-1450 nm. In a specific embodiment, the photovoltaic device is selected from the group consisting of Ge, GalnAs, CuInSe2, Si, GaAs, CdTe and GalnP PV’s. An advantage of a Ge PV is its 16 large absorption range. In an embodiment, the term PV may relate to a plurality of PV’s, and in another embodiment, the term PV may also relate to a plurality of PV’s with two or more different types, such as a GaAs PV and Ge PV.

The term “luminescent” material may also refer to a plurality of luminescent 5 materials. A plurality of luminescent materials may comprise luminescent materials comprising different activator contents, may comprise luminescent materials comprising different activators, may comprise chemically different materials comprising identical activators, may comprise (luminescent) materials with a sensitizer and (luminescent) materials without such sensitizer but with at least an activator, 10 luminescent materials which differ in the number of activators, etc. Also combinations of two or more of those options may be applied.

The luminescent material is provided as particulate material. The particulate material is embedded in the matrix. In a specific embodiment, the luminescent material particles have particle sizes in the range of 5 nm - 5 mm, especially 1 gm - 0.5 mm. 15 Therefore, in an embodiment, the invention provides a waveguide comprising a transmissive viscous matrix contains luminescent material particles having particle sizes in the range of 5 nm - 5 mm, especially 1 gm - 0.5 mm. The phrase “having particle sizes in the range of’ especially refers to the situation wherein 80 % or more, or preferably 90% or more of the particles have particles sizes in that range. The particle 20 size may be determined from SEM and/or TEM. For larger particle size, also laser light scattering measurements may be applied, such as with Malvern type apparatus.

In a specific embodiment, the transmissive viscous matrix contains luminescent material particles having particle sizes smaller than the wavelength of visible light. This may be beneficial for the transparency of the viscous matrix.

25 The invention may especially be applied as “conventional” luminescent material paste, such as used for high or low pressure luminescent lamps. A difference however may be that in conventional procedures, the paste is applied as coating and then dried/hardened/evaporated, whereas in the present invention, the matrix is contained in a cavity and may be kept there as viscous liquid. Further, the matrix is transmissive and 30 may thus have a lower particle content than such convention pastes.

Herein, the luminescent material is especially described with respect to inorganic materials comprising lanthanide ions, transition metal ions and s2 ions, especially as dopant. However, in a specific embodiment, the luminescent material comprises 17 quantum dots, such as cadmium selenide, cadmium sulphide, indium arsenide, and indium phosphide. Quantum dots also have dimensions smaller than the wavelength of visible light which lowers scattering, and their absorption edge may be tuned by tuning the particle size and the absorption of the quantum dots is very broad and covers the 5 whole range from the emission wavelength to higher energy. In this way, a broad solar light absorption spectrum may be created.

Herein, the luminescent material is especially indicated as inorganic luminescent material. The inorganic luminescent material is contained by the matrix as particles of inorganic luminescent material, such as YAG:Ce3+ particles and/or CdS particles.

10

The matrix

The matrix is indicated herein as viscous. The viscosity may for instance be comparable to the viscosity of motor oil (SAE 40) or higher, like the viscosity of honey, or of molten chocolate. In a preferred embodiment, the viscosity may be in the 15 range of about 0.1-106 mPa.s at 20 °C, preferably in the range of about 1-106, even more especially 10-106 , yet even more especially 102 -106 mPa.s, like in the order of 10-50000 mPa.s at 20 °C. These viscosities may be obtained at a shear rate of about 10 s-1 (at 20 °C).

The transmissive viscous matrix may comprise a transmissive viscous liquid, 20 such as for instance a transmissive silicone viscous liquid. The transmissive viscous matrix may also comprise a transmissive gel or a paste or an oil. Combinations of two or more different matrices may also be applied. The matrix itself is preferably transparent. When combined with the luminescent material, the matrix containing the luminescent material may be transparent or translucent. Therefore, the matrix is herein 25 indicated as transmissive.

The transmissive viscous matrix has an index of refraction and the luminescent material has an index of refraction. Preferably, within the wavelength range of the luminescence of the luminescent material the difference between the index of refraction of the transmissive viscous matrix and the index of refraction of the luminescent 30 material is within the range of ± 50%, especially within the range of ± 25%, even more especially within the range of ± 10%, yet even more within the range of ± 1% of the index of refraction of the luminescent material. In this way, loss by light scattering is minimized. The difference in indices of refraction may for instance refer to part or 18 preferably substantially the entire range of 350-1450 nm. However, the index of refraction matching is especially over the wavelength range of the luminescence of the luminescent material. Hence, assuming a luminescent material having an emission/luminescence in the range of 800-1000 nm an index of refraction of 1.8 over 5 this entire wavelength range, then the index of refraction of the viscous matrix is for instance preferably within the range of 1.62-1.98 over this 800-1000 nm range, assuming a tolerance of ± 10%. The better the match of index of refraction is, the lower scattering may be.

Examples of suitable matrices are GE silicone viscasil 60M and Cargill 10 Immersion Oil (also mentioned above). Especially, gel like matrices or viscous liquids such as viscasil are desired.

In an embodiment, the index of refraction of the viscous matrix is equal or smaller than the index of refraction of the waveguide (facilitating luminescence light to leave the matrix layer and enter the waveguide).

15

Production of solar concentrator

In a further aspect, the invention provides a method for the production of the solar concentrator. The method may involve (1) providing a mixture of a transmissive viscous matrix and a particulate luminescent material, and (2) enclosing the mixture in 20 a cavity of a body, wherein at least part of the body is transmissive for solar light and wherein at least part of the body is transmissive for luminescent material light. The body is at least partly transmissive and comprises a waveguide (for luminescent light of the luminescent material). In this way, a solar concentrator may be provided.

In a specific embodiment, the invention provides a method for the production of 25 the solar concentrator comprising (1) providing the mixture of the transmissive viscous matrix and the particulate inorganic luminescent material, (2a) applying the mixture to a first waveguide (“support”) to provide a layer of the transmissive viscous matrix containing the inorganic luminescent material, and (2b) arranging a second waveguide to the layer of the transmissive viscous matrix containing the inorganic luminescent 30 material. Here, the cavity is the space between the first and the second waveguide, which may substantially be occupied by the transmissive viscous matrix containing the inorganic luminescent material.

19

As indicated above, the term “luminescent material” may also refer to a plurality of luminescent material. In an embodiment, wherein a plurality of luminescent materials is applied, the luminescent materials may be mixed prior to mixing the mixed luminescent materials with the viscous matrix. In a further embodiment, the method 5 may further include applying a wavelength selective mirror to at least part of the body. The wavelength selective mirror may be applied before enclosing the mixture in the cavity of the body or after enclosing the mixture in the cavity of the body.

In yet a further embodiment, the body consists of parts (such as a first part comprising the first surface, a second part comprising the second surface and a third 10 part comprising the edge, wherein at least part of one or more of the first part, the second part and the third part are provided with a wavelength selective mirror. This may especially be executed prior to providing the mixture in the cavity of the body (formed of the parts). The parts may for instance be transmissive supports (waveguides).

15 In an embodiment, prior to enclosing the mixture in the cavity, the transmissive viscous matrix and particulate luminescent material are mixed. Alternatively, after enclosing the mixture in the cavity, the combination of viscous matrix and luminescent material may be mixed. Yet alternatively, before and after providing the mixture of viscous matrix and particulate luminescent material are mixed. In general, the 20 luminescent material and viscous matrix are at least mixed before providing to the cavity, but in principle, these materials may also be provided separately and then be mixed.

In a specific embodiment, prior to enclosing the mixture in the cavity, the mixture is subjected to (ultra-)centrifuge. This may provide a more dense mixture with less air 25 or other gas inclusions that may act as scattering centers. After providing the viscous matrix and particulate luminescent material to the cavity, the mixture in the cavity may be subjected to ultrasound, which may help deagglomertion of the luminescent material particles. The phrase “enclosing the mixture in a cavity of a body” will in general include providing a closed cavity with matrix and luminescent material. As will be 30 clear to the person skilled in the art, an open cavity with mixture of viscous matrix and luminescent material may, after optional ultrasound treatment, be closed.

The body thus provided may be the waveguide. For instance, a transparent plate with cavity may be provided, wherein the cavity is filled with a mixture of luminescent 20 material and viscous matrix. Then, for instance a mirror plate may be used to close the cavity.

Modular device 5 Outcoupling surfaces of solar concentrator may be connected with shared PV’s or with different PV’s or with a suitable transparent medium (for optical coupling) (like viscasil) or with a reflective element (like a mirror or a white powder). Hence, in an embodiment, the solar concentrator comprises a plurality of photovoltaic devices. Further, in an embodiment, the solar device comprises a plurality of solar concentrators 10 (arranged in a “module”).

In a specific embodiment, the solar device comprises a plurality of solar concentrators and a plurality of PV’s, wherein outcoupling surfaces of two or more adjacent solar concentrator are optically coupled to each other, wherein optionally outcoupling surfaces of two or more adjacent solar concentrator are separated by 15 reflectors, and wherein one or more outcoupling surfaces of solar concentrator are optically coupled to one or more PV’s.

In a specific embodiment, bi-facial PV’s are applied. Hence, in an embodiment, the solar device comprises such plurality solar concentrators and such plurality of PV’s, wherein a subset of two adjacent solar concentrators is optically coupled to a bi-facial 20 PV. As will be clear to the person skilled in the art, also a plurality of subsets of two adjacent solar concentrators may be optically coupled to a plurality of bi-facial PV’s, respectively.

At least three coupling types of the solar concentrators may be distinguished: (1) optical coupling with a PV, either a single (face) PV or a bifacial PV, (2) an optical 25 coupling with a transparent medium, like viscasil, and (3) optical coupling with a reflector (like a mirror or a white power).

As an example the module may comprise a plurality of waveguides or subsets of optically coupled waveguides that may be coupled with above mentioned coupling types 1, 2 and/or 3.

Solar concentrator

As indicated above, the invention is also directed to a solar concentrator per se. Hence, the above described embodiments, and the below described embodiments, do 30 21 not only relate to solar devices comprising such solar concentrator, but may also relate to the solar concentrator per se.

The present solar concentrator is an alternative to or an improvement upon known solar concentrators, which generally suffer from a lack of power yield per area, a too 5 high production cost per area and stability.

The term “substantially” herein, such as in “substantially all emission” or in “substantially consists”, will be understood by the person skilled in the art. The term “substantially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially may also be removed. Where 10 applicable, the term “substantially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. The term “comprise” includes also embodiments wherein the term “comprises” means “consists of’.

Furthermore, the terms first, second, third and the like in the description and in 15 the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

20 The devices herein are amongst others described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation or devices in operation.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many 25 alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such 30 elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in 22 mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Brief description of the drawings 5 Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Figure la schematically depicts an embodiment of the solar device;

Figures lb-Id schematically depict some aspects of the invention; 10 Figures 2a-2e schematically depict some aspects of the solar device; and

Figures 3a-3b schematically depict some further aspects of the invention.

Description of preferred embodiments

Figure la schematically depicts a solar device 1 comprising a solar concentrator 15 100, wherein the solar concentrator 100 comprises a 150 (“waveguide”) comprising a transmissive viscous matrix 140 containing an inorganic luminescent material 170 and a photovoltaic device 200 (“PV”), which may be optically coupled to an outcoupling surface 162 (of an edge part 150c).

Here, the cavity 18 is provided by a first part 150a (with first surface 151), a 20 second part 150b (with second surface 152) and a third part or edge part 150c (with edge surface 153). Actually, those parts are also waveguides or waveguide plates, like glass or other transparent plates. Hence, here the waveguide 150 constitutes of a first waveguide (first part 150a), the transmissive viscous matrix 140, and a second waveguide (second part 150b). The first and second part, or the first and second 25 waveguide sandwich the transmissive viscous matrix 140.

Especially the first and the second parts may also be indicated as supports or support parts. These parts will in general be waveguides (having waveguiding function for at least part of the luminescent material light), and are herein thus also sometimes indicated as first waveguide and second waveguide.

30 At the edge(s), edge part(s) 150c may be provided. Optionally, the edge part 150c is not present (see also below), or is partly not present, and is (partly replaced by the photovoltaic cell 200 (i.e. the PV functions as (part of the) edge surface 153). Embodiments without edge part 150c are for instance shown in figures lb-Id. When an 23 edge part 150c is applied, at least part of it is transmissive, for transport of concentrated luminescence light to the PV 200.

Solar light 2 is coupled into the waveguide 150 via an incoupling surface 161 and is at least partly absorbed by the luminescent material 170. At least part of the absorbed 5 sunlight is converted into luminescent material light 3. The waveguide 150 comprises matrix 140 containing particulate luminescent material 170. At least part of the luminescent material light 3 and part of the optionally remaining solar light in the waveguide 150 is optically coupled out from the outcoupling surface 162 as outcoupled light 4 and provided to the PV 200. In this way, electrical energy is provided 10 (symbolized by the lamp).

The waveguide 150 may be considered in embodiments as a body or container comprising a cavity 18, which contains the viscous matrix 140 containing the luminescent material 170 (in the form of particles). At least part of the body is transmissive for solar light, i.e. the incoupling surface 161 (here first surface 151), and 15 preferably at least part of the body is at least transmissive for luminescent material light, i.e. the outcoupling surface 162 (here edge 153).

Here, by way of example the waveguide 150 (comprising transmissive viscous matrix 140 containing inorganic luminescent material 170) comprises a planar plate with first surface 151 and opposite second surface 152 and edge surface 153. Here, the 20 PV 200 is optically coupled to at least part of the edge surface 153. At least part of the edge surface 153 may be used as outcoupling surface. Here, the waveguide 150 comprises a plate comprising first surface 151, second surface 152 and edge surface 153, and the cavity 18 is surrounded by the first surface 151, the second surface 152 and the edge surface 153.

25 In figure la, the cavity 18 is provided by a first part 150a (with first surface 151), a second part 150b (with second surface 152) and a third part or edge part 150c (with edge surface 153). As indicated above, the edge part 150c may not necessarily be provided. Note that the viscous matrix material will in general be applied as thin layer, for instance of 0.1 mm.

30 Further, optional mirrors 190 may be provided to assist keeping luminescent material light 3 in the concentrator 100. To this end, the first surface 151 (incoupling surface 161) may be provided with a wavelength selective mirror 191. The second surface 152 may be provided with a mirror 192, which is not necessarily wavelength 24 selective (dependent whether or not also light incoupling via the second surface 152 is desired).

It is preferred that the particles are index matched with the viscous matrix and preferably have a cubic crystalline symmetry and/or are nanoparticles since otherwise 5 light may scatter. With scattering no real waveguiding takes place and as a consequence light only reaches the solar cells after a random walk of the light, which is schematically shown in figure lb. In this case high quality wavelength selective mirrors are of high importance to still have a high efficiency LSC device.

Assuming scattering is minimal by taking the viscous matrix as defined above, 10 we can distinguish two configurations:

Configuration 1: The matrix 140 with luminescent particles 170 act themselves as the waveguide; this is schematically depicted by 3 in figure lc. This is the case when the index of refraction of the viscous matrix is larger than of body/parts surrounding the viscous matrix. The larger the difference of the index of refraction of the matrix and the 15 support parts (which can transparent glass or plastic) the more light is trapped in the matrix. Light that escapes from the matrix (escape cone) is not lost but is internally reflected in the support part and enters the matrix again, escapes from the matrix on the other side, is internally reflected in the support part on the other side, etc, etc. So for light that escapes from the matrix it is the matrix+support part(s) that has the main 20 waveguiding function as indicated on the left in fig lc.

Configuration 2: In another configuration the index of refraction of the viscous matrix is equal or smaller than the support parts like glass plates. In that case less or no light is trapped in the matrix (see figure Id). After emission, the light escapes from the matrix and enters the support part where it is internally reflected, enters the matrix 25 again, escapes from the matrix on the other side, is internally reflected in the support part on the other side, etc. , etc. So in this case the matrix+support parts acts as the waveguide 150.

It is expected that the matrix containing the luminescent particles has a layer that is much thinner that the supporting glass or plastic. Typically 10 times thinner or even 30 thinner like 100 times thinner.

Figs, lb-Id schematically depicts embodiments wherein the solar concentrator 100 comprises a sandwich structure, with first part or waveguide 150a (with first surface 151), and second part or waveguide 150b sandwiching the transmissive viscous 25 matrix 140. The space between the waveguides 150a, 150b is indicated as cavity 18. At least part of the edge 153 of this sandwich structured solar concentrator 100 may be used as outcoupling surface 162 and may optically be coupled to the PV 200.

Figure 2a schematically depicts an embodiment wherein the waveguide 150 5 comprises a sensitizer material 171 that may convert at least part of the solar light 2 coupled into the waveguide 150 to luminescent material light 3. Part of the luminescent material light 3 may be converted by another luminescent material 170 into other luminescent material light 3’. At least part of the luminescent material light (3’ and optionally remaining 3) may be coupled out as outcoupled light 4.

10 Figure 2b schematically depicts an embodiment wherein solar cell 200 surface 201 of the solar cell 200 makes contact with at least part of the outcoupling surface 162. This is a way to optically couple the waveguide outcoupling surface 162 and the solar cell 200.

Figure 2c schematically depicts an embodiment wherein a transparent medium or 15 optical coupler 300 is applied, arranged between the outcoupling surface 162 of the waveguide 150 and the solar cell surface 201. Such optical coupler 300 is a material being transmissive for at least part of the outcoupled light. For instance, this may be an optical coupler paste. Preferably, the outcoupling surface 162 and the optical coupler 300 are in physical contact with each other. Preferably, the optical coupler 300 and the 20 solar cell 200 (solar cell surface 201) are in physical contact with each other. Edge part 150c (see fig. la) may also be used as optical coupler 300.

Figure 2d schematically depicts a cross-section of an embodiment. Here, a plate like waveguide 150 is assumed, wherein both the first surface 151 and the second surface 152 are used as incoupling surfaces 161 and 162, respectively, for instance 25 dependent upon which surface is direct to the sun (during a day) or for instance to allow to utilize indirect sunlight by the waveguide 150. To couple in solar light into the waveguide but to increase trapping of converted light (luminescence) in the waveguide 150, a wavelength selective mirror may be applied. The wavelength selective mirror increases the light trapping efficiency by also reflecting that part of the luminescence 30 light inside of the escape cone of the waveguide. The wavelength selective mirror is indicated with reference 101. At the edge 153, PV 200 is arranged.

Figure 2e schematically depicts an embodiment wherein the solar device 1 comprises a plurality of solar concentrators 100 and a plurality of photovoltaic devices 26 200, wherein the PV’s 200 are optically coupled to the solar concentrators 100. By way of example, each solar concentrators 100 (more precisely the outcoupling surface thereof) is optically coupled to two PV’s.

Figure 3a schematically depicts a variant on the module of which another 5 embodiment has been schematically depicted in figure 2e. Figure 3a schematically depicts an embodiment of the solar device 1, here comprising a plurality of solar concentrators 100 and a plurality of PV’s 200. Some adjacent solar concentrators 100 are optically coupled to each other via optical coupler 300. Some of the solar concentrators 100 are optically coupled to PVs 200. Here, one or more subsets of two 10 adjacent solar concentrators 100 are optically coupled to at least one bi-facial PV (also indicated with reference 200). Reflectors 400 may separate subsets of solar concentrators 100. Hence, a subset of solar concentrators 100(1) and 100(2) can be optically coupled to each other via optical coupler 300; likewise is the subset of solar concentrators 100(3) and 100(4) can be optically coupled to each other. A subset of 15 solar concentrators 100(4) and 100(6) can be optically coupled to the bifacial PV(‘s) 200. Solar concentrators 100(3) and 100(5) are separated by reflector 400. For instance, this may be Teflon or BaSCL powder, etc. The two solar concentrators 100(3) and 100(5) may share such reflector 400. One could say that the subset of solar concentrators 100(3) and 100(5) are optically coupled to the reflector 400. Hence, fig. 20 3a schematically shows an embodiment of a luminescent solar concentrator plate (LSC plate). The LSC plate comprises a plurality of solar concentrators 100 and a plurality of PV’s 200.

Figure 3b schematically depicts how the waveguide 100 may be produced. First a body 5 with cavity 18 may be filled with the viscous matrix 140 containing the 25 luminescent material 170. After optional (further) removal of gas and de-agglomeration of particles, for instance via an ultrasonic treatment or (ultra) centrifuging, the cavity 18 may be closed. This may be with a plug, or as shown here, with a plate 180. In this way the waveguide 100 may be produced. In embodiment, the closure 180 may function as incoupling surface 151, but all kind of other arrangements may also be possible. In an 30 embodiment, the closure 180 may comprise or be a wavelength selective mirror. The photovoltaic cell 200 (not displayed) may be optically coupled to part of the outcoupling surface 153, which is by way of example indicated at the edge 153 on the right.

27

Instead of filling a cavity (with edges), also (plane) support may for instance be spin coated with the matrix containing the luminescent material, after which the coating is protected from the environment by a body or second support, and optionally other means to enclose the coating. In this way, a (sandwich-type) body may be created, i.e. a 5 waveguide, containing a cavity with the matrix containing the luminescent material, wherein the cavity is here the space between a first waveguide and a second waveguide.

The presence of a viscous liquid plays an important role. Without a viscous liquid air will be present between the luminescent material particles causing much stronger scattering losses. In addition, because of the larger refractive index difference between 10 waveguide-air in comparison with waveguide-viscous liquid, more light can be coupled into the waveguide from the luminescent material layer. The measured efficiency achieved with a particular LSC as shown in Fig. lc, is increased by a factor of 10 or more when a viscous liquid (as host of the luminescent particulate material) is used.

15

Claims

A sunlight apparatus comprising (1) a sunlight concentrator, wherein the sunlight concentrator comprises a waveguide comprising a transmissive viscous matrix containing an inorganic luminescent material and (2) a photovoltaic apparatus optically coupled to a coupling surface of the sunlight concentrator .

2. The sunlight apparatus of claim 1, wherein the transmissive viscous matrix has a refractive index and the luminescent material has a refractive index, and wherein within the wavelength range of the luminescence of the luminescent material the difference between the refractive index of the transmissive viscous matrix and the refractive index of the luminescent material is within the range of ± 5% of the refractive index of the luminescent material. 15

The sunlight apparatus of any one of the preceding claims, wherein the transmissive viscous matrix comprises a transmissive viscous liquid.

4. The sunlight apparatus according to any of the preceding claims, wherein the transmissive viscous matrix comprises a transmissive silicone.

The sunlight device according to any of the preceding claims, wherein the transmissive viscous matrix comprises a transmissive gel or paste.

The sunlight apparatus according to any of the preceding claims, wherein the transmissive viscous matrix comprises luminescent material particles which particles have a size in the range of 5 nm-5 mm, in particular 1 µm-0.5 mm.

7. The sunlight apparatus according to any of the preceding claims, wherein the trans-missive viscous matrix contains luminescent material particles which have a particle size that is smaller than the wavelength of the visible light.

The sunlight apparatus according to any of the preceding claims, wherein the luminescence of the luminescent material has a Stoke shift of at least the full width at half the maximum of the excitation line or band of the luminescent material. 5

The sunlight apparatus according to any of the preceding claims, wherein the luminescent material has a cubic crystal symmetry.

The sunlight apparatus according to any of the preceding claims, wherein the luminescent material comprises one or more of lanthanide ions, transition metal ions, and s2 ions.

11. The sunlight apparatus according to any one of the preceding claims, wherein the luminescent material comprises a garnet doped with one or more of cerium, neodymium, ytterbium and chromium.

The sunlight apparatus according to any of the preceding claims, wherein the luminescent material comprises a sesquioxide doped with one or more of cerium, ytterbium, neodymium and chromium. 20

The sunlight apparatus according to any of the preceding claims, wherein the luminescent material comprises one or more of a nitride material and an oxy-nitride material doped with one or more of europium, manganese and cerium.

The sunlight apparatus according to any of the preceding claims, wherein the luminescent material comprises a sentisizer baptism.

The sunlight apparatus according to any of the preceding claims, wherein the luminescent material comprises quantum dots. 30

The sunlight apparatus according to any of the preceding claims, wherein the luminescent material comprises nanoparticles.

The sunlight apparatus of any preceding claim, wherein the sun light concentrator comprises a sandwich structure of a first semiconductor, the transmissive viscous matrix containing the inorganic luminescent material, and a second waveguide. 5

18. The sunlight apparatus according to any one of the preceding claims, further comprising a wavelength selective mirror arranged upstream of a sunlight coupling surface of the sunlight concentrator, the wavelength selective mirror being transmissive for at least a portion of the sunlight and reflective for at least a portion of the luminescence that is generated in the sunlight concentrator.

The sunlight apparatus of claim 18, wherein the wavelength selective mirror is a modulated photonic crystal structured broadband reflector. 15

The sunlight apparatus according to any of the preceding claims, further comprising a second wavelength-selective mirror arranged opposite the first wavelength-dependent mirror with the sunlight concentrator in between.

The sunlight apparatus of any one of the preceding claims, wherein the transmissive viscous matrix has a viscosity in the range of 0.1-106 mPa.s at 20 ° C.

22. The sunlight apparatus according to any one of the preceding claims, comprising a plurality of sunlight concentrators.

The sunlight device according to any of the preceding claims, wherein the sunlight device comprises a plurality of photovoltaic devices.

The sunlight apparatus according to any of the preceding claims, comprising a plurality of sunlight concentrators and a plurality of PVs, wherein a subset of two adjacent sunlight concentrators are optically coupled to a bifacial PV.

The sunlight apparatus according to any of the preceding claims, comprising a plurality of sunlight concentrators, wherein two or more adjacent sunlight concentrators are optically coupled to each other by means of a transparent medium. 5

A sunlight concentrator as described in any one of the preceding claims 1-25.

A method of producing a sunlight concentrator, comprising (1) providing a mixture of a transmissive viscous matrix and a particulate inorganic luminescent material, (2) enclosing the mixture in a cavity of a body, wherein at least a portion of the body is transmissive to sunlight and wherein at least a portion of the body is transmissive to light of the luminescent material. 15

The method of claim 27, comprising (1) providing a mixture of the transmissive viscous matrix and the particulate inorganic luminescent material, (2a) and applying the mixture to a first waveguide about a layer of the transmissive viscous matrix containing the inorganic luminescent material, and (2b) arranging a second waveguide to this layer of the transmissive viscous matrix containing the inorganic luminescent material.

29. The method according to any of claims 27-28, wherein to enclose the mixture in the cavity, the transmissive viscous matrix and particulate luminescent material are mixed.

The method according to claim 28, wherein to enclose the mixture in the cavity, the mixture is subjected to (ultra) centrifuge. 30

The method of any one of claims 27-30, wherein the mixture in the cavity is subjected to ultrasound.