Abstract:
A solar element with increased efficiency and also a method for increasing the efficiency of a solar cell are provided. The solar cell comprises a luminescent element, an upconverter, and also at least one selectively reflecting structure.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a solar element with increased efficiency and also to a method for increasing the efficiency of a solar cell. 
     More than 20% of the energy contained in the solar spectrum is lost during conversion of the incident radiation by means of silicon solar cells as a result of so-called sub-band gap losses. There are herewith described losses which are produced because the energy of a single photon does not suffice to produce a freely moveable electron. They occur when the energy of a photon falls below the so-called band gap energy. Losses of this type occur in all types of solar cells. Their level is dependent upon the band gap energy of the solar cell material which is used. The described losses can be reduced in that a plurality of photons with too low energy are converted with so-called upconverting materials into one photon with sufficient energy. 
     The following problems hereby occur: firstly the absorption range of upconverters is very narrow. Upconversion is possible at all only for the few photons in this spectral range. 
     In order to make more photons usable for the upconversion, a substance with special luminescent properties can be used. This absorbs in the entire or in a part of the spectral range between band gap and absorption range of the upconverter and emits radiation at a wavelength which can be used by the upconverting material for upconversion. However, possible materials with corresponding luminescent properties also absorb in the spectral range in which the upconverters radiate the upconverted light. As a result, only a small part of the upconverted radiation reaches the solar cell. Without further measures, a large part of the upconverted radiation is therefore lost by undesired absorption. 
     A further problem resides in the fact that the efficiency of the conversion of two low energy photons into a higher energy photon is very low. The upconversion is a non-linear process since more than one photon is involved therein. The upconversion efficiency therefore increases, at least at low intensities, linearly with the flow density of the photons which can be used in principle by the upconverter (A. Shalav, B. S. Richards, T. Trupke et al., Appl. Phys. Lett. 86 (2005) 13505). The problem of very low upconversion efficiencies can therefore be reduced by concentration of the incident sunlight. In T. Trupke, M. A. Green, P. Würfel, Journal of Applied Physics, 92, 71 (2002), the possibility of concentration by means of a lens is mentioned. For experimental investigation of the upconversion effects, generally lasers with high intensity are used (in comparison with the sun spectrum in the corresponding spectral range). 
     The widening of the spectral range which can be used for the upconversion with fluorescent colourants is described briefly in C. Strümpel, M. McCann, C del Cañizo et al., Proceedings of the 20 th  EUPVSEC (2005), Barcelona. However no concrete conversions are proposed. 
     BRIEF SUMMARY OF THE INVENTION 
     It is firstly the object of the present invention to minimise the loss of the upconverted radiation by absorption. This object is achieved by the solar element and also by the method according to the present invention. Advantageous developments of the solar cell according to the invention and of the method according to the invention are provided. 
     The idea underlying the invention is to separate the upconverters from the luminescent elements spatially and to dispose, between the upconverters and the luminescent elements, first selectively reflecting and/or selectively transmitting layers or structures. Preferably, these selectively reflecting structures reflect electromagnetic radiation with energies which are higher than the band gap energy of the solar cell. Electromagnetic radiation, the frequency of which is between the smallest frequency which can be converted by the upconverter and the band gap frequency of the solar cell, should preferably be transmitted from the selectively reflecting structure. In this way, radiation with a frequency less than the band gap frequency of the solar cell impinges on the luminescent elements. These then emit radiation with frequencies which can be converted by the upconverter. This radiation passes through the selectively reflecting layer and impinges on the upconverter. The latter converts the incident radiation and emits radiation with frequencies greater than the band gap frequency of the solar cell. Since the selectively reflecting layer reflects radiation with frequencies greater than the band gap energy, the radiation emitted by the upconverter cannot pass back into the luminescent element and is therefore not absorbed there. 
     According to the invention, the solar element now has at least one solar cell which has a band gap frequency v BG . Furthermore, the solar element according to the invention has at least one luminescent element which absorbs electromagnetic radiation with frequencies between two frequencies v LE1  and v LE2  and emits electromagnetic radiation with at least a frequency v LE3 . The luminescent elements are preferably disposed such that radiation coming from a light source, such as for example the sun, which is not absorbed in the solar cell because of its low energy, radiates through the latter onto the luminescent elements. Solar cells and luminescent elements are therefore disposed in succession respectively in the direction of the beam path of incident radiation. 
     The solar element according to the invention has in addition at least one upconverter which absorbs electromagnetic radiation with frequencies between two frequencies v HK1  and v HK2  and emits radiation with at least a frequency v HK3 . This upconverter is disposed such that light which is emitted by the luminescent element impinges on the upconverter. 
     According to the invention, there is now disposed between the luminescent elements and the upconverters at least one first selectively reflecting layer which reflects a part of the incident electromagnetic radiation and transmits a part of this radiation. 
     The luminescent elements are now chosen such that the smallest frequency v LE1  absorbed by them is smaller than the band gap energy of the solar cell v BG . The radiation v LE3  emitted by the luminescent elements is according to the invention in the frequency range which can be absorbed by the upconverter, i.e. between v HK1  and v HK2 . The upconverter then emits radiation with a frequency v HK3  which is greater than or equal to the band gap frequency of the solar cell v BG . 
     If light from a light source therefore impinges on the solar cell, then the proportion of the light which has a frequency greater than the band gap frequency of the solar cell is absorbed by the solar cell, whilst that radiation with lower frequencies passes through the solar cell. This radiation passing through the solar cell now impinges on the luminescent elements which absorb a part of this radiation. The luminescent elements then radiate light and/or radiation with at least a specific frequency or in at least one preferably narrow, frequency range. This emitted radiation is normally emitted non-directionally. At least a part of this radiation now impinges on upconverters which convert the radiation into radiation with a higher frequency. Since the first selectively reflecting layers are disposed between the luminescent elements and the upconverters, the radiation to be upconverted can pass from the luminescent element into the upconverter, on the one hand, on the other hand however, the upconverted radiation cannot radiate into the luminescent element. The unconverted radiation is therefore radiated in the direction of the solar cell and then impinges at least partially on the solar cell. 
     It is therefore essential to the invention that the luminescent elements and the upconverters respectively are disposed adjacently separated by a first selectively reflecting structure. In addition, the upconverters are disposed adjacent to the solar cells so that the upconverted radiation can impinge on the solar cell. Preferably, the luminescent elements are likewise disposed adjacent to the solar cell in order that that radiation not absorbed by the solar cell can pass through the latter onto the luminescent elements. 
     It is preferred if the luminescent elements, wherever they are disposed not adjacent to the solar cell, the first reflecting layer, the upconverter or other elements, are surrounded by mirrors. These mirrors can cover partially or completely the corresponding surfaces of the luminescent elements. The lateral surfaces of the luminescent elements are preferably flat so that the mirrors which are likewise flat are disposed parallel to these lateral faces. It is achieved by these mirrors that the radiation of a suitable frequency is absorbed completely by the luminescent element and radiation emitted by the luminescent element passes completely into the upconverters. 
     As already described, it is advantageous if the first reflecting structure reflects electromagnetic radiation with frequencies which are greater than the band gap frequency of the solar cell v BG  and transmits electromagnetic radiation with frequencies between the smallest upconvertible radiation v HK1  and the band gap frequency v BG . At least the first selectively reflecting structure should transmit electromagnetic radiation with frequencies between v HK1  and v HK2 . Advantageously, as large a proportion of the light as possible with frequencies greater than v BG  should be reflected from the first selectively reflecting structure. 
     It is hereby particularly advantageous if the first selectively reflecting structures transmit electromagnetic radiation with frequencies which are between v HK2  and v BG . 
     Basically it is not necessary for the mentioned components of the solar element to abut directly against each other, i.e. in a form fit. However, in particular for simple production and a small constructional size, it is advantageous if at least some or all of the mentioned elements abut directly against each other, i.e. touch each other. Thus the at least one luminescent element can be disposed abutting directly against a first selectively reflecting structure and/or at least one first selectively reflecting structure can be disposed directly abutting against at least one upconverter and/or at least one upconverter can be disposed directly abutting against a solar cell. Between the solar cell and the luminescent elements, an air gap can be present, light- or radiation-permeable materials can be disposed or the solar cell and the luminescent elements can be disposed touching each other. 
     As already portrayed, the efficiency of the upconverters increases with the intensity of the radiated radiation. It is therefore advantageous for the present invention if the radiation can be concentrated onto the upconverter. According, to the invention, this can be achieved in that the sum of that part of the surface area of the upconverters which is orientated towards the solar cell plus the parts of the surface are of the luminescent elements which are orientated towards the solar cell but not covered by an upconverter is greater than the sum of the surface areas between luminescent elements and upconverters. It is particularly advantageous if the luminescent elements are surrounded by reflecting layers or structures. Preferably, at least one second selectively reflecting structure is disposed hereby on the sides of the luminescent element directly adjacent to the solar cell, which second selectively reflecting structure reflects a part of the radiation impinging thereon and transmits a part of the radiation impinging thereon. This second selectively reflecting structure is therefore disposed between the luminescent elements and the solar cell. On those sides of the luminescent elements which are orientated neither towards the solar cell nor towards an upconverting element, preferably mirrors are disposed, which reflect at least that light which has frequencies between the smallest frequency which can be absorbed by the luminescent element and/or the frequency emitted by the luminescent elements, according to which is smaller, and the band gap frequency of the solar cell. 
     Preferably, the second selectively reflecting structure reflects electromagnetic radiation with at least a frequency v LE3  which is emitted by the corresponding luminescent element. Electromagnetic radiation with frequencies between v LE1  and v LE2  is transmitted. Alternatively, also electromagnetic radiation with frequencies between v LE1  and v BG  can be transmitted and reflected above v BG , however that radiation with at least a frequency v LE3  which is emitted by the luminescent element is reflected. 
     It is now particularly preferred if the first reflecting layers completely cover the respective boundary regions between the luminescent elements and the correspondingly adjacent upconverters. This means that, of each luminescent element, that side orientated towards the corresponding upconverter is completely covered by the first selectively reflecting layer. In addition, it is particularly preferred if the second selectively reflecting layer completely covers those regions of the surface of the luminescent elements which are orientated towards the solar cell and directly adjacent to the solar cell. It is hereby of importance that those regions of the luminescent elements are covered by the second selectively reflecting layer, which regions abut directly against the solar cell via this second selectively reflecting layer, via intermediate layers which are completely transparent in the relevant frequency range or via air. Wherever the luminescent elements abut against a first selectively reflecting layer and/or an upconverter, no second selectively reflecting layer is disposed. 
     The object of concentrating the radiation onto the upconverters can now be achieved in that those regions of the luminescent elements which are orientated towards the incident radiation, i.e. towards the solar cell, have a larger surface area than those regions which are orientated towards the upconverters. In this way, the radiation impinges on a large surface area into the luminescent element and leaves the latter however in the direction of the upconverter only via a small surface area. All other surfaces of the luminescent elements as those orientated towards the upconverters are preferably delimited by mirrors or second selectively reflecting structures. In this way, a concentration of the light emitted by the luminescent element onto the upconverter is assisted. 
     Preferably, a mirror is disposed on each surface of the luminescent elements on which no selectively reflecting layer is disposed. 
     The solar element according to the invention can be produced in various ways. One possibility resides in disposing upconverters between a luminescent element and the solar cell. If the upconverter is disposed over the entire surface of the luminescent element, orientated towards the solar cell, between the solar cell and the luminescent element, then in fact no concentration effect is achieved but the problem of absorption of the converted radiation is resolved. The first selectively reflecting layer is hereby disposed between the upconverter and the luminescent element. A second selectively reflecting layer is not provided here. 
     The upconverters can however also cover only a part of that surface of the luminescent element orientated towards the solar cell. In this case, the first selectively reflecting layer is disposed between the upconverter and the luminescent element, whilst preferably a second selectively reflecting layer is applied wherever the luminescent element is adjacent to the solar cell without an upconverter being disposed therebetween. In this case, the above-described concentration effect is achieved. Basically, a concentration effect can be achieved if the sum of all those surface areas of the upconverters which are orientated towards the solar cell so that light emanating from them impinges on the solar cell plus the surface areas of the luminescent elements which are orientated towards the solar cell but not towards an upconverter is greater than the sum of those surface regions with which the luminescent elements are orientated towards the upconverters, where therefore the luminescent elements are covered by upconverters. 
     Another possibility for producing the solar element according to the invention resides in disposing one or more upconverters between luminescent elements or surrounded by luminescent elements. In this case, the upconverters abut with respectively one lateral surface against the solar cell, whilst they abut with the preferably vertical lateral surfaces, which are not parallel thereto, via respectively a first selectively reflecting layer against the luminescent elements. The luminescent elements are then preferably provided on the sides orientated towards the solar cell with second reflecting structures and are provided with mirrors on those sides which are orientated neither towards the solar cell nor towards an upconverter. 
     Again, a concentration can be achieved if those surface areas of the luminescent elements which are orientated towards the incident radiation, i.e. towards the solar cell, are in total greater than the surface areas abutting against the upconverters. 
     Both for the upconverters and for the luminescent elements, in particular cuboids, cubes, cylinders and similar normal geometric objects are suitable. Cuboid structures with lateral surfaces which are at right angles to each other are particularly suitable. 
     The upconverter preferably has or comprises sodium yttrium fluoride doped with erbium (NaYF 4 : Er 3+ ). 
     In addition, inorganic crystals are possible as upconverters (as bulk, micro- or nanocrystalline material) such as e.g. Al 2 O 3 , NaYF 4 , Er(PO 3 ) 3 , Lu(PO 3 ), Lu(PO 3 ) 3 , Y(PO 3 ) 3 , Gd(PO 3 ) 3 , doped with rare earths, such as e.g. erbium, ytterbium, dysprosium or transition metals, or co-doped, e.g. with erbium and ytterbium or transition metals and rare earths. 
     There can be used as colourants for the luminescent element cyanine, polymethine, Wurster- and Weitz-type radicals, polymethine violenes, monoquinone anions, tetrathiotetracenes, rare earth-doped crystals or organometallic compounds, transition metal compounds, quantum dots, e.g. made of PbSe, PbS (also in so-called core-shell configurations). 
     These are embedded in PMMA, polycarbonate, other highly transparent polymers, glass or glass ceramics. 
     The invention is intended to be explained subsequently with reference to some examples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       There are shown 
         FIG. 1  a first embodiment of the solar element according to the invention and 
         FIG. 2  a second embodiment of the solar element according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a possible embodiment of the present invention. A cuboid luminescent element  2  is hereby provided on five sides with mirrors  5 . A solar cell is disposed next to the sixth side of the luminescent element  2  which is not provided with mirrors  5  with a plane parallel to this side. Between the solar cell  1  and the side of the luminescent material  2  not provided with mirrors, upconverters  3   a ,  3   b  and  3   c  are disposed. These upconverters touch the solar cell  1  directly. Between the luminescent element  2  and the upconverters  3   a ,  3   b  and  3   c , respectively one first selectively reflecting layer  9   a ,  9   b  and  9   c  is disposed. 
     Section 22 shows the permeability of the first selectively reflecting layer  9   b . Radiation  6 , the frequency of which is above the band gap frequency v BG  of the solar cell  1 , is reflected from the selectively reflecting layer  9   b . Radiation  8 , which is absorbed by the luminescent material, i.e. has a frequency between v LE1  and v LE2 , is transmitted from the selectively reflecting structure  9   b . Likewise, radiation  7 , the frequency of which is in a range which can be converted by the upconverter, i.e. which is between v HK1  and v HK2 , likewise is transmitted. 
     In regions in which the side of the luminescent element  2  which is not provided with a mirror  5  does not abut against an upconverter  3   a ,  3   b  or  3   c , the surface of the luminescent element  2  is provided with second selectively reflecting structures  10   a ,  10   b ,  10   c  and  10   d . These structures  10   a  to  10   d  are applied in a form fit on the luminescent material  2 . Enlargement  21  shows how radiation is transmitted and/or reflected from this second selectively reflecting structure. Radiation  6  which has a frequency greater than the band gap frequency v BG  of the solar cell  1  is reflected from the selectively reflecting layer  10   a  to  10   d.    
     Radiation  7  is likewise reflected which has a frequency which can be converted by the upconverter  3   a ,  3   b  or  3   c . In contrast, radiation  8 , the frequency of which is between v LE1  and v BG , i.e. which can be absorbed by the luminescent material but cannot be converted by the upconverter, is transmitted from the selectively reflecting layer  10   a  to  10   d.    
     The solar cell is electrically connected via terminals  4   a  and  4   b.    
     If now radiation  6  which can be used by the solar cell impinges on the solar cell  1 , then this is absorbed. Radiation  8 , the frequency of which is below the band gap frequency v BG  of the solar cell  1  is transmitted from the solar cell  1  and impinges on the luminescent material  2 . The luminescent material  2  thereupon emits light  7  with a frequency which can be converted by the upconverter  3   a ,  3   b  or  3   c . If this radiation  7  impinges on the second selectively reflecting structure, then it is reflected back into the luminescent material  2 . If however it impinges on the first selectively reflecting structure  9   a ,  9   b  or  9   c , then it is transmitted into the upconverter  3   a ,  3   b  or  3   c . In this upconverter  3   a ,  3   b  or  3   c , it is then converted into radiation  6  with a frequency greater than v BG  which is then radiated into the solar cell  1 . The sum of the surface areas of the luminescent element, which are orientated towards the solar cell  1  but are not covered by an upconverter  3   a ,  3   b  or  3   c , and the surface areas of the upconverters  3   a ,  3   b  and  3   c  which are orientated towards the solar cell is greater than the surface area of the first selectively reflecting structures  9   a ,  9   b  and  9   c  together. As a result, the efficiency of the upconverters is increased. 
       FIG. 2  shows another embodiment of the present invention. Cuboid luminescent elements  2   a ,  2   b ,  2   c  and  2   d  and also cuboid upconverters  3   a ,  3   b  and  3   c  are disposed here surrounded by five mirrors  5  which are disposed one upon the other vertically. The upconverters  3   a  to  3   c  are hereby situated between the luminescent elements  2   a  to  2   d . The lateral surfaces of the upconverters  3   a  to  3   c  and of the luminescent elements  2   a  to  2   d  are parallel to each other and parallel to the mirrors  5  which are perpendicular to the solar cell. Respectively one upconverter  3   a ,  3   b  or  3   c  is disposed between respectively two luminescent elements  2   a ,  2   b ,  2   c  or  2   d . Parallel to the plane in which the luminescent elements and the upconverters are disposed adjacently, a solar cell  1  is disposed, which touches that side of the respective upconverter which is not in contact with a luminescent element or a mirror  5 . This solar cell  1  is again electrically connected by contacts  4   a ,  4   b.    
     If now radiation  6  impinges on the solar cell  1 , the frequency of which is higher than the band gap frequency v BG  of the solar cell  1 , then this is absorbed by the solar cell. However, radiation  8 , the frequency of which is below the band gap frequency v BG  of the solar cell  1  is transmitted from the latter and impinges on a luminescent element  2   a  to  2   d . The latter thereupon emits radiation  7  which passes through a first selectively reflecting layer  9   a  to  9   f  onto an upconverter  3   a  to  3   c . The latter converts the radiation  7  into radiation  6 , the frequency of which is above the band gap frequency of the solar cell and therefore can be converted by the latter into electrical energy. That side of the luminescent elements  2   a  to  2   d  which abuts neither against a mirror nor via a first selectively reflecting structure against an upconverter is coated with a second selectively reflecting layer  10   a ,  10   b ,  10   c  or  10   d.    
     The permeability of the first selectively reflecting layer is again shown in enlargement  22 . The permeability of the second selectively reflecting layer is shown in enlargement  21 . The functions of these selectively reflecting layers  9   a  and  10   a  correspond to those in  FIG. 1 .