WO2010102345A1

WO2010102345A1 - Improved photo-voltaic device and system

Info

Publication number: WO2010102345A1
Application number: PCT/AU2010/000287
Authority: WO
Inventors: Shaun Joseph Cunningham
Original assignee: Shaun Joseph Cunningham
Priority date: 2009-03-12
Filing date: 2010-03-12
Publication date: 2010-09-16

Abstract

A photo-voltaic cell and system for converting concentrated light into electricity, and a method of manufacturing thereof. A photovoltaic cell comprising: a substrate, and at least two silicon germanium subcells. The at least two silicon germanium subcells can have a different material composition. A photovoltaic panel assembly can comprise a plurality of optical concentrating elements, a plurality of support structures, and a plurality of photovoltaic receiver assemblies. An optical concentrating element can include a lens element and/or a plurality of regions. A modular photovoltaic power conversion system can comprise one or more cell modules having one or more photovoltaic cells, and one or more cell interconnection modules, which do not contain photovoltaic cells. A method of monitoring a modular photovoltaic power conversion system is also disclosed.

Description

IMPROVED PHOTO-VOLTAIC DEVICE AND SYSTEM

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. The p-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 there from. 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.

Wfiether 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. JJI-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 multifunction 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. Tlte 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 multifunction 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 of GaInPl and GaAs are about 1.85 eV and about 1.424 eV respectively (Ge has an indirect bandgap of about 0.66 eV). Ηieoretical 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, GalnP2 may form the first junction, GaAs can form the second junction, a new 1 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 eVhas 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 multijunction 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 8x10¹⁹ cmS. In the prior art an electro-optically active germanium junction is formed as a consequence of arsenic diffusion into thep-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. Tliis 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. Tiie 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 (InGaP) 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 multi-junction photovoltaic cell comprising: a substrate; and at least two silicon germanium subcells.

Preferably, each of the at least two silicon germanium subcells have a different material composition. More preferably, each of the at least two silicon germanium subcells define a different bandgap.

Preferably, the substrate is primarily comprised of germanium. More preferably, the substrate is inactive. Alternatively, the substrate can be active.

Preferably, the photovoltaic cell further comprises a plurality of Group IH-V semiconductor subcells.

Preferably, the photovoltaic cell further comprises a transition layer between each of the at least two silicon germanium subcells. More preferably, each transition layer comprises silicon germanium. Most preferably, each transition layer comprises a graded composition silicon germanium.

Preferably, a graded composition varies from the material composition of a first adjoining silicon germanium subcell to the material composition of a second adjoining silicon germanium subcell. Preferably, the photovoltaic cell further comprises a germanium subcell. More preferably, the germanium subcell is deposited on the substrate; a first silicon germanium subcell is deposited on the germanium subcell; a second silicon germanium subcell is deposited on the first silicon germanium subcell; and Group III- V subcells are deposited on the second silicon germanium subcell.

The first silicon germanium subcell preferably has a material composition Si_yGe_1-yj where y is between 0 and 30%. The second silicon germanium subcell preferably has a material composition Si₂Ge_1-2, where z is between 0 and 50%. The first and second silicon germanium subcells more preferably have a composition of about Sio.₁₅Geo_.s₅

Preferably, the photovoltaic cell comprising a transition layer between the first and second silicon germanium subcells, this transition layers comprise a graded composition silicon germanium graded composition that varies from the material composition of the first silicon germanium subcell to the material composition of a second silicon germanium subcell

Preferably, the photovoltaic cell comprises: three silicon germanium subcells; wherein: a first silicon germanium subcell is deposited on the substrate; a second silicon germanium subcell is deposited on the first silicon germanium subcell; a third silicon germanium subcell is deposited on the second silicon germanium subcell; and a plurality of Group III-V subcells are deposited on the third silicon germanium subcell.

The first silicon germanium subcell preferably has a material composition Si_xGe_1-x, where x is between 0 and 5%. The second silicon germanium subcell preferably has a material composition Si_yGe_1-y, where y is between 0 and 30%. The third silicon germanium subcell preferably has a material composition Si₂Ge_1-2, where z is between 0 and 50%.

Preferably, at least two of the three silicon germanium subcells have a composition of about Sio.^Geo.ss and Sio.1₉Go.s1. Preferably, the photovoltaic cell further comprises a plurality of Group III-V semiconductor subcells.

Preferably, the plurality of III-V subcells include a first III-V subcell comprise GaAsP and a second III-V subcell comprising InGaP.

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

According to an aspect of the invention there is provided a method of manufacturing a photovoltaic cell, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

According to an aspect of the invention there is provided a method of manufacturing a photovoltaic cell, the method comprising the steps of: providing a substrate; and depositing at least two silicon germanium subcells.

Preferably, the method further comprises the step of depositing a transition layer between each of the at least two silicon germanium subcells.

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 multi-junction photovoltaic cell structure comprising: a cell substrate, at least two silicon germanium subcells; and a plurality of Group III- V semiconductor subcells; wherein each of the at least two silicon germanium subcells has a different material composition and a different bandgap.

According to an aspect of the invention there is provided a five-junction photovoltaic cell structure comprising: a an inactive substrate; 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.

According to an aspect of the invention there is provided a multi-junction photovoltaic cell structure with improved conversion efficiency comprising a cell substrate, at least two silicon germanium subcells and a plurality of Group III- V semiconductor subcells wherein each of the at least two silicon germanium subcells has a different material composition and different bandgap.

Preferably, cell conversion efficiency can be improved and lattice stress and crystal defects can be reduced by using a plurality of Group IV semiconductor subcells and a plurality of Group III-IV semiconductor subcells.

According to an aspect of the invention there is provided a panel assembly for concentrator photovoltaic power systems.

According to an aspect of the invention there is provided a photovoltaic panel assembly comprising: a plurality of optical concentrating elements; a plurality of support structures; and a plurality of photovoltaic receiver assemblies.

Preferably, the assembly further comprises: a substantially optically-transparent panel; wherein the support structures are mounted onto, and are supported by, the transparent panel.

According to an aspect of the invention there is provided a photovoltaic panel assembly comprising: one or more photovoltaic receiver assemblies, wherein the receiver assemblies include a photovoltaic cell as herein disclosed; one or more optical concentrating elements; one or more support structures; and wherein the photovoltaic receiver assemblies are mounted onto the support structures at a location corresponding to the focal point of the optical concentrating elements.

Preferably, the optical concentrating elements are planar lenses. The planar lenses preferably comprise a polymer sheet bonded to the transparent panel. The planar lenses preferably comprise a polymer which is deposited onto the transparent panel and patterned in-situ to from planar lenses. The planar lenses are preferably formed on the surface of the transparent panel by deforming the surface of the panel itself.

Preferably, the optical concentrating elements are mounted on the same side of the transparent panel as the photovoltaic cell assemblies.

Preferably, the support structures are mounted onto the transparent panel using adhesive and/or a mechanical means. More preferably, mechanical means is a mechanical fixing such as screws or clips or the like.

Preferably, the support structures are environmentally sealed at an interface to the transparent panel using a deformable material. More preferably, the deformable material is a polymer gasket or O-ring. Alternatively, the deformable material is the material used to form the optical concentrating elements.

The assembly preferably further comprises a rigid panel structure. Preferably, the panel comprises glass. Alternatively, the panel comprises a polymer.

Preferably, the optical concentrating elements are lenses. More preferably, the optical concentrating elements are planar lenses. Most preferably, the optical concentrating elements are Fresnel lenses.

Preferably, the support structures comprise a surface which encloses the volume of air between the optical concentrating elements and the photovoltaic receiver assemblies. More preferably, the support structures comprise any one or more materials selected from the set comprising: a metal; an aluminium alloy; a polymer; and a metal coated polymer.

Preferably, fabrication of the support structures uses any one or more methods selected from the set comprising: a cold forming process. a pressing or stamping process. a casting process.

Preferably, the support structures comprises a mounting flange (or collar) used to mount the photovoltaic receiver assemblies. More preferably, the photovoltaic receiver assemblies comprise a photovoltaic cell assembly and a heatsink element. Most preferably, the heatsink element comprises mounting features which are used to secure the photovoltaic receiver assemblies to the support structure.

Preferably, the photovoltaic receiver assemblies comprise securing features used to locate and retain the photovoltaic receiver assemblies against mounting surfaces of the support structures. More preferably, the securing features form either a bayonet style locking arrangement or a threaded arrangement. Most preferably, the bayonet style locking arrangement is engaged or disengaged by a rotational movement of the photovoltaic receiver assemblies with respect to the support structures.

Preferably, the assembly further comprises a rigid panel structure. More preferably, the support structures has a fixed, rigid, orientation with respect to the optical concentrating elements and the rigid panel structure; the photovoltaic receiver assemblies are mounted on the support structures; and the photovoltaic receiver assemblies are able to be detached and reattached to the support structures without dismantling the support structures.

Alternatively, the support structures can have fixed, rigid, orientation with respect to the optical concentrating elements and the rigid panel structure; and each of the support structures form a surface which encloses a single optical element, a single photovoltaic receiver assembly and the optical path there between.

Preferably, the support structures comprise an aperture which allows air to pass into and out of the volume enclosed by the support structure. More preferably, a filter element is positioned next to the aperture for the purpose of reducing dust, airborne pollutants or contaminants from entering the volume enclosed by the support structure. Most preferably, the filter element is replaceable.

Preferably, a coyer is provided over the filter element to prevent moisture or water ingress.

According to an aspect of the invention there is provided a photovoltaic 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.

Preferably, the panel assembly is a modular panel structure that facilitates lower costs, and eases panel assembly and maintenance. More preferably, the modular panel structure can reduces costs and facilitates the easy removal of PV cell assemblies for the purpose of repair or system upgrade.

According to an aspect of the invention there is provided a manufacturing and maintenance methods for a panel assembly of concentrator photovoltaic power systems, and the panel assembly having a modular panel structure.

According to an aspect of the invention there is provided a modular cell assembly which can be attached to and detached from a CPV panel without disturbing the structural integrity of either the panel or the CPV system in which the panel is mounted.

According to an aspect of the invention there is provided a Fresnel Lens assembly, and manufacturing methods thereof. It would be appreciated that these lenses can be used in applications such as Concentrator Photo-Voltaic (CPV) power systems. However, it will be appreciated that the invention is not limited to this particular field of use.

According to an aspect of the invention there is provided a lens element comprising a plurality of facets, wherein at least one of the plurality of facets is perpendicular to the plane of the lens in at least one region and all of the plurality of facets are non- opposing.

According to an aspect of the invention there is provided a lens element operatively associated with the assembly as herein disclosed, the lens element forming at least part of the optical concentrating elements, the lens element further comprising a plurality of facets wherein at least one of the plurality of facets is perpendicular to the plane of the lens in at least one region and all of the plurality of facets are non- opposing.

Preferably, the lens structure is comprised of any one or more materials selected from the set comprising: a polymer, a glass.

According to an aspect of the invention there is provided a composite lens structure comprising at least two lens elements as herein disclosed.

According to an aspect of the invention there is provided a composite lens structure operatively associated with the assembly as herein disclosed, the lens element being each of the optical concentrating elements, composite lens element comprising at least two lens elements as herein disclosed.

Preferably, the lens element is a composite planar Fresnel lens.

Preferably, the lens elements form a parquet having the lens elements arranged in an array. More preferably, the parquet array is a 1 by N array and wherein N is an integer. Most preferably, the lens structure comprises two or more of the lens parquets arrays.

Preferably, the lens elements are all fabricated simultaneously using a single process operation. Preferably, the lens structure is mounted on a substantially optically transparent substrate. More preferably, the transparent substrate is glass. Most preferably, the lens structure is mounted using transparent adhesive].

Preferably, the lens structure is used in a CPV panel assembly. More preferably, the CPV panel assembly is as herein disclosed.

According to an aspect of the invention there is provided a lens structure comprising a plurality of regions, wherein: each region comprises a plurality of refracting facets; within each region the refracting facets are distributed uniformly over the region and have an approximately constant facet angle relative to the plane of the' lens; and the facet angle and orientation is different from region to region to achieve superposition of refracted light from all of the regions at the focal point of the lens.

Preferably, the region is rectangular or square.

Preferably, the lens structure is operatively associated with a PV cell, and wherein the region is about the same size and shape as a photovoltaic cell being illuminated by the lens structure. ^•

Preferably, the lens structure comprises a plurality of facets, wherein at least one of the plurality of facets is perpendicular to the plane of the lens in at least one region and all of the plurality of facets are non-opposing.

Preferably, the lens structure is used to define a convolutional Fresnel lens structure.

Preferably, the lens structure is used to define a convolutional Fresnel lens parquet.

Preferably, the lens structure comprises a plurality of regions, wherein facets in the regions produce a spatial convolution of an optical source with the regions to produce approximately uniform illumination at the focal point of the lens in the shape of the regions.

According to an aspect of the invention there is provided a lens structure, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples. According to an aspect of the invention there is provided an optical lens comprising at least two lens structures as herein disclosed.

Preferably, the optical lens comprises a plurality of lens structures, wherein the lens structures are arranged to form a concentric lens structure. More preferably, the plurality of lens elements are arranged in an array.

Preferably, the plurality of lens elements are arranged as a lens parquet, wherein the array is a 1 by N array where N is an integer.More preferably, the array is a convolutional Fresnel lenses comprising two or more lens parquets. Most preferably, the lens structures are mounted on a transparent substrate.

Preferably, lens elements are all fabricated simultaneously using a single process operation.

According to an aspect of the invention there is provided a CPV panel assembly comprising a Fresnel lens structure as herein disclosed. The CPV panel assembly preferably comprising a composite Fresnel lens structure as herein disclosed. Alternatively, the CPV panel assembly can comprising a convolutional Fresnel lens structure as herein disclosed.

According to an aspect of the invention there is provided a method of manufacturing a lens structure comprising at least one facet which is perpendicular to the plane of the lens in at least one region, the method comprising the steps of:

(a) 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;

(b) causing the polymer to solidify, and

(c) separating the portions of the mould at the parting line in a direction such that the portion of the mould in contact with the facets of the lens is moved in a direction which separates it from all lens facets simultaneously.

According to an aspect of the invention there is provided a method of manufacturing a lens structure comprising at least one facet which is perpendicular to the plane of the lens in at least one region, the method comprising the steps of: (a) pressing a first mould into the surface of a deformable material to form facets on one region of the deformable material;

(b) pressing a second mould into the surface of a deformable material to form facets on a second region of the deformable material; wherein facets produced by the first and second moulds are all non-opposing.

Preferably, the first mould is used for the second moulding operation by rotating the deformable material relative to the mould or rotating the mould relative to the material.

Preferably, the lens structure is a Fresnel lens structure or a convolutional Fresnel lens structure.

According to an aspect of the invention there is provided a method of manufacturing a lens structure, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

According to an aspect of the invention there is provided an improved structure for a Fresnel lens which can reduce optical loss.

According to an aspect of the invention there is provided a structure for a convolutional Fresnel lens that produces approximately uniform rectangular illumination across the image plane of the lens.

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

According to an aspect of the invention there is provided a Fresnel Lens parquet assembly which facilitates the simultaneous manufacture of multiple lens components.

According to an aspect of the invention there is provided a structure for a CPV panel that utilises improved Fresnel lens elements. Preferably, rectangular or square photovoltaic cells are disclosed herein.

According to an aspect of the invention there is provided a method of manufacturing a lens structure, the method composing the step of: patterning of the surface of a glass substrate. Preferably, patterning includes using a moulding or embossing tool. More preferably, the glass substrate is heated to a prescribed temperature above 500 degrees Celsius, wherein the glass substrate is readily deformable using an embossing or moulding tool. Most preferably, the embossing or moulding tool is made of a material which maintains its surface characteristics and does not distort at these elevated temperatures. The embossing or moulding tool is preferably made from glassy carbon or vitreous carbon.

According to an aspect of the invention there is provided a method of manufacturing a Fresnel lens parquet, the method comprising the steps of:

(a) forming a lens on a polymer substrates

(b) attaching the polymer substrate to a glass substrates.

Preferably, lenses manufactured on polymer substrates have perpendicular lens facets. More preferably, polymer lens parquets are preferably attached to glass substrates using adhesive. Alternatively, polymer lens parquets are formed by first attaching an unpatterned polymer sheet to a glass substrate using an adhesive, and then patterning said polymer sheet to form Fresnel lenses.

According to an aspect of the invention there is provided a photovoltaic subsystem device for use in solar energy converters. Preferably, the devices can 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 cell interconnection module comprising: a plurality of input ports; and a single output port.

According to an aspect of the invention there is provided a cell interconnection module operatively associated with the assembly as herein disclosed, the cell interconnection module comprising: a plurality of input ports; and a single output port.

Preferably, the input ports are bipolar input ports. More preferably, the output port is a bipolar output port. Preferably, the input ports are intended to be coupled to one or more cell modules comprising a photovoltaic cell; and the output port is intended to be coupled to either another cell interconnect module, or an electrical output terminals of a photovoltaic system panel.

Preferably, the cell modules comprise a photovoltaic cell as herein disclosed.

Preferably, the photovoltaic system panel is a panel as herein disclosed.

Preferably, the module comprises electronic circuitry for monitors voltage and/or current of a plurality of coupled photovoltaic cells. More preferably, the electronic circuitry comprises a microprocessor or a microcontroller.

Most preferably, the electronic circuitry encodes photovoltaic cell performance data produced by the cell interconnection module into a transmit signal which is suitable for transmission to a remote unit. The electronic circuitry preferably comprises an electro-optic isolator.

Preferably, the cell interconnection modules comprise electronic circuitry which encodes photovoltaic cell performance data produced by the cell interconnection module into a transmit signal which is suitable for transmission to a remote unit. More preferably, the transmit signal passes through the electro-optic isolator.

Preferably, 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 RF 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 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.

According to an aspect of the invention there is provided a modular photovoltaic power conversion system comprising: one or more cell modules having one or more photovoltaic cells; and one or more cell interconnection modules, which do not contain photovoltaic cells.

Preferably, the cell modules are as herein disclosed. More preferably, the cell modules comprise photovoltaic cell as herein disclosed.

Preferably, the cell interconnection modules are cell interconnection modules as herein disclosed.

Preferably, a plurality of the cell modules are coupled to each of a plurality of the cell interconnection modules; and the cell interconnection modules operatively couples 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, outputs of a plurality of cell interconnection modules are connected to a signal receiver module. More preferably, the signal receiver module receives data sent from a plurality of performance monitoring circuits contained in a plurality of cell interconnection modules. Most preferably, the signal receiver module produces a collated summary of data received from a plurality of performance monitoring circuits contained in a plurality of cell interconnection modules and encodes the summary into a signal which is suitable for transmission to additional data collection and collation units or computer equipment. Preferably, the cell interconnection modules comprise electronic circuitry which encodes photovoltaic cell performance data produced by the cell interconnection module into a transmit signal which is suitable for transmission to a remote unit. More 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. Most preferably, the transmit signal burst duration is less than 1 percent of the average time interval between bursts. The time interval between bursts is preferably random or pseudo-random. Alternatively, 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.

According to an aspect of the invention there is provided a modular photovoltaic power conversion system, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

According to an aspect of the invention there is provided a method of coupling together a modular photovoltaic power conversion system, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

According to an aspect of the invention there' is provided a method of monitoring a modular photovoltaic power conversion system, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

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 herein disclosed. More preferably, the interconnection module is as herein disclosed. According to an aspect of the invention there is provided 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.

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.

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

According to an aspect of the invention there is provided a 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.

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. 2 A 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 shows a simplified view of a multijunction cell according to an embodiment; FIG. 7 shows a simplified view of a multijunction cell according to an embodiment, showing material compositions;

FIG. 8 is a perspective view of a CPV panel assembly using Fresnel lenses; FIG. 9 is a detailed perspective view of the assembly of FIG.8; FIG. 10 is a perspective view of a modular CPV panel; FIG. 1 IA is a schematic view of a modular CPV subsystem;

FIG. 1 IB is a cross-section view of an embodiment of the modular CPV subsystem of FIG. HA;

FIG. 12A is a cross-section view of an embodiment of the modular CPV subsystem of FIG. 1 IA, showing an attachment element;

FIG. 12B is a plan view of an embodiment of the modular CPV subsystem of FIG. 1 IA, showing an attachment element;

FIG. 13 is a cross-section view of an embodiment of the modular CPV subsystem of FIG. 1 IA, showing a filter assembly;

FIG. 14 is a schematic diagram showing the derivation of Fresnel lens structure;

FIG. 15A is a schematic cross section view of a portion of an ideal Fresnel lens, showing lens surface features and associated refraction of light rays;

FIG. 15B is a schematic cross section view of a portion of an ideal Fresnel lens of FIG. 15 A, showing draft angles on lens facets with associated refraction and reflection of light rays;

FIG. 16 shows a schematic side view of a prismatic lens used to produce uniform illumination across a rectangular image area;

FIG. 17A is a schematic cross section view of a Fresnel lens, which is comprised of at least two physically separate regions;

FIG. 17B is apian view of physically separate portions of a Fresnel lens;

FIG. 17C is a plan view of physically separate portions of a Fresnel lens, shown partially assembled for forming a complete lens; FIG. 18 is a schematic side view of a glass panel comprising a Fresnel lens, shown fixed to the panel surface;

FIG. 19A is a schematic side view of a moulding cavity containing a Fresnel lens, showing a cavity parting line;

FIG. 19B is a schematic side view of a moulding cavity containing a Fresnel lens, showing the separation of the mould to remove the lens;

FIG. 2OA - 2OD is a schematic sequence diagram showing a Fresnel lens being formed in two separate manufacturing steps;

FIG. 21 A - 2 IB show composite Fresnel lens parquets;

FIG. 22 is a schematic cross section of a Fresnel lens;

FIG. 23 is a schematic plan view of a simplified Fresnel lens;

FIG. 24 is a block diagram of an embodiment modular photovoltaic power conversion system, showing a cell modules and a cell interconnection module;

FIG. 25 is a block diagram of an embodiment modular photovoltaic power conversion system, showing cell modules and multiple cell interconnection modules;

FIG. 26 is a block diagram of an embodiment of modular photovoltaic power conversion system, showing termination devices which are used to connect cell modules and cell interconnection modules;

FIG. 27 is a block diagram of an embodiment of modular photovoltaic power conversion system, showing internal architecture of a cell interconnection module;

FIG. 28 is a block diagram of an embodiment of modular photovoltaic power conversion system, showing internal architecture of a cell interconnection module;

FIG. 29 is a block diagram of an embodiment of modular photovoltaic power conversion system, showing an arrangement of cell modules and cell interconnection modules in a PV panel; and FIG. 30 is a flow chart of an embodiment method for measurement and 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; y 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. International Patent Application No. PCT/AU2009/001683, entitled "Improved Photo-Voltaic Device", is herein incorporated by reference.

Multi-Junction Photovoltaic Cell

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 choose 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 TV 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 IH-V subcells is fixed and does not change.

Much effort has been invested in the selection of Group HI-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 UI-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 W and Group HI-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 HI-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 Si_o._O2Geo.₉₈. are suitable in certain circumstances.

An embodiment can provide a multijunction 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.₁₇Geo.s₃ (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. 2A, 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 maybe 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 maybe Sio.πGeo,₈₃ (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, GaAs_0-83Pc₁₇ 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 Ino.₄Gao.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.

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. 3 A through 3D, the growth process starts with an inactive substrate 300 onto which group W 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 TV 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 Sio.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 recrystaϊlises 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 My 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 IIOOC to recrystallise SiGe or Ge Surface layer 409 to form surface layer 419. Then group rv and group HI-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 IV 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 maybe 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 may be Si_o.₁₇Geo.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_o.8₃P_o.i7 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 Ino.4Gao.0P 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.

Referring to FIG. 6, an embodiment provides a multijunction photovoltaic cell structure comprising a plurality of silicon germanium (SiGe) subcells. The composition of each subcell is defined by the formula Si_xGe_1-x where x is a number between 0 and 1 and represents the atomic percentage of silicon in the SiGe subcell material.

According to this embodiment, a first SiGe subcell 602 is deposited on substrate 600 by known epitaxial growth techniques. The substrate may be "inactive" meaning that it does not contain a photovoltaic junction and only serves as a template for crystal growth and as a means of making an electrical connection to the lowest subcell. Alternatively the substrate maybe "active" meaning it also comprises a photovoltaic subcell. Preferably cell substrate 600 comprises germanium.

Preferably the first SiGe subcell has a small percentage of silicon. For example the composition of the first subcell is preferably Si_xGe_1-x where x: 0 < x < 5%. The first subcell may be entirely comprised of Ge (i.e. x=0). A transition layer 601 may optionally be grown on the surface of substrate 600 to improve crystal quality or subcell energy band structure. The composition of this transition layer is preferably graded from the substrate material to Si_xGei_-x.

A second SiGe subcell 604 is then preferably deposited on top of the first SiGe subcell. The composition of the second subcell is preferably SIyGe₁ __y where y: x < y < 30% and x is defined above. A second transition layer 603 optionally may be grown on the surface of the first subcell 602 to improve crystal quality or subcell energy band structure. This second transition layer is preferably graded from the material composition of the first subcell Si_xGβι-_x to the material composition of the second subcell Si_yGe_1-y.

Similarly, a third SiGe subcell 606 is then preferably deposited on top of the second SiGe subcell. The composition of the third subcell is preferably Si₂Ge_1-2 where z: y < z < 50% and y is defined above. A third transition layer 605 optionally may be grown on the surface of the second subcell 604 to improve crystal quality or subcell energy band structure. This third transition layer is preferably graded from the material composition of the second subcell Si_yGe_1-y to the material composition of the third subcell Si₂Ge_1-2.

Additional SiGe subcells and transition layers may be added in a similar fashion according to the band structure and spectral absorption requirements of the overall cell.

According to an embodiment, a plurality of Group III-V subcells 608 is preferably deposited on top of the uppermost SiGe subcell. For example a first Group III-V subcell preferably comprising GaAs or GaAsP is deposited on top of the uppermost SiGe subcell and a second Group III-V subcell preferably comprising InGaP is deposited on top of the first Group III-V subcell. A buffer layer 607 may optionally be grown on top of the uppermost SiGe layer to improve crystal quality or subcell energy band structure.

According to an embodiment, the material composition and thickness of each of the subcells is chosen such that each of the Group IV and Group III-V subcells generates approximately the same current when illuminated by the incident solar spectrum.

Also according to an embodiment, the overall cell structure comprises at least two SiGe subcells. These subcells are specifically chosen to have different material compositions and different bandgaps.

Multiple Group IV subcells have been proposed in prior art multijunction cells. For example in the above referenced 6340788 patent, the use of two identical SiGe subcells is disclosed. The inventor has realised that this structure has inherent problems. Cell structures comprising two layers of the same material are not optimal for cell conversion efficiency for the following reasons.

Firstly, it will be appreciated that the ideal structure for a multijunction cell comprises a number of subcells having a plurality of different bandgaps with different spectral absorption characteristics. A cell structure which has multiple subcells comprising the same material composition and hence the same bandgap is therefore inherently less efficient.

Secondly, it will be appreciated that subcell crystal quality is critical for optimal conversion efficiency. When SiGe materials are combined with Group III- V materials it is difficult to achieve good crystal quality because of the different lattice constants of these materials. For example, the lattice constant of SiGe subcells is significantly less than that of a Ge substrate and preferred Group III- V subcells. When two or more SiGe subcells are grown on a Ge substrate it is advantageous to distribute crystal mismatch over a significant crystal thickness in order to reduce stresses which can lead to crystal defects. In prior art structures using two identical SiGe layers, all of the crystal mismatch is concentrated in the transition layer between the bottom SiGe layer and the substrate and no crystal mismatch exists between the two SiGe subcell layers.

By way of example only, an embodiment is provided in FIG. 7 which includes an example of subcell material compositions.

It will be appreciated that disclosed embodiments can use at least two SiGe subcells having different material compositions and different bandgaps. By using different materials, crystal mismatch stresses can be lowered by distributing stresses across multiple transition layers. Optical absorption characteristics and conversion efficiency also can be improved because of the use of different materials with different bandgaps.

CPV Panel Assemblies

United States patent US5,118,361 describes a CPV array using Fresnel lenses as the primary concentrating element. FIG. 8 and FIG. 9 provides an overview of the design described in this patent including overall panel housing 800, cell module assemblies 801 and Fresnel lenses 802. In this type of system, sunlight is concentrated approximately 500 times by the Fresnel lens and focused directly on the multijunction PV cell.

The teaching of this patent relates to the use of metallised flexible circuit "tapes" which have apertures distributed along their length in which cells are mounted. In this design, as best shown in FIG. 9, cell substrates 901 are bonded directly to electrical conductors 903 formed on the surface of the flexible tape 900. 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. In order to compete with other forms of energy, there is an ongoing need 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. 8, conventional panels rely on side structures 803 as well as front 805 and rear 804 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.

From another perspective, conventional CPV panels give little consideration to the need to replace or upgrade photovoltaic cells. For example, it is likely that cells will fail during extended operation in the field over many years. In current state-of-the-art CPV systems, as marketed for example by SolFocus Inc and Concentrix Solar GmbH, an entire panel needs to be replaced if a single cell fails. This adds substantially to operational overheads and system reliability and impacts on the profitability of CPV installations. Furthermore, current state-of-the-art systems do not provide a means of upgrading panel assemblies as new, higher efficiency cells are developed.

Accordingly, there is a need in the art for a new modular panel design that reduces costs and facilitates the easy removal of PV cell assemblies for the purpose of repair or system upgrade. There is also a need in the art for manufacturing and maintenance methods for such panels.

According to an embodiment there is provided a modular CPV panel assembly comprising: a plurality of concentrating optical elements mounted on a transparent panel; a plurality of support structures which are mounted either directly or indirectly onto the transparent panel and aligned to the optical elements; and a plurality of PV receiver assemblies which are mounted on the support structures. The transparent panel therefore provides an alignment reference plane for panel elements and is the main structural member for the CPV panel.

To reduce the considerable cost associated with conventional CPV panel housings, an embodiment can utilises the transparent panel as the central structural member that other components are mounted onto. Preferably the transparent panel is glass. The following description refers to glass panels. However, polymers may also be used as the transparent panel.

Glass can be used for the front surface of a PV panel because it is optically transparent and environmentally robust. Glass is also a relatively low cost material that is made by a process that inherently results in flat panels. Given that glass is generally needed in CPV assemblies as a transparent, protective front cover and because glass is intrinsically flat and rigid, the inventor has realised that it is cost effective to use a glass panel as the core structural member and the mounting reference plane of a CPV panel. By using the glass panel as the core structural member of the panel, the remainder of the CPV system structure can be simplified and minimised to reduce costs.

The glass panel may need to be thicker than it would otherwise need to be as a simple cover-glass element, 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 - 2 square metres in area. This particular example should not be regarded as restricting the scope of the present disclosure.

An embodiment can comprise a glass panel with a plurality of concentrating optical elements mounted on the surface, preferably in a regular array. Preferably the optical elements are planar structures such as Fresnel lenses. These optical elements preferably comprise a thermoset or thermoplastic polymer sheet which is bonded to the glass panel using transparent adhesive. Alternatively, the optical elements may comprise a polymer which is coated onto the glass panel and patterned in-situ to form an optical concentrating element such as a Fresnel lens. Alternatively the surface of the glass (or polymer) panel may be patterned by a process such as pressing, moulding or embossing to form optical concentrating elements on the surface of the panel itself.

Preferably, the concentrating optical elements are 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 environmental protection for the optical elements.

In the following description, the concentrating optical elements are described as lenses for the sake of simplicity. This should not be interpreted as limiting the scope of the present teaching to only lenses. An aspect of an embodiment can provide a CPV panel comprising support structures which are used to position PV cell assemblies at a certain distance form the surface of the panel corresponding to the focal point of the lenses. These support structures are preferably made of either metal or metallised plastic to resist possible thermal damage caused by misalignment of the panel to the sun. The support structures are mounted on the glass panel which serves as an alignment reference plane. The support structures may be fixed to the glass panel using adhesive or by mechanical fasteners such as screws. The support structures may also be fixed indirectly to the glass panel by being bonding to the lenses which are in turn bonded to the surface of the glass panel.

The support structures are preferably produced as a single seamless structure using a process such as cold forming, stamping, pressing or diecasting. The support structures are preferably made of aluminium or an aluminium alloy.

The support structures preferably comprise side walls that provide an environmental seal to keep the internal lens and PV cell surfaces clean. The support structures therefore preferably enclose the optical path from the lens to the cell to prevent ingress of atmospheric contaminants and/or water or moisture.

FIG. 10 shows an example embodiment of a CPV panel. Optical elements 1001 such as Fresnel lenses are arranged in an array on the surface of the panel 1000 and focus light onto cell assemblies 1003 that are mounted on support structures 1002. These support structures are mounted on panel 1000 and enclose the optical path from lens to cell and provide environmental protection. To reveal internal details, sidewall of the bottom support structure is not shown in FIG. 10.

FIG. 1 IA shows an schematic perspective view of a CPV panel subsystem. A CPV panel is made from an array of such modular subsystems. An optical element such as a Fresnel lens 1101a is located on a surface of the glass panel, a portion of which 1100a is shown in FIG. 1 IA. A support structure 1102a is mounted on the surface of the glass panel 1100a using adhesive or mechanical means such as screws. A gasket or a similar deformable medium may be used to provide an environmental seal in the region 1105a between the panel and the support structure. The support structure 1102a is either mounted directly onto the panel 1100a or is mounted onto the material that forms the lens 1101a. In this latter case, the lens material may provide the deformable medium that seals the edges of the support structure where they contact the panel 1105a.

The CPV subsystem preferably comprises a PV receiver assembly 1120a which is mounted on the support structure 1102a. The PV receiver assembly preferably comprises a heat sink element 1104a and cell assembly 1103 a which is mounted on a surface of the heatsink to form the PV receiver assembly. The PV receiver assembly preferably comprises a mounting surface 1106a for mounting the PV receiver assembly on the support structure. The heatsink element 1104a preferably comprises features such as fins which increase the heatsink surface area for the purpose of increasing heat dissipation. The heatsink element preferably comprises aluminium and is preferably formed using a diecasting or extrusion process.

Alternatively, the PV receiver assembly 1120a may comprise only a cell assembly 1103a without a separate heatsink element. In this case the cell assembly is mounted directly onto the support structure 1102a which provides heatsink features.

The cell assembly preferably comprises a photovoltaic cell and associated packaging which provides physical and environmental protection for the cell as well as features which cater for thermal, optical and electrical requirements of the cell assembly.

FIG. 1 IB shows a cross section view of the CPV subsystem including PV receiver assembly 1120b, comprising heatsink 1104b and cell assembly 1103b and PV receiver assembly mounting surface 1106b. FIG. 1 IB also shows cross sectional views of structure 1102b, lens 1101b and glass panel 1100b.

The support structure 1102a,b provides a mounting surface for mounting and aligning the PV receiver assembly 1120a,b to the support structure. The PV cell assembly 1103b (or cell) is preferably aligned to the heatsink element 1104a,b which is mounted on and aligned to support structure 1102a,b, which in turn is mounted on the glass panel in a fixed alignment to the lens 1101a,b. The cell assembly is therefore aligned to the CPV subsystem lens.

The support structure 1102a,b preferably comprises a mounting surface 1106a,b against which the PV receiver assembly presses to form an environmental seal. Compliant elements such as gaskets or O-rings may also be used at the mounting surface to ensure environmental sealing. The mounting surface is preferably a flange or collar that is formed on the end of the support structure 1102a,b during the manufacturing process that forms the support structure.

The mounting means comprises a securing means that prevents the PV receiver assembly from moving after it is fitted to the support structure. The securing means is reversible, meaning that it can be released or removed to allow the PV receiver assembly to be removed from the support structure. For example, securing means may comprise screws, bolts or clips. Preferably the securing means comprises features integrated into the PV receiver assembly and support structure that provide alignment and mechanical retention features which lock or unlock the cell-heatsink assembly from its mounting location.

For example, features on the PV receiver assembly and support structure may comprise a bayonet-style retention mechanism where the PV receiver assembly is rotated one direction to engage the retention mechanism and is rotated in the opposite direction to release the mechanism. These features or other independent features preferably also provide alignment references that ensure the cell-heatsink assembly is correctly aligned to the support structure when it is fitted.

As. another example, the PV receiver assembly may comprise a threaded feature which engages with a corresponding feature on the support structure and allows the PV receiver assembly to be screwed into position by rotating the PV receiver assembly with respect to the support structure.

By comprising a securing means that allows the PV receiver assembly to be replaced, there is provided a structure and method for repairing or upgrading the CPV panel in the field by simply replacing modular cell assemblies that are separate from panel support structures, concentrating optics and the front coverglass.

The preceding description has made reference to a support structure which is mounted on a transparent panel. An aspect of an embodiment can relate to detachable PV receiver assemblies, and is not restricted to support structures mounted in the herein disclosed manner. When referring to a detachable PV receiver assembly, the term "support structure" can mean any physical structure that is rigidly fixed to the CPV panel assembly in normal operation.

FIG. 12A shows a cross section view of an example embodiment modular CPV subsystem. Cell assembly 1203 a is mounted on heatsink 1204a to form PV receiver assembly 1220a which is attached to support structure 1202a at mounting surface 1206a. The PV receiver assembly comprises mounting features such as pins or tabs 1207a which protrude form the PV receiver assembly surface and engage with corresponding holes or slots in the support structure 1202a. The pins 1207a are shaped to have a large head section and narrow neck so as to provide a means of retaining the PV receiver assembly on the support structure.

FIG. 12B shows details of the mounting surface 1206b of support structure 1202b according to the above example. Mounting surface 1206b comprises apertures 1208b in support structure 1202b that allow the head section of the mounting pins to be inserted. The pins are rotated in the direction of the arrows shown to locations 1209b where the pins are locked into place, thereby fixing the PV receiver assembly in place. A secondary locking mechanism may engage at this location 1209b which stops the heatsink assembly from being rotated and removed accidentally.

It will be appreciated that an embodiment can provide a modular subsystem for a CPV panel comprising an aperture which allows air to pass into the closed volume of the CPV subsystem in a controlled manner. By allowing air to pass between the atmosphere and the closed volume of the CPV subsystem, excessive moisture accumulation inside the CPV system can be avoided. Such accumulation can occur if the CPV subsystem is sealed and small imperfections in the seal exist which allow ingress of moisture or humid air.

FIG. 13 provides an example embodiment of a CPV subsystem comprising an atmospheric aperture. An opening 1310 is provided in support structure 1302 to allow air to pass into and out of the subsystem. A filter 1312 is located adjacent to the opening to prevent airborne contaminants such as dust or pollution, or insects from entering the subsystem. The subsystem also comprises a filter cover structure 1311 which prevents water from entering the filter. The filter and filter cover are preferably designed to allow easy removal and replacement of the filter at regular maintenance intervals.

Fresnel Lens Assemblies

The term Fresnel Lens is typically used in its most general sense to mean a planar lens structure comprising a plurality of facets that refract light to form an image at the focal point of the lens. The term Fresnel Lens therefore does not imply any restriction on the shape or orientation of the refracting facets.

As previously discussed, a useful reference to Fresnel-based CPV systems is provided by United States patent US 5, 118,361. This patent describes a CPV array using Fresnel lenses as the primary concentrating element. FIG. 8 and FIG. 9 provides an overview of the design described in this patent including overall panel housing 800, cell module assemblies 801 and Fresnel lenses 802. In this type of system, sunlight is concentrated approximately 500 times by the Fresnel lens and focused directly on the multifunction PV cell.

FIG. 14 shows a schematic diagram of a derivation of a Fresnel lens structure. It will be appreciated that a principal 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 1400 can be separated into segments that have the same surface shape but reduced thickness. This results in a pseudo planar lens 1401 that is relatively low cost and easy to make by processes such as moulding, casting or stamping etc.

Any process that is able to shape a surface to form lens facets will be generally referred to in this specification as a "moulding" process. The use of the term "moulding" should therefore not be interpreted as restricting the scope of the invention to one kind of lens-forming process.

FIG. 15A is a schematic cross section view of a portion of an ideal Fresnel lens 1500a, showing lens surface features and associated refraction of light rays. The lens comprises facets on the surface of the Lens which refract incoming light 1504a to produce convergent beam 1505a. The arrangement shown is highly simplified for descriptive purposes and is not meant to restrict the scope of the invention. The facets comprise an angled surface 1502a which is responsible for refracting light toward the focal point of the lens. Typically these refracting facets are arranged in circular patterns around the central axis of the lens. The facets also comprise an orthogonal surface 1503a which ideally is perpendicular to the plane of the lens 1501a, 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 Figure 4b, Fresnel lenses in the prior art are modified so that facet surfaces 1503b are at an angle of e.g. 1-2 degrees from the normal of the plane of the lens.

Although introducing a draft angle on perpendicular facets 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 draft angled facets 1503b are reflected to form stay (e.g. divergent) rays 1506b. These rays 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. There is also a need for a corresponding manufacturing process which can fabricate lenses easily using conventional manufacturing equipment.

From a different perspective, conventional circularly symmetric Fresnel lenses are not well suited to CPV applications where light is focussed onto square or rectangular photovoltaic cells. Because the cell is square and the image of the sun is round, it is not straight forward to illuminate the surface of the cell evenly with minimal loss of light.

By way of example only, FIG. 16 shows an approach used to produce uniform illumination across a rectangular image area. A prismatic lens 1600 is used comprising one flat surface 1604 and a plurality of refracting regions 1601 that have flat surfaces. These refracting regions are defined by projecting a rectangular region on the planar surface of the lens 1604 onto a curved surface which sets the focal length of the lens. The rectangular region on the fiat surface of the lens 1604 is chosen to have the same size as the photovoltaic cell being illuminated 1602. In this way, the image formed at the focal point of the lens has rectangular shape and relatively uniform intensity.

It will be appreciated that there are problems associated with this design. The prior art lens is thick which means that it is physically large, heavy, costly and lossy due to the significant optical path length through the lens material. It also not suited to high volume, low cost manufacturing processes. Therefore there is a need for a new type of lens that efficiently illuminates the surface of square or rectangular photovoltaic cells and which overcomes the problems noted above.

Referring to FIG. 17A, an embodiment provides a Fresnel lens 1700 comprising at least two physically separate parts 1701. This figure is highly simplified and is not meant to restrict the scope of the invention. Figure 17A is a cross section, side view of the lens 1700 and two of its constituent elements 1701. Figure 17B is apian view showing the lens comprising four constituent elements. Figure 17C shows the assembly of the multiple elements of the lens to form a complete lens. The peripheral shape of the individual lens elements maybe 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. Most preferably each lens is composed of two constituent elements.

By separating the lens into multiple elements, it is possible to avoid the need for a draft angle on surfaces that are ideally perpendicular to the plane of the lens (herein referred to as "perpendicular facets"). The reason for this is that if a lens is separated into parts that do not have opposing (i.e. opposite-facing) surfaces, the mould used to make the part can be separated in a direction which takes it away form the perpendicular facets on the moulded part. By eliminating the draft angle on perpendicular facets, the optical loss of the lens can be reduced. This is particularly important in photovoltaic applications where lens losses directly effects system efficiency.

In order to avoid the presence of opposing portions of a perpendicular facet, a Fresnel lens preferably is made from 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 invention.

A lens element is said to have "opposing facets" when a moulding tool used to form the facets cannot be moved in a direction which separates it from, all surfaces of the lens element simultaneously. Conversely, a lens element is said to have "non- opposing facets" when a moulding tool used to form the facets can be moved in a direction which separates it from all surfaces of the lens element simultaneously. When a Fresnel lens is assembled from multiple parts or is used as an individual part, 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.

An embodiment can further provide a Fresnel lens, comprising at least two physically separate parts, which are assembled to form a complete lens by bonding the lens parts onto a transparent substrate.

FIG. 18 shows a simplified cross section view of an embodiment. In this example, a concentric Fresnel lens 1800 is formed from multiple lens parts assembled onto a substrate such as glass. This structure comprises moulded Fresnel lens parts 1803 and 1804, a glass substrate 1801 and bonding agent 1802 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.

In an alternative embodiment there is provided a method of fabricating a Fresnel lens element with perpendicular facets.

FIG. 19A shows a cross section view of a Fresnel lens element 1900 in a moulding cavity which comprises upper and lower portions 1901 and 1902. According to an embodiment, when the mould is separated to release the moulded part, the portion of the mould which forms the lens facets is moved in a direction away from both perpendicular facets and other refracting facets of the lens, as indicated by arrows in FIG. 19B. In this way the mould is released simultaneously from all facets and the optical quality of the lens is maximised.

An alternative embodiment can provides a method of fabricating a Fresnel lens by sequentially pressing a plurality of separate moulds into the surface of a deformable lens material. Each mould is designed to avoid opposing perpendicular facets and is capable of producing facets which have no draft angle and which are perpendicular to the plane of the lens. During the pressing process, the deformable lens material adopts the shape of the features of the mould which are preferably facets of a Fresnel lens. While the mould is pressed against the deformable lens material a processing step is applied to render the deformable material rigid so that it maintains the shape of the mould. The processing step is preferably a cooling operation or a curing process that may be initiated by heat, light or delayed chemical reaction. The deformable material is preferably a polymer or glass or a polymer applied onto a glass substrate.

FIGs 2OA though 2OD show an example of simplified manufacturing process sequence of a Fresnel lens. Firstly, mould 2002a is heated and pressed into deformable material 2000a creating lens facets 2001a. The mould and lens material are cooled and the mould is removed in a direction away from the perpendicular facets of the moulded surface as indicated by arrow 2003b. To facilitate this separation, the mould does not have any opposing perpendicular facets. Then, a second mould 2005c is heated and pressed into adjacent regions of the deformable lens material 2000c to form lens facets 2007c The mould and lens material are cooled and the mould is removed in a direction away from the perpendicular facets of the lens as indicated by arrow 2006d. The second moulding operation may be performed by the first mould 2002a by repositioning it or the lens material to achieve the orientation shown in FIG. 2OC and FIG. 2OD.

It would be appreciated that a similar processing sequence is used if the deformable lens material is a light cured polymer. In this example the polymer is selectively cured in each separate moulding step by selective illumination of the polymer. This can be achieved by masking the polymer to prevent illumination and curing in areas yet to be moulded.

An alternative manufacturing process would comprise the patterning of the surface of a glass substrate itself using a moulding or embossing tool. For example a glass substrate could be heated to a prescribed temperature above 500 degrees Celsius where the glass is readily deformable using an embossing or moulding tool. The embossing or moulding tool is preferably made of a material which maintains its surface characteristics and does not distort at these elevated temperatures. The preferred material also does not wet or stick to the glass surface at elevated temperatures. Preferably the embossing or moulding tool is made from glassy carbon or vitreous carbon or a material with similar characteristics. An alternative embodiment can provide a composite Fresnel lens parquet comprising a plurality of Fresnel lens elements each of which are arranged such that the parquet has no opposing perpendicular facets. A parquet is defined to be a single structure containing an array of more than one Fresnel lens elements.

FIG. 21 A shows a lens parquet 2100a according to an embodiment. Three lens elements are shown as an example but in practice any number can be used. In accordance with an embodiment, each of the lens elements do not have opposing perpendicular facets and are arranged so that the overall parquet also does not have any opposing perpendicular facets. This allows the overall parquet to be manufactured with perpendicular facets in a single moulding operation. This lowers manufacturing costs and simplifies lens handling and assembly operations in subsequent subsystems. FIG. 2 IB shows an example of two lens parquets 2100b and 2102b arranged to form an array of Fresnel lenses.

Another embodiment method of manufacturing a Fresnel lens parquet wherein lenses are formed on polymer substrates and then attached to glass substrates in a similar orientation as shown in Figure 2 IB. Such lenses on polymer substrates are manufactured as herein described and have perpendicular lens facets. These polymer lens parquets are preferably attached to glass substrates using adhesive. Alternatively these lens parquets are formed by first attaching an unpatterned polymer sheet to a glass substrate using an adhesive and then patterning said polymer sheet to form Fresnel lenses.

An alternative embodiment can provide a Fresnel lens comprising a plurality of refracting regions, each region having plurality of refracting facets. Within each region the facets have an approximately constant facet angle relative to the plane of the lens and are distributed uniformly over each refracting region. The facet angle and orientation is chosen to be different from region to region to achieve superposition of refracted light at the focal point of the lens. The refracting regions produce a spatial convolution of the optical source with the shape of the refracting regions of the lens to produce approximately uniform illumination in the shape of the refracting region at the focal point of the lens.

In a CPV application, the refracting regions are preferably the same shape and approximately the same size as the photovoltaic cell being illuminated by the lens. For example if the PV cell is square and measures lcm x lcm, the lens would preferably comprise lcm x lcm regions, each region containing refracting Fresnel-style facets. Lens regions can also be made smaller than the size of the illumination target (e.g. the PV cell) without departing from the scope of the invention. The facets within each region preferably have approximately the same facet angle with respect to the plane of the lens and are distributed evenly over each region in a linear, parallel orientation.

FIG. 22 provides a simplified cross section view of an embodiment Fresnel lens 2200, as used in a CPV application. The lens 2200 is formed from regions 2201 which have the same shape and size in the XY plane as the PV cell 2202. Each region of the lens has facets 2205 which are arranged in a linear, parallel orientation and which have a constant angle with respect to the plane of the lens.

FIG. 23 provides a simplified plan view of an example embodiment Fresnel lens 2300. This lens comprises 25 regions 2301 each comprising linear parallel facets. Facets within each region have a constant facet angle with respect to the plane of the lens. The orientation and angle of facets is different in each reason and is chosen to achieve superposition at the focal point of the lens.

Because an embodiment lens design is a planar lens, it has low volume, low mass and low cost. It is also able to be produced in volume using industry standard techniques used to produce conventional Fresnel lenses. The present embodiments can therefore overcomes problems associated with the prior art.

Monitoring Photo-Voltaic Cells

It will be appreciated 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.

United States patent No. 6,545,211 entitled "Solar Cell Module, Building Material With Solar Cell Module, Solar Cell Module Framing Structure, And Solar Power Generation Apparatus" provides a useful summary of the field.

US 6,545,211 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".

Preferred embodiments disclosed in US 6,545,211 patent include a solar cell module comprising a solar cell element, bypass diode, communication unit and parameter detection unit - which measures cell current. The parameter detection unit is shown to be formed by current sensor, voltage sensor and arithmetic unit. Importantly, US 6,545,211 teaches co-locating 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. The patent preferentially claims signalling frequencies below IMHz because of the intrinsic shunt capacitance of the solar cell array. However, it will be appreciated that a number of issues exist with this technique.

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, the patent 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 proposed architecture results in the entire module needing to be replaced. This is 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 teaches use of 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 proposed signal transmission technique unreliable or even impractical.

Finally, US 6,545,211 teaches use of 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 is therefore large and disproportionately costly.

Accordingly, there is a demonstrable need for a new cell monitoring scheme that overcomes the shortcomings of the prior art. In light of the foregoing, an embodiment provides 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. In this way, if a single cell module becomes faulty, it can be replaced without disturbing the rest of the photovoltaic system.

An alternative embodiment can, by way of example only, provide 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. By placing the performance monitoring circuitry in the cell interconnection module, multiple cells are monitored by a single circuit. This reduces cost overhead of performance monitoring and ensures that sufficient cell voltage is available to power monitoring circuitry.

An alternative embodiment can, by way of example only, provide 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 an electro-optic isolation device which couples signals generated by the performance monitoring circuit to a signalling interface located on the cell interconnection module. The performance monitoring circuit creates a 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 particular cell interconnection module.

An alternative embodiment can, by way of example only, provide 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. In this way the connection of a computing device for collecting and analysing cell performance data is simplified.

Referring to FIG. 24, an embodiment provides a cell interconnection module 2400 comprising at least two bipolar electrical connection ports 2402 which are used to connect to at least two cell modules 2401, and a single bipolar output port 2403. Cell interconnection module 2400 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 2401 are manufactured to achieve optimal thermal and optical performance without needing to accommodate conventional electronics.

In this, and following embodiments, 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 invention. Embodiments can include a plurality of at least two 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 important advantage of the present embodiment is the modular nature of the photovoltaic system which facilitates the replacement of individual cell modules with minimal disruption to the remainder of the system.

FIG. 25 provides an example of how multiple cell interconnection modules 2500a and 2500b are used to connect a plurality of cell modules 2501a and 2501b in series, according to an embodiment.

Referring to FIG. 26, an alternative embodiment provides a plurality of cell modules and cell interconnection modules each comprising flexible wires 2602a-b, 2603a-b and 2604a-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 2601 a-b are connected to flexible wires from interconnection units 2600a-b using terminating devices 2605a-b. These terminating devices are, for example, crimp style connections which also meet appropriate environmental requirements. Similar terminating devices 2606a are used to connect the outputs of cell interconnection units 2600a and 2600b. 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 removed or 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.

According to an alternative embodiment there is provided a cell interconnection module comprising a performance monitoring circuit which monitors the voltages , and/or currents of each of the at least two cell modules, and an electro-optic isolation device which couples signals generated by the performance monitoring circuit to a signalling interface located on the cell interconnection module. The performance monitoring circuit creates a 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 particular cell interconnection module.

FIG. 27 shows a cell interconnection module 2700 preferably comprises: at least two interconnection points 2707a,b for connecting the cell interconnection module to at least two cell modules 2701a,b; a bipolar output port 2703a-b for connecting the cell interconnection module to other cell interconnection modules or CPV panel connections; a performance monitoring circuit 2709; an electro-optic isolation device 2710; and a signalling interface connector 2713. The performance monitoring circuit monitors voltages and or currents of the cells connected to the cell interconnection module and encodes this data together with an identifying code which is unique to each cell monitoring unit to form a transmit signal. This transmit signal is applied to an electro-optic isolator 2710 for transmission along the photovoltaic panel's signalling network 2715. The electro-optic isolator allows the common mode voltage present at the cell terminals, which can be hundreds or even thousands of volts, to be isolated from the signalling network which is generally referenced to ground potential. The electro-optic isolator therefore forms an important isolation barrier between these two elements of the photovoltaic system and is generally rated to withstand a voltage of at least twice the open circuit voltage of the photovoltaic system.

The transmit signal is preferably a digital signal comprising synchronisation and error checking components. The performance monitoring circuit applies the transmit signal to the light generating element 2711 of the electro-optic isolator 2710 where it is converted to a time varying optical signal. This optical signal is received by a light receiving element 2712 within the electro-optic modulator which converts the signal back to a time varying electrical signal. The electrical output of the electro-optic isolator is preferably connected to a socket 2713 on the cell interconnection module. This socket preferably accepts a corresponding plug 2714 that facilitates connection ' of the signalling network within the photovoltaic panel.

FIG. 28 provides additional details of an example embodiment. The cell interconnection module's performance monitoring circuit preferably comprises: a current sensing element 2820 through which cell current flows; a voltage regulating element 2821 which stabilises the power supply of the monitoring circuitry; an amplifying and/or level shifting element 2822 which senses cell operating parameters and converts these parameters to levels suitable for analog to digital conversion (ADC); and a microprocessor element 2823. The microprocessor 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 its ADC converter into a form suitable for transmission along the systems signalling network and appends identification codes which are unique to each cell interconnection module. Preferably the microprocessor sends data in short bursts so as not to block transmission from other units. The microprocessor may also be replaced by a customised control circuit such as an ASIC, FPGA or other part capable of performing the same function This circuit description is only an example and certain circuit functions can be added, modified or omitted without departing from the scope of the present invention.

FIG. 29 is a simplified schematic view of an embodiment of a photovoltaic panel circuit schematic showing cell interconnection modules. A plurality of "n" (where n is an integer) cell interconnection modules 2900a,b...n are preferably connected in series to provide the high potential output from the panel 2931, with one input of the first module 2900a preferably grounded 2932. Each cell interconnection module connects to at least two cell modules 2901a,b..n and comprises cell monitoring circuitry and an electro-optic isolator. The signalling outputs of the cell interconnection units 2900a,b..n are preferably connected in parallel using an interconnection cable 2930. This cable preferably comprises plugs 2914a,b..n distributed along its length which suit signalling interface sockets on each cell ^; interconnection module.

The signalling network cable is also connected to a signal receiver module 2936 comprising an amplifying and/or level shifting interface circuit 2933 and a microprocessor unit 2934. The signal receiver module decodes the data transmitted from each cell interconnection module and collates performance data on each cell module. The signal receiver module then encodes collated data into a form that is suitable for transmission to other data collecting devices located elsewhere in the photovoltaic installation. This data may be transmitted through a temporary or permanent cable connection or by a wireless communication link.

It will be appreciated that other signalling interconnection schemes are also possible within the scope of the invention. For example each cell interconnection module may be individually connected to the central receiver module using separate cables.

Providing that performance monitoring circuits transmit short bursts of data, in an embodiment, it is likely that only one module will be active at any one time and therefore multiple modules can share a single signalling network. For example signal burst duration is preferably less than 1% of the average time interval between bursts. If two modules happen to transmit at the same time, the signal receiver module 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).

In an embodiment there can be provided a protocol for transmitting cell performance data across the signalling network, 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 can make use of a random transmission time protocol where data is sent infrequently as short bursts at pseudo-random time intervals. In 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 signal receiver module 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 can preferably comprise 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 corresponding to this algorithm is provided in FIG. 30.

It will be appreciated 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 been 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. m alternative embodiments, the one or more processors operate as a standalone device or may be connected, e.g., networked to other processors), 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 maybe 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: a substrate; and at least two silicon germanium subcells.

2. The photovoltaic cell according to claim 1, wherein each of the at least two silicon germanium subcells have a different material composition.

3. The photovoltaic cell according to any one of the preceding claims, wherein each of the at least two silicon germanium subcells define a different bandgap.

4. The photovoltaic cell according to any one of the preceding claims, wherein the substrate is primarily comprised of germanium.

5. The photovoltaic cell according to any one of the preceding claims, wherein the substrate is inactive.

6. The photovoltaic cell according to any one claims 1 to 4, wherein the substrate is active.

7. The photovoltaic cell according to any one of the preceding claims, the photovoltaic cell further comprising: a plurality of Group III-V semiconductor subcells.

8. The photovoltaic cell according to any one of the preceding claims, the photovoltaic cell further comprising: a transition layer between each of the at least two silicon germanium subcells.

9. The photovoltaic cell according to claim 8, wherein each transition layer comprises silicon germanium.

10. The photovoltaic cell according to claim 8, wherein each transition layer comprise a graded composition silicon germanium.

11. The photovoltaic cell according to claim 10, wherein graded composition varies from the material composition of a first adjoining silicon germanium subcell to the material composition of a second adjoining silicon germanium subcell.

12. The photovoltaic cell according to any one of the preceding claims, the photovoltaic cell further comprising: a germanium subcell.

13. The photovoltaic cell according to claim 12, wherein the germanium subcell is deposited on the substrate; a first silicon germanium subcell is deposited on the germanium subcell; a second silicon germanium subcell is deposited on the first silicon germanium subcell; and

Group III-V subcells are deposited on the second silicon germanium subcell.

14. The photovoltaic cell according to claim 13, wherein the first silicon germanium subcell has a material composition SI_yGe_{1 -y,} where y is between 0 and 30%.

15. The photovoltaic cell according to any one of claims 13 to 14, wherein the second silicon germanium subcell has a material composition Si_zGe_1-z> where z

, , is between 0 and 50%.

16. The photovoltaic cell according to claim 13, wherein the first and second silicon germanium subcells have a composition of about Sio.i₅Geo.s₅ and Sio.₁₉Go.s₁.

17. The photovoltaic cell according to any one of claims 13 to 16, the photovoltaic cell comprising a transition layer between the first and second silicon germanium subcells, this transition layers comprise a graded composition silicon germanium graded composition that varies from the material composition of the first silicon germanium subcell to the material composition of a second silicon germanium subcell

18. The photovoltaic cell according to any one claims 1 to 12, the photovoltaic cell comprising: three silicon germanium subcells; wherein: a first silicon germanium subcell is deposited on the substrate; a second silicon germanium subcell is deposited on the first silicon germanium subcell; a third silicon germanium subcell is deposited on the second silicon germanium subcell; and a plurality of Group III-V subcells are deposited on the third silicon germanium subcell.

19. The photovoltaic cell according to claim 18, wherein the first silicon germanium subcell has a material composition Si_xGe_1-x, where x is between 0 and 5%.

20. The photovoltaic cell according to any one of claims 18 to 19, wherein the second silicon germanium subcell has a material composition Si_yGe_1-y> where y is between 0 and 30%.

21. The photovoltaic cell according to any one of claims 18 to 20, wherein the third silicon germanium subcell has a material composition Si_zGe_1-Zj where z is between 0 and 50%.

22. The photovoltaic cell according to claim 18, wherein at least two of the three silicon germanium subcells have a composition of about Sio.ϊsGeo.ss and

23. The photovoltaic cell according to any one of the preceding claims, the photovoltaic cell further comprising: a plurality of Group III-V semiconductor subcells.

24. The photovoltaic cell according to claim 23, wherein the plurality of III-V subcells include a first III-V subcell comprising GaAsP and a second III-V subcell comprising InGaP.

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

26. A method of manufacturing a photovoltaic cell according to any one of the preceding claims, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

27. A method of manufacturing a photovoltaic cell, the method comprising the steps of: providing a substrate; and depositing at least two silicon germanium sύbcells.

28. The method according to claim 27, further comprising the step of: depositing a transition layer between each of the at least two silicon germanium subcells.

29. A method of manufacturing a photovoltaic cell, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

30. A photovoltaic panel assembly comprising: one or more photovoltaic receiver assemblies, wherein the receiver assemblies include a photovoltaic cell according to any one of claims 1 to 25; one or more optical concentrating elements; one or more support structures; and wherein the photovoltaic receiver assemblies are mounted onto the support structures at a location corresponding to the focal point of the optical concentrating elements.

31. A photovoltaic panel assembly comprising: a plurality of optical concentrating elements; a plurality of support structures; and a plurality of photovoltaic receiver assemblies.

32. The assembly according to any one of claims 30 to 31 , the assembly further comprising: a substantially optically-transparent panel; wherein the support structures are mounted onto, and are supported by, the transparent panel.

33. The assembly according to claim 32, wherein the optical concentrating elements are planar lenses.

34. The assembly according to claim 33, wherein the planar lenses comprise a polymer sheet bonded to the transparent panel.

35. The assembly according to claim 33, wherein the planar lenses comprise a polymer which is deposited onto the transparent panel and patterned in-situ to from planar lenses.

36. The assembly according to claim 33, wherein the planar lenses are formed on the surface of the transparent panel by deforming the surface of the panel itself.

37. The assembly according to any one of claims 32 to 36, wherein the optical concentrating elements are mounted on the same side of the transparent panel as the photovoltaic cell assemblies.

38. The assembly according to any one of claims 32 to 37, wherein the support structures are mounted onto the transparent panel using adhesive and/or a mechanical fixing. ^:

39. The assembly according to any one of claims 32 to 38, wherein the support structures are environmentally sealed at an interface to the transparent panel using a deformable material.

40. The assembly according to claim 39, wherein the deformable material is a polymer gasket or O-ring.

41. The assembly according to claim 39, wherein the deformable material is the material used to form the optical concentrating elements.

42. The assembly according to any one of claims 30 to 41 , the assembly further comprising: a rigid panel structure.

43. The assembly according to claim 42, wherein the panel comprises glass.

44. The assembly according to claim 42, wherein the panel comprises a polymer.

45. The assembly according to any one of claims 30 to 44, wherein the optical concentrating elements are lenses.

46. The assembly according to any one of claims 30 to 45, wherein the optical concentrating elements are planar lenses.

47. The assembly according to any one of claims 30 to 46, wherein the optical concentrating elements are Fresnel lenses.

48. The assembly according to any one of claims 30 to 47, wherein the support structures comprise a surface which encloses the volume of air between the optical concentrating elements and the photovoltaic receiver assemblies.

49. The assembly according to any one of claims 30 to 48, wherein the support structures comprise any one or more materials selected from the set comprising: a metal; an aluminium alloy; a polymer; and a metal coated polymer. ^■

50. The assembly according to any one of claims 30 to 49, wherein fabrication of the support structures uses any one or more methods selected from the set comprising: a cold forming process. a pressing or stamping process. a casting process.

50. The assembly according to any one of claims 30 to 49, wherein the support structures comprises a mounting flange (or collar) used to mount the photovoltaic receiver assemblies.

51. The assembly according to any one of claims 30 to 50, wherein the photovoltaic receiver assemblies comprise a photovoltaic cell assembly and a heatsink element.

52. The assembly according to claim 51, wherein the heatsink element comprises mounting features which are used to secure the photovoltaic receiver assemblies to the support structure.

53. The assembly according to any one of claims 30 to 52, wherein the photovoltaic receiver assemblies comprise securing features used to locate and retain the photovoltaic receiver assemblies against mounting surfaces of the support structures.

54. The assembly according to claim 53, wherein the securing features form either a bayonet style locking arrangement or a threaded arrangement.

55. The assembly according to claim 54, wherein the bayonet style locking arrangement is engaged or disengaged by a rotational movement of the photovoltaic receiver assemblies with respect to the support structures.

56. The assembly according to any one of claims 30 to 52, the assembly further comprising: a rigid panel structure.

55. The assembly according to claim 54, wherein: the support structures has a fixed, rigid, orientation with respect to the optical concentrating elements and the rigid panel structure; the photovoltaic receiver assemblies are mounted on the support structures; and the photovoltaic receiver assemblies are able to be detached and reattached to the support structures without dismantling the support structures.

56. The assembly according to claim 54, wherein: the support structures have fixed, rigid, orientation with respect to the optical concentrating elements and the rigid panel structure; and each of the support structures form a surface which encloses a single optical element, a single photovoltaic receiver assembly and the optical path there between.

57. The assembly according to claim 56, wherein the support structures comprise an aperture which allows air to pass into and out of the volume enclosed by the support structure.

58. The assembly according to claim 57, wherein a filter element is positioned next to the aperture for the purpose of reducing dust, airborne pollutants or contaminants from entering the volume enclosed by the support structure.

59. A photovoltaic 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.

60. A lens element operatively associated with the assembly according to any one of claims 30 to 59, the lens element forming at least part of the optical concentrating elements, the lens element further comprising a plurality of facets wherein at least one of the plurality of facets is perpendicular to the plane of the lens in at least one region and all of the plurality of facets are non-opposing.

61. A lens element comprising a plurality of facets, wherein at least one of the plurality of facets is perpendicular to the plane of the lens in at least one region and all of the plurality of facets are non-opposing.

62. The lens element according to any one of claims 60 to 62, wherein the lens element is comprised of any one or more materials selected from the set comprising: a polymer, a glass.

63. A composite lens structure operatively associated with the assembly according to any one of claims 30 to 59, the lens element being each of the optical concentrating elements, composite lens element comprising at least two lens elements according to any one of claims 61 or 62.

64. A composite lens structure comprising at least two lens elements according to any one of claims 61 or 62.

65. The lens structure according to claim 63 or claim 64, wherein the lens element is a composite planar Fresnel lens.

66. The lens structure according to any one of claims 63 to 65, wherein the lens elements form a parquet having the lens elements arranged in an array.

67. The lens structure according to any one of claims 63 to 66, wherein the lens structure is mounted on a substantially optically transparent substrate.

68. A lens structure comprising a plurality of regions, wherein: each region comprises a plurality of refracting facets; within each region the refracting facets are distributed uniformly over the region and have an approximately constant facet angle relative to the plane of the lens; and the facet angle and orientation is different from region to region to achieve superposition of refracted light from all of the regions at the focal point of the lens.

69. The lens structure according to claim 68$ wherein the lens structure being operatively associated with a PV cell, and wherein the region is about the same size and shape, as a photovoltaic cell being illuminated by the lens structure.

70. The lens structure according to any one of claims 68 to 69, wherein the lens structure comprises a plurality of facets, wherein at least one of the plurality of facets is perpendicular to the plane of the lens in at least one region and all of the plurality of facets are non-opposing.

71. The lens structure according to any one of claims 68 to 70, when used to define a convolutional Fresnel lens structure.

72. The lens structure according to any one of claims 68 to 71, when used to define a convolutional Fresnel lens parquet.

73. The lens structure according to any one of claims 68 to 72, comprising a plurality of regions, wherein facets in the regions produce a spatial convolution of an optical source with the regions to produce approximately uniform illumination at the focal point of the lens in the shape of the regions.

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

75. A method of manufacturing a lens structure comprising at least one facet which is perpendicular to the plane of the lens in at least one region, the method comprising the steps of:

(b) causing the polymer to solidify, and

76. A method of manufacturing a lens structure comprising at least one facet which is perpendicular to the plane of the lens in at least one region, the method comprising the steps of:

(a) pressing a first mould into the surface of a deformable material to form facets on one region of the deformable material;

77. The method according to claim 76 wherein the first mould is used for the second moulding operation by rotating the deformable material relative to the mould or rotating the mould relative to the material.

78. The method according to any one of claims 15 to 77 wherein the lens structure is a Fresnel lens structure or a convolutional Fresnel lens structure.

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

80. A cell interconnection module operatively associated with the assembly according to any one of claims 30 to 59, the cell interconnection module comprising: a plurality of input ports; and a single output port.

81. A cell interconnection module comprising : a plurality of input ports; and a single output port.

82. The cell interconnection module according to claim 80 or claim 81, wherein: the input ports are intended to be coupled to one or more cell modules comprising a photovoltaic cell; and the output port is intended to be coupled to either another cell interconnect module, or an electrical output terminals of a photovoltaic system panel.

82. The cell interconnection module according to claim 82, wherein cell modules comprises a photovoltaic cell according to any one of claims 1 to 25.

83. The cell interconnection module according to claim 82, wherein the photovoltaic system panel is a panel according any one of claims 30 to 59.

84. The cell interconnection module according to any one of claims 80 to 83, the module comprising electronic circuitry for monitors voltage and/or current of a plurality of coupled photovoltaic cells.

85. The cell interconnection module according to claim 84, wherein the electronic circuitry comprises a microprocessor or a microcontroller.

86. The cell interconnection module according to any one of claims 84 to 85, wherein the electronic circuitry encodes photovoltaic cell performance data produced by the cell interconnection module into a transmit signal which is suitable for transmission to a remote unit.

87. The cell interconnection module according to any one of claims 84 to 86, wherein the electronic circuitry comprises an electro-optic isolator.

88. The cell interconnection module according to any one of claims 84 to 86, wherein the cell interconnection modules comprise electronic circuitry which encodes photovoltaic cell performance data produced by the cell interconnection module into a transmit signal which is suitable for transmission to a remote unit.

89. The cell interconnection module according to any one of claims 80 to 88, wherein each of the plurality of input ports are bipolar input ports.

90. The cell interconnection module according to any one of claims 80 to 89, wherein the output port is a bipolar output port.

91. 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.

92. A modular photovoltaic power conversion system comprising: one or more cell modules having one or more photovoltaic cells; and one or more cell interconnection modules, which do not contain photovoltaic cells.

93. The power conversion system according to claim 92, wherein cell modules are according to any one of claims 1 to 25.

94. The power conversion system according to claim 92 or claim 93, wherein the cell interconnection modules are according to any one of claims 80 to 89.

95. The power conversion system according to any one of claims 92 to 94, wherein a plurality of the cell modules are coupled to each of a plurality of the cell interconnection modules; and the cell interconnection modules operatively couples the outputs of the cell modules in series.

96. The power conversion system according to any one of claims 92 to 95, wherein electrical connection ports of the cell modules and the cell interconnection modules comprise flexible cabling which is permanently attached to the modules.

97. The power conversion system according to claim 96, wherein the flexible cabling is joined to interconnect cell modules and cell interconnect modules using environmentally sealed terminating devices.

98. The power conversion system according to any one of claims 92 to 97, wherein outputs of a plurality of cell interconnection modules are connected to a signal receiver module.

99. The power conversion system according to claim 98, wherein the signal receiver module receives data sent from a plurality of performance monitoring circuits contained in a plurality of cell interconnection modules.

100. The power conversion system according to claim 98 or claim 99, wherein the signal receiver module produces a collated summary of data received from a plurality of performance monitoring circuits contained in a plurality of cell interconnection modules and encodes the summary into a signal which is suitable for transmission to additional data collection and collation units or computer equipment.

101. The power conversion system according to any one of claims 92 to 100, wherein the cell interconnection modules comprise electronic. circuitry which encodes photovoltaic cell performance data produced by the cell interconnection module into a transmit signal which is suitable for transmission to a remote unit.

102. The power conversion system according to claim 101, 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.

103. The power conversion system according to claim 102, wherein the transmit signal burst duration is less than 1 percent of the average time interval between bursts.

104. The power conversion system according to claim 102 or claim 103, wherein the time interval between bursts is random or pseudo-random.

105. The power conversion system according to claim 102 or claim 103, 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.

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

107. A method of coupling together a modular photovoltaic power conversion system, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

108. A method of monitoring a modular photovoltaic power conversion system, substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.

109. 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

110. The user access interface according to claim 109, wherein the photovoltaic cell is according to any one of claims 1 to 25.

111. The user access interface according to claim 109 or claim 110, wherein the interconnection module is according to any one of claims 80 to 91.

112. 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.

113. 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.

114. 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.

115. The computer program product according to claim 113 or claim 114, wherein the photovoltaic cell is according to any one of claims 1 to 25.

116. The computer program product according to any one of claims 113 to 115, wherein the interconnection module is according to any one of claims 80 to 91.

117. A 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.