WO2006130920A1

WO2006130920A1 - Scattering elongate photovoltaic cell

Info

Publication number: WO2006130920A1
Application number: PCT/AU2006/000795
Authority: WO
Inventors: Leone Spiccia; Simon Jaime Thompson; Yi-Bing Cheng; David Brian Menzies; George Philip Simon
Original assignee: Monash University
Priority date: 2005-06-08
Filing date: 2006-06-08
Publication date: 2006-12-14

Abstract

This invention relates to a photovoltaic cell (60) comprising a light incident surface and at least one elongated light transmitting fibre comprising a longitudinally extending core (62) having an end for receiving light from the light incident surface, said fibre extending away from the light incident surface to guide light received therefrom within the core and comprising a counter electrode (62a) surrounding the core and a dye-sensitive semiconductor (63) extending about the core and an electrolyte (64) and wherein light passing longitudinally through the core from the light incident surface is scattered laterally onto the dye nanoparticles (63) by bubbles, defects, particles etc.

Description

SCATTERING ELONGATE PHOTOVOLTAIC CELL

Field

This invention relates to photovoltaic cells and more particularly to photovoltaic cells comprising an elongated element, such as fibre coated with a semiconductor. The invention further relates to a photovoltaic cell assembly formed from a plurality of the elements and to a method of manufacture and use thereof.

Background

There have been significant developments in recent years relating to the efficiency and design of photovoltaic cells. Monocrystalline photovoltaic cells remain as the most efficient solar energy converters and can achieve greater than 20% conversion of the incident light to electricity. The expense of monocrystalline cells makes them impractical for commercial production. Thin film silicon solar cells can now achieve energy conversions of around 15% and are in wide commercial use.

In the last fifteen years dye-sensitised solar cells (DSSCs) have been developed. DSSCs use an organic dye (photosensitiser) to convert absorbed light into electrons which are transported through nanoporous materials to a collector electrode which is usually a conductive flat substrate. The potential for significant reduction in manufacturing cost of DSSC has enabled photovoltaic cell design to be advanced.

A number of workers have investigated the use of fibre substrates in design of photovoltaic cells.

Fletcher et al (US Patent 3984256) describe photovoltaic cells prepared from parallel silicon filament arrays and having conventional p and n-type semiconductor regions. The resulting arrays are said to be flexible allowing them to be wound on drums for storage of "blanket" photovoltaic panels. Cole in US Patent 5437736 describes photovoltaic panels formed of arrays of parallel fibres embedded into a superstructure. The contiguous arches formed on the surface of the panel superstructure are said to provide the advantage of focusing the radiation on the embedded semiconductor.

Photovoltaic cells in fibre form using DSSC are described in Baps et al {Key Engineering Materials Vol. 206-213(2002) 937-940), and Chittibabu et al (WO 03/065471 and US Pub 2005/0040374). The fibres may be formed into flexible fabric panels so that a large surface area of fibre is exposed. The fibres have a transparent outer surface and light transmissible outer conductive layer and a conductive inner layer or core. Although DSSCs are significantly less costly and provide flexibility in design and manufacture they are also less efficient than silicon based photovoltaic cells.

Although photovoltaic cells have improved in efficiency of power conversion, panels presenting a very large area are still required to fulfil any significant power requirements. Even for relatively efficient silicon cells a surface area of about seventeen square metres or more is required to generate one kilowatt of power. Aside from the capital expense involved in constructing and maintaining such large panels the panels tend to be aesthetically unattractive and in many cases can not be used in cities or congested areas. The size of the panels also means they cannot readily be manoeuvred to track the path of the sun. Typically, they are only efficient for part of the available period of sunlight during the day.

Attempts have been made to make better use of the exposed photovoltaic area. An increase in thickness of silicon cells and DSSCs has had a very limited effect on conversion of the incident light to electricity. Silicon is opaque and there is a rapid drop in efficiency increase when thickness is increased to above the optimum of about 300 microns. Semiconductors used in DSSCs are generally applied to a thickness of about 20 to 30 microns and while the thickness may be increased this has the deleterious effect of also dramatically increasing resistance to electron conduction. Durr et al (Applied Physics Letters 2004, 84, 3397) and Kubo et al {Journal of Photochemistry and Photobiology a Chemistry 2004, 164, 33) report a device in which a second solar cell is constructed on the back of the counter electrode of the first. Such devices have resulted increased performances up to 10.5% due to increases in the current from the addition of both components. However, this effectively doubles the cost of the devices as the amount of conductive glass, interconnects, dye and semiconductors are approximately double.

There is a need for a photovoltaic cell which makes more efficient use of the exposed surface area.

Summary

Accordingly we provide a photovoltaic cell comprising: a light incident surface; and at least one elongated light-transmitting element comprising a longitudinally extending core having an end for receiving light from the light incident surface, said light-transmitting element extending away from the light incident surface to guide light received therefrom within the core and comprising a semiconductor extending about at least a portion of length of the elongated light-transmitting element; and wherein light passing longitudinally through the core from the light conduit surface is scattered laterally onto the semiconductor.

The semiconductor may be silicon or comprise a Group III element based material coupled with group V element based material such as a GaAs material.

Such silicon and Ill/V semiconductors comprise p and n-type layers. It is particularly preferred, however, that the semiconductor is a nanostructured semiconductor such as dye-sensitised nanoparticulate metal oxide. When the semiconductor comprises a dye-sensitised semiconductor the cell will generally further comprise an electrolyte composition for carrying a charge between the semiconductor and a counter electrode.

The outer surface of the elongated light-transmitting element may be clear but will generally be contained in an opaque material or housing, which may optionally provide the counter electrode. The semiconductor will typically be associated with an electrode which may for example be in the form of a transparent conductive coating, such as indium tin oxide or the like, which may be formed between the light transmitting element and semiconductor.

In one embodiment the photovoltaic cell of the invention comprises: (i) a light incident surface;

(ii) a multiplicity of elongated light-transmitting elements comprising a longitudinally extending core and comprising an end for receiving light from the light-incident surface said light-transmitting element extending away from the light incident surface to guide light received therefrom within the core and comprising a dye-sensitised semiconductor about at least a portion of the elongated light- transmitting element; (iii) wherein the core causes scattering light onto the semiconductor;

(iv) a counter electrode; and

(v) an electrolyte composition for carrying a charge between the multiplicity of light transmissible elements and the counter electrode.

When the cell comprises a dye-sensitised semiconductor it will typically also comprise peripheral wall about the electrolyte and which may provide a housing for the electrolyte so as to maintain in contact with the dye-sensitised semiconductor portions of the multiplicity of elongated light transmissible elements. The housing may provide the counter electrode by means of a conductive internal lining or one or more associated conductive components in functioning contact with the electrolyte. For example the counter electrode may be provided by one or more conductive elements in functional contact with the electrolyte. Fibres formed of a conductive material or comprising a conductive coating may be used.

Alternatively the photovoltaic cell may comprise one or more individual cells each comprising a distinct light-transmitting element associated with an electrolyte and counter electrode. The elongated light-transmitting elements are preferably fibres or rods comprising a conductive coating and a semiconductor layer about at a portion thereof.

An advantage of the invention is that it may be used with a solar concentrator to take advantage of the compactness and efficiency of exposed surface area. Further, the photovoltaic cell of the invention may, if desired, be readily combined with a solar concentrator that is able to track the sun thereby optimising the conversion of solar energy to electricity throughout the day.

In accordance with this, a further aspect the invention provides a photovoltaic cell as herein above described and further comprising a solar collector such as a lens, reflector or the like adapted to increase the intensity of light passing through the light incident surface. The collector will serve to enhance the light intensity passing through the light incident surface relative to the background or ambient light.

Detailed Description

The photovoltaic cell of the invention comprises: (a) a light incident surface; and

(b) at least one elongated light-transmitting element comprising a core having an end for receiving light from the light incident surface, said light-transmitting element extending away from the light incident surface to guide light therefrom with the core and comprising a semiconductor about at least a portion of the elongated light- transmitting element; and

(c) light scattering features within the core for scattering light onto the semiconductor.

The light incident surface may be formed by one of more ends of the light- transmitting elements. Alternatively the cell may comprise of a layer suitable transparent material such as glass or polymeric materials. The surface may be rigid or flexible. The photovoltaic cell will preferably comprise a transparent layer forming the light incident surface which may also form the upper wall of the cell. The light incident surface may also be a solar concentrator which through a suitable lens system directs the light into the light transmitting element(s).

The elongated light-transmitting element may be in the form of a rod, tube, elongated plate or fibre, and may be rigid or flexible. Typically the light- transmitting element will be of length at least twice, and preferably at least five times its average cross-section width. Preferably the elements are cylindrical in cross section and have an average width of no more than 10 mm, preferably at no more than 2 mm and most preferably no more than about 1 mm.

Elongated elements comprising fibre cores are particularly preferred. The fibres may be formed of any suitable material having regard to the need to allow a semiconductor to form a layer about at least a portion of the fibre core of the light-transmitting element.

Light is transmitted longitudinally through the core of the light-transmitting element from the light-receiving end. Light scattering may be defined as divergence of photons from their original path as a result of physical interactions. Scattering of light is produced in at least a portion of the core length by any suitable means such as the presence of defects, imperfections, bubbles, particles or any other features within the core, such as variation in composition or density, or on its surface which causes the light to exit the core and enter the film. When the core is in tube form a gas or a particle mixed liquid or gelled polymer could be filled to produce scattering. The light scattering features within the core cause light to be scattered onto the semiconductor around the core. The end of the light-transmitting element remote from the light- incident surface may be coated with an opaque layer, which can have dual purposes for electrical connection and for reflecting the transmitted light on to the semiconductor.

The light scattering feature may be evenly distributed throughout the core however the invention also allows the light to be transmitted for a desired distance without significant scattering so that power is produced remotely from the light incident surface. Preferably, scattering is produced by colloidal dispersion, particle inclusions (particle size greater than the visible light wavelength (400-800nm)), air bubbles, surface roughness, material imperfections or impurities or density variations. The fibre surface may be roughened or contoured to encourage passage of light out of the core of the elongated light-transmitting element. If the core of the elongated light- transmitting element is a polymer, scattering can also be produced by internal strain introduced by stretching or folding the material. Changes in the refractive index caused by strain in polymers can cause significant light scattering. Furthermore the lower refractive index of polymer fibres may allow easier loss of light to the coated semiconductor layer and surrounding matrix. Scattering results in attenuation of transmitting light intensity along the pathway.

In a preferred embodiment the light scattering will be sufficient so that for a 45 mm length sample of the light transmitting element provides transmission of no more than 20% preferably no more than 10%, still more preferably no more than 5% and most preferably no more than 1 % of light from the end remote from the light incident surface. Conversely this embodiment provides for at least 80% preferably at least 90% more preferably at least 95% and most preferably at least 99% of light entering the core being scattered. Suitable candidate core components and resulting light transmitting elements may be tested for use in the present invention by using a 45 mm sample shining a light at one end thereof and measuring the light emerging from the remote end. The amount of light scattered radially from the core of the fibre may be determined as a function of length of the element by measuring the change in light transmission from the end into which the light is coupled, to that emanating from the other end. The intensity of light at each end may be measured using a light sensor such as a silicon diode and the percentage difference of light in and out determined.

Equipment well known in the art may be used to conduct such tests. In this way suitable surface modifications and/or modifications to the preparation of the core and/or light transmitting element can be made to produce a light transmitting element which transmits no more than 20% (preferably no more than 10% more preferably no more than 5% and most preferably no more than 1%) from the remote end.

In selecting a core component it may be convenient to test the transmission properties in the same way and use a core from which no more than 20%, preferably no more than 10%, more preferably no more than 5% and most preferably no more than 1% of incident light at the light incident end is transmitted out of the remote end. The invention contemplates the use of additional components such as optical fibres to allow operation of the photovoltaic cell remote from the source of light. In such cases the optical fibres may produce little loss of light through scattering and the remotely located photovoltaic cell will preferably produce the significant scattering of this embodiment.

The photovoltaic cell of the invention relies on the scattering of light transmitted through the core of the light-transmitting element to generate energy. It is not necessary that the peripheral surface of the light-transmitting element is exposed to light and indeed the peripheral surface may be covered by an opaque layer or material or it may be transparent or enclosed in a housing, optionally together with one or more other light-transmitting elements. A housing may be in the form of a sheath, tube or conduit and may cooperate with the underside of a layer providing the light incident surface. Where the power is to be generated by a dye-sensitised semiconductor coating, the housing will generally also contain an electrolyte in functional contact with the dye-sensitised semiconductor and the counter electrode.

In a preferred embodiment the light-transmitting element comprises a dye- sensitised semiconductor layer and a conductive layer forming an electrode. The conductive layer is preferably located inboard of the semiconductor layer and generally between the core and semiconductor layer and is transparent so that scattered light radiating outward from the core passes through the conductive layer into the semiconductor with minimal visible light loss. A conductive layer may be provided on the surface on the outside of the semiconductor and the semiconductor is required to be porous to allow for electrolyte permeation.

A range of materials will be known by those skilled in the art for use as conductive layer of the housing. Examples of suitable materials include such as copper, silver, gold, platinum, nickel, palladium, iron, titanium and alloys thereof.

Metal oxides may also be used such as indium tin oxide (ITO), fluorine-doped tin oxide, tin oxide, zinc oxide and the like. Suitable conductive polymers may also be used such as polyaniline, polyacetylene, polypyrrole, pedot and polyacetylene doped with arsenic pentafluoride. Carbon black, carbon nanotubes, other conductive carbon based materials or other nanoparticulate filled polymers may also be used if desired.

The photovoltaic cell will comprise a counter electrode which may be associated with individual light-transmitting elements. Alternatively, and more preferably, the cell will comprise an array of light-transmitting elongated elements contained within a housing. The housing may comprise a top wall forming the light incident surface and a bottom and peripheral wall formed by any suitable material which may be transparent or opaque. One of the functions of the housing may be to retain the electrolyte to provide an operational connection between the semiconductor and the counter electrode. The counter electrode may be formed on the interior of the housing such as a coating on the interior wall of the housing or may be provided by a separate conductive element in functional contact with the electrolyte to become the counter electrode. It is desirable to minimise the distance between the electrodes to maximise the efficiency. A conductive grid, such as a carbon or metal foil membrane, may be placed in the housing in order to optimise the closeness of the electrodes. As described above a particularly preferred embodiment of the invention comprises a dye-sensitised semiconductor. Dye-sensitised semiconductor will generally be in the form of nanoparticulate materials sensitised by an organic dye.

When the semiconductor is of the DSSC type a layer of semiconductor is formed about at least a portion of the elongated light transmissible element. In the preferred embodiment the semiconductor is in the form of a photosensitised nanomatrix. Suitable nanoparticle materials include, but are not limited to, the oxides, sulfides, selenides, and tellurides of titanium, zirconium, zinc, lanthanum, niobium, strontium, tantalum, tin, terbium and tungsten, or one or more combinations thereof. For example, TiO₂, SrTiO₃, CaTiO₃, SnO₂, ZrO₂, ZnO, WO₃, La₂O₃, Nb₂O₅, sodium titanate, and potassium niobate are suitable nanoparticle materials, in particularly preferred embodiments of dye-sensitised solar cells, in particular, the photosensitised nanomatrix materials include nanoparticles with an average size between about 2 nm and about 100 nm. Most preferably, the nanoparticles are titanium dioxide particles. Most preferably the average particle size is about 20 nm. However, larger particles may be used within or on top of the semiconductor layer to prevent light from escaping the photoactive layer and to provide light scattering within the semiconductor layer to enhance the efficiency of light harvesting in the sensitiser.

In the preferred embodiment of DSSCs the semiconductor is a nanomatrix which is photosensitised by an organic dye. Suitable photosensitizing agents may include, for example, dyes that include functional groups, such as carboxyl and/or hydroxyl groups, that can chelate to the nanoparticles, e.g., to Ti(IV) sites on a TiO₂ surface. Examples of suitable dyes including one selected from the group consisting of porphyrins including metalloporphyrins, carotenes, pyrans, cumenes, phthalocyanines including metallophthalocyanines, merocyanines, cyanines, squarates, eosins, xanthines, photoactive polymeric materials such as materials obtained from pyrroles, particularly polypyrroles, and a variety of photoactive metal complexes including well characterised and widely used ruthenium(ll) dyes. Such dyes include cis- bis(isothiocyanato)bis(2,2¹-bipyridyl-4,4'-dicarboxylic acid)-ruthenium(ll); tris(isothiocyanato)-ruthenium(ll)-2,2^I:6',2"-terpyridine-4,4',4"-tricarboxylic acid; bis(tetrabutylammonium) cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'- dicarboxylato)-ruthenium(ll) tris(isothiocyanato)-ruthenium(ll)-2,2':6^I,2"- terpyridine-4'-phosphonic acid and tris(2,2'-bipyridyl-4,4'-dicarboxylic acid) ruthenium (II) dichloride and any other suitable number of the ruthenium family of dyes and derivatives thereof. Some of the dyes are responsive to particular wavelengths of light and suitable dyes may be chosen so that the photovoltaic cell produces electricity and/or a specific voltage in response to certain wavelengths of light.

Quantum dots such as those consisting of Cadmium Selenide (CdSe) and gold can be used as the sensitising agent.

A wide variety of photosensitising agents may be applied to and/or associated with the nanoparticles to produce the photosensitised interconnected nanoparticle material. The photosensitising agent facilitates conversion of incident visible light into electricity to produce the desired photovoltaic effect. It is believed that the photosensitising agent absorbs incident light resulting in the excitation of electrons in the photosensitising agent. The excited electron is then transferred from the excitation levels of the photosensitising agent into a conduction band of the interconnected nanoparticles. This electron transfer results in an effective separation of charge and the desired photovoltaic effect.

Accordingly, the electrons in the conduction band of the interconnected nanoparticles are made available to drive an external load electrically connected to the photovoltaic cell. In one illustrative embodiment, the photosensitising agent is sorbed (e.g., chemisorbed and/or physisorbed) on the interconnected nanoparticles. The photosensitising agent may be sorbed on the surfaces of the interconnected nanoparticles, throughout the interconnected nanoparticles or both. The photosensitising agent is selected, for example, based on its ability to absorb photons in a wavelength range of operation, its ability to produce free electrons (or electron holes) in a conduction band of the interconnected nanoparticles, and its effectiveness in complexing with or sorbing to the interconnected nanoparticles.

In the preferred embodiment of DSSCs an electrolyte will be present. The photovoltaic cell comprises an electrolyte composition for carrying a charge between the dye/semiconductor and counter electrode. The electrolyte may and generally will include a redox system. Suitable redox systems include, for example, organic and/or inorganic redox systems, including coordination complexes and organometallic compounds with appropriate redox potential and electron transfer properties. More particularly, the redox system may be for example, iodine/iodide, bromide/bromide and other main group compounds, the complexes of transition metal and rare earth metals ions, such as v^2+/3+, v^3+/4+, CR^2+/3+, Mn^2+/3+, Mn^3+/4+, Fe^2+/3\ Co^2+/3+, Ni^2+/3+, Cu^1+/2+, Cu^2+/3+, Ce^3+/4+, V^2+/3+, V^2+/3+, oxometallates (including polyoxometallates formed by metals such as vanadium, chromium, manganese, tungsten, molybdenum, niobium and which include doped polyoxometallates; and organic compounds such as viologens.

The electrolyte may comprise a polymeric polyelectrolyte. The polyelectrolyte may include between about 5% and 100% (e.g., 5-60%, 5-40%, or 5-20%) by weight of a polymer, e.g., an ion-conducting polymer; about 5% to about 95%, e.g., about 35-95%, 60-95%, or 80-95%, by weight of a plasticizer; and about 0.01 M to about 10 M of a redox electrolyte, e.g., 0.01-2 M, 0.01-1 M, or 0.01- 0.5 M, of organic or inorganic iodides, and about 0.005 M to about 1 M, e.g., 0.01-5 M, 0.01-2 M, or 0.01-1 M, of iodine. An ion-conducting polymer may include, for example, polyethylene oxide (PEO)₁ polyacrylonitrile (PAN), polymethylmethacrylate (acrylic) (PMMA), polyethers, and polyphenols. Examples of suitable plasticizers include, but are not limited to, ethyl carbonate, propylene carbonate, mixtures of carbonates, organic phosphates, and dialkylphthalates.

The electrolyte composition may include a gelling compound having a metal ion and an organic compound capable of complexing with the metal ion at a plurality of sites. In one embodiment such a gelling compound may be drawn from suitably functionalised metal-organic compounds, such as those derived from metal alkoxides, e.g., alkoxysilanes, titanium alkoxides and tin alkoxides, and functionalised or non-functionalised nanoparticles derived from, e.g, silicon, titanium, tin, indium-tin oxides as well as nanotubes, nanowhiskers, nanoplates, etc. of oxide, sulphide, nitride, carbide and carbonaceous materials. In a further embodiment, the complexing organic compound may be drawn from the class of polymeric compounds. The metal ion may be lithium. In various embodiments, the organic compound includes, for example, poly(4-vinyl pyridine), poly(2-vinyl pyridine), polyethylene oxide, polyurethanes, polyamides, and/or other suitable compounds. The gelling compound may be a lithium salt having the chemical formula LiX, where X is a suitable anion, such as, for example, a halide, perchlorate, thiocyanate, trifluoromethyl sulfonate, or hexafluorophosphate. In one embodiment, the electrolyte composition includes an iodide salt (eg., LiI) at a concentration of more than 0.05M and iodine at a concentration of at least about 0.05 M.

The electrolyte solution may include a compound of the formula MjXj. The i and j variables are ≥ 1. X is a suitable monovalent or polyvalent anion such as a halide, perchlorate, thiocyanate, trifluoromethyl sulfonate, hexafluorophosphate, sulfate, carbonate, or phosphase, and M is a monovalent or polyvalent metal cation such as Li, Cu, Ba, Zn, Ni, lanthanides, Co, Ca, Al, Mg, or other suitable metals.

The electrolyte solution may include a passivating agent such as t-butylpyridine, methyl-benzimidazole, or other species that have free electron pairs and are capable of adsorbing onto titania.

The electrolye solution may include an ionic-liquid which contains a redox couple and may contain the passivating agent.

In one embodiment the electrolyte includes a mixture including up to 90 wt % of an ionic liquid. The electrolyte may in one embodiment be in the form of an iodide salt, from 0 to 10 wt % water or other solvent, iodine at a concentration of at least 0.01 M, and methyl-benzimidazole.

In one embodiment, the imidazoliumiodide-based ionic liquid is selected from butylmethylimidazolium iodide, propylmethylimidazolium iodide, hexylmethylimidazolium iodide, or combinations thereof and the like. In another embodiment, the electrolyte composition includes LiI. In various embodiments, the amount of LiI ranges from about 1 wt % LiI and 6 wt % LiI, is at least about 1 wt % LiI, or is less than about 6 wt % LiI. The charge carrier material may include a polymeric electrolyte. In one version, the polymeric electrolyte includes polyvinyl imidazolium halide) and lithium iodide. In another version, the polymeric electrolyte includes polyvinyl pyridinium salts). In still another embodiment, the charge carrier material includes a solid electrolyte. In one version, the solid electrolyte includes lithium iodide and pyridinium iodide. In another version, the solid or non-volatile liquid electrolyte includes substituted imidazolium iodide.

The charge carrier material may include various types of polymeric polyelectrolytes. In one version, the polyelectrolyte includes between about 5% and 100% (e.g., 5-60%, 5-40%, or 5-20%) by weight of a polymer, e.g., an ion- conducting polymer, about 5% to about 95%, e.g., about 35-95%, 60-95%, or 80-95%, by weight of a plasticizer and about 0.01 M to about 10 M of a redox electrolyte, e.g., about 0.01 M to about 10 M, e.g., 0.01-2 M, 0.01-1 M, or 0.01- 0.5 M, of organic or inorganic iodides, and about 0.0005 M to about.1 M, e.g., 0.01-5 M, 0.05-2 M, or 0.05-1 M of iodine. The ion-conducting polymer may include, for example, polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethylmethacrylate (acrylic) (PMMA), polyethers, and polyphenols. Examples of suitable plasticizers include, but are not limited to, ethyl carbonate, propylene carbonate, mixtures of carbonates, organic phosphates, butyrolactone, and dialkylphthalates.

Preferred Embodiments

Embodiments of the invention will now be described with reference to the attached drawings. In the attached drawings:

Figure 1 shows a photovoltaic cell in accordance with the invention containing a single fibre;

Figure 1a is a cross-section of the coated fibre shown in Figure 1 along the line A-A';

Figure 2 shows a further embodiment of the photovoltaic cell of the invention comprising a three dimensional array of fibres/rods. Figure 3 is a longitudinal section of a photovoltaic cell of a further embodiment of the invention.

Figure 4 is a schematic view showing a section of single fibre solar cell in accordance with an embodiment of the invention.

Figure 5 is a graph showing the effect of the fibre length of the photovoltaic parameters in the photovoltaic cell of the invention illuminated at 1Sun.

Figure 6 is a graph showing the variation of voltage, current and efficiency with fibre length for the test described in Example 9.

Figure 7 is a graph showing the variation of current density with different fibre lengths for the test described in example 9.

Figure 8 is a graph showing the variation of voltage and efficiency with fibre length for a light intensity of 200 mWcm^"2 as described in Example 10.

Figure 9 is a graph showing the cement density at different voltages for the test described in Example 10.

Figure 10 is a graph showing the effect on voltage current and efficiency of cells of 10 mm and 20 mm elements of the invention of varying light intensity.

Figure 11 is a schematic drawing of a cross section of a photovoltaic cell of the invention containing 7 fibres as described in Example 11.

Figure 12 is a schematic drawing of a longitudinal section of a photovoltaic cell of the invention containing 19 fibres as described in Example 11.

Figure 13 is a schematic drawing of a longitudinal section of a photovoltaic cell of Figure 12 through the line A-A' showing the electrical interconnection of electrodes. Figure 14 is a graph showing the current density generated over time from the seven electrode ceil of Figure 11 as described in Example 11.

Referring to Figure 1 , there is shown a photovoltaic cell in accordance with the invention (1) comprising a light incident surface (2) and fibre/rod (3) extending downward from light incident surface and having an exposed end (4) for receiving light transmitted through the light incident surface (2). The fibre/rod is a drawn glass fibre/rod comprising a transparent core formed of a common silicate or other glass that contains imperfections and air bubbles allowing light scattering. The core (5) is surrounded by a transparent layer of a conducting material (6) forming an electrode. Outboard of the electrode (6) is a coating (7) of nanoparticulate titanium dioxide onto which has been adsorbed an N3 dye (for example). A housing (8) containing an electrolyte (9) provides functional contact between the dye-sensitised nanoparticulate semiconductor and a counter electrode (10) formed of a suitable conductive material such as platinum wire. The light-incident surface in the form of a glass plate may be sealed upon the housing to form a top wall of the photovoltaic cell.

In operation light falling upon the light-incident surface is transmitted into the core (5) of the fibre and scattered on to the circumferential coatings of the fibre.

Referring to Figure 2 there is shown a photovoltaic cell (20) comprising a housing (21) comprising a top wall (22) formed of a transparent glass sheet and an opaque body (23) which may be formed of a metal or other suitable material. The underside (24) of light incident surface is coated with a transparent conductive oxide (TCO) layer (25) such as tin doped indium oxide or fluoride doped tin oxide coating. Fibres (3) of the type shown in Figure 1a extend downward from the underside of the light incident surface and may be retained in place by a conductive resin (26) such as a conductive epoxy or a conductive solder. As shown in Figure 1a the fibres (3) contain a transparent conductive coating (27) such as transparent conductive oxide (TCO) coating and a coating of nanoparticulate semiconductor such as titanium dioxide (28) onto which dye (schematically shown 29) as being absorbed. The bottom end surface of the fibres may be coated with an opaque coating to minimise light transmission. The body of the housing (23) is separated from the conductive coating on the underside (24) of the light incident surface (22) by an insulating seal (30). The inside of the housing has a conductive metal coating (31) such as platinum to provide a counter electrode. The housing can also be made from a conductive material such as metal or carbon based material. The housing contains an electrolyte (32) which provides a functional connection between the semiconductor electrode and counter electrode (31).

In the embodiment of Figure 3 there is shown a photovoltaic cell (40) comprising a light incident surface (41) formed of a metal sheet (42) interrupted by holes (43) through which are exposed the ends (44) of an array of glass fibres (45).

The ends (44) of the glass fibres (45) are exposed to form part of the light incident surface 41 of the cell (40). The fibres are fixed to the metal sheet, by a collar of conductive solder (46) and extend below the metal sheet. The other ends (56) of the glass fibres are coated with a light reflective/scattering layer.

The glass fibre has a transparent coating (47) of a conductive oxide which extends about the complete length of the fibre (45). A coating of nanoparticulate titanium oxide (48) onto which a dye (schematically shown 49) is absorbed is provided about the transparent conductive oxide (47).

Side walls (50) and a bottom wall (51 ) of the cell are formed of a metal or other material shell (52) having a platinum inner coating (53). The side walls (50) are electrically insulated from the metal sheet (42) by an insulting seal (54).

The top wall (41 ), side walls (50) and bottom wall (51 ) together form a housing containing an electrolyte (55) in functional contact with the dye-sensitised nanoparticle coating (48) and platinum coating (53) which forms a counter electrode for providing an electricity generating circuit (54) between top wall (41) and side walls (50) (and/or) bottom wall (51 ) of the cell. Referring to Figure 4 the solar cell of the invention comprises single fibre (60) having a light incident surface (61 ) on at least one end. The fibre may be thin in diameter or relatively thick depending on the desired application. The fibre solar cell (60) comprises a glass core (62) having a thin conductive coating of indium tin oxide (ITO) on the cylindrical surface (62a) which has a dye-sensitised mesoporous annular coating layer of a dye sensitised nanoparticulate metal oxide (63) outbound of which is an annular layer electrolyte (64) which separates the dye sensitised semiconductor (63) from an external counter electrode (65) which may be formed of an opaque conductive material (eg. gold, graphite, platinum).

The electrolyte may be a solid electrolyte for example formed in a polymeric conductive material and form an annular layer about the semiconductor layer (63). Where a solid electrolyte is used, the counter electrode may be coated as an annular layer on the solid electrolyte. Gelled liquid, polymer or ionic liquid electrolytes can also be solidified by the use of insulating nanoparticles which can either be mechanically connected to or separate from the dye-sensitised layer on the ITO-coated fibre. The counter electrode in this case would also be coated as an annular layer. This type of embodiment can be achieved in both single and multi-fibre arrangements with an insulating outer layer to seal the device. In addition, sealed single fibres can be selectively connected in series, parallel and in any combination of the two.

In comparison to the standard two dimensional solar cell configuration which consists of two parallel plates sandwiched together, there is greater scope for specifically designing the cell for specific applications. In this embodiment of the invention the light enters the solar cell through the light incident surface (61) at the exposed and of the glass fibre core. This glass fibre (62) can be quite thin in diameter, from the fine diameter of optic fibres to greater diameters of glass rods. The light is transmitted through the interior of the glass fibre (62) and ultimately scatters to the coating (63) of dye-sensitised mesoporous metal oxide. An electrolyte (64) is in direct contact with the dye-absorbed semiconductor (64). A transparent indium tin oxide layer (62a) is coated onto the cylindrical surface of the glass fibre prior to deposition of the dye-sensitised metal oxide (eg TiO₂) layer (63).

The photoelectrochemical mechanism for generation of electricity is the same as the 2D devices, in that electrons generated from excitation in the sensitiser are injected into the conduction band of the semiconductor (63), which then permeate through the mesoporous film through to the ITO (62a) layer. Electrons are collected at the semiconductor layer (63) on the fibre and then travel through an external circuit (66) back through to the counter electrode (65). At the counter electrode (65), the electrons catalyse the reduction reaction in the redox couple of the electrolyte (64). The redox couple then mediates the charge transport to the dye absorbed on the mesoporous metal oxide surface (63) where the dye is reduced to its ground state. Thus, the 3D solar cell is regenerated to allow for further energy conversion.

Such device engineering allows the overall light capture surface area to be reduced particularly when it is used in combination with a solar concentrator. This is because the surface area for the dye-sensitised metal oxide is increased relative to the light capture area. In addition, the sealing of these devices is simplified with the help of conventional battery technology, not readily applicable to 2D solar cells.

The single fibre cell may be part of a multi-cell assembly to provide a combined circuit. The construction of such a device can be constructed using an interpenetrating array of fibre electrodes to form both working and counter electrodes or an array of working electrode elements with a membrane or casing supporting the counter electrode. Conventional solar cells have a two- dimensional panel design and thus require a large light capture area to generate a sufficient amount of electrical power. The glass fibre solar cell of this embodiment is able to generate much higher electrical power per unit of light capture area compared with existing two-dimensional solar cells.

The photovoltaic cell of the invention may comprise a solar concentrator. In one embodiment the light incident surface of the cell is in the form of a concentrator lens. Suitable lens designs are known in the art for use with conventional thin wafer solar cells. Examples of lenses and lens designs are described in US Patent 6804062 which describes the use of a plurality of Fresnel lenses formed from optical grade silicon rubber.

Alternatively a solar concentrator may be of the type comprising a reflector surface which may for example be of parabolic or spherical shape to concentrate incident solar energy upon the light incident surface of the photovoltaic.

The photovoltaic cell of the invention may provide a light incidence surface for receiving concentrated light at or adjacent the focal point of the reflective surface. In the case of large scale energy conversion, a collector of dish type may be provided with a pedestal adapted to allow movement of the dish in an azimuth rotational plane and elevation rotational plane so as to track the sun during the day.

In a further embodiment the photovoltaic cell assembly has a light incident surface for one or more cells provided by an elongated transparent strip and is flanked by reflecting surfaces for reflecting light onto the light incident surface of one or more cells therebetween.

The reflecting surface may have a parabolic or off-axis parabolic cylinder contour with a focal line along or adjacent the elongated transparent strip which forms the light incident surface of one or more of the cells.

In the three-dimensional solar cell, the fibre cells are typically approximately vertically arranged into an array and concentrated sunlight, from a solar concentrator for example, radiates into one end of the glass fibre core. The glass fibre will guide and scatter the light, which will interact with the dye and produce electricity through the sensitised electrode. The photovoltaic cell of the invention may also be used in other roles known for photovoltaic cells such as a sensor responsive to the presence of light generally and/or light of a select wavelength range.

The invention will now be described with reference to the following examples. It is to be understood that the examples are provided by way of illustration of the invention and that they are in no way limiting to the scope of the invention.

Examples

Example 1

Step (A)- Fibre Preparation

Silica glass fibres were cut to a short length with scissors to a length of 4cm. The protective polymer coating was removed from the surface of the fibre. To achieve this, the fibres were immersed in acetone for at least 5 minutes. The fibres were then taken out of the acetone solution and the polymer protective layer was removed from fibres by simply drawing the polymer layer at the fibre surface. The polymer layer easily separated from the glass core.

Step (b) - Conductive Coatings

Polypyrrole, vapor deposited aluminium and indium tin oxide were each examined as conductive coatings. Polypyrrole (Ppy) was deposited using two methods (i) dip coating in the polymerised Ppy and (ii) soaking the fibres in FeCI₃ and then polymerising the pyrrole at the surface. Vapour deposited aluminium was performed at two thicknesses (3nm and 30nm thick coatings). Indium tin oxide was prepared by two methods, (i) R.F. Magnetron Sputtering - approximately 200nm thick coating and (ii) sol-gel process and subsequent dip coating.

Step (c) - TiO₂ Coating

The fibres were attached to glass rods by an adhesive tape. To prime the surface for good TiO₂ adhesion a 0.05 M ethanolic TiCI₄ solution was prepared and dip coated onto the surface of the conductors and then dried in the air for 30 minutes. The conductive fibres were subsequently coated with the TiO₂. This was achieved by using commercial nanopowder (Degussa P25, 25nm crystal size 75% Anatase TiO₂) in an aqueous slurry containing acetylacetone and "Triton X-100" surfactant to disperse the nanoparticles (approx. 30wt%). This coating solution was applied by successive dip coatings, followed by heating at 7O⁰C for 60 minutes. After 3 coatings a sintering stage was applied to the fibres (placed into crucibles) at 45O⁰C for 30 minutes. This procedure was repeated until 15 TiO₂ coatings were applied.

Example 2 Step (a) - Working Electrode Assembly

To assemble the working electrode into a usable format the fibres were fixed to the F:Snθ₂ glass slides using silver loaded epoxy. For this 2.11 g of Epoxy (Araldite), 0.50 g of Hardener (Ethacure 100 Curing Agent) were mixed together. Then 5.50 g of silver powder was added to the mix and pasted onto the conductive side of the glass plates and placed into a jig to allow fibres to be placed in an upright position.

The metal plates were then placed onto a guiding jig to fix the TiO₂-coated fibres into a perpendicular array onto the conductive glass using the conductive silver solution. The fibres were made to stand perpendicular to the plane of the glass. The jig with the glass and the connecting fibres was then placed into the furnace and heated up to 14O⁰C for 2 hours to cure the epoxy resin. After 2 hours the jig was removed from the oven and placed onto a metal plate to cool down.

Step (b) - Dye Adsorption

The fibres were placed upside down into glass tubes containing 0.3 mM N3 dye solution ([Ru(dcbpy)₂(SCN)₂] where dcbpy=4,4^'-dicarboxylic acid-2,2^'-bipyridine in acetonitrile) for 24 hours. The fibres were then removed and ready for final solar cell construction.

Example 3

Step (a) - Photovoltaic Cell Assembly and testing This example relates to a solar cell of the invention constructed from the assembly of Example 2. To assemble the solar cell for final testing a hollow glass tube (4mm inner diameter) was cut to a length of 5cm. This was sealed to a microscope slide using Surlyn (200 μm, Dupont) hot melt polymer on a hot plate. A Pt wire was inserted to the glass tube to act as the counter electrode and the tube was filled with an electrolyte (0.6 M tetrabutylammonium iodide (Aldrich), 0.1 M lithium iodide (Lancaster), 0.1 M iodine (Lancaster) and 0.5 M 4-ferf-butylpyridine (Aldrich) in acetonitrile). The dye-sensitised fibres were then inserted to the tube and supported by a retort stand.

The short circuit currents were recorded for the cells using a source meter (Keithley 2400). The fibre solar cells were illuminated with a 75W tungsten globe and the currents were recorded with and without the lamp.

Example 4

Step (a) - Conductive Coatings

Colloidal platinum was examined at this stage and it was confirmed upon calcination of the fibres that the polymer protective layer was present. The protective polymer layers were then removed by immersion in acetone.

ITO was then trialled as the conductive coating. Two types of ITO coatings were examined with one by R. F. Magnetron Sputtering from AIST, Japan and one sol-gel coating. The ITO was prepared by sol-gel route, dip coated and then calcined whereas the ITO-coated fibres from AIST, Japan were used as prepared (approximately 200nm thick coating) resulting in a transparent golden colour.

Step (b) -TiO₂ Coating

TiO₂ coatings were applied to fibres with and without the protective polymer coating. It was found that the film forming process was different for fibres with and without the protective polymer. When the Al and ITO conductive coatings were present it was found that a thin film of TiCI₄ was required to achieve adequate adherence of the TiO₂ coatings to the optic fibres. Step (c) - Working Electrode Fabrication

Using the silver-loaded epoxy in conjunction with the fibre placement jig a device suitable for proof-of-concept was assembled. As these fibres are very brittle once the protective polymer coating was removed care is needed when handling the fibres for epoxy mounting and transport of the jig. The silver loaded epoxy was highly conductive with lower resistance recorded to that of the conductive glass. However, due to the inability to have a good electrode contact at the fibre no conductivities could be recorded.

Step (D) Dying and Solar Cell Fabrication

The N3 dye adhered well to the surface of the Tiθ₂ on the fibre. Three devices were constructed, two of which consisted of two single fibre cells with the ITO (Fibre 1) and the third contained three of the ITO fibres coated in Japan (Fibre 3).

The short circuit currents recorded from the tests for one fibre and three fibres can be seen in Table 1 with illumination repetitions.

Table 1. Short Circuit Currents in (nA) recorded for fibre solar cells

Light Condition 1 Fibre 3 Fibres

Light off 0.3 40

Light on (1 ) 0.5 840

Light off 0.3 40

Light on (2) 0.5 800

Light off - 40

Light on (3) - 800

Light off - - 40

Light on (4) - 800

Light off - 40

Light on (5) - 800

Light off - 40

Light on (6) - 800

Light off - 40 From the data shown in Table 1 the three fibres cell with magnetron-sputtered ITO coating (Fibre 3) performed far greater than both fibres 1 and 2. This can be put down to potentially much better ITO conductive film.

Example 5

This example relates to preparation of photovoltaic cells comprising glass rod core elements of about 1 mm diameter.

An indium tin oxide (ITO) layer was deposited using radio frequency (RF) magnetron sputtering. The RF magnetron sputtering was performed in a modified A320 (AJA International) system. An ITO target (90% In₂O₃: 10% SnO₂, Goodfellow Cambridge Ltd.) was fitted into the system to allow for the sputtering of the ITO films. The 1 mm diameter glass rods were sputtered in the chamber for 20 minutes at a pressure of 30 x 10^"3 Torr and a power of 30 W. The rods were then flipped over, to allow sputtering of the shadowed side of the rod (total time 40 minutes).

The final stage of the ITO conductive surface preparation is vacuum annealing. The pressure in the magnetron chamber was brought down to around 3 x 10^'6 Torr and the heater block was increased to 35O⁰C. The samples were annealed at this temperature for 2 hours. When completed, the samples were allowed to cool to room temperatures before the chamber was pressurised with air. Rod resistances were then measured and determined to be 50-200 Ω / cm. Any rods that had resistances above 200 Ω / cm or dark shadowing leading to poor light transmittance were rejected, in order to maintain a high standard for the conductive glass rods for 3D solar cell production.

Wet chemical deposition of fluorine-doped tin oxide may be used as an alternative.

To prime the surface of the ITO for good TIO₂ adhesion, a 0.05 M ethanolic titanium (IV) chloride (TiCI₄, Aldrich) solution was prepared and dip coated onto the rods. The TiCI₄ coating solution was produced by dilution of the precursor down to 2 M with ethanol at O⁰C under N₂. This stock solution of 2 M TiCU in ethanol was further diluted with ethanol to 0.05 M. Rods were dip coated into the coating solution in order to give adequate coverage. The rods were subsequently hydrolysed in a water-saturated atmosphere at 7O⁰C for 30 minutes.

The primed glass rods were subsequently coated with the TiO₂ layer. This was achieved by dip coating the rods in a aqueous slurry using commercial nanopowder (Degussa P25, 25 nm average crystal size 75% Anatase TiO₂), as well as acetylacetone and Triton X-100 to disperse the nanoparticles (approx. 33 wt%). After coating the rods, they were dried at 7O⁰C for 30 minutes and cooled down to room temperature under N₂ gas.

Once at room temperature, the TiO₂-coated glass rods were hydrolysed in a water-saturated atmosphere at 7O⁰C for 30 minutes. After the rods were cooled to room temperature the rods underwent a final sintering stage at 45O⁰C for 30 minutes, while placed in crucibles. The electrodes were then cooled down to

100⁰C under N₂ gas and placed into a 0.3 mM ethanolic N719 dye solution ((n-

Bu₄N)₂[Ru(Hdcbpy)₂(NCS)₂] where dcbpy is 4,4^'-dicarboxylate-2,2^'-bipyridine) for at least 24 hours at room temperature. After dye absorption the rods were carefully rinsed in acetonitrile to remove excess dye solution in the pores of the

TiO₂ films.

A tinned joint was then applied to the end of the dye absorbed working electrode rod using an ultrasonic solder station (USS-9200, MBR Electronics). A take-off was attached to the tinned joint, to allow for charge collection and a reproducible charge collection position.

The final component that is required for the testing of the fibre solar cells is the counter electrode which, in the design chosen also served as the casing for the whole device. Precision machining was used to fabricate the casing. The inner diameter of the counter electrode defines the diffusion distance between the working and counter electrodes. A large distance between the two electrodes will lead to a low device performance at the probability of the redox couple to regenerate the dye and recombine at the counter electrode will be low due to the large pathway for ionic transport. As the rate of ionic diffusion in the redox couple is inversely related to the distance between the electrodes, smaller pathways result in better device performance.

Another important factor is the type of material from which the counter electrode is machined. The test was performed with a N719-adsorbed TiO₂ rod working electrode and a coupon of the counter electrode material immersed in a laboratory vial filled with the electrolyte [0.6 M tetrabutylammonium iodide (Aldrich), 0.1 M LiI (Lancaster), 0.1 M I₂ (Lancaster) and 0.5 M 4-tert- butylpyridine (Aldrich) in acetonitrile]. The electrodes were then clamped and run through the solar cell testing program to determine if appropriate responses were forthcoming.

The counter electrode materials that were trailed for the 3 dimensional dye- sensitised solar cells were graphite, steel, titanium and platinum. Test results were obtained by using 3.5 cm active length on a working electrode rod. The photovoltaic characteristics in Table 2 below tested at 1Sun illumination (lOOmWcm^'2, AM1.5G) and using the standard electrolyte [0.6 M tetrabutylammonium iodide (Aldrich), 0.1 M LiI (Lancaster), 0.1 M I₂ (Lancaster) and 0.5 M 5-tert-butylpyridine (Aldrich) in acetonitrile] show that the counter electrode and the inherent spacing between the working electrode and rod and the counter electrode wall are important parameters.

Table 2

Photovoltaic characteristics of the counter electrode materials using

Current (mAcrrϊ²)

Counter Electrode Inner Diameter (mm)

2 1.1

Platinum 1.62 Graphite 0.24 Titanium 0.98 0.55 Steel 0.45 Voltage (mV)

Counter Electrode Inner Diameter (mm)

2 1.1

Platinum 220

Graphite 120

Titanium 130 130

Steel 120

Fill Factor

Counter Electrode Inner Diameter (mm)

2 1.1

Platinum 0.55

Graphite 0.35

Titanium 0.39 0.38

Steel 0.30

Efficiency (%)

Counter Electrode Inner Diameter (mm)

2 1.1

Platinum 0.20%

Graphite 0.01%

Titanium 0.05% 0.03%

Steel 0.02%

The highest efficiency observed from the tests at 1Sun light illumination gave the platinum counter electrode the greatest efficiency at 0.20%. Two inner diameters of the counter electrodes to determine if the distance between the counter and working electrodes greatly affects the device performance were tested. Platinum and graphite counter electrodes were only tested with an inner diameter of 2 mm. The steel sample did not perform as well and only the smaller inner diameter (1.1 mm) was utilised. The currents and voltages recorded using the platinum counter electrode were generally much greater than those recorded with the other materials up to 220 mV and 1.62 mAcm^"2.

Example 6

This example examines the effect of the rod or fibre length on efficiency and output.

Working electrode rods were produced with varying lengths. These provided active lengths of the dye absorbed T1O₂ coatings on the rods of 0.5, 0.8, 1.4, 1.8, 2.2, 2.6 and 3.5 cm. These working electrode rods were prepared in the same fashion as described in Example 5. The testing jig for this was performed with the graphite counter electrode (inner diameter of 3 mm) and the electrolyte used was an ionic liquid [0.1 M iodine, 0.05 M ethylmethylimidazolium iodide, 0.05 M lithium iodide and 0.45 M N-methylbenzimidazole (NMBI) in ethylmethylimidazolium dicyanamide (emlmDCA)]. This electrolyte was chosen as the acetonitrile-based systems are absorbed into the porous graphite. The cells were illuminated at 100 mWcrrf² AM1.5G (1 Sun) for white light testing.

These results (Figure 7) show that the changes in the fibre length cause differences in voltages, currents, dark currents and the overall cell efficiency. With increasing fibre length, both the currents and dark currents (increasing from a negligible level with small active areas on the fibres) increase. However, the voltage decreases with increasing fibre length. This is likely due to increased electrical resistance and recombination, as seen with an increase in the dark currents, due to the increased path length of the electrons in the longer fibres. This results in a slight increase in efficiency with increasing fibre length (maximum level 0.41% at a fibre length of 3.5 cm).

Example 7

This example examines the effect of different electrolytes. These ionic liquid electrolytes were produced for this test to determine if there is an effect on the electrolyte of choice with the different counter electrodes. The components of the three electrolytes are given below in Table 3. Table 3

Electrolyte Composition

A 1-propyl-3-methylimidazolium iodide, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, iodine and N-methylbenzimidole. B 1-ethyl-3-methylimidazolium iodide, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, iodine and N-methylbenzimidazole

C 1-propyl-3-methylimidazolium iodide, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, iodine and lithium iodide.

These three electrolytes were filled into the counter electrodes using a syringe and between each test. The counter electrodes and working electrodes were cleaned thoroughly using acetonitrile to prevent mixing the electrolytes in subsequent tests. The photovoltaic characteristics show that the different electrolytes have an effect on the performances in the different counter electrodes.

Photovoltaic testing at 1Sun (100 mWcm^"2 Ami .5G) indicates that the efficiencies were lower when using the ionic liquid electrolytes. Electrolyte C gave the best performances in conjunction with either the graphite or platinum counter electrodes, 0.35 and 0.22% respectively. Ionic liquid electrolytes lead to lower performances of the devices due to lower ionic diffusions with respect to the common volatile solvent-based electrolytes in 2D solar cells. This effect is amplified by the fact that the distance between the two is much further with a distance of 0.5 mm or 0.5 mm for the 2 mm and 1.1 mm cells, respectively, in comparison to the common distance of the two electrodes of between 5 and 20 μm in a two dimensional DSSC solar cell. Table 4

Photovoltaic Characteristics of 3-D cells using ionic liquids A, B, C

Current (mAcrrr) Fill Factor

Electrolyte A Electrolye A

Platinum 1.05 Platinum 0.43

Graphite 0.69 Graphite 0.45

Titanium 0.56 Titanium 0.37

Electrolyte B Electrolye B

Platinum 9.22 Platinum 0.21

Graphite 2.00 Graphite 0.43

Titanium 0.69 Titanium 0.35

Electrolyte C Electrolye C

Platinum 20.93 Platinum 0.35

Graphite 2.38 Graphite 0.40

Titanium 0.64 Titanium 0.47

Voltage , (mV) Efficiency (%)

Electrolyte A Electrolye A

Platinum 140 Platinum 0.06%

Graphite 0.69 Graphite 0.06%

Titanium 0.56 Titanium 0.03%

Electrolyte B Electrolye B

Platinum 30 Platinum 0.06%

Graphite 150 Graphite 0.13%

Titanium 120 Titanium 0.03%

Electrolyte C Electrolye C

Platinum 30 Platinum 0.22% Graphite 370 Graphite 0.35% Titanium 270 Titanium 0.08% Example 8

This example examines the use of a system of invention comprising a solar cell in conjunction with a solar concentrator. Photovoltaic results were taken with a higher light intensity (200 mWcm^"2 AM1.5G).

The results indicated that the best performance was for the platinum counter electrode. This 3D solar cell performed at 1.91 % efficiency, mainly due to a very high short circuit current density (J_sc) of 27.1 mAcm^"2. The efficiency remained low due to the low open circuit voltage (V₀₀) and lower fill factor (FF) of 410 mV and 0.34 respectively. In addition, there was a marked improvement in the performance of the titanium counter electrode with a much closer distance to the working electrode rod. This is seen in an improvement from 0.04 up to 0.8% in efficiency.

Table 5 Photovoltaic characteristics under 2Sun light intensity (200 mWcm^"2, AM1.5G)

Current lOO mWcm ^r 200 mWcm _^r

Counter Electrode Inner Diameter (mm) Inner Diameter (mm)

2 1.1 2 1.1

Platinum 1.62 27.05 Graphite 0.24 0.07803 Titanium 0.98 0.55 1.15 11 .59 Steel 0.45 0 .26

Voltage lOO mWcm -2 200 mWcm -2

Counter Electrode Inner Diameter (mm) Inner Diameter (mm)

2 1.1 2 1.1

Platinum 220 410 Graphite 120 170 Titanium 130 130 170 450 Steel 120 220 Filler Factor 100 mWcm -2 200 mWcm -2

Counter Electrode Inner Diameter (mm) Inner Diameter (mm)

2 1.1 2 1.1

Platinum 0.55 0.34 Graphite 0.35 0.25 Titanium 0.39 0.38 0.39 0.31 Steel 0.30 0.32

Efficiency lOO mWcm -2 200 mWcrτϊ²

Counter Electrode Inner Diameter (mm) Inner Diameter (mm)

2 1.1 2 1.1

Platinum 0.20% 1.91%

Graphite 0.01% 0.00%

Titanium 0.05% 0.03% 0.04%

0.80%

Steel 0.02% 0.01%

In comparison to the results observed at an illumination of 1Sun (100 mWcm^"2), the fill factors for all the tests at 2Sun illumination (200 mWcm^"2) have a tighter distribution, as the current-voltage curves are more like the standard diode response. The performances are enhanced from those under 1Sun illumination (100 mWcm^"2). Again the currents and voltages recorded using the platinum counter electrodes were much greater than those recorded with the other materials. In addition, the performance was enhanced from that under 1Sun from 220 mV and 1.62 mAcm^"2 up to 410 mV and 27.05 mAcnrf² devices perform to a higher level with increased incident light intensity. The light transmitted through the core of the glass fibres is then diluted from that of the equivalent 2D light capture area. To make use of increased depth of the device, greater concentration of light is desirable to allow for the dye molecules toward the fibres remote from the light source to harvest the light and produce an electric current. Photovoltaic performances were recorded for the devices in Table 6 using the ionic liquid electrolytes in Table 3. The photovoltaic results are lower than those recorded with the acetonitrile-based electrolyte at 2Sun illumination. This is expected as ionic liquids have a higher viscosity, and the diffusion between the two electrodes will affect the performance greater than that of the organic solvent based systems. The three counter electrodes used in this testing were the platinum, graphite and titanium, each with 2 mm inner diameters.

Table 6

Photovoltaic characteristics using ionic liquid electrolytes A, B and C under 2Sun light intensity (200 mWcm^"2, AM1.5G)

Current 100 mWcm^'z 200 mWcm^"* Filler Factor 100 mWcm^'' 200 Wcm^"^

Electrode A (mAcrrf²) Electrolyte A

Platinum 1.05 13.72 Platinum 0.43 0.41

Graphite 0.69 0.63 Graphite 0.45 0.41

Titanium 0.56 0.62 Titanium 0.37 0.37

Electrode B (mAcrrf²) Electrolyte B

Platinum 9.22 2.01 Platinum 0.21 0.39

Graphite 2.00 1.33 Graphite 0.43 0.40

Titanium 0.69 0.74 Titanium 0.35 0.33

Electrode C (mAcrrf²) Electrolyte C

Platinum 20.93 15.38 Platinum 0.35 0.30

Graphite 2.38 1.15 Graphite 0.40 0.41

Titanium 0.64 0.69 Titanium 0.47 0.45 Voltage lOO mWcm* 200 mWcm^"* Efficiency lOO mWcm^"* 200 Wcm^"*

Electrode A (mV) Electrolyte A (%)

Platinum 140 320 Platinum 0.06% 0.91% Graphite 200 230 Graphite 0.06% 0.03% Titanium 140 150 Titanium 0.03% 0.02%

Electrode B (mV) Electrolyte B (%)

Platinum 30 260 Platinum 0.06% 0.10% Graphite 150 130 Graphite 0.13% 0.03% Titanium 120 110 Titanium 0.03% 0.01%

Electrode C (mV) Electrolyte C (%)

Platinum 30 500 Platinum 0.22% 1.14% Graphite 370 340 Graphite 0.35% 0.08% Titanium 270 270 Titanium 0.08% 0.04%

The best photovoltaic results for each of the three ionic liquid electrolytes was recorded when using the platinum counter electrode. In addition, all the tests were highest when electrolyte C was used with the three 2 mm inner diameter counter electrodes (platinum, graphite and titanium). The fill factors were more consistent from the current-voltage responses for the common diode response. The efficiency of the devices using electrolytes A and C are at a respectable level - 0.91 and 1.14% respectively. The platinum counter electrode provided superior results. Example 9

This example examines the effect of variation in fibre length of the photovoltaic cell of Example 6. Increasing voltages, currents and efficiencies experienced with increased fibre length. Measurements taken at a light intensity of 100 mWcm^"2 AM1.5G, with a 2mm inner diameter platinum counter electrode and using a liquid electrolyte [0.6 M tetrabutylammonium iodide (Aldrich), 0.1 M LiI (Lancaster), 0.1 M I₂ (Lancaster) and 0.5 M 5-tert-butylpyridine (Aldrich) in acetonitrile].

Data 10mm 17mm 20mm

Voc (mV) 360 370 430

Jsc (mAcm^'2) 2.02 2.73 3.83 ff 0.40 0.39 0.34 η (%) 0.29 0.40 0.56 The variation of voltage, current and efficiency with fibre length is shown in Figure 6.

The variation of current density with different fibre lengths is shown in Figure 7.

Example 10

This example demonstrates the variation in voltage, current and efficiency of 10 and 20 mm light transmitting elements referred to in Example 9 under different light intensities. Increasing voltages, currents and efficiencies experienced with increased light intensity, the longer fibres having more of an increase in the device performances. The same experimental conditions were used as for the 'effect of fibre length' section with the addition of 200 mWcnrf² light intensity testing.

Data 10mm 20mm

Light Intensity (mWcm^"2) 100 200 100 200

Voc (mV) 360 470 430 520

Jsc (mAcm^'2) 2.02 4.06 3.83 12.12 ff 0.40 0.50 0.34 0.33 η (%) 0.29 0.48 0.56 1.04 The variation of voltage and efficiency with the fibre length for a light intensity of 200 mWcm^'2 is shown in Figure 8.

The current density at different voltages is shown in Figure 9. The effect of varying the incident light intensity on voltage current and efficiency of each of 10 mm and 20 mm fibres is shown in Figure 10.

Example 11

This example demonstrates the current generated of multi-fibre modules with 7 or 19 fibres of 5 cm length (ITO-coated 4cm length), with 3 and 6 working electrode elements respectively. Referring to Figure 11 the photovoltaic cell (70) is a 7 fibre module and includes three working electrodes (71) and four counter electrodes (72) immersed in an electrolyte (73) which in turn is contained within a glass casing (74). The working (21 ) and counter electrodes (72) are evenly spaced

Referring to Figure 12 the photovoltaic cell (80) has 19 fibre electrodes including six working electrodes (81 ) and thirteen counter electrodes (82) immersed in an electrolyte (83) which is contained in a glass casing (84).

Electrical interconnection of the electrodes is shown schematically in Figure 13. The same electrical connection scheme was used for each of the seven and nineteen electrode cells. Referring to Figure 13 device construction in each case was achieved by setting the fibres of working (81 ) and counter (82) electrodes into a silver loaded epoxy joints (90) at either end (91 , 92) of the fibre lengths containing copper wires forming the interconnects (not shown). The electrical connections for counter electrodes were at one end (90a) and the electrical connections for working electrodes at the other (90b). A 1 cm length of fibre without ITO coating (96a, 96b) on both the working and counter electrode elements (81 , 82) were buried in the opposing conductive epoxy interconnects (90a, 90b) for additional structural support to make a robust device without causing a short-circuit. The working electrodes (81) are in the form of light transmitting elements in accordance with the invention comprising a fibre (93) of 1 mm diameter of the type described in Example 9 and comprising an ITO coating about which a nanoporous dye sensitised TiO₂ coating is provided (shown collectively as 94). The counter electrode (82) is a fibre merely having an ITO coating (95). The outer casing (74, 84) in both configurations (70, 80) was glass forming a sealed device. The electrolyte [0.6 M tetrabutylammonium iodide (Aldrich), 0.1 M LiI (Lancaster), 0.1 M I₂ (Lancaster) and 0.5 M 5-tert-butylpyridine (Aldrich) in acetonitrile] was administered through a gap at one end of the glass which was sealed with parafilm (not shown).

The current generation of the 7 fibre module is shown in Figure 14 illuminated at 100 mWcm^"2 (1Sun) and 200 mWcm^"2 (2Sun) AM1.5G. In both cells the currents generated at 2Sun (200 mWcm^"2) were greater than those generated at 1Sun (100 mWcm^"2) illumination.

Example 12

This example examines the effect of surface roughening on the proportions of light scattered.

As an experiment to test the effect on the transmission of light through an optical core, glass rods of 45 mm length were roughened using a chemical method and a physical method. The transmission of these rods was tested by having a constant light source incident on the top flat surface with surrounding area masked to minimise background light. The light transmitted through the entire fibre to the opposite flat surface was quantified by the current from a photodiode placed there. The transmission of the unaffected rod was ~20 μA, while the chemically roughened rod was ~2.5 μA and the physically roughened rod was ~1 μA. This shows that the roughening was effective at reducing the light transmitted through the fibre and increasing the light scatted to the sides. This was also visibly evident in the experiment as well, with the roughened rods scattering light and the unaltered rods appearing emitting a high proportion of light from the end remote, from the incident light. The transmission for the unaltered core is -30%, while the chem roughened is -3% and the physical roughened is ~1%. These techniques are useful in providing the high propositions of scattering of light within the light transmissible element of the invention.

Claims

Claims:

1. A photovoltaic cell comprising:

(i) a light incident surface; and (ii) at least one elongated light-transmitting element comprising a longitudinally extending core having an end for receiving light from the light incident surface, said light-transmitting element extending away from the light incident surface to guide light received therefrom within the core and comprising a semiconductor extending about at least a portion of length of the elongated light-transmitting element; and wherein light passing longitudinally through the core from the light incident surface is scattered laterally onto the semiconductor.

2. A photovoltaic cell according to claim 1 further comprising a counter electrode and an electrolyte composition for carrying a charge between the at least one light transmissible element and the counter electrode.

3. A photovoltaic cell according to claim 1 wherein the semiconductor is a silicon semiconductor or IHA/ semiconductors.

4. A photovoltaic cell according to claim 1 wherein the semiconductor is a dye-sensitised semiconductor.

5. A photovoltaic cell according to claim 2 comprising a transparent conductive layer forming an electrode between the core and semiconductor.

6. A photovoltaic cell according to claim 1 or claim 2 wherein the core produces scattering by means of at least one feature selected from the group consisting of defects, imperfections, bubbles, particles, variation in density and a variation in the composition of the elongated light- transmitting element.

7. A photovoltaic cell according to claim 1 wherein the core is formed of glass and the core produces scattering by means is selected from colloidal dispersion, particle inclusions of size greater than the wave length of visible light and air bubbles.

8. A photovoltaic cell according to claim 1 wherein the light incident surface is formed by one or more ends of said at least one light transmitting element.

9. A photovoltaic cell according to claim 1 wherein the light incident surface is provided by a transparent layer.

10. A photovoltaic cell according to claim 9 wherein the cell comprises a housing containing an electrolyte in functional relationship with the semiconductor and counter electrode.

11. A photovoltaic cell according to claim 2 wherein the counter electrode is a metallic layer about the electrolyte composition.

12. A photovoltaic cell according to claim 2 wherein the electrolyte is a solid material forming an annular layer about the semiconductor and the counter electrode is an annular coating layer of metal about the solid electrolyte.

13. A photovoltaic cell according to claim 5 wherein the cell comprises an annular layer of solid electrolyte about the semiconductor and an annular layer of counter electrode about the solid electrolyte.

14. A photovoltaic cell according to claim 10 wherein a transparent layer forming the light incident surface and the body form a housing for the electrolyte and a plurality of elongated light-transmitting elements.

15. A photovoltaic cell according to claim 1 wherein at least one light- transmitting element is a glass fibre.

16. A photovoltaic cell according to claim 10 wherein at least one light- transmitting element is a flexible plastic fibre.

17. A photovoltaic cell according to claim 1 wherein the end of the elongated element remote from the light incident surface has a reflective coating.

18. A photovoltaic cell according to claim 2 comprising a solar collector adapted to increase the intensity of light passing through the light incident surface.

19. A photovoltaic cell according to any one of the previous claims wherein the light-transmitting element scatters light such that at least 80% of light entering the end of the core for receiving light from the light incident surface would be scattered within a 45 mm length of light transmitting element of the same construction.

20. A photovoltaic cell according to claim 19 wherein at least 95% is scattered.