GB2434485A

GB2434485A - Solar cell

Info

Publication number: GB2434485A
Application number: GB0601101A
Authority: GB
Inventors: Frederik Christian Krebs; Holger Spanggaard
Original assignee: Riso National Laboratory; Danmarks Tekniskie Universitet
Current assignee: Danmarks Tekniskie Universitet
Priority date: 2006-01-19
Filing date: 2006-01-19
Publication date: 2007-07-25
Also published as: GB0601101D0

Abstract

The device comprises a polymer photovoltaic cell 12 and a super-capacitor 14 which stores the charge generated by the photovoltaic cell. The device is formed as a flexible laminate which can be used for forming a photovoltaic lampshade.

Description

SOLAR CELL

The present invention relates to a laminate comprising a solar cell and a supercapacitor, an assembly comprising such a laminate and a lamp comprising such an assembly.

Renewable energy sources are desirable for a number of reasons. First, such energy sources enable a reduction in consumption of non-renewable energy sources. Second, such energy sources enable the use of electrical devices without the need for a mains power source or for batteries which must periodically be replaced. This is of particular interest in remote locations, for example at sea or in developing countries.

Solar power is an important renewable energy source, and is harvested using solar cells. Various designs of solar cell are known.

JP 11214721 describes a thin film solar cell wherein a series of solar cell elements are formed on a flexible substrate.

JP 10233522 describes a thin film solar cell wherein a solar cell element and a capacitor are laminated on opposite sides of a substrate.

GB 2376344 describes a method of forming a solar cell by printing on a flexible or rigid substrate. The printing materials may be polymers, metalloorganics, buckminsterfullerene-doped conducting polymers, polythiophenes, phenylenvinylenes and mixtures thereof.

JP 61070771 describes an assembly wherein a solar cell and a capacitor for accumulating energy are formed side-by-side on a substrate.

Solar cells may rely on photovoltaic polymers. It has been recognised that potentially such devices have advantages over the conventional, similar devices based on inorganic semiconductors These potential advantages include cheapness of the materials and versatility of processing methods, flexibility (lack of rigidity) and toughness.

Photovoltaic polymers can be derived from chemically doped conjugated polymers, for example partially oxidised (p-doped) polypyrrole. The article Conjugated polymers: New materials for photovoltaics' Wallace et al, Chemical Innovation, April 2000, Vol. 30, No. 1, 14-22 reviews the

field.

Various assemblies relying on solar cells are also known.

In such assemblies, a means for storing energy harvested by the solar cell is required. Supercapacitors are suitable for this purpose.

A supercapacitor is a device which stores energy by polarizing an electrolytic solution. Although it is an electrochemical device, there are no chemical reactions involved in its energy storage mechanism. A supercapacitor typically has higher capacitance than a conventional dielectric capacitor.

The supercapacitor consists of two non-reactive porous electrodes suspended within an electrolyte and usually held apart by a separator. The porous material is typically a high surface area material such as activated carbon.

When a voltage is applied across the electrodes, the applied potential on the positive electrode attracts negatively charged species in the electrolyte while the potential on the negative electrodes attracts positively charged species in the electrolyte. This creates two layers of capacitance storage, since there is separation of charge at each electrode.

As the electrodes are charged, charged species in the electrolyte migrate towards the electrodes to compensate for the charge. Thus, an electrical double layer is formed.

The charged species are not capable of being discharged by the electrode as in a battery.

US 5782552 describes a light assembly comprising a light-emitting diode, a rechargeable capacitor to energise the diode and a solar cell to recharge the capacitor. The assembly is encapsulated in potting material. The light assembly may be used for hazard or marker lighting in remote areas.

US 6525507 describes a solar system for a motor vehicle which has a solar generator and a high capacitance capacitor for supplying power to a fan motor.

WO 00/61873 describes a luminescent tile comprising a plurality of solar cells, an electric double layer capacitor for storing electric power generated by the solar cells and a luminescent panel powered by the capacitor.

US Des. 3888529 describes a lampshade with interior solar cells.

US 4810970 describes a self-charging light having a base provided with a rechargeable nickel-cadmium battery pack and a plurality of solar electric panels.

There remains a need for a solar cell and energy storage assembly of simple construction which can be adapted for use in a variety of applications.

The present invention is based on the realization that a flexible photovoltaic cell and a flexible supercapacitor can be combined to create a flexible laminate which provides the functions of harvesting of solar energy and energy storage.

Accordingly, the present invention provides a flexible laminate comprising the following layers: a set of layers forming a solar cell, the solar cell comprising a photovoltaic polymer; and a set of layers forming a supercapacitor electrically connected to the solar cell to store electrical energy produced by the solar cell.

Preferably, the laminate further comprises means for holding the layers of the laminate together. For example, this means may be an envelope formed by passing the sets of layers forming the solar cell and supercapacitor through a lamination machine which applies a plastics or other layer to each surface and seals the edges of the plastics layers together.

Preferably, a surface of the set of layers formin9 the solar cell conta.cts a surface of the set of layers forming the supercapacitor.

Optionally, the solar cell and the supercapacitor share a common layer, for example an electrode.

Optionally, more than one supercapacitor is provided.

This may be used to give high capacitance. The supercapacitors may be stacked on top of one another.

Preferably, either the surface of the supercapacitor remote from the solar cell is reflective, or the laminate further comprises a reflective layer on the surface of the supercapacitor remote from the solar cell, such that light is reflected by the reflective layer away from the solar cell. In one preferred embodiment, the supercapacitor comprises an aluminium electrode layer on the side remote from the solar cell.

Optionally, one or more protective layers is provided on one or both surfaces of the laminate. The protective layer is suitably a plastics layer.

A laminate with protective layers on both surfaces may be formed by passing the sets of layers forming the solar cell and supercapacitor through a lamination machine wherein the plastics layers act as protective layers. Alternatively, the set of layers forming the solar cell and the set of layers forming the supercapacitor may be passed separately through a lamination machine, and then joined, for example using an adhesive layer.

Preferably, the laminate consists of the solar cell, the supercapacitor and optionally the reflective layer and/or protective layer(s) without a separate substrate.

Alternatively, a further layer may be provided, for example an insulating layer between the solar cell and the supercapacitor, which is suitably an adhesive layer used to join the solar cell and supercapacitor.

Various types of solar cell may be used. For example, the solar cell may contain one photovoltaic polymer only, or may contain two photovoltaic polymers, or may contain a photovoltaic polymer blended with fullerene, or may contain a photovoltaic polymer blended with a dye or a mixture of dyes. The photovoltaic polymer may be a co-polymer, for example a block co-polymer.

Preferably, the solar cell comprises a first electrode in electrical contact with an n-type semiconductor polymer block and a second electrode in electrical connection with a p-type semiconductor polymer block.

Preferably, the work function of each electrode is matched to the work function of the polymer in contact with that electrode so that carrier transfer between each polymer and its respective electrode is obtained with substantially no barrier energy.

Preferably, the first electrode has a relatively low work function as compared to the second electrode.

Preferably, the work function of the first electrode is from 2.5 to 3.8 eV and the work function of the second electrode is from 4.0 to 5. 7 eV.

Preferably, the first electrode is of calcium, aluminium, scandium, neodynium, yttrium, samarium, europium, magnesium or magnesium-indium and the second electrode is of gold, silver, nickel, platinium, tungsten, chromium, indium-tin-oxide (ITO), zinc oxide or tin (IV) oxide.

Suitable conducting polymers include poly(terphenylene- vinylene), polyaniline, polythiopene, poly(2-vinyl- pyridine), poly(N-vinylcarbazole) poly-acetylene, poly(p-phenylenevinylene), poly-o--phenylene, poly-m-phenylene, poly-p-phenylene, poly-2, 6-pyridine, poly(3-alkyl-thiophene) or polypyrrole monomer, said polymers being substituted with electron withdrawing substituents in the case of the n-type polymer block and with electron donating substituents in the case of the p-type polymer block. The polymers may be further substituted to alter physical properties such as solubility, for example with alkyl groups.

Further suitable conducting polymers include those disclosed in W003/078498 which have repeating units of the structure shown in general Formula 1:

R

Formula 1 in which each R independently is alkyl, and n is a number of at least 15, indicating the average degree of polymeri sat ion.

Preferably, each R independently is an alkyl group containing from 6 to 20 carbon atoms. More preferably, each R independently is an alkyl group containing from 6 to 12 carbon atoms. For example, each R independently is alkyl containing approximately 8 carbon atoms.

Preferably, one or both of the alkyl groups R. is linear.

Preferably, one or both of the alkyl groups R is bonded to the respective phenyl ring from a terminal CH2 group.

Preferably, n is at least 30. More preferably, n is at least 50.

Such polymers may be formed by reacting a e.g. 1,4- diformyl-2, 5-dialkylbenzene with e.g. p-xylene--bis-phosponicacidtetraethylester. Suitably, the polymerisation is conducted in a high boiling solvent under basic conditions. l,4-diformyl-2,5-dialkylbenzenes may be formed by reacting a l,4-dibromo-2,5-dialkyl benzene with n- butyllithium and dimethylformamide to produce a l-bromo-4- formyl-2,5-dialkyl benzene, reacting the said l-bromo-4-formyl-2, 5-dialkylbenzene with neopentyl-glycol to protect the formyl group by the production of an acetal, reacting the acetal protected l-.bromo-4-formy1-2,5-dialky1benz with n-butyllithium and dimethylformamide to produce a 1-formyl-4-acetal protected formyl-2,5-dialkylbenzene, reacting the second formyl group with neopentyiglycol to form a second acetal protected formyl group, purification and removal of the protecting groups by hydrolysatjon.

As discussed in PCT/EpO3/1025g, the solar cell may comprise a first electrode and a second electrode separated by a dye linked block polymer material, the molecules of which comprise in electrical connection with said first electrode an n-type semiconductor polymer block linked via a light absorbing monomeric dye moiety to a p-type semiconductor polymer block in electrical connection with said second electrode, the two polymer blocks being phase separated into distinct layers.

The dye moiety may be a phenanthrojmjdazole, trioxatriangulene azadioxatriangulene, diazaoxatriangulene, triazatriangulene, perylene, porphyrin, or phthalocyanine, or a derivative of one of these molecules.

The dye moiety is preferably linked to one or both of the polymer blocks by covalent bonding or by ionic interaction. More preferably, the anode and/or the cathode are covalently bonded to the n-type or p-type polymer blocks respectively.

Preferably, the n-type polymer blocks and the p-type polymer blocks of the diblock polymer material each self assemble into mono- layers separated by said dye moiety.

Preferably, said molecules of the dye linked block polymer material comprise multiple units of n-type semiconductor polymer block -dye moiety -p-type semiconductor polymer block joined head to tail and extending between said electrodes.

The non covalent bonding may be ionic bonding or may be hydrogen bonding. Suitably, non covalent binding is provided by amino groups on the hole conducting polymer interacting with acidic groups on the electron conductive material, or vice versa.

Suitably, the electron conductive material is a derivatised fullerene or carbon nanotube.

Preferably, a dye is also incorporated into the polymer by non-covalent bonding to the hole conducting polymer.

Preferably, the solar cell comprises a transparent electrode layer selected from indium tin oxide, titanium dioxide, zinc oxide and tin (Iv) oxide on the side of the solar cell remote from the supercapacitor. Suitably, the solar cell comprises a very thin evaporated metal electrode layer on the side of the solar cell close to the supercapacitor. The thickness of the electrode layer must be sufficient to give adequate electrical conductivity. For this reason it is preferably at least 10 nm, more preferably nm. A thickness of around 200 nm is suitable.

Preferably, the supercapacitor comprises an aluminium electrode layer on the side remote from the solar cell.

The supercapacitor electrodes are preferably thicker than the solar cell electrodes to contain the electrolyte and to provide adequate mechanical strength during processing. Suitably, the thickness is at least 1 pm, preferably at least 10 pm, more preferably at least 100 pm and most preferably at least 1 mm. However, if the supercapacitor electrode layer is formed on a substrate, a very thin electrode layer could be used. In this case the thickness is preferably at least 10 nm, more preferably 100 nm. A thickness of around 200 nm is suitable.

Preferably, the supercapacitor comprises an electrolyte layer provided with a separator. The separator may be formed of paper.

Preferably, the electrolyte comprises a viscous mixture of salt and solvent and/or polymer. Usually solvent and polymer are both included in the mixture to provide a non-solid viscous electrolyte. This is preferred since polymer increases the viscosity of the electrolyte to achieve a balance between fast movement of ions (achieved with low viscosity electrolyte) and easy handling with no leakage (achieved with high viscosity electrolyte) The salt is preferably a tetraalkylammonjum salt.

Suitable anions include PF, BF4, C104 and Tf (methanesuiphonate) . LiC1Q can also be used.

The polymer should be compatible with the solvent used and sufficiently polar to be compatible with the salt used.

Suitable polymers include PEG (polyethylene glycol), PMMA (poly(methylmethacrylate)) PAN (poly(acrylonjtrile)) and polyimine, for example PEI (poly(ethyleneimjne)) Suitable solvents include acetonitrile, water, DMF (N,N-dimethylformamide) DMSO (dimethyl sulphoxide), HMPA (hexamethylphosphorarnje), THF (tetrahydrofuran), glymes (short chain PEG). and carbonates, for example cyclic carbonates such as PC (propylene carbonate) and EC (ethylene carbonate) and linear carbonates such as methyl ethyl carbonate, or mixtures of these solvents. Carbonates are the preferred solvents.

Where glymes are used as the solvent, polymer need not be included in the electrolyte.

The preferred electrolyte composition is PEG as polymer, PC as solvent and Et4NPF6 as salt.

Alternatively, the electrolyte may be an aqueous solution, for example aqueous sulphuric acid or aqueous potassium hydroxide. In this case, it is important that steps be taken to avoid leakage of the electrolyte.

In a preferred embodiment, the solar cell comprises a first electrode layer, a photovoltaic polymer layer and a second electrode layer in that order, and the supercapacitor comprises a first electrode layer, a first layer of high surface area material, a layer of electrolyte provided with a separator, a second layer of high surface area material and a second electrode layer in that order.

Suitably, the laminate is provided with electrical connections at intervals so that the laminate can be cut into smaller sections of laminate. In this way, sections of laminate can be cut to the desired dimensions for a particular application from a larger laminate, for example a roll of laminate, without damaging the electrical connections.

In a second aspect, the present invention relates to an assembly comprising a laminate as claimed in any one of the preceding claims and a power consuming device electrically connected to the supercapacitor to be powered by discharging the supercapacitor.

Preferably, the power consuming device is mounted to the laminate on the supercapacitor side.

Preferably, the power consuming device is a light source. For example, the light source may be a photodiode.

Optionally, the assembly further comprises means for holding the assembly in a non-planar structure. Examples of suitable means are an adhesive strip, a zip, complementary strips of hook and loop fasteners (Velcro1M), complementary buttons and buttonholes, or complementary hooks and eyes.

Preferably, the assembly is shaped into a non-planar structure. More preferably, the structure is cylindrical, conical or frustoconical.

In a third aspect, the present invention relates to a lamp comprising an assembly as described above and switch means operable to power the light source by discharging of the capacitor.

The invention will be further described with reference to two preferred embodiments, as illustrated in the Figures, in which: Fig. 1 is a cross-section through a laminate of a first preferred embodiment of the invention; Fig. 2 is a cross-section through an assembly of the laminate of Fig. 1 and photodiodes; Fig. 3 is a circuit diagram of the assembly of Fig. 2; Fig. 4 is a perspective view of a lampshade formed from an assembly of Fig. 2; Fig. 5 is a cross-section through a laminate of a second preferred embodiment of the invention.

Fig. 1 shows a flexible laminate 10 comprising a solar cell 12 and a supercapacitor 14.

Starting from its outside surface, the solar cell 12 comprises a transparent indium tin oxide electrode layer 16, a conducting polymer layer 18 (comprising a mixture of polythiophene and fullerene) and an aluminium electrode layer 20 which also forms part of the supercapacitor 14.

Starting from its surface in contact with the solar cell 12, the supercapacitor 14 comprises the first aluminium electrode layer 20, a first layer of carbon fibres 22, a layer of electrolyte 24 (comprising a viscous mixture of NEt4 PF6 salt, PEG polymer and PC solvent), through which a separator 26 passes, a second layer of carbon fibres 28 and a second aluminium electrode layer 30. The second aluminium electrode layer 30 forms a reflective coating on the inner surface 32 of the laminate 10. Optionally, a further reflective coating 34 (Fig. 2) is provided.

An adhesive strip (not shown) is provided at one edge of the laminate 10 and protected by a removable tape (not shown).

Photodiodes 36, 38 are mounted on the inner surface 32 of the laminate 10 to form an assembly 40.

Wires 42 (Fig. 3) link the second aluminium electrode layer 30 of the supercapacitor 14 to the indium tin oxide electrode layer 16 to complete a charging circuit 44.

Wires 46, 48 link the photodiodes 36, 38 to the first aluminium electrode layer 20 and the second aluminium electrode layer 30 respectively to form a discharging circuit 50 wherein the photodiodes 36, 38 are connected across the supercapacitor 14. The photodiodes 36, 38 may be connected in series or in parallel. A switch 52 is provided in the discharging circuit 50.

The assembly 40 is rolled and fixed into a frustoconical structure 54 (Fig. 4) by removing the removable tape from the adhesive strip and pressing the adhesive strip to the laminate. The frustoconical structure 54 is fitted to a lamp base (not shown) and gives the appearance of a lampshade.

Charging occurs when light falls on the laminate 10.

The light passes through the transparent indium tin oxide electrode layer 16 and is absorbed by the conducting polymer layer 18, giving rise to the photovoltaic effect. This causes current to flow through the charging circuit 44.

Charge is stored on the aluminium electrode layers 20, 30 of the supercapacitor 14. Charge carrying species (not shown) within the electrolyte layer 24 migrate towards the charged electrode layers 20, 30 and are stored on the surface of the carbon fibres in layers 22, 28 to compensate the charge stored on the aluminium electrode layers 20, 30. Short-circuiting through the supercapacjtor 14 is prevented by the separator 26.

To use the lamp, the switch 52 is operated to complete the discharging circuit 50. This causes current to flow through the photodjodeg 36, 38 as the capacitor 14 is discharged, producing light. The reflective layer 34 reflects the light.

The assembly of the preferred embodiment of the invention has the advantage that it provides three functions: harvesting of solar energy, energy storage and light emission. These functions are provided in the form of a laminate to which photodjodes are mounted. The laminate is thin and light, since all layers of the laminate have an electrical function and no separate substrate is provided.

This means that the laminate can be stored and transported efficiently.

The solar cell is provided over substantially an entire surface of the laminate, and the supercapacitor Occupies a similar area. Thus, solar energy is harvested and stored efficiently.

The use of conducting polymer in the solar cell gives rise to a number of advantages over conventional solar cells based on inorganic semiconductors such as silicon as discussed above.

The use of a supercapacitor means that large amounts of power can be provided to the photodiodes. By contrast, many

prior art devices rely on capacitors.

The laminate is flexible and can be manipulated and fixed into a variety of structures. Therefore, the laminate provides an easy route to a variety of solar powered devices. The laminate can be rolled for storage and transport. The laminate can be deployed over a surface which is not flat, for example on a boat. The laminate can readily be shaped to catch maximum light.

The use of a reflective layer on the surface of the laminate to which the photodiodes are mounted prevents light from being absorbed by the solar cell. Thus, light efficiency is improved.

The use of a frusto-conical lampshade or a cylindrical lampshade has the advantage that solar energy can be harvested from any direction.

Fig. 5 shows the second preferred embodiment of the invention, a flexible laminate 10 comprising a solai cell 12 and a supercapacitor 14.

Starting from its outside surface, the solar cell 12 comprises a transparent plastics protective layer 15, a transparent indium tin oxide electrode layer 16, a conducting polymer layer 18 (comprising a mixture of polythiophene and fullerene) and an aluminium electrode layer 20.

The solar cell 12 is separated from the supercapacitor 14 by an insulating layer 13 formed of adhesive.

Starting from its surface in contact with the insulating layer 13, the supercapacitor 14 comprises a first aluminium electrode layer 21, a first layer of carbon fibres 22, a layer of electrolyte 24 (comprising a viscous mixture of NEt4PF6 salt, PEG polymer and PC solvent), through which a separator 26 passes, a second layer of carbon fibres 28 and a second aluminium electrode layer 30.

A reflective protective coating 34 (Fig. 2) is provided on the second aluminium electrode layer 30.

The laminate is formed into an assembly and used in the same way as the laminate of Fig. 1.

This second preferred embodiment has various advantages compared with the first preferred embodiment.

First, the solar cell and supercapacitor can be formed separately and then joined together by the adhesive layer 13.

Second, the solar cell and supercapacitor do not share an electrode. This means that different materials can be used for the electrodes, and can therefore be chosen to have

suitable properties.

Third, protective layers are provided to protect the solar cell and supercapacitor. The indium tin oxide layer is commercially provided on a transparent substrate, and this acts as the protective layer.

Whilst the invention has been described with reference to the illustrated preferred embodiments, it will be appreciated that various modifications can be made within the scope of the invention.

Claims

Cia ms 1. A flexible laminate comprising the following layers: a set

of layers forming a solar cell, the solar cell comprising a photovoltaic polymer; and a set of layers forming a supercapacitor electrically connected to the solar cell to store electrical energy produced by the solar cell.

2. A laminate as claimed in Claim 1, wherein a surface of the set of layers forming the solar cell contacts a surface of the set of layers forming the supercapacitor.

3. A laminate as claimed in Claim 2, wherein the solar cell and the supercapacitor share a common layer.

4. A laminate as claimed in Claim 2, wherein an insulating layer is provided between the solar cell and the supercapacitor 5. A laminate as claimed in any one of the preceding claims, wherein more than one supercapacitor is provided.

6. A laminate as claimed in any one of the preceding claims, wherein either the surface of the supercapacitor remote from the solar cell is reflective, or the laminate further comprises a reflective layer on the surface of the supercapacitor remote from the solar cell, such that light is reflected by the reflective layer away from the solar cell.

7. A laminate as claimed in any one of the preceding claims, wherein a protective layer is provided on the surface of the supercapacitor remote from the solar cell and/or the surface of the solar cell remote from the supercapacitor 8. A laminate as claimed in any one of the preceding claims, wherein the solar cell comprises a first electrode in electrical contact with an fl-type semiconductor polymer block and a second electrode in electrical connection with a p-type semiconductor polymer block.

9. A laminate as claimed in any one of the preceding claims, wherein the solar cell comprises a transparent electrode layer selected from indium tin oxide, titanium dioxide, zinc oxide and tin (Iv) oxide on the side of the solar cell remote from the supercapacitor 10. A laminate as claimed in any one of the preceding claims, wherein the supercapacitor comprises an aluminium electrode layer on the side remote from the solar cell.

11. A laminate as claimed in any one of the preceding claims, wherein the supercapacitor comprises an electrolyte layer provided with a separator.

12. A laminate as claimed in Claim 11, wherein the electrolyte layer comprises a ViSCOUS mixture of salt and polymer and/or solvent.

13. A laminate as claimed in any one of the preceding claims, wherein the solar cell comprises a first electrode layer, a photovoltaic polymer layer and a second electrode layer in that order, and the supercapacitor comprises a first electrode layer, a first layer of high surface area material, a layer of electrolyte provided with a separator, a second layer of high surface area material and a second electrode layer in that order.

14. A laminate as claimed in any one of the preceding claims wherein electrical connections are provided at intervals so that the laminate can be cut into smaller sections of laminate.

15. An assembly comprising a laminate as claimed in any one of the preceding claims and a power consuming device electrically connected to the supercapacitor to be powered by discharging the supercapacitor 16. An assembly as claimed in Claim 15, wherein the power consuming device is mounted to the laminate on the supercapacitor side.

17. An assembly as claimed in Claim 15 or Claim 16, wherein the power consuming device is a light Source.

18. An assembly as claimed in Claim 17, wherein the light source is a photodiode.

19. An assembly as claimed in any one of Claims 15 to 18, further comprising means for holding the assembly in a non-planar structure.

20. An assembly as claimed in any one of Claims 15 to 19 shaped into a non-planar structure.

21. An assembly as claimed in Claim 20, wherein the structure is cylindrica', conical or frustoconicai 22. A lamp comprising an assembly as claimed in Claim 17, Claim 18 or any one of Claims 19 to 21 when dependent on Claim 17, and switch means operable to power the light source by discharging of the Supercapacitor