WO2010075606A1

WO2010075606A1 - Improved photo-voltaic device

Info

Publication number: WO2010075606A1
Application number: PCT/AU2009/001683
Authority: WO
Inventors: Shaun Joseph Cunningham
Original assignee: Shaun Joseph Cunningham
Priority date: 2008-12-29
Filing date: 2009-12-22
Publication date: 2010-07-08

Abstract

A photo-voltaic cell for converting concentrated light into electricity, and a method of manufacturing thereof. The photo-voltaic cell comprising: an inactive substrate; and at least one epitaxial layer deposited on the substrate. The photo-voltaic cell comprising one or more metallic contact layers for enabling electrical contact to the cell. The photo-voltaic cell can further comprise gridlines having a relatively high thickness to width aspect ratio. The photo-voltaic cell can be applied to a photovoltaic module. The module can further comprise a light guiding prism and/or adaptive optics and/or a Fresnel lens element. A photovoltaic system can include one or more cell modules, and one or more cell interconnection modules for monitoring the performance of the cell modules.

Description

IMPROVED PHOTO-VOLTAIC DEVICE

FIELD OF THE INVENTION

The present invention relates to photovoltaic devices and methods.

The invention has been developed primarily for use in photovoltaic systems and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.

BACKGROUND OF THE INVENTION

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

As the world's awareness of environmental issues becomes more intense, alternate forms of energy to fossil fuels are being sought and renewable sources of energy such as solar energy have increasing importance and commercial value. As a result, considerable effort is being invested in the development of Photovoltaic (PV) cells which convert sunlight directly into electricity.

PV cells generally fall into one of three classes:

1. Conventional bulk-semiconductor cells made from multi-crystalline or polycrystalline silicon ;

2. Thin film cells made from materials such as silicon or Copper Indium Gallium Selenide (CIGS) deposited on glass or other low cost substrates, and

3. Concentrator cells which are made using sophisticated epitaxial semiconductor structures to achieve superior energy conversion efficiency.

Conventional bulk-semiconductor solar cells have been the focus of much of the world's attention for many decades. The proven performance of these silicon structures and their relatively low cost, due to the abundance of silicon as a raw material and simple processing procedures, has made this the solar technology of choice for many terrestrial applications. Despite the considerable investment that has been made into this technology, the conversion efficiency of these silicon cells has reached a commercially practical limit of around 22%. This limit is not an issue in many "consumer grade" installations and this technology continues to flourish. The cost per watt of electricity generated from these conventional cells is currently around $2 - $3 (USD).

Thin film PV cells are a relatively recent development and are intended to reduce the cost per watt generated of PV installations. Conventional silicon PV cells are made on substrates which have high purity and regular atomic lattice structure. Although these substrates benefit from the silicon semiconductor industry's volumes and price points, the need to have the entire substrate made from high quality material is a significant cost burden. As the name suggests, thin film cells are made by depositing only a thin layer of semiconductor material on low cost substrates such as glass, stainless steel or plastic. Although the use of these substrate materials reduces the amount of semiconductor material needed dramatically (e.g. maybe by a factor of 100), it makes the task of forming defect free crystal structures much more difficult because the thin film layer does not have a uniform crystal template to align to during growth. The relatively poor semiconductor quality of these structures results in conversion efficiencies of barely 10%. Despite this, the cost per watt of thin film installations is generally less than $2 per watt. This means that thin film cells are becoming increasingly popular, even though the poor efficiency means more than twice as much collecting area is needed (compared to bulk silicon cells) per watt generated.

High performance Concentrator Photo- Voltaic (CPV) technologies are the most recent PV innovation. The concept of using low cost optical elements to collect and focus light onto relatively small cells has been known for many years. Using this approach, not only can the semiconductor proportion of an installation's cost be reduced, but a more exotic semiconductor structure can be employed to provide higher conversion efficiencies. Much of the innovation occurring at the present time relates to the design of sophisticated epitaxial structures that increase cell efficiency. These structures generally employ compound semiconductors made from elements such as aluminium, gallium, indium, arsenic, phosphorous and other related elements in groups III and V of the periodic table. The structures are typically grown on high purity, mono- crystalline substrates made from germanium or gallium arsenide. In making these cells, it is common to use a so-called "multi-junction" structure where several different cells are stacked one on top of the other. For example, the top cell in such a multi-junction structure might be made from indium gallium phosphide (InGaP), the middle cell might be made from gallium arsenide (GaAs) and the bottom cell might be made from germanium (as a result of using germanium as the substrate for crystal growth). The top cell converts short wavelength solar radiation to electric current but transparently passes longer wavelengths through to the lower cells. These cells also convert a portion of the solar spectrum to electric current according to the bandgap of the materials used. In being stacked together, the outputs of the individual cells are combined in series to raise the voltage (and hence power) generated from the cell. The key advantage of these multijunction devices over other single junction semiconductor structures is that they convert sunlight into electricity more efficiently. This is achieved by tailoring the semiconductor structure to absorb light in relatively narrow spectral bands. This means that different layers in the cell convert "blue", "green" and "red" portions of the incoming spectrum separately. The terms "blue", "green" and "red" are used here to describe relative portions of the solar spectrum and should not be taken literally. This multijunction approach results in better quantum efficiencies and less waste heat generation from carrier thermalisation in the cell.

The current world record for energy conversion of this type of cell is around 41%, double that of the best conventional bulk silicon-based technologies. Although these semiconductor structures are more complex and costly to produce, this has relatively little impact on CPV systems because the semiconductor area needed is only a small fraction of the optical collecting area. Typically, CPV systems use lenses or mirrors as the primary optical concentrating elements to provide concentration ratios of around 500 times. Therefore, although the cost of the multi-junction cell might be 100 times higher than silicon per unit area, the semiconductor area needed can be reduced by 500 (or more) thereby reducing the semiconductor component of system costs by at least a factor of 5. At the same time, these cells generate twice as much energy per unit area as a result of their higher intrinsic efficiencies. This cost advantage is so substantial that it is predicted that the cost per watt of CPV installation will be lower than thin film technologies, despite the fact that installations need to use mechanical tracking systems to keep cells and associated optical elements pointed at the sun.

Certain features of prior art multijunction cells will now be discussed in the context of the present invention.

Multijunction cells were initially developed for satellite power supply systems and have been used in this market for a number of years. The nature of this application demands the highest possible efficiency and lowest launch weight. The cost of satellite PV systems is generally a secondary consideration and PV cells for space applications are sold at a considerable premium. The resulting mindset seems to have influenced current cell and module designs. In particular, modules presently lack the engineering refinement needed to be successful in the high volume, cost-sensitive terrestrial CPV market.

US patent 7,122,733, filed by Narayanan et al on 6 September 2002, assigned to The Boeing Company and titled "Multi-junction photovoltaic cell having buffer layers for the growth of single crystal boron compounds", includes a useful overview of the art of producing multi-junction solar cells. US 7,122,733provides the following summary:

"In a multiple cell device, semiconductive materials are typically lattice- matched to form multiple p-n (or n-p) junctions. Thep-n (or n-p) junctions can be of the homojunction or heterojunction type. When solar energy is received at a junction, minority carriers (i.e., electrons and holes) are generated in the conduction and valence bands of the semiconductor materials adjacent the junction. A voltage is thereby created across the junction and a current can be utilized therefrom. As the solar energy passes to the next junction, which can be optimized to a lower energy range, additional solar energy at this lower energy range can be converted into a useful current. With a greater number of junctions, there can be greater conversion efficiency and increased output voltage.

Whether in the multiple-junction or single-junction PV device, a conventional characteristic of PV cells has been the use of a single window layer disposed on an emitter layer which is disposed on a base layer. Further, the base layer may be disposed on a back surface field layer which is disposed on a substrate. The window layer and the back surface field layers are of higher bandgap semiconducting material lattice matched to the whole structure. The purpose of the top window layer and the back-surface field layer have been to serve both as a passivation layer and a reflection layer due to high electric fields associated with the high bandgap. The photo-generated carriers, such as the electrons in the emitter layer and the holes in the base layer, can further be reflected towards the p-n junction (which is the emitter and the base layer interface), for recombination and for generating electricity.

For a multiple-cell PV device, efficiency is limited by the requirement of low resistance interfaces between the individual cells to enable the generated current to flow from one cell to the next. Accordingly, in a monolithic structure, tunnel junctions have been used to minimize the blockage of current flow. In a multiple wafer structure, front and back metallization grids or contacts with low coverage fraction and transparent conductors have been used for low resistance connectivity. Since the output power is the product of voltage and current, a multi-junction solar cell can be designed with multiple junctions comprised of materials having different bandgaps, so that each junction can absorb a different part of the wide energy distribution of photons in sunlight. Additionally, uniform current generating characteristics may be produced.

Materials for a solar cell are conventionally grown epitaxially in a metal organic vapor phase epitaxy (MOVPE) system, also known as a metal organic chemical vapor deposition (MOCVD) system. During material growth, the lattice parameter for all of the different cell layers comprising the solar cell should be the same as that of the substrate. III-V compound materials of different compositions, but with the same lattice parameter as that of the substrate, are used to achieve different bandgaps that are typically required for multijunction solar cells. These layers are usually grown on a III-V substrate such as a GaAs wafer. In order to reduce the cost of the substrate material as well as to increase the over all power to weight ratio from the solar cell, a GaAs nucleated Ge substrate can be used. The lattice parameter of the Ge substrate is about 5.64613 Angstroms and that of GaAs is about 5.6533 A with little mismatch between the lattice parameters. Although the Ge atomic structure is of a diamond structure pattern and that of GaAs is of a zinc-blend structure, it is possible to grow GaAs on Ge with minimum defects. For a multijunction solar cell device, a thin layer of GaAs is first grown on the Ge substrate and followed by the growth of various other compositions.

Existing III-V semiconductor multi-junction solar cells are processed from epitaxial gallium indium phosphide/gallium arsenide (GaInP2/GaAs) materials, grown on a GaAs nucleated Ge substrate. By providing active junctions in GaInP2, GaAs, and Ge, a triple-junction solar cell can be processed. These existing triple-junction solar cells have demonstrated a 29.3% efficiency under space solar spectrum that is Air Mass 0 (AMO), 0.1353 W/cm2 at 28° C. Under the concentrator terrestrial spectrum (AM1.5D, 44W/cm2, 25° C), an efficiency of 32.3% has also been demonstrated. The Air Mass value indicates the amount of air in space while the conversion efficiency describes a percentage of conversion from the sun's energy to electrical power. A limitation of such triple-junction solar cells includes the inability of increasing the AMO efficiency above 29.3% (to, for example, 35% or higher). To achieve such an increase, four junctions may be needed to enhance the utilization of the sun's energy spectrum.

Conventional methods to grow a triple-junction solar cell typically use GaInP2, GaAs and Ge cells. The direct bandgaps ofGaInP2 and GaAs are about 1.85 eV and about 1.424 eV respectively (Ge has an indirect bandgap of about 0.66 eV). Theoretical studies have shown that an additional third junction of about a 1.0 eV solar cell disposed on top of the Ge junction may be necessary for building a four junction monolithic solar cell. As such, GaInP2 may form the first junction, GaAs can form the second junction, a new I eV material may form the third junction and Ge can form the fourth junction. Limitations of such materials include a lack of a bandgap around 1.0 eV that may be lattice matched to Ge and a lack of requisite material properties needed to process a solar cell. Some materials such as Gallium Indium Arsenic Nitride (GaInAsN) have been used in an attempt to achieve lattice- matching characteristics, however an ability to produce material with requisite characteristics and with a bandgap around 1.0 eV has not been achieved. "

US 7122733 discloses the use of Boron-containing materials for use in forming IeV cell junctions. However, the use of boron is inherently problematic. Because boron is a small atom, its presence in a regular GaAs / Ge dimensioned crystal lattice causes stresses that can lead to crystal defects and poor carrier transport characteristics. For example, carrier lifetimes can be degraded as a result of these defects. This means that photo-generated carriers can recombine at these crystal defects and convert otherwise useful energy to waste heat, thereby degrading the conversion efficiency of the overall cell. There are also potential problems in the compatibility of source gasses used in MOCVD chambers to deposit boron and other compounds and the claimed innovation of using multiple buffer layers to promote correct growth of the boron containing layers is complex with potentially poor reproducibility.

Given the shortcomings of US 7,122,733, a more simple approach is required which uses conventional materials and growth processes and which offers high degrees of manufacturing certainty and reproducibility.

US patent 5,223,043, filed by Olson et al on 11 May 1992, assigned to US DoE and titled "Current Matched High Efficiency, Multi-Junction Monolithic Solar Cells", includes a general overview of issues relating to current matching in series connected cells.

In order to avoid the need for individual connections to each of the sub-cells in a multijunction cell, the subcells are implicitly connected in series as a result of the epitaxial growth process used to form them. Although this solves the connection problem, it introduces the need to match the currents produced by each subcell. Initial attempts at achieving this current match focused on modifying the material composition (and hence bandgaps) of the subcells so that they absorbed portions of the solar spectrum which resulted in equal photo-currents. US 5,223,043 teaches that this is an overly restrictive means of achieving current matching given the relative difficulty of choosing lattice matched materials of the appropriate bandgap. Instead, US 5,223,043 claims the use of thinned subcell layers in dual layer (tandem) cells. When a subcell is made thinner than the minority carrier diffusion length of the semiconductor material used, the subcell becomes increasingly transparent to incoming light and its photo-generated current is reduced. If the upper subcell in a tandem cell structure generates more photocurrent than the lower subcell, current matching can therefore be achieved by thinning this upper subcell. Thinning the upper subcell not only results in a lowering of the current produced by the upper subcell, it also results in an increase in current produced by the lower cell because more light reaches this subcell.

US 5,223,043 focuses exclusively on dual layer tandem cells comprising InGaP- GaAs, AlGaAs-GaAs and GaAs-Ge material layers. It does not teach skills required to produce multifunction cells comprising more than two subcell layers or techniques for increasing the efficiency of cells above the 27.3% quoted for AM 1.5 illumination.

Given US 5,223,043, there is a need for a more sophisticated multijunction cell design which offers higher conversion efficiency.

To increase cell efficiency, currently cell manufacturers incorporate an additional photo-active junction in the substrate on which the other subcell layers are grown. Germanium is the preferred material used for multijunction manufacture because of its close match to the crystal lattice parameters of GaAs and other related III-V materials and its relatively low cost.

US patent 7,339,109, filed by Stan et al on 19 June 2001, assigned to Emcore Corporation and titled "Apparatus And Method For Optimizing The Efficiency Of Germanium Junctions In Multi- Junction Solar Cells", includes a useful overview of issues relating to formation of Ge subcells in prior art multijunction cells. US 7,339,109 provides the following summary:

"The energy conversion characteristic of a solar cell is dependent on the effective utilization of the available solar spectrum. Currently, a state-of-the- art solar cell is a multi-junction device that uses layers of indium gallium phosphide (InGaP), gallium arsenide (GaAs), and germanium (Ge). This triple-junction structure is based on an older dual-junction solar cell structure made of indium gallium phosphide (InGaP) and gallium arsenide (GaAs) covering the absorption spectrum from UV to 890 nm. The addition of a germanium (Ge) junction to the dual-junction structure extends the absorption edge to 1800 nm. Since the germanium (Ge) junction causes increased access to the solar spectrum, the current generated in the germanium (Ge) junction is usually very high. The germanium (Ge) junction is not likely to limit the overall current of this serially connected multi-junction structure. Thus, the contribution of a germanium (Ge) junction improves the energy conversion efficiency by adding open-circuit voltage. Therefore, it becomes extremely important to optimize the open-circuit voltage of the germanium (Ge) junction without sacrificing the overall performance of the solar cell.

FIG. 1 [not included in this present specification] is a diagram that depicts the formation of a typical diffused germanium (Ge) junction on a p-type substrate. As FIG. 1 illustrates, the junction is formed by the diffusion of arsenic (As) and/or phosphorus (P) into the germanium (Ge) so that the conduction element ofp-type substrate is converted into n-type. Arsenic is an n-type impurity in germanium with a solubility, at metal organic chemical vapor deposition (MOCVD) growth temperatures, of 8><10¹⁹ cm3. In the prior art an electro-optically active germanium junction is formed as a consequence of arsenic diffusion into the p-type germanium substrate during the growth of arsenic-containing overlying epilayers.

A critical factor in maximizing the open circuit voltage characteristic is the control of the depth of the germanium (Ge) junction. As a consequence of the solid state diffusion process, the n-type germanium emitter is highly doped. As a result, most of the photo-generated carriers in this region will recombine before collecting at the n-p junction. This leads to an increased reverse saturation current (or referred to as "dark current ") and in a concomitant reduction in the open circuit voltage (Voc) of the cell. Additionally, one would like to minimize the junction depth because the highly doped emitter region acts as an absorber of the' incident long wavelength solar radiation. The increased absorption of long wavelength radiation causes lower short circuit current (Jsc) in the cell, which in turn, reduces the open circuit current of the stack. This results in less than optimum performance.

The depth of the diffused germanium junction is a function of the thermal load that results from the time-temperature profile of the epilayers grown on top of the p-type germanium substrate. Optimization of the germanium junction cannot be accomplished without affecting the subsequent dual junction epilayer device process. More specifically, to control the arsenic diffusion of the germanium substrate, the growth time and temperature of the overlying dual junction epilayer structure must be minimized. Thus, the integrity of the dual junction epilayer structure may be compromised to obtain an appropriate arsenic diffusion profile on the germanium substrate. "

US 7,339,109 further describe a technique for minimising the diffusion depth of dopants from the middle subcell into the germanium substrate. US 7,339,109 notes that Group V elements are the dominant species that diffuse into Ge and that arsenic diffuses approximately 4 times further into Ge than phosphorous does. The proposed technique therefore uses a layer of phosphorous containing material (MGaP) to form a diffusion barrier for arsenic-containing subcell layers. Instead, this layer provides a source of phosphorous atoms as n-type dopants for the Ge subcell. The advantage of this approach is that for a given heat load (temperature x time) phosphorous diffuses more slowly and forms a shallower junction. Quoted junction depths are reduced by 50%.

However, although this process provides a minor improvement in the control of junction depth, it also potentially introduces unfavourable band alignments in the region of the junction, in particular in the conduction band. This can create a barrier to carrier flow and increase cell resistance, thereby lowering efficiency. Secondly, diffusion processes are generally unreliable and do not produce uniform abrupt junctions. This means, for example, temperature needs to be controlled accurately across the entire wafer to ensure uniform diffusion and if it is not, cell yield can suffer. Finally, given the need for a high conductivity substrate, germanium junctions formed by diffusion are heavily compensated which leads to non-ideal subcell characteristics.

Given these shortcomings, there is a need for a multijunction cell structure which provides accurate control of germanium cell parameters at time of manufacture, which uses materials whose band alignment and carrier transport characteristics are optimised and which allows germanium doping densities to be chosen independently of other cell parameters.

US patent 6,340,788, filed by King et al on 2 December 1999, assigned to Hughes Electronic Corporation and titled "Multi-Junction Photovoltaic Cells and Panels Using a Silicon or Silicon Germanium Active Substrate Cell For Space and Terrestrial Applications", describes the use of substrates other than germanium in multijunction structures.

US 6,340,788 describes the use of silicon and silicon germanium as "active substrates" in multijunction cells. The attraction of silicon related materials is understood to be because they are stronger, less expensive and less dense (which is important in space applications). US 6,340,788 further describes a series of elaborate 3, 4 and 5 junction cells wherein the substrate forms one of the active subcells. US 6,340,788 also describes the use of so-called "transition layers" that are used to adjust the crystal lattice spacing from one value to another to facilitate the subsequent deposition of different materials with different lattice constants and bandgaps. US 6,340,788 describes the use of these transition layers at any place in the multijunction cell structure.

Although US 6,340,788 refers to known techniques for depositing transition layers, it is notably silent on the practicality of using these techniques to achieve low defect densities in subcell crystal lattices. This has been the central problem in prior art cells where materials are chosen from their bandgap properties alone. Without good crystal quality in the subcells, carrier lifetimes and overall cell efficiencies will be degraded in elaborate multijunction structures rather than being enhanced. US 6,340,788 also proposes the use of multiple transition layers which potentially has a significant detrimental effect on crystal lattice quality.

Therefore there is a need for an improved strategy in the use of transition layers to achieve the best possible crystal quality. There is also a need for a new multijunction cell structure which optimises cell efficiency through the use of a more sophisticated choice of subcell materials.

US 6,340,788 also proposes the use of Si or SiGe substrates without consideration of the significant difference between the thermal expansion coefficient of silicon and the III-V semiconductors proposed for the multijunction subcells. For example the thermal expansion coefficient for Si is around 2.5 ppm per degree Celsius and GaAs is around 6ppm. This difference causes considerable degrees of stress in epitaxial films as they cool from growth temperatures of around 600 degrees Celsius to room temperature. In particular, since III-V materials shrink more than Si on cooling, significant crystal defects and even cracks can form.

Therefore there is also a need for a new process for growing III-V multijunction cells on silicon substrates which overcomes the difficulties associated with differences in thermal expansion coefficients.

Accordingly, there is a demonstrable need for a new CPV cell module design that overcomes the shortcomings of the prior art. There is also a need for a method of manufacturing this new design at low cost.

The discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain prior art problems by the inventor and, moreover, any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art on or before the priority date of the disclosure and claims herein.

OBJECT OF THE INVENTION

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

It is an object of the invention in its preferred form to provide an improved CPV device, or method of monitoring an improved CPV device.

SUMMARY OF THE INVENTION

According to an aspect of the invention there is provided a photovoltaic cell comprising: an inactive substrate; and at least one epitaxial layer deposited on the substrate.

Preferably, the photovoltaic cell is a multijunction photovoltaic cell. More preferably, the impurity concentration of the inactive substrate is at least ten times greater than the average impurity concentration of the epitaxial layer or layers.

Preferably, the inactive substrate is primarily comprised of germanium. Alternatively, the inactive substrate is primarily comprised of silicon.

According to an aspect of the invention there is provided a photovoltaic cell comprising: an inactive substrate; and a first epitaxial layer deposited on the substrate; wherein the first epitaxial layer comprises at least 95 percent germanium.

According to an aspect of the invention there is provided a photovoltaic cell comprising: an inactive substrate; and two or more epitaxial layers comprising Group IV semiconductors. According to an aspect of the invention there is provided a multijunction photovoltaic cell including five junction cells and comprising both Group IV and Group III-V semiconductors epitaxially deposited on an inactive substrate.

Preferably, the Group IV semiconductor structures comprise two silicon germanium subcells.

According to an aspect of the invention there is provided a five-junction multijunction photovoltaic cell structure comprising two subcells made from Group III/V compound semiconductors, two SiGe subcells and one Ge subcell on an inactive substrate.

According to an aspect of the invention there is provided photovoltaic cell comprising: an inactive substrate; and at least one epitaxial layer deposited on the substrate.

Preferably, the impurity concentration of the inactive substrate is at least ten times greater than the average impurity concentration of the epitaxial layer or layers.

Preferably, the inactive substrate is primarily comprised of germanium. Alternatively, the inactive substrate is preferably primarily comprised of silicon.

Preferably, the photovoltaic cell comprises two or more epitaxial layers comprising Group IV semiconductors.

Preferably the photovoltaic cell comprises one or more subcells comprising Group IV semiconductors; and one or more subcells comprising Group III-V semiconductors. More preferably, the Group IV subcells and the Group III-V subcells are formed by means of epitaxial growth on the surface of the inactive substrate.

Preferably, a first epitaxial layer deposited on the substrate comprises at least 95 percent germanium.

Preferably, the photovoltaic cell comprises two or more epitaxial layers comprising Group IV semiconductors;

Preferably, the photovoltaic cell comprises one or more subcells comprising Group IV semiconductors; and one or more subcells comprising Group III-V semiconductors; wherein the Group IV subcells and the Group III-V subcells are formed by means of epitaxial growth on the surface of the inactive substrate. Preferably, the photovoltaic cell comprises a first subcell comprising germanium or silicon and germanium wherein silicon represents no more than 5 percent of the atomic composition; and a second subcell comprising silicon and germanium wherein silicon represents more than 5 percent and less than 30 percent of the atomic composition; and one or more subcells comprising Group III-V semiconductors;

Preferably, the photovoltaic cell comprises one or more subcells comprising Group IV semiconductors; one or more subcells comprising Group III-V semiconductors; and at least one transition layer; wherein the at least one transition layer has a non-constant crystal lattice spacing and comprises Group IV semiconductors and the Group III-V semiconductor subcells have a fixed, unchanging crystal lattice spacing.

Preferably, the photovoltaic cell comprises one or more subcells comprising Group IV semiconductors; one or more subcells comprising Group III-V semiconductors; and at least one diffusion barrier layer; wherein the at least one diffusion barrier layer is located between the Group IV semiconductor and the Group III-V semiconductor subcells.

Preferably, the photovoltaic cell comprises a first subcell comprising germanium or. silicon and germanium wherein silicon represents no more than 5 percent of the atomic composition; and a second subcell comprising silicon and germanium wherein silicon represents more than 5 percent and less than 30 percent of the atomic composition; and one or more subcells comprising Group III-V semiconductors.

Preferably, the inactive substrate is an inactive germanium substrate, and the photovoltaic cell comprises a first subcell comprising germanium; a second subcell comprising silicon and germanium; a third subcell comprising gallium, arsenic and phosphorous; and a fourth subcell comprising indium, gallium and phosphorous; wherein the first subcell is deposited on the inactive substrate, the second subcell is deposited on the first subcell, the third subcell is deposited on the second subcell, and the fourth subcell is deposited on the third subcell.

Preferably, the second subcell comprises 17 percent silicon and 83 percent germanium.

Preferably, the third subcell comprises 17 percent phosphorous and 83 percent Arsenic.

Preferably, the fourth subcell comprises 40 percent indium and 60 percent gallium. Preferably, the inactive substrate is an inactive germanium substrate, the photovoltaic cell comprises: a first subcell comprising germanium; a second subcell comprising silicon and germanium; a third subcell comprising a gallium, arsenic and nitrogen; a fourth subcell comprising gallium, arsenic and phosphorous; and a fifth subcell comprising indium, gallium and phosphorous; wherein the first subcell is deposited on the inactive substrate, the second subcell is deposited on the first subcell, the third subcell is deposited on the second subcell, the fourth subcell is deposited on the third subcell and the fifth subcell is deposited on the fourth subcell.

Preferably, the fourth subcell comprises 17 percent phosphorous and 83 percent arsenic.

Preferably, the fifth subcell comprises 40 percent indium and 60 percent gallium.

Preferably, the inactive substrate is an inactive silicon substrate, and the photovoltaic cell comprises: a buffer layer deposited on the silicon substrate; one or more subcells comprising Group IV semiconductors deposited on the buffer layer; and one or more subcells comprising Group III-V semiconductors deposited on the Group IV subcells; wherein the buffer layer comprises a region where the crystal lattice of the buffer layer has been modified after deposition to make it partly of fully amorphous and where the surface of the buffer layer is suitable for the epitaxial growth of the Group rv subcells.

Preferably, the modification is performed by ion implantation. More preferably, the ion implantation is performed at a temperature higher than room temperature. Most preferably, the ion implantation is performed at approximately 120 degrees Celsius.

Preferably, the buffer layer is annealed at a temperature between 600 and 1100 degrees Celsius to reduce surface defects prior to deposition of the Group IV subcells.

Preferably, the buffer layer comprises SiGe. More preferably, the buffer layer comprises SiGe and the proportion of Ge content of the buffer layer increases with distance away from the surface of the substrate. Preferably, the surface of the inactive substrate is at an angle of between 3 and 9 degrees from the substrate's (100) crystal plane.

Preferably, the photovoltaic cell according comprises: a first subcell comprised substantially of germanium and deposited on the substrate; a second subcell comprised substantially of silicon germanium and deposited on the first subcell; a third subcell comprised substantially of silicon germanium and deposited on the second subcell; a fourth subcell comprised substantially of Gallium Arsenide Phosphide and deposited on the third subcell; and a fifth subcell comprised substantially of Indium Gallium Phosphide and deposited on the fourth subcell.

Preferably, the inactive substrate is primarily comprised of germanium.

Preferably, the second and third silicon germanium subcells are comprised of approximately 17% silicon and 83% germanium.

Preferably, the Group V composition of the fourth subcell is approximately 17% phosphorous and 83% arsenic.

Preferably, the Group III composition of the fifth subcell is approximately 40% indium and 60% gallium.

Preferably, the photovoltaic cell comprises a transition layer between the first subcell and the second subcell which has a graded lattice constant.

According to an aspect of the invention there is provided a method of manufacturing a photovoltaic cell, the method comprising the steps of: epitaxially growing Group IV subcells on a substrate in a first growth chamber; transferring the substrate to a second growth chamber; and epitaxially growing Group III-V subcells on the substrate in a second growth chamber.

Preferably, the method further comprising the steps of: depositing an oxidation barrier layer onto the surface of the Group IV subcells; and heating the substrate to remove the oxidation barrier layer.

Preferably, the oxidation barrier layer is germanium. According to an aspect of the invention there is provided a multijunction photovoltaic cell structure with improved conversion efficiency comprising at least three subcells on an inactive substrate.

According to an aspect of the invention there is provided a method of manufacturing the multijunction photovoltaic cell structures.

According to an aspect of the invention there is provided a multijunction photovoltaic cell structure comprising Group IV and Group III-V subcells formed by epitaxial growth on a low cost inactive substrate.

According to an aspect of the invention there is provided a 4 or 5 junction cell design offering improved conversion efficiencies.

According to an aspect of the invention there is provided a multijunction photovoltaic cell structure comprising a silicon substrate and an amorphous stress relieving layer.

According to an aspect of the invention there is provided a method of manufacturing multijunction cells incorporating lattice transition layers and buffer layers which improve manufacturability.

According to an aspect of the invention there is provided a method of manufacturing multijunction cells comprising both Group IV and Group III-V subcells comprising use of separate growth chambers.

According to an aspect of the invention there is provided a method of manufacturing multijunction cells comprising use of an oxidation barrier layer to protect substrate surfaces as they are transported between growth chambers.

According to an aspect of the invention there is provided a metallisation processes which can be used in concentrator photovoltaic cells. Preferably, the process relates to metallisation structures with high aspect ratios and minimal shading loss.

According to an aspect of the invention there is provided a photovoltaic cell comprising metallisation structure gridline having a high aspect ratio. Preferably, the gridline further provides a relatively low shading loss.

Preferable, photovoltaic cell as described herein, comprising gridlines having thickness to width aspect ratio of at least 2:1. According to an aspect of the invention there is provided a photovoltaic cell comprising gridlines having a thickness to width aspect ratio of at least 2:1.

Preferably, the thickness to width aspect ratio is at least 5:1. Preferably, the photovoltaic cell comprises a multijunction cell. Preferably, the gridlines are produced by an electroplating process. Preferably, the gridlines comprise copper. Preferably, the gridlines have a meandering shape.

Preferably, the meandering is less than 10% of the distance separating adjacent gridlines.

Preferably, the gridlines comprise lateral support structures extending perpendicular to the length dimension of the gridline.

According to an aspect of the invention there is provided method of producing a , photovoltaic cell comprising gridlines having a high thickness to width aspect ratio, substantially as herein described herein.

According to an aspect of the invention there is provided a photovoltaic cell comprising metal gridlines with aspect ratios of at least 2:1.

According to an aspect of the invention there is provided a photovoltaic cell comprising gridlines which have a meandering shape.

According to an aspect of the invention there is provided a photovoltaic cell comprising gridlines which have lateral support features.

According to an aspect of the invention there is provided an improved structure for optical prisms used to guide light onto solar cells in concentrating photovoltaic modules.

According to an aspect of the invention there is provided a photovoltaic cell as described herein, further comprising a light guiding prism.

According to an aspect of the invention there is provided a photovoltaic cell as described herein, comprising: a transparent prism material; a first surface; a second surface; and a central light guiding structure; wherein light is focused onto a region of the first surface and propagates along the central light guiding structure to a region of the second surface; and the central light guiding structure is formed by a void in the prism material surrounding the central light guiding structure.

According to an aspect of the invention there is provided a light guiding prism for a photovoltaic cell.

Preferably, the guiding prism includes a transparent prism material. More preferably, the guiding prism includes a first surface; a second surface; and a central light guiding structure. Most preferably, the guiding prism focuses light onto a region of the first surface and propagates along the central light guiding structure to a region of the second surface; and the central light guiding structure is formed by a void in the prism material surrounding the central light guiding structure.

According to an aspect of the invention there is provided a light guiding prism for a photovoltaic cell, the light guiding prism comprising a transparent prism material; a first surface; a second surface; and a central light guiding structure; wherein light is focused onto a region of the first surface and propagates along the central light guiding structure to a region of the second surface; and the central light guiding structure is formed by a void in the prism material surrounding the central light guiding structure.

Preferably, the void extends from the second surface into the prism and separates the second surface into separate regions.

Preferably, the transparent prism material is glass. Preferably, the transparent prism material is a polymer.

According to an aspect of the invention there is provided a photovoltaic system comprising the photovoltaic cell as described herein, the system further comprising a light guiding prism as described herein. According to an aspect of the invention there is provided a photovoltaic system comprising the photovoltaic cell as described herein, the system further comprising: an optical concentrating means; and a prism comprising a transparent material, a first surface and a second surface; wherein the prism further comprises an anti-reflective coating on a first region of the first surface of the prism, and a reflective coating on one or more second regions of the first surface of the prism; the optical concentrating means focuses light onto the first region of the first surface of the prism; and the light exits the prism in a region on the second surface.

According to an aspect of the invention there is provided a photovoltaic system comprising: an optical concentrating means; and a prism comprising a transparent material, a first surface and a second surface; wherein the prism further comprises an anti-reflective coating on a first region of the first surface of the prism, and a reflective coating on one or more second regions of the first surface of the prism; the optical concentrating means focuses light onto the first region of the first surface of the prism; and the light exits the prism in a region on the second surface.

According to an aspect of the invention there is provided a photovoltaic cell further comprising: light guiding prism comprising a first surface and a second surface; and a central light guiding structure formed by a void in the prism surrounding the central light guiding structure; a photovoltaic cell; a support structure; wherein the photovoltaic cell is mounted on the support structure, the light guiding prism is mounted on the support structure and a portion of the second surface of the prism is operatively coupled to the photovoltaic cell, and light is focused onto a region of the first surface and propagates along the central light guiding structure to a region of the second surface.

Preferably, the photovoltaic cell comprises: a light guiding prism comprising a first surface and a second surface; and a central light guiding structure formed by a void in the prism surrounding the central light guiding structure; a photovoltaic cell; a support structure; wherein the photovoltaic cell is mounted on the support structure, the light guiding prism is mounted on the support structure and a portion of the second surface of the prism is operatively coupled to the photovoltaic cell, and light is focused onto a region of the first surface and propagates along the central light guiding structure to a region of the second surface.

Preferably, the void is sealed from the atmosphere when the prism is mounted on the support structure.

According to an aspect of the invention there is provided a method of manufacturing a light guiding prism including the step of moulding or casting.

According to an aspect of the invention there is provided a method of manufacturing a light guiding prism including the step of machining and polishing.

Preferably, the transparent prism material is glass. Alternatively, the transparent prism material is preferably a polymer.

According to an aspect of the invention there is provided a structure for a light guiding prism for concentrator photovoltaic subsystems comprising a central guiding structure and a surrounding protective surface.

According to an aspect of the invention there is provided a method of manufacturing a light guiding prism for concentrator photovoltaic subsystems using industry standard manufacturing processes suited to high volume, automated, low cost manufacture.

According to an aspect of the invention there is provided a structure for a light guiding prism for concentrator photovoltaic subsystems comprising a surface which has an optically reflective coating on portions of the surface and an anti-reflective coating on other portions of the surface.

According to an aspect of the invention there is provided a photovoltaic cell module which is suited to high volume, low cost manufacturing processes and which comprises an improved light guiding prism.

According to an aspect of the invention there is provided a panel assembly for Concentrator Photovoltaic power systems. Preferably, the assembly method enables a lower cost of these systems.

According to an aspect of the invention there is provided a Fresnel lens element, the lens element characterised by a normal vector which is perpendicular to the plane of the lens element. Preferably, the lens comprises a plurality of angled facets, each facet being characterised by normal vectors which are perpendicular to the surface at each point on the surface of the facets. More preferably, the lens comprises a plurality of side facets, each facet being characterised by normal vectors which are perpendicular to the surface at each point on the surface of the facets. Most preferably, each of the normal vectors of the side facets are perpendicular to the normal vector of the plane of the lens element, each of the normal vectors of the angled facets are not perpendicular to the normal vector of the plane of the lens element.

According to an aspect of the invention there is provided a Fresnel lens element when operatively associated with a photovoltaic cell as described herein, the lens element characterised by a normal vector which is perpendicular to the plane of the lens element, the lens comprising: a plurality of angled facets, each facet being characterised by normal vectors which are perpendicular to the surface at each point on the surface of the facets; and a plurality of side facets, each facet being characterised by normal vectors which are perpendicular to the surface at each point on the surface of the facets; wherein each of the normal vectors of the side facets are perpendicular to the normal vector of the plane of the lens element, each of the normal vectors of the angled facets are not perpendicular to the normal vector of the plane of the lens element.

According to an aspect of the invention there is provided a Fresnel lens element, the lens element characterised by a normal vector which is perpendicular to the plane of the lens element, the lens comprising: a plurality of angled facets, each facet being characterised by normal vectors which are perpendicular to the surface at each point on the surface of the facets; and a plurality of side facets, each facet being characterised by normal vectors which are perpendicular to the surface at each point on the surface of the facets; wherein each of the normal vectors of the side facets are perpendicular to the normal vector of the plane of the lens element, each of the normal vectors of the angled facets are not perpendicular to the normal vector of the plane of the lens element.

Preferably, the maximum angle between normal vectors of any two points on the surface of the side facets is 180 degrees.

Preferably, the lens is comprised of a polymer.

According to an aspect of the invention there is provided a Fresnel lens when operatively associated with a photovoltaic cell as prescribed herein, the lens comprising two or more lens elements as described herein.

According to an aspect of the invention there is provided a Fresnel lens comprising two or more lens elements as described herein.

Preferably, the Fresnel lens comprises a plurality of lens elements wherein the lens elements are arranged to form a concentric lens structure.

Preferably, the Fresnel lens comprises a plurality of lens elements wherein the lens elements are mounted on a transparent substrate. More preferably, the transparent substrate is glass.

Preferably, the lens elements are mounted using transparent adhesive.

According to an aspect of the invention there is provided a method of manufacturing a Fresnel lens element, the method comprising the steps of: injecting a polymer into a mould, the mould comprising a first and second portion which from a central cavity and which are separable along a parting line; causing the polymer to solidify, and separating the portions of the mould in a manner such that the portion of the mould in contact with the side facets of the lens element is moved in a direction away from the facets.

According to an aspect of the invention there is provided a CPV panel assembly comprising: at least one photovoltaic receiver module, each including a photovoltaic cell as described herein; at least one Fresnel lens elements as described herein; a glass substrate; wherein the Fresnel lenses and metallic support structures are mounted onto and are supported only by the glass substrate and the photovoltaic receiver modules are mounted onto the metallic support structures.

According to an aspect of the invention there is provided a CPV panel assembly comprising: at least one photovoltaic receiver module, at least one Fresnel lens; a metallic support structures; a glass substrate; wherein the Fresnel lenses and metallic support structures are mounted onto and are supported only by the glass substrate and the photovoltaic receiver modules are mounted onto the metallic support structures.

Preferably, each Fresnel lens comprises a plurality of Fresnel lens elements as described herein.

Preferably, the metallic support structures are manufactured using a casting process. More preferably, the metallic support structures are comprised of aluminium or an aluminium alloy. Preferably, the glass substrate has an area greater than 0.5 square metres. More preferably, the glass substrate is at least 3 millimetres thick. Most preferably, the glass substrate is 6 millimetres thick.

Preferably, the metallic support structure comprises: at least one supporting leg, the at least one leg comprising a mounting feature at one end for attaching the at least one leg to a glass panel and a heat spreading region at the other end of the at least one leg onto which a photovoltaic receiver is mounted.

According to an aspect of the invention there is provided a metallic support structure for a CPV panel assembly comprising: at least one supporting leg, the at least one leg comprising a mounting feature at one end for attaching the at least one leg to a glass panel and a heat spreading region at the other end of the at least one leg onto which a photovoltaic receiver is mounted.

Preferably, the metallic support structure comprises three legs. More preferably, metallic support structure comprises a plurality of legs and side surface structures which enclosed the space between the plurality of legs.

According to an aspect of the invention there is provided an improved structure for a Fresnel lens which eases manufacturing requirements.

According to an aspect of the invention there is provided a method of manufacturing an improved Fresnel lens structure.

According to an aspect of the invention there is provided a structure for a CPV panel that reduces material usage and improves use of modular components.

According to an aspect of the invention there is provided a method of manufacturing a CPV panel using modular components.

According to an aspect of the invention there is provided a concentrating photovoltaic subsystems having adaptive means of optical concentration.

According to an aspect of the invention there is provided a photovoltaic system comprising one or more photovoltaic cell and one or more a movable optical elements. Preferably, the system further comprises one or more primary optical concentrating elements. More preferably, the one or more primary optical elements and the one or more photovoltaic cells are mounted such that they do not move relative to the panel frame; the one or more movable optical elements are mounted such that they can move relative to the panel frame. Most preferably, each of the one or more movable optical elements spatially redirects light after it is concentrated by one of the one or more primary optical elements and before it reaches one of the one or more photovoltaic cells.

According to an aspect of the invention there is provided a photovoltaic system comprising one or more photovoltaic cell as described herein, the system further comprising: a panel frame; one or more primary optical concentrating elements; one or more a movable optical elements; wherein the one or more primary optical elements and the one or more photovoltaic cells are mounted such that they do not move relative to the panel frame; the one or more movable optical elements are mounted such that they can move relative to the panel frame; and each of the one or more movable optical elements spatially redirects light after it is concentrated by one of the one or more primary optical elements and before it reaches one of the one or more photovoltaic cells.

According to an aspect of the invention there is provided a photovoltaic system comprising one or more photovoltaic cell as described herein, the system further comprising: a panel frame; one or more primary optical concentrating elements; one or more a movable optical elements; and one or more actuators; wherein the one or more primary optical elements and the one or more photovoltaic cells are mounted such that they do not move relative to the panel frame; the one or more movable optical elements are mounted such that they can move relative to the panel frame; each of the one or more movable optical elements spatially redirects light after it is concentrated by one of the one or more primary optical elements and before it reaches one of the one or more photovoltaic cells; each of the movable optical elements is operatively coupled to one of the actuators; and each of the actuators sense the relative position of the focused light beam with respect to the centre of the cell and adjusts the orientation of the operatively coupled movable optical elements to keep focused light on the centre of the cell.

According to an aspect of the invention there is provided a photovoltaic system comprising: a panel frame; one or more primary optical concentrating elements; one or more a movable optical elements; one or more photovoltaic cells; wherein the one or more primary optical elements and the one or more photovoltaic cells are mounted such that they do not move relative to the panel frame; the one or more movable optical elements are mounted such that they can move relative to the panel frame; and each of the one or more movable optical elements spatially redirects light after it is concentrated by one of the one or more primary optical elements and before it reaches one of the one or more photovoltaic cells.

According to an aspect of the invention there is provided a photovoltaic system comprising: a panel frame; one or more primary optical concentrating elements; one or more photovoltaic cells; one or more a movable optical elements; and one or more actuators; wherein the one or more primary optical elements and the one or more photovoltaic cells are mounted such that they do not move relative to the panel frame; the one or more movable optical elements are mounted such that they can move relative to the panel frame; each of the one or more movable optical elements spatially redirects light after it is concentrated by one of the one or more primary optical elements and before it reaches one of the one or more photovoltaic cells; each of the movable optical elements is operatively coupled to one of the actuators; and each of the actuators sense the relative position of the focused light beam with respect to the centre of the cell and adjusts the orientation of the operatively coupled movable optical elements to keep focused light on the centre of the cell.

Preferably, the one or more primary optical elements are each a lens. More Alternatively, the one or more primary optical elements are preferably each a mirror.

Preferably, the one or more movable optical elements are each a mirror. Alternatively, the one or more movable optical elements are each preferably a lens.

Preferably, the one or more movable optical elements are moved using attraction or repulsion of magnetic fields. More preferably, at least one of the magnetic fields is produced by current flowing from the photovoltaic cells.

According to an aspect of the invention there is provided an adjustable optical concentrator for a CPV subsystem. Preferably, the sub system has increased optical acceptance angle, improved heat transfer characteristics from the cell, simplified structure and lower cost.

Preferably, the sub-system comprises a primary concentrating optical element and a photovoltaic cell that are fixed and cannot move relative to the CPV panel and a secondary optical element that is movable with respect to the panel and other elements. More preferably, the panel provides rigid mechanical support for the primary optical elements. Most preferably, because the cell does not move, simple heatsinking and interconnection features can also be provided for the cell by the panel housing and panel wiring.

Preferably, as relatively little energy is lost in the movable optical element, the subsystem does not produce significant amounts of heat and heatsinking is not necessary.

According to an aspect of the invention there is provided a photovoltaic subsystems used in solar energy converters, having to devices which monitor the performance of photovoltaic modules during normal operation and convey diagnostic information to a central data collection terminal. According to an aspect of the invention there is provided a photovoltaic system, comprising one or more cell modules, and one or more cell interconnection modules which do not contain photovoltaic cells.

Preferably, the one or more cell modules are coupled to each of the one or more cell interconnection modules and the cell interconnection modules are coupled to operatively connect the outputs of the cell modules in series. More preferably, a plurality of cell modules is coupled to one interconnection modules. Most preferably, the interconnection modules monitor coupled cell modules.

According to an aspect of the invention there is provided a photovoltaic system, comprising one or more cell modules each having at least one photovoltaic cell as described herein, the system further comprising: one or more cell interconnection modules which do not contain photovoltaic cells; wherein the one or more cell modules are connected to each of a plurality of the cell interconnection modules and the cell interconnection modules are connected to operatively connect the outputs of the cell modules in series.

According to an aspect of the invention there is provided a photovoltaic system, comprising: cell modules which contain photovoltaic cells; and cell interconnection modules which do not contain photovoltaic cells; wherein a plurality of the cell modules is connected to each of a plurality of the cell interconnection modules and the cell interconnection modules are connected to operatively connect the outputs of the cell modules in series.

Preferably, electrical connection ports of the cell modules and the cell interconnection modules comprise flexible cabling which is permanently attached to the modules. More preferably, the flexible cabling is joined to interconnect cell modules and cell interconnect modules using environmentally sealed terminating devices. Most preferably, the terminating devices comprise electrical crimp connections.

Preferably, the cell interconnection modules comprise electronic circuitry which monitors the voltage and/or current of each of the plurality of photovoltaic cells connected to the cell interconnection module. More preferably, the cell interconnection modules comprise electronic circuitry which encodes photovoltaic cell performance data produced by the cell interconnection module and produces a transmit signal which propagates along cell interconnection module cabling.

Most preferably, the electronic circuitry comprises an RF signal generator which is modulated by the transmit signal.

Preferably, the frequency of the RF signal generator is greater than 10MHz. More preferably, the frequency of the RF signal generator is greater than 100MHz. Most preferably, the frequency of the RF signal generator is chosen to fall within a designated ISM frequency.

Preferably, the transmit signal is produced by a modulator connected to the electronic circuitry and which modulates the frequency of an RF signal which is generated by an external RF signal source and fed to each cell interconnection module.

Preferably, the modulator comprises a non linear device such as an RF mixer.

Preferably, the modulator comprises a non linear device such as a variable capacitor. More preferably, the variable capacitor comprises a varactor diode.

Preferably, the transmit signal is sent in bursts, the bursts being limited in time such that the burst duration is small compared to the time interval between bursts. More preferably, the transmit signal burst duration is less than 1 percent of the average time interval between bursts.

Preferably, the time interval between bursts is random or pseudo-random.

Preferably, the time interval between bursts is determined by an algorithm comprising the previous transmit time interval value and a unique identification number assigned to each cell interconnection module.

Preferably, the photovoltaic system comprises a central receiver module which receives and decodes RF signals transmitted from the cell interconnection modules and which comprises a digital interface for communication with external computer equipment.

According to an aspect of the invention there is provided a cell interconnection module adapted to connect to one or more cell modules containing photovoltaic cells and having an output port. Preferably, the output port is adapted for connecting to another cell interconnect modules. Alternatively, the output port is preferably adapted for connecting to an electrical output terminal of a photovoltaic system panel.

Preferably, the cell interconnection module had a plurality of input ports, each input port adapted to connect to one cell modules containing a photovoltaic cell.

According to an aspect of the invention there is provided a cell interconnection module comprising: a plurality of bipolar input ports; and a single bipolar output port; wherein the input ports are intended to connect to cell modules containing photovoltaic cells and the output port is intended to connect to other cell interconnect modules or electrical output terminals of a photovoltaic system panel.

Preferably, the bipolar input ports and bipolar output ports comprise flexible cabling which is permanently attached to the cell interconnection module.

Preferably, the cell interconnection module comprises bypass diodes which are connected to each bipolar input port and oriented to provide reverse bias protection for photovoltaic cells connected to the input ports.

Preferably, the cell interconnection module comprises a filter structure which provides RF isolation between the bipolar output ports and the bipolar input ports and which provides RP coupling across the bipolar output port. More preferably, the filter structure comprises inductive elements formed by patterned conductors on printed circuit boards. Most preferably, the patterned conductors on printed circuit boards comprise tracks which are a quarter wavelength long at the RF carrier frequency used for conveying signalling information along cell interconnect module wiring.

According to an aspect of the invention there is provided a user access interface for a processor device, the processor device being adapted to monitors one or more photovoltaic cells, the interface comprising a control program adapted to communicate with a cell interconnection module coupled to one or more photovoltaic cells for receiving data indicative of voltage and/or current associated with each of the one or more photovoltaic cells from an interconnection module

Preferably, the photovoltaic cell is as described herein. More preferably, the interconnection module is as described herein.

According to an aspect of the invention there is provided a computer program product stored on a computer usable medium, the computer program product adapted to provide a method of monitoring one or more photovoltaic cells, the method including the step of receiving data indicative of voltage and/or current associated with each of the one or more photovoltaic cells from an interconnection module.

According to an aspect of the invention there is provided a computer program product stored on a computer usable medium, the computer program product adapted to provide a user access interface for a computer device, the computer device being adapted to receive access data indicative of voltage and/or current associated with each of one or more photovoltaic cells, the computer device being coupleable to an interconnection module; the computer program product comprising: computer readable program means for receiving data indicative of voltage and/or current associated with each of the one or more photovoltaic cells from an interconnection module.

According to an aspect of the invention there is provided a modular photovoltaic system comprising a plurality of cell modules and cell interconnection modules, the cell interconnection modules comprising: at least two bipolar electrical connection ports used to connect to at least two photovoltaic cell modules, and; a single bipolar output connection port used to connect the cell interconnection module to other cell interconnection modules or to photovoltaic panel outputs. Preferably, if a single cell module becomes faulty, it can be replaced without disturbing the rest of the photovoltaic system.

According to an aspect of the invention there is provided a photovoltaic cell interconnection module comprising at least two bipolar electrical connection ports used to connect to at least two photovoltaic cell modules, a single bipolar output connection port used to connect the cell interconnection module to other cell interconnection modules or to photovoltaic panel outputs, and a performance monitoring circuit which monitors the voltages and/or currents of each of the at least two cell modules. Preferably, by placing the performance monitoring circuitry in the cell interconnection module, multiple cells are monitored by a single circuit. More preferably, this reduces cost overhead of performance monitoring and ensures that sufficient cell voltage is available to power monitoring circuitry.

According to an aspect of the invention there is provided a photovoltaic cell interconnection module comprising at least two bipolar electrical connection ports used to connect to at least two photovoltaic cell modules, a single bipolar output connection port used to connect the cell interconnection module to other cell interconnection modules or to photovoltaic panel outputs, a performance monitoring circuit which monitors the voltages and/or currents of each of the at least two cell modules, and a modulating circuit which creates a high frequency time-varying signal which contains data corresponding to the voltages and currents as well as a unique identification code corresponding to the identity of the cell interconnection module. Preferably, high frequency used is, for example greater than 10 MHz. More preferably high frequency greater than 100MHz is used. The chosen frequency preferably resides in a designated ISM (Industrial, Scientific and Medical) radio frequency (RF) band. Most preferably, the signal containing cell performance data is located in a frequency band which is separated from low frequency photovoltaic system noise.

According to an aspect of the invention there is provided a photovoltaic cell interconnection module comprising a low cost filter structure which allows RF signals to be superimposed on the DC output connection port of the cell interconnection module. Preferably, by using a filter structure, the capacitive loading effects of the cell modules can be isolated so that they do not effect the transmission of high frequency performance monitoring signals along DC interconnection wiring.

According to an aspect of the invention there is provided a photovoltaic cell interconnection module comprising circuitry which monitors the performance of a plurality of photovoltaic cells connected to the cell interconnection module and a modulation circuit which generates a radio frequency time varying signal at the DC output of the cell interconnection module, wherein the cell interconnection module does not contain a signal source at or near the centre of the frequency band of the radio frequency signal. Preferably, by avoiding the need for a high frequency oscillator inside each interconnection module, performance monitoring costs can be reduced.

According to an aspect of the invention there is provided a photovoltaic power generation system comprising a plurality of photovoltaic cell modules, a plurality of cell interconnection modules and a central signal analysis module, wherein each of the cell interconnection modules is connected to at least two photovoltaic cell modules and wherein the cell interconnection modules send time varying signals to the central signal analysis module wherein data contained in the time varying signals is converted to a computer compatible digital format. Preferably, the connection of a computing device for collecting and analysing cell performance data is simplified.

According to an aspect of the invention there is provided a signalling protocol for transmission of photovoltaic cell performance data from cell interconnection modules to a central monitoring location without the need to synchronise the transmitted signals to a central reference timing signal or timing controller. Preferably, transmitted signals are bursts of data which are sent at random times. More preferably, a random timing sequence is determined by a unique identification code in each cell interconnection module and is different for each interconnection module. Most preferably, by avoiding the need for synchronisation of signals transmitted from interconnection modules, performance monitoring system costs can be significantly reduced.

These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. Accordingly, further scope of applicability of embodiments of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the disclosure herein will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. IA shows a simplified view of a multijunction cell according to an embodiment;

FIG. IB shows a simplified view of the multijunction cell of FIG IA, including transition layer and diffusion barrier layer;

FIG. 1C shows a simplified view of the multijunction cell of FIG IA, showing individual Group IV subcells;

FIG. ID shows a simplified view of the multijunction cell of FIG IA, showing individual Group IV subcells and transition and diffusion barrier layers;

FIG. 2A shows a detailed view of a four junction cell according to an embodiment; FIG. 2B shows a detailed view of a five junction cell according to an embodiment;

FIG. 3 A shows a simplified view of a multijunction cell according to an embodiment, showing an oxidation barrier layer deposited on top of Group IV subcells after subcell growth and before transfer to a second growth chamber;

FIG. 3B shows a simplified view of a multijunction cell according to an embodiment, showing removal of the oxidation barrier layer prior to growth of Group III-V subcells in a second growth chamber;

FIG. 3 C shows a simplified view of a multijunction cell according to an embodiment, showing the cell after growth of the Group III-V subcells in the second chamber;

FIG. 3D shows a simplified flow chart of the process used to make the multijunction cell according to an embodiment;

FIGs 4A to 4D show the steps of making a multijunction cell on a silicon substrate according to an embodiment;

FIG. 5 shows a simplified view of a five-junction cell according to an embodiment; FIG. 6 is a diagrammatic view of a multijunction cell. FIG. 7 shows a simplified side view of a photovoltaic cell with high aspect ratio gridlines according to an embodiment;

FIG. 8 shows a simplified perspective view of meandering gridlines according to an embodiment;

FIG. 9 shows a simplified perspective view of gridlines incorporating lateral support structures according to an embodiment;

FIG. 1OA shows a perspective view of CPV subsystem assembly using cassegrain reflectors;

FIG. 1OB shows side view of CPV subsystem assembly using cassegrain reflectors; FIG. 11 shows a detailed perspective view of the cell assembly of FIG 1OA;

FIG. 12 shows a side view of a CPV cell module comprising an improved light guiding prism according to an embodiment;

FIG 13A is a side cross section view and a top cross section view of an examples of an improved light guiding prism according to an embodiment;

FIG 13B is a side cross section view and a top cross section view of an examples of an improved light guiding prism according to an embodiment;

FIG 14A is a side cross section view and a top cross section view of an examples of an improved light guiding prism comprising reflective and anti-reflective portions of the surface according to an embodiment;

FIG 14B is a side cross section view and a top cross section view of an examples of an improved light guiding prism comprising reflective and anti-reflective portions of the surface according to an embodiment;

FIG. 15 is a side view and a top cross section view of an improved light guiding prism comprising a central light guiding structure with facets formed on a portion of the structure according to an embodiment;

FIG. 16 is a perspective view of a CPV panel assembly using Fresnel lenses; FIG. 17 is a detailed perspective view of the assembly shown in FIG. 16; FIG. 18 is a schematic view showing the derivation of Fresnel lens structure; FIG. 19A is a simplified cross section view of a portion of an ideal Fresnel lens showing lens surface features and associated refraction of light rays;

FIG. 19B is a simplified cross section view of the same portion of a Fresnel lens incorporating draft angles on lens facets and showing associated refraction and reflection of light rays;

FIG. 2OA is a simplified cross section view of a Fresnel lens which is comprised of at least two physically separate regions, according to an embodiment;

FIG. 2OB is a plan view of physically separate portions of a Fresnel lens according to an embodiment;

FIG. 2OC is a plan view of physically separate portions of a Fresnel lens being assembled to for a complete lens according to an embodiment;

FIG. 21 is a side view of a glass panel comprising a Fresnel lens fixed to the panel surface according to an embodiment;

FIG. 22A is a side view of a moulding cavity containing a Fresnel lens and showing cavity parting line;

FIG. 22B is a side view of a moulding cavity containing a Fresnel lens and showing the separation of the mould to remove the lens according to an embodiment;

FIG. 23 A is a modular component of a CPV panel showing a portion of a glass panel which has a hexagonal periphery and a metallic cell support structure according to an embodiment;

FIG. 23B is a modular component of a CPV panel showing a portion of a glass panel which has a hexagonal periphery and a metallic cell support structure including protective sidewalls according to an embodiment;

FIG. 24A is a plan view of a portion of a CPV panel showing an arrangement of modules as described in FIG. 23, mounted on a glass plate, according to an embodiment;

FIG. 25 is a sectional side view of a prior art CPV subsystem comprising a primary concentrating lens and a movable photovoltaic cell assembly; FIG. 26 is a schematic side view of an embodiment comprising a primary optical concentrating lens, a photovoltaic cell and a movable optical element;

FIG. 27A is a schematic side view of an embodiment comprising a primary and a secondary optical cassegrain reflector, a photovoltaic cell and movable optical element;

FIG. 27B is a schematic side view of an embodiment comprising a primary cassegrain reflector, a photovoltaic cell and a movable secondary cassegrain optical element;

FIG. 28 is a schematic view of an embodiment comprising a photovoltaic cell and a movable optical element coupled to a magnetic actuator;

FIG. 29 is a block diagram of a solar cell module containing performance monitoring circuitry;

FIG. 30 is a block diagram of a solar cell module containing performance monitoring circuitry;

FIG. 31 is a block diagram of an embodiment, showing cell modules and a cell interconnection module;

FIG. 32 is a block diagram of an embodiment, showing cell modules and multiple cell interconnection modules;

FIG. 33 is a block diagram of an embodiment, showing termination devices which are used to connect cell modules and cell interconnection modules.

FIG. 34 is a block diagram of an embodiment, showing internal architecture of a cell interconnection module;

FIG. 35 is a block diagram of an embodiment, showing internal architecture of a cell interconnection module having cell bypass diodes;

FIG. 36 is a block diagram of an embodiment, showing internal architecture of a cell interconnection module having individual circuit elements;

FIG. 37 is a block diagram of an embodiment, showing internal architecture of a cell interconnection module having individual circuit elements;

FIG. 38 is a block diagram of an embodiment, showing an arrangement of cell modules and cell interconnection modules in a PV panel; FIG. 39 is a block diagram of an embodiment, showing arrangement of cell modules and cell interconnection modules in a PV panel;

FIG. 40 is a circuit board layout of an embodiment, showing an implementation of signal filtering elements; and

FIG. 41 is a flow chart of a method for a signal transmission protocol.

PREFERRED EMBODIMENT OF THE INVENTION

In order to provide clarification for the accompanying description and claims, the following definitions are provided, whereby the terms are intended to include the following descriptions:

> The term "CPV" is an abbreviation of Concentrator Photo-Voltaic and refers to optical to electric power conversion systems using optical concentrators to collect and focus light onto photovoltaic cells;

> the term "CPV subsystem" is used to mean the combination of a cell module plus optical concentrating elements which focus light onto the cell module plus mechanical structures used to support and / or house the optical elements and the cell module;

> the term "panel" means an array of subsystems that are assembled and interconnected to form a single rigid structure;

> the term "panel frame" means the framework and protective coverings onto which CPV subsystems are mounted.

> the terms "module" or "cell module" or "receiver module" are used to mean the combination of the cell plus the structure immediately surrounding the cell, including means of making electrical contacts to the cell, means of dissipating waste heat from the cell and means of providing structural support or mounting for the cell and adjoining subsystem elements;

> the term "cell" is used to mean the semiconductor device which converts light into electrical energy;

> the term "cell interconnection module" is used to mean a device which contains means of interconnecting cells in series but which does not contain photovoltaic cells; > the term "sub-cell" is used to mean a particular portion of the overall cell comprising a semiconductor p-n junction that is responsive to a specific range of wavelengths of light;

> the term "multijunction cell" is used to mean a photovoltaic cell comprising multiple semiconductor layers having different doping and material properties and which are layered to form multiple photovoltaic junctions connected in series;

> the term "port" is used to mean an electrical connection point to a cell module or cell interconnection module;

> The terms "dopant" or "doped" refer to elements which are deliberately introduced into a semiconductor crystal lattice to obtain desirable electrical or optical properties;

> The term "impurity" is used to refer to elements that are inadvertently incorporated into a semiconductor material as a result of imperfect refinement or manufacturing processes;

> The term "gridline" is used to refer to a metal contact deposited on the photoactive side of a cell for the purpose of collecting photo-generated current.

> The term "length" when used to refer to a gridline indicates the dimension of the gridline parallel to the surface of the cell which is in the overall direction of current flow along the gridline;

> The term "width" when used to refer to a gridline indicates the dimension of the gridline parallel to the surface of the cell which is perpendicular to the overall direction of current flow along the gridline;

> The term "thickness" when used to refer to a gridline indicates the dimension of the gridline in the direction perpendicular to the surface of the cell.

> The term "aspect ratio" when used to refer to a gridline indicates the ratio of gridline thickness to gridline width;

> The terms "meanders" or "meandering" when used to refer to a gridline indicates a gridline which traverses the surface of the cell in a non-linear shape; > The term "prism" means a three dimensional region of dielectric material with refractive index greater than one.

Embodiments teach an improved CPV device, or method of producing an improved CPV device. International Patent Application No. PCT/AU2009/001350, entitled Photo- Voltaic Device", is herein incorporated by reference.

Multi-Junction Cells

Referring to FIG. IA, an embodiment can provide a multijunction photovoltaic cell structure comprising Group IV and Group III-V epitaxial photovoltaic subcell layers 101 and 106 respectively deposited on an inactive cell substrate 100. "Inactive" means that the substrate does not contain a photovoltaic junction and provides only a crystal template for growing epitaxial subcell layers and a means of connecting to the lowest subcell.

Conventionally, cell manufacturers utilise a germanium substrate to form the bottom multijunction subcell. The reason for doing this is that the Ge junction is perceived to come for "free" as part of the growth of upper multijunction cell layers. Instead, it was noted that there is a significant cost associated with these conventional Ge subcell layers and a number of significant production and performance advantages can be obtained by forming the lowest bandgap subcell layer epitaxially rather than as part of the substrate.

Presently the cost of an epi-ready germanium substrate is around $80 - $100 (USD). The cost of depositing multijunction cell layers epitaxially on the surface of the substrate is around $55 - $70. This means that around 60% of the cost of the multijunction substrate is associated with the germanium substrate itself. Unlike silicon, germanium is a relatively rare element in the earth's crust and is expensive to extract and refine to semiconductor grade quality. For example, the cost of unrefined germanium is around $1000 per kilogram, or $1 per gram. A 4 inch diameter Ge wafer which is 150 microns thick therefore contains around $6.50 of unrefined germanium. The difference between this base price and the $80-$100 cost of the epi ready wafer is associated with the purification and physical preparation of the wafer. If the bottom subcell is formed in the substrate, the entire substrate (and the crystal boule it is cut from) has to be produced to exacting standards which are costly. In particular, performance of the germanium subcell is critically dependent on minority carrier lifetimes in the material which need to be maximised for optimal efficiency. This means that impurities in the germanium material need to be reduced to a minimum which increases refining and production costs dramatically. Instead, it was noted that it is advantageous to relax the requirements for substrate material quality to lower costs. It is then possible to use the costs saved to form the germanium subcell epitaxially on the surface of the substrate. In forming the germanium junction this way, a much wider choice of subcell parameters is available and the subcells can be produced with high degrees of accuracy (which improves performance) and reproducibility (which increases manufacturing yield and lowers wastage costs).

A second advantage in using an inactive substrate relates to the freedom to choose a "n-on-p" (i.e. p-type substrate) or "p-on-n" structure for the multijunction subcells. In prior art devices comprising Ge junctions formed by diffusion, an n-on-p structure was needed because of the tendency for Group V elements to diffuse into the substrate, thereby doping it n-type. The use of epitaxial techniques to form the bottom subcell allows the freedom to choosέ dopant polarity such that the minority carrier transport, and hence conversion efficiency, in upper subcells is optimised.

An embodiment can provide a multijunction photovoltaic cell structure comprising epitaxial subcell layers made from multiple elements selected from Group IV of the Periodic Table of the Elements.

Although germanium is preferred as the photoactive layer of the bottom subcell in the multijunction cell structure, silicon germanium compound semiconductors may also be used. For example, by introducing 2 percent silicon into the germanium epitaxial layer the lattice constant of the material is reduced so that it exactly matches the lattice constant of GaAs without significantly changing the bandgap. Adding 2 percent silicon to the germanium also helps to stop diffusion between the SiGe layer and adjoining III- V semiconductor layers, thereby forming more abrupt, idealised junctions.

An embodiment can provide a multijunction photovoltaic cell structure comprising Group III-V subcells and one or more epitaxial layers made from elements selected from Group IV of the Periodic Table wherein the composition of the Group IV epitaxial layers is changed to alter the lattice constant of the crystal structure between two predefined values and where the lattice constant of Group III- V subcells is fixed and does not change.

Much effort has been invested in the selection of Group III- V materials and bandgaps in prior art multijunction cells. Many proposals have been made regarding the use of metamorphic epitaxial structures and transition layers where Group III-V material compositions are changed during the growth of epitaxial layers to achieve desired subcell bandgap characteristics. However, given the nature of Group III-V semiconductors, such transitions can lead to the formation of crystal defects which act as recombination centres for photo-generated carriers. Although defects can also be created in transition layers formed in Group IV semiconductors, it is understood that it is advantageous for cell efficiency to restrict the use of transition layers to layers comprising Group IV materials. It will be appreciated that the disclosed cells can have improved crystal quality and higher conversion efficiencies.

In forming the Group III-V subcells, it is advantageous for the inactive substrate surface to have a specific orientation to the crystal planes of the semiconductor. For example, if the substrate is comprised of germanium, it is preferable for the substrate surface to be oriented at between 3 and 9 degrees to the (100) crystal plane.

An embodiment can provide a multijunction photovoltaic cell structure comprising first epitaxial subcell layers made from Group IV elements and second epitaxial subcell layers made from Group III and Group V elements, wherein a diffusion barrier layer is deposited between the first and second subcell layers.

Referring to FIG. IB, Group IV subcells 111 are preferably deposited on inactive substrate 110. The Group IV subcells comprise a transition layer 114 which adjusts the crystal lattice constant from one value to another either as a discrete layer on top of the upper Group IV subcell or as one of the subcell layers themselves, for example the emitter layer. An optional diffusion barrier layer 115 is deposited at the interface between Group IV and Group III-V subcells. This layer may be combined with the transition layer as a single layer.

Unlike conventional multijunction cells where the bottom cell is formed by diffusion of elements into the cell substrate, an embodiment can provide improved cell performance and manufacturing reproducibility by minimising diffusion between adjacent subcell layers. Although inter-diffusion can be controlled to some extent by epitaxial growth conditions, an embodiment optionally comprises a diffusion barrier layer between Group IV and Group III-V subcells. The choice of suitable diffusion barriers depends on the materials used in adjacent subcells. For example, silicon or specific compositions of SiGe such as Sio._O2Ge_o.98. are suitable in certain circumstances.

An embodiment can provide a multij unction photovoltaic cell structure comprising a plurality of epitaxial subcell layers made from Group IV elements.

In order to increase cell efficiency above the level currently achieved with triple junction cells, additional subcells need to be added to the multijunction structure. It is noted that it is advantageous to use two subcells formed from Group IV elements to achieve this. An embodiment preferably comprises germanium or a SiGe compound semiconductor incorporating a small percentage of Si (e.g. < 5% Si) as the bottom (or first) subcell of the multijunction cell and SiGe with a higher Si content (e.g. up to 30% Si) as the second subcell deposited on top of the bottom subcell. Most preferably, the second subcell is Si_o.πGeo₈₃ (i.e. 17% Si 83% Ge). The reason for choosing this particular SiGe composition is that the bandgap of SiGe increases rapidly as the Si percentage increases from 0 to 17% and then increases more slowly. Therefore, a composition of 17% Si provides a relatively large bandgap (0.92eV) with a relatively small crystal lattice offset from germanium (5.619 A compared to 5.658 A for Ge).

Referring to FIG. 1C, bottom subcell 121 is deposited on inactive substrate 120. Second subcell 123 is deposited on top of bottom subcell 121. A transition layer (not shown in FIG. 1C) is included in either the bottom subcell, the second subcell or in between the subcells to adjust the lattice constant from the bottom cell value to the second cell value. FIG. ID shows possible locations of transition layers 132 and 134 relative to bottom subcell 131 and second subcell 133. Optional diffusion barrier layer 135 is also shown.

Referring now to FIG. 2 A, an embodiment of a 4 junction cell is described. Inactive substrate 200 is preferably comprised of germanium which is heavily doped to provide low resistivity between front and back surfaces of the substrate. For example the doping concentration is greater than IeI 8. The dopant type (n or p) is chosen to achieve optimal minority carrier transport characteristics in the overall multijunction cell. The impurity concentration of the substrate is relaxed to reduce substrate costs. For example the impurity concentration in the substrate might be at least an order of magnitude higher that conventional "semiconductor grade" germanium substrates.

Germanium subcell 201 is grown on top of the inactive substrate using conventional epitaxial techniques. A small percentage of silicon may be included in the germanium subcell to improve material characteristics or to form subcell junctions. For pure germanium, the lattice constant of this layer is 5.658 angstroms and the bandgap is 0.67eV.

A transition layer 202 and second subcell 203 are grown on top of the first subcell. The transition layer is either as a discrete layer positioned on top of the first subcell or is incorporated into the subcell structure of the first or second subcell. The transition layer may be a stepped transition layer where the lattice constant changes abruptly or a graded layer where the lattice constant changes gradually. The material composition of the second subcell is chosen such that it has a larger bandgap than the first subcell. For example, the second subcell may be Si_{0 17}Ge_0.^ (i.e. 17% Si 83% Ge) which has a lattice constant of 5.619 angstroms and a bandgap of 0.92eV. Transition layer 202 is used to adjust the lattice constant from 5.658 to 5.619 angstroms. This transition layer may also be combined with subcell layers such as tunnel junctions or emitter layers of either cell.

A third subcell 205 is grown on top of the second subcell and has a material composition that provides the same lattice constant as the second cell but a higher bandgap. For example, GaAso.₈₃Po.i7 has the same lattice constant as the second subcell (5.619A) and bandgap of 1.623eV.

A fourth subcell 206 is then grown on top of the third subcell in a similar manner. Again, the lattice constant is the same as the subcells below but the bandgap is increased. For example, the fourth subcell may preferably be comprised of In_{O 4}Ga₀₆P and have a bandgap of 2.015eV.

Importantly, the thickness of each subcell layer is preferably adjusted to achieve current matching between each of the subcells.

Tunnel junctions are preferably grown between each subcell to achieve series connection of the subcells. Anti reflection coatings are preferably deposited on top of the fourth subcell using conventional techniques to optimise absorption of the cell. FIG. 2B shows an embodiment of an example 5 junction cell. The subcell layers of this embodiment are equivalent to those of FIG. 2 A except that a fifth subcell is introduced between the second and third subcells. This fifth subcell preferably has the same lattice constant as the subcells above and below it and has a bandgap which is larger than the subcell below and smaller than the subcell above. For example this fifth subcell may preferably be comprised of a dilute nitride material such as InGaAsN, GaAsN with lattice constant 5.619 angstroms and a bandgap of approximately 1.3eV. This fifth subcell may also include elements from groups III or V such as bismuth which act as isoelectronic codopants and improve minority carrier transport characteristics in the subcell.

An embodiment can provide a manufacturing method for producing multij unction cells comprising group IV and group III-V semiconductors.

It is known in prior art that there are significant problems associated with growing group IV and group III-V semiconductors in the same growth chamber. For example in MOCVD systems germanium creates a memory effect in growth chambers and is a significant source of contamination. To overcome this problem an embodiment preferably comprises growth of group IV and group III-V materials in separate chambers. To overcome possible surface contamination when substrates are transferred from one growth chamber to another, an embodiment can also comprises the use of an oxide-forming surface layer on the group IV subcell layers which is removed in-situ in the group III-V growth chamber by heating. For example, a the group IV subcell layers may be capped with a Ge layer which oxidises on exposure to the atmosphere to form GeO₂. When the substrate is heated in the group III-V growth chamber, this GeO₂ layer sublimes to leave a clean surface ready for epitaxial growth.

Referring to FIG. 3A through 3D, the growth process starts with an inactive substrate 300 onto which group IV subcell layers 301 are grown. Before the substrate is removed from the growth chamber, an oxidising barrier layer 307 is formed on the surface of the top subcell. The substrate is then transferred to the group III-V growth chamber and the oxidising barrier layer is removed by heating as shown in FIG. 3B. Group III-V subcells 116 are then deposited onto the surface of the group IV subcells 301. This sequence is summarised in FIG. 3D. An embodiment can provide a structure and manufacturing method for a multijunction photovoltaic cell comprising an inactive silicon substrate.

Many attempts have been made at growing multijunction cell structures on silicon using germanium or germanium compounds such as SiGe as buffer layers. Although some prior art graded buffer techniques provide reasonable substrate crystal quality for the growth of group III-V subcells, a fundamental problem remains: the thermal expansion coefficient of the silicon substrate is much less than the expansion coefficient of the III-V epitaxial layers. This means that significant stress is introduced in the III-V epilayers as substrates are cooled to room temperature after growth.

In an embodiment, a SiGe buffer layer is grown on an inactive silicon substrate. Since the silicon substrate is inactive (i.e. it does not from a photoactive portion of the cell) its material purity can be reduced to lower cost. For example so-called Upgraded Metallurgical Grade (UMG) silicon would be a suitable substrate. The substrate is preferably heavily doped and is used to provide electrical connection to the bottom of the lowest subcell. The polarity of the substrate doping (p or n) is selected to optimise minority carrier transport characteristics in the overall multijunction cell structure.

Preferably, the SiGe buffer layer has either a graded or fixed composition. The top surface of the SiGe layer is preferably predominantly germanium, for example Si_o.o₂Ge_o.98 , or 100% Ge. The preferred SiGe / Ge buffer layer thickness is less than 1 micron. Because of the significant lattice mismatch between the substrate and buffer layer crystal lattice constants, the SiGe layer as grown will have a large number of defects.

To overcome this problem, ion implantation is first used to create a damaged crystal layer below the surface of the SiGe buffer layer. The substrate is heated during the ion implantation process to minimise damage of the Ge or SiGe surface layer. For example the substrate is preferably heated to around 120 degrees Celsius. After implantation, the substrate is annealed at temperatures between 600 and 1100 degrees Celsius. During the annealing process, the SiGe or Ge surface layer recrystallises starting from the top surface and pushes defects down towards the implant damaged regions which is largely amorphous. As a result, the surface crystal quality is improved and the amorphous damaged layer provides a means of lattice slippage and stress relief as the wafer is cooled from annealing or subsequent growth temperatures.

US patent 6,703,293 filed by Tweet et al on 11 July 2002 describes a similar technique for forming SiGe layers on silicon wafers for the purpose of making CMOS integrated circuits. It is understood that this technique can be adapted for use in the unrelated art of multijunction solar cell design and production.

FIG. 4A though 4D show the steps in the manufacturing sequence of a multijunction cell according to an embodiment. In the first step, a buffer layer 409 is deposited on an inactive silicon substrate 400. Then the substrate is heated to a specific temperature, for example 120 degrees Celsius, and then it is implanted with ions such as H, Si or Ge to form an implant damaged buried layer 408 which is largely amorphous. Then the substrate is annealed at a temperature between 600C and 1 IOOC to recrystallise SiGe or Ge Surface layer 409 to form surface layer 419. Then group IV and group III- V subcells 401 and 406 are grown in a manner as described elsewhere in,this specification.

Referring to FIG. 5, an embodiment can provide a five junction multijunction photovoltaic cell structure comprising Group FV and Group III-V epitaxial photovoltaic subcell layers deposited on an inactive substrate. In this embodiment, "inactive" means that the substrate does not contain a photovoltaic junction and provides only a crystal template for growing epitaxial subcell layers and a means of connecting to the lowest subcell.

Inactive substrate 500 is preferably comprised of germanium which is heavily doped to provide low resistivity between front and back surfaces of the substrate. For example the doping concentration is greater than IeI 8. The dopant type (n or p) is chosen to achieve optimal minority carrier transport characteristics in the overall multijunction cell. The impurity concentration of the substrate is relaxed to reduce substrate costs. For example the impurity concentration in the substrate might be at least an order of magnitude higher that conventional "semiconductor grade" germanium substrates.

Germanium subcell 501 is grown on top of the inactive substrate using conventional epitaxial techniques. A small percentage of silicon may be included in the germanium subcell to improve material characteristics or to form subcell junctions. For pure germanium, the lattice constant of this layer is 5.658 angstroms and the bandgap is 0.67eV.

A transition layer 502 and second subcell 503 are grown on top of the first subcell. The transition layer is either as a discrete layer positioned on top of the first subcell or is incorporated into the subcell structure of the first or second subcell. The transition layer may be a stepped transition layer where the lattice constant changes abruptly or a graded layer where the lattice constant changes gradually. The material composition of the second subcell is chosen such that it has a larger bandgap than the first subcell. For example, the second subcell maybe Sio.πGe_o.83 (i.e. 17% Si 83% Ge) which has a lattice constant of 5.619 angstroms and a bandgap of 0.92eV. Transition layer 502 is used to adjust the lattice constant from 5.658 to 5.619 angstroms. This transition layer may also be combined with subcell layers such as tunnel junctions or emitter layers of either cell.

A third subcell 504 is grown on top of the second subcell and is preferably a second SiGe layer. This layer may have the same composition as the first SiGe subcell or may have a slightly larger mole fraction of silicon. This layer preferably has substantially the same lattice constant as the second cell.

A fourth subcell 505 is grown on top of the third subcell and is preferably

GaAs_{0 S3}P_0-17 which has substantially the same lattice constant as the second and third subcells (5.619A) and bandgap of 1.623eV.

A fifth subcell 506 is then grown on top of the fourth subcell in a similar manner. Again, the lattice constant is substantially the same as the subcells below but the bandgap is increased. For example, the fourth subcell may preferably be comprised of Ino₄Gao ₆P and have a bandgap of 2.015eV.

Importantly, the thickness of each subcell layer is preferably adjusted to achieve current matching between each of the subcells. It is noted that the use of two SiGe subcell layers allows close current matching for each of the five subcells. Improved conversion efficiency is achieved by careful choice of each of the subcell layer thicknesses. Accordingly, detailed balance calculations indicate conversion efficiencies of around 60% which is significantly higher than prior art devices. Tunnel junctions are preferably grown between each subcell to achieve series connection of the subcells. Anti reflection coatings are preferably deposited on top of the fourth subcell using conventional techniques to optimise absorption of the cell.

High Aspect Ratio Metallisation

Metallisation techniques for applying contacts to the cells can also be considered in this design. The challenges in designing metallisation structures relate to forming low resistance contacts to both the front (i.e. the photoactive side) of the multijunction cell and the back of the cell substrate and to patterning the front metallisation so as to shadow as little of the active area as possible.

FIG. 6 shows a simplified structure of a prior art multijunction cell 600 (not to scale). The cell structure comprises a substrate 601 which is typically germanium and a multijunction epitaxial structure 602 which performs the optical to electrical energy conversion function. The surface of the multijunction cell 603 is optically transparent and allows light to enter the cell structure below. Metallic fingers 607, sometimes referred to as "gridlines", are deposited on the active surface of the cell to provide electrical contact. These fingers need to carry the operating current of the cell and hence are made from relatively thick metal. For example, these fingers may be 5 - 10 microns thick.

A recent paper by Cotal et al, published in The Journal of Energy and Environmental Science, 10 December 2008, reports that: "At the high current densities obtained in high CPV, the area covered by metal gridlines is still almost 10% of the semiconductor surface".

Gridlines are commonly 10 microns wide and are placed approximately 100 microns apart which results in a so called "obscuration loss" of 10% of the incident light. In order increase cell efficiency it is evident that this obscuration or shadowing loss needs to be reduced to a minimum.

Therefore, there is a need in the art for a metallisation process for multijunction photovoltaic cells which that minimises obscuration loss.

Referring back to FIG. 6, the cell structure comprises a substrate 601 which is typically germanium and a multijunction epitaxial structure 602 which performs the optical to electrical energy conversion function. The surface of the multijunction cell 603 is optically transparent and allows light to enter the cell structure below. Metallic fingers 607 are deposited on the active surface of the cell to provide electrical contact. These fingers need to carry the operating current of the cell and hence are made from relatively thick metal. For example, these fingers may be 10 microns thick.

In order for these contact fingers to make stable, low resistance ohmic contacts with the semiconductor structure beneath them, a multilayer contact structure is used. In this structure, a low bandgap semiconductor layer 604 is grown on the surface of the multijunction cell structure. The bandgap of this material is chosen to facilitate the formation of ohmic contacts with the surface. GaAs or InGaAs has been used for this layer in prior art devices. Before the contact finger metal 607 is deposited, an alloying metal layer 605 is deposited on the surface of ohmic contact layer 604. In prior art devices this alloying metal 605 is a eutectic alloy of gold and germanium which melts at approximately 400 degrees Celsius. A diffusion barrier layer 606 is deposited on top of this alloying metal layer to prevent contact metal 607, which is typically gold or silver, from diffusing downwards into the cell structure. This diffusion barrier layer is generally nickel in prior art devices. An ohmic contact is formed when the overall structure is heated to approximately the melting temperature of the eutectic alloy. At this time the constituents of the alloy diffuse rapidly into the ohmic contact layer 604 and produce an ohmic contact. :

Similarly, a second metal layer 609 is deposited on the back of the wafer to provide the other contact to the cell, hi prior art cells, if the substrate is made from germanium, a gold interface layer 608 may be used to form an alloying contact in a similar fashion to the front contact. Alternatively, the back metal itself can be gold.

Referring to FIG. 7, an embodiment can provide a high aspect ratio gridline structure for photovoltaic cells. Substrate 701 is comprised of semiconductor material and is preferably a multijunction cell intended for use in a CPV system. Gridlines 707 are deposited on the surface using techniques which allow high aspect ratios to be made (i.e. the gridline thickness is significantly greater than the gridline width). For example, the gridline aspect ratio is at least 2:1 and is preferably greater than 5:1. hi general, the aspect ratio is made as large as possible to minimise cell obscuration. For example, aspect ratios of 10:1 may be chosen if manufacturing processes can produce such features reliably. The high aspect ratio gridlines are preferably produced using a specialised photoresist material such as SU-8 or KMPR which are manufactured by MicroChem Corporation. These photoresists can be developed to produce very high aspect ratio slots in relatively thick photoresist layers. For example the photoresist can be developed to form slots that are 4 microns wide in layers of photoresist that are 40 microns thick. High aspect ratio gridlines are produced by electroplating metal such as copper into the photoresist slots.

When the aspect ratio of gridlines is increased, there is a potential problem with mechanical strength of the gridline structures. For example, lateral stresses can potentially bend the metal structures sideways and convey stress to the metal-to- semiconductor connections.

To overcome this difficulty, an embodiment can provide high aspect ratio gridlines which have a meandering shape, thereby providing lateral stability.

FIG. 8 provides an example of two meandering gridlines 807 on the surface of a photovoltaic cell 801. This example shows gridlines with a sinusoidal meandering shape, however other meandering shapes such as triangular or rectangular are also included. As an example, the gridlines may meander a distance equivalent to 10% or less of the separation between gridlines.

An advantage of the meandering structure is that stresses induced by thermal expansion mismatches between the gridline metal and the underlying semiconductor are not concentrated along the axis of the gridline, as would be the case if the gridline was linear.

An embodiment can provide a photovoltaic cell incorporating high aspect ratio gridlines which have lateral support structures at multiple points along their length. These lateral structures serve the same purpose as the meandering feature of gridlines described above but may be deposited in a way that lowers the obscuration loss of the cells.

Referring to FIG. 9, lateral supports 910 are provided at multiple locations along gridlines 907. These lateral supports provide improved mechanical strength for the gridline structures. Cover Prism

It will be appreciated that conventional CPV subsystems typically use lenses or mirrors as the primary optical concentrating elements.

By way of example, US patent application 2006/0266408 filed by Home et al on 26 May 2005 describes a CPV module made using a cassegrain optical concentrator.

FIG 10a shows the elements of the design including the overall cassegrain assembly 1000a, primary focusing mirror 1001a, secondary focusing mirror 1002a and cell module assembly 1003a. FIG. 10b provides a side view of these same elements. Multij unction cell 1004b is located inside cell module assembly 1003b. In this design, sunlight is focused by the primary mirror onto the secondary mirror and then onto the PV cell. One advantage of this design is that the optical path is folded so that the vertical dimension of the optical system 1005b is reduced and therefore the CPV panel can be reduced in thickness and in cost. Another advantage is that the use of a guiding optical prism increases the effective acceptance angle of the photovoltaic cell.

Referring to FIG. 11, the cell assembly 1103 proposed in US 2006/0266408 comprises a cell 1104 mounted on a substrate 1107 and a guiding optical prism 1106 which is attached to the surface of the cell at one end and exposed to concentrated solar energy at the other end. The cell, substrate and prism are housed in metallic cylindrical cover 1108. The optical prism 1106 is held in place by metallic clip 1109 which is attached to cover 1108.

It was recognised that the design proposed in US 2006/0266408 has a number of shortcomings. Firstly, the design involves non-standard assembly processes that are not compatible with high volume automated assembly. The design is therefore not cost effective. Secondly, the sides of the optical prism are not environmentally sealed. This means that airborne contamination can build up on the sides of the prism over time. The efficiency of the prism is fundamentally determined by the total internal reflection that occurs at the dielectric-air boundary. Contamination of the prism surface can therefore create loss at the points of total internal reflection. This means that the loss of the prism can increase over time and degrade the efficiency of the photovoltaic conversion process. Further, the metallic clip used to hold the prism in place in the assembly presses on the top surface of the prism at certain points. The clip obscures the surface at these points, creating shadowing that lowers efficiency. Accordingly, there is a demonstrable need for a new light guiding prism structure for CPV cell assemblies that overcomes the shortcomings of the prior art. There is also a need for a method of manufacturing this new design at low cost.

Referring to FIG. 12, an embodiment can provide a CPV cell module 1200 comprising an improved light guiding prism 1201. The light guiding prism 1201 comprises central light guiding structure 1210 which is surrounded by cavity 1203 and outer protective surface 1211. The light guiding prism 1210 is preferably mounted on the surface of the cell module substrate 1202. The light guiding prism is preferably fixed in place with adhesive 1209 or by mechanical means (not shown). The region between the light guiding prism 1201 and the cell 1205 is filled with a transparent dielectric material 1206 that minimises the refractive index discontinuities within the region and hence minimises reflections. This material 1206 is preferably an adhesive or a refractive index matching gel or grease.

The light guiding prism 1201 is preferably formed from a single piece of dielectric material such as glass or plastic and comprises a cavity 1203 surrounding central light guiding structure 1210. This cavity preferably contains air so that light 1204 propagating in the central light guiding structure 1210 encounters an abrupt refractive index discontinuity at the surface, and is totally internally reflected. The cavity 1203 is preferably sealed from the atmosphere at time of manufacture thereby permanently protecting the side walls of the central light guiding structure from contamination. For descriptive purposes, this cavity can be referred to as a "void" in the prism material.

The top surface of the prism 1207 is preferably curved or flat according to the optical characteristics of the solar concentrating system. An advantage offered by a curved top surface is that it can provide magnification and form part of the optical concentrator of the PV subsystem.

FIGs 13 A and 13B shows two examples 1301a-b of light guiding prisms according to embodiments. Prism 1301a has a flat top surface whereas prism 1301b has a curved top surface.

In these examples, the central light guiding structures 1310a-b have circular cross sections, however other cross section shapes, for example square, elliptical etc, are also considered appropriate. Similarly, the cross section of the outer wall of the prisms 131 la-b are shown as round (i.e. cylindrical walls). Other wall cross sections such as square or elliptical etc are also considered appropriate. As shown in FIG. 15, the central light guiding structure may also comprise facets 1530 or other tapered features which change the shape of the cross section of the light guiding structure along its length. For example, FIG. 15 shows a light guiding structure with a circular cross section at the top (i.e. at the end light is focused onto) and a square cross section at the bottom (i.e. at the at the end which is adjacent to the cell).

The light guiding prisms are preferably made using a moulding process or by machining and polishing a cavity 1303a-b into a solid prism. If the prisms are manufactured by moulding, draft angles are included on prism surfaces to facilitate the moulding process.

An embodiment can provide a light guiding prism comprising a surface which is covered by a reflective coating in some areas and covered by an anti-reflective coating in other areas.

Referring to FIGs 14A and 14B, light guiding prisms are preferably coated with an anti-reflective coating (ARC) 1421a-b on the top surface 1407a-b (i.e. the surface onto which light is focussed). This ARC is preferably located at least in the region above the central light guiding structure 1421a-b in order to maximise transmission of light into the prism in this region. The ARC preferably is deposited over the entire top surface of the prism in order simplify the ARC deposition process. An example of an suitable ARC coating comprises thin layers selected from aluminium oxide, titanium dioxide or magnesium fluoride.

A reflective coating is preferably deposited on the top surface of the prism 1407a-b in a region 1420a-b outside of the central light guiding structure 1410a-b. This reflective coating protects areas adjacent to the PV cell from damage if sunlight is inadvertently focused on them. This may happen, for example, as the CPV system is brought into alignment with the sun. The reflective coating is preferably comprised of a metal such as aluminium or chrome.

Accordingly, an embodiment can provide a light guiding prism which is easy to assemble, which protects the surfaces of light guiding structures and which does not obscure the optical aperture if the light guiding structure. Fresnel Optics

US patent 5,118,361, filed 21 May 1990 by Fraas et al and assigned to The Boeing Company, describes a CPV array using Fresnel lenses as the primary concentrating element. US 5,118,361 describes the structure and manufacturing method of a CPV module and panel. FIG. 16 provides an overview of the design described in US 5,118,361, including overall panel housing 1600, cell module assemblies 1601 and Fresnel lenses 1602. In this type of system, sunlight is concentrated approximately 500 times by the Fresnel lens and focused directly on the multijunction PV cell.

US 5,118,361 relates to the use of metallised flexible circuit "tapes" which have apertures distributed along their length in which cells are mounted, hi this design, as shown in FIG. 17, cell substrates 1701 are bonded directly to electrical conductors 1703 formed on the surface of the flexible tape 1700. After bonding, the cell and tape assembly is glued to panel heat spreaders at points along its length where cells are located. Since the flexible circuit tape allows a degree of movement along the assembly, the cells can be optically aligned within each subsystem before the bonding agent sets thereby fixing them permanently to the rigid heat spreader. US 5,118,361 therefore provides a useful reference to prior art in the field of Fresnel-based CPV systems.

FIG. 18 provides a summary of the concept of a Fresnel lens. The central concept of a Fresnel lens relates to the fact that the optical properties of the lens are determined by the surface shape of the lens and not the thickness of the lens. Therefore the curved surface 1800 can be separated into segments that have the same surface shape but reduced thickness. This results in a pseudo planar lens 1801 that is relatively low cost and easy to make by processes such as moulding, casting or stamping etc. These processes will be referred to in this specification generically as "moulding" processes.

FIG. 19A shows a magnified cross section view of a portion of an ideal Fresnel lens 1900a. The lens comprises prisms on the surface of the Lens which refract incoming light 1904a to produce convergent beam 1905a. It will be appreciated that the arrangement shown is highly simplified for descriptive purposes and a person skilled in the art would recognise the broader scope of this disclosure. There are also many different types of Fresnel lens other than the one shown including linear, elliptic and where the prisms are oriented toward the incoming light. It will be appreciated that other variants are also suitable. The prisms comprise an angled surface 1902a which refracts light and which is typically curved. The prisms also comprise an orthogonal surface 1903a which ideally is perpendicular to the plane of the lens 1901a, and which therefore is parallel to the incoming light rays.

The preferred technique for manufacturing Fresnel lenses is moulding. A key requirement of any moulding process is that so called draft angles are provided on each surface which is perpendicular to the parting plane of the moulding cavity. The intention of the draft angles is to allow parts to be removed easily from the mould. Removal of parts is problematic if surfaces are exactly perpendicular to the parting plane.

Referring to FIG. 19B, known Fresnel lenses are modified so that prism surfaces 1903b are at an angle of e.g. 1-2 degrees from the normal of the plane of the lens. Although this solves the manufacturing difficulties, it creates optical effects that degrade the performance of the lens. Incoming light rays which strike the inner surface of the angled prism facets 1903b and reflected to form stay (e.g. divergent) rays 1906b. This represents an optical loss which is undesirable in many applications, and particularly in CPV power generating systems.

Therefore there is a need for an improved design of a Fresnel lens which avoids optical losses and a corresponding manufacturing process and which can fabricate lenses easily using conventional manufacturing equipment.

There is a desire to reduce the costs of CPV systems. In conventional CPV systems the mechanical structure of the CPV panel represents a significant proportion of the overall costs. Referring to FIG. 16, conventional panels rely on side structures 1603 as well as front 1605 and rear 1604 panel surfaces to provide the mechanical strength and rigidity needed by the panel assembly. This means that relatively heavy gauge metal needs to be used which adds to costs. In addition, the panel needs to be assembled as a relatively large unit and little use is made of high volume modular components that can reduce costs. Accordingly, there is a need for a new panel design and assembly method that reduces costs and simplifies assembly methods.

Referring to FIG. 2OA, an embodiment can provide a Fresnel lens 2000 comprising at least two physically separate parts 2001. This figure is highly simplified, showing only one type of Fresnel lens and is not meant to restrict the spirit and scope of the disclosure herein. FIG. 2OA is a cross section, side view of the lens 2000 and two of its constituent elements 2001. FIG. 2OB is a plan view showing the lens comprising four constituent elements. FIG. 2OC shows the assembly of the multiple portions of the lens to form a complete lens. The peripheral shape of the individual lens elements may be square as shown, or may be triangular, hexagonal or a sector of a circle. Preferably the periphery of the lens elements are a polygon which can completely cover a planar surface without gaps.

By separating the lens into multiple elements, it is possible to avoid the need for a draft angle on surfaces perpendicular to the plane of the lens (herein referred to as "vertical" surfaces). The reason for this is that if a lens is separated into parts that do not have opposing (i.e. opposite-facing) vertical surfaces, the mould used to make the part can be separated in a direction which takes it away form the "vertical" surfaces of the moulded part. In order to avoid the presence of opposing surfaces, a Fresnel lens preferably is separated into at least two parts. These parts may subsequently be arranged to form a complete lens or may be used individually. The same technique can be applied to linear Fresnel or lenticular lenses within the scope of the present disclosure.

The "opposing" nature of the vertical surfaces can be defined as occurring when vectors normal to the vertical surface are pointing toward each other, i.e. with an angle of 180 degrees or more between them. If the angle between the normal vectors is less than 180 degrees, the vertical surfaces are not opposing.

By creating a Fresnel lens with vertical facets, the optical losses of the lens can be significantly reduced. This is particularly important in photovoltaic applications where lens losses directly effects system efficiency. When a Fresnel lens is assembled from multiple parts or is used as an individual part according to an embodiment, a physical boundary exists between each of the parts or around an individual part. Although these boundaries can introduce optical defects which lower efficiency, the relative area of these boundary defects is a very small percentage of the lens area and hence these boundaries have minimal effect on lens efficiency.

From another aspect, an embodiment can provide a Fresnel lens comprising at least two physically separate parts which is assembled to form a complete lens by bonding the lens parts onto a transparent substrate. FIG. 21 shows a side view of an example embodiment. In this example, a concentric Fresnel lens is formed from multiple lens parts assembled onto a substrate such as glass. This structure comprises moulded Fresnel lens parts 2100, a glass substrate 2101 and bonding agent 2102 such as transparent adhesive. The Fresnel lens parts are preferably made of thermoplastic or thermosetting polymers which are easy to shape by moulding or other techniques. The lens structure is relatively thin e.g. l-2mm thick so as to minimise material usage and lower cost. The substrate provides a strong rigid surface which supports the flexible lens parts and provides a planar surface against which the lenses are aligned. The substrate is preferably glass.

FIG. 22 A shows a cross section view of a Fresnel lens element 2200 in a moulding cavity comprising upper and lower portions 2201 and 2202. According to an embodiment, when the mould is separated to remove the moulded part, the mould portion which forms the lens facets is moved in a direction away from the vertical features of the lens, as indicated by arrows in FIG. 22B.

An embodiment can provide a CPV panel assembly comprising a plurality of Fresnel lenses mounted on a glass panel and aligned to modular PV receiver assemblies which are also mounted on the glass panel.

In order to avoid the considerable cost associated with conventional CPV panel housings, an embodiment can utilises a glass panel as the central structural member that other components are mount onto. Glass is typically needed for the front surface of a PV panel because it is transparent and environmentally robust. Glass is also a relatively low cost material that readily provides a flat reference plane that can serve to align system components. Given that a glass front cover is needed in CPV assemblies, it will be appreciated that it can be cost effective to use this glass panel as the core structural member. The glass panel may need to be slightly thicker that it would normally be, but there is relatively little cost associated with making the panel thicker. For example, the glass panel preferably may be 6mm thick and may be approximately 1 square metre in area. This particular example should not be regarded as restricting the scope of the present disclosure.

An example embodiment comprises a glass panel with a plurality Fresnel lenses mounted on the surface preferably in a regular array. These lenses are preferably formed by creating Fresnel facets on a thermoset or thermoplastic polymer sheet which is bonded to the glass panel using adhesive. According to an embodiment, these polymer Fresnel lenses are fabricated in separate parts and have vertical side facets, as previously described in this specification.

Alternatively the Fresnel lenses are formed using a polymer that is deposited on the surface of the glass panel and textured using a pattern which is pressed onto the polymer while it is cured.

The Fresnel lenses are preferably located on the bottom surface of the glass panel (i.e. on the surface away from the sun). This allows the glass panel to provide protection for the lenses.

Preferably, photovoltaic cells are mounted in metallic support structures that are used to position the cells at a certain distance form the surface of the panel corresponding to the focal point of the Fresnel lenses. These metallic support structures are mounted on the glass panel and preferably comprise one or more legs which provide both mechanical support and heat sinking for the PV cell. The metallic support structures may be fixed to the glass panel using adhesive or by mechanical fasteners such as screws. The metallic support structures are preferably produced using a diecasting process and are preferably made of aluminium or an aluminium alloy. The metallic support structures may also be fixed to Fresnel lenses that are bonded to the surface of the glass panel. The PV cells are preferably mounted in sealed modules which are mounted onto the bottom surface of the metallic support structures.

The metallic support structures may also comprise side walls that provide an environmental seal to keep the lens and PV cell surfaces clean.

Examples of these elements/features are provided in FIG. 23 A and FIG. 23B. The glass panel is divided into regions corresponding to the solar collection area of each cell. The shape of these regions are chosen such that they cover the glass panel without intervening gaps. FIG. 23 A shows for example one of a plurality of hexagonal regions 2300a of the overall panel. These regions comprise Fresnel lenses on the bottom surface. Metallic support structures 2302a are mounted onto the glass panel and comprise legs 2301a that provide both mechanical support and heat sinking for the PV cell, which is housed in receiver module 2303 a. The metallic support structure also comprises an optical aperture 2304a that allows light to pass through to the PV cell. FIG. 23B is similar to FIG. 23 A, except it provides an example of a metallic support structure comprising protective side walls 2306b.

Preferably the metallic support structures comprise multiple legs to provide optimal mechanical rigidity and accurate alignment of the cells to the lenses. For example, three legged structures are shown in FIG. 23A and FIG. 23B. FIG. 24 provides a plan view example of how such hexagonal lenses 2401 and metallic support structures 2402 might be arranged on the glass panel 2400.

Adaptive Optics

Conventional CPV subsystems typically use lenses or mirrors as the primary optical concentrating elements.

US patent 5,118,361, filed 21 May 1990 by Fraas et al and assigned to The Boeing Company, describes a CPV array using Fresnel lenses as the primary concentrating element. US 5,118,361 describes the structure and manufacturing method of a CPV module and panel. FIG. 16 provides an overview of the design described in US 5,118,361, including overall panel housing 1600, cell module assemblies 1601 and Fresnel lenses 1602. In this type of system, sunlight is concentrated approximately 500 times by the Fresnel lens and focused directly on the multij unction PV cell.

US patent application 2006/0266408, filed by Home et al on 26 May 2005, describes a CPV module made using a cassegrain optical concentrator. FIG. 1OA shows the elements of a design including the overall cassegrain assembly 1000a, primary focusing mirror 1001a, secondary focusing mirror 1002 a and cell module assembly 1003a. FIG. 1OB provides a side view of these same elements. Multijunction cell 1004b is located inside cell module assembly 1003b. In this design, sunlight is focused by the primary mirror onto the secondary mirror and then onto the PV cell. The advantage of this design is that the optical path is folded so that the vertical dimension of the optical system 1005b is reduced and therefore the CPV panel can be reduced in thickness and in cost. The disadvantage of this design is that the secondary reflector 1002 creates a shadow on the primary reflector and reflects light away from the cell. This shadowing loss is typically around 3%.

There is a significant trade-off that needs to be assessed when setting the concentration ratio of a CPV system. Decreasing concentration ratios increases the semiconductor area needed per watt generated and hence increases cost. Increasing the concentration ratio reduces the acceptance angle of solar radiation and increases the required pointing accuracy of the system and subsystems. For example the pointing accuracy for a system with a concentration ratio of 500 is around +/- 0.1 degree. This means that panels need to incorporate highly rigid frames so that they do not bend under their own weight or under stresses such as wind loads. Such requirements increase costs of tracking systems for CPV systems with high concentration ratios.

US patent 5,707,458 filed by Nagashima et al on 23 May 1996 and assigned to Toyota Jidosha Kabushiki Kaisha describes the use of mechanisms that allow the cell in a CPV system to be moved relative to the primary concentrating optics. US 5,707,458 teaches that it is advantageous for cells to be moved to keep light centrally focused on the cell so that the maximum amount of energy is produced. The technique disclosed in US 5,707,458 relies on moving the cell itself and several techniques for doing this are proposed including magnetically, electromechanically and thermally using shape memory alloys.

Referring to FIG. 25, a preferred embodiment described in US 5,707,458 comprises primary concentrating optical element 2500 which focuses light on cell assembly 2501. US 5,707,458 the concept of moving cell assembly 2501 relative to members that are fixed to the system housing 2504 so that focused light 2505 is centred on the cell. US 5,707,458 proposes moving the cell assembly using various techniques such as magnetic field attraction 2502.

However there are fundamental problems with the techniques proposed in US 5,707,458 that renders them impractical in most systems. Firstly, a significant amount of waste heat is generated at the CPV cell and this needs to be efficiently coupled to a heat-spreader or heatsink. It is difficult and impractical in low maintenance systems to couple this heat out of a moving cell and into a large area heatsink (which is typically the panel housing). Secondly, the electrical conductors that connect to the cell terminals typically have large cross sections to carry cell current and are relatively rigid and are not easily moved. Substantial forces would be needed to move the cell assembly with these conductors attached. This is incompatible with the capabilities of the movement mechanisms proposed. Even if this problem is overcome, there is a another problem relating to the reliability of these cell connections after repeated movement of the cell over the operational lifetime of the system. Therefore, there is a need for an improved means of implementing an adjustable optical CPV subsystem.

An embodiment can provide a CPV subsystem with increased acceptance angle of solar radiation comprising a movable optical element such as a mirror or lens.

In conventional concentrator solar modules a large primary mirror or lens is used to collect sunlight and focus the light onto a cell. Fresnel lenses are commonly used for this purpose. For example the company Amonix Inc produces panels which use 7 inch diameter Fresnel lenses with a focal length of around 20 inches. This means that the solar cell panels are relatively thick which increases costs and weight. Another company, SolFocus produces cassegrain reflector modules where sunlight is focused using a primary and secondary mirror onto the solar cell. This approach folds the optical path length and reduces panel thickness.

The importance of increasing acceptance angle for solar panels relates to the need for all CPV subsystems in a panel to track the sun accurately as it moves across the sky; As panels move and flex under gravity or wind loads, it is possible that the optical alignment of subsystems relative to each other will degrade and light will not be focussed centrally on all cells. Normally, system manufacturers incur significant cost in designing panels that have the mechanical strength necessary to resist flexing and bending. The only way of increasing acceptance angle with passive optics such as mirrors or lenses is to reduce concentration ratios which is unattractive. Hence an active adaptive optical system is needed. Such systems allow less precise optical elements and panel structures to be used thereby lowering system costs.

An embodiment can provide a means of focusing sunlight onto a cell using a passive optical element positioned in the optical path between the primary optical concentrating element and the cell and which is moved relative to the cell and primary optical element.

It is typically impractical to move a primary element such as lens or mirror because of its size. These primary elements instead are preferably held in fixed position in the panel. It is also unattractive to move the cell itself because of the need for good heatsinking and electrical connectivity. In an embodiment, the primary concentrating optical element and solar cell can be fixed and an intermediate optical element is movable to keep the sunlight centred on the solar cell.

Referring to FIG. 26, an embodiment comprises a lens 2601 as the primary concentrating optical element. This lens is mounted in a fixed relationship with panel housing 2600. Light from the lens is focused onto a movable mirror 2603 which reflects light onto a photovoltaic cell 2602. This cell is also fixed to the panel housing which provides heatsinking. The electrical connections on the surface of PV cell 2602 are divided into multiple regions each with an independent output. These independent outputs are applied to movement actuator 2604 which adjusts the position of mirror 2603 so that currents from each output are equal, thereby ensuring that light is focused centrally on the cell.

Referring to FIG. 27A, an embodiment comprises cassegrain reflectors where sunlight is collected by a primary mirror 2701, is focused on secondary mirror 2706, then onto a movable mirror 2703 and then onto the solar cell 2702. The advantage of this arrangement is that the movable mirror is relatively small, is easy to move and can be located near the cell.

Referring to FIG. 27B, an embodiment comprises cassegrain reflectors where sunlight is collected by primary mirror 2711 and focused on secondary mirror 2713 which in turn focuses light onto cell 2712. hi this embodiment, the secondary cassegrain reflector 2713 is attached to movement actuator 2714 which ensures that light is focussed centrally onto the cell. The advantage of this embodiment is that 3 mirrors are used instead of 4 and hence optical losses are minimised.

An embodiment can provide a means of adjusting the position of a movable optical element using magnetic fields. To achieve this, the electrodes on the surface of the solar cell are arranged into a number of individual regions (e.g. quadrants) and current is from each region is applied separately to the movement actuator. This current is passed through a number of inductive elements that generate magnetic fields. One or more permanent magnets are mechanically coupled to the movable optical element (e.g. at the rear of a mirror) and the magnetic poles are arranged such that the magnetic fields generated by the inductors push the magnet (and hence the mirror) in a direction that causes the current in the inductors to be equalised. For example, if the focused sunlight has been displaced due to alignment inaccuracies in the system so that only two of four quadrants are illuminated on the solar cell, the magnetic field produced from inductors connected to these illuminated cell quadrants push the movable mirror in direction that causes the light to be centrally located on all quadrants. At this time, the opposing fields generated by the previously unilluminated quadrants provide feedback to stabilise the position of the focused sunlight.

FIG. 28 shows an embodiment including PV cell 2802 and movable mirror 2803. The cell is divided into a number of regions, for example 4 and individual electrical outputs 2850a-d are produced. These outputs are coupled to inductors 2851 which produce a magnetic field that interacts with permanent magnets 2852 positioned at the rear of the moveable mirror 2803. hi an embodiment, other means are used to adjust the orientation of the movable optical element. For example, electro-mechanical means comprising motors or other servo mechanisms can be used.

Monitoring Systems

Now that photovoltaic systems are becoming cost competitive for utility-grade power generation applications, system reliability is becoming a core requirement. As a result, there is a need for sophisticated performance monitoring and fault finding capabilities which facilitate system optimisation and maintenance.

US patent 6,545,211 (filed 12 January 2000 by Mimura and titled "Solar Cell Module, Building Material With Solar Cell Module, Solar Cell Module Framing Structure, And Solar Power Generation Apparatus") describes a "solar cell module" comprising a solar cell element, a parameter detection unit and a communication unit. The parameter detection unit is located within the solar cell module and generates signals relating to cell operating parameters such as voltage and current. The signals generated by the parameter detection unit are fed to the communication unit which is also located within the solar cell module. This communication unit superimposes the signals onto the module's DC interconnection wiring. The signals then propagate to a remote display unit housed in a "non-solar cell member".

US 6,545,211 discloses modules as shown in FIG. 29 and FIG. 30. Referring to FIG. 29, solar cell module 2905 comprises solar cell element 2901, bypass diode 2904, communication unit 2903 and parameter detection unit 2902 which measures cell current. FIG. 30 is similar except that the parameter detection unit is formed by current sensor 3002, voltage sensor 3007 and arithmetic unit 3006. Importantly, it is understood that the intention of US 6,545,211 is to co-locate cells, parameter detection units and communication units within each solar cell module.

US 6,545,211 also describes a means of conveying signals from the solar cell module to a remote display unit by superimposing specific frequency domain or time domain signals onto the DC power interconnection wiring. US 6,545,211 teach using preferred signalling frequencies below IMHz because of the intrinsic shunt capacitance of the solar cell array.

However, upon substantial analysis, a number of deficiencies were identified within the techniques proposed by US 6,545,211.

Firstly, there is no consideration of the appropriate modularity of the overall solar energy collection system. For example, it is clearly unattractive to have separate performance monitoring circuitry for each PV cell because of the substantial cost involved. Also, assuming the monitoring circuitry is powered from the cell itself, the voltage available from a single cell is limited and is potentially insufficient to power the circuitry, particularly if the cell begins to degrade and output voltage falls. To avoid these difficulties, US 6,545,211 proposes modules comprising multiple cells which are connected in series and provide power for performance monitoring circuitry. However, if one cell element fails or degrades in the module, the architecture proposed in US 6,545,211 results in the entire module needing to be replaced. This can be very wasteful if only one cell is faulty and significantly increases system maintenance costs and impacts on system reliability.

Secondly, it is inefficient and costly to combine the dissimilar assembly and packaging requirements of photovoltaic cells with the requirements of electronic performance monitoring circuitry in a single module. For example, in CPV systems, cell packaging needs to address issues relating to thermal management of cells and ceramic substrates are typically used. However, ceramic substrates are not well suited to the requirements of complex microcontroller-based cell monitoring circuitry and separate substrates need to be used for this purpose. Furthermore, manufacturing processes for cell modules need to address issues relating to clarity and stability of certain materials which affect optical efficiency of the module. This requirement is irrelevant for production of performance monitoring electronic circuitry. It is therefore inefficient to impose all of these different requirements on a single unit that contains both cells and performance monitoring circuitry.

Thirdly, US 6,545,211 attempts to use relatively low frequencies (e.g. IMHz) to transmit signals over cell interconnect wiring to a remote display unit. These frequencies are used so that the capacitive reactance of the cells does not shunt signalling frequencies to ground. However, the use of low frequencies is potentially problematic. The output from solar panels is generally connected to a high power inverter which converts the DC output of the panels to an appropriate AC voltage and current. The inverter is usually a switching power converter that runs at a frequency somewhere between 20KHz and 20OkHz. Given the magnitude of the power being switched by the inverter and the distributed inductance and resistance of the cell array, significant levels of switching noise generally appear on the DC output of the panel and across each cell. This interference includes harmonics of the switching frequency which might extend to tens of Megahertz. This interference potentially makes the signal transmission technique proposed in US 6,545,211 unreliable or even impractical.

Finally, US 6,545,211 teaches an inductive element such as a transformer to superimpose signals onto cell interconnection wiring. As noted above, a low frequency is chosen so that the capacitive loading of the cells does not significantly attenuate the signal. This implies that a relatively large, ferro-magnetic based transformer is needed to provide sufficient inductance at the signalling frequency. This transformer also needs to carry the full DC operating current of the cells which can be tens of amps. The transformer proposed in US 6,545,211 is therefore large and disproportionately costly.

Referring to FIG. 31, an embodiment can provide a cell interconnection module 3100 comprising at least two bipolar electrical connection ports 3102 which are used to connect to at least two cell modules 3101, and a single bipolar output port 3103. Cell interconnection module 3100 thus provides a means of interconnecting a plurality of cell modules. It does not contain any photovoltaic cells and is free from associated manufacturing complications (such as thermal and optical requirements) and is manufactured using conventional low cost electronic assembly techniques. Conversely, cell modules 3101 are manufactured to achieve optimal thermal and optical performance without needing to accommodate conventional electronics.

In this figure and in the following figures, two cell modules are shown connected to each cell interconnection module. The depiction of two cell modules has been used for descriptive purposes only and is not meant to restrict the scope of the disclosure herein, and may include a plurality of cell modules connected to a single interconnection module.

In photovoltaic systems, particularly concentrator photovoltaic (CPV) systems, cells are subjected to considerable operational stresses. It is likely that over the operational lifetime of the PV system, which is typically greater than 20 years, some cells will fail or degrade to the point where they must be replaced. An advantage of using a modular photovoltaic system is that it can facilitate the replacement of individual cell modules with minimal disruption to the remainder of the system.

FIG. 32 provides an example of how multiple cell interconnection modules 3200a and 3200b are used to connect a plurality of cell modules 3201a and 3201b in series, according to an embodiment.

Referring to FIG. 33, an embodiment can provide a plurality of cell modules and cell interconnection modules each comprising flexible wires 3302a-b and 3304a-b which extend from each module and provide electrical interconnection ports. These flexible wires are attached to each module at time of manufacture and have prescribed lengths according to the requirements of the system for which they are intended. These wires are preferably fixed to the modules in a way which meets environmental requirements (e.g. preventing moisture ingress) and are preferably sealed to the module housing with an adhesive compound. Flexible wires from cell modules 3301a-b are connected to flexible wires from interconnection units 3300a-b using terminating devices 3305a- b. These terminating devices are, for example, crimp style connections which also meet appropriate environmental requirements. Similar terminating devices 3306a are used to connect the outputs of cell interconnection units 3300a and 3300b. These terminating devices are preferably used to connect modules after they have been installed in the PV system panel. If a cell module needs to be replaced, the terminating devices are cut off and the wire ends reconnected using another terminating device. Accordingly, flexible wires extending from each module are made long enough the accommodate multiple re-terminations if the need arises.

An embodiment can provide a cell interconnection module comprising a filter structure which allows DC current to flow between the module's output port and connected cell modules but which isolates the output port from the capacitive loading of the cell modules.

Referring to FIG. 34, cell interconnect module 3400 comprises inductive elements L3410 and L3411 connected to output port 3403. These inductive elements allow DC current to flow between output port 3403 and cell modules 3401 a-b but isolate the capacitance of cell modules 3401 from the output port at high frequencies. Capacitor C3408 provides a low impedance connection between the output port terminals of the interconnection module so that high frequency signals pass through the interconnection module and on to signal receivers located elsewhere in the PV panel. In this way, the combination of inductors L3410 and L3411 and capacitor C3408 allow signalling information to be carried along interconnect, wiring connected to the output ports of cell interconnect modules. In order to improve signal transmission characteristics, additional capacitors C3407 are preferably connected to the cell module side of inductors L3410 and L3411. These capacitors may be located outside of the cell interconnection module but are preferably located inside the cell interconnection module. Performance monitoring circuitry 3409 measures cell parameters and generates an RF signal that is injected through C3408 and propagates along cell interconnection module wiring connected to port 3403.

An embodiment can provide a cell interconnection module comprising cell bypass diodes. Conventionally, bypass diodes are fitted to each cell to protect them from possible damage under conditions of reverse bias. Normally, these diodes are mounted adjacent to the photovoltaic cell. This potentially creates a manufacturing difficulty because the photovoltaic cell assembly is usually not suited to the requirements of automated assembly of conventional semiconductor devices (e.g. surface mount diodes). An embodiment may overcome this difficulty by accommodating the bypass diodes in the cell interconnection module.

Referring to FIG. 35, in an embodiment, bypass diodes D3512 are connected across cell module connection ports of the cell interconnection module 3500. An embodiment can provide a cell interconnection module comprising a performance monitoring circuit which monitors the voltage and/or current of at least two series connected photovoltaic cells and which is powered by the cells it is monitoring. Because at least two cells are connected in series, if one cell fails or degrades, there is sufficient voltage provided by the remaining cell or cells to power the performance monitoring circuit.

Referring to FIG. 36, the performance monitoring circuit preferably comprises a voltage regulator 3621, a current sensing element 3620, an amplifying and/or level shifting element 3622, a microprocessor 3623 including at least one analog to digital converter (ADC) and a high frequency signal source 3624. The voltage amplifying and/or level shifting element 3622 senses cell operating parameters and converts these parameters to levels suitable for analog to digital conversion. The microprocessor device is preferably a single chip stand-alone microcontroller containing program and data memory, at least one ADC and assorted peripheral functions. The microprocessor encodes the cell performance data generated by the ADC converter into a form suitable for transmission along cell interconnection module wiring and appends identification codes which are unique to each cell interconnection module. The microprocessor then drives an RF signal generator which modulates the data sequence onto a high frequency carrier signal. This carrier frequency is chosen to be above the frequency band where the photovoltaic system generates noise from power inverters. The chosen frequency band is preferably above 10MHz, and most preferably above 100MHz. The frequency is also preferably chosen to fall within an ISM RF frequency band. For example, the frequency band may be 15.36MHz or 2.4GHz. The RF signal generated is preferably coupled to the output port of the interconnection module using capacitive elements C3608a-b.

FIG. 38 is a preferred, simplified panel circuit schematic showing cell interconnection modules incorporating features described above. A plurality of cell interconnection modules 3800a,b...n are connected in series with one input of the first module 3800a grounded using inductor L3840, and one output from the last module 3800n fed through inductor L3841 to provide the high potential output from the panel 3847. A capacitor C3848 is provided at the panel output to ground and terminate RF signals. Each cell interconnection module comprises an RF signal generator that is used to send performance monitoring signals along cell interconnect module wiring in short bursts. These signals are fed to a receiver housed in a central termination module 3860 which preferably comprises an amplifier 3842 and a mixer 3844 which is driven by an independent RF signal source 3843. These components mix the RF signal down to baseband where a microprocessor 3845 decodes the data and provides a digital interface 3846 to computer based analysis equipment. This interface may be provided through a temporary or permanent cable connection or by a wireless communication link.

An embodiment can provide a cell interconnection module comprising a performance monitoring circuit which generates an RF signal containing performance monitoring data without using an RF signal generator inside the cell interconnection module. Instead, the performance monitoring circuit preferably uses a modulator to modulate an external RF signal source which is fed to each cell interconnection module from a central RF source. By eliminating the need for an RF signal generator inside each cell interconnection unit, the cost of providing performance monitoring is reduced and the reliability of the system is increased because only one oscillator is required for each panel.

Referring to FIG. 37, according to an embodiment, a non-linear element such as a variable capacitor (e.g. a varactor diode) C3708 or a mixer is connected across the cell interconnect module output port 3703. This non-linear element provides a low impedance path for RF signals to propagate along cell interconnect module wiring. When this non-linear element is driven by a modulating signal from the microprocessor 3723 in the performance monitoring circuit, it creates signal sidebands on the RF signal as it passes through the cell interconnect module. Providing that performance monitoring circuits transmit short bursts of data it is likely that only one module will be active at any one time and therefore multiple modules can share a single external signal generator. For example signal burst duration may be less than 1% of the average time interval between bursts. If two modules happen to transmit at the same time, the receiver will detect the signal "collision" by analysing data check sum information in the signal and will discard the corrupted signal burst and wait for the next transmission attempt. A protocol of this nature is suitable for the transmission of cell performance monitoring data because of the relatively slow update requirements (e.g. once per minute). For example, the sidebands created on the RF carrier signal may be at IMHz from the RF carrier. FIG. 39 shows a preferred, simplified panel circuit schematic incorporating cell interconnection modules which incorporate these features. A plurality of cell interconnection modules 3900a,b...n are connected in series with one input of the first module 3900a grounded using inductor L3940, and one output from the last module 390On fed through inductor L3941 to provide the high potential output from the panel 3947. A capacitor C3948 is provided at the panel output to ground and terminate RF signals. An RF signal is generated from a central RF signal source 3943, is fed to the output of the last cell interconnection module using RF coupling capacitor C3949 and propagates along the cell module interconnection wiring. As the signal passes through each interconnection module it encounters non-linear elements which can be modulated by associated microcontrollers to create sidebands on the RF signal so that cell performance data can be carried to the central receiver. This receiver is housed in a central termination module 3960 and preferably comprises an amplifier 3942 and mixer 3944. A key feature of this design is that the mixer is driven by the same RF signal source that is fed to cell interconnection modules. This means that the output of the mixer has a zero frequency offset which considerably simplifies signal demodulation. Microprocessor 3945 decodes the data and provides a digital interface 3946 to computer based analysis equipment. This interface may be provided through a temporary or permanent cable connection or by a wireless communication link.

An embodiment can provide a structure for implementing RF signal isolation filters at very low cost. As previously described, there is an advantage in using high frequencies to send cell performance data across cell interconnect module wiring because system noise in low frequency bands is avoided. A second important benefit is also obtained by using high signalling frequencies: the inductive elements needed for isolation filters can be provided by simple low cost features such as circuit board tracks. These circuit board tracks are chosen to be a quarter wavelength long at the signal frequency and thus provide an "open circuit" for RF signals and a "short circuit for DC current. For example, at 2.4GHz, quarter wave tracks are approximately 20 millimetres long and can easily be accommodated on interconnection module circuit boards.

FIG. 40 shows an example of a circuit board layout according to an embodiment. RF isolating inductors L4010 and L4011 are implemented as linear circuit tracks on circuit board 4070 which run between cell interconnect module output port terminals 4003 a-b and cell module connection ports 4004a.

An embodiment can provide a protocol for transmitting cell performance data across cell interconnect module wiring without the use of transmission synchronisation. In order to reduce system complexity and cost, it is desirable to avoid the need for transmission synchronisation schemes. Instead, an embodiment makes use of a random transmission time protocol where data is sent infrequently as short bursts at pseudo-random time intervals, hi this way, the likelihood of a "collision" between signals generated from independent cell interconnection modules is minimised. If a collision occurs because two or more modules attempt to transmit at the same time, the central receiver system identifies that data is corrupted by analysing the received frame structure and/or checksum data generated by the transmitting microprocessor. The receiver can therefore discard signals that have been corrupted by a transmission collision and can wait for retransmission. To avoid repeated transmit collisions, an embodiment preferably comprises an algorithm for determining the time of the next transmission attempt according to the time of the last attempt and the unique identification code assigned to each cell interconnection module.

A simplified flow chart for a method corresponding to this algorithm is provided in FIG. 41. This method comprises the steps of:

(a) measuring photovoltaic cell parameters

(b) using an cell interconnection module ID code and previous transmit time interval value to determine a current transmit time interval value;

(c) waiting according to the current time interval value; and

(d) transmitting data indicative of the unit ID code and measures cell parameters.

It will be appreciate that disclosed embodiments can provide an improved CPV device, a method of providing an improved CPV device, or a method or system of monitoring an improved CPV device. Interpretation

As noted above, while this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification(s). This application is intended to cover any variations, uses or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth.

Preferred embodiments of the present invention have be described in relation to the drawings. Where possible, unique numbers have been used to identify the same element in each drawing or sub-drawing.

It would be appreciated that, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.

In alternative embodiments, the one or more processors operate as a standalone device or may be connected, e.g., networked to other processor(s), in a networked deployment, the one or more processors may operate in the capacity of a server or a client machine in server-client network environment, or as a peer machine in a peer- to-peer or distributed network environment.

Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as "processing", "computing", "calculating", "determining" or the like, can refer to the action and/or processes of a computer or computing system, or similar electronic computing device, that manipulate and/or transform data represented as physical, such as electronic, quantities into other data similarly represented as physical quantities.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

The terms "upper" or "top" and "lower" or "bottom" are intended to aid description of the drawings as shown and are not meant to restrict the scope of the invention. "Upper" or "top" generally refers to subcell layers closest to the light-receiving side of the cell and "lower" or "bottom" refers to layers closest to the substrate side of the cell.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may refer to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description. Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A photovoltaic cell comprising: an inactive substrate; and at least one epitaxial layer deposited on the substrate.

2. The photovoltaic cell according to claim 1, wherein the impurity concentration of the inactive substrate is at least ten times greater than the average impurity concentration of the epitaxial layer or layers.

3. The photovoltaic cell according to any one of claims 1 to 2, wherein the inactive substrate is primarily comprised of germanium.

4. The photovoltaic cell according to any one of claims 1 to 2, wherein the inactive substrate is primarily comprised of silicon.

5. The photovoltaic cell according to any one of claims 1 to 4, wherein the photovoltaic cell comprises two or more epitaxial layers comprising Group IV semiconductors.

6. The photovoltaic cell according to any one of claims 1 to 4, wherein the photovoltaic cell comprises: one or more subcells comprising Group IV semiconductors; and one or more subcells comprising Group III-V semiconductors.

7. The photovoltaic cell according to claim 6, wherein the Group IV subcells and the Group III-V subcells are formed by means of epitaxial growth on the surface of the inactive substrate.

8. The photovoltaic cell according to claim 1, wherein a first epitaxial layer deposited on the substrate comprises at least 95 percent germanium.

9. The photovoltaic cell according to claim 1 , comprising two or more epitaxial layers comprising Group IV semiconductors;

10. The photovoltaic cell according to claim 1, comprising: one or more subcells comprising Group IV semiconductors; and one or more subcells comprising Group III-V semiconductors; wherein the Group IV subcells and the Group III-V subcells are formed by means of epitaxial growth on the surface of the inactive substrate.

11. The photovoltaic cell according to claim 1, comprising: a first subcell comprising germanium or silicon and germanium wherein silicon represents no more than 5 percent of the atomic composition; and a second subcell comprising silicon and germanium wherein silicon represents more than 5 percent and less than 30 percent of the atomic composition; and one or more subcells comprising Group III-V semiconductors;

12. The photovoltaic cell according to claim 1, comprising: one or more subcells comprising Group IV semiconductors; one or more subcells comprising Group III-V semiconductors; and at least one transition layer; wherein the at least one transition layer has a non-constant crystal lattice spacing and comprises Group IV semiconductors and the Group III-V semiconductor subcells have a fixed, unchanging crystal lattice spacing.

13. The photovoltaic cell according to claim 1, comprising: one or more subcells comprising Group IV semiconductors; one or more subcells comprising Group III-V semiconductors; and at least one diffusion barrier layer; wherein the at least one diffusion barrier layer is located between the Group rv semiconductor and the Group III-V semiconductor subcells.

14. The photovoltaic cell according to claim 1, comprising: a first subcell comprising germanium or silicon and germanium wherein silicon represents no more than 5 percent of the atomic composition; and a second subcell comprising silicon and germanium wherein silicon represents more than 5 percent and less than 30 percent of the atomic composition; and one or more subcells comprising Group III-V semiconductors.

15. The photovoltaic cell according to claim 1, wherein the inactive substrate is an inactive germanium substrate, the photovoltaic cell comprising: a first subcell comprising germanium; a second subcell comprising silicon and germanium; a third subcell comprising gallium, arsenic and phosphorous; and a fourth subcell comprising indium, gallium and phosphorous; wherein the first subcell is deposited on the inactive substrate, the second subcell is deposited on the first subcell, the third subcell is deposited on the second subcell, and the fourth subcell is deposited on the third subcell.

16. The photovoltaic cell according to claim 15, wherein the second subcell comprises 17 percent silicon and 83 percent germanium.

17. The photovoltaic cell according to claim 15, wherein the third subcell comprises 17 percent phosphorous and 83 percent Arsenic.

18. The photovoltaic cell according to claim 15, wherein the fourth subcell comprises 40 percent indium and 60 percent gallium.

19. The photovoltaic cell according to claim 1, wherein the inactive substrate is an inactive germanium substrate, the photovoltaic cell comprising: a first subcell comprising germanium; a second subcell comprising silicon and germanium; a third subcell comprising a gallium, arsenic and nitrogen; a fourth subcell comprising gallium, arsenic and phosphorous; and a fifth subcell comprising indium, gallium and phosphorous; wherein the first subcell is deposited on the inactive substrate, the second subcell is deposited on the first subcell, the third subcell is deposited on the second subcell, the fourth subcell is deposited on the third subcell and the fifth subcell is deposited on the fourth subcell.

20. The photovoltaic cell according to claim 19, wherein the second subcell comprises 17 percent silicon and 83 percent germanium.

21. The photovoltaic cell according to claim 19, wherein the fourth subcell comprises 17 percent phosphorous and 83 percent arsenic.

22. The photovoltaic cell according to claim 19, wherein the fifth subcell comprises 40 percent indium and 60 percent gallium.

23. The photovoltaic cell according to claim 1, wherein the inactive substrate is an inactive silicon substrate, the photovoltaic cell comprising: a buffer layer deposited on the silicon substrate; one or more subcells comprising Group IV semiconductors deposited on the buffer layer; and one or more subcells comprising Group III-V semiconductors deposited on the Group IV subcells; wherein the buffer layer comprises a region where the crystal lattice of the buffer layer has been modified after deposition to make it partly of fully amorphous and where the surface of the buffer layer is suitable for the epitaxial growth of the Group IV subcells.

24. The photovoltaic cell according to claim 23, wherein the modification is performed by ion implantation.

25. The photovoltaic cell according to claim 24, wherein the ion implantation is performed at a temperature higher than room temperature.

26. The photovoltaic cell according to claim 25, wherein the ion implantation is performed at approximately 120 degrees Celsius.

27. The photovoltaic cell according to claim 23, wherein the buffer layer is annealed at a temperature between 600 and 1100 degrees Celsius to reduce surface defects prior to deposition of the Group IV subcells.

28. The photovoltaic cell according to claim 23, wherein the buffer layer comprises SiGe.

29. The photovoltaic cell according to claim 23, wherein the buffer layer comprises SiGe and the proportion of Ge content of the buffer layer increases with distance away from the surface of the substrate.

30. The photovoltaic cell according to any one of the preceding claims, wherein the surface of the inactive substrate is at an angle of between 3 and 9 degrees from the substrate's (100) crystal plane.

31. The photovoltaic cell according to claim 1 , comprising: a first subcell comprised substantially of germanium and deposited on the substrate; a second subcell comprised substantially of silicon germanium and deposited on the first subcell; a third subcell comprised substantially of silicon germanium and deposited on the second subcell; a fourth subcell comprised substantially of Gallium Arsenide Phosphide and deposited on the third subcell; and a fifth subcell comprised substantially of Indium Gallium Phosphide and deposited on the fourth subcell.

32. The photovoltaic cell according to claim 31 , wherein the inactive substrate is primarily comprised of germanium.

33. The photovoltaic cell according to claim 31 or claim 32, wherein the second and third silicon germanium subcells are comprised of approximately 17% silicon and 83% germanium.

34. The photovoltaic cell according to any one of claims 31 to 33, wherein the Group V composition of the fourth subcell is approximately 17% phosphorous and 83% arsenic.

35. The photovoltaic cell according to any one of claims 31 to 34, wherein the Group III composition of the fifth subcell is approximately 40% indium and 60% gallium.

36. The photovoltaic cell according to any one of claims 31 to 35, comprising a transition layer between the first subcell and the second subcell which has a graded lattice constant.

37. A method of manufacturing a photovoltaic cell, the method comprising the steps of: epitaxially growing Group IV subcells on a substrate in a first growth chamber; transferring the substrate to a second growth chamber; and epitaxially growing Group III-V subcells on the substrate in a second growth chamber.

38. The method according to claim 37, further comprising the steps of: depositing an oxidation barrier layer onto the surface of the Group IV subcells; and heating the substrate to remove the oxidation barrier layer.

39. The method according to claim 38, wherein the oxidation barrier layer is germanium.

40. The photovoltaic cell according to any one of claims 31 to 34, comprising gridlines having thickness to width aspect ratio of at least 2:1.

41. A photovoltaic cell comprising gridlines having a thickness to width aspect ratio of at least 2:1.

42. The photovoltaic cell according to any one of claims 40 to 41, wherein thickness to width aspect ratio is at least 5:1.

43. The photovoltaic cell according to any one of claims 40 to 42, wherein the photovoltaic cell comprises a multijunction cell.

44. The photovoltaic cell according to any one of claims 40 to 43, wherein the gridlines are produced by an electroplating process.

45. The photovoltaic cell according to any one of claims 40 to 44, wherein the gridlines comprise copper.

46. The photovoltaic cell according to any one of claims 40 to 45, wherein the gridlines have a meandering shape.

47. The photovoltaic cell according to claim 46, wherein the meandering is less than 10% of the distance separating adjacent gridlines.

48. The photovoltaic cell according to any one of claims 40 to 47, wherein the gridlines comprise lateral support structures extending perpendicular to the length dimension of the gridline.

49. A method of producing a photovoltaic cell comprising gridlines having a high thickness to width aspect ratio, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

50. The photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48, comprising a light guiding prism.

51. A light guiding prism for a photovoltaic cell, the light guiding prism comprising a transparent prism material; a first surface; a second surface; and a central light guiding structure; wherein light is focused onto a region of the first surface and propagates along the central light guiding structure to a region of the second surface; and the central light guiding structure is formed by a void in the prism material surrounding the central light guiding structure.

52. The light guiding prism according to claim 51, wherein the void extends from the second surface into the prism and separates the second surface into separate regions.

53. The light guiding prism according to any one of claims 51 to 52, wherein the transparent prism material is glass.

54. The light guiding prism according to any one of claims 51 to 52, wherein the transparent prism material is a polymer.

55. A light guiding prism substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

56. The photovoltaic system comprising the photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48, the system further comprising a light guiding prism according to any one of claims 51 to 54.

57. The photovoltaic system comprising the photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48, the system further comprising: an optical concentrating means; and a prism comprising a transparent material, a first surface and a second surface; wherein the prism further comprises an anti-reflective coating on a first region of the first surface of the prism, and a reflective coating on one or more second regions of the first surface of the prism; the optical concentrating means focuses light onto the first region of the first surface of the prism; and the light exits the prism in a region on the second surface.

58. A photovoltaic system comprising: an optical concentrating means; and a prism comprising a transparent material, a first surface and a second surface; wherein the prism further comprises an anti-reflective coating on a first region of the first surface of the prism, and a reflective coating on one or more second regions of the first surface of the prism; the optical concentrating means focuses light onto the first region of the first surface of the prism; and the light exits the prism in a region on the second surface.

59. A photovoltaic system substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

60. The photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48, comprising: light guiding prism comprising a first surface and a second surface; and a central light guiding structure formed by a void in the prism surrounding the central light guiding structure; a photovoltaic cell; a support structure; wherein the photovoltaic cell is mounted on the support structure, the light guiding prism is mounted on the support structure and a portion of the second surface of the prism is operatively coupled to the photovoltaic cell, and light is focused onto a region of the first surface and propagates along the central light guiding structure to a region of the second surface.

61. The photovoltaic cell comprising: light guiding prism comprising a first surface and a second surface; and a central light guiding structure formed by a void in the prism surrounding the central light guiding structure; a photovoltaic cell; a support structure; wherein the photovoltaic cell is mounted on the support structure, the light guiding prism is mounted on the support structure and a portion of the second surface of the prism is operatively coupled to the photovoltaic cell, and light is focused onto a region of the first surface and propagates along the central light guiding structure to a region of the second surface.

62. The photovoltaic cell according to claim 58 or 59, wherein the void extends from the second surface into the prism and separates the second surface into separate regions.

63. The photovoltaic cell according to any one of claim 58 to 60, wherein the void is sealed from the atmosphere when the prism is mounted on the support structure.

64. A method of manufacturing a light guiding prism according to any one of claims 51 to 55 including the step of moulding or casting.

65. A method of manufacturing a light guiding prism according to any one of claims 51 to 55 including the step of machining and polishing.

66. A method of manufacturing according to claim 64 or claim 65, wherein the void extends from the second surface into the prism and separates the second surface into separate regions.

67. A method of manufacturing according to claim 64 or claim 65, wherein the transparent prism material is glass.

68. A method of manufacturing according to claim 64 or claim 65, wherein the transparent prism material is a polymer.

69. A method of manufacturing a light guiding prism substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

70. A Fresnel lens element when operatively associated with a photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48, or any one of claims 60 to 63, the lens element characterised by a normal vector which is perpendicular to the plane of the lens element, the lens comprising: a plurality of angled facets, each facet being characterised by normal vectors which are perpendicular to the surface at each point on the surface of the facets; and a plurality of side facets, each facet being characterised by normal vectors which are perpendicular to the surface at each point on the surface of the facets; wherein each of the normal vectors of the side facets are perpendicular to the normal vector of the plane of the lens element, each of the normal vectors of the angled facets are not perpendicular to the normal vector of the plane of the lens element.

71. A Fresnel lens element, the lens element characterised by a normal vector which is perpendicular to the plane of the lens element, the lens comprising: a plurality of angled facets, each facet being characterised by normal vectors which are perpendicular to the surface at each point on the surface of the facets; and a plurality of side facets, each facet being characterised by normal vectors which are perpendicular to the surface at each point on the surface of the facets; wherein each of the normal vectors of the side facets are perpendicular to the normal vector of the plane of the lens element, each of the normal vectors of the angled facets are not perpendicular to the normal vector of the plane of the lens element.

72. The lens element according to claim 70 or claim 71 , wherein the maximum angle between normal vectors of any two points on the surface of the side facets is 180 degrees.

73. The lens element according to any one of claims 70 to 72, wherein the lens is comprised of a polymer.

74. A Fresnel lens when operatively associated with a photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48, or any one of claims 60 to 63, the lens comprising two or more lens elements according to any one of claims 70 to 73.

75. A Fresnel lens comprising two or more lens elements according to any one of claims 70 to 73.

76. The Fresnel lens according to claim 74 or claim 75, comprising a plurality of lens elements wherein the lens elements are arranged to form a concentric lens structure.

77. The Fresnel lens according to any one of claims 74 to 76, comprising a plurality of lens elements wherein the lens elements are mounted on a transparent substrate.

78. The Fresnel lens according to claim 77, wherein the transparent substrate is glass.

79. The Fresnel lens according to claim 77 or claim 78, wherein the lens elements are mounted using transparent adhesive.

80. The Fresnel lens substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

81. A method of manufacturing a Fresnel lens element, the method comprising the steps of: injecting a polymer into a mould, the mould comprising a first and second portion which from a central cavity and which are separable along a parting line; causing the polymer to solidify, and separating the portions of the mould in a manner such that the portion of the mould in contact with the side facets of the lens element is moved in a direction away from the facets.

82. A method of manufacturing a Fresnel lens element, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

83. A CPV panel assembly comprising: at least one photovoltaic receiver module, each including a photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48; at least one Fresnel lens elements according to any one of claims 74 to 80; a metallic support structures; a glass substrate; wherein the Fresnel lenses and metallic support structures are mounted onto and are supported only by the glass substrate and the photovoltaic receiver modules are mounted onto the metallic support structures.

84. A CPV panel assembly comprising: at least one photovoltaic receiver module, at least one Fresnel lens; a metallic support structures; a glass substrate; wherein the Fresnel lenses and metallic support structures are mounted onto and are supported only by the glass substrate and the photovoltaic receiver modules are mounted onto the metallic support structures.

85. The CPV panel assembly according to claim 84, wherein each Fresnel lens comprises a plurality of Fresnel lens elements according to any one of claims 74 to 80.

86. The CPV panel assembly according to any one of claims 83 to 85, wherein the metallic support structures are manufactured using a casting process.

87. The CPV panel assembly according to any one of claims 83 to 86, wherein the metallic support structures are comprised of aluminium or an aluminium alloy.

88. The CPV panel assembly according to any one of claims 83 to 87, wherein the glass substrate has an area greater than 0.5 square metres.

89. The CPV panel assembly according to any one of claims 83 to 88, wherein the glass substrate is at least 3 millimetres thick.

90. The CPV panel assembly according to any one of claims 83 to 89, wherein the glass substrate is 6 millimetres thick.

91. The CPV panel assembly according to any one of claims 83 to 90, wherein the metallic support structure comprises: at least one supporting leg, the at least one leg comprising a mounting feature at one end for attaching the at least one leg to a glass panel and a heat spreading region at the other end of the at least one leg onto which a photovoltaic receiver is mounted.

92. The CPV panel assembly according to any one of claims 83 to 91, wherein the metallic support structure comprising three legs.

93. The CPV panel assembly according to any one of claims 83 to 92, wherein metallic support structure comprising a plurality of legs and side surface structures which enclosed the space between the plurality of legs.

94. A CPV panel assembly, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

95. A metallic support structure for a CPV panel assembly comprising at least one supporting leg, the at least one leg comprising a mounting feature at one end for attaching the at least one leg to a glass panel and a heat spreading region at the other end of the at least one leg onto which a photovoltaic receiver is mounted.

96. The metallic support structure according to claim 95, comprising three legs.

97. The metallic support structure according to claim 95 or claim 96, comprising a plurality of legs and side surface structures which enclosed the space between the plurality of legs.

98. A metallic support structure for a CPV panel assembly, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

99. The photovoltaic system comprising one or more photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48, the system further comprising: a panel frame; one or more primary optical concentrating elements; one or more a movable optical elements; wherein the one or more primary optical elements and the one or more photovoltaic cells are mounted such that they do not move relative to the panel frame; the one or more movable optical elements are mounted such that they can move relative to the panel frame; and each of the one or more movable optical elements spatially redirects light after it is concentrated by one of the one or more primary optical elements and before it reaches one of the one or more photovoltaic cells.

100. The photovoltaic system comprising one or more photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48, the system further comprising: a panel frame; one or more primary optical concentrating elements; one or more a movable optical elements; and one or more actuators; wherein the one or more primary optical elements and the one or more photovoltaic cells are mounted such that they do not move relative to the panel frame; the one or more movable optical elements are mounted such that they can move relative to the panel frame; each of the one or more movable optical elements spatially redirects light after it is concentrated by one of the one or more primary optical elements and before it reaches one of the one or more photovoltaic cells; each of the movable optical elements is operatively coupled to one of the actuators; and each of the actuators sense the relative position of the focused light beam with respect to the centre of the cell and adjusts the orientation of the operatively coupled movable optical elements to keep focused light on the centre of the cell.

101. A photovoltaic system comprising: a panel frame; one or more primary optical concentrating elements; one or more a movable optical elements; one or more photovoltaic cells; wherein the one or more primary optical elements and the one or more photovoltaic cells are mounted such that they do not move relative to the panel frame; the one or more movable optical elements are mounted such that they can move relative to the panel frame; and each of the one or more movable optical elements spatially redirects light after it is concentrated by one of the one or more primary optical elements and before it reaches one of the one or more photovoltaic cells.

102. A concentrating photovoltaic system comprising: a panel frame; one or more primary optical concentrating elements; one or more photovoltaic cells; one or more a movable optical elements; and one or more actuators; wherein the one or more primary optical elements and the one or more photovoltaic cells are mounted such that they do not move relative to the panel frame; the one or more movable optical elements are mounted such that they can move relative to the panel frame; each of the one or more movable optical elements spatially redirects light after it is concentrated by one of the one or more primary optical elements and before it reaches one of the one or more photovoltaic cells; each of the movable optical elements is operatively coupled to one of the actuators; and each of the actuators sense the relative position of the focused light beam with respect to the centre of the cell and adjusts the orientation of the operatively coupled movable optical elements to keep focused light on the centre of the cell.

103. The photovoltaic system according to any one of claims 99 to 102, wherein the one or more primary optical elements is a lens.

104. The photovoltaic system according to any one of claims 99 to 101, wherein the one or more primary optical elements is a mirror.

105. The photovoltaic system according to any one of claims 99 to 102, wherein the one or more movable optical elements is a mirror.

106. The photovoltaic system according to any one of claims 99 to 102, wherein the one or more movable optical elements is a lens.

107. The photovoltaic system according to any one of claims 99 to 104, wherein the one or more movable optical elements is moved using attraction or repulsion of magnetic fields.

108. The photovoltaic system according to claim 105, wherein at least one of the magnetic fields is produced by current flowing from the photovoltaic cells.

109. A photovoltaic system, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

110. A photovoltaic system, comprising one or more cell modules each having at least one photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48, the system further comprising: one or more cell interconnection modules which do not contain photovoltaic cells; wherein the one or more cell modules are connected to each of the one or more cell interconnection modules and the cell interconnection modules are connected to operatively connect the outputs of the cell modules in series.

111. A photovoltaic system, comprising: one or more cell modules which contain photovoltaic cells; and one or more cell interconnection modules which do not contain photovoltaic cells; wherein the one or more cell modules is connected to each of the one or more cell interconnection modules and the cell interconnection modules are connected to operatively connect the outputs of the cell modules in series.

112. The photovoltaic system according to any one of claims 110 to 111, wherein electrical connection ports of the cell modules and the cell interconnection modules comprise flexible cabling which is permanently attached to the modules.

113. The photovoltaic system according to claim 112, wherein the flexible cabling is joined to interconnect cell modules and cell interconnect modules using environmentally sealed terminating devices .

114. The photovoltaic system according to 113, wherein the terminating devices comprise electrical crimp connections.

115. The photovoltaic system according to any one of claims 110 to 114, wherein the cell interconnection modules comprise electronic circuitry which monitors the voltage and/or current of each of the plurality of photovoltaic cells connected to the cell interconnection module.

116. The photovoltaic system according to claim 115, wherein the cell interconnection modules comprise electronic circuitry which encodes photovoltaic cell performance data produced by the cell interconnection module and produces a transmit signal which propagates along cell interconnection module cabling.

117. The photovoltaic system according to claim 116, wherein the electronic circuitry comprises an RF signal generator which is modulated by the transmit signal.

118. The photovoltaic system according to claim 117, wherein the frequency of the RP signal generator is greater than 10MHz.

119. The photovoltaic system according to claim 117, wherein the frequency of the RF signal generator is greater than 100MHz.

120. The photovoltaic system according to claim 117, wherein the frequency of the RF signal generator is chosen to fall within a designated ISM frequency.

121. The photovoltaic system according to any one of claims 116 to 120, wherein the transmit signal is produced by a modulator connected to the electronic circuitry and which modulates the frequency of an RF signal which is generated by an external RF signal source and fed to each cell interconnection module.

122. The photovoltaic system according to claim 121, wherein the modulator comprises a non linear device such as an RF mixer.

123. The photovoltaic system according to claim 121, wherein the modulator, comprises a non linear device such as a variable capacitor.

124. The photovoltaic system according to claim 123, wherein the variable capacitor comprises a varactor diode.

125. The photovoltaic system according to any one of claims 116 to 124, wherein the transmit signal is sent in bursts, the bursts being limited in time such that the burst duration is small compared to the time interval between bursts.

126. The photovoltaic system according to claim 125, wherein the transmit signal . burst duration is less than 1 percent of the average time interval between bursts.

127. The photovoltaic system according to claim 125, wherein the time interval between bursts is random or pseudo-random.

128. The photovoltaic system according to claim 125, wherein the time interval between bursts is determined by an algorithm comprising the previous transmit time interval value and a unique identification number assigned to each cell interconnection module.

125. The photovoltaic system according to any one of claims 110 to 128, comprising a central receiver module which receives and decodes RF signals transmitted from the cell interconnection modules and which comprises a digital interface for communication with external computer equipment.

126. A cell interconnection module comprising: a plurality of bipolar input ports; and a single bipolar output port; wherein the input ports are intended to connect to cell modules containing photovoltaic cells and the output port is intended to connect to other cell interconnect modules or electrical output terminals of a photovoltaic system panel.

127. The cell interconnection module according to claim 126, wherein the bipolar input ports and bipolar output ports comprise flexible cabling which is permanently attached to the cell interconnection module.

128. The cell interconnection module according to claim 126 or claim 127, comprising bypass diodes which are connected to each bipolar input port and oriented to provide reverse bias protection for photovoltaic cells connected to the input ports.

129. The cell interconnection module according to any one of claim 126 to 128, comprising a filter structure which provides RF isolation between the bipolar output ports and the bipolar input ports and which provides RF coupling across the bipolar output port.

130. The cell interconnection module according to claim 129, wherein the filter structure comprises inductive elements formed by patterned conductors on printed circuit boards.

131. The cell interconnection module according to claim 130, wherein the patterned conductors on printed circuit boards comprise tracks which are a quarter wavelength long at the RF carrier frequency used for conveying signalling information along cell interconnect module wiring.

132. A cell interconnection module, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

133. A user access interface for a processor device, the processor device being adapted to monitors one or more photovoltaic cells, the interface comprising a control program adapted to communicate with a cell interconnection module coupled to one or more photovoltaic cells for receiving data indicative of voltage and/or current associated with each of the one or more photovoltaic cells from an interconnection module

133. The user access interface according to claim 133, wherein the photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48.

134. The user access interface according to claim 133, wherein the interconnection module according to any one of claims 126 to 132.

135. A user access interface, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

136. A computer program product stored on a computer usable medium, the computer program product adapted to provide a method of monitoring one or more photovoltaic cells, the method including the step of receiving data indicative of voltage and/or current associated with each of the one or more photovoltaic cells from an interconnection module.

137. A computer program product stored on a computer usable medium, the computer program product adapted to provide a user access interface for a computer device, the computer device being adapted to receive access data indicative of voltage and/or current associated with each of one or more photovoltaic cells, the computer device being coupleable to an interconnection module; the computer program product comprising: computer readable program means for receiving data indicative of voltage and/or current associated with each of the one or more photovoltaic cells from an interconnection module.

138. The computer program product according to claim 136 or claim 137, wherein the photovoltaic cell according to any one of claims 31 to 34 or any one of claims 41 to 48.

139. The computer program product according to claim 136 or claim 139, wherein the interconnection module according to any one of claims 126 to 132.

140. The computer program product, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.