GB2456660A

GB2456660A - A solar cell

Info

Publication number: GB2456660A
Application number: GB0900091A
Authority: GB
Inventors: Keith Sloan
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-01-07
Filing date: 2009-01-05
Publication date: 2009-07-29
Anticipated expiration: 2029-01-05
Also published as: GB2456660B; GB0800229D0; GB0900091D0

Abstract

A solar cell arrangement 14 comprises first and second parabolic mirrors 32, 34 which collect light and focuses the light into a beam of light (16). The first parabolic mirror 32 has an aperture 37 through which the beam of light 16 passes. A prism means 18 receives the beam of light 16 and causes light 20 to emerge from the prism means 18. Optical means 22 for example, a diffraction grating or a plurality of convex lenses, receive the light 20 from the prism 18 and causes light to emerge from the optical means 22 as light 24 with different wavelengths. A semiconductor means 26 has a plurality of p-n junctions 28 which have energy levels at light peaks of the light 24 arriving at the surface of the p-n junctions 28.

Description

1 2456660

A SOLAR CELL

This invention relates to a solar cell.

A solar cell is made from a semiconductor material having a p-n junction. A p-type semiconductor is one where the semiconductor material has been doped with acceptor impurities. An n-type semiconductor is one where the semi-conductor material has been doped with donor impurities.

The p-n junction is where the p-type semiconductor material joins the n-type semiconductor material. The join forms a p-n junction. The p-n junction has an electric field and a potential energy barrier for electrons. When light enters the p-n junction, there is a possibility that a photon will use its energy to free an electron from the lattice at the p-n junction. The electron will then be subject to the electric field of the p-n junction, and will flow through the p-n junction. The energy of the phqton is given by the formula: h xf where h is Planks Constant, and f is the frequency of the light.

In order for the electron to be released, the energy of the photon must exceed the energy required to release the electron from the semiconductor lattice. The end result is that the energy of the photon is converted into electrical energy. The electrical energy is given by the formula: lxV where I is current, and V is voltage or potential energy.

Current (I) is dependent on the rate of photon absorption, that is the number of photons being converted into electrons and flowing across the p-n junction. The potential energy barrier (V) depends upon the base semiconductor material.

Currently, the best solar cells available only have an efficiency of about 25% in terms of changing the energy of light into electrical energy. It is an aim of the present invention to provide an improved solar cell with a greater efficiency.

Accordingly, in one non-limiting embodiment of the present invention there is provided a solar cell comprising: (i) a mirror arrangement which comprises first and second parabolic mirrors and which collects light and focuses the light into a beam of light, the first parabolic mirror having an aperture through which the beam of light passes; (ii) prism means which receives the beam of light and causes light to emerge from the prism means with a greater spectrum than in the beam of light; (iii) optical means which receives the light from the prism means and causes light to emerge from the optical means as light with different wavelengths; and (iv) semiconductor means comprising a plurality of p-n junctions of different semiconductor materials; and the solar cell being such that: (a) energy levels of the p-n junctions match energy of light arriving at the p-n junctions from the optical means; (b) the energy levels of the p-n junctions are at light peaks of the light arriving at the p-n junctions from the optical means; and (c) surfaces of the p-n junctions are directly exposed to the light arriving at the p-n junctions from the optical means.

The solar cell of the present invention is able to operate at improved efficiency compared with known solar cells by converting light into different wavelengths, which also corresponds to different energy levels En the semiconductor means. By matching the energy of light with the different semiconductor materials in the' semiconductor means, more energy is able to be transferred firstly due to the fact that there is befter absorption of photons which are converted into electrons and hence current, and secondly due to the fact that the different semiconductor materials in the semiconductor means provide different contact potentials, which increases the voltage. The first and second parabolic mirrors are advantageous if the solar cell is to track the path of the sun.

The solar cell may be one in which the optical means and the p-n junctions are at right angles.

The prism means may be a single prism. If desired the prism means may comprise two or more prisms.

The optical means may be a diffraction grating. The diffraction grating may be a variable diffraction grating. Alternatively the diffraction grating may be a non-variable diffraction grating.

The optical means may alternatively be a plurality of convex lenses.

The convex lenses are preferably arranged in a line.

The present invention also extends to a solar panel when comprising a plurality of the solar cells of the invention.

The solar panels may be used in a wide variety of situations where it is desired to convert solar energy into electrical energy. Thus, for example, the solar panels may be used a solar collectors for homes, commercial buildings, boats, yachts and caravans. The solar panels may also be used to supply power to roadside installations such for example as motorway telephones or road speed control signs.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 illustrates the operation of a typical known solar cell; Figure 2 shows a first solar cell of the present invention; Figure 3 shows a second solar cell of the present invention; and Figure 4 illustrates the operation of the semiconductor means shown in Figures 2 and 3.

Referring to Figure 1, there is shown a typical known solar cell 2 made out of a semiconductor device 4. The semiconductor device 4 comprises a semiconductor p-n junction 6. The p-n junction 6 is formed where a p-type semiconductor material 8 is in contact with an n-type semiconductor material 10. The join of the p-type semiconductor material 8 and the n-type semiconductor material 10 causes the formation of the p-n junction 6. The p-type semiconductor material 8 is formed by doping a semiconductor with acceptor impurities. The n-type semiconductor material 10 is formed by doping the semiconductor material with donor impurities.

The p-n junction 6 has an electric field and a potential energy barrier for electrons. When light enters the p-n junction 6, there is a possibility that a photon will use its energy to free an electron from the lattice in the junction.

The electron will then be subject to the electric field of the p-n junction 6, and the electron will flow through the p-n junction 6. In order for the electron to be released, the energy of the photon must exceed the energy required to release the electron from the semiconductor lattice. The end result is that the energy of the photon is converted into electrical energy.

Referring now to Figure 2, there is shown a solar cell 12 comprising a mirror arrangement 14 which collects light and focuses the light into a beam of light 16.

The solar cell 12 also comprises prism means 18 which receives the beam of light 16 and causes light 20 to emerge from the prism means 18 with a greater spectrum than in the beam of light 16.

The solar cell 12 further comprises optical means 22 which receives the light 20 from the prism means 18 and causes the light 20 to emerge from the optical means 22 as light 24 with separated wavelengths.

The solar cell 12 further comprises semiconductor means 26 comprising a plurality of p-n junctions 28 of different semiconductor materials.

The solar cell 12 is such that energy levels of the p-n junctions 28 match the energy of the light 24 arriving at the p-n junctions 28 from the optical means 22. The solar cell 12 is also such that the energy levels of the p-n junctions 28 are at light peaks of the light 24 arriving at the p-n junctions 28 from the optical means 22. The solar cell 12 is further such that surfaces of the p-n junctions 28 are directly exposed to the light 24 arriving at the p-n junctions 28 from the optical means 22.

The mirror arrangement 14 comprises a first mirror 32 and a second mirror 34. The first and the second mirrors 32, 34 are parabolic mirrors as shown. The first mirror 32 is positioned in front of the second mirror 34. The mirror 32, 34 are able to follow the path of the sun which provides incident light 36. The first mirror 32 has an aperture 37 through which the beam of light 36 passes. The aperture 37 is at the centre of the first mirror 32.

The prism means 18 is a single prism 38.

The optical means 22 is a variable diffraction grating 40. The light 24 emerging from the variable diffraction grating 40 has a waveform 42 as shown. It will be seen that the diffraction grating 40 produces light with a waveform having peaks and troughs. These peaks and troughs arrive at the p-n junctions 28 such that it is the peaks of the waveform 42 that strike the p-n junctions. Thus the conversion of the light energy into electrical energy is maximized.

Referring now to Figure 4, there is shown a solar cell 44 which is similar to the solar cell 12. Similar parts have been given the same reference numerals for ease of comparison and understanding.

In Figure 3, the optical means 22 in the form of the variable diffraction grating 40 has been replaced by optical means 22 in the form of a series of convex lenses 46. The convex lenses 46 are arranged in a line and they are shown focusing the light 24 on the p-n junctions 28. The convex lenses 46 are at right angles to the p-n junctions 28.

Figure 4 shows the p-n junctions 28 in more detail. More specifically, as shown in Figure 4, light 24 of different wavelengths is shown being concentrated at p-n junctions 28. The p-n junctions 28 are shown as being formed by the joining of p-type semiconductor material 30 and n-type semiconductor material 32. The different semiconductor materials are shown as different semiconductor materials 48, 50, 52, 54, 56. These different semiconductor materials 48, 50, 52, 54, 56 are separated by metal layers 58 as shown. The p-n junctions 28 of the different semiconductor materials 48, 50, 52, 54, 56 are such that the energy levels at the p-n junctions 28 match the light energy of the incoming light 24. The surfaces of the p-n junctions 28 are directly exposed to the light 24. This has a benefit over a standard solar cell in that photons do not have to travel through a layer of semiconductor before they reach a junction. If the light photon has enough energy to release electrons, then the light photon will contribute to the electrical energy of the solar cell.

The energy of incoming photons of light is h x f as described above.

The solar cells of the present invention match this energy with the energy of the base semiconductor material. As one photon can best release one electron, then the current (I) that the cell produces is related to the rate of arrival of the photons. The voltage (V) that the solar cell exhibits is directly related to the base semiconductor material. By matching the energy of the light (h x f) with the electrical energy of the semiconductor (V x I) the maximum amount of energy is converted into electrical energy and is not wasted as heat. At the red end of the spectrum, the light has less energy and the semiconductor used preferably has a low energy level to release electrons and hence a lower voltage. At the blue and ultraviolet end of the spectrum, the semiconductor used preferably has a higher energy level to release an electron and therefore a higher voltage. By matching the energy levels of photons to the base semiconductor materials, the maximum voltage for any one frequency of light is able to be obtained. Also, the current is maximised by the fact the electrons are more likely to be released if the energy of the light matches the energy required to release the electrons.

it is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Thus, for example, the prism means 18 could be made of more than one prism. The variable diffraction grating 40 could be a non-variable diffraction grating.

Claims

A solar cell comprising: (i) a mirror arrangement which comprises first and second parabolic mirrors and which collects light and focuses the light into a beam of light, the first parabolic mirror having an aperture through which the beam of light passes; (ii) prism means which receives the beam of light and causes light to emerge from the prism means with a greater spectrum than in the beam of light; (iii) optical means which receives the light from the prism means and causes light to emerge from the optical means as light with different wavelengths: and (iv) semiconductor means comprising a plurality of p-n junctions of different semiconductor materials; and the solar cell being such that: (a) energy levels of the p-n junctions match energy of light arriving at the p-n junctions from the optical means; (b) the energy levels of the p-n junctions are at light peaks of the light arriving at the p-n junctions from the optical means; and (C) surfaces of the p-n junctions are directly exposed to the right arriving at the p-n junctions from the optical means.
2. A solar cell according to claim 1 in which the optical means and the p-n junctions are at right angles.
3. A solar cell according to claim I or claim 2 in which the prism means is a single prism.
4. A solar cell according to any one of the preceding claims in which the optical means is a diffraction grating.
5. A solar cell according to claim 4 in which the diffraction grating is a variable diffraction grating.
6. A solar cell according to claim 4 in which the diffraction grating is a non-variable diffraction grating.
7. A solar cell according to any one of claims 1 -3 in which the optical means is a plurality of convex lenses.
8. A solar cell according to claim 7 in which the convex lenses are arranged in a line.
9. A solar cell substantially as herein described with reference to Figures 2, 3 and 4 of the accompanying drawings.
10. A solar panel comprising a plurality of solar cells according to any one of the preceding claims.