Solar cell formed on an optical fibre

SOLAR CELLS

Field of the Invention

This invention relates to solar cells and is concerned with the provision of an improved method of making a solar cell.

The invention is also concerned with the provision of an improved form of solar cell.

Conventional solar cell technology includes the manufacture of flat panels that face the sun or the use of a solar concentrator to focus light on to a high efficiency cell.

It is an object of the present invention to provide a method of making solar cells more efficiently than has heretofore been possible.

Summary of the Invention

According to a first aspect of the present invention there is provided a method of making a solar cell that comprises depositing a photovoltaic material onto a fibre optic or light tube.

Surface texturing or optical engineering (optimising the refractive index) of the fibre optic or light tube may be effected prior to deposition of the photovoltaic material and the surface texturing or optical engineering is typically effected by a reel-to-reel process. The surface texturing or optical engineering may be effected by a sandblasting or acid etching operation or by modification of the refractive index or by optical matching of the coating applied to the fibre optic or light tube.

A conductive transparent metal oxide layer is preferably applied to the fibre optic or light tube prior to deposition of the photovoltaic material.

A protective coating is preferably applied on top of the deposited photovoltaic material and the protective coating is preferably a heatabsorbing film.

Deposition of the photovoltaic material may be effected by a reel-toreel process.

The fibre optic or light tube may be doped with an electroluminescent material.

The method of manufacture may include mounting a lens at one end of the fibre optic or light tube and providing a reflector at the other end of the fibre optic or light tube.

According to a second aspect of the present invention there is provided a solar cel, made by the method defined above.

Brief Description of the Drawings

Figure 1 shows the monolithic connection of a solar cell,

Figure 2 shows a photovoltaic-coated and monolithically connected fibre optic or light tube,

Figure 3 shows a fibre optic or light tube as shown in Figure 2 with a protective heat-absorbing coating applied to it, and

Figure 4 shows a column of photovoltaic fibre optics or light tubes provided with water cooling.

Description of the Preferred Embodiments

By coating a fibre optic with a thin film solar cell, which has been optically or mechanically optimised to leak light into the surrounding solar cell, light can be effectively completely trapped. Coating all around the light means that the surface area is dramatically increased compared to conventional solar cell technology.

The coated fibre optics can be bundled together into a column with light focused onto the ends of the fibres. The amount of light entering the fibres will depend on how much is collected by an arrangement of optics and mirrors.

To put this in perspective, in a column 0.5 meters in diameter and 1 meter deep, you will have a total surface area of more than 300 square meters. If you assume a relatively low efficiency cell of 6% this would represent about 15 KW at 1 sun/M². However, depending on the thin film technology used, the amount of light entering the column (collection system) could be greater than 1 sun/M².

As a secondary advantage, it is possible to cool the column of solar cells by inserting pipes (as shown in Figure 4) through the middle or around the column, which not only cools the cells - which increases performance, but will also produce hot water, which can be used for heating or to drive a generator. It will be much easier to do this as compared to a conventional flat solar panel.

As an alternative, a heat-reflecting mirror may be placed in front of the column of fibres at an angle of 45° to the axis of the column, with substantially the same result.

Separating the actual solar cell from the light collection system will dramatically reduce the cost of material associated with manufacturing and installing solar panels. You will simply need to install a light collection array, with a fibre optic cabling system to transfer the light. Alternatively, a column of coated fibres may be placed directly under the light collection system.

In normal use, a fibre optic is designed to trap light. This is achieved by a process called total internal reflection (TIR). It works by the fact that light in the fibre is at a very high angle, trapping light inside.

In the case of a fibre coated by the method of the present invention we want the light to escape into the surrounding solar cell. We achieve this by texturing the surface, using an acid etch process or sand blasting process, breaking the TIR surface. It may also be possible to achieve this in the fibre optic manufacturing process or by applying an optical coating that will control the light coming through the fibre.

The fibre can be coated with any thin film technology such as, but not exclusive to, amorphous silicon (a-Si) single/dual/triple junction, micromorph (a-Si/c-Si), thin-film crystalline (c-Si), Cadmium telluride (CdTe), Copper indium diselenide (CIS or CIGS) or organic cells.

The voltage may be generated by making individual cells down the length of the fibre monolithically interconnected, by offset spacing or laser scribing down the length. The generated voltage can, therefore, be determined by either the length of each cell or the length of the fibre. This allows the positive and negative connections to be made at each end of the fibre.

The generated current will depend on how many fibres are placed together. These may be connected either by pressure in a column or by laser/ultrasonically bonding (fusing) the fibres together at each end of the column. Anti-reflection coatings may be made on the end of the fibres where light enters and a reflective coating will normally be provided at the other end.

In order to minimise the heat load, each cell is preferably coated with a heat-absorbing material that will transfer heat out to a water cooling piping system, which would run through the column of cells.

Manufacture

The manufacturing process will be a reel-to-reel process and in volume production, this will be multi-reel to reel.

Surface preparation

The first process will involve texturing the fibre. For example, this may include the use of a sandblasting machine. The fibre will pass from reel to reel through a sandblasting machine, which has a continuous flow of sand blowing through the machine.

Transparent oxide layer

This layer acts as a conductor and needs to be transparent to let the light pass through into the solar cell. The method of application of this layer may be an Atmospheric Pressure Chemical Vapour Deposition process (APCVD). The material deposited will typically be Tin Oxide (SnO₂). An alternative method would be to use a sputtering process depositing Indium tin oxide (ITO₂) or Zinc oxide (ZnO₂).

In the case of the APCVD process, fibres will be fed into a SnO₂vapour cloud at 400 - 500 degrees Centigrade using a reel-to-reel process.

in the case of a sputtering process, the fibres will be fed into a vacuum system via a vacuum seal through a rotating sputtering target. This will apply to both ITO2& ZnO₂

Photovoltaic solar cell

The fibre can be coated using any thin film technology such as, but not exclusive to, amorphous silicon (a-Si:H) single/dual/triple junction, micro-morph(a-Si/c-Si), thin-film crystalline (c-Si;H), Cadmium telluride (CdTe), Copper indium diselenide (CIS or CIGS) or organic cells.

Any of these technologies may use a reel-to-reel process. For illustration only we will look at single junction amorphous (a-Si:H). In this case one can use a Plasma Enhanced Chemical Vapour Deposition process (PECVD) using 13.56 MHz or 40.68 MHz. The structure deposited will be a PIN (Positive-Intrinsic-Negative). The fibres will pass into the chamber via a vacuum seal. Chamber one will deposit the ‘P’ layer, chamber two the T layer and chamber three the ‘Ν’ layer. The fibre will pass to each chamber via a vacuum seal. The fibre will be heated to around 200° C. locally as it enters the vacuum system - and cooled as it exits the system. In production there will be multiple fibres passing through the system at any one time or/and looped between the electrodes.

Rear contact

The nature rear contact will depend on the technology applied on the fibre. For illustration only we will look at a-Si, which in this case will be applicable to single, dual and triple junction devices. The technology used in this case will be a sputtering system. This may be a magnetron, Microwave, ion beam or any other sputtering system. Again it may be a reel-to-reel process with the fibres passing through a seal into the vacuum system. The material deposited may be a Zinc Oxide/aiuminium mixture with the fibres passing either through or between targets made from this material.

Monolithic interconnection in order to generate the required voltage and reduce series resistance, the length of the fibre will be made up of a series of cells that will be monolithically interconnected. In simple terms, the top of one cell (Negative connection) will be connected to the bottom of the adjacent cell making a series connection.

This can be achieved by masking a line on the cell, for example but not limited to every 2 cm. (the length of each cell). The interconnection occurs by overlapping to the next cell. As an alternative to this, a laser may be positioned at the end of each process to scribe a line on the fibre. This may include using a galvanometer on the laser to scan round the fibre, or a ring optic may be used. The laser may be a double-yag laser operating at 532 nm. This process may be used for the front contact, solar cell deposition and the rear contact.

Protective coating

In order to protect the fibre after the deposition process and to help collect the heat from the fibre, a heat-absorbing film will preferably be applied. This can be achieved by running the fibre through a bath of the material, which may be an acrylic paint or resin, which can be instantly heat or UV cured. The top and bottom cell will remain exposed to allow the connection to be made.

Connection

To collect the power generated, the fibres will normally be bundled together. The amount of fibres that are bundled together will determine the current. As each fibre has been coated with a protective film apart from the first and last cell, they will not short out down the length of the fibre, which will typically be 1 meter in length, though it may be shorter or longer, depending on the voltage requirement and space available. It may well be that, when squeezed together at the top and bottom of the fibres, they will make contact without any further operations. However, it may be desirable to fuse all the fibres together at this point. This may be done using a conductive paste, or ultrasonic or laser bonding technology. A simple wire may then be wrapped round and through the positive and negative end of the column of coated fibres to allow a connection to be made.

Cooling heat collection system

It is desirable to keep the solar cells cool, as heat is a negative aspect. Cooling pipes may be placed within the column of fibres. In this way water can be passed between the fibres, extracting the heat.

Light collection

A focusing lens or lens system may be mounted at one end of the column of coated fibres with a mirror at the other end or the other end may be coated with a reflector.

Fibre design - Electro-luminescence

The fibre material can be of varied thickness and composition, but will preferably be optimised to ensure maximum light collection. The fibre material may also be doped with an electro-luminescence material to optimise it for the wavelength over which the solar cell operates and to enhance light scatter. It is to be understood that the term “fibres” as used above includes light tubes.

Light collection array

It is envisaged that this will be a simple array of solar collectors/mirrors that focus light onto a fibre optic bundle. The light may be focused into the bundle and then expanded out again using lenses to ensure even distribution into the bundle of coated fibres.