NL2005711C2

NL2005711C2 - Luminescent solar concentrator and solar device comprising such luminescent solar concentrator.

Info

Publication number: NL2005711C2
Application number: NL2005711A
Authority: NL
Inventors: Erik Kolk
Original assignee: Univ Delft Tech
Priority date: 2010-11-18
Filing date: 2010-11-18
Publication date: 2012-05-22

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 silicon. 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.

2

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 WO2010067296, 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 a solar device comprising a solar concentrator, wherein the solar concentrator comprises a transparent 20 polycrystalline inorganic luminescent ceramic waveguide, and a photovoltaic device optically coupled to an outcoupling surface of the a transparent polycrystalline inorganic luminescent ceramic waveguide. Advantages of such solar device may for instance be that such a solar device does not suffer from loss of efficiency due to photo-bleaching. Also such a solar device may employ inorganic luminescent materials with a 25 large Stokes shift, which may result in reduction or even absence of re-absorption losses or self-absorption losses. Also the use of a transparent ceramics, with comparable optical quality as single crystals, may prevent light scattering out of the waveguide. Further, such a solar device based on a transparent poly crystalline inorganic luminescent ceramic may have a very low or even neglectable waveguide 30 parasitic absorption and surface losses. In addition, it has been known that crystalline materials, like a transparent polycrystalline inorganic luminescent ceramic, may possess a higher refractive index compared with glasses, glass ceramics or plastic materials. Therefore they may have a higher fraction of light trapped in the waveguide.

3

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 5 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 10 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.

Other developments include organic materials, such as polymer ("plastic") solar 15 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.

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 20 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.

Also luminescent solar concentrators (LSC) are suggested. LSC are under development in various forms. They have the promise of a cheap high efficiency solar 25 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 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 30 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.

4

The development of the LSC has recently shown 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 5 only important requirement for market introduction. Lifetime and cost are 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 10 generation applications, for which a minimum lifetime of 20 years might be 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 use of quantum 15 dots or dye molecules in luminescence solar concentrators may be the effect of reabsorption 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.

Hence, disadvantages of prior art solutions for solar devices and solar 20 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 of above-described drawbacks.

25 Unlike other luminescent solar concentrator concepts this invention makes use of a transparent luminescent ceramic (TLC) material that may be highly transparent for the emitted luminescence and may not suffer from photo-degradation.

A transparent luminescent ceramic is a transparent ceramic material that shows luminescence. Most ceramic materials are formed by compressing and heating fine 30 powders and are generally opaque / translucent as opposed to highly transparent single crystals that are highly transparent. However, when using nanoscale crystalline inorganic (luminescent) materials in general, or a microscale crystalline inorganic (luminescent) materials with a cubic crystal structure, the production of poly crystalline 5 substantially fully transparent (luminescent) ceramics is possible. These TLC have made it possible to develop for example cheap high power lasers that perform better than those made out of large single crystals that can only be obtained by expensive complex crystal growth techniques (see for instance Messing et al., Science vol. 322 5 (2008), p 383-384, and Ikesue et al., Nature photonics, vol. 21 (2008), 721-727).

Hence, for cubic materials, microscale particles may be used, although nanoscale particles may be advantageous. For non-cubic systems, nanoscale particles are preferred.

Herein, the term “nanoscale” refers to particles having dimensions smaller than 10 1000 nm. The number averaged particle size may for instance be in the range of 0.5- 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.

15 Such TLC now surprisingly appears to be very suitable to be used as a LSC. The TLC 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, (or the largest possible wavelength for which the applied solar cells still have a high conversion efficiency). Since the TLC is highly transparent it may thus also serve 20 as a very good waveguide that transports the emitted light to the solar cell(s) (i.e. photovoltaic cell). Alternatively the TLC may absorb between 300 and 950 nm and emit light at for instance 1000 nm, 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.

25 Optionally a wavelength selective mirror can be deposited on both sides of the LTC that reflects the long wavelength luminescence light (such as 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 infra-red cut-off mirror) may especially serve the purpose of increasing light trapping efficiency. Alternatively the 30 wavelength selective mirror reflects light at 1000 nm and longer but is transparent for incoming sunlight between 300 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 TLC, such as to at least one of the edges of a TLC

6 plate. 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.

5 Hence, this invention combines the luminescent solar concentrator (LSC) and transparent luminescent ceramic into a TLCSC (transparent luminescent ceramic solar concentrator).

Therefore, in a first aspect, the invention provides a solar device comprising (1) a solar concentrator, wherein the solar concentrator comprises a transparent 10 polycrystalline inorganic luminescent ceramic waveguide (herein further also indicated as “luminescent ceramic waveguide” “ceramic waveguide” or “waveguide”), and (2) a photovoltaic device (herein further also indicated as “PV”) optically coupled to an outcoupling surface of the luminescent ceramic waveguide.

Such solar device may couple solar light in and transfer to the PV efficiently, 15 without too much loss of (converted) light and against reasonable costs. Further, such solar device, especially the transparent polycrystalline inorganic luminescent ceramic waveguide may be stable, in contrast to some polymer or quantum particle comprising luminescent solar concentrators. Other advantages may be a large Stokes shift, which may result in no re-absorption losses and no self-absorption losses, no light scattering 20 out of the waveguide, very low or even neglectable waveguide parasitic absorption or surface losses.

A TLCSC based on for example YAG:Ce,Cr may for example have a photon-to-electron efficiency of 8% which is considerably higher than current state-of-the-art photon-to-electron efficiencies of LSC based on dye’s or quantum particles (see for 25 example US2009126778) that may be not larger than 2%.

General

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 30 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 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.

7

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 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; 5 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 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 10 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 edge).

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 waveguide and of the PV make 15 physical contact. In another embodiment, a transparent medium (or optical coupler) may be arranged between part of the surface of the two items, such as a transparent (second) 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 20 type A Cargille Immersion Oil. The transparent medium may be used to optically couple two items, such as the outcoupling surface of the waveguide and the PV.

The term “transparent” 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. Herein, the term 25 “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, especially up to 1350 nm.

The transmission can be determined by providing a parallel beam of light at a 30 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 the light provided at that specific wavelength to the material (see also E-208 and E-406 8 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 5 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. The fact that the waveguide is 10 transparent does not exclude that converted light might travel in all kind of directions.

However, non-converted light might travel through the waveguide substantially unscattered. In translucent ceramic materials, light might be transmitted, but substantially only in a scattered way.

Transparent ceramics are markedly different from glasses or glass-ceramic 15 materials in that glasses or glass-ceramic materials have an amorphous phase (fully amorphous for glasses and partly amorphous with one or more polycrystalline phases for glass-ceramics). Transparent ceramics can easily be distinguished from glass or glass ceramics using standard laboratory characterisation techniques like optical or electron spectroscopy techniques or even x-ray diffraction techniques. Also, the 20 presence of glass domains and (multiple) crystalline domains inside the amorphous glass, which is typical for glass ceramics, causes scattering of light, which lowers the efficiency of the waveguide because part of the luminescence light scattered in the direction of the escape cone and is lost. Further, because of the high transparency and the absence of light scattering, larger surface area waveguides can be made, which 25 means much less solar cell surface area needs to be used so that cheaper energy generation becomes possible. In addition, it has been known that crystalline materials possess higher refractive index compared with glasses or plastic materials. Therefore TLC have higher index of refraction n which causes the fraction of light trapped in the LSC to be bigger.

Wavelength selective mirror

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 30 9 incoupling surface of the transparent polycrystalline inorganic luminescent ceramic waveguide, wherein the wavelength selective mirror is further reflective for at least part of the luminescence generated in the luminescent ceramic waveguide. As indicated above, such mirror may also be known as hot-mirror, dichroic mirror or infra-red cut-5 off mirror. Examples of such mirrors and configuration are for instance described in W02006088369 and W02006088370.

The terms “upstream” and “downstream” relate to an arrangement of items or 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 10 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 waveguide plate 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 15 surface, and the latter is downstream of the former.

By arranging such wavelength selective mirror upstream of the light incoupling surface of the waveguide, solar light might enter the waveguide, but solar light and luminescence may essentially not escape from the solar device, unless through the light outcoupling surface. For instance, the wavelength selective mirror may be a (multi-20 )layer on the light incoupling surface.

Assuming a waveguide plate, there may be a first 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, especially configured to reflect the 25 (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 transparent polycrystalline inorganic luminescent ceramic waveguide plate, the plate comprising a first surface, a second surface and an edge surface, wherein the wavelength selective mirror is arranged 30 upstream of the first surface, and wherein the photovoltaic device is optically coupled to at least part of the edge surface. Optionally, a mirror may thus be configured downstream from the second surface, to reflect light back into the waveguide.

10

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 a transparent polycrystalline inorganic luminescent ceramic waveguide plate, 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, wherein the photovoltaic device is optically coupled to at least part of the edge surface, and further 10 comprising a second wavelength selective mirror, arranged opposite of the first wavelength selective mirror (with the transparent polycrystalline inorganic luminescent ceramic waveguide plate in between). 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 luminescent ceramic 15 waveguide. The phrase “reflective for at least part of the luminescence generated in the luminescent ceramic waveguide” 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 20 luminescence that tries 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.

Waveguide plate

Luminescent ceramic material are for instance described in U.S. patent application Ser. No. 10/861,172 (US2005/0269582), U.S. patent application Ser. No. 30 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).

11

An advantage of using the luminescent ceramic based solar concentrator is that the quantum efficiency (QE) of luminescence of rare earth ions and transition metal ions in glass ceramics is as a rule lower than in crystalline material. It is known that a glass doped with a luminescent inorganic ion, such that the ion is within an amorphous 5 environment, such ion generally does not show efficient luminescence.

Preferably, the index of refraction of the waveguide material is at least 1.8. This may further facilitate keeping luminescence light in the waveguide as according to Snell’s law for light refraction more light can be trapped in an optical waveguide when the index of refraction of the waveguide becomes higher. Luminescent glass 10 concentrators or polymer based luminescent solar concentrators may have an index of refraction close to 1.5, while the transparent poly crystalline inorganic luminescent ceramic waveguide may have an index of refraction higher than 1.8 (for instance Y2O3 (1.9), YAG (1.82)), or even higher than 2 (for instance ZrC>2/Y2Ο3 or ZnS). This causes the percentage of light trapped in the solar concentrator to increase from 75% (n=1.5) 15 to 92% (n=2.5) which is an increase of 22%. The above indicated indices of refraction are at 1000 nm.

The use of wavelength selective coatings (see also above) have the purpose to enhance the efficiency of the solar concentrator by reflecting the luminescence within the waveguide that otherwise would be lost out of the escape cone of the LSC. With the 20 high index of refraction of the polycrystalline inorganic luminescent ceramic waveguide the loss fraction from this cone is lower and therefore such coatings, that may complicate the solar concentrator and may make it more expansive, are no longer necessarily needed (but as indicated above, may of course be applied).

Further, the porosity of the transparent polycrystalline inorganic luminescent 25 ceramic waveguide may be less than 1 vol.% (see also below). Due to the low porosity, the waveguide plate can be transparent, instead of many state of the art ceramics that are translucent. As indicated in the Messing et al., Science vol. 322 (2008), p 383-384, and Ikesue et al., Nature photonics, vol. 21 (2008), 721-727 (see also above), the high transparency may especially be obtained by pressing and sintering nano particles of the 30 luminescent material (and optionally a filler material). Preferably, the filler material comprises the same material as the luminescent material, but without luminescent ions, such as Y3AI5O12 as filler and Y^AkO^Cc31 as luminescent material.

12

Herein, the transparent polycrystalline inorganic luminescent ceramic waveguide is essentially 100% polycrystalline fully dense ceramic, pore free, with optical properties substantially identical to that of a single crystal. Pore-free ceramics with grain sizes in the nanometer range may have very good optical, mechanical, electrical, 5 and other properties for use in lasers, health care, and electrical devices.

Transparent ceramics is markedly distinct from conventional translucent ceramics. On the macroscopic scale, there may be no double refraction or refractive-index fluctuation, indicating that the optical quality of transparent ceramics is very high. On the microscopic scale, substantially no residual pores, secondary phases or 10 optically inhomogeneous parts at grain boundaries may be observed.

In terms of cost and performance, the use of ceramics instead of single crystals enables mass production, which may lead to low-cost commercial production.

The transparent ceramics may be produced by forming a (mix of) nanopowder(s) (or for an inorganic crystalline material with cubic symmetry, optionally also a 15 micropowder of such inorganic crystalline material may be applied) of the desired composition into a specific preform shape. The preform structure is then fired in a vacuum at high temperature for many hours (sintering) resulting in the particles fusing together and pores to squeezed. Because the sintering process may leave a few trapped pores, the ceramic parts may be treated 0.5- to 4 hours, such as 1-2 hours in a hot (such 20 as above 1500 °C, like at about 1600 °C, or even higher) isostatic press at many MPa’s, such as at 100 Mpa.

Optical transparency in polycrystalline materials is limited by the amount of light which is scattered by their micro structural features. Primary scattering centres in polycrystalline materials include micro structural defects such as pores and grain 25 boundaries. The volume fraction of microscopic pores has to be less than 2 % (preferably better), such as less than 1 %, for high-quality optical transmission. Preferably, the material density should be 99.9% or more, or even 99.99 % or more of the theoretical crystalline density.

High quality transparent ceramics are especially chosen from cubic crystal 30 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 general unavoidable when light transmits through a grain boundary. So, their in-line transmission may be very low (~15% for AI2O3). A solution may be to arrange the grains in term of their 13 optical axes. Hence, in an alternative embodiment, the transparent ceramics waveguide may be produced from a non-cubic micro crystalline material of a powder thereof with micro dimensions, wherein the particles (grains) are ordered along their optical axes and then produced into a ceramic waveguide (J. Am. Ceram. Soc., 91 [10] 3431-3433 5 (2008)). In addition to pores, grain boundary act as scattering centres but when the size of the crystallites is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent. The size of the crystalline grains may largely be determined by the size of the crystalline particles present in the raw material during formation (or pressing) of the object.

10 Examples of suitable materials are for instance YAG; YSAG; YSAG; Y2O3,

Sc203, Lu203, Hf02-Y203, Y203-Zr02, A203, Al203-Mg0 etc.

Therefore, in an embodiment, the transparent polycrystalline inorganic luminescent ceramic waveguide comprises inorganic luminescent material have a cubic crystalline symmetry.

15 Suitable ceramic 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 (manganese), Bi (bismuth), Pb (lead), Tl (tellurium), Sn (tin), Sb (antimony), Ce (cerium), Pr (praseodymium), Nd (neodymium), Sm (samarium), Eu (europium), Tb 20 (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 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 25 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.

Especially, the transparent polycrystalline inorganic luminescent ceramic waveguide comprises a garnet doped with one or more of the above indicated dopants, 30 especially one or more of cerium, neodymium, ytterbium, and chromium. Garnets are for instance YAG (Y3AI5O12) or derivatives 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

14 (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. No. 11/080,801 (US2006/0202105), W02006/097868, W02007/080555, US2007/0126017 and 5 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 WOO 108452 and WOO 108453, which are all herein incorporated 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 % 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 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 15 in YAG for N, at least part of the Al may be substituted with Si, as known in the art.

In another embodiment, the transparent polycrystalline inorganic luminescent ceramic waveguide comprises a sesquioxide doped 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 20 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 above cited documents.

Optionally, also materials like MgA^CL, or variants thereon in which Mg is at least partly replaced by Sr or Ba, may be applied. Such materials may for instance be 25 doped with one or more lanthanides and/or one or more transition metal ions (dopants), such as indicated above.

Further, also materials like GOS (Gd202S) or variants thereon (for instance with Gd at least partly replaced by one or more of Sc, La, Gd, and Lu) may be applied. Such materials may for instance be doped with one or more lanthanides (dopants), such as 30 indicated above, or alternatively or additionally with transition metal ions, or alternatively or additionally with or s2-ions.

Another material that may be applied is La2Hf2C>7, or variants thereon (for instance wherein La is at least partly replaced by one or more of Sc, Y, Gd and Lu).

15

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.

Preferably, the luminescence light is at a substantial longer wavelength than the 5 wavelength of absorption. This may prevent substantial self-absorption. Hence, in an embodiment, the transparent polycrystalline inorganic luminescent ceramic waveguide comprises inorganic luminescent material having a Stokes-shift that is preferably equal to or larger than FWHM (full width half maximum) of the excitation line or band, preferably at least 2 times or larger than the FWHM, even more especially equal to or 10 larger than 5 times the FWHM.

The luminescent material may comprise activators, such as Nd and Ce, that absorb the solar light and emit the luminescence, but the luminescent material may also comprises a sensitizer (or sensitizer dopant), which absorb the solar light and transfer energy to activators. For instance, a suitable sensitizer may be Ce (thus besides 15 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 transparent polycrystalline inorganic luminescent ceramic waveguide comprises inorganic luminescent material comprising a sensitizer dopant.

20 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 large absorption range. In an embodiment, the term PV may relate to a plurality of 25 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.

Modular device

In an embodiment, the solar concentrator comprises a plurality of transparent 30 polycrystalline inorganic luminescent ceramic waveguides (arranged in a “module”). The outcoupling surfaces 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 16 solar concentrator comprises a plurality of photovoltaic devices. Further, in an embodiment, the solar device comprises a plurality of solar concentrators.

In a specific embodiment, the solar device comprises a plurality polycrystalline inorganic luminescent ceramic waveguides and a plurality of PV’s, wherein 5 outcoupling surfaces of two or more adjacent polycrystalline inorganic luminescent ceramic waveguides are optically coupled to each other, wherein optionally outcoupling surfaces of two or more adjacent polycrystalline inorganic luminescent ceramic waveguides are separated by reflectors, and wherein one or more outcoupling surfaces of polycrystalline inorganic luminescent ceramic waveguides are optically 10 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 polycrystalline inorganic luminescent ceramic waveguides and such plurality of PV’s, wherein a subset of two adjacent polycrystalline inorganic luminescent ceramic waveguides is optically coupled to a bi-15 facial PV. As will be clear to the person skilled in the art, also a plurality of subsets of two adjacent poly crystalline inorganic luminescent ceramic waveguides may be optically coupled to a plurality of bi-facial PV’s, respectively.

At least three coupling types of the waveguide may be distinguished: (1) optical coupling with a PV, either a single (face) PV or a bifacial PV, (2) an optical coupling 20 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.

25 Because of the high transparency and the substantially absence of light scattering due to grains or pores, larger surface area solar concentrators can be made, such as the above indicated module, which means higher solar concentration effect and cheaper energy generation.

30 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 17 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 and 5 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 18 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 1 schematically depicts an embodiment of the solar device;

Figures 2a-2e schematically depict some aspects of the solar device; 10 Figures 3a-3b schematically depict some aspects of luminescent ceramics; and

Figure 4 schematically depicts an embodiment of the solar device.

Description of preferred embodiments

Figure 1 schematically depicts a solar device 1 comprising a solar concentrator 15 100, wherein the solar concentrator 100 comprises a transparent polycrystalline inorganic luminescent ceramic waveguide 150 (“waveguide”) and a photovoltaic device 200 (“PV”) optically coupled to an outcoupling surface 162 of the luminescent ceramic waveguide 150.

Solar light 2 is coupled into the waveguide 150 via an incoupling surface 161 and 20 is at least partly absorbed by the waveguide 150. At least part of the absorbed sunlight is converted into luminescence 170 within the waveguide 150 into luminescent material light 3. The waveguide 150 comprises a ceramic of luminescent material 170 which functions as waveguide. In fact, the luminescent material is made into the waveguide, and the waveguide could be considered luminescent material. At least part of the 25 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 (symbolized by the lamp).

Here, by way of example the transparent polycrystalline inorganic luminescent 30 ceramic waveguide 150 comprises a planar plate with first surface 151 and opposite second surface 152 and edge 153. Here, the PV 200 is optically coupled to at least part of the edge 153. At least part of the edge 153 may be used as outcoupling surface.

19

Figure 2a schematically depicts an embodiment wherein the waveguide 150 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 5 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.

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 10 solar cell 200.

Figure 2c schematically depicts an embodiment wherein a transparent medium or 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 15 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 solar cell 200 (solar cell surface 201) are in physical contact with each other.

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 20 surface 152 are used as incoupling surfaces 161 and 162, respectively, for instance 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 25 increases the light trapping efficiency by also reflecting that part of the luminescence 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 poly crystalline inorganic luminescent ceramic waveguides 150 30 and a plurality of photovoltaic devices 200, wherein the PV’s 200 are optically coupled to the polycrystalline inorganic luminescent ceramic waveguides 150. By way of example, each poly crystalline inorganic luminescent ceramic waveguides 150 (more precisely the outcoupling surface thereof) is optically coupled to two PV’s.

20

Figures 3a and 3b schematically depict how a translucent polycrystalline inorganic luminescent ceramic may function as scatterer (and converter) of (solar) light 2 (fig. 2a), whereas a transparent polycrystalline inorganic luminescent ceramic waveguides 150 allows light to travel through the waveguide 150 without substantial 5 scattering (fig. 2b). Both may be transmissive, but the former may be translucent and the latter may be transparent. Reference 21 refers to particles and reference 22 refers to pores, which can be distinguished in the translucent polycrystalline inorganic luminescent ceramic, but which are substantially or even entirely absent in the transparent poly crystalline inorganic luminescent ceramic waveguide 150. Reference 10 20 indicates a ceramic body. All waveguides according to the invention are ceramic bodies.

Figure 4 schematically depicts a variant on the module of which another embodiment has been schematically depicted in figure 2e. Figure 4 schematically depicts an embodiment of the solar device 1, here comprising a plurality of 15 polycrystalline inorganic luminescent ceramic waveguides 150 and a plurality of PV’s 200. Some adjacent waveguides 150 are optically coupled to each other via optical coupler 300. Some of the waveguides 150 are optically coupled to PVs 200. Here, one or more subsets of two adjacent polycrystalline inorganic luminescent ceramic waveguides 150 are optically coupled to at least one bi-facial PV (also indicated with 20 reference 200). Reflectors 400 may separate subsets of solar concentrators 100. Hence, a subset of polycrystalline inorganic luminescent ceramic waveguides 150(1) and 150(2) can be optically coupled to each other via optical coupler 300; likewise is the subset of polycrystalline inorganic luminescent ceramic waveguides 150(3) and 150(4) can be optically coupled to each other. A subset of polycrystalline inorganic 25 luminescent ceramic waveguides 150(4) and 150(6) can be optically coupled to the bifacial PV(‘s) 200. Polycrystalline inorganic luminescent ceramic waveguides 150(3) and 150(5) are separated by reflector 400. For instance, this may be Teflon or BaSC>4 powder, etc. The two polycrystalline inorganic luminescent ceramic waveguides 150(3) and 150(5) may share such reflector 400. One could say that the subset of 30 polycrystalline inorganic luminescent ceramic waveguides 150(3) and 150(5) are optically coupled to the reflector 400.

21

Experimental

Good results for example were obtained by using a square plate LSC consisting of the TLC Lu2C>3:Eu3+. At 250 nm, a LSC quantum efficiency of 80% was measured for a geometric concentration factor of 50. This is very close to the theoretical 5 maximum of 85% that is expected for a perfect/ideal LSC with an index of refraction of n=1.9. This high efficiency combined with a high concentration factor demonstrates the advantages of using TLC as a LSC’s. Other LSC based on organic dye or quantum dots suffer from waveguide absorption losses, dye re-absorption losses, surface reflection losses, light scattering losses and photo bleaching losses. These losses appear to be very 10 low in a LSC based on TLC.

Claims

A solar cell comprising (1) a sunlight concentrator, the sunlight concentrator comprising a transparent polycrystalline inorganic luminescent ceramic waveguide, and (2) a photovoltaic cell optically coupled to a coupling surface of the transparent polycrystalline inorganic luminescent ceramic waveguide.

2. The solar cell according to claim 1, wherein the refractive index of the waveguide material is at least 1.8.

The solar cell according to any of the preceding claims, wherein the porosity of the transparent polycrystalline inorganic luminescent ceramic waveguide is less than 1 volume%. 15

4. The solar cell according to any of the preceding claims, wherein the transparent polycrystalline inorganic luminescent ceramic waveguide comprises an inorganic luminescent material that has a Stokes shift of at least the full width at the height of half the maximum of the absorption line or band.

5. The solar cell according to any of the preceding claims, wherein the transparent polycrystalline inorganic luminescent ceramic waveguide comprises an inorganic fluorescent material with a cubic crystalline symmetry.

The solar cell according to any of the preceding claims, wherein the transparent polycrystalline inorganic luminescent ceramic waveguide comprises one or more of lanthanide ions, transition metal ions, and s2 ions. 30

The solar cell according to any of the preceding claims, wherein the transparent polycrystalline inorganic luminescent ceramic waveguide comprises a garnet doped with one or more cerium, neodymium, ytterbium and chromium.

8. The solar cell according to any of the preceding claims, wherein the transparent polycrystalline inorganic luminescent ceramic waveguide comprises a sesquioxide doped with one or more of cerium, ytterbium, neodymium and chromium.

9. The solar cell according to any of the preceding claims, wherein the transparent polycrystalline inorganic luminescent ceramic waveguide comprises an inorganic luminescent material which comprises a sensitizer baptismal material.

10. The solar cell according to any of the preceding claims, further comprising a wavelength selective mirror that is transmissive to at least a portion of the sunlight arranged upstream of a sunlight-engaging surface of the transparent polycrystalline inorganic luminescent ceramic waveguide, the wavelength selective mirror is furthermore reflective for at least a portion of the luminescence generated in the luminescent ceramic waveguide.

The solar cell according to claim 10, comprising a transparent polycrystalline inorganic luminescent ceramic waveguide plate, 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 cell is optically coupled is on at least a portion of the edge surface.

12. The solar cell according to claim 11, further comprising a second wavelength-selective mirror arranged opposite the first wavelength-selective mirror with the transparent polycrystalline inorganic luminescent ceramic waveguide plate in between.

The solar cell according to any of the preceding claims, wherein the solar concentrator comprises a plurality of transparent polycrystalline inorganic luminescent ceramic waveguides.

The solar cell according to any of the preceding claims, comprising a plurality of solar concentrators.

The solar cell according to any of the preceding claims, wherein the solar concentrator comprises a plurality of photovoltaic cells. 10

16. The solar cell according to any one of the preceding claims, comprising a plurality of polycrystalline inorganic luminescent ceramic waveguides and a plurality of photovoltaic cells, wherein a subset of two adjacent polycrystalline inorganic luminescent ceramic waveguides are optically coupled to a bi-facial photovoltaic cell.

17. The solar cell according to any of the preceding claims, comprising a plurality of polycrystalline inorganic luminescent ceramic waveguides, wherein two or more adjacent polycrystalline inorganic luminescent ceramic waveguides are optically coupled to each other by means of a transparent medium.

A solar concentrator as described in any one of claims 1-17.