WO2010051599A1

WO2010051599A1 - A photovoltaic cell

Info

Publication number: WO2010051599A1
Application number: PCT/AU2009/001459
Authority: WO
Inventors: John Beavis Lasich; Pierre Jacques Verlinden
Original assignee: Solar Systems Pty Ltd
Priority date: 2008-11-10
Filing date: 2009-11-10
Publication date: 2010-05-14

Abstract

A photovoltaic cell comprising: a solar radiation facing photovoltaic surface for receiving concentrated incident solar radiation; a plurality of conductive regions having at least light reflective surface portions, the conductive regions deposited in at least an upper section thereof by metal vapour deposition on the solar radiation facing surface; and a transparent upper layer encapsulating the solar radiation facing surface and conductive regions; the conductive regions and upper layer being shaped and arranged such that sufficient incident radiation reflected off the light reflective surface portions is directed by total internal reflection onto the photovoltaic surface such that in operation electric power produced by the photovoltaic cell is increased by at least 3 percent relative to electric power produced by a corresponding photovoltaic cell without the transparent upper layer.

Description

Title

A PHOTOVOLTAIC CELL Field

The invention relates to a photovoltaic cell, a photovoltaic module, a receiver, and a method of producing electricity.

Background

Photovoltaic cells are an important part of solar power generation systems. Many solar cells incorporate conductive portions on the solar radiation facing surface in the form of conductive fingers extending over the surface to conduct charge. A problem with such conductive fingers is that they are usually opaque and shade the underlying photovoltaic surface reducing the number of photons which can be converted into electric charge. However, there is a competing problem that having a smaller area covered by fingers increases the resistive losses of the cell.

Accordingly, there is a need to address these problems.

Summary

In a first aspect, the invention provides a photovoltaic cell comprising: a solar radiation facing photovoltaic surface for receiving concentrated incident solar radiation; a plurality of conductive regions having at least light reflective surface portions, the conductive regions deposited in at least an upper section thereof by metal vapour deposition on the solar radiation facing surface; and a transparent upper layer encapsulating the solar radiation facing surface and conductive regions; the conductive regions and upper layer being shaped and arranged such that sufficient incident radiation reflected off the light reflective surface portions is directed by total internal reflection onto the photovoltaic surface such that in operation electric power produced by the photovoltaic cell is increased by at least 3 percent relative to electric power produced by a corresponding photovoltaic cell without the transparent upper layer.

In an embodiment, the transparent upper layer comprises an outer transparent layer and an encapsulant layer which adheres the outer transparent layer to the photovoltaic surface.

In an embodiment, the outer transparent layer is substantially flat and formed of a low loss glass

In an embodiment, the transparent upper layer consists of an encapsulant layer.

In an embodiment, the encapsulant is a silicone.

In an embodiment, at least the surface portions are formed of silver.

In an embodiment, at least portions of the light reflective portions of the conductive regions are sloped at an angle relative to an interface between the transparent layer and atmosphere sufficient to cause total internal reflection of light reflected from the sloped portions at the interface.

In an embodiment, at least an upper portion of the conductive regions has a trapezoidal cross-section. In an embodiment, a width of a flat top surface of the trapezoidal portion is less than half a bottom width.

In an embodiment, at least an upper portion of the conductive regions has a triangular cross-section.

In an embodiment, the angle of the slope is less than 70 degrees to the interface.

In an embodiment, the angle of the slope is less than 60 degrees to the interface.

In an embodiment, the angle of the slope is less than 70 and greater than 30 degrees to the interface.

In an embodiment, at least part of the surface of the light reflective surface portions is rough so as to provide a diffuse component of reflection.

In an embodiment, the photovoltaic cell is a multi- junction cell.

In a second aspect, the invention provides a photovoltaic cell comprising: a solar radiation facing photovoltaic surface for receiving concentrated incident solar radiation; a plurality of conductive regions having at least light reflective surface portions, the conductive regions deposited in at least an upper section thereof by metal vapour deposition on the solar radiation facing surface; and a transparent upper layer encapsulating the solar radiation facing surface; wherein at least portions of the reflective portions of conductive regions are sloped at an angle relative to an interface between the transparent layer and atmosphere sufficient to cause total internal reflection of light reflected from the sloped portions at the interface.

In an embodiment, at least an upper portion of the conductive regions has a trapezoidal cross -section.

In an embodiment, a lower portion of the conductive regions has substantially vertical sides.

In an embodiment, a width of a flat top surface of the trapezoidal portion is less than half a bottom width.

In a third aspect, the invention provides a multi- junction photovoltaic cell comprising: a plurality of layers of material with different band gaps to absorb energy of photons of differing energy, the highest band gap material arranged nearest the surface of the cell to absorb high-energy photons while allowing lower-energy photons to be absorbed by at least one lower band gap material below, the highest band gap material providing a solar radiation facing photovoltaic surface for receiving concentrated incident solar radiation; a plurality of conductive regions having at least light reflective surface portions disposed on the photovoltaic surface; and a transparent upper layer encapsulating the front side and conductive regions; the conductive regions and upper layer being shaped and arranged such that sufficient light reflected off the conductive regions is directed by total internal reflection onto the photovoltaic surface such that in operation electric power produced by the photovoltaic cell is increased by at least 3 percent relative to electric power produced by a corresponding photovoltaic cell without the transparent upper layer.

In a fourth aspect, the invention provides a photovoltaic cell comprising: a solar radiation facing photovoltaic surface for receiving concentrated incident solar radiation; a plurality of conductive regions having at least light reflective surface portions, the conductive regions deposited in at least an upper section thereof by metal vapour deposition on the solar radiation facing surface; and a transparent upper layer encapsulating the solar radiation facing surface; wherein at least portions of the reflective portions of conductive regions have a texture which produces a substantially non- specular reflection distribution.

In an embodiment, the textured portions are flat regions substantially parallel to the solar radiation facing surface.

In an embodiment, the conductive regions have a generally rectangular cross-section with perpendicular sides meeting the flat region.

In an embodiment, the conductive regions have a generally trapezoidal cross -section with sloping sides meeting the flat region.

In a fifth aspect, the invention provides a photovoltaic cell comprising: a solar radiation facing photovoltaic surface for receiving concentrated incident solar radiation; a plurality of conductive regions having at least light reflective surface portions deposited in at least an upper section thereof by metal vapour deposition on the solar radiation facing surface; and a transparent upper layer encapsulating the solar radiation facing surface and conductive regions; the conductive regions and upper layer being shaped and arranged such that light reflected off at least part of the conductive regions is directed by total internal reflection onto the photovoltaic surface, wherein a geometrical shading fraction of such conductive regions is larger than an unenscapsulated geometrical shading fraction that minimises the sum of losses from shading and electrical resistance for a corresponding unencapsulated cell without the transparent upper layer.

The invention also provides a photovoltaic module comprising at least one photovoltaic cell as described above.

The invention also provides a photovoltaic module comprising a plurality of cells arranged side by side in a densely packed array.

The invention also provides a receiver comprising a plurality of photovoltaic modules as described above.

The invention also provides a receiver comprising at least one photovoltaic cell as described above.

The invention also provides a method of producing electricity comprising concentrating solar radiation onto a receiver as described above.

In another aspect, the invention provides a method of manufacturing a photovoltaic cell comprising: depositing a plurality of at least top portions of light reflective conductive regions by metal vapour deposition on a solar radiation facing surface; controlling the deposition of the at least top portions of the conductive regions to create sloped portions of the conductive regions; and encapsulating the conductive regions and the solar radiation facing surface with a transparent upper layer, wherein the sloped portions are sloped at an angle relative to an interface between the transparent layer and atmosphere sufficient to cause total internal reflection of light reflected from the sloped portions at the interface.

In another aspect, the invention provides a method of manufacturing a photovoltaic cell comprising: depositing a plurality of at least top portions of light reflective conductive regions by metal vapour deposition on a solar radiation facing surface; controlling the rate of deposition of at least a portion of the at least top portions of the conductive regions to create a substantially non- specular reflection distribution; and encapsulating the conductive regions and the solar radiation facing surface with a transparent upper layer.

In another aspect, the invention provides a method of manufacturing a photovoltaic cell comprising: depositing a plurality of at least top portions of light reflective conductive regions by metal vapour deposition on a solar radiation facing surface; mechanically roughening a portion of the at least top portions of the conductive regions to create a substantially non-specular reflection distribution; and encapsulating the conductive regions and the solar radiation facing surface with a transparent upper layer. Brief Description of the Drawings

Embodiments of the invention are described further by way of example with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of an exemplary system for generating electrical power from solar radiation;

Figure 2 is a front view of the receiver of the system shown in Figure 1 which illustrates the exposed surface area of the photovoltaic cells of the receiver;

Figure 3 is an exploded perspective view of a photovoltaic cell module that forms part of the receiver;

Figure 4 is a schematic diagram illustrating one exemplary cross-section of conductive regions;

Figure 5 is a an example of a masking technique for producing the conductive regions of Figure 5;

Figure 6 is schematic diagram of another cross-section of conductive regions .

Detailed Description

The embodiments provide a photovoltaic cell as well as a photovoltaic module incorporating a plurality of photovoltaic cells and a receiver comprising a plurality of photovoltaic modules. In one embodiment, the photovoltaic cell has a transparent layer and opaque conductive regions which are arranged and shaped so that at least some light reflected off the conductive regions is directed by total internal reflection at the cover glass interface with air onto the surface of the photovoltaic cell. The embodiments are of particular use in solar power generation systems which employ a concentrator and a receiver in electricity generation. For example, systems which employ a parabolic mirror concentrator or a heliostat field as a concentrator. However, the embodiment can be employed in other contexts, for example, in one- dimensional or two-dimensional concentrators using mirrors or lenses. Specific embodiments relate to multi- junction solar cells. It will be appreciated that the photovoltaic cells can be used in other applications, for example, as space solar cells.

Exemplary power generation system

An exemplary solar radiation-based electric power generating system shown in Figure 1 includes a concentrator 3 in the form of a parabolic array of mirrors that reflects solar radiation that is incident on the mirrors towards a plurality of photovoltaic cells 5 as shown in Figure 2.

The cells 5 form part of a solar radiation receiver 7 that includes an integrated coolant circuit. The surface area of the concentrator 3 that is exposed to solar radiation is substantially greater than the surface area of the photovoltaic cells 5 that is exposed to reflected solar radiation. The photovoltaic cells 5 convert reflected solar radiation into DC electrical energy. The receiver 7 includes an electrical circuit (not shown) for the electrical energy output of the photovoltaic cells.

The concentrator 3 is mounted to a framework 9. A series of arms 11 extend from the framework 9 to the receiver 7 and locate the receiver as shown in Figure 1. The system further includes: (a) a support assembly 13 that supports the concentrator and the receiver in relation to a ground surface and for movement to track the Sun; and (b) a tracking system (not shown) that moves the concentrator 3 and the receiver 7 as required to track the Sun.

As described in further detail in WO 02/080286 which is owned by the present applicant, Solar Systems Pty Ltd, the amount of heat generated by the concentrated light can lead to problems with the operating temperature and performance of the cells 5. To this end, the receiver 7 includes a coolant circuit such as described in WO

02/080286 which can be applied to a wide range of solar cells, including multi- junction solar cells.

The coolant circuit cools the photovoltaic cells 5 of the receiver 7 with a coolant, preferably water, in order to minimise the operating temperature and to maximise the performance (including operating life) of the photovoltaic cells 5.

A number of other components of the receiver 7, such as components that make up the electrical circuit of the receiver 7, are not included in the Figures 1 to 3 for clarity.

Figure 3 illustrates the basic construction of each module 23. As is indicated above, each module 23 includes an array of twenty four closely packed photovoltaic cells 5.

Each module 23 includes a substrate 27, on which the cells 5 are mounted. Each module 23 also includes a glass cover

37 that is mounted on the exposed surface of the array of photovoltaic cells 5. The glass cover 37 may be formed to optimise transmission of useful wavelengths of solar radiation and minimise transmission of un-wanted wavelengths of solar radiation. Optionally, the upper surface of the cover glass can be coated with an anti- reflection coating and/or a coating reflecting light of such wavelengths that are not used by the cell for the photovoltaic conversion.

Each module 23 also includes a coolant member 35 that is mounted to the surface of the substrate 27 that is opposite to the array of photovoltaic cells 5.

The size of the coolant member 35 and the material from which it is made are selected so that the coolant member 35 acts as a heat sink. An exemplary material for the coolant member is copper.

Furthermore, the coolant member 35 is formed to define a set of flow paths for coolant for cooling the photovoltaic cells 5.

Further details of a receiver are found in WO 02/080286, the disclosure of which is incorporated herein. A further module with alternative coolant flow channels defined by sintered rods is described in WO 2005/022652 and can be adapted for use with this embodiment.

In the embodiment, the cell employs a multi- junction photovoltaic cell as such cells have been demonstrated to have relatively high efficiency. However, the skilled person will appreciate that while the invention is advantageously applied to multi- junction cells, it may also be applied to other photovoltaic cells with conductive opaque regions deposited on the solar radiation facing surface of the photovoltaic cell. A typical multi- junction photovoltaic cell has a plurality of layers of material with different band gaps to absorb energy of photons of differing energy, the highest band gap material arranged nearest the surface of the cell to absorb high- energy photons while allowing lower-energy photons to be absorbed by at least one lower band gap material below, the highest band gap material providing a photon source facing photovoltaic surface for receiving incident photons .

Further details of multi- junction cells, their materials and manufacture, are available from their manufacturers, for example from Spectrolab, Inc of Sylmar, California, USA.

Referring to Figure 4, there is shown a photovoltaic cell 600 having a plurality of conductive regions 620 (known as "fingers") having sloped portions 621. The fingers 620 and the photon source facing surface 611 of the upper layer 610 of the multi- junction cell 600 are encapsulated by a transparent layer 650 formed from a layer 630 of an encapsulant, such as silicone or any transparent encapsulant suitable for the application, such as EVA or PVB, and optionally an outer transparent layer for example a cover glass 640 which may be a low- loss glass such as low iron glass. Cover glass 640 may also be coated with an anti-reflective coating (not shown) . If no cover glass is used and only a transparent encapsulant is used, the encapsulant must be thick enough, significantly thicker than the conductive regions, to be substantially non- conforming on its upper surface to the shape of the conductive regions, advantageously flat. In other embodiments, the outer transparent layer 640 could be an outer plastic layer formed of Tefzel, ETFE, PTFE, Teflon, FET or acrylic for example. A layer of low iron glass is appropriate in part because the refractive index is appropriate and the material of the outer transparent layer should be chosen with this in mind.

The sloped portions are sloped at an angle α such that light (as indicated by arrow A) which impinges on the sloped portions is reflected by total internal reflection at the interface 641 between the cover glass and the atmosphere 660 and finally impinges the photovoltaic cell surface 611. In the embodiment shown in Figure 4, the conductive portions are generally trapezoidal in cross- section such that some light as indicated by arrow B is reflected off the top surface 622 of the fingers 620 and escapes.

The metal fingers are preferably silver to provide a low electrical resistance to the photogenerated current and to provide a good reflectance, and while the above embodiment shows a trapezoidal cross -section on at least a top part and triangular cross-section could also be provided on at least a top part thereof. It will be appreciated that vapour deposition, such as vacuum evaporation with help of thermal power or electron beam, or sputtering, is particular suited to creating a sloped surface as the fingers can be laid down in layers of decreasing width to provide the desired slope as shown in Figure 5.

To enhance total internal reflection, it is advantageous if a width of a flat top surface of the fingers is less than half a bottom width. To further enhance total internal reflection, the sloped surface of the fingers makes an angle of less than 70 degrees to the surface, and lower angles such as 60 degrees or even 40 degrees provide a high degree of total internal reflection and have the benefit of in practice allowing a narrower flat top region or in the case of triangular shapes, no flat topped region at all. The design of the angle needs to take into a number of competing considerations: the range of angles over which the incident photons impinge on the cell from the typically reflective or refractive optics of the concentrator cell, the width of any flat top, and the total electrical resistance of the shape. In the case that the maximum angle of incidence of the incoming rays to the upper surface is 30 degrees, for typical silicone and glass compositions the minimum angle of the sloped portions to the photovoltaic surface is about 32.4 degrees to capture all radiation specularly reflected off the sloped portions. For a maximum angle of incidence of 45 degrees, the minimum angle is 36.8 degrees.

In some embodiments, surfaces of the metal fingers may be roughened so as to provide a substantially non- specular component of reflection. Roughness of the finger surface will affect the number of rays that are totally internally reflected and, to the extent it can be manipulated, can be used to enhance total internal reflection from the generally flat region on the top, by supplying a more diffuse component of reflection. An example of a technique to produce a rougher surface is to use a high vapour deposition rate. Another example is to mechanically roughen the surface after deposition.

In Figure 4, the fingers are about 10 micron in bottom width (c) and 5 micron in height (h) . The fingers 620 occupy about 8% of the surface 611 of the cell and are adapted to concentrator applications. As described above, total internal reflection occurs when on angle or the ray to the normal of the cover glass interface 641 exceeds a critical angle controlled by the refractive index of the cover glass and air, about 44 degrees for suitable cover glass materials.

As shown in Figure 5, the fingers 620 are deposited preferably by a technique called lift-off. The lift-off technique consists of depositing the fingers 620 through a mask 710, preferably of photoresist, as indicated by arrows 720. Build-up 735 of deposited material 730 is allowed to occur on the edge of the mask to narrow the gap between the mask edges as part of the deposition process to produce the trapezoidal cross-section shown in Figure 5. At the end of the process the mask is chemically etched away or mechanically lifted off to leave the cell with metal fingers as shown in Fig. 4. Persons skilled in the art will appreciate that plural masking steps can be employed to control the shape of the vapour deposition process. In this respect it will be appreciated that the slope may not be entirely even in gradient, for example it may be slightly stepped, while still being generally disposed at an angle to the upper surface of the transparent upper layer providing an atmosphere/glass interface.

Also, fingers 620 can be deposited by a different technique not shown in Figure 5. For example, the material 730 is deposited onto the entire surface 611. And then a photoresist mask is deposited and patterned onto the material 730. Exposed material 730 in undesired locations can be chemically etched away and the remaining photoresist can be chemically removed.

As discussed above, the angled sides of the trapezoid 620 limit the size of flat area 622 on top of the finger, increasing the number of rays that are totally internally reflected. An advantageous design will involve trading-off minimization of any flat region 622 with the extra current that can be carried by a tall finger with higher cross sectional area. Advantageous shapes can thus be determined by geometric design and trial and error based on what can in practice be produced with vapour deposition techniques. For example, a shape with steep lower sides and a trapezoidal or triangular top cross-section 623 such as finger 620C shown in Figure 6 will also be highly advantageous. In this embodiment, which could be manufactured in a number of steps, a high total cross- section enables high current carrying capacity and the top triangular shape having surface angles of about 45 degrees enables efficient light trapping in comparison to smooth flat tops.

It will be appreciated that controlling the shape of the vapour deposited fingers as described by way of example in the above embodiments, enables other shapes and characteristics of the fingers to be designed which affect the optimization of the cross -sectional shape and size of the vapour deposited fingers on front contact cells. The above techniques enables the skilled person to modify the cross-section to enhance the amount of total internal reflection even further, and secondly to provide the optimum balance of current carrying capacity (and also of resistive losses) and geometric shading fraction that is now altered by the much reduced effective shading fraction provided by the embodiment potentially favouring larger geometric shading fractions. In this respect, the effective shading fraction is a measure of how much of the incident light is effectively lost and does not impinge on the photovoltaic surface 611 taking into account all possible reflection of the light including total internal reflection at the upper surface interface with air, whereas the geometric shading fraction is a measure of the fraction of the area of the photovoltaic surface covered by the fingers. Thus more efficient cells can be constructed by designing a cell according to the current invention with a geometrical shading fraction larger than would minimise the sum of losses from shading and electrical resistance for a corresponding unencapsulated cell without the transparent upper layer. The geometrical shading fraction can be varied keeping other parameters constant by varying number and hence the lateral separation of the fingers.

It will be appreciated that to take advantage of the above techniques it is essential to have a transparent layer of sufficient thickness relative to the height of the conductive regions such that the upper surface of the transparent layer is an interface to air or other low refractive index material and is disposed relative to the shape of the silver fingers and the photovoltaic surface to provide the benefit of the total internal reflection, for example by being substantially flat.

In contrast, prior art techniques teach against the use of a thick transparent layer for concentrator cells with vapour deposited fingers, recommending thin conformal coatings, which would not produce total internal reflection, or no coating at all. Either the existence or scale of the net efficiency benefit of the current invention for vapour deposited fingers has not been appreciated by concentrator system or cell designers in their goal to produce the most efficient and sufficiently durable system at the lowest cost per watt output, for which 1% gains are generally considered substantial. The above embodiments described in further below show surprising efficiency gains, which are at least 3-5% and up to 6% relative to the energy output of an uncovered cell, and this makes a more expensive cell with an appropriate transparent layer much more economical over its life-cycle.

Experimental results

A photovoltaic module comprising a plurality of III -V multi- junction cells with a cover glass and encapsulant covering generally trapezoidal fingers was tested. The efficiency was dramatically improved compared with the uncovered module from 33.5 percent to 35.5 percent, an absolute improvement of 2.0% and a relative improvement of 6.0%. The improvement is very surprising, considering that the total geometric shading area of the fingers is 7.9%, which means that about 75% of the photons impinging the fingers are reflected toward the cell surface, directly or by total internal reflectance at the cover glass interface with air, and is so great that design of photovoltaic systems will now be changed to include rather than avoid encapsulation, and to have specific regard for this effect in the shaping of the fingers, as described above. It means also that, due to total internal reflection of the light at the cover glass or encapsulant interface with air, the effective shading fraction can be as low as 25% of the geometric shading fraction due to the fingers.

A contribution to the relative benefit due to an antireflection coating on the cover glass in comparison to the uncovered module is possible, but likely to be no more than 2 relative percent.

Further many variations may be made without departing from the scope of the invention. In particular, features of the above embodiments may be employed to form further embodiments.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as

"comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

CLAIMS :

1. A photovoltaic cell comprising: a solar radiation facing photovoltaic surface for receiving concentrated incident solar radiation; a plurality of conductive regions having at least light reflective surface portions, the conductive regions deposited in at least an upper section thereof by metal vapour deposition on the solar radiation facing surface; and a transparent upper layer encapsulating the solar radiation facing surface and conductive regions; the conductive regions and upper layer being shaped and arranged such that sufficient incident radiation reflected off the light reflective surface portions is directed by total internal reflection onto the photovoltaic surface such that in operation electric power produced by the photovoltaic cell is increased by at least 3 percent relative to electric power produced by a corresponding photovoltaic cell without the transparent upper layer.

2. A photovoltaic cell as claimed in claim 1, wherein the transparent upper layer comprises an outer transparent layer and an encapsulant layer which adheres the outer transparent layer to the photovoltaic surface.

3. A photovoltaic cell as claimed in claim 2, wherein the outer transparent layer is substantially flat and formed of a low loss glass

4. A photovoltaic cell as claimed in claim 1 wherein the transparent upper layer consists of an encapsulant layer.

5. A photovoltaic cell as claimed in any one of claims 1 to 4, wherein the encapsulant is a silicone.

6. A photovoltaic surface as claimed in any one of claims 1 to 5, wherein at least the surface portions are formed of silver.

7. A photovoltaic cell as claimed in any one of claims 1 to 6, wherein at least portions of the light reflective portions of the conductive regions are sloped at an angle relative to an interface between the transparent layer and atmosphere sufficient to cause total internal reflection of light reflected from the sloped portions at the interface.

8. A photovoltaic cell as claimed in claim 7, wherein at least an upper portion of the conductive regions has a trapezoidal cross-section.

9. A photovoltaic cell as claimed in claim 8, wherein a width of a flat top surface of the trapezoidal portion is less than half a bottom width.

10. A photovoltaic cell as claimed in claim 7, wherein at least an upper portion of the conductive regions has a triangular cross -section.

11. A photovoltaic cell as claimed in any one of claims 7 to 10, wherein the angle of the slope is less than 70 degrees to the interface.

12. A photovoltaic cell as claimed in any one of claims 7 to 10, wherein the angle of the slope is less than 60 degrees to the interface.

13. A photovoltaic cell as claimed in any one of claims 7 to 10, wherein the angle of the slope is less than 70 and greater than 30 degrees to the interface.

14. A photovoltaic cell as claimed in any one of claims 1 to 13, wherein at least part of the surface of the light reflective surface portions is rough so as to provide a diffuse component of reflection.

15. A photovoltaic cell as claimed in any one of claims 1 to 14, wherein the photovoltaic cell is a multi- junction cell.

16. A photovoltaic cell comprising: a solar radiation facing photovoltaic surface for receiving concentrated incident solar radiation; a plurality of conductive regions having at least light reflective surface portions, the conductive regions deposited in at least an upper section thereof by metal vapour deposition on the solar radiation facing surface; and a transparent upper layer encapsulating the solar radiation facing surface; wherein at least portions of the reflective portions of conductive regions are sloped at an angle relative to an interface between the transparent layer and atmosphere sufficient to cause total internal reflection of light reflected from the sloped portions at the interface.

17. A photovoltaic cell as claimed in claim 16, wherein at least an upper portion of the conductive regions has a trapezoidal cross-section.

18. A photovoltaic cell as claimed in claim 17, wherein a lower portion of the conductive regions has substantially vertical sides.

19. A photovoltaic cell as claimed in claim 17, wherein a width of a flat top surface of the trapezoidal portion is less than half a bottom width.

20. A photovoltaic cell as claimed in claim 13, wherein at least an upper portion of the conductive regions has a triangular cross -section.

5

21. A multi- junction photovoltaic cell comprising: a plurality of layers of material with different band gaps to absorb energy of photons of differing energy, the highest band gap material arranged nearest the surface0 of the cell to absorb high-energy photons while allowing lower-energy photons to be absorbed by at least one lower band gap material below, the highest band gap material providing a solar radiation facing photovoltaic surface for receiving concentrated incident solar radiation; 5 a plurality of conductive regions having at least light reflective surface portions disposed on the photovoltaic surface; and a transparent upper layer encapsulating the front side and conductive regions; 0 the conductive regions and upper layer being shaped and arranged such that sufficient light reflected off the conductive regions is directed by total internal reflection onto the photovoltaic surface such that in / operation electric power produced by the photovoltaic cell5 is increased by at least 3 percent relative to electric power produced by a corresponding photovoltaic cell without the transparent upper layer.

22. A photovoltaic cell comprising: 0 a solar radiation facing photovoltaic surface for receiving concentrated incident solar radiation; a plurality of conductive regions having at least light reflective surface portions, the conductive regions deposited in at least an upper section thereof by metal5 vapour deposition on the solar radiation facing surface; and a transparent upper layer encapsulating the solar radiation facing surface; wherein at least portions of the reflective portions of conductive regions have a texture which produces a substantially non- specular reflection distribution.

23. A photovoltaic cell as claimed in claim 22, wherein the textured portions are flat regions substantially parallel to the solar radiation facing surface.

24. A photovoltaic cell as claimed in claim 23, wherein the conductive regions have a generally rectangular cross- section with perpendicular sides meeting the flat region.

25. A photovoltaic cell as claimed in claim 23, wherein the conductive regions have a generally trapezoidal cross- section with sloping sides meeting the flat region.

26. A photovoltaic cell comprising: a solar radiation facing photovoltaic surface for receiving concentrated incident solar radiation; a plurality of conductive regions having at least light reflective surface portions deposited in at least an upper section thereof by metal vapour deposition on the solar radiation facing surface; and a transparent upper layer encapsulating the solar radiation facing surface and conductive regions; the conductive regions and upper layer being shaped and arranged such that light reflected off at least part of the conductive regions is directed by total internal reflection onto the photovoltaic surface, wherein a geometrical shading fraction of such conductive regions is larger than an unenscapsulated geometrical shading fraction that minimises the sum of losses from shading and electrical resistance for a corresponding unencapsulated cell without the transparent upper layer.

27. A photovoltaic module comprising at least one photovoltaic cell as claimed in any one of claims 1 to 26.

28. A photovoltaic module as claimed in claim 27, comprising a plurality of cells arranged side by side in a densely packed array.

29. A receiver comprising a plurality of photovoltaic modules as claimed in claim 27.

30. A receiver comprising at least one photovoltaic cell as claimed in any one of claims 1 to 26.

31. A method of producing electricity comprising concentrating solar radiation onto a receiver of claim 29 or claim 30.

32. A method of manufacturing a photovoltaic cell comprising: depositing a plurality of at least top portions of light reflective conductive regions by metal vapour deposition on a solar radiation facing surface; controlling the deposition of the at least top portions of the conductive regions to create sloped portions of the conductive regions; and encapsulating the conductive regions and the solar radiation facing surface with a transparent upper layer, wherein the sloped portions are sloped at an angle relative to an interface between the transparent layer and atmosphere sufficient to cause total internal reflection of light reflected from the sloped portions at the interface.

33. A method of manufacturing a photovoltaic cell comprising: depositing a plurality of at least top portions of light reflective conductive regions by metal vapour deposition on a solar radiation facing surface; controlling the rate of deposition of at least a portion of the at least top portions of the conductive regions to create a substantially non- specular reflection distribution; and encapsulating the conductive regions and the solar radiation facing surface with a transparent upper layer.

34. A method of manufacturing a photovoltaic cell comprising: depositing a plurality of at least top portions of light reflective conductive regions by metal vapour deposition on a solar radiation facing surface; mechanically roughening a portion of the at least top portions of the conductive regions to create a substantially non- specular reflection distribution; and encapsulating the conductive regions and the solar radiation facing surface with a transparent upper layer .