Improvements In and Relating to Textiles Incorporating Photovoltaic Cells

FIELD OF THE INVENTION The present invention relates to photovoltaic or solar cells and in particular to photovoltaic cells incorporated in or attached to textiles.

BACKGROUND TO THE INVENTION Photovoltaic cells are devices in which photons release valence electrons from the constituent atoms causing a current to flow. In a solid state photovoltaic cell, valence electrons are freed into the solid to travel to the contacts when light below a wavelength specific to that solid is absorbed. This threshold wavelength must be selected to provide an optimum match to the spectrum of the light source: too short a wavelength will allow much of the radiation to pass through unabsorbed; too long a wavelength will only extract part of the energy of the shorter- wavelength photons (the part equivalent to the threshold wavelength) .

The typical solar spectrum requires a threshold absorption wavelength of -800 nm in the near infrared to provide the maximum electrical conversion. However, even with maximum electrical conversion, some 40% of the solar radiation is unused. Further losses occur through incomplete absorption or reflection of higher energy radiation. The current that is delivered by a solar cell depends directly on the number of photons that is absorbed. A closer match to the solar spectrum and a larger cell area will give a higher current.

As well as a current (i.e. number of electrons per second), the voltage is determined by the internal construction of the cell, although it is ultimately limited by the threshold energy for electron release.

Figures IA and IB show a silicon solar cell 1 comprising a contact layer 3 which is transparent to sunlight, a p-type doped silicon layer 5, a junction layer 7, an n-type layer 9 and a metal contact 11. An electrical field is built into the cell by the addition of minute amounts of intentional impurities ("dopants") to the two cell halves to create the PIN device which generates a current upon the action of incident photons 15. The junction separates the photon- generated electrons from the effective positive charges (known as "holes") that pull them back and in so doing, generates a potential of, in this case -0.5 volts. This potential depends on the amounts of dopants and on the actual semiconductor itself. Multiple cells have integrated layers stacked together during manufacture, producing greater voltages than single junctions

The power developed by a solar cell depends on the product of current and voltage. To operate any load will require a certain voltage and current which can be achieved by adding cells in series and/or parallel, just as with conventional dry-cell batteries .

Conventional crystalline silicon cells have a thick metal contact on the back surface, and a gridded metal contact on the front surface. They do not require complete coverage of the front surface because the top layer of silicon is sufficiently conducting ("heavily doped") to deliver the photo-current without significant resistance losses. Amorphous silicon cells, like most thin-film cells, have a more insulating top semiconductor layer and so require the whole surface to be contacted. This is achieved without blocking the incoming light, by a layer of transparent conducting oxide (TCO) such as ZnO or Indium tin oxide (ITO) , often with other elements added to enhance the conductivity without reducing the transparency. In addition,

a fine gridded contact is often superimposed to further improve the current collection efficiency.

Despite the many attractions of solar cells as vehicles for providing energy, the way in which they are constructed provides problems in application. Typically, solar cells are either encased between glass plates, which are rigid and heavy, or the cells are covered by glass. Glass plates are fragile, and so care has to be taken with their storage and transport. The rigid nature of solar cells requires their attachment to flat surfaces and, since they are used outdoors, they have to be protected from any atmospheric pollution and adverse weather.

Therefore, increasing attention has been turned to the construction of lighter, flexible cells, which can still withstand unfavourable environments, yet nevertheless maintain the durability required. Several examples have appeared of solar cells in plastic films. However, whilst the successful incorporation of solar cells into plastics represents an important step in the expansion of solar cell technology, still greater expansion of the technology would be achieved through their incorporation into textile fabrics .

A textile may be defined as a flexible material which comprises a network of natural or artificial fibres. Woven textiles can be created by a number of well known techniques such as weaving, knitting, crocheting or knotting. In woven textiles, the fibres are interlaced. Non-woven textiles are textiles which are neither woven nor knitted such as felt. They are made by putting together a sheet of fibres and binding them together typically by the use of an adhesive or by interlocking them with serrated needle. Other commonly used methods of binding fibres for nonwovens are spunlacing (sometimes called hydroentanglement) , whereby jets of water

are used to entangle the fibres, and thermal bonding, in which loose fibres of thermoplastics (such as polyester) can be bonded. Textiles have a huge range of applications and markets and there are very many different types of woven, knitted and nonwoven textile constructions .

Currently, solar cells are incorporated into textiles by attaching the cell to a glass or plastic substrate and fixing the substrate to the textile by lamination, welding or sewing. In addition, photovoltaic cells in the form of fibres are also known.

It is an object of the present invention to provide an improved photovoltaic cell.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention there is provided a device comprising a photovoltaic cell deposited onto a textile, the textile being treated such that:

I. the area where the photovoltaic cell is deposited conducts electricity; and II. the fibres forming the textile in said area are stuck together.

Preferably, the fibres are stuck together to enhance electrical conduction in the device.

Preferably, the area of the textile is treated with a conductive material .

Preferably, the conductive material is a conductive polymer.

Preferably the fibres are stuck together at their cross-over points.

Preferably, the fibres are fused together.

Preferably, the fibres are stuck together by applying heat.

Preferably, the fibres are stuck together by applying pressure .

Preferably, a calendering apparatus is used to stick the fibres together.

Optionally, a press is used to stick the fibres together.

Preferably, the textile is a woven textile.

Preferably, the textile is a non-woven textile.

Preferably, the textile is made from synthetic and/or natural fibres.

Preferably, device further comprises a protective cover layer.

Preferably, the cover layer is a laminated layer.

Preferably, the cover layer is a plasma coating.

Preferably, the photovoltaic cell comprises p-type and n- type doped semiconductor.

Preferably, the semiconductor is a nanocrystalline semiconductor .

Optionally, the semiconductor is a polycrystalline semiconductor .

Optionally, the semiconductor is an amorphous semiconductor.

Preferably, the device comprises an array of photovoltaic cells as described in accordance with the first aspect of the invention.

In accordance with a second aspect of the invention there is provided a method of manufacturing a photovoltaic cell, the method comprising the steps of: treating a textile such that the fibres of the textile in the treated area stick together; applying a conducting material to the fibre; and depositing a photovoltaic cell onto the textile.

Preferably, the step of applying a conducting material to the fibre comprises applying a conducting material once the fibre has been formed into the textile.

Optionally, the step of applying a conducting material comprises applying a conducting material to the fibres of the textile before the textile has been made.

Preferably, the conducting material is a conducting polymer.

Preferably the fibres are stuck together at their cross-over points .

Preferably, the step of sticking the fibres together comprises fusing the fibres.

Preferably, the step of sticking the fibres together comprises heating the fibres.

Preferably, the step of sticking the fibres together comprises applying pressure.

Preferably, the step of sticking the fibres together comprises using a calendering apparatus.

Preferably, the step of sticking the fibres together comprises using a press.

Preferably, the textile is a woven textile.

Preferably, the textile is a non-woven textile.

Preferably, the textile is made from synthetic and/or natural fibres.

Preferably, the step of depositing the photovoltaic cell is a cold process.

Preferably, the step of depositing a photovoltaic cell comprises plasma enhanced chemical vapour deposition.

More preferably, the step of depositing a photovoltaic cell comprises microwave plasma enhanced chemical vapour deposition.

Preferably, the step of depositing a photovoltaic cell comprises: depositing a conducting layer on the textile; depositing a PIN device on the layer; depositing a transparent conducting layer on top of the PIN device .

Preferably, the process further comprises applying a protective cover layer.

Preferably, the cover layer is applied using a lamination process.

Alternatively, the cover layer is applied using a plasma coating process.

Preferably, the protective cover layer encapsulates the photovoltaic cell.

Preferably the photovoltaic cell comprises p-type and n-type doped semiconductor.

Preferably the semiconductor is a nanocrystalline semiconductor .

Optionally, the semiconductor is a polycrystalline semiconductor .

Optionally, the semiconductor is an amorphous semiconductor.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figures IA and IB are schematic diagrams of a known type of photovoltaic cell;

Figure 2 is a schematic diagram of a first embodiment of the present invention;

Figure 3 is a schematic diagram of a second embodiment of the present invention;

Figure 4 is a schematic diagram of a plurality of devices in accordance with the present invention arranged on a textile;

Figure 5 is a flow diagram of a first embodiment of a process in accordance with the present invention;

Figure 6 is a flow diagram of a second embodiment of a process in accordance with the present invention;

Figure 7 is a schematic diagram of a calendering process in accordance with the present invention; and

Figures 8A and 8B show fibres in a textile before and after treatment by the process in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 2 shows a first embodiment of a device in accordance with the present invention. The device 21 comprises a transparent top layer 23 made of a conducting material which may suitably be formed from a transparent conducting oxide such as indium tin oxide (ITO) or zinc oxide. The next layer in the structure 25 is formed from a p-type semiconductor such as silicon. Below the layer of p-type semiconductor there is a junction layer 27, a layer of n-type semiconductor 29 and a conducting contact 31 which is positioned on top of the treated textile 35.

In this example of the present invention treatment of the textile comprises applying heat and pressure to a predetermined area of textile and treating the textile area with a conductive polymer as shown by reference numeral 37.

Figure 3 shows a second embodiment of a device in accordance with the present invention which is similar in structure to the embodiment shown in figure 2. The device 41 comprises a transparent top layer 45 made of a conducting material which may suitably be formed from a transparent conducting oxide such as indium tin oxide (ITO) or zinc oxide. The next layer in the structure 47 is formed from a p-type semiconductor

such as silicon. Below the layer of p-type semiconductor there is a junction layer 49, a layer of n-type semiconductor 51 and a conducting contact 53 which is positioned on top of the treated textile 35. The embodiment of figure 3 further comprises a cover layer 43 which encapsulates the photovoltaic cell to prevent damage. In this embodiment of the present invention encapsulation is achieved by laminating the photovoltaic cell. Alternatively a plasma coating may be used.

Figure 4 shows a plurality of devices made in accordance with the present invention applied to a sheet of textile. Figure 4 shows a textile 63 and three cells 65, 67 and 69 connected in series 71 to provide an output. In this embodiment of the present invention each of the devices is deposited onto the textile at a predetermined area. The textile may be folded, rolled up or other-wise used in the same manner as any textile for its intended purpose.

A series of solar cells of this type allows an electrical supply to be made that meet a user's particular needs. Large-scale users can couple a cell array to either an active load-matching circuit or to a buffer energy store (e.g. electrochemical battery) . Small-scale users may need more choice of array size and shape and this can be provided by flexible textile cell arrays of the type shown in figure 4.

The active cell components, the semiconducting layers, have a great part to play in determining the cell performance, not only from an efficiency point of view but also from the load matching perspective. The separate improvement of both output current and voltage demands effective optical absorption and efficient photo-generated charge separation. Textile substrates may improve the optical absorption in

thin layers by scattering light that passes through the layer back into the layer for a second chance of absorption.

Figure 5 shows a first embodiment of the process in accordance with the present invention. The process 75 comprises selecting a woven or nonwoven fabric such as a polyester fabric 77, subjecting a predetermined area of the fabric to a process of heat and pressure treatment 79 in order to stick or fuse together fibres of the fabric then treating the fused fabric with a conductive polymer 81.

After treatment, the conducting layer of the photovoltaic cell is deposited 83 on top of the treated fabric. In this embodiment of the present invention the conducting layer of the photovoltaic cell comprises a layer of aluminium which is sputtered or evaporated onto the treated layer of fabric. A contact mask can be used to define the area upon which the aluminium is to be coated.

Advantageously, treatment of the fused fabric using a conductive polymer provides a conduction pathway in addition to the conducting layer of the photovoltaic cell. This additional layer is of particular use where breaks or discontinuities occur in the conducting layer.

In an alternative embodiment, the fibres of the textile may be treated with a conducting material prior to being made into a textile.

The PIN device is deposited on top of the aluminium layer using a cold processing technique which in this example is microwave plasma enhanced chemical vapour deposition. The process comprises depositing a layer of n-type silicon 85, depositing a layer of undoped or intrinsic silicon 87 then depositing a layer of p-type silicon 89.

Finally the transparent layer which in this example of the present invention comprises a transparent conducting oxide namely indium tin oxide is sputtered 91 onto the top of the PIN device through a contact mask.

Microwave plasma enhanced chemical vapour deposition (MPECVD) is used because it allows the process of depositing the PIN layers to be carried out at a reduced temperature. MPECVD supplies the energy needed to etch or deposit materials as an electrical discharge instead of heating the gas with a flame or oven.

PECVD occurs in a closed chamber containing two parallel plate electrodes, one of which is grounded and the other is connected to a high voltage power supply. When the pressure is reduced inside the chamber the high electric field across the plates provides accelerated electrons that can activate the remaining gas (i.e. ionise, dissociate or decompose it) and produce a plasma. Although the electrons provide sufficiently high energy for the process they are light and in relatively small concentration so they carry very little "heat"; the ions and neutral species gain even less speed (energy) than the electrons. Thus gas plasma processes are known as cold processes.

The high voltage source is usually AC and not DC because the latter can lead to one of the electrode plates becoming charged by the electrons, especially if there is an insulating substrate being processed. It is more common to use RF sources and one internationally assigned frequency is 13.56MHz. This produces a field that alternates direction so rapidly that only the electrons, and not the heavier ions, can follow it. If an even higher frequency such as 2,45GHz microwave energy is used, the pressure in the gas can be much higher than at lower frequencies and still produce a stable plasma.

Of especial interest to the silicon film deposition process is that the more efficient use of microwave power enables faster film growth from the preferred gas mixture of silane diluted in hydrogen and /or argon. Pure silane is too easily decomposed and readily produces dusty silicon films whereas heavily diluted silane produces nanocrystalline silicon rather than amorphous silicon, which retains the good optical properties of amorphous silicon and provides somewhat better electrical properties. (Heavily diluted silane mixtures lead to slow deposition rates at the standard 13.56 MHz plasma frequency)

The dilution ratio of silane to hydrogen (or indeed to another gas such as argon, or to argon and hydrogen) depends on the degree of crystallization intended and on other preparative conditions. It is possible to use 1:100 silane to hydrogen if the reaction is driven by 13.56MHz RF in a parallel plate plasma reactor. At VHF frequencies the dilution varies but may be around to 1:20 or 1:5 for the purpose of obtaining a nano-crystalline product.

Figure 6 shows a second embodiment of the process in accordance with the present invention. The process 95 comprises selecting a woven or nonwoven fabric such as a polyester fabric 97 subjecting a predetermined area of the fabric to a process of heat and pressure treatment 99 in order to stick or fuse together fibres of the fabric then treating the fused fabric with a conductive polymer 101. After treatment of the fabric, the conducting layer of the photovoltaic cell is deposited 103 on top of the treated fabric.

The PIN device is deposited on top of the aluminium layer using a cold processing technique which in this example is microwave plasma enhanced chemical vapour deposition. The

process comprises depositing a layer of n-type silicon 105, depositing a layer of undoped silicon 107 then depositing a layer of p-type silicon 109.

Next, the transparent layer, which in this example present invention comprises a transparent conducting oxide namely indium tin oxide is sputtered 111 onto the top of the PIN device through a contact mask.

Finally, the photovoltaic cell is encapsulated 113 to prevent it from being contaminated using either a laminate or plasma coating.

Figure 7 illustrates one way of applying heat and pressure to textile for the purpose of the present invention. Figure 7 is a schematic diagram of a calendering machine 115 which comprises a pair of heated rollers 117 and 119 through which the textile is fed such that heat and pressure are applied to the textile at predetermined parts of the textile surface. In the example of figure 7 the change in thickness of the textile is shown with the textile being thicker before it is fed through the rollers 121 and thinner after it is fed rollers 123.

Figures 8A and 8B illustrate the changes that may occur to the textile structure after the application of heat and pressure. Figure 8A shows a woven textile 131 before treatment and it can be seen that the fibres 133 and 135 form a loose mesh. Treatment using the process of the present invention ensures that the fibres 133, 135 are stuck together at points 137 to form a continuous interconnected surface.

The selection of a type of fibre is strongly influenced by its ability to withstand prolonged irradiation by ultraviolet (uv) light. Fibre selection is also governed by the

temperatures required to lay down the thin films comprising the solar cell. Nanocrystalline silicon thin films can be successfully deposited at temperatures as low as 200 ⁰ C.

Polyethylene terephthalate (PET) fibres melt at 260-270 ⁰ C and exhibit good stability to uv light. They are commercially attractive too because of their existing widespread use. Thus, fabrics composed of a large variety of PET grades are currently available. PET fibres possess good mechanical properties and are resistant to most forms of chemical attack. Fabrics constructed from PET fibres should, therefore, be suitable as substrates for solar cells, whilst also possessing flexibility and conformability to any desired shape.

Many high-performance fibres have excellent mechanical properties and readily withstand temperatures up to 300- 400 ⁰ C. Examples of suitable high-performance fibres could be polybenzimidazole (PBI) fibres, polyimide (PI) fibres and polyetheretherketone (PEEK) fibres. Textiles of this type are suitable for use in the device and method of the present invention.

The successful integration of solar cells into textiles opens up a whole range of fresh applications. Many of these applications are likely to exploit the flexibility and light weight of textile fabrics. They can, for example, be placed over the curved surfaces of buildings. They can also be installed in spaces which may otherwise be inaccessible, as in automotive, marine and aerospace equipment. In addition, textile fabrics can be rolled up, transported to a desired location and then unrolled at that location. Thus, the technology would then be beneficial to those living in remote areas, where there is no supply of electricity from the grid, fuel may scarce and expensive, and maintenance of equipment is uncertain. Moreover, it could be used to get

power quickly to disaster areas, hit by earthquakes, hurricanes, floods or fire.

Improvements and modifications may be incorporated herein without deviating from the scope of the invention.