GB2474292A

GB2474292A - Planar arrangement of solar cell elements with luminescent concentrator elements

Info

Publication number: GB2474292A
Application number: GB0917755A
Authority: GB
Inventors: Tomas Markvart; Pattareeya Kittidachachan; Lefteris Danos
Original assignee: University of Southampton
Current assignee: University of Southampton
Priority date: 2009-10-09
Filing date: 2009-10-09
Publication date: 2011-04-13
Also published as: WO2011042708A3; WO2011042708A2; GB0917755D0

Abstract

A solar cell assembly has one or more solar cell elements 21, 31 and light collector elements 15, 17, 33, the collector arranged adjacent at least one edge of the solar cell element. The solar cell adjacent edge may be an edge that does not form a plane surface perpendicular to a growth direction of the cell element. The adjacent edge may also or instead be a relatively smaller side edge of a substantially planar element. The light collector is preferably a luminescent light concentrator absorbing and re-emitting light. A mirror 27 may be provided on a rear of the assembly, and a photonic mirror 25, reflecting re-emitted light from the collector, may be provided on a light collecting front of the assembly. The assembly may comprise multiple layers of alternating solar cell elements 31a,31b and collector elements 33a,33b,33c, the layers staggered so a cell in one layer overlaps a collector in the other layer. The elements may for a 2-D pattern of lines or a chequered pattern. A further alternative arrangement (see fig 5) has recesses (45) formed in the light receiving surface of a solar cell into which one or more light collector elements (46) may be placed.

Description

Solar Cell Assembly The present invention is concerned with solar cells. More specifically, the present invention is concerned with solar cell assemblies which comprise a solar cell with a collecting element.

Solar cell technology allows conversion of solar radiation into electricity in order to power a large range of items from calculators, to terrestrial power generation arrays and space craft In order to collect enough light to be useful, a solar cell needs to be relatively large. When manufactured from crystalline silicon the solar cell also has to be relatively thick to absorb a significant fraction of the incident light, in view of the poor optical absorption properties of crystalline silicon. Producing a large area thick solar cell is expensive in terms of the material required. A solution provided to this problem was the introduction of collectors which are used to collect radiation and direct it to the solar cells. A particularly successful type of collector is the so-called luminescent solar concentrator (LSC). Luminescent solar concentrators comprise a plate of transparent material such as glass or plastic adjacent to the solar cell which contain luminescent centres which absorb and emit radiation. The LSC is dimensioned such that the radiation emitted by the luminescent centres is directed via total internal reflection to an adjacent solar cell. These collectors are made of inexpensive material and hence their use considerably reduces the cost of collecting solar radiation over a given area without compromising efficiency.

Traditionally, solar cells have been manufactured from semiconductors and either comprise a semiconductor wafer which is (usually) diffused in order to form a pn junction and contacts (these are deposited in some form) or comprises layers formed overlying a semiconductor wafer or another substrate. Previously, such wafers have been joined to collectors by providing the large flat surface of the solar cell, i.e. the surface which is perpendicular to the direction of growth, against the collector. This arrangement then sets certain requirements on the size of the collector and the material used in the whole system since the collector must at least extend over the whole area of the solar cell.

The present invention challenges this arrangement and provides a different orientation for connecting the collector and the solar cell.

Therefore, in a first aspect, the present invention provides a solar cell assembly comprising a solar cell and light collector, said solar cell comprising a photoconducting material which has a generally planar structure comprising first and second opposing sides having the largest surface area of the sides of said structure with remaining sides forming the edges of said structure; said light collector being provided adjacent to at least one of the edges of said solar cell, said light collector being configured to collect solar radiation and provide radiation to said solar cell.

In the present invention, radiation is directed into the edges of the solar cell from the collector.

In many cases the edges of the cell will be perpendicular to the growth direction of the wafer from which the cell was formed. Thus, in a second aspect, the present invention provides a solar cell assembly comprising a solar cell and light collector said solar cell comprising a photoconducting material which has been formed in a first growth direction; said light collector being provided adjacent to at least one of the edges of said solar cell, the edges of said solar cell being the sides of said solar cell which are not perpendicular to said growth direction of said solar cell, said light collector being configured to collect solar radiation and provide radiation to said solar cell.

By providing the solar cell edge on to the collector, it is possible to considerably decrease the size of the solar cell. This arrangement considerably reduces the amount of material necessary for producing an efficient solar cell while still maintaining performance with that of existing solar cells.

When radiation is incident on a solar cell, it penetrates into the solar cell until it is absorbed. Therefore, it is important to ensure that the radiation penetrates far enough into the cell to stand a chance of being absorbed. In a conventional solar cell, the maximum penetration depth is set by the thickness of the wafer and/or the layers formed overlying the wafer. Therefore conventional solar cells need to have a relatively thick wafer in order to allow a sufficient radiation penetration depth. If the cell is too thin then it will not efficiently absorb radiation.

However, in the present invention, since radiation is collected at the edge of the cell, the maximum penetration depth is governed by the lateral extent of the cell in a direction perpendicular to the edge where radiation is collected. Thus, the wafer and/or layers formed on the wafer can be made much thinner than in conventional cells.

For example, the thickness of a conventional solar cell is typically over 200 p.m. The thickness being measured as the thickness of the edge of the cell. This will usually be the thickness of the wafer and/or layers formed thereon. However, in the present invention cells may be made as thin as 40 p.m or less, more preferably 30 p.m or less.

Cells may even be made as thin as 10p.m or less.

The collector and the cell will form a common light collection surface, in a preferred embodiment, said light collection surface will comprise 1 to 20% solar cell and the remainder of the surface will be the light collector.

Generally, said light collector will comprise a luminescent material which will luminesce upon irradiation by sunlight and is configured such that radiation produced by said luminescent material will be directed to said solar cell. Such a collector may comprise organic dye molecules embedded in a plastic (e.g. PMMA), inorganic molecules or ions in glass or fluorescent nanocrystals/quantum dots.

In an embodiment, said solar cell is provided adjacent to one collector or between two collectors, said collectors being provided at the edges of said cell. Other arrangements are possible. For example, the solar cell may be surrounded by light collectors.

In a further embodiment, a multilayer structure is provided a first layer and a second layer, said first layer comprising a plurality of light collectors and solar cells, said light collectors being disposed at the edges of said solar cells, and said second layer comprising a plurality of light collectors and solar cells, said light collectors being disposed at the edges of said solar cells, said first and second layers being arranged overlying one another such that there is an overlap between at least one of the solar cells of the first layer and a light collector of the second layer.

In said arrangement, the two layer solar cell assembly will still be thinner than conventional solar cell assemblies since the collectors direct radiation into the sides of the solar cell and hence the solar cells can be thinner than conventional solar cells as described above.

In said multilayer structure, the edge to edge arrangement of the collectors and solar cells allows the solar cells and collectors to jointly form a light collecting surface.

Therefore, in a third aspect, the present invention provides a solar cell assembly comprising a solar cell and light collector, said solar cell assembly having a light collection surface through which radiation is collected, said solar cell comprising a photoconducting material, said light collector being provided adjacent to at least one of the edges of said solar cell, said light collector being configured to collect solar radiation and provide radiation to said solar cell, wherein said light collection surface being provided both by a surface of said solar cell and a surface of said light collector.

In an embodiment, a solar cell is provided in a separate layer to said collector such that radiation which has passed once through said solar cell may enter said collector.

In a further embodiment, said solar cell has recesses provided in its light collection surface and said light collector is provided within said recesses, said light collector directing radiation to the sidewalls of said recesses.

Even in this arrangement where collectors are provided in recesses in trie solar cells, the solar cell assembly may be made thinner than a conventional solar cell as the collectors direct radiation into edges of the cell.

One or more mirrors may be provided, for example a mirror may be provided on the opposing side of the assembly to the side where light is collected i.e. the back surface of the assembly. The back surface reflector can be provided by a thin oxide layer.

Also, instead of a reflecting back surface, the bottom surface of the fluorescent collector can be reflecting.

Mirrors may also be provided on a side of the collector substantially perpendicular to the light collection surface. This arrangement would allow radiation which had not been absorbed to be reflected back into the collector or solar cell for absorption.

Mirrors may be provided in any of the above arrangements.

A photonic mirror may be provided overlying the light collection surface, said mirror being configured to transmit radiation into the collector in the absorption range of the collector and reflect radiation in the emission range of the collector. Thus, the photonic mirror reflects most of the (narrow band) luminescent radiation emitted by the collector but only a small part of the broad spectrum of incident light. Such a photonic mirror may be provided in any of the above arrangements.

Preferably, in all aspects of the invention, the assembly is covered with a layer of antireflection coating.

In a fourth aspect, the present invention provides a method of manufacturing a solar cell assembly, the method comprising: forming a solar cell by growing a photoconductive material, wherein said solar cell has a generally planar structure comprising first and second opposing sides having the largest surface area of the sides of said structure with remaining sides forming the edges of said structure, forming a p region and n region in said photoconductive material by diffusion or implantation; forming a light collector; providing said light collector adjacent to an edge of said solar cell,; providing contacts to said photoconductive material.

In a fifth aspect, the present invention provides a method or manutacturing a solar cell assembly, the method comprising: forming a solar cell by growing a photoconductive material, said solar cell having a first growth direction; forming a p region and n region in said photoconductive material by diffusion or implantation; forming a light collector; providing said light collector adjacent to said solar cell, said light collector being direction substantially perpendicular to the growth direction of said solar cell; providing contacts to said photoconductive material.

In sixth aspect, the present invention provides a method of manufacturing a solar cell assembly, the method comprising: forming a solar cell from photoconductive material; forming a p region and n region in said photoconductive material by diffusion or implantation; forming recesses in a light collecting surface of said photoconducting material; forming a light collector in said recesses; and providing contacts to said photoconductive material.

In order to support the arrangement of the solar cell and collector, the solar cell and collector may be formed on a substrate. In a preferred embodiment, the substrate is a mirror.

The present invention will now be described with reference to the following non-limiting embodiments in which: Figure 1 is a schematic of a solar cells in combination with a luminescent

collector in accordance with the prior art;

Figure 2 is a schematic of a solar cell assembly comprising a solar cell and luminescence collectors in accordance with the first embodiment of the present invention; Figure 3 is a solar cell assembly in accordance with a further embodiment of the present invention comprising mirrors; Figure 4 is a solar cell assembly in accordance with a yet further embodiment of the present invention comprising multiple layers; and Figure 5 is a solar cell assembly in accordance with a further embodiment of the present invention having a recessed structure.

Figure 1 is a schematic of a known solar cell assembly 1 comprising a light collector 3 and a solar cell 5.

The solar cell is formed from a photoconductive material. A typical material which is often used in solar cell manufacture is silicon. To manufacture the solar cells 5, the silicon is grown in the form of an ingot. The ingot is then sliced to form wafers. Solar cells are then either directly formed from the wafers or layers are grown on top of the wafers to form the solar cells. In either situation, the direction of growth for a standard solar cell is shown by the arrow 7 in figure 1.

A typical solar cell of the type 5 shown in figure 1 comprises a p-n junction which provides an electric field across the device. Upon illumination, photo-excited charged carriers flow giving rise to a current between the p-n type regions which can be harvested. For simplicity, the contacts or the p-n region are not shown in figure 1.

More complicated solar cells exist with more exotic photoconductive materials and also more than one p-n junction. However, the above is the basic principle of the solar cell.

The solar cell 5 will, upon illumination, provide an electric current, However, in order to produce a useful amount of energy for many applications, the solar cell needs to have a relatively large surface area over which radiation will be collected. As the material used to produce a solar cell is relatively expensive, it is not economical to provide such large solar cells. Therefore, the solar cell 5 is combined with a light collector 3. Light collector 3 collects radiation over a large area and directs it towards the solar cell 5.

The light collector in figure 1 is a so-called luminescent collector (LSC).

A luminescent collector comprises a plate of transparent material for example glass or plastic which contains luminescing centres which absorb and then emit light. Possible luminescing centres are phosphors or organic laser dyes such as rhodamine or coumarine, or inorganic luminescent centres. Sunlight enters the LSC 3 and is partially or fully absorbed by a luminescing centre. A fraction of the resulting luminescence is trapped by total internal reflection. Successive reflections transmit the luminescence to solar cell 5.

There are many types of luminescent collectors. Different substrate materials and different dyes or combinations of dyes have been suggested. It can be seen that luminescent couector 3 is disposed adjacent to solar cell b. However, It Is clisposea against the side of the cell with the largest surface area. In this example, the collector is disposed along the first direction of growth 7.

Figure 2 shows a solar cell assembly in accordance with an embodiment of the present invention. The solar cell assembly 11 comprises a solar cell 13 and first collector 15 and a second collector 17. The composition of the solar cell 13 and the first collector and second collector 17 can be the same as those described with reference to figure 1. However, the arrangement of the solar cell 13 and the collectors 15 and 17 is completely different. Here, the solar cell 13 was manufactured such that it has a growth direction shown by arrow 19. Contrary to figure 1, the solar collectors 15 and 17 are provided adjacent the solar cell 13 but are disposed adjacent the edge of the cell. In the cell of figure 2, the edges are the sides of the cell which are not the growth plane. In other words, they are the sides of the cell which are not substantially perpendicular to the growth plane. In many cases the edges of the cell will be parallel to the growth direction. However, depending on the orientation of the wafer, the cleaved edges of the wafer may not be parallel to the growth direction.

The solar cell may be manufactured by other methods such as ribbon growth techniques for example edge defined film fed growth techniques. In such techniques, the edge of the eventually produced wafer may be substantially parallel to the growth direction. A cell produced from this wafer or from further layers formed upon this wafer will have a major surfaces defined by the opposing sides of the cell with the largest surface area. The edges of the cell being the sides other than the major surface.

The collectors will be optically coupled to the solar cell. Any gaps between the solar cell and collector will be filled with material of similar refractive index as the collector or the solar cell (e.g. optical gel).

Typically, collectors can be quite large e.g. tens of cm to a metre and have a thickness from 1 to about 10 cm to accommodate conventional solar cells. In other situations, small collectors can be combined with small solar cells several hundred micrometers wide. The collector of figure 2 will be thinner than a conventional collector, for example from a fraction of micrometer to 1 mm. The surface area of the collector can be anything and will typically be from 1 cm2 to 10 m2 or even larger.

To support this iarge thin structure, the soiar assembiy may be formed on a substrate for example a mirror.

A p-type contact 21 and an n-type contact 23 are provided in solar cell 13. Typically solar cell 13 will be formed from either p or n type silicon. If the solar cell is formed from p type silicon, then an n type region will be formed in the p type region by diffusion. An n type contact is then provided to this n type region. Similarly, if the substrate is formed from n type silicon then a p type region will be formed in the n type substrate by diffusion and contact will be made appropriately.

In the example of figure 2, the contacts 21 and 23 are placed on the upper surface, If they are placed on the surface of the device where light is received, then they should be transparent.

Light incident on the device of figure 2 will either be captured by light collectors 15 and 17 or will be incident on solar cell 13. Photons with a higher energy than the silicon band gap may be absorbed by solar cell 14 directly. The dyes in or other luminescence centres in the collectors 15 and 17 are configured to absorb across the whole spectrum in the arrangement shown in figure 2. However, as will be described with reference to figures 3 and 4, different collector arrangements may require collectors that absorb at different wavelengths. Radiation collected by collectors 15 and 17 is then re-emitted by luminescence centres at a wavelength that will allow it to be absorbed by silicon solar cell 13. The total internal reflection properties of the collectors 15 and 17 will allow radiation emitted by the luminescence centres to be directed towards solar cell 13.

The solar cell and collector can have a thickness of 40p.m or less and still efficiently absorb radiation. Although the solar cell assembly may also be thicker.

The assembly is covered with a layer of antireflection coating (not shown).

Figure 3 shows a variation on the structure of figure 2. A photonic mirror 25 overlies the structure, The photonic mirror transmits short wave length photons above a certain threshold and reflects long wavelength light. This helps to recycle longer wave length photons in the structure to give them a second or third chance of interacting with luminescence A standara mirror wnicn rellects all wavelengths is provKled underneath the structure.

Figure 4 shows a further variation on the structure of figures 2 and 3. The structure of figure 4 comprises a plurality of solar cells 31a, 31b etc. The light collectors 33a, 33b are also provided. The solar cells 31 and light collectors 33 are provided in a first layer 35 and a second layer 37. The first layer comprises alternating solar cells 31a and collectors 33a. However, other arrangements are possible. The pattern between solar cells 31 and collectors 33 is shown as a 1 D pattern. However, the pattern could extend in two dimensions to form a chequered pattern or could be provided in lines.

The second layer 37 also comprises alternating solar cells 31 and collectors 33. In this particular arrangement, a solar cell 31 on the second layer 37 lies underneath a collector 33 on the upper layer 35. Similarly, a collector 33 on the lower layer underlies a solar cell 31 on the upper layer. However, other overlap arrangements are possible.

In the variation of figure 4, it is preferable if only the red/near infra-red part of the spectrum is absorbed and emitted by the dye. These wavelengths will pass right through the silicon solar cells 31 but will then be absorbed in the collector. The collector through reemission via the luminescence centres and total internal reflection will direct the radiation towards solar cells 31. The solar cells will absorb most of the radiation which is incident on them lying above the silicon bandgap. In this arrangement, the fluorescent collectors 33 absorb in a fairly narrow wavelength band.

The band could be quite broad, depending on dimensions (thickness) of the structure.

This increases their efficiency and the efficiency of the collector of the device as a whole.

In the assembly of figure 4, as the collectors direct radiation into the edges of the solar cell, the thickness of the structure, even though it contains multiple layers, can be substantially reduced over that of conventional assemblies. The thickness of the assembly of figure 4 measured perpendicular to the plane of the layers can be 100 pm or less if desired. The assembly may even have a thickness of 50 pm or less if desired.

Figure 5 shows a solar cell assembly in accordance with a further embodiment of the invention. The solar cell 41 comprises a photo-conductive material of the type previously described with reference to figures 1 to 4. Radiation enters the assembly through light collection surface 43.

The light collection surface of solar cell 41 is patterned to form recesses 45. Recesses have a cuboid shape with straight side-walls 46. Light collector 46 is formed in said cuboid recesses 45 such that a common light collecting surface 43 is formed from the non-recessed parts of the upper surface of said solar cell 41 and the upper surface of said light collector.

The light collectors 46 are configured to direct radiation towards the sidewalls 46 of said recesses 45. Thus the radiation from the collectors enters the solar cell via the edges of the solar cell provided by the sidewalls of the recesses 45.

The areas 47 of the solar cell which are located between the recesses 45 are doped to form a p-n junction. Contact 48 is made to the top these sections. However, it should be noted that the p-n junctions may be formed anywhere.

A mirror 49 is formed on the opposing side of the solar cell to the light collection surface.

The operation of this device is similar to that of the cell described with reference to figure 4. The absorption I fluorescent properties of the fluorescent collector will absorb in the red or near infra-red wavelength range. Blue light and higher wavelength light is absorbed directly in the solar cell. The structure is designed to enhance the red response of the solar cell.

Even though the light collectors are recessed into the solar cell, the structure can be made thinner than conventional solar cells. This is because radiation is directed into the edges of the cells. The thickness of the assembly of figure 5 measured perpendicular to the plane of the layers can be 100 im or less if desired. The assembly may even have a thickness of 50 tm or less if desired.

The photonic mirror and back surface mirror of figure 3 maybe provided in any of the structures shown in figures 4 and 5.

The above description has referred to crystalline silicon cells, but other types of solar cell may be used. The cell may be formed from a wafer or by layers which have been grown or otherwise deposited on a wafer or other substrate.

If a wafer is used in the production of the solar cell it may be formed as an ingot using a technique such as a czokralski technique. However, the wafer may also be produced by other techniques for example, from thin ribbons of silicon grown or pulled from melt or edge defined film fed growth techniques.

Claims

CLAIMS: 1. A solar cell assembly comprising a solar cell and light collector, said solar cell comprising a photoconducting material which has a generally planar structure comprising first and second opposing sides having the largest surface area of the sides of said structure with remaining sides forming the edges of said structure; said light collector being provided adjacent to at least one of the edges of said solar cell, said light collector being configured to collect solar radiation and provide radiation to said solar cell.
2. A solar cell assembly comprising a solar cell and light collector, said solar cell comprising a photoconducting material which has been formed in a first growth direction; said light collector being provided adjacent to at least one of the edges of said solar cell, the edges of said solar cell being the sides of said solar cell which are not perpendicular to said growth direction of said solar cell, said light collector being configured to collect solar radiation and provide radiation to said solar cell.
3. A solar cell assembly according to any preceding claim, wherein said solar cell is provided between at least two collectors, said collectors being provided adjacent edges of said solar cell.
4. A solar cell assembly according to any preceding claim, comprising a first layer and a second layer, said first layer comprising a plurality of light collectors and solar cells, said light collectors being provided at the edges of said solar cells, and said second layer comprising a plurality of light collectors and solar cells, said light collectors being provided at the edges of said solar cells, said first and second layers being arranged overlying one another such that there is an overlap between at least one of the solar cells of the first layer and a light collector of the second layer.
5. A solar cell assembly according to any of claims 1 to 4, wherein said light collector completely surrounds the edges of said solar cell.
6. A solar cell assembly comprising a solar cell and light collector, said solar cell assembly having a light collection surface through which radiation is collected, said solar cell comprising a photoconducting material, said light collector being provided adjacent to at least one of the edges of said solar cell, said light collector being configured to collect solar radiation and provide radiation to said solar cell, wherein said light collection surface being provided both by a surface of said solar cell and a surface of said light collector.
7. A solar cell assembly according to claim 6, wherein said solar cell has recesses provided in its light collection surface and said light collector is provided within said recesses, said light collector directing radiation to the sidewalls of said recesses.
8. A solar cell assembly according to any preceding claim, further comprising at least one mirror provided extending along a surface of said solar cell and light collector.
9. A solar cell assembly according to claim 8, further comprising a second mirror provided on an opposite side of said solar cell and light collector to said at least one mirror.
10. A solar cell assembly according to any preceding claim, further comprising a photonic mirror provided overlying the a surface of the assembly where light is collected, said mirror being configured to transmit radiation into the collector in the absorption range of the collector and reflect radiation in the emission range of the collector.
11. A solar cell assembly according to any preceding claim, wherein said light collector comprises a luminescent material which will luminecse upon irradiation by sunlight and is configured such that radiation produced by said luminescent material will be directed to said solar cell.
12. A method of manufacturing a solar cell assembly, the method comprising: forming a solar cell by growing a photoconductive material, wherein said solar cell has a generally planar structure comprising first and second opposing sides having the largest surface area of the sides of said structure with remaining sides forming the edges of said structure, forming a p region and n region in said photoconductive material by diffusion or implantation; forming a light collector; providing said light collector adjacent to an edge of said solar cell,; providing contacts to said photoconductive material.
13. A method of manufacturing a solar cell assembly, the method comprising: forming a solar cell by growing a photoconductive material, said solar cell having a first growth direction; forming a p region and n region in said photoconductive material by diffusion or implantation; forming a light collector; providing said light collector adjacent to said solar cell, said light collector being direction substantially perpendicular to the growth direction of said solar cell; providing contacts to said photoconductive material.
14. A method of manufacturing a solar cell assembly, the method comprising: forming a solar cell from photoconductive material; forming a p region and n region in said photoconductive material by diffusion or implantation; forming recesses in a light collecting surface of said photoconducting material; forming a light collector in said recesses; and providing contacts to said photoconductive material.